Thin steel sheet, member, and production methods for these

Abstract

The present invention provides: a thin steel sheet which has high strength and excellent formability; a member; and production methods for these.　Provided is a thin steel sheet which has a component composition containing specific amounts of C, Si, Mn, P, S, Al, N, Cu, Sn, and Ni, and satisfying specific formula (1) and formula (2), with the balance being made up with Fe and inevitable impurities, and which has a steel structure containing, by area ratio, a total of 55-95% of ferrite and bainite, 5-45% of martensite, and a total of 0-5% of pearlite and cementite, with the total of the area ratios of pearlite and cementite being 1 / 3 or less of the area ratio of martensite. In cases where the area ratio of ferrite is more than 0%, the Vickers hardness of martensite is 4.00 times the Vickers hardness of ferrite or less, and the precipitation density of MnS having an equivalent circle diameter of 5 µm or more is 100 per mm2 or less.

Description

Thin Steel Sheet, Component, and Method for Producing the Same

[0001] The present invention relates to a thin steel sheet, a component, and a method for producing the same.

[0002] As the awareness of the crisis regarding climate change caused by greenhouse gases has increased globally in recent years, the demand for reducing CO ２ emissions has been intensifying. In the automotive field, efforts have been actively made to reduce CO ２ emissions by improving fuel efficiency through vehicle body weight reduction. As a means of vehicle body weight reduction, in addition to thinning by increasing the strength of the thin steel sheet used for parts, it is effective to increase the degree of freedom of part shapes by improving the formability of the thin steel sheet. Basic formabilities required for thin steel sheets for automobiles include flanging formability and shape freezing property. However, in order to withstand forming into parts with more complex shapes, it is important to improve bending formability and stretch flange formability.

[0003] Moreover, recently, the requirement for reducing CO ２ emissions in the manufacturing process of thin steel sheets has become more stringent. Therefore, a thin steel sheet manufacturing process that suppresses CO ２ emissions by utilizing an electric furnace has been attracting attention. In the steelmaking process using an electric furnace, since iron scrap is used as the main raw material, compared with the process using a blast furnace that reduces iron ore with coke, it is possible to suppress CO ２ emissions. In addition, the steelmaking process using an electric furnace also contributes to global environmental conservation in terms of resource recycling. However, iron scrap, which is the main raw material of the steel material (electric furnace steel) obtained by the steelmaking process using an electric furnace, contains elements that are difficult to remove in the steelmaking process called tramp elements. In particular, Cu, Ni, and Sn are typical tramp elements contained in iron scrap, and it is important to establish a thin steel sheet with excellent formability and a method for producing the same on the premise that these three elements remain in the steel simultaneously.

[0004] There is little disclosure regarding thin steel sheets for automobiles and their manufacturing technologies that actively utilize Cu, Ni, and Sn, which are typical tramp elements contained in iron scrap, on the premise that all of them are included.

[0005] For example, Patent Document 1 discloses a technology for a hot-rolled steel sheet with excellent chemical conversion treatment properties, which utilizes Cu compounds on the surface of the steel sheet as cathode points, and a method for manufacturing the same.

[0006] Furthermore, Patent Document 2 discloses technology for a high-strength steel sheet with excellent formability that utilizes Cu, Ni, and Sn as optional additive elements, and a method for manufacturing the same.

[0007] Furthermore, Patent Document 3 discloses a technology for a steel plate that achieves both strength and formability through press working, and a method for manufacturing the same.

[0008] Japanese Patent Publication No. 2020-84325, Japanese Patent Publication No. 2016-216808, Japanese Patent No. 7078186

[0009] Patent Document 1 only discloses a technology for improving chemical treatment properties by utilizing Cu in steel derived from iron scrap used as a raw material, and does not provide any technology for material design of thin steel sheets utilizing trumpet elements.

[0010] Patent Document 2 discloses suitable Cu, Ni, and Sn content for improving the formability of high-strength steel sheets. However, it only mentions the contribution of Cu and Ni to increasing strength, and the contribution of Sn to improving plateability. It does not provide technology for appropriate thin steel sheets and their manufacturing methods that take into account in detail the material advantages and disadvantages of containing these three elements.

[0011] Patent Document 3 discloses Cu, Ni, and Sn content suitable for improving the strength and press formability of steel sheets. However, it only mentions the contribution of Cu and Ni to strength increase through solid solution strengthening, and the effect of Sn on suppressing decarburization of the steel sheet surface. It does not provide technology for appropriate thin steel sheets and their manufacturing methods that take into account in detail the material advantages and disadvantages of containing these three elements.

[0012] The present invention was developed to solve the above-mentioned problems, and aims to provide thin steel sheets, components, and methods for manufacturing them that are high-strength and have excellent formability, taking into account not only conventional blast furnace manufacturing but also cases where iron scrap is used as the main raw material.

[0013] Here, "high strength" refers to a tensile strength (TS) of 590 MPa or more and less than 980 MPa, as determined by a tensile test conducted in accordance with the provisions of JIS Z2241 (2011). Furthermore, "excellent formability" refers to excellent ultimate deformation capacity. Excellent ultimate deformation capacity means that ε, calculated from the following equation (5), is 0.55 or greater. ε = -ln(t / t) ０ ) ...Equation (5) In Equation (5), t ０ : is the thickness (mm) of the thin steel plate before the tensile test, and t: is the thickness (mm) of the fractured portion after the tensile test in accordance with JIS Z 2241 (2011).

[0014] The inventors focused on the ultimate deformability, which has a strong correlation with bendability and stretch flange formability, in addition to the tensile strength of steel sheets, and conducted extensive research. As a result, they found that by simultaneously incorporating the three elements Cu, Ni, and Sn in predetermined amounts and in predetermined balances, it is possible to suppress red-hot brittleness caused by Cu and Sn during casting and hot rolling, while reducing coarse MnS, which is a factor that degrades the ultimate deformability, and to obtain a steel sheet with superior ultimate deformability compared to conventional steels. Furthermore, they found that by including the three elements Cu, Ni, and Sn in predetermined amounts and controlling the cooling rate during annealing within a predetermined range according to the amounts of these three elements, it is possible to suppress the decomposition of austenite into ferrite and cementite, which causes a decrease in strength, and to obtain a steel sheet that achieves both high strength and excellent ultimate deformability.

[0015] This invention is based on the above findings, and its gist is as follows.

