Thin steel sheet, member, and production methods for same
A controlled composition and manufacturing process for thin steel sheets, focusing on microstructural enhancements, addresses the limitations of electric furnace steelmaking by improving ductility and ultimate deformation capacity, suitable for complex part forming and environmental sustainability.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- JFE STEEL CORP
- Filing Date
- 2025-11-27
- Publication Date
- 2026-06-04
AI Technical Summary
Existing thin steel sheets, particularly those produced using electric furnace steelmaking with iron scrap as the main raw material, lack sufficient bend formability and stretch flange formability due to the presence of trace elements like Cu, Ni, and Sn, which are difficult to remove during the steelmaking process, limiting their ultimate deformation capacity and ductility.
A thin steel sheet composition with controlled amounts of C, Si, Mn, P, S, Al, N, Ti, Nb, Cu, Ni, Sn, and optional elements, along with a manufacturing process involving hot rolling, cold rolling, and annealing, results in a microstructure with a high ferrite area ratio, controlled grain size, and precipitation density of carbides, enhancing ductility and ultimate deformation capacity.
The solution provides thin steel sheets with excellent ductility and ultimate deformation capacity, meeting the demands for complex part forming and reducing CO2 emissions through resource recycling in steel production.
Smart Images
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Abstract
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] In recent years, awareness of the crisis caused by greenhouse gases and climate change has been growing globally, ２ The demand for reducing emissions is getting stronger. 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 recycling.
[0004] However, iron scrap, the main raw material for steel produced by electric furnace steelmaking processes (electric furnace steel), contains elements known as trump elements, which are difficult to remove during the steelmaking process. In particular, Cu, Ni, and Sn are representative trump elements found in iron scrap, and it is important to establish a thin steel sheet with excellent formability and a manufacturing method thereof, assuming that these three elements remain simultaneously in the steel.
[0005] Patent Document 1 describes a material containing, by mass%, C: 0.0010 to 0.0040%, Si: 0.005 to 0.05%, Mn: 0.1 to 0.8%, P: 0.01 to 0.07%, S: 0.001 to 0.01%, Al: 0.01 to 0.08%, N: 0.0010 to 0.0050%, Nb: 0.002 to 0.020%, and Mo: 0.005 to 0.050%, where the Mn content is [Mn%] and the P content is [P%], and the value of [Mn%] / [P%] is between 1.6 and 45, where the C content is [C%] and the Nb content is [Nb%], and it can be calculated as [C%] - (12 / 93) × [Nb%]. A high-strength bake-hardenable cold-rolled steel sheet has been proposed, characterized in that the amount of solid solution carbon is 0.0005% or more and 0.0025% or less, with the remainder being Fe and unavoidable impurities, and the X-ray diffraction integral intensity ratios X(222), X(110), and X(200) of the {222} plane, {110} plane, and {200} plane parallel to the surface at a depth position of 1 / 4 thickness of the high-strength bake-hardenable cold-rolled steel sheet satisfy the following formula (1), and the tensile strength is 300 MPa or more and 450 MPa or less, exhibiting excellent bake-hardenability, room-temperature aging resistance, and deep-draw workability, and low in-plane anisotropy. X(222) / {X(110)+X(200)}≧3.0 ...Formula (1)
[0006] International Publication No. 2012 / 073538
[0007] The above-mentioned conventional technology has the following problems. The technology described in Patent Document 1 yields a high-strength bake-hardened cold-rolled steel sheet with excellent deep-drawability and low in-plane anisotropy, but it does not specify any formability other than deep-drawability. In particular, the ultimate deformation capacity, which will be described later, is strongly correlated with bendability and stretch flange formability, and improving the ultimate deformation capacity is important in order to manufacture thin steel sheets with excellent formability. Thus, there has been a strong demand for the establishment of a technology for thin steel sheets that have excellent ultimate deformation capacity and also excellent ductility.
[0008] 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.
[0009] Here, excellent formability refers to excellent ductility and ultimate deformation capacity. Excellent ductility means that the total elongation determined by a tensile test in accordance with JIS Z 2241 (2011) is 30.0% or more. Excellent ultimate deformation capacity means that ε, determined from the following equation (4), is 1.4 or more. ε = -ln(t / t) ０ ) ...Equation (4) In Equation (4), 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).
