Hot-dip galvanized steel sheet, member formed using hot-dip galvanized steel sheet, automobile frame structural component or automobile reinforcement component comprising member, and method for producing hot-dip galvanized steel sheet and member

Abstract

Provided is a hot-dip galvanized steel sheet that has a TS value of at least 980 MPa, high ductility, high stretch-flangeability and bendability, and high local ductility. This hot-dip galvanized steel sheet comprises a base steel sheet and a hot-dip galvanized layer on the surface of the base steel sheet. The hot-dip galvanized steel sheet has prescribed components and the steel structure in which, at a 1 / 4 sheet thickness position of the base steel sheet, the area ratio of martensite is at least 30%, the area ratio of ferrite is not more than 70%, the area ratio of residual austenite is not more than 10%, and the area ratio of martensite is at least 5% in a region at a depth of up to 10 μm from the surface of the base steel sheet, and the area ratio of martensite at a 1 / 4 sheet thickness position of the base steel sheet is not more than 80%. The low-temperature region diffusible hydrogen quantity in the base steel sheet, which is the hydrogen quantity released when the base steel sheet is heated to 50°C, is 0.015 ppm by mass or less.

Description

Hot-dip galvanized steel sheet, member made using hot-dip galvanized steel sheet, automobile frame structural part or automobile reinforcing part made of member, and method for manufacturing hot-dip galvanized steel sheet and member

[0001] The present invention relates to a hot-dip galvanized steel sheet, a member made using the hot-dip galvanized steel sheet, an automobile frame structural part or automobile reinforcing part made of the member, and a method for manufacturing the hot-dip galvanized steel sheet and the member.

[0002] CO2 emissions from lighter vehicles ２ With the aim of reducing emissions and improving crashworthiness, efforts are being made to increase the strength of steel sheets for automobiles. Furthermore, against the backdrop of the successive introduction of new legal regulations, there has been an increase in the number of cases in which high-strength steel sheets are used in the main structural parts and reinforcing parts that form the framework of an automobile cabin (hereinafter also referred to as "automotive framework structural parts") in order to increase the strength of the automobile body, and in particular there has been an increase in the number of cases in which high-strength steel sheets with a tensile strength (hereinafter also referred to as "TS") of 980 MPa or more are used.

[0003] High-strength steel sheets used for automotive structural parts and the like are required to have high stretch flangeability when formed into a desired shape. Furthermore, among automotive structural parts, crash boxes, for example, have bending parts. Therefore, from the viewpoint of formability, it is preferable to apply steel sheets having high bendability to such parts.

[0004] As a technique relating to such high-strength steel sheets, for example, claim 1 of Patent Document 1 describes a high-strength 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, the high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more, the base steel sheet containing, in mass %, C: 0.050% or more and 0.200% or less, Si: 0.10% or more and 0.90% or less, Mn: 2.00% or more and 3.50% or less, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, Ca: 0.0200% or less, and Cr: 0.300% or less, and the relationship [%Mn] / [%Si] is 2.9 or more and 11.7 or less. "A high-strength hot-dip galvanized steel sheet having a component composition that satisfies the above requirements, with the balance consisting of Fe and unavoidable impurities, and having a steel structure in which one or two types selected from the group consisting of bainite and ferrite have an area ratio of 5% to 85% in total, an area ratio of tempered martensite is 65% or less, an area ratio of quenched martensite is 5% to 40% and a volume ratio of retained austenite is 5.0% or less, a ratio of the amount of concentrated Si to the amount of concentrated Mn in the surface layer of the base steel sheet is 0.7 to 1.3 and the amount of diffusible hydrogen in the base steel sheet is 0.80 ppm by mass or less, where [%Mn] and [%Si] respectively represent the contents (% by mass) of Mn and Si in the steel."

[0005] International Publication No. 2020 / 170542

[0006] However, the hot-dip galvanized steel sheet described in Patent Document 1 does not take into consideration ultimate deformability and local ductility. Therefore, from the viewpoint of increasing the application ratio of high-strength steel sheets having a TS of 980 MPa or more to automotive frame structural parts and the like, particularly hot-dip galvanized steel sheets, there is currently a demand for the development of hot-dip galvanized steel sheets having a TS of 980 MPa or more that have high ultimate deformability, high ductility, high stretch-flange formability and bendability, and also high local ductility.

[0007] The present invention has been developed in view of the above-mentioned current situation, and aims to provide a hot-dip galvanized steel sheet having a TS of 980 MPa or more, which has high ultimate deformability, high ductility, high stretch flangeability and bendability, and high local ductility. Another object of the present invention is to provide a method for manufacturing the above hot-dip galvanized steel sheet. Another object of the present invention is to provide a member made using the above hot-dip galvanized steel sheet.

[0008] Here, "high ultimate deformability" means that the ultimate deformability measured by the evaluation method described below is 0.10 or more. Here, "high ductility" means that the product (TS × El) of TS and total elongation (hereinafter also referred to as "El") measured in accordance with JIS Z 2241 is 10,000 MPa·% or more. "High stretch flangeability" means that the hole expansion ratio (hereinafter also referred to as "λ") measured in accordance with JIS Z 2256 is 20% or more. "High bendability" means that in a bending test performed in accordance with JIS Z 2248 (for details, see the Examples described below), none of the five samples evaluated crack, or at least one of the five samples exhibits microcracks of less than 200 μm. Here, "high local ductility" means that the product (TS x L.El) of TS and local elongation (hereinafter also referred to as "L.El") measured in accordance with JIS Z 2241 is 4500 MPa·% or more.

[0009] The present inventors conducted extensive research to achieve the above-mentioned objectives, and as a result, have made the following discoveries. (1) The base steel sheet has a predetermined chemical composition and a steel structure containing martensite (quenched martensite, tempered martensite, and bainite). This results in a TS of 980 MPa or more. (2) High ductility is obtained by making the steel structure of the base steel sheet contain at least one of ferrite and retained austenite. (3) High ultimate deformability and high bendability are obtained by making the area fraction of martensite in the region from the surface to a depth of 10 μm of the base steel sheet 5% or more and 80% or less of the area fraction of martensite at a quarter-thickness position of the base steel sheet. (4) High local ductility, stretch flangeability, and bendability are obtained by making the amount of diffusible hydrogen in the low-temperature region of the base steel sheet 0.015 ppm by mass or less.

[0010] [1] A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer on a surface of the base steel sheet, wherein the base steel sheet has a component composition containing, in mass %, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, 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, with the balance being Fe and unavoidable impurities, and at a 1 / 4 position of the sheet thickness of the base steel sheet, an area ratio of martensite is 30% or more, an area ratio of ferrite is 70% or less, and an area ratio of retained austenite is 10% or less, and at least one of ferrite and retained austenite is included, a steel structure in which, in a region from the surface to a depth of 10 μm of the base steel sheet, an area ratio of martensite is 5% or more and is 80% or less of an area ratio of martensite at a position of 1 / 4 of the sheet thickness of the base steel sheet; and an amount of low-temperature diffusible hydrogen in the base steel sheet, which is the amount of hydrogen released when the base steel sheet is heated to 50°C, is 0.015 mass ppm or less. [2] The composition further includes, in mass%, 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 [3] The hot-dip galvanized steel sheet according to [1], wherein the hot-dip galvanized layer is a galvannealed layer.[4] The base steel plate has a surface soft layer which is a region within 200 μm from the surface of the base steel plate in the thickness direction, in which the Vickers hardness is 85% or less of the Vickers hardness at a position 1 / 4 of the thickness of the base steel plate, and when the nanohardness is measured at 300 or more points in a 50 μm × 50 μm region of the plate surface at each of a position 1 / 4 of the thickness direction depth of the surface soft layer and a position 1 / 2 of the thickness direction depth from the surface of the base steel plate, the ratio of the number of measurements where the nanohardness of the plate surface at a position 1 / 4 of the thickness direction depth of the surface soft layer from the surface of the base steel plate is 7.0 GPa or more to the total number of measurements is 0.10 or less, the standard deviation σ of the nanohardness of the plate surface at a position 1 / 4 of the thickness direction depth of the surface soft layer from the surface of the base steel plate is 1.8 GPa or less,

[0014] The hot-dip galvanized steel sheet of any one of [1] to [3], wherein the standard deviation σ of nano-hardness of the sheet surface at a position halfway from the surface of the base steel sheet to a depth in the sheet thickness direction of the soft surface layer is 2.2 GPa or less. [5] The hot-dip galvanized steel sheet of any one of [1] to [4], wherein a metal plating layer is formed between the base steel sheet and the hot-dip galvanized layer on one or both sides of the hot-dip galvanized steel sheet. [6] A member made using the hot-dip galvanized steel sheet of any one of [1] to [5]. [7] An automobile frame structural part or automobile reinforcing part made of the member of [6]. [8] A steel slab having the chemical composition of [1] or [2] is hot-rolled under the condition of a coiling temperature of 400°C or more and 700°C or less to form a hot-rolled steel sheet, and then the hot-rolled steel sheet is pickled, and then the hot-rolled steel sheet is subjected to a cumulative rolling reduction of 30% or more and a unit tension of 10 kgf / mm between a final pass and a pass immediately before the final pass. ２ The cold-rolled steel sheet is then subjected to cold rolling under the above conditions to obtain a cold-rolled steel sheet, and then the cold-rolled steel sheet is subjected to annealing at a temperature T of 750°C to 950°C in which the oxygen concentration is set to 0.5% by volume or more and 5.0% by volume or less in a temperature range of 250°C to 700°C inclusive. １ Heating to over 700°C T １[9] A method for producing a hot-dip galvanized steel sheet, comprising: annealing the annealed cold-rolled steel sheet under conditions of a dew point of -30°C or higher in the following temperature range, and retaining the steel sheet in the temperature range for a residence time of 10 seconds to 500 seconds; subsequently, subjecting the annealed cold-rolled steel sheet to a hot-dip galvanizing treatment to obtain a plated steel sheet; and subsequently, cooling the plated steel sheet to obtain a hot-dip galvanized steel sheet. [9] A method for producing a high-strength steel sheet according to [8], wherein, during cooling of the plated steel sheet, the steel sheet is maintained at a temperature range of 100°C to 450°C for 5 seconds or more, and then cooled.

[10] A method for producing a high-strength steel sheet according to [8], wherein, during cooling of the plated steel sheet, cooling is stopped at 300°C or lower, and the plated steel sheet is then reheated to a temperature range of (cooling stop temperature + 50°C) to 450°C, then maintained at that temperature for 5 seconds or more, and then cooled.

[11] A method for producing a hot-dip galvanized steel sheet according to any of [8] to

[10] , wherein, after the hot-dip galvanizing treatment, the plated steel sheet is subjected to an alloying treatment.

[12] A method for producing a hot-dip galvanized steel sheet according to any one of [8] to

[11] , comprising a metal plating step of forming a metal plating layer on one or both sides of the cold-rolled steel sheet before the annealing step.

[13] A method for producing a member, comprising a step of forming or joining the hot-dip galvanized steel sheet according to any one of [1] to [5], to produce a member.

[0011] According to the present invention, it is possible to provide a hot-dip galvanized steel sheet and member having high ultimate deformability, high ductility, high stretch flangeability and bendability, and high local ductility of 980 MPa or more, together with a manufacturing method thereof.

[0012] 1(a) and 1(b) are schematic diagrams illustrating the preparation of a sample for a U-bend + close bending test in the examples. FIG. 1(a) relates to U-bend processing (primary bending), and FIG. 1(b) relates to bending (secondary bending). FIG. 2(a) and 2(b) relate to V-bend processing (primary bending), and FIG. 2(b) relates to orthogonal VDA bending (secondary bending). FIG. 3(a) and 3(b) are schematic diagrams illustrating an axial crush test sample and test in the examples. FIG. 3(a) and 3(b) are front views of a test member, and FIG. 3(c) is a schematic diagram illustrating an axial crush test.

[0013] The present invention will be described based on the following embodiments, but the present invention is not limited to the following embodiments.

[0014] [1] Hot-dip galvanized steel sheet [1-1] Base steel sheet The hot-dip galvanized steel sheet of the present invention comprises a base steel sheet and a hot-dip galvanized layer on the surface of the base steel sheet. First, the base steel sheet will be described.

[0015] [1-1-1] Elemental Composition The elemental composition of the base steel sheet will be described. In the following description, "%" representing the content of the elemental elements in the steel sheet means "mass %" unless otherwise specified. Furthermore, in this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits.

