Hot-dip galvanized steel sheet, member using hot-dip galvanized steel sheet, automobile framework structure member comprising member or automobile

Abstract

Provided is a hot-dip galvanized steel sheet having high strength, high ductility, high stretch flangeability and bendability, and reduced anisotropy of stretch flange cracks. A hot-dip galvanized steel sheet provided with a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, the base steel sheet having a prescribed component composition and a steel structure in which the area ratio of martensite is 10-80% and the area ratio of ferrite is 20-90% at a position 1 / 4 of the sheet thickness in the depth direction from the surface of the base steel sheet. The area ratio of retained austenite is 10% or less, and the ratio of the area ratio of ferrite having {001} orientation to the area ratio of all ferrite is 0.50 or less; the amount of diffusible hydrogen in a low-temperature region in the base steel sheet, which is the amount of hydrogen released when the base steel sheet is heated to 50 DEG C, is 0.015 mass ppm or less.

Description

Technical Field

[0001] The present invention relates to a hot-dip galvanized steel sheet, a member made of the hot-dip galvanized steel sheet, a skeleton structure member or a reinforcing member of an automobile composed of the member, and a method for manufacturing the hot-dip galvanized steel sheet and the member. Background Art

[0002] In order to reduce CO2 emissions and improve collision resistance performance by lightening vehicles, the high-strength of steel sheets for automobiles is being promoted. In addition, against the background of the continuous introduction of new laws and regulations, in order to increase the body strength, the cases of applying high-strength steel sheets to the main structural members and reinforcing members (hereinafter also referred to as "skeleton structure members of automobiles", etc.) forming the skeleton of an automobile cab are increasing. In particular, the increase in the application cases of high-strength steel sheets with a tensile strength (hereinafter also referred to as "TS") of 780 MPa or more is remarkable.

[0003] For high-strength steel sheets used in skeleton structure members of automobiles, etc., high stretch flange formability is required when formed into a desired shape. Further, in skeleton structure members of automobiles, etc., for example, a collision box, etc. has a bent portion, and thus, from the viewpoint of formability, a steel sheet having high bendability is preferably used.

[0004] As a technique related to such high-strength steel sheets, for example, Patent Document 1 discloses a high-strength hot-dip galvanized steel sheet, in which the base steel sheet has the following composition: containing a specified amount of C, Si, Mn, P, S, Al, N, Ca, and Cr, and [%Mn] / [%Si] satisfies the relationship of 2.9 to 11.7, and the balance is composed of Fe and inevitable impurities; the base steel sheet has the following steel structure: one or two selected from bainite and ferrite, tempered martensite, quenched martensite, and retained austenite are in specified amounts; the ratio of the Si enrichment amount to the Mn enrichment amount on the surface layer of the base steel sheet is 0.7 to 1.3, and the diffusible hydrogen amount in the above base steel sheet is 0.80 mass ppm or less.

[0005] Prior Art Documents

[0006] Patent Documents

[0007] Patent Document 1: International Publication No. 2020 / 170542 Summary of the Invention

[0008] However, in the hot-dip galvanized steel sheet described in Patent Document 1, the anisotropy of stretch flange cracks is not considered. Therefore, from the viewpoint of increasing the application ratio of the hot-dip galvanized steel sheet in skeleton structure members of automobiles, etc., at present, it is required to develop a hot-dip galvanized steel sheet having high strength, high ductility, high stretch flange formability and bendability, and reduced anisotropy of stretch flange cracks.

[0009] The present invention has been developed in view of the above situation, and an object thereof is to provide a hot-dip galvanized steel sheet having high strength, high ductility, high flange stretchability and bendability, and reduced anisotropy of flange stretch cracks, and a method for manufacturing the same.

[0010] In addition, an object of the present invention is to provide a member using the above hot-dip galvanized steel sheet and a method for manufacturing the same.

[0011] Here, "high strength" means that the tensile strength (hereinafter also referred to as "TS") measured according to JIS Z 2241 is 780 MPa or more.

[0012] "High ductility" means that the product (TS×El) of the TS and the total elongation (hereinafter also referred to as "El") measured according to JIS Z 2241 is 13,000 MPa·% or more.

[0013] "High flange stretchability" means that the hole expansion ratio (hereinafter also referred to as "λ") measured according to JIS Z 2256 is 20% or more.

[0014] "High bendability" means that in the bend test described in the following examples, the case where none of the 5 evaluated samples are broken or the case where one or more of the 5 samples have fine cracks less than 200 μm are generated. This bend test is based on JIS Z 2248.

[0015] "Reduction of anisotropy of flange stretch cracks" means that in the hole expansion test described in the following examples, for the rolling direction (L direction) of the steel sheet, the 45-degree direction (D direction) with respect to the rolling direction of the steel sheet, and the direction perpendicular to the rolling direction of the steel sheet (C direction), the crack generation rate in each direction is 60% or less.

[0016] The inventors of the present invention repeatedly conducted in-depth research in order to achieve the above object, and as a result, obtained the following insights.

[0017] That is, in order to obtain a hot-dip galvanized steel sheet with high ductility, high flange stretchability and bendability, and reduced anisotropy of flange stretching cracks and having high strength, from the viewpoint of high strength, for the base steel sheet, on the basis of making it have a specified composition, it is important to make a steel structure containing martensite (quenched martensite, tempered martensite and bainite). Furthermore, in terms of controlling ductility, it is important to ensure a certain amount of ferrite in the steel structure of the base steel sheet. In addition, in order to reduce the anisotropy of flange stretching cracks, it is important that the ratio of the area fraction of ferrite with a {001} orientation to the area fraction of all ferrite (area fraction of ferrite with a {001} orientation / area fraction of all ferrite) and the amount of hydrogen released in the low temperature region (temperature region up to 50°C) when heating the base steel sheet (diffusible hydrogen amount in the low temperature region) are within a specified numerical range. Specifically, the inventors of the present invention found that, in achieving the above object, particularly in reducing the anisotropy of flange stretching cracks, it is important that the ratio of the area fraction of ferrite with a {001} orientation to the area fraction of all ferrite is 0.50 or less, and the diffusible hydrogen amount in the low temperature region is 0.015 mass ppm or less.

[0018] The gist of the present invention is as follows.

[0019] (1) A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet,

[0020] The base steel sheet has the following composition and steel structure,

[0021] The composition contains, by mass%, C: 0.030% to 0.500%, Si: 0.01% to 2.50%, Mn: 0.10% to 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, and the balance consists of Fe and inevitable impurities.

[0022] In the steel structure, at the 1 / 4 thickness position of the sheet, the area fraction of martensite is 10% to 80%, the area fraction of ferrite is 20% to 90%, the area fraction of retained austenite is 10% or less, and the ratio of the area fraction of ferrite with a {001} orientation to the area fraction of all ferrite is 0.50;

[0023] And the amount of hydrogen released when heating the base steel sheet to 50°C, that is, the diffusible hydrogen amount in the low temperature region in the base steel sheet, is 0.015 mass ppm or less.

[0024] (2) The hot-dip galvanized steel sheet according to (1) above, wherein the above composition further contains at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less by mass.

[0025] (3) The hot-dip galvanized steel sheet according to (1) or (2) above, wherein a metal coating is provided between the above base steel sheet and the above hot-dip galvanized layer.

[0026] (4) The hot-dip galvanized steel sheet according to any one of (1) to (3) above, wherein the above base steel sheet has a surface soft layer, and the surface soft layer is a region where the Vickers hardness is 85% or less with respect to the Vickers hardness at the 1 / 4 position of the plate thickness of the above base steel sheet and is within 200 μm in the plate thickness direction from the surface of the above base steel sheet.

[0027] (5) The hot-dip galvanized steel sheet according to (4) above, wherein when measuring the nano-hardness at more than 300 points in a 50 μm × 50 μm region on the plate surface at each of the 1 / 4 position and the 1 / 2 position of the plate thickness direction depth of the surface soft layer from the surface of the above base steel sheet,

[0028] the proportion of the number of measurements with a nano-hardness of 7.0 GPa or more at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the surface of the above base steel sheet with respect to the total number of measurements is 0.10 or less,

[0029] the standard deviation σ of the nano-hardness at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the surface of the above base steel sheet is 1.8 GPa or less,

[0030] moreover, the standard deviation σ of the nano-hardness at the 1 / 2 position of the plate thickness direction depth of the surface soft layer from the surface of the above base steel sheet is 2.2 GPa or less.

[0031] (6) The hot-dip galvanized steel sheet according to any one of (1) to (5) above, wherein the above hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

[0032] (7)A component made of the hot-dip galvanized steel sheet described in any one of the above (1) to (6).

[0033] (8)A skeleton structure part or a reinforcing part of an automobile, composed of the component described in the above (7).

[0034] (9)A method for manufacturing a hot-dip galvanized steel sheet, which hot-rolls a steel billet having the composition described in the above (1) or (2) under the condition of a coiling temperature of 400°C to 700°C to obtain a hot-rolled steel sheet.

[0035] Then, pickling is performed on the above hot-rolled steel sheet.

[0036] Then, cold rolling is performed on the above hot-rolled steel sheet under the conditions of a total reduction ratio of 40% or more and the number of passes from reaching an accumulated reduction ratio of 30% or more to the total reduction ratio of 2 passes or more to obtain a cold-rolled steel sheet.

[0037] Then, annealing is performed on the above cold-rolled steel sheet under the conditions of an average heating rate in the temperature range of 250°C to 600°C of 1.0°C / s to 100°C / s and an annealing temperature of 750°C to 900°C.

[0038] Then, hot-dip galvanizing treatment is performed on the above cold-rolled steel sheet to obtain a coated steel sheet.

[0039] Then, the above coated steel sheet is cooled.

[0040] (10)According to the method for manufacturing a hot-dip galvanized steel sheet described in the above (9), during the cooling of the above coated steel sheet, heat preservation is carried out for 5 s or more in the temperature range of 100°C to 450°C, and then it is cooled.

[0041] (11)According to the method for manufacturing a hot-dip galvanized steel sheet described in the above (9), during the cooling of the above coated steel sheet, cooling is stopped at a cooling stop point below 300°C, and then it is reheated to a temperature range above (cooling stop temperature + 50°C) and below 450°C, heat preservation is carried out for 5 s or more in this temperature range, and then it is cooled.

[0042] (12)According to the method for manufacturing a galvanized steel sheet described in any one of the above (9) to (11), the above annealing is carried out in an atmosphere with a dew point of -25°C or higher.

[0043] (13)According to the method for manufacturing a galvanized steel sheet described in any one of the above (9) to (12), a metal plating process of forming a metal coating on one or both sides of the above cold-rolled steel sheet is included before the above annealing process.

[0044] (14) The manufacturing method of the hot-dip galvanized steel sheet according to any one of (9) to (13) above, wherein an alloying treatment is performed on the steel sheet after the hot-dip galvanizing treatment.

[0045] (15) A manufacturing method of a member, which has a process of manufacturing a member by performing at least one of forming processing or joining processing on the hot-dip galvanized steel sheet according to any one of (1) to (8) above.

[0046] According to the present invention, it is possible to provide a hot-dip galvanized steel sheet having high strength, high ductility, high flanging elongation, and high bendability, and anisotropy with reduced flanging cracks. In addition, it is possible to provide a member using the above hot-dip galvanized steel sheet.

[0047] Furthermore, according to the present invention, it is possible to provide a manufacturing method of the above hot-dip galvanized steel sheet and a member using the hot-dip galvanized steel sheet. Description of the drawings

[0048] Figure 1 It is a schematic view related to the production of a sample for V-bending + orthogonal VDA bending test of an embodiment. Figure 1 (a) relates to V-bending (primary bending), Figure 1 (b) relates to orthogonal VDA bending (secondary bending).

[0049] Figure 2 It is a schematic view related to a sample for axial crush test and the test of an embodiment. Figure 2 (a) is a front view of a test member, Figure 2 (b) is a front view of a test member. Figure 2 (c) is a schematic diagram showing an axial crush test. Detailed description of the invention

[0050] The present invention will be described based on the following embodiments. The present invention is not limited to the following embodiments.

