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

Abstract

Invention details;

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 subjected to zinc plating, for example, hot-dip galvanization.

[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 states that "C: 0.03 to 0.13% (meaning mass %; hereinafter the same applies to chemical composition), Si: 0.02 to 0.8%, Mn: 1.0 to 2.5%, P: 0.03% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.01% or less, Ti: 0.004 to 0.1% and / or Nb: 0.004 to 0.07%, the balance: iron and inevitable impurities, the structure has a ferrite parent phase structure and a second phase structure of bainite and martensite, the ratios of these in the total structure are 50 to 86 area % of ferrite, 10 to 30 area % of bainite, and 4 to 20 area % of martensite, and the relationship of (bainite area ratio) > (martensite area ratio) is satisfied, The disclosure states that "a high-strength steel sheet with excellent workability is characterized in that the average grain size of the ferrite is 2.0 to 5.0 μm, and the average hardness (Hv) of the ferrite / tensile strength (MPa) of the steel sheet is ≧0.25."

[0007] JP 2002-069574 A JP 2010-156031 A JP 2010-065316 A

[0008] Generally, increasing the strength of a steel sheet reduces its ductility. However, steel sheets used as raw materials for automotive components are required to have both high strength and excellent ductility, specifically, improved total elongation (hereinafter simply referred to as El) in a tensile test.

[0009] Furthermore, among automobile components, steel sheets used in automobile frame structural components, in particular, are required to have high component strength when press-formed. To improve the strength of automobile components, for example, it is effective to increase the yield stress (hereinafter also simply referred to as YS) of the steel sheet.

[0010] Furthermore, steel sheets used for automotive structural members and the like are formed into complex shapes, and therefore require excellent formability, particularly excellent hole expandability (stretch flangeability).

[0011] However, the steel sheets disclosed in Patent Documents 1 to 3 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, high YS, excellent ductility, and excellent hole expandability, 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.

[0013] Here, high strength and high YS mean that the tensile strength (hereinafter also referred to as TS) and YS measured in a tensile test in accordance with JIS Z 2241 satisfy the following formulas, respectively: TS 780 MPa≦TS YS 420 MPa≦YS when 780 MPa≦TS<980 MPa 550 MPa≦YS when 980 MPa≦TS

[0014] 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.

[0015] Excellent hole expandability means that the limiting hole expansion ratio (λ) measured in a hole expansion test in accordance with JIS Z 2256 is 20% or more.

[0016] To achieve the above-mentioned objectives, the inventors conducted extensive research and obtained the following findings. (1) After adjusting the chemical composition of the substrate steel sheet to a predetermined range, the total area fraction of ferrite and bainite and the area fraction of martensite are controlled to 20% to 90% and 10% to 80%, respectively. This makes it possible to achieve both high strength and excellent ductility. (2) The amount of coarse MnS particles in the substrate steel sheet, particularly near the thickness center of the substrate steel sheet, is reduced. This suppresses the occurrence of microcracks during punching and stress concentration during press forming, improving hole expandability. (3) The area fraction of martensite in the thickness direction of the substrate steel sheet is made as uniform as possible, particularly by reducing the difference between the area fraction of martensite at the half-thickness position and the area fraction of martensite at the quarter-thickness position of the substrate steel sheet. This improves hole expandability. A high YR is also obtained. The vicinity of the center of the thickness of the substrate steel sheet (i.e., the half-thickness position of the substrate steel sheet) is a region where martensite is likely to form due to the enrichment of Mn. (4) The amount of diffusible hydrogen in the substrate steel sheet is reduced. This further improves hole expandability. The present invention was completed based on the above findings and further investigations.

[0017] That is, the gist of the present invention is as follows: 1. A galvanized steel sheet having a substrate steel sheet and a galvanized layer on 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: 0.1% to 1.8%, Mn: 1.5% to 3.0%, P: 0.001% to 0.100%, S: 0.0500% or less, Al: 0.010% to 1.000%, N: 0.0100% or less, and one or both of Nb and Ti: 0.005% to 0.200% in total, satisfying the relationship of the following formula (1), with the balance being Fe and unavoidable impurities: At a quarter-thickness position of the base steel sheet, the area ratio of one or both of ferrite and bainite is 20% or more and 90% or less in total, and the area ratio of martensite is 10% or more and 80% or less, and the number density of MnS particles having a length of 150 μm or more in the rolling direction is 7 particles / mm ２ is less than or equal to, and M Ｃ / M Ｑ is 0.85 or more and 2.00 or less, and M Ｃ and M Ｑ are the area fractions of martensite at the 1 / 2 and 1 / 4 thickness positions of the substrate steel sheet, respectively, and 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. 0.50 ≦ [C] + [Mn] / 5 ≦ 0.68 (1) where [C] and [Mn] are the contents (mass%) of C and Mn in the chemical composition of the substrate steel sheet, respectively.

[0018] 2. The chemical composition of the substrate steel sheet further includes, in mass%, V: 0.45% or less, B: 0.010% or less, Cr: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Sb: 0.1% or less, Sn: 0.1% or less, Cu: 1.0% or less, Ta: 0.1% or less, W: 0.2% or less, Mg: 0.01% or less, Zn: 0.02% or less, Co: 0.02% or less, Zr: 0.2% or less, Ca: 0.02% or less, Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, 2. The galvanized steel sheet according to 1 above, containing at least one selected from Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less, and REM: 0.02% or less.

[0019] 3. The galvanized steel sheet according to 1 or 2 above, wherein the zinc plating layer is a hot-dip galvanized layer or a galvannealed hot-dip galvanized layer.

[0020] 4. A member made using the galvanized steel sheet according to any one of 1 to 3 above.