[0010] This invention was made to solve the aforementioned problems, and its gist is as follows. [1] A material having a composition in mass% of the following: C: 0.0005% or more and 0.0100% or less, Si: 0.10% or less, Mn: 0.01% or more and 0.50% or less, P: 0.050% or less, S: 0.020% or less, Al: 0.100% or less, N: 0.0200% or less, Ti: 0.001% or more and 0.150% or less, Nb: 0.0001% or more and 0.0500% or less, Cu: 0.01% or more and 0.50% or less, Ni: 0.01% or more and 0.50% or less, Sn: 0.001% or more and 0.050% or less, satisfying the following formulas (1) and (2), with the remainder being Fe and unavoidable impurities, and having a structure in which the area ratio of ferrite is 90% or more, The ferrite has an average grain size of 10 μm or more, a solid solution carbon content represented by the following formula (3) is 0.0030 mass% or less, and the precipitation density of the carbide, which has an equivalent circle diameter of 5 μm or more, is 350 particles / mm². ２The following is true for thin steel sheets: 0.1 × [%Cu] + 1.5 × [%Sn] ≤ [%Ni] ≤ 5.0 × [%Cu] + 75 × [%Sn] ... Equation (1) [%Ti] - ([%C] - [%Nb] × 12 / 93) × 48 / 12 - [%N] × 48 / 14 - ([%S] - [%Cu] × 32 / 64 - [%Mn] × 32 / 55) × 48 / 32 ≥ -0.100 ... Equation (2) (Solid solution C amount (mass%)) = (Total C content (mass%)) - (Amount of C present as carbides (mass%)) ... Equation (3) In equations (1) and (2), [%M] is the content (mass%) of element M in the thin steel sheet. [2] The above component composition is further defined in mass% as follows: Cr: 0.40% or less, Mo: 0.100% or less, V: 0.050% or less, B: 0.0020% or less, Sb: 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 at least one surface, wherein the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electro-galvanized layer, a tin-plated layer, or an aluminum-plated layer. [4] A member made using a 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, comprising: a hot rolling step of heating a steel material having the above component composition at 1050°C to 1300°C, hot rolling it at a finish rolling completion temperature of 840°C to 960°C, cooling it from the finish rolling completion temperature to a winding temperature of 550°C to 750°C at an average cooling rate CR1 of 20°C / s to 150°C / s, and winding it at the winding temperature; a cold rolling step of performing cold rolling on the hot-rolled steel sheet obtained after the hot rolling step under the condition of a rolling ratio of 70% to 95%; and an annealing step of annealing the steel sheet obtained after the cold rolling step at a maximum attainable temperature of 730°C to 900°C, and then cooling it from 730°C to 350°C at an average cooling rate CR2 of 80°C / s or less. [6] A method for manufacturing a thin steel sheet according to [5], comprising a plating step of applying a plating treatment to the surface of the steel sheet after the annealing step, wherein the plating step involves applying a hot-dip galvanizing treatment, alloyed hot-dip galvanizing treatment, electro-galvanizing treatment, tin plating treatment, or aluminum plating treatment to the surface of the thin steel sheet. [7] A method for manufacturing a component, comprising a step of applying at least one of forming and joining processes to the thin steel sheet according to any one of [1] to [3] to make a component.
[0011] This invention makes it possible to provide thin steel sheets, components, and methods for manufacturing them, all of which exhibit excellent formability.
[0012] Thin Steel Sheet The thin steel sheet of the present invention has the following composition by mass%, C: 0.0005% or more and 0.0100% or less, Si: 0.10% or less, Mn: 0.01% or more and 0.50% or less, P: 0.050% or less, S: 0.020% or less, Al: 0.100% or less, N: 0.0200% or less, Ti: 0.001% or more and 0.150% or less, Nb: 0.0001% or more and 0.0500% or less, Cu: 0.01% or more and 0.50% or less, Ni: 0.01% or more and 0. The material has a composition that contains 50% or less of Sn: 0.001% to 0.050%, satisfies the following formulas (1) and (2), with the remainder being Fe and unavoidable impurities, and has a structure in which the area ratio of ferrite is 90% or more, the average crystal grain size of ferrite is 10 μm or more, the amount of solid-solution carbon represented by the following formula (3) is 0.0030 mass% or less, and the precipitation density of carbides with an equivalent circle diameter of 5 μm or more is 350 particles / mm².２ The following holds: 0.1×[%Cu] + 1.5×[%Sn] ≤ [%Ni] ≤ 5.0×[%Cu] + 75×[%Sn]... Equation (1) [%Ti] - ([%C] - [%Nb]×12 / 93)×48 / 12 - [%N]×48 / 14 - ([%S] - [%Cu]×32 / 64 - [%Mn]×32 / 55)×48 / 32 ≥ -0.100... Equation (2) (Amount of solid-solved C (mass %)) = (Total C content (mass %)) - (Amount of C present as carbide (mass %))... Equation (3) In Equation (1) and Equation (2), [%M] is the content (mass %) of element M in the thin steel sheet.
[0013] Hereinafter, embodiments of the thin steel sheet of the present invention will be described. First, the reasons for limiting the range of the component composition of the thin steel sheet of the present invention will be explained. Note that % regarding the component content is "mass %".
[0014] C: 0.0005% or more and 0.0100% or less, amount of solid-solved C: 0.0030 mass % or less (Amount of solid-solved C (mass %)) = (Total C content (mass %)) - (Amount of C present as carbide (mass %))... Equation (3) When the C content exceeds 0.0100%, precipitation of coarse carbides is promoted, resulting in a decrease in the ultimate deformation ability. Therefore, the C content is set to 0.0100% or less. When the C content is less than 0.0005%, the decrease in the ultimate deformation ability due to grain boundary segregation elements such as P and Sn becomes significant, so the C content is set to 0.0005% or more. The C content is preferably 0.0007% or more, and more preferably 0.0015% or more. Also, when the amount of solid-solved C exceeds 0.0030 mass %, the ductility of the thin steel sheet decreases, so the amount of solid-solved C is set to 0.0030 mass % or less. The amount of solid-solved C is preferably 0.0025 mass % or less. The lower limit does not particularly need to be limited, and the amount of solid-solved C may be 0%. Note that the amount of solid-solved C is obtained by subtracting the amount of C present as carbide in the thin steel sheet from the C content (total C content) in the thin steel sheet, as shown by the above Equation (3).
