Galvanized steel sheet, part, and method for manufacturing the same

Abstract

The present invention provides a zinc-plated steel sheet having a tensile strength of 980 MPa or more and high YS, and excellent ductility, work hardening ability, and hole expandability. The base steel sheet is provided with a prescribed composition, the area ratio of ferrite, bainite ferrite, tempered martensite, residual austenite, and fresh martensite in the steel structure of the base steel sheet is a prescribed value, the ratio S MA1 / S MA of the area ratios is 0.80 or less, and the ratio S MA2 / S MA of the area ratios is 0.20 or more. Here, S MA is the area ratio of hard second phases composed of residual austenite and fresh martensite, S MA1 is the total area ratio of island-shaped regions in which the equivalent circle diameter is 2.0 μm or more and 20% or less of the circumference is in contact with tempered martensite among island-shaped regions constituting the above-mentioned hard second phases, and S MA2 is the total area ratio of island-shaped regions in which the circumference is 1% or more in contact with bainite ferrite among island-shaped regions constituting the above-mentioned hard second phases.

Description

TECHNICAL FIELD

[0001] The present application relates to a galvanized steel sheet, a member using the same as a base material, and a method for manufacturing the same. BACKGROUND

[0002] In recent years, from the viewpoint of protecting the global environment, improving the fuel efficiency of automobiles has become an important issue. Therefore, the trend of making the automobile body lightweight by increasing the strength and reducing the thickness of the steel sheet as a base material of an automobile member has become more active.

[0003] In addition, the social demand for improving the collision safety of automobiles has become higher. Therefore, it is desirable to develop a steel sheet which not only has high strength but also has excellent impact resistance when an automobile collides during driving (hereinafter, also referred to simply as impact resistance). In particular, from the viewpoint of the rust resistance of the automobile body, a galvanized steel sheet is often used as a base material of an automobile member. Therefore, it is desirable to develop a galvanized steel sheet which not only has high strength but also has excellent impact resistance.

[0004] As a steel sheet which is a base material of such an automobile member, for example, in Patent Literature 1, the following is disclosed:

[0005] "A high-strength steel sheet having excellent tensile flange properties and collision resistance, characterized by containing, in mass%, 0.04 to 0.22% of C, 1.0% or less of Si, 3.0% or less of Mn, 0.05% or less of P, 0.01% or less of S, 0.01 to 0.1% of Al, and 0.001 to 0.005% of N, with the remainder consisting of Fe and inevitable impurities, and being composed of a ferrite phase as a main phase and a martensite phase as a second phase, and the maximum particle diameter of the martensite phase being 2 μm or less and the area ratio thereof being 5% or more."

[0006] In Patent Literature 2, the following is disclosed:

[0007] "A high-strength hot-dip galvanized steel sheet having excellent plating adhesion and formability, characterized in that a cold-rolled steel sheet in which a surface layer is removed by 0.1 μm or more has a hot-dip galvanized layer on the surface of the cold-rolled steel sheet to which 0.2 g / m 2 ~ 2.0 g / m 2 of Ni is pre-plated,

[0008] a hot-dip galvanized layer containing less than 7% of Fe and the remainder consisting of Zn, Al, and inevitable impurities is provided on the surface of a steel sheet,

[0009] The steel sheet contains, in 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%, and the remainder consisting of Fe and inevitable impurities,

[0010] The microstructure of the steel sheet contains, in volume%, ferrite 40% or more as a main phase, residual austenite 8% or more, two or more kinds of martensite [3] including the following specified three kinds of martensite [1], [2], [3], 1% or more of bainite, and 0 to 10% of pearlite, and the above three kinds of martensite [1], [2], [3] are respectively, in volume%, martensite [1]: 0% to 50%, martensite [2]: 0% or more and less than 20%, and martensite [3]: 1% to 30%,

[0011] In terms of tensile strength TS (MPa), total elongation EL (%), and hole expansion ratio λ (%), TS x EL is 18000 MPa% or more, and TS x λ is 35000 MPa% or more, and the tensile strength is 980 MPa or more.

[0012] Martensite [1]: C concentration (CM1) is less than 0.8%, and hardness Hv1 is:

[0013] Hv1 / (-982.1 x CM12 + 1676 x CM1 + 189) ≤ 0.60

[0014] Martensite [2]: C concentration (CM2) is 0.8% or more, and hardness Hv2 is:

[0015] Hv2 / (-982.1 x CM22 + 1676 x CM2 + 189) ≤ 0.60

[0016] Martensite [3]: C concentration (CM3) is 0.8% or more, and hardness Hv3 is:

[0017] Hv3 / (-982.1 x CM32 + 1676 x CM3 + 189) ≥ 0.80.

[0018] In Patent Literature 3, the following is disclosed:

[0019] "A high-strength hot-dip galvanized steel sheet having the following composition and steel sheet structure,

[0020] The above composition consists of, in mass%, C: 0.15 to 0.25%, Si: 0.50 to 2.5%, Mn: 2.3 to 4.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01 to 2.5%, and the remainder of Fe and inevitable impurities,

[0021] The above steel sheet has, in area%, a tempered martensite phase: 30 to 73%, a ferrite phase: 25 to 68%, a residual austenite phase: 2 to 20%, and other phases: 10% or less (including 0%), and as the other phases, a martensite phase: 3% or less (including 0%) and a bainite ferrite phase: less than 5% (including 0%), the above tempered martensite phase has an average crystal grain size of 8 μm or less, and the C amount in the above residual austenite phase is less than 0.7 mass%.

[0022] Prior art documents

[0023] Patent documents

[0024] Patent document 1: Japanese Patent No. 3887235

[0025] Patent document 2: Japanese Patent No. 5953693

[0026] Patent document 3: Japanese Patent No. 6052472 SUMMARY

[0027] However, the current situation is that the impact energy absorbing members of automobiles represented by the front side members and the rear side members are limited to use steel sheets having a tensile strength (hereinafter, also referred to as TS) of 590 MPa class.

[0028] That is, in order to increase the absorbed energy at the time of impact (hereinafter, also referred to as impact absorbed energy), it is effective to increase the yield stress (hereinafter, also referred to as YS). However, in general, if the TS and the YS of a steel sheet are increased, properties such as ductility, work hardening ability, hole expandability, and the like are deteriorated. These properties are properties related to the difficulty of cracking of a member in a bending crush test and an axial crush test of a simulated collision test. Therefore, if such a steel sheet having increased TS and YS is used for the above-described impact energy absorbing members of automobiles, not only is the molding difficult, but also the member cracks in the test of the simulated collision test, in other words, the actual impact absorbed energy is not as high as expected from the value of the YS. Therefore, the current situation is that the above-described impact energy absorbing members are limited to use steel sheets having a TS of 590 MPa class. It should be noted that the work hardening ability and the hole expandability are respectively related to protrusion property and stretch flange property.

[0029] In fact, for the steel sheets disclosed in Patent Documents 1 to 3, it cannot be said that TS: 980 MPa or more and high YS and excellent ductility, work hardening ability, and hole expandability are obtained.

[0030] The present application was developed in view of the above-described circumstances, and aims to provide a galvanized steel sheet having TS: 980 MPa or more and high YS and excellent ductility, work hardening ability, and hole expandability, and a favorable production method thereof.

[0031] In addition, the present application aims to provide a member using the above-described galvanized steel sheet as a base material and a production method thereof.

[0032] wherein high YS and excellent ductility, work hardening ability, and hole expandability mean that:

[0033] • YS determined in a tensile test based on JIS Z 2241 satisfies the following formula depending on TS determined in the tensile test,

[0034] 980 MPa ≤ TS < 1180 MPa, 550 MPa ≤ YS

[0035] 1180 MPa ≤ TS < 1310 MPa, 700 MPa ≤ YS

[0036] 1310 MPa ≤ TS, 800 MPa ≤ YS

[0037] • total elongation (El) determined in a tensile test based on JIS Z 2241 satisfies the following formula depending on TS determined in the tensile test,

[0038] 980 MPa ≤ TS < 1180 MPa, 13.0% ≤ El

[0039] 1180 MPa ≤ TS < 1310 MPa, 12.0% ≤ El

[0040] 1310 MPa ≤ TS, 10.0% ≤ El

[0041] • n value / YR determined in a tensile test based on JIS Z 2241 satisfies the following formula,

[0042] n value / YR ≥ 0.070

[0043] • and limit hole expansion ratio (λ) determined in a hole expansion test based on JIS Z 2256 is 20% or more.

[0044] Thus, the present inventors and others have repeatedly conducted intensive research in order to achieve the above-described object.

[0045] As a result, it was found that by appropriately adjusting the composition of the base steel sheet of the galvanized steel sheet, and making the steel structure of the base steel sheet of the galvanized steel sheet as follows, a galvanized steel sheet having TS: 980 MPa or more, and having high YS, and excellent ductility, work hardening ability, and hole expandability can be obtained. That is,

[0046] area fraction of ferrite: 65.0% or less (including 0%),

[0047] area fraction of bainite ferrite: 5.0% to 40.0%,

[0048] area fraction of tempered martensite: 0.5% to 80.0%,

[0049] area fraction of retained austenite: 3.0% or more,

[0050] area fraction of fresh martensite: 20.0% or less (including 0%),

[0051] S BF + S TM + 2 x S MA : 65.0% or more,

[0052] S MA1 / S MA : 0.80 or less, and

[0053] S MA2 / S MA : 0.20 or more.

[0054] The present application was completed based on the above findings plus further research.

[0055] That is, the gist of the present application is as follows.

[0056] 1. A galvanized steel sheet having a base steel sheet and a galvanized layer on the surface of the base steel sheet,

[0057] and a tensile strength of 980 MPa or more,

[0058] the base steel sheet having a composition and a steel structure as follows,

[0059] the composition being, in mass%, C: 0.050% to 0.400%, Si: 0.20% to 3.00%, Mn: 1.00% or more and less than 3.50%, P: 0.001% to 0.100%, S: 0.0200% or less, Al: 0.010% to 2.000%, and N: 0.0100% or less, the carbon equivalent Ceq being 0.540% or more, the remainder being Fe and unavoidable impurities,

[0060] The area ratio of ferrite in the steel structure: 65.0% or less (including 0%), the area ratio of bainite ferrite: 5.0% to 40.0%, the area ratio of tempered martensite: 0.5% to 80.0%, the area ratio of residual austenite: 3.0% or more, the area ratio of fresh martensite: 20.0% or less (including 0%), S BF +S TM +2×S MA : 65.0% or more, S MA1 / S MA : 0.80 or less, and S MA2 / S MA : 0.20 or more,

[0061] wherein,

[0062] S BF : the area ratio of the above bainite ferrite

[0063] S TM : the area ratio of the above tempered martensite

[0064] S MA : the area ratio of a hard second phase composed of the above residual austenite and the above fresh martensite

[0065] S MA1 : the total area ratio of island-shaped regions in which the equivalent circle diameter is 2.0 μm or more and 20% or less of the circumference is in contact with the tempered martensite in island-shaped regions constituting the above hard second phase

[0066] S MA2 : the total area ratio of island-shaped regions in which 1% or more of the circumference is in contact with the bainite ferrite in island-shaped regions constituting the above hard second phase.

[0067] 2. The galvanized steel sheet according to 1 above, wherein the base steel sheet further contains at least one kind selected from the group consisting of 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, in mass%.

[0068] 3. The galvanized steel sheet according to 1 or 2 above, wherein S MA3 / S MA is 0.05 or more.

[0069] wherein S MA3 : the total area ratio of island-like regions of which 1% or more of the circumference is in contact with the bainite ferrite phase and more than 20% of the circumference is in contact with the tempered martensite phase in the island-like regions constituting the hard second phase.

[0070] 4. The galvanized steel sheet according to any one of 1 to 3 above, wherein the diffusible hydrogen amount of the base steel sheet is 0.50 mass ppm or less.

[0071] 5. The galvanized steel sheet according to any one of 1 to 4 above, having a decarburized layer.

[0072] 6. The galvanized steel sheet according to any one of 1 to 5 above, wherein at least one of the base steel sheet and the galvanized layer has a metallic plated layer.

[0073] 7. The galvanized steel sheet according to 6 above, wherein the metallic plated layer is an Fe-based plated layer.

[0074] 8. The galvanized steel sheet according to any one of 1 to 7 above, wherein the galvanized layer is a hot-dip galvanized layer or an alloyed hot-dip galvanized layer.

[0075] 9. A member made using the galvanized steel sheet according to any one of 1 to 8 above.

[0076] 10. A method of manufacturing a galvanized steel sheet, comprising the steps of: a hot rolling step of hot-rolling a steel slab having the composition according to claim 1 or 2 to produce a hot-rolled steel sheet, a cold rolling step of cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet, an annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 760°C to 900°C and 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 to 550°C, a holding step of holding the cold-rolled steel sheet at a temperature range of 300°C to 550°C for 3 seconds to 600 seconds, a plating step of galvanizing the cold-rolled steel sheet to produce a galvanized steel sheet, 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) to 500°C, and holding the galvanized steel sheet at a temperature range of (the second cooling stop temperature + 50°C) to 500°C for 10 seconds to 2000 seconds.