[0016] [C: 0.030% or More and 0.500% or Less] C is one of the important basic components of steel, and in the present invention, it is an important element that affects the area fraction of martensite and ferrite. If the C content is less than 0.030%, the martensite fraction decreases, making it difficult to achieve the desired tensile strength. On the other hand, if the C content exceeds 0.500%, the area fraction of retained austenite increases, which does not promote hydrogen desorption from the plated steel sheet and increases the amount of low-temperature diffusible hydrogen in the base steel sheet, making it difficult to achieve the desired local ductility. Furthermore, retained austenite transforms to martensite during punching, which increases the generation of voids during hole expansion, reducing λ and potentially degrading bendability. Therefore, the C content is set to 0.030% or more and 0.500% or less. The C content is preferably 0.050% or more, more preferably 0.070% or more, and is preferably 0.400% or less, more preferably 0.300% or less.

[0017] [Si: 0.01% or more and 2.50% or less] Si is one of the important basic components of steel, and in the present invention, it is an important element that affects the area ratios of martensite and ferrite. If the Si content is less than 0.01%, the area ratio of ferrite in the surface layer decreases, making it impossible to control the area ratio of martensite in the region from the surface to a depth of 10 μm within the desired range. As a result, it becomes difficult to obtain high ultimate deformability, high local ductility, stretch flangeability, and bendability. On the other hand, if the Si content exceeds 2.50%, the formation of carbides during continuous annealing is suppressed and the formation of retained austenite, which has high hydrogen solid solubility, is promoted. This prevents hydrogen desorption from the plated steel sheet, increases the amount of diffusible hydrogen in the low-temperature region of the base steel sheet, and makes it difficult to achieve the desired local ductility. Furthermore, retained austenite transforms to martensite during punching, increasing the formation of voids during hole expansion, thereby reducing λ. Bendability also deteriorates. Therefore, the Si content is set to 0.01% or more and 2.50% or less. The Si content is preferably 0.05% or more, more preferably 0.10% or more, and is preferably 2.00% or less, more preferably 1.80% or less.

[0018] [Mn: 0.10% or more and 5.00% or less] Mn is one of the important basic components of steel, and in the present invention, it is an important element that affects the area ratios of martensite and ferrite. If the Mn content is less than 0.10%, the area ratio of martensite decreases, making it difficult to achieve the desired TS. On the other hand, if the Mn content exceeds 5.00%, the structure becomes mainly quenched martensite, and the amount of low-temperature diffusible hydrogen in the base steel sheet increases, making it difficult to achieve the desired local ductility, and also reducing λ and bendability. Therefore, the Mn content is set to 0.10% or more and 5.00% or less. The Mn content is preferably 0.80% or more, more preferably 1.00% or more, and preferably 4.50% or less, more preferably 4.00% or less.

[0019] [P: 0.100% or less] P is an element that segregates at prior austenite grain boundaries, embrittling the grain boundaries and reducing the ultimate deformability of the steel sheet, and may also reduce local ductility and λ. Therefore, the P content is set to 0.100% or less, preferably 0.070% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more.

[0020] [S: 0.0200% or less] S exists as sulfide and is an element that reduces the ultimate deformability of the steel sheet, which can reduce local ductility and λ. Therefore, the S content is 0.0200% or less, preferably 0.0050% or less. There is no particular lower limit for the S content, but due to constraints on production technology, it is preferably 0.0001% or more.

[0021] [Al: 1.000% or less] Al is A ３ Al is an element that raises the transformation point, and the amount of ferrite in the steel structure increases, which can make it difficult to achieve the desired TS. Therefore, the Al content is set to 1.000% or less, and preferably 0.100% or less. There is no particular lower limit for the Al content, but since Al suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite, it is preferably 0.001% or more.

[0022] [N: 0.0100% or less] N exists as a nitride and is an element that reduces the ultimate deformability of the steel sheet, which can reduce local ductility and λ. Therefore, the N content is 0.0100% or less, preferably 0.0050% or less. There is no particular lower limit for the N content, but due to constraints on production technology, the N content is preferably 0.0001% or more.

[0023] [O: 0.0100% or less] O exists as an oxide and is an element that reduces the ultimate deformability of the steel sheet, which can reduce local ductility and λ. Therefore, the O content is set to 0.0100% or less, and preferably 0.0050% or less. There is no particular lower limit for the O content, but due to constraints on production technology, the O content is preferably 0.0001% or more.

[0024] [Optional Components] In addition to the above-mentioned chemical composition, the base steel sheet of the present invention further contains, in mass%, 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: 0.020% or less, Te: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less These elements may be contained alone or in combination of two or more.

[0025] When Ti, Nb, or V is contained, a large amount of coarse precipitates or inclusions are generated, which reduces the ultimate deformability of the steel sheet and reduces the local ductility and λ. To avoid this, the content of Ti, Nb, or V is preferably 0.200% or less, and more preferably 0.100% or less. There are no particular restrictions on the lower limit of the content of Ti, Nb, or V, but since these elements form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet, the content of Ti, Nb, or V is preferably 0.001% or more.

[0026] When Ta or W is contained, large amounts of coarse precipitates or inclusions are generated, which reduces the ultimate deformability of the steel sheet and reduces the local ductility and λ. To avoid this, the content of Ta or W is preferably 0.10% or less, more preferably 0.08% or less. There are no particular restrictions on the lower limit of the content of Ta or W, but since they form fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing, thereby increasing the strength of the steel sheet, the content of Ta or W is more preferably 0.01% or more.

[0027] When B is contained, cracks are generated inside the steel sheet during casting or hot rolling, which reduces the ultimate deformability of the steel sheet, and ultimately reduces the local ductility and λ. To avoid this, the B content is preferably 0.0100% or less, and more preferably 0.0080% or less. There is no particular lower limit for the B content, but since B is an element that segregates to austenite grain boundaries during annealing and improves hardenability, the B content is preferably 0.0003% or more.

[0028] When Cr, Mo, or Ni is contained, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and ultimately reduces the local ductility and λ. To avoid this, the amount of Cr, Mo, or Ni is preferably 1.00% or less, and more preferably 0.80% or less. Although there are no particular restrictions on the lower limit of the amount of Cr, Mo, or Ni, the amount of Cr, Mo, or Ni is preferably 0.01% or more because these elements improve hardenability.

[0029] When Co is contained, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and ultimately reduces the local ductility and λ. To avoid this, the Co content is preferably 0.010% or less, and more preferably 0.008% or less. Although there is no particular lower limit for the Co content, since Co is an element that improves hardenability, the Co content is preferably 0.001% or more.

[0030] When Cu is contained, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and ultimately reduces the local ductility and λ. To avoid this, the Cu content is preferably 1.00% or less, and more preferably 0.80% or less. Although the lower limit of the Cu content is not particularly limited, since Cu is an element that improves hardenability, the Cu content is preferably 0.01% or more.

[0031] When Sn is contained, cracks are generated inside the steel sheet during casting or hot rolling, which reduces the ultimate deformability of the steel sheet, and ultimately reduces the local ductility and λ. To avoid this, the Sn content is preferably 0.200% or less, and more preferably 0.100% or less. Although there is no particular restriction on the lower limit of the Sn content, since Sn is an element that improves hardenability, the Sn content is preferably 0.001% or more.

[0032] When Sb is contained, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and ultimately reduces the local ductility and λ. To avoid this, the Sb content is preferably 0.200% or less, and more preferably 0.100% or less. There is no particular lower limit for the Sb content, but since Sb is an element that controls the area ratio of martensite in a region from the surface of the base steel sheet to a depth of 10 μm, and enables strength adjustment, the Sb content is preferably 0.001% or more.

[0033] When Ca, Mg, or REM is contained, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and ultimately reduces the local ductility and λ. To avoid this, the content of Ca, Mg, or REM is preferably 0.0100% or less, and more preferably 0.0050% or less. There are no particular lower limits for the content of Ca, Mg, or REM, but since these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the content of Ca, Mg, or REM is more preferably 0.0005% or more.

[0034] When Zr or Te is contained, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and ultimately reduces the local ductility and λ. Therefore, the content of Zr or Te is preferably 0.100% or less, and more preferably 0.080% or less. Although there are no particular restrictions on the lower limit of the content of Zr or Te, since these elements spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, the content of Zr or Te is preferably 0.001% or more.

[0035] When Hf is contained, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and ultimately reduces the local ductility and λ. To avoid this, the Hf content is preferably 0.10% or less, and more preferably 0.08% or less. There is no particular lower limit for the Hf content, but since Hf is an element that spheroidizes the shapes of nitrides and sulfides and improves the ultimate deformability of the steel sheet, the Hf content is preferably 0.01% or more.

[0036] When Bi is contained, the amount of coarse precipitates and inclusions increases, which reduces the ultimate deformability of the steel sheet, and ultimately reduces the local ductility and λ. To avoid this, the Bi content is preferably 0.200% or less, and more preferably 0.100% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is preferably 0.001% or more.

[0037] The base steel sheet of the present invention has a composition containing essential elements and, in some cases, optional elements, with the balance being Fe and unavoidable impurities. Examples of the unavoidable impurities include Zn, Pb, As, Ge, Sr, and Cs. The unavoidable impurities are permitted to be contained in a total amount of 0.100% or less.

[0038] [1-1-2] Steel Structure The steel structure of the base steel plate in the present invention will be described.

[0039] [Area Fraction of Martensite at 1 / 4 Thickness of Base Steel Plate: 30% or More] If the area fraction of martensite at the 1 / 4 thickness of the base steel plate is less than 30%, it becomes difficult to achieve the desired TS. Therefore, the area fraction of martensite is set to 30% or more. The area fraction of martensite at the 1 / 4 thickness of the base steel plate is preferably 35% or more, more preferably 40% or more, and even more preferably 45% or more. There is no particular upper limit to the area fraction of martensite at the 1 / 4 thickness of the base steel plate, but from the viewpoint of obtaining high ductility, the area fraction of martensite at the 1 / 4 thickness of the base steel plate is preferably 95% or less, more preferably 90% or less, and even more preferably 85% or less. The martensite referred to here includes tempered martensite and bainite in addition to quenched martensite (fresh martensite).

[0040] [Area fraction of ferrite at the 1 / 4 thickness position of the base steel plate: 70% or less] If the area fraction of ferrite at the 1 / 4 thickness position of the base steel plate exceeds 70%, it becomes difficult to achieve the desired TS. Therefore, the area fraction of ferrite at the 1 / 4 thickness position of the base steel plate is set to 70% or less. The area fraction of ferrite at the 1 / 4 thickness position of the base steel plate is preferably 65% ​​or less, more preferably 60% or less, and even more preferably 55% or less. Note that there is no particular restriction on the lower limit of the area fraction of ferrite at the 1 / 4 thickness position of the base steel plate, but from the viewpoint of obtaining high ductility, the area fraction of ferrite at the 1 / 4 thickness position of the base steel plate is preferably 2% or more, more preferably 5% or more, and even more preferably 10% or more. The ferrite referred to here includes polygonal ferrite as well as bainitic ferrite.

[0041] Here, the methods for measuring the area ratio of martensite (quenched martensite, tempered martensite, and bainite) at the 1 / 4 position in the thickness direction of the base steel plate and the area ratio of ferrite (polygonal ferrite and bainitic ferrite) at the 1 / 4 position in the thickness direction of the base steel plate are as follows.

[0042] A sample is cut out from the steel plate so that the plate thickness cross section (L cross section) parallel to the rolling direction serves as the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste, then finish-polished using alumina, and then etched with 3% by volume of nital to reveal the structure. Next, the sample is observed at 1 / 4 of the plate thickness of the steel plate, and under an acceleration voltage of 10 kV, it is observed at 3000x magnification using a scanning electron microscope (SEM), and three SEM images (1 field of view 40 μm × 30 μm) are obtained. From the obtained SEM image, the area ratio of each structure (ferrite (polygonal ferrite and bainitic ferrite), martensite (quenched martensite, tempered martensite and bainite)) is calculated using Adobe Photoshop (manufactured by Adobe Systems). Specifically, the area ratio of each tissue is calculated by dividing the area of ​​each tissue by the measured area. The area ratio of each tissue is calculated for three fields of view, and the average value is used as the area ratio of each tissue.

[0043] In the SEM image, ferrite (polygonal ferrite and bainitic ferrite) is a flat structure with recesses and no carbides, tempered martensite and bainite are a structure with recesses and fine carbides, and quenched martensite is a structure with protrusions and fine irregularities inside the structure, and they are all distinguishable from one another. Note that tempered martensite and bainite do not need to be distinguishable from one another because the total area ratio is calculated as the area ratio of martensite.

[0044] [Area Fraction of Retained Austenite at 1 / 4 Thickness of Base Steel Sheet: 10% or Less] Because retained austenite is a phase with high hydrogen solid solubility, if the area fraction of retained austenite exceeds 10%, hydrogen desorption from the plated steel sheet is not promoted, the amount of diffusible hydrogen in the low-temperature region of the base steel sheet increases, and it becomes difficult to achieve the desired local ductility. Furthermore, retained austenite transforms to martensite during punching, which increases the generation of voids during hole expansion, reduces λ, and may also deteriorate bendability. The area fraction of retained austenite at the 1 / 4 thickness of the base steel sheet is preferably as small as possible, preferably 7% or less, more preferably 5% or less. The lower limit of the area fraction of retained austenite is not particularly limited; even if the area fraction is 0%, the desired properties can be obtained as long as the steel structure contains ferrite.