[0051] [1] Hot-dip galvanized steel sheet

[0052] First, the hot-dip galvanized steel sheet of the present invention will be described.

[0053] [1-1] Base metal steel sheet

[0054] The hot-dip galvanized steel sheet includes a base metal steel sheet and a hot-dip galvanized layer formed on the surface of the base metal steel sheet. The base metal steel sheet in the hot-dip galvanized steel sheet will be described.

[0055] [1-1-1] Composition

[0056] The appropriate range of the component composition of the base metal steel plate and the reasons for its limitation will be described. It should be noted that in the following description, unless otherwise specified, "%" indicating the content of the component elements of the steel plate refers to "mass%". In addition, in this specification, the numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value.

[0057] [C: 0.030% to 0.500%]

[0058] C is one of the important basic components of steel. Especially in the present invention, it is an important element affecting the area ratio of martensite and ferrite. If the content of C is less than 0.030%, the fraction of martensite decreases, and it is difficult to achieve the desired TS. On the other hand, if the content of C is greater than 0.500%, the area ratio of martensite embrittles, and it is difficult to achieve the desired ductility. Therefore, the content of C is 0.030% to 0.500%. The content of C is preferably 0.050% or more, more preferably 0.070% or more. In addition, it is preferably 0.400% or less, more preferably 0.300% or less.

[0059] [Si: 0.01% to 2.50%]

[0060] Si is one of the important basic components of steel. Especially in the present invention, it is an important element affecting the area ratio of martensite and ferrite. If the content of Si is less than 0.01%, the area ratio of ferrite decreases, and it is difficult to achieve the desired ductility. On the other hand, if the content of Si is greater than 2.50%, the formation of carbides in continuous annealing is inhibited, and the formation of retained austenite, which is a phase with a large hydrogen solubility, is promoted. Therefore, the detachment of hydrogen from the plated steel plate is not promoted, the amount of diffusible hydrogen in the low-temperature region of the base metal steel plate increases, and the anisotropy of the stretch flange crack becomes larger. In addition, during blanking, a phase transformation from retained austenite to martensite occurs, the generation of voids during hole expansion increases, which may reduce λ and may reduce bendability. Therefore, the content of Si is 0.01% to 2.50%. The content of Si is preferably 0.05% or more, more preferably 0.10% or more. In addition, it is preferably 2.00% or less, more preferably 1.80% or less.

[0061] [Mn: 0.10% to 5.00%]

[0062] Mn is one of the important basic components of steel, especially in the present invention, it is an important element that affects the area ratio of martensite and ferrite. If the content of Mn is less than 0.10%, the area ratio of martensite decreases, and it is difficult to achieve the desired TS. On the other hand, if the content of Mn is greater than 5.00%, the area ratio of ferrite decreases, and it is difficult to achieve the desired ductility. Therefore, the content of Mn is 0.10% - 5.00%. The content of Mn is preferably 0.80% or more, more preferably 1.00% or more, and preferably 4.50% or less, more preferably 4.00% or less.

[0063] [P: 0.100% or less]

[0064] P is an element that segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and reduces the ultimate deformation ability of the steel plate. It may reduce λ and may also reduce bendability. Therefore, the content of P is 0.100% or less. The lower limit of the content of P is not particularly limited, but P is a solid solution strengthening element and can improve the strength of the steel plate. Therefore, it is preferably 0.001% or more. The content of P is preferably 0.001% or more, and preferably 0.070% or less.

[0065] [S: 0.0200% or less]

[0066] S is an element that exists in the form of sulfide and reduces the ultimate deformation ability of the steel plate. It may reduce λ and may also reduce bendability. Therefore, the content of S is 0.0200% or less. The lower limit of the content of S is not particularly limited, but due to production technical constraints, it is preferably 0.0001% or more. The content of S is preferably 0.0001% or more, and preferably 0.0050% or less.

[0067] [Al: 1.000% or less]

[0068] Al is an element that raises the A3 transformation point, makes a large amount of ferrite in the steel structure, and it may be difficult to achieve the desired TS. Therefore, the content of Al is 1.000% or less. The lower limit of the content of Al is not particularly limited, but due to suppressing carbide formation during continuous annealing and promoting the formation of retained austenite, the content of Al is preferably 0.001% or more. The content of Al is preferably 0.001% or more, and preferably 0.100% or less.

[0069] [N: 0.0100% or less]

[0070] N is an element that exists in the form of nitride and reduces the ultimate deformation ability of the steel plate, which may reduce λ and may also reduce bendability. Therefore, the content of N is 0.0100% or less. The lower limit of the content of N is not particularly limited, but due to production technology constraints, the content of N is preferably 0.0001% or more. The content of N is preferably 0.0001% or more, and further preferably 0.0050% or less.

[0071] [O: 0.0100% or less]

[0072] O is an element that exists in the form of oxide and reduces the ultimate deformation ability of the steel plate, which may reduce λ and may also reduce bendability. Therefore, the content of O is 0.0100% or less. The lower limit of the content of O is not particularly limited, but due to production technology constraints, the content of O is preferably 0.0001% or more. The content of O is preferably 0.0001% or more, and further preferably 0.0050% or less.

[0073] [Optional component]

[0074] In addition to the above composition, the base steel plate may further contain, by mass%, selected from

[0075] Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less,

[0076] Ta: 0.10% or less, W: 0.10% or less,

[0077] B: 0.0100% or less,

[0078] Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less,

[0079] Co: 0.010% or less,

[0080] Cu: 1.00% or less,

[0081] Sn: 0.200% or less,

[0082] Sb: 0.200% or less,

[0083] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less,

[0084] Zr: 0.020% or less, Te: 0.020% or less,

[0085] Hf: 0.10% or less and

[0086] Bi: 0.200% or less

[0087] At least one of the elements. These elements can be used alone or in combinations of two or more.

[0088] When containing Ti, Nb or V, in order to avoid the formation of a large amount of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate is reduced, and thus λ is reduced and the bendability is reduced. The content of Ti, Nb or V is preferably 0.200% or less, more preferably 0.100% or less, respectively. There is no particular limitation on the lower limit of the content of Ti, Nb or V, but since the strength of the steel plate is increased by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing, the content of Ti, Nb or V is preferably 0.001% or more, respectively.

[0089] When containing Ta or W, in order to avoid the formation of a large amount of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate is reduced, and thus λ is reduced and the bendability is reduced. The content of Ta or W is preferably 0.10% or less, more preferably 0.08% or less, respectively. There is no particular limitation on the lower limit of the content of Ta or W, but since the strength of the steel plate is increased by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing, the content of Ta or W is preferably 0.01% or more, respectively.

[0090] When containing B, in order to avoid cracking inside the steel plate during casting or hot rolling, the ultimate deformation ability of the steel plate is reduced, and thus λ is reduced and the bendability is reduced. The content of B is preferably 0.0100% or less, more preferably 0.0080% or less. There is no particular limitation on the lower limit of the content of B, but since B is an element that segregates at the austenite grain boundary during annealing to improve hardenability, the content of B is preferably 0.0003% or more.

[0091] When containing Cr, Mo or Ni, in order to avoid an increase in coarse precipitates and inclusions, the ultimate deformation ability of the steel plate is reduced, and thus λ is reduced and the bendability is reduced. The content of Cr, Mo or Ni is preferably 1.00% or less, more preferably 0.80% or less, respectively. There is no particular limitation on the lower limit of the content of Cr, Mo or Ni, but since they are elements that improve hardenability, the content of Cr, Mo or Ni is preferably 0.01% or more, respectively.

[0092] When containing Co, in order to avoid an increase in coarse precipitates and inclusions, the ultimate deformation ability of the steel plate is reduced, and thus λ is reduced and the bendability is reduced. The content of Co is preferably 0.010% or less, more preferably 0.008% or less. There is no particular limitation on the lower limit of the content of Co, but since it is an element that improves hardenability, the content of Co is preferably 0.001% or more.

[0093] When containing Cu, in order to avoid the increase of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate decreases, and then λ decreases, and the bendability decreases. The content of Cu is preferably 1.00% or less, more preferably 0.80% or less. There is no particular limitation on the lower limit of the content of Cu, but since it is an element that improves hardenability, the content of Cu is preferably 0.01% or more.

[0094] When containing Sn, in order to avoid the generation of cracks inside the steel plate during casting or hot rolling, the ultimate deformation ability of the steel plate decreases, and then λ decreases, and the bendability decreases. The content of Sn is preferably 0.200% or less, more preferably 0.100% or less. There is no particular limitation on the lower limit of the content of Sn, but since it is an element that improves hardenability, the content of Sn is preferably 0.001% or more.

[0095] When containing Sb, in order to avoid the increase of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate decreases, and then λ decreases, and the bendability decreases. The content of Sb is preferably 0.200% or less, more preferably 0.100% or less. There is no particular limitation on the lower limit of the content of Sb, but since it is an element that controls the thickness of the surface softened layer and can adjust the strength, the content of Sb is preferably 0.001% or more.

[0096] When containing Ca, Mg or REM, in order to avoid the increase of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate decreases, and then λ decreases, and the bendability decreases. The content of Ca, Mg or REM is preferably 0.0100% or less, more preferably 0.0050% or less respectively. There is no particular limitation on the lower limit of the content of Ca, Mg or REM, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformation ability of the steel plate, the content of Ca, Mg or REM is preferably 0.0001% or more respectively.

[0097] When containing Zr or Te, in order to avoid the increase of coarse precipitates and inclusions, the ultimate deformation ability of the steel plate decreases, and then λ decreases, and the bendability decreases. The content of Zr or Te is preferably 0.100% or less, more preferably 0.080% or less respectively. There is no particular limitation on the lower limit of the content of Zr or Te, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformation ability of the steel plate, the content of Zr or Te is preferably 0.001% or more respectively.

[0098] When Hf is contained, in order to avoid an increase in coarse precipitates and inclusions, the ultimate deformation capacity of the steel sheet decreases, and thus λ decreases and bendability decreases. The content of Hf is preferably 0.10% or less, more preferably 0.08% or less. The lower limit of the content of Hf is not particularly limited, but since it is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformation capacity of the steel sheet, the content of Hf is preferably 0.01% or more.

[0099] When Bi is contained, in order to avoid an increase in coarse precipitates and inclusions, the ultimate deformation capacity of the steel sheet decreases, and thus λ decreases and bendability decreases. The content of Bi is preferably 0.200% or less, more preferably 0.100% or less. The lower limit of the content of Bi is not particularly limited, but since it is an element that reduces segregation, the content of Bi is preferably 0.001% or more.

[0100] The base steel sheet of the hot-dip galvanized steel sheet of the present invention has a composition containing essential components and optional components as appropriate, and the remaining part is composed of Fe and inevitable impurities. Here, as inevitable impurities, Zn, Pb, As, Ge, Sr, and Cs can be cited. These inevitable impurities are allowed to be contained in a total amount of 0.100% or less.

[0101] [1-1-2] Steel structure

[0102] The steel structure of the base steel sheet will be described.

[0103] [Area ratio of martensite: 10% - 80%]

[0104] If the area ratio of martensite is less than 10%, it is difficult to achieve a TS of 780 MPa or more. On the other hand, if the area ratio of martensite is greater than 80%, it is difficult to achieve the desired ductility. Therefore, the area ratio of martensite is 10% - 80%. The area ratio of martensite is preferably 15% or more, more preferably 25% or more. Additionally, it is preferably 75% or less, preferably 70% or less.

[0105] In the martensite mentioned here, in addition to quenched martensite (fresh martensite), it also includes tempered martensite and bainite. As described later, the observation position of the area ratio of martensite is at the 1 / 4 position of the plate thickness of the base steel sheet.

