Galvanized steel sheet and member, and method for producing same

Abstract

The present invention provides a galvanized steel sheet which combines high strength, excellent ductility, and excellent delayed fracture resistance. A base steel sheet is given a prescribed composition, steel structure, and diffusible hydrogen content. Specifically, M1 / Mt is restricted to 0.30 or less in the steel structure of the base steel sheet, and the diffusible hydrogen content in the base steel plate is reduced to 0.50 mass ppm or less.

Description

Galvanized steel sheets and components, and their manufacturing methods

[0001] The present invention relates to a galvanized steel sheet and member suitable for use in automobile parts and the like, and to a method for manufacturing the same.

[0002] In recent years, from the perspective of global environmental conservation, the automobile industry has ２ Attempts are being made to reduce exhaust gases such as those mentioned above. Specifically, by increasing the strength and thinning of steel sheets, which are the raw material for automobile parts, the weight of the vehicle body can be reduced and fuel efficiency can be improved. In this way, attempts are being made to reduce exhaust gases.

[0003] Furthermore, from the viewpoint of rust prevention performance of the vehicle body, the steel sheets that are used as raw materials for automobile parts are sometimes plated with zinc.

[0004] As an example of a steel sheet that can be used as a material for such automotive parts, Patent Document 1 discloses the following: "A low-yield-ratio, high-strength cold-rolled steel sheet having excellent hole expandability, containing, 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%, and N: 0.006% or less, and further satisfying (%Ti) / (%S)≧5 when %S and %Ti are the S and Ti contents, respectively, with the remainder being Fe and unavoidable impurities."

[0005] Patent Document 2 discloses "a hot-dip galvanized high-strength steel sheet with excellent formability, characterized in that it contains, by mass%, 0.07 to 0.22% C, 0.005 to 1.0% Si, 1.5 to 2.8% Mn, 0.001 to 0.1% P, 0.001 to 0.01% S, 0.0005 to 0.01% N, and 0.02 to 1.0% Al, with the balance being Fe and unavoidable impurities, and has a microstructure in which ferrite accounts for 20 to 70% in area fraction, retained austenite accounts for 1 to 5% or less in area fraction, martensite accounts for 20% to 70% or less in area fraction, and the balance is bainite, and the steel sheet satisfies the formulas (A-1), (A-2), and (B)."

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

[0007] Patent Document 4 states, "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 unavoidable impurities, "A high-strength steel plate having a tensile ultimate strength of 900 MPa or more and good ductility and delayed fracture resistance, characterized in that the steel plate structure is mainly ferrite and contains martensite composed of block sizes of 1 μm or less, the volume fraction of ferrite is 60% or more, the C concentration in the martensite is 0.3% to 0.9%, and the yield ratio (YR), which is the ratio of the tensile ultimate strength (TS) to the yield stress (YS), is 0.75 or less."

[0008] JP 2002-069574 A JP 2010-156031 A JP 2013-108154 A JP 2011-111671 A

[0009] Generally, increasing the strength of a steel sheet reduces its ductility. However, steel sheets used as raw materials for automotive components (hereinafter also referred to as automotive steel sheets) are required to have both high strength and excellent ductility, specifically, improved total elongation (hereinafter also referred to simply as El) in a tensile test.

[0010] Furthermore, with the recent worldwide spread of automobiles, further improvements in delayed fracture resistance are required in view of their use in a wide variety of regions and climates. Here, delayed fracture refers to a phenomenon in which, when high stress is applied to a steel sheet (or a part made of steel sheet), hydrogen in the steel sheet reduces the interatomic bonding strength and causes local deformation, resulting in the formation of microcracks, which then propagate and lead to fracture.

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

[0012] The present invention has been developed to meet the above-mentioned demands, and aims to provide a galvanized steel sheet that combines high strength, excellent ductility, and excellent delayed fracture resistance, together with an advantageous method for manufacturing the same. Another aim of the present invention is to provide a member made from the above-mentioned galvanized steel sheet, and a method for manufacturing the same. Note that in this disclosure, any numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits, respectively.

[0013] In order to achieve the above object, the inventors have conducted extensive research and have come to the following findings.

[0014] (1) After adjusting the chemical composition of the base steel sheet to a predetermined range, the steel structure of the base steel sheet is controlled so that the total area ratio of ferrite and bainite and the area ratio of martensite are 20% to 90% and 10% to 80%, respectively, thereby achieving both high strength and excellent ductility.

[0015] (2) To further improve delayed fracture resistance, it is effective to suppress the formation of voids when high stress is applied to a galvanized steel sheet. In particular, it is effective to suppress the formation and connection of voids at the interface between the region of martensite with high C and Mn concentrations and ferrite or bainite. To achieve this, it is important to control the total area fraction of ferrite and bainite and the area fraction of martensite in the steel structure of the substrate steel sheet within the above-mentioned ranges, while controlling M1 / Mt to 0.30 or less. This ensures high strength and excellent ductility while achieving excellent delayed fracture resistance.

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

[0017] The first region is a region that satisfies the relationships of the following expressions (3) and (4): [C] Ｍ / [C]≧2.4...(3) [Mn] Ｍ / [Mn]≧1.5 (4) where [C] and [Mn] are the contents (mass%) of C and Mn in the chemical composition of the base steel sheet, respectively. Ｍ and [Mn] Ｍ are the concentrations (mass%) of C and Mn in the martensite region, respectively.

[0018] (3) The amount of diffusible hydrogen in the base steel sheet is reduced, which further improves delayed fracture resistance.