[0015] Si: 0.10% or less. Si forms oxides on the surface during annealing, which inhibits plating performance. Therefore, the Si content must be 0.10% or less. Preferably, the Si content is 0.07% or less. There is no particular lower limit, but preferably the Si content is 0.01% or more.
[0016] Mn: 0.01% or more and 0.50% or less. During manufacturing, Mn combines with S in the steel to form MnS, which contributes to preventing surface defects caused by FeS. Therefore, the Mn content should be 0.01% or more. Preferably, the Mn content should be 0.02% or more. On the other hand, if the Mn content exceeds 0.50%, ductility decreases due to solid solution strengthening. Therefore, the Mn content should be 0.50% or less. Preferably, the Mn content should be 0.40% or less.
[0017] P: 0.050% or less. P is an element that easily segregates at grain boundaries, and if present in excess, it segregates in large quantities at ferrite grain boundaries, degrading ductility and ultimate deformability. Therefore, the P content should be 0.050% or less. Preferably, the P content should be 0.040% 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 0.004% or more, and more preferably 0.006% or more.
[0018] S: 0.020% or less. S combines with Mn in the steel to form MnS. If there is an excess amount of S, a large amount of coarse sulfides will be generated, degrading the ultimate deformability. Therefore, the S content should be 0.020% or less. Preferably, the S content should be 0.015% or less. It is preferable to reduce the S content as much as possible, and there is no particular lower limit, but it is preferable that the S content be 0.001% or more, and more preferably 0.002% or more.
[0019] Al: 0.100% or less. Al is an element contained to remove oxygen in steel. However, when the Al content in steel exceeds 0.100%, the amounts of Al oxides and Al nitrides formed in the steel become excessive, and the ultimate deformation ability decreases. Therefore, the Al content should be 0.100% or less. Preferably, the Al content is 0.07% or less. The lower limit is not particularly limited, but from the perspective of reducing the manufacturing load in Al-based inclusion treatment, it is preferably 0.010% or more.
[0020] N: 0.0200% or less. N may form coarse nitrides and become the starting point of void generation, reducing the ultimate deformation ability. The N content that can suppress this effect is 0.0200% or less. Preferably, the N content is 0.0100% or less. The lower limit is not particularly limited, but from the perspective of reducing the manufacturing load in denitrification treatment, the N content is preferably 0.0005% or more.
[0021] Ti: 0.001% or more and 0.150% or less. Ti is an element that forms fine carbides. In addition to reducing the amount of solid-solved C and improving the ductility of thin steel sheets, by suppressing the precipitation of coarse carbides, the ultimate deformation ability is improved. Therefore, the Ti content should be 0.001% or more. Preferably, the Ti content is 0.005% or more, and more preferably 0.010% or more. On the other hand, when the Ti content exceeds 0.150%, the precipitation strengthening amount due to fine precipitates becomes excessive, and the ductility of the thin steel sheet decreases. Therefore, the Ti content should be 0.150% or less. Preferably, the Ti content is 0.100% or less.
[0022] Nb: 0.0001% or more and 0.0500% or less. Nb is an element that suppresses the precipitation of coarse carbides by forming fine carbides and contributes to the improvement of the ultimate deformation ability. Therefore, the Nb content should be 0.0001% or more. Preferably, the Nb content is 0.0003% or more. On the other hand, when the Nb content exceeds 0.0500%, the precipitation strengthening amount due to fine precipitates becomes excessive, and the ductility of the thin steel sheet decreases. Therefore, the Nb content should be 0.0500% or less. Preferably, the Nb content is 0.0300% or less.
[0023] Cu: 0.01% or more and 0.50% or less. Cu is an element that combines with S in steel. Due to this effect, the precipitation of TiS is suppressed and the formation of Ti carbide is promoted, contributing to the reduction of the amount of dissolved C and the suppression of the precipitation of coarse carbides. Therefore, the Cu content should be 0.01% or more. On the other hand, when Cu is contained in excess, red-hot brittleness occurs during casting or hot rolling, and fine cracks remain in the thin steel sheet, reducing ductility and the ultimate deformation ability. Therefore, the Cu content should be 0.50% or less. It is preferable that the Cu content is 0.30% or less.
[0024] Ni: 0.01% or more and 0.50% or less. Ni is an element effective in improving the ultimate deformation ability by suppressing red-hot brittleness caused by Cu and Sn. The Ni content required to obtain this effect is 0.01% or more. On the other hand, when Ni is contained in excess, it causes deterioration of the ultimate deformation ability due to the non-uniformity of the steel structure and the increase of inclusions. Also, Ni causes an increase in cost. Therefore, the Ni content should be 0.50% or less. It is preferable that the Ni content is 0.30% or less.
