JP2010209428A - Hot dip galvanized steel sheet or hot dip galvannealed steel sheet having excellent bending workability and fatigue strength - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength plated steel sheet with a tensile strength in a class of 780 MPa having excellent bending workability and fatigue strength. <P>SOLUTION: The hot dip galvanized steel sheet or galvannealed steel sheet (1) comprises prescribed components in the steel and (2) has a structure including a polygonal ferrite structure and a structure formed by low-temperature transformation, wherein the structure formed by low-temperature transformation at least includes bainite, further may include martensite, and when a plane located in the range of from a surface of the steel sheet to a depth of 0.1 mm therefrom is examined with a scanning electron microscope with respect to twenty sights in total in different positions in the sheet-width direction, and then, image analysis is performed in a 50 μm×50 μm area in the sights, all the following requirements are satisfied: (a) the maximum value (Fmax) of the area rate of the polygonal ferrite ≤80%; (b) the minimum value (Fmin) of the area rate of the polygonal ferrite ≥10%; (c) Fmax-Fmin≤40%; and (d) the maximum value (Mmax) of the area rate of the martensite occupied in the structure formed by low temperature transformation ≤50%. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having an excellent bending workability and fatigue strength and a tensile strength of 780 MPa or more. The plated steel sheet of the present invention is suitably used for, for example, automobile structural members (for example, body skeleton members such as pillars, members, and reinforcements; strength members such as bumpers, door guard bars, seat parts, and suspension parts). .

  In recent years, the demand for high-strength galvanized steel sheets that have been hot-dip galvanized or alloyed hot-dip galvanized for the purpose of reducing fuel consumption by reducing the body weight of automobiles and ensuring safety in the event of a collision is increasing. is doing. Accordingly, demands for the tensile strength of steel sheets are increasing, and high strength plated steel sheets of 780 MPa class or higher are required from low strength plated steel sheets of 590 MPa class. However, when the tensile strength is 780 MPa or higher, a decrease in formability is unavoidable, and in particular, a decrease in bending workability becomes a problem. Bending is performed in a rolling direction (L direction) [bending in which the bending axis is perpendicular to the rolling direction] and sheet width direction (C direction) bending [bending in which the bending axis is parallel to the rolling direction]. It is divided roughly into. With a low-strength steel plate of 590 MPa class, any of the bending processes can be performed relatively easily, but as the tensile strength increases, the bending process in the C direction becomes difficult, and it is easier to perform the bending process than in the C direction. The so-called bending in the L direction tends to be difficult.

  In addition, a high-strength steel sheet has a problem that it is inferior in bending workability of a shear end face (shearing edge) where the base steel sheet is exposed by shearing. When the shearing edge is subjected to bending, cracks occur even if the bending radius is the same due to the influence of voids and work hardening during shearing, compared to the case where the unmachined part that is not subjected to shearing is bent. It is because it becomes easy to generate | occur | produce.

  As a high-strength steel sheet excellent in bending workability, a steel sheet having a composite structure in which a ferrite phase and a low-temperature transformation phase such as martensite and bainite coexist is used. A composite steel sheet is intended to simultaneously improve strength and workability by dispersing a hard low-temperature transformation phase in soft ferrite ground. For example, methods of Patent Documents 1 to 5 have been proposed.

  Patent Document 1 is proposed by the applicant of the present application, and describes a method for improving bending workability by controlling the number of oxide inclusions present on the fracture surface. Patent Document 2 describes a method of preventing cracking during bending by generating bainite containing carbide and / or martensite containing carbide. In Patent Document 3, by optimizing the ferrite grain size, the fraction of the low-temperature transformation generation phase and the hardness, in addition to elongation and stretch flangeability, bending workability when bent in the rolling direction (L direction) is improved. It is stated that it will be. Patent Document 4 describes a method of securing bending workability by reducing the hardness of the surface layer from the inside and suppressing the variation in the internal Vickers hardness in the high-strength steel sheet mainly composed of bainite or martensite. . In Patent Document 5, steel having a specific chemical composition is heated, hot rolling conditions (especially hot finish rolling temperature, subsequent cooling rate, and winding temperature) and annealing conditions (annealing temperature and subsequent cooling rate). By appropriately controlling, bending workability is excellent in any direction of bending in the rolling direction, bending in the width direction, and bending at 45 ° (bending in which the bending axis is inclined at 45 ° with respect to the rolling direction). A high strength steel sheet is disclosed.

  On the other hand, in order to apply the above-described high-strength steel sheet to automotive parts and the like to make it thinner, it is necessary to have excellent fatigue strength. This is because the stress during running of an automobile increases due to the thinning of the wall, and the risk of fatigue failure increases when the fatigue strength is low. However, in the above-mentioned patent documents, fatigue strength is not considered.

JP 2002-363694 A JP 2004-68050 A JP-A-2005-171321 JP 2006-70328 A JP 2001-335890 A

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-strength plated steel sheet having a tensile strength of 780 MPa which is excellent in both bending workability and fatigue strength. Specifically, the object of the present invention is excellent in the bending workability of the L-direction bending and the C-direction bending of the unprocessed portion that is not subjected to the shearing process, and the bending workability of the shearing edge subjected to the shearing process. Another object of the present invention is to provide a high-strength plated steel sheet having a tensile strength of 780 MPa that is excellent in fatigue strength.

A high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more according to the present invention, which has been able to solve the above problems,
(1) Components in steel are
C: 0.05 to 0.20% (in the case of a chemical component, it represents mass%, the same applies hereinafter),
Si: 0.01 to less than 0.6%,
Mn: 1.6 to 3.5%
P: 0.05% or less,
S: 0.01% or less,
sol. Al: 1.5% or less,
N: a steel plate containing 0.01% or less, the balance being iron and inevitable impurities,
(2) The structure has a polygonal ferrite structure and a low temperature transformation generation structure, the low temperature transformation generation structure includes at least bainite, and may further include martensite,
When a plate surface having a depth of 0.1 mm from the surface of the steel plate was changed in the plate width direction, a total of 20 fields of view were observed with a microscope, and image analysis was performed on a 50 μm × 50 μm region in each field of view (a) It has a gist in the place where all the requirements of (d) are satisfied.
(A) Maximum value of polygonal ferrite area ratio (Fmax) ≦ 80%
(B) Minimum value of polygonal ferrite area ratio (Fmin) ≧ 10%
(C) Fmax−Fmin ≦ 40%
(D) Maximum value of martensite area ratio in the low temperature transformation formation structure (Mmax) ≦ 50%

In a preferred embodiment, the hot dip galvanized steel sheet further satisfies the following requirement (e).
(E) Minimum value of martensite area ratio in the low temperature transformation formation structure (Mmin) ≧ 5%

  In a preferred embodiment, the steel component of the plated steel sheet further contains at least one selected from the group consisting of Cr: 1.0% or less, Mo: 0.5% or less, and B: 0.005% or less. To do.

  In a preferred embodiment, the steel component of the plated steel sheet further contains at least one selected from the group consisting of Nb: 0.1% or less, Ti: 0.2% or less, and V: 0.2% or less. To do.