[0016] [1] In mass %, C: 0.010 to 0.200%, Si: 2.00% or less, Mn: 0.80 to 3.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.10% or less, N: 0.0200% or less, Cu: 0.010 to 0.500%, Sn: 0.0010 to 0.0500%, Ni: 0.010 to 0.500%, satisfying the following formulas (1) and (2), with the balance being Fe and inevitable impurities, in area ratio, total of ferrite and bainite: 55 to 95%, martensite: 5 to 45%, and total of pearlite and cementite: 0 to 5%, and the total area ratio of pearlite and cementite being 1 / 3 or less of the area ratio of martensite, having a steel structure, and further, when the area ratio of ferrite exceeds 0%, the Vickers hardness of martensite is 4.00 times or less of the Vickers hardness of ferrite, and the precipitation density of MnS with a circle equivalent diameter of 5 μm or more is 100 pieces / mm ２ or less, and a thin steel sheet having a tensile strength of 590 MPa or more and less than 980 MPa. [%Cu] + 15 × [%Sn] - 0.15 × [%Mn] ０．８ - 0.4 × [%Si] ０．４ - 8 × [%Ni] ≤ 0.00... Formula (1) [%Mn] × [%S] / (0.5 × [%Cu] + [%Sn] ０．５) ≤ 1.00 ...Equation (2) In Equations (1) and (2), [%M] is the mass %) content of element M in the thin steel sheet. [2] In the above component composition, further, in mass%, Cr: 1.00% or less, Mo: 0.500% or less, B: 0.0100% or less, Sb: 0.0500% or less, V: 0.100% or less, Ti: 0.200% or less, Nb: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Zr: 0.020% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less [1] A thin steel sheet according to [1], 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 (excluding Ce): 0.0200% or less. [3] A thin steel sheet according to [1] or [2], having a plating layer of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electro-galvanized layer, or an aluminum plating layer on one or both surfaces of the thin steel sheet. [4] A member made using the thin steel sheet according to any one of [1] to [3]. [5] A method for manufacturing a thin steel sheet as described in [1] or [2] above, wherein a steel slab having the above component composition is hot-rolled at a heating temperature of 1050 to 1350°C, the finishing rolling completion temperature is 1000°C or less, and the steel sheet is obtained by winding it at a temperature of 400 to 700°C. ＦＤＴ +350×[%C]+75×([%Mn]-1)+30×[%Cu]+100×[%Ni]+40×[%Sn] ０．５ ...Equation (4) In Equation (4), T ＦＤＴ is the finish rolling completion temperature (°C), and [%M] is the element M content (mass%) of the steel slab. [6] The steel sheet after winding is pickled and then cold-rolled, and heated under the conditions of annealing temperature: 700 to 900°C, holding time at annealing temperature: 1 to 500 s, and average cooling rate V ＣA method for manufacturing a thin steel sheet as described in [5] above, wherein the temperature satisfies formula (3) and is 120°C / s or less, and cooling is performed under the condition that the cooling stop temperature is 550°C or less. 6.0 ≤ V Ｃ ×(1+1.5×[%Cu]+5×[%Ni]+2×[%Sn] ０．５ ) / (1 + 0.02 × [%Mn]) ...Equation (3) In Equation (3), [%M] is the content (mass%) of element M in the steel slab. [7] A method for manufacturing a thin steel sheet as described in [1] or [2] above, wherein a steel slab having the above component composition is hot-rolled at a heating temperature of 1050 to 1350°C and a finishing rolling completion temperature of 850 to 1000°C, then wound at a temperature of 400 to 700°C to form a steel sheet, the obtained steel sheet is pickled and further cold-rolled, and heated at an annealing temperature of 700 to 900°C and a holding time at the annealing temperature of 1 to 500 s, with an average cooling rate V Ｃ A method for manufacturing thin steel sheets, wherein the temperature satisfies equation (3), is 120°C / s or less, and cooling is performed under conditions of a cooling stop temperature of 550°C or less. 6.0 ≤ V Ｃ ×(1+1.5×[%Cu]+5×[%Ni]+2×[%Sn] ０．５ ) / (1 + 0.02 × [%Mn]) ... Equation (3) In Equation (3), [%M] is the content (mass%) of element M in the steel slab. [8] After hot rolling, the steel sheet that has been coiled and pickled is heated under the conditions of annealing temperature: 700 to 900°C, holding time at annealing temperature: 1 to 500 s, and average cooling rate V Ｃ A method for manufacturing a thin steel sheet as described in [5] above, wherein the following conditions are met: the temperature satisfies formula (3) and is 120°C / s or less, and the cooling is stopped at a temperature of 550°C or less; and the surface of the steel sheet is further plated with hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, or aluminum plating. 6.0 ≤ V Ｃ ×(1+1.5×[%Cu]+5×[%Ni]+2×[%Sn] ０．５) / (1 + 0.02 × [%Mn]) ... Formula (3) In Formula (3), [%M] is the mass %) content of element M in the steel slab. [9] A method for manufacturing a thin steel sheet according to [6] or [7], wherein the surface of the cooled thin steel sheet is further plated with hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, or aluminum plating.

[10] A method for manufacturing a component, comprising the step of forming and joining a thin steel sheet according to any one of [1] to [3] to make a component.

[0017] According to the present invention, it is possible to provide high-strength thin steel sheets and components with excellent formability, along with advantageous manufacturing methods for them.

[0018] The embodiments of the present invention will be described in detail below.

[0019] Thin Steel Sheet The thin steel sheet of the present invention contains, by mass%, C: 0.010 to 0.200%, Si: 2.00% or less, Mn: 0.80 to 3.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.10% or less, N: 0.0200% or less, Cu: 0.010 to 0.500%, Sn: 0.0010 to 0.0500%, Ni: 0.010 to 0.500%, and satisfies the following formulas (1) and (2), with the remainder being Fe and unavoidable impurities, and has a component composition with area ratio of Fe A steel structure having a total of lite and bainite: 55-95%, martensite: 5-45%, and a total of pearlite and cementite: 0-5%, and the sum of the area percentages of pearlite and cementite being 1 / 3 or less of the area percentage of martensite, and furthermore, when the area percentage of ferrite is greater than 0%, the Vickers hardness of martensite is 4.00 times or less of the Vickers hardness of ferrite, and the precipitation density of MnS with an equivalent circle diameter of 5 μm or more is 100 particles / mm ２ The following is true, and the tensile strength is between 590 MPa and less than 980 MPa: [%Cu] + 15 × [%Sn] - 0.15 × [%Mn] ０．８ -0.4 × [%Si] ０．４ -8×[%Ni]≦0.00...Formula (1) [%Mn]×[%S] / (0.5×[%Cu]+[%Sn] ０．５) ≤ 1.00 ...Equation (2) In Equations (1) and (2), [%M] is the mass %) content of element M in the thin steel sheet.

[0020] The tensile strength (TS) of the thin steel sheet in this invention is 590 MPa or more and less than 980 MPa.

[0021] The following describes the preferred component composition of the thin steel sheet of the present invention. In the following description, "%" indicating the content of a component means "mass%" unless otherwise specified.

[0022] C: 0.010-0.200% C is an element that contributes to the formation of austenite at the annealing temperature and the formation of martensite obtained by cooling it. If the C content is less than 0.010%, a sufficient amount of austenite cannot be obtained at the annealing temperature, and ferrite grows excessively during cooling, so the desired strength cannot be achieved in the thin steel sheet that is ultimately obtained. Therefore, the C content is 0.010% or more, preferably 0.030% or more, more preferably 0.045% or more, and even more preferably 0.060% or more. On the other hand, if the C content exceeds 0.200%, a large amount of carbides such as cementite are formed in the ferrite, and cracks that occur at the interface deteriorate the ultimate deformability of the steel sheet. Therefore, the C content is 0.200% or less, preferably 0.170% or less, and more preferably 0.140% or less.

[0023] Si: 2.00% or less. Si is an element that contributes to suppressing pearlite formation. By including an appropriate amount, the decrease in martensite associated with pearlite formation can be suppressed, which can contribute to increasing the strength of the steel sheet. It is also an element that contributes to suppressing red-hot brittleness caused by Cu and Sn during casting and hot rolling. By including an appropriate amount, it also contributes to preventing the deterioration of the ultimate deformability of the final thin steel sheet. Furthermore, Si dissolves in ferrite, reducing the strength difference between martensite and ferrite, thereby suppressing the occurrence of cracks at the interface and improving the ultimate deformability. On the other hand, if the Si content exceeds 2.00%, intermetallic compounds consisting of Fe and Si are formed, and the ductility and ultimate deformability of the steel sheet deteriorate. Therefore, the Si content is 2.00% or less, preferably 1.90% or less. In this invention, there are no particular adverse effects from reducing the Si content, and there is no need to set a lower limit; it may be 0.00%. However, the Si content is preferably 0.01% or more, and more preferably 0.02% or more.

[0024] Mn: 0.80-3.00% Mn is an element that contributes to the formation of austenite at the annealing temperature and the formation of martensite obtained by cooling it. If the Mn content is less than 0.80%, a sufficient amount of austenite cannot be obtained at the annealing temperature, and ferrite grows excessively during cooling, so the desired strength cannot be achieved in the thin steel sheet that is ultimately obtained. Therefore, the Mn content is 0.80% or more, preferably 1.20% or more, and more preferably 1.40% or more. On the other hand, if the Mn content exceeds 3.00%, coarse MnS and Mn segregation will occur in the material, and excellent ultimate deformability cannot be obtained. In addition, a large amount of Mn is dissolved in the martensite, and the strength difference between martensite and ferrite becomes large, so cracks that occur at the interface deteriorate the ultimate deformability of the steel sheet. Therefore, the Mn content is 3.00% or less, preferably 2.80% or less, and more preferably 2.40% or less.

[0025] P: 0.100% or less. P is an element that is very prone to segregation at grain boundaries, and if present in excess, it segregates in large quantities at ferrite grain boundaries, degrading the ultimate deformability. Therefore, the P content is 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less. On the other hand, in the present invention, there are no particular adverse effects from reducing the P content, and it is not necessary to set a lower limit, but the P content is preferably 0.001% or more, and more preferably 0.002% or more.

