WO2013157222A1

WO2013157222A1 - High-strength hot-dip galvanized steel sheet and process for producing same

Info

Publication number: WO2013157222A1
Application number: PCT/JP2013/002420
Authority: WO
Inventors: 善継鈴木; 大輔原子; 長滝　康伸
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2012-04-18
Filing date: 2013-04-09
Publication date: 2013-10-24
Also published as: CN104245999A; JP5630588B2; KR20150008112A; JPWO2013157222A1; CN104245999B; KR101657866B1

Abstract

A hot-dip galvannealed steel sheet which comprises a steel sheet that has a composition containing, in terms of mass%, 0.02-0.30% C, 0.01-2.5% Si, 0.1-3.0% Mn, 0.003-0.08% P, up to 0.01% S, 0.001-0.20% Al, and 0.03-0.40% Ti, with the remainder comprising Fe and unavoidable impurities, and a zinc coating layer formed on each of the surfaces thereof in an amount of 20-120 g/m² per surface, wherein each coating layer contains carbide grains having an average grain diameter of 10 nm or smaller in a number of 5-50 per section and oxide grains having an average grain diameter of 50 nm or larger in a number of 5-50 per section. The section means an area (t₁×1 (µm²)) defined by the coating-layer thickness (t₁ µm) and segments obtained by dividing a cross-section of the coating layer in the direction perpendicular to the thickness direction at an interval of 1 µm.

Description

High-strength hot-dip galvanized steel sheet and manufacturing method thereof

The present invention relates to an alloyed hot-dip galvanized steel sheet suitable as an automotive rust-proof surface-treated steel sheet and a method for producing the same.

Conventionally, hot rolled steel sheets of TS440 MPa class or lower have been used for members such as automobile and truck frames and undercarriages. However, recently, for the purpose of improving the anti-collision properties of automobiles and preserving the global environment, high strength and thinning of steel sheets for automobiles has been promoted, and high strength hot rolled steel sheets of TS590MPa class, TS780MPa class, and more than TS980MPa class. The use of is beginning to be considered.

Automobile members often have complicated shapes obtained by press molding, and materials that are high in strength but excellent in workability are required. On the other hand, from the viewpoint of securing the rust prevention power of the car body due to the thinning of the steel sheet, the surface treated steel sheet provided with rust resistance to the material steel sheet, especially alloyed molten zinc that is excellent in corrosion resistance and weldability after painting and can be manufactured at low cost. There is a need for plated steel sheets.

Conventionally, several high-strength hot-rolled steel sheets or hot-dip galvanized high-strength steel sheets excellent in workability and their manufacturing methods have been proposed. For example, in Patent Document 1, in mass%, C: 0.02 to 0.06%, Si ≦ 0.3%, Mn: 0.5 to 2.0%, P ≦ 0.06%, S ≦ 0 0.005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.14%, the balance being substantially made of Fe By melting steel and performing hot rolling under conditions of finish rolling finish temperature of 880 ° C. or higher and coiling temperature of 570 ° C. or higher, Ti and Mo having substantially a ferrite single phase structure and an average particle size of less than 10 nm are obtained. There is disclosed a high-strength steel sheet excellent in workability having a tensile strength of 590 MPa or more, and a method for producing the same, characterized in that a carbide containing is dispersed and precipitated.

Further, in Patent Document 2, by mass, C: 0.01 to 0.1%, Si ≦ 0.3%, Mn: 0.2 to 2.0%, P ≦ 0.04%, S ≦ 0 0.02%, Al ≦ 0.1%, N ≦ 0.006%, Ti: 0.03 to 0.2%, and one or more of Mo ≦ 0.5% and W ≦ 1.0% Steel, the balance being Fe and inevitable impurities are melted, hot-rolled in the austenite single-phase region, wound at 550 ° C. or more, and after producing a single-phase ferrite hot-rolled steel sheet, the scale is further removed. By applying the hot dip galvanizing as it is, it is 4.8C + 4.2Si + 0.4Mn + 2Ti ≦ 2.5 by mass%, the structure is ferrite with an area ratio of 98% or more, and the atomic ratio is (Mo + W) / In the range satisfying (Ti + Mo + W) ≧ 0.2, at least one of Ti, Mo and W Precipitates of less than 10nm, including a is characterized by the presence in the dispersion method of producing a hot-dip galvanized high-strength hot-rolled steel sheet is disclosed.