[0106] [Area ratio of ferrite: 20% - 90%]

[0107] If the area ratio of ferrite is less than 20%, it is difficult to achieve the desired ductility. On the other hand, if the area ratio of ferrite is greater than 90%, it is difficult to achieve a TS of 780 MPa or more. Therefore, the area ratio of ferrite is 20% to 90%. The area ratio of ferrite is preferably 25% or more, more preferably 30% or more. In addition, it is preferably 85% or less, more preferably 80% or less.

[0108] The ferrite mentioned here includes bainitic ferrite in addition to polygonal ferrite. As described later, the observation position of the area ratio of ferrite is at the 1 / 4 position of the plate thickness of the base metal steel plate.

[0109] Here, the measurement methods for the area ratios of martensite (quenched martensite, tempered martensite, and bainite) and ferrite (polygonal ferrite and bainitic ferrite) are as follows.

[0110] A sample is cut out from the base metal steel plate in such a way that the plate thickness section (L section at the 1 / 4 position of the plate thickness) parallel to its rolling direction becomes the observation surface. The observation surface of the sample is mirror-polished using diamond polishing paste, then fine-polished using alumina, and further etched with a 3 vol% nitric acid ethanol solution to expose the microstructure.

[0111] Next, under the condition of an acceleration voltage of 10 kV, the observation surface of the sample is observed at a magnification of 3000 times using a scanning electron microscope (SEM) to obtain SEM images of 3 fields of view (1 field of view is 40 μm × 30 μm).

[0112] From the obtained SEM images, using Adobe Photoshop (manufactured by Adobe Systems), the area ratios of each microstructure (ferrite (polygonal ferrite and bainitic ferrite), martensite (quenched martensite, tempered martensite, and bainite)) are calculated. Specifically, the value obtained by dividing the area of each microstructure by the measurement area is used as the area ratio of each microstructure. The area ratios of each microstructure in 3 fields of view are calculated, and their average value is used as the area ratio of each microstructure.

[0113] In the SEM images, ferrite (polygonal ferrite and bainitic ferrite) is a flat microstructure in the concave part and does not contain carbides, tempered martensite and bainite are microstructures in the concave part and contain fine carbides, and quenched martensite is a convex part and has fine unevenness inside the microstructure, and they can be distinguished from each other. Since the total area ratio is obtained as the area ratio of martensite for tempered martensite and bainite, they cannot be distinguished from each other.

[0114] [Area ratio of retained austenite: 10% or less]

[0115] Retained austenite is a phase with a large hydrogen solubility. Therefore, if the area ratio of retained austenite is greater than 10%, it does not promote the detachment of hydrogen from the plated steel sheet, the amount of diffusible hydrogen in the low-temperature region of the base steel sheet increases, and the anisotropy of the stretch flange crack becomes larger. In addition, during blanking, austenite transforms into martensite, increasing the generation of voids during reaming, which may further reduce λ and bendability. Therefore, the area ratio of retained austenite is 10% or less. The smaller the area ratio of retained austenite, the better. It is preferably 7% or less, more preferably 5% or less, and may also be 0%. The observation position of the area ratio of retained austenite is at the 1 / 4 thickness position of the base steel sheet, and the measurement method is as follows.

[0116] First, grind the base steel sheet so that the 1 / 4 thickness position (the position equivalent to 1 / 4 of the sheet thickness in the depth direction from the steel sheet surface) becomes the measurement surface, and then further grind 0.1 mm by chemical polishing to obtain a sample.

[0117] For the measurement surface of the sample, using an X-ray diffractometer and a Co Kα ray source, measure the integrated reflection intensities of the (200), (220), and (311) planes of fcc iron (austenite) and the (200), (211), and (220) planes of bcc iron.

[0118] Calculate the intensity ratio of the integrated reflection intensity of each plane of fcc iron to that of each plane of bcc iron. Take the average value of the 9 intensity ratios as the volume ratio of retained austenite. Consider this volume ratio of retained austenite as three-dimensional uniform and regard it as the area ratio of retained austenite at the 1 / 4 thickness position of the base steel sheet.

[0119] [Remaining microstructure]

[0120] The steel microstructure of the present invention may have a microstructure (remaining microstructure) other than the above-mentioned martensite, ferrite, and retained austenite.

[0121] As the remaining microstructure, microstructures other than martensite, ferrite, and retained austenite and microstructures well-known as the microstructure of the steel sheet can be cited, such as carbides including pearlite, cementite, metastable carbides (ε-carbide, η-carbide, χ-carbide, etc.). The identification of the remaining microstructure can be carried out, for example, by observation using SEM.

[0122] The area ratio of the remaining microstructure at the 1 / 4 thickness position of the base steel sheet is preferably 5% or less. The area ratio of the remaining microstructure can be calculated by the following formula.

[0123] [Area ratio of remaining microstructure (%)] = 100 - [Area ratio of martensite (%)] - [Area ratio of ferrite (%)] - [Area ratio of retained austenite (%)]

[0124] [Area ratio of ferrite with {001} orientation / Area ratio of all ferrite: 0.50 or less]

[0125] The ratio of the area ratio of ferrite with {001} orientation to the area ratio of all ferrite (Area ratio of ferrite with {001} orientation / Area ratio of all ferrite) is an extremely important component in the present invention. That is, the inventors have found that the anisotropy of the stretch flange crack depends to a great extent on the area ratio of ferrite with {001} orientation in the annealed sheet after cold rolling. If this ratio (Area ratio of ferrite with {001} orientation / Area ratio of all ferrite) is greater than 0.50, it is difficult to significantly reduce the anisotropy of the stretch flange crack. Therefore, this ratio (Area ratio of ferrite with {001} orientation / Area ratio of all ferrite) is 0.50 or less. The smaller this ratio (Area ratio of ferrite with {001} orientation / Area ratio of all ferrite) is, the better. It is preferably 0.45 or less, more preferably 0.43 or less, and can also be 0.

[0126] Here, the calculation method of this ratio (Area ratio of ferrite with {001} orientation / Area ratio of all ferrite) is as follows.

[0127] A specimen is cut out such that the plate thickness cross-section (L-section) parallel to the rolling direction of the base metal steel plate becomes the observation surface. The observation surface of the specimen is polished using diamond polishing paste, then smoothed by polishing with a colloidal silica solution, and then etched with a 0.1 vol.% nitric acid ethanol solution to minimize the unevenness on the specimen surface and remove the processed deformed layer. Then, for the position at 1 / 4 of the plate thickness (the position equivalent to 1 / 4 of the plate thickness in the depth direction from the steel plate surface), the crystal orientation is measured using the SEM-EBSD (Electron Back-Scatter Diffraction) method. Using OIM Analysis of AMETEK EDAX, phases other than ferrite including martensite are excluded using CI (Confidence Index) and IQ (Image Quality), and the texture of only ferrite is extracted. By obtaining the area ratio of ferrite with {001} orientation from the obtained orientation data of only ferrite, this ratio (Area ratio of ferrite with {001} orientation / Area ratio of all ferrite) can be calculated.

[0128] [Diffusible hydrogen content in the low-temperature region of the base metal steel plate: 0.015 mass ppm or less]

[0129] The diffusible hydrogen content in the low-temperature region of the base metal steel sheet is an extremely important component in the present invention. That is, the inventors have found that the anisotropy of the stretch flange crack depends more on the diffusible hydrogen content released in the low-temperature region, specifically in the temperature range up to 50°C (diffusible hydrogen content in the low-temperature region), compared to the hydrogen content released from the base metal steel sheet in the high-temperature region when heating the base metal steel sheet. If the diffusible hydrogen content in the low-temperature region of the base metal steel sheet is greater than 0.015 mass ppm, it is difficult to have high ductility, high stretch flange formability, and bendability, and the anisotropy of the stretch flange crack is significantly reduced. Therefore, the diffusible hydrogen content in the low-temperature region of the base metal steel sheet is 0.015 mass ppm or less. The less the diffusible hydrogen content in the low-temperature region, the better. It is preferably 0.010 mass ppm or less, more preferably 0.006 mass ppm or less, and may also be 0 ppm.

[0130] Here, the method for measuring the diffusible hydrogen content in the low-temperature region of the base metal steel sheet is as follows.

[0131] A test piece with a length of 30 mm and a width of 5 mm is taken from the central position of a sample of a hot-dip galvanized steel sheet by shearing. After taking, the test piece is immediately immersed in liquid nitrogen. While controlling the temperature of the treatment liquid so that the surface temperature of the test piece is 10°C or less, the hot-dip galvanized layer of the test piece is removed with an alkali. Then, the test piece is loaded into a thermal desorption analysis device and left to stand for 5 minutes in a state where Ar gas is flowing, and then heating is started. Specifically, the test piece is heated under the conditions of a heating temperature reaching 300°C and a heating rate of 200°C / hr, and then cooled to room temperature for measurement. The surface temperature of the test piece at the start of heating is 10°C or less.

[0132] Measure the hydrogen content released from the test piece in the temperature range from the starting temperature (room temperature) to 50°C at the start of heating (hereinafter also referred to as the cumulative released hydrogen content) obtained here, and calculate the diffusible hydrogen content in the low-temperature region of the base metal steel sheet by the following formula.

[0133] [Diffusible hydrogen content in the low-temperature region of the base metal steel sheet (mass ppm)] = [Cumulative released hydrogen content (g)] ÷ [Mass of the test piece (g)] × 10 6

[0134] For steel sheets obtained by subjecting hot-dip galvanized steel sheets to processing such as blanking, stretch flange forming, and bending, and products (components) manufactured by welding the processed steel sheets, it is only necessary to measure the diffusible hydrogen content in the low-temperature region of the base metal steel sheet part in the same manner as above.

[0135] [1-1-3] Others

[0136] The thickness of the base metal steel sheet is not particularly limited and can be set according to the thickness of the final hot-dip galvanized steel sheet. The thickness can be, for example, 0.3 mm to 3.0 mm.

[0137] The surface layer of the base metal steel plate is preferably a soft layer (surface soft layer). Since the above-mentioned surface soft layer helps to inhibit the propagation of bending cracks during press forming and when colliding with the vehicle body, the bending fracture resistance characteristics can be further improved.

[0138] The surface layer refers to the region corresponding to a thickness of 200 μm from the surface of the base metal steel plate along the plate thickness direction.

[0139] The soft layer refers to the region where the Vickers hardness of the cross-section (the plane parallel to the steel plate surface) at the 1 / 4 position of the plate thickness of the base metal steel plate is 85% or less. The soft layer includes the decarburized layer of the surface layer of the base metal steel plate.

[0140] The surface soft layer refers to the soft layer contained in the surface layer. It can be that the entire surface layer is a soft layer, or a part of the surface layer is a soft layer. The surface soft layer can be the region corresponding to a thickness within 200 μm from the surface of the base metal steel plate along the plate thickness direction.

[0141] For example, assuming that a region with a Vickers hardness of 85% or less of the cross-section (the plane parallel to the steel plate surface) at the 1 / 4 position of the plate thickness of the base metal steel plate is formed at a specified depth from the surface of the base metal steel plate along the plate thickness direction, when the specified depth along the plate thickness direction is within 200 μm, the region corresponding to the thickness from the surface to the specified depth along the plate thickness direction is the surface soft layer; when the specified depth along the plate thickness direction is greater than 200 μm, the region corresponding to a thickness of 200 μm from the surface of the base metal steel plate to a depth of 200 μm along the plate thickness direction is the surface soft layer.

[0142] When there is a surface soft layer, the lower limit of the thickness of the surface soft layer is not particularly limited, preferably 8 μm or more, more preferably greater than 17 μm.

[0143] The Vickers hardness is measured based on JIS Z 2244-1(2020) with a load of 10 gf.

[0144] When there is a surface soft layer, when measuring the nano-hardness at more than 300 points in a 50 μm × 50 μm region on the plate surface at the 1 / 4 position of the plate thickness direction of the surface soft layer from the surface of the base metal steel plate (the position at a depth of 1 / 4 of the thickness of the surface soft layer from the surface of the base metal steel plate along the depth direction), the proportion of the nano-hardness of 7.0 GPa or more is preferably 0.10 or less. When the proportion of the nano-hardness of 7.0 GPa or more is 0.10 or less, it means that the proportion of hard tissues (such as martensite) and inclusions is small, and the generation, connection, and crack propagation of voids during press forming and collision of hard tissues (such as martensite) and inclusions can be further inhibited, and excellent bendability during press forming and excellent bending fracture characteristics during collision can be easily obtained.

