Thin steel sheet, member, and production methods thereof

Abstract

The present invention provides: a thin steel sheet which has excellent formability; a member; and production methods thereof.　Provided is a thin steel sheet which has a component composition containing specific amounts of C, Si, Mn, P, S, Al, N, Cu, Ni and Sn, and satisfying 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, 60-98% of ferrite and 2-35% of pearlite, and having an average crystal grain diameter of ferrite of 5 µm to 35 µm inclusive and an average crystal grain diameter of pearlite of 15 µm or less, wherein the precipitation density of MnS having an equivalent circle diameter of 5 µm or more is 100 per mm2 or less. Formula (1): 0.10 × (%Cu) + 1.5 × (%Sn) ≤ (%Ni) ≤ 5.0 × (%Cu) + 75 × (%Sn) Formula (2): (%Mn) × (%S) / (0.5 × (%Cu) + (%Sn)0.5) ≤ 1.00

Description

Thin steel sheets, components, and methods for manufacturing them.

[0001] This invention relates to thin steel sheets, components, and methods for manufacturing them.

[0002] A global awareness of the crisis caused by greenhouse gases and climate change is increasing, ２ The demand for reducing emissions has become even stronger in recent years. In the automotive sector, CO2 emissions are being reduced through improved fuel efficiency achieved by making vehicles lighter. ２ Efforts to reduce emissions are being actively pursued. As a means of reducing vehicle weight, in addition to making the thin steel sheets used in parts thinner by increasing their strength, it is also effective to increase the freedom of part shapes by improving the formability of the thin steel sheets. Basic formability required for thin steel sheets for automobiles includes bulge formability and shape freezing properties, but in order to withstand the forming of parts with more complex shapes, it is important to improve bend formability and stretch flange formability.

[0003] Furthermore, in the most recent case, CO2 in the manufacturing process of thin steel sheets ２ As demands for reducing emissions become stricter, utilizing electric furnaces can reduce CO2 emissions. ２ A steelmaking process that reduces emissions is attracting attention. In steelmaking processes using electric furnaces, iron scrap is used as the main raw material, so compared to processes using blast furnaces that reduce iron ore with coke, CO2 emissions are lower. ２ It is possible to reduce emissions. Furthermore, the steelmaking process using electric furnaces contributes to global environmental protection in terms of resource reuse. However, the iron scrap that is the main raw material for steel products obtained by the steelmaking process using electric furnaces (electric furnace steel) contains elements called trump elements, which are difficult to remove during the steelmaking process. In particular, Cu, Ni, and Sn are typical trump elements contained in iron scrap, and it is important to establish thin steel sheets with excellent formability and a method for manufacturing them, assuming that these three elements remain in the steel simultaneously.

[0004] Assuming that iron scrap contains all of the representative trump elements Cu, Ni, and Sn, there is little disclosure regarding thin steel sheets for automobiles and their manufacturing technologies that actively utilize these elements.

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

[0008] 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.

[0009] 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.

[0010] 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 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.

[0011] Here, excellent formability refers to excellent ultimate deformation ability. Excellent ultimate deformation ability means that ε, which can be obtained from the following equation (5), is 0.60 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).

[0012] The inventors of the present invention have intensively studied with a focus on the ultimate deformation ability, which has a strong correlation with bending formability and stretch flange formability. As a result, by simultaneously containing three elements of Cu, Ni, and Sn in a predetermined amount and a predetermined balance, while suppressing the red hot brittleness caused by Cu and Sn during casting and hot rolling, it is possible to reduce coarse MnS, which is a factor deteriorating the ultimate deformation ability, and a steel sheet having an ultimate deformation ability superior to the conventional ones has been found.

[0013] The present invention has been made based on such findings, and the gist thereof is as follows.