[0045] Here, the method for measuring the area fraction of retained austenite at the 1 / 4 position in the plate thickness direction of the base steel plate is as follows. The steel plate is ground so that the observation surface is the 1 / 4 position in the plate thickness direction from the surface of the steel plate (a position corresponding to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate), and then further polished by 0.1 mm by chemical polishing to obtain a sample. For the measurement surface of the sample, an X-ray diffractometer is used with a Co Kα radiation source to 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. The intensity ratio of the integrated reflection intensity from each plane of fcc iron (austenite) to the integrated reflection intensity from each plane of bcc iron is calculated. The average of the nine intensity ratios is taken as the volume fraction of austenite from retained austenite. This volume fraction of austenite is then considered to be three-dimensionally uniform and is taken as the area fraction of retained austenite at the 1 / 4 position in the plate thickness direction of the base steel plate.

[0046] The steel structure at the 1 / 4 position of the plate thickness of the base steel plate contains at least one of ferrite and retained austenite, and the total area ratio of ferrite and retained austenite is preferably 2% or more, more preferably 5% or more, and further preferably 10% or more.

[0047] [Remaining structure] The steel structure at the 1 / 4 plate thickness position of the base steel plate may have a structure (remaining structure) other than martensite, ferrite, and retained austenite. Examples of the remaining structure include structures other than martensite, ferrite, and retained austenite that are known as steel plate structures, such as pearlite, cementite, and carbides such as metastable carbides (epsilon (ε) carbide, eta (η) carbide, chi (χ) carbide, etc.). The remaining structure can be identified, for example, by observation using a SEM.

[0048] The area ratio of the remaining structure at the 1 / 4 position of the plate thickness of the steel plate is preferably 5% or less, and may be 0%. The area ratio of the remaining structure is calculated by the following formula: [Area ratio of remaining structure (%)] = 100 - [Area ratio of martensite (%)] - [Area ratio of ferrite (%)] - [Area ratio of retained austenite (%)]

[0049] [Area Fraction of Martensite in the Region from the Surface to a Depth of 10 μm of the Base Steel Plate: 5% or More and 80% or Less of the Area Fraction of Martensite at a Quarter Thickness of the Base Steel Plate] In the present invention, the area fraction of martensite in the surface layer portion of the base steel plate is an extremely important component. By reducing the area fraction of martensite in the surface layer portion of the base steel plate, in other words, by increasing the area fraction of phases with low hydrogen solid solubility, such as ferrite, bainitic ferrite, and bainite, in the surface layer portion of the base steel plate, desired bendability can be achieved. Furthermore, increasing the area fraction of phases with low hydrogen solid solubility in the surface layer portion of the base steel plate also contributes to reducing the amount of diffusible hydrogen in the low-temperature region of the base steel plate, as described below. To achieve this effect, the area fraction of martensite in the region from the surface to a depth of 10 μm of the base steel plate is set to 80% or less of the area fraction of martensite at a Quarter Thickness of the base steel plate. The area ratio of martensite in the region from the surface to a depth of 10 μm of the base steel sheet is preferably 75% or less, more preferably 70% or less, of the area ratio of martensite at a quarter-thickness position of the base steel sheet. From the viewpoint of realizing a desired TS, the area ratio of martensite in the region from the surface to a depth of 10 μm of the base steel sheet is 5% or more, preferably 10% or more. The martensite referred to here includes tempered martensite in addition to quenched martensite (fresh martensite).

[0050] Here, the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm is measured as follows. A sample is cut out so that the plate thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet serves as the observation surface. Next, the observation surface of the sample is polished. Next, the observation surface of the sample is etched with 3 volume % nital to reveal the structure. Next, the region from the surface of the base steel sheet to a depth of 10 μm is set as the observation position, and three fields of view (40 μm × 10 μm, 10 μm corresponds to the depth) are observed using an SEM at a magnification of 3000 times. From the obtained structure image, the area of ​​martensite for the three fields of view is calculated using Adobe Photoshop from Adobe Systems. Next, the area of ​​martensite calculated for each field of view is divided by the area of ​​each field of view, and the arithmetic average of these values ​​is taken as the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm. In the above SEM image, martensite exhibits a white structure.

[0051] The area ratio of structures other than martensite in the region from the surface of the base steel plate to a depth of 10 μm is preferably 20% or more, more preferably 25% or more, and even more preferably 30% or more, and is preferably 95% or less, more preferably 85% or less, and even more preferably 70% or less.

[0052] The structure other than martensite in the region from the surface of the base steel sheet to a depth of 10 μm is basically composed of ferrite (polygonal ferrite and bainitic ferrite). The area ratio of ferrite in the region from the surface of the base steel sheet to a depth of 10 μm is preferably 5% or more, more preferably 15% or more, and even more preferably 30% or more, and is preferably 80% or less, more preferably 75% or less, and preferably 70% or less.

[0053] However, retained austenite may be contained as long as its area fraction is 5% or less, or it may be 0%. Furthermore, other structures known as steel plate structures, such as bainite, pearlite, cementite, and metastable carbides (epsilon (ε) carbide, eta (η) carbide, chi (χ) carbide, etc.), may be contained as long as their area fraction is 5% or less. The structure can be identified, for example, by observation using a SEM (Scanning Electron Microscope). The area fraction of structures other than martensite is calculated using the following formula: [Area Fraction (%)] = 100 - [Area Fraction (%) of Martensite]

[0054] [1-1-3] Amount of diffusible hydrogen in low temperature region of base steel sheet The amount of diffusible hydrogen in low temperature region of base steel sheet is an extremely important constituent element of the invention. That is, the present inventors have conducted extensive research to obtain a hot-dip galvanized steel sheet having high ductility, high stretch flangeability and bendability, and high local ductility, and having a TS of 980 MPa or more, and have found that the amount of diffusible hydrogen in low temperature region of the base steel sheet, i.e., the amount of hydrogen released from the base steel sheet when the base steel sheet is heated from room temperature to 50°C, has a significant effect on the above properties, particularly local ductility. According to the findings of the present inventors, local ductility is largely dependent on the amount of hydrogen released in the low temperature range, specifically, the temperature range from room temperature to 50°C (low temperature diffusible hydrogen amount), rather than the amount of hydrogen released from the base steel sheet in the high temperature range when the base steel sheet is heated. If the amount of hydrogen released exceeds 0.015 ppm by mass, it is difficult to achieve high local ductility while maintaining high ductility, high stretch flangeability, and high bendability. Therefore, the low temperature diffusible hydrogen amount of the base steel sheet is set to 0.015 ppm by mass or less. The lower the low temperature diffusible hydrogen amount of the base steel sheet, the better, and it is preferably 0.010 ppm by mass or less, more preferably 0.006 ppm by mass or less, and may be 0 ppm.

[0055] The method for measuring the amount of diffusible hydrogen in a low-temperature region of a base steel sheet is as follows. A test piece 30 mm long and 5 mm wide is obtained by shearing from the center of a hot-dip galvanized steel sheet sample. After obtaining the test piece, the test piece is immediately immersed in liquid nitrogen. The hot-dip galvanized layer of the test piece is alkali-removed while controlling the temperature of the treatment solution so that the surface temperature of the test piece is 10°C or less. The test piece is then loaded into a thermal desorption spectroscopy apparatus, and after waiting for 5 minutes with Ar gas flowing, heating is initiated. Specifically, the test piece is heated to a temperature of 300°C at a heating rate of 200°C / hr, and then cooled to room temperature. The surface temperature of the test piece at the start of heating is 10°C or less. The amount of hydrogen released from the test piece in the temperature range from the temperature at the start of heating (room temperature) to 50°C (hereinafter also referred to as the cumulative released hydrogen amount) is measured, and the amount of diffusible hydrogen in a low-temperature region of the base steel sheet is calculated using the following formula: [Amount of diffusible hydrogen in the low temperature region of the base steel plate (ppm by mass)] = [Cumulative amount of released hydrogen (g)] ÷ [Mass of test piece (g)] × 10 ６

[0056] For steel sheets obtained by subjecting hot-dip galvanized steel sheets to processing such as punching, stretch flange forming, and bending, and for products (components) manufactured by welding steel sheets after the above processing, the amount of low-temperature diffusible hydrogen in the base steel sheet portion may be measured in the same manner as above.

[0057] [1-1-4] Plate Thickness The plate thickness of the base steel plate is not particularly limited and can be set according to the plate thickness of the final hot-dip galvanized steel plate. The plate thickness can be, for example, 0.3 mm or more and 3.0 mm or less.

[0058] [1-1-5] Soft surface layer The surface layer of the base steel sheet is preferably a soft layer (soft surface layer). The soft surface layer contributes to suppressing the propagation of bending cracks during press forming and vehicle collisions, thereby further improving bending fracture resistance.

[0059] The surface layer refers to a region corresponding to a thickness of 200 μm from the surface of the base steel sheet to 200 μm in the sheet thickness direction. The soft layer refers to a region having a Vickers hardness of 85% or less of the Vickers hardness of a cross section (a plane parallel to the steel sheet surface) at 1 / 4 of the sheet thickness of the base steel sheet. The soft layer encompasses a decarburized layer in the surface layer of the base steel sheet. The surface soft layer refers to a soft layer contained in the surface layer, and the entire surface layer may be a soft layer, or only a part of the surface layer may be a soft layer. The surface soft layer can be a region corresponding to a thickness of 200 μm or less from the surface of the base steel sheet in the sheet thickness direction. For example, if a region having a Vickers hardness of 85% or less of the cross section (plane parallel to the steel plate surface) at 1 / 4 of the plate thickness of the base steel plate is formed at a predetermined depth from the surface of the base steel plate in the plate thickness direction, when the predetermined depth is within 200 μm in the plate thickness direction, the region corresponding to the thickness from the surface to the predetermined depth in the plate thickness direction is the surface soft layer, and when the predetermined depth is more than 200 μm in the plate thickness direction, the region corresponding to the thickness of 200 μm from the surface of the base steel plate to a depth of 200 μm in the plate thickness direction is the surface soft layer. When a surface soft layer is present, the lower limit of the thickness of the surface soft layer is not particularly limited, but is preferably 8 μm or more, and more preferably more than 17 μm. Vickers hardness is measured based on JIS Z 2244-1 (2020) at a load of 10 gf.

[0060] When a soft surface layer is provided, when the nanohardness is measured at 300 or more points in a 50 μm × 50 μm region on the sheet surface at a position ¼ of the depth of the soft surface layer in the sheet thickness direction from the surface of the base steel sheet (a position ¼ of the thickness of the soft surface layer in the depth direction from the surface of the base steel sheet), it is preferable that the proportion of nanohardnesses of 7.0 GPa or more is 0.10 or less. When the proportion of nanohardnesses of 7.0 GPa or more is 0.10 or less, this means that the proportion of hard structures (martensite, etc.), inclusions, etc. is small, and it is possible to further suppress the generation and connection of voids and crack propagation in hard structures (martensite, etc.) and inclusions during press forming and collision, and to easily obtain excellent bendability during press forming and excellent bending fracture properties during collision.

[0061] In the present invention, in order to obtain excellent bendability during press forming and excellent bending fracture properties during collision, it is preferable that the standard deviation σ of the nanohardness of the sheet surface at a position ¼ of the thickness direction depth of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less, and further, the standard deviation σ of the nanohardness of the sheet surface at a position ½ of the thickness direction depth of the soft surface layer from the surface of the base steel sheet is 2.2 GPa or less. When the standard deviation σ of the nanohardness of the sheet surface at a position ¼ of the thickness direction depth of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less, and further, the standard deviation σ of the nanohardness of the sheet surface at a position ½ of the thickness direction depth of the soft surface layer from the surface of the base steel sheet is 2.2 GPa or less, this means that the difference in microstructural hardness in the micro region is small, and it is possible to further suppress the generation and connection of voids and the propagation of cracks during press forming and collision, and therefore excellent bendability and bending fracture properties during collision can be easily obtained.

[0062] Furthermore, the standard deviation σ of the nano-hardness of the sheet surface at a position ¼ of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is more preferably 1.7 GPa or less. The standard deviation σ of the nano-hardness of the sheet surface at a position ½ of the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet is more preferably 2.1 GPa or less.

