Steel sheet, plated steel sheet, and method for manufacturing steel sheet

Abstract

To provide a steel sheet having excellent high temperature strength while suppressing the amount of alloy addition by utilizing a microalloy.SOLUTION: Containing, by mass%, C:0.010% or more and 0.050% or less, Si: 0% or more and 1.00% or less, Mn: 0% or more and 0.80% or less, Al: 0.01% or more and 0.10% or less, Ti: 0.04% or more and 0.20% or less, B: 1ppm or more and 50ppm or less, and N: 0ppm or more and 80ppm or less, the balance being Fe and an inevitable impurity, a mass ratio (Ti) / (C) of Ti to C being 3.6 or more, An area ratio of a recrystallized structure in a plane perpendicular to a rolled direction is 25% or less, and circle-equivalent diameters of precipitates are 26nm or less when a position of 1 / 4 of a sheet thickness in the plane perpendicular to the rolled direction is observed by a transmission electron microscopy (TEM) or a scanning transmission electron microscopy (STEM).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel sheet, a plated steel sheet, and a method for manufacturing a steel sheet. [Background technology]

[0002] The Building Standards Act stipulates the fire resistance required for each part of a building to prevent damage from spreading in the event of a fire. Specifically, fire resistance refers to performance such as damage resistance, heat insulation, and flame resistance. Steel components (pillars, walls, floor underlayment, etc.) are used as structural materials to maintain the structure of a building, but general-purpose steel loses strength when exposed to high temperatures caused by fire, etc., and is no longer able to function as a structural member. For this reason, insulation materials are used in combination to prevent the temperature of steel components from becoming too high even in the event of a fire.

[0003] On the other hand, if the high-temperature strength of steel can be increased, it will be possible to maintain its structure even when exposed to high temperatures such as those caused by a fire. In other words, if the high-temperature strength of steel can be increased, it is expected that the same current structure will be able to maintain fire resistance and fire prevention for a long period of time, or that the same fire resistance and fire prevention performance can be maintained for the same period of time even with a reduced amount of insulation used. For this reason, improving the high-temperature strength of steel has long been studied.

[0004] For example, by adding Cu (copper) to a steel material, the high-temperature strength of the steel material can be improved by cluster strengthening and precipitation of Cu. Specifically, Patent Document 1 describes a low-yield-ratio, high-strength hot-dip galvanized cold-rolled steel sheet for construction, in which a steel slab contains, by weight, 0.02 to 0.1% C, Si≦0.3%, Mn: 0.3 to 1.5%, P≦0.05%, Al≦0.05%, and Cu: 0.6 to 2.0%, with the balance being Fe and unavoidable impurities.

[0005] Furthermore, high-temperature strength can be ensured by adding Mo (molybdenum) or W (tungsten) to the steel material to precipitate carbides of Mo or W. Specifically, Patent Document 2 describes a fire-resistant high-tensile hot-dip Zn-Al alloy-plated steel sheet that uses steel containing, by weight percent, C: 0.01 to 0.25, Si: 1.5 or less, Mn: 0.05 to 2.5, P: 0.1 or less, S: 0.02 or less, Al: 0.005 to 0.1, Mo: 0.05 to 1.0, and the balance being iron and unavoidable impurities. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 3-226520 [Patent Document 2] Japanese Patent Application Publication No. 5-306411 Summary of the Invention [Problem to be solved by the invention]

[0007] However, to fully obtain the precipitation strengthening effect of elements such as Cu, Mo, and W, it is necessary to add large amounts of these elements. For example, Patent Document 1 requires 0.6% or more of Cu, and Patent Document 2 requires 0.05% or more of Mo (actually 0.4% or more), or if W is added alone, it requires a large amount of 0.10% or more (actually 0.33% or more). Even if the effect of improving high-temperature strength can be obtained by adding large amounts of Cu, Mo, or W, there is a problem that the effect does not justify the increase in cost because these elements are expensive.

[0008] On the other hand, from the viewpoint of precipitation strengthening, elements called microalloys, such as Ti and Nb, are known to be effective. These elements are relatively inexpensive, and even adding trace amounts of 0.2% or less can achieve precipitation strengthening effects.

