Galvanized steel sheets and components, and methods for manufacturing them.

Abstract

Invention details;

Description

Galvanized steel sheets and components, and their manufacturing methods

[0001] The present invention relates to a zinc-plated steel sheet, a member made from the zinc-based plated steel sheet, and a method for manufacturing the same.

[0002] In recent years, improving the fuel efficiency of automobiles has become an important issue from the perspective of protecting the global environment. Therefore, there has been an active movement to reduce the weight of automobile bodies by increasing the strength and reducing the thickness of steel sheets, which are the raw materials for automobile parts.

[0003] In addition, social demands for improved collision safety of automobiles are becoming increasingly stronger. Therefore, there is a demand for the development of steel sheets that not only have high strength but also have excellent impact resistance properties in the event of a collision while the automobile is in motion (hereinafter simply referred to as impact resistance properties). In particular, from the viewpoint of corrosion prevention performance of the automobile body, steel sheets that are used as raw materials for automobile components are often zinc-plated. Therefore, there is a demand for the development of zinc-plated steel sheets that not only have high strength but also have excellent impact resistance properties.

[0004] As an example of a steel sheet that can be used as a material for such automotive parts, Patent Document 1 discloses the following: "A high-strength steel sheet having a component composition containing, expressed by mass%, 0.04 to 0.22% C, 1.0% or less Si, 3.0% or less Mn, 0.05% or less P, 0.01% or less S, 0.01 to 0.1% Al, and 0.001 to 0.005% N, with the balance being Fe and unavoidable impurities, and being composed of a ferrite phase as a main phase and a martensite phase as a second phase, the maximum grain size of the martensite phase being 2 μm or less and the area ratio thereof being 5% or more."

[0005] Patent Document 2 states that "a surface layer of 0.1 μm or more is ground off and a nickel layer of 0.2 g / m is applied to a cold-rolled steel sheet." 2 2.0g / m or more 2A hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of a pre-galvanized cold-rolled steel sheet, containing, by mass%, C: 0.05% or more and 0.4% or less, Si: 0.01% or more and 3.0% or less, Mn: 0.1% or more and 3.0% or less, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Al: 0.01% or more and 2.0% or less, Si + Al > 0.5%, with the balance being Fe and unavoidable impurities, and the microstructure contains, by volume fraction, ferrite as a main phase of 40% or more, retained austenite of 8% or more, two or more of three types of martensite [1] [2] [3] specified below, including martensite [3], 1% or more of bainite, and 0 to 10% of pearlite, and the three types of martensite [1] [2] [3] each have, by volume fraction: A high-strength hot-dip galvanized steel sheet with excellent coating adhesion and formability, characterized in that the steel sheet has a martensite [1] of 0% or more and 50% or less, martensite [2] of 0% or more and less than 20%, and martensite [3] of 1% or more and 30% or less, and has a hot-dip galvanized layer on the surface thereof, the hot-dip galvanized layer containing less than 7% Fe and the balance being Zn, Al, and unavoidable impurities, and the tensile strength TS (MPa), total elongation EL (%), and hole expansion ratio λ (%) are such that TS×EL is 18,000 MPa·% or more and TS×λ is 35,000 MPa·% or more, and the tensile strength is 980 MPa or more. The following is disclosed: Martensite [1]: C concentration (CM1) is less than 0.8%, and hardness Hv1 is Hv1 / (-982.1 x CM12 + 1676 x CM1 + 189) ≦ 0.60 Martensite [2]: C concentration (CM2) is 0.8% or more, and hardness Hv2 is Hv2 / (-982.1 x CM22 + 1676 x CM2 + 189) ≦ 0.60 Martensite [3]: C concentration (CM3) is 0.8% or more, and hardness Hv3 is Hv3 / (-982.1 x CM32 + 1676 x CM3 + 189) ≧ 0.80

[0006] Patent Document 3 states, "A high-strength hot-dip galvanized steel sheet having a chemical composition consisting, in mass%, of C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, with the balance being Fe and unavoidable impurities; and having, in area percentages, a tempered martensite phase: 30% or more and 73% or less, a ferrite phase: 25% or more and 68% or less, a retained austenite phase: 2% or more and 20% or less, and other phases: 10% or less (inclusive of 0%), and the other phases being a martensite phase: 3% or less (inclusive of 0%) and a bainitic ferrite phase: less than 5% (inclusive of 0%), wherein the tempered martensite phase has an average grain size of 8 μm or less, and the retained austenite phase has a C content of less than 0.7% by mass." has been disclosed.

[0007] Patent No. 3887235 Patent No. 5953693 Patent No. 6052472

[0008] Incidentally, currently, the use of steel sheets having a tensile strength (hereinafter also referred to as TS) of 590 MPa class is limited to the impact energy absorbing members of automobiles, such as front side members and rear side members.

[0009] That is, improving the yield stress (hereinafter also referred to as YS) is effective in increasing the energy absorption during impact (hereinafter also referred to as impact absorption energy). However, increasing the TS and YS of a steel sheet generally reduces formability, particularly properties such as ductility, work hardening ability, and hole expandability. These properties correlate with the resistance of components to cracking in bending crush tests and axial crush tests that simulate crash tests. Therefore, if a steel sheet with such increased TS and YS is applied to the above-mentioned automotive impact energy absorption component, not only will it be difficult to form, but the component will crack in tests that simulate crash tests. In other words, the actual impact absorption energy will not be as high as expected from the YS value. Therefore, the current situation is that the above-mentioned impact energy absorption component is limited to steel sheets with a TS of 590 MPa or so. Note that work hardening ability and hole expandability are correlated with bulging ability and stretch flangeability, respectively.

[0010] In fact, the steel sheets disclosed in Patent Documents 1 to 3 also have a TS of 980 MPa or more, and cannot be said to have a high YS, excellent ductility, work hardening ability, and hole expandability.

[0011] The present invention has been developed in view of the above-mentioned current situation, and aims to provide a galvanized steel sheet having a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability, and hole expandability, together with an advantageous manufacturing method thereof. Another aim of the present invention is to provide a member made from the above-mentioned galvanized steel sheet, and a manufacturing method thereof.

[0012] Here, having a high YS and excellent ductility, work hardening ability and hole expandability means that the YS measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula in accordance with the TS measured in the tensile test: 550 MPa ≦ YS when 980 MPa ≦ TS < 1180 MPa, 700 MPa ≦ YS when 1180 MPa ≦ TS < 1310 MPa, and 800 MPa ≦ YS when 1310 MPa ≦ TS. The total elongation (El) measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula in accordance with the TS measured in the tensile test: 13.0% ≦ El when 980 MPa ≦ TS < 1180 MPa, and 12.0% ≦ El when 1180 MPa ≦ TS < 1310 MPa. When 1310 MPa≦TS, 10.0%≦El The n value / YR measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula: n value / YR≧0.070 Furthermore, the critical hole expansion ratio (λ) measured in a hole expansion test in accordance with JIS Z 2256 is 20% or more.

[0013] The inventors have conducted extensive research to achieve the above object, and as a result, have found that the composition of the base steel sheet of a galvanized steel sheet is appropriately adjusted, and the steel structure of the base steel sheet of a galvanized steel sheet is set to have the following: Area ratio of ferrite: 65.0% or less (including 0%), Area ratio of bainitic ferrite: 5.0% to 40.0%, Area ratio of tempered martensite: 0.5% to 80.0%, Area ratio of retained austenite: 3.0% or more, Area ratio of fresh martensite: 20.0% or less (including 0%), S ＢＦ +S ＴＭ +2 x S ＭＡ :65.0% or more, S ＭＡ１ / S ＭＡ : 0.80 or less, and S ＭＡ２ / S ＭＡ It has been found that by setting the tensile strength to 0.20 or more, a galvanized steel sheet having a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability, and hole expandability can be obtained. The present invention has been completed based on the above findings and further studies.

[0014] That is, the gist of the present invention is as follows: 1. A galvanized steel sheet having a substrate steel sheet and a galvanized layer on a surface of the substrate steel sheet, wherein the substrate steel sheet has a chemical composition, in mass %, of C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more but less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, with a carbon equivalent Ceq of 0.540% or more, and the balance being Fe and unavoidable impurities, and wherein the substrate steel sheet has an area ratio of ferrite: 65.0% or less (including 0%), an area ratio of bainitic ferrite: 5.0% or more and 40.0% or less, Area ratio of tempered martensite: 0.5% or more and 80.0% or less, Area ratio of retained austenite: 3.0% or more, Area ratio of fresh martensite: 20.0% or less (including 0%), S ＢＦ +S ＴＭ +2 x S ＭＡ:65.0% or more, S ＭＡ１ / S ＭＡ : 0.80 or less, and S ＭＡ２ / S ＭＡ : 0.20 or more, and a tensile strength of 980 MPa or more. ＢＦ : Area ratio of the bainitic ferrite S ＴＭ : Area ratio of the tempered martensite S ＭＡ : Area ratio of the hard second phase consisting of the retained austenite and the fresh martensite S ＭＡ１ S: the total area ratio of island regions constituting the hard second phase, which have a circle equivalent diameter of 2.0 μm or more and are in contact with tempered martensite over 20% or less of their perimeter ＭＡ２ : the total area ratio of island regions constituting the hard second phase, the island regions having 1% or more of their perimeter in contact with bainitic ferrite.

[0015] 2. The chemical composition of the substrate steel sheet further contains, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Ce: 0.0200% or less, 2. The galvanized steel sheet according to 1 above, containing at least one selected from Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

[0016] 3. In the steel structure of the base steel sheet, SＭＡ３ / S ＭＡ 3. The galvanized steel sheet according to 1 or 2 above, wherein S is 0.05 or more. ＭＡ３ : the total area ratio of island regions constituting the hard second phase, which have 1% or more of their circumferential length in contact with bainitic ferrite and more than 20% of their circumferential length in contact with tempered martensite.

[0017] 4. The galvanized steel sheet according to any one of 1 to 3 above, wherein the substrate steel sheet has a diffusible hydrogen content of 0.50 ppm by mass or less.

[0018] 5. The galvanized steel sheet according to any one of the above items 1 to 4, which has a decarburized layer.

[0019] 6. The galvanized steel sheet according to any one of the above items 1 to 5, which has a metal plating layer at least on one side between the base steel sheet and the galvanized layer.

[0020] 7. The zinc-plated steel sheet according to 6 above, wherein the metal plating layer is an Fe-based plating layer.

[0021] 8. The galvanized steel sheet according to any one of the above items 1 to 7, wherein the galvanized layer is a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer.

[0022] 9. A member formed using the zinc-plated steel sheet according to any one of 1 to 8 above.

[0023] 10. A hot rolling process in which a steel slab having the chemical composition according to 1 or 2 is hot rolled to obtain a hot rolled steel sheet, a cold rolling process in which the hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet, an annealing process in which the cold rolled steel sheet is annealed at an annealing temperature of 760°C or more and 900°C or less for an annealing time of 20 seconds or more, a first cooling process in which the cold rolled steel sheet is cooled to a first cooling stop temperature of 300°C or more and 550°C or less, a holding process in which the cold rolled steel sheet is held in a temperature range of 300°C or more and 550°C or less for 3 seconds or more and 600 seconds or less, a plating process in which the cold rolled steel sheet is galvanized to obtain a galvanized steel sheet, and a second cooling process in which the galvanized steel sheet is cooled to a second cooling stop temperature of 100°C or more and less than 300°C. and a reheating step of reheating the galvanized steel sheet to a reheating temperature of (the second cooling stop temperature + 50°C) or more and 500°C or less, and holding the galvanized steel sheet in a temperature range of (the second cooling stop temperature + 50°C) or more and 500°C or less for 10 seconds or more and 2000 seconds or less, wherein the first cooling stop temperature and the temperature of the galvanizing bath in the galvanizing treatment satisfy the relationship of the following formula (1): -150°C≦T ０ -T １ ≦50° C. (1) where T ０ is the first cooling stop temperature (°C), T １ is the temperature of the zinc plating bath in the zinc plating process (°C).