[0145] In the present invention, in order to obtain excellent bendability during press forming and excellent bend fracture characteristics during collision, it is preferable that the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the base metal steel plate is 1.8 GPa or less, and further, the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 2 of the plate thickness direction depth of the surface soft layer from the surface of the base metal steel plate is 2.2 GPa or less. When the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the base metal steel plate is 1.8 GPa or less, and further, the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 2 of the plate thickness direction depth of the surface soft layer from the surface of the base metal steel plate is 2.2 GPa or less, it indicates that the difference in tissue hardness in the microscopic region is small, and the generation, connection, and propagation of voids during press forming and collision can be further suppressed, and excellent bendability and bend fracture characteristics during collision can be easily obtained.

[0146] In addition, a more preferable range of the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the base metal steel plate is 1.7 GPa or less. A more preferable range of the standard deviation σ of the nano-hardness of the plate surface at a position 1 / 2 of the plate thickness direction depth of the surface soft layer from the surface of the base metal steel plate is 2.1 GPa or less.

[0147] Here, the nano-hardness of the plate surface at the 1 / 4 position and 1 / 2 position of the plate thickness direction depth refers to the hardness measured by the following method.

[0148] First, in the case where a coating layer is formed, after peeling the coating layer, mechanical polishing is performed until a position 1 / 4 of the plate thickness direction depth of the surface soft layer from the surface of the base metal steel plate is reached, polishing using diamond and alumina is performed, and further colloidal silica polishing is performed. The nano-hardness is measured using a diamond indenter in the shape of a triangular pyramid (Vickers) under the conditions of load: 500 μN, measurement area: 50 μm × 50 μm, and dot interval: 2 μm.

[0149] In addition, mechanical polishing is performed until a position 1 / 2 of the plate thickness direction depth of the surface soft layer is reached, polishing using diamond and alumina is performed, and further colloidal silica polishing is performed. Then, the nano-hardness is measured using a diamond indenter in the shape of a triangular pyramid under the conditions of load: 500 μN, measurement area: 50 μm × 50 μm, and dot interval: 2 μm.

[0150] Here, the thickness of the surface soft layer can be measured by the following method. After smoothing the plate thickness cross-section (L cross-section) parallel to the rolling direction of the base metal steel plate by wet grinding, a Vickers hardness tester is used to measure at intervals of 1 μm from a position 1 μm in the plate thickness direction from the surface of the base metal steel plate to a position 100 μm in the plate thickness direction. Then, measurements are made at intervals of 20 μm up to the plate thickness center. The region where the hardness is reduced to 85% or less compared to the hardness at the 1 / 4 position of the plate thickness is defined as the soft layer (surface soft layer), and the thickness in the plate thickness direction of this region is set as the thickness of the soft layer.

[0151] [1-2] Metal coating

[0152] The base metal steel plate preferably has a metal coating on one or both sides. The metal coating helps to suppress the generation of bending cracks during press forming and when colliding with the vehicle body, so the bending fracture resistance characteristics can be further improved.

[0153] The metal coating is directly formed on the surface of the base metal steel plate and is a metal coating containing a total of more than 50% by mass of one or more metals selected from Al, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi, excluding hot-dip galvanized layers, alloyed hot-dip galvanized layers, and electro-galvanized layers. The first coating is preferably a metal electroplated layer, and the following will be described taking the metal electroplated layer as an example.

[0154] By forming a metal electroplated layer on the steel plate surface, the outermost metal electroplated layer helps to suppress the generation of bending cracks during press forming and when colliding with the vehicle body, so the bending fracture resistance characteristics are further improved.

[0155] As the metal type of the metal electroplated layer, it can be any one of Al, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, Bi, but more preferably Fe. The following will be described taking the Fe-based electroplated layer as an example.

[0156] The adhesion amount of the Fe-based electroplated layer is greater than 0 g / m 2 , preferably 2.0 g / m 2 or more. There is no particular limitation on the upper limit of the adhesion amount on one side of the Fe-based electroplated layer, but from the perspective of cost, it is preferably that the adhesion amount on one side of the Fe-based electroplated layer is 60 g / m 2 or less. The adhesion amount of the Fe-based electroplated layer is preferably 50 g / m 2 or less, more preferably 40 g / m 2 or less, and further preferably 30 g / m2 as follows

[0157] The adhesion amount of the Fe-based electroplated layer is measured as follows. A sample with a size of 10×15 mm is taken from the Fe-based electroplated steel sheet and embedded in resin to prepare a cross-section embedded sample. Using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV, any three places of this cross-section are observed at a magnification of 2000 to 10000 times according to the thickness of the Fe-based coating layer, and the average value of the thickness of the three fields of view is multiplied by the specific gravity of iron, thereby converting it into the adhesion amount in one side of the Fe-based coating layer.

[0158] As the Fe-based electroplated layer, in addition to pure Fe, alloy coatings such as Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, and Fe-W alloy can also be used. The composition of the Fe-based electroplated layer is not particularly limited, but it is preferably a composition containing 1 or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in a total amount of 10% by mass or less, and the remaining part is composed of Fe and inevitable impurities. By making the amount of elements other than Fe 10% by mass or less in total, a decrease in electrolysis efficiency can be prevented, and the Fe-based electroplated layer can be formed at low cost. In the case of the Fe-C alloy, the content of C is preferably 0.08% by mass or less.

[0159] The base steel sheet for hot-dip galvanizing can be only the base metal steel sheet, or a base metal steel sheet with a metal coating layer, and is preferably a base metal steel sheet with a metal coating layer formed on the surface of the base metal steel sheet having a surface soft layer.

[0160] [1-3] Hot-dip galvanized layer

[0161] The hot-dip galvanized layer in the hot-dip galvanized steel sheet will be described. The hot-dip galvanized layer mentioned here also includes an alloyed hot-dip galvanized layer (a coating layer obtained by subjecting hot-dip galvanizing to an alloying treatment). In addition, the hot-dip galvanized layer can be provided on both sides of the surface of the base metal steel sheet. In this case, the hot-dip galvanized layer can be directly formed on the surface of the base metal steel sheet. When the base metal steel sheet has a surface soft layer, a metal electroplated layer, etc. on the surface, it can be formed on these layers.

[0162] The hot-dip galvanized layer usually has Zn (zinc) as the main component (Zn content is 50.0 mass% or more). The composition is not particularly limited and can be a well-known composition. The hot-dip galvanized layer is preferably composed of, for example, Zn and 20.0 mass% or less of Fe, and 0.001 mass% to 1.0 mass% of Al. In addition, the hot-dip galvanized layer may optionally contain one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with a total content greater than 0.0 mass% and 3.5 mass% or less. Additionally, the Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. It should be noted that the remainder other than the above elements is inevitable impurities.

[0163] In addition, the alloyed hot-dip galvanized layer is preferably composed of, for example, 20 mass% or less of Fe and 0.001 mass% to 1.0 mass% of Al. In addition, the alloyed hot-dip galvanized layer may optionally contain one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with a total content greater than 0 mass% and 3.5 mass% or less. The Fe content of the alloyed hot-dip galvanized layer is more preferably 7.0 mass% or more, and further preferably 8.0 mass% or more. Additionally, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15.0 mass% or less, and further preferably 12.0 mass% or less. It should be noted that the remainder other than the above elements is inevitable impurities.

[0164] The plating adhesion amount on one side of the hot-dip galvanized layer is not particularly limited. For example, it can be 20 g / m 2 ~80 g / m 2 .

[0165] The plating adhesion amount of the above galvanized layer is measured as follows. Prepare a treatment solution by adding 0.6 g of a corrosion inhibitor for Fe ("IBIT 700BK" (Japanese registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass% hydrochloric acid aqueous solution. Then, immerse a sample of a steel plate with a galvanized layer in this treatment solution to dissolve the galvanized layer. Then, measure the mass reduction amount of the sample before and after dissolution, and divide this value by the surface area of the base steel plate (the surface area of the part to be plated), thereby calculating the plating adhesion amount (g / m 2 ).

[0166] The hot-dip galvanized layer preferably has cracks. By intentionally imparting cracks to the hot-dip galvanized layer, a further reduction in the amount of diffusible hydrogen in the low-temperature region of the base steel plate can be achieved.

[0167] Here, the presence or absence of cracks in the hot-dip galvanized layer is determined as follows. For the surfaces (front and back) of the hot-dip galvanized layer of the hot-dip galvanized steel sheet, two fields of view (one field of view: 30 μm × 40 μm) are observed for each surface using SEM at a magnification of 3000 times, for a total of four fields of view. If there is one or more cracks penetrating the hot-dip galvanized layer in any of the above four fields of view, it is determined that there are cracks. In addition, if there are no cracks penetrating the hot-dip galvanized layer in all of the above four fields of view, it is determined that there are no cracks.

[0168] [1-4] Others

[0169] The thickness of the hot-dip galvanized steel sheet of the present invention is not particularly limited and may be 0.3 mm to 3.0 mm.

[0170] [2] Method for manufacturing hot-dip galvanized steel sheet

[0171] Next, the method for manufacturing the hot-dip galvanized steel sheet of the present invention (hereinafter, for convenience, also referred to as "the manufacturing method of the present invention") will be described. The manufacturing method of the present invention is also a method for manufacturing the above-mentioned hot-dip galvanized steel sheet of the present invention. Here, the temperature related to the manufacturing method is based on the surface temperature of the steel billet or steel sheet unless otherwise specified.

[0172] [2-1] Hot rolling process

[0173] The manufacturing method of the present invention includes a step of subjecting a steel billet having the composition of the base metal steel sheet to hot rolling under the condition of a coiling temperature of 400°C to 700°C to produce a hot-rolled steel sheet. The steel billet is not particularly limited as long as it has the above composition. For example, a steel billet obtained by melting a steel billet material to obtain molten steel having the above composition and solidifying the obtained molten steel can be used.

[0174] The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. The method for manufacturing a steel billet from molten steel is not particularly limited, and known methods such as continuous casting, ingot casting, and thin slab casting can be used. From the viewpoint of preventing macrosegregation, continuous casting is preferred.

[0175] After manufacturing the steel billet, it can be temporarily cooled to room temperature and then reheated and rolled. From the viewpoints of carbide dissolution and reduction of rolling load, the slab heating temperature is preferably 1100°C or higher. In addition, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. Here, the slab heating temperature is the temperature of the slab surface.

[0176] In this process, energy-saving processes such as direct feeding rolling and direct rolling can be applied. Here, direct feeding rolling is a process in which the manufactured steel billet is not cooled to room temperature but is charged into the heating furnace in a warm sheet state for rolling, and direct rolling is a process in which the manufactured steel billet is slightly heat-retained and then immediately rolled.

[0177] The steel billet is hot-rolled to obtain a hot-rolled steel sheet. The hot rolling can be composed of rough rolling and finish rolling.

[0178] For example, the steel billet can be subjected to rough rolling to produce a thin steel sheet. The conditions for rough rolling can be well-known conditions.

[0179] Next, the thin steel sheet can be subjected to finish rolling. In the case of reducing the slab heating temperature, from the viewpoint of preventing failures during rolling, it is preferable to heat the thin steel sheet using a bar heater or the like before finish rolling. The finish rolling temperature is preferably above the Ar3 transformation point. If the finish rolling temperature is too low, the rolling load increases and the reduction ratio in the non-recrystallized state of austenite increases. As a result, abnormal structures elongated in the rolling direction develop, and sometimes the workability of the steel sheet obtained after annealing is reduced. Here, the Ar3 transformation point is obtained by the following formula.