[0077] a hot rolling step of hot-rolling a steel slab having the composition according to claim 1 or 2 to produce a hot-rolled steel sheet,

[0078] a cold rolling step of cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet,

[0079] an annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 760°C to 900°C and an annealing time of 20 seconds or more,

[0080] a first cooling step of cooling the cold-rolled steel sheet to a first cooling stop temperature of 300°C to 550°C,

[0081] a holding step of holding the cold-rolled steel sheet at a temperature range of 300°C to 550°C for 3 seconds to 600 seconds,

[0082] a plating step of galvanizing the cold-rolled steel sheet to produce a galvanized steel sheet,

[0083] 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,

[0084] a reheating step of reheating the galvanized steel sheet to a reheating temperature of (the second cooling stop temperature + 50°C) to 500°C, and holding the galvanized steel sheet at a temperature range of (the second cooling stop temperature + 50°C) to 500°C for 10 seconds to 2000 seconds,

[0085] the first cooling stop temperature and the temperature of the galvanizing bath in the galvanizing step satisfy the following relation (1).

[0086] -150°C ≤ To - Ti ≤ 50°C (1)

[0087] wherein To is the first cooling stop temperature (°C) and Ti is the temperature of the galvanizing bath in the galvanizing step (°C).

[0088] 11. The method of manufacturing a galvanized steel sheet according to claim 10, wherein the dew point of the annealing step is greater than -30°C.

[0089] 12. The method of manufacturing a galvanized steel sheet according to claim 10 or 11, further comprising a metal plating 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.

[0090] 13. The method of manufacturing a galvanized steel sheet according to claim 12, wherein the metal plating layer is an Fe-based plating layer.

[0091] 14. The method of producing a galvanized steel sheet according to any one of 10 to 13, wherein the galvanizing treatment is a hot-dip galvanizing treatment or an alloyed hot-dip galvanizing treatment.

[0092] 15. A method of producing a member, comprising a step of subjecting the galvanized steel sheet according to any one of 1 to 8 to at least one of a forming process or a joining process to produce a member.

[0093] According to the present application, a galvanized steel sheet having TS: 980 MPa or more and high YS, and excellent ductility, work hardening ability, and hole expandability can be obtained. In addition, a member using the galvanized steel sheet of the present application as a base material is high in strength and has excellent impact resistance, and thus can be extremely favorably applied to an impact energy absorbing member of an automobile or the like. BRIEF DESCRIPTION OF DRAWINGS

[0094] Figure 1 (A) of FIG. 1 is an example of a microstructure image obtained by SEM used in the identification of the microstructure, Figure 1 (B) of FIG. 1 is a figure in which the microstructure image of (A) is color-coded using Adobe Photoshop of Adobe Systems Inc.

[0095] Figure 2 (A) of FIG. 2 is an example of a microstructure image obtained by SEM used in the identification of island-like regions of the hard second phase, particularly a microstructure image including island-like regions identified as MA1, Figure 2 (B) of FIG. 2 is a figure in which the microstructure image of (A) is color-coded using Adobe Photoshop of Adobe Systems Inc.

[0096] Figure 3 (A) of FIG. 3 is an example of a microstructure image obtained by SEM used in the identification of island-like regions of the hard second phase, particularly a microstructure image including island-like regions identified as MA2, Figure 3 (B) of FIG. 3 is a figure in which the microstructure image of (A) is color-coded using Adobe Photoshop of Adobe Systems Inc.

[0097] Figure 4 (A) of FIG. 4 is an example of a microstructure image obtained by SEM used in the identification of island-like regions of the hard second phase, particularly a microstructure image including island-like regions identified as MA3, Figure 4 (B) of FIG. 4 is a figure in which the microstructure image of (A) is color-coded using Adobe Photoshop of Adobe Systems Inc.

[0098] Figure 5 (A) is a schematic diagram illustrating the gist of an evaluation method of the resistance spot welding crack resistance of the welded portion, Figure 5 The upper drawing in (B) is a plan view of a plate set after resistance spot welding used in the evaluation, Figure 5 The lower drawing in (B) is an A-A sectional view of the upper drawing. DETAILED DESCRIPTION

[0099] The present application will be described based on the following embodiments.

[0100] [1] Galvanized steel sheet

[0101] First, the composition of the base steel sheet of the galvanized steel sheet based on one embodiment of the present application will be described. Note that the unit of the composition is "mass%" and, hereinafter, simply represented by "%" unless otherwise specified.

[0102] C: 0.050% to 0.400%

[0103] C is an element effective to generate appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and residual austenite to ensure TS of 980 MPa or more and high YS. When the C content is less than 0.050%, the area ratio of ferrite increases, and it is difficult to make TS 980 MPa or more. In addition, it also leads to a decrease in YS. On the other hand, if the C content exceeds 0.400%, the carbon concentration in the residual austenite excessively increases. Therefore, if the steel sheet is subjected to blanking processing, the hardness of fresh martensite generated from the residual austenite greatly increases. As a result, for the steel sheet after the blanking processing, the crack propagation at the time of reaming is promoted (i.e., leading to a decrease in reamability).

[0104] Therefore, the C content is 0.050% to 0.400%. The C content is preferably 0.100% or more. In addition, the C content is preferably 0.300% or less.

[0105] Si: 0.20% to 3.00%

[0106] Si suppresses carbide generation in annealing and promotes the generation of residual austenite. That is, Si is an element that affects the area ratio of residual austenite and the carbon concentration in the residual austenite. When the Si content is less than 0.20%, the area ratio of residual austenite decreases, and the ductility decreases. On the other hand, if the Si content exceeds 3.00%, the area ratio of ferrite excessively increases, and it is difficult to make TS 980 MPa or more. In addition, it also leads to a decrease in YS. Also, the carbon concentration in the residual austenite excessively increases. Therefore, if the steel sheet is subjected to blanking processing, the hardness of fresh martensite generated from the residual austenite greatly increases. As a result, for the steel sheet after the blanking processing, the crack propagation at the time of reaming is promoted (i.e., leading to a decrease in reamability).

[0107] Therefore, the Si content is 0.20% to 3.00%. The Si content is preferably 0.40% or more. In addition, if the Si content exceeds 2.00%, there is a concern that the resistance to electric resistance welding cracking characteristics will decrease, so the Si content is preferably 2.00% or less.

[0108] Mn: 1.00% or more and less than 3.50%

[0109] Mn is an element that adjusts the area ratio of bainite ferrite, tempered martensite, and the like. If the Mn content is less than 1.00%, the area ratio of ferrite excessively increases, and it is difficult to make the TS 980 MPa or more. In addition, 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 bainite ferrite decreases, and the area ratio of tempered martensite excessively increases. As a result, the desired ductility is not obtained.

[0110] Therefore, the Mn content is 1.00% or more and less than 3.50%. The Mn content is preferably 1.80% or more. In addition, the Mn content is preferably less than 3.20%.

[0111] P: 0.001% to 0.100%

[0112] P is an element that increases the strength of the steel sheet by having a solid solution strengthening effect. In order to obtain such an effect, the P content is made 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates to the old austenite grain boundaries to make the grain boundaries brittle. Therefore, if the steel sheet is subjected to blanking processing, the amount of voids generated increases, leading to a decrease in hole expansion.

[0113] Therefore, the P content is 0.001% to 0.100%. The P content is preferably 0.030% or less.

[0114] S: 0.0200% or less

[0115] S exists in the form of sulfides in steel. In particular, if the S content exceeds 0.0200%, the limit ductility of the steel sheet decreases. Therefore, if the steel sheet is subjected to blanking processing, the amount of voids generated increases, leading to a decrease in hole expansion.

[0116] Therefore, the S content is 0.0200% or less. The S content is preferably 0.0080% or less. It should be noted that the lower limit of the S content is not particularly specified, and the S content is preferably 0.0001% or more from the viewpoint of limitations in production technology.

[0117] Al: 0.010% to 2.000%

[0118] Al suppresses the generation of carbides in annealing and promotes the generation of residual austenite. That is, Al is an element that has an influence on the area ratio of residual austenite and the carbon concentration in residual austenite. In order to obtain such an effect, the Al content is made to be 0.010% or more. On the other hand, if the Al content exceeds 2.000%, the area ratio of ferrite excessively increases, and it is difficult to make the TS 980 MPa or more. In addition, it also leads to a decrease in YS.

[0119] Therefore, the content of Al is 0.010% to 2.000%. The Al content is preferably 0.015% or more. In addition, the Al content is preferably 1.000% or less.

[0120] N: 0.0100% or less

[0121] N exists in the steel in the form of nitride. In particular, if the N content exceeds 0.0100%, the limit ductility of the steel sheet decreases. Therefore, if the steel sheet is subjected to blanking processing, the amount of generation of voids increases, leading to a decrease in hole expansibility.

[0122] Therefore, the N content is 0.0100% or less. In addition, the N content is preferably 0.0050% or less. It should be noted that the lower limit of the N content is not particularly specified, and the N content is preferably 0.0005% or more from the limitation in production technology.

[0123] Carbon equivalent Ceq: 0.540% or more

[0124] The carbon equivalent Ceq has an influence on TS. In particular, if the carbon equivalent Ceq is less than 0.540%, it is difficult to make the TS 980 MPa or more. Therefore, the carbon equivalent Ceq is made to be 0.540% or more.

[0125] wherein the carbon equivalent Ceq is defined by the following formula.

[0126] Carbon equivalent Ceq = [C%] + ([Si%] / 24) + ([Mn%] / 6) + ([Ni%] / 40) + ([Cr%] / 5) + ([Mo%] / 4) + ([V%] / 14)

[0127] It should be noted that [element symbol %] in the above formula indicates the content (mass %) of the element in the composition of the base steel sheet. In addition, an element not contained in the composition of the base steel sheet is counted as 0.

[0128] The above describes the basic components of the base steel sheet of the galvanized steel sheet according to one embodiment of the present application, but the base steel sheet of the galvanized steel sheet according to one embodiment of the present application has a composition including the above basic components and the remaining portion other than the above basic components includes Fe (iron) and inevitable impurities. Here, the base steel sheet of the galvanized steel sheet according to one embodiment of the present application preferably has a composition including the above basic components and the remaining portion consisting of Fe and inevitable impurities. The base steel sheet of the galvanized steel sheet according to one embodiment of the present application can contain at least one selected from any of the following components other than the above basic components. Note that the following any of the components can achieve the effects of the present application as long as they are contained in an amount equal to or less than the upper limit described below, and thus the lower limit is not particularly set. Note that the following any of the components is contained as inevitable impurities when it is contained in an amount less than the lower limit described below.

[0129] 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

[0130] Ti: 0.200% or less

[0131] Ti forms fine carbides, nitrides, or carbonitrides at the time of hot rolling or at the time of annealing, thereby increasing TS. In order to achieve such an effect, it is preferable that the Ti content be 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%, a large amount of coarse precipitates or inclusions are sometimes generated. In such a case, if diffusible hydrogen is present in the steel sheet, the coarse precipitates or inclusions become the starting points of cracks at the time of a hole expansion test, that is, can cause a decrease in hole expandability. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.

[0132] Nb: 0.200% or less

[0133] Nb, like Ti, improves TS by forming fine carbides, nitrides or carbonitrides at the time of hot rolling and at the time of annealing. In order to obtain such an effect, it is preferable to make the Nb content 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%, sometimes a large amount of coarse precipitates, inclusions are generated. In such a case, if diffusible hydrogen exists in the steel sheet, the coarse precipitates, inclusions become the starting point of cracking at the time of hole expansion test, i.e., can cause a decrease in hole expansibility. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.

[0134] V: 0.100% or less

[0135] V, like Ti and Nb, improves TS by forming fine carbides, nitrides or carbonitrides at the time of hot rolling and at the time of annealing. In order to obtain such an effect, it is preferable to make the V content 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%, sometimes a large amount of coarse precipitates, inclusions are generated. In such a case, if diffusible hydrogen exists in the steel sheet, the coarse precipitates, inclusions become the starting point of cracking at the time of hole expansion test, i.e., can cause a decrease in hole expansibility. Therefore, when V is contained, the V content is preferably 0.100% or less. The V content is more preferably 0.060% or less.

[0136] B: 0.0100% or less

[0137] B is an element that improves hardenability by segregating to the grain boundaries of austenite. In addition, B is an element that suppresses the generation and grain growth of ferrite at the time of cooling after annealing. In order to obtain such an effect, it is preferable to make the B content 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%, it can cause cracks in the steel sheet at the time of hot rolling, and decrease the limit ductility of the steel sheet. In addition, as the limit ductility of the steel sheet decreases, the amount of voids generated at the time of punching processing of the steel sheet increases, causing a decrease in hole expansibility. Therefore, when B is contained, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.

[0138] Cu: 1.000% or less

[0139] Cu is an element that improves the hardenability. In particular, Cu is an element effective for adjusting the area ratio of fresh martensite and the like to a more preferable range and adjusting the TS to a more preferable range. In order to obtain such effects, 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 excessively increases, and the TS becomes excessively high. In addition, a large amount of coarse precipitates and inclusions are sometimes generated. In such a case, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions become the starting points of cracks at the time of a tensile test, i.e., can cause a decrease in the hole expandability. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.200% or less.

[0140] Cr: 1.000% or less

[0141] Cr is an element that improves the hardenability, and in addition, Cr is an element effective for generating residual austenite and fresh martensite. In order to obtain such effects, the Cr content is preferably 0.0005% or more. In particular, from the viewpoint of adjusting the TS to a more preferable 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 fresh martensite excessively increases, and can cause a decrease in the hole expandability. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less. In addition, the Cr content is more preferably 0.250% or less, and further preferably 0.100% or less.