[0009] However, to obtain the precipitation strengthening effect of elements called microalloys, it is necessary to heat the steel to a high temperature of, for example, 1000°C or higher to form a solid solution, and then finely precipitate the microalloys during subsequent cooling or reheating after cooling. Cold-rolled steel sheets and plated steel sheets are produced by hot-rolling a steel material, followed by cold-rolling, and then heating it to approximately 700 to 900°C in a continuous annealing furnace and then cooling it. With this production method, the microalloys cannot be solid-dissolved during heating in the annealing furnace, and the precipitates coarsen during heating. In other words, the amount of precipitation strengthening decreases, making it difficult to ensure high-temperature strength. Therefore, cold-rolled steel sheets and plated steel sheets using microalloys and having excellent high-temperature strength have not been realized.

[0010] In view of this situation, an object of the present invention is to provide a steel sheet that utilizes microalloys and has excellent high-temperature strength while reducing the amount of alloy added. [Means for solving the problem]

[0011] As a result of extensive research to solve the above problems, the present inventors have found that the above problems can be solved by the following configuration, and have completed the present invention through further research based on this finding.

[0012] A steel sheet according to one embodiment of the present invention contains, by mass%, C: 0.010% to 0.050%, Si: 0% to 1.00%, Mn: 0% to 0.80%, Al: 0.01% to 0.10%, Ti: 0.04% to 0.20%, B: 1 ppm to 50 ppm, and N: 0 ppm to 80 ppm, with the balance being iron and inevitable impurities, a mass ratio of Ti to C (Ti) / (C) of 3.6 or more, an area fraction of recrystallized structures in a plane perpendicular to the rolling direction of 25% or less, and when a position at 1 / 4 of the sheet thickness in the plane perpendicular to the rolling direction is observed with a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM), the circle equivalent diameter of precipitates is 26 nm or less. [Effects of the Invention]

[0013] According to the present invention, by utilizing microalloys, it is possible to provide a steel sheet that has excellent high-temperature strength while reducing the amount of alloy added. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to these.

[0015] <Steel plate> The steel sheet according to this embodiment will be described below. The type of steel sheet according to this embodiment is not particularly limited, but examples thereof include cold-rolled steel sheet and plated steel sheet. Regarding the chemical composition of the steel sheet according to this embodiment, first, the basic elements will be described, and then elements that may be selectively added will be described. Note that in the following description of the chemical composition, "%" always means "% by mass."

[0016] [Chemical composition] The steel plate of this embodiment contains C (carbon): 0.010% or more and 0.050% or less, Si (silicon): 0% or more and 1.00% or less, Mn (manganese): 0% or more and 0.80% or less, Al (aluminum): 0.01% or more and 0.10% or less, Ti (titanium): 0.04% or more and 0.20% or less, B (boron): 1 ppm or more and 50 ppm or less, and N (nitrogen): 0 ppm or more and 80 ppm or less, and the mass ratio of Ti to C (Ti) / (C) is 3.6 or more.

[0017] (C: 0.010% or more and 0.050% or less) C is an element necessary for bonding with microalloys and forming fine precipitates that contribute to precipitation strengthening, thereby ensuring the high-temperature strength of the steel sheet. If the C content is insufficient, the high-temperature strength of the steel sheet will decrease. In order to ensure the high-temperature strength of the steel sheet, the C content is set to 0.010% or more. The lower limit of the C content is preferably 0.015% or more, more preferably 0.020% or more. On the other hand, if the C content is excessive, the microalloys will become too stable, and some of the microalloys will remain undissolved during hot rolling, preventing the formation of sufficient fine precipitates around them. In other words, this may result in a decrease in high-temperature strength. Therefore, the C content is set to 0.050% or less. The upper limit of the C content is preferably 0.045% or less, more preferably 0.040% or less.

[0018] (Si: 0% or more and 1.00% or less) Si is a solid solution strengthening element and is an element that effectively improves the high-temperature strength of steel sheet. However, excessive Si content may promote the formation of microalloy precipitates during hot rolling, causing them to coarsen, which may result in a decrease in high-temperature strength. Therefore, the upper limit of the Si content is set to 1.00% or less. The upper limit of the Si content is preferably 0.80% or less, more preferably 0.60% or less. Furthermore, the lower limit of the Si content is 0% or more, but may be 0.02% by mass or more, or may be 0.05% or more.