[0014] [1] In mass %, C: 0.040 to 0.200%, Si: 0.30% or less, Mn: 0.10 to 2.50%, P: 0.100% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.0200% or less, Cu: 0.010 to 0.500%, Ni: 0.010 to 0.500%, Sn: 0.0010 to 0.0500%, and satisfying the following formulas (1) and (2), with the balance being composed of Fe and inevitable impurities, and having a ferrite area ratio of 60 to 98% and a pearlite area ratio of 2 to 35% in terms of area ratio, and the average crystal grain size of ferrite being 5 μm or more and 35 μm or less, and the average crystal grain size of pearlite being 15 μm or less, and having a steel structure, and the precipitation density of MnS having an equivalent circle diameter of 5 μm or more is 100 pieces / mm ２ The following is a thin steel sheet. 0.10 × [%Cu] + 1.5 × [%Sn] ≤ [%Ni] ≤ 5.0 × [%Cu] + 75 × [%Sn] ··· 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: 0.0500% 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 [1] A thin steel sheet according to [1], containing one or more selected from As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM (excluding Ce): 0.0200% or less. [3] A thin steel sheet according to [1] or [2], having a plating layer on one or both surfaces of the thin steel sheet, wherein the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electroplated galvanized layer, or an aluminum plated layer. [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 heated to a temperature of 1050 to 1350°C, then hot-rolled at a temperature of 1000°C or less, satisfying the following formula (4) for the end of finishing rolling, and then wound at a temperature of 400 to 700°C to obtain a steel sheet. 850 ≤ (end of finishing rolling temperature (°C)) + 350 × [%C] + 75 × ([%Mn] - 1.5) + 30 × [%Cu] + 100 × [%Ni] + 40 × [%Sn] ０．５　...Equation (4) In Equation (4), [%M] is the mass %) content of element M in the steel slab. [6] The method for manufacturing a thin steel sheet as described in [5] above, wherein the steel sheet after winding is pickled and then cold-rolled, the atmosphere in the temperature range of 600°C or higher is set to a dew point of -55 to 25°C and a hydrogen concentration of 0.2 to 30 volume%, the annealing temperature is set to 700 to 900°C and the holding time at the annealing temperature is set to 1 to 500 s, and when Vc is defined by the following Equation (3), the heated steel sheet is cooled at an average cooling rate of Vc to 70°C / s and a cooling stop temperature of 550°C or lower. Vc = 2.5 × [%Mn] ０．１ + (10 × [%S]) ２．０ -[%Cu]-0.5×[%Sn] ０．５ ...Equation (3) In Equation (3), [%M] is the mass %) content of element M in the steel slab. [7] A method for manufacturing a thin steel sheet as described in [1] or [2] above, comprising: heating a steel slab having the above component composition to a temperature of 1050 to 1350°C; hot rolling it at a finish rolling completion temperature of 850 to 1000°C; 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 in an atmosphere in the temperature range of 600°C or higher with a dew point of -55 to 25°C and a hydrogen concentration of 0.2 to 30% by volume; annealing at a temperature of 700 to 900°C and holding time at the annealing temperature of 1 to 500 s; and, when Vc is defined by the following formula (3), cooling the heated steel sheet at an average cooling rate of Vc to 70°C / s and a cooling stop temperature of 550°C or lower. Vc = 2.5 × [%Mn] ０．１ + (10 × [%S]) ２．０ -[%Cu]-0.5×[%Sn] ０．５・・・Formula (3) In Formula (3), [%M] is the content (mass %) of element M in the steel slab. [8] After winding and pickling the steel sheet after the hot rolling, the atmosphere in the temperature range of 600°C or higher is set to the conditions of dew point: -55 to 25°C, hydrogen concentration: 0.2 to 30% by volume, annealing temperature: 700 to 900°C, holding time at the annealing temperature: 1 to 500 s, and when Vc is defined by the following Formula (3), the steel sheet after heating is cooled under the conditions of average cooling rate: Vc to 70°C / s, cooling stop temperature: 550°C or lower, and further, as a plating treatment, hot-dip galvanizing treatment, alloyed hot-dip galvanizing treatment, electro-galvanizing treatment, or aluminum plating treatment is performed. The method for manufacturing a thin steel sheet according to [5] above. Vc = 2.5×[%Mn] ０．１ +(10×[%S]) ２．０ - [%Cu] - 0.5×[%Sn] ０．５ ・・・Formula (3) In Formula (3), [%M] is the content (mass %) of element M in the steel slab. [9] As a further plating treatment on the surface of the thin steel sheet after the cooling, hot-dip galvanizing treatment, alloyed hot-dip galvanizing treatment, electro-galvanizing treatment, or aluminum plating treatment is performed. The method for manufacturing a thin steel sheet according to [6] or [7] above.

[10] A method for manufacturing a member including a step of subjecting the thin steel sheet according to any one of [1] to [3] above to at least one of forming and joining to form a member.

[0015] According to the present invention, it is possible to provide a thin steel sheet and a member having excellent formability together with their advantageous manufacturing methods.

[0016] Hereinafter, embodiments of the present invention will be described in detail.

[0017] The thin steel sheet of the present invention contains, in mass%, C: 0.040 to 0.200%, Si: 0.30% or less, Mn: 0.10 to 2.50%, P: 0.100% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.0200% or less, Cu: 0.010 to 0.500%, Ni: 0.010 to 0.500%, Sn: 0.0010 to 0.0500%, satisfies the following formulas (1) and (2), and the balance consists of Fe and unavoidable impurities. In terms of area ratio, it has ferrite: 60 to 98% and pearlite: 2 to 35%, the average crystal grain size of ferrite is 5 μm or more and 35 μm or less, and the average crystal grain size of pearlite is 15 μm or less. It has a steel structure, and the precipitation density of MnS with an equivalent circle diameter of 5 μm or more is 100 pieces / mm ２ or less. 0.10 × [%Cu] + 1.5 × [%Sn] ≤ [%Ni] ≤ 5.0 × [%Cu] + 75 × [%Sn] ··· Formula (1) [%Mn] × [%S] / (0.5 × [%Cu] + [%Sn] ０．５ ) ≤ 1.00 ··· Formula (2) In Formulas (1) and (2), [%M] is the content (mass%) of element M in the thin steel sheet.

