TWI646218B - Hot-dip galvanized steel sheet - Google Patents

Abstract

In the range of 1/8 thickness to 3/8 thickness centered on the surface of the base material steel plate with a thickness of 1/4 of the thickness of the base material, the ferrite grain iron phase is 50% or more and 97% or less by volume fraction, and the toughened iron is used. The sum of the hard structures composed of one or more of the phase, the toughened ferrite phase, the new Matian iron phase and the tempered Matian iron phase is 3% or more by volume fraction, and the remaining Worthite iron phase The volume fraction is 0% to 8%, and the total of the ferrite phase and the coarse snowy carbon phase is 0% to 8% by volume fraction, and has an average at the interface between the hot-dip galvanized layer and the parent metal plate. a Fe-Al alloy layer having a thickness of 0.1 μm to 2.0 μm, and a difference between a maximum thickness and a minimum thickness in a width direction of the steel sheet of 0.5 μm or less; and a micronized layer in direct contact with the Fe-Al alloy layer The difference between the maximum thickness and the minimum thickness of the micronized layer in the width direction of the steel sheet is within 2.0 μm.

Description

Hot-dip galvanized steel sheet

The present invention relates to a high-strength hot-dip galvanized steel sheet which is excellent in plating adhesion at the time of ductility, bendability, hole expandability, and bending deformation, and is excellent in continuous bending fatigue resistance.

Background of the Invention High-strength has been demanded for steel sheets mainly used for automobile frame members. In order to obtain high strength and excellent formability, these high-strength steel sheets generally contain alloying elements represented by Si and Mn which contribute to strength improvement. However, alloying elements such as Si and Mn also reduce plating adhesion. Moreover, since the steel sheets for automobiles are generally used outdoors, they are usually required to have excellent corrosion resistance.

Regarding the use of the outer panel of the automobile, the peripheral portion of the panel is subjected to severe bending processing (bending and bending) by press working. Moreover, not only the outer panel of the automobile but also other applications are often subjected to severe bending or reaming processing by press working. However, when the conventional hot-dip galvanized steel sheet is subjected to severe bending processing or hole expanding processing, the plating portion may be peeled off from the base material steel sheet in the processed portion. In this case, if the plating layer is peeled off, the corrosion resistance is lost at this point, and there is a problem that the base material steel plate is corroded and rusted from the early stage. Even if the plating layer is not peeled off, the adhesion between the plating layer and the base material steel plate may be lost, resulting in some micro-voids in the portion, causing external air or moisture to intrude from the space, causing the plating layer to lose the corrosion prevention function. . As a result, the base metal sheet is corroded and rusted from the early stage. Based on these problems, it is strongly desired to use a plated steel sheet having a hot-dip galvanized layer having a plating layer with excellent adhesion to the base material steel sheet, which is used for a high-strength steel sheet which is subjected to severe bending work or the like.

In order to improve the adhesion of the plating layer, for example, as disclosed in Patent Documents 1 to 3, it has been proposed to form an oxide inside the steel sheet to reduce oxides at the interface between the base iron and the plating layer which cause plating peeling. The method. However, when the oxide is formed on the surface layer of the steel sheet, the carbon of the surface layer of the steel sheet is combined with oxygen to be gasified. As a result, the carbon is detached from the steel sheet, so that the region where the carbon is detached has a situation in which the strength is remarkably lowered. When the strength of the surface layer of the steel sheet is lowered, the fatigue resistance characteristic which is strongly influenced by the characteristics of the surface layer portion is deteriorated, and the fatigue strength is greatly lowered.

Or in order to improve the adhesion of the plating layer, Patent Document 4 proposes a new annealing step and a pickling step to be performed before the general annealing step to modify the surface of the base metal sheet to improve the plating adhesion. The method of sex. However, the method described in Patent Document 4 requires an additional step for the production method of a general high-strength plated steel sheet, and has a problem in terms of cost.

Further, Patent Document 5 proposes a method of removing carbon from the surface layer portion of the base material steel sheet to improve plating adhesion. However, the method described in Patent Document 5 significantly reduces the strength of the carbon-removed region. Therefore, in the method described in Patent Document 5, the fatigue resistance which is strongly influenced by the characteristics of the surface layer portion is deteriorated, and the fatigue strength is largely lowered.

Further, in Patent Documents 6 and 7, a steel sheet in which the amount of Mn, Al, and Si in the plating layer is controlled within an appropriate range to improve plating adhesion is proposed. However, in the steel sheets described in Patent Documents 6 and 7, it is necessary to control the amount of the elements in the plating layer with high precision at the time of manufacture, which causes an increase in work load and a cost problem.

Regarding the method of improving the adhesion of plating, Patent Document 8 proposes a high-strength steel sheet in which the microstructure of the steel sheet is composed only of ferrite. However, the steel sheet described in Patent Document 8 has a microstructure which is only soft and ferrite, so that a sufficiently high strength cannot be obtained.

Nowadays, alloyed hot-dip galvanized steel sheets subjected to alloying treatment after hot-dip galvanizing treatment are widely used. The alloying treatment is a treatment in which a plating layer is heated to a temperature higher than a melting point of Zn, a large amount of Fe atoms are diffused from a base material steel sheet into a plating layer, and a plating layer is formed into a layer of a Zn-Fe alloy body. For example, Patent Documents 9, 10, and 11 propose an alloyed hot-dip galvanized steel sheet excellent in plating adhesion. However, in order to sufficiently alloy the plating layer, it is necessary to heat the steel sheet to a high temperature. When the steel sheet is heated to a high temperature, the microstructure inside the steel sheet is deteriorated, and in particular, coarse iron-based carbides are easily formed to impair the characteristics of the steel sheet, which is not preferable.

On the other hand, for example, the hot-dip galvanized steel sheet described in Patent Document 12 has a problem that the plating layer has uneven appearance due to uneven Fe content in the width direction.

CITATION LIST Patent Literature Patent Literature 1: Japanese Laid-Open Patent Publication No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. Japanese Patent Laid-Open Publication No. JP-A-2003-096751, Patent Document No. JP-A-2003-096751, Patent Document No. JP-A-2003-096541, Patent Document 8: JP-A-2005-200750 Japanese Patent Laid-Open Publication No. Hei No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei. No. Hei.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention In view of the above circumstances, the present invention provides an excellent formability of a steel sheet which is represented by ductility, flexibility, and stretch flange formability, and uniformity of appearance, fatigue resistance, and fusion of plating. High-strength hot-dip galvanized steel sheet excellent in properties, corrosion resistance and plating adhesion.

Means for Solving the Problems The inventors of the present invention have obtained excellent formability, ductility, and stretch flange formability, and have fatigue resistance, weldability, corrosion resistance, and plating adhesion. Excellent high-strength galvanized steel sheet and active efforts to review it. As a result, the present inventors improved ductility and hole expandability (extension flangeability) by controlling the microstructure of the steel sheet to an appropriate tissue fraction. Further, the inventors of the present invention improved the flexibility and the fatigue resistance by controlling the volume fraction of the hard phase on the side of the base material steel plate from the boundary between the plating layer and the base material steel sheet. Further, the present inventors have found that even when a steel sheet containing a large amount of Si and Mn is used as a plating original plate, it is formed directly under the Fe-Al alloy layer formed at the interface between the plating layer and the base material steel sheet. The specific micronized layer composed of the extremely fine particles of the ferrite-grained iron phase suppresses generation and propagation of cracks during processing, and can suppress plating peeling from the starting point. In addition, by controlling the thickness of the fine layer and the Fe-Al alloy layer in the width direction of the steel sheet within a specific range, it is possible to obtain a hot-dip galvanized steel sheet which is excellent not only in plating adhesion but also in excellent appearance uniformity.

The present invention has been completed in view of the above findings, and its state is as follows.

(1) A hot-dip galvanized steel sheet having a hot-dip galvanized layer on at least one side of a base material steel sheet, and the hot-dip galvanized steel sheet is characterized in that the base material steel sheet has the following chemical composition: % by mass, C: 0.040 %～0.280%, Si:0.05%～2.00%, Mn:0.50%～3.50%, P:0.0001%～0.1000%, S:0.0001%～0.0100%, Al:0.001%～1.500%, N:0.0001%～ 0.0100%, O: 0.0001% to 0.0100%, Ti: 0% to 0.150%, Nb: 0% to 0.100%, V: 0% to 0.300%, Cr: 0% to 2.00%, Ni: 0% to 2.00% Cu: 0% to 2.00%, Mo: 0% to 2.00%, B: 0% to 0.0100%, W: 0% to 2.00%, and Ca, Ce, Mg, Zr, La, and REM: 0% in total 0.0100%, and the remainder is composed of Fe and impurities; and has the following microstructure: 1/8 thickness ～3 centering on the surface of the base material steel sheet with a thickness of 1/4 of the total thickness of the base material steel sheet In the thickness range of /8, the ferrite iron phase is 50% or more and 97% or less in volume fraction, and is composed of a toughened iron phase, a toughened ferrite phase, a new Matian iron phase, and a tempered Matian iron phase. Total of hard tissues composed of one or more types The volume fraction is 3% or more, and the residual Worthfield iron phase is 0% to 8% by volume fraction, and the total of the Borne iron phase and the coarse snowy carbon iron phase is 0% by volume fraction. 8%, in the surface layer portion from the interface between the hot-dip galvanized layer and the base material steel sheet to the depth of the steel sheet of 20 μm, the remaining Worthite iron is 0% to 3% by volume fraction, and the surface layer portion is the aforementioned The ratio of the volume fraction V1 of the hard structure to the volume fraction V2 of the hard structure in the range of 1/8 thickness to 3/8 thickness centering on the 1/4 thickness of the steel sheet surface, and V1/V2 is set to 0.10 or more. And a range of 0.90 or less; the Fe content in the hot-dip galvanized layer is greater than 0% and less than 3.0%, and the Al content is greater than 0% and less than 1.0%, and has an average thickness at the interface between the hot-dip galvanized layer and the parent metal steel sheet. It is an Fe-Al alloy layer of 0.1 μm to 2.0 μm, and the difference between the maximum thickness and the minimum thickness in the width direction of the steel sheet is within 0.5 μm, and the base material steel sheet has fine contact with the Fe-Al alloy layer directly. The average thickness of the micronized layer is 0.1 μm to 5.0 μm, and the ferrite phase in the micronized layer The average particle diameter is 0.1 μm to 3.0 μm, and the fine layer has one or two or more kinds of oxides of Si and Mn, and the maximum diameter of the oxide is 0.01 μm to 0.4 μm in the width direction of the steel sheet. The difference between the maximum thickness and the minimum thickness of the aforementioned micronized layer is within 2.0 μm. (2) The hot-dip galvanized steel sheet according to (1), wherein the single-sided plating adhesion amount of the hot-dip galvanized layer is 10 g/m ² or more and 100 g/m ² or less. (3) The hot-dip galvanized steel sheet according to (1) or (2), wherein the base material steel sheet further contains, in mass%, from Ti: 0.001% to 0.150%, Nb: 0.001% to 0.100%, and V : 0.001% to 0.300% of one or more of the groups formed. (4) The hot-dip galvanized steel sheet according to any one of (1) to (3), wherein the base material steel sheet further contains, in mass%, from: 0.01% to 2.00%, and Ni: 0.01% to 2.00. %, Cu: 0.01% to 2.00%, Mo: 0.01% to 2.00%, B: 0.0001% to 0.0100%, and W: 0.01% to 2.00% of one or more of the groups. (5) The hot-dip galvanized steel sheet according to any one of (1) to (4), wherein the base material steel sheet further contains, in mass%, from 0.0001% to 0.0100%, selected from the group consisting of Ca, Ce, Mg, and Zr. One or two or more of the groups consisting of La and REM.

Advantageous Effects of Invention According to the aspect of the invention, it is possible to provide a hot-dip galvanized steel sheet which is excellent in appearance uniformity, moldability, fatigue resistance, weldability, corrosion resistance and plating adhesion of plating.

EMBODIMENT OF THE INVENTION The hot-dip galvanized steel sheet according to the embodiment of the present invention is formed by forming a hot-dip galvanized layer (hereinafter, simply referred to as a plating layer) on the surface of a base material steel sheet (hereinafter, simply referred to as a steel sheet) having the following chemical composition. The chemical composition of the base material steel plate contains C: 0.040% to 0.280%, Si: 0.01% to 2.00%, Mn: 0.50% to 3.50%, P: 0.0001% to 0.100%, and S: 0.0001% to 0.0100, by mass%. %, Al: 0.001% to 1.500%, O: 0.0001% to 0.0100%, N: 0.0001% to 0.0100%, and the remainder is composed of Fe and impurities. Further, the thickness of the base material steel sheet is preferably 0.6 mm or more and less than 5.0 mm. When the thickness of the base material steel sheet is less than 0.6 mm, it is difficult to keep the shape of the base material steel sheet flat and unsuitable. In addition, when the thickness of the base material steel sheet is 5.0 mm or more, it is difficult to control the cooling during the production process, and the predetermined microstructure may not be obtained, and the formability may be deteriorated. The Fe content of the plating layer is more than 0% and less than 3.0%, and the Al content is more than 0% and less than 1.0%.

First, the chemical composition (composition) of the base material steel sheet constituting the hot-dip galvanized steel sheet according to the embodiment of the present invention will be described. In addition, [%] in the following description means [% by mass] unless otherwise specified.

[C: 0.040% to 0.280%] The C system is contained to improve the strength of the base material steel plate. However, if the C content is more than 0.280%, the spot weldability may be deteriorated, so that the C content is 0.280% or less. Further, from the viewpoint of spot weldability, the C content is preferably 0.250% or less, more preferably 0.220% or less. On the other hand, when the C content is less than 0.040%, the strength is lowered, and it is difficult to ensure sufficient tensile maximum strength, so the C content is made 0.040% or more. Further, in order to obtain higher strength, the C content is preferably 0.055% or more, more preferably 0.070% or more.

[Si: 0.05% to 2.00%] The Si system can suppress the formation of iron-based carbides in the base material steel plate and improve the strength and formability. However, Si is also an element which causes the steel plate to be embrittled. Therefore, when the Si content is more than 2.00%, problems such as cracking of the cast steel are likely to occur. Therefore, the Si content is made 2.00% or less. Further, Si forms an oxide on the surface of the base material steel sheet during the annealing step, which significantly impairs the adhesion of the plating. From this point of view, the Si content is preferably 1.500% or less, more preferably 1.200% or less. On the other hand, when the Si content is less than 0.05%, coarse iron-based carbides are formed in a large amount in the plating step of the hot-dip galvanized steel sheet, and the strength and moldability are deteriorated. Therefore, the Si content is 0.05% or more. From the viewpoint of suppressing the formation of iron-based carbides, the Si content is preferably 0.10% or more, more preferably 0.25% or more.

