Hot-dip galvanized steel sheet, member using the hot-dip galvanized steel sheet, skeletal structure part or reinforcing part of an automobile made of the member, and method for producing the hot-dip galvanized steel sheet and the 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

Technical Field

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

Background Art

[0002] In order to reduce CO2 emissions by lightening vehicles while improving collision resistance performance, the strength of steel sheets for automobiles has been increased. In addition, against the background of successive introduction of new regulations, the number of cases where high-strength steel sheets are applied to main structural parts and reinforcing parts (hereinafter also referred to as "skeletal structural parts of automobiles", etc.) that form the skeleton of automobile cabins has been increasing. In particular, the number of cases where high-strength steel sheets with a tensile strength (hereinafter also referred to as "TS") of 980 MPa or more are applied has been increasing.

[0003] High-strength steel sheets used for skeletal structural parts of automobiles and the like are required to have high elongation flangeability when being formed into a desired shape. Furthermore, among skeletal structural parts of automobiles and the like, for example, a crash box has a bent portion. Therefore, from the viewpoint of formability, it is preferable to apply a steel sheet having high bendability to such parts.

[0004] As a technology related to such high-strength steel sheets, for example, in claim 1 of Patent Document 1, it is described that "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, having a tensile strength of 780 MPa or more, wherein the base steel sheet contains, by 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 satisfies the relationship of [%Mn] / [%Si] being 2.9 or more and 11.7 or less, with the balance being Fe and inevitable impurities, having a steel structure in which one or two selected from the group consisting of bainite and ferrite have a total area ratio of 5% or more and 85% or less, the area ratio of tempered martensite is 65% or less, the area ratio of quenched martensite is 5% or more and 40% or less, and the volume ratio of retained austenite is 5.0% or less, the ratio of the Si enrichment amount to the Mn enrichment amount in the surface layer of the base steel sheet is 0.7 or more and 1.3 or less, and the diffusible hydrogen amount in the base steel sheet is 0.80 mass ppm or less. However, [%Mn] and [%Si] respectively represent the contents (mass%) of Mn and Si in the steel."

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in the hot-dip galvanized steel sheet described in Patent Document 1, no consideration has been given to the ultimate deformability and local ductility. Therefore, from the perspective of increasing the application ratio of high-strength steel sheets with a TS of 980 MPa or more, particularly hot-dip galvanized steel sheets, to automotive skeletal structure parts and the like, there is a current demand for the development of a hot-dip galvanized steel sheet with a TS of 980 MPa or more that has high ultimate deformability, high ductility, high stretch flange formability, and bendability, and also has high local ductility.

[0007] The present invention has been developed in view of the above-described situation, and an object thereof is to provide a hot-dip galvanized steel sheet with a TS of 980 MPa or more that has high ultimate deformability, high ductility, high stretch flange formability, and bendability, and also has high local ductility. Another object of the present invention is to provide a method for manufacturing the above-described hot-dip galvanized steel sheet. Furthermore, an object of the present invention is to provide a member using the above-described 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 the TS and the total elongation (hereinafter also referred to as "El") measured in accordance with JIS Z 2241 is 10,000 MPa·% or more. "High stretch flange formability" 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 the bending test performed in accordance with JIS Z 2248 (for details, refer to the description of the examples below), none of the 5 samples evaluated are cracked, or minute cracks less than 200 μm occur in one or more of the 5 samples. Here, "high local ductility" means that the product (TS × L.El) of the TS and the local elongation (hereinafter also referred to as "L.El") measured in accordance with JIS Z 2241 is 4,500 MPa·% or more.

Means for Solving the Problems

[0009] In order to achieve the above-described problems, the inventors have conducted intensive studies and as a result, obtained the following findings. (1) For the base steel plate, after setting a predetermined component composition, the steel structure contains martensite (quenched martensite, tempered martensite, and bainite). Thereby, TS of 980 MPa or more can be obtained. (2) By making the steel structure of the base steel plate contain at least one of ferrite and retained austenite, high ductility can be obtained. (3) By setting the area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm to 5% or more and the area ratio of martensite at the 1 / 4 position of the plate thickness of the base steel plate to 80% or less, high ultimate deformability and high bendability can be obtained. (4) By setting the amount of diffusible hydrogen in the low temperature range of the base steel plate to 0.015 mass ppm or less, high local ductility, stretch flangeability, and bendability can be obtained.

[0010] [1] A hot-dip galvanized steel sheet including a base steel plate and a hot-dip galvanized layer on the surface of the base steel plate, wherein the base steel plate 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 and having a component composition in which the balance consists of Fe and unavoidable impurities, at the 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, containing at least one of ferrite and retained austenite, in the 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 80% or less of the area ratio of martensite at the 1 / 4 thickness position of the base steel plate, and having, the amount of diffusible hydrogen in the low temperature range in the base steel plate, which is the amount of hydrogen released when the base steel plate is heated to 50 °C, is 0.015 mass ppm or less, a hot-dip galvanized steel sheet. [2] The component composition is further, 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 Bi: 0.200% or less The hot-dip galvanized steel sheet according to [1], containing at least one element selected from the group consisting of. [3] The hot-dip galvanized steel sheet according to [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 where the Vickers hardness is 85% or less with respect to the Vickers hardness at the 1 / 4 position of the plate thickness of the base steel plate and is within 200 μm in the plate thickness direction from the surface of the base steel plate. When measuring the nano-hardness at 300 points or more in a 50 μm × 50 μm region on the plate surface at each of the 1 / 4 position and the 1 / 2 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate, The ratio of the number of measurements where the nano-hardness on the plate surface at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 7.0 GPa or more is 0.10 or less with respect to the total number of measurements. The standard deviation σ of the nano-hardness on the plate surface at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 1.8 GPa or less. Further, the standard deviation σ of the nano-hardness on the plate surface at the 1 / 2 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 2.2 GPa or less, and the hot-dip galvanized steel plate according to any one of [1] to [3]. [5] The hot-dip galvanized steel plate according to any one of [1] to [4], having a metal plating layer formed between the base steel plate and the hot-dip galvanized layer on one or both sides of the hot-dip galvanized steel plate. [6] A member made of the hot-dip galvanized steel plate according to any one of [1] to [5]. [7] An automotive skeletal structure part or an automotive reinforcement part made of the member according to [6]. [8] On a steel slab having the component composition of [1] or [2], Hot rolling is performed under the condition that the coiling temperature is 400 °C or more and 700 °C or less to obtain a hot-rolled steel plate. Next, pickling is performed on the hot-rolled steel plate. Next, on the hot-rolled steel plate, The cumulative reduction rate is 30% or more, The unit tension between the final pass and the pass immediately before the final pass is 10 kgf / mm 2 or more, and cold rolling is performed under these conditions to obtain a cold-rolled steel plate. Next, the cold-rolled steel plate is heated to an annealing temperature T1 of 750 °C or more and 950 °C or less with the oxygen concentration in the temperature range of 250 °C or more and 700 °C or less being 0.5 vol% or more and 5.0 vol% or less. Anneal under the condition that the dew point in the temperature range below 700 °C and below T1 is -30 °C or higher and the residence time in this temperature range is 10 s or longer and 500 s or shorter. Next, subject the annealed cold-rolled steel sheet to hot-dip galvanizing treatment to obtain a galvanized steel sheet. A method for manufacturing a hot-dip galvanized steel sheet, including cooling the galvanized steel sheet to obtain a hot-dip galvanized steel sheet. [9] During the cooling of the galvanized steel sheet, keep it in a temperature range of 100 °C or higher and 450 °C or lower for 5 s or longer and then cool it. A method for manufacturing the high-strength steel sheet according to [8].

[10]

[10] During the cooling of the galvanized steel sheet, stop the cooling at 300 °C or lower, then reheat it to a temperature range of (cooling stop temperature + 50 °C) or higher and 450 °C or lower, then keep it for 5 s or longer and then cool it. A method for manufacturing the high-strength steel sheet according to [8].

[11] After the hot-dip galvanizing treatment, subject the galvanized steel sheet to an alloying treatment. A method for manufacturing a hot-dip galvanized steel sheet according to any one of [8] to

[10] .

[12] Before the annealing step, include a metal plating step of performing metal plating on one or both sides of the cold-rolled steel sheet to form a metal plating layer. A method for manufacturing a hot-dip galvanized steel sheet according to any one of [8] to

[11] .

[13] A method for manufacturing a member, which has a step of subjecting a hot-dip galvanized steel sheet according to any one of [1] to [5] to at least one of a forming process or a joining process to obtain a member. [Effect of the Invention]

[0011] According to the present invention, it is possible to provide a hot-dip galvanized steel sheet of 980 MPa or higher and a member, which have high ultimate formability, high ductility, high stretch flangeability and bendability, and also have high local ductility, together with a manufacturing method thereof. [Brief Description of the Drawings]

[0012]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0013] The present invention will be described based on the following embodiments. 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 includes 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] Component composition The component composition of the base steel sheet will be described. In the following description, "%" representing the content of the component elements of the steel sheet means "mass%" unless otherwise specified. Also, in this specification, the numerical range represented by "~" means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value.

[0016] [C: 0.030% or more and 0.500% or less] C is one of the important basic components of steel and, in particular, in the present invention, it is an important element that affects the area ratios of martensite and ferrite. When the C content is less than 0.030%, the fraction of martensite decreases, making it difficult to achieve the desired TS. On the other hand, when the C content exceeds 0.500%, the area ratio of retained austenite increases, so the hydrogen desorption from the plated steel sheet is not promoted, the amount of diffusible hydrogen in the base steel sheet in the low temperature range increases, and it becomes difficult to achieve the desired local ductility. Also, during punching, it transforms from retained austenite to martensite, increasing the generation of voids during hole expansion, decreasing λ, and the bendability may also decrease. Therefore, the C content shall be 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 particular, in the present invention, it is an important element that affects the area ratios of martensite and ferrite. When the Si content is less than 0.01%, the area ratio of ferrite in the surface layer decreases, and it is impossible 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 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, when the Si content exceeds 2.50%, it suppresses carbide formation during continuous annealing and promotes the formation of retained austenite, which is a phase with a large hydrogen solubility, so the hydrogen desorption from the plated steel sheet is not promoted, the amount of diffusible hydrogen in the base steel sheet in the low temperature range increases, and it becomes difficult to achieve the desired local ductility. Also, during punching, it transforms from retained austenite to martensite, increasing the generation of voids during hole expansion and decreasing λ. Also, the bendability decreases. Therefore, the Si content shall be 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: Above 0.10% and below 5.00%] Mn is one of the important basic components of steel, and in particular in the present invention, it is an important element that affects the area ratios of martensite and ferrite. When 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, when the Mn content exceeds 5.00%, the structure mainly consists of quenched martensite, and the amount of diffusible hydrogen in the low-temperature range of the base steel plate increases, making it difficult to achieve the desired local ductility. Also, λ decreases and the bendability deteriorates. Therefore, the Mn content is set to be above 0.10% and below 5.00%. The Mn content is preferably 0.80% or more, more preferably 1.00% or more, and is preferably 4.50% or less, more preferably 4.00% or less.