[0018] The preferable tensile strength (TS) of the thin steel sheet in the present invention is 440 MPa or more.

[0019] Component composition: The preferable component composition of the thin steel sheet of the present invention will be described below. In the following description, "%" representing the content of a component means "mass%" unless otherwise specified.

[0020] C: 0.040 to 0.200% C is an element that contributes to the formation of pearlite. When the C content exceeds 0.200%, a large amount of coarse pearlite is generated, which contributes to promoting Mn segregation and the formation of coarse MnS. Therefore, the C content is 0.200% or less, preferably 0.190% or less. On the other hand, when the C content is less than 0.040%, the formation of pearlite becomes too little, resulting in excessive coarsening of ferrite and making it easy to generate coarse MnS. Therefore, the C content is 0.040% or more, preferably 0.045% or more.

[0021] Si: 0.30% or less. Si is an element that contributes to suppressing cementite formation, and by including an appropriate amount, it can be used to control carbide formation. It is also an element that contributes to suppressing red-hot brittleness caused by Cu and Sn during casting and hot rolling, and by including an appropriate amount, it contributes to preventing deterioration of the ultimate deformability of the final thin steel sheet. On the other hand, Si forms oxides on the surface during annealing, which can be an inhibitory factor for plating. If the Si content exceeds 0.30%, pearlite formation becomes insufficient, causing ferrite to become excessively coarse and making it easier to produce coarse MnS. Also, surface oxide formation during annealing becomes significant, which may cause surface defects when plating is performed. Therefore, the Si content is 0.30% or less, preferably 0.25% or less. In this invention, there are no particular adverse effects from reducing the Si content, and it is not necessary to set a lower limit, but the Si content is preferably 0.001% or more, and more preferably 0.002% or more.

[0022] Mn: 0.10-2.50% During manufacturing, Mn combines with S in the steel to form MnS, which contributes to preventing surface defects caused by FeS. This effect is not sufficiently obtained when the Mn content is less than 0.10%, and if defects caused by FeS occur, the ultimate deformability of the resulting thin steel sheet may be impaired. Therefore, the Mn content is 0.10% or more, preferably 0.15% or more, and more preferably 0.20% or more. On the other hand, if the Mn content exceeds 2.50%, coarse MnS and Mn segregation are generated in the steel, and excellent ultimate deformability cannot be obtained. Therefore, the Mn content is 2.50% or less, preferably 2.40% or less, and more preferably 2.30% or less.

[0023] 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 0.0010% or more, and more preferably 0.0020% or more.

[0024] S: 0.020% 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 0.020% or less, preferably 0.015% or less, and more preferably 0.010% or less. It is preferable to reduce the S content as much as possible, and there is no need to specifically set a lower limit, but the S content is preferably 0.0001% or more, and more preferably 0.0002% or more.

[0025] 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, preferably 0.080% or less.

[0026] 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.

[0027] Cu: 0.010 to 0.500% Cu is an element necessary to suppress 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. The Cu content required to obtain this effect 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.

[0028] Ni: 0.010 to 0.500% Ni is an effective element for improving ultimate deformability by suppressing red-hot brittleness caused by Cu and Sn. The Ni content required to obtain this effect 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, it can cause deterioration of ultimate deformability due to non-uniformity of the steel structure and an increase in inclusions. Ni also increases costs. Therefore, the Ni content is 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.

[0029] Sn: 0.0010 to 0.0500% Sn is an element that segregates at grain boundaries and contributes to suppressing the coarsening of MnS. The Sn content required to obtain this effect 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 or 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.

[0030] Equation (1): 0.10 × [%Cu] + 1.5 × [%Sn] ≤ [%Ni] ≤ 5.0 × [%Cu] + 75 × [%Sn] 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 is effective in suppressing red-hot brittleness, but it causes an increase in inclusions. In order to effectively bring out the benefits of containing these three elements and obtain excellent ultimate deformability, it is necessary to satisfy Equation (1). If [%Ni] is less than 0.10 × [%Cu] + 1.5 × [%Sn], red-hot brittleness is not sufficiently suppressed, and excellent ultimate deformability cannot be obtained. Therefore, [%Ni] should be 0.10 × [%Cu] + 1.5 × [%Sn] or greater. Preferably, [%Ni] should be 0.12 × [%Cu] + 1.8 × [%Sn] or greater, and more preferably, [%Ni] should be 0.15 × [%Cu] + 2.0 × [%Sn] or greater. On the other hand, if [%Ni] is greater than 5.0 × [%Cu] + 75 × [%Sn], the effect of suppressing red-hot brittleness saturates, and the influence of increased inclusions due to Ni becomes large, so excellent ultimate deformability cannot be obtained. Therefore, [%Ni] should be 5.0 × [%Cu] + 75 × [%Sn] or less. Preferably, [%Ni] is 4.0 × [%Cu] + 50 × [%Sn] or less, and more preferably, [%Ni] is 3.0 × [%Cu] + 25 × [%Sn] or less.

[0031] 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.

[0032] The remainder of the composition 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 a predetermined amount of at least one element selected from the following group of elements as an optional element (selected element).