[0025] Sn: 0.001% or more and 0.050% or less. Sn is an element that segregates at grain boundaries and contributes to the suppression of the coarsening of carbides. The Sn content required to obtain this effect is 0.001% or more. On the other hand, when Sn is contained in excess, red-hot brittleness occurs during casting or hot rolling, and fine cracks remain in the thin steel sheet, reducing ductility and the ultimate deformation ability. Therefore, the Sn content should be 0.050% or less. It is preferable that the Sn content is 0.015% or less.
[0026] In equation (1), which satisfies equation (1): 0.1 × [% Cu] + 1.5 × [% Sn] ≤ [% Ni] ≤ 5.0 × [% Cu] + 75 × [% Sn], [% M] is the mass %) content of element M in the thin steel sheet. If the Ni content is less than 0.1 × (Cu content) + 1.5 × (Sn content), red-hot brittleness is not sufficiently suppressed, and the ultimate deformability decreases. Therefore, the Ni content should be 0.1 × (Cu content) + 1.5 × (Sn content) or more. Preferably, the Ni content should be 0.1 × (Cu content) + 2.0 × (Sn content) or more, and more preferably, the Ni content should be 0.1 × (Cu content) + 2.5 × (Sn content) or more. On the other hand, the crack suppression effect of Ni addition saturates at a Ni content of 5.0 × (Cu content) + 75 × (Sn content), and the influence of Ni inclusions becomes large, so the Ni content should be 5.0 × (Cu content) + 75 × (Sn content) or less. Therefore, the Ni content needs to be within the range of formula (1). Preferably, the Ni content is 3.0 × (Cu content) + 30 × (Sn content) or less, and more preferably, the Ni content is 1.0 × (Cu content) + 20 × (Sn content) or less.
[0027] In formula (2), which satisfies the condition: [%Ti] - ([%C] - [%Nb] × 12 / 93) × 48 / 12 - [%N] × 48 / 14 - ([%S] - [%Cu] × 32 / 64 - [%Mn] × 32 / 55) × 48 / 32 ≥ -0.100, [%M] is the mass %) content of element M in the thin steel sheet. If formula (2) is not satisfied, the Ti content, which contributes to reducing the amount of solid-solution C by forming carbides, becomes insufficient, resulting in an excess of solid-solution C and a decrease in ductility. Therefore, in the present invention, it is necessary to satisfy formula (2). That is, the left side of equation (2) ([%Ti] - ([%C] - [%Nb] × 12 / 93) × 48 / 12 - [%N] × 48 / 14 - ([%S] - [%Cu] × 32 / 64 - [%Mn] × 32 / 55) × 48 / 32) shall be -0.100 or greater. The left side of equation (2) is preferably 0.045 or greater, and more preferably 0.050 or greater. The upper limit of the left side of equation (2) is not particularly limited, but the left side of equation (2) is preferably 0.650 or less, and more preferably 0.600 or less.
[0028] 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 to the extent that they do 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). In addition, in the present invention, one or more elements selected from the following elements may be included as optional elements (selected elements) in addition to the above component composition.
[0029] Cr: 0.40% or less By adding Cr, the overall strength of the steel structure can be adjusted to a high and stable level, reducing the number of void initiation points and obtaining superior ultimate deformability. To obtain this effect, the Cr content is preferably 0.01% or more, and more preferably 0.02% or more. On the other hand, if the Cr content exceeds 0.40%, the ductility of the thin steel sheet decreases. Therefore, when Cr is included, the Cr content should be 0.40% or less. It is preferable that the Cr content be 0.25% or less.
[0030] Mo: 0.100% or less By adding Mo, the overall strength of the steel structure can be adjusted to a high and stable level, reducing the number of void initiation points and obtaining superior ultimate deformation capacity. To obtain this effect, it is preferable that the Mo content be 0.001% or more. On the other hand, if the Mo content exceeds 0.100%, the ductility of the thin steel sheet decreases. Therefore, when Mo is included, the Mo content should be 0.100% or less. It is preferable that the Mo content be 0.050% or less.
[0031] V: 0.050% or less V reduces the amount of coarse carbide precipitation by forming fine carbides, contributing to an improvement in ultimate deformability. To obtain this effect, the V content is preferably 0.002% or more, and more preferably 0.004% or more. On the other hand, if the V content exceeds 0.050%, the amount of precipitation strengthening due to fine precipitates becomes excessive, and the ductility of the thin steel sheet decreases. Therefore, when V is included, the V content should be 0.050% or less. It is preferable that the V content be 0.045% or less.
[0032] B: 0.0020% or less. B combines with N in the steel to form BN, which helps suppress the precipitation of coarse nitrides such as AlN, which cause deterioration of the ultimate deformability. To obtain this effect, it is preferable that the B content be 0.0001% or more. On the other hand, if the B content exceeds 0.0020%, B may excessively segregate at the ferrite grain boundaries, which may deteriorate the ultimate deformability. Therefore, if B is included, the B content should be 0.0020% or less. It is preferable that the B content be 0.0010% or less.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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 0.0003% or more, more preferably 0.0005% or more, and even more preferably 0.0010% 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.