[0063] Here, the nanohardness of the sheet surface at the 1 / 4 and 1 / 2 positions in the sheet thickness direction refers to the hardness measured by the following method. First, when a plating layer is formed, after peeling off the plating layer, mechanical polishing is performed from the surface of the base steel sheet to the 1 / 4 position in the sheet thickness direction of the soft surface layer, buffing with diamond and alumina, and further colloidal silica polishing. The nanohardness is measured using a Berkovich-shaped diamond indenter under the following conditions: load: 500 μN, measurement area: 50 μm × 50 μm, and dot spacing: 2 μm. Furthermore, mechanical polishing is performed to the 1 / 2 position in the sheet thickness direction of the soft surface layer, buffing with diamond and alumina, and further colloidal silica polishing. The nanohardness is then measured using a Berkovich-shaped diamond indenter under the following conditions: load: 500 μN, measurement area: 50 μm × 50 μm, and dot spacing: 2 μm.

[0064] Here, the thickness of the surface soft layer can be measured by the following method. After smoothing a thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet by wet polishing, measurements were made using a Vickers hardness tester at 1 μm intervals from a position 1 μm from the surface of the base steel sheet in the thickness direction to a position 100 μm in the thickness direction under a load of 10 gf. Thereafter, measurements were made at 20 μm intervals to the center of the thickness. The region where the hardness was reduced to 85% or less compared to the hardness at the 1 / 4 position in the thickness direction was defined as the soft layer (surface soft layer), and the thickness of this region in the thickness direction was taken as the thickness of the soft layer.

[0065] [1-2] Metal plating layer The base steel sheet preferably has a metal plating layer on one or both sides. The metal plating layer contributes to suppressing the occurrence of bending cracks during press forming and vehicle collisions, thereby further improving bending fracture resistance.

[0066] The metal plating layer is formed directly on the surface of the base steel sheet and contains one or more elements 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 in a total amount of more than 50 mass%, excluding zinc plating layers such as hot-dip galvanized layers, alloyed hot-dip galvanized layers, and electrogalvanized layers. The first plating layer is preferably a metal electroplated layer, and the following description will be given taking a metal electroplated layer as an example.

[0067] By forming a metal electroplated layer on the surface of the steel sheet, the outermost metal electroplated layer contributes to suppressing the occurrence of bending cracks during press forming and vehicle collisions, thereby further improving bending fracture resistance.

[0068] The metal species of the metal electroplating layer may be any 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, and Bi, but Fe is more preferred. In the following, an Fe-based electroplating layer will be described as an example.

[0069] The deposition amount of the Fe-based electroplating layer is 0 g / m ２ More than 2.0 g / m ２Although there is no particular upper limit to the amount of the Fe-based electroplated layer deposited on one side, from the viewpoint of cost, the amount of the Fe-based electroplated layer deposited on one side is set to 60 g / m or more. ２ The coating weight of the Fe-based electroplating layer is preferably 50 g / m or less. ２ More preferably, it is 40 g / m or less. ２ More preferably, it is 30 g / m or less. ２ The following applies.

[0070] The deposition weight of the Fe-based electroplated layer is measured as follows: A 10 × 15 mm sample is taken from the Fe-based electroplated steel sheet and embedded in resin to prepare a cross-section embedded sample. Three randomly selected locations on the cross-section are observed using a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and a magnification of 2,000 to 10,000 times depending on the thickness of the Fe-based plating layer. The average thickness of the three fields of view is multiplied by the specific gravity of iron to convert it into the deposition weight of the Fe-based plating layer per side.

[0071] The Fe-based electroplating layer may be made of pure Fe or an alloy such as an Fe—B alloy, an Fe—C alloy, an Fe—P alloy, an Fe—N alloy, an Fe—O alloy, an Fe—Ni alloy, an Fe—Mn alloy, an Fe—Mo alloy, or an Fe—W alloy. The composition of the Fe-based electroplating layer is not particularly limited, but it preferably contains one or more elements selected from the group consisting of 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, with the remainder consisting of Fe and unavoidable impurities. By limiting the total amount of elements other than Fe to 10% by mass or less, a decrease in electrolysis efficiency can be prevented and the Fe-based electroplating layer can be formed at low cost. In the case of an Fe—C alloy, the C content is preferably 0.08% by mass or less.

[0072] The base steel sheet for hot-dip galvanizing may be a base steel sheet alone or a base steel sheet with a metal plating layer, and is preferably a base steel sheet in which a metal plating layer is formed on the surface of a base steel sheet having a soft surface layer.

[0073] [1-3] Hot-dip galvanized layer The hot-dip galvanized layer in a hot-dip galvanized steel sheet will now be described. The hot-dip galvanized layer referred to here also includes an alloyed hot-dip galvanized layer (a plating layer obtained by performing an alloying treatment on hot-dip galvanization). The hot-dip galvanized layer can be formed on both surfaces of the base steel sheet. In this case, the hot-dip galvanized layer may be formed directly on the surface of the base steel sheet, or, if the base steel sheet has a surface soft layer, a metal electroplated layer, or the like, the hot-dip galvanized layer may be formed on these layers.

[0074] The hot-dip galvanized layer typically contains Zn (zinc) as its main component (Zn content of 50.0% by mass or more). The composition is not particularly limited and may be a known composition. The hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0% by mass or less of Fe, and 0.001% by mass to 1.0% by mass of Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of more than 0.0% by mass to 3.5% by mass or less. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0% by mass. The remainder other than the above elements is unavoidable impurities. The galvannealed layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of more than 0% by mass and 3.5% by mass or less. The Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more. The Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less. The remainder other than the above elements is unavoidable impurities.

[0075] The coating weight of the hot-dip galvanized layer per side is not particularly limited, and may be, for example, 20 g / m ２ 80g / m or more２ It can be:

[0076] The coating weight of the zinc plating layer is measured as follows. A treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ivit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous hydrochloric acid solution. Next, a steel sheet sample having a zinc plating layer is immersed in the treatment solution to dissolve the zinc plating layer. The mass loss of the ampoule S before and after dissolution is measured, and this value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the portion that was covered with the plating) to determine the coating weight (g / m ２ ) is calculated.

[0077] The hot-dip galvanized layer preferably has cracks. By intentionally providing cracks in the hot-dip galvanized layer, the amount of diffusible hydrogen in the low-temperature region of the base steel sheet can be further reduced. Here, the presence or absence of cracks in the hot-dip galvanized layer is determined as follows. The surfaces (front and back) of the hot-dip galvanized layer of the hot-dip galvanized steel sheet are observed using an SEM at a magnification of 3000x, with two fields of view (each field: 30 μm × 40 μm) on each side, for a total of four fields of view. If one or more cracks penetrating the hot-dip galvanized coating are present in any of the four fields of view, it is determined that cracks are present. Furthermore, if no cracks penetrating the hot-dip galvanized coating are present in all of the four fields of view, it is determined that no cracks are present.

[0078] [1-4] Others The thickness of the hot-dip galvanized steel sheet of the present invention is not particularly limited, but is usually 0.3 mm or more and 2.8 mm or less.

[0079] The hot-dip galvanized steel sheet of the present invention has a TS of 980 MPa or more. The TS is measured in accordance with JIS Z 2241 as described in the examples below.

[0080] The hot-dip galvanized steel sheet of the present invention can have an ultimate deformability of 0.10 or more. The local ductility and stretch-flange formability of a hot-dip galvanized steel sheet are largely dependent on the ultimate deformability of the annealed sheet after cold rolling, and setting the ultimate deformability of the hot-dip galvanized steel sheet to 0.10 or more is effective in terms of achieving high local ductility while maintaining high ductility, high stretch-flange formability, and bendability. The higher the ultimate deformability, the better, and it is preferably 0.30 or more, more preferably 0.50 or more. There is no upper limit, but it is usually 2.10 or less.

[0081] Here, the ultimate deformability (ε ｌ ) is calculated by the method described in "Mizunuma et al.: Report of the Institute of Physical and Chemical Research, 45-4 (1969), 79", and specifically, the limiting deformation capacity (ε ｌ ) uses the following formula: ｗ = ln(W / W ０ ) ε ｔ = ln(T / T ０ ) ε ｌ = - (ε ｗ +ε ｔ ) In the formula, W ０ : Initial plate width, W: Broken part plate width, T: Initial plate thickness, T ０ : The thickness at the broken portion. The width and thickness at the broken portion were measured according to JIS Z 2241, as described in the examples below.

[0082] [2] Manufacturing Method of Hot-Dip Galvanized Steel Sheet The manufacturing method of the hot-dip galvanized steel sheet of the present invention will be described. The manufacturing method of the hot-dip galvanized steel sheet of the present invention is also a method for manufacturing the hot-dip galvanized steel sheet of the present invention described above.

[0083] The method for producing a hot-dip galvanized steel sheet of the present invention includes: hot rolling a steel slab having the above-described chemical composition under a coiling temperature condition of 400°C or more and 700°C or less to obtain a hot-rolled steel sheet; then, pickling the hot-rolled steel sheet; and then, pickling the hot-rolled steel sheet under a cumulative rolling reduction of 30% or more and a unit tension of 10 kgf / mm between a final pass and a pass immediately before the final pass. ２The cold-rolled steel sheet is then subjected to cold rolling under the above conditions to obtain a cold-rolled steel sheet, and then the cold-rolled steel sheet is subjected to annealing at a temperature T of 750°C to 950°C in which the oxygen concentration is set to 0.5% by volume or more and 5.0% by volume or less in a temperature range of 250°C to 700°C inclusive. １ Heating to over 700°C T １ the dew point in the following temperature range is set to -30°C or higher, and the steel sheet is annealed under the conditions of a residence time of 10 seconds to 500 seconds in the temperature range, the annealed cold-rolled steel sheet is then subjected to a hot-dip galvanizing treatment to obtain a plated steel sheet, and the plated steel sheet is then cooled to obtain a hot-dip galvanized steel sheet. Unless otherwise specified, the above temperatures are based on the surface temperature of the steel slab or steel sheet.

[0084] [2-1] Hot Rolling Process A steel slab (slab) is hot-rolled to produce a hot-rolled steel sheet. The method for producing a steel slab is not particularly limited, and any known method, such as a converter or electric furnace, is suitable. Furthermore, to prevent macrosegregation, a steel slab is preferably produced by a continuous casting method, but it can also be produced by an ingot casting method or a thin slab casting method. After producing a steel slab, in addition to the conventional method of cooling it to room temperature and then reheating it, energy-saving processes such as direct rolling and direct rolling can also be applied without any problems. Direct rolling is a process in which a hot slab is charged into a heating furnace without being cooled to room temperature. Direct rolling is a process in which a short period of heat retention is followed by immediate rolling.

[0085] When heating a steel slab, the slab heating temperature is preferably 1100°C or higher from the viewpoint of dissolving carbides and reducing the rolling load. 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 refers to the temperature of the slab surface.

[0086] Next, the slab is hot rolled. Hot rolling can consist of rough rolling and finish rolling. For example, the slab can be made into a sheet bar by rough rolling. The conditions for rough rolling are not particularly limited and can be known conditions. Next, the sheet bar can be subjected to finish rolling. When the slab heating temperature is set low, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling in order to prevent problems during rolling. The finish rolling temperature is set at a temperature of Ar ３ The transformation point or higher is preferable. ３ If the temperature is equal to or higher than the transformation point, it is possible to avoid an increase in the rolling load and an increase in the reduction ratio in the non-recrystallized state of austenite, and in turn, it is possible to suppress the development of an abnormal structure elongated in the rolling direction, and it is possible to easily avoid a decrease in the workability of the steel sheet obtained after annealing.

[0087] Ar ３ The transformation point is calculated using the following formula: Ar ３ (°C) = 868 - 396 x [%C] + 24.6 x [%Si] - 68.1 x [%Mn] - 36.1 x [%Ni] - 20.7 x [%Cu] - 24.8 x [%Cr]. In the above formula, the [% element symbol] represents the content (mass%) of the corresponding element in the above composition.

[0088] Finish rolling may be performed continuously by joining sheet bars together. Alternatively, the sheet bars may be temporarily wound before finish rolling. Furthermore, in order to reduce the rolling load, part or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the viewpoint of uniforming the shape of the steel sheet and the material properties. The coefficient of friction during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0089] [Coiling Temperature: 400°C or Higher and 700°C or Lower] In the manufacturing method of the present invention, control of the coiling temperature in the hot rolling process is particularly important. In the hot rolling process, after finish rolling, the hot-rolled sheet is coiled and recovered, and then cooled. In this process, by setting the coiling temperature to 400°C or higher, C diffuses into the oxide scale formed during rolling, i.e., decarburization of the surface layer of the hot-rolled steel sheet is promoted, making it possible to control the area fraction of martensite in the region from the surface to a depth of 10 μm of the base steel sheet within a desired range. By reducing the area fraction of martensite in the region from the surface to a depth of 10 μm of the base steel sheet, the amount of low-temperature diffusible hydrogen in the base steel sheet also decreases. As a result, it is possible to obtain high ultimate deformability, high local ductility, stretch flangeability, and bendability. However, if the coiling temperature exceeds 700°C, the area fraction of martensite in the region from the surface to a depth of 10 μm of the base steel sheet decreases, making it difficult to achieve a desired TS. Therefore, the coiling temperature is set to 400° C. or higher and 700° C. or lower. The coiling temperature is preferably 430° C. or higher, more preferably 450° C. or higher, and is preferably 670° C. or lower, more preferably 650° C. or lower.