[0026] S: 0.0200% or less. During manufacturing, S combines with Mn in the steel to form MnS. If the S content is excessive, a large amount of coarse MnS is generated, degrading the ultimate deformability. Therefore, the S content is 0.0200% or less, preferably 0.0150% or less, and more preferably 0.0100% or less. It is preferable to reduce the S content as much as possible, and there is no particular lower limit, but the S content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0027] Al: 0.10% or less. Al is an element used as a deoxidizing agent. To obtain this effect, it is preferable to have an Al content of 0.01% or more. On the other hand, if the Al content exceeds 0.10%, coarse oxides and nitrides are generated during steelmaking, leading to deterioration of slab quality and potentially adversely affecting the ultimate deformability of the resulting thin steel sheet. Therefore, the Al content is 0.10% or less, and preferably 0.08% or less.

[0028] N: 0.0200% or less. N can form coarse nitrides, which can be the starting point for void formation and reduce the ultimate deformability. The N content that can suppress this effect is 0.0200% or less, preferably 0.0150% or less, and more preferably 0.0100% or less. The lower limit is not particularly limited, but due to production technology constraints, an N content of 0.0005% or more is preferred. An N content of 0.0010% or more is more preferred.

[0029] Cu: 0.010-0.500% Cu is an element necessary for suppressing Mn segregation in the steel structure. By suppressing Mn segregation, the precipitation density of coarse MnS is reduced, contributing to an improvement in ultimate deformability. Furthermore, by suppressing the decomposition of austenite into ferrite and pearlite during cooling from the annealing temperature, the desired structure can be obtained in the final thin steel sheet. The Cu content necessary to obtain these effects is 0.010% or more, preferably 0.025% or more, and more preferably 0.050% or more. On the other hand, if Cu is present in excess, red-hot brittleness occurs during casting or hot rolling, leaving fine cracks in the final thin steel sheet, and excellent ultimate deformability cannot be obtained. Therefore, the Cu content is 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.

[0030] Sn: 0.0010 to 0.0500% Sn is an element that segregates at grain boundaries and contributes to suppressing the coarsening of MnS. It also segregates at the interface between carbides and ferrite during annealing and contributes to suppressing carbide growth. The Sn content required to obtain these effects is 0.0010% or more, preferably 0.0015% or more, and more preferably 0.0020% or more. On the other hand, if Sn is present in excess, red-hot brittleness occurs during casting and hot rolling, leaving fine cracks in the final thin steel sheet, and excellent ultimate deformability cannot be obtained. Therefore, the Sn content is 0.0500% or less, preferably 0.0400% or less, and more preferably 0.0250% or less.

[0031] Ni: 0.010-0.500% Ni is an effective element for improving ultimate deformability by suppressing red-hot brittleness caused by Cu and Sn. Furthermore, by suppressing the decomposition of austenite into ferrite and pearlite during cooling from the annealing temperature, the desired microstructure can be obtained in the final thin steel sheet. The Ni content required to obtain these effects is 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, if Ni is present in excess, a large amount of Ni will dissolve in the martensite, increasing the strength difference between martensite and ferrite, and cracks formed at the interface will degrade the ultimate deformability of the steel sheet. In addition, the addition of Ni will increase costs. Therefore, the Ni content is 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.

[0032] Formula (1): [%Cu]+15×[%Sn]-0.15×[%Mn] ０．８ -0.4 × [%Si] ０．４ -8 × [%Ni] ≤ 0.00 In equation (1), [%M] is the mass %) content of element M in the thin steel sheet. In the thin steel sheet of the present invention, the three essential elements Cu, Ni, and Sn must satisfy equation (1) in addition to the above-mentioned content in the steel. As described above, Cu is effective in reducing the precipitation density of MnS, and Sn is effective in suppressing the coarsening of MnS, while both of these elements cause red-hot brittleness. Ni contributes to suppressing red-hot brittleness by increasing the solid solubility limit of Cu, and Si and Mn contribute to suppressing red-hot brittleness by removing Cu concentrated on the surface in the scale during the hot rolling process. Therefore, in order to effectively realize the benefits of containing Cu and Sn and obtain excellent ultimate deformability, it is necessary to satisfy equation (1). [%Cu] + 15 × [%Sn] - 0.15 × [%Mn] ０．８ -0.4 × [%Si] ０．４ If -8 × [%Ni] is greater than 0.00, red-hot brittleness is not sufficiently suppressed, and excellent ultimate deformability cannot be obtained. Therefore, [%Cu] + 15 × [%Sn] - 0.15 × [%Mn] ０．８ -0.4 × [%Si] ０．４-8 × [%Ni] shall be 0.00 or less. Preferably, [%Cu] + 15 × [%Sn] - 0.15 × [%Mn] ０．８ -0.4 × [%Si] ０．４ -8 × [%Ni] shall be -0.05 or less, more preferably -0.15 or less. The lower limit of the left side of equation (1) is not particularly limited, but the left side of equation (1) is preferably -3.00 or more, more preferably -2.50 or more, and even more preferably -2.00 or more.

[0033] Formula (2): [%Mn]×[%S] / (0.5×[%Cu]+[%Sn] ０．５ ) ≤ 1.00 In equation (2), [%M] is the mass %) content of element M in the thin steel sheet. In addition to the above, the four elements Mn, S, Cu, and Sn must satisfy equation (2). If equation (2) is not satisfied, the suppression effect of coarse MnS by Cu and Sn will be insufficient, and excellent ultimate deformability cannot be obtained. Therefore, the left side of equation (2) ([%Mn] × [%S] / (0.5 × [%Cu] + [%Sn]) ０．５ )) shall be 1.00 or less. The left side of equation (2) is preferably 0.900 or less, and more preferably 0.850 or less. The lower limit of the left side of equation (2) is not particularly limited, but the left side of equation (2) is preferably 0.0001 or more, and more preferably 0.0003 or more.

[0034] The remainder of the material consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that are inevitably introduced from raw materials, manufacturing processes, or manufacturing equipment, and are permitted to be included in a range that does not hinder the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, or scrap. Examples of impurities include O (oxygen) and H (hydrogen). The component composition may further include at least one element selected from the following group of elements as an optional element (selected element) in a predetermined amount. Note that if the following optional elements are included in amounts below a suitable lower limit, the optional elements may be included as unavoidable impurities.

[0035] Cr: 1.00% or less By adding Cr, the overall strength of the steel structure is adjusted to a high level of stability, reducing the number of void initiation points and obtaining superior ultimate deformability. To obtain this effect, the Cr content is preferably 0.005% or more, more preferably 0.008% or more, and even more preferably 0.010% or more. The Cr content is even more preferably 0.015% or more, and even more preferably 0.030% or more. On the other hand, when Cr is included, from the viewpoint of preventing cost increases, the Cr content is 1.00% or less. The Cr content is preferably 0.80% or less, and more preferably 0.50% or less.

[0036] Mo: 0.500% or less By adding Mo, the overall strength of the steel structure can be adjusted to a high level of stability, thereby reducing the number of void initiation points and obtaining superior ultimate deformability. To obtain this effect, it is preferable that the Mo content be 0.002% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, when Mo is included, from the viewpoint of preventing cost increases, the Mo content should be 0.500% or less. It is more preferable that the Mo content be 0.400% or less, and even more preferably 0.300% or less.

[0037] B: 0.0100% or less. B combines with N in the steel to form BN, which contributes to suppressing the precipitation of coarse nitrides such as AlN, which cause deterioration of the ultimate deformability. In addition, since BN tends to precipitate with MnS as a nucleus, it also contributes to suppressing the coarsening of MnS. To obtain this effect, when B is included, the B content is preferably 0.0001% or more. On the other hand, if the B content exceeds 0.0100%, B may excessively segregate at the ferrite grain boundaries, which may deteriorate the ultimate deformability. Therefore, when B is included, the B content should be 0.0100% or less.

[0038] Sb: 0.0500% or less. Sb is an element that contributes to suppressing decarburization that occurs on the surface of the steel sheet during manufacturing. Sb may be included as needed to suppress excessive coarsening of surface ferrite grains associated with decarburization. To obtain such an effect, the Sb content is preferably 0.0001% or more, and more preferably 0.0005% or more. On the other hand, excessive Sb content may degrade toughness. Therefore, when Sb is included, the Sb content is preferably 0.0500% or less, and preferably 0.0200% or less.

[0039] V: 0.100% or less. V is an element that contributes to increasing the strength of ferrite by forming fine carbides, and may be included as needed to obtain suitable ferrite strength. To obtain this effect, the V content is preferably 0.0005% or more, and more preferably 0.001% or more. On the other hand, if the V content exceeds 0.100%, excessive carbide formation may occur, which may cause deterioration of the ultimate deformability. Therefore, when V is included, the V content should be 0.100% or less, and preferably 0.060% or less.