[0019] [P: 0.100% or less] P is an element that segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and reduces the ultimate deformability of the steel plate, and can also reduce the local ductility and λ. Therefore, the P content is set to 0.100% or less, preferably 0.070% or less. The lower limit of the P content is not particularly limited, but since P is a solid solution strengthening element and can increase the strength of the steel plate, 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 plate, and the local ductility and λ can decrease. Therefore, the S content is set to 0.0200% or less, preferably 0.0050% or less. The lower limit of the S content is not particularly limited, but due to production technology constraints, it is preferably 0.0001% or more.

[0021] [Al: 1.000% or less] Al is an element that raises the A3 transformation point, leading to a large amount of ferrite in the steel structure and making it difficult to achieve the desired TS. Therefore, the Al content is set to 1.000% or less, preferably 0.100% or less. The lower limit of the Al content is not particularly limited, but from the perspective of suppressing carbide formation during continuous annealing and promoting the formation of retained austenite, it is preferably 0.001% or more.

[0022] [N: 0.0100% or less] N exists as nitrides and is an element that reduces the ultimate deformability of the steel sheet, and can reduce local ductility and λ. Therefore, the N content is set to 0.0100% or less, preferably 0.0050% or less. The lower limit of the N content is not particularly limited, but due to production technology constraints, the N content is preferably 0.0001% or more.

[0023] [O: 0.0100% or less] O exists as oxides and is an element that reduces the ultimate deformability of the steel sheet, and can reduce local ductility and λ. Therefore, the O content is set to 0.0100% or less, preferably 0.0050% or less. The lower limit of the O content is not particularly limited, but due to production technology constraints, the O content is preferably 0.0001% or more.

[0024] [Optional components] In addition to the above component 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: below 0.020%, Te: below 0.020%, Hf: below 0.10% and Bi: below 0.200% It may contain at least one element selected from the group consisting of. These elements may be used alone or in combination of two or more.

[0025] When containing Ti, Nb or V, in order to avoid the generation of a large amount of coarse precipitates and inclusions, the reduction of the ultimate deformability of the steel sheet, and the reduction of local ductility and λ, the content of Ti, Nb or V is preferably 0.200% or less respectively, more preferably 0.100% or less. The lower limit of the content of Ti, Nb or V is not particularly limited, but since the strength of the steel sheet is increased by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing, the content of Ti, Nb or V is preferably 0.001% or more respectively.

[0026] When containing Ta or W, in order to avoid the generation of a large amount of coarse precipitates and inclusions, the reduction of the ultimate deformability of the steel sheet, and the reduction of local ductility and λ, the content of Ta or W is preferably 0.10% or less respectively, more preferably 0.08% or less. The lower limit of the content of Ta or W is not particularly limited, but since the strength of the steel sheet is increased by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing, the content of Ta or W is more preferably 0.01% or more respectively.

[0027] When containing B, in order to avoid the generation of cracks inside the steel sheet during casting or hot rolling, the reduction of the ultimate deformability of the steel sheet, and thus the reduction of local ductility and λ, the content of B is preferably 0.0100% or less, more preferably 0.0080% or less. The lower limit of the content of B is not particularly limited, but since it is an element that segregates at the austenite grain boundary during annealing and improves hardenability, the content of B is preferably 0.0003% or more.

[0028] When containing Cr, Mo or Ni, in order to avoid the increase of coarse precipitates and inclusions, the decrease of the ultimate deformability of the steel sheet, and thus the decrease of local ductility and λ, the content of Cr, Mo or Ni is preferably 1.00% or less respectively, more preferably 0.80% or less. The lower limit of the content of Cr, Mo or Ni is not particularly limited, but since they are elements that improve hardenability, the content of Cr, Mo or Ni is preferably 0.01% or more respectively.

[0029] When containing Co, in order to avoid the increase of coarse precipitates and inclusions, the decrease of the ultimate deformability of the steel sheet, and thus the decrease of local ductility and λ, the content of Co is preferably 0.010% or less, more preferably 0.008% or less. The lower limit of the content of Co is not particularly limited, but since it is an element that improves hardenability, the content of Co is preferably 0.001% or more.

[0030] When containing Cu, in order to avoid the increase of coarse precipitates and inclusions, the decrease of the ultimate deformability of the steel sheet, and thus the decrease of local ductility and λ, the content of Cu is preferably 1.00% or less, more preferably 0.80% or less. The lower limit of the content of Cu is not particularly limited, but since it is an element that improves hardenability, the content of Cu is preferably 0.01% or more.

[0031] When containing Sn, in order to avoid the generation of cracks inside the steel sheet during casting or hot rolling, the decrease of the ultimate deformability of the steel sheet, and thus the decrease of local ductility and λ, the content of Sn is preferably 0.200% or less, more preferably 0.100% or less. The lower limit of the content of Sn is not particularly limited, but since Sn is an element that improves hardenability, the content of Sn is preferably 0.001% or more.

[0032] When containing Sb, in order to avoid the increase of coarse precipitates and inclusions, the decrease of the ultimate deformation capacity of the steel plate, and thus the decrease of local ductility and λ, the content of Sb is preferably 0.200% or less, more preferably 0.100% or less. Although the lower limit of the content of Sb is not particularly limited, since it is an element that controls the area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm and enables strength adjustment, the content of Sb is preferably 0.001% or more.

[0033] When containing Ca, Mg or REM, in order to avoid the increase of coarse precipitates and inclusions, the decrease of the ultimate deformation capacity of the steel plate, and thus the decrease of local ductility and λ, the content of Ca, Mg or REM is preferably 0.0100% or less, more preferably 0.0050% or less, respectively. Although the lower limit of the content of Ca, Mg or REM is not particularly limited, since they are elements that spheroidize the shapes of nitrides and sulfides and improve the ultimate deformation capacity of the steel plate, the content of Ca, Mg or REM is more preferably 0.0005% or more, respectively.

[0034] When containing Zr or Te, in order to avoid the increase of coarse precipitates and inclusions, the decrease of the ultimate deformation capacity of the steel plate, and thus the decrease of local ductility and λ, the content of Zr or Te is preferably 0.100% or less, more preferably 0.080% or less, respectively. Although the lower limit of the content of Zr or Te is not particularly limited, since they are elements that spheroidize the shapes of nitrides and sulfides and improve the ultimate deformation capacity of the steel plate, the content of Zr or Te is preferably 0.001% or more, respectively.

[0035] When containing Hf, in order to avoid the increase of coarse precipitates and inclusions, the decrease of the ultimate deformation capacity of the steel plate, and thus the decrease of local ductility and λ, the content of Hf is preferably 0.10% or less, more preferably 0.08% or less. Although the lower limit of the content of Hf is not particularly limited, since it is an element that spheroidizes the shapes of nitrides and sulfides and improves the ultimate deformation capacity of the steel plate, the content of Hf is preferably 0.01% or more.

[0036] When Bi is contained, coarse precipitates and inclusions increase, the ultimate deformability of the steel sheet decreases, and consequently, local ductility and λ decrease. Therefore, in order to avoid this, the content of Bi is preferably 0.200% or less, more preferably 0.100% or less. The lower limit of the content of Bi is not particularly limited, but since it is an element that reduces segregation, the content of Bi is preferably 0.001% or more.

[0037] The base metal steel sheet of the present invention contains essential components and optional components as the case may be, and has a component composition in which the balance consists of Fe and inevitable impurities. Here, examples of the inevitable impurities include Zn, Pb, As, Ge, Sr, and Cs. The total amount of these inevitable impurities is allowed to be contained in an amount of 0.100% or less.

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

[0039] [Area ratio of martensite at 1 / 4 thickness position of base metal steel sheet: 30% or more] When the area ratio of martensite at the 1 / 4 thickness position of the base metal steel sheet is less than 30%, it becomes difficult to achieve the desired TS. Therefore, the area ratio of martensite is set to 30% or more. The area ratio of martensite at the 1 / 4 thickness position of the base metal steel sheet is preferably 35% or more, more preferably 40% or more, still more preferably 45% or more. The upper limit of the area ratio of martensite at the 1 / 4 thickness position of the base metal steel sheet is not particularly limited, but from the viewpoint of obtaining high ductility, the area ratio of martensite at the 1 / 4 thickness position of the base metal steel sheet is preferably 95% or less, more preferably 90% or less, still more preferably 85% or less. The martensite referred to here includes tempered martensite and bainite in addition to quenched martensite (fresh martensite).

[0040] [Area ratio of ferrite at 1 / 4 thickness position of base metal steel sheet: 70% or less] When the area ratio of ferrite at the 1 / 4 thickness position of the base metal steel plate exceeds 70%, it becomes difficult to achieve the desired TS. Therefore, the area ratio of ferrite at the 1 / 4 thickness position of the base metal steel plate is set to 70% or less. The area ratio of ferrite at the 1 / 4 thickness position of the base metal steel plate is preferably 65% or less, more preferably 60% or less, and even more preferably 55% or less. Note that the lower limit of the area ratio of ferrite at the 1 / 4 thickness position of the base metal steel plate is not particularly limited, but from the viewpoint of obtaining high ductility, the area ratio of ferrite at the 1 / 4 thickness position of the base metal 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 bainitic ferrite in addition to polygonal ferrite.

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

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

[0043] In the SEM image, ferrite (polygonal ferrite and bainitic ferrite) is a flat tissue in the concave part that does not contain carbides, tempered martensite and bainite are tissues in the concave part that contain fine carbides, and quenched martensite is a tissue in the convex part with fine unevenness inside the tissue, and they are distinguishable from each other. Note that since the total area ratio is obtained as the area ratio of martensite for tempered martensite and bainite, they do not have to be distinguishable from each other.

[0044] [Area ratio of retained austenite at 1 / 4 of the thickness of the base steel plate: 10% or less] Since retained austenite is a phase with a high hydrogen solubility, when the area ratio of retained austenite exceeds 10%, the desorption of hydrogen from the plated steel plate is not promoted, the amount of diffusible hydrogen in the low-temperature region of the base steel plate increases, and it becomes difficult to achieve the desired local ductility. In addition, during punching, retained austenite transforms into martensite, increasing the generation of voids during hole expansion, decreasing λ, and potentially reducing bendability. The area ratio of retained austenite at 1 / 4 of the thickness of the base steel plate is preferably as small as possible, preferably 7% or less, more preferably 5% or less. The lower limit of the area ratio of retained austenite is not particularly limited, and even if it is 0%, if the steel structure contains ferrite, the desired properties can be obtained.