[0038] 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. More preferably, it is 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. It is preferable that the Mg content be 0.0100% or less, and more preferably 0.0050% or less.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] The microstructure of the thin steel sheet of the present invention will be described below. Ferrite area ratio: 90% or more In order to obtain a thin steel sheet with excellent ductility and ultimate deformability, the ferrite area ratio should be 90% or more. Preferably, the ferrite area ratio should be 93% or more, and more preferably 95% or more. On the other hand, the upper limit of the ferrite area ratio is not particularly limited and may be 100%, but it is preferable that the ferrite area ratio is 98% or less.
[0045] The tissue other than ferrite (remaining tissue) is not particularly limited and may be 0%, but it may also contain second-phase tissues such as cementite or pearlite as the remaining tissue. The area ratio of the remaining tissue is preferably 10% or less, more preferably 7% or less, and even more preferably 5% or less.
[0046] Average grain size of ferrite: 10 μm or more. If the average grain size of ferrite is less than 10 μm, the ultimate deformability of the thin steel sheet decreases. Therefore, the average grain size of ferrite should be 10 μm or more. Preferably, the average grain size of ferrite should be 12 μm or more, and more preferably 15 μm or more. There is no particular upper limit to the average grain size of ferrite, but from the viewpoint of manufacturing load, preferably, the average grain size of ferrite should be 30 μm or less, and more preferably 25 μm or less.
[0047] Precipitation density of carbides with an equivalent circular diameter of 5 μm or more: 350 particles / mm２ From the perspective of improving the ultimate deformability of thin steel sheets, the precipitation density of carbides with an equivalent circular diameter of 5 μm or more is 350 particles / mm². ２ The following applies: The precipitation density is 340 particles / mm³. ２ Preferably, the following: 310 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 25 particles / mm ２ Preferably, the number is 50 or more per mm. ２ It is more preferable that the above conditions are met.
[0048] In the present invention, the area ratio of each phase in the above structure, the average grain size of ferrite, the equivalent circle diameter of the carbide, and the precipitation density can be determined by the method described in the examples.
[0049] The thin steel sheet in this invention may have a plating layer on one or both sides. Examples of the plating layer include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electroplated zinc layer, a tin plating layer, a Cr plating layer, and an aluminum plating layer (hot-dip aluminum plating layer). The type of plating layer is not limited to the above examples, and the conditions are not limited.
[0050] The thickness of the thin steel sheet of the present invention is not particularly limited, but it is preferably 0.30 mm or more. Furthermore, the thickness of the thin steel sheet of the present invention is preferably 4.0 mm or less.
[0051] 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 includes a hot rolling step in which a steel material having the above-mentioned component composition is heated to 1050°C or more and 1300°C or less, hot-rolled at a finish rolling completion temperature of 840°C or more and 960°C or less, cooled from the finish rolling completion temperature to a winding temperature of 550°C or more and 750°C or less at an average cooling rate CR1 of 20°C / s or more and 150°C / s or less, and wound at the winding temperature; a cold rolling step in which the hot-rolled steel sheet obtained after the hot rolling step is cold-rolled under the condition of a rolling ratio of 70% or more and 95% or less; and an annealing step in which the steel sheet obtained after the cold rolling step is annealed at a maximum attainable temperature of 730°C or more and 900°C or less, and then cooled from 730°C to 350°C at an average cooling rate CR2 of 80°C / s or less. In addition, the temperatures used when heating or cooling steel slabs (steel materials) or steel plates, etc., as described below, refer to the surface temperature of the steel slabs (steel materials), steel plates, etc., unless otherwise specified.
[0052] Manufacturing Method for Steel Slabs (Steel Material) The method for melting steel slabs having the above-described component composition is not particularly limited. Any known melting method, such as an electric furnace or 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 steel slabs (steel material) by a continuous casting method, but 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.
[0053] Hot rolling process heating temperature: 1050°C or higher and 1300°C or lower. If the heating temperature exceeds 1300°C, excessive surface oxidation occurs, and a large amount of coarse granular oxide is generated inside the surface layer of the steel sheet, which may degrade the ultimate deformability of the thin steel sheet that is ultimately obtained. For this reason, the heating temperature should be 1300°C or lower. It is preferable that the heating temperature be 1250°C or lower. On the other hand, if the heating temperature is below 1050°C, coarse carbides precipitated in the slab remain in the thin steel sheet that is ultimately obtained, reducing the ultimate deformability. For this reason, the heating temperature should be 1050°C or higher. It is preferable that the heating temperature be 1100°C or higher.
[0054] Finish Rolling Completion Temperature: 840°C to 960°C In order to improve the ultimate deformability of the thin steel sheet obtained in the end, it is important to homogenize the microstructure within the steel sheet during hot rolling. By setting the finish rolling completion temperature to 840°C or higher, the microstructure within the steel sheet can be sufficiently homogenized. Therefore, the finish rolling completion temperature should be 840°C or higher. Preferably, the finish rolling completion temperature should be 870°C or higher. On the other hand, if the finish rolling completion temperature exceeds 960°C, the austenite grains during hot rolling become coarser, which tends to produce coarse carbides that adversely affect the ultimate deformability. Therefore, the finish rolling completion temperature should be 960°C or lower. Preferably, the finish rolling completion temperature should be 930°C or lower.
