Galvanized steel sheet and member, and method for producing same

Abstract

To provide a galvanized steel sheet having high strength, a high yield ratio, excellent hole expandability, and excellent plating quality. In the present invention: a base steel sheet has a prescribed component composition; at a position at 1 / 4 of the sheet thickness of the base steel sheet, the area ratio of ferrite is 35% or less, the area ratio of martensite is 10%-90%, the area ratio of bainite is 10%-90%, and the area ratio of the balance structure is 10% or less; M1 / Mt is 0.30 or less and the average particle diameter of the bainite is 12 μm or less; and the number density of oxides in the surface layer part of the base steel sheet is at least 3.0 grains / μm2.

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, improving fuel efficiency. In this way, attempts are being made to reduce exhaust gases.

[0003] 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 hot-dip galvanized steel sheet having excellent formability and a tensile strength of 980 MPa or more, the steel sheet containing, by mass%, C: 0.05 to 0.4%, Si: 0.01 to 3.0%, Mn: 0.1 to 3.0%, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Al: 0.01 to 2.0%, Si + Al > 0.5%, the balance being Fe and unavoidable impurities, and the steel sheet having a microstructure which contains, by volume fraction, one or more of three types of martensite and bainite in total at a rate of 40% or more as main phases, and contains 8% or more austenite, the balance being ferrite, and which may contain 10% or less pearlite, and on the surface of which is provided a hot-dip galvanized layer which contains less than 7% by mass of Fe, the balance being Zn, Al, and unavoidable impurities."

[0004] Patent Document 2 discloses "a high-strength cold-rolled steel sheet excellent in steel sheet shape and shape freezability, characterized in that it satisfies, in mass %, C: 0.08 to 0.20%, Si: 0.2 to 2%, Mn: 1.0 to 3%, P: 0.05% or less, S: 0.01% or less, Ti: 0.001 to 0.2%, Al: 0.01 to 0.1%, B: 0.0002 to 0.01%, and N: 0.01% or less, with the balance being Fe and unavoidable impurities, and in which the ratios of bainite to the entire metal structure are 10 to 40 area % and the balance is tempered martensite, and which may further contain retained austenite in the ranges of 5 area % or less (including 0 area %) and ferrite in the ranges of 10 area % or less (including 0 area %), and which has a tensile strength of 980 MPa or more and a yield ratio of 70 to 80%."

[0005] Patent No. 5699889 Patent No. 6291289

[0006] Recently, further improvements in collision safety have been required from the viewpoint of ensuring the safety of passengers in the event of a collision of an automobile. In order to increase the energy absorption during a collision, it is effective to improve the yield ratio (YR = yield strength (YS) / tensile strength (TS)). Therefore, steel sheets used as materials for automotive structural members, particularly those around the cabin of an automobile, are required to have an improved yield ratio in addition to tensile strength.

[0007] Furthermore, since steel sheets used as raw materials for automotive structural members and the like are formed into complex shapes, the steel sheets are also required to have excellent formability, particularly excellent hole expandability (stretch flangeability). In addition, from the viewpoint of car body rust prevention performance, the steel sheets used as raw materials for automotive structural members and the like are sometimes plated with zinc, and therefore are also required to have excellent plating quality.

[0008] However, neither of the steel sheets disclosed in Patent Documents 1 and 2 can be said to satisfy all of the above-mentioned required properties, and there is currently a demand for the development of a galvanized steel sheet that combines high strength, a high yield ratio, excellent hole expandability, and excellent plating quality.

[0009] The present invention was developed in consideration of the above-mentioned current situation, and aims to provide a galvanized steel sheet that combines high strength, a high yield ratio, excellent hole expandability, and excellent plating quality, together with an advantageous manufacturing method thereof. The present invention also aims to provide a member made from the above-mentioned galvanized steel sheet and a manufacturing method thereof. 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.

[0010] To achieve the above-mentioned object, the inventors have conducted extensive research and have discovered the following. (A) After adjusting the chemical composition of the substrate steel sheet to a predetermined range, the area fractions of ferrite, martensite, and bainite in the steel structure of the substrate steel sheet are appropriately controlled. This makes it possible to simultaneously achieve high strength and a high yield ratio. (B) In the steel structure of the substrate steel sheet, the proportion of hard martensite with a high Mn concentration is reduced, particularly to set M1 / Mt to 0.30 or less. This reduces the difference in hardness between martensite and ferrite and bainite, making it possible to simultaneously achieve high strength, a high yield ratio, and excellent hole expandability. Here, Mt is the area fraction (%) of martensite. M1 is the area fraction (%) of the region constituting martensite that satisfies the relationship of the following formula (1): [Mn] Ｍ / [Mn]≧1.5 (1) In the formula, [Mn] is the Mn content (mass%) in the chemical composition of the base steel sheet. Ｍ is the Mn concentration (mass%) of each region constituting martensite. (C) By refining bainite, specifically by making the average grain size of bainite 12 μm or less, the yield ratio is further improved. (D) By reducing the oxide size in the surface layer of the base steel sheet to 3.0 particles / μm ２ By ensuring the above, it is possible to obtain excellent plating quality while ensuring high strength, a high yield ratio, and excellent hole expandability.