[0090] The cooling conditions after coiling are not particularly limited, and known conditions can be used. For example, the cooling rate is preferably 0.001°C / s or more and 1°C / s or less, and the cooling stop temperature is preferably 20°C or more and 200°C or less.

[0091] [2-2] Pickling Process After the hot rolling process, the hot-rolled steel sheet is pickled. Pickling can remove oxides from the steel sheet surface, ensuring good chemical conversion treatability and plating quality. Pickling may be performed once or multiple times. There are no particular restrictions on the pickling conditions, and known conditions can be applied.

[0092] [2-3] Heat Treatment Step (Optional) After pickling, the hot-rolled sheet may be subjected to a heat treatment (hot-rolled sheet annealing). The heat treatment conditions are not particularly limited, and the following conditions may be mentioned.

[0093] [Heat treatment temperature: 450°C or higher and 650°C or lower (preferred conditions)] By subjecting a hot-rolled steel sheet to heat treatment, decarburization of the surface layer of the hot-rolled steel sheet is promoted, and it becomes possible to control the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm within a more preferable range. Therefore, the heat treatment temperature is preferably 450°C or higher. On the other hand, if the heat treatment temperature exceeds 650°C, the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm decreases, and TS decreases. Therefore, the heat treatment temperature is preferably 450°C or higher and 650°C or lower. The heat treatment temperature is more preferably 460°C or higher, even more preferably 470°C or higher, and more preferably 600°C or lower, even more preferably 550°C or lower.

[0094] [Dwell Time in a Temperature Range of 400°C or Higher and Less than the Heat Treatment Temperature (hereinafter also referred to as Dwell Time in the Heat Treatment Temperature Range): 10 Minutes or More (Preferred Condition)] By setting the dwell time in a temperature range of 400°C or higher and less than the heat treatment temperature (hereinafter also referred to as "dwell time in the heat treatment temperature range") to 10 minutes or more, decarburization of the surface layer of the hot-rolled steel sheet is promoted, and it becomes possible to control the area ratio of martensite in a region from the surface of the base steel sheet to a depth of 10 µm within a more suitable range. Therefore, the dwell time in the heat treatment temperature range is preferably 10 minutes or more, more preferably 100 minutes or more, and even more preferably 500 minutes or more. There is no particular limitation on the upper limit of the dwell time in the heat treatment temperature range, but from the viewpoint of controlling the area ratio of martensite in a region from the surface of the base steel sheet to a depth of 10 µm within a more suitable range, it is preferably 3000 minutes or less, more preferably 2000 minutes or less.

[0095] [2-4] Cold Rolling Step Next, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. In the manufacturing method of the present invention, it is important to satisfy the following conditions.

[0096] [Cold Rolling Cumulative Reduction Ratio: 30% or More] Increasing the cold rolling cumulative reduction ratio promotes decarburization of the surface layer during annealing, making it possible to control the area ratio of martensite in the region from the surface to a depth of 10 μm of the base steel sheet within a desired range. By reducing the area ratio of martensite in the region from the surface to a depth of 10 μm of the base steel sheet, the amount of low-temperature diffusible hydrogen in the base steel sheet also decreases. As a result, it becomes possible to obtain high ultimate deformability, high local ductility, stretch flangeability, and bendability. Therefore, the cold rolling cumulative reduction ratio is set to 30% or more. The cold rolling cumulative reduction ratio is preferably set to 35% or more, more preferably 40% or more. The upper limit of the cold rolling cumulative reduction ratio is not particularly limited, but due to production technology constraints, it is preferably set to 90% or less, more preferably 85% or less.

[0097] [Unit tension between the final pass of cold rolling and the pass immediately before the final pass: 10 kgf / mm ２ By increasing the tension in the unit between the final pass of cold rolling and the pass immediately before the final pass, strain can be introduced into the surface layer of the steel sheet during cold rolling, which promotes decarburization of the surface layer during annealing and makes it possible to control the area ratio of martensite in the region from the surface to a depth of 10 μm of the base steel sheet within a desired range. By reducing the area ratio of martensite in the region from the surface to a depth of 10 μm of the base steel sheet, the amount of low-temperature diffusible hydrogen in the base steel sheet also decreases. As a result, it becomes possible to obtain high ultimate deformability, high local ductility, stretch flangeability and bendability. From this point of view, the unit tension between the final pass of cold rolling and the pass immediately before the final pass: 10 kgf / mm ２ or more, preferably 15 kgf / mm ２ The upper limit of the unit tension between the final pass of cold rolling and the pass immediately before the final pass is not particularly limited, but is preferably 100 kgf / mm due to constraints on production technology. ２ Less than or equal to 50 kgf / mm ２ The following is the result.

[0098] Other cold rolling conditions are not particularly limited, and known conditions can be used. For example, cold rolling can be performed by tandem multi-stand rolling or reverse rolling. Furthermore, the number of rolling passes and the reduction rate of each pass are not particularly limited, and known conditions can be used.

[0099] [2-5] Metal Plating Step (Optional) The surface of the cold-rolled steel sheet obtained as described above may be subjected to a metal plating treatment to obtain a pre-annealing metal-plated steel sheet having a metal plating layer (pre-annealing metal plating layer) formed on at least one surface. The pre-annealing metal-plated steel sheet is preferably a pre-annealing metal electroplated steel sheet having a pre-annealing metal electroplated layer.

[0100] Although the metal electroplating method is not particularly limited, as described above, it is preferable to form a metal electroplating layer on the base steel sheet, and therefore metal electroplating is preferably performed. For example, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of the two can be used as an Fe-based electroplating bath. The coating weight of the pre-annealed metal electroplating layer can be adjusted by, for example, the current application time. Note that the term "pre-annealed metal electroplated steel sheet" means that the metal electroplating layer has not undergone an annealing process, and does not exclude hot-rolled steel sheets, pickled steel sheets after hot rolling, or cold-rolled steel sheets that have been annealed in advance before metal electroplating.

[0101] Here, the metal species of the electroplating layer may be any 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, and Bi, but Fe is more preferable, so a method for producing Fe-based electroplating will be described below.

[0102] The Fe ion content in the Fe-based electroplating bath before the start of current application is Fe ２＋ The Fe ion content in the Fe-based electroplating bath is preferably 0.5 mol / L or more. ２＋A sufficient Fe deposition weight can be obtained if the Fe ion content in the Fe-based electroplating bath before the start of current application is 0.5 mol / L or more. To obtain a sufficient Fe deposition weight, the Fe ion content in the Fe-based electroplating bath before the start of current application is preferably 2.0 mol / L or less. In addition to Fe ions, the Fe-based electroplating bath may contain at least one element selected from the group consisting of 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 10 mass% or less in the Fe-based electroplated layer before annealing. Metal elements may be contained as metal ions, and nonmetal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. The iron sulfate plating solution may also contain conductivity aids such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0103] Other conditions for the Fe-based electroplating bath are not particularly limited. The temperature of the Fe-based electroplating solution is preferably 30°C or higher, and preferably 85°C or lower, from the viewpoint of maintaining a constant temperature. The pH of the Fe-based electroplating bath is also not particularly limited, but is preferably 1.0 or higher, from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, and is preferably 3.0 or lower, from the viewpoint of the electrical conductivity of the Fe-based electroplating bath. The current density is 10 A / dm or lower, from the viewpoint of productivity. ２ It is preferable that the current flow rate is 150 A / dm or more, and from the viewpoint of facilitating control of the deposition amount of the Fe-based electroplating layer, ２ The sheet threading speed is preferably 5 mpm or more from the viewpoint of productivity, and is preferably 150 mpm or less from the viewpoint of stable control of the amount of adhesion.

[0104] As a treatment prior to the Fe-based electroplating treatment, a degreasing treatment and water washing to clean the surface of the cold-rolled steel sheet can be performed, and further, a pickling treatment and water washing to activate the surface of the cold-rolled steel sheet can be performed. These pretreatments are followed by the Fe-based electroplating treatment. The methods for the degreasing treatment and water washing are not particularly limited, and conventional methods can be used. In the pickling treatment, various acids can be used, such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof. Among these, sulfuric acid, hydrochloric acid, and mixtures thereof are preferred. The concentration of the acid is not particularly limited, but is preferably 1% by mass or more and 20% by mass or less from the viewpoints of the ability to remove oxide films and the prevention of rough surfaces (surface defects) due to excessive pickling. The pickling treatment solution may also contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, etc.

[0105] [2-6] Annealing Step The cold-rolled steel sheet obtained as described above is annealed. At this time, it is important to satisfy the following conditions: the oxygen concentration in the temperature range of 250°C to 700°C is 0.5% by volume to 5.0% by volume, and the annealing temperature T is 750°C to 900°C. １ Heating to over 700°C T １ The dew point in the following temperature range is set to -30°C or higher, and the water is allowed to stay in the temperature range for a residence time of 10 seconds or more and 500 seconds or less.

[0106] [Oxygen Concentration in the Temperature Range of 250°C to 700°C: 0.5% by Volume to 5.0% by Volume] By increasing the oxygen concentration in the heating temperature range, decarburization proceeds via oxygen in the atmosphere, making it possible to control the area fraction of martensite in the region from the surface to a depth of 10 μm within a desired range. By reducing the area fraction of martensite in the region from the surface to a depth of 10 μm, the amount of low-temperature diffusible hydrogen in the base steel sheet also decreases. As a result, it becomes possible to obtain high ultimate deformability, high local ductility, stretch flangeability, and bendability. To achieve these effects, the oxygen concentration in the heating temperature range is set to 0.5% by volume or more. On the other hand, if the oxygen concentration in the heating temperature range exceeds 5.0% by volume, the area fraction of martensite in the region from the surface to a depth of 10 μm of the base steel sheet decreases, making it difficult to achieve the desired TS. Therefore, the oxygen concentration in the heating temperature range is set to 0.5% by volume or more and 5.0% by volume or less. The oxygen concentration in the heating temperature range is preferably 1.0 vol% or more, more preferably 1.5 vol% or more, and is preferably 4.5 vol% or less, more preferably 4.0 vol% or less. Here, the temperature in the heating temperature range is based on the surface temperature of the steel sheet. That is, when the surface temperature of the steel sheet is within the heating temperature range, the oxygen concentration may be adjusted to the above range.

[0107] [Annealing temperature T １ : 750°C or more and 950°C or less] If the annealing temperature is less than 750°C, the austenite fraction during annealing cannot be sufficiently ensured, resulting in a decrease in the area fraction of martensite, making it difficult to achieve the desired TS. On the other hand, if the annealing temperature exceeds 950°C, annealing occurs in the austenite single-phase region. If the surface layer structure during annealing is austenite single-phase, decarburization does not progress, making it difficult to control the area fraction of martensite in the region from the surface of the base steel sheet to a depth of 10 μm within the desired range. Also, it becomes difficult to include ferrite and retained austenite. Therefore, the annealing temperature T １The annealing temperature is 750°C or higher and 950°C or lower. The annealing temperature is preferably 770°C or higher, more preferably 780°C or higher. The annealing temperature is preferably 900°C or lower, more preferably 880°C or lower. Here, the annealing temperature is the maximum temperature reached in the annealing step.

[0108] The heat retention time in the annealing temperature range (hereinafter also referred to as annealing time) is not particularly limited, but is preferably 10 seconds or more and 600 seconds or less from the viewpoint of controlling the area ratio of ferrite and martensite in the base steel sheet within a predetermined range. In addition, the temperature during heat retention does not need to be always constant.

[0109] [Over 700℃T １ Residence time in the following temperature range: 10 s or more and 500 s or less] 700 ° C. or more T １ By increasing the residence time in the temperature range below, decarburization progresses via oxygen in the atmosphere, and it becomes possible to control the area ratio of martensite in the region from the surface to a depth of 10 μm within a desired range. By reducing the area ratio of martensite in the region from the surface to a depth of 10 μm, the amount of diffusible hydrogen in the low temperature region of the base steel sheet also decreases. As a result, it becomes possible to obtain high ultimate deformability, high local ductility, stretch flangeability and bendability. In order to obtain these effects, it is necessary to perform a test at a temperature above 700°C. １ The residence time in the temperature range below 700°C is set to 10 seconds or more. １ If the residence time in the temperature range below 700°C exceeds 500 s, the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm decreases, making it difficult to achieve the desired TS. １ The residence time in the temperature range below is 10 seconds or more and 500 seconds or less. １ The residence time in the following temperature range is preferably 15 seconds or more, more preferably 20 seconds or more. １ The residence time in the following temperature range is preferably 400 seconds or less, more preferably 300 seconds or less. Here, the temperature in the heating temperature range is based on the surface temperature of the steel sheet.