[0024] 11. The method for producing a galvanized steel sheet according to 10 above, wherein the dew point in the annealing step is higher than −30° C.

[0025] 12. The method for producing a galvanized steel sheet according to 10 or 11, further comprising a metal plating treatment step of forming a metal plating layer on at least one surface of the cold-rolled steel sheet after the cold-rolling step and before the annealing step.

[0026] 13. The method for producing a zinc-plated steel sheet according to 12 above, wherein the metal plating layer is an Fe-based plating layer.

[0027] 14. The method for producing a galvanized steel sheet according to any one of the above items 10 to 13, wherein the galvanizing treatment is a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.

[0028] 15. A method for producing a component, comprising the step of subjecting the zinc-plated steel sheet according to any one of 1 to 8 above to at least one of forming and joining to form a component.

[0029] According to the present invention, a galvanized steel sheet having a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability, and hole expandability can be obtained. Furthermore, a member made from the galvanized steel sheet of the present invention has high strength and excellent impact resistance, and can therefore be extremely advantageously used for automobile impact energy absorbing members, etc.

[0030] (A) is an example of a structural image taken by SEM used to identify the structure, and (B) is the structural image of (A) color-coded using Adobe Photoshop from Adobe Systems Incorporated. (A) is an example of a structural image taken by SEM used to identify the island regions of the hard second phase, particularly an example of a structural image containing island regions determined to be MA1, and (B) is the structural image of (A) color-coded using Adobe Photoshop from Adobe Systems Incorporated. (A) is an example of a structural image taken by SEM used to identify the island regions of the hard second phase, particularly an example of a structural image containing island regions determined to be MA2, and (B) is the structural image of (A) color-coded using Adobe Photoshop from Adobe Systems Incorporated. (A) is an example of a structural image taken by SEM used to identify island regions of the hard second phase, particularly an example of a structural image containing island regions identified as MA3, and (B) is a color-coded version of the structural image of (A) using Adobe Photoshop from Adobe Systems Inc. (A) is a diagram explaining a method for evaluating resistance weld cracking resistance in a weld, the upper diagram of (B) is a top view of a plate assembly after resistance spot welding used in the evaluation, and the lower diagram of (B) is a cross-sectional view taken along the line A-A in the upper diagram.

[0031] The present invention will be described based on the following embodiments. [1] Galvanized steel sheet First, the chemical composition of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention will be described. Note that although the unit of chemical composition is always "mass %", hereinafter, unless otherwise specified, it will be simply expressed as "%".

[0032] C: 0.050% or more and 0.400% or less. C is an effective element for generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite to ensure a TS of 980 MPa or more and a high YS. Here, if the C content is less than 0.050%, the area ratio of ferrite increases, making it difficult to achieve a TS of 980 MPa or more. This also results in a decrease in YS. On the other hand, if the C content exceeds 0.400%, the carbon concentration in the retained austenite increases excessively. Therefore, when the steel sheet is punched, the hardness of the fresh martensite formed from the retained austenite increases significantly. As a result, crack propagation during hole expansion is promoted in the punched steel sheet (i.e., resulting in a decrease in hole expandability). Therefore, the C content is set to 0.050% or more and 0.400% or less. The C content is preferably 0.100% or more. The C content is preferably 0.300% or less.

[0033] Si: 0.20% or more and 3.00% or less. Si suppresses carbide formation during annealing and promotes the formation of retained austenite. That is, Si is an element that affects the area fraction of retained austenite and the carbon concentration in the retained austenite. Here, if the Si content is less than 0.20%, the area fraction of retained austenite decreases, resulting in reduced ductility. On the other hand, if the Si content exceeds 3.00%, the area fraction of ferrite increases excessively, making it difficult to achieve a TS of 980 MPa or more. This also results in a decrease in YS. In addition, the carbon concentration in the retained austenite increases excessively. Therefore, when the steel sheet is punched, the hardness of the fresh martensite formed from the retained austenite increases significantly. As a result, crack propagation during hole expansion is promoted in the punched steel sheet (i.e., resulting in a decrease in hole expandability). Therefore, the Si content is set to 0.20% or more and 3.00% or less. The Si content is preferably 0.40% or more. Furthermore, if the Si content exceeds 2.00%, there is a concern that resistance weld crack resistance may be reduced, so the Si content is preferably 2.00% or less.

[0034] Mn: 1.00% or more and less than 3.50% Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, etc. Here, if the Mn content is less than 1.00%, the area ratio of ferrite increases excessively, making it difficult to achieve a TS of 980 MPa or more. It also leads to a decrease in YS. On the other hand, if the Mn content is 3.50% or more, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite increases excessively. As a result, the desired ductility cannot be obtained. Therefore, the Mn content is set to 1.00% or more and less than 3.50%. The Mn content is preferably 1.80% or more. The Mn content is also preferably less than 3.20%.

[0035] P: 0.001% or more and 0.100% or less P is an element that has a solid solution strengthening effect and increases the strength of steel sheet. To achieve this effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries, embrittling the grain boundaries. Therefore, when the steel sheet is punched, the amount of voids generated increases, leading to a decrease in hole expandability. Therefore, the P content is set to 0.001% or more and 0.100% or less. The P content is preferably 0.030% or less.

[0036] S: 0.0200% or less S exists as sulfides in steel. In particular, if the S content exceeds 0.0200%, the ultimate deformability of the steel sheet decreases. Therefore, when the steel sheet is punched, the amount of voids generated increases, resulting in a decrease in hole expandability. Therefore, the S content is set to 0.0200% or less. The S content is preferably 0.0080% or less. Note that there is no particular lower limit for the S content, but due to constraints on production technology, the S content is preferably 0.0001% or more.

[0037] Al: 0.010% or more and 2.000% or less Al suppresses the formation of carbides during annealing and promotes the formation of retained austenite. That is, Al is an element that affects the area fraction of retained austenite and the carbon concentration in the retained austenite. To achieve this effect, the Al content is set to 0.010% or more. On the other hand, if the Al content exceeds 2.000%, the ferrite area fraction increases excessively, making it difficult to achieve a TS of 980 MPa or more. This also leads to a decrease in YS. Therefore, the Al content is set to 0.010% or more and 2.000% or less. The Al content is preferably 0.015% or more. The Al content is preferably 1.000% or less.

[0038] N: 0.0100% or less N exists as nitride in steel. In particular, if the N content exceeds 0.0100%, the ultimate deformability of the steel sheet decreases. Therefore, when the steel sheet is punched, the amount of voids generated increases, resulting in a decrease in hole expandability. Therefore, the N content is set to 0.0100% or less. Furthermore, the N content is preferably 0.0050% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0005% or more.

[0039] Carbon equivalent Ceq: 0.540% or more The carbon equivalent Ceq affects TS. In particular, if the carbon equivalent Ceq is less than 0.540%, it becomes difficult to achieve TS of 980 MPa or more. Therefore, the carbon equivalent Ceq is set to 0.540% or more. Here, the carbon equivalent Ceq is defined by the following formula: Carbon equivalent Ceq = [C%] + ([Si%] / 24) + ([Mn%] / 6) + ([Ni%] / 40) + ([Cr%] / 5) + ([Mo%] / 4) + ([V%] / 14) In the above formula, the [element symbol %] represents the content (mass %) of the corresponding element in the chemical composition of the substrate steel sheet. Elements not contained in the chemical composition of the substrate steel sheet are calculated as 0.

[0040] The basic components of the substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention have been described above. However, the substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention has a composition containing the above basic components, with the balance other than the basic components including Fe (iron) and unavoidable impurities. Here, the substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention preferably contains the above basic components, with the balance consisting of Fe and unavoidable impurities. The substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention may contain, in addition to the above basic components, at least one selected from the optional components listed below. Note that the effects of the present invention can be achieved as long as the optional components listed below are contained in amounts up to the upper limit amounts listed below, so no lower limit is particularly set. Note that when the optional components listed below are contained in amounts less than the preferred lower limit values ​​described below, the optional components are considered to be included as unavoidable impurities. Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less

[0041] Ti: 0.200% or less Ti increases TS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. To achieve this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting points for cracks during a hole expansion test, i.e., the hole expandability may be reduced. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0042] Nb: 0.200% or less Like Ti, Nb increases TS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. To achieve this effect, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting point of cracks during a hole expansion test, that is, the hole expandability may be reduced. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0043] V: 0.100% or less Like Ti and Nb, V increases TS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, if the V content exceeds 0.100%, large amounts of coarse precipitates and inclusions may be formed. In such cases, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting point of cracks during a hole expansion test, i.e., the hole expandability may be reduced. Therefore, when V is contained, the V content is preferably 0.100% or less. The V content is more preferably 0.060% or less.

[0044] B: 0.0100% or less B is an element that segregates at austenite grain boundaries to improve hardenability. Furthermore, B is an element that suppresses the formation and grain growth of ferrite during cooling after annealing. To achieve this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, if the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling, reducing the ultimate deformability of the steel sheet. Furthermore, as the ultimate deformability of the steel sheet decreases, the amount of voids generated when the steel sheet is punched increases, resulting in a decrease in hole expandability. Therefore, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.

[0045] Cu: 1.000% or less Cu is an element that improves hardenability. In particular, Cu is an effective element for adjusting the area ratio of hard fresh martensite and the like to a more suitable range, thereby adjusting TS to a more suitable range. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.000%, the area ratio of fresh martensite increases excessively, resulting in an excessively high TS. In addition, large amounts of coarse precipitates and inclusions may be generated. In such cases, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting point for cracks during tensile testing, i.e., the hole expandability may be reduced. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

[0046] Cr: 1.000% or less Cr is an element that improves hardenability and is also effective in generating retained austenite and fresh martensite. To achieve this effect, the Cr content is preferably 0.0005% or more. In particular, from the viewpoint of setting the TS in a more suitable range, the Cr content is more preferably 0.010% or more. On the other hand, if the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, which may result in a decrease in hole expandability. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less. Furthermore, the Cr content is more preferably 0.250% or less, and even more preferably 0.100% or less.

[0047] Ni: 1.000% or less Ni is an element that improves hardenability. Ni is also an element that is effective in adjusting the area fraction of retained austenite and fresh martensite to a more suitable range, thereby adjusting TS to a more suitable range. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, if the Ni content exceeds 1.000%, the area fraction of fresh martensite increases excessively, which may result in a decrease in ductility and dimensional accuracy during forming. Furthermore, large amounts of coarse precipitates and inclusions may be generated. In such cases, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting point for cracks during a hole expansion test, i.e., the hole expandability may be reduced. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.

[0048] Mo: 0.500% or less Mo is an element that improves hardenability. Mo is also an effective element for generating hard fresh martensite. To achieve this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, if the Mo content exceeds 0.500%, the area ratio of fresh martensite increases excessively, which may result in a decrease in hole expandability. Therefore, when Mo is contained, the Mo content is preferably 0.500% or less. The Mo content is more preferably 0.450% or less, and even more preferably 0.400% or less.