[0033] 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.

[0034] Mo: 0.500% or less By adding Mo, 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, 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 preferable that the Mo content be 0.400% or less, and more preferably 0.300% or less.

[0035] 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. When B is included to obtain this effect, 0.0001% or more is preferable. 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.

[0036] 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.

[0037] V: 0.0500% or less. V is an element that contributes to the refinement of the steel structure by forming fine carbides, and may be included as needed to obtain a suitable ferrite grain size. To obtain this effect, the V content is preferably 0.0005% or more, and more preferably 0.0010% or more. On the other hand, if the V content exceeds 0.0500%, excessive carbide formation may occur, which may cause deterioration of the ultimate deformability. Therefore, when V is included, the V content should be 0.0500% or less, and preferably 0.0300% or less.

[0038] Ti: 0.200% or less. Ti is an element that contributes to the refinement of the steel structure by forming fine carbides, and may be included as needed to obtain a suitable ferrite grain size. 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.

[0039] Nb: 0.200% or less. Nb is an element that contributes to the refinement of the steel structure by forming fine carbides, and may be included as needed to obtain a suitable ferrite grain size. 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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 0.020% or less.

[0047] 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.

[0048] 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.

[0049] Any of the above optional additive elements may be present in a 0% concentration. Furthermore, if the content of any of the above optional additive elements is below the preferred lower limit, it can be said that the element is present as an unavoidable impurity.

[0050] 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 average grain size of ferrite, the average grain size of pearlite, the equivalent circle diameter of MnS, and the precipitation density are determined by the method described in the examples.

[0051] Ferrite area ratio: 60-98% Ferrite is the main phase of the thin steel sheet of the present invention and has excellent workability. In order to obtain excellent ultimate deformability, the ferrite area ratio needs to be 60% or more, and preferably 65% ​​or more. On the other hand, in order to control the ferrite to the grain size described later, it is necessary to include a certain amount of the second phase, so the ferrite is 98% or less, and preferably 95% or less.

[0052] Average grain size of ferrite: 5 μm or more and 35 μm or less. As the grain size of ferrite increases, it becomes softer, which is advantageous for improving moldability and contributes to improving ultimate deformability. Therefore, the average grain size of ferrite is 5 μm or more, preferably 10 μm or more. On the other hand, if the ferrite is excessively coarse, voids tend to connect during molding, which is unfavorable for ultimate deformability. Therefore, the average grain size of ferrite is 35 μm or less, preferably 30 μm or less.

[0053] Pearlite area ratio: 2-35% By including 2% or more pearlite in a steel structure with ferrite as the main phase, a suitable tensile strength can be stably obtained without impairing the ultimate deformability of the ferrite mentioned above. Therefore, the pearlite area ratio should be 2% or more, and preferably 5% or more. On the other hand, including an excessive amount of pearlite impairs the ultimate deformability. Therefore, the pearlite area ratio should be 35% or less, and preferably 30% or less.

[0054] Average grain size of pearlite: 15 μm or less. Pearlite has a microstructure in which layered cementite and ferrite are alternately stacked. When the average grain size of pearlite exceeds 15 μm, voids formed at the interface between the layered cementite and ferrite tend to connect, and excellent ultimate deformability cannot be obtained. Therefore, the average grain size of pearlite is 15 μm or less, and preferably 10 μm or less. The lower limit is not particularly limited, but the average grain size of pearlite is preferably 1 μm or more, and more preferably 2 μm or more.

[0055] 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, and a lower value is preferable, but the precipitation density should be 0.01 particles / mm ２ It may be greater than or equal to 0.1 pieces / mm ２ That's fine too.

[0056] 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, martensite (including tempered martensite). 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%.

[0057] 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, an electro-galvanized layer, and a vapor-deposited zinc plating 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.

[0058] 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 more than 2.3 mm 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 0.3 mm or more and 2.3 mm or less.