[0011] The present invention was completed based on the above findings and further studies. 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: 0.04% to 0.13%, Si: 0.2% to 0.8%, Mn: 2.20% to 3.50%, P: 0.100% or less, S: 0.0500% or less, Al: 0.010% to 0.100%, N: 0.0100% or less, and one or both of Nb and Ti: 0.005% or more in total, with the balance being Fe and unavoidable impurities. At a 1 / 4 position in the sheet thickness of the substrate steel sheet, the area ratio of ferrite is 35% or less, the area ratio of martensite is 10% or more and 90% or less, the area ratio of bainite is 10% or more and 90% or less, the area ratio of the remaining structure is 10% or less, M1 / Mt is 0.30 or less, and the average grain size of the bainite is 12 μm or less, and the number density of oxides in the surface layer portion of the substrate steel sheet is 3.0 particles / μm ２ A galvanized steel sheet having a tensile strength of 780 MPa or more, wherein the surface layer portion of the substrate steel sheet is a region from the surface of the substrate steel sheet to a depth of 100 μm in the sheet thickness direction, and wherein Mt is the area fraction (%) of the martensite. M1 is the area fraction (%) of the region constituting the martensite that satisfies the relationship of the following formula (1): [Mn] Ｍ / [Mn]≧1.5 (1) In the formula, [Mn] is the Mn content (mass%) in the chemical composition of the base steel sheet. Ｍ is the Mn concentration (mass%) of each region constituting the martensite.

[0012] 2. The chemical composition of the substrate steel sheet further includes, in mass%, 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, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, 2. The galvanized steel sheet according to 1 above, containing at least one selected from Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

[0013] 3. A member made using the galvanized steel sheet according to 1 or 2 above.

[0014] 4. A hot rolling process in which a steel slab having the chemical composition described in 1 or 2 above 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, and then a cold rolling process in which the hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet, and then an annealing process in which the cold rolled steel sheet is annealed under the conditions of an annealing temperature of 770°C to 900°C, an annealing time of 1 second to 30 seconds, and an atmospheric dew point of -25°C or higher, and then the cold rolled steel sheet is cooled under the conditions of an intermediate cooling temperature satisfying the relationship of the following formula (2), a second half cooling rate of 1.0°C / second or more, and a cooling stop temperature of 550°C or less, the intermediate cooling temperature being a temperature at a point halfway through the time from the start of cooling to the cooling stop temperature, A method for producing a galvanized steel sheet, comprising: a cooling step in which the latter cooling rate is an average cooling rate in a temperature range from the intermediate cooling temperature to the cooling stop temperature; a retention step in which the cold-rolled steel sheet is retained in a temperature range of 550°C or less for a retention time of 5 seconds or more; and a galvanizing step in which the cold-rolled steel sheet is subjected to a galvanizing treatment to obtain a galvanized steel sheet. ｈ ≦1 / 2×(T+T Ｑ ) (2) In the formula, T: annealing temperature (°C) in the annealing step, T ｈ : Intermediate cooling temperature (°C) in the cooling step, and T Ｑ : Cooling stop temperature (°C) in the cooling step.

[0015] 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.

[0016] According to the present invention, a galvanized steel sheet having high strength, a high yield ratio, excellent hole expandability, and excellent plating quality can be obtained. The galvanized steel sheet of the present invention has high strength, a high yield ratio, excellent hole expandability, and excellent plating quality, and can be produced with high productivity without introducing new equipment, so it can be extremely advantageously used as a material for automotive frame structural members, etc.

[0017] 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 "%".

[0018] C: 0.04% or more and 0.13% 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 0.04%, ferrite increases and the desired strength cannot be obtained. On the other hand, if the C content exceeds 0.13%, the yield ratio (hereinafter also referred to as YR) decreases. Therefore, the C content is set to 0.04% or more and 0.13% or less. The C content is preferably 0.05% or more, more preferably 0.06% or more. The C content is preferably 0.12% or less, more preferably 0.11% or less.

[0019] Si: 0.2% or more and 0.8% or less Si is an element that improves the strength of steel sheet through solid solution strengthening. Si also improves the YR by increasing the strength of ferrite. Furthermore, Si is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. That is, Si is an element that affects the area ratio of ferrite. Here, if the Si content is less than 0.2%, the YR decreases. On the other hand, if the Si content is excessive, particularly if it exceeds 0.8%, the ferrite becomes excessive and the YR decreases. Therefore, the Si content is set to 0.2% or more and 0.8% or less. The Si content is preferably 0.3% or more, more preferably 0.4% or more. The Si content is preferably 0.7% or less, more preferably 0.6% or less.

[0020] Mn: 2.20% or more and 3.50% or less Mn is an element that improves the hardenability of steel. Mn is added to ensure a predetermined amount of martensite. If the Mn content is less than 2.20%, the hardenability is insufficient and ferrite is excessively formed. This makes it difficult to ensure the desired TS and YR. On the other hand, if Mn is added in excess, the weldability deteriorates. Therefore, the Mn content is set to 2.20% or more and 3.50% or less. The Mn content is preferably 2.30% or more, more preferably 2.40% or more. The Mn content is preferably 3.20% or less, more preferably 3.00% or less.

[0021] 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%, weldability 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.

[0022] S: 0.0500% or less S causes hot embrittlement. S also reduces weldability. Furthermore, S forms coarse MnS and the like, reducing workability. 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. There is no particular lower limit for the S content. The S content is preferably 0.0001% or more, and more preferably 0.0002% or more.

[0023] Al: 0.010% or more and 0.100% or less Al is added to deoxidize and reduce inclusions in steel. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.015% or more, more preferably 0.030% or more. On the other hand, Al is an element that promotes ferrite transformation in the annealing process and the subsequent cooling process. Therefore, if the Al content exceeds 0.100%, ferrite becomes excessive and the YR decreases. Therefore, the Al content is set to 0.100% or less. The Al content is preferably 0.080% or less, more preferably 0.060% or less.

[0024] N: 0.0100% or less If the N content exceeds 0.0100%, nitride precipitates such as AlN become coarse, resulting in a decrease in ductility and toughness. This may also lead to a decrease in surface quality. 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. The lower limit of the N content is not particularly limited. Due to production technology constraints, the N content is preferably 0.0006% or more.