  In a preferred embodiment, the steel component of the plated steel sheet further contains Ca: 0.003% or less and / or REM: 0.003% or less.

  In a preferred embodiment, the plated steel sheet is further subjected to an alloying treatment. That is, the present invention includes both hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets.

  According to the present invention, not only the unprocessed portion that is not subjected to the shearing process is excellent in the bending workability of the L direction bending and the C direction bending, but also the bending workability of the shearing end face (shearing edge) is excellent. In addition, a 780 MPa grade hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet having high fatigue strength could be provided.

FIG. 1 is a diagram schematically showing a distribution state of a microstructure on a plate surface of a composite structure steel plate. FIG. 2 is a schematic diagram showing a heat treatment pattern in the annealing process. FIG. 3 is a diagram for explaining a rolling direction and a bending ridge line of a bending test piece used for bending an unprocessed portion not subjected to shearing. FIG. 4 is a diagram for explaining a rolling direction and a bending ridge line of a bending test piece used for bending a shearing edge. FIG. 5 is a diagram schematically showing a bending workability test method. FIG. 6 is a view showing a plane bending test piece used for measurement of fatigue strength.

  In particular, the present inventor has studied in order to provide a plated steel sheet excellent in both bending workability and fatigue strength in a high strength plated steel sheet having a tensile strength of 780 MPa which is suitably used as an automobile structural component. Specifically, not only unprocessed parts that are not subjected to shearing, but also excellent bending workability (including both L direction and C direction) of the shearing edge, and high fatigue strength. In order to provide plated steel sheets, studies have been repeated. As a result, (a) in the composite structure steel plate having polygonal ferrite and a low-temperature transformation structure, in particular, the maximum and minimum values of the polygonal ferrite area ratio observed in a predetermined region of the plate surface, and the maximum and minimum values. If the difference (variation) between the two and the maximum value (preferably the minimum value) of the martensite area ratio in the low-temperature transformation formation structure is appropriately controlled, the intended purpose can be achieved. In order to produce such a high-strength plated steel sheet, it is particularly effective to perform the annealing process after hot rolling by a predetermined three-stage cooling method (rapid cooling → slow cooling → rapid cooling) with different cooling rates, The present invention has been completed.

  That is, the feature of the present invention is that the area ratio of the microstructure on the plate surface is finely controlled to achieve both bending workability and fatigue strength. Conventionally, for example, as represented by the above-mentioned patent document, the area ratio of the microstructure existing in the cross section in the plate thickness direction is defined to improve the properties such as bending workability, and the present invention No attention was paid to the microstructure present on the plate surface. However, according to the study results of the present inventors, the microstructure on the plate surface varies widely in the plate width direction, and the area ratio of the microstructure has a great influence on the improvement of bending workability and fatigue strength. Since it became clear, it is as soon as the above requirements are specified.

  This point will be explained in a little more detail.

  The present inventor first clarifies the mechanism of occurrence of cracks (cracks) and fatigue cracks during bending in a composite structure steel plate of 780 MPa class or higher composed of polygonal ferrite and a low temperature transformation formation structure (hard phase). Focusing on the vicinity of the surface layer of the plate surface (a plate surface polished about 0.1 mm in the depth direction from the outermost layer surface of the steel plate, a surface perpendicular to the plate thickness direction), the microstructure was observed in detail.

  FIG. 1 is a schematic diagram showing a microstructure distribution state in the vicinity of the surface layer of the plate. According to this schematic diagram, polygonal ferrite is expressed in white, and low temperature transformation formation structures such as bainite and martensite are expressed in black (gray). The size of the polygonal ferrite and the low temperature transformation formation structure is generally 10 μm or less.

  As shown in FIG. 1A, the plate surface has regions A that appear gray overall and regions B that appear white overall, which are alternately arranged in the plate width direction at intervals of several tens of μm to several hundreds of μm. I understand. Of these, the region A is enlarged as shown in FIG. 1 (b). In the region A, many low-temperature transformation generation phases such as martensite are distributed, and there is little polygonal ferrite. On the other hand, FIG. 1C is an enlarged view of the region B. In the region B, a large amount of polygonal ferrite is distributed, and there are few low-temperature transformation generation phases such as martensite. Thus, it can be seen that the microstructure near the surface of the plate surface varies greatly in the plate width direction, and there are wide regions in the plate surface where the area ratios of polygonal ferrite and low-temperature transformation generation phase are different.

  Although not shown in the figure, when martensite and bainite are included as the low-temperature transformation formation structure, not only the area ratio of polygonal ferrite largely varies in the plate width direction as described above, but also martensite and It was also found that the ratio of bainite greatly varies in the plate width direction.

  When bending such as L-direction bending or C-direction bending is performed on a composite structure steel plate having such a plate surface microstructure, the strain concentrates on a portion where there is a large amount of polygonal ferrite in the vicinity of the surface layer, resulting in a low-temperature transformation generation phase. The deformation of the main area is very small. As a result, near the boundary between the polygonal ferrite and the low-temperature transformation generation phase and inside the polygonal ferrite, the strain difference becomes large and cracks are likely to occur.

  In addition, when the above-mentioned composite structure steel plate is subjected to bending of the shearing edge, generation of cracks in the subsequent bending is promoted by voids generated during the shearing. Therefore, in this case, it is necessary to suppress not only the generation of voids during shearing and bending, but also the development of cracks during subsequent bending. It was found that the area ratio of bainite and martensite constituting the material greatly affected.

  In addition, fatigue cracks due to repeated loads occur in a ferrite-rich region, but the propagation of initial cracks can be suppressed by the coexisting hard low-temperature transformation generation phase. However, when there are few hard phases, the said effect | action will become inadequate and it will also have a bad influence also on fatigue strength.

  From the above results, the following can be derived.

  First, it has been found that unless the area ratio (maximum area ratio and minimum area ratio) of polygonal ferrite in the surface layer portion of the plate is controlled within a certain range, it is impossible to achieve both improvement in bending workability and fatigue strength. Also, the difference in polygonal ferrite area ratio (difference between the maximum and minimum values) should be as small as possible. This suppresses the strain that occurs near the boundary between the polygonal ferrite and the low-temperature transformation generation phase. It also became clear that processability was improved.

  Secondly, it has been clarified that, in addition to the polygonal ferrite as the parent phase, the structure of the low-temperature transformation formation structure of the second phase is deeply involved in improving both the bending workability and the fatigue strength. Specifically, it was found that the bending workability and fatigue strength including the shearing edge vary greatly depending on the ratio of bainite and martensite constituting the low temperature transformation formation structure. For example, bainite is effective in improving the bending workability including the shearing edge, and the ratio of martensite in the low-temperature transformation formation structure is reduced to reduce the bainite-based structure (martensite is not included). It is found that the bending workability of the shearing edge tends to be improved. On the other hand, martensite is more useful than bainite for improving fatigue strength, and if the ratio of martensite in the low temperature transformation structure is increased to a mixed structure of bainite and martensite, the fatigue strength can be further enhanced. It has been found.

    Based on these results, in the present invention, the above requirements are specified.