[0059] Next, a method for manufacturing a thin steel sheet according to one embodiment of the present disclosure will be described. In the method for manufacturing a thin steel sheet of the present invention, a steel slab having the above-described component composition is heated to a temperature of 1050 to 1350°C, then hot-rolled at a finish rolling completion temperature satisfying the following formula (4) and below 1000°C, and then wound at a temperature of 400 to 700°C to form a steel sheet, thereby obtaining the thin steel sheet (hot-rolled steel sheet (hot-rolled thin steel sheet)) of the present invention. 850 ≤ (Finish rolling completion temperature (°C)) + 350 × [%C] + 75 × ([%Mn] - 1.5) + 30 × [%Cu] + 100 × [%Ni] + 40 × [%Sn] ０．５　...Equation (4) In Equation (4), [%M] is the mass %) content of element M in the steel slab. Alternatively, the steel sheet after winding is pickled and then cold-rolled, and the atmosphere in the temperature range of 600°C or higher is set to a dew point of -55 to 25°C and a hydrogen concentration of 0.2 to 30 volume%, and the annealing temperature is set to 700 to 900°C and the holding time at the annealing temperature is set to 1 to 500 s. When Vc is defined by the following Equation (3), the heated steel sheet is cooled at an average cooling rate of Vc to 70°C / s and a cooling stop temperature of 550°C or lower, thereby also obtaining the thin steel sheet of the present invention (cold-rolled steel sheet (cold-rolled thin steel sheet)). Vc = 2.5 × [%Mn] ０．１ + (10 × [%S]) ２．０ -[%Cu]-0.5×[%Sn] ０．５ ...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 temperature of 1050 to 1350°C, hot-rolling it at a finish rolling completion temperature of 850 to 1000°C, 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 in an atmosphere in the temperature range of 600°C or higher with a dew point of -55 to 25°C and a hydrogen concentration of 0.2 to 30 volume%, annealing at a temperature of 700 to 900°C, holding time at the annealing temperature of 1 to 500 s, and then cooling the heated steel sheet at an average cooling rate of Vc to 70°C / s and a cooling stop temperature of 550°C or lower. This also yields the thin steel sheet of the present invention (cold-rolled steel sheet (cold-rolled thin steel sheet)). In addition, the temperatures mentioned below when heating or cooling steel slabs (steel materials) or steel plates refer to the surface temperature of the steel slabs (steel materials), steel plates, etc., unless otherwise specified.

[0060] 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.

[0061] 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.

[0062] Finish rolling completion temperature: 850 to 1000°C, or satisfying the following formula (4) and being 1000°C or less: 850 ≤ (Finish rolling completion temperature (°C)) + 350 × [%C] + 75 × ([%Mn] - 1.5) + 30 × [%Cu] + 100 × [%Ni] + 40 × [%Sn] ０．５　...Equation (4) In Equation (4), [%M] is the mass %) of element M in 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 component composition and structural framework within the steel sheet during hot rolling, and this must be done in the austenite single-phase region. If the finish rolling end temperature is less than 850°C, a complete austenite single-phase region is not reached, and as a result of uneven concentrations of Mn within the steel sheet, the formation of coarse MnS may be promoted. Therefore, it is preferable that the finish rolling end temperature be 850°C or higher. More preferably, the finish rolling end temperature is 870°C or higher. However, if Equation (4) is satisfied, the finish rolling end temperature can be less than 850°C. This is because steel containing austenite-stabilizing elements such as C, Mn, Cu, Cu, and Ni in predetermined amounts and balances tends to maintain the austenite single-phase state. Therefore, the finish rolling end temperature should be set to satisfy Equation (4). The right-hand side of equation (4) is preferably 860°C 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.

[0063] 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 will coarseen, resulting in a heterogeneous steel structure, and MnS will also tend to coarseen, adversely affecting the ultimate deformability of the final thin steel sheet. Therefore, the winding temperature should be 700°C or lower, preferably 680°C or lower.

[0064] Cold Rolling Process Pickling and Cold Rolling In the method for manufacturing thin steel sheets of the present invention, the hot-rolled steel sheet obtained by the above method may be pickled and cold-rolled before annealing. Pickling is performed to remove the mill scale 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 is obtained, but it is usually performed so that the cold rolling rate (cumulative rolling rate) is 95% or less. Furthermore, 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.

[0065] Annealing process Dew point of the atmosphere in the temperature range of 600°C or higher: -55 to 25°C If the dew point of the atmosphere in the temperature range of 600°C or higher is greater than 25°C, decarburization becomes significant, the ferrite structure on the surface of the steel sheet becomes excessively coarse, and the steel sheet of the present invention may not be obtained. Therefore, it is preferable to set the dew point to 25°C or lower. More preferably, the dew point is 20°C or lower, and even more preferably 15°C or lower. On the other hand, trying to stably obtain conditions with a dew point below -55°C greatly impairs productivity and is therefore not industrially suitable. Therefore, it is preferable to set the dew point to -55°C or higher. More preferably, the dew point is -50°C or higher, and even more preferably -45°C or higher.

[0066] Hydrogen concentration: 0.2 to 30 volume% In order to ensure the minimum surface quality required for automotive applications, it is necessary to suppress oxidation of the steel sheet surface during annealing. If annealing is performed under conditions of significant surface oxidation, decarburization may be excessive, causing the ferrite structure on the surface of the steel sheet to become excessively coarse, and the steel sheet of the present invention may not be obtained. Furthermore, when plating is performed after annealing, the iron oxide film on the surface of the steel sheet may hinder the formation of a uniform plating layer, so it is preferable to reduce the surface of the steel sheet during annealing by using a hydrogen-containing atmosphere. To suppress surface oxidation, it is necessary to perform annealing in a hydrogen-containing atmosphere of a certain concentration. For the reasons above, it is preferable that the hydrogen concentration be 0.2 volume% or more. More preferably, the hydrogen concentration is 1.0 volume% or more, and even more preferably 2.0 volume% or more. On the other hand, if the hydrogen concentration exceeds 30 volume%, it is excessive for suppressing surface oxidation and may lead to a decrease in productivity due to increased costs. Therefore, it is preferable that the hydrogen concentration be 30 volume% or less. More preferably, the hydrogen concentration is 25 volume% or less, and even more preferably 20 volume% or less.