[0040] Ti: 0.200% or less. Ti is an element that contributes to increasing the strength of ferrite by forming fine carbides, and may be included as needed to obtain suitable ferrite strength. To obtain this effect, the Ti content is preferably 0.001% or more, and more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, excessive carbide formation may occur, which may cause deterioration of the ultimate deformability. Therefore, when Ti is included, the Ti content should be 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less.

[0041] Nb: 0.200% or less. Nb is an element that contributes to increasing the strength of ferrite by forming fine carbides, and may be included as needed to obtain suitable ferrite strength. To obtain this effect, the Nb content is preferably 0.001% or more, and more preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.200%, excessive carbide formation may occur, which may cause deterioration of the ultimate deformability. Therefore, when Nb is included, the Nb content should be 0.200% or less, preferably 0.100% or less, and more preferably 0.050% or less.

[0042] Ta: 0.100% or less By adding Ta, the overall strength of the steel structure is adjusted to a high level of stability, which reduces the number of void initiation points and allows for superior ultimate deformability. To obtain this effect, it is preferable that the Ta content be 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more. On the other hand, when Ta is included, from the viewpoint of preventing cost increases, the Ta content should be 0.100% or less. It is preferable that the Ta content be 0.050% or less, and more preferably 0.020% or less.

[0043] W: 0.500% or less By adding W, the overall strength of the steel structure is adjusted to a high level of stability, which reduces the number of void initiation points and allows for superior ultimate deformability. To obtain this effect, it is preferable that the W content be 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more. On the other hand, when W is included, from the viewpoint of preventing cost increases, it is preferable that the W content be 0.500% or less, more preferably 0.450% or less, and more preferably 0.400% or less.

[0044] Zr: 0.020% or less. The addition of Zr improves the ultimate deformability of the steel sheet and enhances its elongation flange properties. To obtain this effect, it is preferable that the Zr content be 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more. On the other hand, when Zr is included, from the viewpoint of preventing cost increases, the Zr content should be 0.020% or less. It is preferable that the Zr content be 0.010% or less, and more preferably 0.0050% or less.

[0045] Ca: 0.0200% or less. By including Ca, the morphology of the sulfide can be controlled and the ultimate deformability can be improved. To obtain this effect, it is preferable that the Ca content be 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more. On the other hand, in order to obtain better ultimate deformability, if Ca is included, the Ca content should be 0.0200% or less. It is preferable that the Ca content be 0.0100% or less, more preferably 0.0050% or less. It is even more preferable that the Ca content be 0.0040% or less, and even more preferably 0.0030% or less.

[0046] Mg: 0.0200% or less By including 0.0001% or more of Mg, the morphology of the sulfide can be controlled and the ultimate deformability can be improved. The Mg content is preferably 0.0001%, more preferably 0.0005% or more, and even more preferably 0.001% or more. On the other hand, in order to obtain better ultimate deformability, if Mg is included, the Mg content should be 0.0200% or less. The Mg content is preferably 0.0100% or less, and more preferably 0.0050% or less.

[0047] Zn: 0.020% or less. Adding Zn can improve the ultimate deformability of the steel sheet. To obtain this effect, it is preferable that the Zn content be 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more. On the other hand, when Zn is included, from the viewpoint of preventing cost increases, the Zn content should be 0.020% or less. It is preferable that the Zn content be 0.010% or less, and more preferably 0.008% or less.

[0048] Co: 0.200% or less. Adding Co can improve the ultimate deformability of the steel sheet. To obtain this effect, it is preferable that the Co content be 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more. On the other hand, when Co is included, from the viewpoint of preventing cost increases, the Co content should be 0.200% or less. It is preferable that the Co content be 0.100% or less, and more preferably 0.080% or less.

[0049] The content of each of the following elements (excluding Ce): 0.0200% or less. By adding these elements, it is possible to improve the ultimate deformability of the steel sheet. To obtain this effect, it is preferable to include at least one of these elements in an amount of 0.0001% or more. On the other hand, from the viewpoint of preventing cost increases, when at least one of these elements is included, the content of each should be 0.0200% or less.

[0050] In this context, REM refers to the collective term for 16 elements: the 14 lanthanide elements from lanthanum (La, atomic number 57) to lutetium (Lu, atomic number 71), excluding Ce; scandium (Sc, atomic number 21); and yttrium (Y, atomic number 39). These 16 elements can be included individually or in combination. The REM content refers to the total content of these 16 elements. Among the REM elements, La is particularly preferred.

[0051] The following describes the preferred steel structure in the thin steel sheet of the present invention. In the following description, "%" referring to the area ratio of each constituent phase of the structure means "area %" unless otherwise specified. In the present invention, the area ratio of the constituent phases of the steel structure, the hardness of martensite, the hardness of ferrite, the equivalent circle diameter of MnS, and the precipitation density are determined by the method described in the examples.

[0052] Total area ratio of ferrite and bainite: 55-95% Ferrite and bainite are the main phases of the thin steel sheet of the present invention and have excellent workability. In order to obtain excellent ultimate deformability, the total area ratio of ferrite and bainite must be 55% or more, and preferably 65% ​​or more. On the other hand, in order to obtain the desired high strength, it is necessary to include a certain amount of martensite, which will be described later, as the second phase, so the total area ratio of ferrite and bainite is 95% or less, and preferably 90% or less. The area ratio of ferrite is preferably more than 0%, more preferably 30% or more, even more preferably 45% or more, and even more preferably 55% or more, in order to obtain excellent ultimate deformability. Also, the area ratio of ferrite is preferably 95% or less, and more preferably 90% or less. In order to obtain the desired high strength and excellent ultimate deformability, bainite may be included, and the area ratio of bainite is preferably 0.5% or more, more preferably 1% or more, and even more preferably 3% or more. Furthermore, the area ratio of bainite is preferably 50% or less, more preferably 25% or less, and even more preferably 15% or less.

[0053] Area ratio of martensite: 5-45% Martensite is the main second phase in the thin steel sheet of the present invention and has excellent strength. In order to obtain the desired high strength, the area ratio of martensite needs to be 5% or more, and preferably 10% or more. On the other hand, in order to obtain excellent ultimate deformability, the main phase needs to be ferrite, so the area ratio of martensite is 45% or less, preferably 40% or less, and more preferably 36% or less.

[0054] Total area ratio of pearlite and cementite: 0-5% Pearlite and cementite are formed when austenite decomposes during cooling from the annealing temperature. Therefore, if they are present in excess, the martensite necessary to obtain high strength in the final thin steel sheet will not be obtained. Accordingly, the total area ratio of pearlite and cementite is 5% or less, preferably 3% or less. On the other hand, in the present invention, there are no particular adverse effects from reducing the total area ratio of pearlite and cementite, and no lower limit is set, and it may be 0%.

[0055] Total area ratio of pearlite and cementite: Less than or equal to 1 / 3 of the area ratio of martensite. As described above, pearlite and cementite are formed when austenite decomposes during cooling from the annealing temperature. In order to obtain high strength in the final thin steel sheet, it is necessary to obtain a sufficient amount of high-strength martensite, and therefore it is necessary to prevent the decomposition of austenite. Accordingly, the total area ratio of pearlite and cementite is less than or equal to 1 / 3 of the area ratio of martensite, preferably less than or equal to 1 / 5. The lower limit may be set to 0%.

[0056] Vickers hardness of martensite: 4.00 times or less than the Vickers hardness of ferrite In the thin steel sheet of the present invention, the main phase is soft ferrite and the second phase is hard martensite. When the thin steel sheet is processed, strain concentrates particularly on the softer ferrite side at the interface between these phases. Voids are formed near the interface where strain is concentrated, becoming the starting point for cracks and degrading the ultimate deformability. The greater the difference in hardness between ferrite and martensite, the more likely strain is to concentrate; therefore, the hardness of martensite should not be excessively high compared to ferrite. Accordingly, when the area ratio of ferrite is greater than 0%, the hardness of martensite (Vickers hardness) is 4.00 times or less than the hardness of ferrite (Vickers hardness), preferably 3.50 times or less, and more preferably 3.00 times or less. When the area ratio of ferrite is greater than 0%, the hardness (Vickers hardness) of martensite is preferably 1.00 times or more than the hardness (Vickers hardness) of ferrite, and more preferably 1.10 times or more. In order to reduce the hardness of martensite, it is effective to reduce the carbon (C) content and manganese (Mn) content.