  In this specification, “excellent bending workability” means 90 ° bending in the L direction (rolling direction = longitudinal direction of the test piece) and C direction (direction perpendicular to the rolling direction) in an unprocessed part that is not subjected to shearing. This means that the fracture end face (shearing edge) subjected to shearing is also excellent in bending workability of the shearing edge when bent in the C direction.

  These bendability evaluations are based on the value (Rmin / t) obtained by dividing the minimum bending radius (Rmin) obtained by 90 ° bending in the L and C directions by the thickness (t) of the steel sheet. And an acceptance criterion of “Rmin / t” was set according to the strength class of the steel sheet. This is because the bending workability changes depending on the plate thickness and strength class of the steel plate. Details are as described in the column of Examples described later.

  In this specification, “excellent fatigue strength” means that the fatigue limit ratio (ratio of fatigue strength / tensile strength) is approximately when a plane bending fatigue test is performed by the method described in the column of Examples described later. , 0.40 or more (preferably more than 0.45).

  In the present specification, the “plate surface” means not a surface (outermost surface) of a steel plate but a plate surface (surface perpendicular to the plate thickness direction) having a depth of about 0.1 mm from the surface. Whereas the area ratio of the microstructure of the outermost layer plate surface is easy to change, the area ratio of the microstructure existing on the plate surface is about 0.1 mm deep from the outermost surface. This is because it hardly changes. Note that “0.1 mm depth” is not strictly defined, and in the case of a thin steel plate having a thickness of about 0.8 to 2.3 mm as in the present invention, about 1 to the plate thickness. A plate surface at the position of / 20 to 1/8 is also acceptable. This is because within the above range, the area ratio of the microstructure of the plate surface hardly changes.

  The present invention includes both hot-dip galvanized steel sheets and galvannealed steel sheets. Since all of these plated steel sheets have polygonal ferrite (matrix) and low-temperature transformation formation structure (second phase), they may be collectively referred to as “composite structure steel sheets”. Alternatively, it may be abbreviated as “steel plate”.

  The steel sheet of the present invention is substantially composed of soft polygonal ferrite and a hard low-temperature transformation structure. Polygonal ferrite is a structure useful for securing elongation and bending workability, and both strength and elongation can be increased by coexistence with a low temperature transformation generation structure. On the other hand, the low temperature transformation generation structure is a structure useful for ensuring the strength, and also contributes to improvement of bending workability and fatigue characteristics including a shearing edge (details will be described later).

  Here, “substantially” means that the ratio of polygonal ferrite and low-temperature transformation formation structure in the entire structure is 95% in total (meaning area%, hereinafter “%” means “area%” for the structure) This means the above, and the balance is a structure (pearlite, retained austenite, etc.) that is inevitably generated during the manufacturing process of the steel sheet. The larger the total area of the polygonal ferrite and the low-temperature transformation formation structure, the better, and it is most preferable that it is composed of only the polygonal ferrite and the low-temperature transformation formation structure (100% in total).

  Examples of the “low temperature transformation formation structure” include bainite, martensite (including tempered martensite), bainitic ferrite, and the like. In the present invention, at least bainite is included as the low temperature transformation formation structure, and martensite may further be included. The meaning of “including martensite” will be described in detail later. According to the study results of the present inventors, bainite is a structure useful for improving bending workability such as a shearing edge, and even if it is a low-temperature transformation generation structure consisting essentially of bainite without containing martensite, It was found that a plated steel sheet having excellent bending workability and fatigue strength can be obtained if the polygonal ferrite has a properly controlled composite structure (see No. 1 in Table 3 of Examples described later). On the other hand, martensite is a very useful structure for improving fatigue properties. If the low temperature transformation formation structure is a mixed structure of bainite and martensite, the fatigue strength is lower than that of a steel sheet whose low temperature transformation formation structure is substantially composed of bainite. (See Nos. 2 to 12 in Table 3 of Examples described later).

  In the present specification, “the low temperature transformation formation phase structure contains bainite” or “the low temperature transformation formation phase structure contains martensite” means that the ratio of bainite or martensite in the low temperature transformation formation phase structure is, It means bainite ≧ 5% or more, or martensite ≧ 5% or more. In other words, those having a bainite ratio of less than 5% (including 0) in the low-temperature transformation formation phase structure are judged as “the low-temperature transformation formation structure does not contain bainite”. Similarly, when the ratio of martensite in the low-temperature transformation formation structure is less than 5% (including 0), it is determined that “low-temperature transformation formation structure does not contain martensite”.

The plated steel sheet of the present invention is a composite structure steel sheet containing a predetermined steel component and having a polygonal ferrite structure and a low temperature transformation structure, and in particular, a plate surface (hereinafter referred to as 0.1 mm depth from the surface of the steel sheet). Then, it may be simply referred to as “plate surface”.) With respect to a total of 20 fields (1 field: about 60 μm × about 80 μm) by changing the position in the plate width direction, a magnification of 1000 to 1000 using a microscope (SEM or optical microscope). It is characterized by satisfying all the following requirements (a) to (d) when image analysis is performed on a 50 μm × 50 μm region in each visual field when observed at 2000 times.
(A) Maximum value of polygonal ferrite area ratio (Fmax) ≦ 80%
(B) Minimum value of polygonal ferrite area ratio (Fmin) ≧ 10%
(C) Fmax−Fmin ≦ 40%
(D) Maximum value of martensite area ratio in the low temperature transformation formation structure (Mmax) ≦ 50%

Preferably, the steel sheet of the present invention further satisfies the following requirement (e) when the above image analysis is performed.
(E) Minimum value of martensite area ratio in the low temperature transformation formation structure (Mmin) ≧ 5%

  Hereinafter, the requirements (a) to (e) will be described in detail. In this specification, the units of Fmax, Fmin, (Fmax−Fmin), Mmax, and Mmin are all area%, and are abbreviated as “%” below.

  First, requirements (a) to (c) relating to polygonal ferrite will be described.

(A) Minimum value of polygonal ferrite area ratio Fmin ≧ 10%
The minimum value (Fmin) of the polygonal ferrite area ratio is an important requirement for ensuring good bending workability and obtaining excellent elongation characteristics. As shown in Examples to be described later, when Fmin was less than 10%, bending workability and elongation also tended to decrease (see Nos. 13, 19, and 21 in Table 3). Fmin is preferably 15% or more, and more preferably 20% or more.

(B) Maximum value of polygonal ferrite area ratio Fmax ≦ 80%
The maximum value (Fmax) of the polygonal ferrite area ratio ensures high strength with a tensile strength of 780 MPa or more, and secures a predetermined amount of hard phase that suppresses the propagation of fatigue cracks on the surface layer to ensure excellent fatigue strength. It is an important parameter to do. As shown in the examples described later, when Fmax exceeds 80%, the tensile strength and the fatigue strength decrease (see Nos. 15, 16, and 19 in Table 3). Fmax is preferably 75% or less, and more preferably 70% or less.