[0021] 5. A slab heating step of heating a steel slab having the chemical composition described in 1 or 2 above under the conditions of an average heating rate of 0.5°C / second or less in a temperature range from 300°C to 1220°C, a slab heating temperature of 1220°C or higher, and a slab heating time of 1.0 hour or longer; a hot rolling step of hot rolling the steel slab under the conditions of a finish rolling end temperature of 840°C to 1000°C, an average cooling rate of 5°C / second or more in a temperature range from the finish rolling end temperature to 700°C, and a coiling temperature of 620°C or lower to obtain a hot rolled steel sheet; a cold rolling step of cold rolling the hot rolled steel sheet under the conditions of a rolling reduction of 20% to 80% to obtain a cold rolled steel sheet; a heating step of heating the cold-rolled steel sheet at an average heating rate of 1°C / second or more in a temperature range from 600°C to 750°C; an annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 750°C to 920°C for an annealing time of 1 second to 30 seconds in an atmosphere with a hydrogen concentration of less than 10% by volume; a cooling step of cooling the cold-rolled steel sheet at an average cooling rate of 5°C / second to 200°C / second in a temperature range from an annealing temperature of -30°C to 600°C; and a galvanizing step of galvanizing the cold-rolled steel sheet.

[0022] 6. The method for producing a galvanized steel sheet according to 5 above, wherein the galvanizing treatment is a hot-dip galvanizing treatment or a hot-dip galvannealing treatment.

[0023] 7. The method for producing a galvanized steel sheet according to 5 or 6 above, further comprising a temper rolling step of performing temper rolling at an elongation rate of 0.10% or more after the galvanizing treatment step.

[0024] 8. A method for manufacturing a component, comprising the step of subjecting the galvanized steel sheet according to any one of 1 to 3 above to at least one of forming and joining to form a component.

[0025] According to the present invention, a galvanized steel sheet having high strength, high YS, excellent ductility, and excellent hole expandability can be obtained. Furthermore, since the galvanized steel sheet of the present invention has high strength, high YS, excellent ductility, and excellent hole expandability, it can be extremely advantageously used as a material for automotive frame structural members having complex shapes.

[0026] 1 is an example of an observation image used to measure the number density of MnS particle groups.

[0027] 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 although the unit of chemical composition is always "mass %", hereinafter, unless otherwise specified, it will be simply expressed as "%".

[0028] 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 high strength and high YS. 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%, TS becomes excessively high and El decreases. In addition, the stability of austenite increases, making it difficult for bainite to form. 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, and even more preferably 0.09% or more. The C content is preferably 0.18% or less, more preferably 0.17% or less.

[0029] Si: 0.1% 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 0.1%, 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 0.1% 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.

[0030] Mn: 1.5% or more and 3.0% or less. Mn is added to improve the hardenability of the steel and ensure a predetermined area ratio of martensite and bainite. Here, if the Mn content is less than 1.5%, the hardenability is insufficient and ferrite is excessively formed. This makes it difficult to achieve a TS of 780 MPa or more. 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 1.5% or more and 3.0% or less. The Mn content is preferably 1.65% or more, more preferably 1.8% or more. The Mn content is preferably 2.85% or less, more preferably 2.7% or less.

[0031] P: 0.001% or more and 0.100% or less P is an element that has the effect of solid solution strengthening and increases the TS of steel sheet. To achieve this effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries, embrittling the grain boundaries. Therefore, when the steel sheet is subjected to hole expansion processing (stretch flange forming), voids are generated and cracks propagate along the prior austenite grain boundaries, preventing the desired hole expandability. Therefore, the P content is set to 0.100% or less. Due to production technology constraints, the P content is preferably 0.002% or more. Furthermore, the P content is preferably 0.050% or less, more preferably 0.030% or less.

[0032] S: 0.0500% or less S forms MnS and the like, reducing ductility. Furthermore, when Ti is contained together with S, TiS, Ti(C,S), and the like are formed, which may reduce hole expandability. In particular, coarse MnS particles degrade hole expandability. Here, if the S content exceeds 0.0500%, the number of coarse MnS particles increases, making it impossible to obtain excellent ductility and even excellent hole expandability. 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. The S content is more preferably 0.0002% or more.

[0033] Al: 0.010% or more and 1.000% or less 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, 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 achieve a TS of 780 MPa or more. 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. Moreover, the Al content is preferably 0.500% or less, more preferably 0.100% or less.

[0034] 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.

[0035] One or both of Nb and Ti: 0.005% or more and 0.200% or less in total. Nb and Ti contribute to an increase in TS through the formation of fine precipitates, such as carbides, nitrides, and carbonitrides, during the hot rolling and annealing processes. Furthermore, Nb and Ti reduce the amount of diffusible hydrogen in the base steel sheet by forming fine precipitates that act as hydrogen trapping sites, contributing to increased strength and improved hole expandability. To achieve these effects, the total content of Nb and Ti is set to 0.005% or more. On the other hand, excessive Nb and Ti content results in coarse precipitates and inclusions remaining without sufficient solid solution during the slab heating process. Furthermore, the composite precipitation of these coarse precipitates and inclusions with MnS reduces hole expandability. Therefore, the total content of Nb and Ti is set to 0.200% or less. The total content of Nb and Ti is preferably 0.010% or more, more preferably 0.015% or more. The total content of Nb and Ti is preferably 0.130% or less, more preferably 0.065% or less.

[0036] The respective contents of Nb and Ti are not particularly limited as long as the total content of one or both of Nb and Ti is 0.005% or more and 0.200% or less. For example, the Nb content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The Nb content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. The Ti content is preferably 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. The Ti content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less.