[0057] Precipitation density of MnS with an equivalent circle diameter of 5 μm or more: 100 particles / mm ２ As described above, in order to obtain excellent ultimate deformability, it is necessary to suppress the formation of coarse MnS. In particular, MnS with an equivalent circle diameter of 5 μm or more has a major adverse effect, so in order to obtain excellent ultimate deformability, the precipitation density of MnS should be 100 particles / mm ２ The following is required: The precipitation density should be 95 particles / mm³. ２ Preferably, the number is 90 pieces / mm ２ The following is more preferable. On the other hand, the lower limit of the precipitation density is not particularly limited, but the precipitation density is 0.01 particles / mm ２ It may be greater than or equal to 0.1 pieces / mm ２ That's fine too.

[0058] The steel structure necessary to obtain the effects of the present invention is as described above, but other phases may be included as long as they do not impair the effects of the present invention. Other structures (remaining structures) may include, for example, retained austenite. From the viewpoint of obtaining the predetermined effects more advantageously, the area ratio of other structures is preferably 10% or less, and more preferably 5% or less. The lower limit of the area ratio of other structures is not particularly limited and may be 0%.

[0059] The thin steel sheet in the present invention may have a plating layer on one or both of its surfaces. Examples of plating layers include zinc plating layers such as a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electroplated zinc layer. Examples of plating layers other than zinc plating layers include aluminum plating layers such as a hot-dip aluminum plating layer. Among these, a hot-dip galvanized layer is preferred.

[0060] The thickness of the thin steel sheet of the present invention is not particularly limited, but if the thin steel sheet of the present invention is a hot-rolled steel sheet, the thickness is preferably 2.3 mm or more and 30.0 mm or less. If the thin steel sheet of the present invention is a cold-rolled steel sheet, the thickness is preferably 0.30 mm or more and less than 2.3 mm.

[0061] Next, a method for manufacturing a thin steel sheet according to one embodiment of the present disclosure will be described. The method for manufacturing a thin steel sheet according to the present invention involves hot rolling a steel slab having the above-described component composition at a heating temperature of 1050 to 1350°C and a finish rolling completion temperature of 850 to 1000°C, then winding it at a temperature of 400 to 700°C to form a steel sheet, pickling the obtained steel sheet, further cold rolling it, and heating it at an annealing temperature of 700 to 900°C and a holding time at the annealing temperature of 1 to 500 s, with an average cooling rate V Ｃ The thin steel sheet of the present invention can be obtained by performing cooling under the conditions that satisfy equation (3), the temperature is 120°C / s or less, and the cooling stop temperature is 550°C or less. Ｃ ×(1+1.5×[%Cu]+5×[%Ni]+2×[%Sn] ０．５) / (1 + 0.02 × [%Mn]) ... Equation (3) In Equation (3), [%M] is the content (mass%) of element M in the steel slab. Alternatively, the manufacturing method of the thin steel sheet of the present invention involves heating a steel slab having the above-mentioned component composition to a heating temperature of 1050 to 1350°C, and finishing rolling completion temperature T ＦＤＴ The thin steel sheet of the present invention can also be obtained by hot rolling under conditions where the temperature is 1000°C or less and equation (4) is satisfied, followed by winding at a temperature of 400 to 700°C. ＦＤＴ +350×[%C]+75×([%Mn]-1)+30×[%Cu]+100×[%Ni]+40×[%Sn] ０．５ ...Equation (4) In Equation (4), T ＦＤＴ is the finish rolling completion temperature (°C), and [%M] is the element M content (mass%) of the steel slab. Alternatively, the steel sheet after winding is pickled and then cold-rolled, and heated under the conditions of annealing temperature: 700-900°C, holding time at annealing temperature: 1-500 s, and average cooling rate V Ｃ The thin steel sheet of the present invention can also be obtained by cooling under conditions that satisfy formula (3), have a temperature of 120°C / s or less, and have a cooling stop temperature of 550°C or less. In addition, the temperature when heating or cooling the steel slab (steel material) or steel sheet shown below refers to the surface temperature of the steel slab (steel material), steel sheet, etc., unless otherwise specified.

[0062] The method for manufacturing steel slabs having the above-mentioned component composition is not particularly limited. Any known melting method, such as an electric furnace or a converter, can be used, and secondary refining may be performed in a vacuum degassing furnace. Subsequently, from the viewpoint of productivity and quality stability, it is preferable to manufacture the steel slabs (steel material) by a continuous casting method, but the steel slabs may also be manufactured by known casting methods such as ingot-parting rolling. Since the thin steel sheet of the present invention has Cu, Ni, and Sn as essential elements, it is possible to manufacture it without going through the iron ore reduction process by melting iron scrap containing these three elements in an electric furnace, which is also beneficial from the viewpoint of carbon neutrality and resource recycling.

[0063] Hot rolling process slab heating temperature: 1050 to 1350°C. If the slab heating temperature is below 1050°C, coarse MnS and Mn segregation precipitated in the slab will remain in the final thin steel sheet, preventing the acquisition of excellent ultimate deformability. Therefore, the slab heating temperature is 1050°C or higher, preferably 1080°C or higher, and more preferably 1100°C or higher. On the other hand, if the slab heating temperature is excessively high, excessive surface oxidation occurs, resulting in a large amount of coarse granular oxides forming inside the surface layer of the steel sheet, which may degrade the ultimate deformability of the final thin steel sheet. Therefore, the slab heating temperature is 1350°C or lower, preferably 1300°C or lower, and more preferably 1250°C or lower. The temperature history from the completion of continuous casting of the slab to the slab heating in hot rolling is not particularly limited. The slab may be cooled to room temperature and then reheated to the above temperature range before hot rolling, or the slab may be cast and then reheated to the above temperature range while still hot without cooling to room temperature before hot rolling.

[0064] Finish rolling completion temperature: 850 to 1000°C, or satisfying equation (4) and 1000°C or less 850 ≤ T ＦＤＴ +350×[%C]+75×([%Mn]-1)+30×[%Cu]+100×[%Ni]+40×[%Sn] ０．５ ...Equation (4) In Equation (4), T ＦＤＴ is the finish rolling completion temperature (°C), and [%M] is the element M content (mass%) of the steel slab. In order to improve the ultimate deformability of the thin steel sheet that is finally obtained, it is important to homogenize the structure within the steel sheet during hot rolling, and this must be done in the austenite single-phase region. Therefore, the finish rolling completion temperature is preferably 850°C or higher. More preferably, the finish rolling completion temperature is 870°C or higher. In this regard, in the present invention, the steel component composition contains a large amount of austenite-stabilizing elements C, Mn, Cu, Ni and a strongly segregating element Sn, which suppresses the precipitation of ferrite during cooling, so that finish rolling can be performed at an even lower temperature. Therefore, in the finish rolling of the present invention, the C, Mn, Ni, and Sn contained in the steel component composition and the finish rolling completion temperature T ＦＤＴ The equation (4) relating to T is satisfied.ＦＤＴ +350×[%C]+75×([%Mn]-1)+30×[%Cu]+100×[%Ni]+40×[%Sn] ０．５ If the value is less than 850, ferrite will precipitate before the end of the finishing roll, resulting in a non-uniform structure and making it impossible to obtain the desired ultimate deformability. Therefore, T ＦＤＴ +350×[%C]+75×([%Mn]-1)+30×[%Cu]+100×[%Ni]+40×[%Sn] ０．５ The temperature is 850 or higher, preferably 870 or higher. On the other hand, if the finish rolling completion temperature exceeds 1000°C, the austenite grains become coarser during hot rolling, which can lead to the formation of coarse MnS that negatively affects the ultimate deformability. Therefore, the finish rolling completion temperature is 1000°C or lower, preferably 950°C or lower.

[0065] Winding temperature: 400 to 700°C. If the winding temperature is below 400°C, the ferrite grains may not grow uniformly, which can adversely affect the ultimate deformability of the final thin steel sheet. Furthermore, winding temperatures below 400°C also impair industrial manufacturability. Therefore, the winding temperature should be 400°C or higher, preferably 450°C or higher. On the other hand, if the winding temperature exceeds 700°C, the ferrite and pearlite grains become coarse, resulting in a heterogeneous steel structure, and MnS grains tend to coarseen, which adversely affects the ultimate deformability of the final thin steel sheet. Therefore, the winding temperature should be 700°C or lower, preferably 680°C or lower.