(C) Difference between maximum value (Fmax) and minimum value (Fmin) of polygonal ferrite area ratio ≦ 40%
The difference (variation) between the maximum value (Fmax) and the minimum value (Fmin) of the polygonal ferrite area ratio is an important parameter for ensuring the desired bending workability, and when the above variation exceeds 40%, Deformation concentrates in a region where the ferrite area ratio is large at the time of bending, and bending workability (particularly, bending workability in the C direction) is reduced (see Nos. 18, 19, and 21 in Table 3 of Examples described later). . The smaller the variation, the better. For example, it is preferably 30% or less, and most preferably 0%.

  Next, the requirements (d) and (e) regarding the composition ratio of the low-temperature transformation formation structure will be described.

(D) Maximum value of martensite area ratio in the low temperature transformation formation structure (Mmax) ≦ 50%
This is based on the results of experiments by the inventors that bending workability (especially bending workability of the shearing edge) decreases when the martensite ratio in the low-temperature transformation formation structure increases and the bainite ratio decreases. Based on this. In the present invention, those containing martensite (the ratio of martensite in the low-temperature transformation formation structure is 5% or more) and those not containing martensite (martensite area ratio in the low-temperature transformation formation structure is 5). In any case, when Mmax is calculated by performing image analysis described in detail below, at least Mmax is set to 50% or less. This ensures good bending workability. As shown in Examples described later, those having Mmax exceeding 50% are inferior in bending workability (see Nos. 14 and 17 to 19 in Table 3).

  The reason is unknown in detail, but it is assumed as follows. That is, if the ratio of martensite in the hard low-temperature transformation structure (mainly bainite and martensite in the present invention) is large, voids are likely to be generated from the boundary between martensite and ferrite during strong processing. Subsequently, when bending is further performed, the voids near the outer surface of the bending propagate mainly hard martensite and a hard phase other than martensite to the strain concentrated portion of the surface, thereby generating cracks. As a result, the bending workability of the shearing end face is significantly reduced. On the other hand, in the region where the low-temperature transformation formation structure is mainly bainite, the generation of voids is small, and even if voids are present, it is difficult for cracks to be connected to the strain-concentrated portion of the surface layer. As described above, in the present invention, in the region where there are many martensites, the maximum area ratio Mmax of martensite in the low-temperature transformation-generated structure is particularly suppressed from the viewpoint that many voids are generated and easily propagated. It depends on ensuring workability.

  From the viewpoint of improving the bending workability (particularly the bending workability of the shearing edge), Mmax should be as small as possible. The preferred Mmax is about 45% or less, more preferably about 40% or less. The present invention includes a case where Mmax is zero.

(E) Minimum value of martensite area ratio in the low temperature transformation formation structure (Mmin) ≧ 5%
This stipulates the requirements for effectively exerting the fatigue property improving action by martensite, thereby further increasing the fatigue strength of the plated steel sheet (No. 2 in Table 3 of Examples described later). 12). According to the examination results of the present inventors, it has been found that the effect of stopping the propagation of fatigue cracks generated in ferrite and improving the fatigue characteristics is higher in martensite than in bainite. In order to effectively exhibit the action of such martensite, the larger the Mmin, the better. The more preferable Mmin is 10% or more, and further preferably 15% or more.

  The method for measuring the maximum and minimum values of the above-mentioned polygonal ferrite area ratio and the maximum and minimum values of the martensite area ratio in the low-temperature transformation formation structure is as follows.

  First, a steel sheet for microstructure measurement (size is approximately 20 mm long × 20 mm wide × 1.6 mm thick) is prepared and polished from the surface of the steel sheet to a depth of about 0.1 mm in the plate thickness direction. Next, polygonal ferrite, bainite, and martensite present on the plate surface (in the plate width direction) at the above positions were observed at a magnification of 1000 to 2000 times using a scanning electron microscope (SEM) or an optical microscope. Determine. Specifically, a microstructure of a total of 20 fields (1 field: about 60 μm × about 80 μm) is observed at a pitch of 0.1 mm in the plate width direction, and a photograph is taken at a magnification of 1000 to 2000 times.

  Here, the microscope observation may be either SEM observation or optical microscope observation as described above. Depending on the steel sheet, it is difficult to distinguish between bainite and martensite, but even in that case, these can be determined by observation with one or both of the SEM and optical microscope, which will be described later. This is because Mmin and Mmax can be calculated. In Examples described later, ferrite was determined by SEM observation, and bainite and martensite were determined by optical microscope observation. In addition, when observing with an optical microscope, it is preferable to observe the test piece which carried out the repeller corrosion with an optical microscope. When optical microscope observation is performed after repeller corrosion, bainite is black, martensite is white, polygonal ferrite is gray, and each structure is observed in different colors, making it easy to distinguish between bainite and martensite. Because.

  Next, an area of 50 μm × 50 μm is designated in the photograph, and image analysis is performed using an image analysis apparatus of “LUZEX F” manufactured by Nireco to obtain the area ratios of polygonal ferrite, bainite, and martensite. Image analysis was performed in the same manner as described above for the total 20 visual fields, and the polygonal ferrite area ratio was measured. These minimum values were Fmin and the maximum values were Fmax. Similarly, for a total of 20 visual fields, image analysis was performed in the same manner as described above to measure the martensite area ratio, and the minimum value was Mmin and the maximum value was Mmax.

  In the above, the maximum and minimum values of the polygonal ferrite area ratio that greatly affect the bending workability and fatigue strength, and the maximum and minimum values of the martensite area ratio in the low-temperature transformation structure are explained in detail. In the steel sheet, as long as these requirements are satisfied, the ratio of the polygonal ferrite and the low temperature transformation formation structure contained in the entire structure (average value in the entire structure) is not particularly limited. Preferred ratios are polygonal ferrite: about 20-70%, low temperature transformation formation structure: about 30-80%, more preferred ratios are polygonal ferrite: about 35-75%, low temperature transformation formation structure: about 25- 75%.

  The organization that most characterizes the present invention has been described above.

  Next, the components in steel of the present invention will be described.

C: 0.05-0.20%
C is an element necessary for securing a predetermined amount of low-temperature transformation generation phase and obtaining a high strength of 780 MPa or more. For this reason, the amount of C is set to 0.05% or more. However, if added excessively, the formation of polygonal ferrite is insufficient and the minimum value of the polygonal ferrite area ratio becomes small. In addition, the maximum value of the martensite area ratio is increased, bending workability including duct edges and ductility are reduced (see the examples described later), and spot weldability is reduced. 0.20%. The C content is preferably 0.07% or more and 0.17% or less.

Si: 0.01 to less than 0.6% Si is an element effective for improving fatigue strength because it solidifies and strengthens ferrite and suppresses the occurrence of fatigue cracks. Moreover, it has the effect of making the carbides in bainite fine and improving stretch flangeability. In order to exhibit such an effect effectively, Si is contained 0.01% or more. A preferable Si amount is 0.05% or more. However, when the amount of Si increases, ferrite transformation is promoted, C concentration to untransformed γ progresses, the maximum value of the martensite area ratio in the low-temperature transformation formation structure increases, and especially the bending formability of the shearing edge decreases. To do. In addition, in order to use it for hot dip galvanizing, dedicated equipment such as a redox furnace and pre-Fe plating is required, which increases costs. Therefore, in the present invention, the Si content is less than 0.6%. A preferable Si amount is 0.5% or less, and more preferably 0.3% or less.