[0025] Nb and / or Ti: Total 0.005% or More Nb and Ti contribute to improving TS and YR by forming fine precipitates and refining the steel structure. To achieve this effect, the total content of Nb and / or Ti is 0.005% or more, preferably 0.008% or more, and more preferably 0.011% or more. Excessive Nb and Ti may result in the formation of large amounts of coarse precipitates. These coarse precipitates may precipitate in combination with MnS, potentially resulting in reduced workability. Furthermore, they may increase deformation resistance during cold rolling, potentially resulting in reduced productivity. Therefore, the total content of Nb and / or Ti is preferably 0.400% or less, more preferably 0.120% or less, and even more preferably 0.060% or less.

[0026] Note that, as long as the total content of one or both of Nb and Ti is 0.005% or more, the respective contents of Nb and Ti are not particularly limited. 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.

[0027] 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, the substrate steel sheet of a galvanized steel sheet according to one embodiment of the present invention preferably contains 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 element selected from the following as an optional element, either alone or in combination. Note that the effects of the present invention can be obtained as long as each of the following optional elements is contained in an amount equal to or less than the upper limit, and therefore no lower limit is particularly set. Note that if the content of each of the following optional elements is less than the preferred lower limit described below, the element may be treated as an unavoidable impurity.

[0028] V: 0.45% or less Like Nb and Ti, V forms fine precipitates during the hot rolling process and annealing process, thereby increasing TS and YR. To achieve this effect, the V content is preferably 0.001% or more, 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 generated, which may result in a decrease in formability. Therefore, when V is contained, the V content is preferably 0.45% or less, more preferably 0.060% or less.

[0029] B: 0.0100% 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, more preferably 0.0002% or more. On the other hand, if the B content exceeds 0.0100%, weldability may be reduced. 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.

[0030] Cr: 1.00% or less Cr is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS and YR. To achieve this effect, the Cr content is preferably 0.0005% or more, more preferably 0.010% or more. On the other hand, if the Cr content exceeds 1.00%, the above effect saturates. In addition, this leads to an increase in costs. Therefore, when Cr is contained, the Cr content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.60% or less.

[0031] Ni: 1.00% or less Ni is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS and YR. To achieve this effect, the Ni content is preferably 0.005% or more, more preferably 0.020% or more. On the other hand, if the Ni content exceeds 1.00%, the above effect saturates. In addition, this leads to an increase in costs. Therefore, when Ni is contained, the Ni content is preferably 1.00% or less, more preferably 0.50% or less.

[0032] Mo: 1.00% or less Mo is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS and YR. To achieve this effect, the Mo content is preferably 0.010% or more, more preferably 0.030% or more. On the other hand, if the Mo content exceeds 1.00%, the above effect saturates. It also leads to an increase in costs. 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.

[0033] 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, more preferably 0.005% or more. On the other hand, an Sb content exceeding 0.100% may result in a decrease in castability. Therefore, when Sb is contained, the Sb content is preferably 0.100% or less, more preferably 0.060% or less, and even more preferably 0.040% or less.

[0034] 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.

[0035] Cu: 1.00% or less Cu is an element that improves hardenability and promotes the formation of martensite, thereby increasing TS and YR. To achieve this effect, the Cu content is preferably 0.005% or more, more preferably 0.020% or more. On the other hand, if the Cu content exceeds 1.00%, cracks may be induced during hot rolling, which may cause surface defects. Therefore, when Cu is contained, the Cu content is preferably 1.00% or less, more preferably 0.20% or less.

[0036] Ta: 0.100% or less Like Ti, Nb, and V, Ta increases TS and YR 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. This further increases TS and YR. 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 reduced workability. Therefore, when Ta is contained, the Ta content is preferably 0.100% or less, more preferably 0.050% or less.

[0037] W: 0.200% or less Like Ti, Nb, and V, W forms fine precipitates during the hot rolling and annealing processes, thereby increasing TS and YR. To achieve this effect, the W content is preferably 0.001% or more, 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 is formed, resulting in a decrease in workability. Therefore, when W is contained, the W content is preferably 0.200% or less, more preferably 0.060% or less.

[0038] Mg: 0.010% or less Mg is an element that spheroidizes the shape of inclusions such as sulfides and oxides, thereby improving the workability 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.

[0039] Zn: 0.020% or less Zn is an element that spheroidizes the shape of inclusions and improves the workability of the steel sheet. 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 workability. Therefore, when Zn is contained, the Zn content is preferably 0.020% or less.

[0040] Co: 0.500% or less Like Zn, Co is an element that spheroidizes the shape of inclusions and improves the workability 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 decrease in workability. Therefore, when Co is contained, the Co content is preferably 0.500% or less.

[0041] Zr: 0.20% or less Zr contributes to improving TS and YR by, for example, refining prior austenite grains. Zr is also an effective element for improving castability. To achieve this effect, 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 increases during heating of the steel slab before the hot rolling process, resulting in a decrease in workability. 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.

[0042] 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 workability. Furthermore, the 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. The Ca content is, for example, preferably 0.0005% or more.

[0043] 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: 0.0200% or less. Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective in increasing the strength and improving the workability of steel sheet. To achieve these 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 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 may be formed, which may actually reduce workability. Therefore, when these elements are contained, the Se, Te, Ge, Sr, Cs, Hf, Pb, Bi, and REM contents are preferably 0.0200% or less, and the As content is preferably 0.0500% or less. Note that 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 17 elements, including 15 lanthanoid elements from La (lanthanum) with atomic number 57 to Lu (lutetium) with atomic number 71, Sc (scandium) with atomic number 21, and Y (yttrium) with atomic number 39. These 17 elements can be contained alone or in combination. The REM content refers to the total content of these 17 elements. Among the REMs, La is particularly preferred.

[0044] 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) and H (hydrogen). 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.

[0045] Next, the steel structure of the base steel sheet of a 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 of the base steel sheet.