[0049] Sb: 0.200% or less Sb is an element that suppresses the diffusion of C near the steel sheet surface during annealing and is effective in controlling the formation of a soft layer near the steel sheet surface. To achieve this effect, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.200%, a soft layer is not formed near the steel sheet surface, which may result in a decrease in hole expandability. Therefore, when Sb is contained, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less.

[0050] Sn: 0.200% or less Like Sb, Sn is an element that suppresses the diffusion of C near the steel sheet surface during annealing and is effective in controlling the formation of a soft layer near the steel sheet surface. In order to achieve this effect, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more. On the other hand, if the Sn content exceeds 0.200%, a soft layer is not formed near the steel sheet surface, which may result in a decrease in hole expandability. Therefore, when Sn is contained, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.020% or less.

[0051] Ta: 0.100% or less Like Ti, Nb, and V, Ta increases TS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This improves TS and even YS. To achieve this effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content exceeds 0.100%, large amounts of coarse precipitates and inclusions may be formed. In such cases, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting points for cracks during hole expansion tests, i.e., the hole expandability may be reduced. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less.

[0052] W: 0.500% or less W is an element effective for improving hardenability and adjusting TS to a more suitable range. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.030% or more. On the other hand, if the W content exceeds 0.500%, the area ratio of hard fresh martensite increases excessively, which may result in a decrease in hole expandability. Therefore, when W is contained, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less.

[0053] Mg: 0.0200% or less Mg is an element that effectively spheroidizes the shape of inclusions such as sulfides and oxides, thereby improving the ultimate deformability and hole expandability of steel sheets. To achieve this effect, the Mg content is preferably 0.0001% or more. On the other hand, if the Mg content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting point for cracks during a hole expansion test, i.e., the hole expandability may be reduced. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less.

[0054] Zn: 0.0200% or less Zn is an element that effectively spheroidizes the shape of inclusions and improves the ultimate deformability and hole expandability of the steel sheet. To achieve this effect, the Zn content is preferably 0.0010% or more. On the other hand, if the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting point of cracks during a hole expansion test, that is, the hole expandability may be reduced. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less.

[0055] Co: 0.0200% or less Like Zn, Co is an effective element for spheroidizing the shape of inclusions and improving the ultimate deformability and hole expandability of steel sheets. To achieve this effect, the Co content is preferably 0.0010% or more. On the other hand, if the Co content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be generated. In such cases, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting point of cracks during a hole expansion test, that is, the hole expandability may be reduced. Therefore, when Co is contained, the Co content is preferably 0.0200% or less.

[0056] Zr: 0.0200% or less Like Zn and Co, Zr is an element that effectively spheroidizes the shape of inclusions and improves the ultimate deformability and hole expandability of steel sheets. To achieve this effect, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be generated. In such cases, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting point for cracks during a hole expansion test, i.e., the hole expandability may be reduced. Therefore, when Zr is contained, the Zr content is preferably 0.0200% or less.

[0057] Ca: 0.0200% or less. Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, the coarse inclusions may become the starting point of cracks during a hole expansion test, that is, the hole expandability may be reduced. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. The lower limit of the Ca content is not particularly limited, but the Ca content is preferably 0.0005% or more. Furthermore, due to constraints on production technology, the Ca content is more preferably 0.0010% or more.

[0058] Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less and REM: 0.0200% or less Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are all effective elements for improving the ultimate deformability and hole expandability of steel sheet. To obtain such effects, it is preferable that the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are each 0.0001% or more. On the other hand, if the content of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be generated. In such cases, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become the starting point of cracks during hole expansion tests, that is, there is a risk of reducing hole expandability. Therefore, when at least one of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is contained, it is preferable that the content of each be 0.0200% or less.

[0059] That is, the substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention contains, in mass %, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, and N: 0.0100% or less, and a carbon equivalent Ceq of 0.540% or more, and optionally Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, The steel sheet has a chemical composition containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less, with the balance being Fe and unavoidable impurities.

[0060] Next, the steel structure of the substrate steel sheet of the galvanized steel sheet according to one embodiment of the present invention will be described. The steel structure of the substrate steel sheet of the galvanized steel sheet according to one embodiment of the present invention has: Area ratio of ferrite: 65.0% or less (including 0%), Area ratio of bainitic ferrite: 5.0% to 40.0%, Area ratio of tempered martensite: 0.5% to 80.0%, Area ratio of retained austenite: 3.0% or more, Area ratio of fresh martensite: 20.0% or less (including 0%), S ＢＦ +S ＴＭ +2 x S ＭＡ:65.0% or more, S ＭＡ１ / S ＭＡ : 0.80 or less, and S ＭＡ２ / S ＭＡ : 0.20 or more. ＢＦ : Area ratio of the bainitic ferrite S ＴＭ : Area ratio of the tempered martensite S ＭＡ : Area ratio of the hard second phase consisting of the retained austenite and the fresh martensite S ＭＡ１ S: the total area ratio of island regions constituting the hard second phase, which have a circle equivalent diameter of 2.0 μm or more and are in contact with tempered martensite over 20% or less of their perimeter ＭＡ２ : the total area ratio of the island regions constituting the hard second phase, the island regions having 1% or more of their perimeter in contact with bainitic ferrite. The reasons for each of these limitations will be explained below.

[0061] Ferrite area fraction: 65.0% or less (including 0%) Soft ferrite is a phase that improves ductility and work hardening ability. However, from the viewpoint of ensuring a TS of 980 MPa or more, a high YS, and good hole expandability, the ferrite area fraction is set to 65.0% or less. The ferrite area fraction is preferably 35.0% or less, more preferably 25.0% or less. The lower limit of the ferrite area fraction is not particularly limited and may be 0%. In particular, when 980 MPa ≦ TS < 1180 MPa is required, the ferrite area fraction is preferably 5.0% or more.

[0062] Area fraction of bainitic ferrite: 5.0% or more and 40.0% or less Bainitic ferrite has a hardness intermediate between soft ferrite and hard fresh martensite, and is an important phase for ensuring good hole expandability. Bainitic ferrite is also a useful phase for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite. Therefore, the area fraction of bainitic ferrite is 5.0% or more. The area fraction of bainitic ferrite is preferably 10.0% or more. On the other hand, if the area fraction of bainitic ferrite increases excessively, the hole expandability will actually decrease. Therefore, the area fraction of bainitic ferrite is 40.0% or less. The area fraction of bainitic ferrite is preferably 35.0% or less.

[0063] Area fraction of tempered martensite: 0.5% or more and 80.0% or less Tempered martensite has a hardness intermediate between soft ferrite and hard fresh martensite, and is an important phase for ensuring good hole expandability. Therefore, the area fraction of tempered martensite is set to 0.5% or more. The area fraction of tempered martensite is preferably 40.0% or more. On the other hand, from the viewpoint of ensuring good ductility, the area fraction of tempered martensite is set to 80.0% or less. Furthermore, the area fraction of tempered martensite is preferably 75.0% or less.

[0064] Area fraction of retained austenite: 3.0% or more From the viewpoint of obtaining good ductility, the area fraction of retained austenite is set to 3.0% or more. The area fraction of retained austenite is preferably 5.0% or more. There is no particular upper limit to the area fraction of retained austenite, but the area fraction of retained austenite is preferably 20.0% or less.

[0065] Area fraction of fresh martensite: 20.0% or less (including 0%) From the viewpoint of ensuring good hole expandability, the area fraction of fresh martensite is set to 20.0% or less. The lower limit of the area fraction of fresh martensite is not particularly limited and may be 0%. Furthermore, from the viewpoint of ensuring a TS of 980 MPa or more, the area fraction of fresh martensite is preferably 3.0% or more. Note that fresh martensite is as-quenched (untempered) martensite.

[0066] The area ratio of the remaining structure other than the above is preferably 10.0% or less. The area ratio of the remaining structure is more preferably 5.0% or less. The area ratio of the remaining structure may also be 0%. The remaining structure is not particularly limited, and examples thereof include carbides such as lower bainite, pearlite, and cementite. The type of the remaining structure can be confirmed, for example, by observation using a scanning electron microscope (SEM).

[0067] Here, the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard second phase (retained austenite + fresh martensite) are measured at a quarter-thickness position of the substrate steel sheet as follows. That is, a sample is cut out from the substrate steel sheet so that the observation surface is the thickness cross section parallel to the rolling direction of the substrate steel sheet. Next, the observation surface of the sample is mirror-polished using diamond paste. Next, the observation surface of the sample is finish-polished using colloidal silica, and then etched with 3 vol. % nital to reveal the structure. Then, five fields of view of 25.6 μm × 17.6 μm on the observation surface of the sample are observed using a SEM (Scanning Electron Microscope) under conditions of an acceleration voltage of 15 kV and a magnification of 5000x. From the obtained structural image (see, for example, FIG. 1(A)), ferrite, bainitic ferrite, tempered martensite, and a hard second phase (retained austenite + fresh martensite) are identified as follows.

[0068] Ferrite: A region that is black and has a blocky shape. It contains almost no iron-based carbides. However, if iron-based carbides are contained, the area of ​​the ferrite includes the area of ​​the iron-based carbides. The same applies to bainitic ferrite and tempered martensite, which will be described later. Bainitic ferrite: A region that is black to dark gray and has a blocky or amorphous shape. It contains no iron-based carbides or a relatively small number of iron-based carbides. Tempered martensite: A region that is gray and has an amorphous shape. It contains a relatively large number of iron-based carbides. Hard second phase (retained austenite + fresh martensite): A region that is white to light gray and has an amorphous shape. It does not contain iron-based carbides. When the size is relatively large, the color gradually darkens as it moves away from the interface with other structures, and the interior may be dark gray. Remaining structure: The above-mentioned lower bainite, pearlite, cementite and other carbides are included, and the forms thereof are as known.

[0069] In addition to the above-mentioned SEM observation, it is also possible to appropriately add carbide observation at higher magnification, detailed structural analysis using EBSD (electron backscatter diffraction) in the same field of view, component analysis using EPMA (electron probe microanalyzer), and local hardness measurement using a microhardness tester. For example, if it is difficult to identify the structure using the above-mentioned SEM observation, it is effective to appropriately add these measurements. For example, in EBSD analysis, ferrite does not have a substructure (it is not observed). On the other hand, bainitic ferrite, tempered martensite, and fresh martensite have a substructure and have a specific crystal orientation relationship with retained austenite. Furthermore, it is possible to reproduce and confirm the austenite structure during the annealing process from these structures. These points serve as criteria for structural identification. Furthermore, in component analysis using EPMA, the fact that the C concentration and Mn concentration vary depending on the structure serves as criteria for structural identification. For example, the C concentration in ferrite and bainitic ferrite is lower than that in the region mainly composed of tempered martensite (including fine hard second phases and carbides). Furthermore, the Mn concentration in ferrite may be lower than that in other structures. In hardness measurements using a microhardness tester, the fact that hardness varies depending on the structure is a factor in determining structure identification. For example, among ferrite, bainitic ferrite, tempered martensite, and the hard second phase, ferrite has the lowest hardness, and the hard second phase has the highest hardness. Furthermore, bainitic ferrite and tempered martensite exhibit hardness between that of ferrite and that of the hard second phase.

[0070] Next, the regions of each phase identified in the structural image are color-coded (quaternary imaged) using Adobe Photoshop from Adobe Systems Incorporated (see, for example, FIG. 1(B)), and the area of ​​each phase is calculated. Next, the area of ​​each phase (total area for each phase) is divided by the area of ​​the observation region (25.6 μm × 17.6 μm) and multiplied by 100 to calculate the value for five fields of view. The average of these values ​​is then used as the area ratio of each phase (ferrite, bainitic ferrite, tempered martensite, and hard second phase). Note that FIG. 1(A) shows a portion extracted from one field of view of the observation region (25.6 μm × 17.6 μm) of the sample for the above explanation.

