Zinc-plated steel sheet and member, and method for producing the same

Abstract

Provided is a galvanized steel sheet having high strength, excellent ductility, and excellent delayed fracture resistance properties. The base steel sheet has a predetermined component composition, steel structure, and diffusible hydrogen content. In particular, in the steel structure of the base steel sheet, M1 / Mt is controlled to 0.30 or less, and the diffusible hydrogen content of the base steel sheet is reduced to 0.50 mass ppm or less.

Description

Technical Field

[0001] The present invention relates to a galvanized steel sheet and a member suitably used for automotive members and the like, and a method for manufacturing them.

Background Art

[0002] In recent years, from the viewpoint of global environmental conservation, in the automotive industry, attempts have been made to reduce exhaust gases such as CO2. Specifically, by increasing the strength and reducing the thickness of the steel sheet used as a material for automotive members, the vehicle body is lightened and the fuel efficiency is improved. Thereby, attempts are being made to reduce the amount of exhaust gas.

[0003] Also, from the viewpoint of the rust prevention performance of the vehicle body, the steel sheet used as a material for automotive members may be galvanized.

[0004] As such a steel sheet used as a material for automotive members, for example, Patent Document 1 discloses "A low yield ratio high-strength cold-rolled steel sheet excellent in hole expansion property, which contains, by mass%, C: 0.04 to 0.14%, Si: 0.4 to 2.2%, Mn: 1.2 to 2.4%, P: 0.02% or less, S: 0.01% or less, Al: 0.002 to 0.5%, Ti: 0.005 to 0.1%, N: 0.006% or less, and further satisfies (%Ti) / (%S)≧5 when %S and %Ti are the S and Ti contents respectively, and the balance consists of Fe and unavoidable impurities." is disclosed.

[0005] Patent Document 2 discloses "A hot-dip galvanized high-strength steel sheet excellent in formability, which contains, by mass%, C: 0.07 to 0.22%, Si: 0.005 to 1.0%, Mn: 1.5 to 2.8%, P: 0.001 to 0.1%, S: 0.001 to 0.01%, N: 0.0005 to 0.01%, Al: 0.02 to 1.0%, the balance being Fe and inevitable impurities, and whose microstructure consists of ferrite with an area ratio of 20 to 70%, retained austenite with an area ratio of 1 to 5% or less, martensite with an area ratio of 20% or more and 70% or less, and the balance being bainite, and which satisfies the formula (A-1)(A-2)(B). " is disclosed.

[0006] Patent Document 3 discloses "A hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet, wherein the steel sheet contains, by mass%, C: 0.11% or more and 0.20% or less, Si: 0.001% or more and 0.35% or less, Mn: 2.0% or more and 3.0% or less, P: 0.1% or less, S: 0.01% or less, sol.Al: 0.001% or more and 1.5% or less, Ti: 0.001% or more and 0.30% or less, N: 0.02% or less, B: 0.0021% or more and 0.0080% or less, and further has a chemical composition satisfying the following formula (1) and a metal structure in which the retained austenite is 7% by volume or less, wherein the hot-dip galvanized steel sheet has a tensile strength in the rolling transverse direction of 1180 MPa or more, characterized by a hot-dip galvanized steel sheet. 15×sol.Al + 100×Ti ≧ 1.5 ···(1)" is disclosed.

[0007] Patent Document 4 discloses "a steel containing, by mass%, C: 0.07 to 0.25%, Si: 0.3 to 2.50%, Mn: 1.5 to 3.0%, Ti: 0.005 to 0.09%, B: 0.0001 to 0.01%, P: 0.001 to 0.03%, S: 0.0001 to 0.01%, Al: 2.5% or less, N: 0.0005 to 0.0100%, O: 0.0005 to 0.007%, and the balance being iron and inevitable impurities, A high-strength steel plate having a tensile maximum strength of 900 MPa or more, excellent ductility and stress corrosion cracking resistance characteristics, characterized in that the steel plate structure mainly consists of ferrite and contains martensite composed of block sizes of 1 μm or less, the volume ratio of ferrite is 60% or more, the C concentration in martensite is 0.3% to 0.9%, and the yield ratio (YR) consisting of the ratio of the tensile maximum strength (TS) to the yield stress (YS) is 0.75 or less. is disclosed.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0009] By the way, when the steel plate is strengthened, the ductility generally decreases. However, steel plates used as materials for automobile members (hereinafter also referred to as automobile steel plates) are required to have both high strength and excellent ductility, specifically, an improvement in the total elongation (hereinafter also simply referred to as El) in a tensile test.

[0010] In recent years, automobiles have been spreading globally, and from the viewpoint of being used for various purposes in various regions and climates, further improvement in stress corrosion cracking resistance characteristics is required. Here, stress corrosion cracking refers to a phenomenon in which, when a high stress is applied to a steel plate (a component made of a steel plate), hydrogen in the steel plate reduces the interatomic bonding force or causes local deformation, resulting in the generation of microcracks, and the progress of these microcracks leads to fracture.

[0011] However, the steel sheets disclosed in Patent Documents 1 to 4 cannot be said to satisfy all of the above required characteristics.

[0012] The present invention has been developed to meet the above requirements, and an object thereof is to provide a galvanized steel sheet having high strength, excellent ductility, and excellent hydrogen embrittlement resistance, together with an advantageous manufacturing method therefor. Another object of the present invention is to provide a member made of the above galvanized steel sheet and a manufacturing method therefor. In the present disclosure, any numerical range represented by "~" means a range including the numerical values described before and after "~" as a lower limit value and an upper limit value, respectively.

Means for Solving the Problems

[0013] Now, the inventors have intensively studied to achieve the above object and obtained the following findings.

[0014] (1) After adjusting the component composition of the base steel sheet to a predetermined range, control the total area ratio of ferrite and bainite, and the area ratio of martensite in the steel structure of the base steel sheet to 20% or more and 90% or less, and 10% or more and 80% or less, respectively. Thereby, it becomes possible to achieve both high strength and excellent ductility.

[0015] (2) In order to further improve the hydrogen embrittlement resistance, it is effective to suppress voids generated when a high stress is applied to the galvanized steel sheet. In particular, it is effective to suppress the generation and connection of voids at the interface between a region having a high C and Mn concentration and ferrite or bainite in the region constituting martensite. For this purpose, it is important to control M1 / Mt to 0.30 or less while controlling the total area ratio of ferrite and bainite and the area ratio of martensite in the steel structure of the base steel sheet within the above range. Thereby, excellent hydrogen embrittlement resistance can be obtained while ensuring high strength and excellent ductility.

[0016] Here, Mt is the area ratio (%) of martensite, M1 is the area ratio (%) of the first region among the regions constituting martensite. It is.

[0017] Moreover, the first region is a region that satisfies the relationships of the following equations (3) and (4). [C] M / [C] ≥ 2.4 ··· (3) [Mn] M / [Mn] ≥ 1.5 ··· (4) Here, [C] and [Mn] are the contents (mass%) of C and Mn in the component composition of the base steel plate, respectively. Also, [C] M and [Mn] M are the concentrations (mass%) of C and Mn in the region constituting martensite, respectively.

[0018] (3) Reduce the diffusible hydrogen content of the base steel plate. Thereby, the stress corrosion cracking resistance is further improved.

[0019] The present invention has been completed through further study based on the above findings. That is, the gist configuration of the present invention is as follows.

[0020] 1. A galvanized steel sheet having a base steel plate and a zinc plating layer on the surface of the base steel plate, wherein the base steel plate in mass%, C: 0.05% or more and 0.20% or less, Si: 0.2% or more and 1.8% or less, Mn: 1.50% or more and 3.00% or less, P: 0.100% or less, S: 0.0500% or less, Al: 0.010% or more and 1.000% or less and N: 0.0100% or less and satisfies the relationships of the following equations (1) and (2), with the balance being Fe and inevitable impurities, component composition, and at the 1 / 4 position of the plate thickness of the base steel plate, The total area ratio of one or both of ferrite and bainite: 20% or more and 90% or less, The area ratio of martensite: 10% or more and 80% or less and M1 / Mt: 0.30 or less wherein Mt is the area ratio (%) of the martensite, M1 is the area ratio (%) of the first region among the regions constituting the martensite, The first region is a region satisfying the relationships of the following formulas (3) and (4), has a steel structure, The diffusible hydrogen content of the base steel sheet is 0.50 mass ppm or less, A galvanized steel sheet having a tensile strength of 780 MPa or more. [C]+[Si] / 24+[Mn] / 6≦0.65 ···(1) 0.13≦[Si] / [Mn]≦0.75 ···(2) [C] M / [C]≧2.4 ···(3) [Mn] M / [Mn]≧1.5 ···(4) Here, [C], [Si], and [Mn] are the contents (mass %) of C, Si, and Mn in the component composition of the base steel sheet, respectively. Also, [C] M and [Mn] M are the concentrations (mass %) of C and Mn in the region constituting the martensite, respectively.