[0037] 0.50 ≦ [C] + [Mn] / 5 ≦ 0.68 (1) Excessive C and Mn increase the amount of coarse carbides and MnS, resulting in poor hole expandability. On the other hand, insufficient C and Mn may prevent a TS of 780 MPa or more. Therefore, the relationship of [C] + [Mn] / 5 must satisfy the above formula (1). [C] + [Mn] / 5 is preferably 0.51 or more, more preferably 0.52 or more. Furthermore, [C] + [Mn] / 5 is preferably 0.67 or less, more preferably 0.66 or less. [C] and [Mn] are the C and Mn contents (mass%) of the base steel sheet, respectively.

[0038] The basic chemical composition of the substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention has been described above, but 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 chemical components, with the balance other than the above basic chemical components including Fe (iron) and unavoidable impurities. Here, the substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention preferably has a chemical composition containing the above basic chemical components, with the balance consisting of Fe and unavoidable impurities. The substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention may contain, in addition to the above basic chemical components, at least one optional additional element selected from the following: V: 0.45% or less, B: 0.010% or less, Cr: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Sb: 0.1% or less, Sn: 0.1% or less, Cu: 1.0% or less, Ta: 0.1% or less, W: 0.2% or less, Mg: 0.01% or less, Zn: 0.02% or less, Co: 0.02% or less, Zr: 0.2% or less, Ca: 0.02% or less, Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less and REM: 0.02% or less. Note that the effects of the present invention can be obtained as long as the optional elements are contained in amounts not exceeding the upper limits, so no lower limits are particularly set. Note that when the optional elements are contained in amounts less than the preferred lower limits described below, the elements are considered to be contained as unavoidable impurities.

[0039] V: 0.45% or less Like Nb and Ti, V increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, 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 serve as hydrogen trapping sites, thereby contributing to improved hole expandability. 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. The V content is more preferably 0.060% or less.

[0040] B: 0.010% or less B is an element that segregates at austenite grain boundaries to improve hardenability. Furthermore, B is an element that 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 0.010%, 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 0.010% or less. The B content is more preferably 0.0050% or less, and even more preferably 0.0030% or less.

[0041] Cr: 1.0% 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 1.0%, the area ratio of martensite increases, which may result in a decrease in ductility. Therefore, when Cr is contained, the Cr content is preferably 1.0% or less. The Cr content is more preferably 0.60% or less, and even more preferably 0.30% or less.

[0042] Ni: 1.0% 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 1.0%, the area ratio of martensite increases, which may result in a decrease in ductility. Therefore, when Ni is contained, the Ni content is preferably 1.0% or less. The Ni content is more preferably 0.5% or less.

[0043] Mo: 1.0% 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 base steel sheet by forming fine precipitates that serve as hydrogen trap sites, contributing to improved hole expandability. 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 1.0%, the area ratio of martensite increases, and the desired ductility may not be achieved. Therefore, when Mo is contained, the Mo content is preferably 1.0% or less. The Mo content is more preferably 0.5% or less, and even more preferably 0.3% or less.

[0044] Sb: 0.1% or less Sb is an element that suppresses the diffusion of C near the steel sheet surface during annealing and is effective in controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it may be difficult to achieve a TS of 780 MPa or more. Therefore, 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 0.1%, castability decreases. Therefore, when Sb is contained, the Sb content is preferably 0.1% or less. The Sb content is more preferably 0.06% or less, and even more preferably 0.04% or less.

[0045] Sn: 0.1% or less Sn suppresses oxidation and nitriding near the steel sheet surface, thereby suppressing a decrease in the C and B contents near the steel sheet surface. This suppresses excessive ferrite formation near the steel sheet surface, contributing to achieving a TS of 780 MPa or more. From this perspective, the Sn content is preferably 0.002% or more. However, if the Sn content exceeds 0.1%, castability decreases. Therefore, when Sn is contained, the Sn content is preferably 0.1% or less. The Sn content is more preferably 0.04% or less, and even more preferably 0.02% or less.

[0046] Cu: 1.0% 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 1.0%, the area ratio of martensite may increase excessively, which may reduce ductility. In addition, large amounts of coarse precipitates and inclusions may be formed, which may reduce ductility. Therefore, when Cu is contained, the Cu content is preferably 1.0% or less. The Cu content is more preferably 0.2% or less.

[0047] Ta: 0.1% or less Like Ti, Nb, and V, Ta increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, during hot rolling and annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This further improves TS. To achieve this effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content exceeds 0.1%, large amounts of coarse precipitates and inclusions may form. In such cases, the coarse precipitates and inclusions may reduce ductility and even hole expandability. Therefore, when Ta is contained, the Ta content is preferably 0.1% or less. The Ta content is more preferably 0.05% or less.

[0048] W: 0.2% or less Like Ti and Nb, W increases TS by forming fine precipitates, such as carbides, nitrides, and carbonitrides, 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 trapping sites, thereby contributing to improved hole expandability. 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 0.2%, a large amount of coarse precipitates and inclusions are formed, resulting in a decrease in ductility. Therefore, when W is contained, the W content is preferably 0.2% or less. The W content is more preferably 0.060% or less.

[0049] Mg: 0.01% or less Mg is an element that is effective in spheroidizing the shape of inclusions such as sulfides and oxides to improve the hole expandability of steel sheets. To achieve this effect, the Mg content is preferably 0.0001% or more. However, if the Mg content exceeds 0.01%, the surface quality deteriorates. Therefore, when Mg is contained, the Mg content is preferably 0.01% or less. The Mg content is more preferably 0.005% or less, and even more preferably 0.001% or less.

[0050] Zn: 0.02% or less Zn is an element that is effective in spheroidizing the shape of inclusions and improving the hole expandability 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 0.02%, a large amount of coarse precipitates and inclusions may be generated, which may actually result in a decrease in hole expandability. Therefore, when Zn is contained, the Zn content is preferably 0.02% or less.