[0071] The area fraction of retained austenite is measured as follows. Specifically, the substrate steel sheet is mechanically ground in the thickness direction (depth direction) to a position one-quarter of the sheet thickness, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. CoKα rays are used as the incident X-rays, and the ratios of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of bcc iron are calculated. The volume fraction of retained austenite is calculated from the ratio of the diffraction intensities of each plane. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0072] The area fraction of fresh martensite is calculated by subtracting the area fraction of retained austenite from the area fraction of the hard second phase calculated as described above: [Area fraction of fresh martensite (%)] = [Area fraction of hard second phase (%)] - [Area fraction of retained austenite (%)]

[0073] The area ratio of the remaining structure is calculated by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of the hard second phase calculated as described above from 100%. [Area ratio of remaining structure (%)] = 100 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Area ratio of hard second phase (%)]

[0074] S ＢＦ +S ＴＭ +2 x S ＭＡ : 65.0% or more From the viewpoint of ensuring TS of 980 MPa or more, S ＢＦ +S ＴＭ +2 x S ＭＡ is 65.0% or more. ＢＦ +S ＴＭ +2 x S ＭＡ Although there is no particular limitation on the upper limit of S, it is preferably 130.0% or less. ＢＦ : Area ratio of bainitic ferrite S ＴＭ : Area ratio of tempered martensite S ＭＡ : Area ratio of the hard second phase consisting of retained austenite and the fresh martensite.

[0075] S ＭＡ１ / S ＭＡ : 0.80 or less The hard second phase (hereinafter also referred to as MA) consisting of retained austenite and fresh martensite is composed of multiple island regions. Among these island regions, island regions (hereinafter also referred to as MA1) having a circle equivalent diameter of 2.0 μm or more and having 20% ​​or less of their perimeter in contact with tempered martensite have a low solute C concentration. In other words, the stability of the retained austenite contained in MA1 is low. Therefore, MA1 does not contribute to ensuring good ductility. Furthermore, since the proportion of fresh martensite is high in MA1, MA1 reduces hole expandability. Therefore, S, which is the ratio of the area fraction of MA1 to the area fraction of the hard second phase, ＭＡ１ / S ＭＡ is set to 0.80 or less. In particular, when 980 MPa ≦ TS < 1180 MPa is required, S ＭＡ１ / S ＭＡ is preferably 0.75 or less, more preferably 0.40 or less. ＭＡ１ / S ＭＡ is preferably 0.50 or less, more preferably 0.30 or less. ＭＡ１ / S ＭＡThe lower limit of is not particularly limited and may be 0. Each island region is separated from other island regions of the hard second phase by a phase other than the hard second phase (each island region is in contact with a phase other than the hard second phase over its entire periphery). The specific shape of each island region is not particularly limited and may be, for example, a circle, an ellipse, a polygon, an amoeba shape (a shape elongated in multiple irregular directions), or the like.

[0076] S ＭＡ２ / S ＭＡ : 0.20 or more. Among the island regions constituting the hard second phase consisting of retained austenite and fresh martensite, the island regions (hereinafter also referred to as MA2) that are in contact with bainitic ferrite over 1% or more of their perimeter have a high solute C concentration. In other words, the stability of the retained austenite contained in MA2 is high. Therefore, MA2 plays an extremely important role in ensuring good work hardening ability and ductility. That is, when bainitic ferrite is formed under appropriate conditions during cooling after annealing, the solute C that diffuses from bainitic ferrite to the surrounding untransformed austenite does not sufficiently diffuse into the untransformed austenite. In other words, it is possible to locally achieve a high solute C content only around the bainitic ferrite in the untransformed austenite. Then, by performing reheating treatment under appropriate conditions in this state, a hard second phase with a high solute C concentration, i.e., MA2, is formed around the bainitic ferrite. Therefore, the stability of the retained austenite contained in MA2 is increased, and MA2 plays an extremely important role in ensuring good work hardening ability and ductility. From the above, S, which is the ratio of the area fraction of MA2 to the area fraction of the hard second phase, ＭＡ２ / S ＭＡ is 0.20 or more. ＭＡ２ / S ＭＡ is preferably 0.25 or more, more preferably 0.30 or more. ＭＡ２ / S ＭＡ The upper limit of S is not particularly limited and may be 1. ＭＡ２ / S ＭＡIn order to ensure high YS and excellent hole expandability, when 980 MPa ≦ TS < 1180 MPa is required, S is preferably 0.98 or less. ＭＡ２ / S ＭＡ is preferably 0.70 or less.

[0077] In addition, the steel structure of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention further comprises S ＭＡ３ / S ＭＡ is preferably 0.05 or more. ＭＡ３ : the total area ratio of island regions constituting the hard second phase, which have 1% or more of their circumferential length in contact with bainitic ferrite and more than 20% of their circumferential length in contact with tempered martensite.

[0078] S ＭＡ３ / S ＭＡ : 0.05 or more Among the island regions constituting the hard second phase consisting of retained austenite and fresh martensite, the island regions (hereinafter also referred to as MA3) in which 1% or more of the circumferential length is in contact with bainitic ferrite and more than 20% of the circumferential length is in contact with tempered martensite have a particularly high solute C concentration, even among MA2. That is, in MA3, solute C diffuses not only from bainitic ferrite but also from tempered martensite, so the solute C concentration is particularly high. Therefore, MA3 contributes particularly effectively to ensuring good work hardenability and ductility. Therefore, S, which is the ratio of the area fraction of MA3 to the area fraction of the hard second phase, ＭＡ３ / S ＭＡ is preferably 0.05 or more. ＭＡ３ / S ＭＡ is preferably 0.07 or more, more preferably 0.10 or more. ＭＡ３ / S ＭＡ The upper limit of S is not particularly limited and may be 1. ＭＡ３ / S ＭＡ is preferably 0.70 or less.

[0079] Here, S ＭＡ１ , S ＭＡ２ and S ＭＡ３are measured as follows. That is, in the structural image (see, for example, FIGS. 2(A), 3(A), and 4(A)), ferrite, bainitic ferrite, tempered martensite, and the hard second phase (retained austenite + fresh martensite) are identified as described above. Next, after color-coding (quaternary imaging) using Adobe Photoshop from Adobe Systems, island-like regions of the hard second phase are extracted, and the circle-equivalent diameter of each island-like region, the perimeter of each island-like region, and the length of contact between each island-like region and bainitic ferrite and tempered martensite are determined using the open-source ImageJ. Note that the pixel density of the structural image when determining the perimeter is 30 pixels / μm or more and 100 pixels / μm or less. Then, from the obtained values, each island region is judged to correspond to MA1, MA2, or MA3, and color-coded using Adobe Photoshop from Adobe Systems (see, for example, Figures 2(B), 3(B), and 4(B)), and the area of ​​each is calculated. Next, the total area of ​​each island region judged to be MA1, MA2, and MA3 is divided by the area of ​​the observation region (25.6 μm × 17.6 μm), and the result is multiplied by 100 to calculate the value (area ratio) for five visual fields. The average value of the values ​​(area ratio) for the five visual fields for each of MA1, MA2, and MA3 is then calculated as S ＭＡ１ , S ＭＡ２ and S ＭＡ３ For island regions that fall under both MA1 and MA2, the area is counted for both MA1 and MA2. The same applies to MA1 and MA3, and MA2 and MA3. For the above explanation, each of Figures 2(A), 3(A), and 4(A) shows a portion extracted from one field of view of the observation area (25.6 μm × 17.6 μm) of the sample.

[0080] In addition, in the substrate steel sheet of the galvanized steel sheet according to one embodiment of the present invention, it is preferable that the amount of diffusible hydrogen is 0.50 mass ppm or less.

[0081] Diffusible hydrogen content of substrate steel sheet: 0.50 ppm by mass or less From the viewpoint of obtaining better hole expandability, the diffusible hydrogen content of the substrate steel sheet is preferably 0.50 ppm by mass or less. Furthermore, the diffusible hydrogen content of the substrate steel sheet is more preferably 0.35 ppm by mass or less. Note that the lower limit of the diffusible hydrogen content of the substrate steel sheet is not particularly specified, and it may be 0 ppm by mass. Furthermore, due to constraints on production technology, the diffusible hydrogen content of the substrate steel sheet is more preferably 0.01 ppm by mass or more.

[0082] Here, the amount of diffusible hydrogen in the substrate steel sheet is measured as follows. Specifically, a test piece 30 mm long and 5 mm wide is taken from a galvanized steel sheet, and the galvanized layer is removed with alkali. Next, the amount of hydrogen released from the test piece is measured by thermal desorption analysis. Specifically, the test piece is continuously heated from room temperature to 300°C at a heating rate of 200°C / h, and then cooled to room temperature. During this process, the amount of hydrogen released from the test piece (cumulative amount of hydrogen) is measured in the temperature range from room temperature to 210°C during the continuous heating. The measured amount of hydrogen is then divided by the mass of the test piece (the test piece after removal of the galvanized layer and before continuous heating), and the value converted to ppm by mass is taken as the amount of diffusible hydrogen in the substrate steel sheet.

[0083] In addition, for products (components) obtained after forming or joining a galvanized steel sheet, a test piece is cut out from the product in a typical usage environment, and the amount of diffusible hydrogen in the substrate steel sheet portion is measured in the same manner as above. If the measured value is 0.50 mass ppm or less, it can be considered that the amount of diffusible hydrogen in the substrate steel sheet of the galvanized steel sheet at the material stage before forming or joining is also 0.50 mass ppm or less.

[0084] Furthermore, the galvanized steel sheet according to one embodiment of the present invention preferably has a decarburized layer. In particular, the substrate steel sheet of the galvanized steel sheet according to one embodiment of the present invention preferably has a decarburized layer. In a galvanized steel sheet using a steel sheet containing Si, particularly a steel sheet with a high Si content as the substrate steel sheet, cracking due to liquid metal embrittlement (LME) during resistance spot welding may become a problem. However, when the galvanized steel sheet has a decarburized layer, particularly in the surface layer of the substrate steel sheet, resistance weld cracking resistance can be improved even when the substrate steel sheet has a high Si content.

[0085] The thickness of the decarburized layer, in other words, the depth from the surface of the substrate steel sheet in the sheet thickness direction, is preferably 30 μm or more, more preferably 40 μm or more. There is no particular upper limit to the thickness of the decarburized layer, but in order to keep the tensile strength within a good range, the thickness of the decarburized layer is preferably 130 μm or less. Here, the decarburized layer is defined as the region where the C concentration of the substrate steel sheet is analyzed from the surface in the sheet thickness direction, and the C concentration is 80% or less of the C content of the chemical composition of the substrate steel sheet, and the thickness of the decarburized layer is defined as the thickness of this region.

[0086] The thickness of the decarburized layer was measured by area or line analysis of the element distribution near the surface of the substrate steel sheet using an electron probe microanalyzer (EPMA) on a cross-section-processed sample. First, the resin-embedded galvanized steel sheet was polished to prepare a cross section perpendicular to the rolling direction for observation, and then removed from the resin to prepare the measurement sample. The acceleration voltage was 7 kV and the probe current was 50 nA. Area or line analysis of the sample cross section was performed in 1-μm increments over a 300 × 300 μm area, including the outermost layer (surface) of the substrate steel sheet, to measure the C intensity. To prevent contamination, a plasma cleaner was used to remove hydrocarbons from the surface and periphery of the sample before starting the measurement in two locations: the measurement chamber and the sample preparation chamber. To prevent hydrocarbon accumulation during the measurement, the sample temperature was heated and maintained at a maximum of 100°C on the stage during the measurement. The C intensity is converted to a C concentration (mass%) using a calibration curve prepared by measuring a separate standard sample. It is confirmed that the lower limit of C detection is sufficiently lower than 0.10 mass% due to the effect of contamination suppression. Details of the equipment used and the method of contamination suppression are as explained in Reference 1 below.