[0110] [Over 700℃T １ Dew point in the following temperature range: -30°C or higher] Over 700°C１ By increasing the dew point in the following temperature range, decarburization proceeds via oxygen in the atmosphere, making it possible to control the area fraction of martensite in the region from the surface to a depth of 10 μm of the base steel sheet within a desired range. By reducing the area fraction of martensite in the region from the surface to a depth of 10 μm of the base steel sheet, the amount of diffusible hydrogen in the low-temperature region of the base steel sheet also decreases. As a result, it becomes possible to obtain high ultimate deformability, high local ductility, stretch flangeability, and bendability. To achieve these effects, the dew point in the annealing temperature range is set to −30° C. or higher. While the upper limit of the dew point in the annealing temperature range is not particularly limited, from the viewpoint of ensuring a desired TS, the dew point in the annealing temperature range is preferably 15° C. or lower, more preferably 5° C. or lower. Here, the temperature in the annealing temperature range is based on the surface temperature of the steel sheet. That is, when the surface temperature of the steel sheet is within the annealing temperature range, the dew point may be adjusted to the above range.

[0111] The oxygen concentration in the annealing temperature range is not particularly limited, but is preferably 2 ppm by volume or more and 30 ppm by volume or less from the viewpoint of controlling the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm within a predetermined range.

[0112] After the annealing, the cold-rolled steel sheet is cooled. The conditions for this are not particularly limited, and known conditions can be adopted. For example, T １ The average cooling rate in the temperature range of 500°C or higher is not particularly limited, but is preferably 5°C / s or more and 50°C / s or less from the viewpoint of controlling the area ratio of ferrite and martensite in the base steel plate within a predetermined range.

[0113] [2-7] Hot-dip galvanizing process The cold-rolled steel sheet is then subjected to hot-dip galvanizing. After the hot-dip galvanizing process, an alloying process may be performed.

[0114] Hot dip galvanizing treatment is T １ Alternatively, the cold-rolled steel sheet may be cooled to below 400°C, and then the steel sheet temperature may be increased again to 400°C or higher before the treatment.

[0115] The annealing, cooling, and plating treatments may be performed continuously in one line (CGL (Continuous Galvanizing Line)). For example, after annealing, the cold-rolled steel sheet is cooled to a temperature range of about 500°C. Next, the cold-rolled steel sheet is passed through the steel strip outlet side of the cooling zone, and further cooled while being moved into the hot-dip galvanizing bath via a snout whose tip is immersed 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 is preferably 300 seconds or less from the viewpoint of controlling the area ratios of ferrite and martensite within a predetermined range. Incidentally, a roll for changing the traveling direction of the cold-rolled steel sheet so that it enters the snout is provided immediately before the connection part between the cooling zone and the snout, and the cold-rolled steel sheet enters the snout after passing through the roll. Next, the cold-rolled steel sheet is introduced into a hot-dip galvanizing bath through the snout and immersed in the hot-dip galvanizing bath to be subjected to hot-dip galvanizing treatment, thereby obtaining a plated steel sheet.

[0116] In the hot-dip galvanizing treatment, for example, the cold-rolled steel sheet is immersed in a hot-dip galvanizing bath at 440° C. or more and 500° C. or less. It is preferable to use a hot-dip galvanizing bath having an Al content of 0.10 mass % or more and 0.23 mass % or less, with the balance being Zn and unavoidable impurities.

[0117] Furthermore, after the above-described hot-dip galvanizing treatment, an alloying treatment may be performed at a temperature range of 460°C or higher and 600°C or lower. If the alloying treatment temperature is lower than 460°C, the Zn-Fe alloying rate becomes excessively slow, resulting in reduced productivity. On the other hand, if the alloying treatment temperature exceeds 600°C, untransformed austenite may transform into pearlite, resulting in reduced TS and El. Therefore, the alloying treatment temperature is preferably 460°C or higher and 600°C or lower. The alloying treatment temperature is preferably 470°C or higher. Furthermore, the alloying treatment temperature is preferably 560°C or lower.

[0118] The plating weight is 20 to 80 g / m per side. ２(Double-sided plating) is preferred. Furthermore, the alloyed hot-dip galvanized steel sheet (GA) is preferably subjected to the alloying treatment described below to adjust the Fe concentration in the coating layer to 7 to 15 mass %. The coating weight can be adjusted by performing gas wiping or the like after the hot-dip galvanizing treatment.

[0119] [2-8] Cooling Step After the hot-dip galvanizing treatment step, i.e., the plated steel sheet that has been subjected to hot-dip galvanizing treatment or hot-dip galvanizing treatment and alloying treatment, is cooled. The cooling conditions are not particularly limited, and known conditions can be used. For example, after the hot-dip galvanizing treatment or alloying treatment is completed, the average cooling rate to the heat retention temperature or cooling stop temperature described below, and the cooling rate after heat retention or reheating described below are not particularly limited, but from the viewpoint of further improving TS, they are preferably 2°C / s or more, more preferably 5°C / s or more. In addition, due to production technology constraints, the average cooling rate is preferably 50°C / s or less, more preferably 40°C / s or less. The cooling method is not particularly limited, and gas jet cooling, mist cooling, water cooling, air cooling, etc. can be applied.

[0120] [Heat retention temperature during cooling of plated steel sheet: 100°C or higher and 450°C or lower (preferred conditions)] During cooling of the plated steel sheet, the plated steel sheet may be retained at a temperature range of 100°C or higher and 450°C or lower for 5 seconds or longer, and then cooled. Retaining the temperature during cooling of the plated steel sheet promotes hydrogen desorption from the plated steel sheet, and the amount of low-temperature region diffusible hydrogen in the base steel sheet is reduced. From this perspective, the heat retention temperature during cooling of the plated steel sheet is preferably 100°C or higher, more preferably 130°C or higher, and even more preferably 150°C or higher. On the other hand, if the heat retention temperature during cooling of the plated steel sheet exceeds 450°C, the area ratio of martensite decreases, and TS decreases. Therefore, the heat retention temperature during cooling of the plated steel sheet is preferably 450°C or lower, more preferably 400°C or lower, and even more preferably 350°C or lower.

[0121] [Heat retention time during cooling of plated steel sheet: 5 seconds or more (preferred condition)] By retaining heat during cooling of the plated steel sheet, hydrogen desorption from the plated steel sheet is promoted, and the amount of diffusible hydrogen in the low-temperature region of the base steel sheet is reduced. From this point of view, the heat retention time during cooling of the plated steel sheet is preferably 5 seconds or more, more preferably 10 seconds or more, and even more preferably 15 seconds or more. There is no particular upper limit to the heat retention time during cooling of the plated steel sheet, but from the viewpoint of controlling the area ratios of ferrite and martensite in the base steel sheet within a predetermined range, it is preferably 300 seconds or less, more preferably 100 seconds or less.

[0122] [Cooling Stop Temperature During Cooling of Plated Steel Sheet: 300°C or Less (Preferred Condition)] During cooling of a plated steel sheet, cooling may be stopped at 300°C or less, and then the plated steel sheet may be reheated to a temperature range of (cooling stop temperature + 50°C) or more and 450°C or less, and then maintained at that temperature for 5 seconds or more, followed by cooling. During cooling of a plated steel sheet, cooling to a temperature below the martensitic transformation start temperature and then reheating promotes hydrogen desorption from the plated steel sheet, thereby reducing the amount of low-temperature diffusible hydrogen in the base steel sheet. From this perspective, the cooling stop temperature during cooling of a plated steel sheet is preferably 300°C or less, more preferably 250°C or less, and even more preferably 150°C or less. There is no particular restriction on the lower limit of the cooling stop temperature during cooling of a plated steel sheet, but due to constraints on production technology, it is preferably 10°C or more, and more preferably 30°C or more.

[0123] [Reheating temperature of plated steel sheet: (cooling stop temperature + 50°C) or higher and 450°C or lower (preferred conditions)] Reheating a plated steel sheet after cooling promotes hydrogen desorption from the plated steel sheet, thereby reducing the amount of low-temperature diffusible hydrogen in the base steel sheet. From this perspective, the reheating temperature of the plated steel sheet is preferably (cooling stop temperature + 50°C) or higher, more preferably (cooling stop temperature + 80°C) or higher, and even more preferably (cooling stop temperature + 100°C) or higher. If the reheating temperature of the plated steel sheet exceeds 450°C, the area fraction of martensite decreases, lowering the TS. Therefore, the reheating temperature of the plated steel sheet is preferably 450°C or lower, more preferably 400°C or lower, and even more preferably 350°C or lower.

[0124] [Heat retention time at the reheating temperature of the plated steel sheet: 5 seconds or more (preferred condition)] By retaining the plated steel sheet at the reheating temperature after cooling, hydrogen desorption from the plated steel sheet is promoted, and the amount of low-temperature diffusible hydrogen in the base steel sheet is reduced. From this perspective, the heat retention time at the reheating temperature of the plated steel sheet is preferably 5 seconds or more, more preferably 10 seconds or more, and even more preferably 15 seconds or more. There is no particular upper limit to the heat retention time at the reheating temperature of the plated steel sheet, but from the viewpoint of controlling the area ratios of ferrite and martensite in the base steel sheet within a predetermined range, it is preferably 300 seconds or less, more preferably 100 seconds or less.

[0125] After the above-mentioned cooling, the plated steel sheet is preferably rolled at an elongation rate of 0.05% or more and 1.00% or less. By rolling at an elongation rate of 0.05% or more after cooling, cracks can be introduced into the hot-dip galvanized layer, and as a result, a further reduction in the amount of low-temperature diffusible hydrogen in the base steel sheet can be expected. When rolling is performed after cooling, the elongation rate of rolling is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, when rolling at an elongation rate of more than 1.00% after cooling, the area ratio of ferrite having {001} orientation increases, and there is a risk of worsening the anisotropy of stretch flange cracking. When rolling is performed after cooling, the elongation rate of rolling is preferably 1.00% or less, more preferably 0.50% or less.

[0126] The rolling after cooling may be performed on an apparatus continuous with the continuous hot-dip galvanizing apparatus (online), or on an apparatus discontinuous with the continuous hot-dip galvanizing apparatus (offline). The target elongation may be achieved in a single rolling pass, or multiple rolling passes may be performed to achieve a total elongation of 0.05% to 1.00%. Here, the rolling after cooling generally refers to temper rolling, but may also be a processing method involving repeated bending using a tension leveler or rolls, as long as it can impart an elongation equivalent to that of temper rolling. The rolling treatment may be performed after the plated steel sheet has been cooled to near room temperature, or may be performed when the cooling of the plated steel sheet is stopped, followed by a reheating treatment.

[0127] When hot-dip galvanized steel sheets are traded, they are usually cooled to room temperature before being traded.

[0128] The production conditions other than those mentioned above are not particularly limited, and known conditions can be used.

[0129] [3] Member and manufacturing method thereof The member of the present invention and its manufacturing method will be described. The member of the present invention is a member made using the hot-dip galvanized steel sheet of the present invention described above. The member can be manufactured, for example, by forming the hot-dip galvanized steel sheet of the present invention into a desired shape by press working or the like.

[0130] The hot-dip galvanized steel sheet of the present invention has high ductility, high stretch flangeability and bendability, and high local ductility, with a TS of 980 MPa or more. Therefore, by applying the hot-dip galvanized steel sheet of the present invention or a member made using the hot-dip galvanized steel sheet to, for example, an automobile frame structural part or an automobile reinforcing part, it is possible to reduce the weight of the automobile body and thereby improve fuel efficiency, and the industrial utility value is extremely great.

[0131] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0132] [Test Nos. 1 to 47] A steel slab (steel material) having the chemical composition shown in Table 1, with the balance consisting of Fe and unavoidable impurities, was melted in a converter and obtained by continuous casting. The obtained steel slab was heated to 1250°C and rough rolled to obtain a sheet bar. The obtained sheet bar was then subjected to finish rolling at a finish rolling temperature of 900°C and coiled under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was pickled and then cold-rolled under the conditions shown in Table 2 to obtain a cold-rolled steel sheet with a thickness of 1.4 mm.