However, in Patent Documents 1 and 2, since fine carbides containing Ti and Mo are precipitated in ferrite, a coiling temperature of 550 ° C. or higher (hereinafter sometimes referred to as CT) after finishing rolling is finished. It is necessary to wind up with. When performing a winding process under such high CT conditions on a hot-rolled base material containing an element that is easier to oxidize than Fe, such as Si and Mn (hereinafter also referred to as an easily oxidizable element). By forming an internal oxide containing an easily oxidizable element on the surface layer of the steel plate base material, the Zn-Fe alloying reaction is excessively promoted in the subsequent hot dip galvanizing treatment and alloying treatment, and the plating adhesion There is a problem of deterioration. In addition, if there is a large amount of internal oxide in the base steel plate surface layer, the internal oxide becomes the starting point during stretch flange processing, and fine cracks occur in the steel plate surface layer and plating layer. There is a problem of deterioration.

On the other hand, in order to suppress internal oxides generated during hot rolling, when performing a winding process with a reduced CT, the precipitation of carbides is insufficient, and a structure such as pearlite grows to increase strength and In addition to causing a decrease in workability, there is a problem in that hydrogen is occluded and the hydrogen embrittlement resistance deteriorates when the steel sheet is annealed in the subsequent continuous galvanizing equipment.

JP 2002-322543 A JP 2003-321736 A

The present invention has been made in view of such circumstances, and while ensuring good workability, the plating adhesion of the bent portion and the corrosion resistance after painting of the stretch flange portion are excellent, and further, the hydrogen embrittlement resistance is excellent. An object is to provide a high-strength hot-dip galvanized steel sheet.

The present inventors obtained the following knowledge as a result of intensive studies on the plating treatment of high-strength steel sheets.

First, the present inventors have found that in order to obtain a plated steel sheet having excellent hydrogen embrittlement resistance, the average particle size of the components in the plating layer, particularly oxides and carbides, is extremely important. The reason for this is that when a carbide having an average particle diameter of 10 nm or less and an oxide having an average particle diameter of 50 nm or more exist in the surface layer portion of the steel sheet, it acts as a hydrogen intrusion trap site and suppresses the concentration of diffusible hydrogen into the steel sheet. It is thought that the sensitivity of delayed fracture is suppressed. Furthermore, cracks are generated and propagated in the plating layer when subjected to compressive strain during press molding. In the present invention, the presence of fine carbides and oxides allows the fine carbides and oxides to have a pinning effect at the crack generation portion. It is presumed that this pinning effect stops the propagation of cracks and does not lead to large peeling, but improves the plating adhesion during bending. As a result, the corrosion resistance after painting is improved.

Next, regarding the manufacturing method, while controlling the CT to suppress the internal oxide generated during hot rolling, the solid solution Ti present in the steel sheet surface layer portion is defined by prescribing the heating temperature when annealing the steel sheet thereafter. The present inventors have found that it is important to precipitate as carbide. In addition, in order to stably precipitate Ti oxide during annealing, the oxide layer obtained by oxidizing the steel sheet surface layer in the heating zone is used as a source of oxygen that internally oxidizes Ti during reductive annealing in the soaking zone. The inventors have found that this is essential. In addition, it is extremely important to control the water vapor partial pressure (P _H2O ) and the hydrogen partial pressure (P _H2 ) in the furnace atmosphere in order to stably precipitate Ti carbide during reductive annealing in the soaking zone. The present inventors have found. As a result, these carbides and oxides are taken into the plating layer during the hot dip galvanizing treatment and alloying treatment and are present in the plating layer, thereby improving post-coating corrosion resistance, plating adhesion, and hydrogen embrittlement resistance.