[0142] Ni: 1.000% or less

[0143] Ni is an element that improves the hardenability. In addition, Ni is an element effective for adjusting the area ratio of residual austenite and fresh martensite to a more preferable range and adjusting the TS to a more preferable range. In order to obtain such effects, 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 ratio of fresh martensite excessively increases, and the ductility and the dimensional accuracy at the time of molding can decrease. In addition, a large amount of coarse precipitates and inclusions are sometimes generated. In such a case, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions become the starting points of cracks at the time of a hole expansion test, i.e., can cause a decrease in the hole expandability. Therefore, when Ni is contained, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.

[0144] Mo: 0.500% or less

[0145] Mo is an element for improving the hardenability. In addition, Mo is an effective element for generating hard fresh martensite and the like. In order to obtain such an 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 excessively increases, which can result in a decrease in the 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 further preferably 0.400% or less.

[0146] Sb: 0.200% or less

[0147] Sb is an element effective for suppressing the diffusion of C in the vicinity of the surface of the steel sheet during annealing to control the formation of a soft layer in the vicinity of the surface of the steel sheet. In order to obtain such an 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 in the vicinity of the surface of the steel sheet, which can result in a decrease in the 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.

[0148] Sn: 0.200% or less

[0149] Sn, like Sb, is an element effective for suppressing the diffusion of C in the vicinity of the surface of the steel sheet during annealing to control the formation of a soft layer in the vicinity of the surface of the steel sheet. In order to obtain such an 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 in the vicinity of the surface of the steel sheet, which can result in a decrease in the 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.

[0150] Ta: 0.100% or less

[0151] Ta, like Ti, Nb, and V, improves the TS by forming fine carbides, nitrides, or carbonitrides at the time of hot rolling and at the time of annealing. Also, a portion of Ta is dissolved in the Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). Thus, the coarsening of the precipitates is suppressed, and the precipitation strengthening is stabilized. Thus, the TS is improved, and further, the YS is improved. In order to obtain such effects, it is preferable that the Ta content be 0.001% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions are sometimes generated. In such a case, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions become the starting points of cracking at the time of the expansion test, i.e., the expansion property can be degraded. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less.

[0152] W: 0.500% or less

[0153] W is an element effective for improving the hardenability and adjusting the TS to a more preferable range. In order to obtain such effects, it is preferable that the W content be 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 excessively increases, and the expansion property can be degraded. 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 further preferably 0.400% or less.

[0154] Mg: 0.0200% or less

[0155] Mg is an element effective for spheroidizing the shape of inclusions such as sulfides and oxides, improving the limit deformation capacity of the steel sheet, and further improving the expansion property. In order to obtain such effects, it is preferable that the Mg content be 0.0001% or more. On the other hand, if the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are sometimes generated. In such a case, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions become the starting points of cracking at the time of the expansion test, i.e., the expansion property can be degraded. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less.

[0156] Zn: 0.0200% or less

[0157] Zn is an element effective for making the shape of inclusions spherical, improving the limit deformation capacity of the steel sheet, and further improving the expandability. In order to obtain such effects, 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 are sometimes generated. In such a case, if diffusible hydrogen exists in the steel sheet, the coarse precipitates and inclusions become the starting points of cracks at the time of the expansion test, i.e., can cause a decrease in the expandability. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less.

[0158] Co: 0.0200% or less

[0159] Co, like Zn, is an element effective for making the shape of inclusions spherical, improving the limit deformation capacity of the steel sheet, and further improving the expandability. In order to obtain such effects, the Co content is preferably 0.0010% or more. On the other hand, if the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are sometimes generated. In such a case, if diffusible hydrogen exists in the steel sheet, the coarse precipitates and inclusions become the starting points of cracks at the time of the expansion test, i.e., can cause a decrease in the expandability. Therefore, when Co is contained, the Co content is preferably 0.0200% or less.

[0160] Zr: 0.0200% or less

[0161] Zr, like Zn and Co, is an element effective for making the shape of inclusions spherical, improving the limit deformation capacity of the steel sheet, and further improving the expandability. In order to obtain such effects, the Zr content is preferably 0.0010% or more. On the other hand, if the Zr content exceeds 0.0200%, a large amount of coarse precipitates and inclusions are sometimes generated. In such a case, if diffusible hydrogen exists in the steel sheet, the coarse precipitates and inclusions become the starting points of cracks at the time of the expansion test, i.e., can cause a decrease in the expandability. Therefore, when Zr is contained, the Zr content is preferably 0.0200% or less.

[0162] Ca: 0.0200% or less,

[0163] Ca exists in the form of inclusions in the steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions are sometimes generated. In such a case, if diffusible hydrogen exists in the steel sheet, the coarse inclusions become the starting points of cracks at the time of the expansion test, i.e., can cause a decrease in the expandability. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The Ca content is preferably 0.0020% or less. It should be noted that the lower limit of the Ca content is not particularly limited, and the Ca content is preferably 0.0005% or more. Further, the Ca content is more preferably 0.0010% or more from the viewpoint of the limitation in production technology.

[0164] 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

[0165] Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective to improve the limit deformation capacity of the steel sheet, and further improve the hole expandability. In order to obtain such effects, the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more. On the other hand, if the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM each exceed 0.0200%, sometimes a large amount of coarse precipitates and inclusions are generated. In such a case, if diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions become the starting points of cracks at the time of the hole expansion test, i.e., can cause a decrease in the hole expandability. Therefore, when at least one of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is contained, the content thereof is each preferably 0.0200% or less.

[0166] That is, the base steel sheet of the galvanized steel sheet according to one embodiment of the present application has the following composition:

[0167] C: 0.050% to 0.400%, Si: 0.20% to 3.00%, Mn: 1.00% or more and less than 3.50%, P: 0.001% to 0.100%, S: 0.0200% or less, Al: 0.010% to 2.000%, and N: 0.0100% or less in mass%, and the carbon equivalent Ceq is 0.540% or more,

[0168] Arbitrarily contains at least one selected from 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, and the remainder is Fe and inevitable impurities.

[0169] Next, the steel structure of the base steel sheet of the galvanized steel sheet based on one embodiment of the present application is described.

[0170] The steel structure of the base steel sheet of the galvanized steel sheet based on one embodiment of the present application is:

[0171] Area fraction of ferrite: 65.0% or less (including 0%),

[0172] Area fraction of bainite ferrite: 5.0% to 40.0%,

[0173] Area fraction of tempered martensite: 0.5% to 80.0%,

[0174] Area fraction of retained austenite: 3.0% or more,

[0175] Area fraction of fresh martensite: 20.0% or less (including 0%),

[0176] S BF +S TM +2×S MA : 65.0% or more,

[0177] S MA1 / S MA : 0.80 or less, and

[0178] S MA2 / S MA : 0.20 or more

[0179] steel structure.

[0180] wherein,

[0181] S BF : area ratio of the above-described bainite ferrite

[0182] S TM : area ratio of the above-described tempered martensite

[0183] S MA : area ratio of the hard second phase composed of the above-described residual austenite and the above-described fresh martensite

[0184] S MA1 : area ratio of the sum of island-shaped regions in which the equivalent circle diameter is 2.0 μm or more and 20% or less of the circumference is in contact with the tempered martensite, among island-shaped regions constituting the above-described hard second phase

[0185] S MA2 : area ratio of the sum of island-shaped regions in which 1% or more of the circumference is in contact with the bainite ferrite, among island-shaped regions constituting the above-described hard second phase.

[0186] Hereinafter, reasons for the respective limitations will be described.

[0187] Area ratio of ferrite: 65.0% or less (including 0%)

[0188] The 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 expansibility, the area ratio of the ferrite is made 65.0% or less. The area ratio of the ferrite is preferably 35.0% or less, and more preferably 25.0% or less. The lower limit of the area ratio of the ferrite is not particularly limited, and can be 0%. In particular, when 980 MPa ≤ TS < 1180 MPa is required, the area ratio of the ferrite is preferably 5.0% or more.

[0189] Area ratio of bainite ferrite: 5.0% to 40.0%

[0190] The bainite ferrite has a hardness that is intermediate between the soft ferrite and the hard fresh martensite or the like, and is an important phase for ensuring good hole expansibility. In addition, the bainite ferrite is a useful phase for obtaining an appropriate amount of residual austenite by diffusing C from the bainite ferrite to the untransformed austenite. Therefore, the area ratio of the bainite ferrite is made 5.0% or more. In addition, the area ratio of the bainite ferrite is preferably 10.0% or more. On the other hand, if the area ratio of the bainite ferrite is excessively increased, the hole expansibility is instead decreased. Therefore, the area ratio of the bainite ferrite is made 40.0% or less. In addition, the area ratio of the bainite ferrite is preferably 35.0% or less.

[0191] Area ratio of tempered martensite: 0.5% to 80.0%

[0192] The tempered martensite has a hardness that is intermediate between the soft ferrite and the hard fresh martensite, and is an important phase for ensuring good hole expansibility. Therefore, the area fraction of the tempered martensite is made to be 0.5% or more. The area fraction of the tempered martensite is preferably 40.0% or more. On the other hand, from the viewpoint of ensuring good ductility, the area fraction of the tempered martensite is made to be 80.0% or less. In addition, the area fraction of the tempered martensite is preferably 75.0% or less.

[0193] Area fraction of residual austenite: 3.0% or more

[0194] From the viewpoint of obtaining good ductility, the area fraction of the residual austenite is made to be 3.0% or more. The area fraction of the residual austenite is preferably 5.0% or more. Note that the upper limit of the area fraction of the residual austenite is not particularly limited, and the area fraction of the residual austenite is preferably 20.0% or less.

[0195] Area fraction of fresh martensite: 20.0% or less (including 0%)

[0196] From the viewpoint of ensuring good hole expansibility, the area fraction of the fresh martensite is made to be 20.0% or less. Note that the lower limit of the area fraction of the fresh martensite is not particularly limited, and can be 0%. In addition, from the viewpoint of ensuring a TS of 980 MPa or more, the area fraction of the fresh martensite is preferably 3.0% or more.

[0197] Note that the fresh martensite refers to the martensite in the quenched state (without being tempered).

[0198] Note that the area fraction of the remaining portion of the microstructure other than the above is preferably 10.0% or less. The area fraction of the remaining portion of the microstructure is more preferably 5.0% or less. In addition, the area fraction of the remaining portion of the microstructure can be 0%.

[0199] Note that the remaining portion of the microstructure is not particularly limited, and for example, lower bainite, pearlite, carbides such as cementite, and the like can be cited. Note that the type of the remaining portion of the microstructure can be confirmed, for example, by observation using an SEM (Scanning Electron Microscope).

[0200] Note that the area fractions of the ferrite, the bainitic ferrite, the tempered martensite, and the hard second phase (residual austenite + fresh martensite) are measured at a position of 1 / 4 of the thickness of the base steel sheet.

[0201] That is, a sample was cut out from the base steel sheet in a manner that a plate thickness section parallel to the rolling direction of the base steel sheet becomes an observation surface. Next, mirror polishing was performed on the observation surface of the sample using a diamond polishing paste. Next, after performing final polishing on the observation surface of the sample using colloidal silica, etching was performed using 3 vol.% nitric acid alcohol to make the structure appear.

[0202] Then, under the conditions of an acceleration voltage of 15 kV and a magnification of 5000 times, the field of view of 25.6 μm x 17.6 μm of the observation surface of the five samples was observed using an SEM (Scanning Electron Microscope).

[0203] Based on the obtained structure image (for example, refer to Figure 1 (A)), ferrite, bainite ferrite, tempered martensite, and hard second phase (retained austenite + fresh martensite) were identified as follows.

[0204] Ferrite: a region appearing black, and having a blocky shape. In addition, it hardly contains iron-based carbides. However, in the case where it contains iron-based carbides, the area of the ferrite also includes the area of the iron-based carbides. In addition, the same applies to the bainite ferrite and the tempered martensite described later.

[0205] Bainite ferrite: a region appearing black to dark gray, and having a blocky shape, amorphous shape, or the like. In addition, it does not contain iron-based carbides or contains a small amount of iron-based carbides.

[0206] Tempered martensite: a region appearing gray, and having an amorphous shape. In addition, it contains a large amount of iron-based carbides.

[0207] Hard second phase (retained austenite + fresh martensite): a region appearing white to light gray, and having an amorphous shape. In addition, it does not contain iron-based carbides. Note that when the size is large, the color gradually becomes darker as it moves away from the interface with other structures, and the inside is sometimes dark gray.

[0208] Remaining structure: examples include the above-described lower bainite, pearlite, cementite, and the like, and their shapes and the like are as known.

[0209] Note that in addition to the above-described observation using an SEM, observation of carbides at a higher magnification, detailed structure analysis using EBSD (Electron Backscatter Diffraction) for the same field of view, composition analysis using an EPMA (Electron Probe Micro-Analyzer), local hardness measurement using a micro hardness tester, and the like can be appropriately added. For example, in the case where it is difficult to identify the structure by the above-described observation using an SEM, it is effective to appropriately add these measurements.

[0210] For example, in analysis using EBSD, ferrite lacks a lower microstructure (not observed). On the other hand, bainitic ferrite, tempered martensite, and fresh martensite possess a lower microstructure, exhibiting retained austenite and specific crystal orientation relationships. Furthermore, these microstructures can be used to reconstruct the austenite microstructure from the annealing process, thus confirming its presence. Such points serve as criteria for microstructure identification.

[0211] Furthermore, in compositional analysis using EPMA, the C and Mn concentrations vary depending on the microstructure, serving as crucial criteria for microstructural identification. For instance, the C concentration in ferrite and bainitic ferrite is lower than that in regions dominated by tempered martensite (including fine hard second phases and carbides). Additionally, the Mn concentration in ferrite is sometimes lower than in other microstructures.