[0045] Here, the method for measuring the area ratio of retained austenite at 1 / 4 of the thickness of the base steel plate is as follows. The steel plate is ground so that the position at 1 / 4 of the plate thickness from the steel plate surface (the position corresponding to 1 / 4 of the plate thickness in the depth direction from the steel plate surface) becomes the observation surface, and further polished by 0.1 mm by chemical polishing to obtain a sample. Regarding the measurement surface of the sample, using a Co Kα ray source with an X-ray diffractometer, 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. Obtain 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. The average value of the nine intensity ratios is taken as the volume fraction of austenite from retained austenite. Then, regarding this volume fraction of austenite as being three-dimensionally uniform, it is taken as the area fraction of retained austenite at the 1 / 4 position of the plate thickness 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. The total of the area fractions of ferrite and retained austenite is preferably 2% or more, more preferably 5% or more, and even more preferably 10% or more.

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

[0048] The area fraction 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 fraction 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] [The area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm is 5% or more, and is 80% or less of the area ratio of martensite at the 1 / 4 plate thickness position of the base steel plate] In the present invention, the area ratio of martensite in the surface layer portion of the base steel plate is an extremely important configuration. By reducing the area ratio of martensite in the surface layer portion of the base steel plate, in other words, by increasing the area ratio of phases with low hydrogen solubility such as ferrite, bainitic ferrite, and bainite in the surface layer portion of the base steel plate, the desired bendability can be realized. Further, by increasing the area ratio of phases with low hydrogen solubility in the surface layer portion of the base steel plate, it also contributes to the reduction of the amount of diffusible hydrogen in the low temperature range of the base steel plate described later. To obtain such effects, the area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm is set to 80% or less of the area ratio of martensite at the 1 / 4 plate thickness position of the base steel plate. The area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm is preferably 75% or less of the area ratio of martensite at the 1 / 4 plate thickness position of the base steel plate, more preferably 70% or less. From the viewpoint of realizing the desired TS, the area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm 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 plate 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 plate becomes 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 plate to a depth of 10 μm is set as the observation position, and it is observed by SEM at a magnification of 3000 times in three fields of view (40 μm × 10 μm. 10 μm corresponds to the depth). From the obtained tissue images, using Adobe Photoshop of Adobe Systems, the area of martensite is calculated for three fields of view. Next, the area of martensite calculated for each field of view is divided by the area of each field of view range, and the arithmetic mean value of these values is taken as the area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm. In the above SEM image, martensite exhibits a white tissue.

[0051] The area ratio of the tissue 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, still more preferably 30% or more, and is preferably 95% or less, more preferably 85% or less, still more preferably 70% or less.

[0052] The tissue other than martensite in the region from the surface of the base steel plate 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 plate to a depth of 10 μm is preferably 5% or more, more preferably 15% or more, still more preferably 30% or more, preferably 80% or less, more preferably 75% or less, preferably 70% or less.

[0053] However, if the retained austenite has an area ratio of 5% or less, it may be included, or it may be 0%. Also, if the area ratio is 5% or less, other tissues known as steel plate tissues, such as bainite, pearlite, cementite, and metastable carbides (epsilon (ε) carbide, eta (η) carbide, chi (χ) carbide, etc.) may be included. Identification of the tissue can be performed, for example, by observation using SEM (Scanning Electron Microscope). The area ratio other than martensite is calculated by the following formula. [Area ratio (%)] = 100 - [Area ratio of martensite (%)]

[0054] [1-1-3] Diffusible hydrogen content in the base steel sheet at low temperature The diffusible hydrogen content in the base steel sheet is an extremely important invention constituent element. That is, the inventors have intensively studied to obtain a hot-dip galvanized steel sheet with a TS of 980 MPa or more, having high ductility, high elongation flangeability and bendability, and high local ductility. As a result, it has been found that the diffusible hydrogen content in the base steel sheet, that is, the amount of hydrogen released from the base steel sheet when the base steel sheet is heated from room temperature to 50 °C, greatly affects the above characteristics, particularly the local ductility. According to the findings obtained by the inventors, the local ductility depends greatly on the amount of hydrogen released from the base steel sheet in the low temperature range, specifically, the amount of hydrogen released in the temperature range from room temperature to 50 °C (diffusible hydrogen content in the low temperature range), rather than the amount of hydrogen released from the base steel sheet in the high temperature range when the base steel sheet is heated. When it exceeds 0.015 mass ppm, it is difficult to have high local ductility while having high ductility, high elongation flangeability and bendability. Therefore, the diffusible hydrogen content in the base steel sheet should be 0.015 mass ppm or less. The smaller the diffusible hydrogen content in the base steel sheet, the more preferable it is. Preferably, it is 0.010 mass ppm or less, more preferably 0.006 mass ppm or less, and it may be 0 ppm.

[0055] Here, the measurement method of the diffusible hydrogen content in the base steel sheet is as follows. A test piece with a length of 30 mm and a width of 5 mm is sampled by shearing from the central position of the hot-dip galvanized steel sheet sample. After sampling, immediately immerse the test piece in liquid nitrogen. While controlling the temperature of the treatment liquid so that the surface temperature of the test piece becomes 10 °C or less, remove the hot-dip galvanized layer of the test piece with alkali. Next, load the test piece into a temperature-programmed desorption analyzer, wait for 5 minutes in a state where Ar gas is flowing, and then start heating. Specifically, the test piece is heated under the measurement conditions of a temperature rise arrival temperature: 300 °C and a temperature rise rate: 200 °C / hr, and then cooled to room temperature. The surface temperature of the test piece at the start of heating should be 10 °C or less. Measure the amount of hydrogen released from the test piece in the temperature range from the temperature at the start of heating (room temperature) obtained here to 50°C (hereinafter also referred to as the cumulative hydrogen release amount), and calculate the diffusible hydrogen amount in the low temperature range of the base steel plate according to the following formula. [Diffusible hydrogen amount in the low temperature range of the base steel plate (mass ppm)] = [Cumulative hydrogen release amount (g)] ÷ [Mass of test piece (g)] × 10 6

[0056] For steel plates subjected to processing such as punching, stretch flange forming, and bending of hot-dip galvanized steel plates, and products (members) manufactured by welding the processed steel plates, the diffusible hydrogen amount in the low temperature range of the base steel plate 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] Surface soft layer The surface layer of the base steel plate is preferably a soft layer (surface soft layer). Since the surface soft layer contributes to suppressing the progress of bending cracks during press forming and vehicle body collision, the bending fracture resistance characteristics can be further improved.

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

[0060] When having a surface soft layer, when measuring the nano-hardness at 300 points or more in a 50 μm × 50 μm region of the plate surface at the 1 / 4 position of the thickness direction of the surface soft layer from the surface of the base steel plate (the 1 / 4 position of the thickness of the surface soft layer in the depth direction from the surface of the base steel plate), the ratio of the nano-hardness of 7.0 GPa or more is preferably 0.10 or less. When the ratio of the nano-hardness of 7.0 GPa or more is 0.10 or less, it means that the ratio of hard structures (such as martensite), inclusions, etc. is small, and it becomes possible to further suppress the generation and connection of voids during press forming and collision, as well as the propagation of cracks, and it is possible to easily obtain excellent bendability during press forming and excellent bend fracture characteristics during collision.

[0061] In the present invention, in order to obtain excellent bendability during press forming and excellent bend fracture characteristics during collision, the standard deviation σ of the nano-hardness of the plate surface at the 1 / 4 position in the plate thickness direction from the surface of the base metal steel plate of the surface soft layer is 1.8 GPa or less, and further, the standard deviation σ of the nano-hardness of the plate surface at the 1 / 2 position in the plate thickness direction from the surface of the base metal steel plate of the surface soft layer is preferably 2.2 GPa or less. When the standard deviation σ of the nano-hardness of the plate surface at the 1 / 4 position in the plate thickness direction from the surface of the base metal steel plate of the surface soft layer is 1.8 GPa or less, and further, the standard deviation σ of the nano-hardness of the plate surface at the 1 / 2 position in the plate thickness direction from the surface of the base metal steel plate of the surface soft layer is 2.2 GPa or less, it means that the tissue hardness difference in the micro region is small, and it is possible to further suppress the generation, connection, and crack propagation of voids during press forming and collision, and excellent bendability and bend fracture characteristics during collision can be easily obtained.

[0062] Also, a more preferable range of the standard deviation σ of the nano-hardness of the plate surface at the 1 / 4 position in the plate thickness direction from the surface of the base metal steel plate of the surface soft layer is 1.7 GPa or less. A more preferable range of the standard deviation σ of the nano-hardness of the plate surface at the 1 / 2 position in the plate thickness direction from the surface of the base metal steel plate of the surface soft layer is 2.1 GPa or less.

[0063] Here, the nano-hardness of the plate surface at the 1 / 4 position and 1 / 2 position in the plate thickness direction is the hardness measured by the following method. First, when a plating layer is formed, after peeling the plating layer, mechanical polishing is performed from the surface of the base metal steel plate to the 1 / 4 position in the plate thickness direction of the surface soft layer, buff polishing with diamond and alumina is performed, and further colloidal silica polishing is performed. The nano-hardness is measured under the conditions of a load of 500 μN, a measurement area of 50 μm × 50 μm, and a dot interval of 2 μm using a Berkovich-shaped diamond indenter. Also, mechanical polishing is performed to the 1 / 2 position in the plate thickness direction of the surface soft layer, buff polishing with diamond and alumina is performed, and further colloidal silica polishing is performed. Then, the nano-hardness is measured under the conditions of a load of 500 μN, a measurement area of 50 μm × 50 μm, and a dot interval of 2 μm using a Berkovich-shaped diamond indenter.

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

[0065] [1-2] Metal plating layer The base steel plate preferably has a metal plating layer on one side or both sides. Since the metal plating layer contributes to suppressing the occurrence of bending cracks during press forming and vehicle body collision, the bending fracture resistance characteristics can be further improved.

[0066] The metal plating layer is directly formed on the surface of the base steel plate, and is a metal plating layer containing in total more than 50 mass% of one or more selected from Al, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi, excluding zinc plating layers such as hot-dip galvanized layer, alloyed hot-dip galvanized layer, and electro-galvanized layer. The first plating layer is preferably a metal electroplating layer, and hereinafter, the metal electroplating layer will be described as an example.

[0067] When the metal electroplating layer is formed on the steel plate surface, the outermost metal electroplating layer contributes to suppressing the occurrence of bending cracks during press forming and vehicle body collision, so the bending fracture resistance characteristics are further improved.

[0068] As the metal species of the metal electroplating layer, 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 may be used, but Fe is more preferable. Hereinafter, the Fe-based electroplating layer will be described as an example.