The present invention is based on the above findings, and the features thereof are as follows.
[1] By mass%, C: 0.02% to 0.30%, Si: 0.01% to 2.5%, Mn: 0.1% to 3.0%, P: 0.00. 00% or more and 0.08% or less, S: 0.01% or less, Al: 0.001% or more and 0.20% or less, Ti: 0.03% or more and 0.40% or less, with the balance being Fe and A steel plate surface having a component composition consisting of inevitable impurities has a galvanized layer with a coating amount of 20 to 120 g / m ^{2 on} one side, and carbide with an average particle size of 10 nm or less per section 5. An alloyed hot-dip galvanized steel sheet characterized in that 5 or more and 50 or less oxides having an average particle diameter of 50 nm or more are present in a ratio of 5 or more and 50 or less per section. The one section is the plating layer thickness (t ₁ μm) and the area (t ₁ × 1 (μm ² )) obtained by dividing the plating layer cross section at intervals of 1 μm in the direction orthogonal to the thickness direction. It is.
[2] The carbide includes Ti, and the oxide includes one or more oxides selected from TiO ₂ , MnO, MnO ₂ , SiO ₂ , Al ₂ O ₃ , Mn ₂ SiO ₄ , and MnSiO _3. The high-strength hot-dip galvanized steel sheet according to [1].
[3] The steel sheet further has a component composition of mass%, Nb: 0.001% to 0.2%, V: 0.001% to 0.5%, Mo: 0.01% or more. The high-strength hot-dip galvanized coating according to [1] or [2], containing 0.5% or less, W: one or more of 0.001% or more and 0.2% or less steel sheet.
[4] The steel sheet according to any one of [1] to [3], wherein the steel sheet further contains B: 0.0005% or more and 0.005% or less as a component composition by mass%. The high-strength hot-dip galvanized steel sheet as described.
[5] The high-strength hot-dip galvanized steel sheet according to any one of [1] to [4], wherein the steel sheet is a hot-rolled steel sheet.
[6] The steel having the component composition according to any one of [1], [3], and [4] is hot-rolled, and after finishing rolling, cooling and winding are performed, and then continuous annealing and A method for producing a high-strength hot-dip galvanized steel sheet, characterized in that when performing hot-dip galvanizing treatment, the finish rolling finish temperature is 850 ° C. or higher, the coiling temperature is 540 ° C. or lower, and continuous annealing is performed under the following conditions.
(A) The composition of the combustion gas in the heating zone of the annealing furnace is H ₂ ≧ 40 vol% or more, CH ₄ ≧ 20 vol%, CO ₂ ≧ 1 vol%, the balance CO, N ₂ , C _x H _y (x ≧ 2, y ≧ 4), the steel plate is heated to 520 ° C. or higher and 650 ° C. or lower at a furnace temperature of 500 ° C. or higher and 1000 ° C. or lower, and an oxidation treatment is performed to form an oxide layer having a thickness of 6 to 60 nm on the steel plate surface layer.
(B) Then, soaking zone atmosphere is balance _{N 2} include: hydrogen 5 vol% or more 50 vol%, and water vapor partial pressure _{(P H2 O)} and hydrogen partial pressure _{(P H2)} satisfies the equation (1) below Then, reduction annealing is performed at a temperature reached from 630 ° C. to 780 ° C. in the soaking zone.
10 ⁻³ ≦ P _H2O / P _H2 ≦ 10 ⁻¹ (1)
However, _PH2O shows water vapor partial pressure (Pa), and _PH2 shows hydrogen partial pressure (Pa).
[7] After the hot dipping treatment, the steel plate is further heated to a temperature of 450 ° C. or higher and 510 ° C. or lower to be alloyed, and cooled to 400 ° C. at a rate of 20 ° C./s or less. [7] The method for producing a high-strength hot-dip galvanized steel sheet according to [6], characterized by being in the range of 7 to 15%.

In the present invention, the high strength means that the tensile strength TS is 590 MPa or more. The alloyed hot-dip galvanized steel sheet of the present invention includes both cold-rolled steel sheets and hot-rolled steel sheets, and hot-rolled steel sheets are particularly preferable from the viewpoints of stretch flangeability and hole-expandability.

According to the present invention, it is possible to obtain a high-strength hot-dip galvanized steel sheet that is excellent in plating adhesion at the bent portion and corrosion resistance after coating at the stretch flanged portion, and further excellent in hydrogen embrittlement resistance while ensuring good workability.

Hereinafter, the present invention will be specifically described. In the following description, the unit of the content of each element of the steel component composition is “mass%”, and hereinafter, simply indicated by “%” unless otherwise specified.

Hereinafter, the present invention will be described in detail.
(1) Component composition C of steel sheet: 0.02% or more and 0.30% or less C is an element necessary for precipitating carbide in the steel sheet, and for that purpose, 0.02% or more is necessary. On the other hand, if it exceeds 0.30%, the weldability deteriorates, so the upper limit is made 0.30%.

Si: 0.01% or more and 2.5% or less Si is effective as a solid solution strengthening element. For the strengthening effect to appear, it is necessary to contain 0.01% or more. On the other hand, if the content exceeds 2.5%, Si oxide concentrates on the surface of the steel sheet during the annealing process, causing non-plating defects and plating adhesion deterioration, so the upper limit is 2.5%. To do.

Mn: 0.1% or more and 3.0% or less Mn is added for increasing the strength, and it is necessary to contain 0.1% or more for the strengthening effect to appear. On the other hand, if the content exceeds 3.0%, the Mn oxide is concentrated on the surface of the steel sheet during the annealing process, causing non-plating defects and plating adhesion deterioration, so the upper limit is made 3.0%.