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

[0020] 1. A galvanized steel sheet having a substrate steel sheet and a galvanized layer on a surface of the substrate steel sheet, wherein the substrate steel sheet has a chemical composition, in mass %, of C: 0.05% 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, satisfying the relationships of the following formulas (1) and (2), with the balance being Fe and unavoidable impurities; and at a 1 / 4 position of the thickness of the substrate steel sheet, a total area ratio of one or both of ferrite and bainite: 20% or more and 90% or less, an area ratio of martensite: 10% or more and 80% or less, and M1 / Mt: 0.30 or less, 1. A galvanized steel sheet having a steel structure in which Mt is an area fraction (%) of the martensite, M1 is an area fraction (%) of a first region among regions constituting the martensite, and the first region is a region that satisfies the relationships of the following formulas (3) and (4), wherein the diffusible hydrogen content of the substrate steel sheet is 0.50 mass ppm or less, and the tensile strength is 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) where [C], [Si] and [Mn] are the contents (mass%) of C, Si and Mn in the chemical composition of the base steel sheet, respectively. Ｍ and [Mn] Ｍ are the concentrations (mass%) of C and Mn in the region constituting the martensite, respectively.

[0021] 2. The chemical composition of the substrate steel sheet further contains, 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, 2. The galvanized steel sheet according to 1 above, containing at least one selected from 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.

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

[0023] 4. A steel slab having the chemical composition according to 1 or 2 above is hot rolled at a finish rolling end temperature of 840°C to 1000°C and a coiling temperature of 620°C or less to obtain a hot rolled steel sheet, and then a cold rolling process is performed on the hot rolled steel sheet at a rolling reduction of 20% to 80% to obtain a cold rolled steel sheet, and then an annealing process is performed on the cold rolled steel sheet at an annealing temperature of 750°C to 900°C and an annealing time of 1 second to 30 seconds, and then a cooling process is performed on the cold rolled steel sheet at an atmospheric hydrogen concentration of less than 30% by volume and a cooling stop temperature of 600°C or less, and then a galvanizing process is performed on the cold rolled steel sheet, 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) where, 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.

[0024] 5. A method for manufacturing a component, comprising the step of subjecting the galvanized steel sheet according to 1 or 2 above to at least one of forming processing and joining processing to form a component.

[0025] According to the present invention, a galvanized steel sheet having high strength, excellent ductility, and excellent resistance to delayed fracture can be obtained. In addition, since the galvanized steel sheet of the present invention has high strength, excellent ductility, and excellent resistance to delayed fracture, it can be extremely advantageously used as a material for automotive frame structural members and the like.

[0026] The present invention will be described based on the following embodiments. First, the chemical composition of the substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention will be described. Note that the unit of chemical composition is "mass %", and hereinafter, unless otherwise specified, it will be simply referred to as "%".

[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 added from the viewpoint of ensuring the desired strength. If the C content is less than 0.05%, the area ratio of ferrite increases, making it impossible to obtain the desired strength. On the other hand, if the C content exceeds 0.20%, the tensile strength (hereinafter also referred to as TS) becomes excessively high and El decreases. In addition, martensite becomes excessively hard, and the delayed fracture resistance decreases. Therefore, the C content is set to 0.05% or more and 0.20% or less. The C content is preferably 0.06% or more, more preferably 0.07% or more. 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 steel sheet through solid solution strengthening. Si also increases the strength of ferrite, thereby improving ductility while suppressing strength degradation. Furthermore, Si promotes ferrite transformation during the annealing process and the subsequent cooling process. That is, Si is an element that affects the ferrite area ratio. Here, if the Si content is less than 0.2%, the ferrite area ratio decreases and ductility decreases. On the other hand, if the Si content is excessive, particularly if it exceeds 1.8%, it results in a significant increase in the rolling load during hot rolling and cold rolling. It also results in a decrease in toughness. Therefore, the Si content is set to 0.2% or more and 1.8% or less. The Si content is preferably 0.3% or more, more preferably 0.5% or more. 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 added 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 formed. This makes it difficult to ensure the desired strength. On the other hand, if Mn is added excessively, the ferrite and bainite transformation is delayed, resulting in 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. The Mn content is also preferably 2.85% or less, more preferably 2.70% or less.

[0030] P: 0.100% or less P is an element that has the effect of solid solution strengthening and increases strength. To achieve this effect, the P content is preferably 0.001% or more. Furthermore, due to constraints on production technology, the P content is more preferably 0.002% or more. On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries and embrittles the grain boundaries. As a result, the delayed fracture resistance decreases. Therefore, 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 coarse particles such as MnS, which reduces ductility. Furthermore, coarse particles such as MnS also reduce delayed fracture resistance. In particular, if the S content exceeds 0.0500%, excellent delayed fracture 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, and even more preferably 0.0030% or less. 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 deoxidize and reduce inclusions in steel. Furthermore, Al is an element that promotes ferrite transformation during the annealing process and the subsequent cooling process. That is, Al is an element that affects the area ratio of ferrite. Here, if the Al content is less than 0.010%, the area ratio of ferrite decreases, and ductility decreases. On the other hand, if 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 0.010% or more and 1.000% or less. The Al content is preferably 0.015% or more, more preferably 0.030% or more. 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 generates nitride-based precipitates such as AlN that pin grain boundaries, and can be added to improve elongation. However, if the N content exceeds 0.0100%, the nitride-based precipitates such as AlN become coarse, 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. There is no particular lower limit for the N content. Due to constraints in production technology, the N content is preferably 0.0006% or more.

[0034] [C] + [Si] / 24 + [Mn] / 6 ≦ 0.65 (1) Excessive C, Si, and Mn contents, particularly when [C] + [Si] / 24 + [Mn] / 6 exceeds 0.65, result in a decrease in delayed fracture resistance. This is believed to be due to an increase in coarse inclusions and the promotion of intergranular fracture due to 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. There is no particular lower limit for [C] + [Si] / 24 + [Mn] / 6. For example, [C] + [Si] / 24 + [Mn] / 6 is preferably 0.30 or more.

[0035] 0.13≦[Si] / [Mn]≦0.75 (2) To achieve both high strength and ductility, it is important to control the [Si] / [Mn] ratio to an appropriate value. If the [Si] / [Mn] ratio is less than 0.13, the appropriate amounts of ferrite and bainite may not be obtained, resulting in excessive strength. On the other hand, if the [Si] / [Mn] ratio exceeds 0.75, excessive ferrite and bainite are formed, making it difficult to obtain the desired strength. Therefore, the [Si] / [Mn] ratio is set to 0.13 or more and 0.75 or less. The [Si] / [Mn] ratio is preferably 0.14 or more, more preferably 0.15 or more. The [Si] / [Mn] ratio is also preferably 0.65 or less, more preferably 0.55 or less.