[0067] 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. In addition, 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. 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 the 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. On the other hand, if the holding time is too long, the ferrite may become coarser and the amount of coarse MnS may increase, and excellent ultimate deformability may not be obtained. Therefore, the holding time is preferably 500 s or less, more preferably 450 s or less, and even more preferably 400 s or less.

[0069] In steel sheets cooled and annealed under conditions of average cooling rate: Vc ~ 70°C / s and cooling stop temperature: 550°C or lower, ferrite grain growth progresses in the temperature range above 700°C, and pearlite is formed in the temperature range above 550°C. Therefore, in order to obtain a suitable steel structure in the present invention, it is preferable to control cooling within the above range in the temperature range above 550°C. Accordingly, it is preferable to set the cooling stop temperature to 550°C or lower. The lower limit of the cooling stop temperature is not particularly limited, but 300°C or higher is preferable. Furthermore, if the average cooling rate is too low, the pearlite will coarseen and its area ratio will become excessive, and ferrite grain growth will also progress excessively, potentially leading to the generation of a large amount of coarse MnS. Also, as mentioned above, Mn and S contribute to an increase in the amount of coarse MnS precipitated, while Cu and Sn contribute to a decrease in the amount of coarse MnS precipitated. Therefore, the preferred average cooling rate Vc (°C / s) to avoid the excessive formation of coarse MnS varies depending on the amounts of these four elements and is expressed by the following equation (3): Vc = 2.5 × [%Mn] ０．１ + (10 × [%S]) ２．０ -[%Cu] -0.5×[%Sn] ０．５...Equation (3) In Equation (3), [%M] is the content (mass%) of element M in the steel slab. Therefore, the average cooling rate is preferably Vc°C / s or higher, more preferably Vc + 1.0°C / s or higher, and even more preferably Vc + 2.0°C / s or higher. On the other hand, if the cooling stop temperature is too high, the pearlite area ratio may be too low. Therefore, the average cooling rate is preferably 70°C / s or lower, and more preferably 50°C / s or lower. Here, the average cooling rate (°C / s) is defined as "(cooling start temperature (annealing temperature: 700 to 900°C) - cooling stop temperature (550°C or lower)) / cooling time from cooling start to cooling stop (seconds)".

[0070] The equipment used for annealing is not particularly limited and can be carried out using a continuous annealing line (CAL), a hot-dip galvanizing line (CGL), or a batch annealing furnace (BAF), etc.

[0071] 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 under the above conditions, the obtained hot-rolled steel sheet may be coiled and pickled to remove the mill scale, and then the aforementioned annealing, cooling, and plating treatments may be performed without cold rolling. That is, after hot-rolling the steel sheet is coiled and pickled, the atmosphere in the temperature range of 600°C or higher is set to a dew point of -55 to 25°C and a hydrogen concentration of 0.2 to 30 volume%, the annealing temperature is set to 700 to 900°C, and the holding time at the annealing temperature is set to 1 to 500 s. After heating, the steel sheet is cooled at an average cooling rate of Vc to 70°C / s and a cooling stop temperature of 550°C or lower. Furthermore, as a plating treatment, hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, or aluminum plating may be performed. In this case, there are no particular limitations on the type of pickling bath or immersion conditions used; it is sufficient to remove scale to a degree that is adequate to obtain a surface condition suitable for plating.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] Other than the conditions mentioned above, there are no specific limitations; you may follow the usual law.

[0079] 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.

[0080] The thin steel sheet of the present invention has excellent formability. Therefore, components obtained using the thin steel sheet of the present invention also exhibit excellent formability, at least except for the processing-affected zone and heat-affected zone after processing. Furthermore, using components of the present invention allows for weight reduction. Accordingly, components of the present invention can be suitably used, for example, in vehicle body frame components.

[0081] 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.

[0082] 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.