[0069] The deposition amount of the Fe-based electroplated layer is 0 g / m 2 or more, preferably 2.0 g / m 2 or more. The upper limit of the deposition amount per side of the Fe-based electroplated layer is not particularly limited, but from the perspective of cost, the deposition amount per side of the Fe-based electroplated layer is set to 60 g / m 2 or less. It is preferable that the deposition amount of the Fe-based electroplated layer is preferably 50 g / m 2 or less, more preferably 40 g / m 2 or less, and still more preferably 30 g / m 2 or less.

[0070] The deposition amount of the Fe-based electroplated layer is measured as follows. A sample with a size of 10×15 mm is taken from the Fe-based electroplated steel sheet and embedded in resin to obtain a cross-sectionally embedded sample. Any three places on the same cross-section are observed at an acceleration voltage of 15 kV using a scanning electron microscope (SEM) at magnifications of 2000 to 10000 times according to the thickness of the Fe-based plating layer, and the average value of the thickness of three fields of view is multiplied by the specific gravity of iron to convert it to the deposition amount per side of the Fe-based plating layer.

[0071] As the Fe-based electroplated layer, in addition to pure Fe, alloy plating layers such as Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, and Fe-W alloy can be used. The component composition of the Fe-based electroplated layer is not particularly limited, but it preferably has a component composition containing 1 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 balance being Fe and unavoidable impurities. By setting the total amount of elements other than Fe to 10% by mass or less, a decrease in electrolysis efficiency can be prevented, and an Fe-based electroplated layer can be formed at low cost. In the case of an Fe-C alloy, the content of C is preferably 0.08% by mass or less.

[0072] The base steel plate for hot-dip galvanizing may be only the base metal steel plate or a base metal steel plate provided with a metal plating layer, and preferably, it is a base metal steel plate having a metal plating layer formed on the surface of a base metal steel plate having a surface soft layer.

[0073] [1-3] Hot-dip galvanized layer The hot-dip galvanized layer in the hot-dip galvanized steel plate will be described. The hot-dip galvanized layer here shall include an alloyed hot-dip galvanized layer (a plating layer obtained by subjecting hot-dip galvanizing to an alloying treatment). Further, the hot-dip galvanized layer can be provided on both surfaces of the surface of the base metal steel plate. In that case, the hot-dip galvanized layer may be directly formed on the surface of the base metal steel plate, or when the base metal steel plate is provided with a surface soft layer, a metal electroplating layer, etc. on the surface, it may be formed on those layers.

[0074] The hot-dip galvanized layer usually has Zn (zinc) as the main component (Zn content is 50.0 mass% or more). The composition is not particularly limited and can be a known composition. The hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% or more and 1.0 mass% or less of Al. Further, the hot-dip galvanized layer may optionally contain a total of more than 0.0 mass% and 3.5 mass% or less of 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. Further, the Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The balance other than the above elements is inevitable impurities. Also, the alloyed hot-dip galvanized layer is preferably composed of, for example, Fe of 20 mass% or less and Al of 0.001 mass% or more and 1.0 mass% or less. Further, the alloyed hot-dip galvanized layer may optionally contain, in total, more than 0 mass% and 3.5 mass% or less of 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. The Fe content of the alloyed hot-dip galvanized layer is more preferably 7.0 mass% or more, and even more preferably 8.0 mass% or more. Also, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15.0 mass% or less, and even more preferably 12.0 mass% or less. Note that the balance other than the above elements is inevitable impurities.

[0075] The plating adhesion amount per one side of the hot-dip galvanized layer is not particularly limited, and can be, for example, 20 g / m 2 or more and 80 g / m 2 or less.

[0076] The plating adhesion amount of the above galvanized layer is measured as follows. A treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe ("Ibit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass% hydrochloric acid aqueous solution. Next, a sample of a steel sheet provided with a galvanized layer is immersed in the treatment solution to dissolve the galvanized layer. Then, the mass reduction amount of the sample before and after dissolution is measured, and the value is divided by the surface area of the base steel sheet (the surface area of the portion covered by plating) to calculate the plating adhesion amount (g / m 2 ).

[0077] The hot-dip galvanized layer preferably has cracks. By intentionally imparting cracks to the hot-dip galvanized layer, it is possible to further reduce the amount of diffusible hydrogen in the base steel sheet in the low temperature range. Here, the presence or absence of cracks in the hot-dip galvanized layer is determined as follows. The surface (front and back surfaces) of the hot-dip galvanized layer of the hot-dip galvanized steel sheet is observed by SEM at a magnification of 3000 times, with two fields of view (one field of view: 30 μm × 40 μm) for each surface, for a total of four fields of view. If there is one or more cracks penetrating the hot-dip galvanized layer in any of the above four fields of view, it is determined that there are cracks. Also, if there are no cracks penetrating the hot-dip galvanized layer in all of the above four fields of view, it is determined that there are no cracks.

[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 TS of the hot-dip galvanized steel sheet of the present invention is 980 MPa or more. The method for measuring TS conforms to JIS Z 2241 and is as described in the examples below.

[0080] The hot-dip galvanized steel sheet of the present invention can have a limiting drawing ratio of 0.10 or more. The local ductility and stretch flangeability of the hot-dip galvanized steel sheet greatly depend on the limiting drawing ratio of the annealed sheet after cold rolling. Making the limiting drawing ratio of the hot-dip galvanized steel sheet 0.10 or more is effective in terms of having high ductility, high stretch flangeability and bendability while having high local ductility. The higher the limiting drawing ratio, the more preferable it is, preferably 0.30 or more, more preferably 0.50 or more. The upper limit is not limited, but is usually 2.10 or less.

[0081] Here, the limiting drawing ratio (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". Specifically, the limiting drawing ratio (ε l ) uses the following formula. ε w =ln(W / W0) ε t =ln(T / T0) ε l =-(ε w +ε t ) In the formula, W0 is the initial plate width, W is the plate width of the fracture part, T is the initial plate thickness, and T0 is the plate thickness of the fracture part. The plate width and plate thickness of the fracture part are as described in the following examples according to the measurement method conforming to JIS Z 2241.

[0082] [2] Method for manufacturing hot-dip galvanized steel sheet The method for manufacturing the hot-dip galvanized steel sheet of the present invention will be described. The method for manufacturing the hot-dip galvanized steel sheet of the present invention is also the method for manufacturing the hot-dip galvanized steel sheet of the present invention described above.

[0083] The method for manufacturing the hot-dip galvanized steel sheet of the present invention is For the steel slab having the above component composition, Hot rolling is performed under the conditions of a coiling temperature of 400°C or higher and 700°C or lower to obtain a hot-rolled steel sheet, Next, pickling is performed on the hot-rolled steel sheet, Next, on the hot-rolled steel sheet, The cumulative reduction rate is 30% or more, The unit tension between the final pass and the pass immediately before the final pass is 10 kgf / mm 2 Cold rolling is performed under the above conditions to obtain a cold-rolled steel sheet, Next, the cold-rolled steel sheet is The oxygen concentration in the temperature range of 250°C or higher and 700°C or lower is set to 0.5% by volume or more and 5.0% by volume or less, and it is heated to an annealing temperature T1 of 750°C or higher and 950°C or lower, The dew point in the temperature range exceeding 700°C and T1 or lower is -30°C or higher, and annealing is performed under the condition of staying in this temperature range for 10 s or more and 500 s or less, Next, hot-dip galvanizing treatment is performed on the annealed cold-rolled steel sheet to obtain a galvanized steel sheet, Next, cooling the galvanized steel sheet to obtain a hot-dip galvanized steel sheet. Unless otherwise specified, the above temperatures are all based on the surface temperature of the steel slab or steel sheet.

[0084] [2-1] Hot rolling process The steel slab (slab) is hot-rolled to obtain a hot-rolled steel sheet. The melting method of the steel slab is not particularly limited, and any known melting method such as a converter or an electric furnace is applicable. Further, in order to prevent macrosegregation, the steel slab is preferably manufactured by a continuous casting method, but it can also be manufactured by an ingot casting method, a thin slab casting method, or the like. After manufacturing the steel slab, in addition to the conventional method of once cooling to room temperature and then reheating, energy-saving processes such as direct rolling and direct hot rolling can be applied without problems. Direct rolling is a process of charging the hot slab directly into the heating furnace without cooling to room temperature. Direct hot rolling is a process of immediately rolling after performing slight heat retention.

[0085] When heating the steel slab, from the viewpoints of carbide dissolution and reduction of rolling load, the slab heating temperature is preferably 1100 °C or higher. Further, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300 °C or lower. Here, the slab heating temperature is the temperature of the slab surface.

[0086] Next, the slab is hot-rolled. The 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, finish rolling can be performed on the sheet bar. When the slab heating temperature is lowered, from the viewpoint of preventing troubles during rolling, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling. The finish rolling temperature is preferably above the Ar3 transformation point. If it is above the Ar3 transformation point, an increase in rolling load and an increase in the reduction ratio in the unrecrystallized state of austenite can be avoided. As a result, the development of abnormal structures elongated in the rolling direction can be suppressed, and a decrease in the workability of the steel sheet obtained after annealing can be easily avoided.

[0087] The Ar3 transformation point is obtained by the following formula. Ar3(°C) = 868 - 396×[%C] + 24.6×[%Si] - 68.1×[%Mn] - 36.1×[%Ni] - 20.7×[%Cu] - 24.8×[%Cr] In the above formula, [%element symbol] represents the content (mass %) of the element in the above component composition.

[0088] The finish rolling may be continuously performed by joining sheet bars together. Also, the sheet bars may be wound up once before the finish rolling. Further, in order to reduce the rolling load, part or all of the finish rolling may be lubricated rolling. Performing lubricated rolling is also effective from the viewpoints of uniformizing the shape of the steel sheet and the material. The friction coefficient during lubricated rolling is preferably in the range of 0.10 or more and 0.25 or less.

[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 the finish rolling, the hot rolled sheet is wound up and recovered, and then cooled. At that time, by setting the coiling temperature to 400°C or higher, C diffuses into the oxide scale generated during rolling, that is, decarburization of the surface layer portion of the hot rolled steel sheet is promoted, and the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm can be controlled within a desired range. When the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm decreases, the amount of diffusible hydrogen in the low temperature range 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. However, when the coiling temperature exceeds 700°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 it becomes difficult to realize the 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 winding are not particularly limited, and known conditions can be adopted. 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. By pickling, the oxides on the surface of the steel sheet can be removed, ensuring good chemical conversion treatment properties and plating quality. Pickling may be performed only once or may be performed in multiple steps. The pickling conditions are not particularly limited, and known conditions can be applied.

[0092] [2-3] Heat treatment process (optional) After pickling, heat treatment (hot rolled sheet annealing) may be performed on the hot rolled sheet. The heat treatment conditions are not particularly limited, and the following conditions can be mentioned.