Mn: 1.6 to 3.5%
Mn is an element necessary for suppressing the excessive formation of polygonal ferrite to ensure a predetermined low-temperature transformation generation phase and to ensure a high strength of 780 MPa or more. Mn, like Si, is an element that solidifies and strengthens ferrite to suppress the occurrence of fatigue cracks and contributes to the improvement of fatigue strength. In order to effectively exhibit these actions, the lower limit of the Mn amount is set to 1.6%. However, if added excessively, it becomes difficult to secure a predetermined amount of ferrite, workability is lowered, and spot weldability and delayed fracture resistance are also lowered. Therefore, the upper limit of Mn amount is set to 3.5%. . A preferable amount of Mn is 1.8% or more and 3.0% or less, and more preferably 2.0% or more and 3.0% or less.

P: 0.05% or less P is an element that deteriorates workability and spot weldability, so the upper limit is made 0.05%. The smaller the amount of P, the better.

S: 0.01% or less Since S is an element that reduces stretch flangeability and bending formability, the upper limit is made 0.01%. The smaller the amount of S, the better.

sol. Al: 1.5% or less sol. Al (soluble Al) has a function of generating ferrite in addition to a deoxidizing action. In order to effectively exhibit such an action, sol. It is preferable to add 0.005% or more of Al. However, sol. When Al is added excessively, inclusions increase and stretch flangeability and bending workability deteriorate, so the upper limit is made 1.5%. Preferred sol. The upper limit of the amount of Al is 0.8%.

N: 0.01% or less If N is excessively present, ductility may be deteriorated, so the upper limit is made 0.01%. The N content is preferably as small as possible and is preferably 0.006% or less. The lower limit of the N amount is about 0.001% when considering the balance with cost at the actual operation level.

  The components in steel of the present invention contain the above elements, and the balance is iron: unavoidable impurities. However, the following elements can also be positively added for the purpose of imparting other characteristics within the range not impairing the action of the present invention.

At least one selected from the group consisting of Cr: 1.0% or less, Mo: 0.5% or less, and B: 0.005% or less. These elements are effective elements for improving the strength. In addition to difficulty in securing a predetermined amount of polygonal ferrite, delayed fracture resistance and spot weldability are reduced, so the upper limits are Cr: 1.0%, Mo: 0.5%, and B: It is preferable to set it as 0.005%. More preferably, Cr: 0.05% or more and 0.8% or less, Mo: 0.01% or more and 0.4% or less, and B: 0.0005% or more and 0.003% or less. These elements may be added alone or in combination of two or more.

At least one selected from the group consisting of Nb: 0.1% or less, Ti: 0.2% or less, and V: 0.2% or less. These elements are useful for increasing strength through precipitation or refinement of the structure. It is. However, if added in excess, the elongation and stretch flangeability deteriorate, so the upper limit is preferably Nb: 0.1%, Ti: 0.2%, and V: 0.2%. More preferably, Nb: 0.005% to 0.08%, Ti: 0.005% to 0.16%, and V: 0.005% to 0.15%. These elements may be added alone or in combination of two or more.

Ca: 0.003% or less, and / or REM: 0.003% or less These elements contribute to the improvement of stretch flangeability, but even if added in excess, the effect is saturated and economical. Therefore, it is preferable to set the upper limit to Ca: 0.003% and REM: 0.003%, respectively. More preferably, Ca: 0.0005% to 0.0025%, REM: 0.0005% to 0.0025%. These elements may be added alone or in combination of two or more.

  In this specification, REM means a lanthanoid element (a total of 15 elements from La to Lu in the periodic table). Among these elements, it is preferable to contain La and / or Ce. Moreover, the form of REM added to molten steel is not specifically limited, For example, as REM, pure La, pure Ce, or the like, or Fe-Si-La alloy, Fe-Si-Ce alloy, Fe-Si-La-Ce alloy Etc. may be added. Moreover, you may add misch metal to molten steel. Misch metal is a mixture of cerium group rare earth elements, and specifically contains about 40 to 50% of Ce and about 20 to 40% of La.

  In addition to the above elements, for example, Cu, Ni, and Mg may be added for the purpose of improving delayed fracture resistance. The upper limit of these elements is preferably approximately Cu: 1.0%, Ni: 1.0%, and Mg: 0.001%, so that the above-described action can be achieved without impairing the action of the present invention. Can be improved. Further, Sn, Zn, Zr, W, As, Pb, and Bi may be added for the purpose of improving corrosion resistance and delayed fracture resistance. The total amount of these elements is preferably about 0.01% or less, whereby the above-described action can be improved without impairing the action of the present invention.

  Next, a method for producing the steel sheet of the present invention will be described.

  The steel sheet of the present invention in which the area ratio of polygonal ferrite (Fmax, Fmin, variation) existing on the plate surface and the area ratio of martensite in the low-temperature transformation formation structure (Mmax, further Mmin) satisfy all the above requirements. In order to obtain, it is necessary to strictly control the cooling conditions after annealing in the annealing process after hot rolling / cold rolling (in the case of using a hot dip galvanizing line), in the present invention, FIG. The three-stage cooling pattern of rapid cooling (CR1 in the figure) → slow cooling (CR2 in the figure) → rapid cooling (CR3 in the figure) as shown in FIG. In the case where the above three-stage cooling is not performed, since the microstructure of the plate surface does not satisfy the requirements of the present invention, at least one of bending workability (including bending workability of the shear edge) and fatigue strength is reduced (described later). See examples).

  Further, even if the above-mentioned patent documents are referred to, the three-stage cooling method as in the present invention is not disclosed. For example, in the example of Patent Document 2, the annealing process is performed as follows: “Hold for 5 seconds or more in a temperature range of 720 to 900 ° C. → 550 to 760 ° C. at an average cooling rate (first stage cooling rate) of 4 to 7 ° C./s. Cooling to quenching → quenching to 200 to 420 ° C. at an average cooling rate (second stage cooling rate) of 60 to 90 ° C./s ”is disclosed. The steel sheet of the present invention was not obtained even when a cooling pattern simulating the above was obtained, and in particular, the bending workability in the C direction was lowered (see Examples described later). Moreover, in the Example of patent document 3, after cooling the temperature to 650-450 degreeC with the average cooling rate of 60 degreeC / sec, it describes cooling to the cooling stop temperature range of 200-450 degreeC. The average cooling rate up to the cooling stop temperature range is not specifically described.

  As described above, the method for producing a high-strength plated steel sheet according to the present invention is characterized in that the cooling conditions in the annealing process are appropriately controlled. General methods for manufacturing can be employed. The plated steel sheet of the present invention is manufactured, for example, by continuous casting → hot rolling → pickling → cold rolling → hot dip galvanizing line, and the annealing step is performed in the hot dip galvanizing line. In the hot dip galvanizing line, it is necessary to strictly control the cooling conditions after annealing as described above, but other conditions other than the cooling conditions in each hot dip galvanizing process and annealing process (for example, the heating rate and annealing). The temperature is not particularly limited. Further, the present invention includes an alloyed hot dip galvanized steel sheet, but the alloying conditions are not particularly limited. In the above hot dip galvanizing line, a method usually used is appropriately selected and alloyed. Can be done.