[0021] 2. The component composition of the base steel sheet further satisfies, in mass %, Nb: 0.40% or less, Ti: 0.40% or less, V: 0.45% or less, B: 0.0100% or less, Cr: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, Sb: 0.100% or less, Sn: 0.100% or less, Cu: 1.00% or less, Ta: less than 0.100%, W: less than 0.200%, Mg: less than 0.010%, Zn: less than 0.020%, Co: less than 0.500%, Zr: less than 0.20%, Ca: less than 0.0200%, Ce: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0500%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200% and REM (excluding Ce): less than 0.0200% containing at least one selected from among the above, the galvanized steel sheet according to 1 above.

[0022] 3. A member made using the galvanized steel sheet according to 1 or 2 above.

[0023] 4. The steel slab having the component composition according to 1 or 2 above is finish rolling end temperature: 840°C or higher and 1000°C or lower and coiling temperature: 620°C or lower hot-rolled under the conditions above to obtain a hot-rolled steel sheet, a hot-rolling process; and then, the hot-rolled steel sheet is rolling reduction: 20% or higher and 80% or lower cold-rolled under the conditions above to obtain a cold-rolled steel sheet, a cold-rolling process; and then, the cold-rolled steel sheet is annealing temperature: 750°C or higher and 900°C or lower and annealing time: 1 second or longer and 30 seconds or shorter annealed under the conditions above, an annealing process; and then, the cold-rolled steel sheet is Atmospheric hydrogen concentration: less than 30% by volume and Cooling stop temperature: 600 °C or lower Cooling in the condition of, cooling process, and Then, applying a zinc plating treatment to the cold-rolled steel sheet, zinc plating treatment process, and having, A method for manufacturing a zinc-plated steel sheet that satisfies the relationship of the following formula (5). 1.5 ((CT-350) / 100) ×T×log 10 (t + 10)×1.15 HA ×1.05 HB ≦6800 ···(5) Here,[[]] CT: Coiling temperature (°C) in the hot rolling process,[[]] T: Annealing temperature (°C) in the annealing process,[[]] t: Annealing time (seconds) in the annealing process,[[]] HA: Atmospheric hydrogen concentration (volume %) in the annealing process and HB: Atmospheric hydrogen concentration (volume %) in the cooling process is.[[]]

[0024] 5. A method for manufacturing a member, comprising a step of subjecting the zinc-plated steel sheet according to the above 1 or 2 to at least one of a forming process or a joining process to form a member.[[]]

Advantages of the Invention

[0025] According to the present invention, a zinc-plated steel sheet having high strength, excellent ductility, and excellent delayed fracture resistance can be obtained. Further, since the zinc-plated steel sheet of the present invention has high strength, excellent ductility, and excellent delayed fracture resistance, it can be extremely advantageously applied as a material for skeletal structure members of automobiles and the like.[[]]

Embodiments for Carrying Out the Invention

[0026] The present invention will be described based on the following embodiments. First, the component composition of the base steel sheet of the zinc-plated steel sheet according to an embodiment of the present invention will be described. In addition, the unit in the component composition is all "mass %", and hereinafter, unless otherwise specified, it is simply indicated by "%".[[]]

[0027] C: 0.05% or more and 0.20% or less C is an element that increases the strength of martensite and bainite. Therefore, C is included from the viewpoint of ensuring the desired strength. If the C content is less than 0.05%, the area ratio of ferrite increases and the desired strength cannot be obtained. On the other hand, when the C content exceeds 0.20%, the tensile strength (hereinafter also referred to as TS) becomes excessively high and El decreases. Also, martensite is excessively hardened and the stress corrosion cracking resistance deteriorates. Therefore, the C content is 0.05% or more and 0.20% or less. The C content is preferably 0.06% or more, more preferably 0.07% or more. Also, the C content is preferably 0.18% or less, more preferably 0.17% or less.

[0028] Si: 0.2% or more and 1.8% or less Si is an element that improves the strength of the steel sheet by solid solution strengthening. Also, Si is an element that improves ductility while suppressing a decrease in strength by increasing the strength of ferrite. Furthermore, Si is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. That is, Si is an element that affects the area ratio of ferrite. Here, if the Si content is less than 0.2%, the area ratio of ferrite decreases and the ductility deteriorates. On the other hand, when the Si content becomes excessive, particularly when it exceeds 1.8%, a significant increase in the rolling load during hot rolling and cold rolling occurs. Also, a decrease in toughness occurs. Therefore, the Si content is 0.2% or more and 1.8% or less. The Si content is preferably 0.3% or more, more preferably 0.5% or more. Also, the Si content is preferably 1.5% or less, more preferably 1.0% or less.

[0029] Mn: 1.50% or more and 3.00% or less Mn is an element that improves the hardenability of steel. Mn is contained to ensure a predetermined area ratio of martensite. If the Mn content is less than 1.50%, the hardenability is insufficient, and ferrite and bainite are excessively generated. This makes it difficult to ensure the desired strength. On the other hand, if Mn is contained in excess, the transformation of ferrite and bainite is delayed, leading to a decrease in ductility. Therefore, the Mn content is set to 1.50% or more and 3.00% or less. The Mn content is preferably 1.65% or more, more preferably 1.80% or more. Also, the Mn content is preferably 2.85% or less, more preferably 2.70% or less.

[0030] P: 0.100% or less P is an element that has a solid solution strengthening effect and increases strength. To obtain such an effect, the P content is preferably 0.001% or more. Also, due to production technology constraints, the P content is more preferably 0.002% or more. On the other hand, when the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries and embrittles the grain boundaries. Therefore, the stress corrosion cracking resistance decreases. Thus, the P content is set to 0.100% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less.

[0031] S: 0.0500% or less S forms particles such as coarse MnS and decreases ductility. Also, particles such as coarse MnS decrease the stress corrosion cracking resistance. In particular, when the S content exceeds 0.0500%, excellent stress corrosion cracking resistance cannot be obtained. Therefore, the S content is set to 0.0500% or less. The S content is preferably 0.0100% or less, more preferably 0.0050% or less, still more preferably 0.0030% or less. Note that the lower limit of the S content is not particularly limited. The S content is preferably 0.0001% or more, more preferably 0.0002% or more.

[0032] Al: 0.010% or more and 1.000% or less Al is added to remove oxygen and reduce inclusions in the steel. Also, Al is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. That is, Al is an element that affects the area ratio of ferrite. Here, when the Al content is less than 0.010%, the area ratio of ferrite decreases and the ductility decreases. On the other hand, when the Al content exceeds 1.000%, the area ratio of ferrite increases excessively, making it difficult to obtain the desired strength. Therefore, the Al content is set to be 0.010% or more and 1.000% or less. The Al content is preferably 0.015% or more, more preferably 0.030% or more. Also, the Al content is preferably 0.500% or less, more preferably 0.100% or less.

[0033] N: 0.0100% or less N is an element that forms nitride-based precipitates such as AlN that pin the grain boundaries and can be added to improve elongation. However, when the N content exceeds 0.0100%, the nitride-based precipitates such as AlN coarsen, resulting in a decrease in elongation. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0070% or less, more preferably 0.0050% or less. Note that the lower limit of the N content is not particularly limited. Due to production technology constraints, the N content is preferably 0.0006% or more.

[0034] [C] + [Si] / 24 + [Mn] / 6 ≤ 0.65 ···(1) When the contents of C, Si, and Mn are excessive, especially when [C] + [Si] / 24 + [Mn] / 6 exceeds 0.65, the stress corrosion cracking resistance deteriorates. The reason is considered to be that coarse inclusions increase and intergranular fracture is promoted by the grain boundary segregation of C, Si, and Mn. Therefore, [C] + [Si] / 24 + [Mn] / 6 is set to 0.65 or less. [C] + [Si] / 24 + [Mn] / 6 is preferably 0.62 or less, more preferably 0.58 or less. The lower limit of [C] + [Si] / 24 + [Mn] / 6 is not particularly limited. For example, [C] + [Si] / 24 + [Mn] / 6 is preferably 0.30 or more.