[0036] The basic elements (hereinafter also referred to as basic element) of the chemical composition of the substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention have been described above. The substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention contains the above basic element, with the balance other than the above basic element including Fe (iron) and unavoidable impurities. Here, it is preferable that the substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention has a chemical composition containing the above basic element, with the balance consisting of Fe and unavoidable impurities. The substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention may contain, in addition to the above basic element, at least one optional element selected from the following: 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

[0037] The effects of the present invention can be obtained as long as the optional elements are contained in an amount equal to or less than the upper limit, so no lower limit is set. When the optional elements are contained in an amount less than the preferred lower limit described below, the elements are considered to be contained as inevitable impurities.

[0038] Nb: 0.40% or less Nb increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, during the hot rolling and annealing processes. Furthermore, Nb reduces the amount of diffusible hydrogen in the base steel sheet by forming fine precipitates that act as hydrogen trapping sites, thereby contributing to improved delayed fracture resistance. To achieve this effect, the Nb content is preferably 0.002% or more. On the other hand, if 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 in an undissolved state during slab heating in the hot rolling process increases, resulting in a decrease in delayed fracture resistance. 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: 0.40% or less Ti increases TS by forming fine precipitates during the hot rolling and annealing processes. Ti also reduces the amount of diffusible hydrogen in the base steel sheet by forming fine precipitates that act as hydrogen trap sites, thereby contributing to improved delayed fracture resistance. To achieve this effect, the Ti content is preferably 0.002% or more. On the other hand, if the Ti content exceeds 0.40%, the amount of coarse Ti-based precipitates, such as TiN, Ti(C,N), Ti(C,S), and TiS, remaining in an undissolved state during slab heating in the hot rolling process increases, resulting in a decrease in delayed fracture resistance. 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 Like Nb and Ti, V increases TS by forming fine precipitates during the hot rolling and annealing processes. Furthermore, V reduces the amount of diffusible hydrogen in the base steel sheet by forming fine precipitates that act as hydrogen trap sites, thereby contributing to improved delayed fracture resistance. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, if the V content exceeds 0.45%, a large amount of coarse precipitates and inclusions may be formed, potentially reducing 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 segregates at austenite grain boundaries to improve hardenability. Furthermore, B controls the formation and grain growth of ferrite in the cooling process after the annealing process. To achieve this effect, the B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, if the B content exceeds 0.0100%, the amount of nitride-based precipitates such as BN becomes excessive, which may result in a decrease in 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: 1.00% or less Cr is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Cr content is preferably 0.0005% or more. The Cr content is more preferably 0.010% or more. On the other hand, if the Cr content exceeds 1.00%, the area ratio of martensite increases, which may result in a decrease in ductility. 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: 1.00% or less Ni is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, if the Ni content exceeds 1.00%, the area ratio of martensite increases, which may result in a decrease in ductility. Therefore, when Ni is contained, the Ni content is preferably 1.00% or less, more preferably 0.50% or less.

[0044] Mo: 1.00% or less Mo is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS. Furthermore, Mo reduces the amount of diffusible hydrogen in the substrate steel sheet by forming fine precipitates that serve as hydrogen trap sites, thereby contributing to improved delayed fracture resistance. To achieve this effect, the Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, if the Mo content exceeds 1.00%, the area fraction of martensite increases, and the desired ductility may not be achieved. 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: 0.100% or less Sb is an element that is effective in suppressing the diffusion of C near the steel sheet surface during annealing and controlling the formation of a soft layer near the steel sheet surface. To achieve this effect, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.100%, castability may be reduced. Therefore, when Sb is contained, the Sb content is preferably 0.100% or less, more preferably 0.060% or less, and even more preferably 0.040% or less.

[0046] Sn: 0.100% or less Sn suppresses oxidation and nitridation near the steel sheet surface, thereby suppressing the resulting decrease in the C and B contents near the steel sheet surface. This suppresses excessive ferrite formation near the steel sheet surface, contributing to improved strength. To achieve this effect, the Sn content is preferably 0.002% or more. However, if the Sn content exceeds 0.100%, castability may be reduced. Therefore, when Sn is contained, the Sn content is preferably 0.100% or less, more preferably 0.040% or less, and even more preferably 0.020% or less.

[0047] Cu: 1.00% or less Cu is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS. To achieve this effect, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.00%, the area ratio of martensite increases excessively. Furthermore, a large amount of coarse precipitates and inclusions is formed, resulting in a decrease in ductility and delayed fracture 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 Like Ti, Nb, and V, Ta increases TS by forming fine precipitates during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. As a result, TS is further increased. To achieve this effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions are formed, resulting in a decrease in ductility and delayed fracture 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 Like Ti, Nb, and V, W increases TS by forming fine precipitates during the hot rolling and annealing processes. Furthermore, W reduces the amount of diffusible hydrogen in the base steel sheet by forming fine precipitates that act as hydrogen trap sites, thereby contributing to improved delayed fracture resistance. To achieve this effect, the W content is preferably 0.001% or more. The W content is more preferably 0.005% or more. On the other hand, if the W content exceeds 0.200%, a large amount of coarse precipitates and inclusions are formed, resulting in a decrease in ductility and delayed fracture 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 element that is effective in spheroidizing the shape of inclusions such as sulfides and oxides to improve the delayed fracture resistance of steel sheets. To achieve this effect, the Mg content is preferably 0.0001% or more. However, if 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 that is effective in spheroidizing the shape of inclusions and improving the delayed fracture resistance of steel sheets. To achieve this effect, the Zn content is preferably 0.001% or more. On the other hand, if the Zn content exceeds 0.020%, a large amount of coarse precipitates and inclusions may be formed, which may actually lead to a decrease in delayed fracture resistance. Therefore, when Zn is contained, the Zn content is preferably 0.020% or less.