[Mn: 0.50% to 3.50%] Mn can be contained in order to increase the strength by improving the hardenability of the base material steel sheet. However, when the Mn content is more than 3.50%, a coarse Mn-concentrated portion is formed in the center portion of the base material of the base material steel sheet, which is liable to cause embrittlement, and is liable to cause problems such as cracking of the cast steel. Therefore, the Mn content is set to 3.50% or less. Further, if the content of Mn is increased, the spot weldability of the hot-dip galvanized steel sheet is deteriorated. From this point of view, the Mn content is preferably 3.00% or less, more preferably 2.80% or less. On the other hand, when the Mn content is less than 0.50%, a large amount of soft structure is formed during cooling after annealing, and it is difficult to ensure a sufficiently high tensile maximum strength. Therefore, the Mn content must be 0.50% or more. In order to further increase the strength of the hot-dip galvanized steel sheet, the Mn content is preferably 0.80% or more, more preferably 1.00% or more.

[P: 0.0001% to 0.1000%] P is an element which causes embrittlement of steel, and when the P content is more than 0.1000%, it is likely to cause cracking of steel blanks during casting, cracking of steel blanks during rolling, etc. The problem is therefore that the P content is 0.1000% or less. Further, P is an element which embrittles the molten portion which is formed by welding the transmission point. Therefore, in order to obtain sufficient fusion bonding strength, the P content is preferably 0.0400% or less, more preferably 0.0200% or less. On the other hand, when the P content is less than 0.0001%, the manufacturing cost is greatly increased. Therefore, the lower limit of the P content is 0.0001%, preferably 0.0010% or more.

[S: 0.0001% to 0.0100%] The S system combines with Mn to form coarse MnS, which reduces the formability such as ductility, hole expandability (extension flangeability) and flexibility. Therefore, the S content is 0.0100% or less. . Further, S is an element which deteriorates the spot weldability. Therefore, the S content is preferably set to 0.0060% or less, more preferably 0.0035% or less. On the other hand, when the S content is less than 0.0001%, the manufacturing cost is greatly increased. Therefore, the lower limit of the S content is 0.0001%, preferably 0.0005% or more, and more preferably 0.0010% or more.

[Al: 0.001% to 1.500%] Al is an element that makes the steel brittle. When the Al content is more than 1.500%, problems such as cracking of the cast steel preform are liable to occur, so that the Al content is 1.500% or less. Further, if the Al content is increased, the spot weldability is deteriorated, so the Al content is preferably 1.200% or less, more preferably 1.000% or less. On the other hand, the effect of the present embodiment can be exhibited without particularly limiting the lower limit of the Al content. On the other hand, Al is a trace amount of impurities present in the raw material, and when the content is less than 0.001%, the manufacturing cost is greatly increased. Therefore, the Al content is made 0.001% or more. Further, Al is also an element which can be effectively used as a deoxidizing material, and in order to more fully obtain a deoxidizing effect, the Al content is preferably set to 0.010% or more.

[N: 0.0001% to 0.0100%] N-forms form coarse nitrides, and elements which deteriorate formability such as ductility, hole expandability (extension flangeability) and flexibility are necessary to suppress the content thereof. When the N content is more than 0.0100%, the formability is remarkably deteriorated, so the upper limit of the N content is made 0.0100%. Further, when N is excessively contained, pores are generated at the time of welding, so the content thereof is preferably small. From these viewpoints, the N content is preferably 0.0070% or less, more preferably 0.0050% or less. On the other hand, the effect of the present embodiment can be exhibited without particularly limiting the lower limit of the N content. However, if the N content is less than 0.0001%, the manufacturing cost is greatly increased. Based on this, the lower limit of the N content is made 0.0001% or more. The N content is preferably 0.0003% or more, more preferably 0.0005% or more.

[O: 0.0001% to 0.0100%] O forms an oxide and deteriorates the formability such as ductility, hole expandability (stretch flangeability) and flexibility of the hot-dip galvanized steel sheet. Therefore, it is necessary to suppress the O content. When the O content is more than 0.0100%, the formability is remarkably deteriorated, so the upper limit of the O content is set to 0.0100%. Further, the O content is preferably 0.0050% or less, more preferably 0.0030% or less. The effect of the present embodiment can be exhibited without particularly limiting the lower limit of the O content. However, when the O content is less than 0.0001%, the manufacturing cost is greatly increased, so the lower limit is made 0.0001%. Further, the O content is preferably 0.0003% or more, more preferably 0.0005% or more.

Further, the base material steel sheet of the hot-dip galvanized steel sheet according to the present embodiment may contain the following elements as needed.

First, the base material steel sheet of the present embodiment may further contain one selected from the group consisting of Ti: 0.001% to 0.150%, Nb: 0.001% to 0.100%, and V: 0.001% to 0.300%. 2 or more types.

[Ti: 0.001% to 0.150%] The Ti-based system can enhance the growth of the galvanized steel sheet by strengthening the precipitates, suppressing the growth of the ferrite grains, and strengthening the fine particles and inhibiting the recrystallization. The element of strength. However, when the Ti content is more than 0.150%, the precipitation of carbonitrides is increased, and the formability is deteriorated. Therefore, the Ti content is made 0.150% or less. Further, from the viewpoint of moldability, the Ti content is preferably 0.080% or less. On the other hand, the effect of the present embodiment can be exhibited without particularly limiting the lower limit of the Ti content. However, in order to sufficiently obtain the effect of improving the strength by using Ti, the Ti content is preferably 0.001% or more. In order to increase the strength of the hot-dip galvanized steel sheet, the Ti content is more preferably 0.010% or more.

[Nb: 0.001% to 0.100%] Nb can enhance the precipitation of ferrite grains by inhibiting the growth of precipitates and suppressing the growth of ferrite grains, and inhibiting the recrystallization of the grains. The element of strength. However, when the Nb content is more than 0.100%, the precipitation of carbonitrides is increased, and the formability of the hot-dip galvanized steel sheet is deteriorated. Therefore, the Nb content is made 0.100% or less. Further, from the viewpoint of moldability, the Nb content is preferably 0.060% or less. On the other hand, the effect of the present embodiment can be exhibited without particularly limiting the lower limit of the Nb content. However, in order to sufficiently obtain the effect of improving the strength by using Nb, the Nb content is preferably 0.001% or more. In order to increase the strength of the hot-dip galvanized steel sheet, the Nb content is more preferably 0.005% or more.

[V: 0.001% to 0.300%] The V-based system can enhance the growth of the galvanized steel sheet by strengthening the precipitates, suppressing the growth of the ferrite grains, and strengthening the fine particles and inhibiting the recrystallization. The element of strength. However, when the V content is more than 0.300%, precipitation of carbonitrides is increased, and formability is deteriorated. Therefore, the V content is set to 0.300% or less. And the V content is preferably 0.200% or less. On the other hand, the effect of the present embodiment can be exhibited without particularly limiting the lower limit of the V content. In order to sufficiently obtain the effect of improving the strength by using V, the V content is preferably 0.001% or more, more preferably 0.010% or more.

Further, the base material steel sheet according to the present embodiment may further contain, from Cr: 0.01% to 2.00%, Ni: 0.01% to 2.00%, Cu: 0.01% to 2.00%, and Mo: 0.01% to 2.00%, B. One or two or more of the group consisting of 0.0001% to 0.0100% and W: 0.01% to 2.00%.

[Cr: 0.01% to 2.00%] Cr is an element which is effective for suppressing the phase transformation at a high temperature and is effective for increasing the strength of the hot-dip galvanized steel sheet, and may be contained in place of a part of C and/or Mn. However, when the Cr content is more than 2.00%, the workability in the heat treatment is impaired and the productivity is lowered. Therefore, the Cr content is made 2.00% or less. And the Cr content is preferably 1.20% or less. On the other hand, the effect of the present embodiment is not particularly limited as long as the lower limit of the Cr content is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by using Cr, the Cr content is preferably 0.01% or more, and more preferably 0.10% or more.

[Ni: 0.01% to 2.00%] The Ni-based element which suppresses the phase transformation at a high temperature and is effective for increasing the strength of the hot-dip galvanized steel sheet may be contained in place of a part of C and/or Mn. However, when the Ni content is more than 2.00%, the weldability is impaired, so that the Ni content is 2.00% or less. And the Ni content is preferably 1.20% or less. On the other hand, the effect of the present embodiment is not particularly limited, and the effect of the present embodiment is not particularly limited. However, in order to sufficiently obtain the effect of increasing the strength by using Ni, the Ni content is preferably 0.01% or more, and more preferably 0.10% or more.

[Cu: 0.01% to 2.00%] Cu may be used as an element in which fine particles are present in steel to increase the strength of the hot-dip galvanized steel sheet, and may be contained in place of a part of C and/or Mn. However, when the Cu content is more than 2.00%, the weldability is impaired, so that the Cu content is 2.00% or less. And the Cu content is preferably 1.20% or less. The effect of the present embodiment is not particularly limited, and the effect of the present embodiment can be exhibited. However, in order to sufficiently obtain the effect of improving the strength of the hot-dip galvanized steel sheet by using Cu, the Cu content is preferably 0.01% or more, and more preferably 0.10% or more.

[Mo: 0.01% to 2.00%] Mo is an element which suppresses the phase transformation at a high temperature and is effective for increasing the strength of the hot-dip galvanized steel sheet. Therefore, it may be substituted for a part of C and/or Mn. . However, when the Mo content is more than 2.00%, the workability in the heat treatment is impaired and the productivity is lowered. Therefore, the Mo content is 2.00% or less. And the Mo content is preferably 1.20% or less. On the other hand, the effect of the present embodiment is not particularly limited, and the effect of the present embodiment is not particularly limited. However, in order to sufficiently obtain the effect of increasing the strength by Mo, the Mo content is preferably 0.01% or more, and more preferably 0.05% or more.

[B: 0.0001% to 0.0100%] B is an element which suppresses the phase transformation at a high temperature and is effective for increasing the strength of the hot-dip galvanized steel sheet. Therefore, it may be substituted for a part of C and/or Mn. . However, when the B content is more than 0.0100%, the workability in the heat treatment is impaired and the productivity is lowered. Therefore, the B content is made 0.0100% or less. From the viewpoint of productivity, the B content is preferably 0.0050% or less. On the other hand, the effect of the present embodiment is not particularly limited as long as the lower limit of the B content is exhibited. However, in order to sufficiently obtain the effect of increasing the strength by B, the B content is preferably made 0.0001% or more. Further, in order to further increase the strength of the hot-dip galvanized steel sheet, the B content is more preferably 0.0005% or more.

[W: 0.01% to 2.00%] W is an element which suppresses the phase transformation at a high temperature and is effective for increasing the strength of the hot-dip galvanized steel sheet. Therefore, it may be substituted for a part of C and/or Mn. . However, when the W content is more than 2.00%, the workability in the heat treatment is impaired and the productivity is lowered. Therefore, the W content is made 2.00% or less. Further, the W content is preferably set to 1.20% or less. On the other hand, the effect of the present embodiment is not particularly limited, and the effect of the present embodiment is not particularly limited. However, in order to sufficiently obtain the effect of increasing the strength by using W, the W content is preferably 0.01% or more, and more preferably 0.10% or more.

Further, the base material steel sheet of the hot-dip galvanized steel sheet according to the present embodiment may contain a total of 0.0001% to 0.0100% of one or two selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM. Above as other elements. The reasons for containing these elements are as follows. Among them, REM is the abbreviation of Rare Earth Metal, which refers to the elements of the genus. In the embodiment of the present invention, REM and Ce are mostly contained in a rare earth metal alloy, and in addition to La and Ce, a lanthanoid compound is contained in combination. The effect of the present embodiment can also be exhibited by containing the lanthanoid elements other than such La or Ce as impurities. Further, the effect of the present embodiment can be exhibited by containing the metal La or Ce.

Ca, Ce, Mg, Zr, La, and REM are elements which can effectively improve the formability of the hot-dip galvanized steel sheet, and therefore may be contained in one type or two or more types. However, when the total content of one or more selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM is more than 0.0100%, the ductility is impaired, so that the total content of each element is set. It is 0.0100% or less. Further, the total content of the respective elements is preferably 0.0070% or less. On the other hand, the lower limit of the content of one or more selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM is not particularly limited, and the effect of the present embodiment can be exhibited. The effect of improving the formability of the hot-dip galvanized steel sheet is preferably 0.0001% or more in total of the respective elements. In view of the formability, the total content of one or more selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM is preferably 0.0010% or more.

In the chemical composition of the hot-dip galvanized steel sheet according to the present embodiment, the remainder of each element described above is Fe and impurities. Further, the above Ti, Nb, V, Cr, Ni, Cu, Mo, B, and W are all allowed to be contained as impurities in a trace amount lower than each of the above lower limits. Further, Ca, Ce, Mg, Zr, La, and REM are also allowed to be contained as impurities in a very small amount below the lower limit of the total amount thereof.

The reason for the base material steel sheet structure of the hot-dip galvanized steel sheet according to the embodiment of the present invention is as follows.

(Microstructure) The microstructure of the base material steel sheet of the hot-dip galvanized steel sheet according to the embodiment of the present invention will be described. The characteristics of the steel sheet change with the microstructure. When the microstructure is to be quantified, it is impossible to quantify the entire area of the steel sheet in reality. Therefore, the present invention is defined by quantifying a microstructure representative of a steel material, that is, a microstructure having a thickness of 1/8 to 3/8 centering on the surface of the base material steel sheet at a thickness of 1/4. The microstructure at the center of the plate thickness changes due to strong solidification segregation and cannot be called a microstructure representative of the steel sheet. The microstructure of the portion near the surface of the steel sheet may change due to local temperature changes or reaction with external gases, and therefore cannot be referred to as a microstructure representative of the steel sheet. The microstructure of the base material steel sheet of the hot-dip galvanized steel sheet according to the embodiment of the present invention is in the range of 1/8 thickness to 3/8 thickness centering on the surface of the base material steel sheet, and the ferrite grain phase (hereinafter referred to as fat) Granular iron) is 40% or more and 97% or less by volume fraction, and is composed of a toughened iron phase (hereinafter referred to as toughened iron), a toughened ferrite iron phase (hereinafter referred to as a toughened ferrite iron), and a new generation of Matian iron. The total number of hard structures consisting of one or more of the phase (hereinafter referred to as the nascent granulated iron) and the tempered granulated iron phase (hereinafter referred to as the tempered granulated iron) is 3% or more in terms of volume fraction, and the remaining Worthfield The iron phase (hereinafter referred to as residual Worthite iron) is 0% to 8% (including 0%) in terms of volume fraction, and the Borne iron phase (hereinafter referred to as Borne iron) and the coarse snowy carbon iron phase (hereinafter referred to as snow) The total amount of carbon iron is 0% to 8% (including 0%) in terms of volume fraction.