[0035] 0.13 ≤ [Si] / [Mn] ≤ 0.75 ···(2) In order to achieve both high strength and ductility, it is important to control [Si] / [Mn] to an appropriate value. If [Si] / [Mn] is less than 0.13, an appropriate amount of ferrite and bainite cannot be obtained, and there is a risk that the strength will be excessive. On the other hand, if [Si] / [Mn] exceeds 0.75, ferrite and bainite will be produced in excess, making it difficult to obtain the desired strength. Therefore, [Si] / [Mn] is set to be 0.13 or more and 0.75 or less. [Si] / [Mn] is preferably 0.14 or more, more preferably 0.15 or more. Also, [Si] / [Mn] is preferably 0.65 or less, more preferably 0.55 or less.

[0036] As described above, the elements that form the basis of the component composition of the base steel sheet of the galvanized steel sheet according to an embodiment of the present invention (hereinafter also referred to as basic component elements) have been explained. The base steel sheet of the galvanized steel sheet according to an embodiment of the present invention contains the above basic component elements, and the balance has a component composition containing Fe (iron) and unavoidable impurities. Here, it is preferable that the base steel sheet of the galvanized steel sheet according to an embodiment of the present invention contains the above basic component elements and the balance consists of Fe and unavoidable impurities. The base steel sheet of the galvanized steel sheet according to an embodiment of the present invention may contain at least one selected from the following as optional additive elements in addition to the above basic component elements. Nb: 0.40% or less, Ti: 0.40% or less, V: 0.45% or less, B: 0.0100% or less, Cr: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, Sb: 0.100% or less, Sn: 0.100% or less, Cu: 1.00% or less, Ta: 0.100% or less, W: 0.200% or less, Mg: 0.010% or less, Zn: below 0.020%, Co: below 0.500%, Zr: below 0.20%, Ca: below 0.0200%, Ce: below 0.0200%, Se: below 0.0200%, Te: below 0.0200%, Ge: below 0.0200%, As: below 0.0500%, Sr: below 0.0200%, Cs: below 0.0200%, Hf: below 0.0200%, Pb: below 0.0200%, Bi: below 0.0200% and REM (excluding Ce): below 0.0200%

[0037] In addition, for any of the above-mentioned optional additive elements, if they are contained below the upper limit amounts described above, the effects of the present invention can be obtained, so no lower limit is particularly provided. When any of the above-mentioned optional additive elements are contained below the preferable lower limit values described later, such elements shall be regarded as being contained as inevitable impurities.

[0038] Nb: below 0.40% Nb increases the TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, during the hot rolling process and the annealing process. Further, Nb contributes to the improvement of the delayed fracture resistance characteristics by forming fine precipitates that serve as trap sites for hydrogen and reducing the amount of diffusible hydrogen in the base steel sheet. To obtain such effects, the Nb content is preferably 0.002% or more. On the other hand, when the Nb content exceeds 0.40%, the amount of coarse Nb-based precipitates such as NbN, Nb(C,N), and (Nb,Ti)(C,N) that remain undissolved during slab heating in the hot rolling process increases, and the delayed fracture resistance characteristics deteriorate. Therefore, when Nb is contained, the Nb content is preferably 0.40% or less, more preferably 0.20% or less, and even more preferably 0.10% or less.

[0039] Ti: below 0.40% Ti increases the TS by forming fine precipitates in the hot rolling process and annealing process. In addition, Ti contributes to the improvement of the hydrogen-induced cracking resistance by forming fine precipitates that serve as hydrogen trap sites and reducing the diffusible hydrogen content in the base steel plate. In order to obtain such effects, the Ti content is preferably 0.002% or more. On the other hand, when the Ti content exceeds 0.40%, the amount of coarse precipitates of Ti-based compounds such as TiN, Ti(C,N), Ti(C,S), and TiS that remain undissolved during slab heating in the hot rolling process increases, and the hydrogen-induced cracking resistance deteriorates. Therefore, when Ti is contained, the Ti content is preferably 0.40% or less, more preferably 0.20% or less, and even more preferably 0.10% or less.

[0040] V: 0.45% or less Similar to Nb and Ti, V increases the TS by forming fine precipitates in the hot rolling process and annealing process. In addition, V contributes to the improvement of the hydrogen-induced cracking resistance by forming fine precipitates that serve as hydrogen trap sites and reducing the diffusible hydrogen content in the base steel plate. In order to obtain such effects, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, when the V content exceeds 0.45%, a large amount of coarse precipitates and inclusions are generated, which may reduce the ductility. Therefore, when V is contained, the V content is preferably 0.45% or less, more preferably 0.060% or less.

[0041] B: 0.0100% or less B is an element that enhances hardenability by segregating at the austenite grain boundaries. In addition, B is an element that controls the formation and grain growth of ferrite in the cooling process after the annealing process. In order to obtain such effects, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, when the B content exceeds 0.0100%, the amount of nitride-based precipitates such as BN becomes excessive, which may reduce the ductility. Therefore, when B is contained, the B content is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less.

[0042] Cr: Below 1.00% Cr is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. To obtain such an effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. On the other hand, when the Cr content exceeds 1.00%, the area ratio of martensite may increase and the ductility may decrease. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less, more preferably 0.60% or less, and even more preferably 0.30% or less.

[0043] Ni: Below 1.00% Ni is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. To obtain such an effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, when the Ni content exceeds 1.00%, the area ratio of martensite may increase and the ductility may decrease. Therefore, when Ni is contained, the Ni content is preferably 1.00% or less, more preferably 0.50% or less.

[0044] Mo: Below 1.00% Mo is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. Also, Mo contributes to the improvement of the hydrogen-induced cracking resistance by reducing the diffusible hydrogen amount in the base steel plate through the formation of fine precipitates that serve as hydrogen trap sites. 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, when the Mo content exceeds 1.00%, the area ratio of martensite may increase and the desired ductility may not be obtained. Therefore, when Mo is contained, the Mo content is preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.30% or less.

[0045] Sb: Below 0.100% Sb is an element effective for suppressing the diffusion of C near the surface of the steel sheet during annealing and controlling the formation of a soft layer near 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, when the Sb content exceeds 0.100%, it may cause a decrease in castability. Therefore, when Sb is contained, the Sb content is preferably 0.100% or less, more preferably 0.060% or less, and still more preferably 0.040% or less.

[0046] Sn: 0.100% or less Sn suppresses oxidation and nitridation near the surface of the steel sheet, thereby suppressing a decrease in the contents of C and B near the surface of the steel sheet. As a result, excessive formation of ferrite near the surface of the steel sheet is suppressed, contributing to an improvement in strength. In order to obtain such an effect, the Sn content is preferably 0.002% or more. However, when the Sn content exceeds 0.100%, it may cause a decrease in castability. Therefore, when Sn is contained, the Sn content is preferably 0.100% or less, more preferably 0.040% or less, and still more preferably 0.020% or less.

[0047] Cu: 1.00% or less Cu is an element that enhances hardenability and promotes the formation of martensite, thereby increasing TS. In order to obtain such an effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.020% or more. On the other hand, when the Cu content exceeds 1.00%, the area ratio of martensite increases excessively. Furthermore, a large amount of coarse precipitates and inclusions are generated, leading to a decrease in ductility and stress corrosion cracking resistance. Therefore, when Cu is contained, the Cu content is preferably 1.00% or less, more preferably 0.20% or less.

[0048] Ta: 0.100% or less Similar to Ti, Nb, and V, Ta increases the TS by forming fine precipitates during the hot rolling process and annealing process. In addition, Ta partially dissolves into Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb,Ta)(C,N). This suppresses the coarsening of the precipitates and stabilizes the precipitation strengthening. As a result, the TS is further increased. To obtain such an effect, the Ta content is preferably 0.001% or more. On the other hand, when the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, leading to a decrease in ductility and SCC resistance. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less, more preferably 0.050% or less.

[0049] W: 0.200% or less Similar to Ti, Nb, and V, W increases the TS by forming fine precipitates during the hot rolling process and annealing process. Also, W reduces the diffusible hydrogen content of the base steel sheet by forming fine precipitates that serve as hydrogen trap sites, contributing to the improvement of SCC resistance. To obtain such an effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more. On the other hand, when the W content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated, leading to a decrease in ductility and SCC resistance. Therefore, when W is contained, the W content is preferably 0.200% or less, more preferably 0.060% or less.