P: 0.003% or more and 0.08% or less P is one of the elements inevitably contained, and in order to make it less than 0.003%, there is a concern about an increase in cost, so 0.003% That's it. On the other hand, when P exceeds 0.08%, weldability deteriorates. Furthermore, the surface quality deteriorates. In addition, a desired degree of alloying cannot be achieved unless the alloying treatment temperature is raised during the alloying treatment. When the alloying treatment temperature is raised to obtain a desired degree of alloying, the ductility deteriorates and at the same time the adhesion of the alloyed plating film deteriorates. If the amount of P added is too high, the alloying temperature rises excessively. From the above, in order to achieve a desired degree of alloying, good ductility, and adhesion of the alloyed plating film, the content is made 0.08% or less.

S: 0.01% or less S segregates at grain boundaries. Or when MnS produces | generates abundantly, toughness will be reduced. From the above, it is necessary to be 0.01% or less. The lower limit of the S content is not particularly limited, and may be about the impurity level.

Al: 0.001% to 0.20% Al is added for the purpose of deoxidizing molten steel. However, when the content is less than 0.001%, the object is not achieved. On the other hand, if the content exceeds 0.20%, a large amount of inclusions are generated, which causes wrinkling of the steel sheet. From the above, Al is made 0.001% or more and 0.20% or less.

Ti: 0.03% or more and 0.40% or less Ti is an element necessary for increasing the strength by precipitating carbide in the steel sheet, and is also an effective element from the viewpoint of cost. However, if the addition amount is less than 0.03%, the amount of precipitates necessary for increasing the strength is insufficient, and if it exceeds 0.40%, the effect is saturated and the cost increases. Accordingly, Ti is set to 0.03% or more and 0.40% or less.

In addition to the above elements, the following elements may be added to control the strength and workability.

Nb: 0.001% to 0.2%, V: 0.001% to 0.5%, Mo: 0.01% to 0.5%, W: 0.001% to 0.2% One or more of the following, Nb, V, Mo, W are elements that are effective for precipitating fine carbides in a stable manner by precipitating as a composite carbide containing Ti in the steel sheet. 1 type (s) or 2 or more types are added. However, if the addition amount is less than the specified range, the effect of increasing the strength due to precipitation is insufficient, and if it exceeds the specified range, the effect is saturated and the cost is increased. Therefore, when contained, Nb is 0.001% to 0.2%, V is 0.001% to 0.5%, Mo is 0.01% to 0.5%, and W is 0.001%. 001% to 0.2%.

B: 0.0005% to 0.005% B is an effective element for improving the hardenability. However, if it is less than 0.0005%, it is difficult to obtain the quenching promoting effect. On the other hand, when it exceeds 0.005%, the effect is saturated and the cost is increased. Therefore, when B is contained, the content of B is 0.0005% or more and 0.005% or less.

The balance is Fe and inevitable impurities.

(2) Carbides and oxides present in the plating layer In the high-strength hot-dip galvanized steel sheet of the present invention, the average particle size of carbides present in the plating layer is 10 nm or less, and the average particle size of oxides is 50 nm or more. It is characterized by being. If the average particle size of the carbide exceeds 10 nm, the effect of suppressing crack propagation is small and the adhesion of plating during processing is deteriorated, and the hydrogen trap effect is small and the hydrogen embrittlement resistance is deteriorated. When the average particle size of the oxide is less than 50 nm, the hydrogen trap effect is small and the hydrogen embrittlement resistance deteriorates. At the same time, cracks occur and the corrosion resistance after painting of the processed parts deteriorates. The carbide is present at a rate of 5 or more and 50 or less per section. If it is less than 5 per section, the hydrogen trap effect is small and the hydrogen embrittlement resistance deteriorates. When it exceeds 50 per section, the workability of the plating film is deteriorated and the plating adhesion is lowered.
Oxides are present at a rate of 5 or more and 50 or less per section. If it is less than 5 per section, the hydrogen trap effect is small and the hydrogen embrittlement resistance deteriorates. When it exceeds 50 per section, the workability of the plating film is deteriorated and the plating adhesion is lowered.
In addition, said 1 division is a fixed area of a plating cross section, and is an area obtained by dividing a plating layer thickness (t ₁ μm) and a plating layer cross section at a 1 μm interval in a direction perpendicular to the thickness direction. (T ₁ × 1 (μm ² )).

In the present invention, the carbide preferably contains Ti. In addition, it is effective that the oxide contains Ti, Si, Mn, and Al. Specifically, from among TiO ₂ , MnO, MnO ₂ , SiO ₂ , Al ₂ O ₃ , Mn ₂ SiO ₄ , and MnSiO ₃ One or more selected oxides are preferred. This is because the strengthening element added in the steel is precipitated as an oxide as much as possible to soften the surface layer portion of the steel plate immediately below the plating layer and promote stress relaxation during processing.