[0066] Cold Rolling Process Pickling and Cold Rolling In the method for manufacturing thin steel sheets of the present invention, the steel sheet (hot-rolled steel sheet) obtained by the above method may be pickled. Furthermore, cold rolling and annealing may be performed. Pickling is performed for the purpose of removing the mill scale that has formed on the surface of the steel sheet during hot rolling. The method and conditions of pickling are not particularly limited, as long as the surface of the steel sheet before cold rolling is uniformly scale-free. The conditions of cold rolling are also not particularly limited, as long as a steel sheet of the desired thickness can be obtained, but it is usually performed so that the cold rolling rate (cumulative rolling rate) is 95% or less. In addition, in order to stably obtain excellent ultimate deformability, it is preferable to promote the recrystallization of ferrite during annealing and not leave an excess of unrecrystallized ferrite, so a suitable cold rolling rate is 20% or more.

[0067] Annealing process annealing temperature: 700 to 900°C. If the annealing temperature exceeds 900°C, coarse MnS is likely to form, which may degrade the ultimate deformability. Also, if the annealing temperature exceeds 900°C, the microstructure of the present invention may not be obtained. Therefore, the annealing temperature is preferably 900°C or lower, and more preferably 880°C or lower. On the other hand, if the annealing temperature is below 700°C, there is a risk that an excess of unrecrystallized ferrite will remain, adversely affecting the ultimate deformability. Therefore, the annealing temperature is preferably 700°C or higher, and more preferably 720°C or higher. Note that the annealing temperature may be kept constant, or it may be varied within the above range.

[0068] Holding time: 1 to 500 s. The holding time is the time the material remains within the above annealing temperature range. If the holding time is too short, the recrystallization of ferrite may not proceed sufficiently, and excellent ultimate deformability may not be obtained. Therefore, the holding time is preferably 1 s or more, and more preferably 5 s or more. The holding time is even more preferably 7 s or more, and even more preferably 15 s or more. On the other hand, if the holding time is too long, the austenite becomes coarser and the amount of coarse MnS increases, and excellent ultimate deformability cannot be obtained. Therefore, the holding time is preferably 500 s or less, and more preferably 400 s or less. The holding time is even more preferably 300 s or less, and even more preferably 120 s or less.

[0069] Cooling stop temperature: In steel sheets cooled and annealed under conditions of 550°C or lower, ferrite growth occurs in the temperature range of 700°C or higher, and pearlite is formed in the temperature range of 550°C or higher, both of which reduce the amount of martensite in the final thin steel sheet. Thus, in order to obtain a suitable steel structure in the present invention, the average cooling rate V in the temperature range of 550°C or higher is Ｃ It is preferable to perform controlled cooling by limiting the range. Therefore, it is preferable that the cooling stop temperature be 550°C or lower. The lower limit of the cooling stop temperature is not particularly limited, but it is preferable that the cooling stop temperature be 300°C or higher.

[0070] Average cooling rate V Ｃ: Satisfies equation (3) and is 120°C / s or less Equation (3): 6.0 ≤ V Ｃ ×(1+1.5×[%Cu]+5×[%Ni]+2×[%Sn] ０．５ ) / (1 + 0.02 × [%Mn]) In equation (3), [%M] is the content (mass%) of element M in the steel slab. In the steel plate of the present invention, the average cooling rate V from the annealing temperature to the cooling stop temperature Ｃ The material must satisfy equation (3). If the average cooling rate is too low, austenite decomposes into ferrite and pearlite, generating pearlite and cementite and reducing martensite, which may prevent the acquisition of the high strength desired in this invention. In addition, if the average cooling rate is too low, as ferrite grows, Mn becomes concentrated in the austenite, increasing the Vickers hardness of martensite and widening the difference in Vickers hardness between ferrite and martensite, which may prevent the acquisition of the desired high ultimate deformability. Furthermore, the coarsening of MnS may degrade the ultimate deformability. Therefore, it is preferable to set a lower limit on the average cooling rate, and the three elements Cu, Ni, and Sn contribute to suppressing the growth of pearlite and cementite. In particular, Sn segregates at the interface between ferrite and carbide, significantly suppressing the growth of carbide, so even a very small amount contributes greatly. Also, since Sn segregates at the interface between ferrite and MnS, it also suppresses the coarsening of MnS. On the other hand, since Mn is concentrated in austenite and widens the difference in Vickers hardness between martensite and ferrite, if it is present in excess, the average cooling rate needs to be increased. To obtain excellent ultimate deformability and high strength, the contributions of the three elements Cu, Ni, and Sn, as well as Mn, should be considered, and the average cooling rate V Ｃ It is preferable that equation (3) is satisfied. V Ｃ ×(1+1.5×[%Cu]+5×[%Ni]+2×[%Sn] ０．５ If ) / (1 + 0.02 × [%Mn]) is less than 6.0, pearlite may form from austenite during cooling, making it impossible to obtain the desired steel structure. Therefore, V Ｃ ×(1+1.5×[%Cu]+5×[%Ni]+2×[%Sn] ０．５ It is preferable that ) / (1 + 0.02 × [%Mn]) be 6.0 or greater.Ｃ ×(1+1.5×[%Cu]+5×[%Ni]+2×[%Sn] ０．５ ) / (1 + 0.02 × [%Mn]) is more preferably 8.0 or higher, and even more preferably 10.0 or higher. On the other hand, the average cooling rate V Ｃ If the average cooling rate V is excessive, residual stress will be generated in the steel plate due to rapid volume contraction, causing the steel plate to bend and degrade in shape, which may reduce its ultimate deformability during processing. Ｃ The average cooling rate V is preferably 120°C / s or less, and more preferably 100°C / s or less. Ｃ The average cooling rate V is more preferably 70°C / s or less, and even more preferably 50°C / s or less. Ｃ (°C / s) is defined as "(cooling start temperature (annealing temperature: 700-900°C) - cooling stop temperature (below 550°C)) / cooling time from cooling start to cooling stop (s)".

[0071] The equipment used for annealing is not particularly limited and can be used in continuous annealing lines (CAL), hot-dip galvanizing lines (CGL), or batch annealing furnaces (BAF), etc.

[0072] Plating Process After annealing and cooling under the above conditions, plating may be performed as needed to form a plating layer on at least one surface of the steel sheet. Alternatively, after hot rolling, the steel sheet may be coiled and pickled to remove the mill scale, and then, without cold rolling, the aforementioned annealing, cooling, and plating processes may be performed to form a plating layer on at least one surface of the steel sheet. That is, after hot rolling, the steel sheet that has been coiled and pickled is heated under the conditions of annealing temperature: 700 to 900°C, holding time at annealing temperature: 1 to 500 s, and average cooling rate V Ｃ The following conditions may be met: the temperature satisfies formula (3), the temperature is 120°C / s or less, and the cooling is stopped at a temperature of 550°C or less. The surface of the steel plate may then be further plated with hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, or aluminum plating.

[0073] The type of plating treatment is not particularly limited. Examples of plating treatments include zinc plating treatments such as hot-dip galvanizing, alloyed hot-dip galvanizing (hot-dip galvanizing and alloying), electro-galvanizing, and vapor deposition galvanizing. Examples of plating treatments other than zinc plating include aluminum plating treatments such as hot-dip aluminum plating. Among these, hot-dip galvanizing is preferred.

[0074] The plating conditions are not particularly limited and can be those of a standard method. For example, in the case of hot-dip galvanizing, the hot-dip galvanizing bath consists of Zn, Al, and unavoidable impurities. In one example, the Al concentration in the hot-dip galvanizing bath (hereinafter also referred to as the Al concentration in the bath) may be 0.05% by mass or more, or 0.220% by mass or less. If the Al concentration in the bath is 0.05% by mass or more, the occurrence of bottom dross can be more effectively prevented. Also, if the Al concentration in the bath is 0.220% by mass or less, the occurrence of top dross can be more effectively prevented. From a cost perspective, it is preferable to set the Al concentration in the bath to 0.220% by mass or less. The temperature of the plating bath (hereinafter also referred to as the plating bath temperature) is not particularly limited. In one example, the plating bath temperature may be 440°C or more, or 500°C or less. The amount of plating deposited on one side is also not particularly limited. In one example, the amount of plating deposited on one side is 20 g / m². ２ This may be the case, preferably 25 g / m² ２ The above is possible, and 120 g / m ２ It may be less than or equal to 80 g / m², preferably 80 g / m². ２ The following is possible: Plating adhesion amount per side is 20 g / m². ２ If the above conditions are met, corrosion resistance will be particularly good, and the amount of plating adhesion will be particularly easy to control. Also, the amount of plating adhesion per side will be 120 g / m². ２ The following conditions result in particularly good plating adhesion. The method for adjusting the amount of plating adhesion is not particularly limited. For example, gas wiping can be used, and the amount of plating adhesion can be adjusted by the gas pressure and the distance between the wiping nozzle and the steel plate.