[0133] The obtained cold-rolled steel sheets were annealed under the conditions shown in Table 2. Next, the cold-rolled steel sheets were subjected to the types of plating treatments shown in Table 2 to obtain plated steel sheets having hot-dip galvanized layers on both sides. In the type of plating treatment in Table 2, hot-dip galvanized steel sheets (GI) mean that only hot-dip galvanizing treatment was performed, without alloying treatment. Also, GA means that hot-dip galvanizing treatment and alloying treatment were performed. For GI, a hot-dip galvanizing bath containing 0.20 mass% Al, with the balance being Zn and unavoidable impurities, was used as the plating bath. For GA, a hot-dip galvanizing bath containing 0.14 mass% Al, with the balance being Zn and unavoidable impurities, was used. The plating bath temperature was 470°C in both cases. The coating weight for GI was 45 to 72 g / m per side. ２ (Both sides plating) and for GA, 45 g / m per side ２ The alloying temperature for GA was approximately 550°C. The composition of the hot-dip galvanized layer for GI was 0.1 to 1.0 mass% Fe, 0.2 to 1.0 mass% Al, and the balance being Zn and unavoidable impurities. The composition of the (alloyed) hot-dip galvanized layer for GA was 7 to 15 mass% Fe, 0.1 to 1.0 mass% Al, and the balance being Zn and unavoidable impurities.

[0134] Next, some of the galvanized steel sheets were subjected to a heat-holding treatment or a reheating treatment after cooling under the conditions shown in Table 2. Conditions not specified were those according to conventional methods. For the hot-dip galvanized steel sheets obtained in this manner, the structure was identified in the region of the base steel sheet at 1 / 4 of the sheet thickness and in the region from the surface to a depth of 10 μm, using the method described above, and the low-temperature diffusible hydrogen content of the base steel sheet was measured. The results are shown in Table 3. The chemical compositions of the base steel sheets obtained were substantially the same as those at the steel slab stage, and all of the conforming steels were within the chemical composition range of the present invention, while all of the comparative steels were outside the chemical composition range of the present invention.

[0135] The obtained hot-dip galvanized steel sheets were evaluated for tensile properties, ultimate deformability, stretch flangeability and bendability according to the following test methods. The results are shown in Table 3.

[0136] [Tensile Test] The tensile test was performed in accordance with JIS Z 2241. That is, a JIS No. 5 test piece was taken from the obtained hot-dip galvanized steel sheet so that the direction perpendicular to the rolling direction (C direction) of the steel sheet was the longitudinal direction. Next, using the taken test piece, a crosshead speed of 1.67 × 10 －１ A tensile test was performed under conditions of 1000 psi / s, and the YS, TS, El, and L.El were measured. TS of 980 MPa or more was judged to be acceptable. The product of TS and El (TS × El) was calculated from the measured TS and El, and TS × El was calculated. TS × El was judged to be acceptable when it was 13,000 MPa·% or more. TS × L.El was also calculated from the measured TS and L.El, and TS × L.El was calculated. TS × L.El was judged to be acceptable when it was 4,500 MPa·% or more.

[0137] [Ultimate deformability] Test pieces were taken in the same manner as above, and a tensile test was carried out. ０ : Initial plate width, W: Broken part plate width, T: Initial plate thickness, T ０ : The thickness of the fractured part is calculated and the ultimate deformability ε is calculated from the following formula: ｌ was calculated. ｗ = ln(W / W ０ ) ε ｔ = ln(T / T ０ ) ε ｌ = - (ε ｗ +ε ｔ )

[0138] [Hole Expanding Test] The hole expanding test was carried out in accordance with JIS Z 2256. That is, the obtained hot-dip galvanized steel sheet was sheared to 100 mm x 100 mm, and then a hole of 10 mm diameter was punched into the sheared steel sheet with a clearance of 12.5%. Next, the steel sheet was held down using a die with an inner diameter of 75 mm with a blank holding force of 9 ton (88.26 kN), and in this state, a conical punch with an apex angle of 60° was pressed into the hole to measure the hole diameter at the crack initiation limit. Then, the (limit) hole expanding ratio (%) was calculated using the following formula. (Limit) hole expanding ratio: λ (%) = {(D ｆ -D ０ ) / D ０}×100 where D ｆHole diameter at crack initiation (mm), D ０ is the initial hole diameter (mm). The (limiting) hole expansion ratio of three sheared steel sheets was measured, and the average value was taken as λ. When λ was 30% or more, the stretch flangeability was judged to be acceptable.

[0139] [Bending Test] The bending test was performed in accordance with JIS Z 2248. From the obtained hot-dip galvanized steel sheet, strip-shaped test pieces with a width of 30 mm and a length of 100 mm were taken so 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 performed under conditions of an indentation load of 100 kN and a holding time of 5 seconds. In the present disclosure, bendability was evaluated based on the pass rate of the bending test. Five samples were subjected to a bending test at the maximum R at which the value R / t, obtained by dividing the bending radius (R) by the sheet thickness (t), was 5 or less (e.g., for a sheet thickness of 1.4 mm, the bending radius was 7.0 mm). Next, the presence or absence of cracks at the ridgeline of the bend apex was evaluated. If none of the five samples exhibited cracks, the bendability was judged to be "excellent." Furthermore, if microcracks of less than 200 μm occurred in one or more of the five samples, the bendability was judged to be "good." Furthermore, if microcracks of 200 μm or more occurred in one or more of the five samples, the bendability was judged to be "poor." The presence or absence of cracks was evaluated by measuring the ridgeline at the apex of the bend using a digital microscope (RH-2000, manufactured by Hirox Corporation) at a magnification of 40x.

[0140]

[0141]

[0142]

[0143] As shown in Table 3, all of the inventive examples had a TS of 980 MPa or more, and had high ductility, high stretch flangeability and bendability, as well as high local ductility. On the other hand, in the comparative examples, at least one of TS, ductility, stretch flangeability, bendability, and local ductility was insufficient.

[0144] [Test Nos. 48 to 71] A steel slab (steel material) having the composition shown in Table 1, with the balance consisting of Fe and unavoidable impurities, was melted in a converter and obtained by continuous casting. The obtained steel slab was heated to 1250°C and rough rolled to obtain a sheet bar. The obtained sheet bar was then subjected to finish rolling at a finish rolling temperature of 900°C and coiled under the conditions shown in Table 4 to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was pickled and then cold-rolled under the conditions shown in Table 4 to obtain a cold-rolled steel sheet with a thickness of 1.4 mm.

[0145] Some cold-rolled steel sheets were subjected to a metal electroplating process. In Table 4, the column for metal electroplating (plating type) indicates that Fe-based electroplating was performed when "Yes" is indicated, and that Ni-based electroplating was performed when "(Ni)" is indicated. The composition of the metal electroplating layer was 95 to 100 mass% Fe for the Fe-based electroplating and 95 to 100 mass% Ni for the Ni-based electroplating, with the remainder consisting of unavoidable impurities. The cold-rolled steel sheets were then subjected to the types of plating (GI, GA) shown in Table 4 to obtain plated steel sheets having hot-dip galvanized layers on both sides. The conditions for GI and GA were the same as those described above.

[0146] Next, some of the plated steel sheets were subjected to a heat-holding treatment or a reheating treatment after cooling under the conditions shown in Table 4. Conditions not specified were those according to conventional methods.

[0147] The composition of the base steel plate of the obtained steel plate was substantially the same as the composition of the steel slab, and all of the suitable steels were within the range of the composition according to the embodiment described above.

[0148] The hot-dip galvanized steel sheets thus obtained were measured for ultimate deformability, tensile properties, stretch flangeability, bendability, and low-temperature diffusible hydrogen content. The results are shown in Tables 5 and 6.

[0149] Furthermore, the thickness of the soft surface layer and the coating weight of the metal plating layer were measured for the obtained hot-dip galvanized steel sheets. The results are shown in Table 5.

[0150] [Thickness of Surface Soft Layer] A thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet was smoothed by wet polishing, and then measurements were made using a Vickers hardness tester at 1 μm intervals from a position 1 μm from the surface of the base steel sheet in the thickness direction to a position 100 μm in the thickness direction under a load of 10 gf. Measurements were then made at 20 μm intervals to the center of the sheet thickness. The region where the hardness was reduced to 85% or less compared to the hardness at the 1 / 4 position in the sheet thickness was defined as the soft layer (surface soft layer), and the thickness of this region in the thickness direction was taken as the thickness of the soft layer.

[0151] [Metal plating layer deposition mass] A 10 mm × 15 mm sample was taken from the hot-dip galvanized steel sheet and embedded in resin to prepare a cross-section embedded sample. Three randomly selected locations on the cross-section were observed using a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and a magnification of 2,000 to 10,000 times depending on the thickness of the Fe-based plating layer. The average thickness of the three fields of view was multiplied by the specific gravity of iron to convert it into the deposition mass of the Fe-based plating layer per side.

[0152] [Nanohardness] The nanohardness of the soft surface layer of the base steel sheet of the obtained hot-dip galvanized steel sheet was measured. The results are shown in Table 5. The 1 / 4 position of the soft surface layer is as follows. After the coating was peeled off from the obtained hot-dip galvanized steel sheet, mechanical polishing, buffing with diamond and alumina, and colloidal silica polishing were performed from the surface of the base steel sheet to a position 1 / 4 of the way down the thickness direction of the soft surface layer. Using a nanoindentation device (Hysitron's Tribo-950), nanohardness was measured at a total of 512 points using a Berkovich-shaped diamond indenter under the following conditions: loading rate and unloading rate: 50 μN / s, maximum load: 500 μN, measurement area: 50 μm × 50 μm, data collection pitch: 5 msec, and impact spacing: 2 μm. Next, the soft surface layer was mechanically polished to a position halfway down the thickness direction, buffed with diamond and alumina, and polished with colloidal silica. Using a Tribo-950 manufactured by Hysitron, a Berkovich-shaped diamond indenter was used to measure the nano-hardness at a total of 512 points under the same conditions as above.

[0153] Hot-dip galvanized steel sheets were subjected to a U-bend + contact bending test, a V-bend + orthogonal VDA bending test, and an axial crush test. These tests were all performed on steel sheets with a thickness of 1.2 mm, taking into account the influence of sheet thickness. Steel sheets with a thickness of more than 1.2 mm were ground on one side to reduce the thickness to 1.2 mm. Because grinding may affect the bendability of the steel sheet surface, the ground surface was placed on the inside (valley side) of the bend in the U-bend + contact bending test, and the ground surface was placed on the outside (crown side) of the bend in the V-bend + orthogonal VDA bending test, and then on the inside (valley side) of the bend in the subsequent VDA bending test. On the other hand, the U-bend + contact bending test, V-bend + orthogonal VDA bending test, and axial crush test on galvanized steel sheets with a thickness of less than 1.2 mm were performed without grinding because the influence of sheet thickness was small.

[0154] [U-Bend + Contact Bending Test] The U-bend + contact bending test was performed as follows. 60 mm x 30 mm test specimens were obtained from the obtained galvanized steel sheets by shearing and edge grinding. Here, the 60 mm side was parallel to the width (C) direction. Test specimens were prepared by U-bending (primary bending) in the width (C) direction with the rolling (L) direction as the axis, with a curvature radius / sheet thickness of 4.2. In the U-bend (primary bending), as shown in Figure 1(a), a punch B1 was pressed into a steel sheet placed on a roll A1 to obtain test specimen T1. Next, as shown in Figure 1(b), contact bending (secondary bending) was performed by crushing test specimen T1 placed on a lower die A2 with an upper die B2. In Figure 1(a), D1 indicates the width (C) direction, and D2 indicates the rolling (L) direction. Note that spacers S, described below, were inserted between the test specimens.

[0155] The U-bending conditions for the U-bending + close bending test were as follows: Test method: Roll support, punch pushing Punch tip R: 5.0 mm Clearance between roll and punch: Sheet thickness + 0.1 mm Stroke speed: 10 mm / min Bending direction: Orthogonal to rolling (C) direction

[0156] The conditions for the close bending in the U-bend + close bending test were as follows: Spacer thickness: Vary at 0.5 mm intervals Test method: Die support, punch pressing Forming load: 10 tons Test speed: 10 mm / min Holding time: 5 seconds Bending direction: Orthogonal to rolling (C) direction

[0157] The above U-bend + close bending test was performed three times, and the critical spacer thickness (ST) was determined when no cracks occurred in any of the three tests. Using a Leica stereo microscope, cracks of 200 μm or more in length at 25x magnification were judged to be cracks. Note that ST is an index for evaluating fracture resistance during a collision (the fracture resistance of the vertical wall portion in an axial crushing test). The results are shown in Table 6.