[0046] Ferrite area ratio (hereinafter also referred to as F area ratio): 35% or less Ferrite is soft, so it reduces the YR. Furthermore, if excessive ferrite is produced, the interface between ferrite and martensite increases. Since the difference in hardness between ferrite and martensite is large, the increase in the interface between ferrite and martensite reduces the hole expandability. Therefore, in order to obtain the desired YR and hole expandability, the F area ratio is set to 35% or less. The F area ratio is preferably 30% or less, more preferably 25% or less. There is no lower limit for the F area ratio, and the F area ratio may be 0%. The F area ratio is preferably 3% or more.

[0047] Martensite area ratio (hereinafter also referred to as M area ratio): 10% to 90%. Martensite is hard and is a structure necessary for obtaining high TS and high YR. Here, if the M area ratio is less than 10%, the desired TS and YR cannot be obtained. On the other hand, to achieve an M area ratio of more than 90%, the cooling capacity in the cooling process after the annealing process must be significantly increased, which increases capital investment costs. Therefore, the M area ratio is set to 10% to 90%. The M area ratio is preferably 20% or more, more preferably 30% or more. The M area ratio is also preferably 80% or less, more preferably 70% or less. Here, martensite is a hard structure generated by transformation from austenite below the martensite transformation point (also simply referred to as the Ms point). In addition, martensite includes both so-called fresh martensite, which is as-quenched, and so-called tempered martensite, which is obtained by tempering the fresh martensite.

[0048] Bainite area fraction (hereinafter also referred to as B area fraction): 10% or more and 90% or less Bainite is a structure necessary for obtaining a high YR and excellent hole expandability. Therefore, the B area fraction is set to 10% or more. The B area fraction is preferably 15% or more, more preferably 20% or more. On the other hand, if the B area fraction exceeds 90%, the desired M area fraction cannot be obtained, and the desired TS cannot be obtained. Therefore, the B area fraction is set to 90% or less. The B area fraction is preferably 80% or less, more preferably 70% or less.

[0049] Area ratio of the remaining structure: 10% or less The area ratio of the remaining structure other than martensite, ferrite, and bainite is 10% or less. The area ratio of the remaining structure is preferably 5% or less, and more preferably 3% or less. There is no lower limit for the area ratio of the remaining structure, and the area ratio of the remaining structure may be 0%.

[0050] Examples of the residual structure include pearlite, retained austenite, and unrecrystallized ferrite. Here, pearlite is formed from austenite at relatively high temperatures and is a lamellar structure of ferrite and (acicular) cementite. Retained austenite is austenite that remains without transforming from austenite to ferrite, martensite, bainite, or the like. Furthermore, retained austenite is formed, 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). Unrecrystallized ferrite is ferrite that has not been recrystallized, and subgrain boundaries exist within the grains. The type of residual structure can be confirmed, for example, by observation using a scanning electron microscope (SEM, hereinafter simply referred to as SEM).

[0051] Here, the F area ratio, M area ratio, and B area ratio are measured at a 1 / 4 position in the sheet thickness direction 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 sheet 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 SEM image (structure image) using Adobe Photoshop from Adobe Systems. Then, the F area ratio, M area ratio, and B area ratio 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, martensite, and bainite was then counted. The number of grid points on ferrite, martensite, and bainite was then divided by the total number of grid points, and the result was multiplied by 100 to calculate the F area ratio, M area ratio, and B area ratio.

[0052] In SEM images, ferrite, martensite, and bainite are observed, for example, as follows: Ferrite: Ferrite is a black region and has a blocky shape. Ferrite is a structure consisting of crystal grains with a BCC lattice. Ferrite is formed by transformation from austenite. Martensite: Martensite is a white to light gray region. As described above, martensite is a hard structure formed by transformation from austenite in a temperature range below the Ms point. Martensite includes both as-quenched, so-called fresh martensite, and so-called tempered martensite obtained by tempering the fresh martensite. Bainite: Bainite is a black region and is a lath-shaped structure composed of bainitic ferrite and fine carbides. Bainite includes upper bainite and lower bainite. In upper bainite, carbides precipitate at the interface of the lath-shaped structure. In lower bainite, carbides are precipitated inside the lath-like structure. Ferrite and bainite can be distinguished from each other based on the shape and carbide precipitation morphology. In bainite, the crystal orientation relationship between bainitic ferrite and carbides is usually one type, so the carbides are elongated in one direction.

[0053] The area ratio of the remaining structure is calculated by subtracting the F area ratio, M area ratio, and B area ratio calculated as above from 100% according to the following formula: [Area ratio of remaining structure (%)] = 100 - [F area ratio (%)] - [M area ratio (%)] - [B area ratio (%)]

[0054] M1 / Mt: 0.30 or less Martensite is hard and is a structure necessary for increasing the strength of steel sheets. However, if the proportion of hard martensite with a high Mn concentration in martensite increases, the difference in hardness between martensite and ferrite and bainite increases, resulting in a decrease in YR. Furthermore, voids are more likely to form at the interface between bainite and martensite, resulting in a decrease in hole expandability. 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 M1 / Mt may be 0.

[0055] Here, Mt is the area ratio (%) of martensite, and M1 is the area ratio (%) of the region constituting martensite that satisfies the relationship of the following formula (1). [Mn] Ｍ / [Mn]≧1.5 (1) In the formula, [Mn] is the Mn content (mass%) in the chemical composition of the base steel sheet. Ｍ is the Mn concentration (mass%) of each region constituting martensite.