[0093] [Heat treatment temperature: 450 °C or more and 650 °C or less (preferred conditions)] By subjecting the hot rolled steel sheet to heat treatment, decarburization of the surface layer of the hot rolled steel sheet is promoted, and the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm can be controlled within a more suitable range. Therefore, the heat treatment temperature is preferably 450 °C or more. On the other hand, when 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 the TS decreases. Therefore, the heat treatment temperature is preferably 450 °C or more and 650 °C or less. The heat treatment temperature is more preferably 460 °C or more, even more preferably 470 °C or more, and also more preferably 600 °C or less, even more preferably 550 °C or less.

[0094] [Residence time in the temperature range of 400 °C or more and the heat treatment temperature or less (hereinafter also referred to as the residence time in the heat treatment temperature range): 10 minutes or more (preferred conditions)] By setting the residence time in the temperature range of 400 °C or higher and below the heat treatment temperature (hereinafter also referred to as "residence 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 the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm can be controlled within a more suitable range. Therefore, the residence 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. The upper limit of the residence time in the heat treatment temperature range is not particularly limited, but 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 more suitable range, it is preferably 3000 minutes or less, more preferably 2000 minutes or less.

[0095] [2-4] Cold rolling process Next, cold rolling is performed on the hot-rolled steel sheet to obtain a cold-rolled steel sheet. In the manufacturing method of the present invention, it is important to satisfy the following conditions at that time.

[0096] [Cumulative reduction ratio of cold rolling: 30% or more] By increasing the cumulative reduction ratio of cold rolling, decarburization of the surface layer during annealing is promoted, and the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm can be controlled within a desired range. By decreasing the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm, the amount of diffusible hydrogen in the low-temperature range 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. Therefore, the cumulative reduction ratio of cold rolling is 30% or more. The cumulative reduction ratio of cold rolling is preferably 35% or more, more preferably 40% or more. The upper limit of the cumulative reduction ratio of cold rolling is not particularly limited, but due to production technology constraints, it is preferably 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 or more] 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. As a result, decarburization of the surface layer is promoted during annealing, and the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm can be controlled within a desired range. When the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm decreases, the amount of diffusible hydrogen in the low-temperature range 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. 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 or more. 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 due to production technology constraints, it is preferably 100 kgf / mm 2 or less, more preferably 50 kgf / mm 2 or less.

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

[0099] [2-5] Metal plating treatment process (optional) A pre-annealing metal-plated steel sheet having a metal plating layer (pre-annealing metal plating layer) formed on at least one side may be obtained by subjecting the surface of the cold-rolled steel sheet obtained as described above to a metal plating treatment. The pre-annealing metal-plated steel sheet is preferably a pre-annealing electroplated steel sheet provided with a pre-annealing electroplated metal layer.

[0100] The method of electroplating the metal is not particularly limited. However, as described above, since it is preferable that the metal plating layer formed on the base steel sheet is an electroplated metal layer, it is preferable to perform electroplating treatment. For example, in an Fe-based electroplating bath, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both can be applied. Further, the adhesion amount of the electroplated metal layer before annealing can be adjusted by the energization time or the like. Note that the electroplated steel sheet before annealing means that the electroplated metal layer has not undergone the annealing process, and does not exclude the pre-annealed state of the hot-rolled steel sheet, the pickled sheet after hot rolling, or the cold-rolled steel sheet before the electroplating treatment.

[0101] Here, as the metal species of the electroplated layer, 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 may be used, but since Fe is more preferable, the manufacturing method of Fe-based electroplating will be described below.

[0102] The Fe ion content in the Fe-based electroplating bath before the start of energization is preferably 0.5 mol / L or more in terms of Fe. 2+ If the Fe ion content in the Fe-based electroplating bath is 0.5 mol / L or more in terms of Fe, a sufficient Fe adhesion amount can be obtained. Further, in order to obtain a sufficient Fe adhesion amount, the Fe ion content in the Fe-based electroplating bath before the start of energization is preferably 2.0 mol / L or less. 2+ In addition to Fe ions, the Fe-based electroplating bath can 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 such that the total content of these elements in the Fe-based electroplating layer before annealing is 10% by mass or less. The metal elements may be contained as metal ions, and the non-metal elements can be contained as a part of boric acid, phosphoric acid, nitric acid, organic acids, etc. Further, the ferrous sulfate plating solution may contain conductivity aids such as sodium sulfate and potassium sulfate, a chelating agent, and a pH buffer.

[0103] Other conditions of the Fe-based electroplating bath are not particularly limited. From the viewpoint of constant temperature holding property, the temperature of the Fe-based electroplating solution is preferably 30°C or higher, and preferably 85°C or lower. The pH of the Fe-based electroplating bath is 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 preferably 3.0 or lower from the viewpoint of the electrical conductivity of the Fe-based electroplating bath. The current density is preferably 10 A / dm 2 or higher from the viewpoint of productivity, and preferably 150 A / dm 2 or lower from the viewpoint of facilitating the control of the deposition amount of the Fe-based electroplating layer. The plating speed is preferably 5 mpm or higher from the viewpoint of productivity, and preferably 150 mpm or lower from the viewpoint of stably controlling the deposition amount.

[0104] As pretreatment before the Fe-based electroplating treatment, degreasing treatment and water washing for cleaning the surface of the cold-rolled steel sheet, and further pickling treatment and water washing for activating the surface of the cold-rolled steel sheet can be performed. Subsequently, the Fe-based electroplating treatment is carried out after these pretreatments. The methods of degreasing treatment and water washing are not particularly limited, and ordinary methods can be used. In pickling, various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used. Among them, sulfuric acid, hydrochloric acid, and mixtures thereof are preferred. The concentration of the acid is not particularly limited, but from the viewpoints of the removal ability of the oxide film and prevention of surface roughness (surface defects) due to over-pickling, it is preferably 1% by mass or more and 20% by mass or less. In addition, the pickling treatment solution may contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, and the like.

[0105] [2-6] Annealing process The cold-rolled steel sheet obtained as described above is annealed. At this time, it is important to satisfy the following conditions. That is, the oxygen concentration in the temperature range of 250°C or more and 700°C or less is 0.5% by volume or more and 5.0% by volume or less, heated to an annealing temperature T1 of 750°C or more and 900°C or less, the dew point in the temperature range of more than 700°C and T1 or less is -30°C or more, and retained in this temperature range for a residence time of 10 s or more and 500 s or less.

[0106] [Oxygen concentration in the temperature range of 250°C or more and 700°C or less: 0.5% by volume or more and 5.0% by volume or less] By increasing the oxygen concentration in the heating temperature range, decarburization proceeds through oxygen in the atmosphere, 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 desired range. When the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm decreases, the amount of diffusible hydrogen in the low-temperature range 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 such effects, the oxygen concentration in the heating temperature range is set to 0.5% by volume or more. On the other hand, when the oxygen concentration in the heating temperature range exceeds 5.0% by volume, the area ratio of martensite in the region from the surface of the base steel sheet to a depth of 10 μm decreases, and it becomes difficult to realize the desired TS. Therefore, the oxygen concentration in the heating temperature range is 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% by volume or more, more preferably 1.5% by volume or more, and is preferably 4.5% by volume or less, more preferably 4.0% by volume 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 T1: 750°C or higher and 950°C or lower] When the annealing temperature is less than 750°C, the fraction of austenite during annealing cannot be sufficiently ensured. As a result, the area ratio of martensite decreases, and it becomes difficult to achieve the desired TS. On the other hand, when the annealing temperature exceeds 950°C, annealing occurs in the austenite single-phase region. When the surface structure during annealing is austenite single-phase, decarburization does not progress, and it becomes difficult 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 the desired range. Also, it becomes difficult to contain ferrite and retained austenite. Therefore, the annealing temperature T1 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 process.

[0108] The holding time in the annealing temperature range (hereinafter also referred to as the annealing time) is not particularly limited, but from the viewpoint of controlling the area ratios of ferrite and martensite in the base steel sheet within a predetermined range, it is preferably 10 s or longer and 600 s or shorter. Also, the temperature during holding does not necessarily have to be constant at all times.

[0109] [Residence time in the temperature range above 700°C and below T1: 10 s or longer and 500 s or shorter] By increasing the residence time in the temperature range above 700°C and below T1, decarburization proceeds through oxygen in the atmosphere, and it becomes possible to control the area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm within a desired range. When the area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm decreases, the amount of diffusible hydrogen in the low-temperature range of the base steel plate also decreases. As a result, it becomes possible to obtain high ultimate deformability, high local ductility, stretch flangeability, and bendability. To obtain such effects, the residence time in the temperature range above 700°C and below T1 is set to 10 s or more. On the other hand, when the residence time in the temperature range above 700°C and below T1 exceeds 500 s, the area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm decreases, making it difficult to achieve the desired TS. Therefore, the residence time in the temperature range above 700°C and below T1 is set to 10 s or more and 500 s or less. The residence time in the temperature range above 700°C and below T1 is preferably 15 s or more, more preferably 20 s or more. The residence time in the temperature range above 700°C and below T1 is preferably 400 s or less, more preferably 300 s or less. Here, the temperature in the heating temperature range is based on the surface temperature of the steel plate.

[0110] [Dew point in the temperature range above 700°C and below T1: -30°C or higher] By increasing the dew point in the temperature range above 700°C and below T1, decarburization proceeds through oxygen in the atmosphere, and it becomes possible to control the area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm within a desired range. When the area ratio of martensite in the region from the surface of the base steel plate to a depth of 10 μm decreases, the amount of diffusible hydrogen in the low-temperature range of the base steel plate also decreases. As a result, it becomes possible to obtain high ultimate deformability, high local ductility, stretch flangeability, and bendability. To obtain such effects, the dew point in the annealing temperature range is set to -30°C or higher. The upper limit of the dew point in the annealing temperature range is not particularly limited, but from the viewpoint of ensuring the 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 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 to a predetermined range, it is preferably 2 to 30 volume ppm. Also, the temperature during heat retention does not necessarily have to be constant at all times.

[0112] After the above annealing, the cold-rolled steel sheet is cooled. The conditions at this time are not particularly limited, and known conditions can be adopted. For example, the average cooling rate in the temperature range of 500°C or higher and T1 or lower is not particularly limited, but from the viewpoint of controlling the area ratio of ferrite and martensite in the base steel sheet to a predetermined range, it is preferably 5°C / s or higher and 50°C / s or lower.

[0113] [2-7] Hot-dip galvanizing process Next, the cold-rolled steel sheet is subjected to a hot-dip galvanizing process. An alloying process may be performed after the hot-dip galvanizing process.

[0114] The hot-dip galvanizing process can be performed in the temperature range of 400°C or higher and T1 or lower. The cold-rolled steel sheet may be once cooled to less than 400°C and then the steel sheet temperature may be raised again to 400°C or higher before performing the process.