[0051] Co: 0.02% or less Like Zn, Co is an effective element for spheroidizing the shape of inclusions and improving the hole expandability 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 0.02%, a large amount of coarse precipitates and inclusions may be generated, which may actually result in a decrease in hole expandability. Therefore, when Co is contained, the Co content is preferably 0.02% or less.

[0052] Zr: 0.2% or less Zr contributes to high strength by refining prior austenite grains. Zr also contributes to high 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, adding a large amount of Zr increases the amount of coarse ZrN-based and ZrS-based precipitates that remain in an undissolved state during the slab heating process, thereby reducing ductility. Therefore, when Zr is contained, the Zr content is preferably 0.2% or less. The Zr content is more preferably 0.05% or less, and even more preferably 0.01% or less.

[0053] Ca: 0.02% or less Ca exists as inclusions in steel. If the Ca content exceeds 0.02%, a large amount of coarse inclusions may be generated, which may reduce ductility and hole expandability. Furthermore, surface quality may also deteriorate. Therefore, when Ca is contained, the Ca content is preferably 0.02% 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 constraints in production technology, the Ca content is more preferably 0.0010% or more.

[0054] Se: 0.02% or less, Te: 0.02% or less, Ge: 0.02% or less, As: 0.05% or less, Sr: 0.02% or less, Cs: 0.02% or less, Hf: 0.02% or less, Pb: 0.02% or less, Bi: 0.02% or less, and REM: 0.02% or less Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all effective elements for improving the hole expandability of steel sheet. To obtain such effects, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more. On the other hand, if the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM exceed 0.02% each, or if the content of As exceeds 0.05%, large amounts of coarse precipitates and inclusions may be generated, which may actually reduce hole expandability. Therefore, when these elements are contained, it is preferable that the contents of Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM are each 0.02% or less, and the content of As is 0.05% or less. Note that Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM may be contained alone or in combination.

[0055] The balance other than the above elements is Fe and unavoidable impurities.

[0056] 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 steel structure of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention has, at a quarter-thickness position of the base steel sheet, an area ratio of one or both of ferrite and bainite: 20% or more and 90% in total, and an area ratio of martensite: 10% or more and 80% or less, and a number density of MnS particles having a length of 150 μm or more in the rolling direction of 7 particles / mm ２ is less than or equal to, and M Ｃ / M Ｑ is 0.85 or more and 2.00 or less, and M Ｃ and M Ｑare the area fractions of martensite at the 1 / 2 and 1 / 4 thickness positions of the base steel sheet, respectively. The reasons for each limitation will be explained below. The area fraction of each phase is the ratio of the area occupied by each phase to the area of ​​the entire steel structure.

[0057] 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 achieve a TS of 780 MPa or more. 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. Ferrite and bainite may be contained alone, or both may be contained.

[0058] 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 TS 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. The area fraction of martensite is preferably 75% or less, more preferably 60% or less. Note that martensite is a hard structure formed by transformation from austenite below the martensitic transformation point (also simply referred to as the Ms point). Martensite includes both as-quenched so-called fresh martensite and tempered fresh martensite.

[0059] 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%. Note that retained austenite refers to austenite that remains without transforming from austenite to ferrite, martensite, bainite, or other metallic phases. Retained austenite is generated, for example, when elements such as C are concentrated in austenite, causing the martensite transformation point to fall below room temperature (austenite remains without transforming).

[0060] The area ratio of the remaining structure other than the above 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%. 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 relatively high temperatures and is a structure consisting of lamellar ferrite and cementite.

[0061] 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 as follows. Specifically, a sample is cut out from the galvanized steel sheet so that the observation surface is the thickness cross section (L cross section) parallel to the rolling direction of the substrate steel sheet. The observation surface of the sample is then polished using diamond paste, and then finish-polished using alumina. The observation surface of the sample is then etched with nital to reveal the structure. Then, five fields of view of the observation surface of the sample are observed using a scanning electron microscope (SEM) at a magnification of 1500x. The following regions are then color-coded (defined) from the obtained structure image using Adobe Photoshop from Adobe Systems. The total area ratio of ferrite and bainite, as well as the area ratio of martensite, was then calculated using the point counting method. Specifically, 16 × 15 lattice points were placed at 4.8 μm intervals in an area of ​​82 μm × 57 μm in actual length of each SEM image. The number of lattice points on ferrite, bainite, and martensite was then counted. The number of lattice points on ferrite, bainite, and martensite was then divided by the total number of lattice points, and multiplied by 100 to calculate the total area ratio of ferrite and bainite, as well as the area ratio of martensite. Ferrite: A black region with a blocky morphology. Ferrite is a structure consisting of crystal grains with a BCC lattice. Ferrite is formed by transformation from austenite at relatively high temperatures. Bainite: A black to dark gray region with a blocky or amorphous morphology. As described above, bainite is a hard structure in which fine carbides are dispersed in needle-like or plate-like ferrite. Bainite is formed from austenite at relatively low temperatures (above the Ms point). Bainite also contains a relatively small number of carbides. Martensite: A region that is white to light gray in color. As described above, martensite is a hard structure that is formed by transformation from austenite at or below the Ms point.Martensite includes both so-called fresh martensite, which is as quenched, and so-called tempered martensite, which is obtained by tempering the fresh martensite.

[0062] 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.

[0063] 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 (%)]

[0064] Number density of MnS particle groups (hereinafter simply referred to as MnS particle groups) having a length in the rolling direction of 150 μm or more: 7 particles / mm ２ Coarse MnS particles near the center of the thickness of the base steel sheet cause microcracks when punching holes in the galvanized steel sheet. Furthermore, such coarse MnS particles cause stress concentration during press forming. In particular, the presence of coarse MnS particles extending in the rolling direction or multiple MnS particles closely spaced in a dotted row results in significant adverse effects. In other words, suppressing the number density of MnS particle groups is important for achieving excellent hole expandability. Therefore, the number density of MnS particle groups should be kept at 7 particles / mm. ２ The number density of the MnS particles is preferably 5 particles / mm ２ Below, 3 pieces / mm２ The number density of the MnS particles is 0 particles / mm ２ may be.