The composition of carbides and oxides in the plating layer can be confirmed by the following method. For example, after processing a cross section of a steel sheet into a thin piece so as to include a plating layer with a focused ion beam processing apparatus (FIB), observation with a transmission electron microscope (TEM) and composition with an energy dispersive X-ray detector (EDX) Examples of methods include analysis and electron beam analysis. Moreover, as a measuring method of the average particle diameter of the carbide | carbonized_material and oxide in a plating layer, the method of measuring and averaging the maximum diameter and the minimum diameter from the photograph by the said observation method was used.

Furthermore, the high-strength hot-dip galvanized steel sheet of the present invention has a galvanized layer having a plating adhesion amount of 20 to 120 g / m ^{2 on} one surface of the steel sheet. If it is less than 20 g / m ^2, it will be difficult to ensure corrosion resistance after coating, and if it exceeds 120 g / m ² , plating adhesion will be reduced.

The high-strength hot-dip galvanized steel sheet of the present invention is preferably a hot-rolled steel sheet for reasons of stretch flangeability and hole expansion.

(3) Manufacturing method of high-strength hot-dip galvanized steel sheet Next, the manufacturing method of the high-strength hot-dip galvanized steel sheet according to the present invention and the reason for limitation will be described.
First, hot rolling conditions will be described.

When the finish rolling finish temperature is 850 ° C. or more and the finish rolling finish temperature is less than 850 ° C., the cumulative amount of distortion that occurs due to the progress of rolling with non-recrystallization increases, leading to an increase in rolling load. Therefore, the finish rolling end temperature is set to 850 ° C. or higher. The upper limit is not particularly limited. In this invention, 1100 degrees C or less is preferable.

When the coiling temperature exceeds 540 ° C, an internal oxide is formed by an easily oxidizable element, and the Zn-Fe alloying reaction is excessively promoted during the subsequent hot dip galvanizing and alloying processes. As a result, the appearance deterioration due to the occurrence of uneven alloying, the plating adhesion of the bent portion is lowered, and the corrosion resistance of the stretch flange portion after coating is deteriorated. Further, since internal oxidation proceeds, Ti necessary for carbide generation is consumed, so that a carbide forming element such as Ti is consumed by internal oxidation, and a Ti-deficient layer is formed. Therefore, it is difficult for sufficient Ti carbide to be present in the plating layer. Therefore, the coiling temperature is 540 ° C. or lower.

Next, continuous annealing and hot dip galvanizing will be described.

The component composition of the gas in the heating zone of the annealing furnace is H ₂ ≧ 40 vol% or more, CH ₄ ≧ 20 vol%, CO ₂ ≧ 1 vol%, the balance CO, N ₂ , C _x H _y (x ≧ 2, y ≧ 4)
If the amount of H ₂ , CH ₄ , and CO ₂ is small, the surface activation effect after redox is small, and carbides and oxides formed immediately below the plating layer during the reduction annealing are difficult to be taken into the plating layer. Therefore, the carbide and oxide supply effect into the plating layer which is the most important in the present invention cannot be obtained. There is no particular limitation on the upper limit. For the remaining gas, the same effect can be obtained if these gases are mixed even in a very small amount. The combustion gas may be formed by mixing hydrogen gas into natural gas, industrial methane, ethane, propane gas, or the like, or coke gas generated by so-called water gas reaction may be used. However, the calorie of the coke gas changes depending on the operating rate of the mountain base and the coke oven that produce the raw material coal. Therefore, since it may be necessary to adjust components by adding hydrogen gas or the like, coke gas cannot always be used as it is.

Furnace temperature in the heating zone of 500 ° C. or more and 1000 ° C. or less If the furnace temperature is less than 500 ° C., the steel plate surface is not sufficiently oxidized and uneven oxidation occurs, so the effect of incorporating carbide oxide into the plating layer does not progress uniformly. . If it exceeds 1000 ° C., the surface of the steel sheet is excessively oxidized, the interface between the plating layer and the steel sheet is roughened, and the plating adhesion during processing deteriorates.

If the heating temperature of the steel sheet in the heating zone is 520 ° C. or more and 650 ° C. or less and less than 520 ° C., the steel plate surface is not sufficiently oxidized, so that the effect of incorporating carbides and oxides into the plating layer is small. If it exceeds 650 ° C., it will be excessively oxidized, and the interface between the plating layer and the steel sheet will be roughened and the plating adhesion during processing will deteriorate.