[0115] Annealing, cooling, and plating processes may be continuously performed 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 to the molten zinc plating bath through a snout with its tip immersed in the molten zinc plating bath. The time from the end of cooling of the cold-rolled steel sheet to its entry into the molten zinc plating bath is not particularly limited, but from the viewpoint of controlling the area ratio of ferrite and martensite within a predetermined range, it is preferably 300 s or less. Note that immediately before the connection part between the cooling zone and the snout, a roll is provided to change the traveling direction of the cold-rolled steel sheet and make it enter the snout. The cold-rolled steel sheet enters the snout after passing through the roll. Next, the cold-rolled steel sheet guided to the molten zinc plating bath through the snout is immersed in the molten zinc plating bath to perform a molten zinc plating process, resulting in a plated steel sheet.

[0116] In the molten zinc plating process, for example, the cold-rolled steel sheet is immersed in a molten zinc plating bath at 440°C or higher and 500°C or lower. Also, it is preferable to use a molten zinc plating bath having a composition in which the Al content is 0.10 mass% or more and 0.23 mass% or less, and the balance is Zn and unavoidable impurities.

[0117] Also, after the molten zinc plating process as described above, an alloying process may be performed in a temperature range of 460°C or higher and 600°C or lower. If the alloying process temperature is less than 460°C, the Zn-Fe alloying rate becomes excessively slow, resulting in a decrease in productivity. On the other hand, if the alloying process temperature exceeds 600°C, untransformed austenite transforms into pearlite, and TS and El may decrease. Therefore, the alloying process temperature is preferably 460°C or higher and 600°C or lower. The alloying process temperature is preferably 470°C or higher. Also, the alloying process temperature is preferably 560°C or lower.

[0118] Also, the plating adhesion amount is 20 - 80 g / m per side 2Double-sided plating is preferred. Also, for galvannealed steel sheet (GA), it is preferable to perform the following alloying treatment to make the Fe concentration in the plating layer 7 to 15% by mass. Note that the plating adhesion amount can be adjusted by performing gas wiping or the like after the hot-dip galvanizing treatment.

[0119] [2-8] Cooling process After the hot-dip galvanizing treatment process, that is, the plated steel sheet subjected to the hot-dip galvanizing treatment or the hot-dip galvanizing treatment and the alloying treatment is cooled. The cooling conditions are not particularly limited, and known conditions can be adopted. For example, after the hot-dip galvanizing treatment or the alloying treatment is completed, the average cooling rate to the holding temperature or the cooling stop temperature described later, and the cooling rate after the holding or the reheating described later are not particularly limited, but from the viewpoint of further improving TS, it is preferably 2°C / s or more, more preferably 5°C / s or more. Also, 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] [Holding temperature during cooling of the plated steel sheet: 100°C or more and 450°C or less (preferred conditions)] During the cooling of the plated steel sheet, it may be held in the temperature range of 100°C or more and 450°C or less for 5 s or more, and then cooled. By holding during the 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 range of the base steel sheet is reduced. From this point, the holding temperature during the cooling of the plated steel sheet is preferably 100°C or more, more preferably 130°C or more, still more preferably 150°C or more. On the other hand, when the holding temperature during the cooling of the plated steel sheet exceeds 450°C, the area ratio of martensite decreases and TS decreases. Therefore, the holding temperature during the cooling of the plated steel sheet is preferably 450°C or less, more preferably 400°C or less, still more preferably 350°C or less.

[0121] [Heat preservation time during cooling of the plated steel sheet: 5 s or more (preferred condition)] By performing heat preservation during the 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 range of the base steel sheet decreases. From this point of view, the heat preservation time during the cooling of the plated steel sheet is preferably 5 s or more, more preferably 10 s or more, and even more preferably 15 s or more. The upper limit of the heat preservation time during the cooling of the plated steel sheet is not particularly limited, but from the viewpoint of controlling the area ratios of ferrite and martensite in the base steel sheet within a predetermined range, it is preferably 300 s or less, more preferably 100 s or less.

[0122] [Cooling stop temperature during cooling of the plated steel sheet: 300 °C or less (preferred condition)] During the cooling of the plated steel sheet, after stopping the cooling at 300 °C or less, it may be reheated in a temperature range of (cooling stop temperature + 50 °C) or more and 450 °C or less, then heat-preserved for 5 s or more, and then cooled. By cooling the plated steel sheet to a temperature below the martensite transformation start temperature during cooling and then reheating it, hydrogen desorption from the plated steel sheet is promoted, and the amount of diffusible hydrogen in the low temperature range of the base steel sheet decreases. From this point of view, the cooling stop temperature during the cooling of the plated steel sheet is preferably 300 °C or less, more preferably 250 °C or less, and even more preferably 150 °C or less. The lower limit of the cooling stop temperature during the cooling of the plated steel sheet is not particularly limited, but due to production technology constraints, it is preferably 10 °C or more, more preferably 30 °C or more.

[0123] [Reheating temperature of the plated steel sheet: (cooling stop temperature + 50 °C) or more and 450 °C or less (preferred condition)] By reheating the plated steel sheet after cooling, hydrogen desorption from the plated steel sheet is promoted, and the amount of diffusible hydrogen in the base steel sheet in the low-temperature range decreases. From this point of view, 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. When the reheating temperature of the plated steel sheet exceeds 450°C, the area ratio of martensite decreases and the TS decreases. 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] [Holding time at the reheating temperature of the plated steel sheet: 5 s or more (preferred condition)] After cooling the plated steel sheet and holding it at the reheating temperature, hydrogen desorption from the plated steel sheet is promoted, and the amount of diffusible hydrogen in the base steel sheet in the low-temperature range decreases. From this point of view, the holding time at the reheating temperature of the plated steel sheet is preferably 5 s or more, more preferably 10 s or more, and even more preferably 15 s or more. The upper limit of the holding time at the reheating temperature of the plated steel sheet is not particularly limited, but from the viewpoint of controlling the area ratios of ferrite and martensite in the base steel sheet within a predetermined range, it is preferably 300 s or less, more preferably 100 s or less.

[0125] After the above cooling, it is preferable to perform rolling on the plated steel sheet with an elongation rate of 0.05% or more and 1.00% or less. By performing rolling with an elongation rate of 0.05% or more after cooling, cracks can be introduced into the hot-dip galvanized layer, and as a result, further reduction in the amount of diffusible hydrogen in the base steel sheet in the low-temperature range 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 is performed with an elongation rate exceeding 1.00% after cooling, the area ratio of ferrite having the {001} orientation increases, and the anisotropy of elongation flange cracking may deteriorate. 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 above-mentioned rolling after cooling may be performed (online) on a device continuous with the continuous hot-dip galvanizing apparatus, or may be performed (offline) on a device discontinuous with the continuous hot-dip galvanizing apparatus. Also, the target elongation rate may be achieved by a single rolling, or multiple rollings may be performed to achieve a total elongation rate of 0.05% or more and 1.00% or less. Here, the above-mentioned rolling after cooling generally refers to temper rolling, but as long as an elongation rate equivalent to that of temper rolling can be imparted, a processing method such as tension leveling or repeated bending by rolls may be used. The rolling process may be performed after cooling the galvanized steel sheet to near room temperature, or the rolling process may be performed at the time of stopping the cooling of the galvanized steel sheet, and then a reheat treatment may be performed.

[0127] When the hot-dip galvanized steel sheet becomes a trading object, it is usually cooled to room temperature and then becomes a trading object.

[0128] The manufacturing conditions other than the above are not particularly limited, and known conditions can be adopted.

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

[0130] The hot-dip galvanized steel sheet of the present invention is a hot-dip galvanized steel sheet having a TS of 980 MPa or more, which has high ductility, high elongation flangeability and bendability, and also has high local ductility. 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, the skeletal structure parts or reinforcing parts of an automobile, it is possible to improve the fuel efficiency by reducing the vehicle body weight, and the industrial utility value is extremely large.

Examples

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

[0132] [Test Nos. 1 to 47] A steel slab (steel material) having the component composition shown in Table 1 and the balance consisting of Fe and unavoidable impurities was melted in a converter, and a steel slab was obtained by continuous casting. The obtained steel slab was heated to 1250 °C and rough rolled to obtain a sheet bar. Next, finish rolling was performed on the obtained sheet bar at a finish rolling temperature of 900 °C, and coiling was performed under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. After pickling the obtained hot-rolled steel sheet, cold rolling was performed 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 sheet was annealed under the conditions shown in Table 2. Next, the cold-rolled steel sheet was subjected to a plating treatment of the type shown in Table 2 to obtain a plated steel sheet having a hot-dip zinc plating layer on both sides. In the type of plating treatment in Table 2, the hot-dip zinc plated steel sheet (GI) means that only the hot-dip zinc plating treatment is performed without alloying treatment. Also, GA means that both the hot-dip zinc plating treatment and the alloying treatment have been performed. In the case of GI, as the plating bath, a hot-dip zinc plating bath containing 0.20% by mass of Al and the balance consisting of Zn and unavoidable impurities was used. Also, in the case of GA, a hot-dip zinc plating bath containing 0.14% by mass of Al and the balance consisting of Zn and unavoidable impurities was used. The plating bath temperature was 470 °C in both cases. The plating adhesion amount was about 45 to 72 g / m 2 (double-sided plating) per side in the case of GI, and about 45 g / m 2 (double-sided plating) per side in the case of GA. Also, in the case of GA, the alloying treatment temperature was about 550 °C. The composition of the hot-dip zinc plating layer of GI was Fe: 0.1 to 1.0% by mass, Al: 0.2 to 1.0% by mass, and the balance was Zn and unavoidable impurities. Also, the composition of the (alloyed) hot-dip zinc plating layer of GA was Fe: 7 to 15% by mass, Al: 0.1 to 1.0% by mass, and the balance was Zn and unavoidable impurities.

[0134] Next, some of the plated steel sheets were subjected to heat treatment or post-heat treatment after heat preservation or cooling under the conditions shown in Table 2. For the conditions not specified, the conventional methods were followed. For the hot-dip galvanized steel sheets thus obtained, the structure was identified in the region from the 1 / 4 position of the base steel sheet thickness to a depth of 10 μm from the surface by the method described above, and the amount of diffusible hydrogen in the low-temperature range of the base steel sheet was measured. The results are shown in Table 3. The component composition of the base steel sheet of the obtained steel sheet was substantially the same as the component composition at the steel slab stage. For all the compliant steels, they were within the range of the component composition of the present invention, and for all the comparative steels, they were outside the range of the component composition of the present invention.