[0019] (Mn: 0% or more and 0.80% or less) Mn has the effect of delaying transformation during cooling after hot rolling and cooling after annealing, thereby ensuring the strength of the steel sheet. The lower limit of the Mn content is 0% or more, preferably 0.02% or more, and more preferably 0.05% or more. On the other hand, in the present invention, it is preferable to minimize the precipitation of precipitates during cooling after hot rolling, or to make the precipitation fine, if any. To achieve this, it is preferable to maximize the speed of interfacial migration of the ferrite phase during transformation during cooling after hot rolling. Here, Mn has the effect of lowering the transformation temperature and delaying the interfacial migration speed during transformation. Adding a large amount of Mn reduces the interfacial migration speed of the ferrite phase during hot rolling, making it easier for coarse precipitates to form on the interface during the interfacial migration. This makes it difficult to sufficiently form fine precipitates after annealing, which may result in a decrease in high-temperature strength. Therefore, the Mn content is set to 0.80% or less. The upper limit of the Mn content is preferably 0.6% or less, more preferably 0.5% or less.

[0020] (Al: 0.01% or more and 0.10% or less) Al is an element that acts as a deoxidizer in steel. To effectively exert this function, the Al content is set to 0.01% or more. The lower limit of the Al content is preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, if the Al content is excessive, a large amount of inclusions such as alumina is generated in the steel sheet, which may cause fracture during processing of the steel sheet. Therefore, the Al content is set to 0.10% or less. The upper limit of the Al content is preferably 0.09% or less, and more preferably 0.08% or less.

[0021] (Ti: 0.04% or more and 0.20% or less) Ti is an element that improves high-temperature strength by combining with carbon to form precipitates, and is a type of microalloy. Even when the amount of Ti necessary for precipitation strengthening is added, the Ti combined with carbon can be solid-dissolved at a heating temperature that can be achieved during hot rolling. Then, by uniformly and finely precipitating Ti in the subsequent manufacturing process, high-temperature strength can be ensured. To effectively exert this effect, the Ti content is set to 0.04% or more. The lower limit of the Ti content is preferably 0.06% or more, more preferably 0.08% or more. On the other hand, an excessive Ti content can result in excess Ti carbide, which may cause cracking during processing of the steel sheet. Therefore, the Ti content is set to 0.20% or less, preferably 0.18% or less, more preferably 0.15% or less.

[0022] (B: 1ppm or more and 50ppm or less) B is an element that segregates to austenite grain boundaries during heating during hot rolling, thereby delaying the nucleation of the ferrite phase during cooling and lowering the temperature of the ferrite transformation. However, unlike alloying elements such as Mn, B does not affect the interfacial migration speed of the ferrite phase. Therefore, the formation of coarse precipitates on the interface during transformation can be avoided, and fine precipitates can be secured after annealing, ensuring high-temperature strength. To effectively exert this effect, the B content is set to 1 ppm or more. The B content is preferably 3 ppm or more, and more preferably 5 ppm or more. On the other hand, if the B content is excessive, the effect saturates and only costs increase. Therefore, the B content is set to 50 ppm or less. The B content is preferably 30 ppm or less, and more preferably 25 ppm or less.

[0023] (N: 0ppm or more and 80ppm or less) N strongly bonds with microalloys to form nitrides that do not dissolve even when heated during hot rolling. This reduces the amount of microalloys that can contribute to precipitation strengthening, adversely affecting high-temperature strength. Therefore, it is preferable to reduce the N content as much as possible, with an upper limit of 80 ppm. The N content is more preferably 70 ppm or less, and even more preferably 60 ppm or less. The N content may be 0 ppm, and although there is no particular limit on the lower limit, it is usually contained at 0.0001% or more.

[0024] (Mass ratio of Ti to C: (Ti) / (C): 3.6 or more) The mass ratio of Ti to C (Ti) / (C) is an indicator of which component remains in a state other than TiC precipitates when titanium (microalloy) and carbon combine. If (Ti) / (C) is 4.0 or higher, Ti remains; if it is lower than 4.0, C remains. Even if coarsening of precipitates is avoided by hot rolling, if reverse transformation occurs during annealing beyond the A1 point, the precipitates in the reverse-transformed region will coarsen. These coarsened precipitates will remain even after subsequent cooling and retransformation. This results in insufficient high-temperature strength. As the amount of residual C increases, the A1 point decreases, making reverse transformation more likely during annealing. This leads to coarsening of the precipitates in the reverse-transformed region, resulting in insufficient high-temperature strength. Considering the allowable amount of residual C from the perspective of the A1 point, we found that a mass ratio of Ti to C (Ti) / (C) of 3.6 or higher can achieve the desired microstructure without reverse transformation during annealing. Therefore, the mass ratio of Ti to C (Ti) / (C) is 3.6 or more, preferably 3.8 or more, and more preferably 4.0 or more.