"Fat Grain Iron" A fermented iron system with excellent ductility. However, since the ferrite iron is soft, the strength is low, and if the volume fraction of the ferrite is more than 97%, the molten galvanized steel sheet having the maximum tensile strength can not be obtained. Therefore, the volume fraction of the ferrite iron is set to be 97% or less. In order to increase the maximum tensile strength of the hot-dip galvanized steel sheet, the volume fraction of the ferrite iron should be 92% or less, and more preferably 85% or less. Further, in order to obtain a hot-dip galvanized steel sheet having a tensile maximum strength of more than 950 MPa, the volume fraction of the ferrite iron is preferably 80% or less, and more preferably 70% or less. On the other hand, if the volume fraction of the ferrite iron is less than 50%, sufficient ductility cannot be obtained, so the volume fraction of the ferrite iron is set to 50% or more. The volume fraction of ferrite iron should be set to 55% or more, and more preferably 60% or more.

"Residual Vostian Iron" Residual Worth Iron is a structure that enhances the strength-ductility balance of hot-dip galvanized steel sheets. On the other hand, the residual Worthite iron is deformed into a hard granulated loose iron, which has an influence on the starting point of the fracture, and the stretch flangeability is deteriorated. Therefore, the upper limit of the volume fraction of the remaining Worthite iron is 8%. From the viewpoint of moldability of the hot-dip galvanized steel sheet, the volume fraction of the remaining Worth iron is preferably small, and is preferably 5% or less, and more preferably 0% to 3% (including 0%). The volume fraction of the residual Worthite iron in the hot-dip galvanized steel sheet should be as small as possible, and 0% is also no problem.

"Hard tissue" In order to increase the maximum tensile strength of the hot-dip galvanized steel sheet, the volume fraction of the hard structure composed of one or more of the toughened iron, the toughened ferrite iron, the new Matian loose iron, and the tempered Matian loose iron The total ratio must be set to 3% or more. In order to increase the maximum tensile strength of the hot-dip galvanized steel sheet, the volume fraction of the hard structure is preferably 7% or more, and more preferably 15% or more. On the other hand, if the volume fraction of the hard structure is excessively increased, the ductility of the hot-dip galvanized steel sheet is deteriorated, so that the volume fraction of the hard structure is limited to 60% or less. From this point of view, the volume fraction of the hard tissue is preferably set to 55% or less, more preferably 50% or less.

"Toughened ferrite iron and toughened iron" The structure of the toughened ferrite and the toughened iron-based hot-dip galvanized steel sheet with excellent balance between strength and formability, and the steel sheet structure may contain 60% or less by volume fraction. Toughened ferrite iron and / or toughened iron. Moreover, the toughened ferrite iron and the toughened iron system have a strength in the middle of the soft ferrite iron and the hard Ma Tian loose iron, the tempered Ma Tian loose iron and the residual Worth iron, and the stretch flanged The viewpoint is more preferably 5% or more, and more preferably 10% or more. On the other hand, if the volume fraction of the toughened ferrite iron and/or the toughened iron exceeds 60%, the lodging stress is excessively increased, and the shape freezing property is deteriorated, which is not preferable.

"Returning to Ma Tian loose iron" The tempering Ma Tian loose iron system can greatly increase the tensile strength of the hot-dip galvanized steel sheet, and the steel sheet structure may also contain 60% or less by volume fraction. From the viewpoint of tensile strength, the volume fraction of the tempered granulated iron should be set to 5% or more. On the other hand, if the volume fraction of the tempered iron in the steel sheet structure exceeds 60%, the stress of the undulation will increase excessively, and the shape freezing property may be deteriorated.

"New Ma Tian San Shi" The new Ma Tian loose iron can greatly increase the tensile strength of the hot-dip galvanized steel sheet, but on the other hand, it will become the starting point of the damage and the stretch flangeability will be deteriorated. Therefore, the steel sheet structure should contain the volume fraction. It is less than 30%. In particular, in order to improve the hole expandability, the volume fraction of the new Ma Tian loose iron is more preferably set to 20% or less, and more preferably 10% or less.

"Other Microstructures" The steel sheet structure of the hot-dip galvanized steel sheet according to the embodiment of the present invention may contain a structure other than the above, such as a pulverized iron and/or a coarse stellite. However, if the amount of ferrite and/or coarse swarf carbon iron in the steel sheet structure of the hot-dip galvanized steel sheet increases, the ductility deteriorates. From this point of view, it is assumed that the volume fraction of the pulverized iron and/or the coarse swarf carbon iron contained in the steel sheet structure is 8% or less in total. Further, the volume fraction of the pulverized iron and/or the coarse swarf carbon iron is preferably 5% or less in total.

In the steel sheet structure of the hot-dip galvanized steel sheet according to the embodiment of the present invention, the surface portion of the surface layer having a depth of 20 μm from the interface between the plating layer and the base material steel sheet (base iron) is retained, and the Worthite iron remains. The volume fraction is limited to 3% or less, and the volume fraction "V1" of the hard structure in the surface layer portion is in the range of 1/8 thickness to 3/8 thickness centering on the 1/4 thickness of the steel sheet surface. The volume fraction of the tissue "V2" is in the range of 0.10 times or more and 0.90 times or less.

"Remaining Worthfield iron near the interface between the plating layer and the base iron" The residual Worthite iron near the interface between the plated layer of the hot-dip galvanized steel sheet and the base metal plate is deformed into a hard Ma Tian loose iron. On the other hand, when a large strain is applied to the vicinity of the surface of the hot-dip galvanized steel sheet, the deformation starts to be affected, and the bending property and the fatigue resistance are deteriorated. From this point of view, in the surface layer portion from the interface between the plating layer and the base material steel sheet to the depth of the steel sheet direction of 20 μm, the volume fraction of the residual Worthite iron must be limited to 0% to 3% (including 0%). ). Further, the volume fraction of the remaining Worthite iron in the surface layer portion is preferably as low as possible, and 0% is also not preferable.

"Hard structure near the interface between the plating layer and the base iron" The hard structure near the interface between the plating layer of the hot-dip galvanized steel sheet and the base material steel sheet (base iron) can improve the strength of the surface layer portion of the hot-dip galvanized steel sheet and cause fatigue. The ultimate strength is greatly improved, and the tissue contributes to the improvement of fatigue resistance. From this point of view, the volume fraction of the hard structure in the surface layer portion from the interface between the plating layer and the base iron to the depth in the steel sheet direction of 20 μm is "V1", and the thickness of the steel sheet is 1/4. When the total volume fraction of the 1/8 thickness to 3/8 thickness range of the center is "V2", the ratio V1/V2 must be 0.10 or more to sufficiently increase the strength of the surface layer portion of the hot-dip galvanized steel sheet. Further, in order to sufficiently improve the fatigue resistance, V1/V2 is preferably 0.20 or more, more preferably 0.30 or more, and more preferably 0.40 or more. On the other hand, the hard tissue fraction in the surface layer portion from the interface between the plating layer and the base iron to the depth of the steel sheet of 20 μm is suppressed to some extent, and the strength near the surface of the hot-dip galvanized steel sheet is lowered to improve the local ductility. It can also improve the flexibility. From this viewpoint, in order to obtain good bendability, V1/V2 is set to 0.90 or less, and is preferably set to 0.85 or less, and more preferably set to 0.80 or less.

Further, in the hot-dip galvanized steel sheet according to the present embodiment, the surface layer portion having a depth from the interface between the plating layer and the base iron to the depth of the steel sheet of 20 μm may be contained in the BCC grain boundary and/or the crystal grains of the iron. A fine oxide of Si and/or Mn. By forming a fine oxide in the steel sheet before the surface layer portion, it is possible to suppress the formation of Si and/or Mn at the interface of the steel sheet, that is, at the interface between the plating layer and the base material steel sheet, which is the starting point of the peeling of the plating layer. Oxide.

The volume fraction of each structure contained in the base material steel sheet of the hot-dip galvanized steel sheet according to the embodiment of the present invention is measured, for example, by the method described below.

The volume fraction of the fermented iron, the toughened ferrite iron, the toughened iron, the tempering Ma Tian loose iron, the new Ma Tian loose iron, the Bora iron and the coarse snowy carbon iron may be included in the steel sheet structure of the hot-dip galvanized steel sheet of the present invention. It was measured by the following method. First, after taking a sample with a plate thickness section parallel to the rolling direction of the steel sheet as a viewing surface, the observation surface is polished and subjected to a nital etchant etching. Then, the thickness is 1/8 to 3/8 of the thickness of 1/4 of the thickness of the sheet, or the depth of the steel sheet is 20 μm from the interface between the plating layer and the base steel sheet (base iron). In the range, the area fraction was observed and measured by a field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron Microscope), and this was regarded as a volume fraction. When the plating layer is etched away by the oxidizing agent, the surface of the sample can also be regarded as the interface between the plating layer and the base iron.

The volume fraction of the remaining Worthite iron contained in the steel sheet structure of the hot-dip galvanized steel sheet according to the present embodiment was evaluated by FE-SEM and analysis of the direction of the high decomposition energy crystal by the EBSD (Electron Bach-Scattering Diffraction) method. First, the plate thickness section parallel to the rolling direction is processed into a mirror surface, and the surface of the base material steel plate is 1/8 thickness to 3/8 thickness centered on the 1/4 thickness, or the interface between the plating layer and the base iron. In the range from the starting point to the depth of the steel sheet direction of 20 μm, the measurement pitch is 0.15 μm or less, and the crystal direction is measured in a total area of 10000 μm ² or more. Then, it is judged that each measurement point is iron of BCC (body-centered cubic structure) or iron of FCC (face-centered cubic structure), and the point of FCC iron is judged as residual Worthite iron, and the residual Worthite iron is determined. Area fraction, which can be used as the volume fraction. In addition, if a wide area is measured, the area fraction will be equivalent to the volume fraction. Therefore, in the above case, the area fraction of the residual Worthite iron can be regarded as a volume by measuring the crystal direction in a total area of 10000 μm ² or more. The rate.

As shown in Fig. 1, the hot-dip galvanized steel sheet according to the present embodiment has an Fe-Al alloy layer 30 at the interface between the hot-dip galvanized layer 10 and the base material steel sheet 20, and has the following fine layer in the base material steel sheet 20. 40 and decarburization layer 50. The micronized layer and the decarburized layer are layers which are formed by performing a decarburization reaction under a specific gas atmosphere in a specific temperature range as described later in the annealing step. Therefore, in addition to the oxide and the intervening particles, the structure in the fine layer and the decarburized layer is substantially a structure mainly composed of the ferrite-grained iron phase 60. Specifically, it means that the volume fraction of the iron phase of the fertiliser is above 70%, and the remaining part of the structure is one of the Worthfield iron phase, the toughened iron phase, the Matian iron phase, and the Borne iron phase. Two or more layers of mixed tissue. The micronized layer is defined as a micronized layer which exists when the average particle diameter of the ferrite-grain phase in the outermost portion of the base material steel sheet is 1/2 or less of the average particle diameter of the ferrite-grain iron phase in the decarburized layer. The boundary between the average particle diameter of the ferrite iron phase in the micronized layer and the average particle diameter of the ferrite iron phase in the decarburized layer is defined as the boundary between the micronized layer and the decarburized layer.

The micronized layer is in direct contact with the aforementioned Fe-Al alloy layer. The average thickness of the fine layer is 0.1 μm to 5.0 μm, and the average particle diameter of the ferrite iron phase in the fine layer is 0.1 μm to 3.0 μm, and the fine layer contains one or more oxides of Si and Mn. Further, the maximum diameter of the oxide is from 0.01 μm to 0.4 μm.

The average thickness of the micronized layer is from 0.1 μm to 5.0 μm. When the average thickness of the micronized layer is less than 0.1 μm, the effect of suppressing the occurrence of cracks and stretching cannot be obtained, and the effect of improving the plating adhesion cannot be obtained. When it is more than 5.0 μm, the plating layer is promoted to be alloyed (the Zn-Fe alloy is formed), and the Fe content in the plating layer is increased to cause a decrease in plating adhesion. The average thickness of the finely refined layer is preferably 0.2 μm to 4.0 μm, more preferably 0.3 μm to 3.0 μm.

The difference between the maximum thickness and the minimum thickness of the aforementioned fine layer in the width direction of the steel sheet is preferably within 2.0 μm. Here, the maximum thickness and the minimum thickness of the micronized layer in the width direction of the steel sheet mean the thickness of the micronized layer which is measured at a position of 50 mm from both ends and a total of 8 divisions which are divided into 7 equal parts. Maximum thickness and minimum thickness. The thicker the thickness of the fine layer, the easier the alloying of the plating layer (forming a Zn-Fe alloy) is. Therefore, if the thickness difference of the fine layer in the width direction of the steel sheet is large, uneven alloying may occur. There is a case where the plating adhesion and the uniformity of the plating appearance are adversely affected. Therefore, the difference between the maximum thickness and the minimum thickness of the fine layer in the width direction of the steel sheet is preferably 1.5 μm or less and 1.0 μm or less from the viewpoint of plating adhesion and uniformity of plating appearance.

The average grain size of the ferrite iron phase in the micronized layer is from 0.1 μm to 3.0 μm. When the average particle diameter of the ferrite-particle iron phase is less than 0.1 μm, the effect of suppressing generation and elongation of cracks cannot be obtained, and the effect of improving plating adhesion cannot be obtained. When the average particle diameter of the ferrite-grained iron phase exceeds 3.0 μm, the effect of improving the plating adhesion cannot be obtained. Therefore, the preferred ferrite iron phase has an average particle diameter of 0.1 μm to 2.0 μm.