[0087] Reference 1: Yamashita et al., "Carbon Distribution in the Early Stage of Proeutectoid Ferrite Transformation in Low Carbon Steel Using High-Precision FE-EPMA," Iron and Steel, Vol. 103 (2017) No. 11, pp. 14-20. However, the necessity for contamination prevention measures during measurement depends on the model and conditions used, so the above configuration is not necessarily required. In other words, it is sufficient as long as it is confirmed that sufficient measurement accuracy is obtained, and the measurement conditions are not essentially related to the effects of the present invention.

[0088] From the obtained C concentration map, a line profile in the sheet thickness direction is extracted from the surface of the substrate steel sheet, and this is averaged over 300 points in a direction parallel to the surface of the substrate steel sheet to obtain a C concentration profile in the sheet thickness direction. The obtained C concentration profile in the sheet thickness direction is subjected to a smoothing process using a simple moving average method. In this case, it is preferable to smooth approximately 21 points. Next, in the strength profile after the smoothing process, a range in the sheet thickness direction where the C concentration is 80% or less of the C content in the chemical composition of the substrate steel sheet is identified and used as the thickness of the decarburized layer.

[0089] Next, the mechanical properties of the zinc-based plated steel sheet according to one embodiment of the present invention will be described.

[0090] Tensile strength (TS): 980 MPa or more The tensile strength of the zinc-based plated steel sheet according to one embodiment of the present invention is 980 MPa or more. The tensile strength of the zinc-based plated steel sheet according to one embodiment of the present invention is preferably 1180 MPa or more.

[0091] The yield stress (YS), total elongation (El), work hardening exponent (n value) / yield ratio (YR), and limiting hole expansion ratio (λ) of the zinc-based plated steel sheet according to one embodiment of the present invention are as described above.

[0092] The tensile strength (TS), yield stress (YS), total elongation (El), and work hardening index (n value) / yield ratio (YR) are measured by a tensile test conforming to JIS Z 2241, which will be described later in the examples. The limiting hole expansion ratio (λ) is measured by a hole expansion test conforming to JIS Z 2256, which will be described later in the examples.

[0093] Furthermore, the zinc plating layer of the zinc-plated steel sheet according to one embodiment of the present invention may be formed on only one surface of the base steel sheet, or on both surfaces. The zinc plating layer here refers to a plating layer containing Zn as the main component (Zn content of 50% or more), such as a hot-dip galvanized layer or a galvannealed layer. The hot-dip galvanized layer is preferably composed of Zn, 20% by mass or less of Fe, and 0.001% by mass to 1.0% by mass of Al. The hot-dip galvanized layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0% by mass to 3.5% by mass. The Fe content of the hot-dip galvanized layer is more preferably less than 7% by mass. The remainder other than the above elements is unavoidable impurities. The galvannealed layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al. The galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0% by mass or more and 3.5% by mass or less. The Fe content of the galvannealed layer is more preferably 7% by mass or more, and even more preferably 8% by mass or more. The Fe content of the galvannealed layer is more preferably 15% by mass or less, and even more preferably 12% by mass or less. The remainder other than the above elements is unavoidable impurities.

[0094] In addition, the plating weight of the zinc plating layer per side is not particularly limited, but is preferably 20 to 80 g / m ２ It is preferable to set the following.

[0095] The coating weight of the zinc plating layer is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe ("Ivit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous hydrochloric acid solution. Next, a zinc-plated steel sheet to be used as a test material is immersed in the treatment solution to dissolve the zinc plating layer. The mass loss of the test material before and after dissolution is measured, and this value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the part that was covered with the plating) to determine the coating weight (g / m ２ ) is calculated.

[0096] Furthermore, the galvanized steel sheet according to one embodiment of the present invention may have a metal plating layer other than the galvanized layer at least on one side between the substrate steel sheet and the galvanized layer. The metal plating layer contributes to improving resistance weld crack resistance. Furthermore, the formation of the metal plating layer can suppress resistance weld crack resistance even when the substrate steel sheet has a high Si content. The mechanism by which the metal plating layer improves resistance weld crack resistance is not necessarily clear. However, the inventors believe that when a metal plating layer is provided between the substrate steel sheet and the galvanized layer, in other words, on the surface of the substrate steel sheet, the metal plating layer acts as a barrier layer that suppresses the penetration of zinc in the galvanized layer into the substrate steel sheet during resistance spot welding, making resistance weld cracks less likely to occur (zinc penetration suppression effect). Note that when a galvanized layer is provided on both sides of the substrate steel sheet, the metal plating layer may be provided only on one side between the substrate steel sheet and the galvanized layer, or may be provided on both sides between the substrate steel sheet and the galvanized layer.

[0097] Here, the coating weight of the metal plating layer is preferably 0 g / m ２ More preferably, more than 2.0 g / m ２ Although there is no particular upper limit to the amount of metal plating layer deposited on one side, from the viewpoint of cost, the amount of metal plating layer deposited is set to 60 g / m ２ The coating weight of the metal plating layer is preferably 50 g / m or less. ２ More preferably 40 g / m or less ２ More preferably, 30 g / m or less ２The amount of metal plating layer mentioned here is per side.

[0098] The deposition weight of the metal plating layer is measured as follows. Specifically, a 10 x 15 mm sample is taken from a zinc-plated steel sheet and embedded in resin to form a cross-section-embedded sample. Three random locations on the cross-section of the sample are observed using a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and a magnification of 2,000 to 10,000 times depending on the thickness of the metal plating layer. The thicknesses of the metal plating layer at the three locations are measured, and the average value is calculated. The calculated average value is then multiplied by the specific gravity of the metal constituting the metal plating layer to convert it into the deposition weight per side of the metal plating layer.

[0099] The metal used for the metal plating layer is preferably a metal with a melting point higher than that of Zn, such as Fe and Ni. In addition to the above-mentioned zinc penetration suppression effect, an Fe-based plating layer is desirable because it can be expected to have the following toughness reduction suppression effect.

[0100] That is, when the Si content near the surface of the substrate steel sheet is high, the toughness of the weld is reduced, and the resistance weld cracking resistance of the weld is thought to deteriorate. In contrast, when an Fe-based plating layer is provided between the substrate steel sheet and the zinc-plated layer, i.e., on the surface of the substrate steel sheet, the Fe-based plating layer acts as a solute Si-depleted layer, reducing the amount of Si dissolved in the weld. This is thought to suppress the decline in toughness of the weld and improve the resistance weld cracking resistance of the weld (toughness decline suppression effect). Furthermore, the Fe-based plating layer functions as a soft layer, mitigating the stress applied to the steel sheet surface during resistance spot welding. This is thought to reduce the residual stress in the weld and improve the resistance weld cracking resistance (stress relaxation effect).

[0101] Examples of the Fe-based plating layer include a pure Fe plating layer, as well as 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. The composition of the Fe-based plating layer is not particularly limited as long as the Fe content is 50% by mass or more. However, a composition consisting of Fe and unavoidable impurities, or a composition containing one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in a total amount of 10% by mass or less, with the balance being Fe and unavoidable impurities, is particularly preferred. When elements other than Fe are contained, by limiting the total content of these elements to 10% by mass or less, a decrease in electrolysis efficiency can be prevented and an Fe-based plating layer, particularly an Fe-based electroplated layer, can be formed at low cost. In the case of an Fe--C alloy, the C content is preferably 0.08 mass % or less.

[0102] Furthermore, the galvanized steel sheet according to one embodiment of the present invention may have both a metal plating layer and a decarburized layer (i.e., the order from the surface of the galvanized steel sheet is the galvanized layer, the metal plating layer, and the decarburized layer (on the surface of the base steel sheet)). This can further improve resistance weld cracking resistance. When the galvanized steel sheet has a metal plating layer, the C concentration may be analyzed in the thickness direction from the surface of the metal plating layer or the interface between the galvanized layer and the cold-rolled steel sheet by the above-mentioned method, and the thickness of the decarburized layer (depth in the thickness direction from the surface of the base steel sheet) may be evaluated.

[0103] The thickness of the galvanized steel sheet according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more and 3.0 mm or less.

[0104] [2] Member Next, a member according to one embodiment of the present invention will be described. The member according to one embodiment of the present invention is a member made using (using as a raw material) the above-mentioned galvanized steel sheet. For example, the raw material galvanized steel sheet is subjected to at least one of forming and joining to form a member. Here, the above-mentioned galvanized steel sheet has a TS of 980 MPa or more, a high YS, excellent ductility, work hardening ability, and hole expandability. Therefore, the member according to one embodiment of the present invention has high strength and excellent impact resistance. Therefore, the member according to one embodiment of the present invention is particularly suitable for application to impact energy absorbing members used in the automotive field.

[0105] [3] Method for Manufacturing Galvanized Steel Sheet Next, a method for manufacturing a galvanized steel sheet according to one embodiment of the present invention will be described.

[0106] A method for producing a galvanized steel sheet according to one embodiment of the present invention includes: a hot rolling step of hot rolling a steel slab having the above-described chemical composition to obtain a hot-rolled steel sheet; a cold rolling step of cold-rolling the hot-rolled steel sheet to obtain a cold-rolled steel sheet; an annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 760°C or more and 900°C or less for an annealing time of 20 seconds or more; a first cooling step of cooling the cold-rolled steel sheet to a first cooling stop temperature of 300°C or more and 550°C or less; a holding step of holding the cold-rolled steel sheet in a temperature range of 300°C or more and 550°C or less for 3 seconds or more and 600 seconds or less; a plating step of plating the cold-rolled steel sheet with zinc to obtain a galvanized steel sheet; and a second cooling step of cooling the galvanized steel sheet to a second cooling stop temperature of 100°C or more and less than 300°C. and a reheating step of reheating the galvanized steel sheet to a reheating temperature of (the second cooling stop temperature + 50°C) or more and 500°C or less, and holding the galvanized steel sheet in a temperature range of (the second cooling stop temperature + 50°C) or more and 500°C or less for 10 seconds or more and 2000 seconds or less, wherein the first cooling stop temperature and the temperature of the galvanizing bath in the galvanizing treatment satisfy the relationship of the following formula (1): -150°C≦T ０ -T １ ≦50° C. (1) where T ０ is the first cooling stop temperature (°C), T１ is the temperature (°C) of the zinc plating bath in the zinc plating treatment. Note that, unless otherwise specified, the above temperatures refer to the surface temperatures of the steel slab and steel plate.

[0107] First, a steel slab having the above-mentioned composition is prepared. For example, a steel material is melted to obtain molten steel having the above-mentioned composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. Next, the obtained molten steel is solidified to obtain a steel slab. The method for obtaining a steel slab from molten steel is not particularly limited, and for example, a continuous casting method, an ingot casting method, or a thin slab casting method can be used. From the viewpoint of preventing macrosegregation, a continuous casting method is preferred.