[0055] Average cooling rate CR1 from the end of finish rolling temperature to the winding temperature: 20°C / s or more and 150°C / s or less. If the average cooling rate CR1 from the end of finish rolling temperature to the winding temperature is less than 20°C / s, the coarsening of Ti carbides becomes excessive, and these coarse carbides remain in the final thin steel sheet, reducing the ultimate deformability of the thin steel sheet. Therefore, the average cooling rate CR1 from the end of finish rolling temperature to the winding temperature should be 20°C / s or more. Preferably, the average cooling rate CR1 should be 25°C / s or more, and more preferably 30°C / s or more. On the other hand, if the average cooling rate CR1 exceeds 150°C / s, the amount of Ti carbide precipitation decreases and the amount of solid-solution carbon increases. Therefore, the average cooling rate CR1 from the end of finish rolling temperature to the winding temperature should be 150°C / s or less. The average cooling rate CR1 is preferably 120°C / s or less, and more preferably 100°C / s or less. Here, the average cooling rate CR1 (°C / s) is defined as "(cooling start temperature (finishing rolling end temperature) - cooling stop temperature (winding temperature)) / cooling time from cooling start to cooling stop (seconds)".
[0056] Winding temperature: 550°C or higher and 750°C or lower. If the winding temperature is below 550°C, the amount of Ti carbide precipitated decreases, increasing the amount of dissolved carbon in the steel and reducing the ductility of the thin steel sheet. Therefore, the winding temperature should be 550°C or higher. Preferably, the winding temperature should be 600°C or higher. On the other hand, if the winding temperature exceeds 750°C, the amount of coarse Ti carbide precipitated increases, reducing the ultimate deformability of the thin steel sheet. Therefore, the winding temperature should be 750°C or lower. Preferably, the winding temperature should be 720°C or lower.
[0057] After winding, H ２ SO ４ HCl, H ３ PO ４ Pickling may be performed with aqueous solutions such as those listed above.
[0058] Cold Rolling Process Rolling ratio in cold rolling: 70% to 95% After the hot rolling process described above, cold rolling is performed. In order to stably obtain excellent ultimate deformability, it is necessary to promote the recrystallization of ferrite during annealing and to avoid leaving an excess of unrecrystallized ferrite. In order to increase the driving force for recrystallization, the rolling ratio in cold rolling (cold rolling ratio (cumulative rolling ratio)) should be 70% or more. It is preferable that the cold rolling ratio be 75% or more. On the other hand, if the cold rolling ratio exceeds 95%, the ferrite grains of the thin steel sheet become finer, and the ductility decreases. Therefore, the cold rolling ratio should be 95% or less. It is preferable that the cold rolling ratio be 90% or less.
[0059] The maximum temperature reached during annealing is 730°C to 900°C, with an average cooling rate CR2 of 80°C / s or less from 730°C to 350°C. Annealing is performed after the cold rolling process described above. To ensure sufficient ductility and improve ultimate deformability by promoting ferrite recrystallization and grain growth, the maximum temperature reached during annealing is 730°C or higher. Preferably, the maximum temperature reached during annealing is 770°C or higher. On the other hand, if the maximum temperature reached during annealing exceeds 900°C, coarse carbides are formed, and the ultimate deformability decreases. Therefore, the maximum temperature reached during annealing is 900°C or lower. Preferably, the maximum temperature reached during annealing is 860°C or lower.
[0060] If the average cooling rate CR2 from 730°C to 350°C exceeds 80°C / s, the amount of dissolved carbon becomes excessive, reducing the ductility of the thin steel sheet. Therefore, the average cooling rate from 730°C to 350°C should be 80°C / s or less. There is no particular need to limit the lower limit, but it is preferable to set it at 2°C / s or more for improved manufacturing efficiency. Here, the average cooling rate (°C / s) is defined as "(cooling start temperature (730°C) - cooling stop temperature (350°C)) / cooling time from cooling start to cooling stop (seconds)".
[0061] Furthermore, the thin steel sheet in this invention may be subjected to temper rolling before or after plating, as needed. When temper rolling is performed, the conditions can be set as appropriate, but in order to ensure the ductility of the thin steel sheet, it is preferable that the rolling ratio be 5% or less. It is more preferable that the rolling ratio be 2% or less.
[0062] Plating Process After the annealing process described above, a plating process may be performed as needed to form a plating layer on at least one surface of the steel sheet.
[0063] 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, and electroplating. Examples of plating treatments other than zinc plating include aluminum plating treatments such as hot-dip aluminum plating, tin plating, and chromium plating. Among these, hot-dip galvanizing is preferred.
[0064] 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 0.190% 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 0.190% 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 0.190% 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. ２The 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.
[0065] 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 to 15% by mass.
[0066] 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.
[0067] In the case of molten aluminum plating, for example, a 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.