[0212] In hardness testing using a microhardness tester, points with varying hardness based on the microstructure become the criteria for microstructure identification. For example, among ferrite, bainitic ferrite, tempered martensite, and hard second phase, ferrite has the lowest hardness, while hard second phase has the highest. Furthermore, bainitic ferrite and tempered martensite exhibit hardness levels between those of ferrite and hard second phase.

[0213] Next, using Adobe Photoshop from Adobe Systems, the regions of each phase identified in the tissue image will be color-coded (image quaternization) (for example, refer to...). Figure 1 (B) The area of ​​each phase was calculated. Next, the area of ​​each phase (the total area of ​​all phases) was calculated for each of the five fields of view, divided by the area of ​​the observation region (25.6 μm × 17.6 μm), and multiplied by 100. The average of these values ​​was then used as the area fraction of each phase (ferrite, bainitic ferrite, tempered martensite, and hard second phase). It should be noted that... Figure 1 (A) was obtained by extracting a portion of the observation area (25.6 μm × 17.6 μm) of the sample for the purpose of the above description.

[0214] In addition, the area ratio of the retained austenite was determined as follows.

[0215] That is, the base steel plate is mechanically ground along its thickness (depth direction) to 1 / 4 of its thickness, and then chemically ground with oxalic acid to create an observation surface. Next, the observation surface is observed using X-ray diffraction. CoKα rays are used as the incident X-rays. The ratio of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to those of the (200), (211), and (220) planes of bcc iron is calculated, and the volume fraction of the retained austenite is calculated from the ratio of the diffraction intensities of each plane. Then, the retained austenite is considered three-dimensionally homogeneous, and the volume fraction of the retained austenite is used as the area fraction of the retained austenite.

[0216] In addition, the area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite from the area ratio of the hard second phase as described above.

[0217] [Area percentage of fresh martensite (%)] = [Area percentage of hard second phase (%)] - [Area percentage of retained austenite (%)]

[0218] In addition, the area ratio of the remaining part of the structure is obtained by subtracting the area ratio of ferrite, bainitic ferrite, tempered martensite, and hard second phase as calculated above from 100%.

[0219] [Area fraction of remaining microstructure (%)] = 100 - [Area fraction of ferrite (%)] - [Area fraction of bainitic ferrite (%)] - [Area fraction of tempered martensite (%)] - [Area fraction of hard second phase (%)]

[0220] S BF +S TM +2×S MA : 65.0% or more

[0221] From the perspective of ensuring a TS of over 980 MPa, S BF +S TM +2×S MA It is above 65.0%. BF +S TM +2×S MA There is no particular upper limit, but it is preferred to be below 130.0%.

[0222] in,

[0223] S BF : Area ratio of bainitic ferrite

[0224] S TM Area ratio of tempered martensite

[0225] S MA : The area ratio of the hard second phase composed of residual austenite and the aforementioned fresh martensite.

[0226] S MA1 / S MA : 0.80 or less

[0227] The hard second phase composed of residual austenite and fresh martensite (hereinafter, also referred to as MA) is composed of a plurality of island-like regions. Among such island-like regions, the island-like region in which the equivalent circle diameter is 2.0 μm or more and 20% or less of the circumference is in contact with tempered martensite (hereinafter, also referred to as MA1) has a low solid solution C concentration. In other words, the residual austenite contained in MA1 has low stability. Therefore, MA1 is not favorable for securing good ductility. In addition, since the ratio of fresh martensite in MA1 is high, MA1 deteriorates the hole expansibility. Therefore, the ratio of the area of MA1 to the area of the hard second phase, that is, S MA1 / S MA is 0.80 or less. Particularly, in the case where 980 MPa ≤ TS < 1180 MPa is required, S MA1 / S MA is preferably 0.75 or less, and more preferably 0.40 or less. In addition, in the case where 1180 MPa ≤ TS is required, S MA1 / S MA is preferably 0.50 or less, and more preferably 0.30 or less. Note that the lower limit of S MA1 / S MA is not particularly limited, and can be 0.

[0228] Note that each island-like region is separated from other island-like regions of the hard second phase by a phase other than the hard second phase (the entire circumference of each island-like region is in contact with the phase other than the hard second phase). In addition, the specific shape of each island-like region is not particularly limited, and can be any shape such as a circular shape, an elliptical shape, a polygonal shape, an amoeboid shape (a shape extending in a plurality of irregular directions), and the like.

[0229] S MA2 / S MA : 0.20 or more

[0230] The island-like region in which 1% or more of the circumference in the island-like region constituting the hard second phase composed of residual austenite and fresh martensite is in contact with bainitic ferrite (hereinafter, also referred to as MA2) has a high solid solution C concentration. In other words, the residual austenite contained in MA2 has high stability. Therefore, MA2 plays an extremely important role in securing good work hardening ability and ductility.

[0231] That is, if bainite ferrite is generated under appropriate conditions during cooling after annealing, solid-solution C diffusing from the bainite ferrite to the surrounding untransformed austenite cannot sufficiently diffuse to the inside of the untransformed austenite. That is, only the periphery of the bainite ferrite in the untransformed austenite can be made to be in a state in which the solid-solution C amount is locally high. Thereafter, by performing a re-heating treatment under appropriate conditions in this state, a hard second phase, i.e., MA2, in which the solid-solution C concentration is high around the bainite ferrite is generated. Therefore, the stability of the residual austenite contained in MA2 is high, and MA2 plays an extremely important role in ensuring good work hardening ability and ductility.

[0232] In view of the above, the ratio of the area ratio of MA2 to the area ratio of the hard second phase, i.e., S MA2 / S MA is preferably 0.20 or greater. S MA2 / S MA is more preferably 0.30 or greater. Note that S MA2 / S MA is not particularly limited and can be 1. In addition, from the viewpoint of ensuring a high YS and excellent hole expandability, in the case where 980 MPa ≤ TS < 1180 MPa is required, S MA2 / S MA is preferably 0.98 or less. In addition, in the case where 1180 MPa ≤ TS is required, S MA2 / S MA is preferably 0.70 or less.

[0233] In addition, in the steel structure of the base steel sheet of the galvanized steel sheet according to one embodiment of the present application, it is further preferable that S MA3 / S MA be 0.05 or greater.

[0234] wherein

[0235] S MA3 : the total area ratio of island-shaped regions in which 1% or greater of the perimeter is in contact with bainite ferrite and more than 20% of the perimeter is in contact with tempered martensite in the island-shaped regions constituting the hard second phase composed of residual austenite and fresh martensite.

[0236] S MA3 / S MA : 0.05 or greater

[0237] In the island-shaped regions constituting the hard second phase composed of residual austenite and fresh martensite, island-shaped regions in which 1% or greater of the perimeter is in contact with bainite ferrite and more than 20% of the perimeter is in contact with tempered martensite (hereinafter, also referred to as MA3.) are particularly high in solid-solution C concentration in MA2.

[0238] In other words, in MA3, dissolved carbon diffuses not only from bainitic ferrite but also from tempered martensite, resulting in a particularly high concentration of dissolved carbon. Therefore, MA3 is particularly effective in ensuring good work hardening ability and ductility.

[0239] Therefore, the ratio of the area fraction of MA3 to the area fraction of the hard second phase, i.e., S... MA3 / S MA Preferably, it is 0.05 or higher. S MA3 / S MA Preferably, it is 0.07 or higher, more preferably 0.10 or higher. It should be noted that S MA3 / S MA There is no specific upper limit for S; it can be 1. Additionally, S... MA3 / S MA Preferably, it is below 0.70.

[0240] Among them, S MA1 S MA2 and S MA3 The measurements were taken as follows.

[0241] That is, following the above principles, in organizing the image (for example, referring to...) Figure 2 (A) Figure 3 (A) and Figure 4 In (A), ferrite, bainitic ferrite, tempered martensite, and hard second phase (retained austenite + fresh martensite) were identified. Next, after color coding (image quaternization) using Adobe Photoshop (Adobe Systems), the island regions of the hard second phase were extracted. Using the open-source ImageJ, the equivalent circle diameter, perimeter, and the length of the connection between each island region and the bainitic ferrite and tempered martensite were calculated. It should be noted that the pixel density of the microstructure image used to calculate the perimeter was 30 pixels / μm to 100 pixels / μm. Then, based on the calculated values, it was determined whether each island region belonged to MA1, MA2, and MA3, respectively. Color coding was performed using Adobe Photoshop (Adobe Systems) (for example, refer to...). Figure 2 (B) Figure 3 (B) and Figure 4 (B) Calculate the area of ​​each region. Next, for the five fields of view, calculate the total area of ​​each island region identified as MA1, MA2, and MA3, divided by the area of ​​the observation area (25.6 μm × 17.6 μm), and multiplied by 100 to obtain the value (area ratio). Then, set the average value (area ratio) of the five fields of view corresponding to MA1, MA2, and MA3 as S. MA1 S MA2 and S MA3It should be noted that, for the island-shaped region belonging to both MA1 and MA2, the area is calculated using both MA1 and MA2. The same applies to MA1 and MA3, and MA2 and MA3. In addition, Figure 2 (A) of FIG. 1, Figure 3 (A) of FIG. 1, and FIG. 4(A) are each extracted from a part of one field of view of the observation region (25.6 μm x 17.6 μm) of the sample.

[0242] In addition, in the base steel sheet of the galvanized steel sheet according to one embodiment of the present application, the diffusible hydrogen amount is preferably 0.50 mass ppm or less.

[0243] Diffusible hydrogen amount of the base steel sheet: 0.50 mass ppm or less

[0244] The diffusible hydrogen amount of the base steel sheet is preferably 0.50 mass ppm or less from the viewpoint of obtaining more excellent hole expandability. In addition, the diffusible hydrogen amount of the base steel sheet is more preferably 0.35 mass ppm or less. It should be noted that the lower limit of the diffusible hydrogen amount of the base steel sheet is not particularly specified, and can be 0 mass ppm. In addition, the diffusible hydrogen amount of the base steel sheet is more preferably 0.01 mass ppm or more from the viewpoint of the limitation in production technology.

[0245] The diffusible hydrogen amount of the base steel sheet is measured as follows.

[0246] That is, a test piece having a length of 30 mm and a width of 5 mm is taken from the galvanized steel sheet, and the alkali is removed from the galvanized layer. Subsequently, the amount of hydrogen released from the test piece is measured by a temperature programmed desorption analysis method. Specifically, the test piece is continuously heated from room temperature to 300°C at a temperature increasing rate of 200°C / h, and then cooled to room temperature. At this time, the amount of hydrogen released from the test piece (cumulative hydrogen amount) is measured in the temperature region from room temperature to 210°C in the continuous heating. The measured amount of hydrogen is then divided by the mass of the test piece (the test piece after the removal of the galvanized layer and before the continuous heating) and converted to a value in mass ppm units as the diffusible hydrogen amount of the base steel sheet.

[0247] It should be noted that, for a product (member) in which the galvanized steel sheet is subjected to a forming process or a joining process, a test piece is cut from the product under a general use environment, and the diffusible hydrogen amount of the base steel sheet portion is measured in accordance with the same procedure as described above, and if the value is 0.50 mass ppm or less, the diffusible hydrogen amount of the base steel sheet of the galvanized steel sheet at the stage of the blank before the forming process or the joining process can also be regarded as 0.50 mass ppm or less.

[0248] In addition, the galvanized steel sheet according to one embodiment of the present application preferably has a decarburized layer. In particular, the base steel sheet of the galvanized steel sheet according to one embodiment of the present application preferably has a decarburized layer. For a plated steel sheet in which a steel sheet containing Si, particularly a steel sheet having a high Si content, is used as a base steel sheet, cracks due to liquid metal embrittlement (LME) at the time of resistance spot welding sometimes become a problem. However, when the surface layer of the galvanized steel sheet, particularly the base steel sheet, has a decarburized layer, even in the case where the base steel sheet has a high Si content, the resistance welding crack resistance can be improved.

[0249] The thickness of the decarburized layer, in other words, the depth in the sheet thickness direction from the surface of the base steel sheet, is preferably 30 μm or more, and more preferably 40 μm or more. The upper limit of the thickness of the decarburized layer is not particularly limited, and in order to be in a range in which the tensile strength is good, the thickness of the decarburized layer is preferably 130 μm or less. In this case, when the C concentration of the base steel sheet is analyzed in the sheet thickness direction from the surface of the base steel sheet, the region in which the C concentration is 80% or less of the C content of the composition of the base steel sheet is defined as the decarburized layer, and the thickness of the decarburized layer is defined as the thickness of this region.

[0250] In addition, the thickness of the decarburized layer is measured by, for a sample subjected to cross-section processing, performing surface analysis or line analysis of the elemental distribution in the vicinity of the surface layer of the base steel sheet using an electron probe micro analyzer (EPMA). First, the galvanized steel sheet embedded in resin is ground, and after final processing of a cross-section perpendicular to the rolling direction, the sample is taken out from the resin and used as a sample for measurement. The acceleration voltage is set to 7 kV, the irradiation current is set to 50 nA, and surface analysis or line analysis of the cross-section of the sample is performed in a range of 300 x 300 μm including the surface layer (surface) of the base steel sheet at a step of 1 μm, and measurement of the C intensity is performed. At this time, in order to suppress contamination, a plasma cleaning machine is used to remove hydrocarbons from the surface and the periphery of the sample in both the measurement chamber and the sample preparation chamber before the start of measurement. In addition, in order to suppress the accumulation of hydrocarbons during measurement, the measurement is performed on a stage in a state in which the temperature of the sample is heated and maintained at a maximum of 100°C. The C intensity is converted to the C concentration (mass%) using a calibration curve prepared using a standard sample for measurement. Due to the effect of suppressing contamination, it is confirmed that the C detection limit is much lower than 0.10 mass%. The details of the device used and the above-described method for suppressing contamination are explained in Reference Literature 1 below.