[0108] [Hot Rolling Process] Next, the steel slab is hot-rolled to produce a hot-rolled steel sheet. Hot rolling may be performed using an energy-saving process. Examples of energy-saving processes include direct rolling (a method in which the steel slab is charged into a heating furnace as a hot slab without being cooled to room temperature and then hot-rolled) and direct rolling (a method in which the steel slab is briefly kept at room temperature and then immediately rolled). The hot rolling conditions are not particularly limited, and can be performed, for example, under the following conditions. That is, the steel slab is once cooled to room temperature, then reheated, and then rolled. The slab heating temperature (reheating temperature) is preferably 1100°C or higher from the viewpoints of dissolving carbides and reducing the rolling load. Furthermore, the slab heating temperature is preferably 1300°C or lower to prevent an increase in scale loss. Note that the slab heating temperature is based on the temperature of the steel slab surface. Next, the steel slab is roughly rolled according to a conventional method to produce a rough-rolled sheet (hereinafter also referred to as a sheet bar). Next, the sheet bar is subjected to finish rolling to obtain a hot-rolled steel sheet. When the slab heating temperature is set to a low value, it is preferable to heat the sheet bar using a bar heater or the like before the finish rolling in order to prevent problems during the finish rolling. The finish rolling temperature is set to a value lower than Ar in order to reduce the rolling load. ３ It is preferable that the rolling reduction is set to a temperature equal to or higher than the transformation point. In addition, if the rolling reduction rate is high in the non-recrystallized state of austenite, an abnormal structure elongated in the rolling direction may develop, which may deteriorate the workability of the annealed sheet.３ It is preferable to set the temperature at the transformation point or higher. ３ The transformation point is calculated using the following formula: Ar ３ (°C) = 868 - 396 × [C%] + 25 × [Si%] - 68 [Mn%]. In the above formula, [element symbol %] represents the content (mass %) of the element in question in the chemical composition of the base steel sheet.

[0109] Note that sheet bars may be joined together during hot rolling and continuous finish rolling may be performed. The sheet bars may also be temporarily wound before finish rolling. Furthermore, in order to reduce the rolling load during hot rolling, some or all of the finish rolling may be performed as lubricated rolling. Lubricated rolling is also effective from the viewpoint of uniforming the shape and material properties of the steel sheet. Note that the friction coefficient during lubricated rolling is preferably in the range of 0.10 to 0.25. In a hot rolling process including rough rolling and finish rolling, a steel slab is generally formed into a sheet bar by rough rolling and then into a hot-rolled steel sheet by finish rolling. However, depending on the mill capacity, etc., such classification is not essential, and it is not a problem as long as the desired size is achieved. The finish rolling temperature is preferably in the range of 800°C to 950°C. By setting the finish rolling temperature at 800°C or higher, the steel structure at the hot-rolled steel sheet stage and, ultimately, the steel structure of the final product, tend to be uniform. Note that a non-uniform steel structure tends to reduce bendability. On the other hand, if the finish rolling temperature exceeds 950°C, the amount of oxide (scale) generated increases. As a result, the interface between the base steel and the oxide becomes rough, which may deteriorate the surface quality of the steel sheet after pickling and cold rolling. In addition, the crystal grains become coarse, which may cause a decrease in the strength and bendability of the steel sheet. After finish rolling, the hot-rolled steel sheet is coiled. The coiling temperature is preferably 450°C or higher and 750°C or lower.

[0110] [Pickling Process] The hot-rolled steel sheet after the hot rolling process is optionally pickled. Pickling can remove oxides from the steel sheet surface, ensuring good chemical conversion treatability and plating quality. Pickling may be carried out once or multiple times. The pickling conditions are not particularly limited, and may be carried out in accordance with conventional methods.

[0111] [Cold Rolling Process] Next, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. Cold rolling is performed, for example, by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling. The cold-rolling reduction is not particularly limited, but is preferably 20% to 80%. If the cold-rolling reduction is less than 20%, the steel structure is likely to become coarse and non-uniform during the annealing process, which may result in reduced strength and workability in the final product. On the other hand, if the cold-rolling reduction exceeds 80%, the steel sheet is likely to have a defective shape and the amount of zinc coating may become non-uniform. Optionally, the cold-rolled steel sheet obtained after cold rolling may be pickled.

[0112] [Metal Plating Process] In addition, in the method for producing a galvanized steel sheet according to one embodiment of the present invention, a metal plating process may be optionally performed after the cold rolling process and before the annealing process described below to form a metal plating layer on at least one surface of the cold-rolled steel sheet obtained as described above. Hereinafter, a cold-rolled steel sheet having a metal plating layer on at least one surface before undergoing the annealing process described below may be referred to as a metal-plated steel sheet. While the metal plating process method is not particularly limited, electroplating is preferred from the viewpoint of manufacturability. Examples of a metal plating bath that can be used include a sulfuric acid bath, a hydrochloric acid bath, and a mixture of both. In the case of electroplating, the coating weight of the metal plating layer can be adjusted by, for example, the current application time. As described above, the metal-plated steel sheet refers to a steel sheet having a metal plating layer on at least one surface of the cold-rolled steel sheet before undergoing the annealing process described below. This does not exclude a cold-rolled steel sheet that has been annealed before the metal plating process.

[0113] The metal used in the metal plating treatment is preferably a metal with a melting point higher than that of Zn, such as Fe and Ni. In addition, it is preferable to form the above-mentioned Fe-based plating layer by metal plating treatment, as this is expected to have a greater effect of improving resistance weld crack resistance.

[0114] In addition to Fe ions, the plating bath for forming the Fe-based plating layer may contain one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The total content of these elements in the plating bath is preferably 10 mass% or less in the composition of the metal plating layer of the metal-plated steel sheet. Metal elements may be contained as metal ions, and nonmetal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. The iron sulfate plating solution may also contain conductivity enhancers such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0115] As pretreatments prior to metal plating, the cold-rolled steel sheet may optionally be degreased and rinsed to clean the surface, and further may be pickled and rinsed to activate the surface. Following these pretreatments, the above-described metal plating is carried out. The degreasing and rinsing methods are not particularly limited, and conventional methods can be used. Various acids can be used in pickling, such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof. Among these, sulfuric acid, hydrochloric acid, and mixtures thereof are preferred. The acid concentration is not particularly specified, but a concentration of approximately 1 to 20 mass% is preferred, taking into consideration the ability to remove oxide films and the prevention of surface roughness (surface defects) due to excessive pickling. The pickling solution may also contain antifoaming agents, pickling accelerators, pickling inhibitors, etc.

[0116] [Annealing step] Next, the cold-rolled steel sheet (including the case of a metal-plated steel sheet) obtained as described above is annealed at an annealing temperature of 760°C to 900°C for an annealing time of 20 seconds or more. Note that the number of annealing steps may be two or more, but one step is preferred from the viewpoint of energy efficiency.

[0117] Annealing temperature: 760°C or higher and 900°C or lower When the annealing temperature is lower than 760°C, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, resulting in a decrease in YS. In addition, the hole expandability also decreases. Furthermore, it becomes difficult to achieve a TS of 980 MPa or higher. On the other hand, when the annealing temperature exceeds 900°C, excessive grain growth of austenite occurs, slowing down the rate of bainitic ferrite generation in the subsequent process. As a result, it becomes impossible to obtain an appropriate amount of area ratio of bainitic ferrite and retained austenite. Therefore, S ＭＡ２ / S ＭＡ The annealing temperature is preferably 780°C or higher, more preferably higher than 790°C. The annealing temperature is preferably 880°C or lower. The annealing temperature is the maximum temperature reached in the annealing process.

[0118] Annealing time: 20 seconds or more If the annealing time is less than 20 seconds, the austenite generation rate during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area ratio of ferrite increases excessively after annealing, resulting in a decrease in YS. In addition, the hole expandability also decreases. Furthermore, it becomes difficult to achieve a TS of 980 MPa or more. Therefore, the annealing time is set to 20 seconds or more. Although the upper limit of the annealing time is not particularly limited, it is preferably set to 900 seconds or less. The annealing time refers to the holding time in a temperature range of (annealing temperature -40°C) or more and the annealing temperature or less. In other words, the annealing time includes not only the holding time at the annealing temperature, but also the residence time in a temperature range of (annealing temperature -40°C) or more and the annealing temperature or less during heating and cooling before and after reaching the annealing temperature.

[0119] Dew point: greater than -30°C Furthermore, in a method for producing a galvanized steel sheet according to one embodiment of the present invention, the dew point of the annealing atmosphere in the annealing step is preferably greater than -30°C. By setting the dew point to greater than -30°C, the decarburization reaction is promoted, reducing the C concentration in the surface layer of the cold-rolled steel sheet (base steel sheet), making it possible to form a decarburized layer. The dew point is preferably -20°C or higher, more preferably -5°C or higher. Setting the dew point to -5°C or higher makes it possible to further improve the resistance weld cracking resistance characteristics of the weld. There is no particular upper limit to the dew point, but from the viewpoints of suitably preventing oxidation of the cold-rolled steel sheet or the surface of the metal plating layer and improving plating adhesion when a galvanized layer is formed, the dew point is preferably 30°C or lower.

[0120] [First Cooling Step] Next, the cold-rolled steel sheet that has been annealed as described above is cooled to a first cooling stop temperature of 300°C or higher and 550°C or lower.

[0121] First cooling stop temperature: 300°C or higher and 550°C or lower If the first cooling stop temperature is lower than 300°C, the area ratio of tempered martensite increases excessively, and appropriate amounts of bainitic ferrite and retained austenite cannot be obtained. In addition, in the subsequent galvanizing process, untransformed austenite may decompose into pearlite or carbides. Therefore, S ＭＡ２ / S ＭＡ and S ＭＡ３ / S ＭＡ On the other hand, if the first cooling stop temperature exceeds 550°C, the area fraction of bainitic ferrite decreases and the area fraction of tempered martensite increases excessively. ＭＡ２ / S ＭＡ , and even S ＭＡ３ / S ＭＡ Therefore, the first cooling stop temperature is set to 300°C or higher and 550°C or lower. The first cooling stop temperature is preferably 350°C or higher. Furthermore, the first cooling stop temperature is preferably 510°C or lower.

[0122] [Holding Step] Next, the cold-rolled steel sheet is held in a temperature range of 300° C. or more and 550° C. or less (hereinafter also referred to as a holding temperature range) for 3 seconds or more and 600 seconds or less.

[0123] Holding time in the holding temperature range: 3 seconds to 600 seconds During the holding step, bainitic ferrite is generated, and C diffuses from the generated bainitic ferrite to the untransformed austenite adjacent to the bainitic ferrite. As a result, a predetermined area ratio of retained austenite is secured, and S ＭＡ２ / S ＭＡ , and even S ＭＡ３ / S ＭＡ Here, if the holding time in the holding temperature range is less than 3 seconds, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite increases excessively. ＭＡ２ / S ＭＡ , and even S ＭＡ３ / S ＭＡ On the other hand, if the holding time in the holding temperature range exceeds 600 seconds, the area ratio of bainitic ferrite increases excessively, which may result in a decrease in YS. In addition, excessive diffusion of C from bainitic ferrite to untransformed austenite occurs, which may result in an increase in S. ＭＡ１ / S ＭＡ Furthermore, excessive diffusion of C occurs inside the untransformed austenite, making it impossible to locally achieve a high amount of dissolved C only in the untransformed austenite around the bainitic ferrite. ＭＡ２ / S ＭＡ , and even S ＭＡ３ / S ＭＡThe holding time in the holding temperature range is preferably 3 seconds or more and 600 seconds or less. The holding time in the holding temperature range is preferably 5 seconds or more, more preferably 10 seconds or more. The holding time in the holding temperature range is preferably less than 200 seconds, more preferably less than 80 seconds. The holding time in the holding temperature range includes the residence time in that temperature range until the first cooling stop temperature is reached in the first cooling step, and the residence time of the cold-rolled steel sheet in that temperature range until the start of galvanization treatment in the plating step described below (for example, the residence time in that temperature range until the cold-rolled steel sheet is immersed in the galvanization bath). However, the holding time in the holding temperature range does not include the residence time of the galvanized steel sheet in that temperature range after hot-dip galvanization treatment in the plating step.