One or two or more kinds of oxides of Si and Mn contained in the fine layer are, for example, one selected from the group consisting of SiO ₂ , Mn ₂ SiO ₄ , MnSiO ₃ , Fe ₂ SiO ₄ , FeSiO ₃ , and MnO. Or two or more. The maximum diameter of one or two or more kinds of oxides of Si and Mn contained in the fine layer is 0.01 μm to 0.4 μm. As will be described later, the oxide is formed inside the base material steel sheet at a specific temperature range during annealing, and the oxide particles can suppress the growth of the ferrite-grained iron phase crystal in the surface layer of the base material steel sheet to form a fine layer. When the maximum diameter of the oxide is less than 0.01 μm, the fine layer may not be sufficiently formed, and the plating adhesion may be deteriorated. When the maximum diameter of the oxide is larger than 0.4 μm, the ferrite-grained iron phase is coarsened, and the fine-grained layer is not sufficiently formed, and the oxide itself becomes a starting point of plating peeling, which deteriorates plating adhesion. Therefore, the preferred range of the maximum diameter of the oxide is from 0.05 μm to 0.2 μm.

The average thickness of the fine layer and the average particle diameter of the ferrite iron phase in the fine layer were measured by the following methods. The sample was taken from the hot-dip galvanized steel sheet with a plate thickness section parallel to the rolling direction of the base material steel plate as an observation surface. The observation surface of the sample was processed by a CP (Cross section polisher) apparatus, and measured by a reflection electron image in FE-SEM (Field Emission Scanning Electron Microscopy) at 5000 times.

The maximum diameter of one or two or more oxides of Si and Mn contained in the fine layer was measured by the following method. The sample was taken from the hot-dip galvanized steel sheet with a plate thickness section parallel to the rolling direction of the base material steel plate as an observation surface. The sample observation surface was processed by FIB (Focused Ion Beam) to prepare a film sample. Thereafter, the film sample was observed at 30,000 times by FE-TEM (Field Emission Transmission Electr on Microscopy). Five fields of view were observed for each film sample, and the maximum 値 of the oxide diameter obtained by the full-field measurement was taken as the maximum diameter of the oxide of the film sample.

(Fe-Al alloy layer) In the embodiment of the present invention, an Fe-Al alloy layer is formed at the interface between the plating layer and the steel sheet. By forming the Fe—Al alloy layer, it is possible to suppress the alloying of the plating layer (the formation of the Zn—Fe alloy), and it is possible to suppress the deterioration of the plating adhesion. Further, it is possible to suppress uneven appearance due to uneven alloying. Compared with the alloyed hot-dip galvanized steel sheet which has been alloyed after the hot-dip galvanizing treatment, the hot-dip galvanized steel sheet which has not been alloyed is more likely to be uneven in appearance due to uneven alloying. Therefore, the thickness of the Fe-Al alloy layer is set to be 0.1 μm to 2.0 μm. When it is less than 0.1 μm, the plating adhesion and the appearance may be deteriorated, and when it is more than 2.0 μm, the plating adhesion may be lowered. Therefore, it is preferably from 0.1 μm to 1.0 μm.

The difference between the maximum thickness and the minimum thickness of the aforementioned Fe-Al alloy layer in the width direction of the steel sheet was set to be within 0.5 μm. Here, the maximum thickness and the minimum thickness of the aforementioned Fe-Al alloy layer in the width direction of the steel sheet refer to a position where the position of 50 mm from both ends is respectively measured and a total of 8 portions of Fe-Al alloy layer divided into 7 equal portions are formed. Thickness, where maximum thickness and minimum thickness. The thinner the thickness of the Fe-Al alloy layer, the easier the alloying of the plating layer (the formation of the Zn-Fe alloy) is. Therefore, if the thickness difference of the Fe-Al alloy layer in the width direction of the steel sheet is large, it may cause uneven alloying. There is a case where the plating adhesion and the uniformity of the plating appearance are adversely affected. Therefore, the difference between the maximum thickness and the minimum thickness of the Fe-Al alloy layer in the width direction of the steel sheet is preferably 0.4 μm or less and more preferably 0.3 μm or less from the viewpoint of plating adhesion and uniformity of plating appearance.

(Placing layer) In the embodiment of the present invention, the Fe content of the hot-dip galvanized layer is more than 0% and not more than 3.0%, and the Al content is more than 0% and not more than 1.0%. And the molten galvanized layer may contain or mix with Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, I, K, La, Li, Mg, Mn, Mo, Na, Nb One or more of Ni, Pb, Rb, Sb, Si, Sn, Sr, Ta, Ti, V, W, Zr, and REM. In the above, even if one or two or more kinds of the above-mentioned elements are contained or mixed in the hot-dip galvanized layer, the effects of the present invention are not impaired, and the content thereof is preferably improved in corrosion resistance and workability. Further, in the present embodiment, the hot-dip galvanized layer may contain columnar crystals composed of a ruthenium phase (FeZn ₁₃ ), and the ruthenium phase of the entire interface between the plating layer and the base material steel sheet from the viewpoint of plating adhesion The coverage ratio should be less than 20%. Further, the amount of adhesion of the hot-dip galvanized layer on one side of the base material steel sheet is preferably 10 g/m ² or more and 100 g/m ² or less.

[Fe content of the hot-dip galvanized layer: more than 0% and not more than 3.0%] The Fe content of the hot-dip galvanized layer is more than 0% and not more than 3.0%. It is difficult to produce a Fe content of 0% in practice. If the Fe content exceeds 3.0%, the plating adhesion is lowered. When the Fe content is less than 0.3%, the plating adhesion may be lowered. Therefore, the Fe content is preferably in the range of 0.3% to 2.5% and 0.5% to 2.0 from the viewpoint of ensuring plating adhesion. % is better.

[Al content of the hot-dip galvanized layer: more than 0% and less than 1.0%] The Al content of the hot-dip galvanized layer is more than 0% and not more than 1.0%. When the plating bath does not contain Al or contains a very small amount, the plating layer is alloyed and the plating adhesion is lowered. Therefore, the Al content in the plating layer is preferably 0.1% or more. The alloying of the plating layer means that Fe atoms diffuse into the plating layer to form a Zn-Fe alloy. When the Al content is more than 1.0%, the plating adhesion is lowered. Therefore, from the viewpoint of ensuring plating adhesion, the Al content is preferably in the range of 0.1% to 0.8%, more preferably 0.2% to 0.5%.

[Amount of adhesion by hot-dip galvanizing: 10 g/m ² to 100 g/m ² ] If the amount of adhesion of the hot-dip galvanized layer to one side of the base material steel sheet is small, sufficient corrosion resistance cannot be obtained. From this point of view, the adhesion amount of the plating layer to the single side of the base material steel sheet is preferably set to 10 g/m ² or more. On the other hand, from the viewpoint of corrosion resistance, the adhesion amount is more preferably 20 g/m ² or more, and more preferably 30 g/m ² or more. On the other hand, when the amount of adhesion of the plating layer is large, the electrode loss becomes intense when the spot welding is performed, and the melting block diameter decreases and the fusion bonding strength deteriorates when the welding is continuously performed. Therefore, the amount of adhesion of the plating layer is preferably set to 100 g/m ² or less. The continuous weldability viewpoint, the adhesion amount of 93g / m ² or less good, and 85g / m ² or less more preferably.

(Manufacturing Method of Hot-Dip Galvanized Steel Sheet) Next, a method of producing a hot-dip galvanized steel sheet according to an embodiment of the present invention will be described in detail.

The method for producing a hot-dip galvanized steel sheet according to the present embodiment includes the following steps: heating the steel slab having the chemical composition to 1080 ° C or higher, and then performing hot rolling at a rolling end temperature of 850 ° C to 980 ° C. The step of forming a hot-rolled steel sheet and then winding it into a coil, controlling the temperature of the hot-rolled steel sheet during the cooling process from the hot rolling to 300 ° C to a hot rolling step satisfying the following formula (1); continuing the hot rolling step After the pickling, a cold rolling step of a cold rolling reduction in which the total rolling reduction ratio is 85% or less is performed; and the average heating rate between the steel sheets after the cold rolling step is between 600 ° C and 750 ° C is 1.0 ° C / s or more, the highest The heating temperature is (Ac1+25) °C or higher and Ac3 °C or lower, and heating is performed in a temperature range of 750 ° C or higher, and then the average cooling rate between 760 ° C and 700 ° C is set to 0.1 ° C / s to 5.0 ° C / s, an annealing step of cooling at an average cooling rate between 650 ° C and 500 ° C of 1.0 ° C / s or more; after the annealing step, when the temperature of the plating bath is 440 ° C to 470 ° C, entering the plating bath The steel plate temperature is set to 430 ° C ~ 480 ° C, the effective Al amount in the plating bath is set to 0.180 a plating step of immersing a steel sheet in a plating bath under 0.250 mass% plating conditions, and performing a hot-dip galvanization on the surface of the steel sheet to form a plating layer; and, after the plating step, cooling the steel sheet to 100 ° C or lower and then using A roll having a diameter of 50 mm to 800 mm is subjected to a bending/returning deformation of a total of two or more processing steps. Hereinafter, each manufacturing step will be described in detail.

In order to manufacture the hot-dip galvanized steel sheet according to the embodiment of the present invention, first, a base material steel sheet is produced. The base material steel plate is produced by casting a steel embryo of the alloy element in accordance with the characteristics, and performing hot rolling and rolling to produce a cold rolling. Hereinafter, each manufacturing step will be described in detail.

"Casting Step" First, the steel is to be cast for hot rolling. The chemical composition (composition) of the steel embryo is preferably the above component. Steel blanks to be used for hot rolling may be produced by continuously casting steel or by using a thin steel continuous casting machine or the like.

"Hot Rolling Step" In the hot rolling step, in order to suppress anisotropy in the crystal direction caused by casting, the heating temperature of the steel blank is preferably set to 1080 ° C or higher. The heating temperature of the steel embryo is more preferably set to be above 1150 °C. On the other hand, the upper limit of the heating temperature of the steel blank is not limited. Heating the steel embryo above 1300 °C requires a lot of energy, which leads to a substantial increase in manufacturing costs. From this point of view, the heating temperature of the steel blank should be set to 1300 ° C or less.

Hot rolling is performed after heating the steel embryo. When the end temperature (rolling end temperature) of the hot rolling is lower than 850 ° C, the reaction force of the rolling is increased, and it is difficult to stably obtain the specified thickness. Therefore, the end temperature of the hot rolling is preferably set to 850 ° C or higher, and more preferably set to 870 ° C or higher. On the other hand, in order to set the end temperature of the hot rolling to be higher than 980 ° C, a device for heating the steel sheet is required in the step from the end of the heating of the steel bristles to the end of the hot rolling, which causes a high cost. Therefore, the end temperature of the hot rolling is set to 980 ° C or lower, and more preferably 950 ° C or lower.

Next, the hot rolled steel sheet which has been subjected to hot rolling is wound into a coil. Further, the average cooling rate during the cooling from the hot rolling to the winding is preferably set to 10 ° C /sec or more. This is because the particle size of the hot-rolled steel sheet can be made fine by the transformation at a lower temperature, and the effective crystal grain size of the base material steel sheet after cold rolling and annealing can be made fine.

The coiling temperature of the hot-rolled steel sheet is preferably set to 450 ° C or more and 650 ° C or less. This is because the ferrite and/or the coarse stellite having a long diameter of 1 μm or more are dispersed and formed in the microstructure of the hot-rolled steel sheet, and the strain introduced into the hot-rolled steel sheet due to the cold rolling is localized. Thereby, the Worstian iron inversion state in various crystal directions can be made in the annealing step, and the effective crystal grains of the annealed base material steel plate can be made fine. If the coiling temperature is lower than 450 ° C, it may not be possible to generate a wave of iron and/or a large snowy carbon. On the other hand, if the coiling temperature is higher than 650 ° C, the ferrite and the ferrite iron will form a long strip shape in the rolling direction. Therefore, it is not preferable to form a coarse material in which the effective crystal grains of the base material steel sheet which is formed after the cold rolling and annealing are formed in the rolling direction from the ferrite portion. Further, after the hot-rolled steel sheet is wound up, unevenness in formation of the internal oxide layer (the center portion is thicker than the end portion) may occur under the scale layer. This phenomenon is more pronounced if the coiling temperature is higher than 650 °C. When the internal oxide layer cannot be removed in the subsequent step (acid pickling or cold rolling) to be described later, the unevenness of the formation of the fine layer and the Fe-Al alloy layer is affected, and the plating adhesion and the uniformity of appearance are poor. The possibility of influence. Therefore, it is preferable to lower the coiling temperature to 650 ° C or less from the viewpoint of plating adhesion and appearance uniformity.

Here, in order to control the hard structure to a predetermined volume fraction on the surface of the base metal sheet after annealing, it is necessary to appropriately decarburize the surface of the base material steel sheet in the hot rolling step. Although the decarburization behavior from the base metal sheet can be controlled by controlling the gas environment, large-scale equipment must be used, which is burdensome in terms of cost. Therefore, in the present embodiment, the decarburization behavior is controlled by controlling the cooling rate to control the temperature of the steel sheet during the period from the completion of the completion rolling (end of hot rolling) to 300 °C. The temperature control of the base material steel sheet is performed in a range from the end of the completion rolling to 300 ° C, and the surface of the base material steel sheet is at a temperature at which the BCC phase of iron can be stabilized at a temperature of Ae3*° C. or lower. This is because the decarburization ratio of the BCC phase from iron is faster than the decarburization of the FCC phase which is a stable phase at a high temperature. Further, in the present embodiment, when the temperature of the base material steel sheet is lower than 300 ° C, the diffusion rate of oxygen becomes slow, and the decarburization progress does not affect the decarburization behavior, so heat The temperature range of the temperature control of the base material steel sheet in the rolling step is set to be between 300 °C. And Ae3*[°C] can be calculated by the following formula.

Ae3*[°C]=885+31.7Si-29.3Mn+123.2Al-18.2Cr-40.0Ni-21.0Cu+12.6Mo

In the above formula, C, Si, Mn, Al, Cr, Ni, Cu, and Mo are each an element content [% by mass].

Further, the decarburization behavior of the steel sheet is controlled from the first period from the completion of the rolling to the winding of the coil, and the second period from the winding of the coil to the arrival of the room temperature. This is because the first stage is decarburized in the atmosphere, and the second stage is wound into a coil to make the steel sheets adhere to each other, and decarburization is carried out under the condition that almost no external gas is invaded, so the two are decarburized. There is a big difference in speed.