[0050] Mg: 0.010% or less Mg is an effective element for improving the SCC resistance of the steel sheet by spheroidizing the shape of inclusions such as sulfides and oxides. To obtain such an effect, the Mg content is preferably 0.0001% or more. However, when the Mg content exceeds 0.010%, the surface quality deteriorates. Therefore, when Mg is contained, the Mg content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.001% or less.

[0051] Zn: 0.020% or less Zn is an element effective for spheroidizing the shape of inclusions and improving the delayed fracture resistance of the steel sheet. To obtain such an effect, the Zn content is preferably 0.001% or more. On the other hand, when the Zn content exceeds 0.020%, a large amount of coarse precipitates and inclusions are generated, which may conversely cause a decrease in the delayed fracture resistance. Therefore, when Zn is contained, the Zn content is preferably 0.020% or less.

[0052] Co: 0.500% or less Co, like Zn, is an element effective for spheroidizing the shape of inclusions and improving the delayed fracture resistance of the steel sheet. To obtain such an effect, the Co content is preferably 0.001% or more. On the other hand, when the Co content exceeds 0.500%, a large amount of coarse precipitates and inclusions are generated, which may conversely cause a decrease in the delayed fracture resistance. Therefore, when Co is contained, the Co content is preferably 0.500% or less.

[0053] Zr: 0.20% or less Zr contributes to high strength through refinement of prior austenite grains. Also, Zr contributes to high strength through reduction of block size, bainite grain size, etc., which are internal structural units of martensite and bainite, due to refinement of prior austenite grains. Furthermore, Zr improves castability. To obtain such an effect, the Zr content is preferably 0.001% or more. However, when a large amount of Zr is contained, the amount of coarse precipitates of ZrN-based and ZrS-based that remain undissolved during heating of the steel slab before the hot rolling process increases, and the ductility decreases. Therefore, when Zr is contained, the Zr content is preferably 0.20% or less, more preferably 0.05% or less, and even more preferably 0.01% or less.

[0054] Ca: 0.0200% or less Ca exists as an inclusion in steel. Here, when the Ca content exceeds 0.0200%, a large amount of coarse inclusions are generated, which may reduce ductility and stress corrosion cracking resistance. Also, the surface quality deteriorates. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less. The lower limit of the Ca content is not particularly limited. The Ca content is preferably, for example, 0.0005% or more. Also, due to production technology constraints, the Ca content is more preferably 0.0010% or more.

[0055] Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% 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 (excluding Ce): 0.0200% or less Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective in improving the delayed fracture resistance of steel sheets. To obtain such an effect, the content of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is preferably 0.0001% or more respectively. On the other hand, when the content of Ce, Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceeds 0.0200% respectively, and when the As content exceeds 0.0500%, a large amount of coarse precipitates and inclusions are generated, and the delayed fracture resistance may instead decrease. Therefore, when these elements are contained, the content of Ce, Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM is preferably 0.0200% or less respectively, and the As content is preferably 0.0500% or less. Note that Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM may be contained individually or in combination. In addition, REM as used herein is a general term for 16 elements including 14 lanthanoid elements from La (lanthanum) with atomic number 57 to Lu (lutetium) with atomic number 71 excluding Ce, Sc (scandium) with atomic number 21, and Y (yttrium) with atomic number 39. These 16 elements can be contained individually or in combination. Note that the REM content means the total content of these 16 elements.

[0056] The balance other than the above elements is Fe and unavoidable impurities. Note that any of the above optional addition elements may be 0%. Unavoidable impurities are impurities that are unavoidably mixed in from raw materials, manufacturing processes, manufacturing equipment, etc., and are allowed to be contained within a range that does not inhibit the object of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of impurities include O (oxygen). Also, when the content of each of the above optional addition elements is less than the preferred lower limit value, it can be said that the element is included as an unavoidable impurity.

[0057] Next, the steel structure of the base steel plate of the zinc-plated steel plate according to an embodiment of the present invention will be described. The area ratio of each phase is the ratio of the area occupied by each phase to the total area of the steel structure.

[0058] Total area ratio of one or both of ferrite and bainite (hereinafter, also referred to as the total area ratio of ferrite and bainite): 20% or more and 90% or less Since ferrite and bainite are soft, they are effective in obtaining excellent ductility. In order to obtain the desired ductility, the total area ratio of ferrite and bainite is set to 20% or more. On the other hand, when the area ratio of ferrite and bainite becomes excessive, it becomes difficult to obtain the desired strength. Therefore, the total area ratio of ferrite and bainite is set to 90% or less. The total area ratio of ferrite and bainite is preferably 25% or more, more preferably 30% or more. Also, the total area ratio of ferrite and bainite is preferably 75% or less, more preferably 65% or less. Note that ferrite and bainite may be contained individually, or both of them may be contained. Since both ferrite and bainite adjust the strength and ductility to the desired range, it is not necessary to specify the area ratio of each of ferrite and bainite in the present disclosure.

[0059] Area ratio of martensite: 10% or more and 80% or less Martensite is hard and is a structure necessary for increasing the strength of the steel plate. Here, when the area ratio of martensite is less than 10%, the desired strength cannot be obtained. On the other hand, an excessive increase in the area ratio of martensite causes a decrease in ductility. Therefore, the area ratio of martensite is set to 10% or more and 80% or less. The area ratio of martensite is preferably 20% or more, more preferably 30% or more. Also, the area ratio of martensite is preferably 70% or less, more preferably 60% or less.

[0060] Martensite is a hard structure formed by transformation from austenite below the martensite transformation point (also simply referred to as the Ms point). Martensite includes both so-called fresh martensite as it is after quenching and so-called tempered martensite obtained by tempering the fresh martensite.

[0061] In addition, the steel structure of the base steel plate may contain retained austenite. Retained austenite improves the balance between strength and ductility. However, when the amount of retained austenite becomes excessive, for example, when forming the steel plate into a component, the retained austenite transforms into martensite, increasing the initiation points of cracks. Therefore, the area ratio of retained austenite is preferably 20% or less, more preferably 15% or less, still more preferably 10% or less. The area ratio of retained austenite may be 0%.

[0062] Retained austenite is austenite that remains without transforming into ferrite, martensite, bainite or other metal phases. Retained austenite is generated, for example, when the concentration of elements such as C in austenite causes the martensite transformation point to be below room temperature (so that austenite remains without transforming).

[0063] The area ratio of the remaining structure other than the above-described ferrite, bainite, martensite and retained austenite is preferably 10% or less. The area ratio of the remaining structure is more preferably 5% or less. The area ratio of the remaining structure may also be 0%.

[0064] The remaining structure is not particularly limited and includes, for example, pearlite and carbides such as cementite. The type of the remaining structure can be confirmed by observation using, for example, SEM (Scanning Electron Microscope). Pearlite is a structure composed of layered ferrite and cementite, which is formed from austenite at a relatively high temperature.

[0065] Here, the total area ratio of ferrite and bainite, as well as the area ratio of martensite, are measured, for example, as follows at the 1 / 4 plate thickness position of the base steel plate. That is, a sample is cut out from the galvanized steel plate so that the cross-section (L cross-section) parallel to the rolling direction and the plate thickness direction of the base steel plate of the galvanized steel plate becomes the observation surface. Then, the observation surface of the sample is polished using diamond paste, and then the observation surface of the sample is finish-polished using alumina. Then, the observation surface of the sample is etched with nital to reveal the structure. And, using SEM, the observation surface of the sample is observed in 5 fields of view under the condition of a magnification of 1500 times. Then, from the obtained structure images, the following regions are color-separated (defined) using Adobe Photoshop of Adobe Systems. And, by the point counting method, the total area ratio of ferrite and bainite, as well as the area ratio of martensite, are calculated. Specifically, in a region with an actual length of 82 μm × 57 μm of each SEM image, 16 × 15 grid points are set at intervals of 4.8 μm. Then, the number of grid points on ferrite and bainite, as well as martensite, are counted respectively. Then, the number of grid points on ferrite and bainite, as well as martensite, are each divided by the number of all grid points and multiplied by 100 to calculate the total area ratio of ferrite and bainite, as well as the area ratio of martensite.