[0124] [Plating Step] Next, the cold-rolled steel sheet is subjected to a galvanizing treatment to obtain a galvanized steel sheet. Examples of the galvanizing treatment include a hot-dip galvanizing treatment and a galvannealing treatment. In this plating step, the first cooling stop temperature in the first cooling step and the temperature of the galvanizing bath in the galvanizing treatment (hereinafter also referred to as the plating bath temperature) must satisfy the relationship of the following formula (1): -150°C≦T ０ -T １ ≦50° C. (1) where T ０ is the first cooling stop temperature (°C), T １ is the temperature of the zinc plating bath in the zinc plating process (°C).

[0125] That is, from the viewpoint of ensuring excellent work hardening ability, it is necessary to appropriately control the difference between the first cooling stop temperature and the coating bath temperature, specifically, to satisfy the relationship of the above formula (1). ０ -T １ When the temperature exceeds 50°C or becomes lower than -150°C, S ＭＡ２ / S ＭＡ and S ＭＡ３ / S ＭＡ decreases, resulting in a decrease in work hardening ability and ductility. ０ -T １ is preferably −120° C. or higher, more preferably −100° C. or higher. ０ -T１ is preferably 45°C or less, more preferably 40°C or less.

[0126] Other conditions are not particularly limited and may be conventional. For example, in the case of hot-dip galvanizing, it is preferable to immerse a cold-rolled steel sheet in a galvanizing bath and then adjust the coating weight by gas wiping or the like. The galvanizing bath temperature is 440°C or higher and 500°C or lower. The galvanizing bath is not particularly limited as long as it provides the above-described composition of the galvanized layer. For example, it is preferable to use a galvanizing bath having an Al content of 0.10% by mass or higher and 0.23% by mass or lower, with the balance consisting of Zn and unavoidable impurities. In the case of alloyed galvanizing, it is preferable to perform alloying by heating the galvanized steel sheet to an alloying temperature of 450°C or higher and 600°C or lower after hot-dip galvanizing as described above. If the alloying temperature is lower than 450°C, the Zn-Fe alloying rate may be slow, making alloying difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite may transform to pearlite, resulting in reduced TS and ductility. The alloying temperature is more preferably 470° C. or higher, and more preferably 570° C. or lower.

[0127] The coating weight of both the hot-dip galvanized steel sheet (GI) and the galvannealed steel sheet (GA) is 20 to 80 g / m per side. ２ The plating weight can be adjusted by gas wiping or the like.

[0128] In addition to the holding step described above (holding the cold-rolled steel sheet before the plating step), an additional holding step may be performed in which the galvanized steel sheet is held in a temperature range of 300°C to 550°C (hereinafter also referred to as an additional holding temperature range) for 3 seconds to 600 seconds. The additional holding step is a step that achieves the same effect as the holding step. The additional holding step may be performed after or during the plating step, as long as it is performed before the second cooling step described below. In addition, when the plating step is a galvannealed plating treatment, the additional holding step may be performed during the plating step. In other words, the plating step may also serve as the additional holding step. In addition, when the additional holding step is performed, the total holding time of the holding step and the additional holding step is preferably 3 seconds to 600 seconds. The total holding time of the holding step and the additional holding step is more preferably less than 200 seconds.

[0129] [Second Cooling Step] Next, the galvanized steel sheet is cooled to a second cooling stop temperature of 100°C or higher and lower than 300°C.

[0130] Second cooling stop temperature: 100°C or higher and lower than 300°C The second cooling process is a necessary process for controlling the area ratio of tempered martensite and the area ratio of retained austenite generated in the subsequent reheating process within a predetermined range. Here, if the second cooling stop temperature is lower than 100°C, the untransformed austenite present in the steel in the second cooling process is almost entirely transformed into martensite. This causes an excessive increase in the area ratio of tempered martensite and a decrease in the area ratio of retained austenite. As a result, ductility and work hardening ability decrease. On the other hand, if the second cooling stop temperature is 300°C or higher, the area ratio of tempered martensite decreases and the area ratio of fresh martensite increases. With this increase in the area ratio of fresh martensite, the amount of diffusible hydrogen in the steel sheet increases, and hole expandability decreases. In addition, S ＭＡ１ / S ＭＡ An increase in the temperature also reduces the hole expandability. Therefore, the second cooling stop temperature is set to 100°C or higher and lower than 300°C. The second cooling stop temperature is preferably 120°C or higher. In addition, the second cooling stop temperature is preferably 280°C or lower.

[0131] [Reheating Step] Next, the galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature + 50°C) or more and 500°C or less, and the galvanized steel sheet is held in a temperature range of (the second cooling stop temperature + 50°C) or more and 500°C or less (hereinafter also referred to as the reheating temperature range) for 10 seconds or more and 2000 seconds or less. This tempers the martensite present in the steel at the end of the second cooling step. In addition, C supersaturated in the martensite is diffused into the untransformed austenite, thereby generating austenite that is stable at room temperature, i.e., retained austenite.

[0132] Reheating temperature: (Second cooling stop temperature + 50°C) or higher and 500°C or lower. If the reheating temperature is lower than (cooling stop temperature + 50°C), the diffusion of C from martensite present in the steel to untransformed austenite at the end of the second cooling step does not proceed sufficiently, and the desired area ratio of retained austenite is not achieved. This results in a decrease in ductility. Furthermore, fresh martensite increases. Furthermore, the external release of hydrogen contained in the substrate steel sheet becomes insufficient, increasing the amount of diffusible hydrogen in the substrate steel sheet. This results in a decrease in hole expandability. On the other hand, if the reheating temperature exceeds 500°C, excessive tempering of martensite present in the steel at the end of the second cooling step progresses, making it difficult to achieve a TS of 980 MPa or higher. Furthermore, the untransformed austenite present in the steel at the end of the second cooling step decomposes as carbides (pearlite), resulting in a decrease in ductility. Furthermore, the external release of hydrogen contained in the substrate steel sheet becomes insufficient, increasing the amount of diffusible hydrogen in the substrate steel sheet. This results in a decrease in hole expandability. Therefore, the reheating temperature is set to (cooling stop temperature + 50°C) or more and 500°C or less. The reheating temperature is preferably (cooling stop temperature + 70°C) or more. The reheating temperature is preferably 450°C or less. The reheating temperature is the maximum temperature reached in the reheating step.

[0133] Holding time in the reheating temperature range: 10 seconds or more and 2000 seconds or less. If the holding time in the reheating temperature range is less than 10 seconds, the diffusion of C from martensite to untransformed austenite present in the steel at the end of the second cooling step does not proceed sufficiently, and the desired area ratio of retained austenite is not achieved. This results in reduced ductility. Furthermore, in addition to an increase in fresh martensite, the external release of hydrogen contained in the base steel sheet becomes insufficient, increasing the amount of diffusible hydrogen in the base steel sheet. This may also result in reduced hole expandability. On the other hand, if the holding time in the reheating temperature range exceeds 2000 seconds, excessive tempering of the martensite present in the steel at the end of the second cooling step progresses, making it difficult to achieve a TS of 980 MPa or more. Furthermore, the untransformed austenite present in the steel at the end of the second cooling step decomposes as carbide (pearlite), resulting in reduced ductility. Therefore, the holding time in the reheating temperature range is set to 10 seconds or more and 2000 seconds or less. The holding time in the reheating temperature range is preferably 15 seconds or more. The holding time in the reheating temperature range is preferably 1200 seconds or less. The holding time in the reheating temperature range includes not only the holding time at the reheating temperature but also the residence time in the temperature range during heating and cooling before and after reaching the reheating temperature.

[0134] The cooling conditions after holding in the reheating temperature range are not particularly limited and may be conventional. Examples of cooling methods that can be used include gas jet cooling, mist cooling, roll cooling, water cooling, and air cooling. From the viewpoint of preventing surface oxidation, the material is preferably cooled to 50°C or less after holding in the reheating temperature range, more preferably to room temperature. The average cooling rate after holding in the reheating temperature range is preferably, for example, 1°C / sec or more and 50°C / sec or less.

[0135] The galvanized steel sheet obtained as described above may further be subjected to temper rolling. If the temper rolling reduction exceeds 2.00%, the yield stress increases, which may result in a decrease in dimensional accuracy when the galvanized steel sheet is formed into a component. Therefore, the temper rolling reduction is preferably 2.00% or less. The lower limit of the temper rolling reduction is not particularly limited, but is preferably 0.05% or more from the viewpoint of productivity. Furthermore, temper rolling may be performed on an apparatus continuous with the annealing apparatuses used for the above-mentioned processes (online), or may be performed on an apparatus discontinuous with the annealing apparatuses used for the above-mentioned processes (offline). The number of times temper rolling may be one, two, or more. Furthermore, rolling using a leveler or the like may be performed as long as it can impart an elongation rate equivalent to that of temper rolling.

[0136] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0137] [4] Manufacturing Method of Member Next, a manufacturing method of a member according to one embodiment of the present invention will be described. The manufacturing method of a member according to one embodiment of the present invention includes a step of subjecting the above-mentioned galvanized steel sheet (e.g., a galvanized steel sheet manufactured by the above-mentioned manufacturing method of a galvanized steel sheet) to at least one of forming and joining to form a member. Here, the forming method is not particularly limited, and for example, a general processing method such as press working can be used. In addition, the joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, riveting, caulking, etc. can be used. Note that the forming conditions and joining conditions are not particularly limited, and may be conventional methods.

[0138] Example 1 A steel material having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and formed into a steel slab by continuous casting. The obtained steel slab was heated to 1250°C, and after heating, the steel slab was subjected to hot rolling consisting of rough rolling and finish rolling to form a hot-rolled steel sheet. The obtained hot-rolled steel sheet was then pickled and cold-rolled (reduction: 50%) to form a cold-rolled steel sheet having a thickness shown in Table 3. The obtained cold-rolled steel sheet was then subjected to an annealing step, a first cooling step, a holding step, a galvanizing step, a second cooling step, and a reheating step under the conditions shown in Table 2 to obtain a galvanized steel sheet. The dew point in the annealing step was set to -35°C to -30°C.

[0139] Here, in the plating step, a hot-dip galvanizing treatment or a galvannealed treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a hot-dip galvannealed steel sheet (hereinafter also referred to as GA). In Table 2, the type of plating step is also indicated as "GI" or "GA." When performing the galvannealed treatment, except for Nos. 20, 27, and 28, the holding time in the holding temperature range and the holding time in the temperature range of 300°C to 550°C in the alloying treatment were set to a total of 3 seconds to 600 seconds.

[0140] The zinc plating bath used for producing GI was one containing 0.20 mass% Al, with the balance consisting of Zn and unavoidable impurities. The zinc plating bath used for producing GA was one containing 0.14 mass% Al, with the balance consisting of Zn and unavoidable impurities. The plating weight was 45 to 72 g / m per side for producing GI. ２ When manufacturing GA, the thickness is 45 g / m per side. ２ The composition of the zinc plating layer of the finally obtained zinc-plated steel sheet was as follows: GI: 0.1 to 1.0 mass% Fe, 0.2 to 1.0 mass% Al, and the balance being Zn and unavoidable impurities. GA: 7 to 15 mass% Fe, 0.1 to 1.0 mass% Al, and the balance being Zn and unavoidable impurities. The zinc plating layers were formed on both sides of the substrate steel sheet in all cases.