[0025] If the amount of residual Ti increases, Ti becomes less likely to dissolve during heating in hot rolling, and the precipitates tend to become coarse during this process, which may result in coarse Ti precipitates remaining in the final product. Therefore, the mass ratio of Ti to C, (Ti) / (C), is preferably 5.0 or less, and more preferably 4.5 or less.

[0026] (Remainder) The basic components of the steel sheet according to this embodiment are as described above, with the remainder consisting of iron and inevitable impurities. However, impurities (e.g., P (phosphorus), S (sulfur), O (oxygen), etc.) inevitably introduced due to the conditions of raw materials, materials, manufacturing equipment, etc. are permitted to be mixed in.

[0027] (P:0.03% or less) P segregates at grain boundaries and causes embrittlement, so it is preferable to reduce its content as much as possible. Therefore, the upper limit of the P content is preferably set to 0.03%. The upper limit of the P content is more preferably 0.02% or less. Note that P is an impurity that is inevitably mixed into steel, and it is impossible to reduce its content to 0% in industrial production, so it is usually contained at 0.0005% or more.

[0028] (S:0.01% or less) S forms sulfide-based inclusions in steel that can cause fracture and reduce performance, so it is preferable to reduce the S content as much as possible. Therefore, the upper limit of the S content is preferably 0.01%. The upper limit of the S content is more preferably 0.005% or less. Since the S content is best as low as possible, there is no particular lower limit, but reducing the S content to 0% is impossible in industrial production, so the S content is usually 0.0001% or more.

[0029] (O: 0.002% or less) O forms oxide-based inclusions in steel, which reduces performance, so it is preferable to reduce the O content as much as possible. Therefore, the upper limit of the O content is preferably set to 0.002%. The upper limit of the O content is more preferably 0.001% or less. Since the O content is best as low as possible, there is no particular lower limit, but reducing the O content to 0% is impossible in industrial production, so the O content is usually 0.0001% or more.

[0030] (Other elements) Any other elements may be further contained as long as the properties of the steel sheet according to this embodiment can be maintained. Examples of other elements that can be selectively contained in this way are listed below.

[0031] If necessary, the steel sheet of this embodiment may also effectively contain one or more other microalloy elements, which are equivalent to Ti, selected from the group consisting of V (vanadium), Nb (niobium), W (tungsten), Ta (tantalum), Zr (zirconium), and Hf (hafnium). Like Ti, these elements bond with carbon to form precipitates, improving high-temperature strength. When adding these elements, it is preferable to set the upper limit of the amount of each element to 0.05% or less. These elements will be described in more detail below.

[0032] (V:0.05% or less) V is an element that contributes to increasing the strength of steel sheets. To achieve this effect, the V content is preferably 0.01% or more. However, if the V content is excessive, the effect saturates and costs increase. Therefore, the V content is preferably 0.05% or less, and more preferably 0.03% or less.

[0033] (Nb:0.05% or less) Nb is an element that contributes to increasing the strength of steel sheets. To achieve this effect, the Nb content is preferably 0.01% or more. However, excessive Nb content deteriorates the hardenability of the steel sheets. Therefore, the Nb content is preferably 0.05% or less, and more preferably 0.03% or less.

[0034] (W: 0.05% or less) W is an element that contributes to increasing the strength of steel sheets. To achieve this effect, the W content is preferably 0.01% or more. However, if the W content is excessive, the effect saturates and costs increase. Therefore, the W content is preferably 0.05% or less, and more preferably 0.03% or less.

[0035] (Ta:0.05% or less) Ta is an element that contributes to increasing the strength of steel sheets. To achieve this effect, the Ta content is preferably 0.01% or more. However, if the Ta content is excessive, the effect saturates and costs increase. Therefore, the Ta content is preferably 0.05% or less, and more preferably 0.03% or less.