[0068] In the case of tin plating, it is preferable to form a tin-iron alloy layer by performing an alloying treatment (reflow treatment) after plating. The conditions for the plating and reflow treatments are not particularly limited and should be followed according to conventional methods. When performing tin plating, the amount of plating deposited per side is 1.0 g / m². ２The above is preferable. Furthermore, from the viewpoint of reducing the amount of Sn mixed into the iron scrap, the amount of plating deposited on one side should be 30 g / m². ２ The following is preferable.
[0069] In the case of chromium plating, there are no particular restrictions on the conditions; standard methods should be followed. When performing chromium plating, the amount of plating deposited per side should be 5 mg / m². ２ The above is preferable. The amount of plating deposited on one side is 200 mg / m². ２ The following is preferable.
[0070] Other than the conditions mentioned above, there are no specific limitations; you may follow the usual law.
[0071] Next, the component and its manufacturing method will be described. The component of the present invention is obtained by subjecting a thin steel sheet of the present invention to at least one of forming and joining processes. Furthermore, the manufacturing method of the component of the present invention includes the step of forming and joining a thin steel sheet of the present invention to obtain a component.
[0072] 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 corner portion after processing. Furthermore, when the component of the present invention is welded in a 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.
[0073] 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 corners and heat-affected zones 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.
[0074] 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.
[0075] Examples of the present invention are shown below. The present invention is not limited to the examples shown herein.
[0076] Steel slabs with the component compositions shown in Table 1, No. 1 to 24 (the remainder being Fe and unavoidable impurities), obtained by casting molten steel produced using an electric furnace, were subjected to hot rolling, cold rolling, and annealing under the conditions shown in Table 2 to obtain steel plates No. 1 to 29. In Table 1, "○" in formula (1) indicates steel that satisfies formula (1), and "×" indicates steel that does not satisfy formula (1). The thickness of the obtained steel plates (thin steel plates) was 0.7 mm.
[0077] Furthermore, for some steel sheets, various plating treatments were performed after annealing under the following conditions, and some of the thin steel sheets that underwent hot-dip galvanizing were further alloyed. In Table 2, thin steel sheets that were not plated are indicated as CR, thin steel sheets that underwent hot-dip galvanizing are indicated as GI, thin steel sheets that underwent both hot-dip galvanizing and alloying are indicated as GA, thin steel sheets that underwent electro-galvanizing are indicated as EG, thin steel sheets that underwent tin plating are indicated as Sn, and thin steel sheets that underwent hot-dip aluminum plating are indicated as Al. Unless otherwise specified, the conventional method was followed. [Hot-dip galvanizing (GI)] ・Plating bath composition: Zn, Al and unavoidable impurities (Al concentration in the bath: 0.10 to 0.20 mass%, remainder: Zn and unavoidable impurities) ・Plating bath temperature: 460℃ ・Plating adhesion amount per side: 30 to 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 / m ２ [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[Tin Plating (Sn)] - Plating amount per side: 3.0 g / m ２ • Alloying treatment (reflow treatment) temperature: 280°C • Alloying treatment (reflow treatment) time: 4 seconds
[0078] From the steel plate obtained for microstructural observation, a test specimen for microstructural observation was taken. After polishing the plate thickness cross section parallel to the rolling direction, the microstructure was revealed by etching with a 3 mass% concentration Nital solution. Using an optical microscope, the microstructure was photographed in three fields of view, each covering 1 / 4 of the plate thickness from one of the surfaces, at a magnification of 200x and a field of view of 0.42 mm × 0.33 mm. From the obtained optical microscope images, the area ratio of ferrite surrounded by grain boundaries was determined, and the steel plate microstructure was evaluated using the average value (sum of the area ratios of ferrite in the three fields (%) / 3). The remainder other than ferrite was evaluated as a second phase such as cementite. The average grain size of ferrite was determined by the sectioning method described in JIS G0551 (2020) and the average value of the three fields was used. More specifically, in each field of view, the average grain size per grain of the test line crossing the grain was determined, and the average of the average grain sizes of the three fields of view (sum of the average grain sizes of the three fields of view (μm) / 3) was defined as the average ferrite grain size.
[0079] From the aforementioned steel plate, specimens were taken for analysis of the extraction residue, measurement of the precipitation density of carbides, and tensile testing.
[0080] The amount of solid-solution carbon was calculated by analyzing the extraction residue using the following procedure. After electrolytic extraction with 10 vol% AA, filtering, and mixed acid decomposition of the sample taken from the steel plate, the amount of precipitated Fe, Ti, Nb, and Mo was measured by ICP-AES. From the amount of precipitated Fe, Ti, Nb, and Mo, the amount of carbide (Fe ３ C, TiC, NbC, Mo ２ The amount of C precipitated as C (precipitated C) was calculated, and the difference between the total C content (mass%) and the precipitated C amount (mass%) was defined as the amount of dissolved C (mass%). Note that the amount of C precipitated as carbides calculated from the precipitate amounts of elements other than Fe, Ti, Nb, and Mo is very small and may be included in the amount of dissolved C defined in this invention.