  Hereinafter, preferred manufacturing conditions of the present invention will be described in detail with reference to the heat treatment pattern of continuous annealing shown in FIG.

  First, molten steel satisfying the composition of the present invention is melted by a known melting method such as a converter or an electric furnace, and is formed into a steel slab such as a slab by continuous casting or casting-slab rolling.

  Next, the steel slab is hot rolled. In detail, hot rolling may be performed directly after continuous casting, or, in the case of manufacturing by continuous casting or casting-bulk rolling, after cooling to a suitable temperature and then heating in a heating furnace Hot rolling may be performed.

In the hot rolling step, it is preferable that after heating to a temperature of about 1200 ° C. or higher, the hot rolling is finished at a temperature of about Ac ₃ points or higher and wound at 650 ° C. or lower (more preferably 600 ° C. or lower). By performing hot rolling as described above, in particular, variations in the polygonal ferrite area ratio of the plate surface can be suppressed.

  Subsequently, after performing cold rolling and pickling according to a conventional method, annealing → cooling → plating (→ alloying if necessary) is performed in a continuous hot dip galvanizing line.

In the annealing step, first, it is preferable to set the annealing temperature (soaking temperature, T1 in the figure) to Ac ₃ point or higher and hold (anneal) at that temperature for about 5 seconds or longer. When T1 is less than Ac ₃ point or the annealing time is less than 5 seconds, the variation of the polygonal ferrite area ratio on the plate surface is particularly large. Preferable annealing conditions are T1: Ac ₃ points + 20 ° C. or more, and annealing time: 10 seconds or more. In addition, although these upper limits are not specifically limited, When the load of an installation is considered, it is preferable to set it as T1 <= 950 degreeC and annealing time <= 5 minutes.

In this specification, Ac ₃ points are calculated based on the following equation.
Ac ₃ points (℃)
= 910−203√ [C] −15.2 [Ni] +44.7 [Si]
+104 [V] +31.5 [Mo] -30 [Mn] -11 [Cr]
−20 [Cu] +700 [P] +400 [Al] +400 [Ti]
[In the formula, [] means the content (%) of each element].

  Cool after annealing. In the present invention, as shown in FIG. 2, after annealing (T1 in the figure), the temperature (T1 to T3) ranges from about 460 ° C. to about 700 ° C. (T3 in the figure), and the temperature of T2 is the boundary. Thus, it is very important to perform three-stage cooling of rapid cooling (CR1) → slow cooling (CR2) → rapid cooling (CR3), that is, cooling so as to satisfy the relationship of CR1> CR2 and CR3> CR2. Here, both T2 and T3 are positioned as the cooling rate change temperature. Specifically, after annealing (T1), the temperature range from T1 to T2 is rapidly cooled at an average cooling rate (CR1) of about 10 ° C./s or more, and then the temperature range from T2 to T3 is about 10 ° C. / Slowly cool at an average cooling rate (CR2) of s or less. Thus, after annealing, the temperature range from T1 to T2 is rapidly cooled at a cooling rate capable of suppressing ferrite transformation, and then the temperature range from T2 to T3 (temperature range near the ferrite nose) is applied for about 2 to 30 seconds. By gradually cooling, all the polygonal ferrite area ratios of the plate surface can be appropriately controlled, and a uniform microstructure can be obtained. Moreover, the martensite area ratio (especially Mmax) in a low-temperature transformation production | generation structure | tissue can be controlled appropriately. T2 is preferably in the range of approximately 500 to 700 ° C. within the temperature range of T1 and T3.

  As shown in the examples described later, when CR1 is small, the maximum value (Fmax) and variation of the polygonal ferrite area ratio increase, fatigue strength and bending workability decrease, and when CR2 is large, the polygonal ferrite area ratio As the minimum value of becomes smaller, bending workability and elongation (EL) are lowered.

  In order to obtain a high-strength plated steel sheet having excellent bending workability and fatigue strength, CR1 is preferably as large as possible. For example, it is preferably about 10 ° C./s or more, more preferably about 15 ° C./s or more. Preferably, it is about 20 ° C./s or more. On the other hand, CR2 is preferably as small as possible, and is, for example, preferably about 10 ° C./s or less, more preferably about 5 ° C./s or less, within a range satisfying the relationship of “CR2 <CR1”. The upper limit of CR1 is not particularly limited, but is preferably about 100 ° C./s in view of the cooling capacity of facilities at the actual operation level. Also, the lower limit of CR2 is not particularly limited, but it is preferably about 1 ° C./s in view of the fact that when CR2 becomes extremely low, a heat insulation facility is separately required.

  In the present invention, the temperature of T3 is also important. As shown in the examples described later, when T3 becomes too low, the maximum value (Fmax) of the polygonal ferrite area ratio increases and the fatigue strength decreases. Preferable T3 is generally 480 to 680 ° C., although it varies depending on the components.

  After cooling as described above, it is cooled to T4 at the cooling rate CR3. When CR3 is small, ferrite transformation proceeds excessively and Fmax increases. Moreover, since it takes a long time to reach the bainite transformation temperature range, it is necessary to lengthen the production equipment. Otherwise, the bainite area ratio in the hard phase is insufficient, and the martensite maximum exceeds the upper limit. The larger CR3 is, the better, and within a range satisfying the relationship of “CR2 <CR3”, for example, preferably about 10 ° C./s or more, more preferably about 15 ° C./s or more, and further preferably about 20 ° C./s or more.

  Subsequently, in order to advance a bainite transformation, it hold | maintains more than fixed time (t) with holding temperature (T4 in the figure). T4 is a temperature range corresponding to a bainite transformation nose. Although specific holding conditions differ depending on the types of components in the steel, etc., it is generally preferable to control T4 within a range of about 350 to 500 ° C. and t to 30 seconds or more. When the holding temperature T4 exceeds about 500 ° C., Mmax increases, and when it is below about 350 ° C., Mmin decreases. Further, when the holding time t is less than 30 seconds, Mmax increases. During the holding time t, T4 may be a constant temperature (isothermal) or may gradually change within a range of about 5 ° C./s or less within the above temperature range. Specifically, for example, in order to obtain a bainite-based hard phase, it is generally preferable to hold at a holding temperature of 350 to 450 ° C. for about 30 seconds or more. On the other hand, for example, in order to obtain a hard phase having a mixed structure of bainite and martensite, it is generally preferable to hold at a holding temperature of 450 to 550 ° C. for about 20 seconds or longer. In this way, a hot dip galvanized steel sheet is obtained.

  In addition, when manufacturing an alloyed hot dip galvanized steel sheet, for example, after annealing and cooling under the above-described conditions, the steel sheet is immersed in a plating bath and a temperature of 400 to 750 ° C. (preferably about 500 to 600 ° C.). The region may be heated and alloyed. In the examples described later, alloying was performed at a temperature of 550 ° C. as shown in FIG.

  The plated steel sheet of the present invention may be further subjected to an organic film such as a film laminate, a chemical conversion treatment such as a phosphate treatment, or a coating treatment. In particular, a plated steel sheet that has been subjected to a chemical conversion treatment is suitably used as a base treatment before painting.