[0052] Co: 0.500% or less Like Zn, Co is an element that is effective in spheroidizing the shape of inclusions and improving the delayed fracture resistance of steel sheets. To achieve this effect, the Co content is preferably 0.001% or more. On the other hand, if the Co content exceeds 0.500%, a large amount of coarse precipitates and inclusions may be formed, which may actually lead to a deterioration in 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 increasing strength by refining prior austenite grains. Zr also contributes to increasing strength by reducing the block size and vein grain size, which are the internal structural units of martensite and bainite, due to the refinement of prior austenite grains. Furthermore, Zr improves castability. To achieve these effects, the Zr content is preferably 0.001% or more. However, if a large amount of Zr is added, the amount of coarse ZrN-based and ZrS-based precipitates remaining in an undissolved state during heating of the steel slab before the hot rolling process increases, resulting in reduced ductility. Therefore, when Zr is added, 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 inclusions in steel. If the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be generated, which may reduce ductility and delayed fracture resistance. Furthermore, surface quality may also be reduced. 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. For example, the Ca content is preferably 0.0005% or more. Furthermore, 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 effective elements for improving the delayed fracture resistance of steel sheet. To achieve 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 Ce, Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM contents exceed 0.0200% each, or if the As content exceeds 0.0500%, large amounts of coarse precipitates and inclusions are formed, which may actually deteriorate the delayed fracture resistance. Therefore, when these elements are contained, it is preferable that the Ce, Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM contents are each 0.0200% or less, and the As content is 0.0500% or less. Note that Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM may be contained alone or in combination. Furthermore, REM as used herein is a collective term for 16 elements, including 14 lanthanoid elements from La (lanthanum) with atomic number 57 to Lu (lutetium) with atomic number 71, excluding Ce, as well as Sc (scandium) with atomic number 21 and Y (yttrium) with atomic number 39. These 16 elements can be contained alone or in combination. The REM content refers to the total content of these 16 elements.

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

[0057] Next, the steel structure of the base steel sheet of the galvanized steel sheet according to one 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 area of ​​the entire 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. Ferrite and bainite are soft, and therefore 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, if the area ratios of ferrite and bainite are 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. Furthermore, 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 alone, or both may be contained. Since both ferrite and bainite adjust strength and ductility to the desired ranges, it is not necessary to specify the area ratios of ferrite and bainite in the present disclosure.

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

[0060] Martensite is a hard structure that is formed by transformation from austenite at or below the martensite transformation point (also simply referred to as the Ms point). Martensite includes both as-quenched so-called fresh martensite and tempered fresh martensite.

[0061] Furthermore, the steel structure of the base steel sheet may contain retained austenite. Retained austenite improves the balance between strength and ductility. However, if the retained austenite is excessive, for example, when the steel sheet is formed into a part, the retained austenite transforms into martensite, increasing the number of crack initiation points. Therefore, the area fraction of retained austenite is preferably 20% or less, more preferably 15% or less, and even more preferably 10% or less. The area fraction of retained austenite may be 0%.

[0062] The term "retained austenite" refers to austenite that remains without transforming into ferrite, martensite, bainite, or other metallic phases. For example, retained austenite is generated when elements such as carbon become concentrated in austenite, causing the martensite transformation point to drop below room temperature (residual austenite without transforming).

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

[0064] The remaining structure is not particularly limited, and examples thereof include pearlite and carbides such as cementite. The type of the remaining structure can be confirmed, for example, by observation using a scanning electron microscope (SEM). Pearlite is formed from austenite at a relatively high temperature and is a structure consisting of lamellar ferrite and cementite.

[0065] Here, the total area ratio of ferrite and bainite, and the area ratio of martensite are measured at a quarter-thickness position of the substrate steel sheet, for example, as follows. That is, a sample is cut out from the galvanized steel sheet so that the cross section (L cross section) parallel to the rolling direction and thickness direction of the substrate steel sheet of the galvanized steel sheet serves as the observation surface. Next, the observation surface of the sample is polished using diamond paste, and then the observation surface of the sample is finish-polished using alumina. Next, the observation surface of the sample is etched with nital to reveal the structure. Then, five fields of view of the observation surface of the sample are observed using an SEM at a magnification of 1500x. Next, the following regions are color-coded (defined) from the obtained structure image using Adobe Photoshop from Adobe Systems. Then, the total area ratio of ferrite and bainite, and the area ratio of martensite are calculated using the point counting method. Specifically, 16 × 15 grid points were set at 4.8 μm intervals in an area of ​​82 μm × 57 μm in actual length in each SEM image. The number of grid points on ferrite, bainite, and martensite was then counted. The number of grid points on ferrite, bainite, and martensite was then divided by the total number of grid points, and the result was multiplied by 100 to calculate the total area ratio of ferrite and bainite, and the area ratio of martensite.

[0066] Ferrite: A region that is black and has a blocky shape. Ferrite is a structure consisting of crystal grains with a BCC lattice. Ferrite is formed by transformation from austenite. Bainite: A region that is black to dark gray and has a blocky or amorphous shape. As described above, bainite is a hard structure in which fine carbides are dispersed in needle-like or plate-like ferrite. Bainite is formed by transformation from austenite at a temperature lower than that of ferrite (above the Ms point). Martensite: A region that is white to light gray. As described above, martensite is a hard structure formed by transformation from austenite at a temperature below the Ms point. Martensite includes both so-called fresh martensite, which is as-quenched, and so-called tempered martensite, which is formed by tempering fresh martensite. Of these, tempered martensite contains carbides.

[0067] The area fraction of retained austenite is measured at a quarter-thickness position of the substrate steel sheet as follows. Specifically, the substrate steel sheet is mechanically ground in the thickness direction (depth direction) to a quarter-thickness position, and then chemically polished with oxalic acid to obtain an observation surface. The observation surface is then observed by X-ray diffraction. CoKα rays are used as the incident X-rays, and the ratios of the diffraction intensities of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensities of the (200), (211), and (220) planes of bcc iron are calculated. The volume fraction of retained austenite is then calculated from the ratio of the diffraction intensities of each plane. The retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite.