[0065] Here, an MnS particle group is composed of one or more MnS particles with a major axis of 0.3 μm or more. Furthermore, when the group is composed of two or more MnS particles, the distance between the nearest MnS particles with a major axis of 0.3 μm or more is 10 μm or less. That is, for a given MnS particle (hereinafter also referred to as the MnS particle in question), if the distance between the MnS particle in question and the nearest MnS particle with a major axis of 0.3 μm or more (hereinafter also referred to as the nearest MnS particle) is 10 μm or less, the MnS particle in question and the nearest MnS particle are determined to belong to the same MnS particle group.

[0066] The number density of MnS particles is determined as follows. Specifically, a sample is cut out from a galvanized steel sheet so that the observation surface is a thickness cross section (L cross section) parallel to the rolling direction of the base steel sheet of the galvanized steel sheet. The observation surface of the sample is then observed using a scanning electron microscope (SEM) at a magnification of 1500x, with the center at half the thickness of the base steel sheet, to obtain a backscattered electron image. Next, the image is analyzed using ImageJ with a phase map, and the major axis and center of gravity of each observed MnS particle are determined using the particle analysis function. For reference, an example of an observed image is shown in Figure 1. In the observed image, the white areas extending in the rolling direction are MnS particles. In addition to MnS, this region may contain precipitates and inclusions such as TiS and Ti(C,S), but these can also be considered MnS particles and the following measurements can be performed. Next, for the observed MnS particles with a major axis of 0.3 μm or more, the MnS particle group to which each MnS particle with a major axis of 0.3 μm or more belongs is determined based on the distance to the nearest MnS particle with a major axis of 0.3 μm or more (the distance between the centers of gravity of the particles). Then, the rolling direction length of each MnS particle group is measured, and the number of MnS particle groups with a rolling direction length of 150 μm or more is counted. Note that when an MnS particle group is composed of two or more MnS particles, the rolling direction length of the MnS particle group is the rolling direction length between the outer ends of the MnS particles located at both ends of the MnS particle group in the rolling direction. Furthermore, when the MnS particle group is composed of one MnS particle, the rolling direction length of the MnS particle group is the rolling direction length of this MnS particle. Next, the number of MnS particle groups with a rolling direction length of 150 μm or more was counted by the area of ​​the observation region (approximately 1.6 mm ２ (thickness direction: 650 μm, rolling direction: 2500 μm region) to determine the number density of MnS particles.

[0067] M Ｃ / M Ｑ : 0.85 or more and 2.00 or less The difference between the area ratio of martensite at the 1 / 2 position of the sheet thickness of the base steel sheet and the area ratio of martensite at the 1 / 4 position of the sheet thickness is reduced. This improves the hole expandability. Also, a high YR is obtained. Here, MＣ / M Ｑ If M is less than 0.85 or exceeds 2.00, the difference in strength between the 1 / 2 position and the 1 / 4 position of the sheet thickness of the base steel sheet becomes large, and the starting point of cracks is likely to occur during punching. Ｃ / M Ｑ is set to 0.85 or more and 2.00 or less. Ｃ / M Ｑ is preferably 0.90 or more, more preferably 0.95 or more. Ｃ / M Ｑ is preferably 1.90 or less, more preferably 1.80 or less. Ｃ / M Ｑ is most preferably near 1.00, at which point the steel structure of the substrate steel sheet becomes uniform in the thickness direction.

[0068] Here, M Ｃ / M Ｑ is determined as follows. That is, the area ratio of martensite is measured at the 1 / 2 position of the plate thickness of the substrate steel sheet in the same manner as the measurement of the area ratio of martensite at the 1 / 4 position of the plate thickness of the substrate steel sheet described above. Then, the area ratios of martensite at the 1 / 2 position and the 1 / 4 position of the plate thickness of the substrate steel sheet are respectively determined as M Ｃ and M Ｑ As M Ｃ M Ｑ By dividing by M Ｃ / M Ｑ Ask for.

[0069] Diffusible hydrogen content of substrate steel sheet: 0.50 ppm by mass or less From the viewpoint of obtaining excellent hole expandability, 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 the lower limit of the diffusible hydrogen content of the substrate steel sheet is not particularly limited and 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.

[0070] Here, the amount of diffusible hydrogen in the substrate steel sheet is measured as follows. Specifically, a test piece 30 mm long and 5 mm wide is taken from a galvanized steel sheet, and the galvanized layer is removed 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.

[0071] In addition, for products (components) obtained after forming (punching, stretch flange forming, etc.) or joining of galvanized steel sheets, test pieces are cut out from the products in a typical usage environment and the amount of diffusible hydrogen in the substrate steel sheet portion is measured in the same manner as above. If the measured value is 0.50 ppm by mass or less, it can be considered that the amount of diffusible hydrogen in the substrate steel sheet of the galvanized steel sheet at the material stage before forming or joining is also 0.50 ppm by mass or less.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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:

[0076] 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 steel sheet (the surface area of ​​the part that was covered with plating) to determine the coating weight (g / m２ ) is calculated.

[0077] Next, the mechanical properties of the galvanized steel sheet according to one embodiment of the present invention will be described.

[0078] Tensile strength (TS): 780 MPa or more The tensile strength of the galvanized steel sheet according to one embodiment of the present invention is 780 MPa or more. There is no particular upper limit to the tensile strength of the galvanized steel sheet according to one embodiment of the present invention. However, for example, the tensile strength of the galvanized steel sheet according to one embodiment of the present invention is preferably less than 1180 MPa.