  As the paint used for the coating treatment, known resins such as epoxy resin, fluororesin, silicon acrylic resin, polyurethane resin, acrylic resin, polyester resin, phenol resin, alkyd resin, melamine resin and the like can be used. From the viewpoint of corrosion resistance, an epoxy resin, a fluororesin, and a silicon acrylic resin are preferable. A curing agent may be used together with the resin. The paint may contain known additives such as coloring pigments, coupling agents, leveling agents, sensitizers, antioxidants, ultraviolet stabilizers, flame retardants and the like.

  In the present invention, the form of the paint is not particularly limited, and any form of paint such as solvent-based paint, water-based paint, water-dispersed paint, powder paint, and electrodeposition paint can be used. The coating method is not particularly limited, and a dipping method, a roll coater method, a spray method, a curtain flow coater method, an electrodeposition coating method, and the like can be used. What is necessary is just to set the thickness of a coating layer (a plating layer, an organic membrane | film | coat, a chemical conversion treatment film, a coating film etc.) suitably according to a use.

  The plated steel sheet of the present invention includes automotive strength parts, for example, collision parts such as front and rear side members and crash boxes, pillars such as center pillar reinforcements, roof rail reinforcements, side sills, floor members, and kick parts. It can be used for car body components such as. These parts obtained by forming and processing the high-strength plated steel sheet of the present invention have sufficient material characteristics (such as strength) and exhibit excellent fatigue characteristics.

  Hereinafter, the present invention will be described in more detail with reference to experimental examples.However, the present invention is not limited by the following experimental examples, and should be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all included in the technical scope of the present invention.

Example 1
(Production method of plated steel sheet)
Steels having various composition shown in Table 1 (unit: mass%, balance: iron and inevitable impurities) were melted and continuously cast, and then hot-rolled under the following conditions (finished thickness: 2. 6 mm), then pickling and cold rolling to a thickness of 1.2 mm.
Heating temperature: 1250 ° C for 30 minutes, finishing temperature: 880 ° C, winding temperature: 550 ° C

  Next, after annealing was performed under the heat treatment conditions shown in Table 2, alloying was performed by reheating to a temperature of 550 ° C. as shown in FIG. 2 to obtain an alloyed hot-dip galvanized steel sheet. Specifically, after heating to a predetermined temperature (T1 in FIG. 2) and holding for 180 seconds, gas cooling was performed with various cooling patterns shown in Table 2.

(Observation of microstructure)
The microstructure of the steel sheet thus obtained is observed based on the method described above, and the maximum value (Fmax) and minimum value (Fmin) of the polygonal ferrite area ratio are measured, and the difference between the maximum value and the minimum value is measured. (Dispersion) was calculated, and the maximum area ratio (Mmax) of martensite and the minimum area ratio (Mmin) of martensite in the low-temperature transformation generation structure were measured. Here, ferrite was observed with an SEM, and bainite and martensite were observed with an optical microscope. Further, the ratio (average value) of polygonal ferrite and low-temperature transformation formation structure in the entire structure was obtained by performing image analysis as described above and calculating the average value of the visual field at a total of 20 locations.

(Characteristic evaluation)
The tensile strength, bending workability, and fatigue strength of the steel sheet were measured as follows.

(Measurement of tensile strength)
Tensile strength (TS) was measured in accordance with JIS Z 2241 by collecting JIS No. 5 tensile test pieces from the direction perpendicular to the rolling direction of the steel sheet. In this example, those having a tensile strength of 780 MPa or more were evaluated as ◯ (passed). For reference, elongation (El) and yield stress (YP) were also measured.

(Evaluation of bending workability)
(1) Bending workability of the unprocessed part As shown in FIG. 3, the bending processability of the unprocessed part is obtained by mechanically grinding the end face so as not to generate cracks, and then in the L direction (rolling direction = longitudinal direction of the specimen). And 90 ° bending in the C direction (direction perpendicular to the rolling direction) to calculate the minimum bending radius, and the value obtained by dividing the obtained minimum bending radius (Rmin) by the thickness (t) of the steel sheet (Rmin / t).

  Specifically, using the No. 1 test piece (thickness 1.2 mm) defined in JIS Z 2204 and the tool shown in FIG. 5 and changing the die shoulder radius Dp in units of 0.5 mm, 90 in the L and C directions. ° Bending was performed. Specifically, as shown in FIG. 5, after fixing the test piece 2 with the die 1, the test piece 2 is adjusted to the shoulder of the die 1 by moving the punch 3 downward (direction A in FIG. 5). It was. In FIG. 5, the clearance 4 is the distance (gap) between the die 1 and the punch 3 and is set to the thickness of the test piece +0.1 mm. In this embodiment, since a test piece having a plate thickness of 1.2 mm is used, the clearance 4 is 1.3 mm. After performing the 90 ° bending process as described above, the minimum bending radius (minimum value of the die shoulder radius Dp, mm) that can be bent without generating a crack was determined. In addition, the presence or absence of the crack was observed using a loupe, and the determination was made based on the absence of occurrence of hair cracks.

As described above, the bending workability varies depending on the strength and thickness of the steel plate. Therefore, in this embodiment, the minimum bending radius Rmin (mm) / the plate thickness t (mm) of the steel plate (in this embodiment, the plate thickness t = 1.2 mm) is calculated for both the L direction and the C direction. According to the strength level, bending workability was evaluated according to the following criteria.
780 MPa level: Pass Rmin / t ≦ 0.3 (more than 780 MPa and less than 980 MPa)
980 MPa level: Pass Rmin / t ≦ 0.5 (980 MPa or more and less than 1180 MPa)

(2) Bending workability of shearing edge The bending workability of the shearing edge was evaluated as follows. First, the end face of the bending test piece with the C direction as the longitudinal direction of the test piece was subjected to shearing (clearance 10%), and then the burred surface was bent in the state of shearing as shown in FIG. 90 ° bending was performed as the outside, and the minimum bending radius (Rmin) at which no cracks occurred at the burr portion was obtained. A value (Rmin / t) obtained by dividing the minimum bending radius (Rmin) obtained in this way by the plate thickness of the steel plate (in this embodiment, plate thickness t = 1.2 mm) is calculated, and according to the strength level of the steel plate. The bending workability of the shearing edge was evaluated according to the following criteria.

780 MPa level: Pass Rmin / t ≦ 0.5 (more than 780 MPa and less than 980 MPa)
980 MPa level: Pass Rmin / t ≦ 1.5 (980 MPa or more and less than 1180 MPa)

  In this example, the bending workability of the unprocessed part (L direction and C direction) and the bending workability of the shearing edge (C direction) are all evaluated as being “excellent in bending workability” Those in which either one failed were evaluated as “inferior in bending workability”.

(Measurement of fatigue strength)
The fatigue strength was calculated by conducting a plane bending test by the method described in JIS Z 2275 using the plane bending test piece shown in FIG. Here, the repetition rate was 1500 times / minute (frequency 25 Hz), and the stress ratio (R) was -1. A ratio between the fatigue strength and the tensile strength thus obtained was determined as a fatigue limit ratio, and the fatigue strength was evaluated according to the following criteria.
Fatigue limit ratio> 0.45: ○
Fatigue limit ratio> 0.40 to <0.45: △
Fatigue limit ratio 0.40 or less: ×

  In this example, the fatigue limit ratio was “good” or “bad”, and “x” was rejected.