[0251] Reference Literature 1: Yamanaka et al., "Distribution of Carbon at the Initial Stage of Proeutectoid Ferrite Transformation in Low Carbon Steel by High Precision FE-EPMA", Iron and Steel, Vol. 103 (2017) No. 11. p14-20

[0252] The necessity of the countermeasure against contamination at the time of measurement is determined by the model or conditions of use, and the above-described configuration is not essential. That is, the measurement conditions are not essential to the effects of the present application, and it is only necessary to be able to confirm that sufficient precision is obtained.

[0253] In the obtained C concentration profile, the spectral line distribution in the plate thickness direction is extracted from the surface of the base steel sheet, and is averaged in the parallel direction to the surface of the base steel sheet at 300 points, whereby a profile of the C concentration in the plate thickness direction is obtained. The profile of the C concentration in the plate thickness direction obtained is smoothed using a simple moving average method. At this time, the number of smoothing points is preferably about 21 points. Next, in the intensity distribution after the smoothing, a range in the plate thickness direction in which the C concentration is 80% or less of the C content of the composition of the base steel sheet is confirmed as the thickness of the decarburized layer.

[0254] Next, the mechanical properties of the galvannealed steel sheet based on one embodiment of the present application will be described.

[0255] Tensile strength (TS): 980 MPa or more

[0256] The tensile strength of the galvannealed steel sheet based on one embodiment of the present application is 980 MPa or more. The tensile strength of the galvannealed steel sheet based on one embodiment of the present application is preferably 1180 MPa or more.

[0257] Note that the yield stress (YS), total elongation (El), work hardening exponent (n value) / yield ratio (YR), and limit hole expansion ratio (λ) of the galvannealed steel sheet based on one embodiment of the present application are as described above.

[0258] In addition, the tensile strength (TS), yield stress (YS), total elongation (El), and work hardening exponent (n value) / yield ratio (YR) are measured by the tensile test based on JIS Z 2241 described later in the examples. The limit hole expansion ratio (λ) is measured by the hole expansion test based on JIS Z 2256 described later in the examples.

[0259] In addition, the galvannealed layer of the galvannealed steel sheet based on one embodiment of the present application can be provided only on one surface of the base steel sheet, or can be provided on both surfaces.

[0260] Note that the galvannealed layer referred to herein means a plated layer in which Zn is the main component (Zn content is 50% or more), and for example, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer can be given.

[0261] Here, the hot-dip galvanized layer is, for example, preferably composed of Zn with 20 mass% or less of Fe, 0.001 mass% to 1.0 mass% of Al. In addition, the hot-dip galvanized layer can arbitrarily contain 1 or 2 or more kinds of 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 of 0 mass% to 3.5 mass%. Further, the Fe content of the hot-dip galvanized layer is more preferably less than 7 mass%. Note that the remainder other than the above elements is an unavoidable impurity.

[0262] In addition, the alloyed hot-dip galvanized layer is, for example, preferably composed of 20 mass% or less of Fe, 0.001 mass% to 1.0 mass% of Al. In addition, the alloyed hot-dip galvanized layer can arbitrarily contain 1 or 2 or more kinds of 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 of 0 mass% to 3.5 mass%. The Fe content of the alloyed hot-dip galvanized layer is more preferably 7 mass% or more, and further preferably 8 mass% or more. In addition, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15 mass% or less, and further preferably 12 mass% or less. Note that the remainder other than the above elements is an unavoidable impurity.

[0263] In addition, the plating adhesion amount per one face of the zinc-plated layer is not particularly limited, and is preferably 20 to 80 g / m 2 .

[0264] Note that the plating adhesion amount of the zinc-plated layer is determined as follows.

[0265] That is, a treatment solution in which 0.6 g of a corrosion inhibitor for Fe (IBIT 700BK (registered trademark) manufactured by Asahi Denka Kogyo K.K.) is added to 1 L of 10 mass% hydrochloric acid aqueous solution is prepared. Next, a zinc-plated steel sheet serving as a test material is immersed in the treatment solution, and the zinc-plated layer is dissolved. Then, the mass reduction amount of the test material before and after the dissolution is measured, and the value is divided by the surface area of the base steel sheet (the surface area of the portion covered by plating), to calculate the plating adhesion amount (g / m 2 ).

[0266] Further, the galvanized steel sheet according to one embodiment of the present application has a metal plated layer other than the zinc plated layer at least one of the base steel sheet and the zinc plated layer. The metal plated layer contributes to improvement of the resistance spot welding crack resistance. Moreover, by forming the metal plated layer, the resistance spot welding crack resistance can be suppressed even when the Si content of the base steel sheet is high. The mechanism of improvement of the resistance spot welding crack resistance by the metal plated layer is not clear, but the present inventors believe that when the metal plated layer is present between the base steel sheet and the zinc plated layer, in other words, on the surface of the base steel sheet, the metal plated layer functions as a barrier layer that suppresses the penetration of zinc in the zinc plated layer into the base steel sheet at the time of resistance spot welding, and the resistance spot welding crack is less likely to occur (effect of suppression of zinc penetration). Note that when the zinc plated layer is provided on both surfaces of the base steel sheet, the metal plated layer can be provided on only one of the base steel sheet and the zinc plated layer, or on both of the base steel sheet and the zinc plated layer.

[0267] The attachment amount of the metal plated layer is preferably more than 0 g / m2 2 More preferably, it is 2.0 g / m2 2 The upper limit of the attachment amount of the metal plated layer per one surface is not particularly limited, and from the viewpoint of cost, the attachment amount of the metal plated layer is preferably 60 g / m2 2 More preferably, the attachment amount of the metal plated layer is 50 g / m2 2 Further preferably, it is 40 g / m2 2 More further preferably, it is 30 g / m2 2 Note that the attachment amount of the metal plated layer referred to herein is the attachment amount per one surface.

[0268] The attachment amount of the metal plated layer is determined as follows. That is, a test piece of 10 x 15 mm in size is taken from the galvanized steel sheet and embedded in resin to prepare a cross-section embedded test piece. The thickness of the metal plated layer is measured at three arbitrary positions of the cross-section of the test piece using a scanning electron microscope (SEM) at a magnification of 2000 to 10000 times at an acceleration voltage of 15 kV based on the thickness of the metal plated layer, and the average value thereof is calculated. Next, the calculated average value is multiplied by the specific gravity of the metal constituting the metal plated layer, and converted into the attachment amount of the metal plated layer per one surface.

[0269] As the metal used in the metal plated layer, a metal having a higher melting point than Zn is preferred, and for example, metals such as Fe and Ni can be used. In addition, from the viewpoint of the effect of suppression of decrease in toughness in addition to the above-mentioned effect of suppression of zinc penetration, an Fe-based plated layer is preferred.

[0270] That is, it is considered that in the case where the amount of Si near the surface of the base steel sheet is large, the toughness of the welded portion is decreased and the resistance spot welding crack resistance of the welded portion is deteriorated. On the contrary, in the case where the base steel sheet has the Fe-based plated layer between the base steel sheet and the zinc plating layer, that is, the surface of the base steel sheet has the Fe-based plated layer, the Fe-based plated layer functions as a solid solution Si-deficient layer, and the amount of solid solution Si in the welded portion is decreased. Thus, it is considered that the decrease in toughness of the welded portion can be suppressed, and the resistance spot welding crack resistance of the welded portion can be improved (toughness decrease suppression effect). In addition, the Fe-based plated layer functions as a soft layer, and the stress imparted to the surface of the steel sheet at the time of resistance spot welding is moderated. Thus, it is considered that the residual stress of the welded portion can be decreased, and the resistance spot welding crack resistance can be improved (stress moderation effect).

[0271] As the Fe-based plated layer, in addition to the plated layer of pure Fe, for example, there are alloy plated layers of Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, Fe-W alloy, and the like. The composition of the Fe-based plated layer is not particularly limited as long as the Fe content is 50 mass% or more, and it is particularly preferable that the composition consists of Fe and inevitable impurities, or that the composition contains 1 or 2 or more kinds of elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in a total of 10 mass% or less and the remainder is Fe and inevitable impurities. When elements other than Fe are contained, by making the content of these elements a total of 10 mass% or less, it is possible to prevent a decrease in electrolytic efficiency, and it is possible to form the Fe-based plated layer, particularly the Fe-based electroplated layer, at low cost. Note that in the case of the Fe-C alloy, the C content is preferably 0.08 mass% or less.

[0272] In addition, the zinc-plated steel sheet according to one embodiment of the present application can have both the metallic plated layer and the decarburized layer (i.e., in order from the surface of the zinc-plated steel sheet, the zinc plating layer, the metallic plated layer, the decarburized layer (of the surface layer of the base steel sheet)). Thus, it is possible to further improve the resistance spot welding crack resistance. When the metallic plated layer is present, the C concentration can be analyzed from the surface of the metallic plated layer or from the interface between the zinc plating layer and the cold-rolled steel sheet in the thickness direction of the sheet, and the thickness of the decarburized layer (the depth in the thickness direction from the surface of the base steel sheet) can be evaluated.

[0273] Note that the thickness of the zinc-plated steel sheet according to one embodiment of the present application is not particularly limited, and is preferably 0.5 mm to 3.0 mm.

[0274] [2] Component

[0275] Next, a component according to one embodiment of the present application will be described.

[0276] The component based on one embodiment of the present application is a component using the above-described galvanized steel sheet (as a base material). For example, the galvanized steel sheet as a base material can be subjected to at least one of a forming process or a joining process to produce a component.

[0277] wherein the above-described galvanized steel sheet has TS: 980 MPa or more, and has high YS and excellent ductility, work hardening ability, and hole expandability. Therefore, the component based on one embodiment of the present application is high in strength, and also excellent in impact resistance. Therefore, the component based on one embodiment of the present application is particularly suitable for an impact energy absorbing component used in the automobile field.

[0278] [3] Method for manufacturing a galvanized steel sheet

[0279] Next, a method for manufacturing a galvanized steel sheet based on one embodiment of the present application will be described.

[0280] The method for manufacturing a galvanized steel sheet based on one embodiment of the present application has the following steps:

[0281] a hot rolling step of subjecting a steel billet having the above-described composition to hot rolling to produce a hot rolled steel sheet,

[0282] a cold rolling step of subjecting the above-described hot rolled steel sheet to cold rolling to produce a cold rolled steel sheet,

[0283] an annealing step of annealing the above-described cold rolled steel sheet at an annealing temperature: 760°C to 900°C and an annealing time: 20 seconds or more,

[0284] a first cooling step of cooling the above-described cold rolled steel sheet to a first cooling stop temperature of 300°C to 550°C,

[0285] a holding step of holding the above-described cold rolled steel sheet at a temperature range of 300°C to 550°C for 3 seconds to 600 seconds,

[0286] a plating step of subjecting the above-described cold rolled steel sheet to a galvanizing treatment to produce a galvanized steel sheet,

[0287] a second cooling step of cooling the above-described galvanized steel sheet to a second cooling stop temperature of 100°C or more and less than 300°C, and

[0288] a reheating step of reheating the above-described galvanized steel sheet to a reheating temperature of (the above-described second cooling stop temperature + 50°C) to 500°C, and holding the above-described galvanized steel sheet at a temperature range of (the above-described second cooling stop temperature + 50°C) to 500°C for 10 seconds to 2000 seconds,

[0289] the above-described first cooling stop temperature and the temperature of a galvanizing bath in the above-described galvanizing treatment satisfy the following formula (1).

[0290] - 150°C < To - Ti < 50°C... (1)

[0291] wherein To is a first cooling stop temperature (°C) and Ti is a temperature of a galvanizing bath in a galvanizing treatment (°C).

[0292] Note that the above temperatures represent the surface temperature of the billet and the steel sheet unless otherwise specified.

[0293] First, a billet having the above composition is prepared. For example, a steel material is melted to produce a molten steel having the above composition. The melting method is not particularly limited, and a known melting method such as converter melting, electric furnace melting, or the like can be used. Next, the resulting molten steel is solidified to produce a billet. The method for obtaining a billet from a molten steel is not particularly limited, and for example, a continuous casting method, an ingot casting method, or a thin slab casting method, or the like can be used. From the viewpoint of preventing macro-segregation, the continuous casting method is preferred.

[0294] [Hot rolling step]

[0295] Next, the billet is subjected to hot rolling to produce a hot-rolled steel sheet.

[0296] The hot rolling can employ an energy saving process. As the energy saving process, direct charging rolling (a method in which a billet is not cooled to room temperature but is directly charged into a heating furnace in a hot billet state to perform hot rolling) or direct rolling (a method in which a billet is immediately rolled after being slightly heat-insulated) or the like can be given.

[0297] The hot rolling conditions are not particularly limited, and for example, the following conditions can be employed.

[0298] That is, the billet is temporarily cooled to room temperature, and then, after being reheated, is subjected to rolling. From the viewpoint of reducing the dissolution of carbides and the rolling load, the slab heating temperature (reheating temperature) is preferably 1100°C or higher. In addition, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. Note that the slab heating temperature is based on the temperature of the surface of the billet.