[0079] The yield stress (YS), total elongation (El), and limiting hole expansion ratio (λ) of the galvanized steel sheet according to one embodiment of the present invention are as described above. The tensile strength (TS), yield stress (YS), total elongation (El), and limiting hole expansion ratio (λ) are measured as described later in the examples.

[0080] The thickness of the galvanized steel sheet according to one embodiment of the present invention is preferably 0.5 mm or more and 3.5 mm or less.

[0081] [2] Member Next, a member according to one embodiment of the present invention will be described. The member according to one embodiment of the present invention is a member made using (using as a raw material) the above-mentioned galvanized steel sheet. For example, the raw material galvanized steel sheet is subjected to at least one of forming and joining to form a member. Here, the above-mentioned galvanized steel sheet has a TS of 780 MPa or more, a high YS, excellent ductility, and excellent hole expandability. Therefore, the member according to one embodiment of the present invention has high strength and is particularly suitable for application to complex-shaped members used in the automotive field.

[0082] [3] Method for Manufacturing Galvanized Steel Sheet Next, a method for manufacturing a galvanized steel sheet according to one embodiment of the present invention will be described.

[0083] A method for producing a galvanized steel sheet according to one embodiment of the present invention includes: a slab heating step of heating a steel slab having the above-described chemical composition under the conditions of an average heating rate of 0.5°C / second or less in a temperature range from 300°C to 1220°C, a slab heating temperature of 1220°C or more, and a slab heating time of 1.0 hour or more; a hot rolling step of hot rolling the steel slab under the conditions of a finish rolling end temperature of 840°C to 1000°C, an average cooling rate of 5°C / second or more in a temperature range from the finish rolling end temperature to 700°C, and a coiling temperature of 620°C or less to obtain a hot-rolled steel sheet; and a cold rolling step of cold-rolling the hot-rolled steel sheet under the conditions of a rolling reduction of 20% to 80% to obtain a cold-rolled steel sheet. The method includes a heating step of heating the cold-rolled steel sheet at an average heating rate of 1°C / sec or more in a temperature range from 600°C to 750°C, an annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 750°C to 920°C, an annealing time of 1 second to 30 seconds, and an atmospheric hydrogen concentration of less than 10% by volume, a cooling step of cooling the cold-rolled steel sheet at an average cooling rate of 5°C / sec to 200°C / sec in a temperature range from an annealing temperature of -30°C to 600°C, and a galvanizing step of galvanizing the cold-rolled steel sheet. Note that the above temperatures refer to the surface temperatures of the steel slab and the steel sheet unless otherwise specified.

[0084] 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.

[0085] [Slab Heating Step] Next, the steel slab is heated. At this time, in the method for producing a galvanized steel sheet according to one embodiment of the present invention, it is important to satisfy the following conditions.

[0086] Average heating rate in the temperature range from 300°C to 1220°C: 0.5°C / sec or less After casting, the steel slab is cooled to 300°C or less. The steel slab is then heated at an average heating rate in the temperature range from 300°C to 1220°C (hereinafter also referred to as the slab heating rate) of 0.5°C / sec or less. This dissolves coarse precipitates and inclusions (hereinafter simply referred to as precipitates), such as MnS and Nb- and Ti-based carbonitrides, generated during casting, thereby reducing the number of coarse precipitates that could become crack initiation sites. Furthermore, this method reduces Mn segregation generated during continuous casting and significantly contributes to uniforming the area fraction of martensite in the thickness direction of the base steel sheet. This improves the hole expandability of the final galvanized steel sheet. Therefore, the slab heating rate is set to 0.5°C / sec or less. The slab heating rate is preferably 0.4°C / sec or less, more preferably 0.3°C / sec or less. The slab heating rate is not particularly limited. However, even if the slab heating rate is less than 0.1°C / sec, the effect will saturate. Therefore, the slab heating rate is preferably 0.1°C / sec or more. The slab heating rate is calculated using the following formula: Slab heating rate (°C / sec) = (1220 (°C) - 300 (°C)) / [time (sec) for the surface temperature of the steel slab to reach 1220°C from 300°C]

[0087] Slab heating temperature: 1220°C or higher When the slab heating temperature is 1220°C or higher, coarse Nb- and Ti-based precipitates formed during casting are sufficiently dissolved. This reduces the amount of coarse precipitates. As a result, the hole expandability of the final galvanized steel sheet product is improved. Therefore, the slab heating temperature is set to 1220°C or higher. The slab heating temperature is preferably 1230°C or higher, more preferably 1240°C or higher. There is no particular upper limit to the slab heating temperature. For example, the slab heating temperature is preferably 1400°C or lower. The slab heating temperature is the maximum temperature reached by the steel slab during the slab heating process.

[0088] Slab heating time: 1.0 hour or more When the slab heating temperature is 1220°C or more and the slab heating time is 1.0 hour or more, coarse Nb- and Ti-based precipitates formed during casting are sufficiently dissolved. This reduces the amount of coarse precipitates. As a result, the hole expandability of the final galvanized steel sheet product is improved. Therefore, the slab heating time is set to 1.0 hour or more. The slab heating time is preferably 1.1 hours or more, more preferably 1.2 hours or more. There is no particular upper limit to the slab heating time. For example, the slab heating time is preferably 3.0 hours or less. The slab heating time is the holding time in the temperature range of 1220°C or more.

[0089] [Hot Rolling Step] Next, the steel slab is hot rolled to obtain a hot rolled steel sheet. In this hot rolling step, it is important to satisfy the following conditions.