[0180] Ar3 (°C) = 868 - 396×[%C] + 24.6×[%Si] - 68.1×[%Mn] - 36.1×[%Ni] - 20.7×[%Cu] - 24.8×[%Cr]

[0181] In the above formula, [%element symbol] represents the content (mass%) of the element in the above composition.

[0182] The thin steel sheets can be joined to each other and finish rolling can be carried out continuously. In addition, the thin steel sheet can be temporarily wound before finish rolling. Furthermore, in order to reduce the rolling load, part or all of the finish rolling can be set as lubricated rolling. From the viewpoints of the uniformity of the steel sheet shape and the uniformity of the material quality, lubricated rolling is also effective. The friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0183] In the hot rolling process, after finish rolling, the hot-rolled steel sheet is wound and collected, and then cooled. At this time, it is particularly important to control the coiling temperature within the range of 400°C to 700°C.

[0184] By making the coiling temperature 400 °C or higher, the dissolved C in the hot-rolled steel sheet can be reduced, and the area ratio of ferrite with a {001} orientation can be reduced during annealing after cold rolling. Thereby, the anisotropy of the stretch flange crack can be reduced. However, if the coiling temperature is greater than 700 °C, coarse pearlite is generated in the hot-rolled steel sheet structure, and the nucleation sites of austenite during annealing are reduced. As a result, the fraction of martensite is reduced, and it is difficult to achieve the desired TS. Therefore, the coiling temperature is 400 °C to 700 °C. The coiling temperature is preferably 430 °C or higher, more preferably 450 °C or higher. Additionally, it is preferably 670 °C or lower, more preferably 650 °C or lower.

[0185] The cooling conditions after coiling are not particularly limited, and known conditions can be adopted. For example, the cooling rate is preferably 0.001 °C / s to 1 °C / s, and the cooling stop temperature is preferably 20 °C to 200 °C.

[0186] [2-2] Pickling process

[0187] After the hot rolling process, the hot-rolled steel sheet is pickled. By pickling to an appropriate degree, the oxides on the steel sheet surface can be removed, ensuring good chemical conversion treatability and plating quality. Pickling can be carried out only once or in multiple times. For the pickling conditions, there is no particular limitation, and known conditions can be applied.

[0188] [2-3] Heat treatment process (optional)

[0189] After the pickling process, heat treatment can be performed on the hot-rolled steel sheet. By performing heat treatment on the hot-rolled steel sheet, fine carbides are uniformly generated in the steel structure of the hot-rolled steel sheet, and the area ratio of martensite does not decrease. Therefore, a decrease in TS can be suppressed. From this point of view, the heat treatment temperature is preferably 450 °C or higher. On the other hand, if the heat treatment temperature is greater than 650 °C, the carbides become globular and coarse or coarse pearlite is generated, which may reduce the TS. Therefore, the heat treatment temperature is preferably 450 °C to 650 °C. The heat treatment temperature is more preferably 460 °C or higher, further preferably 470 °C or higher. Additionally, it is more preferably 600 °C or lower, further preferably 550 °C or lower.

[0190] The residence time at the heat treatment temperature is not particularly limited, but from the perspective of obtaining the effect of the heat treatment process where fine carbides are uniformly generated in the steel structure of the hot-rolled steel sheet and the area ratio of martensite does not decrease, thereby suppressing a decrease in TS, the residence time in the heat treatment temperature range is preferably 10 minutes or more. The residence time in the heat treatment temperature range is more preferably 100 minutes or more, further preferably 500 minutes or more. The upper limit of the residence time is not particularly limited, but from the perspective of uniformly generating fine carbides in the steel structure of the hot-rolled steel sheet, it is preferably 3000 minutes or less, more preferably 2000 minutes or less.

[0191] [2-4] Cold rolling process

[0192] A cold-rolled steel sheet is produced by subjecting a hot-rolled steel sheet after the hot rolling process or a hot-rolled steel sheet after the hot rolling process and the heat treatment process to cold rolling. At this time, it is important to satisfy the following conditions.

[0193] [Total reduction ratio: 40% or more]

[0194] By increasing the total reduction ratio of cold rolling from the thickness of the steel slab until the final thickness of the cold-rolled steel sheet is reached, the area ratio of ferrite with a {001} orientation can be reduced during annealing, and thus, the anisotropy of the stretch flange crack can be reduced. Therefore, the total reduction ratio of cold rolling until the final thickness is reached is 40% or more. The total reduction ratio is preferably 45% or more, more preferably 50% or more. There is no particular limitation on the upper limit of the total reduction ratio, but due to production technical constraints, it is preferably 90% or less, more preferably 85% or less.

[0195] The final thickness of the cold-rolled steel sheet can be set according to the thickness of the hot-dip galvanized steel sheet, for example, it can be 0.3 mm to 3.0 mm.

[0196] [Number of passes from the cumulative reduction ratio of 30% or more to the total reduction ratio: 2 passes or more]

[0197] By increasing the number of passes from when the cumulative reduction ratio reaches 30% or more until the final thickness is reached, the area ratio of ferrite with a {001} orientation can be reduced during annealing. Thus, the anisotropy of the stretch flange crack can be reduced. Therefore, the number of passes from the cumulative reduction ratio of 30% or more to the total reduction ratio is 2 passes or more, preferably 3 passes or more, more preferably 4 passes or more. There is no particular limitation on the upper limit of the number of passes from 30% or more to the total reduction ratio, but due to production technical constraints, it is preferably 100 passes or less, more preferably 50 passes or less.

[0198] Here, the number of passes from the cumulative reduction ratio of 30% or more to the total reduction ratio means the number of passes from the first pass when the cumulative reduction ratio first reaches 30% or more to the last pass when the final thickness is reached (including the first and last passes).

[0199] Other cold rolling conditions are not particularly limited, and known conditions can be adopted. Cold rolling can be carried out, for example, by tandem multi-stand rolling or reverse rolling. The total number of passes from the thickness of the steel slab until the final thickness is reached and the reduction ratio of each pass are not particularly limited, and known conditions can be adopted.

[0200] [2-5] Metal coating treatment process (optional)

[0201] The surface of the cold-rolled steel sheet obtained as described above can be subjected to a metal plating treatment to produce a pre-annealing metal-plated steel sheet having a metal coating layer (pre-annealing metal coating layer) formed on at least one side. The pre-annealing metal-plated steel sheet is preferably a pre-annealing metal electroplated steel sheet having a pre-annealing metal electroplating layer.

[0202] The method of the metal electroplating treatment is not particularly limited. However, as described above, as the metal coating layer formed on the base steel sheet, a metal electroplating layer is preferred. Therefore, it is preferable to perform a metal electroplating treatment. For example, an Fe-based electroplating bath can be a sulfuric acid bath, a hydrochloric acid bath, or a mixture of the two. In addition, the adhesion amount of the pre-annealing metal electroplating layer can be adjusted according to the energization time and the like. It should be noted that the pre-annealing metal electroplated steel sheet means that the metal electroplating layer has not undergone an annealing process, and a method in which the hot-rolled steel sheet, the pickled sheet after hot rolling, or the cold-rolled steel sheet before the metal electroplating treatment is pre-annealed is not excluded.

[0203] Here, as the metal type of the electroplating layer, it can be any one of Al, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, Bi, etc., but Fe is more preferred. Therefore, the manufacturing method of Fe-based electroplating is described below.

[0204] The content of Fe ions in the Fe-based electroplating bath before the start of energization is preferably 0.5 mol / L or more in terms of Fe 2+ If the content of Fe ions in the Fe-based electroplating bath is 0.5 mol / L or more in terms of Fe 2+ , a sufficient amount of Fe adhesion can be obtained. In addition, in order to obtain a sufficient amount of Fe adhesion, the content of Fe ions in the Fe-based electroplating bath before the start of energization is preferably 2.0 mol / L or less.

[0205] In addition, in the Fe-based electroplating bath, in addition to Fe ions, it can contain at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The total content of these elements in the Fe-based electroplating bath is preferably such that the total content of these elements in the Fe-based electroplating layer before annealing is 10% by mass or less. It should be noted that the metal element can be contained in the form of a metal ion, and the non-metal element can be contained as a part of boric acid, phosphoric acid, nitric acid, organic acid, etc. In addition, a conductive aid, a chelating agent, and a pH buffer such as sodium sulfate and potassium sulfate can be contained in the iron sulfate plating solution.

[0206] Other conditions of the Fe-based electroplating bath are not particularly limited. From the viewpoint of temperature maintenance, the temperature of the Fe-based electroplating solution is preferably 30 °C or higher, and further preferably 85 °C or lower. The pH of the Fe-based electroplating bath is not particularly limited either, but from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, it is preferably 1.0 or higher, and from the viewpoint of the conductivity of the Fe-based electroplating bath, it is preferably 3.0 or lower. Regarding the current density, from the viewpoint of productivity, it is preferably 10 A / dm 2 or higher, and from the viewpoint of easily controlling the adhesion amount of the Fe-based electroplating layer, it is preferably 150 A / dm 2 or lower. Regarding the sheet passing speed, from the viewpoint of productivity, it is preferably 5 mpm or higher, and from the viewpoint of stably controlling the adhesion amount, it is preferably 150 mpm or lower.

[0207] As the treatment before the Fe-based electroplating treatment, degreasing treatment and water washing for cleaning the surface of the cold-rolled steel sheet, and pickling treatment and water washing for activating the surface of the cold-rolled steel sheet can be carried out. After these pretreatment steps, the Fe-based electroplating treatment is carried out. The methods of degreasing treatment and water washing are not particularly limited, and usual methods can be used.

[0208] In the pickling treatment, various acids such as sulfuric acid, hydrochloric acid, nitric acid, and their mixtures can be used. Among them, sulfuric acid, hydrochloric acid, and their mixtures are preferred. The concentration of the acid is not particularly limited, but from the viewpoints of the removal ability of the oxide film and preventing surface roughness (surface defects) caused by over-pickling, it is preferably 1 mass% to 20 mass%.

[0209] In addition, defoamers, pickling accelerators, pickling inhibitors, etc. can be contained in the pickling treatment solution.

[0210] [2 - 6] Annealing process

[0211] The cold-rolled steel sheet obtained as described above is annealed. The cold-rolled steel sheet may or may not have been subjected to metal plating treatment. When annealing, it is important to satisfy the following conditions.

[0212] [Average heating rate in the temperature range of 250 °C to 600 °C: 1 °C / s to 100 °C / s]

[0213] By controlling the average heating rate in the heating temperature range during annealing, the area ratio of ferrite with a {001} orientation can be reduced during annealing. As a result, the anisotropy of the stretch flange crack can be reduced. Therefore, the average heating rate in the temperature range of 250°C to 600°C is 100°C / s or less. On the other hand, if the average heating rate in the temperature range of 250°C to 600°C is less than 1.0°C / s, the fraction of martensite decreases, and it is difficult to achieve the desired TS. Therefore, the average heating rate in the temperature range of 250°C to 600°C is 1.0°C / s or more. Therefore, the average heating rate in the temperature range of 250°C to 600°C is 1.0°C / s to 100°C / s. The average heating rate is preferably 3.0°C / s or more, more preferably 5.0°C / s or more. Additionally, the average heating rate is preferably 80°C / s or less, more preferably 50°C / s or less.

[0214] [Annealing temperature: 750°C to 900°C]

[0215] The cold-rolled steel sheet is heated to an annealing temperature of 750°C to 900°C. If the annealing temperature is 750°C or lower, the fraction of austenite during annealing cannot be sufficiently ensured. As a result, the area ratio of martensite decreases, and it is difficult to achieve the desired TS. On the other hand, if the annealing temperature is greater than 900°C, annealing is performed in the austenite single-phase region. Therefore, the area ratio of ferrite decreases, and it is difficult to achieve the desired ductility. Additionally, the amount of diffusible hydrogen in the low-temperature region of the base metal steel sheet increases, and it is difficult to achieve the desired ductility, stretch flange property, bendability, and anisotropy of the stretch flange crack. Therefore, the annealing temperature is 750°C to 900°C. The annealing temperature is preferably 770°C or higher, more preferably 780°C or higher. Additionally, it is preferably 880°C or lower, more preferably 860°C or lower.