[0299] Next, the billet is subjected to rough rolling according to a conventional method to produce a rough-rolled sheet (hereinafter, also referred to as a thin steel sheet). Next, the thin steel sheet is subjected to finish rolling to produce a hot-rolled steel sheet. Note that when the slab heating temperature is low, from the viewpoint of preventing failure during finish rolling, it is preferred to heat the thin steel sheet using a bar heater or the like before finish rolling. In order to reduce the rolling load, the finish rolling temperature is preferably the Ar3 transformation point or higher. In addition, if the reduction rate in the non-recrystallized state of austenite is high, abnormal structure grows in the rolling direction, which can cause a decrease in the workability of the annealed sheet, and from this point of view, the finish rolling temperature is also preferably the Ar3 transformation point or higher. Note that the Ar3 transformation point is calculated by the following formula.

[0300] Ar3(°C) = 868 - 396 x [C%] + 25 x [Si%] - 68 x [Mn%]

[0301] Note that [element symbol %] in the above formula indicates the content (mass %) of the element in the composition of the base steel sheet.

[0302] Note that the thin steel sheets can be joined to each other at the time of hot rolling, and the finish rolling can be performed continuously. Alternatively, the thin steel sheets can be temporarily wound before the finish rolling. Further, in order to reduce the rolling load at the time of hot rolling, a part or all of the finish rolling can be lubrication rolling. It is also effective to perform the lubrication rolling from the viewpoint of uniformization of the shape of the steel sheet and uniformization of the material. Note that the coefficient of friction at the time of lubrication rolling is preferably in the range of 0.10 to 0.25.

[0303] In the hot rolling process including rough rolling and finish rolling, a general billet becomes a thin steel sheet by the rough rolling, and becomes a hot-rolled steel sheet by the finish rolling. However, regardless of such division depending on the capacity of the mill or the like, it is not a problem as long as the prescribed size is reached.

[0304] The finish rolling temperature is preferably in the range of 800°C to 950°C. By making the finish rolling temperature 800°C or higher, the steel structure at the stage of the hot-rolled steel sheet, and further the steel structure of the final product, also easily becomes uniform. Note that if the steel structure is not uniform, there is a tendency that the bendability decreases. On the other hand, if the finish rolling temperature exceeds 950°C, the amount of oxide (scale) generation becomes large. As a result, the interface between the base iron and the oxide becomes rough, and the surface quality of the steel sheet after pickling and cold rolling can be deteriorated. Further, since the grains become coarse, it can also become a cause of decrease in the strength or bendability of the steel sheet.

[0305] After the finish rolling, the hot-rolled steel sheet is wound. The winding temperature is preferably in the range of 450°C to 750°C.

[0306] [Pickling process]

[0307] The hot-rolled steel sheet after the hot rolling process is optionally subjected to pickling. By the pickling, the oxide on the surface of the steel sheet can be removed, and good chemical conversion treatment property and plating quality can be ensured. Note that the pickling can be performed only once, or can be performed in multiple times. The pickling conditions are not particularly limited, and can be performed according to the conventional method.

[0308] [Cold rolling process]

[0309] Next, the hot-rolled steel sheet is subjected to cold rolling to produce a cold-rolled steel sheet. The cold rolling is performed, for example, by multi-stand rolling of a tandem type or multi-pass rolling requiring 2 or more passes.

[0310] The cold-rolling reduction is not particularly limited, and is preferably 20 to 80%. When the cold-rolling reduction is less than 20%, coarsening and non-uniformity of the steel structure are likely to occur in the annealing step, and the strength and workability of the final product can decrease. On the other hand, if the cold-rolling reduction exceeds 80%, shape defects of the steel sheet are likely to occur, and the amount of zinc plating becomes non-uniform.

[0311] In addition, the obtained cold-rolled steel sheet is optionally subjected to pickling after the cold-rolling.

[0312] [metal plating treatment step]

[0313] In addition, in the method for producing a zinc-plated steel sheet according to one embodiment of the present application, a metal plating treatment of forming a metal plating layer on at least one surface of the cold-rolled steel sheet obtained as described above can be optionally performed after the cold-rolling step and before the annealing step described later. Hereinafter, the cold-rolled steel sheet having a metal plating layer on at least one surface in a state before the annealing step described later is sometimes referred to as a metal-plated steel sheet. The method of the metal plating treatment is not particularly limited, and from the viewpoint of manufacturability, electroplating is preferred. As the metal plating bath, a sulfuric acid bath, a hydrochloric acid bath, or a mixed solution of both, or the like can be used. In the case of electroplating, the amount of the metal plating layer can be adjusted according to the current application time, or the like. Note that, as described above, the metal-plated steel sheet refers to a steel sheet having a metal plating layer on at least one surface in a state before the annealing step described later, and does not exclude a method in which the cold-rolled steel sheet before the metal plating treatment is subjected to pre-annealing.

[0314] As the metal used in the metal plating treatment, a metal having a higher melting point than Zn is preferred, and metals such as Fe and Ni can be used, for example. In addition, from the viewpoint of being able to expect a higher effect of improving the resistance to welding crack characteristics, it is preferred that the Fe-based plating layer described above be formed by the metal plating treatment.

[0315] In addition, in the plating bath for forming the Fe-based plating layer, in addition to Fe ions, one or two 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 can be contained. The total content of these elements in the plating bath is preferably such that the total content of these elements becomes 10% by mass or less in the component composition of the metal plating layer of the metal-plated steel sheet. Note that the metal elements can be contained as metal ions, and the non-metal elements can be contained as a part of boric acid, phosphoric acid, nitric acid, organic acid, or the like. In addition, in the iron sulfate plating solution, a conductivity aid such as sodium sulfate, potassium sulfate, a chelating agent, and a pH buffer can be contained.

[0316] Note that, as the pretreatment before the metal plating treatment, a degreasing treatment and water washing for cleaning the surface of the cold-rolled steel sheet, and an acid pickling treatment and water washing for activating the surface of the cold-rolled steel sheet can be arbitrarily performed. The metal plating treatment described above can be performed after these pretreatments. The method of the degreasing treatment and water washing is not particularly limited, and a general method can be used. Various acids such as sulfuric acid, hydrochloric acid, nitric acid, and a mixture thereof can be used in the acid pickling treatment. Among them, sulfuric acid, hydrochloric acid, or a mixture thereof is preferred. The concentration of the acid is not particularly specified, and is preferably about 1 to 20 mass% from the viewpoint of the ability to remove the oxide film and to prevent surface roughening (surface defects) caused by over-pickling. In addition, a defoaming agent, a pickling accelerator, a pickling inhibitor, or the like can be contained in the acid pickling treatment solution.

[0317] [Annealing Step]

[0318] Next, the cold-rolled steel sheet (including the case of the metal-plated steel sheet) obtained as described above is annealed at an annealing temperature of 760 to 900°C and an annealing time of 20 seconds or more. Note that the number of times of annealing can be 2 or more, but from the viewpoint of energy efficiency, 1 time is preferred.

[0319] Annealing temperature: 760 to 900°C

[0320] When the annealing temperature is less than 760°C, the proportion of the generation of austenite in the heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite excessively increases after annealing, and the YS decreases. In addition, the hole expandability also decreases. Furthermore, it is difficult to make the TS 980 MPa or more. On the other hand, if the annealing temperature exceeds 900°C, the grain growth of austenite excessively proceeds, and the generation speed of bainite ferrite in the subsequent step becomes slow. Thus, the area ratio of the appropriate amount of bainite ferrite and residual austenite is not obtained. Therefore, the S MA2 / S MA decreases, and the ductility and work hardening ability decrease. Therefore, the annealing temperature is made 760 to 900°C. The annealing temperature is preferably 780°C or more, and more preferably exceeds 790°C. In addition, the annealing temperature is preferably 880°C or less. Note that the annealing temperature is the maximum reached temperature in the annealing step.

[0321] Annealing time: 20 seconds or more

[0322] If the annealing time is less than 20 seconds, the proportion of austenite generated in the heating in the ferrite and austenite two-phase region becomes insufficient. Therefore, the area ratio of ferrite after annealing excessively increases, and YS decreases. In addition, hole expansibility also decreases. And it is difficult to make TS be 980 MPa or more. Therefore, the annealing time is made to be 20 seconds or more. It should be noted that the upper limit of the annealing time is not particularly limited, and is preferably 900 seconds or less. It should be noted that the annealing time means the holding time in the temperature range of (annealing temperature - 40°C) to the annealing temperature. That is, the annealing time includes, in addition to the holding time at the annealing temperature, the residence time in the temperature range of (annealing temperature - 40°C) to the annealing temperature in the heating and cooling before and after reaching the annealing temperature.

[0323] Dew point: more than -30°C

[0324] In addition, in the method for producing a galvanized steel sheet according to one embodiment of the present application, the dew point of the annealing atmosphere in the annealing step is preferably more than -30°C. By making the dew point more than -30°C, the decarburization reaction can be promoted, the C concentration of the surface layer of the cold-rolled steel sheet (base steel sheet) can be reduced, and a decarburized layer can be formed. The dew point is preferably -20°C or more, and more preferably -5°C or more. By making the dew point -5°C or more, the resistance welding crack resistance of the welded portion can be further improved. The upper limit of the dew point is not particularly limited, and from the viewpoint of appropriately preventing oxidation of the surface of the cold-rolled steel sheet or the metal plated layer and making the plating adhesion good when the galvanized layer is provided, the dew point is preferably 30°C or less.

[0325] [First cooling step]

[0326] Next, the cold-rolled steel sheet on which the annealing has been performed as described above is cooled to a first cooling stop temperature of 300°C to 550°C.

[0327] First cooling stop temperature: 300°C to 550°C

[0328] If the first cooling stop temperature is less than 300°C, the area ratio of tempered martensite excessively increases, and the area ratios of bainite ferrite and residual austenite are not appropriately obtained. In addition, in the galvanizing treatment as a subsequent step, untransformed austenite sometimes decomposes into pearlite and carbide. Therefore, S MA2 / S MA and S MA3 / S MA decrease, and the ductility and work hardening ability decrease. On the other hand, if the first cooling stop temperature exceeds 550°C, the area ratio of bainite ferrite decreases, and the area ratio of tempered martensite excessively increases. In addition, S MA2 / S MA , and S MA3 / S MAdecrease in ductility and work hardening ability. Therefore, the first cooling stop temperature is set to 300°C to 550°C. The first cooling stop temperature is preferably 350°C or higher. In addition, the first cooling stop temperature is preferably 510°C or lower.

[0329] [holding step]

[0330] Next, the cold-rolled steel sheet is held at a temperature region of 300°C to 550°C (hereinafter, also referred to as a holding temperature region) for 3 seconds to 600 seconds.

[0331] holding time in the holding temperature region: 3 seconds to 600 seconds

[0332] In the holding step, bainite ferrite is generated, and C diffusion from the generated bainite ferrite to untransformed austenite adjacent to the bainite ferrite occurs. As a result, the area ratio of the prescribed amount of residual austenite, S MA2 / S MA , and S MA3 / S MA increases.

[0333] wherein if the holding time in the holding temperature region is less than 3 seconds, the area ratio of bainite ferrite decreases, and the area ratio of tempered martensite excessively increases. In addition, S MA2 / S MA , and S MA3 / S MA decreases, and the ductility and work hardening ability decrease. On the other hand, if the holding time in the holding temperature region exceeds 600 seconds, the area ratio of bainite ferrite excessively increases, and YS can decrease. In addition, C diffusion from the bainite ferrite to untransformed austenite can excessively occur, S MA1 / S MA increases, and the hole expandability decreases. And C diffusion inside the untransformed austenite excessively occurs, and it is not possible to become a state in which the untransformed austenite around the bainite ferrite has a locally high solid-solution C amount. As a result, S MA2 / S MA , and S MA3 / S MAThe decrease in the ductility. Therefore, the holding time in the holding temperature range is set to 3 seconds to 600 seconds. The holding time in the holding temperature range is preferably 5 seconds or more, and more preferably 10 seconds or more. In addition, the holding time in the holding temperature range is preferably 200 seconds or less, and more preferably 80 seconds or less. Note that the holding time in the holding temperature range includes the residence time of the cold-rolled steel sheet in the temperature range before reaching the first cooling stop temperature in the first cooling step, and the residence time of the cold-rolled steel sheet in the temperature range before the start of the galvanizing treatment in the plating step described later (for example, the residence time of the cold-rolled steel sheet in the temperature range before immersion in the galvanizing bath). However, the holding time in the holding temperature range does not include the residence time of the galvanized steel sheet in the temperature range after the hot dip galvanizing treatment is performed in the plating step.

[0334] [Plating Step]

[0335] Next, the cold-rolled steel sheet is subjected to a galvanizing treatment to produce a galvanized steel sheet. As the galvanizing treatment, for example, a hot dip galvanizing treatment, an alloyed galvanizing treatment can be given. Also in the plating step, the first cooling stop temperature in the first cooling step described above and the temperature of the galvanizing bath (hereinafter, also referred to as the plating bath temperature) in the galvanizing treatment need to satisfy the following relation of formula (1).

[0336] -150°C ≤ To - Ti ≤ 50°C... (1)

[0337] wherein To is the first cooling stop temperature (°C), and Ti is the temperature of the galvanizing bath (°C) in the galvanizing treatment.

[0338] That is, from the viewpoint of securing excellent work hardening ability, it is necessary to appropriately control the difference between the first cooling stop temperature and the plating bath temperature, and specifically, it is necessary to satisfy the relation of formula (1) described above. On the other hand, if To - Ti exceeds 50°C or is less than -150°C, the S MA2 / S MA and the S MA3 / S MA decrease, and the work hardening ability and the ductility decrease. To - Ti is preferably -120°C or more, and more preferably -100°C or more. In addition, To - Ti is preferably 45°C or less, and more preferably 40°C or less.