[0141] Using the galvanized steel sheets thus obtained, the steel structure of the substrate steel sheet was identified and the amount of diffusible hydrogen was measured in the same manner as described above. The results are shown in Table 3. In Table 3, F represents ferrite, BF represents bainitic ferrite, TM represents tempered martensite, RA represents retained austenite, FM represents fresh martensite, LB represents lower bainite, P represents pearlite, and θ represents cementite.

[0142] In addition, tensile tests and hole expansion tests were performed according to the following procedures, and the tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR), and limiting hole expansion ratio (λ) were evaluated according to the following criteria.・TS 〇 (Pass): 980 MPa or more × (Fail): Less than 980 MPa ・YS 〇 (Pass): 550 MPa ≦ YS when 980 MPa ≦ TS < 1180 MPa 700 MPa ≦ YS when 1180 MPa ≦ TS < 1310 MPa 800 MPa ≦ YS × (Fail): 550 MPa > YS when 980 MPa ≦ TS < 1180 MPa 700 MPa > YS when 1180 MPa ≦ TS < 1310 MPa 800 MPa > YS when 1310 MPa ≦ TS ・El 〇 (Pass): 13.0% ≦ El when 980 MPa ≦ TS < 1180 MPa If 1180MPa≦TS<1310MPa, 12.0%≦El If 1310MPa≦TS, 10.0%≦El × (Fail): If 980MPa≦TS<1180MPa, 13.0%>El If 1180MPa≦TS<1310MPa, 12.0%>El If 1310MPa≦TS, 10.0%>El n-value / YR ◯ (Pass): n-value / YR≧0.070 × (Fail): n-value / YR<0.070 λ ◯ (Pass): 20% or more × (Fail): Less than 20%

[0143] (1) Tensile Test The tensile test was conducted in accordance with JIS Z 2241. Specifically, JIS No. 5 test specimens were prepared from the obtained galvanized steel sheets so that their longitudinal direction was perpendicular to the rolling direction of the substrate steel sheet. Using the prepared test specimens, tensile tests were conducted at a crosshead speed of 10 mm / min to measure TS, YS, El, and n-values. The n-value was calculated from the elongation and strength at 0.4 and 0.8 times the uniform elongation (U-El). Furthermore, the yield ratio YR (= YS / TS) and the ratio of n-value to YR were calculated from the measured YS, TS, and n-value. The ratio of n-value to YR represents the work hardening capacity and serves as an index for comprehensively evaluating the formability and impact resistance of the steel sheet. The results are also shown in Table 3.

[0144] (2) Hole Expanding Test The hole expanding test was conducted in accordance with JIS Z 2256. Specifically, a 100 mm x 100 mm test piece was obtained from the obtained galvanized steel sheet by shearing. A 10 mm diameter hole was punched into the test piece with a clearance of 12.5%. Next, a blank holding force of 9 ton (88.26 kN) was applied around the hole using a die with an inner diameter of 75 mm. In this state, a conical punch with an apex angle of 60° was pressed into the hole, and the diameter of the hole in the test piece at the crack initiation limit (when a crack occurred) was measured. The limiting hole expanding ratio: λ (%) was then calculated using the following formula. λ is an index for evaluating stretch flangeability. The results are also shown in Table 3. λ (%) = {(D ｆ -D ０ ) / D ０}×100 where, D ｆ D: diameter of the hole in the test piece when a crack occurs (mm) ０ : initial test piece hole diameter (mm)

[0145]

[0146]

[0147]

[0148]

[0149] As shown in Table 3, in all of the inventive examples, the tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR), and limiting hole expansion ratio (λ) were all acceptable. On the other hand, in the comparative examples, at least one of the tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR), and limiting hole expansion ratio (λ) was insufficient. Furthermore, it was found that the members obtained by forming or joining the steel sheets of the inventive examples had the excellent properties characteristic of the present invention in terms of tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR), and limiting hole expansion ratio (λ).

[0150] Example 2 A steel material having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and formed into a steel slab by continuous casting. The obtained steel slab was heated to 1250°C, and after heating, the steel slab was subjected to hot rolling consisting of rough rolling and finish rolling to form a hot-rolled steel sheet. The obtained hot-rolled steel sheet was then subjected to pickling and cold rolling (reduction rate: 50%) to form a cold-rolled steel sheet with a thickness of 1.6 mm.

[0151] Next, among the obtained cold-rolled steel sheets, Nos. 8 to 10 were subjected to Fe-based electroplating as a metal plating treatment to form a metal plating layer (Fe-based plating layer) on the surface of the cold-rolled steel sheet. Specifically, the cold-rolled steel sheet was first subjected to a degreasing treatment using an alkali. Then, under the conditions shown below, an electrolytic treatment was performed using the cold-rolled steel sheet as the cathode to form a metal plating layer on the surface of the cold-rolled steel sheet. [Electrolysis conditions] Bath temperature: 50°C pH: 2.0 Current density: 45 A / dm ２ Plating bath: Fe ２＋ Sulfuric acid bath containing 1.5 mol / L of ions Anode: iridium oxide electrode The deposition weight of the metal plating layer was controlled by the current application time.

[0152] Next, the obtained cold-rolled steel sheet (including a metal-plated steel sheet having a metal plating layer formed on the surface of the cold-rolled steel sheet) was subjected to an annealing step, a first cooling step, a holding step, a plating step, a second cooling step, and a reheating step under the conditions shown in Table 5, thereby obtaining a galvanized steel sheet.

[0153] In the plating step, a galvannealed steel sheet (GA) was obtained by performing a galvannealed steel sheet plating treatment. The treatment conditions other than those described in Table 5 were the same as those in Example 1. In addition, the galvanized layers were formed on both sides of the base steel sheet in all cases.

[0154] Using the galvanized steel sheets thus obtained, the steel structure of the substrate steel sheet was identified, and the thickness of the decarburized layer, the coating weight of the metal coating layer, and the amount of diffusible hydrogen were measured in the same manner as described above. The results are shown in Table 6. In Table 6, F represents ferrite, BF represents bainitic ferrite, TM represents tempered martensite, RA represents retained austenite, FM represents fresh martensite, LB represents lower bainite, P represents pearlite, and θ represents cementite. In Table 6, for the thickness of the decarburized layer and the coating weight of the metal coating layer, "-" indicates that there is no decarburized layer and no metal coating layer, respectively.

[0155] In addition, tensile tests and hole expansion tests were conducted in the same manner as in Example 1, and the tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR), and limiting hole expansion ratio (λ) were evaluated according to the same criteria as in Example 1. The results are also shown in Table 7.

[0156] Furthermore, the resistance weld crack resistance characteristics of the welded portion were evaluated in the following manner.

[0157] <Evaluation of resistance weld crack resistance at welded portion> A test piece 2 measuring 150 mm in the longitudinal direction and 50 mm in the transverse direction was cut out of the obtained galvanized steel sheet, with the direction perpendicular to the rolling (TD) as the longitudinal direction and the rolling direction as the transverse direction, and the test piece 2 was stacked on a galvannealed steel sheet 1 (sheet thickness: 1.6 mm, TS: 980 MPa class) to form a sheet assembly. The galvannealed steel sheet 1 for test had a coating weight of 50 g / m2 per side of the galvannealed layer. ２and was cut out to the same size as the test piece 2. The sheet set was assembled so that the evaluation surface of the test piece 2 (if the test piece 2 had a zinc-plated layer and a metal-plated layer on only one side, the zinc-plated layer on that side) faced the zinc-plated layer of the test galvannealed steel sheet 1. The sheet set was fixed to a fixing base 4 via a spacer 3 having a thickness of 2.0 mm. The spacer 3 was a pair of steel plates measuring 50 mm in the longitudinal direction, 45 mm in the transverse direction, and 2.0 mm in thickness, and was arranged so that the longitudinal end faces of each of the pair of steel plates were aligned with the transverse end faces of the sheet set, as shown in Figure 5(A). Therefore, the distance between the pair of steel plates was 60 mm. The fixing base 8 was a single plate with a hole in the center.

[0158] Next, using a servomotor-driven, single-phase AC (50 Hz) resistance welding machine, the sheet assembly was pressed with a pair of electrodes 5 (tip diameter: 6 mm) while being deflected. Resistance spot welding was performed with a welding current that resulted in a nugget diameter r of 5.9 mm under the following conditions: a pressure of 3.5 kN, a hold time of 0.12 seconds, 0.18 seconds, or 0.24 seconds, and a welding time of 0.36 seconds, to obtain a sheet assembly with a weld. The pair of electrodes 5 pressed the sheet assembly from above and below in the vertical direction, and the lower electrode pressed the test piece 2 through a hole in the fixture 4. During pressing, the lower electrode and the fixture 4 were fixed so that the lower electrode of the pair of electrodes 5 was in contact with a plane extending from the contact surface between the spacer 3 and the fixture 4, and the upper electrode was movable. The upper electrode was also in contact with the center of the test galvannealed steel sheet 1. The plate assembly was welded with the plate assembly tilted 5° in the longitudinal direction of the plate assembly relative to the horizontal. The hold time refers to the time from the end of the welding current flow to the start of the electrode release. Referring to the lower diagram of Figure 5(B), the nugget diameter r refers to the distance between the ends of the nugget 6 in the longitudinal direction of the plate assembly.

[0159] Next, the plate assembly with the weld was cut along line A-A in the upper diagram of Figure 5(B) so as to include the center of the weld including nugget 6. The cross section of the weld was observed with an optical microscope (200x magnification) and the resistance weld crack resistance of the weld was evaluated according to the following criteria. A+, A, or B was determined to be excellent in the resistance weld crack resistance of the weld. A C was determined to be poor in the resistance weld crack resistance of the weld. The results are also shown in Table 7. A+: No cracks of 0.1 mm or more were observed in any of the cases where the hold time was 0.12 seconds, 0.18 seconds, or 0.24 seconds. A: A crack of 0.1 mm or more was observed in the case where the hold time was 0.12 seconds, but no cracks of 0.1 mm or more were observed in the case where the hold time was 0.18 seconds or 0.24 seconds. B: Cracks of 0.1 mm or more in length were observed at hold times of 0.12 seconds and 0.18 seconds, but no cracks of 0.1 mm or more in length were observed at a hold time of 0.24 seconds. C: Cracks of 0.1 mm or more in length were observed at hold times of 0.12 seconds, 0.18 seconds, and 0.24 seconds.

[0160] In the lower diagram of Fig. 5(B), a crack that occurred in the test piece 2 is schematically indicated by the reference symbol 7. If a crack occurs in the mating steel sheet (galvannealed steel sheet for testing), the stress will be dispersed to the steel sheets to be evaluated (steel sheets of each invention example and comparative example), and an appropriate evaluation will not be possible. For this reason, data in which no cracks occurred in the mating steel sheet was used as an example.

[0161]

[0162]

[0163]

[0164] As shown in Table 7, all of the inventive examples passed the tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR), and limiting hole expansion ratio (λ). They also had excellent resistance weld cracking resistance in the welds. In addition, inventive examples Nos. 1, 6 to 10, and especially inventive examples Nos. 8 and 9, the resistance weld cracking resistance in the welds was extremely excellent. Furthermore, it was found that members obtained by forming or joining the steel sheets of the inventive examples had the excellent properties characteristic of the present invention, including tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR), limiting hole expansion ratio (λ), and resistance weld cracking resistance in the welds.

[0165] REFERENCE SIGNS LIST 1 galvannealed steel sheet for test 2 test piece 3 spacer 4 fixing base 5 electrode 6 nugget 7 crack