[0068] Furthermore, the area ratio of the remaining structure is calculated by subtracting the total area ratio of ferrite and bainite, the area ratio of martensite, and the area ratio of retained austenite calculated as described above from 100%: [Area ratio of 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 steel sheets. However, when stress is applied to a steel sheet, voids are generated at the interface between martensite and soft ferrite or bainite, particularly at the interface between the martensite region with high C and Mn concentrations and the ferrite or bainite. These voids then coalesce, resulting in delayed fracture. To suppress the generation and coalescence of such voids and achieve excellent delayed fracture resistance, it is effective to reduce the proportion of the first region, which corresponds to the region with high C and Mn concentrations, in the martensite region and increase the proportion 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 determined, for example, in accordance with the procedure described in Reference 1. Reference 1: Yamashita et al., "Carbon Distribution in the Early Stage of Proeutectoid Ferrite Transformation in Low Carbon Steels Using High-Precision FE-EPMA," Iron and Steel, Vol. 103 (2017) No. 11, pp. 14-20

[0071] That is, using the sample used in measuring the area fraction of each phase, quantitative analysis of C and Mn was performed at a quarter-thickness position of the substrate steel sheet using a field emission electron probe microanalyzer (FE-EPMA), and two-dimensional distributions of C and Mn (C mapping and Mn mapping) were created. Here, quantitative analysis of C was performed while preventing carbon contamination on the surface, and then quantitative analysis of Mn was performed in the same field of view. For quantitative analysis of C, an acceleration voltage of 7 kV and a probe current of 5 nA were used. For quantitative analysis of Mn, an acceleration voltage of 9 kV and a probe current of 10 nA were used. Next, the structural image used in measuring the area fraction of each phase was compared with the two-dimensional distribution of C and Mn. Then, in the region constituting martensite, the region where the concentrations of C and Mn satisfy the relationships in the following equations (3) and (4) was defined as the first region, and the remaining region was defined as the second region. Next, the total area of ​​the regions defined as the first region is calculated, and M1 is calculated by dividing the total area by the total area of ​​the observation field. [C] Ｍ / [C]≧2.4...(3) [Mn] Ｍ / [Mn]≧1.5 (4) where [C] and [Mn] are the contents (mass%) of C and Mn in the chemical composition of the base steel sheet, respectively. Ｍ and [Mn] Ｍ are the concentrations (mass%) of C and Mn in the martensite region, respectively.

[0072] [C] in the above formulas (3) and (4) Ｍ / [C] and [Mn] Ｍ The upper limit of / [Mn] is 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 substrate steel sheet: 0.50 ppm by mass or less From the viewpoint of obtaining excellent delayed fracture resistance, the diffusible hydrogen content of the substrate steel sheet is set to 0.50 ppm by mass or less. Furthermore, the diffusible hydrogen content of the substrate steel sheet is preferably 0.45 ppm by mass or less, more preferably 0.40 ppm by mass or less, and even more preferably 0.35 ppm by mass or less. Note that there is no particular restriction on the lower limit of the diffusible hydrogen content of the substrate steel sheet, and it may be 0 ppm by mass. However, due to constraints on production technology, the diffusible hydrogen content of the substrate steel sheet is preferably 0.01 ppm by mass or more.

[0074] Here, the amount of diffusible hydrogen in the substrate steel sheet is measured, for example, as follows. Specifically, a test piece 30 mm long and 5 mm wide is taken from a galvanized steel sheet, and the galvanized layer is removed using a router (precision grinder). Next, the amount of hydrogen released from the test piece is measured using thermal desorption analysis. 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. During this process, the amount of hydrogen released from the test piece (cumulative amount of hydrogen) is measured in the temperature range from 25°C to 210°C during the continuous heating. The measured amount of hydrogen is then divided by the mass of the test piece (the test piece after removal of the galvanized layer and before continuous heating), and the value converted to ppm by mass is taken as the amount of diffusible hydrogen in the substrate steel sheet.

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

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

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

[0078] In addition, the plating weight of the zinc plating layer per side is not particularly limited, but is preferably 20 g / m ２ 80g / m or more ２ It is preferable to do the following:

[0079] The coating weight of the zinc plating layer is measured as follows. That is, a treatment solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe ("Ivit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10 mass % aqueous hydrochloric acid solution. Next, a steel sheet to be used as a test material is immersed in the treatment solution to dissolve the zinc plating layer. The mass loss of the test material before and after dissolution is measured, and this value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the part that was covered with the plating) to determine the coating weight (g / m２ ) is calculated.

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

[0081] TS: 780 MPa or more The TS of the galvanized steel sheet according to one embodiment of the present invention is 780 MPa or more. There is no particular upper limit for the TS of the galvanized steel sheet according to one embodiment of the present invention. For example, the TS of the galvanized 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 in accordance with JIS Z 2241 (2022) (hereinafter also simply referred to as JIS Z 2241).

[0083] Excellent ductility means that the total elongation (El) measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula: 15%≦El when 780 MPa≦TS<980 MPa, and 9%≦El when 980 MPa≦TS.

[0084] Excellent delayed fracture resistance means that, in a delayed fracture test in accordance with SEP1970, a stress equivalent to the yield strength (YS) of the steel plate is applied to a steel plate (test specimen), and no fracture occurs in the steel plate 96 hours after the stress is applied.

[0085] In the techniques disclosed in the aforementioned Patent Documents 3 and 4, delayed fracture resistance is evaluated by forcibly injecting hydrogen into a steel sheet by immersing the steel sheet in an acidic aqueous solution such as hydrochloric acid for a certain period of time under a predetermined stress. However, such a test involves forcibly injecting hydrogen into the steel sheet (base steel sheet), and the effect of hydrogen intrusion during the steel sheet manufacturing process cannot be evaluated. Therefore, in this specification, delayed fracture resistance is evaluated by the delayed fracture test in accordance with the aforementioned SEP 1970.