[0339] The conditions other than the above are not particularly limited, and can be performed according to a conventional method.

[0340] For example, in the case of the hot-dip galvanizing treatment, after the cold-rolled steel sheet is immersed in the galvanizing bath, the plated adhesion amount is preferably adjusted by gas blowing or the like. As the plating bath temperature, 440°C to 500°C is set. Further, as the galvanizing bath, there is no particular limitation as long as it becomes the composition of the above-described galvanized layer, and, for example, a plating bath of a composition in which the Al content is 0.10 mass% to 0.23 mass% and the remaining portion is composed of Zn and unavoidable impurities is preferably used.

[0341] Further, in the case of the alloyed galvanizing treatment, after the hot-dip galvanizing treatment is performed in accordance with the above-described essentials, the galvanized steel sheet is preferably heated to an alloying temperature of 450°C to 600°C to perform the alloying treatment. When the alloying temperature is lower than 450°C, the Zn-Fe alloying speed becomes slow, and sometimes the alloying becomes difficult. On the other hand, if the alloying temperature exceeds 600°C, the untransformed austenite transforms to pearlite phase, and sometimes the TS and the ductility decrease. It should be noted that the alloying temperature is more preferably 470°C or higher. Further, the alloying temperature is more preferably 570°C or lower.

[0342] Further, the plated adhesion amount of the hot-dip galvanized steel sheet (GI) and the alloyed hot-dip galvanized steel sheet (GA) is preferably 20 to 80 g / m 2 It should be noted that the plated adhesion amount can be adjusted by gas blowing or the like.

[0343] It should be noted that, in addition to the above-described holding process (holding of the cold-rolled steel sheet before the plating process), an additional holding process of holding the galvanized steel sheet at a temperature region of 300°C to 550°C (hereinafter, also referred to as an additional holding temperature region) for 3 seconds to 600 seconds can be performed. The additional holding process is a process of obtaining the same effect as the holding process. Further, the additional holding process can be performed after or in the middle of the plating process as long as it is before the second cooling process described later. Further, in the case of the alloyed galvanizing treatment, the additional holding process can be performed in the middle of the plating process. That is, the plating process can serve as the additional holding process. Further, when the additional holding process is performed, the holding time of the holding process and the additional holding process is preferably 3 seconds to 600 seconds in total. The holding time of the holding process and the additional holding process is more preferably less than 200 seconds in total.

[0344] [Second cooling process]

[0345] Next, the galvanized steel sheet is cooled to a second cooling stop temperature of 100°C or higher and lower than 300°C.

[0346] Second cooling stop temperature: 100°C or higher and lower than 300°C

[0347] The second cooling step is a step necessary to control the area ratio of the tempered martensite and the area ratio of the residual austenite generated in the reheating step, which is a subsequent step, within a prescribed range. When the second cooling stop temperature is lower than 100°C, almost all of the untransformed austenite present in the steel in the second cooling step is transformed into martensite. As a result, the area ratio of the tempered martensite excessively increases, and the area ratio of the residual austenite decreases. As a result, the ductility and the work hardening ability decrease. On the other hand, when the second cooling stop temperature is 300°C or higher, the area ratio of the tempered martensite decreases, and the area ratio of the fresh martensite increases. With the increase in the area ratio of the fresh martensite, the amount of diffusible hydrogen in the steel sheet increases, and the hole expansibility decreases. In addition, since S MA1 / S MA increases, the hole expansibility also decreases. 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.

[0348] [Reheating Step]

[0349] Next, the galvanized steel sheet is reheated to a reheating temperature of (the above-mentioned second cooling stop temperature + 50°C) to 500°C, and the above-mentioned galvanized steel sheet is held at a temperature range of (the above-mentioned second cooling stop temperature + 50°C) to 500°C (hereinafter, also referred to as a reheating temperature range) for 10 seconds to 2000 seconds.

[0350] As a result, the martensite present in the steel at the end of the second cooling step is tempered. In addition, by diffusing C supersaturated in the martensite to the untransformed austenite, stable austenite at room temperature, i.e., residual austenite, can be generated.

[0351] Reheating temperature: (the above-mentioned second cooling stop temperature + 50°C) to 500°C

[0352] If the reheating temperature is lower than (cooling stop temperature + 50°C), diffusion of C from the martensite present in the steel at the end of the second cooling step to the untransformed austenite cannot be sufficiently performed, and the area fraction of the residual austenite cannot be obtained in a prescribed amount. Thus, the ductility decreases. In addition, fresh martensite increases. And external release of hydrogen contained in the base steel sheet becomes insufficient, and the diffusible hydrogen amount of the base steel sheet increases. Thus, the hole expansibility decreases. On the other hand, if the reheating temperature exceeds 500°C, tempering of the martensite present in the steel at the end of the second cooling step excessively proceeds, and thus it is difficult to make the TS 980 MPa or more. In addition, the untransformed austenite present in the steel at the end of the second cooling step is decomposed into carbides (pearlite), and thus the ductility decreases. And external release of hydrogen contained in the base steel sheet becomes insufficient, and the diffusible hydrogen amount of the base steel sheet increases. Thus, the hole expansibility decreases. Therefore, the reheating temperature is made (cooling stop temperature + 50°C) to 500°C. The reheating temperature is preferably 70°C or more than the cooling stop temperature. In addition, the reheating temperature is preferably 450°C or less. It should be noted that the reheating temperature is the maximum temperature reached in the reheating step.

[0353] The holding time in the reheating temperature region: 10 seconds to 2000 seconds

[0354] If the holding time in the reheating temperature region is less than 10 seconds, diffusion of C from the martensite present in the steel at the end of the second cooling step to the untransformed austenite cannot be sufficiently performed, and the area fraction of the residual austenite cannot be obtained in a prescribed amount. Thus, the ductility decreases. In addition, fresh martensite increases, and external release of hydrogen contained in the base steel sheet becomes insufficient, and the diffusible hydrogen amount of the base steel sheet increases. Thus, the hole expansibility can also decrease. On the other hand, if the holding time in the reheating temperature region exceeds 2000 seconds, tempering of the martensite present in the steel at the end of the second cooling step excessively proceeds, and it is difficult to make the TS 980 MPa or more. In addition, the untransformed austenite present in the steel at the end of the second cooling step is decomposed into carbides (pearlite), and thus the ductility decreases. Therefore, the holding time in the reheating temperature region is made 10 seconds to 2000 seconds. The holding time in the reheating temperature region is preferably 15 seconds or more. In addition, the holding time in the reheating temperature region is preferably 1200 seconds or less. It should be noted that the holding time in the reheating temperature region includes, in addition to the holding time at the reheating temperature, the residence time in the temperature region in heating and cooling before and after reaching the reheating temperature.

[0355] The cooling conditions after the temperature zone of the reheating is maintained are not particularly limited and can be performed according to a conventional method. As the cooling method, for example, jet cooling, water mist cooling, roll cooling, water cooling, air cooling, or the like can be used. In addition, from the viewpoint of preventing oxidation of the surface, after the temperature zone of the reheating is maintained, it is preferable to cool to 50°C or lower, and more preferably to room temperature or thereabout. The average cooling rate after the temperature zone of the reheating is maintained is, for example, preferably 1°C / sec to 50°C / sec.

[0356] In addition, the galvanized steel sheet obtained as described above can be further subjected to quenching and tempering rolling. If the reduction rate of the quenching and tempering rolling exceeds 2.00%, the yield stress increases, and the dimensional accuracy when the galvanized steel sheet is formed into a member can decrease. Therefore, the reduction rate of the quenching and tempering rolling is preferably 2.00% or less. Note that the lower limit of the reduction rate of the quenching and tempering rolling is not particularly limited, and is preferably 0.05% or more from the viewpoint of productivity. In addition, the quenching and tempering rolling can be performed on-line in a device continuous to the annealing device used for performing each of the above-described processes, or can be performed off-line in a device discontinuous to the annealing device used for performing each of the processes. In addition, the number of times of rolling of the quenching and tempering rolling can be one or more than two. Note that as long as an elongation rate equivalent to that of the quenching and tempering rolling can be imparted, rolling using a leveler or the like can also be used.

[0357] The conditions other than the above are not particularly limited and can be performed according to a conventional method.

[0358] [4] Method for manufacturing member

[0359] Next, a method for manufacturing a member based on one embodiment of the present application will be described.

[0360] The method for manufacturing a member based on one embodiment of the present application has a process of manufacturing a member by subjecting the above-described galvanized steel sheet (for example, a galvanized steel sheet manufactured by the above-described method for manufacturing a galvanized steel sheet) to at least one of forming processing or joining processing.

[0361] Note that the forming processing method is not particularly limited and, for example, a general processing method such as press processing can be used. In addition, the joining processing method is not particularly limited and, for example, a general welding such as spot welding, laser welding, arc welding, a riveting, caulking, or the like can be used. Note that the forming conditions and the joining conditions are not particularly limited and can be performed according to a conventional method.

[0362] Example

[0363] Example 1

[0364] A steel billet having a composition shown in Table 1 (the remainder being Fe and inevitable impurities) was melted by a converter, and a steel billet was produced by a continuous casting method. The obtained steel billet was heated to 1250°C, and after the heating, hot-rolled by a hot-rolling process composed of rough rolling and finish rolling to produce a hot-rolled steel sheet. Subsequently, the obtained hot-rolled steel sheet was subjected to pickling and cold-rolling (reduction: 50%) to produce a cold-rolled steel sheet having a sheet thickness shown in Table 3. Subsequently, the obtained cold-rolled steel sheet was subjected to an annealing process, a first cooling process, a holding process, a plating process, a second cooling process, and a reheating process under the conditions shown in Table 2 to obtain a galvanized steel sheet. Note that the dew point in the annealing process was -35°C to -30°C.

[0365] Here, the plating process was performed as hot-dip galvanizing treatment or galvannealing treatment to obtain a hot-dip galvanized steel sheet (hereinafter, also referred to as GI) or a galvannealed steel sheet (hereinafter, also referred to as GA). Note that in Table 2, the type of the plating process is indicated as "GI" and "GA". Note that when the galvannealing treatment was performed, the total of 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 was 3 seconds to 600 seconds except for Nos. 20, 27, and 28.

[0366] Further, as the galvanizing bath, when GI was produced, a plating bath having a composition containing Al: 0.20 mass% and the remainder consisting of Zn and inevitable impurities was used. When GA was produced, a plating bath having a composition containing Al: 0.14 mass% and the remainder consisting of Zn and inevitable impurities was used.

[0367] As for the plating adhesion amount, when GI was produced, 45 to 72 g / m 2 per one side, and when GA was produced, 45 g / m 2 per one side.

[0368] Note that as for the composition of the galvanized layer of the finally obtained galvanized steel sheet, when GI, Fe: 0.1 to 1.0 mass%, Al: 0.2 to 1.0 mass%, and the remainder Zn and inevitable impurities were contained. Further, when GA, Fe: 7 to 15 mass%, Al: 0.1 to 1.0 mass%, and the remainder Zn and inevitable impurities were contained.

[0369] Further, the galvanized layer was formed on both surfaces of the base steel sheet.

[0370] Using the galvanized steel sheet thus obtained, the identification of the steel structure of the base steel sheet and the measurement of the diffusible hydrogen amount were performed in accordance with the above-described protocols. The results are shown in Table 3. In Table 3, F is ferrite, BF is bainite ferrite, TM is tempered martensite, RA is retained austenite, FM is fresh martensite, LB is lower bainite, P is pearlite, and θ is cementite.

[0371] In addition, the tensile test and the hole expansion test were performed in accordance with the following protocols, and the tensile strength (TS), the yield stress (YS), the total elongation (El), the work hardening exponent (n value) / yield ratio (YR), and the limit hole expansion ratio (λ) were evaluated in accordance with the following criteria.

[0372] TS

[0373] O (Pass): 980 MPa or more

[0374] X (Fail): Less than 980 MPa

[0375] YS

[0376] O (Pass):

[0377] 980 MPa≤TS<1180 MPa, 550 MPa≤YS

[0378] 1180 MPa≤TS<1310 MPa, 700 MPa≤YS

[0379] 1310 MPa≤TS, 800 MPa≤YS

[0380] X (Fail):

[0381] 980 MPa≤TS<1180 MPa, 550 MPa>YS

[0382] 1180 MPa≤TS<1310 MPa, 700 MPa>YS

[0383] 1310 MPa≤TS, 800 MPa>YS

[0384] El

[0385] O (Pass):

[0386] 980 MPa≤TS<1180 MPa, 13.0%≤El

[0387] 1180 MPa≤TS<1310 MPa, 12.0%≤El

[0388] 1310 MPa≤TS, 10.0%≤El

[0389] X (Fail):

[0390] 980 MPa≤TS<1180 MPa, 13.0%>El

[0391] 1180 MPa≤TS<1310 MPa, 12.0%>El

[0392] 1310 MPa < TS, 10.0% < El

[0393] • n value / YR

[0394] O (Pass): n value / YR ≥ 0.070

[0395] X (Fail): n value / YR < 0.070

[0396] • λ

[0397] O (Pass): 20% or more

[0398] X (Fail): less than 20%

[0399] (1) Tensile Test

[0400] The tensile test was performed based on JIS Z 2241. That is, a JIS No. 5 test piece was taken from the obtained galvanized steel sheet in such a manner that the long side direction was made perpendicular to the rolling direction of the base steel sheet. Using the taken test piece, a tensile test was performed at a cross head speed of 10 mm / min, and TS, YS, El, and n value were measured. Among them, the n value was calculated from the elongation and strength at 0.4 times and 0.8 times the uniform elongation (U-El). In addition, the yield ratio YR (= YS / TS) and the n value / YR value were calculated from the measured YS, TS, and n value. It should be noted that the n value / YR value indicates the work hardening ability, and serves as an index for comprehensively evaluating the formability and impact resistance of the steel sheet. The results were collectively recorded in Table 3.