[0036] (Zr:0.05% or less) Zr is an element that contributes to increasing the strength of steel sheets. To achieve this effect, the Zr content is preferably 0.01% or more. However, if the Zr content is excessive, the effect saturates and costs increase. Therefore, the Zr content is preferably 0.05% or less, and more preferably 0.03% or less.

[0037] (Hf:0.05% or less) Hf is an element that contributes to increasing the strength of steel sheets. To achieve this effect, the Hf content is preferably 0.01% or more. However, if the Hf content is excessive, the effect saturates and costs increase. Therefore, the Hf content is preferably 0.05% or less, and more preferably 0.03% or less.

[0038] The steel sheet of this embodiment may optionally contain one or more elements selected from the group consisting of Cr (chromium), Mo (molybdenum), Cu (copper), Ni (nickel), and Co (cobalt), which are elements similar to Mn that are involved in ferrite transformation. These elements may slow the interfacial migration speed during ferrite transformation and cause coarsening of precipitates, so it is preferable to avoid adding them as much as possible, but each element may be added in an amount of 0.20% or less. These elements will be described in more detail below.

[0039] (Cr:0.20% or less) Cr contributes to improving the toughness of the base material. However, excessive Cr content may slow the interface migration speed during ferrite transformation and cause coarsening of precipitates. Therefore, the upper limit of the Cr content is preferably 0.20% or less. The Cr content is more preferably 0.15% or less. Furthermore, it is preferable to avoid adding Cr as much as possible, but if added, the lower limit of the Cr content may be 0.01% or more.

[0040] (Mo: 0.20% or less) Mo has the effect of increasing the strength and toughness of the base material. However, excessive inclusion of Mo may slow down the interfacial migration speed during ferrite transformation and cause coarsening of precipitates. Therefore, the upper limit of the Mo content is preferably 0.20% or less. The Mo content is more preferably 0.15% or less. Furthermore, it is preferable to avoid adding Mo as much as possible, but if added, the lower limit of the Mo content may be 0.01% or more.

[0041] (Cu:0.20% or less) Cu has the effect of improving the strength of the base material through solid solution strengthening and precipitation strengthening. However, excessive Cu content may slow the interface migration speed during ferrite transformation and cause coarsening of precipitates. Therefore, the upper limit of the Cu content is preferably 0.20% or less. The Cu content is more preferably 0.15% or less. Furthermore, it is preferable to avoid adding Cu as much as possible, but if added, the lower limit of the Cu content may be 0.01% or more.

[0042] (Ni:0.20% or less) Ni has the effect of enhancing the toughness of the base material. However, excessive Ni content may slow down the interfacial migration speed during ferrite transformation and cause coarsening of precipitates. Therefore, the upper limit of the Ni content is preferably 0.20% or less. The Ni content is more preferably 0.15% or less. Furthermore, it is preferable to avoid adding Ni as much as possible, but if added, the lower limit of the Ni content may be 0.01% or more.

[0043] (Co:0.20% or less) Co has the effect of enhancing the toughness of the base material. However, excessive Co content may slow down the interfacial migration speed during ferrite transformation and cause coarsening of precipitates. Therefore, the upper limit of the Co content is preferably 0.20% or less. The Co content is more preferably 0.15% or less. Furthermore, it is preferable to avoid adding Co as much as possible, but if Co is added, the lower limit of the Co content may be 0.01% or more.

[0044] (Mg, Li, Na, Ca, REM: 30ppm or less each) Furthermore, the steel sheet of this embodiment may contain, as necessary, one or more elements selected from the group consisting of Mg (magnesium), Li (lithium), Na (sodium), Ca (calcium), and REM (rare earth elements). These elements contribute to improving the formability of the steel sheet. Furthermore, these elements contribute to refining inclusions and are expected to suppress deterioration of properties due to the presence of inclusions. To achieve this effect, the amount of each element is preferably 1 ppm or more. On the other hand, excessive amounts of these elements deteriorate the pickling properties, weldability, and hot workability of the steel sheet. Furthermore, costs increase. Therefore, the upper limit of the amount of each element is preferably 30 ppm or less, and more preferably 20 ppm or less.