Oxidation treatment in which an oxide layer having a thickness of 6 to 60 nm is formed on the surface layer of the steel sheet In the present invention, if the thickness is less than 6 nm, the oxidation amount on the surface of the steel sheet is insufficient, so that the effect of incorporating carbide and oxide into the plating layer is small. If it exceeds 60 nm, excessive oxidation occurs, the interface between the plating layer and the steel sheet becomes rough, and the plating adhesion during processing deteriorates. Note that the oxide layer in the present invention is an oxide layer mainly composed of an Fe oxide layer and substantially free of Ti (refers to an oxide layer having Ti of 0.001% or less). When the heating temperature of the steel plate exceeds 650 ° C., Ti is taken into the iron oxide as an oxide. This is not preferable because it remains at the interface during the reduction annealing and roughens the interface between the plating layer and the steel sheet, and the plating adhesion during processing deteriorates.

Soaking zone atmosphere, a balance _{N 2} include: hydrogen 5 vol% or more 50 vol%, and water vapor partial pressure _{(P H2 O)} and hydrogen partial pressure _{(P H2)} satisfies the equation (1).
If H ₂ is less than 5 vol%, the steel sheet surface is not sufficiently reduced, so that oxide remains at the interface and the corrosion resistance after coating deteriorates. If it exceeds 50 vol%, the steel sheet occludes a large amount of hydrogen, so that blisters of the plating film are generated and the surface quality is deteriorated. The balance and _{N 2.} In addition, the water vapor partial pressure (P _H2O ) and hydrogen partial pressure (P _H2 ) ratio needs to satisfy the following formula (1).
10 ⁻³ ≦ P _H2O / P _H2 ≦ 10 ⁻¹ (1)
When P _H2O / P _H2 is less than 10 ⁻³ (0.0010), Ti is nitrided and does not become a carbide. On the other hand, if P _H2O / P _H2 exceeds 10 ⁻¹ (0.1000), Ti is internally oxidized and consumed during annealing, and carbide cannot be formed.

If the ultimate temperature of the steel sheet in the soaking zone during reduction annealing is not less than 630 ° C. and not more than 780 ° C. and less than 630 ° C., the surface is not activated and the effect of introducing carbides and oxides into the plating layer is not preferable. If it exceeds 780 ° C, Ti is selectively oxidized externally and consumed, and carbide cannot be formed. Control of H ₂ O composition is performed by installing a bubbling device outside the annealing furnace, passing N ₂ gas at a predetermined flow rate through a water tank maintained at room temperature, mixing it with N ₂ gas not previously humidified, and introducing it into the furnace. carry out. At this time, it is necessary to flow gas from the lower part of the annealing furnace. This is because H ₂ O stays in the upper part of the furnace because the specific gravity of H ₂ O is light. Here, the furnace body lower part indicates a height of 1/10 of the entire furnace body height.

Measurement method of H _{2 O} and H ₂ partial pressure from the dew point are not particularly limited. For example, a predetermined amount of gas is sampled, and the dew point is measured by using a dew point measuring device such as Dew Cup to determine the H ₂ O partial pressure. Similarly, the H ₂ partial pressure is measured with a commercially available H ₂ partial pressure gauge. Alternatively, if the pressure in the atmosphere is measured, the partial pressures of H ₂ O and H ₂ are calculated from the concentration ratio.

Moreover, the hot dip galvanized steel sheet of the present invention can be made into an alloyed hot dip galvanized steel sheet by subjecting it to an alloying treatment after the hot dip plating process. In this case, the steel sheet is heated to a temperature of 450 ° C. or more and 510 ° C. or less to be alloyed, and cooled to 400 ° C. at 20 ° C./s or less. The Fe content in the plating layer thus obtained is 7 to 15%. If the Fe content is less than 7%, not only a uniform surface appearance cannot be obtained due to the occurrence of unevenness in alloying, but also the Zn—Fe alloying reaction is insufficient, so that a soft ζ phase is formed thick on the plating surface layer. The flaking that causes the plating layer to flake off during bending is caused. On the other hand, if it exceeds 15%, the Zn—Fe alloying reaction proceeds excessively, a brittle Γ phase is formed in the vicinity of the interface between the plating layer and the steel sheet, and the plating adhesion deteriorates.
If the alloying temperature is less than 450 ° C., the alloying reaction does not proceed sufficiently. If it exceeds 510 ° C., the Γ phase is formed thick and the plating adhesion of the processed part is deteriorated. After alloying, it is cooled to 400 ° C. at 20 ° C./s or less. If the cooling rate is slow, the Γ phase is formed thick and the plating adhesion deteriorates.

After heating the slab having the composition shown in Table 1 at 1250 ° C., hot rolling is performed under the conditions shown in Table 2, and further black scale removal is performed by pickling, and a hot-rolled steel sheet having a thickness of 2.3 mm is obtained. did.