[0066] Ferrite: It is a region showing black and has a massive form. Also, ferrite is a structure composed of crystal grains of BCC lattice. Ferrite is generated by the transformation from austenite. Bainite: It is a region showing black to dark gray and has a form such as massive or amorphous. Also, as described above, bainite is a hard structure in which fine carbides are dispersed in acicular or plate-like ferrite. Bainite is generated by the transformation from austenite in a temperature range lower than ferrite (above the Ms point). Martensite: It is a region that exhibits white to light gray. Also, as described above, martensite is a hard structure formed by the transformation from austenite in the temperature range below the Ms point. Martensite includes both so-called fresh martensite as quenched and so-called tempered martensite obtained by tempering the fresh martensite. Among these, tempered martensite contains carbides.

[0067] In addition, the area ratio of retained austenite is measured as follows at the 1 / 4 position of the plate thickness of the base steel plate. That is, after the base steel plate is machined by grinding to the 1 / 4 position of the plate thickness in the plate thickness direction (depth direction), chemical polishing with oxalic acid is performed to obtain an observation surface. Then, the observation surface is observed by X-ray diffraction method. CoKα ray is used as the incident X-ray, and the ratio of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of bcc iron is determined. Then, the volume ratio of retained austenite is calculated from the ratio of the diffraction intensities of each plane. And assuming that the retained austenite is three-dimensionally homogeneous, the volume ratio of retained austenite is taken as the area ratio of retained austenite.

[0068] Furthermore, the area ratio of the remaining structure is obtained by subtracting the total area ratio of ferrite and bainite, the area ratio of martensite, and the area ratio of retained austenite determined as described above from 100%. [Area ratio of the remaining structure (%)] = 100 - [Total area ratio of ferrite and bainite (%)] - [Area ratio of martensite (%)] - [Area ratio of retained austenite (%)]

[0069] M1 / Mt: 0.30 or less Martensite is hard and is a structure necessary for increasing the strength of the steel sheet. However, when stress is applied to the steel sheet, voids are generated at the interface between martensite and soft ferrite or bainite, particularly at the interface between the region with high C and Mn concentrations in the region constituting martensite and ferrite or bainite. Then, when these voids are connected, delayed fracture occurs. In order to suppress the generation and connection of such voids and obtain excellent delayed fracture resistance characteristics, in the region constituting martensite, it is effective to reduce the ratio of the first region corresponding to the region with high C and Mn concentrations and increase the ratio of the other region (hereinafter also referred to as the second region). Therefore, M1 / Mt is set to 0.30 or less. M1 / Mt is preferably 0.25 or less, more preferably 0.20 or less. The lower limit of M1 / Mt is not particularly limited and may be 0.

[0070] Here, M1 is obtained, for example, in accordance with the method described in Reference 1. Reference 1: Yamashita et al., "Carbon Distribution at the Initial Stage of Primary Ferrite Transformation in Low-Carbon Steel by High-Precision FE-EPMA," Iron and Steel, Vol. 103 (2017) No. 11 P. 14-20

[0071] That is, using the sample used for the measurement of the area ratio of each of the above phases, quantitative analysis of C and Mn is performed by a field emission electron probe microanalyzer (FE-EPMA) at a position 1 / 4 of the thickness of the base steel plate, and two-dimensional distributions of C and Mn (C mapping and Mn mapping) are created. Here, quantitative analysis of C is performed while preventing carbon contamination on the surface, and then quantitative analysis of Mn is performed in the same field of view. In the quantitative analysis of C, the acceleration voltage is 7 kV and the irradiation current is 5 nA. In the quantitative analysis of Mn, the acceleration voltage is 9 kV and the irradiation current is 10 nA. Then, the tissue image used for the measurement of the area ratio of each of the above phases is compared with the two-dimensional distributions of C and Mn. And, in the region constituting martensite, a region where the concentrations of C and Mn satisfy the relationships of the following formulas (3) and (4) is defined as the first region, and other regions are defined as the second region. Then, the total area of the region defined as the first region is obtained, and M1 is obtained by dividing the total area by the total area of the observation field of view. [C] M / [C]≧2.4 ···(3) [Mn] M / [Mn]≧1.5 ···(4) Here, [C] and [Mn] are the contents (mass %) of C and Mn in the component composition of the base steel plate, respectively. Also, [C] M and [Mn] M are the concentrations (mass %) of C and Mn in the region constituting martensite, respectively.

[0072] Note that the upper limits of [C] M / [C] and [Mn] M / [Mn] according to the above formulas (3) and (4) are not particularly limited. For example, [C] M / [C] is preferably 5.0 or less, and [Mn] M / [Mn] is preferably 3.0 or less.

[0073] Diffusible hydrogen content of the base steel plate: 0.50 mass ppm or less From the viewpoint of obtaining excellent delayed fracture resistance characteristics, the diffusible hydrogen content of the base steel plate is set to 0.50 mass ppm or less. Further, the diffusible hydrogen content of the base steel plate is preferably 0.45 mass ppm or less, more preferably 0.40 mass ppm or less, and still more preferably 0.35 mass ppm or less. Note that the lower limit of the diffusible hydrogen content of the base steel plate is not particularly limited and may be 0 mass ppm. However, due to production technology limitations, the diffusible hydrogen content of the base steel plate is preferably 0.01 mass ppm or more.

[0074] Here, the diffusible hydrogen content of the base steel plate is measured as follows, for example. That is, a test piece with a length of 30 mm and a width of 5 mm is taken from a galvanized steel plate, and the galvanized layer is removed by a router (precision grinder). Then, the amount of hydrogen released from the test piece is measured by the temperature-programmed desorption analysis method. Specifically, the test piece is continuously heated from 25°C to 300°C at a heating rate of 200°C / h and then cooled to room temperature. At this time, in the temperature range from 25°C to 210°C during the continuous heating, the amount of hydrogen released from the test piece (integrated hydrogen amount) is measured. Then, the measured hydrogen amount is divided by the mass of the test piece (the test piece after removing the galvanized layer and before continuous heating), and the value converted to mass ppm units is taken as the diffusible hydrogen content of the base steel plate.

[0075] Next, the galvanized layer of the galvanized steel plate according to an embodiment of the present invention will be described. The galvanized layer may be provided on only one surface of the steel plate or on both surfaces. Note that the galvanized layer refers to a plating layer having Zn as the main component (Zn content of 50.0 mass% or more). Examples of the galvanized layer include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer. A steel plate having a galvanized layer can also be referred to as a galvanized steel plate. Further, a steel plate having the above-described hot-dip galvanized layer and alloyed hot-dip galvanized layer can also be referred to as a hot-dip galvanized steel plate (GI) and an alloyed hot-dip galvanized steel plate (GA), respectively.

[0076] Here, the hot-dip galvanized layer is preferably composed of, for example, Zn, 20.0 mass% or less of Fe, and 0.001 mass% or more and 1.0 mass% or less of Al. Further, the hot-dip galvanized layer may optionally contain a total of 0.0 mass% or more and 3.5 mass% or less of one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM. The Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The balance other than the above elements is inevitable impurities.

[0077] Further, the alloyed hot-dip galvanized layer is preferably composed of, for example, Zn, 20 mass% or less of Fe, and 0.001 mass% or more and 1.0 mass% or less of Al. Further, the alloyed hot-dip galvanized layer may optionally contain a total of 0.0 mass% or more and 3.5 mass% or less of one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM. The Fe content of the alloyed hot-dip galvanized layer is more preferably 7.0 mass% or more, and even more preferably 8.0 mass% or more. The Fe content of the alloyed hot-dip galvanized layer is more preferably 15.0 mass% or less, and even more preferably 12.0 mass% or less. The balance other than the above elements is inevitable impurities.

[0078] In addition, the plating adhesion amount per side of the zinc plating layer is not particularly limited, but it is preferably 20 g / m 2 or more and 80 g / m 2 or less.

[0079] The plating adhesion amount of the zinc plating layer is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (Ibit 700BK (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass% hydrochloric acid aqueous solution. Next, a steel sheet to be a test piece is immersed in the treatment solution to dissolve the zinc plating layer. Then, the mass reduction amount of the test piece before and after dissolution is measured, and the value is divided by the surface area of the base steel sheet (the surface area of the portion covered with plating) to calculate the plating adhesion amount (g / m 2 )

[0080] Next, the mechanical properties of the zinc-plated steel sheet according to one embodiment of the present invention will be described.