[0086] Detailed procedures for measuring the above-mentioned properties are as described in the examples below.

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

[0088] [2] Member Next, a member according to one embodiment of the present invention will be described. The member according to one embodiment of the present invention is a member made using (as a raw material) the above-mentioned galvanized steel sheet. For example, the raw material galvanized steel sheet is subjected to at least one of forming and joining to form a member. Here, the above-mentioned galvanized steel sheet has high strength, excellent ductility, and excellent delayed fracture resistance. Therefore, the member according to one embodiment of the present invention is particularly suitable for use in the automotive field.

[0089] [3] Manufacturing Method of Galvanized Steel Sheet Next, a manufacturing method of a galvanized steel sheet according to one embodiment of the present invention will be described. Note that, unless otherwise specified, the temperatures referred to here refer to the surface temperatures of the steel slab and the steel sheet.

[0090] First, a steel slab having the above-mentioned composition is prepared. For example, a steel material is melted to obtain molten steel having the above-mentioned composition. The melting method is not particularly limited, and known melting methods such as converter melting and electric furnace melting can be used. The obtained molten steel is then solidified 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 casting method, or a thin slab casting method can be used. From the viewpoint of preventing macrosegregation, a continuous casting method is preferred.

[0091] In addition to conventional methods, energy-saving processes such as direct rolling and direct rolling can also be applied without any problems. The conventional method involves cooling the resulting steel slab to room temperature, reheating it, and then hot rolling it. Direct rolling involves charging the resulting steel slab as a hot slab into a heating furnace without cooling it to room temperature, and then hot rolling it. Direct rolling involves briefly maintaining the temperature of the steel slab and then immediately hot rolling it. Furthermore, from the viewpoint of dissolving carbides and reducing the rolling load, the slab heating temperature is preferably 1100°C or higher. On the other hand, to prevent an increase in scale loss, the slab heating temperature is preferably 1300°C or lower. The steel slab is then roughly rolled into a sheet bar, for example, under conditions consistent with conventional methods. 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 in order to prevent problems during hot rolling.

[0092] [Hot Rolling Step] Next, the steel slab is subjected 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 lower than 840°C, ferrite formation is promoted, and excessive ferrite is formed before the hot-rolled steel sheet is coiled. This causes C and Mn to concentrate in untransformed austenite. Excessive C concentration in untransformed austenite promotes pearlite transformation. That is, excessive pearlite is formed in the steel structure of the hot-rolled steel sheet obtained after hot rolling. Pearlite is a lamellar structure of ferrite and cementite, and Mn concentrates in cementite. Here, from the viewpoint of suppressing Mn concentration in 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-enriched region in the structure of the cold-rolled steel sheet before the annealing process. Therefore, the finish rolling end temperature is set to 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 is excessively high, it may become difficult to cool the steel sheet to the coiling temperature described below. Therefore, the finish rolling end temperature is set to 1000°C or less. The finish rolling end temperature is preferably 950°C or less.

[0094] Coiling temperature: 620°C or less If the coiling temperature exceeds 620°C, the amount of pearlite produced during coiling will be excessively large, accelerating Mn concentration. The lower the coiling temperature, the less pearlite is produced, so a lower coiling temperature is preferred. Therefore, the coiling temperature is set to 620°C or less. The coiling temperature is preferably 600°C or less, more preferably 580°C or less. On the other hand, if the coiling temperature is less than 400°C, the steel sheet may become excessively hardened, which may cause fracture during cold rolling. Therefore, the coiling temperature is preferably 400°C or more, more preferably 420°C or more.

[0095] In addition, descaling may be performed as appropriate to remove primary and secondary scales formed on the surface of the hot-rolled steel sheet. Before cold rolling, the hot-rolled steel sheet is preferably thoroughly pickled to reduce remaining scale. Furthermore, from the viewpoint of reducing the load during cold rolling, the hot-rolled steel sheet may optionally be subjected to hot-rolled sheet annealing.

[0096] [Cold Rolling Step] Next, the hot rolled steel sheet is subjected to cold rolling 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 is set to 20% or more. That is, if the reduction ratio is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing process, resulting in reduced TS and ductility in the final product. Therefore, the reduction ratio is set to 20% or more. On the other hand, if the reduction ratio exceeds 80%, the steel sheet is likely to have a defective shape. In addition, there is a risk of non-uniformity in the steel structure and non-uniformity in the amount of zinc coating due to temperature unevenness in the annealing process. Therefore, the reduction ratio is set to 80% or less. The reduction ratio is preferably 30% or more. Furthermore, the reduction ratio is preferably 70% or less.

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

[0099] Annealing temperature: 750°C or higher and 900°C or lower If the annealing temperature is lower than 750°C, the proportion of austenite generated during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, the area fraction 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 fractions of ferrite and bainite cannot be obtained, and ductility decreases. Therefore, the annealing temperature is set to 750°C or higher and 900°C or lower. The annealing temperature is preferably 890°C or lower, more preferably 880°C or lower. 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 viewpoint of suppressing the concentration of C and Mn in martensite in the final structure, it is important to suppress the concentration of C and Mn in austenite during the annealing process. To achieve this, the shorter the annealing time, the better. Furthermore, to reduce the amount of diffusible hydrogen in the base steel sheet, the shorter the annealing time, the better. Therefore, the annealing time is set to 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, if the annealing time is less than 1 second, coarse Fe-based precipitates, which serve as the initiation point for delayed fracture, do not dissolve sufficiently, resulting in a decrease in delayed fracture resistance. Therefore, the annealing time is set to 1 second or more. The annealing time is preferably 3 seconds or more, more preferably 5 seconds or more. The annealing time refers to the holding time at the annealing temperature.