[0216] The heat retention time at the annealing temperature (hereinafter also referred to as the annealing time) is not particularly limited, but from the viewpoint of controlling the area ratios of ferrite and martensite in the base metal steel sheet within a specified range, it is preferably 10 s to 600 s.

[0217] The average heating rate from above 600°C to the annealing temperature can be 0.5°C / s, preferably 1.0°C / s.

[0218] [Dew point of the annealing atmosphere in the annealing process: -25°C or higher (preferred condition)]

[0219] The dew point of the annealing atmosphere in the annealing process is preferably -25°C. By conducting the annealing process with the dew point of the annealing atmosphere being -25°C or higher, the decarburization reaction can be promoted, and a surface soft layer can be formed deeper. The dew point of the annealing atmosphere in the annealing process is more preferably -15°C or higher, and even more preferably -5°C or higher. There is no particular limitation on the upper limit of the dew point of the annealing atmosphere in the annealing process, but from the perspective of ensuring good plating adhesion when setting the hot-dip galvanized layer, it is preferably 30°C or lower. If the dew point is within the above range, the cold-rolled steel sheet with an Fe-based electroplated layer can appropriately prevent oxidation of the plating surface.

[0220] [2 - 7] Cooling process after annealing

[0221] After the above annealing, the cold-rolled steel sheet is cooled. There are no particular limitations on the conditions at this time, and known conditions can be adopted. For example, the average cooling rate in the temperature range of 500°C or higher from the annealing temperature is not particularly limited, but from the perspective of controlling the area ratios of ferrite and martensite in the base steel sheet within a specified range, it is preferably 3°C / s to 50°C / s.

[0222] In addition, the average cooling rate in the temperature range below 500°C is not particularly limited, and from the perspective of controlling the area ratio of martensite within an appropriate range, it is preferably 1°C / s to 30°C / s.

[0223] [2 - 8] Plating treatment process

[0224] A hot-dip galvanizing treatment is performed on the cold-rolled steel sheet. After the hot-dip galvanizing treatment, an alloying treatment can be performed.

[0225] The hot-dip galvanizing treatment can be performed in the temperature range below the above annealing temperature and 400°C or higher. The cold-rolled steel sheet can be temporarily cooled to below 400°C, then the steel sheet temperature is raised again to 400°C or higher, and then the treatment is performed.

[0226] Annealing, cooling, and hot-dip galvanizing treatments can be continuously performed in one production line (CGL (Continuous Galvanizing Line)).

[0227] For example, after annealing, the cold-rolled steel sheet can be cooled to a temperature range of about 500°C, and then the cold-rolled steel sheet is passed through the output side of the steel strip of the cooling zone, and moves towards the hot-dip galvanizing bath while being further cooled through the nozzle (snout) at the front end dipped in the hot-dip galvanizing bath. The time from the end of cooling of the cold-rolled steel sheet to the entry of the cold-rolled steel sheet into the hot-dip galvanizing bath is not particularly limited, but from the viewpoint of controlling the area ratio of ferrite and martensite within a specified range, it is preferably 300 s or less. A plurality of rollers for changing the traveling direction of the cold-rolled steel sheet to make it enter the nozzle can be provided immediately in front of the connection portion between the cooling zone and the nozzle. After passing through the rollers, the cold-rolled steel sheet enters the nozzle, and then the cold-rolled steel sheet introduced into the hot-dip galvanizing bath through the nozzle is immersed in the hot-dip galvanizing bath to perform hot-dip galvanizing treatment to produce a coated steel sheet.

[0228] The conditions for the hot-dip galvanizing treatment are not particularly limited. For example, the cold-rolled steel sheet can be immersed in a hot-dip galvanizing bath at 440°C to 500°C. In addition, a hot-dip galvanizing bath having a composition with an Al content of 0.10% by mass to 0.23% by mass and the balance being Zn and inevitable impurities is preferably used. The hot-dip galvanizing bath can further contain at least one selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0% by mass to 3.5% by mass. The hot-dip galvanizing bath can contain Fe dissolved from the steel sheet.

[0229] The hot-dip galvanizing treatment is preferably carried out in such a manner that the coating adhesion amount on one side of the hot-dip galvanized steel sheet (GI) is 20 g / m 2 ~80 g / m 2 . The adhesion amount of the coating can be adjusted by gas wiping or the like after the hot-dip galvanizing treatment.

[0230] After the hot-dip galvanizing treatment, an alloying treatment can be carried out. If the alloying treatment temperature is less than 460°C, the Zn-Fe alloying rate becomes too slow, and there is a risk of reduced productivity. On the other hand, if the alloying treatment temperature is greater than 600°C, the untransformed austenite transforms into pearlite, and sometimes TS and El decrease. Therefore, it is preferable that the alloying treatment temperature is 460°C to 600°C. The alloying treatment temperature is preferably 470°C or higher, and is preferably 560°C or lower.

[0231] The Fe concentration in the alloyed hot-dip galvanized layer after the alloying treatment is preferably 7% by mass to 15% by mass, and more preferably 8% by mass to 13% by mass.

[0232] [2-9] Cooling process after coating treatment

[0233] After the coating treatment (hot-dip galvanizing treatment or hot-dip galvanizing treatment and alloying treatment), the coated steel sheet is cooled.

[0234] The cooling conditions are not particularly limited, and known conditions can be adopted. For example, after hot-dip galvanizing treatment or alloying treatment, the average cooling rate to the cooling stop temperature is not particularly limited. However, from the viewpoint of further improving TS, it is preferably 2°C / s or more, more preferably 5°C / s or more. In addition, due to production technology constraints, it is preferably 50°C / s or less, more preferably 40°C / s or less. The cooling method is also not particularly limited, and gas jet cooling, spray cooling, water cooling, air cooling, etc. can be applied.

[0235] [Heat preservation temperature during cooling of the coated steel sheet: 100°C to 450°C (preferred condition)]

[0236] During the cooling of the coated steel sheet, heat can be preserved in the temperature range of 100°C to 450°C for 5 s or more, and then cooled. By preserving heat during the cooling of the coated steel sheet, the detachment of hydrogen from the coated steel sheet is promoted, and the amount of diffusible hydrogen in the low-temperature region of the base steel sheet is reduced. Therefore, the heat preservation temperature during the cooling of the coated steel sheet is preferably 100°C or more. On the other hand, if the heat preservation temperature during the cooling of the coated steel sheet is greater than 450°C, the area ratio of martensite decreases, and TS is reduced. Therefore, the heat preservation temperature during the cooling of the coated steel sheet is preferably 100°C to 450°C. The heat preservation temperature during the cooling of the coated steel sheet is more preferably 130°C or more, further preferably 150°C or more. In addition, it is more preferably 400°C or less, further preferably 350°C or less.

[0237] [Heat preservation time during cooling of the coated steel sheet: 5 s or more (preferred condition)]

[0238] From the viewpoint of effectively reducing the amount of diffusible hydrogen in the low-temperature region of the base steel sheet by heat preservation, the heat preservation time during the cooling of the coated steel sheet is preferably 5 s or more, more preferably 10 s or more, further preferably 15 s or more. The upper limit of the heat preservation time during the cooling of the coated steel sheet is not particularly limited. However, from the viewpoint of controlling the area ratios of ferrite and martensite of the base steel sheet within a specified range, it is preferably 300 s or less, more preferably 100 s or less.

[0239] [Cooling stop temperature during cooling of the coated steel sheet: 300°C or less (preferred condition)]

[0240] In the cooling of the plated steel sheet, the cooling is stopped at a cooling stop temperature below the start temperature of martensitic transformation, and then reheated, thereby promoting the detachment of hydrogen from the plated steel sheet, and the amount of diffusible hydrogen in the low-temperature region of the base steel sheet can be reduced. From this viewpoint, the cooling stop temperature in the cooling of the plated steel sheet is preferably 300 °C or lower, more preferably 250 °C or lower, and further preferably 150 °C or lower. The lower limit of the cooling stop temperature in the cooling of the plated steel sheet is not particularly limited, but due to production technical constraints, it is preferably 10 °C or higher, more preferably 30 °C or higher.

[0241] [Reheating temperature of the plated steel sheet: (cooling stop temperature + 50 °C) or higher and 450 °C or lower (preferred condition)]

[0242] After reaching the cooling stop temperature, the plated steel sheet is reheated, thereby promoting the detachment of hydrogen from the plated steel sheet, and the amount of diffusible hydrogen in the low-temperature region of the base steel sheet is reduced. From this viewpoint, the reheating temperature of the plated steel sheet is preferably (cooling stop temperature + 50 °C) or higher. On the other hand, if the reheating temperature of the plated steel sheet is greater than 450 °C, the area ratio of martensite decreases, and the TS is reduced. Therefore, the reheating temperature of the plated steel sheet is preferably (cooling stop temperature + 50 °C) or higher and 450 °C or lower, more preferably (cooling stop temperature + 80 °C) or higher, further preferably (cooling stop temperature + 100 °C) or higher. In addition, it is preferably 400 °C or lower, and further preferably 350 °C or lower.

[0243] [Heat retention time at the reheating temperature of the plated steel sheet: 5 s or longer (preferred condition)]

[0244] By retaining heat at the reheating temperature, the detachment of hydrogen from the plated steel sheet is promoted, and the amount of diffusible hydrogen in the low-temperature region of the base steel sheet can be reduced. From this viewpoint, the heat retention time at the reheating temperature of the plated steel sheet is preferably 5 s or longer, more preferably 10 s or longer, and further preferably 15 s or longer. The upper limit of the heat retention time at the reheating temperature of the plated steel sheet is not particularly limited, but from the viewpoint of controlling the area ratios of ferrite and martensite in the base steel sheet within a specified range, it is preferably 300 s or lower, more preferably 100 s or lower.

[0245] [Rolling after cooling: elongation 0.05% - 1.00% (preferred process and condition)]

[0246] After cooling, it is preferable to perform rolling on the plated steel sheet with an elongation rate of 0.05% to 1.00%. By making the elongation rate in the rolling after cooling 0.05% or more, cracks can be introduced into the hot-dip galvanized layer, and as a result, a further reduction in the amount of diffusible hydrogen in the low-temperature region of the base steel sheet can be expected. On the other hand, if the elongation rate in the rolling after cooling is greater than 1.00%, the area ratio of ferrite having a {001} orientation increases, and sometimes the anisotropy of the stretch flange crack deteriorates. Therefore, the elongation rate of the rolling after cooling is preferably 0.05% to 1.00%. The elongation rate of the rolling after cooling is more preferably 0.10% or more, and further preferably 0.50% or less.

[0247] The rolling after cooling can be performed on a device continuous with the continuous hot-dip galvanizing device (online), or can be performed on a device discontinuous with the continuous hot-dip galvanizing device (offline). In addition, the target elongation rate can be achieved by one rolling, or multiple rollings can be performed to achieve a total elongation rate of 0.05% to 1.00%. The rolling described here generally refers to skin pass rolling, but as long as an elongation rate equivalent to that of skin pass rolling can be imparted, it can also be a processing method such as repeated bending using a tension leveling machine or rolls.

[0248] For the rolling treatment, the rolling treatment can be performed after cooling the plated steel sheet to near room temperature, or the rolling treatment can be performed when the cooling of the plated steel sheet stops, and then a reheating treatment can be performed.

[0249] When the hot-dip galvanized steel sheet is the object to be treated, it usually becomes the object to be treated after cooling to room temperature.

[0250] For manufacturing conditions other than those described above, there are no particular limitations, and known conditions can be adopted.

[0251] [3] Component and its manufacturing method

[0252] The component of the present invention and its manufacturing method will be described.

[0253] The component of the present invention is a component using the above-described hot-dip galvanized steel sheet of the present invention. The component can be manufactured, for example, by forming the hot-dip galvanized steel sheet of the present invention into a target shape through pressing processing or the like.