[0081] The precipitation density of carbides was measured using the following procedure. After taking a test piece from the steel plate, the cross section parallel to the rolling direction was polished, and the carbides were exposed by picral etching to obtain a sample for observation. An optical microscope was used to photograph a 1 / 4 thickness section at an observation magnification of 400x with a field of view of 0.32 mm × 0.22 mm. The precipitation density of carbides with an equivalent circle diameter of 5 μm or more was measured in three randomly selected fields of view, and the average value of the three fields of view (precipitation density of the three fields (pieces / mm)) was calculated. ２ The precipitation density of carbides in the steel plate was defined as (1) / 3). The equivalent circle diameter (μm) refers to the diameter of a perfect circle that has the area of one carbide, and the area of one carbide X (μm) ２ ) for 2 × (X / π) １／２ This is how it is determined. Furthermore, for a single carbide, in the microscopic image, a region where the outer edge is surrounded by something other than the carbide and is formed as a single, uninterrupted entity is measured as one unit.
[0082] Tensile tests were conducted according to the method compliant with JIS Z 2241 (2011), and the total elongation was evaluated.
[0083] The ultimate deformation capacity (ε) is the plate thickness (t) before the tensile test. ０ The thickness of the fractured section (t) (mm) after the test was measured and calculated using the following formula. The thickness of the fractured section 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. ε = -ln(t / t) ０ )...Formula (4)
[0084] Table 3 shows the evaluation results for solid solution carbon content, total elongation, and ultimate deformability. In all of the inventive examples in Table 3, the total elongation is 30.0% or more, and the ultimate deformability ε is 1.4 or more. Therefore, the inventive examples can be said to be thin steel sheets with excellent ductility and ultimate deformability.
[0085] On the other hand, comparative examples in which either the component composition or manufacturing conditions were outside the scope of the invention showed inferiority in either total elongation or ultimate deformability, or both, compared to the inventive example.
[0086] 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 corners and heat-affected zones exhibited excellent total elongation and ultimate deformation capabilities, similar to the steel sheet of the present invention, because the thin steel sheet of the present invention exhibited excellent total elongation and ultimate deformation capabilities.
[0087]
[0088]
[0089]
Claims
1. A material having a composition in mass% of the following elements: C: 0.0005% or more and 0.0100% or less, Si: 0.10% or less, Mn: 0.01% or more and 0.50% or less, P: 0.050% or less, S: 0.020% or less, Al: 0.100% or less, N: 0.0200% or less, Ti: 0.001% or more and 0.150% or less, Nb: 0.0001% or more and 0.0500% or less, Cu: 0.01% or more and 0.50% or less, Ni: 0.01% or more and 0.50% or less, Sn: 0.001% or more and 0.050% or less, satisfying the following formulas (1) and (2), with the remainder being Fe and unavoidable impurities, and having a structure in which the area ratio of ferrite is 90% or more. The ferrite has an average grain size of 10 μm or more, a solid solution carbon content represented by the following formula (3) is 0.0030 mass% or less, and the precipitation density of the carbide, which has an equivalent circle diameter of 5 μm or more, is 350 particles / mm². ２ The following is true for thin steel sheets: 0.1 × [%Cu] + 1.5 × [%Sn] ≤ [%Ni] ≤ 5.0 × [%Cu] + 75 × [%Sn] ... Equation (1) [%Ti] - ([%C] - [%Nb] × 12 / 93) × 48 / 12 - [%N] × 48 / 14 - ([%S] - [%Cu] × 32 / 64 - [%Mn] × 32 / 55) × 48 / 32 ≥ -0.100 ... Equation (2) (Solid solution C amount (mass%)) = (Total C content (mass%)) - (Amount of C present as carbides (mass%)) ... Equation (3) In equations (1) and (2), [%M] is the content (mass%) of element M in the thin steel sheet.
2. The above component composition is further defined in mass percent as follows: Cr: 0.40% or less, Mo: 0.100% or less, V: 0.050% or less, B: 0.0020% or less, Sb: 0.0500% 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, Sr: 0.0200% or less. A thin steel sheet according to claim 1, containing one or more elements selected from 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 at least one surface is provided with a plating layer, the plating layer being a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electroplated galvanized layer, a tin-plated 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: a hot rolling step of heating a steel material having the above-mentioned component composition at 1050°C to 1300°C, hot rolling it at a finish rolling completion temperature of 840°C to 960°C, cooling it from the finish rolling completion temperature to a winding temperature of 550°C to 750°C at an average cooling rate CR1 of 20°C / s to 150°C / s, and winding it at the winding temperature; a cold rolling step of performing cold rolling on the hot-rolled steel sheet obtained after the hot rolling step at a rolling ratio of 70% to 95%; and an annealing step of annealing the steel sheet obtained after the cold rolling step at a maximum temperature of 730°C to 900°C, and then cooling it from 730°C to 350°C at an average cooling rate CR2 of 80°C / s or less.
6. The method for manufacturing a thin steel sheet according to claim 5, comprising a plating step after the annealing step, wherein the plating step involves applying a hot-dip galvanizing treatment, alloyed hot-dip galvanizing treatment, electro-galvanizing treatment, tin plating treatment, or aluminum plating treatment to the surface of the thin steel sheet.
7. 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.