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

[0136] [Tensile test] The tensile test was carried out in accordance with JIS Z 2241. That is, from the obtained hot-dip galvanized steel sheet, a JIS No. 5 test piece was taken so that the rolling right angle direction (C direction) of the steel sheet was the longitudinal direction. Next, using the taken test piece, a tensile test was carried out under the condition of a crosshead speed of 1.67×10 -1 mm / s, and YS, TS, El, and L.El were measured. For TS, a value of 980 MPa or more was judged as qualified. Also, from the measured TS and El, the product of TS and El (TS×El) was calculated, and TS×El was obtained. And a value of TS×El of 13000 MPa·% or more was judged as qualified. Furthermore, from the measured TS and L.El, the product of TS and L.El (TS×L.El) was calculated, and TS×L.El was obtained. And a value of TS×L.El of 4500 MPa·% or more was judged as qualified.

[0137] [Ultimate deformation ability] Test pieces were taken in the same manner as above, and a tensile test was carried out to obtain W0: initial plate width, W: plate width at the fracture part, T: initial plate thickness, and T0: plate thickness at the fracture part. The ultimate deformation ability ε was calculated from the following formulal was obtained. ε w = ln(W / W0) ε t = ln(T / T0) ε l = -(ε w + ε t )

[0138] [Hole expansion test] The hole expansion test was carried out in accordance with JIS Z 2256. That is, the obtained hot-dip galvanized steel sheet was sheared into 100 mm × 100 mm, and then a hole with a diameter of 10 mm and a clearance of 12.5% was punched in the sheared steel sheet. Next, using a die with an inner diameter of 75 mm, the steel sheet was pressed with a wrinkling suppression force of 9 tons (88.26 kN), and in this state, a conical punch with an apex angle of 60° was pushed into the hole to measure the hole diameter at the crack generation limit. Then, the (limit) hole expansion rate (%) was obtained by the following formula. (Limit) Hole expansion rate: λ (%) = {(D f - D0) / D0} × 100 Here, D f is the hole diameter (mm) at the time of crack generation, and D0 is the initial hole diameter (mm). The (limit) hole expansion rates of 3 sheared steel types were measured, and the average value was taken as λ. When λ was 30% or more, it was judged that the elongation flange property passed.

[0139] [Bending test] The bending test was conducted 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 sampled such that the direction parallel to the rolling direction (L direction) of the steel sheet was the axial direction of the bending test. Thereafter, a 90° V-bending test was conducted under the conditions of a pressing load of 100 kN and a pressing holding time of 5 seconds. In the present disclosure, the bendability was evaluated by the pass rate of the bending test. At the maximum R (for example, when the sheet thickness is 1.4 mm, the bending radius is 7.0 mm) where the value of R / t obtained by dividing the bending radius (R) by the sheet thickness (t) is 5 or less, a bending test of 5 samples was carried out. Subsequently, the presence or absence of cracks at the ridge line portion of the bending apex was evaluated. When none of the 5 samples cracked, the bendability was judged to be "excellent". In addition, when microcracks less than 200 μm occurred in one or more of the 5 samples, the bendability was judged to be "good". Further, when microcracks of 200 μm or more occurred in one or more of the 5 samples, the bendability was judged to be "poor". Here, the presence or absence of cracks was evaluated by measuring the ridge line portion of the bending apex at a magnification of 40 times using a digital microscope (RH-2000: manufactured by High-Locks Co., Ltd.).

[0140]

Table 1

[0141]

Table 2

[0142]

Table 3

[0143] As shown in Table 3, in all of the invention examples, the TS was 980 MPa or more, and they had high ductility, high elongation flangeability, bendability, and high local ductility. On the other hand, in the comparative examples, at least one of the TS, ductility, elongation flangeability, bendability, and local ductility was not sufficient.

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

[0145] For some of the cold-rolled steel sheets, a metal electroplating treatment process was performed. In the column of the presence or absence (type of plating) of the metal electroplating treatment in Table 4, when it is indicated as "present (Fe)", it is an example in which Fe-based electroplating treatment was performed, and when it is indicated as "(Ni)", it is an example in which Ni-based electroplating treatment was performed. The composition of the metal electroplating layer contained 95 to 100% by mass of Fe in Fe-based electroplating and 95 to 100% by mass of Ni in Ni-based electroplating, and the balance was unavoidable impurities in each case. Next, the cold-rolled steel sheets were subjected to plating treatments (GI, GA) of the types shown in Table 4 to obtain plated steel sheets having a hot-dip galvanized layer on both sides. The conditions for GI and GA were the same as above.

[0146] Next, for some of the plated steel sheets, heat treatment or reheat treatment after cooling was performed under the conditions shown in Table 4. For conditions not specified, the conventional method was followed.

[0147] The component composition of the base steel sheet of the obtained steel sheet was substantially the same as the component composition at the steel slab stage, and all of the compliant steels were within the range of the component composition according to the above-described embodiments.

[0148] For the hot-dip galvanized steel sheets thus obtained, the ultimate deformability, tensile properties, stretch flange formability, bendability, and the amount of diffusible hydrogen in the low-temperature range of bendability were measured. The results are shown in Tables 5 and 6.

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

[0150] [Thickness of the surface soft layer] After smoothing the plate thickness cross-section (L cross-section) parallel to the rolling direction of the base steel plate by wet polishing, using a Vickers hardness tester, at a load of 10 gf, measurements were taken at 1-μm intervals from a position 1 μm in the plate thickness direction from the surface of the base steel plate to a position 100 μm in the plate thickness direction. Thereafter, measurements were taken at 20-μm intervals up to the center of the plate thickness. The region where the hardness decreased to 85% or less compared to the hardness at the 1 / 4 position of the plate thickness was defined as the soft layer (surface soft layer), and the thickness in the plate thickness direction of this region was defined as the thickness of the soft layer.

[0151] [Adhesion amount of the metal plating layer] A sample with a size of 10 mm × 15 mm was taken from the hot-dip galvanized steel plate and embedded in resin to obtain a cross-section embedded sample. Three arbitrary locations on the same cross-section were observed using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV and a magnification of 2000 - 10000 times according to the thickness of the Fe-based plating layer. By multiplying the average value of the thickness of three fields of view by the specific gravity of iron, it was converted to the adhesion amount per side of the Fe-based plating layer.

[0152] [Nano-hardness] The nano-hardness of the surface soft layer of the base steel plate of the obtained hot-dip galvanized steel plate was measured. The results are shown in Table 5. Regarding the 1 / 4 position of the surface soft layer, it is as follows. From the obtained hot-dip galvanized steel plate, after peeling the plating, mechanical polishing, buff polishing with diamond and alumina, and colloidal silica polishing were carried out from the surface of the base steel plate to a depth of 1 / 4 of the plate thickness direction of the surface soft layer. Using a nano-indentation device (tribo-950 from Hysitron), with a Berkovich-shaped diamond indenter, Loading rate and unloading rate: 50 μN / s Maximum load: 500 μN Measurement area: 50 μm × 50 μm Data acquisition pitch: 5 msec Dot interval: 2 μm Under these conditions, a total of 512 nano-hardness measurements were taken. Next, mechanical polishing, buffing with diamond and alumina, and colloidal silica polishing were performed up to the position of 1 / 2 of the depth in the plate thickness direction of the surface soft layer. Using Hysitron's tribo-950, 512 nano-hardness measurements were taken under the same conditions as above with a Berkovich-shaped diamond indenter.

[0153] U-bending + adhesion bending tests, V-bending + orthogonal VDA bending tests, and axial crushing tests were conducted on the hot-dip galvanized steel sheet. In these tests, considering the influence of the plate thickness, all tests were carried out on steel sheets with a plate thickness of 1.2 mm. Steel sheets with a plate thickness exceeding 1.2 mm were ground on one side to make the plate thickness 1.2 mm. Since the bendability of the steel sheet surface may be affected by the grinding process, in the U-bending + adhesion bending test, the ground surface was set as the inner side (valley side) of the bend, in the V-bending + orthogonal VDA bending test, the ground surface was set as the outer side (peak side) of the bend during the V-bending test, and then the ground surface was set as the inner side (valley side) of the bend during the subsequent VDA bending test. On the other hand, in the U-bending + adhesion bending test, V-bending + orthogonal VDA bending test, and axial crushing test of galvanized steel sheets with a plate thickness less than 1.2 mm, since the influence of the plate thickness is small, the tests were conducted without grinding treatment.

[0154] [U-bending + adhesion bending test] The U-bending + adhesion bending test was conducted as follows. Test pieces measuring 60 mm × 30 mm were sampled from the obtained galvanized steel sheet by shearing and end grinding. Here, the 60-mm side is parallel to the width (C) direction. U-bending (primary bending) was performed in the width (C) direction with a radius of curvature / plate thickness of 4.2 around the rolling (L) direction axis to prepare the test pieces. In the U-bending (primary bending), as shown in Fig. 1(a), for the steel sheet placed on roll A1, punch B1 was pushed in to obtain test piece T1. Next, as shown in Fig. 1(b), the test piece T1 placed on the lower die A2 was subjected to adhesion bending (secondary bending) by being crushed with the upper die B2. In Fig. 1(a), D1 indicates the width (C) direction and D2 indicates the rolling (L) direction. A spacer S, which will be described later, was inserted between the test pieces.

[0155] The conditions for U-bending in the U-bending + adhesion bending test are as follows. Test method: Roll support, punch pressing Punch tip R: 5.0 mm Clearance between roll and punch: Plate thickness + 0.1 mm Stroke speed: 10 mm / min Bending direction: Direction perpendicular to rolling (C direction)

[0156] The conditions for the clinching bend in the U-bend + clinching bend test are as follows. Spacer thickness: Varies at a pitch of 0.5 mm Test method: Die support, punch pressing Forming load: 10 ton Test speed: 10 mm / min Holding time: 5 s Bending direction: Direction perpendicular to rolling (C direction)

[0157] The above U-bend + clinching bend test was carried out 3 times, and the limiting spacer thickness (ST) was determined when no cracks occurred in all 3 times. Also, using a Leica stereomicroscope, cracks with a length of 200 μm or more at a magnification of 25 times were judged as cracks. Note that ST is an index for evaluating the fracture resistance characteristics at the time of impact (the fracture resistance characteristics of the vertical wall part in the axial crushing test). The results are shown in Table 6.