[0101] 1.5 ((CT-350) / 100) x T x log １０ (t+10)×1.15 ＨＡ ×1.05 ＨＢ≦6800 ... (5) To obtain excellent delayed fracture resistance, it is important to suppress the concentration of C and Mn in martensite in the final structure and to reduce the amount of diffusible hydrogen in the base steel sheet. To achieve both of these, it is important to appropriately control the hot rolling conditions and annealing conditions, in particular, the relationship between 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, and HA: atmospheric hydrogen concentration (volume %) in the annealing process, and HB: atmospheric hydrogen concentration (volume %) in the cooling process, which will be described later; specifically, it is important to satisfy the relationship of the above 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-hand side value of equation (5)) is 6800 or less. The left-hand side value of equation (5) is preferably 6500 or less, more preferably 6000 or less. The lower limit of the left-hand side value of equation (5) is not particularly limited. The left-hand side value of equation (5) is preferably 2000 or more, for example. In order to prevent oxides from being generated on the surface of the substrate steel sheet, which would result in a decrease in the coating weight, the atmospheric hydrogen concentration (hereinafter also simply referred to as HA) in the annealing process is preferably 1 vol% or more, more preferably 2 vol% or more. In addition, in order to reduce the amount of diffusible hydrogen in the substrate steel sheet, HA is preferably 15 vol% or less, more preferably 10 vol% or less.

[0102] [Cooling Step] 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 simply referred to as HB) during the cooling process is 30% by volume or more, the amount of diffusible hydrogen in the base steel sheet increases, and the delayed fracture resistance decreases. Therefore, HB is less than 30% by volume, preferably less than 26% by volume, and more preferably less than 22% by volume. There is no particular lower limit for HB. Due to constraints on production technology, HB is preferably 0.2% by volume or more, and more preferably 0.5% by volume or more.

[0104] Cooling stop temperature: 600°C or less If the cooling stop temperature exceeds 600°C, ferrite and pearlite are excessively formed, and the desired strength cannot be obtained. Therefore, the cooling stop temperature is 600°C or less, preferably 580°C or less, and more preferably 560°C or less. The lower limit of the cooling stop temperature is not particularly limited. However, when performing the galvanizing treatment described below, particularly the hot-dip galvanizing treatment or the galvannealed hot-dip galvanizing treatment, it is preferable to set the sheet temperature at which the steel enters the galvanizing bath higher than the galvanizing bath temperature. Therefore, if the cooling stop temperature is excessively low, an increase in the equipment capacity for reheating the cold-rolled steel sheet is required, which may increase the capital investment burden. Therefore, the cooling stop temperature is preferably 350°C or more, more preferably 400°C or more.

[0105] The conditions after cooling is stopped (up to the galvanizing treatment step) are not particularly limited. For example, after cooling is stopped, the cold-rolled steel sheet may be held in a 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 galvanizing process and a hot-dip galvannealing process. The treatment conditions may be those of a conventional method.

[0107] For example, in the case of hot-dip galvanizing, it is preferable to immerse a cold-rolled steel sheet in a galvanizing bath at a temperature of 440° C. or higher and 500° C. or lower, and then adjust the coating weight by gas wiping or the like. The galvanizing bath is not particularly limited as long as it provides the above-mentioned composition of the galvanized layer, but it is preferable to use, for example, a plating bath having an Al content of 0.10 mass % or higher and 0.23 mass % or lower, with the balance consisting of Zn and unavoidable impurities.

[0108] Furthermore, in the case of alloying hot-dip galvanizing treatment, after performing the hot-dip galvanizing treatment as described above, it is preferable to perform an alloying treatment in a temperature range of 450°C or higher and 600°C or lower. If the alloying temperature is lower than 450°C, the Zn-Fe alloying rate may be excessively slow, making alloying difficult. On the other hand, if the alloying temperature exceeds 600°C, untransformed austenite may transform to pearlite, resulting in a decrease in strength and ductility. 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. The alloying temperature in the alloying treatment is more preferably 580°C or lower, and even more preferably 560°C or lower.

[0109] The plating weight is 20 g / m per side. ２ 80g / m or more ２ It is preferable that the plating thickness is set to the following: The plating thickness can be adjusted by gas wiping or the like.

[0110] The conditions after the galvanizing treatment step are not particularly limited. For example, when hot-dip galvanizing treatment or galvannealed hot-dip galvanizing treatment is performed, from the viewpoint of productivity, it is preferable to cool the galvanized steel sheet after the completion of the hot-dip galvanizing treatment or galvannealed hot-dip galvanizing treatment to a temperature of 50°C or less at an average cooling rate (hereinafter also referred to as the cooling rate after the galvanizing treatment) of 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. There is no particular upper limit to the cooling rate after the galvanizing treatment, but from the viewpoint of reducing the burden of capital investment, the cooling rate after the galvanizing treatment is preferably 200°C / second or less, more preferably 100°C / second or less.

[0111] [Temper Rolling Step] The galvanized steel sheet obtained as described above may be further subjected to temper rolling. In this case, from the viewpoint of shape correction and surface roughness adjustment, the elongation rate is preferably 0.10% or more. The elongation rate is more preferably 0.12% or more, and even more preferably 0.15% or more. There is no particular upper limit to the elongation rate. However, if the elongation rate exceeds 2.00%, the yield stress increases excessively, which may result in a decrease in dimensional accuracy when the galvanized steel sheet is formed into a component. Therefore, the elongation rate is preferably 2.00% or less.

[0112] Furthermore, temper rolling may be performed on an apparatus continuous with the annealing apparatus for carrying out each of the above-mentioned steps (online), or may be performed on an apparatus discontinuous with the annealing apparatus for carrying out each of the steps (offline). The number of times temper rolling may be one, or two or more. Note that rolling using a leveler or the like may also be used as long as it can impart an elongation rate equivalent to that of temper rolling.

[0113] From the viewpoint of productivity, it is preferable that a series of processes such as the annealing process and the galvanizing process be carried out in a continuous annealing line (CAL) or a hot-dip galvanizing line (CGL). After the hot-dip galvanizing process, wiping can be performed to adjust the coating weight of the coating.

[0114] According to the method for producing a galvanized steel sheet according to one embodiment of the present invention described above, a galvanized steel sheet having high strength, excellent ductility, and excellent delayed fracture resistance can be obtained, and the galvanized steel sheet can be suitably used for, for example, automobile parts.