[0056] Note that M1 may be determined, for example, in accordance with the method described in the following 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. 622-628

[0057] That is, using the sample used in measuring the area ratio of each phase above, quantitative analysis of Mn is performed at a position 1 / 4 of the sheet thickness of the substrate steel sheet using a field emission electron probe microanalyzer (FE-EPMA), and a two-dimensional distribution of Mn (Mn mapping) is created. Here, quantitative analysis of Mn is performed while preventing carbon contamination on the surface. In the quantitative analysis of Mn, the acceleration voltage is 9 kV and the probe current is 10 nA. Next, the structural image used in measuring the area ratio of each phase above is compared with the two-dimensional Mn distribution, and a region that satisfies the relationship of formula (1) above is defined in the region constituting martensite. Next, the total area of ​​the defined regions is calculated, and M1 is calculated by dividing this total area by the total area of ​​the observation field.

[0058] Average grain size of bainite (hereinafter also referred to as B grain size): 12 μm or less To obtain the desired YR, it is necessary to refine the bainite. If the bainite becomes coarse, the YR decreases. Therefore, the B grain size is 12 μm or less. The B grain size is preferably 10 μm or less, more preferably 8 μm or less. There is no particular lower limit for the B grain size. From the viewpoint of production technology, the B grain size is preferably 0.5 μm or more.

[0059] Here, the B grain size is measured at a position 1 / 4 of the sheet thickness of the base steel sheet, for example, as follows. That is, 10 bainite particles (particles) are randomly selected from each of the five fields of view used in the measurement of the area ratio of each phase above, and the major axis of the selected bainite particles is measured. The average of the major axes of the measured bainite particles is then taken as the B grain size. The major axis is the maximum distance between two parallel lines tangent to the outline of the bainite particle. The same applies hereinafter.

[0060] Number density of oxides in the surface layer of the base steel sheet (hereinafter also referred to as surface layer oxide density): 3.0 particles / μm ２ As described above, utilizing Si and Mn is effective in increasing the strength of steel sheets. However, Si and Mn are easily oxidizable elements, and form oxides (hereinafter also referred to as surface oxides) on the surface of the substrate steel sheet. During zinc plating, surface oxides reduce the wettability of the substrate steel sheet in the plating bath, which can cause poor appearance such as unplated areas. Here, by causing internal oxidation in the surface layer of the substrate steel sheet, i.e., by increasing the surface oxide density, the formation of surface oxides is suppressed. As a result, the plating quality is improved. Therefore, the surface oxide density is 3.0 particles / μm ２ The surface oxide density is preferably 5.0 particles / μm ２ More preferably, 7.0 particles / μm ２ The upper limit of the surface oxide density is not particularly limited. However, even if the surface oxide density is excessively increased, the effect of suppressing the formation of surface oxides becomes saturated. Therefore, the surface oxide density is preferably 30.0 particles / μm ２ Here, the surface layer portion of the substrate steel sheet refers to a region from the surface of the substrate steel sheet (the interface between the substrate steel sheet and the zinc-plated layer) to a depth of 100 μm in the sheet thickness direction. In addition, when a zinc-plated layer is provided on only one surface of the substrate steel sheet, it is sufficient to control the surface layer oxide density within the above range in a region from that one surface to a depth of 100 μm in the sheet thickness direction.

[0061] Here, the surface oxide density is measured, for example, as follows, in a region from the surface of the substrate steel sheet to a depth of 100 μm in the sheet thickness direction. That is, a sample is cut out from the zinc-plated steel sheet so that the cross section (L cross section) parallel to the rolling direction and sheet thickness direction of the substrate steel sheet of the zinc-plated steel sheet serves as the observation surface. Next, the observation surface of the sample is polished using diamond paste. Next, using a SEM, backscattered electron images are observed at a magnification of 5000x, with five fields of view in the observation surface of the sample, from the surface of the substrate steel sheet to a depth of 100 μm in the sheet thickness direction (vertical direction) (actual length in the rolling direction (horizontal direction): 113 μm). Next, in each SEM image, the number of oxides present in the observation region is counted, and the surface oxide density is calculated by dividing the number of oxides by the total area of ​​the observation region. Note that in each SEM image, tiny black dots with a major axis of 0.1 μm to 0.6 μm are counted as oxides. The oxides are compounds of O with the basic component elements and optional added elements contained in the base steel sheet, excluding C and N. The oxides are mainly composed of Si oxide and Mn oxide.

[0062] 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 a base 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. 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.

[0063] 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.

[0064] The galvannealed layer is preferably composed of, for example, Zn, 20.0 mass% or less Fe, and 0.001 mass% to 1.0 mass% 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 mass% to 3.5 mass%. The Fe content of the galvannealed layer is more preferably 7.0 mass% or more, and even more preferably 8.0 mass% or more. The Fe content of the galvannealed layer is more preferably 15.0 mass% or less, and even more preferably 12.0 mass% or less. The remainder other than the above elements is unavoidable impurities.

[0065] 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:

[0066] The coating weight of the zinc plating layer is measured, for example, 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. Then, the mass loss of the test material before and after dissolution is measured, and the value is divided by the surface area of ​​the base steel sheet (the surface area of ​​the part that was covered with the plating) to determine the coating weight (g / m２ ) is calculated.

[0067] Next, the mechanical properties of the galvanized steel sheet according to one embodiment of the present invention will be described.

[0068] 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.

[0069] Here, high strength means TS: 780 MPa or more. High yield ratio means YR: 0.70 or more. Excellent hole expandability means λ (critical hole expansion ratio): 20% or more. Excellent plating quality means that defects due to unplated areas are not visually confirmed in the zinc plating layer.

[0070] Here, TS and YR are measured by a tensile test in accordance with JIS Z 2241:2022, and λ is measured by a hole expansion test in accordance with JIS Z 2256:2010.

[0071] Detailed procedures for measuring the above-mentioned properties are as described in the examples below.

[0072] 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.

[0073] [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 combines high strength, a high yield ratio, excellent hole expandability, and excellent plating quality. Therefore, the member according to one embodiment of the present invention is particularly suitable for application to automotive components, for example, automotive frame structural components.