Specifically, in order to appropriately decarburize the surface layer portion of the steel sheet, the steel sheet temperature is controlled to satisfy the following formula (1) during the cooling process from the end of the completion rolling to 300 ° C. Formula (1) is an equation for the degree of progress of the decarburization behavior, and the larger the enthalpy of the formula (1), indicates that decarburization is progressing. In addition, regarding the various times in the formula (1), t[sec] is the elapsed time from the end of the completion rolling, t1 [sec] is the elapsed time from the end of the rolling delay to the temperature of Ae3*, t2 [ Seconds] is the elapsed time from the end of the rolling process to the winding of the coil, and t3 [sec] is the elapsed time from the end of the completion rolling to the temperature of the steel sheet of 300 °C. Further, the T(t)[°C]-based steel sheet temperature, W _Si [% by mass] and W _Mn [% by mass] are the average contents of the respective elements of Si and Mn in the entire steel sheet. Further, α, β, γ, and δ are constant terms, and are 8.35 × 10 ⁸ , 2.20 × 10 ⁴ , 1.73 × 10 ¹⁰ , and 2.64 × 10 ^{4 , respectively} .

[Number 1]

In the above formula (1), the first integral term in the parentheses is the term of the degree of decarburization in the first-stage cooling, and the second integral term is the term of the degree of decarburization in the second-stage cooling. In either case, the decarburization is performed on the base steel sheet at a higher temperature and a longer residence time. In particular, in the second phase, elemental oxygen which is decarburized is hardly present in a gas atmosphere, and is decarburized by oxygen which is attracted from Si and Mn in the steel layer of the surface layer, so the second integral term The influence of Si and Mn content is also included, and the larger the amount of Si and Mn in the steel, the larger the enthalpy of the formula (1), indicating that decarburization is being carried out.

In the cooling process after the completion of the rolling and rolling, if the enthalpy of the above formula (1) is less than 0.8, the surface layer portion of the base material steel sheet is hardly decarburized, and the volume ratio V1 of the hard portion of the surface layer portion is from the surface of the plate thickness. The ratio V1/V2 of the volume ratio V2 of the hard structure centered at 1/4 thickness is more than 0.90, and the bendability is deteriorated. Therefore, the enthalpy of the above formula (1) is set to 0.8 or more to be cooled. From this viewpoint, it is preferable to set the enthalpy of the above formula (1) to 1.0 or more, and to set it to 1.3 or more. On the other hand, if the enthalpy of the above formula (1) exceeds 20.0, the surface layer portion of the steel sheet is excessively decarburized, and V1/V2 is less than 0.30, so that the fatigue resistance of the steel sheet is remarkably deteriorated, so that the above formula (1) is preferable. The crucible is cooled to 20.0 or less. From this viewpoint, it is preferable to set the enthalpy of the above formula (1) to 15.0 or less to be cooled, and it is more preferable to set it to 10.0 or less.

Next, the hot-rolled steel sheet obtained as described above is pickled. Pickling removes the oxide formed on the surface of the hot-rolled steel sheet and is important for improving the plating property of the base material steel sheet. Pickling can be carried out once or in fractions. From the viewpoint that the fine layer and the Fe-Al alloy layer can be uniformly formed and the appearance uniformity achieved can be ensured, it is preferable to strengthen the pickling to remove the internal oxide layer formed under the scale layer as much as possible. The pickling conditions are not particularly limited as long as the internal oxide layer can be removed. For example, hydrochloric acid is preferably used from the viewpoint of pickling efficiency and economy. The conditions for removing the internal oxide layer include, for example, a hydrogen chloride concentration of 5% by mass or more, a pickling temperature of 80° C. or more, and a pickling time of 30 seconds or more.

"Cold rolling step" Next, the hot-rolled steel sheet after pickling is cold-rolled to obtain a cold-rolled steel sheet. In the case of cold rolling, when the rolling reduction ratio is more than 85%, the steel sheet loses ductility and increases the risk of the steel sheet breaking in the cold rolling. Therefore, the total reduction ratio is preferably 85% or less. From this point of view, the total reduction ratio is more preferably set to 75% or less, and more preferably 70% or less. The lower limit of the total reduction ratio in the cold rolling step is not particularly limited. However, when the total reduction ratio is less than 0.05%, the shape of the base material steel sheet may be uneven, so that the plating may not be uniformly adhered and the appearance may be impaired. Therefore, it is preferably set to 0.05% or more, and more preferably set to 0.10% or more. In addition, the cold rolling is preferably carried out in multiple passes, and there is no limitation on the number of passes of the cold rolling and the distribution of the rolling reduction of each pass.

Further, when the total rolling reduction ratio of the cold rolling is in the range of more than 10% and less than 20%, recrystallization may not be sufficiently performed in the subsequent annealing step, and the ductility may be lost in the vicinity of the surface layer due to generation of a large number of poor rows. The coarse crystal grains have a situation in which the bending property and the fatigue resistance are deteriorated. In order to avoid this, the total reduction of the rolling reduction ratio can be reduced, and the ductility of the crystal grains can be effectively reduced and accumulated in the crystal grains. Alternatively, by increasing the total reduction ratio, recrystallization can be sufficiently performed in the annealing step, and the processed structure can be effectively made into recrystallized grains having less internal discharge and less accumulation. The rolling reduction ratio in the cold rolling step is preferably 10% or less, and more preferably 5.0% or less, from the viewpoint of mitigating the difference in the accumulation of the crystal grains. On the other hand, in order to sufficiently carry out the recrystallization in the annealing step, the total reduction ratio is preferably 20% or more, and more preferably 30% or more.

"Annealing step" In the embodiment of the present invention, the cold-rolled steel sheet is annealed. In the embodiment of the present invention, a continuous annealing plating line having a pre-tropical zone, a soaking zone and a plating zone is preferably used. Moreover, it is preferred to carry out the annealing step while passing it through the pre-tropical zone and the soaking zone, and to terminate the annealing step when the plating zone is to be reached, and to perform the plating step on the plating zone.

As described above, when a continuous annealing plating line is used in the annealing step and the plating step, for example, the method shown below is preferably used. In particular, in order to make the fine layer and the Fe-Al alloy layer suitable and uniform, and to ensure plating adhesion and uniformity of appearance, the gas environment of the pre-tropical zone, the control of the heating method, and the gas environment for controlling the tropical zone are very important.

In the preheating zone, based Log Zhi water vapor partial pressure P (H ₂ O) and the hydrogen partial pressure P (H ₂₎ of the ratio of _{Log (P (H 2 O)} / P (H 2)) in the control -1.7 ~ In a gas atmosphere of -0.2, use a preheating burner with an air ratio of 0.7 to 1.0 to heat the steel sheet to a temperature of 400 ° C to 800 ° C while passing the steel sheet. Adjusting the ratio of the water vapor partial pressure (P(H ₂ O) to the hydrogen partial pressure P(H ₂ ) in the pre-tropical zone, the Fe-Al alloy phase is uniformly deposited in the width direction at the interface in the subsequent hot-dip galvanizing and It affects the surface properties of the steel sheet before plating. This is because the pre-tropical air-adjusting ratio suppresses the formation of an oxide film of strong deoxidizing elements such as Si on the surface of the steel sheet. Further, by adjusting the ratio of the partial pressure of water vapor P (H ₂ O) to the partial pressure of hydrogen P (H ₂ ), excessive decarburization on the surface of the steel sheet can be suppressed. Thereby, in the subsequent plating step, the Fe-Al alloy reaction can be selectively performed by suppressing excessive Fe-Zn alloy reaction at the grain boundary of the steel sheet surface. By selectively performing the Fe-Al alloy reaction, formation of a uniform Fe-Al alloy layer can be promoted, and excellent plating adhesion and uniform appearance can be obtained. When Log(P(H ₂ O)/P(H ₂ )) is more than -0.2, it is easy to cause Fe-Zn alloying in the subsequent plating step, so that the Fe concentration in the plating becomes high. As a result, the adhesion of the plating is lowered, and uneven appearance is liable to occur. On the other hand, when Log(P(H ₂ O)/P(H ₂ )) is less than -1.7, a portion having a high carbon concentration is formed on the surface of the steel sheet, and a fine layer cannot be formed on the surface to lower the plating adhesion. . The "air ratio" is a ratio of the volume of the air contained in the mixed gas per unit volume to the theoretically required air volume for completely combusting the fuel gas contained in the mixed gas per unit volume, and can be expressed by the following formula. Air ratio = [air volume (m ³ ) contained in the mixed gas per unit volume] / [the theoretically required air volume (m ³ ) in order to completely burn the fuel gas contained in the mixed gas per unit volume]

When the air ratio is more than 1.0 and is too large, an Fe oxide film is excessively formed in the surface layer portion of the steel sheet, and the decarburized layer after annealing is enlarged, and the fine layer is excessively formed. Therefore, the plating alloying is excessively performed, and the plating adhesion, the chipping property, and the pulverization property are lowered. Therefore, the air ratio is preferably 1.0 or less, more preferably 0.9 or less. When the air ratio is less than 0.7 and is too small, the micronized layer cannot be formed, and the plating adhesion is lowered. Therefore, the air ratio is set to 0.7 or more, and is preferably set to 0.8 or more. Further, if the temperature of the steel sheet passing through the pretropical zone is lower than 400 ° C, a sufficient fine layer cannot be formed. Therefore, the temperature of the steel sheet passing through the pre-tropical zone is set to 400 ° C or higher, and is preferably set to 600 ° C or higher. On the other hand, if the temperature of the steel sheet passing through the pretropical zone is higher than 800 ° C, a coarse oxide containing Si and/or Mn is formed on the surface of the steel sheet to lower the plating adhesion. Therefore, the temperature of the steel sheet passing through the pre-tropical zone is set to 800 ° C or lower, and preferably set to 750 ° C or lower. If the heating rate of the pre-tropical zone is slow, the interior will oxidize and a coarse oxide will be formed inside the steel sheet. In particular, the heating rate of 600 ° C to 750 ° C is particularly important. In order to avoid excessive decarburization of the surface layer of the steel sheet and to suppress the formation of coarse oxides, the average heating rate between them must be 1.0 ° C / sec or more. When the average heating rate between them is less than 1.0 ° C / sec, a coarse oxide is formed in the fine layer, and the plating adhesion and the pulverizability are lowered. In order to avoid excessive decarburization of the surface layer portion of the steel sheet and formation of coarse oxides, the average heating rate between 600 ° C and 750 ° C is preferably 1.5 ° C / sec or more, more preferably 2.0 ° C / sec or more. In order to ensure the treatment time of the pre-tropical zone, the average heating rate at 600 ° C to 750 ° C is preferably set to 50 ° C / sec or less. When the average heating rate is 50 ° C /sec or less, it is easy to obtain a uniform fine layer, and a plating layer excellent in plating adhesion and appearance uniformity can be obtained.

To control the microstructure fraction associated with the formability of the steel sheet to a predetermined range, the highest heating temperature of the annealing step is an important factor. When the maximum heating temperature is low, coarse iron-based carbides are melted in the steel to deteriorate the formability. Further, if the maximum heating temperature is lower than 750 ° C, the coarse iron carbide in the hot-rolled steel sheet cannot be sufficiently melted, and remains in the sheet of the product to be formed, and the ductility is greatly impaired. In order to sufficiently dissolve the iron-based carbide and improve the moldability, the maximum heating temperature is (Ac1 point + 25) ° C or more and 750 ° C or more, and is preferably set to (Ac1 point + 50) ° C or more. On the other hand, if the maximum heating temperature exceeds the Ac3 point, the iron content of the fertilizer in the steel is remarkably reduced, so the maximum heating temperature is set to be equal to or less than Ac3. Further, from the viewpoint of plating adhesion, in order to reduce the oxide on the surface of the base iron, the maximum heating temperature is preferably low. From this point of view, the maximum heating temperature is preferably 850 ° C or lower, and more preferably 830 ° C or lower.

The Ac1 point and the Ac3 point of the steel sheet are the starting point and the ending point of the inverted state of the Worstian iron, respectively. Specifically, the steel sheet after the hot rolling is cut into small pieces and heated to 1200 ° C at 10 ° C / sec. And measuring the volume expansion during this period.

The maximum heating temperature (above 750 °C) in the annealing step will reach the point in the tropics. The soaking tropical gas environment controls Log(P(H ₂ O)/P(H ₂ )) from -1.7 to -0.2. When Log(P(H ₂ O)/P(H ₂ )) is less than -1.7, the micronized layer cannot be formed, and the plating adhesion is lowered. When Log(P(H ₂ O)/P(H ₂ )) is greater than -0.2, the decarburization will be excessively performed, and not only the hard phase of the surface layer of the base metal sheet will be significantly reduced, but also the coarse oxide layer will be formed in the fine layer. The plating adhesion and pulverizability are lowered.

As described above, when the log (P(H ₂ O)/P(H ₂ )) of the homogenous tropic is -1.7 to -0.2, Si and Mn oxide which are the starting points of the plating peeling are not formed on the outermost layer, and A fine oxide of Si and/or Mn having a maximum diameter of 0.05 μm to 0.4 μm is formed inside the surface layer of the steel sheet. The fine oxide of Si and/or Mn can suppress the growth of Fe recrystallization in annealing. Moreover, since the water vapor in the annealing gas environment decarburizes the surface layer of the base material, the surface layer of the base material after annealing becomes ferrite iron. As a result, the surface layer of the base material after annealing is formed to have an average thickness of 0.1 μm to 5.0 μm, the average particle diameter of the ferrite iron phase is 0.1 μm to 3.0 μm, and the maximum diameter is 0.01 μm to 0.4 μm containing Si and/or Mn. A fine layer of oxide.

In the annealing step, after the maximum heating temperature is reached and the plating before the plating bath is cooled (pre-plating cooling step), it is divided into a temperature range of 760 ° C to 700 ° C and 650 ° C to 500 ° C. The temperature domain 2 is controlled to control the temperature of the steel sheet, whereby a predetermined microstructure can be obtained. First, in order to sufficiently generate ferrite iron, an average cooling rate from 760 ° C to 700 ° C is specified. When the average cooling rate from 760 ° C to 700 ° C exceeds 5.0 ° C / sec, the ferrite iron may not be sufficiently formed, so the average cooling rate is set to 5.0 ° C / sec or less. In order to sufficiently form the ferrite iron, the average cooling rate is preferably set to 3.5 ° C / sec or less, more preferably 2.5 ° C / sec or less. On the other hand, when the average cooling rate from 760 ° C to 700 ° C is less than 0.3 ° C / sec, a large amount of ferrite is generated, so the average cooling rate is set to 0.3 ° C / sec or more. In order to avoid the generation of the wave iron, the average cooling rate is preferably set to 0.5 ° C / sec or more, more preferably set to 0.7 ° C / sec or more.