[0090] Finish rolling end temperature: 840°C or higher and 1000°C or lower From the viewpoint of uniforming the area ratio of martensite in the thickness direction in the base steel sheet of the galvanized steel sheet obtained as the final product (hereinafter also referred to as the viewpoint of uniforming martensite in the thickness direction), it is important to suppress Mn enrichment (variation in Mn concentration) in the hot-rolled steel sheet. Here, 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 to enrich in untransformed austenite. Excessive C enrichment 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 enriches in the cementite. As a result, variation in Mn concentration occurs. 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 lower. The finish rolling end temperature is preferably 950°C or lower, more preferably 920°C or lower.

[0091] Average cooling rate in the temperature range from the finish rolling temperature to 700°C (hereinafter also referred to as the first cooling rate): 5°C / sec or more. As described above, from the viewpoint of uniforming martensite in the thickness direction, it is important to suppress Mn enrichment (variation in Mn concentration) in the hot-rolled steel sheet. Here, if the first cooling rate is slow, the amount of ferrite produced during cooling becomes excessive, leading to the enrichment of C in untransformed austenite. Excessive C enrichment in untransformed austenite promotes pearlite transformation. That is, excessive pearlite is produced in the steel structure of the hot-rolled steel sheet obtained after hot rolling. As described above, pearlite is a lamellar structure of ferrite and cementite, and Mn enriches in cementite. As a result, variation in Mn concentration occurs. Furthermore, if the first cooling rate is slow, precipitates such as Nb or Ti carbides and nitrides, which can serve as starting points for crack initiation, become coarse. Therefore, the first cooling rate is set to 5°C / sec or more. The first cooling rate is preferably 10°C / sec or more, more preferably 15°C / sec or more. There is no particular upper limit to the first cooling rate. However, from the viewpoint of energy saving in the cooling equipment, the first cooling rate is preferably 1000°C / sec or less.

[0092] Coiling temperature: 620°C or less If the coiling temperature exceeds 620°C, pearlite will be excessively increased during coiling, 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, and 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. The coiling temperature is more preferably 450°C or more.

[0093] 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.

[0094] [Cold Rolling Step] Next, the hot rolled steel sheet is subjected to cold rolling to obtain a cold rolled steel sheet.

[0095] 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 during the annealing process, resulting in reduced TS and hole expandability 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 during 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.

[0096] [Heating Step] Next, the cold-rolled steel sheet is heated to the annealing temperature. At this time, it is important to appropriately control the average heating rate in the temperature range from 600°C to 750°C.

[0097] Average heating rate in the temperature range from 600°C to 750°C (hereinafter also referred to as heating rate): 1°C / sec or more. The shorter the residence time of the cold-rolled steel sheet in the temperature range from 600°C to 750°C (hereinafter also referred to as heating temperature range) during the heating process, the less diffusible hydrogen there is in the base steel sheet, thereby improving hole expandability. Here, if the heating rate is less than 1°C / sec, the residence time in the heating temperature range increases, and the amount of diffusible hydrogen in the steel increases. As a result, hole expandability decreases. Therefore, the heating rate is set to 1°C / sec or more. The heating rate is preferably 2°C / sec or more, more preferably 3°C / sec or more. The upper limit of the heating rate is not particularly limited. The heating rate is preferably 15°C / sec or less, for example. The heating rate is more preferably 12°C / sec or less, and even more preferably 9°C / sec or less.

[0098] [Annealing Step] Next, the cold-rolled steel sheet is annealed under the conditions of an annealing temperature of 750° C. to 920° C., an annealing time of 1 second to 30 seconds, and an atmospheric hydrogen concentration of less than 10% by volume.

[0099] Annealing temperature: 750°C or higher and 920°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 TS cannot be obtained. On the other hand, if the annealing temperature exceeds 920°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 920°C or lower. The annealing temperature is preferably 900°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 In a manufacturing method of a galvanized steel sheet according to one embodiment of the present invention, the shorter the annealing time, the better in order to reduce the amount of diffusible hydrogen in the substrate steel sheet to 0.50 mass ppm or less. Furthermore, a shorter annealing time is preferable in order to suppress coarsening of Nb- and Ti-based carbides. 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, the coarse Fe-based precipitates do not dissolve, resulting in reduced elongation. Furthermore, from the viewpoint of dissolving as much coarse MnS as possible, a longer annealing time is preferable. 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] Atmospheric hydrogen concentration: less than 10% by volume If the atmospheric hydrogen concentration is 10% by volume or more, the amount of diffusible hydrogen in the base steel sheet exceeds 0.50 ppm by mass, resulting in a decrease in hole expandability. Therefore, the atmospheric hydrogen concentration is set to less than 10% by volume. The atmospheric hydrogen concentration is preferably less than 9% by volume, more preferably less than 8% by volume. The lower limit of the atmospheric hydrogen concentration is not particularly limited. However, if the atmospheric hydrogen concentration is less than 1% by volume, the reducing effect of hydrogen in the annealing process is reduced, and oxides are formed on the surface of the base steel sheet. As a result, there is a risk of a decrease in the coating weight. Therefore, the atmospheric hydrogen concentration is preferably 1% by volume or more. The atmospheric hydrogen concentration is more preferably 2% by volume or more.

[0102] In the temperature-raising step, similarly to the annealing step, it is preferable that the atmospheric hydrogen concentration be less than 10% by volume.

[0103] [Cooling Step] Next, the cold-rolled steel sheet annealed as described above is cooled under the following conditions.