[0083] Thin steel sheets were obtained by hot rolling, cold rolling, annealing, and plating steel slabs with the component compositions shown in Table 1 (the remainder being Fe and unavoidable impurities) obtained by casting molten steel produced using an electric furnace. The slabs were subjected to hot rolling, cold rolling, annealing, and plating under the conditions shown in Table 2. The cooling stop temperature during annealing was 500°C. In this example, the thickness of the steel sheet after hot rolling was 3.2 to 3.3 mm, and the thickness after cold rolling was 1.0 to 1.4 mm. Cold rolling was performed after removing the scale formed on the surface of the steel sheet during hot rolling by pickling. In Table 2, the material classifications are as follows: cold-rolled steel sheets (unplated cold-rolled thin steel sheets) that have undergone cold rolling and annealing treatment but have not been plated are indicated as CR; cold-rolled steel sheets (hot-dip galvanized thin steel sheets) that have undergone hot-dip galvanizing treatment are indicated as GI; cold-rolled steel sheets (alloyed hot-dip galvanized thin steel sheets) that have undergone hot-dip galvanizing treatment and alloying treatment are indicated as GA; cold-rolled steel sheets (electro-galvanized thin steel sheets) that have undergone electro-galvanizing treatment are indicated as EG; and cold-rolled steel sheets (aluminum-plated thin steel sheets) that have undergone hot-dip aluminum plating treatment are indicated as Al. Furthermore, thin steel sheets (hot-rolled steel sheets, unplated hot-rolled thin steel sheets) that have undergone hot rolling but have not undergone cold rolling or annealing treatment and have not been plated are indicated as HR. Furthermore, hot-rolled steel sheets (alloyed hot-dip galvanized thin steel sheets) that have undergone hot rolling but have not undergone cold rolling but have undergone annealing treatment, hot-dip galvanizing treatment and alloying treatment are indicated as GA. Unless otherwise specified, the standard procedures shall apply. [Hot-dip galvanizing (GI)] ・Plating bath composition: Zn, Al and unavoidable impurities (Al concentration in the bath: 0.10-0.20% by mass, remainder: Zn and unavoidable impurities) ・Plating bath temperature: 460°C ・Plating amount per side: 30-60 g / m ２ [Alloying Hot-Dip Zinc Plating (GA) (Hot-dip zinc plating is the same as above)] - Alloying temperature: 450-560°C - Degree of alloying: 8.0-14.0% by mass [Electro-Zinc Plating (EG)] - Divalent zinc ion concentration in plating solution: 80 g / L (Divalent zinc ions were added as sulfate.) - pH of plating solution: 2.0 (The pH of the plating solution was adjusted with sulfuric acid.) - Temperature of plating solution: 55°C - Current density: 50 A / dm ２ • Electrolysis time: 30 s • Plating amount per side: 50 g / m２ [Hot-dip aluminum plating (Al)] • Plating bath composition: Al and unavoidable impurities • Plating bath temperature: 700°C • Plating amount per side: 60 g / m² ２

[0084] <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 a 1% by mass Nital solution. Using a scanning electron microscope (SEM), images were taken of three fields of view each from one of the surfaces: a region at 1 / 8 thickness, a region at 1 / 4 thickness, and a region at 1 / 2 thickness, at a magnification of 2000x with a field of view of 40 μm × 60 μm. For all nine fields of view images obtained, the area ratios of ferrite and pearlite were determined, and the steel plate microstructure was evaluated using the average values. Microstructures with no corrosion marks or cementite observed within the grains were defined as the ferrite phase, and microstructures with two or more layered cementite structures observed as white contrast within the grains were defined as pearlite. The remainder consisted of martensite, bainite, spheroidal cementite, retained austenite, or MnS and other inclusions, and these were evaluated by their total area ratio. For MnS, the size and density were evaluated separately using the method described later. The average grain size of ferrite and pearlite was evaluated as the average of the major and minor axis lengths when approximated as an ellipse. More specifically, for the average grain size of ferrite, for each elliptically approximated ferrite displayed in the images for all nine fields, the average of the major and minor axis lengths was used as the grain size of the ferrite, and the average value of all ferrite grain sizes (sum of ferrite grain sizes / number of elliptically approximated ferrites) was used as the average grain size of the ferrite. Similarly, for the average grain size of pearlite, for each elliptically approximated pearlite displayed in the images for all nine fields, the average of the major and minor axis lengths was used as the grain size of the pearlite, and the average value of all pearlite grain sizes (sum of pearlite grain sizes / number of elliptically approximated pearlites) was used as the average grain size of the pearlite. For each elliptically approximated ferrite, the region where the outer circumference is surrounded by non-ferrite elements such as grain boundaries and formed as a single, uninterrupted entity was counted as one unit for measurement. Furthermore, for each pearlite that approximates an ellipse, the measurement was performed on a region where the outer perimeter was surrounded by materials other than pearlite, forming a continuous, uninterrupted area.

[0085] 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 at 1 / 8 of the plate thickness, a region at 1 / 4 of the plate thickness, and a region at 1 / 2 of the plate thickness, at a magnification of 1000x with a field of view of 80 μm × 120 μm. The composition of the inclusions observed in each field of view was confirmed using energy-dispersive X-ray spectroscopy (EDS) to identify MnS. Of the MnS observed in all nine fields of view, the total number of those with an equivalent circle diameter of 5 μm or more was allocated to the total observed area to determine the precipitation density of MnS. The equivalent circle diameter (μm) refers to the diameter of a perfect circle with the area of ​​one MnS, and the area X (μm) of one MnS is... 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 MnS and is formed as a single, uninterrupted entity is measured as one molecule.

[0086] <Tensile Test> From each obtained steel plate, a JIS No. 5 tensile 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 performed in accordance with the provisions of JIS Z2241 (2011) with a value of / s, and the tensile strength (TS) was determined.

[0087] <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 0.60 or higher were considered to have excellent ultimate deformability and were used as examples of the present invention. 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).

[0088] As shown in Table 3, it can be seen that all of the thin steel sheets of the present invention have superior ultimate deformation ability compared to the thin steel sheets of the comparative examples.