[0081] TS: 780 MPa or more The TS of the zinc-plated steel sheet according to one embodiment of the present invention is 780 MPa or more. The upper limit of the TS of the zinc-plated steel sheet according to one embodiment of the present invention is not particularly limited. For example, the TS of the zinc-plated steel sheet according to one embodiment of the present invention is preferably less than 1300 MPa.

[0082] Here, TS is measured by a tensile test conforming to JIS Z 2241 (2022) (hereinafter, also simply referred to as JIS Z 2241).

[0083] Excellent ductility means that the total elongation (El) measured by a tensile test conforming to JIS Z 2241 satisfies the following formula. When 780 MPa ≤ TS < 980 MPa, 15% ≤ El When 980 MPa ≤ TS, 9% ≤ El

[0084] Excellent delayed fracture resistance means that in a delayed fracture test conforming to SEP1970, a stress corresponding to the yield strength (YS) of the steel sheet (test piece) is applied to the steel sheet, and no fracture occurs in the steel sheet when 96 hours have elapsed after the stress is applied.

[0085] In the techniques disclosed in Patent Documents 3 and 4 described above, while applying a predetermined stress, the steel sheet is immersed in an acidic aqueous solution such as hydrochloric acid for a certain period of time to forcibly introduce hydrogen into the steel sheet and evaluate the hydrogen embrittlement resistance characteristics. However, in such a test, hydrogen is forcibly introduced into the steel sheet (base steel sheet) for evaluation, and the influence of hydrogen that enters during the manufacturing process of the steel sheet cannot be evaluated. Therefore, in this specification, the hydrogen embrittlement resistance characteristics are evaluated by the hydrogen embrittlement test in accordance with SEP1970 described above.

[0086] The detailed measurement procedures for each of the above characteristics are as described in the examples described later.

[0087] Also, the thickness of the galvanized steel sheet according to an embodiment of the present invention is not particularly limited, but is preferably 0.5 mm or more and 3.5 mm or less.

[0088] [2] Member Next, a member according to an embodiment of the present invention will be described. A member according to an embodiment of the present invention is a member (used as a material) made using the above galvanized steel sheet. For example, at least one of forming and joining is performed on the galvanized steel sheet as the material to form a member. Here, the above galvanized steel sheet has high strength, excellent ductility, and excellent hydrogen embrittlement resistance characteristics. Therefore, a member according to an embodiment of the present invention is particularly suitable for application to members used in the automotive field.

[0089] [3] Manufacturing method of galvanized steel sheet Next, a manufacturing method of a galvanized steel sheet according to an embodiment of the present invention will be described. Here, each temperature mentioned means the surface temperature of the steel slab and the steel sheet unless otherwise specified.

[0090] First, prepare a steel slab having the above-described component composition. For example, melt a steel material to obtain molten steel having the above-described component composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. Then, solidify the obtained molten steel to obtain a steel slab. The method for obtaining a steel slab from molten steel is not particularly limited. For example, a continuous casting method, an ingot-making method, or a thin slab casting method can be used. From the viewpoint of preventing macrosegregation, the continuous casting method is preferred.

[0091] Here, in addition to the conventional method, energy-saving processes such as direct delivery rolling and direct rolling can be applied without problems. The conventional method is a method in which the obtained steel slab is once cooled to room temperature, then reheated, and hot-rolled. Direct delivery rolling is a method in which the obtained steel slab is charged into a heating furnace as a warm slab without being cooled to room temperature and hot-rolled. Direct rolling is a method in which the steel slab is slightly heat-insulated and then immediately hot-rolled. Also, from the viewpoints of carbide dissolution and reduction of rolling load, the slab heating temperature is preferably 1100 °C or higher. On the other hand, in order to prevent an increase in scale loss, the slab heating temperature is preferably 1300 °C or lower. The steel slab is made into a sheet bar by rough rolling under conditions according to a conventional method, for example. When the slab heating temperature is lowered, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling from the viewpoint of preventing troubles during hot rolling.

[0092] [Hot Rolling Process] Next, subject the steel slab to hot rolling under the following conditions to obtain a hot-rolled steel sheet.

[0093] Finish rolling end temperature: 840 °C or higher and 1000 °C or lower If the finish rolling end temperature is less than 840°C, the formation of ferrite is promoted, and excessive ferrite is generated before coiling the hot-rolled steel sheet. As a result, C and Mn are concentrated in the untransformed austenite. Excessive concentration of C in the untransformed austenite promotes pearlite transformation. That is, in the steel structure of the hot-rolled steel sheet obtained after hot rolling, excessive pearlite is generated. Pearlite is a lamellar structure of ferrite and cementite, and Mn is concentrated in the cementite. Here, from the viewpoint of suppressing the concentration of Mn into martensite in the steel structure of the base steel sheet of the final product (hereinafter also referred to as the final structure), it is important to minimize the Mn concentration region in the structure of the cold-rolled steel sheet before the annealing process. Therefore, the finish rolling end temperature should be 840°C or higher. The finish rolling end temperature is preferably 850°C or higher. On the other hand, if the finish rolling end temperature becomes excessively high, it may be difficult to cool to the coiling temperature described later. Therefore, the finish rolling end temperature should be 1000°C or lower. The finish rolling end temperature is preferably 950°C or lower.

[0094] Coiling temperature: 620°C or lower If the coiling temperature exceeds 620°C, excessive pearlite is formed during coiling, and Mn concentration is promoted. The lower the coiling temperature, the smaller the amount of pearlite formed. Therefore, the coiling temperature is preferably low. Therefore, the coiling temperature should be 620°C or lower. The coiling temperature is preferably 600°C or lower, more preferably 580°C or lower. On the other hand, if the coiling temperature is less than 400°C, the steel sheet may become excessively hard and cause breakage during cold rolling. Therefore, the coiling temperature is preferably 400°C or higher, more preferably 420°C or higher.

[0095] In addition, descaling may be appropriately performed to remove the primary scale and secondary scale formed on the surface of the hot-rolled steel sheet. Before cold rolling the hot-rolled steel sheet, it is advisable to perform sufficient pickling to reduce the remaining scale. Also, from the viewpoint of reducing the load during cold rolling, optionally, the hot-rolled steel sheet may be subjected to hot-rolled sheet annealing.

[0096] [Cold rolling process] Next, cold rolling is performed on the hot-rolled steel sheet under the following conditions to obtain a cold-rolled steel sheet.

[0097] Reduction ratio: 20% or more and 80% or less The reduction ratio in cold rolling shall be 20% or more. That is, if the reduction ratio is less than 20%, coarsening and non-uniformity of the steel structure are likely to occur in the annealing process, and TS and ductility will decrease in the final product. Therefore, the reduction ratio shall be 20% or more. On the other hand, if the reduction ratio exceeds 80%, shape defects of the steel sheet are likely to occur. Also, non-uniformity of the steel structure due to temperature unevenness in the annealing process and non-uniformity of the zinc plating adhesion amount may occur. Therefore, the reduction ratio shall be 80% or less. The reduction ratio is preferably 30% or more. Also, the reduction ratio is preferably 70% or less.

[0098] [Annealing process] Next, the cold-rolled steel sheet is annealed under the following conditions.

[0099] Annealing temperature: 750°C or more and 900°C or less When the annealing temperature is less than 750°C, the formation ratio of austenite during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite increases excessively after annealing, and the desired strength cannot be obtained. On the other hand, if the annealing temperature exceeds 900°C, the desired area ratios of ferrite and bainite cannot be obtained, and the ductility decreases. Therefore, the annealing temperature shall be 750°C or more and 900°C or less. The annealing temperature is preferably 890°C or less, more preferably 880°C or less. Note that the annealing temperature is the maximum temperature reached in the annealing process.

[0100] Annealing time: 1 second or more and 30 seconds or less From the perspective of suppressing the enrichment of C and Mn into martensite in the final structure, it is important to suppress the enrichment of C and Mn into austenite during the annealing process. For this purpose, the shorter the annealing time, the better. Also, in order to reduce the diffusible hydrogen content in the base steel plate, the shorter the annealing time, the better. Therefore, the annealing time should be 30 seconds or less. The annealing time is preferably 25 seconds or less, more preferably 20 seconds or less, and even more preferably 15 seconds or less. On the other hand, when the annealing time is less than 1 second, coarse Fe-based precipitates that serve as the starting point of delayed fracture do not dissolve sufficiently, resulting in a deterioration of the delayed fracture resistance properties. Therefore, the annealing time should be 1 second or more. The annealing time is preferably 3 seconds or more, more preferably 5 seconds or more. Note that the annealing time is the holding time at the annealing temperature.