Next, in order to avoid excessive generation of wave iron and/or coarse snowy carbon, the cooling rate at 650 ° C to 500 ° C is specified. When the average cooling rate at 650 ° C to 500 ° C is less than 1.0 ° C / sec, a large amount of ferrite and/or coarse snow carbon is generated, so that the average cooling rate is 1.0 ° C / sec or more. Since the steel preferably contains no pulverized iron and/or coarse swarf carbon iron, the average cooling rate is preferably 2.0 ° C / sec or more, and more preferably 3.0 ° C / sec or more, in order to sufficiently avoid such formation. The upper limit of the average cooling rate in the temperature range of 650 ° C to 500 ° C is not particularly set, but it is not suitable to obtain an excessive average cooling rate by using a special cooling device or a refrigerant which does not affect the plating step. From this viewpoint, the average cooling rate in the above temperature range is preferably 100 ° C / sec or less, and more preferably 70 ° C / sec or less.

In order to obtain the tempering pre-plating cooling step, in order to obtain the tempered granulated iron, the metamorphic treatment of the granulated iron can also keep the steel sheet in a predetermined temperature range for a certain period of time after the steel sheet temperature reaches 500 ° C until the plating bath is reached. The metamorphic treatment temperature of the Ma Tian bulk iron should be based on the upper limit of the Ms point of the metamorphic iron metamorphosis of the Ma Tian, and the upper limit is set to be (Ms point -20) ° C. The metamorphic treatment temperature of the Ma Tian iron is preferably 50 ° C as the lower limit, and the lower limit is preferably 100 ° C. Moreover, the metamorphic treatment time of the Ma Tian loose iron should be set to 1 second to 100 seconds, and more preferably set to 10 seconds to 60 seconds. In addition, the methadrite iron produced by the metamorphic treatment of the granulated iron in the field will become tempered granulated iron by entering the high temperature plating bath in the plating step.

The Ms point is calculated according to the following formula. Ms point [°C]=541-474C/(1-VF)-15Si-35Mn-17Cr-17Ni+19Al

In the above formula, VF represents the volume fraction of ferrite iron, and C, Si, Mn, Cr, Ni, and Al are elemental contents [% by mass], respectively. In addition, it is difficult to directly measure the volume fraction of ferrite in the manufacturing process. Therefore, in the determination of the Ms point, the present invention cuts the small piece before the cold-rolled steel sheet is passed through the continuous annealing line, and anneals the piece in the same temperature history as when passing through the continuous annealing line, and then determines the small piece of ferrite iron. The volume change, and the number calculated using the result is taken as the volume fraction VF of the ferrite iron.

Further, in order to generate the toughened iron, the toughening iron metamorphic treatment may also cause the steel sheet to stay in the predetermined temperature range for a certain period of time after the steel sheet temperature reaches 500 ° C until the plating bath. If the toughening iron metamorphic treatment temperature is greater than 500 ° C, the ferrite and/or the coarse swarf carbon iron are generated, so the metamorphic iron metamorphic treatment temperature is set to 500 ° C or lower. Further, if the toughening iron metamorphic treatment temperature is lower than 350 ° C, the deformation cannot be sufficiently performed, so that the toughening iron transformation treatment temperature is set to 350 ° C or higher. In order to sufficiently perform the metamorphosis, the toughening iron metamorphic treatment time is set to 10 seconds or more, and in order to suppress the formation of the ferrite and/or the coarse ferritic carbon, it is set to 500 seconds or less. In addition, after the pre-plating cooling step, when both the toughening iron metamorphic treatment and the 麻田散铁 metamorphic treatment are performed, the execution sequence is set to the toughening iron metamorphic treatment and the granitic iron metamorphosis treatment.

"Plating Step" Next, the base material steel sheet obtained as described above was immersed in a plating bath. The plating bath has a composition mainly composed of zinc and having a total amount of Al in the plating bath minus the total amount of Fe, that is, an effective Al amount of 0.180% to 0.250%. When the amount of effective Al in the plating bath is less than 0.180%, the Fe-Al alloy layer may not be sufficiently formed, and Fe may enter the plating layer to impair the plating adhesion, so it is required to be 0.180% or more. From this point of view, the effective Al amount in the plating bath is more preferably 0.185% or more, and more preferably 0.190% or more. On the other hand, when the amount of effective Al in the plating bath is more than 0.250%, the Fe-Al alloy layer of the base material steel sheet and the plating layer is excessively formed to impair the plating adhesion. From this point of view, the effective Al amount in the plating bath must be 0.250% or less, and is preferably 0.240% or less, and more preferably 0.230% or less.

Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, I, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni may also be mixed in the plating bath. One or two or more elements of Pb, Rb, Sb, Si, Sn, Sr, Ta, Ti, V, W, Zr, and REM, and the content of each element can improve the corrosion resistance of the hot-dip galvanized layer and A better case such as processability.

Further, the plating bath temperature should be set to 440 ° C to 470 ° C. When the temperature of the plating bath is lower than 440 ° C, the viscosity of the plating bath is excessively increased, and it is difficult to control the thickness of the plating layer, which impairs the appearance of the molten galvanized steel sheet. Therefore, the temperature of the plating bath is preferably 440 ° C or higher, more preferably 445 ° C or higher. On the other hand, when the plating bath temperature is higher than 470 ° C, a large amount of smoke is generated, which is difficult to manufacture safely. Therefore, the plating bath temperature is preferably 470 ° C or lower, more preferably 460 ° C or lower.

Further, when the temperature of the steel sheet when the base material steel sheet enters the plating bath is lower than 430 ° C, in order to stabilize the temperature of the plating bath at 440 ° C or higher, it is necessary to give a large amount of heat to the plating bath without being practical. On the other hand, if the temperature of the steel sheet when the base material steel sheet enters the plating bath is higher than 480 ° C, in order to stabilize the temperature of the plating bath below 470 ° C, it is necessary to introduce a device capable of discharging a large amount of heat from the plating bath, and It is not suitable in terms of manufacturing cost. Therefore, in order to stabilize the bath temperature of the plating bath, the temperature at which the base material steel sheet enters the plating bath is preferably set to 430 ° C or more and 480 ° C or less. Further, in order to appropriately control the Fe-Al alloy layer, the temperature at which the base material steel sheet enters the plating bath is more preferably set to 440 ° C or more and 470 ° C or less.

The temperature of the plating bath is preferably stabilized at a temperature in the range of 440 ° C to 470 ° C. If the temperature of the plating bath is unstable, the Fe content in the Fe-Al alloy layer and the plating layer may be uneven, and the appearance and adhesion of the plating layer may be uneven. Therefore, in order to stabilize the temperature of the plating bath, it is preferable to make the temperature of the steel sheet entering the plating bath substantially coincide with the temperature of the plating bath. Specifically, from the viewpoint of limited temperature control of the actual manufacturing equipment, it is preferable to set the temperature of the steel sheet entering the plating bath to within ±10 ° C of the plating bath temperature, and to set the plating bath temperature to within ±5 ° C. good.

Further, in order to obtain an appropriate amount of plating adhesion after immersion in the plating bath, it is preferable to spray a high-pressure gas mainly composed of nitrogen on the surface of the steel sheet to remove excessive zinc in the surface layer. It was then cooled to room temperature. In this case, since the Fe atoms hardly diffuse from the base material steel plate to the plating layer, it is preferable to ensure the cooling rate to be 1 ° C / sec or more from the viewpoint of ensuring the plating adhesion to a temperature at which the enthalpy phase is almost stopped at 350 ° C. .

Further, after cooling to 350 ° C, in order to obtain a hard structure, it may be cooled to 250 ° C or lower at an average cooling rate of 1.0 ° C /sec or more. In order to obtain a new granulated iron phase and/or a tempered granule iron phase, the average cooling rate is preferably set to 3.0 ° C / sec or more, and more preferably 5.0 ° C / sec or more.

Further, after cooling to 250 ° C or lower, reheating treatment may be performed in order to obtain tempered granulated iron. The treatment temperature and treatment time of the reheating treatment can be appropriately set according to the desired characteristics. However, when the reheating temperature is lower than 250 ° C, sufficient effect cannot be obtained, and when it is higher than 350 ° C, the plating layer is deteriorated, and the plating adhesion is deteriorated, so the reheating temperature should be set to be set. 250 ° C or more and 350 ° C or less. Further, if the reheating treatment time exceeds 1000 seconds, the treatment effect is saturated, so the treatment time should be set to 1000 seconds or less.

Further, after cooling to 350 ° C, in order to obtain the residual Worthite iron, a toughening iron metamorphosis treatment may be performed in a temperature range of from 250 ° C to 350 ° C for 15 seconds to 500 seconds. If the toughening iron metamorphic treatment temperature is lower than 250 ° C, the granulated iron is formed and the residual Worth iron is not sufficiently obtained. Therefore, the toughening iron metamorphic treatment temperature is preferably set to 250 ° C or higher, and more preferably set to 300 ° C or higher. When the toughening iron metamorphic treatment temperature is higher than 350 ° C, the Fe atoms are excessively diffused from the base material steel sheet to the plating layer, which deteriorates the plating adhesion. Therefore, the toughening iron metamorphic treatment temperature should be set to 350 ° C or lower, and more preferably set to 330 ° C or lower. If the toughening iron metamorphic treatment time is more than 15 seconds, the effect of the toughening iron metamorphosis treatment can be fully obtained. The toughening iron metamorphic treatment time is more preferably 25 seconds or more. If the toughening iron metamorphic treatment time is less than 500 seconds, the toughening iron metamorphosis treatment can be efficiently performed. The toughening iron metamorphic treatment time is preferably less than 300 seconds.

Further, in order to stabilize the residual Worthite iron, it may be cooled to 250 ° C or lower and then subjected to reheat treatment. The treatment temperature and treatment time of the reheating treatment can be appropriately set according to the desired characteristics. However, if the reheating treatment temperature is lower than 250 ° C, a sufficient effect cannot be obtained. Therefore, the reheating treatment temperature is preferably set to 250 ° C or higher, and more preferably set to 280 ° C or higher. When the reheating treatment temperature is higher than 350 ° C, Fe atoms diffuse from the base material steel sheet to the plating layer, and the plating adhesion is deteriorated. Therefore, the reheating treatment temperature is preferably set to 350 ° C or lower, and more preferably set to 330 ° C or lower. also. If the reheating treatment time exceeds 1000 seconds, the effect is saturated, so the reheat treatment time should be set to 1000 seconds or less.

"Processing Step" Next, after the steel sheet temperature is cooled to 100 ° C or lower, the plated steel sheet is subjected to bending-folding deformation in order to reduce the residual Worth iron in the surface layer of the base material steel sheet. The bending can be carried out using a roller having a diameter of 50 mm to 800 mm. When the roll diameter is less than 50 mm, a large amount of strain is introduced into the surface layer of the base material steel sheet due to the bending deformation, and the formability of the steel sheet is impaired. When the roll diameter is larger than 800 mm, the strain amount of the surface layer of the base material steel plate is small, and the residual Worthite iron cannot be sufficiently reduced. Bending-folding deformation In order to reduce the residual Worthite iron on each surface of the back surface of the base material steel plate, it is necessary to perform one or more deformations of the front and back surfaces to the outside, so that a total of two or more bending-reversing deformations must be performed. . According to the above, the residual Worth iron on both sides of the front and back surfaces of the base material steel sheet can be controlled to a predetermined range.

The hot-dip galvanized steel sheet according to the present embodiment can be produced by the above-described production method, but the present invention is not limited to the above embodiment.

For example, in the embodiment of the present invention, a film composed of a composite oxide of phosphorus oxide and/or phosphorus may be applied to the surface of the galvanized layer of the hot-dip galvanized steel sheet obtained by the above method. The film composed of the phosphorus oxide and/or the phosphorus-containing composite oxide functions as a lubricant when processing the hot-dip galvanized steel sheet, and can protect the galvanized layer formed on the surface of the base material steel sheet.

Further, in the embodiment of the present invention, in order to correct the shape, the hot-dip galvanized steel sheet which has been cooled to room temperature may be subjected to cold rolling at a rolling reduction ratio of 3.00% or less.

Further, the method for producing a hot-dip galvanized steel sheet according to the embodiment of the present invention described above is preferably applied to the production of a hot-dip galvanized steel sheet having a base material steel sheet thickness of 0.6 mm or more and less than 5.0 mm. When the thickness of the base material steel sheet is less than 0.6 mm, it is difficult to keep the shape of the base material steel sheet flat, and it is unfavorable. When the thickness of the base material steel plate is 5.0 mm or more, it may be difficult to control the cooling in the annealing step and the plating step.

EXAMPLES Examples of the invention will be described. Further, the conditions in the examples are those in which the conditions and effects of the present invention are confirmed. The present invention is not limited to this one conditional example. The present invention can be applied to various conditions without departing from the gist of the present invention for the purpose of the invention.

"Example 1" A steel preform having a chemical composition (composition) of A to BY shown in Tables 1 to 3 was cast, and the hot rolling step conditions (steel embryo heating temperature, rolling end temperature) shown in Tables 4 to 7 were cast. The hot rolling is performed, and the hot rolling step conditions (average cooling rate from the end of hot rolling to coiling, coiling temperature, and formula (1)) shown in Tables 4 to 7 are performed to obtain hot rolling. Steel plate. Thereafter, the hot-rolled steel sheet was pickled (immersed in 10% hydrochloric acid at 80 ° C, the immersion time is shown in Tables 4 to 7), and the cold rolling step conditions (rolling reduction ratio) shown in Tables 4 to 7 were performed. Cold rolled steel sheets were produced by rolling.