[0089] 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 exhibit excellent ultimate deformation capabilities, similar to the steel sheet of the present invention, because the thin steel sheet of the present invention exhibits excellent ultimate deformation capabilities.

[0090]

[0091]

[0092]

[0093] According to the present invention, a thin steel sheet with excellent extreme deformation ability can be obtained, which is particularly suitable for automotive parts applications.

Claims

1. A composition comprising, by mass%, C: 0.040 to 0.200%, Si: 0.30% or less, Mn: 0.10 to 2.50%, P: 0.100% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.0200% or less, Cu: 0.010 to 0.500%, Ni: 0.010 to 0.500%, Sn: 0.0010 to 0.0500%, satisfying the following formulas (1) and (2), with the remainder being Fe and unavoidable impurities; and a steel structure having, by area percentage, ferrite: 60 to 98% and pearlite: 2 to 35%, with an average grain size of ferrite of 5 μm or more and 35 μm or less, and an average grain size of pearlite of 15 μm or less. The precipitation density of MnS with an equivalent circle diameter of 5 μm or more is 100 particles / mm². ２ The following is the formula for thin steel plate: 0.10 × [%Cu] + 1.5 × [%Sn] ≤ [%Ni] ≤ 5.0 × [%Cu] + 75 × [%Sn] ... Equation (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: 0.0500% 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 A thin steel sheet according to claim 1, containing one or more elements selected from As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM (excluding Ce): 0.0200% or less.

3. The thin steel sheet according to claim 1 or 2, wherein the thin steel sheet has a plating layer on either one or both of its surfaces, and the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electro-galvanized layer, or an aluminum-plated 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: heating a steel slab having the above-mentioned component composition to a temperature of 1050 to 1350°C; hot rolling under conditions of finishing rolling completion temperature: satisfying the following formula (4) and 1000°C or less; then winding at a temperature of 400 to 700°C to obtain a steel sheet. 850 ≤ (Finishing rolling completion temperature (°C)) + 350 × [%C] + 75 × ([%Mn] - 1.5) + 30 × [%Cu] + 100 × [%Ni] + 40 × [%Sn] ０．５　 ...Equation (4) In Equation (4), [%M] is the mass %) of element M in the steel slab.

6. The method for manufacturing a thin steel sheet according to claim 5, wherein the steel sheet after winding is pickled and then cold-rolled, the atmosphere in the temperature range of 600°C or higher is set to a dew point of -55 to 25°C and a hydrogen concentration of 0.2 to 30 volume%, the annealing temperature is set to 700 to 900°C and the holding time at the annealing temperature is set to 1 to 500 s, and when Vc is defined by the following formula (3), the heated steel sheet is cooled at an average cooling rate of Vc to 70°C / s and a cooling stop temperature of 550°C or lower. Vc = 2.5 × [%Mn] ０．１ + (10 × [%S]) ２．０ -[%Cu]-0.5×[%Sn] ０．５ ...Equation (3) In Equation (3), [%M] is the mass %) content of element M in the steel slab.

7. A method for manufacturing a thin steel sheet according to claim 1 or 2, wherein a steel slab having the above composition is heated to a temperature of 1050 to 1350 °C, hot-rolled under the conditions of a finishing rolling end temperature of 850 to 1000 °C, coiled at a temperature of 400 to 700 °C to obtain a steel sheet, the obtained steel sheet is pickled, further cold-rolled, the atmosphere in the temperature range of 600 °C or higher is set to conditions of a dew point of -55 to 25 °C and a hydrogen concentration of 0.2 to 30 vol%, heated under the conditions of an annealing temperature of 700 to 900 °C and a holding time at the annealing temperature of 1 to 500 s, and when Vc is defined by the following formula (3), cooling is performed on the steel sheet after heating under the conditions of an average cooling rate: Vc to 70 °C / s and a cooling stop temperature of 550 °C or lower. Vc = 2.5×[%Mn] ０．１ +(10×[%S]) ２．０ −[%Cu]−0.5×[%Sn] ０．５ ・・・Formula (3) In formula (3), [%M] is the content (mass%) of element M in the steel slab.

8. The method for manufacturing a thin steel sheet according to claim 5, wherein after hot-rolling the steel sheet is wound and pickled, the atmosphere is set to a temperature range of 600°C or higher with a dew point of -55 to 25°C and a hydrogen concentration of 0.2 to 30% by volume, the annealing temperature is set to 700 to 900°C and the holding time at the annealing temperature is set to 1 to 500 s, and when Vc is defined by the following formula (3), the heated steel sheet is cooled at an average cooling rate of Vc to 70°C / s and a cooling stop temperature of 550°C or lower, and further a plating treatment is performed, such as hot-dip galvanizing, alloyed hot-dip galvanizing, electro-galvanizing, or aluminum plating. Vc = 2.5 × [%Mn] ０．１ + (10 × [%S]) ２．０ -[%Cu]-0.5×[%Sn] ０．５ ...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 subjected to a plating treatment, such as 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.

Images