[0158] The acceptance criteria for the limiting spacer thickness (ST) are as follows. When 980 MPa ≤ TS < 1180 MPa, 4.5 mm ≥ ST When 1180 MPa ≤ TS < 1320 MPa, 5.0 mm ≥ ST When 1320 MPa ≤ TS < 1470 MPa, 5.5 mm ≥ ST When 1470 MPa ≤ TS, 6.0 mm ≥ ST The rejection criteria for the limiting spacer thickness (ST) are as follows. When 980 MPa ≤ TS < 1180 MPa, 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 < SFmax When 1470 MPa ≤ TS, 6.0 mm < SF max

[0159] [V-bending + Orthogonal VDA Bending Test] The V-bending + Orthogonal VDA bending test is conducted as follows. A test piece of 60 mm × 65 mm was sampled from the obtained hot-dip galvanized steel sheet by shearing and end face grinding. Here, the 60-mm side is parallel to the rolling (L) direction. With a radius of curvature / thickness of 4.2, a 90° bending process (primary bending process) was performed in the rolling (L) direction with the width (C) direction as the axis to prepare the test piece. In the 90° bending process (primary bending process), as shown in Fig. 2(a), the punch B3 was pushed into the steel sheet placed on the die A3 with a V-groove to obtain the test piece T1. Next, as shown in Fig. 2(b), the punch B4 was pushed into the test piece T1 placed on the support roll A4 so that the bending direction was perpendicular to the rolling direction to perform orthogonal bending (secondary bending process). In Fig. 2(a) and Fig. 2(b), D1 indicates the width (C) direction and D2 indicates the rolling (L) direction.

[0160] The conditions for V-bending in the V-bending + Orthogonal VDA bending test are as follows. Test method: Die support, punch pushing Forming load: 10 tons Test speed: 30 mm / min Holding time: 5 s Bending direction: Rolling (L) direction

[0161] The conditions for VDA bending in the V-bending + Orthogonal VDA bending test are as follows. Test method: Roll support, punch pushing Roll diameter: φ30 mm Punch tip R: 0.4 mm Roll distance: (Thickness × 2) + 0.5 mm Stroke speed: 20 mm / min Test piece size: 60 mm × 60 mm Bending direction: Perpendicular to rolling (C) direction

[0162] In the stroke-load curve obtained when the VDA bending is applied, the stroke at the maximum load is determined. 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). Note that SF max serves as an index for evaluating the fracture resistance characteristics during collision (the fracture resistance characteristics of the bending ridge part in the axial crushing test). The results are shown in Table 6.

[0163] SF max The passing criteria for SF are as follows. When 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 SF max The failing criteria for SF are as follows. When 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 Crushing Test] The axial crushing test was carried out as follows. A test piece measuring 150 mm × 100 mm was cut from the obtained hot-dip galvanized steel sheet by shearing. Here, the 150-mm side is 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, it was formed (bent) to a depth of 40 mm to produce the hat-shaped member 10 shown in FIGS. 3(a) and 3(b). Also, the steel sheet used as the material for the hat-shaped member was separately cut into a size of 80 mm × 100 mm. Next, the cut steel sheet 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 sheet 20. FIG. 3(b) is a perspective view of the test member 30. As shown in FIG. 3(b), the position of the spot-welded portion 40 was set such that the distance between the end of the steel sheet and the welded portion was 10 mm and the distance between the welded portions was 20 mm. Next, as shown in FIG. 3(c), the test member 30 was joined to the floor plate 50 by TIG welding to produce a sample for axial crushing test. Next, an impactor 60 was collided with the produced sample for axial crushing test at a constant collision speed of 10 mm / min, and the sample for axial crushing test was crushed by 70 mm. As shown in FIG. 3(c), the crushing direction was set to be parallel to the longitudinal direction of the test member 30. The results are shown in Table 6.

[0165] The criteria for evaluating the presence or absence of axial crushing fracture (appearance crack) are as follows. A (pass): No appearance crack was observed in the sample after the axial crushing test. B (pass): One or fewer appearance cracks were observed in the sample after the axial crushing test C (fail): Two or more appearance cracks were observed in the sample after the axial crushing test

[0166]

Table 4

[0167]

Table 5

[0168]

Table 6

[0169] As shown in Table 6, in all of the invention examples, the TS was 980 MPa or more, and they had high ductility, high stretch flangeability and bendability, and high local ductility. On the other hand, in the comparative examples, at least one of the TS, ductility, stretch flangeability, bendability and local ductility was not sufficient. Furthermore, they were also excellent in fracture resistance characteristics (bending fracture characteristics and axial crushing characteristics) during collision.

[0170] As described above, the embodiments of the present invention have been explained, but the present invention is not limited to the description that forms a part of the disclosure of the present invention according to the above embodiments. That is, all other embodiments, examples and operation techniques made by those skilled in the art based on the above embodiments are included in the scope of the present invention. For example, in the series of heat treatments in the manufacturing method described above, as long as the heat history conditions are satisfied, the equipment for heat treating the steel sheet is not particularly limited.

Industrial Applicability

[0171] According to the present invention, a hot-dip galvanized steel sheet having a TS of 980 MPa or more, high ductility, high stretch flangeability and bendability, and high local ductility can be obtained. In particular, since the hot-dip galvanized steel sheet of the present invention is excellent in various properties, it can be applied to skeletal structure parts of automobiles of various sizes and shapes. Thereby, it is possible to improve the fuel efficiency by reducing the vehicle body weight, and the industrial utility value is extremely large.

Explanation of Reference Numerals

[0172] 10 Hat-shaped member 20 Galvanized steel sheet 30 Test member 40 Spot weld part 50 Floor 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 the surface of the base steel sheet, wherein the base steel sheet, 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 and having a component composition consisting of the balance being Fe and inevitable impurities, at the 1 / 4 thickness position of the base steel sheet, the area ratio of martensite is 30% or more, the area ratio of ferrite is 24% or more and 70% or less, the area ratio of retained austenite is 10% or less, including at least one of ferrite and retained austenite, in the region from the surface of the base steel sheet to a depth of 10 μm, the area ratio of martensite is 5% or more and 80% or less of the area ratio of martensite at the 1 / 4 thickness position of the base steel sheet, and having a steel structure, and the amount of diffusible hydrogen in the low temperature range 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. A hot-dip galvanized steel sheet.

2. The component composition further comprises, 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 Bi: 0.200% or less and contains at least one element selected from the group consisting of. The hot-dip galvanized steel sheet according to claim 1.

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. A hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer on the surface of the base steel sheet, wherein the base steel sheet, 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 comprising a component composition in which the balance consists of Fe and inevitable impurities, at the 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, including at least one of ferrite and retained austenite, in the region from the surface of the base steel plate to a depth of 10 μm, the area ratio of martensite is 5% or more and is 80% or less of the area ratio of martensite at the 1 / 4 position of the plate thickness of the base steel plate, a steel structure, having the amount of diffusible hydrogen in the low temperature range in the base steel plate, which is the amount of hydrogen released when the base steel plate is heated to 50 °C, is 0.015 mass ppm or less, the base steel plate has a surface soft layer which is a region where the Vickers hardness is 85% or less with respect to the Vickers hardness at the 1 / 4 position of the plate thickness of the base steel plate and is within 200 μm in the plate thickness direction from the surface of the base steel plate, when measuring the nano-hardness at 300 points or more in a 50 μm × 50 μm region of the plate surface at each of the 1 / 4 position and the 1 / 2 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate, the ratio of the number of measurements where the nano-hardness of the plate surface at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 7.0 GPa or more is 0.10 or less with respect to the total number of measurements, the standard deviation σ of the nano-hardness of the plate surface at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 1.8 GPa or less, Furthermore, a hot-dip galvanized steel sheet in which the standard deviation σ of the nano-hardness of the plate surface at the 1 / 2 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel plate is 2.2 GPa or less.

5. The component composition further comprises, 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 Bi: 0.200% or less The hot-dip galvanized steel sheet according to claim 4, containing at least one element selected from the group consisting of

6. The base steel sheet has a surface soft layer which is a region where the Vickers hardness is 85% or less with respect to the Vickers hardness at the 1 / 4 position of the plate thickness of the base steel sheet and is a region within 200 μm in the plate thickness direction from the surface of the base steel sheet, When measuring the nano-hardness at 300 points or more in a 50 μm × 50 μm region of the plate surface at each of the 1 / 4 position and the 1 / 2 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel sheet, The measurement ratio of the nano-hardness of the plate surface at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel sheet being 7.0 GPa or more is 0.10 or less with respect to the total number of measurements, The standard deviation σ of the nano-hardness of the plate surface at the 1 / 4 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel sheet is 1.8 GPa or less, Furthermore, the standard deviation σ of the nano-hardness of the plate surface at the 1 / 2 position of the plate thickness direction depth of the surface soft layer from the surface of the base steel sheet is 2.2 GPa or less. The hot-dip galvanized steel sheet according to claim 3.

7. The hot-dip galvanized steel sheet according to claim 1 or 2, 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 according to 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.

9. The hot-dip galvanized steel sheet according to claim 4 or 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. The hot-dip galvanized steel sheet according to claim 6, 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.

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

12. A skeletal structure part or a reinforcing part of an automobile comprising the member according to claim 11.

13. On a steel slab having the component composition according to claim 1 or 2, Hot rolling is performed under the conditions of a coiling temperature of 400 °C or more and 700 °C or less to obtain a hot-rolled steel sheet, Next, pickling is performed on the hot-rolled steel sheet, Next, on the hot-rolled steel sheet, The cumulative reduction ratio is 30% or more, The unit tension between the final pass and the pass immediately before the final pass is 10 kgf / mm ２ Cold rolling is carried out under the above conditions to obtain a cold-rolled steel sheet, Next, the cold-rolled steel sheet is The oxygen concentration in the temperature range of 250 °C or higher and 700 °C or lower is set to 0.5% by volume or higher and 5.0% by volume or lower, and annealed up to an annealing temperature T of 750 °C or higher and 950 °C or lower. １ Heat until Above 700°C １ Anneal under the condition that the dew point in the following temperature range is -30°C or higher and the residence time in this temperature range is 10 s or more and 500 s or less. Next, the annealed cold-rolled steel sheet is subjected to a hot-dip galvanizing treatment to obtain a galvanized steel sheet, Next, a method for manufacturing a hot-dip galvanized steel sheet, which includes cooling the galvanized steel sheet to obtain a hot-dip galvanized steel sheet.

14. The method for manufacturing a high-strength steel sheet according to claim 13, wherein during the cooling of the galvanized steel sheet, heat is retained for 5 s or more in a temperature range of 100°C or higher and 450°C or lower, and then cooled.

15. The method for manufacturing a high-strength steel sheet according to claim 13, wherein during the cooling of the galvanized steel sheet, after stopping the cooling at 300°C or lower, it is reheated to a temperature range of (cooling stop temperature + 50°C) or higher and 450°C or lower, then heat is retained for 5 s or more, and then cooled.

16. The method for manufacturing a hot-dip galvanized steel sheet according to claim 13, wherein after the hot-dip galvanizing treatment, an alloying treatment is performed on the galvanized steel sheet.

17. The method for manufacturing a hot-dip galvanized steel sheet according to claim 13, which includes a metal plating step of performing metal plating on one or both surfaces of the cold-rolled steel sheet before the annealing step to form a metal plating layer.

18. A method for manufacturing a member, which has a step of subjecting the hot-dip galvanized steel sheet according to claim 1 or 2 to at least one of forming processing or joining processing to obtain a member.

Images