[0075] When performing hot-dip galvanizing, an alloying treatment may be carried out subsequently to form an alloyed hot-dip galvanized layer. The conditions for the alloying treatment are not particularly limited and can be followed according to conventional methods. For example, the alloying treatment may be performed with an alloying temperature of 440°C to 600°C and an alloying time of 5 s to 60 s. The alloying time is the holding time at the alloying temperature. Furthermore, when performing the alloying treatment, it is preferable that the Fe content in the plating layer (hereinafter also referred to as the degree of alloying) be 7% by mass or more and 15% by mass or less.

[0076] In the case of electroplating, for example, if the divalent zinc ion concentration of the electroplating solution is 80 g / L, and the current density is 10 to 80 A / dm², ２ Adjust the electrolysis time within the specified range. This will result in a plating deposition rate of 20 g / m² per side. ２ 80g / m or more ２ The following control is possible. Divalent zinc ions in the electroplating solution can be added, for example, as sulfates. Furthermore, the processing conditions for vapor deposition zinc plating are not particularly limited and can be followed according to conventional methods.

[0077] In the case of molten aluminum plating, for example, the rolled steel sheet is immersed in an aluminum plating bath at 660 to 730°C. Then, the amount of plating is adjusted by gas wiping or the like. The amount of plating is 20 g / m² per side. ２ 120g / m or more ２ The following is preferable. The molten aluminum plating bath is not particularly limited as long as it has the composition of the molten aluminum plating layer described above, and can be prepared according to conventional methods.

[0078] Temper rolling may be performed on steel sheets before or after plating, as needed, and the conditions for this rolling can be set as appropriate.

[0079] Other than the conditions mentioned above, there are no specific limitations; you may follow the usual law.

[0080] Next, the component and method for manufacturing the component of the present invention will be described. The component of the present invention is obtained by subjecting the thin steel sheet of the present invention to at least one of forming and joining processes. The method for manufacturing the component of the present invention includes the step of subjecting the thin steel sheet of the present invention to at least one of forming and joining processes to form the component. The component of the present invention has the same component composition as the thin steel sheet of the present invention, and has the same structure and properties as the thin steel sheet of the present invention described above, except for the processing-affected zone after processing. Furthermore, when the component of the present invention is welded in the joining process, it has the same structure and properties as the thin steel sheet of the present invention described above, except for the heat-affected zone.

[0081] The thin steel sheet of the present invention has high strength and excellent formability. Therefore, components obtained using the thin steel sheet of the present invention also have high strength and excellent formability, at least except for the processing-affected zone and heat-affected zone after processing. Furthermore, weight reduction is possible when using components of the present invention. Accordingly, components of the present invention can be suitably used, for example, in vehicle body frame components.

[0082] Forming processes can utilize general processing methods such as press working without restriction. Joining processes can utilize general welding methods such as spot welding and arc welding, as well as riveting and crimping without restriction.

[0083] The present invention will be described in detail below based on examples. The technical scope of the present invention is not limited to the following examples.

[0084] For steel slabs with each component composition shown in Table 1 (the balance being Fe and inevitable impurities) obtained by casting molten steel produced using an electric furnace, hot rolling, cold rolling, annealing, and plating were performed under the conditions shown in Table 2 to obtain thin steel sheets. In this example, the thickness of the steel sheet after hot rolling was set to 2.3 to 3.2 mm, and the thickness after cold rolling was set to 1.2 to 1.4 mm. Note that cold rolling was performed after removing the scale generated on the steel sheet surface during hot rolling by pickling. In Table 2, as the material classification, cold-rolled steel sheets that were subjected to cold rolling and annealing and then not plated are indicated as CR, cold-rolled steel sheets that were subjected to hot-dip galvanizing are indicated as GI, cold-rolled steel sheets that were subjected to hot-dip galvanizing and alloying treatment are indicated as GA, furthermore, cold-rolled steel sheets that were subjected to electro-galvanizing are indicated as EG, and cold-rolled steel sheets that were subjected to hot-dip aluminum plating are indicated as Al. Also, thin steel sheets (hot-rolled steel sheets) that were hot-rolled and then not subjected to cold rolling, annealing, or plating are indicated as HOT. Also, thin steel sheets (hot-rolled steel sheets) that were hot-rolled and then annealed and then subjected to hot-dip galvanizing and alloying treatment are indicated as HOT-GA. Note that conditions not specifically specified were in accordance with conventional methods. [Hot-dip galvanizing treatment (GI)] - Plating bath composition: Zn, Al, and inevitable impurities (Al concentration in the bath: 0.10 to 0.20% by mass, balance: Zn and inevitable impurities) - Plating bath temperature: 460 °C - Plating adhesion amount per side: 30 to 60 g / m ２ [Alloying hot-dip galvanizing treatment (GA) (the hot-dip galvanizing treatment is the same as above)] - Alloying temperature: 450 to 560 °C - Alloying degree: 8.0 to 14.0% by mass [Electro-galvanizing treatment (EG)] - Concentration of divalent zinc ions in the plating solution: 80 g / L (the divalent zinc ions were added as sulfate.) - pH of the plating solution: 2.0 (the pH of the plating solution was adjusted with sulfuric acid.) - Temperature of the plating solution: 55 °C - Current density: 50 A / dm ２ - Electrolysis time: 30 s - Plating adhesion amount per side: 50 g / m ２ [Hot-dip aluminum plating treatment (Al)] - Plating bath composition: Al and inevitable impurities - Plating bath temperature: 700 °C - Plating adhesion amount per side: 60 g / m ２

[0085] <Microstructure Observation> Test specimens for microstructure observation were taken from the obtained thin steel sheets (steel plates). After polishing the thickness cross section parallel to the rolling direction, the microstructure was revealed by etching with 3% by mass Nital solution. Using an SEM (Scanning Electron Microscope), five fields of view were observed at a magnification of 1500x at a position 1 / 4 of the plate thickness from either surface. From the obtained microstructure images, the area ratios of ferrite and martensite, as well as the total area ratios of pearlite and cementite, were calculated using the point counting method. Specifically, in a region of actual length: 82 μm × 57 μm in each SEM image, 16 × 15 grid points were placed at intervals of 4.8 μm. Then, the number of grid points on ferrite, bainite, martensite, pearlite, and cementite, as shown below, were counted. Next, the area percentages of ferrite, bainite, and martensite, as well as the total area percentages of pearlite and cementite, were calculated by dividing the number of lattice points on ferrite, bainite, martensite, and pearlite and cementite by the total number of lattice points and multiplying by 100. Ferrite: This is a region that appears black, and its morphology can be film-like or massive. Ferrite is a structure consisting of crystal grains with a BCC lattice. Bainite: This is a region that appears black, and its morphology can be plate-like or lath-like. Cementite, which will be described later, may be present between the plates or laths. It precipitates from austenite grain boundaries, etc., when cooled from the annealing temperature to below 550°C. Martensite: This is a region that appears white to light gray, and its morphology can be massive or amorphous. Depending on the corrosion conditions, a lath-like substructure may be visible within the martensite grains. Martensite includes both so-called fresh martensite (as quenched) and so-called tempered martensite (fresh martensite that has been tempered). Cementite: This region exhibits a bright white color and is granular or linear in form. Its particle size is smaller than that of ferrite and martensite, often less than 1 μm. Perlite: This region consists of layers of cementite and ferrite. It exists as a massive region adjacent to ferrite and martensite.Remaining Austenite, etc.: Regions not classified above were evaluated as remaining austenite. In particular, the area fraction of retained austenite is determined as follows: A test piece taken from a steel plate is ground and polished in the thickness direction so that the measurement surface is at a position corresponding to 1 / 4 of the plate thickness in the thickness depth direction from the surface of the steel plate. The amount of retained austenite is determined by analyzing the measurement surface by X-ray diffraction. The ratio of the peak intensities of the {111}, {200}, and {211} planes of ferrite is determined, and the amount of retained austenite is calculated from the average value of these ratios. In this method, the volume fraction of retained austenite is determined, and this value is taken as the area fraction of retained austenite.