[0158] The criteria for acceptance of the critical spacer thickness (ST) are as follows: If 980 MPa≦TS<1180 MPa, then 4.5 mm≧ST If 1180 MPa≦TS<1320 MPa, then 5.0 mm≧ST If 1320 MPa≦TS<1470 MPa, then 5.5 mm≧ST If 1470 MPa≦TS, then 6.0 mm≧ST The criteria for rejection of the critical spacer thickness (ST) are as follows: If 980 MPa≦TS<1180 MPa, then 4.5 mm<SF ｍａｘ When 1180 MPa≦TS<1320 MPa, 5.0 mm<SF ｍａｘ When 1320 MPa ≦ TS < 1470 MPa, 5.5 mm < SF ｍａｘ When 1470 MPa≦TS, 6.0 mm<SF ｍａｘ

[0159] [V-bend + Orthogonal VDA Bending Test] The V-bend + orthogonal VDA bending test was performed as follows. A 60 mm x 65 mm test specimen was obtained from the obtained hot-dip galvanized steel sheet by shearing and edge grinding. Here, the 60 mm side was parallel to the rolling (L) direction. The test specimens were prepared by bending the steel sheet 90° (primary bending) in the rolling (L) direction with a curvature radius / sheet thickness of 4.2, with the width (C) direction as the axis. In the 90° bending (primary bending), as shown in Figure 2(a), a punch B3 was pressed into a steel sheet placed on a die A3 with a V groove to obtain a test specimen T1. Next, as shown in Figure 2(b), an orthogonal bending (secondary bending) was performed by pressing a punch B4 into the test specimen T1 placed on a support roll A4 so that the bending direction was perpendicular to the rolling direction. In Figures 2(a) and 2(b), D1 indicates the width (C) direction, and D2 indicates the rolling (L) direction.

[0160] The V-bending conditions in the V-bending + orthogonal VDA bending test are as follows: Test method: die support, punch pressing Forming load: 10 ton Test speed: 30 mm / min Holding time: 5 s Bending direction: rolling (L) direction

[0161] The VDA bending conditions for the V-bending + orthogonal VDA bending test are as follows: Test method: roll support, punch pressing Roll diameter: φ30 mm Punch tip R: 0.4 mm Distance between rolls: (sheet thickness × 2) + 0.5 mm Stroke speed: 20 mm / min Test piece size: 60 mm × 60 mm Bending direction: perpendicular (C) direction to rolling

[0162] The stroke at the maximum load is determined from the stroke-load curve obtained when the VDA bending test is performed. The average value of the stroke at the maximum load when the V bending + orthogonal VDA bending test is performed three times is defined as SF ｍａｘ (mm). ｍａｘ is an index for evaluating the fracture resistance characteristics during a collision (the fracture resistance characteristics of the bend ridge line in an axial crushing test). The results are shown in Table 6.

[0163] science fiction ｍａｘThe criteria for passing are as follows: 980 MPa ≦ TS < 1180 MPa, 27.0 mm ≦ SF ｍａｘ When 1180 MPa ≦ TS < 1320 MPa, 26.0 mm ≦ SF ｍａｘ When 1320 MPa ≦ TS < 1470 MPa, 24.5 mm ≦ SF ｍａｘ When 1470 MPa ≦ TS, 24.0 mm ≦ SF ｍａｘ science fiction ｍａｘ The rejection criteria are as follows: 980 MPa ≦ TS < 1180 MPa, 27.0 mm > SF ｍａｘ When 1180 MPa ≦ TS < 1320 MPa, 26.0 mm > SF ｍａｘ When 1320 MPa ≦ TS < 1470 MPa, 24.5 mm > SF ｍａｘ When 1470 MPa≦TS, 24.0 mm>SF ｍａｘ

[0164] [Axial Crush Test] The axial crush test was performed as follows. A 150 mm × 100 mm test piece was obtained from the obtained hot-dip galvanized steel sheet by shearing. Here, the 150 mm side was parallel to the rolling (L) direction. Using a mold with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm, the hat-shaped member 10 shown in FIGS. 3( a) and 3(b) was produced by forming (bending) the hat-shaped member to a depth of 40 mm. The steel plate used as the material for the hat-shaped member was separately cut into a size of 80 mm × 100 mm. Next, the cut-out steel plate 20 and the hat-shaped member 10 were spot-welded to produce a test member 30 as shown in FIGS. 3( a) and 3(b). FIG. 3(a) is a front view of the test member 30 produced by spot-welding the hat-shaped member 10 and the steel plate 20. FIG. 3(b) is a perspective view of the test member 30. As shown in FIG. 3( b), the spot welds 40 were positioned so that the distance between the edge of the steel plate and the weld was 10 mm and the distance between the welds was 20 mm. Next, as shown in FIG. 3( c), the test member 30 was joined to the base plate 50 by TIG welding to prepare a sample for an axial crush test. Next, an impactor 60 was collided with the prepared sample for the axial crush test at a constant velocity of 10 mm / min, crushing the sample for the axial crush test by 70 mm. As shown in FIG. 3( c), the crush direction was parallel to the longitudinal direction of the test member 30. The results are shown in Table 6.

[0165] The evaluation criteria for the presence or absence of axial crush fracture (visual cracks) are as follows: A (pass): No visual cracks were observed on the sample after the axial crush test. B (pass): One or less visual cracks were observed on the sample after the axial crush test. C (fail): Two or more visual cracks were observed on the sample after the axial crush test.

[0166]

[0167]

[0168]

[0169] As shown in Table 6, all of the inventive examples had a TS of 980 MPa or more, and had high ductility, high stretch flangeability and bendability, as well as high local ductility. On the other hand, the comparative examples were insufficient in at least one of TS, ductility, stretch flangeability, bendability, and local ductility. Furthermore, they also had excellent fracture resistance (bending fracture resistance and axial crush resistance) during collision.

[0170] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, which form part of the disclosure of the present invention. In other words, other embodiments, examples, and operational techniques made by those skilled in the art based on the above-described embodiments are all included in the scope of the present invention. For example, in the series of heat treatments in the above-described manufacturing method, the equipment for subjecting the steel sheet to heat treatment is not particularly limited as long as the thermal history conditions are satisfied.

[0171] According to the present invention, a hot-dip galvanized steel sheet having high ductility, high stretch flangeability and bendability, and high local ductility, and a TS of 980 MPa or more, can be obtained. In particular, the hot-dip galvanized steel sheet of the present invention has various excellent properties and can be applied to automotive frame structural parts of various sizes and shapes. This enables the weight of the vehicle body to be reduced, thereby improving fuel efficiency, and has extremely great industrial value.

[0172] REFERENCE SIGNS LIST 10 Hat-shaped member 20 Galvanized steel sheet 30 Test member 40 Spot welded portion 50 Base plate 60 Impactor A1 Die A2 Support roll A3 Die A4 Support roll B1 Punch B2 Punch B3 Punch B4 Punch D1 Width (C) direction D2 Rolling (L) direction S Spacer T1 Test piece T2 Test piece

Claims

1. A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer on a surface of the base steel sheet, The base steel plate is In mass percent, C: 0.030% or more and 0.500% or less, Si: 0.01% or more and 2.50% or less, Mn: 0.10% or more and 5.00% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less; O: 0.0100% or less and the balance being Fe and unavoidable impurities; At a 1 / 4 position of the plate thickness of the base steel plate, The area ratio of martensite is 30% or more, The area ratio of ferrite is 70% or less, The area ratio of retained austenite is 10% or less, Contains at least one of ferrite and retained austenite, In a region from the surface of the base steel plate to a depth of 10 μm, A steel structure in which the area ratio of martensite is 5% or more and the area ratio of martensite at a 1 / 4 plate thickness position of the base steel plate is 80% or less; having The amount of low-temperature diffusible hydrogen in the base steel sheet, which is the amount of hydrogen released when the base steel sheet is heated to 50° C., is 0.015 ppm by mass or less. Hot-dip galvanized steel sheet.

2. The composition further includes, in mass%, 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; Bi: 0.200% or less The hot-dip galvanized steel sheet according to claim 1, further comprising at least one element selected from the group consisting of:

3. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

4. The base steel plate has a surface soft layer which is a region having a Vickers hardness of 85% or less of the Vickers hardness at a 1 / 4 position of the plate thickness of the base steel plate and is a region within 200 μm in the plate thickness direction from the surface of the base steel plate, When the nano hardness was measured at 300 points or more in a 50 μm × 50 μm region of the sheet surface at a 1 / 4 position and a 1 / 2 position of the sheet thickness direction depth of the surface soft layer from the surface of the base steel sheet, The ratio of the number of measurements in which the nano hardness of the sheet surface at a 1 / 4 position of the sheet thickness direction depth of the soft surface layer from the surface of the base steel sheet is 7.0 GPa or more to the total number of measurements is 0.10 or less, The standard deviation σ of the nanohardness of the sheet surface at a ¼ position of the sheet thickness direction depth of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less, The hot-dip galvanized steel sheet according to claim 1 or 2, further comprising a standard deviation σ of nano-hardness of the sheet surface at a position half the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet being 2.2 GPa or less.

5. The base steel plate has a surface soft layer which is a region having a Vickers hardness of 85% or less of the Vickers hardness at 1 / 4 of the plate thickness of the base steel plate and is within 200 μm from the surface of the base steel plate in the plate thickness direction, When the nano hardness was measured at 300 points or more in a 50 μm × 50 μm region of the sheet surface at a 1 / 4 position and a 1 / 2 position of the sheet thickness direction depth of the surface soft layer from the surface of the base steel sheet, The ratio of the number of measurements in which the nano hardness of the sheet surface at a 1 / 4 position of the sheet thickness direction depth of the soft surface layer from the surface of the base steel sheet is 7.0 GPa or more to the total number of measurements is 0.10 or less, The standard deviation σ of the nanohardness of the sheet surface at a ¼ position of the sheet thickness direction depth of the soft surface layer from the surface of the base steel sheet is 1.8 GPa or less, The hot-dip galvanized steel sheet according to claim 3, further comprising a standard deviation σ of nano-hardness of the sheet surface at a position half the depth in the sheet thickness direction of the soft surface layer from the surface of the base steel sheet being 2.2 GPa or less.

6. The hot-dip galvanized steel sheet according to claim 1 or 2, further comprising a metal plating layer formed between the base steel sheet and the hot-dip galvanized layer on one or both sides of the hot-dip galvanized steel sheet.

7. A hot-dip galvanized steel sheet as described in claim 3, having a metal plating layer formed between the base steel sheet and the hot-dip galvanized layer on one or both sides of the hot-dip galvanized steel sheet.

8. The hot-dip galvanized steel sheet as described in claim 4, having a metal plating layer formed between the base steel sheet and the hot-dip galvanized layer on one or both sides of the hot-dip galvanized steel sheet.

9. The hot-dip galvanized steel sheet according to claim 5, having a metal plating layer formed between the base steel sheet and the hot-dip galvanized layer on one or both sides of the hot-dip galvanized steel sheet.

10. A member made using the hot-dip galvanized steel sheet according to claim 1 or 2.

11. A frame structural part of an automobile or a reinforcing part of an automobile, comprising the member according to claim 10.

12. A steel slab having the composition according to claim 1 or 2, The hot-rolled steel sheet is obtained by hot-rolling the steel sheet under the condition of a coiling temperature of 400° C. or more and 700° C. or less. Next, the hot-rolled steel sheet is subjected to pickling, Next, the hot-rolled steel sheet is Cumulative reduction rate of 30% or more, Unit tension between the final pass and the pass immediately before the final pass: 10 kgf / mm ２ Cold rolling is performed under the above conditions to obtain cold-rolled steel sheet. Next, the cold rolled steel sheet is The oxygen concentration in the temperature range of 250° C. to 700° C. is 0.5 vol.% to 5.0 vol.%, and the annealing temperature T １ Heat until Over 700℃T １ The dew point in the following temperature range is set to -30°C or higher, and the retention time in the temperature range is set to 10 s or more and 500 s or less, and annealing is performed under the conditions of Next, the annealed cold rolled steel sheet is subjected to a hot-dip galvanizing treatment to obtain a plated steel sheet, and then cooling the plated steel sheet to obtain a hot-dip galvanized steel sheet.

13. The method for producing a high strength steel sheet according to claim 12, wherein during cooling of the plated steel sheet, the plated steel sheet is kept at a temperature in the range of 100°C or more and 450°C or less for 5 seconds or more, and then cooled.

14. The method for producing a high-strength steel sheet according to claim 12, wherein, during cooling of the plated steel sheet, cooling is stopped at 300°C or less, and then the plated steel sheet is reheated to a temperature range of (cooling stop temperature + 50°C) or more and 450°C or less, and then the temperature is maintained for 5 s or more, and then cooled.

15. The method for producing a hot-dip galvanized steel sheet according to claim 12, further comprising the step of: subjecting the plated steel sheet to an alloying treatment after the hot-dip galvanizing treatment.

16. The method for producing a hot-dip galvanized steel sheet according to claim 12, further comprising a metal plating step of forming a metal plating layer by applying metal plating to one or both sides of the cold-rolled steel sheet before the annealing step.

17. A method for manufacturing a component, comprising a step of subjecting the hot-dip galvanized steel sheet according to claim 1 or 2 to at least one of forming and joining to form a component.