Next, for the obtained cold-rolled steel sheet, the heating step conditions of the annealing step shown in Tables 8 to 11 (pre-tropical air ratio, Log(P(H ₂ O)/P(H) in the pre-tropical gas environment) ₂ )), Annealing is performed in Log (P(H ₂ O)/P(H ₂ )) in a tropical gas environment, an average heating rate in a temperature range of 600 ° C to 750 ° C, and a maximum heating temperature. The steel sheets have the conditions shown in Tables 8 to 11 (cooling rate 1 (average cooling rate in the temperature range of 760 ° C to 700 ° C) and cooling rate 2 (average cooling rate in the temperature range of 650 ° C to 500 ° C) The conditions (process temperature, treatment time) and the conditions of the toughened iron metamorphosis treatment 1 (treatment temperature, treatment time) of the granulated iron were subjected to a cooling treatment to obtain a base material steel sheet for plating treatment. Next, the conditions shown in Tables 12 to 15 (effective Al amount, plating bath temperature, and steel sheet entry temperature) were immersed in a galvanization bath, and the conditions shown in Tables 12 to 15 (cooling rate 3 (to 350 ° C) Average cooling rate), cooling rate 4 (average cooling rate in the temperature range of 350 ° C to 250 ° C), conditions of the toughened iron metamorphosis treatment 2 (treatment temperature, treatment time), reheating treatment conditions (treatment temperature, treatment) Time)) After the plating, it is cooled. Then, the bending-folding process was performed under the conditions shown in Tables 12 to 15 (roll diameter and number of times of processing), and cold rolling was performed under the conditions (rolling reduction ratio) shown in Tables 12 to 15 to obtain an experimental example. A hot-dip galvanized steel sheet of 1 to 202 (in which part of the experimental examples were interrupted).

Next, a sample was taken from each of the hot-dip galvanized steel sheets with a plate thickness section parallel to the rolling direction of the base material steel plate, and a microstructure was observed by a field emission scanning electron microscope (FE-SEM) and high by the EBSD method. Decomposition of crystal direction analysis, measuring 1/8 thickness to 3/8 thickness range (1/4 thickness) centered on 1/4 of the plate thickness, and starting from the interface between the plating layer and the base material steel plate to a depth of 20 μm The volume fraction of the microstructure of the surface layer portion (base iron surface layer). In addition, in Tables 16 to 19, "Mada's loose iron" means the new Ma Tian loose iron, and the "others" in the microstructures in Tables 16 to 19 indicate the Borne iron and/or the coarse swarf carbon iron. Further, the "hard phase" refers to a hard structure composed of one or more of toughened iron, toughened ferrite iron, new Matian loose iron, and tempered Matian loose iron.

The plating adhesion amount was obtained by melting the plating layer with hydrochloric acid doped with an inhibitor and comparing the weights before and after melting. At the same time, Fe and Al were quantitatively determined by ICP to determine the Fe concentration and the Al concentration in the plating layer.

Then, a sample is taken from the plated steel sheet in a plate thickness section parallel to the rolling direction of the base material steel plate, and the Fe-Al alloy layer at the interface between the base material steel plate and the hot-dip galvanized layer is obtained by the above-described measurement method. The difference in thickness between the average thickness and the width direction, and the difference in thickness between the average thickness and the width direction of the micronized layer directly contacting the micro-thin layer, and the average particle diameter of the ferrite-grain iron phase in the micro-layer (fertilizer iron-phase average particle diameter) And the maximum diameter (maximum oxide diameter) of one or more oxides of Si and Mn in the micronized layer. The results are shown in Tables 20 to 23.

Next, in order to investigate the characteristics of the plated steel sheet, tensile test, hole expansion test, bending test, fatigue test, adhesion evaluation test, spot welding test, corrosion test, fragmentation test, pulverization test, and plating were performed. Appearance uniformity assessment. The characteristics of each experimental example are shown in Tables 24 to 31.

In the tensile test, the test piece No. 5 described in JIS Z 2201 was prepared from the plated steel sheet, and the drop strength YS, the maximum tensile strength TS, and the total elongation El were determined according to the method described in JIS Z2241. Further, the tensile properties were evaluated as good when the maximum tensile strength TS was 550 MPa or more. The hole expansion test was carried out in accordance with the method described in JIS Z 2256. In the formability, the ductility (total elongation) El and the hole expandability λ vary depending on the maximum tensile strength TS, but the strength, ductility, and hole expandability are evaluated when the following formula (2) is satisfied. TS ^1.5 × El × λ ^0.5 ≧ 2.0 × 10 ⁶・・・式(2)

In the bending test, the No. 5 test piece described in JIS Z 2201 was prepared from the hot-dip galvanized steel sheet, and the 90° V bending test was carried out in accordance with the V-shaped concave block method described in JIS Z 2248. The radius of the bottom of the V-shaped concave block was changed from 1.0 mm to 6.0 mm on a scale of 0.5 mm, and the minimum radius at which the test piece was not cracked was taken as the minimum bending radius r [mm]. The bending property was evaluated by evaluating the minimum bending radius r as "r/t" normalized by the thickness t [mm], and evaluating "r/t" below 2.0 as good bending property.

In the fatigue test, the No. 1 test piece described in JIS Z 2275 was prepared from the hot-dip galvanized steel sheet, and the pulse plane bending fatigue test was performed in accordance with JIS Z 2273. The fatigue limit DL and the fatigue limit ratio DL/TS were evaluated by setting the maximum number of repetitions to 10 million times, and the fatigue limit ratio was evaluated to be a good fatigue resistance characteristic with a fatigue limit ratio of 0.30 or more.

The plating adhesion was performed on a plated steel sheet to which 5% uniaxial tensile strain was imparted. The plated steel sheet after the smashing test was peeled off with an adhesive tape, and the plating was not peeled off, and it was evaluated as particularly good (?), and the plating was evaluated as poor (×) when peeled by 5% or more, and plating was performed. The case where the peeling was less than 5% was evaluated as good (○). The DuPont Crushing Test uses a punch with a radius of curvature of 1/2 turn at the front end to allow a 3 kg hammer to fall from a height of 1 m.

The point weldability is evaluated by a continuous dot test. The spot welding was carried out 1,000 times in a welding condition in which the diameter of the molten portion became 5.3 to 5.7 times the square root of the plate thickness. Further, the first point d ₁ of the diameter of the molten portion was compared with the 1000th point d ₁₀₀₀ , and the case where d ₁₀₀₀ /d ₁ was 0.90 or more was evaluated as pass (○), and the case of less than 0.90 was evaluated as unacceptable (×).

The corrosion resistance evaluation was performed by cutting a test piece of 150 mm × 70 mm from a plated steel plate. The test piece was subjected to a zinc phosphate-based immersion chemical conversion treatment, followed by a 20 μm cation plating coating, a 35 μm intermediate coating, and a 35 μm surface coating, and then the back surface and the end portion were sealed with an insulating tape. The corrosion resistance test used CCT with SST 6 hr → drying 4 hr → wet 4 hr → freezing 4 hr for 1 cycle. The evaluation of the corrosion resistance after coating was carried out by cutting the coated surface with a cutter to the base metal steel sheet to measure the expansion width after 60 cycles of CCT. The case where the expansion width was 3.0 mm or less was evaluated as pass (○), and the case where the expansion width was more than 3.0 mm was evaluated as unacceptable (×).

Fragmentation was carried out by using a test piece of 70 mm × 150 mm cut out from a plated steel plate. First, the test piece was subjected to steps of degreasing, forming a chemical conversion film, and three-layer coating for automobiles. Next, 10 pieces of gravel (0.3 g to 0.5 g) were vertically irradiated with an air pressure of ² kgf/cm ² while the test piece was cooled and maintained at -20 °C. The irradiation of the crushed stone was carried out five times for each test piece. Thereafter, a total of 50 cracks were observed for each test piece, and the position of the peeling interface was evaluated based on the following criteria. The peeling interface was evaluated to be more qualified than the plating layer (the interface of the plating layer-chemical conversion film or the interface of the plating coating-intermediate coating) (○), and there was one place at the interface of the plating layer-based iron. The peeling evaluation was unacceptable (×).

The pulverizability was evaluated by evaluating the workability of the plating layer using V bending processing (JIS Z 2248). The plated steel sheet was cut into 50 × 90 mm, and a molded body was formed using a 1R-90°V-shaped mold press to prepare a test body. Tape peeling was performed on the valley of each test body. Specifically, after the cellophane tape having a width of 24 mm was pressed against the bent portion of the test piece and torn, the portion of the cellophane tape having a length of 90 mm was visually judged. The evaluation criteria are as follows. The peeling of the plating layer was evaluated as acceptable (○) with respect to the area of the processed portion of less than 5%, and the peeling of the plating layer was evaluated as unacceptable (×) with respect to the area of the processed portion of more than 5%.

Appearance uniformity evaluation is to determine the position of the steel plate in the width direction of 50 mm from the 両 end and the brightness (L*値) of 8 points divided into 7 equal parts. The difference between the maximum 値 minus the minimum 小于 is less than 5 and is evaluated as uniform. (○), 5 or more and less than 10 were evaluated as slightly uneven (Δ), and 10 or more was evaluated as uneven (×).

[Table 1]

[Table 2]

[table 3]

[Table 4]

[table 5]

[Table 6]

[Table 7]

[Table 8]

[Table 9]

[Table 10]

[Table 11]

[Table 12]

[Table 13]

[Table 14]

[Table 15]

[Table 16]

[Table 17]

[Table 18]

[Table 19]

[Table 20]

[Table 21]

[Table 22]

[Table 23]

[Table 24]

[Table 25]

[Table 26]

[Table 27]

[Table 28]

[Table 29]

[Table 30]

[Table 31]

In the experimental example 30 of the embodiment of the present invention, the effective Al amount is 0.180% by mass, but the Fe-Al alloy is uniformly formed in the width direction. Therefore, the plating adhesion described in Patent Document 12 is not lowered, and the improvement is improved.

In Experimental Example 187, when the C content was small, the volume fraction of the hard phase became small, and sufficient strength, ductility, and hole expandability could not be obtained. Experimental Example 188 is an example in which the C content is large and the weldability of the spot is deteriorated. Moreover, the fraction of the granulated iron in the kenian becomes large, and the fatigue resistance, ductility, hole expandability, and bendability are inferior.

In Experimental Example 189, a large amount of Si was formed, and in the annealing step and the plating step, a large amount of ferrite and coarse swarf carbon iron were generated, and the formability of the steel sheet could not be sufficiently obtained. In Experimental Example 190, the Si content was large, and in the hot rolling step, the steel embryo was cracked during heating, so that the experiment was interrupted.

In Experimental Example 191, the Mn content was small, and a large amount of pulverized iron and coarse swarf carbon iron were generated in the annealing step and the plating step, and the formability of the steel sheet could not be sufficiently obtained. In Experimental Example 192, the Mn content was large, and in the hot rolling step, the steel embryo was cracked during heating, so that the experiment was interrupted.

In Experimental Example 193, the content of P was large, and cracking occurred after rolling the steel preform in the hot rolling step, so that the experiment was interrupted. In Experimental Example 194, the S content was large, and a large amount of coarse sulfide was formed, so that ductility, hole expandability, bendability, spot weldability, and fatigue resistance were deteriorated. In Experimental Example 195, the Al content was large, and in the hot rolling step, the steel blank was cracked during transportation, so that the experiment was interrupted.

In Experimental Example 196, a large amount of N was formed, and a large amount of coarse nitride was formed, so that ductility, hole expandability, bendability, spot weldability, and fatigue resistance were deteriorated. In Experimental Example 197, a large amount of O was formed, and a large amount of coarse oxide was formed, so that ductility, hole expandability, bendability, spot weldability, and fatigue resistance were deteriorated.

In Experimental Examples 28, 131, and 155, since the reduction of the formula (1) in the hot rolling step is small, the hard phase fraction (V1/V2) of the surface layer is increased, and sufficient flexibility cannot be obtained. In Experimental Examples 51, 114, and 165, in the hot rolling step, the formula (1) was large, and the decarburization of the surface layer was excessively performed. Therefore, the hard phase fraction (V1/V2) was small, and sufficient fatigue resistance could not be obtained. example.

In the annealing example, in the annealing step, since the average heating rate in the temperature range between 600 ° C and 750 ° C is small, the oxide inside the steel sheet is excessively grown, and a coarse oxide which is a starting point of destruction is formed, so that the bending property is high. An example of deterioration of fatigue resistance. In addition, as the bending property is deteriorated, plating peeling occurs due to bending cracks of the steel sheet in the evaluation test of the plating adhesion and the pulverization property, so that the plating adhesion and the pulverizability are also deteriorated. In the annealing step, in the annealing step, the average heating rate in the temperature range between 600 ° C and 750 ° C is excessively large, so that the difference between the maximum 値 and the minimum 厚度 of the thickness of the fine layer in the width direction of the steel sheet is more than 2.0 μm, and the width direction of the steel sheet The difference between the maximum 値 and the minimum 厚度 of the thickness of the Fe-Al alloy layer is greater than 0.5 μm. Therefore, the plating appearance is slightly uneven.

In Experimental Example 67, in the annealing step, since the maximum heating temperature (TM) was higher than the Ac3 temperature, the volume fraction of the ferrite-grain iron phase at a thickness of 1/4 became small, and the ductility and the bendability were deteriorated. In Experimental Example 183, the maximum heating temperature (TM) was lower than (Ac1 + 20) °C in the annealing step, so that coarse iron-based carbides remained in the melt, and ductility and hole expandability were deteriorated. In Experimental Example 46, the air ratio of the pre-tropical zone was small in the heating step of the annealing step, so that the plating adhesion was deteriorated. Further, since plating adhesion occurs during bending deformation due to deterioration in plating adhesion, pulverizability is also deteriorated. Moreover, it is not plated and also causes poor appearance.

In the experimental example 16, in the heating step of the annealing step, the air ratio of the pre-tropical zone is large, and the decarburization of the surface layer is excessively performed, so that the fraction (V1/V2) of the hard phase is small, and the fatigue resistance is deteriorated. In addition, the average thickness of the fine layer is increased, and the alloying of the plating layer (formation of Zn-Fe alloy) is excessively performed, resulting in an increase in Fe content in the plating layer, and plating adhesion and chipping property. An example of a smashing property.

In Experimental Example 181, the Log (P(H ₂ O)/P(H ₂ )) of the pre-tropical zone was small in the heating step of the annealing step, and the surface layer was not refined, and the plating adhesion was deteriorated. Further, the average grain size of the ferrite iron on the surface was 3.2 μm, and the maximum diameter of the oxide inside the steel sheet in the range of 0.5 μm from the surface was less than 0.01 μm. Moreover, it is not plated and also causes poor appearance. In Experimental Example 168, the log (P(H ₂ O)/P(H ₂ )) in the tropical zone was small in the heating step of the annealing step, so that the particle size of the surface layer of the base material steel plate was not refined, and the plating adhesion was changed. Poor example. Further, the average particle diameter of the ferrite iron on the surface of the base material steel sheet was 3.4 μm, and the maximum diameter of the oxide inside the steel sheet in the range of 0.5 μm from the surface was less than 0.01 μm.