[0101] 1.5 ((CT-350) / 100) ×T×log 10 (t + 10)×1.15 HA ×1.05 HB ≦6800 ···(5) To obtain excellent delayed fracture resistance properties, it is important to suppress the enrichment of C and Mn into martensite in the final structure and reduce the diffusible hydrogen content in the base steel plate. To achieve both of these, it is important to appropriately control the relationship between the hot rolling conditions and the annealing conditions, specifically, CT: the coiling temperature (°C) in the hot rolling process, T: the annealing temperature (°C) in the annealing process, t: the annealing time (seconds) in the annealing process, and HA: the atmospheric hydrogen concentration (volume %) in the annealing process, and HB: the atmospheric hydrogen concentration (volume %) in the cooling process described later. Specifically, it is important to satisfy the relationship of the above-mentioned formula (5). Therefore, 1.5 ((CT-350) / 100) ×T×log 10 (t + 10)×1.15 HA ×1.05 HB(hereinafter also referred to as the left side value of formula (5)) shall be 6800 or less. The left side value of formula (5) is preferably 6500 or less, more preferably 6000 or less. The lower limit of the left side value of formula (5) is not particularly limited. The left side value of formula (5) is preferably 2000 or more, for example. In addition, from the viewpoint of suppressing the formation of an oxide on the surface of the base steel sheet and thereby reducing the plating adhesion amount, the atmospheric hydrogen concentration (hereinafter also simply referred to as HA) in the annealing process is preferably 1% by volume or more, more preferably 2% by volume or more. Also, from the viewpoint of reducing the diffusible hydrogen amount of the base steel sheet, HA is preferably 15% by volume or less, more preferably 10% by volume or less.

[0102] [Cooling process] Next, the cold-rolled steel sheet annealed as described above is cooled under the following conditions.

[0103] Atmospheric hydrogen concentration: less than 30% by volume If the atmospheric hydrogen concentration (hereinafter also simply referred to as HB) in the cooling process is 30% by volume or more, the diffusible hydrogen amount of the base steel sheet increases and the stress corrosion cracking resistance deteriorates. Therefore, HB is less than 30% by volume, preferably less than 26% by volume, more preferably less than 22% by volume. The lower limit of HB is not particularly limited. Due to production technology constraints, HB is preferably 0.2% by volume or more, more preferably 0.5% by volume or more.

[0104] Cooling stop temperature: 600°C or less If the cooling stop temperature exceeds 600°C, excessive ferrite and pearlite are generated, and the desired strength cannot be obtained. Therefore, the cooling stop temperature is 600°C or lower, preferably 580°C or lower, more preferably 560°C or lower. The lower limit of the cooling stop temperature is not particularly limited. However, when performing a zinc plating process described later, especially a hot-dip zinc plating process or an alloyed hot-dip zinc plating process, it is preferable to make the temperature of the plate entering the plating bath higher than the plating bath temperature. Therefore, if the cooling stop temperature becomes excessively low, it is required to increase the equipment capacity for reheating the cold-rolled steel sheet, and there is a possibility of increasing the equipment investment burden. Therefore, the cooling stop temperature is preferably 350°C or higher, more preferably 400°C or higher.

[0105] The conditions after cooling stop (until the zinc plating process) are not particularly limited. For example, after cooling stop, the cold-rolled steel sheet may be held in the temperature range of 350 to 600°C for 10 to 150 seconds.

[0106] [Zinc Plating Process] Next, the cold-rolled steel sheet is subjected to a zinc plating process to obtain a zinc-plated steel sheet. Examples of the zinc plating process include a hot-dip zinc plating process and an alloyed hot-dip zinc plating process. The processing conditions may follow the conventional methods.

[0107] For example, in the case of a hot-dip zinc plating process, after immersing the cold-rolled steel sheet in a zinc plating bath at 440°C or higher and 500°C or lower, it is preferable to adjust the plating adhesion amount by gas wiping or the like. The zinc plating bath is not particularly limited as long as it has the composition of the zinc plating layer described above. For example, it is preferable to use a plating bath having a composition in which the Al content is 0.10 mass% or more and 0.23 mass% or less, and the balance is composed of Zn and inevitable impurities.

[0108] In the case of alloying hot-dip galvanizing treatment, after performing the hot-dip galvanizing treatment in the above manner, it is preferable to perform the alloying treatment in a temperature range of 450°C or higher and 600°C or lower. If the alloying temperature is less than 450°C, the Zn-Fe alloying rate becomes excessively slow, and alloying may be difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite transforms into pearlite, and the strength and ductility may decrease. Therefore, the alloying temperature in the alloying treatment is preferably 450°C or higher and 600°C or lower. The alloying temperature in the alloying treatment is more preferably 460°C or higher, and even more preferably 470°C or higher. Also, the alloying temperature in the alloying treatment is more preferably 580°C or lower, and even more preferably 560°C or lower.

[0109] Also, the coating weight is preferably 20 g / m 2 or more and 80 g / m 2 or less per side. Note that the coating weight can be adjusted by gas wiping or the like.

[0110] The conditions after the galvanizing treatment step are not particularly limited. For example, when performing the hot-dip galvanizing treatment or the alloying hot-dip galvanizing treatment, from the viewpoint of productivity, after the hot-dip galvanized steel sheet or the alloying hot-dip galvanized steel sheet is finished with the hot-dip galvanizing treatment or the alloying hot-dip galvanizing treatment, it is preferably cooled to a temperature of 50°C or lower at an average cooling rate (hereinafter, also referred to as the cooling rate after the galvanizing treatment): 0.5°C / second or more. The cooling rate after the galvanizing treatment is more preferably 1°C / second or more, and even more preferably 2°C / second or more. The upper limit of the cooling rate after the galvanizing treatment is not particularly limited either, but from the viewpoint of reducing the equipment investment burden, the cooling rate after the galvanizing treatment is preferably 200°C / second or lower, and more preferably 100°C / second or lower.

[0111] [Temper rolling process] Further, the zinc-plated steel sheet obtained as described above may be further subjected to temper rolling. In this case, from the viewpoints of shape correction and surface roughness adjustment, the elongation is preferably 0.10% or more. More preferably, the elongation is 0.12% or more, and still more preferably 0.15% or more. The upper limit of the elongation is not particularly limited. However, if the elongation exceeds 2.00%, the yield stress may excessively increase, and the dimensional accuracy may decrease when the zinc-plated steel sheet is formed into a member. Therefore, the elongation is preferably 2.00% or less.

[0112] Further, the temper rolling may be performed on a continuous device (online) with the annealing device for performing the above-described respective steps, or may be performed on a discontinuous device (offline) from the annealing device for performing the respective steps. Also, the number of rolling passes in the temper rolling may be one or two or more. Note that rolling by a leveler or the like may be employed as long as an elongation equivalent to that of the temper rolling can be imparted.

[0113] From the viewpoint of productivity, a series of treatments such as the above-described annealing step and zinc plating treatment step are preferably performed in a continuous annealing line (CAL) or a continuous galvanizing line (CGL). After the hot-dip galvanizing treatment, wiping is possible in order to adjust the coating weight.

[0114] The conditions other than those described above are not particularly limited, and may be in accordance with conventional methods. According to the method for manufacturing a zinc-plated steel sheet according to one embodiment of the present invention described above, a zinc-plated steel sheet having high strength, excellent ductility, and excellent stress corrosion cracking resistance can be obtained, and the zinc-plated steel sheet can be suitably used for, for example, automobile members.

[0115] [4] Method for manufacturing a member Next, a method for manufacturing a member according to an embodiment of the present invention will be described. The method for manufacturing a member according to an embodiment of the present invention includes a step of subjecting the above galvanized steel sheet to at least one of forming and joining to form a member. Here, the forming method is not particularly limited, and for example, a general processing method such as press forming can be used. Also, the joining method is not particularly limited, and for example, general welding such as spot welding, laser welding, arc welding, and rivet joining, caulking joining, etc. can be used. Note that the forming conditions and joining conditions are not particularly limited and may follow conventional methods.