[0254] The hot-dip galvanized steel sheet of the present invention is a high-strength hot-dip galvanized steel sheet having high ductility, high stretch flange property and bendability, and reduced anisotropy of stretch flange cracks. Therefore, by applying the hot-dip galvanized steel sheet of the present invention or a component using the hot-dip galvanized steel sheet to, for example, a skeletal structure component of an automobile or a reinforcing component of an automobile, an improvement in fuel efficiency due to vehicle body lightening can be achieved, and the industrial utilization value is extremely high.

[0255] Examples

[0256] Hereinafter, the present invention will be further described in detail by way of examples, but the present invention is not limited by the examples.

[0257] [Test Nos. 1 to 39]

[0258] A steel slab (steel slab material) having the composition shown in Table 1 and the balance composed of Fe and inevitable impurities was melted using a converter, and a steel slab was obtained by continuous casting. The obtained steel slab was heated to 1250 °C and rough rolled to obtain a thin steel sheet. Then, the obtained thin steel sheet was finish rolled at a finish rolling temperature of 900 °C and wound under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. After pickling the obtained hot-rolled steel sheet, cold rolling was carried out under the conditions shown in Table 2 to obtain a cold-rolled steel sheet with a thickness of 1.4 mm.

[0259] The obtained cold-rolled steel sheet was annealed under the conditions shown in Table 2. Then, the cold-rolled steel sheet was subjected to a plating treatment of the type shown in Table 2 to obtain a plated steel sheet having hot-dip galvanized layers on both sides.

[0260] In the types of plating treatments in Table 2, GI means only hot-dip galvanizing treatment without an alloying treatment. In addition, GA means hot-dip galvanizing treatment and alloying treatment.

[0261] In GI, as the plating bath, a hot-dip galvanizing bath containing 0.20 mass% of Al and the balance composed of Zn and inevitable impurities was used. In addition, in GA, a hot-dip galvanizing bath containing 0.14 mass% of Al and the balance composed of Zn and inevitable impurities was used. The plating bath temperature was 470 °C in both cases.

[0262] The plating adhesion amount was about 45 to 72 g / m per single side in GI, and 2 about 45 g / m in GA 2 (double-sided plating), and plating treatment was carried out on both sides in both cases. In GA, the alloying treatment temperature was about 550 °C.

[0263] The composition of the hot-dip galvanized layer of GI was Fe: 0.1 to 1.0 mass%, Al: 0.2 to 1.0 mass%, and the balance was Zn and inevitable impurities. In addition, the composition of the (alloyed) hot-dip galvanized layer of GA was Fe: 7 to 15 mass%, Al: 0.1 to 1.0 mass%, and the balance was Zn and inevitable impurities.

[0264] Then, a part of the plated steel sheet was subjected to a heat retention treatment under the conditions shown in Table 2 or reheated after cooling. Regarding the conditions not clearly described, they were carried out according to the conventional methods.

[0265] The obtained hot-dip galvanized steel sheet was subjected to the identification of the structure at the position of 1 / 4 of the thickness of the base steel sheet by the above method, and the ratio of the area ratio of ferrite with {001} orientation to the area ratio of all ferrite (area ratio of ferrite with {001} orientation / area ratio of all ferrite) and the diffusible hydrogen amount in the low-temperature region of the base steel sheet were measured. The results are shown in Table 3.

[0266] The composition of the base steel sheet of the obtained steel sheet is substantially the same as the composition at the billet stage. The suitable steels are all within the range of the composition of the present invention, and the comparative steels are all outside the range of the composition of the present invention.

[0267] The obtained hot-dip galvanized steel sheet was evaluated for tensile properties, flange elongation properties, anisotropy of flange elongation cracks, and bendability according to the following test methods. The results are shown in Table 3.

[0268] [Tensile test]

[0269] The tensile test was carried out in accordance with JIS Z 2241. That is, a JIS No. 5 test piece was taken from the obtained hot-dip galvanized steel sheet in such a way that the rolling right angle direction (C direction) of the steel sheet became the long side direction. Then, using the taken test piece, a tensile test was carried out under the condition of a crosshead speed of 1.67×10 -1 mm / s, and YS, TS, and El were measured. Then, regarding TS, a value of 780 MPa or more was judged as qualified. In addition, the product of TS and El (TS×El) was calculated from the measured TS and El, and TS×El was calculated. Then, a value of TS×El of 13000 MPa·% or more was judged as qualified.

[0270] [Hole expansion test]

[0271] The hole expansion test was carried out in accordance with JIS Z 2256. That is, the obtained hot-dip galvanized steel sheet was cut into 100 mm×100 mm. Then, a hole with a diameter of 10 mm was punched out on the cut steel sheet with a clearance of 12.5%. Then, the steel sheet was pressed with a die having an inner diameter of 75 mm and a wrinkling holding force of 9 ton (88.26 kN). In this state, a conical punch with a vertex angle of 60° was pressed into the hole, and the diameter of the hole at the limit of crack generation was measured. Then, the (limit) hole expansion ratio (%) was obtained by the following formula.

[0272] (Limit) hole expansion ratio: λ(%) = {(D f −D0) / D0}×100

[0273] Here, D fLet φ be the pore diameter (mm) when cracks occur, and D0 be the initial pore diameter (mm). The (ultimate) hole expansion ratio of 3 sheared steel plates is measured, and its average value is taken as λ. When λ is 30% or more, it is judged that the flange stretching property is qualified.

[0274] In the hole expansion test, the generation direction and number of cracks when cracks occur are evaluated. The generation direction of cracks is set as the rolling direction (L direction) of the steel plate, the direction 45 degrees relative to the rolling direction of the steel plate (D direction), and the direction perpendicular to the rolling direction of the steel plate (C direction). Furthermore, the direction between L and D is set as the LD direction, and the direction between D and C is set as the DC direction, and they are classified into 5 directions. In addition, when there is 1 crack in the same direction, the number of cracks in that direction is 1, and when there are 2 cracks in the same direction, the number of cracks in that direction is 2, and the number of cracks in each direction is also evaluated. Then, the crack generation rate in each direction is calculated by the following formulas (1) to (3).

[0275] Crack generation rate in the L direction (%) = {Number of cracks in the L direction + (Number of cracks in the LD direction) / 2} / (Total number of cracks) × 100 ··· (1)

[0276] Crack generation rate in the D direction (%) = {(Number of cracks in the LD direction) / 2 + Number of cracks in the D direction + (Number of cracks in the DC direction) / 2} / (Total number of cracks) × 100 ··· (2)

[0277] Crack generation rate in the C direction (%) = {(Number of cracks in the DC direction) / 2 + Number of cracks in the C direction} / (Total number of cracks) × 100 ··· (3)

[0278] Here, when the crack generation rate in each direction for the L, D, and C directions is 60% or less, it is judged that the anisotropy of the flange stretching crack is reduced, that is, it is qualified.

[0279] [Bending test]

[0280] The bending test was conducted in accordance with JIS Z 2248. A strip-shaped test piece with a width of 30 mm and a length of 100 mm was taken from the obtained hot-dip galvanized steel sheet in such a way that the axial direction of the bending test was parallel to the rolling direction (L direction) of the steel sheet. Then, a 90° V-bending test was conducted under the conditions of a pressing load of 100 kN and a pressing holding time of 5 seconds. The bendability was evaluated by the pass rate of the bending test. The bending test was carried out on 5 samples at the maximum R value where the value obtained by dividing the bending radius (R) by the plate thickness (t) was 5 or less (for example, when the plate thickness was 1.4 mm, the bending radius was 7.0 mm). Then, it was evaluated whether cracks occurred in the ridge line part of the bending apex. The case where none of the 5 samples cracked was judged as "excellent" bendability. In addition, the case where one or more of the 5 samples had fine cracks less than 200 μm was judged as "good" bendability. Furthermore, the case where one or more of the 5 samples had fine cracks of 200 μm or more was judged as "poor" bendability. Here, whether cracks occurred was evaluated by measuring the ridge line part of the bending apex at a magnification of 40 times using a digital microscope (RH-2000: manufactured by HIROX Co., Ltd.).

[0281]

[0282] [Table 2]

[0283] Underlined part: indicates out of range

[0284]

[0285] As shown in Table 3, in all the inventive examples, the TS was 780 MPa or more, having high ductility, high flange stretchability, and bendability, and reducing the anisotropy of flange stretch cracks. On the other hand, in the comparative examples, at least one of the TS, ductility, flange stretchability, bendability, and anisotropy of flange stretch cracks was insufficient.

[0286] [Test No. 40 - 63]

[0287] A steel billet (steel billet material) having the composition shown in Table 1 and the remaining part composed of Fe and inevitable impurities was melted using a converter, and a steel billet was obtained by continuous casting. The obtained steel billet was heated to 1250 °C and rough rolled to obtain a thin steel sheet. Then, the obtained thin steel sheet was finish rolled at a finish rolling temperature of 900 °C and wound under the conditions shown in Table 4 to obtain a hot-rolled steel sheet. After pickling the obtained hot-rolled steel sheet, cold rolling was carried out under the conditions shown in Table 4 to obtain a cold-rolled steel sheet with a plate thickness of 1.4 mm.

[0288] A metal electroplating treatment process is performed on a part of the cold-rolled steel sheet. When the column of the presence or absence of metal electroplating treatment (coating type) in Table 4 is "yes" (Fe), it is an example of performing Fe-based electroplating treatment. When it is "(Ni)", it is an example of performing Ni-based electroplating treatment. The composition of the metal electroplating layer contains Fe: 95 to 100% by mass in Fe-based electroplating and Ni: 95 to 100% by mass in Ni-based electroplating, and the remaining parts are inevitable impurities.

[0289] Next, the cold-rolled steel sheet is subjected to the coating treatment (GI, GA) of the type shown in Table 4 to obtain a coated steel sheet having hot-dip galvanized layers on both sides. The conditions of GI and GA are the same as above.

[0290] Next, a part of the coated steel sheet is subjected to a heat retention treatment under the conditions shown in Table 4 or a re-heating treatment after cooling. For the conditions not clearly described, they are carried out according to the conventional method.

[0291] The obtained hot-dip galvanized steel sheet is subjected to the identification of the structure at the position of 1 / 4 of the thickness of the base metal steel sheet according to the above test method, and the ratio of the area ratio of ferrite with {001} orientation to the area ratio of all ferrite (area ratio of ferrite with {001} orientation / area ratio of all ferrite) and the amount of diffusible hydrogen in the low-temperature region of the base metal steel sheet are measured. The results are shown in Table 5.

[0292] The composition of the base metal steel sheet of the obtained steel sheet is substantially the same as the composition at the steel billet stage, and all steels suitable are within the range of the composition of the embodiment of the present invention.

[0293] The obtained hot-dip galvanized steel sheet is evaluated for tensile properties, flanging properties, anisotropy of flanging cracks, and bendability according to the above test method. The results are shown in Table 5.

[0294] Furthermore, the thickness of the surface soft layer and the metal coating adhesion amount of the obtained hot-dip galvanized steel sheet are measured. The results are shown in Table 5.

[0295] [Thickness of surface soft layer]

[0296] After smoothing the plate thickness cross-section (L cross-section) parallel to the rolling direction of the base metal steel sheet by wet grinding, a Vickers hardness tester is used to measure at intervals of 1 μm from the position 1 μm in the plate thickness direction from the surface of the base metal steel sheet to the position 100 μm in the plate thickness direction under a load of 10 gf. Then, the measurement is carried out at intervals of 20 μm to the plate thickness center. The region where the hardness is reduced to less than 85% compared to the hardness at the position of 1 / 4 of the plate thickness is defined as the soft layer (surface soft layer), and the thickness in the plate thickness direction of this region is set as the thickness of the soft layer.