  These results are listed in Table 3. In Table 3, the "Bending workability" column has a "Comprehensive evaluation" column, and all bending workability passes "○", and one of them fails "X". It was. Note that “−” indicates that the bending workability was not evaluated because TS did not reach the acceptance standard of the present invention (above 780 MPa or more). In Table 3, PF means polygonal ferrite and M means martensite.

  From these tables, it can be considered as follows.

  No. in Table 3 No. 1 in Table 2 uses the steel type A in Table 1 that satisfies the composition of the present invention. 1 is an example of the present invention manufactured by a method satisfying the requirements of the present invention shown in No. 1, and satisfies all the requirements (a) to (d) defined in the present invention. In Table 3, No. Nos. 2 to 12 use steel types B to L in Table 1 that satisfy the composition of the present invention. It is the example of this invention manufactured by the method of satisfying the requirements of this invention shown to 2-12, and satisfies all the requirements of (a)-(e) prescribed | regulated by this invention. As shown in Table 3, high strength plated steel sheets having excellent bending workability and good fatigue strength were obtained. Further, when comparing the fatigue strength, the minimum value Mmin of the martensite area ratio occupied in the low-temperature transformation formation structure [No. defined in the present invention (e)] satisfies the preferable range of the present invention. Nos. 2 to 12 are Nos. In Table 3 that do not satisfy the requirement (e). Compared with 1, fatigue strength was improved.

  On the other hand, the following examples that do not satisfy any of the requirements of the present invention have the following problems.

  No. in Table 3 13 is an example using the steel type M in Table 1 with a large amount of C. Polygonal ferrite is not sufficiently formed, and the minimum value (Fmin) of the polygonal ferrite area ratio is reduced, and all bending workability is lowered. .

  No. in Table 3 No. 14 is an example using the steel type N in Table 1 with a large amount of C and Si, and the minimum value (Mmin) of the martensite area ratio in the low-temperature transformation structure is increased and the fatigue strength is good. Further, since the maximum value (Mmax) of the martensite area ratio is increased, the bending workability of the shearing edge is lowered.

  No. in Table 3 No. 15 is an example using the steel type O of Table 1 with a large amount of Si and a small amount of Mn. 16 is an example using the steel type P of Table 1 with a small amount of C and Mn. In either case, the maximum value Fmax of polygonal ferrite was increased, and martensite was not generated (Mmin and Mmax were both 0), so that the strength was insufficient and the fatigue strength was also reduced.

  No. in Table 3 17 is an example using the steel type Q in Table 1 with a large amount of Si, and the maximum martensite area ratio (Mmax) in the low-temperature transformation structure is increased, and the fatigue strength is good. Edge bendability was reduced.

  No. in Table 3 Nos. 18 to 21 are examples using steel types satisfying the component composition of the present invention.

  Of these, No. 3 in Table 3. No. 18 is an example in which the steel type A in Table 1 was used, CR1 was cooled at the same rate as CR2, and the holding time of the holding temperature T4 was shortened. As a result, not only the variation in the area ratio of polygonal ferrite but also the maximum value Mmax of the martensite area ratio constituting the low-temperature transformation formation structure is increased, and the fatigue strength is excellent. The bending workability of the processing edge was lowered.

  No. in Table 3 19 and no. 21 is an example simulating the annealing process (slow cooling → rapid cooling two-stage cooling) described in Patent Document 2 described above.

For details, see No. 3 in Table 3. No. 19 is an example in which the steel type L in Table 1 is used, the annealing temperature T1 is 750 ° C., which is lower than the Ac ₃ point of the steel type L (796 ° C., see Table 1), and the holding time t is shortened. . As a result, all the requirements of the polygonal ferrite area ratio are outside the scope of the present invention, and the maximum value Mmax of the martensite area ratio constituting the low-temperature transformation formation structure is increased, and the C-direction bending workability of the unprocessed portion and Shearing edge bending workability decreased and fatigue strength also decreased.

  On the other hand, no. No. 20 uses the steel type B in Table 1, and the holding temperature T4 is increased and the holding time t is shortened, so that the maximum value Mmax of the martensite area ratio constituting the low-temperature transformation structure is increased, and the fatigue strength is good. The bending workability of the shearing edge decreased.

  No. in Table 3 No. 21 is an example in which the steel type I in Table 1 was used and T2 in the annealing process was increased. The minimum value and variation of the polygonal ferrite area ratio were outside the scope of the present invention, and all bending workability was lowered.

  In addition, although the alloyed hot dip galvanized steel sheet was manufactured in this example, the same tendency as described above was observed even in the hot dip galvanized steel sheet not subjected to alloying, and the one satisfying the requirements of the present invention was bent. It has been confirmed that it is excellent in both strength and fatigue strength.

1 Die 2 Specimen 3 Punch 4 Clearance
A Direction of test force

Claims (6)

(1) Components in steel are
C: 0.05 to 0.20% (in the case of a chemical component, it represents mass%, the same applies hereinafter),
Si: 0.01 to less than 0.6%,
Mn: 1.6 to 3.5%
P: 0.05% or less,
S: 0.01% or less,
sol. Al: 1.5% or less,
N: a steel plate containing 0.01% or less, the balance being iron and inevitable impurities,
(2) The structure has a polygonal ferrite structure and a low-temperature transformation generation structure, the low-temperature transformation generation structure includes at least bainite, and may further include martensite,
When a plate surface having a depth of 0.1 mm from the surface of the steel plate was changed in the plate width direction position, a total of 20 fields of view were observed with a microscope, and image analysis was performed on a 50 μm × 50 μm region in each field of view (a) A hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more, excellent in bending workability and fatigue strength, characterized by satisfying all the requirements of (d).
(A) Maximum value of polygonal ferrite area ratio (Fmax) ≦ 80%
(B) Minimum value of polygonal ferrite area ratio (Fmin) ≧ 10%
(C) Fmax−Fmin ≦ 40%
(D) Maximum value of martensite area ratio in the low temperature transformation formation structure (Mmax) ≦ 50% Furthermore, the hot-dip galvanized steel sheet according to claim 1, which satisfies the following requirement (e).
(E) Minimum value of martensite area ratio in the low temperature transformation formation structure (Mmin) ≧ 5%   The hot dip galvanized steel sheet according to claim 1 or 2, further comprising at least one selected from the group consisting of Cr: 1.0% or less, Mo: 0.5% or less, and B: 0.005% or less. .   The melt according to any one of claims 1 to 3, further comprising at least one selected from the group consisting of Nb: 0.1% or less, Ti: 0.2% or less, and V: 0.2% or less. Galvanized steel sheet.   Furthermore, the hot dip galvanized steel sheet in any one of Claims 1-4 containing Ca: 0.003% or less and / or REM: 0.003% or less.   The hot-dip galvanized steel sheet according to any one of claims 1 to 5, further subjected to alloying treatment.