[0115] [4] Manufacturing Method of Member Next, a manufacturing method of a member according to one embodiment of the present invention will be described. The manufacturing method of a member according to one embodiment of the present invention includes a step of subjecting the above-mentioned galvanized steel sheet 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. Furthermore, the joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, caulking joining, etc. can be used. Note that the forming conditions and joining conditions are not particularly limited, and may be in accordance with conventional methods.

[0116] A steel material having the chemical 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, the steel slab was subjected to hot rolling consisting of rough rolling and finish rolling 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, the obtained cold-rolled steel sheet was subjected to annealing, cooling, and galvanization under the conditions shown in Table 2 to obtain a galvanized steel sheet. In some examples, temper rolling was performed under the conditions shown in Table 2. Note that conditions not specified were those according to conventional methods.

[0117] Here, in the galvanizing treatment step, a hot-dip galvanizing treatment or a galvannealed hot-dip galvanizing treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or a galvannealed hot-dip galvanized steel sheet (hereinafter also referred to as GA). In Table 2, the type of the galvanizing treatment step is also indicated as "GI" or "GA".

[0118] In the hot dip galvanizing treatment, the plating bath contained 0.20 mass% Al, with the remainder consisting of Zn and unavoidable impurities. The plating bath temperature was 470°C. The coating weight was 45 to 72 g / m per side. ２ The composition of the zinc plating layer of the finally obtained GI was Fe: 0.1 to 1.0 mass %, Al: 0.2 to 1.0 mass %, and the remainder was Zn and unavoidable impurities.

[0119] In the galvannealed hot-dip galvanizing treatment, the plating bath contained 0.14 mass% Al, with the remainder consisting of Zn and unavoidable impurities. The plating bath temperature was 470°C. The coating weight was 45 g / m per side. ２ The alloying temperature was 520°C. The composition of the finally obtained zinc-plated layer of GA was Fe: 7 to 15% by mass, Al: 0.1 to 1.0% by mass, and the remainder was Zn and unavoidable impurities.

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

[0121] Further, a tensile test was carried out as follows, and TS and El were evaluated according to the following criteria: Pass (excellent) TS: 780 MPa≦TS Fail (poor): TS<780 MPa Pass (excellent) El: 15%≦El when 780 MPa≦TS<980 MPa, 9%≦El when 980 MPa≦TS Fail (poor): El<15% when 780 MPa≦TS<980 MPa, El<9% when 980 MPa≦TS

[0122] The tensile test was carried out in accordance with JIS Z 2241. That is, JIS No. 5 test pieces were taken from the obtained galvanized steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the substrate steel sheet. Using the taken test pieces, a tensile test was carried out at a crosshead speed of 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 x 30 mm were taken from the obtained galvanized steel sheets so 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 taken test pieces. Next, a stress equivalent to the yield strength (YS) of the steel sheet from which the test pieces were taken was loaded in the longitudinal direction to the test pieces, and they were held in this state for 96 hours. The yield strength (YS) was measured by the tensile test described above. Then, the delayed fracture resistance was evaluated according to the following criteria. Pass (excellent): No fracture occurred after 96 hours had elapsed Fail (poor): Fracture occurred within 96 hours

[0124]

[0125]

[0126]

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

[0128] According to the present invention, a galvanized steel sheet having high strength, excellent ductility, and excellent delayed fracture resistance can be obtained. Furthermore, the galvanized steel sheet can be extremely advantageously used as a material for automotive frame structural members, etc. This can reduce the weight of the vehicle body and improve fuel efficiency, and therefore the industrial utility value is extremely great.

Claims

1. A galvanized steel sheet having a substrate steel sheet and a galvanized layer on the surface of the substrate steel sheet, wherein the substrate steel sheet has a chemical composition, in mass %, of C: 0.05% to 0.20%, Si: 0.2% to 1.8%, Mn: 1.50% to 3.00%, P: 0.100% or less, S: 0.0500% or less, Al: 0.010% to 1.000%, and N: 0.0100% or less, satisfying the relationships of the following formulas (1) and (2), with the balance being Fe and unavoidable impurities; and at a 1 / 4 position in the thickness direction of the substrate steel sheet, a total area ratio of one or both of ferrite and bainite: 20% to 90%, an area ratio of martensite: 10% to 80%, and M1 / Mt: 0.30 or less.

1. A galvanized steel sheet having a steel structure in which Mt is an area fraction (%) of the martensite, M1 is an area fraction (%) of a first region among regions constituting the martensite, and the first region is a region that satisfies the relationships of the following formulas (3) and (4), wherein the diffusible hydrogen content of the substrate steel sheet is 0.50 mass ppm or less, and the tensile strength is 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) where [C], [Si] and [Mn] are the contents (mass%) of C, Si and Mn in the chemical composition of the base steel sheet, respectively. Ｍ and [Mn] Ｍ are the concentrations (mass%) of C and Mn in the region constituting the martensite, respectively.

2. The chemical composition of the substrate steel sheet further contains, 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, 2. The galvanized steel sheet according to claim 1, comprising at least one selected from 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.

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

4. A manufacturing method comprising: a hot rolling process in which a steel slab having the chemical composition according to claim 1 or 2 is hot rolled under the conditions of a finish rolling end temperature of 840°C to 1000°C and a coiling temperature of 620°C or less to obtain a hot rolled steel sheet; a cold rolling process in which the hot rolled steel sheet is cold rolled under the conditions of a rolling reduction of 20% to 80% to obtain a cold rolled steel sheet; an annealing process in which the cold rolled steel sheet is annealed under the conditions of an annealing temperature of 750°C to 900°C and an annealing time of 1 second to 30 seconds; a cooling process in which the cold rolled steel sheet is cooled under the conditions of an atmospheric hydrogen concentration of less than 30% by volume and a cooling stop temperature of 600°C or less; and a galvanizing process in which the cold rolled steel sheet is galvanized, 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) where, 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.

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

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