[0297] [Metal coating adhesion amount]

[0298] A sample with a size of 10 mm × 15 mm was taken from the hot-dip galvanized steel sheet and embedded in resin to prepare a cross-section embedded sample. Using a Scanning Electron Microscope (SEM), at an acceleration voltage of 15 kV, any three locations of this cross-section were observed at magnifications of 2000 - 10000 times according to the thickness of the Fe-based coating. The average value of the thicknesses of the three fields of view was multiplied by the specific gravity of iron, and thus the adhesion amount on one side of the Fe-based coating was converted.

[0299] [Nanohardness of the surface soft layer]

[0300] The nanohardness of the surface soft layer of the base steel sheet of the obtained hot-dip galvanized steel sheet was measured. The results are shown in Table 5.

[0301] For the 1 / 4 position of the surface soft layer, as follows. After peeling the coating from the obtained hot-dip galvanized steel sheet, mechanical grinding, polishing with diamond and alumina, and colloidal silica grinding were carried out until the depth reached 1 / 4 of the thickness direction of the surface soft layer from the surface of the base steel sheet. Using a nanoindentation device (tribo-950 of Hysitron), with a diamond indenter in the shape of a triangular pyramid, the nanohardness of a total of 512 points was measured under the conditions of a loading speed and an unloading speed of 50 μN / s, a maximum load of 500 μN, a measurement area of 50 μm × 50 μm, a data acquisition interval of 5 msec, and a dot interval of 2 μm.

[0302] Next, mechanical grinding, polishing with diamond and alumina, and colloidal silica grinding were carried out until the depth reached 1 / 2 of the thickness direction of the above-mentioned surface soft layer. Using the tribo-950 of Hysitron, with a diamond indenter in the shape of a triangular pyramid, the nanohardness of a total of 512 points was measured under the same conditions as above.

[0303] [V-bending + orthogonal VDA bending test, axial crushing fracture test]

[0304] The obtained hot-dip galvanized steel sheet was subjected to a V-bending + orthogonal VDA bending test and an axial crushing fracture test. The results are shown in Table 5.

[0305] It should be noted that in the V-bending + orthogonal VDA bending test and the axial crushing test of hot-dip galvanized steel sheets with a thickness greater than 1.2 mm, considering the influence of the thickness, all tests were carried out using steel sheets with a thickness of 1.2 mm. The steel sheets with a thickness greater than 1.2 mm were ground on one side to make the thickness 1.2 mm. On the other hand, in the V-bending + orthogonal VDA bending test and the axial crushing test of hot-dip galvanized steel sheets with a thickness less than 1.2 mm, the influence of the thickness is small, so the tests were carried out without grinding treatment.

[0306] The V-bending + orthogonal VDA bending test is carried out as follows.

[0307] A test piece of 60 mm × 65 mm is taken from the obtained hot-dip galvanized steel sheet by shearing and end face grinding. Here, the 60-mm side is parallel to the rolling (L) direction. A 90° bending process (primary bending process) is carried out in the rolling (L) direction with a curvature radius / thickness of 4.2 and with the width (C) direction as the axis to prepare the test piece. In the 90° bending process (primary bending process), as Figure 1 (a) shows, a punch B1 is pressed into the steel sheet placed on a die A1 with a V-shaped groove to obtain a test piece T1. Next, as Figure 1 (b) shows, an orthogonal bending (secondary bending process) is carried out by pressing a punch B2 into the test piece T1 placed on a support roll A2 in such a way that the bending direction is perpendicular to the rolling direction. In Figure 1 (a) and Figure 1 (b), D1 represents the width (C) direction and D2 represents the rolling (L) direction.

[0308] The conditions for V-bending in the V-bending + orthogonal VDA bending test are as follows.

[0309] Test method: Die support, punch pressing

[0310] Forming load: 10 ton

[0311] Test speed: 30 mm / min

[0312] Holding time: 5 s

[0313] Bending direction: Rolling (L) direction

[0314] The conditions for VDA bending in the V-bending + orthogonal VDA bending test are as follows.

[0315] Test method: Roll support, punch pressing

[0316] Roll diameter:

[0317] Radius of punch tip: 0.4 mm

[0318] Distance between rolls: (Thickness × 2) + 0.5 mm

[0319] Stroke speed: 20 mm / min

[0320] Test piece size: 60 mm × 60 mm

[0321] Bending direction: Direction perpendicular to rolling (C) direction

[0322] In the stroke-load curve obtained when performing the above-mentioned VDA bending, find the stroke at the maximum load. The average value of the strokes at the maximum load when performing the above-mentioned V bending + orthogonal VDA bending test three times is taken as SF max (mm). When the obtained SF max satisfies 26.0 mm or more, it is judged that the fracture resistance during collision (resistance to bending fracture) is excellent.

[0323] [Axial crushing test]

[0324] The axial crushing test is carried out as follows.

[0325] Take a test piece of 150 mm × 100 mm from the obtained hot-dip galvanized steel sheet by shearing. Here, the 150-mm side is parallel to the rolling (L) direction. Using a die with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm, perform forming processing (bending processing) so that the depth reaches 40 mm to fabricate Figure 2 (a) and Figure 2 (b) shown hat-shaped member 10.

[0326] In addition, cut out a steel sheet with a size of 80 mm × 100 mm from the steel sheet used as the blank of the hat-shaped member. Next, spot-weld the cut steel sheet 20 and the hat-shaped member 10 to fabricate a test member 30 as shown in Figure 2 (a) and Figure 2 (b). Figure 2 (a) is a front view of the test member 30 fabricated by spot-welding the hat-shaped member 10 and the steel sheet 20. Figure 2 (b) is a perspective view of the test member 30. The position of the spot-welded part 40 is set as shown in Figure 2 (b) so that the end of the steel sheet and the welded part are 10 mm apart and the distance between the welded parts is 20 mm. Next, as shown in Figure 2 (c), join the test member 30 and the bottom plate 50 by TIG welding to fabricate a sample for the axial crushing test. Next, make the impactor 60 collide with the fabricated sample for the axial crushing test at a collision speed of 10 mm / min at a constant speed, and crush the sample for the axial crushing test by 70 mm. As shown in Figure 2 (c), the crushing direction D3 is the direction parallel to the long side direction of the test member 30.

[0327] Observe the appearance of the test member 30 after the test, and confirm the presence or absence of axial crushing fracture (appearance crack).

[0328] When no appearance crack is observed, record "A" in Table 5 below. When 1 or fewer appearance cracks are observed, record "B" in Table 5 below. When 2 or more appearance cracks are observed, record "C" in Table 5 below. In the case of "A" or "B", it is judged that the fracture resistance during collision (resistance to axial crushing fracture) is excellent.

[0329]

[0330]

[0331] As shown in Table 5, in all inventive examples, TS is 780 MPa or more, having high ductility, high flange stretchability and bendability, and reducing the anisotropy of flange stretch cracks. Furthermore, the fracture resistance characteristics during collision (bending fracture characteristics and axial crushing characteristics) are also excellent.

[0332] Industrial Applicability

[0333] According to the present invention, a hot-dip galvanized steel sheet with high strength, high ductility, high flange stretchability and bendability, and reduced anisotropy of flange stretch cracks can be provided. In particular, various characteristics of the hot-dip galvanized steel sheet of the present invention are excellent, so it can be applied to frame structure components of various sizes and shapes of automobiles, etc. Thereby, the improvement of fuel efficiency brought about by vehicle body lightening can be achieved, and the industrial utilization value is extremely large.

[0334] Symbol Explanation

[0335] A1 Die

[0336] A2 Support Roll

[0337] B1 Punch

[0338] B2 Punch

[0339] T1 Test Piece

[0340] D1 Width (C) Direction

[0341] D2 Rolling (L) Direction

[0342] D3 Crushing Direction

[0343] 10 Hat-shaped Member

[0344] 20 Steel Sheet

[0345] 30 Test Member

[0346] 40 Spot Welded Part

[0347] 50 Base Plate

[0348] 60 Impactor

Claims

1. A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, wherein the base steel sheet has the following composition and steel structure, the composition contains, by mass%, C: 0.030% to 0.500%, Si: 0.01% to 2.50%, Mn: 0.10% to 5.00%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, and the balance is composed of Fe and inevitable impurities, in the steel structure, at the 1 / 4 position of the plate thickness, the area ratio of martensite is 10% to 80%, the area ratio of ferrite is 20% to 90%, the area ratio of retained austenite is 10% or less, and the ratio of the area ratio of ferrite with {001} orientation to the total area ratio of ferrite is 0.50 or less; moreover, the amount of hydrogen released when the base steel sheet is heated to 50°C, i.e., the diffusible hydrogen amount in the low-temperature region of the base steel sheet, is 0.015 mass ppm or less.

2. The hot-dip galvanized steel sheet according to claim 1, wherein, The composition further contains, by mass%, at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.

3. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein, A metal coating is provided between the base steel sheet and the hot-dip galvanized layer.

4. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein, The base steel sheet has a surface soft layer, which is a region where the Vickers hardness is 85% or less relative to the Vickers hardness at the 1 / 4 position of the plate thickness of the base steel sheet and is within 200 μm from the surface of the base steel sheet in the plate thickness direction.

5. The hot-dip galvanized steel sheet according to claim 4, wherein, When measuring the nano-hardness at more than 300 points in a 50 μm × 50 μm area on the plate surface at each of the 1 / 4 position and 1 / 2 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel sheet, the proportion of the number of measurements with a nano-hardness of 7.0 GPa or more on the plate surface at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel sheet relative to the total number of measurements is 0.10 or less, the standard deviation σ of the nano-hardness on the plate surface at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel sheet is 1.8 GPa or less, moreover, the standard deviation σ of the nano-hardness on the plate surface at the 1 / 2 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel sheet is 2.2 GPa or less.

6. The hot-dip galvanized steel sheet according to any one of claims 1 to 5, wherein, The hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

7. A component made of the hot-dip galvanized steel sheet described in any one of claims 1 to 6.

8. A skeletal structure component or a reinforcing component of an automobile, composed of the component described in claim 7.

9. A method for manufacturing a hot-dip galvanized steel sheet, which comprises hot-rolling a steel slab having the composition described in claim 1 or 2 to produce a hot-rolled steel sheet under the condition that the coiling temperature is 400°C to 700°C. Then, pickling the hot-rolled steel sheet. Then, cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet under the conditions that the total reduction ratio is 40% or more and the number of passes from reaching an accumulated reduction ratio of 30% or more to the total reduction ratio is 2 passes or more. Then, annealing the cold-rolled steel sheet under the conditions that the average heating rate in the temperature range of 250°C to 600°C is 1.0°C / s to 100°C / s and the annealing temperature is 750°C to 900°C. Then, performing hot-dip galvanizing treatment on the cold-rolled steel sheet to produce a coated steel sheet. Then, cooling the coated steel sheet.

10. The manufacturing method of the hot-dip galvanized steel sheet according to claim 9, wherein, During the cooling of the coated steel sheet, heat is retained for 5 s or more in the temperature range of 100°C to 450°C, and then it is cooled.

11. The manufacturing method of the hot-dip galvanized steel sheet according to claim 9, wherein, During the cooling of the coated steel sheet, cooling is stopped at a cooling stop point below 300°C, and then it is reheated to a temperature range above (cooling stop temperature + 50°C) and below 450°C, and heat is retained for 5 s or more in this temperature range, and then it is cooled.

12. The manufacturing method of the galvanized steel sheet according to any one of claims 9 to 11, wherein, The annealing is carried out in an atmosphere with a dew point of -25°C or higher.

13. The manufacturing method of the galvanized steel sheet according to any one of claims 9 to 12, wherein, Before the annealing process, there is a metal plating process of forming a metal coating on one or both sides of the cold-rolled steel sheet.

14. The manufacturing method of the hot-dip galvanized steel sheet according to any one of claims 9 to 13, wherein, Performing an alloying treatment on the steel sheet after the hot-dip galvanizing treatment.

15. A method for manufacturing a component, which has a process of manufacturing a component by performing at least one of forming processing or joining processing on the hot-dip galvanized steel sheet described in any one of claims 1 to 8.

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