[0086] Furthermore, after polishing the cross-section of the plate thickness perpendicular to the rolling direction, a scanning electron microscope (SEM) was used to observe three fields of view each from one of the surfaces: a region of 1 / 8 thickness, a region of 1 / 4 thickness, and a region of 1 / 2 thickness, at a magnification of 1000x with a field of view of 1.25 mm × 1.00 mm. The composition of the inclusions observed in each field of view was confirmed by energy-dispersive X-ray analysis (EDS) to identify MnS. The precipitation density of MnS was determined by allocating the total number of MnS particles with an equivalent circle diameter of 5 μm or more across the total observed area. The equivalent circle diameter (μm) refers to the diameter of a perfect circle containing the area of ​​one MnS particle, and the area X (μm) of one MnS particle is calculated as follows: ２ ) for 2 × (X / π) １／２ This is how it is determined. Furthermore, for a single MnS molecule, in the microscope image, a region where the outer edge is surrounded by something other than carbides and which is formed as a single, uninterrupted entity is measured as one molecule.

[0087] <Hardness Test> Using the specimens described above for microstructure observation, ferrite and martensite were identified near a point approximately 1 / 4 of the plate thickness from one of the surfaces. For each identified microstructure, a micro-Vickers hardness tester was used to measure the Vickers hardness (HV) at 10 points under a load of 10 gf. The average value (sum of 10 data points (HV) / 10) was used to evaluate the hardness. The Vickers hardness measurement points were chosen to be the positions furthest from grain boundaries or phase interfaces within each microstructure.

[0088] <Tensile Test> From each obtained steel plate, a JIS No. 5 test specimen (JIS Z2201) was taken in a direction parallel to the rolling direction, and the strain rate was set to 10 －３ A tensile test was conducted in accordance with the provisions of JIS Z2241 (2011) with a value of / s, and the tensile strength (TS) was determined.

[0089] <Ultimate Deformability> The ultimate deformability (ε) was determined by measuring the plate thickness (mm) before the tensile test and the plate thickness (mm) at the fracture point after the test, and using the following formula. The plate thickness at the fracture point was measured by observing a cross-section parallel to the tensile direction at the center of the width of the test specimen using an optical microscope. Thin steel plates with an ultimate deformability ε of 0.55 or higher were considered to have excellent ultimate deformability. ε = -ln(t / t) ０ ) ... Equation (5) In Equation (5), t ０ : is the thickness (mm) of the thin steel plate before the tensile test, and t: is the thickness (mm) of the fractured portion after the tensile test in accordance with JIS Z 2241 (2011).

[0090] As shown in Table 3, the thin steel sheets of the present invention all exhibit superior ultimate deformability and high strength compared to the thin steel sheets of the comparative examples.

[0091] Furthermore, it was found that, in members obtained by forming and joining using the thin steel sheet of the present invention, the parts other than the processing-affected zones and heat-affected zones are also high in strength and have excellent ultimate deformability, similar to the steel sheet of the present invention, because the thin steel sheet of the present invention has high strength and excellent ultimate deformability.

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[0097] According to the present invention, it is possible to obtain a thin steel sheet that is particularly suitable for automotive parts applications, possessing high strength and excellent extreme deformability.

Claims

1. A composition comprising, by mass%, C: 0.010 to 0.200%, Si: 2.00% or less, Mn: 0.80 to 3.00%, P: 0.100% or less, S: 0.0200% or less, Al: 0.10% or less, N: 0.0200% or less, Cu: 0.010 to 0.500%, Sn: 0.0010 to 0.0500%, Ni: 0.010 to 0.500%, satisfying the following formulas (1) and (2), with the remainder being Fe and unavoidable impurities, and having, by area percentage, a total of ferrite and bainite: 55 to 95%, martensite: 5 to 45%, and a total of pearlite and cementite: 0 to 5%, Furthermore, the steel structure has a total area ratio of pearlite and cementite that is 1 / 3 or less of the area ratio of martensite, and if the area ratio of ferrite is greater than 0%, the Vickers hardness of martensite is 4.00 times or less of the Vickers hardness of ferrite, and the precipitation density of MnS with an equivalent circle diameter of 5 μm or more is 100 particles / mm ２ A thin steel sheet having a tensile strength of 590 MPa or more and less than 980 MPa, as follows: [%Cu] + 15 × [%Sn] - 0.15 × [%Mn] ０．８ -0.4 × [%Si] ０．４ -8×[%Ni]≦0.00 ...Formula (1) [%Mn]×[%S] / (0.5×[%Cu]+[%Sn] ０．５ ) ≤ 1.00 ...Equation (2) In Equations (1) and (2), [%M] is the mass %) content of element M in the thin steel sheet.

2. In the above component composition, further, in mass%, Cr: 1.00% or less, Mo: 0.500% or less, B: 0.0100% or less, Sb: 0.0500% or less, V: 0.100% or less, Ti: 0.200% or less, Nb: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Zr: 0.020% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Zn: 0.020% or less, Co: 0.200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less The thin steel sheet according to claim 1, 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 (excluding Ce): 0.0200% or less.

3. The thin steel sheet according to claim 1 or 2, wherein one or both surfaces of the thin steel sheet have a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electro-galvanized layer, or an aluminum plated layer as a plating layer.

4. A member made using a thin steel plate as described in any one of claims 1 to 3.

5. A method for manufacturing a thin steel sheet according to claim 1 or 2, comprising: hot rolling a steel slab having the above-mentioned component composition at a heating temperature of 1050 to 1350°C, with a finish rolling completion temperature of 1000°C or less, and satisfying formula (4), and then winding it at a temperature of 400 to 700°C to produce a steel sheet. 850 ≤ T ＦＤＴ +350×[%C]+75×([%Mn]-1)+30×[%Cu]+100×[%Ni]+40×[%Sn] ０．５ ...Equation (4) In Equation (4), T ＦＤＴ is the finishing rolling completion temperature (°C), and [%M] is the element M content (mass%) of the steel slab.

6. Pickle the steel sheet after coiling, further subject it to cold rolling, heat it under the conditions of annealing temperature: 700 to 900 °C, holding time at the annealing temperature: 1 to 500 s, and average cooling rate V Ｃ satisfies formula (3) and is 120 °C / s or less, and perform cooling under the condition of cooling stop temperature: 550 °C or less. The method for manufacturing a thin steel sheet according to claim 5. 6.0 ≤ V Ｃ × (1 + 1.5 × [%Cu] + 5 × [%Ni] + 2 × [%Sn] ０．５ ) / (1 + 0.02 × [%Mn])... formula (3) In formula (3), [%M] is the content (mass%) of element M in the steel slab.

7. A method for manufacturing a thin steel sheet according to claim 1 or 2, comprising: hot rolling a steel slab having the above-mentioned component composition at a heating temperature of 1050 to 1350°C and a finish rolling completion temperature of 850 to 1000°C, then winding it at a temperature of 400 to 700°C to form a steel sheet; pickling the obtained steel sheet, further cold rolling it, heating it at an annealing temperature of 700 to 900°C and holding time at the annealing temperature of 1 to 500 s, and an average cooling rate V Ｃ A method for manufacturing thin steel sheets, wherein the temperature satisfies equation (3), is 120°C / s or less, and cooling is performed under conditions of a cooling stop temperature of 550°C or less. 6.0 ≤ V Ｃ ×(1+1.5×[%Cu]+5×[%Ni]+2×[%Sn] ０．５ ) / (1 + 0.02 × [%Mn]) ... Equation (3) In Equation (3), [%M] is the mass %) content of element M in the steel slab.

8. After hot rolling, the steel sheet that has been coiled and pickled is heated under the conditions of annealing temperature: 700 to 900°C, holding time at annealing temperature: 1 to 500 s, and the average cooling rate V Ｃ A method for manufacturing a thin steel sheet according to claim 5, wherein the following conditions are met: the temperature satisfies formula (3) and is 120°C / s or less, and the cooling is performed under conditions where the cooling stop temperature is 550°C or less, and the surface of the steel sheet is further plated with hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, or aluminum plating. 6.0 ≤ V Ｃ ×(1+1.5×[%Cu]+5×[%Ni]+2×[%Sn] ０．５ ) / (1 + 0.02 × [%Mn]) ... Equation (3) In Equation (3), [%M] is the mass %) content of element M in the steel slab.

9. The method for manufacturing a thin steel sheet according to claim 6 or 7, wherein the surface of the cooled thin steel sheet is further plated with hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, or aluminum plating.

10. A method for manufacturing a component, comprising the step of forming and joining a thin steel sheet according to any one of claims 1 to 3 to form a component.

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