[0401] (2) Hole Expansion Test

[0402] The hole expansion test was performed based on JIS Z 2256. That is, a test piece of 100 mm x 100 mm was taken from the obtained galvanized steel sheet by shearing. A hole of 10 mm in diameter was punched in the test piece with an interval of 12.5%. Next, a press force of 9 ton (88.26 kN) was applied to the periphery of the hole using a punch of 75 mm in inner diameter, and a conical punch of 60° in apex angle was pressed into the hole in this state, and the diameter of the hole of the test piece at the limit of crack generation (at the time of crack generation) was measured. Then, the limit hole expansion ratio λ (%) was calculated using the following equation. It should be noted that λ is an index for evaluating the tensile flangeability. The results were collectively recorded in Table 3.

[0403] λ (%) = {(D f - D0) / D0} x 100

[0404] wherein,

[0405] D f : diameter of the hole of the test piece at the time of crack generation (mm)

[0406] D0: diameter of the hole of the test piece at the initial stage (mm)

[0407] [Table 1]

[0408] Table 1

[0409]

[0410]

[0411]

[0412]

[0413]

[0414] [Table 4]

[0415] Table 4

[0416]

[0417] As shown in Table 3, in the present example, all of the tensile strength (TS), the yield stress (YS), the total elongation (El), the work hardening index (n value) / yield ratio (YR), and the limit of hole expansion ratio (λ) were satisfactory.

[0418] On the other hand, in the comparative example, at least one of the tensile strength (TS), the yield stress (YS), the total elongation (El), the work hardening index (n value) / yield ratio (YR), and the limit of hole expansion ratio (λ) was not satisfactory.

[0419] In addition, it was found that the tensile strength (TS), the yield stress (YS), the total elongation (El), the work hardening index (n value) / yield ratio (YR), and the limit of hole expansion ratio (λ) of a member obtained by performing a forming process or a member obtained by performing a joining process using the steel sheet of the present example all had excellent characteristics as features of the present application.

[0420] Example 2

[0421] A steel billet material having the composition shown in Table 1 (the remainder being Fe and inevitable impurities) was melted by a converter, and a steel billet was produced by a continuous casting method. The obtained steel billet was heated to 1250°C, and after the heating, hot rolling composed of rough rolling and finish rolling was performed on the steel billet to produce a hot-rolled steel sheet. Subsequently, the obtained hot-rolled steel sheet was subjected to pickling and cold rolling (reduction ratio: 50%) to produce a cold-rolled steel sheet having a sheet thickness of 1.6 mm.

[0422] Next, in the obtained cold-rolled steel sheet, No. 8 to 10 were subjected to Fe-based plating 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, first, the cold-rolled steel sheet was subjected to degreasing treatment with an alkali. Next, the cold-rolled steel sheet was subjected to electrolytic treatment under the conditions shown below as a cathode to form a metal plating layer on the surface of the cold-rolled steel sheet.

[0423] [Electrolysis conditions]

[0424] Bath temperature: 50°C

[0425] pH: 2.0

[0426] Current density: 45 A / dm 2

[0427] Plating bath: sulfuric acid bath containing 1.5 mol / L of Fe ion 2+

[0428] Anode: iridium oxide electrode

[0429] Note that the attached amount of the metal plating layer was controlled by the power application time.

[0430] Next, the obtained cold-rolled steel sheet (including a metal-plated steel sheet on which a metal plating layer was 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 to obtain a galvanized steel sheet.

[0431] In the plating step, galvannealing treatment was performed to obtain a galvannealed steel sheet (GA). The treatment conditions other than those described in Table 5 were the same as in Example 1. In addition, a galvanized layer was formed on both surfaces of the base steel sheet.

[0432] Using the galvanized steel sheet thus obtained, the base steel sheet was subjected to evaluation of the steel structure, and measurement of the thickness of the decarburized layer, the attached amount of the metal plating layer, and the diffusible hydrogen amount, in accordance with the above-described protocols. The results are shown in Table 6. In Table 6, F is ferrite, BF is bainite ferrite, TM is tempered martensite, RA is residual austenite, FM is fresh martensite, LB is lower bainite, P is pearlite, and θ is cementite. In addition, in Table 6, the thickness of the decarburized layer and the attached amount of the metal plating layer are indicated by “-” to indicate that the base steel sheet did not have a decarburized layer and a metal plating layer, respectively.

[0433] In addition, tensile tests and hole expansion tests were performed in accordance with the same protocols as in Example 1, and the tensile strength (TS), the yield stress (YS), the total elongation (El), the work hardening index (n value) / the yield ratio (YR), and the limit hole expansion ratio (λ) were evaluated in accordance with the same criteria as in Example 1. The results are collectively shown in Table 7.

[0434] ​Further, the resistance spot welding crack resistance of the welded portion was evaluated in accordance with the following procedure.

[0435] Evaluation of resistance spot welding crack resistance of welded portion

[0436] A test piece 2 of 150 mm in the longitudinal direction by 50 mm in the transverse direction was cut from the obtained galvanized steel sheet with the rolling transverse direction as the longitudinal direction and the rolling direction as the transverse direction, and a sheet set was prepared by overlapping the test piece 2 with a test alloyed hot-dip galvanized steel sheet 1 (sheet thickness: 1.6 mm, TS: 980 MPa grade). Note that the adhering amount of the alloyed hot-dip galvanized layer per one face of the test alloyed hot-dip galvanized steel sheet 1 was 50 g / m 2 , and was cut to the same size as the test piece 2. The sheet set was assembled with the evaluation target face of the test piece 2 (the galvanized layer on the side when the galvanized layer and the metallic plated layer are present on only one side) facing the galvanized layer of the test alloyed hot-dip galvanized steel sheet 1. The sheet set was fixed to a fixing table 4 with a spacer 3 of 2.0 mm in thickness interposed therebetween. The spacer 3 was a pair of steel sheets of 50 mm in the longitudinal direction by 45 mm in the transverse direction by 2.0 mm in thickness, and was arranged with the longitudinal direction end faces of the respective pair of steel sheets aligned with the two end faces in the transverse direction of the sheet set, as shown in (A) of FIG. 1. Figure 5

[0437] Next, a single-phase alternating current (50 Hz) resistance welder of the servo motor pressurization type was used to perform resistance spot welding with a welding current having a nugget diameter r of 5.9 mm under conditions of a pressurization force of 3.5 kN, a holding time of 0.12 seconds, 0.18 seconds, or 0.24 seconds, and a welding time of 0.36 seconds, in a state in which the sheet set was pressed with a pair of electrodes 5 (front end diameter: 6 mm) and the sheet set was bent, to produce a sheet set with a welded portion. At this time, the pair of electrodes 5 pressed the sheet set from above and below in the vertical direction, with the lower electrode pressing the test piece 2 via the hole of the fixing table 4. The lower electrode was fixed to the fixing table 4 so that the lower electrode of the pair of electrodes 5 was in contact with a plane extending from the face of the spacer 3 and the fixing table 4 that were in contact, and the upper electrode was movable, during pressurization. In addition, the upper electrode was in contact with the central portion of the test alloyed hot-dip galvanized steel sheet 1. Furthermore, the welding was performed in a state in which the sheet set was inclined by 5° toward the longitudinal direction side of the sheet set with respect to the horizontal direction. Note that the holding time refers to the time from the end of the flow of the welding current to the start of opening the electrodes. Here, with reference to the lower drawing of (B) of FIG. 1, the nugget diameter r refers to the distance between the end portions of the nugget 6 in the longitudinal direction of the sheet set. Figure 5

[0438] Next, the above-described sheet set with a welded portion was cut in the longitudinal direction of the sheet set with the center of the welded portion including the nugget 6 included, to produce a test piece 2 of 150 mm in the longitudinal direction by 50 mm in the transverse direction. Figure 5 ​​The weld section was cut along line AA in the upper part of diagram (B) and observed using an optical microscope (200x). The resistance to resistance weld cracking characteristics of the weld section were evaluated according to the following criteria. It should be noted that a result of A+, A, or B indicates excellent resistance to resistance weld cracking characteristics. A result of C indicates poor resistance to resistance weld cracking characteristics. The results are recorded in Table 7.

[0439] A+: No cracks longer than 0.1 mm were observed in any of the holding times of 0.12 seconds, 0.18 seconds, and 0.24 seconds.

[0440] A: Cracks longer than 0.1 mm were observed when the holding time was 0.12 seconds, but no cracks longer than 0.1 mm were observed when the holding time was 0.18 seconds and 0.24 seconds.

[0441] B: Cracks longer than 0.1 mm were observed at holding times of 0.12 seconds and 0.18 seconds, but no cracks longer than 0.1 mm were observed at a holding time of 0.24 seconds.

[0442] C: Cracks longer than 0.1 mm were observed in all cases where the holding time was 0.12 seconds, 0.18 seconds, and 0.24 seconds.

[0443] It should be noted that, in Figure 5 In the figure below (B), the cracks produced by test piece 2 are schematically represented by symbol 7. It should be noted that when cracks appear in the target-side steel plate (the alloyed hot-dip galvanized steel plate used in the test), the evaluation of stress dispersion on the target steel plate (the steel plates of each inventive example and comparative example) cannot be adequately assessed. Therefore, data showing no cracks in the target-side steel plate are used as an example.

[0444]

[0445]

[0446] Table 7]

[0447] Table 7

[0448]

[0449] As shown in Table 7, in the invention examples, tensile strength (TS), yield stress (YS), total elongation (E1), work hardening index (n value) / yield ratio (YR), and ultimate hole expansion rate (λ) all meet the requirements. Furthermore, the welded portion exhibits excellent resistance to weld cracking characteristics.

[0450] Furthermore, in Invention Examples No.1, 6-10, and especially Invention Examples No.8 and 9, the resistance to resistance welding cracks of the welded parts are excellent.

[0451] In addition, it is known that the tensile strength (TS), the yield stress (YS), the total elongation (El), the work hardening exponent (n value) / yield ratio (YR), the limit of hole expansion (λ), and the resistance to resistance welding cracking of the welded portion of a member obtained by performing a forming process or a member obtained by performing a joining process using the steel sheet of the example of the present application each have excellent characteristics as features in the present application.

[0452] Explanation of symbols

[0453] 1 Alloyed hot-dip galvanized steel sheet for test

[0454] 2 Test piece

[0455] 3 Spacer

[0456] 4 Fixing table

[0457] 5 Electrode

[0458] 6 Weld nugget

[0459] 7 Alligatoring

Claims

1. A method of manufacturing a galvanized steel sheet, comprising the steps of: a hot rolling step of hot-rolling a steel slab having a composition consisting of, in mass%, C: 0.050 to 0.400%, Si: 0.20 to 3.00%, Mn: 1.00% or more and less than 3.50%, P: 0.001 to 0.100%, S: 0.0200% or less, Al: 0.010 to 2.000%, and N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities, to produce a hot-rolled steel sheet, a cold rolling step of cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet, an annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 760 to 900°C and 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 to 550°C, a holding step of holding the cold-rolled steel sheet at a temperature range of 300 to 550°C for 3 to 600 seconds, a plating step of subjecting the cold-rolled steel sheet to a galvanizing treatment to produce a galvanized steel sheet, 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, a reheating step of reheating the galvanized steel sheet to a reheating temperature of (the second cooling stop temperature + 50°C) to 500°C and holding the galvanized steel sheet at a temperature range of (the second cooling stop temperature + 50°C) to 500°C for 10 to 2000 seconds, the first cooling stop temperature and the temperature of a galvanizing bath in the galvanizing treatment satisfying the following formula (1), -150°C ≤ To - Ti ≤ 50°C • • • (1) To being the first cooling stop temperature in °C and Ti being the temperature of the galvanizing bath in the galvanizing treatment in °C, and the base steel sheet further containing, in mass%, at least one selected from the group consisting of 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. The dew point of the annealing step is greater than -30°C. ​ ​ ​ ​ ​ ​ ​ ​ ​ wherein, ​ 2. The method of producing a galvanized steel sheet according to claim 1, wherein, ​ 3. The method of producing a galvanized steel sheet according to claim 1, wherein, ​ 4. The method of producing a galvanized steel sheet according to claim 2, wherein The dew point of the annealing process is greater than -30°C.

5. The method of producing a galvanized steel sheet according to any one of claims 1 to 4, wherein Further having a metal plating treatment process of forming a metal plating layer on at least one surface of the cold-rolled steel sheet after the cold-rolling process and before the annealing process.

6. The method of producing a galvanized steel sheet according to claim 5, wherein The metal plating layer is an Fe-based plating layer.

7. The method of producing a galvannealed steel sheet according to any one of claims 1 to 4, wherein The zinc plating treatment is a hot-dip galvanizing treatment or an alloyed hot-dip galvanizing treatment.

8. The method of producing a galvanized steel sheet according to claim 5, wherein The zinc plating treatment is a hot-dip galvanizing treatment or an alloyed hot-dip galvanizing treatment.

9. The method of producing a galvanized steel sheet according to claim 6, wherein The zinc plating treatment is a hot-dip galvanizing treatment or an alloyed hot-dip galvanizing treatment.

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