Example

[0116] A steel material having the component composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter, and a steel slab was obtained by continuous casting. Next, hot rolling consisting of rough rolling and finish rolling was performed on the steel slab under the conditions shown in Table 2 to obtain a hot-rolled steel sheet. Next, the obtained hot-rolled steel sheet was subjected to pickling and cold rolling under the conditions shown in Table 2 to obtain a cold-rolled steel sheet. Next, annealing, cooling, and galvanizing treatment were performed on the obtained cold-rolled steel sheet under the conditions shown in Table 2 to obtain a galvanized steel sheet. Also, in some examples, temper rolling was carried out under the conditions shown in Table 2. Note that the conditions not specified were in accordance with conventional methods.

[0117] Here, in the galvanizing treatment step, hot-dip galvanizing treatment or hot-dip galvannealing treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a hot-dip galvannealed steel sheet (hereinafter also referred to as GA). Note that in Table 2, the types of galvanizing treatment steps are also indicated as "GI" and "GA".

[0118] Here, in the hot-dip galvanizing treatment, the plating bath had a composition containing 0.20% by mass of Al, with the balance being Zn and unavoidable impurities. The plating bath temperature was 470°C. The plating adhesion amount was 45 to 72 g / m per side 2It was (double-sided plating). Also, the composition of the zinc plating layer of the finally obtained GI was Fe: 0.1 to 1.0% by mass, Al: 0.2 to 1.0% by mass, and the balance was composed of Zn and unavoidable impurities.

[0119] Also, in the case of the alloying hot-dip zinc plating treatment, the plating bath had a composition containing 0.14% by mass of Al, and the balance was composed of Zn and unavoidable impurities. The plating bath temperature was 470°C. The plating adhesion amount was 45 g / m per side 2 It was (double-sided plating). The alloying temperature was 520°C. Also, the composition of the zinc plating layer of the finally obtained GA was Fe: 7 to 15% by mass, Al: 0.1 to 1.0% by mass, and the balance was composed of Zn and unavoidable impurities.

[0120] Using the steel sheet thus obtained, the steel structure of the base steel sheet was identified, M1 / Mt, and the diffusible hydrogen amount were measured in the above-described manner. The measurement results are shown in Table 3.

[0121] Also, a tensile test was conducted in the following manner, and TS and El were evaluated according to the following criteria. ·TS Qualified (excellent): 780 MPa ≤ TS Unqualified (defective): TS < 780 MPa ·El Qualified (excellent): When 780 MPa ≤ TS < 980 MPa, 15% ≤ El When 980 MPa ≤ TS, 9% ≤ El Unqualified (defective): When 780 MPa ≤ TS < 980 MPa, El < 15% When 980 MPa ≤ TS, El < 9%

[0122] The tensile test was conducted in accordance with JIS Z 2241. That is, a JIS No. 5 test piece was taken from the obtained zinc-plated steel sheet such that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. Using the taken test piece, a tensile test was conducted under the condition that the crosshead speed was 10 mm / min, and TS and El were measured. The results are also shown in Table 3.

[0123] The delayed fracture test was conducted in accordance with SEP1970. That is, test pieces measuring 85 mm × 30 mm were sampled from the obtained galvanized steel sheets such that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. Holes with a radius of 10 mm were punched at the center positions of the major and minor axes of the sampled test pieces. Subsequently, a stress corresponding to the yield strength (YS) of the steel sheet from which the test piece was sampled was applied to the test piece in the longitudinal direction and held in that state for 96 hours. The yield strength (YS) was measured by the tensile test described above. Then, the delayed fracture resistance characteristics were evaluated according to the following criteria. Pass (excellent): No fracture occurred at the time when 96 hours had elapsed. Fail (poor): Fracture occurred before 96 hours had elapsed.

[0124] [Table 1]

[0125] [Table 2]

[0126] [Table 3]

[0127] As shown in Table 3, in all of the inventive examples, all of TS, El, and the delayed fracture resistance characteristics passed. On the other hand, in the comparative examples, at least one of TS, El, and the delayed fracture resistance characteristics failed. [Industrial Applicability]

[0128] According to the present invention, a galvanized steel sheet having high strength, excellent ductility, and excellent delayed fracture resistance characteristics can be obtained. Further, the galvanized steel sheet can be extremely advantageously applied as a material for skeletal structural members of automobiles and the like. Thereby, it is possible to improve the fuel efficiency by reducing the vehicle body weight, and thus the industrial utility value is extremely large.

Claims

1. A galvanized steel sheet having a base steel sheet and a zinc plating layer on the surface of the base steel sheet, wherein the base steel sheet is in mass %, C: 0.05% or more and 0.20% or less, Si: 0.2% or more and 1.8% or less, Mn: 1.50% or more and 3.00% or less, P: 0.100% or less, S: 0.0500% or less, Al: 0.010% or more and 1.000% or less and N: 0.0100% or less and satisfies the relationships of the following formulas (1) and (2), the balance being Fe and inevitable impurities, the component composition, at the 1 / 4 position of the plate thickness of the base steel sheet, the total area ratio of one or both of ferrite and bainite: 20% or more and 90% or less, the area ratio of martensite: 10% or more and 80% or less and M1 / Mt: 0.30 or less wherein Mt is the area ratio (%) of the martensite, M1 is the area ratio (%) of the first region among the regions constituting the martensite, the first region is a region satisfying the relationships of the following formulas (3) and (4), has a steel structure, the diffusible hydrogen content of the base steel sheet is 0.50 mass ppm or less, and the tensile strength is 780 MPa or more. A galvanized steel sheet. [C] + [Si] / 24 + [Mn] / 6 ≦ 0.65... (1) 0.13 ≦ [Si] / [Mn] ≦ 0.75... (2) [C] Ｍ / [C] ≥ 2.4...(3) [Mn] Ｍ / [Mn] ≥ 1.5...(4) Here, [C], [Si], and [Mn] are the contents (mass%) of C, Si, and Mn in the component composition of the base steel sheet, respectively. Also, [C] Ｍ and [Mn] Ｍ are the concentrations (mass%) of C and Mn in the region constituting the martensite, respectively.

2. The component composition of the base steel sheet further comprises, in mass %, Nb: 0.40% or less, Ti: 0.40% or less, V: 0.45% or less, B: 0.0100% or less, Cr: 1.00% or less, Ni: 1.00% or less, Mo: 1.00% or less, Sb: 0.100% or less, Sn: 0.100% or less, Cu: 1.00% or less, Ta: 0.100% or less, W: 0.200% or less, Mg: 0.010% or less, Zn: 0.020% or less, Co: 0.500% or less, Zr: 0.20% 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: 0.0500% 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 (excluding Ce): 0.0200% or less The galvanized steel sheet according to claim 1, containing at least one selected from the above.

3. A member made using the galvanized steel sheet according to claim 1 or 2.

4. A steel slab having the component composition according to claim 1 or 2 is Finish rolling end temperature: 840°C or higher and 1000°C or lower and Coiling temperature: 620°C or lower Hot rolling is performed under these conditions to obtain a hot-rolled steel sheet, and a hot rolling process, Then, on the hot-rolled steel sheet, Reduction ratio: 20% or higher and 80% or lower Cold rolling is performed under these conditions to obtain a cold-rolled steel sheet, and a cold rolling process, Then, the cold-rolled steel sheet is Annealing temperature: 750°C or higher and 900°C or lower and Annealing time: 1 second or longer and 30 seconds or shorter Annealing is performed under these conditions, and an annealing process, Then, the cold-rolled steel sheet is Atmospheric hydrogen concentration: less than 30% by volume and Cooling stop temperature: 600°C or lower Cooling is performed under these conditions, and a cooling process, Then, a galvanizing treatment is performed on the cold-rolled steel sheet, and a galvanizing treatment process, having A method for manufacturing a galvanized steel sheet that satisfies the relationship of the following formula (5). 1.5 （（ＣＴ－３５０）／１００） ×T×log １０ (t + 10)×1.15 ＨＡ ×1.05 ＨＢ ≦6800 ··· (5) Here, CT: Coiling temperature (°C) in the hot rolling process, T: Annealing temperature (°C) in the annealing process, t: Annealing time (seconds) in the annealing process, HA: Atmospheric hydrogen concentration (% by volume) in the annealing process and HB: Atmospheric hydrogen concentration (% by volume) in the cooling process is.

5. A method for manufacturing a member, which has a step of forming at least one of a forming process or a joining process on the galvanized steel sheet according to claim 1 or 2 to obtain a member.

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