Next, continuous annealing treatment and hot dip galvanizing treatment were performed on the CGL line. In CGL, coke gas whose composition is adjusted to a predetermined component in the heating zone is burned and oxidized, and then the furnace atmosphere, water vapor partial pressure, hydrogen partial pressure, and maximum steel plate temperature in the soaking zone are shown in Table 2. The steel sheet was subjected to reduction treatment under the conditions shown in FIG. Note that the control of the dew point in the atmosphere, N ₂ by heating water tank installed in the gas line, wet N ₂ gas flows in advance separately installed piping was, wet N H ₂ gas into _two gas Were mixed and introduced into the furnace to control the dew point of the atmospheric gas. The H ₂ concentration in the atmosphere was controlled by adjusting the amount of H ₂ gas introduced into the N ₂ gas with a gas valve.

Thereafter, it was immersed in an Al-containing Zn bath having a bath temperature of 460 ° C. and subjected to hot dip galvanizing treatment. The plating adhesion amount at this time is 45 g / m ² (plating layer thickness t ₁ : 6 μm), 70 g / m ² (plating layer thickness t ₁ : 10 μm), 140 g / m ² (plating layer thickness) by gas wiping. (T ₁ : 20 μm). The alloyed hot dip galvanized steel sheet was subjected to an alloying treatment after the hot dip galvanizing treatment.

About the hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA) obtained as described above, appearance (plated surface appearance), workability, plating adhesion of bent parts, hydrogen embrittlement resistance, stretch flange The corrosion resistance after painting of the processed part was investigated. The measurement method and evaluation criteria are shown below. The size and composition of carbides and oxides in the plating layer were measured by observing and analyzing FIB-processed plated thin film samples with TEM-EDX and EELS. Moreover, about the steel plate surface layer after a heating, the oxide seed | species was identified and analyzed by the X ray diffraction method.

<Appearance>
Appearance was determined as good appearance (◯) when there was no plating surface appearance defect such as non-plating or alloying unevenness, and when it was present, it was judged as poor appearance (×).

<Processability>
A JIS No. 5 tensile test piece is taken from the sample in the direction of 90 ° with respect to the rolling direction, a tensile test is performed at a constant crosshead speed of 10 mm / min in accordance with the provisions of JIS Z 2241, and the tensile strength (TS (MPa)). Elongation (El (%)) was measured, and TS × El ≧ 15000 was judged good, and TS × El <15000 was judged bad.
In addition, a test piece having a hole punched out with a clearance of 12.5% by a punch of 10 mmφ at the center of a steel plate cut into a 130 mm square is pushed up by a 60 ° conical punch from the opposite direction on the burr side of the punched hole, and cracks are generated. The hole diameter d (mm) at the time of penetrating the steel plate was measured, and the hole expansion rate λ was calculated from the following equation.
λ (%) = [(d−10) / 10] × 100
<Plating adhesion of bent part>
The plating adhesion of the hot dip galvanized steel sheet not subjected to the alloying treatment was evaluated by the following evaluation after the steel sheet was bent at 180 °, the outside of the bent portion was peeled off with tape, the presence or absence of peeling of the plated layer was visually determined.
○: Plating layer is not peeled ×: Plating layer is peeled Also, the plating adhesion of the galvannealed steel sheet was evaluated by the following powdering test.
An adhesive tape was affixed to the plated steel sheet, the tape was applied to the inside, and the tape was bent back 90 ° with a bending radius of 5 mm, and the peeled tape was analyzed with fluorescent X-rays. The Zn count number per unit length at this time was determined as the amount of plating peeling. In terms of anti-powdering property, the amount of plating peeling obtained in the above-mentioned powdering test is good (◎) for rank 1 in light of the following criteria (◎), 2 is generally good (○), and 3 Evaluated as defective (x). ◎ and ○ are acceptable.
Plating peeling amount: Rank 0 less than 3000: 1 (good (◎))
3000 or more and less than −6000: 2 (good (◯))
6000 or more: 3 (defect (x))
<Hydrogen embrittlement resistance>
A strip test piece of 150 mm × 30 mm is bent at a bending radius of 5 mm, a water-resistant strain gauge is attached to the surface, and immersed in 0.5 mol / L sulfuric acid. A current density of 0.1 mA / cm ² is applied to the test piece. The electrolysis was performed by energizing the test piece, hydrogen was allowed to enter the test piece, and the occurrence of cracks after 2 hours of energization was evaluated according to the following criteria.
Good (○): No cracking defect (×): Cracking <corrosion resistance after painting>
Prepare a test piece with a hole punched at a clearance of 12.5% with a 10mmφ punch at the center of a steel plate cut into a 130mm square, and push it up from the opposite side of the burr side of the punched hole with a 60 ° conical punch to expand the hole. gave. At this time, it was pushed up until the hole expansion rate reached 80% at which cracking occurred. The test piece thus processed was subjected to chemical conversion treatment and electrodeposition coating, and a salt spray test based on JIS Z 2371 (2000) was conducted for 10 days to evaluate the presence or absence of swelling in the processed part.
Good (◯): No blistering failure (×): Swollen The results obtained above are shown in Table 2-1, Table 2-2, Table 3-1, and Table 3-2 together with the manufacturing conditions.