[0104] Average cooling rate in the annealing temperature range from −30°C to 600°C: 5°C / sec or more and 200°C / sec or less. In this cooling step, in order to reduce the diffusible hydrogen content of the base steel sheet to 0.50 mass ppm or less, it is necessary to appropriately control the cooling rate, particularly the average cooling rate in the annealing temperature range from −30°C to 600°C (hereinafter also referred to as the second cooling rate). If the second cooling rate is too slow, the residence time at high temperature increases, and the diffusible hydrogen content of the base steel sheet increases. Therefore, the second cooling rate is set to 5°C / sec or more. The second cooling rate is preferably 7°C / sec or more, more preferably 9°C / sec or more. On the other hand, if the second cooling rate exceeds 200°C / sec, the diffusible hydrogen that penetrated the cold-rolled steel sheet during the annealing step cannot be desorbed during cooling, which may actually increase the diffusible hydrogen content of the base steel sheet. Therefore, the second cooling rate is set to 200°C / sec or less. The second cooling rate is preferably 100° C. / sec or less, more preferably 75° C. / sec or less.

[0105] Conditions other than those described above are not particularly limited. For example, the cooling stop temperature may be set to 400°C or higher and 600°C or lower, and the steel sheet may be held at a temperature in the range of 400°C or higher and 600°C or lower for 10 seconds or lower and then subjected to the galvanizing treatment step described below. Alternatively, the cooling stop temperature may be set to 25°C or higher and 400°C or lower, and the steel sheet may be reheated to a temperature in the range of the cooling stop temperature + 50°C or higher and 550°C or lower, and then held at a temperature in the range of 10 seconds or higher and 150 seconds or lower, and then subjected to the galvanizing treatment step described below.

[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 TS 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] [Temper Rolling Process] The galvanized steel sheet obtained as described above may be further subjected to temper rolling. In this case, the elongation rate is preferably 0.10% or more for the purpose of shape correction, surface roughness adjustment, and also for the purpose of stably obtaining a high YS. The elongation rate is more preferably 0.15% or more, and even more preferably 0.20% or more. There is no particular upper limit to the elongation rate. However, if the elongation rate exceeds 2.00%, the YS 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.

[0111] 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.

[0112] 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.

[0113] 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, a high YS, excellent ductility, and excellent hole expandability can be obtained, and the galvanized steel sheet can be suitably used for, for example, automotive parts.

[0114] [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.

[0115] A steel material having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and continuously cast into a steel slab. The steel slab was then heated under the conditions shown in Table 2, and hot-rolled, consisting of rough rolling and finish rolling, to obtain a hot-rolled steel sheet. The slab heating rate and slab heating time for No. 14 are the average heating rate from 300°C to the slab heating temperature and the holding time at the slab heating temperature, respectively. The resulting hot-rolled steel sheet was then pickled and cold-rolled under the conditions shown in Table 2 to obtain a cold-rolled steel sheet. The resulting cold-rolled steel sheet was then subjected to a heating process, annealing process, cooling process, and galvanization process under the conditions shown in Table 2 to obtain a steel sheet as a final product. Conditions not specified were those according to conventional methods.

[0116] 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".

[0117] 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 zinc plating layer of the finally obtained GI had a composition of Fe: 0.1 to 1.0 mass %, Al: 0.2 to 1.0 mass %, and the remainder consisting of Zn and unavoidable impurities.

[0118] 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 balance was Zn and unavoidable impurities.

[0119] Using the steel sheet thus obtained, the steel structure of the base steel sheet was identified, the number density of MnS particles, and M Ｃ / M Ｑ The measurement results are shown in Table 3.

[0120] In addition, tensile tests and hole expansion tests were carried out in the following manner, and the tensile strength (TS), yield stress (YS), total elongation (El) and limiting hole expansion ratio (λ) were evaluated according to the following criteria.・TS Pass: 780MPa≦TS Fail: TS<780MPa ・YS Pass: 420MPa≦YS if 780MPa≦TS<980MPa 550MPa≦YS if 980MPa≦TS Fail: YS<420MPa if 780MPa≦TS<980MPa YS<550MPa if 980MPa≦TS ・El Pass: 15%≦El if 780MPa≦TS<980MPa 9%≦El if 980MPa≦TS Fail: El<15% if 780MPa≦TS<980MPa El<9% if 980MPa≦TS ・λ Pass: 20%≦λ Fail: λ<20%

[0121] 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, YS, and El were measured. The results are also shown in Table 3.

[0122] The hole expansion test was conducted in accordance with JIS Z 2256. Specifically, 100 mm x 100 mm test pieces were cut from the obtained galvanized steel sheets by shearing. A 10 mm diameter hole was punched into the test piece with a clearance of 12.5%. Next, a blank holding force of 9 tons (88.26 kN) was applied around the hole using a die with an inner diameter of 75 mm to hold the test piece. In this state, a conical punch with an apex angle of 60° was pressed into the hole, and the diameter of the hole in the test piece at the crack initiation limit (when a crack occurred) was measured. The critical hole expansion ratio λ (%) was then calculated using the following formula. The results are also shown in Table 3. λ (%) = {(Dｆ -D ０ ) / D ０}×100 where, D ｆ D: diameter of the hole in the test piece when a crack occurs (mm) ０ : initial test piece hole diameter (mm)

[0123]

[0124]

[0125]

[0126] As shown in Table 3, in all of the inventive examples, TS, YS, El, and λ were all acceptable. Furthermore, all of the members obtained by forming or joining the galvanized steel sheets of the inventive examples had the desired shapes without cracking, and all of TS, YS, El, and λ met the acceptable standards. On the other hand, in the comparative examples, at least one of TS, YS, El, and λ was unacceptable.

[0127] According to the present invention, a galvanized steel sheet having high strength, high YS, excellent ductility, and excellent hole expandability can be obtained. Furthermore, the galvanized steel sheet can be extremely advantageously used as a material for automotive frame structural members having complex shapes. This allows for the reduction of vehicle body weight and the improvement of fuel efficiency, and therefore has great industrial utility value.

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