In Experimental Example 198, the Log (P(H ₂ O)/P(H ₂ )) of the pre-tropical zone was large in the heating step of the annealing step, so that the surface layer of the fine layer was excessively thickened, and the plating layer was alloyed (generated). Excessive progress of the Zn-Fe alloy causes an increase in the Fe content in the plating layer and an example in which the plating adhesion is deteriorated. Further, the difference between the maximum 値 and the minimum 厚度 of the thickness of the Fe-Al alloy layer in the width direction of the steel sheet is more than 0.5 μm, so that the plating appearance is not uniform. In the experimental example 75, the log (P(H ₂ O)/P(H ₂ )) of the tropical zone was particularly large in the heating step of the annealing step, and the decarburization of the surface layer was excessively performed to make the fraction of the hard phase (V1/V2). As it becomes smaller, the fatigue resistance is deteriorated. Further, the fine layer of the surface layer of the base material steel sheet is excessively thick, and the alloying of the plating layer (formation of a Zn-Fe alloy) is excessively performed, resulting in an increase in the Fe content in the plating layer, and plating adhesion and chipping. Examples of poor sex and pulverization.

In the heating step of the annealing step, in the heating step of the annealing step, the average cooling rate (cooling rate 1) between 760 ° C and 700 ° C is small, and a large amount of ferrite is generated, so that the ductility and the hole expandability are deteriorated. In Experimental Example 49, in the heating step of the annealing step, the average cooling rate (cooling rate 1) between 760 ° C and 700 ° C was large, and the volume fraction of the ferrite-grain iron phase at 1/4 thickness was small, and it was impossible to Obtain sufficient examples of ductility. In Experimental Example 137, in the heating step of the annealing step, the average cooling rate (cooling rate 2) between 650 ° C and 500 ° C was small, and a large amount of ferrite was generated, so that ductility and hole expandability were deteriorated.

In Experimental Example 2, in the plating step, the effective Al concentration in the plating bath was low, and a Fe-Al alloy layer having a sufficient thickness could not be formed, and the Fe content in the plating layer became large, and plating adhesion could not be obtained. Examples of fragmentation, pulverizability, and uniformity of appearance of plating. In Experimental Example 14, in the plating step, the effective Al concentration in the plating bath was extremely low, and the Fe-Al alloy layer could not be formed, and the Fe content in the plating layer was excessively increased, and sufficient plating adhesion could not be obtained. Examples of properties, fragmentation, pulverizability, and uniformity of plating appearance. In Experimental Example 148, in the plating step, the effective Al concentration in the plating bath was high, and the Al content in the plating layer was excessively increased, and sufficient plating adhesion and spot fusion properties could not be obtained. In Experimental Example 180, the cooling rate after plating in the plating step was small, and the Fe content in the plating layer was excessively increased, and sufficient plating adhesion, chipping property, and pulverizability could not be obtained.

Experimental Examples 8, 17, 31, 81, and 179 are examples in which the metamorphic treatment of the granulated iron is performed in the cooling step of the annealing step, and the formability, the plating adhesion, the weldability, the corrosion resistance, and the fatigue resistance are excellent. High strength hot-dip galvanized steel sheet. Experimental Examples 37, 45, 66, 89, 102, 104, 108, 142, 149, and 162 were subjected to the toughening iron metamorphosis treatment 1 in the cooling step of the annealing step to obtain formability and plating adhesion. High-strength hot-dip galvanized steel sheet with excellent weldability, corrosion resistance and fatigue resistance. In the experimental example 43, an example of the toughening iron metamorphosis treatment 1 was carried out in the cooling step of the annealing step. However, since the treatment temperature was high, the ferrite and the coarse snow-capped carbon iron were generated, and the ductility and the hole expandability were deteriorated. Experimental Example 174 was an example in which the toughening iron metamorphosis treatment 1 was carried out in the cooling step of the annealing step. However, since the treatment time was long, the ferrite and the coarse stellite carbon were generated, and the ductility and the hole expandability were deteriorated.

Experimental Examples 25, 41, 55, 90, 113, 135, 152, 170, and 184 are examples in which the toughening iron metamorphosis treatment 2 is performed in the post-plating cooling step to obtain formability, plating adhesion, and fusion. High-strength hot-dip galvanized steel sheet excellent in properties, corrosion resistance and fatigue resistance. In Experimental Example 163, an example of the toughening iron metamorphosis treatment 2 was carried out in the post-plating cooling step. However, since the treatment temperature was high, a large amount of residual Worthite iron was generated, which was an example of the loss of flexibility. Experimental Examples 5, 20, 48, 53, 62, 73, 76, 87, 94, 112, 129, and 186 were subjected to reheating treatment in the post-plating cooling step to obtain formability and plating adhesion. High-strength hot-dip galvanized steel sheet excellent in properties, weldability, corrosion resistance and fatigue resistance.

In the experimental example 18, the Matian iron treatment and the toughening iron transformation treatment 1 were carried out in the cooling step of the annealing step, and the moldability, the plating adhesion, the weldability, the corrosion resistance and the fatigue resistance were excellent. High-strength galvanized steel sheet. In the experimental examples 9, 110, 132, 138, 154, and 169, after the toughening iron metamorphosis treatment 1 was performed in the cooling step of the annealing step, an example of the toughening iron metamorphosis treatment 2 was performed in the post-plating cooling step. High-strength hot-dip galvanized steel sheet excellent in formability, plating adhesion, weldability, corrosion resistance, and fatigue resistance. In the experimental examples 24, 34, and 96, after the toughening iron metamorphosis treatment 1 was performed in the cooling step of the annealing step, the reheating treatment was performed in the post-plating cooling step to obtain formability, plating adhesion, High-strength hot-dip galvanized steel sheet excellent in weldability, corrosion resistance and fatigue resistance.

In the eleventh example, the toughened iron metamorphosis treatment 2 and the reheat treatment were carried out in the post-plating cooling step, and the moldability, the plating adhesion, the weldability, the corrosion resistance, and the fatigue resistance were excellent. The high-strength hot-dip galvanized steel sheet has a slightly uneven appearance as described later.

In the experimental example 172, after the toughening iron metamorphosis treatment 1 was performed in the cooling step of the annealing step, the toughening iron metamorphosis treatment 2 and the reheating treatment were performed in the post-plating cooling step, and the formability and plating were obtained. A high-strength hot-dip galvanized steel sheet excellent in adhesion, weldability, corrosion resistance, and fatigue resistance.

In the experimental example 79, the diameter of the roll used for the processing in the bending-folding processing step of the processing step was small, and excessive strain was introduced into the surface layer portion of the steel sheet, resulting in an example in which the ductility was deteriorated. In the experimental example 59, the diameter of the roll used for the processing in the bending-folding processing step of the processing step was large, and the strain was not sufficiently introduced into the surface layer portion of the steel sheet, so that a large amount of residual Worth iron was present in the surface layer portion of the steel sheet, and the bending property was changed. Poor example.

In the experimental examples 3, 11, and 116, the coiling temperature in the hot rolling step was 650 ° C or more, so that the difference between the maximum 値 and the minimum 微 of the thickness of the micronized layer in the width direction of the steel sheet was more than 2.0 μm, and in the width direction of the steel sheet. The difference between the maximum 値 and the minimum 厚度 of the thickness of the Fe-Al alloy layer is greater than 0.5 μm, and the plating appearance is slightly uneven. On the other hand, in Experimental Examples 4 and 12, as described above, although the coiling temperature of the hot rolling step was 650 ° C or higher, the maximum thickness of the micronized layer in the width direction of the steel sheet was increased by increasing the pickling time of the pickling step. The difference of the minimum enthalpy was improved to within 2.0 μm, and the difference between the maximum 値 and the minimum 厚度 of the thickness of the Fe-Al alloy layer in the width direction of the steel sheet was improved to within 0.5 μm, and good plating appearance uniformity was obtained. In the embodiment 200, the coiling temperature is lower than 650 ° C, but the pickling time is 15 seconds and the time is short, so that the internal oxide layer which is uneven in formation cannot be completely removed, and the thickness of the micronized layer in the width direction of the steel sheet is the largest. The difference between 値 and the minimum 大于 is more than 2.0 μm, and the difference between the maximum 値 and the minimum 厚度 of the thickness of the Fe—Al alloy layer in the width direction of the steel sheet is more than 0.5 μm, and the plating appearance is slightly uneven.

The embodiments of the present invention have been described in detail above, but the above embodiments are merely examples for carrying out the invention. The technical scope of the present invention is not limited to the embodiments. That is, the present invention can be implemented in various forms without departing from the technical idea of the present invention or its main features.

INDUSTRIAL APPLICABILITY The present invention is a high-strength hot-dip galvanized steel sheet which can be effectively used for plating uniformity of appearance, formability, fatigue resistance, weldability, corrosion resistance and plating adhesion, and its production Method of technology. Further, according to the embodiment of the present invention, it is possible to provide high-strength hot-dip plating which is excellent in ductility, hole expandability, and bendability, and has excellent plating adhesion after molding, has a high fatigue limit, and has excellent spot weldability and corrosion resistance. Zinc steel plate and its manufacturing method.

10‧‧‧Metal galvanized layer

20‧‧‧Basic steel plate

30‧‧‧Fe-Al alloy layer

40‧‧‧Microlayer

50‧‧‧Decarburized layer

60‧‧‧Fat iron phase

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing a sectional structure of a hot-dip galvanized steel sheet according to the present invention.

Claims (9)

A hot-dip galvanized steel sheet having a hot-dip galvanized layer on at least one side of a base material steel sheet, and the hot-dip galvanized steel sheet is characterized in that the base material steel sheet has the following chemical composition: in mass%, C: 0.040% to 0.280 %, Si: 0.05% to 2.00%, Mn: 0.50% to 3.50%, P: 0.0001% to 0.1000%, S: 0.0001% to 0.0100%, Al: 0.001% to 1.500%, N: 0.0001% to 0.0100%, O: 0.0001% to 0.0100%, Ti: 0% to 0.150%, Nb: 0% to 0.100%, V: 0% to 0.300%, Cr: 0% to 2.00%, Ni: 0% to 2.00%, Cu: 0%~2.00%, Mo: 0%~2.00%, B: 0%~0.0100%, W: 0%~2.00%, and Ca, Ce, Mg, Zr, La, and REM: total 0%~0.0100%, And The remaining portion is composed of Fe and impurities; and has a microstructure in which a thickness of 1/8 to 3/8 is centered on the surface of the base material steel sheet with respect to the total thickness of the base material steel sheet; In the middle, the ferrite grain iron phase is 50% or more and 97% or less by volume fraction, and one or more of the toughened iron phase, the toughened ferrite iron phase, the newly formed Matian iron phase, and the tempered Matian iron phase The total amount of the hard structures formed is 3% or more in terms of volume fraction, and the residual Worstian iron phase is 0% to 8% by volume fraction, and the total volume of the Borne iron phase and the coarse snowy carbon iron phase is in volume. The fractional rate is 0% to 8%. From the interface between the hot-dip galvanized layer and the base material steel plate to the depth of the base metal plate in the direction of 20 μm, the remaining Worthite iron is 0 to 3 by volume fraction. %, the ratio of the volume fraction V1 of the hard structure in the surface layer portion to the volume fraction V2 of the hard tissue in the thickness range of 1/8 thickness to 3/8 centering on the surface of the base material steel sheet by 1/4 thickness , V1/V2 is set to a range of 0.10 or more and 0.90 or less; the Fe content in the above-mentioned hot-dip galvanized layer is more than 0% by mass% and 3.0% or less, and the Al content is The amount % is more than 0% and is 1.0% or less, and has an Fe-Al alloy layer having an average thickness of 0.1 μm to 2.0 μm at the interface between the hot-dip galvanized layer and the base material steel sheet, and is in the width direction of the base material steel sheet. The difference between the maximum thickness and the minimum thickness is within 0.5 μm, and the base material steel sheet has direct contact with the aforementioned Fe-Al alloy layer. In the micronized layer, the average thickness of the fine layer is 0.1 μm to 5.0 μm, and the average grain size of the ferrite phase in the fine layer is 0.1 μm to 3.0 μm, and the fine layer contains Si and Mn 1 The oxide or the two or more kinds of oxides have a maximum diameter of 0.01 μm to 0.4 μm, and the difference between the maximum thickness and the minimum thickness of the fine layer in the width direction of the base material steel sheet is within 2.0 μm. The hot-dip galvanized steel sheet according to claim 1, wherein the single-sided plating adhesion amount of the hot-dip galvanized layer is 10 g/m ² or more and 100 g/m ² or less. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the base material steel sheet is selected from the group consisting of Ti: 0.001% to 0.150%, Nb: 0.001% to 0.100%, and V: 0.001% to 0.300% by mass%. One or two or more of the groups formed. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the base material steel sheet contains, by mass%, selected from the group consisting of Cr: 0.01% to 2.00%, Ni: 0.01% to 2.00%, Cu: 0.01% to 2.00%, Mo: one or more of the group consisting of 0.01% to 2.00%, B: 0.0001% to 0.0100%, and W: 0.01% to 2.00%. The hot-dip galvanized steel sheet according to claim 3, wherein the base material steel sheet is selected from the group consisting of Cr: 0.01% to 2.00%, Ni: 0.01% to 2.00%, Cu: 0.01% to 2.00%, and Mo: 0.01% to 2.00%, B: 0.0001% to 0.0100%, and W: 0.01% to 2.00% of one or more of the groups. A molten galvanized steel sheet according to claim 1 or 2, wherein the aforementioned The base material steel plate is contained in a total of 0.0001% to 0.0100% by mass, and one or more selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM. The hot-dip galvanized steel sheet according to claim 3, wherein the base material steel sheet contains 0.0001% to 0.0100% by mass in total of one selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM. Or two or more. The hot-dip galvanized steel sheet according to claim 4, wherein the base material steel sheet contains 0.0001% to 0.0100% by mass in total of one selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM. Or two or more. The hot-dip galvanized steel sheet according to claim 5, wherein the base material steel sheet contains 0.0001% to 0.0100% by mass in total of one selected from the group consisting of Ca, Ce, Mg, Zr, La, and REM. Or two or more.