[0074] [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.

[0075] First, a steel slab having the above-mentioned composition is prepared. For example, a steel material is melted to produce 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 produce a steel slab. The method for obtaining a steel slab from the 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.

[0076] 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 immediately hot rolling the steel slab after heat retention. Furthermore, from the viewpoints 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. In the hot rolling process described below, for example, the steel slab is subjected to rough rolling under conditions according to conventional methods to produce a sheet bar. When the slab heating temperature is low, 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.

[0077] [Hot Rolling Step] Next, the steel slab is subjected to hot rolling under the following conditions to obtain a hot-rolled steel sheet.

[0078] 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, resulting in excessive ferrite formation 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.

[0079] Coiling temperature: 620°C or less If the coiling temperature exceeds 620°C, excessive pearlite is formed during coiling, promoting Mn concentration. The lower the coiling temperature, the less pearlite is formed, 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.

[0080] 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.

[0081] [Cold Rolling Step] Next, the hot-rolled steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The cold rolling conditions are not particularly limited and may be performed according to conventional methods. For example, the reduction is preferably 20% or more, more preferably 30% or more. If the reduction is less than 20%, the steel structure is likely to become coarse and non-uniform in the annealing step, which may result in a decrease in TS and ductility in the final product. On the other hand, if the reduction is more than 80%, the steel sheet is likely to have a defective shape. In addition, temperature unevenness in the annealing step may cause non-uniformity in the steel structure and non-uniformity in the amount of zinc coating. Therefore, the reduction is preferably 80% or less, more preferably 70% or less.

[0082] [Annealing Step] Next, the cold-rolled steel sheet is annealed under the following conditions.

[0083] Annealing temperature: 770°C or higher and 900°C or lower If the annealing temperature is lower than 770°C, the proportion of austenite generated during heating in the two-phase region of ferrite and austenite becomes insufficient. As a result, ferrite increases excessively after annealing, making it difficult to obtain the desired TS and YR. Therefore, the annealing temperature is set to 770°C or higher. The annealing temperature is preferably set to 790°C or higher, more preferably 810°C or higher. On the other hand, if the annealing temperature exceeds 900°C, the effect of reducing ferrite saturates. This also increases costs. Therefore, the annealing temperature is set to 900°C or lower. The annealing temperature is preferably set to 890°C or lower, more preferably 880°C or lower. The annealing temperature is the maximum temperature reached in the annealing process.

[0084] Annealing time: 1 second or more and 30 seconds or less From the viewpoint of suppressing Mn enrichment in martensite in the final structure, it is important to suppress Mn enrichment in austenite during the annealing process. To achieve this, the shorter the annealing time, the better. Furthermore, in order to refine bainite, 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, the proportion of austenite generated during annealing becomes insufficient, making it difficult to obtain the desired TS and YR. Furthermore, the surface oxide density becomes insufficient. 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.

[0085] Dew point of atmosphere: -25°C or higher If the dew point of the atmosphere during annealing is less than -25°C, the surface oxide density will be insufficient. Therefore, the dew point of the atmosphere is -25°C or higher, preferably -20°C or higher, and more preferably -15°C or higher. There is no particular upper limit to the dew point of the atmosphere. However, from the viewpoint of avoiding excessive capital investment for enlarging the size of the equipment, the dew point of the atmosphere is preferably 20°C or lower.

[0086] [Cooling Step] Next, the cold-rolled steel sheet annealed as described above is cooled under the following conditions.

[0087] Intermediate cooling temperature: Satisfying the relationship of the following formula (2) If the residence time at high temperature in the cooling process is long, ferrite is produced in excess, and the desired steel structure cannot be obtained. Here, T on the left side of the following formula (2) which represents the intermediate cooling temperature ｈ is the temperature at the start of cooling, that is, the temperature at the point where half of the time has elapsed from the annealing temperature to the cooling stop temperature. Ｑ ) is the average value of the annealing temperature and the cooling stop temperature. That is, by satisfying the relationship of the following formula (2), the residence time at high temperature is shortened, and it becomes possible to suppress the excessive formation of ferrite and obtain the desired steel structure. Therefore, the intermediate cooling temperature is 1 / 2 × (T + T Ｑ The intermediate cooling temperature is preferably 19 / 40 x (T + T Ｑ) or less, more preferably 9 / 20 × (T + T Ｑ The lower limit of the intermediate cooling temperature is not particularly limited. For example, the intermediate cooling temperature is preferably T Ｑ More preferably, T Ｑ +5°C or higher. ｈ ≦1 / 2×(T+T Ｑ ) (2) In the formula, T: annealing temperature in the annealing step (°C), T ｈ : Intermediate cooling temperature in the cooling process (°C) and T Ｑ : Cooling stop temperature (°C) in the cooling process.

[0088] The time from the start of cooling to the cooling stop temperature is preferably 3 to 20 seconds. In the case of a continuous annealing furnace, for example, the time from the start of cooling to the cooling stop temperature can be calculated from the line length of the cooling zone (the line length from the entrance to the exit of the cooling zone) and the sheet passing speed. In addition, the intermediate cooling temperature (T ｈ ) can be calculated, for example, by determining the thermal history of the steel sheet from the temperature measured by a thermometer installed in a continuous annealing furnace and the sheet passing speed.