From Table 2-1, Table 2-2, Table 3-1, and Table 3-2, the examples of the present invention are the appearance, workability, plating adhesion of the bent portion, hydrogen embrittlement resistance, and coating of the stretch flange processed portion. Good post-corrosion resistance (◯). On the other hand, the comparative example which does not satisfy the scope of the present invention has a low evaluation.

Claims

In mass%, C: 0.02% to 0.30%, Si: 0.01% to 2.5%, Mn: 0.1% to 3.0%, P: 0.003% or more 0.08% or less, S: 0.01% or less, Al: 0.001% or more and 0.20% or less, Ti: 0.03% or more and 0.40% or less, with the balance being Fe and inevitable impurities The surface of the steel sheet having a composition comprising: a galvanized layer having a coating adhesion amount of 20 to 120 g / m 2 on one side, and 5 or more carbides having an average particle size of 10 nm or less per section in the plated layer An alloyed hot-dip galvanized steel sheet, wherein 50 or less oxides having an average particle diameter of 50 nm or more are present in a proportion of 5 or more and 50 or less per section. The one section is the plating layer thickness (t 1 μm) and the area (t 1 × 1 (μm 2 )) obtained by dividing the plating layer cross section at intervals of 1 μm in the direction orthogonal to the thickness direction. It is.
The carbide includes Ti, and the oxide includes one or more oxides selected from TiO 2 , MnO, MnO 2 , SiO 2 , Al 2 O 3 , Mn 2 SiO 4 , and MnSiO 3. The high-strength hot-dip galvanized steel sheet according to claim 1.
The steel sheet further has a component composition of mass%, Nb: 0.001% to 0.2%, V: 0.001% to 0.5%, Mo: 0.01% to 0.5%. % Or less, W: 1 type or 2 types or more of 0.001% or more and 0.2% or less are contained, The high intensity | strength hot-dip galvanized steel sheet of Claim 1 or 2 characterized by the above-mentioned.
The high-strength melt according to any one of claims 1 to 3, wherein the steel sheet further contains, as a component composition, B: 0.0005% to 0.005% by mass%. Galvanized steel sheet.
The high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 4, wherein the steel sheet is a hot-rolled steel sheet.
When hot rolling is performed on the steel having the component composition according to any one of claims 1, 3, and 4, and after finishing rolling, cooling and winding are performed, and then continuous annealing and hot dip galvanizing are performed. The method for producing a high-strength hot-dip galvanized steel sheet, characterized in that the finish rolling finish temperature is 850 ° C. or higher, the winding temperature is 540 ° C. or lower, and the continuous annealing is performed under the following conditions.
(A) The composition of the gas in the heating zone of the annealing furnace is H 2 ≧ 40 vol% or more, CH 4 ≧ 20 vol%, CO 2 ≧ 1 vol%, the balance CO, N 2 , C x H y (x ≧ 2, y ≧ 4), the steel sheet is heated to 520 ° C. or higher and 650 ° C. or lower at a furnace temperature of 500 ° C. or higher and 1000 ° C. or lower in the heating zone, and oxidation treatment is performed to form an oxide layer having a thickness of 6 to 60 nm on the steel plate surface layer
(B) Then, soaking zone atmosphere is balance N 2 include: hydrogen 5 vol% or more 50 vol%, and water vapor partial pressure (P H2 O) and hydrogen partial pressure (P H2) satisfies the equation (1) below Then, reduction annealing is performed by setting the reached temperature of the steel plate in the soaking zone to 630 ° C. or higher and 780 ° C. or lower.
10 −3 ≦ P H2O / P H2 ≦ 10 −1 (1)
However, PH2O shows water vapor partial pressure (Pa), and PH2 shows hydrogen partial pressure (Pa).
After the hot dipping treatment, the steel plate is further heated to a temperature of 450 ° C. or more and 510 ° C. or less to be alloyed, cooled to 400 ° C. at 20 ° C./s or less, and the Fe content in the plating layer is 7 to 15%. %. The method for producing a high-strength hot-dip galvanized steel sheet according to claim 6, wherein