[0089] Second half cooling rate: 1.0°C / sec or more In order to obtain the desired B area ratio and M area ratio, the second half cooling rate, which is the average cooling rate from the intermediate cooling temperature until the cooling stop temperature is reached, is set to 1.0°C / sec or more. If the second half cooling rate is less than 1.0°C / sec, ferrite is produced in excess, making it difficult to obtain the desired YR and hole expandability. Therefore, the second half cooling rate is 1.0°C / sec or more, preferably 2.0°C / sec or more, and more preferably 3.0°C / sec or more. There is no particular limit to the upper limit of the second half cooling rate. However, from the viewpoint of avoiding increases in equipment costs and manufacturing costs, the second half cooling rate is preferably 50.0°C / sec or less, and more preferably 30.0°C / sec or less. The second half cooling rate may be calculated, for example, by the following formula: [Second half cooling rate (°C / sec)] = (T ｈ -T Ｑ ) / [T ｈ From T Ｑ Time (seconds) until T ｈ : Intermediate cooling temperature in the cooling process (°C) and T Ｑ: Cooling stop temperature (°C) in the cooling process.

[0090] Cooling stop temperature: 550°C or less. If the cooling stop temperature exceeds 550°C, excessive ferrite is generated, making it difficult to obtain the desired YR and hole expandability. Therefore, the cooling stop temperature is 550°C or less, preferably 540°C or less, and more preferably 530°C or less. The lower limit of the cooling stop temperature is not particularly limited. Note that, 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, increased equipment capacity is required for reheating the cold-rolled steel sheet, which may increase capital investment costs. Therefore, the cooling stop temperature is preferably 400°C or higher, more preferably 420°C or higher.

[0091] [Dwelling Step] After the cooling step, the cold-rolled steel sheet is allowed to dwell in a temperature range of 550°C or lower for 5 seconds or more.

[0092] Residence time in a temperature range of 550°C or less: 5 seconds or more In order to obtain the desired B area ratio, the residence time (hereinafter also simply referred to as residence time) in a temperature range of 550°C or less (hereinafter also referred to as residence temperature range) is set to 5 seconds or more. If the residence time is less than 5 seconds, the desired B area ratio cannot be obtained and the YR decreases. Therefore, the residence time is 5 seconds or more, preferably 7 seconds or more, and more preferably 10 seconds or more. There is no particular limitation on the upper limit of the residence time. However, from the viewpoint of avoiding an increase in equipment costs and a decrease in productivity, it is preferably 1000 seconds or less, more preferably 100 seconds or less, and even more preferably 80 seconds or less.

[0093] Furthermore, if the residence temperature range exceeds 550°C, excessive ferrite is produced, and the desired TS and YR cannot be obtained. Therefore, the residence temperature range is a temperature range of 550°C or less. The residence temperature range is preferably a temperature range of 540°C or less, more preferably a temperature range of 530°C or less. The lower limit of the residence temperature range is not particularly limited. However, when a hot-dip galvanizing process or a hot-dip galvannealing process is performed as the galvanizing process described below, it is preferable to set the sheet temperature at which the steel enters the coating bath higher than the coating 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 capital investment costs. Therefore, the residence temperature range is preferably a temperature range of 400°C or more, more preferably a temperature range of 420°C or more. When a hot-dip galvanizing process or a hot-dip galvannealing process is performed as the galvanizing process described below, the residence time can be said to be the residence time in a temperature range of 550°C or less from the end of the cooling step (the time when the cooling stop temperature is reached) to the time when the cold-rolled steel sheet enters the plating bath.

[0094] [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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] The cooling conditions after the galvanizing treatment are not particularly limited, and may be in accordance with conventional methods.

[0099] [Temper Rolling Step] Next, in order to stably obtain a high YR, the galvanized steel sheet obtained as described above is preferably subjected to temper rolling under the following conditions.

[0100] Elongation: 0.10% or more To stably obtain a high YR, the elongation is preferably 0.10% or more, more preferably 0.12% or more, and even more preferably 0.14% or more. There is no particular upper limit to the elongation. However, if the elongation exceeds 2.00%, shape defects may occur, which may reduce the dimensional accuracy when the galvanized steel sheet is formed into a component. Therefore, the elongation is preferably 2.00% or less.

[0101] 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.

[0102] 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 galvanizing line (CGL). After the hot-dip galvanizing process, wiping can be performed to adjust the coating weight of the galvanized steel sheet.

[0103] 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 yield ratio, excellent hole expandability, and excellent plating quality can be obtained, and the galvanized steel sheet can be suitably used for automobile components, such as automobile frame structural components.

[0104] [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 a general processing method such as press forming can be used. The joining method is also not particularly limited, and for example, general welding such as spot welding, laser welding, and arc welding, rivet joining, and caulking joining can be used. The forming conditions and joining conditions are not particularly limited, and may be conventional methods.

[0105] 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 addition, some of the obtained galvanized steel sheets were subjected to temper rolling under the conditions shown in Table 2. Note that conditions not specified were those according to conventional methods.

[0106] 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".

[0107] 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.

[0108] 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.

[0109] Using each of the zinc-coated steel sheets thus obtained, the F area ratio, M area ratio, B area ratio, and area ratio of the remaining structure of each substrate steel sheet, as well as M1 / Mt, B particle size, and surface oxide density were measured in the same manner as described above. The measurement results are shown in Table 3. Measurement of the surface oxide density was performed on the surface layer portions of both sides of the substrate steel sheet, and similar results were obtained, so the result from one side is shown as a representative.

[0110] In addition, a tensile test, a hole expansion test, and a plating quality evaluation test were performed in the following manner to evaluate TS, YR, hole expandability, and plating quality.

[0111] (1) Tensile Test The tensile test was conducted in accordance with JIS Z 2241:2022. 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 base steel sheet. Using the taken test pieces, a tensile test was conducted at a crosshead speed of 10 mm / min to measure TS and YS (yield strength). In addition, YR was calculated from the measured TS and YS using the following formula. TS and YR were evaluated according to the following evaluation criteria. The results are also shown in Table 3. YR = YS / TS [Evaluation criteria] Pass (excellent): 780 MPa ≦ TS Fail (poor): TS < 780 MPa Pass (excellent): 0.70 ≦ YR Fail (poor): YR < 0.70

[0112] (2) Hole Expansion Test The hole expansion test was performed in accordance with JIS Z 2256:2010. Specifically, 100 mm × 100 mm test specimens were obtained from the obtained galvanized steel sheets by shearing. A 10 mm diameter hole was punched into the test specimen with a clearance of 12.5%. Next, a die with an inner diameter of 75 mm was used to apply a blank holding force of 9 ton (88.26 kN) around the hole to hold the test specimen. In this state, a conical punch with an apex angle of 60° was pressed into the hole of the test specimen, and the diameter of the hole at the crack initiation limit (when a crack occurred) was measured. The limiting hole expansion ratio λ (%) was then calculated using the following formula, and the hole expandability was evaluated according to the following evaluation criteria. The results are also shown in Table 3. λ (%) = {(Df - D0) / D0} x 100 Where, Df: diameter (mm) of the hole in the test piece when a crack occurs D0: initial diameter (mm) of the hole in the test piece [Evaluation criteria] Pass A (particularly excellent): 40% ≦ λ Pass B (excellent): 20% ≦ λ (except Pass A) Fail (bad): λ < 20%

[0113] (3) Plating Quality Evaluation Test The obtained galvanized steel sheets were visually inspected for the presence or absence of unplated areas (hereinafter also referred to as "visually inspected unplated areas") and appearance defects such as uneven gloss and stains. Furthermore, if any appearance defects were visually inspected, the presence or absence of unplated areas and foreign matter attached at the interface between the galvanized layer and the steel substrate (hereinafter collectively referred to as "unplated areas observed by SEM") was inspected in the appearance defects. The plating quality was then evaluated according to the following evaluation criteria. The results are also shown in Table 3. [Evaluation Criteria] Pass A (particularly excellent): No unplated areas or appearance defects were observed in the galvanized layer by visual inspection. Pass B (excellent): No unplated areas were observed in the galvanized layer by visual inspection, and appearance defects were observed, but no unplated areas were observed by SEM inspection. Fail (poor): At least one of unplated areas by visual inspection and unplated areas by SEM inspection was observed in the galvanized layer. Here, for the SEM observation, a sample was cut out from the obtained galvanized steel sheet so that a cross section (L cross section) parallel to the rolling direction and the sheet thickness direction was the observation surface, and so that the sample included a visually confirmed area with a defective appearance. Then, the above sample was observed by SEM at a magnification of 3000 times in three fields of view, each of which was 35 μm × 45 μm.

[0114]

[0115]

[0116]

[0117] As shown in Table 3, all of the inventive examples were acceptable in terms of TS, YR, hole expandability, and plating quality. On the other hand, the comparative examples were unacceptable in terms of at least one of TS, YR, hole expandability, and plating quality.

[0118] According to the present invention, a galvanized steel sheet having high strength, a high yield ratio, excellent hole expandability, and excellent plating quality can be obtained. Furthermore, the galvanized steel sheet can be extremely advantageously used as a material for automobile components, such as automobile frame structural components. This allows for a reduction in vehicle weight and therefore improved fuel efficiency, making the steel sheet extremely valuable in industry.

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: 0.04% or more and 0.13% or less, Si: 0.2% or more and 0.8% or less, Mn: 2.20% or more and 3.50% or less, P: 0.100% or less, S: 0.0500% or less, Al: 0.010% or more and 0.100% or less, N: 0.0100% or less, and one or both of Nb and Ti: 0.005% or more in total, with the balance being Fe and unavoidable impurities. At a 1 / 4 position in the sheet thickness of the substrate steel sheet, the area ratio of ferrite is 35% or less, the area ratio of martensite is 10% or more and 90% or less, the area ratio of bainite is 10% or more and 90% or less, the area ratio of the remaining structure is 10% or less, M1 / Mt is 0.30 or less, and the average grain size of the bainite is 12 μm or less, and the number density of oxides in the surface layer portion of the substrate steel sheet is 3.0 particles / μm ２ The surface layer portion of the substrate steel sheet is a region from the surface of the substrate steel sheet to a depth of 100 μm in the sheet thickness direction, and the galvanized steel sheet has a tensile strength of 780 MPa or more, where Mt is the area ratio (%) of the martensite. M1 is the area ratio (%) of the region constituting the martensite that satisfies the relationship of the following formula (1): [Mn] Ｍ / [Mn]≧1.5 (1) In the formula, [Mn] is the Mn content (mass%) in the chemical composition of the base steel sheet. Ｍ is the Mn concentration (mass%) of each region constituting the martensite.

2. The chemical composition of the substrate steel sheet further includes, in mass%, 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, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, 2. The galvanized steel sheet according to claim 1, containing at least one selected from the group consisting of Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

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

4. 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 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 770°C to 900°C, an annealing time of 1 second to 30 seconds and an atmospheric dew point of -25°C or more; and an intermediate cooling temperature satisfying the relationship of the following formula (2), a second half cooling rate of 1.0°C / second or more and a cooling stop temperature of 550°C or less, the intermediate cooling temperature being the temperature at the point where half of the time has elapsed since the start of cooling until the cooling stop temperature is reached, A method for producing a galvanized steel sheet, comprising: a cooling step in which the latter cooling rate is an average cooling rate in a temperature range from the intermediate cooling temperature to the cooling stop temperature; a retention step in which the cold-rolled steel sheet is retained in a temperature range of 550°C or less for a retention time of 5 seconds or more; and a galvanizing step in which the cold-rolled steel sheet is subjected to a galvanizing treatment to obtain a galvanized steel sheet. ｈ ≦1 / 2×(T+T Ｑ ) (2) In the formula, T: annealing temperature (°C) in the annealing step, T ｈ : Intermediate cooling temperature (°C) in the cooling step, and T Ｑ : Cooling stop temperature (°C) in the cooling step.

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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