[0045] [Organization] (Recrystallization rate) In the structure of the steel sheet according to this embodiment, the area fraction of the recrystallized structure (recrystallization fraction) in a plane perpendicular to the rolling direction is 25% or less. Even if coarsening of precipitates is avoided by hot rolling, if recrystallization occurs during annealing, the precipitates will coarsen at the grain boundaries along which the recrystallized grains move as they grow. Therefore, when recrystallized grains are formed, coarse precipitates are formed within them, making it impossible to ensure high-temperature strength. Therefore, the area fraction of the recrystallized structure is set to 25% or less. The area fraction of the recrystallized structure is preferably 20% or less, and more preferably 18% or less. Furthermore, the area fraction of the recrystallized structure is preferably close to 0%, and the lower limit is not particularly limited.

[0046] The recrystallization rate is generally evaluated by evaluating the recrystallized area ratio through microstructural observation. However, the evaluation of whether grains are recrystallized or not through microstructural observation may involve subjective observation by the observer. Therefore, in this embodiment, the area ratio of the recrystallized structure (recrystallization rate) was evaluated using hardness evaluation as follows. First, a load was set so that the indentation size was equal to or smaller than the crystal grain size (approximately several tens of μm). Next, hardness measurements were performed at 100 or more points within a range of a certain area or larger on the plate surface, and regions with a certain hardness or less were defined as recrystallized regions. The recrystallization rate (%) was then defined as the proportion of measurement points with a certain hardness or less to all measurement points. More specifically, in a micro-Vickers hardness test using a 200 gf load and a 0.8 mm thick plate, hardness measurements were performed at 6 points with a 0.1 mm pitch from the front surface to 0.1 mm from the back surface, and at 20 points with a 0.1 mm pitch in the direction perpendicular to the plate thickness, for a total of 120 points. Points with a Vickers hardness of less than 160 Hv were defined as recrystallized grains, and the proportion of recrystallized regions among all measurement points (120 points) was calculated as the recrystallization rate.

[0047] (precipitate) In the steel sheet according to this embodiment, when a position at 1 / 4 of the sheet thickness on a surface perpendicular to the rolling direction is observed with a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM), the circle-equivalent diameter of the observed precipitates is 26 nm or less. Examples of the precipitates include precipitates containing microalloys and precipitates not containing microalloys. Examples of the precipitates containing microalloys include precipitates containing Ti (e.g., TiC) and precipitates containing multiple microalloys such as Ti, Nb, and Mo (e.g., TiNb). Examples of the precipitates not containing microalloys include cementite. The position at 1 / 4 of the sheet thickness where the circle-equivalent diameter of the precipitates is observed is because this is the position that shows the most representative characteristics of the steel sheet.

[0048] To ensure high-temperature strength, it is important that fine precipitates are uniformly present. On the other hand, even if a large amount of fine precipitates are present, if coarse precipitates are also present, the microalloys surrounding the coarse precipitates will be concentrated in the coarse precipitates, forming regions where fine precipitates cannot exist. If such regions increase, deformation in regions with few fine precipitates will be promoted, especially at high temperatures, resulting in a decrease in high-temperature strength. Therefore, it is necessary to suppress the circle-equivalent diameter of the precipitates to a certain size or less. The circle-equivalent diameter of the precipitates is preferably 25 nm or less, more preferably 24 nm or less. The circle-equivalent diameter of the precipitates is preferably 5 nm or more, more preferably 8 nm or more.

[0049] The equivalent circle diameter of a precipitate can be determined as follows. Specifically, using a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM), the precipitates are observed at a position 1 / 4 of the sheet thickness on a surface perpendicular to the rolling direction. The average value of the first to ten largest sizes of the observed precipitates is calculated, and this is defined as the equivalent circle diameter of the precipitates in the steel sheet. More specifically, the equivalent circle diameter of the precipitates can be determined by the method described in the examples below.

[0050] [Strength] The high-temperature strength of the steel plate according to this embodiment is preferably 280 MPa or more. In this specification, the high-temperature strength of the steel plate can be determined as the yield strength measured by conducting a tensile test at 500°C. The yield strength at 500°C is measured by conducting a tensile test in accordance with JIS G 0567. If the measured yield strength at 500°C is 280 MPa or more, it can be determined that excellent high-temperature strength is ensured. The yield strength of the steel plate at 500°C is more preferably 290 MPa or more, and even more preferably 300 MPa or more.

[0051] The tensile strength of the steel sheet at room temperature (20°C) is preferably 400 MPa or more, more preferably 420 MPa or more, and even more preferably 440 MPa or more. In this specification, the tensile strength at room temperature is measured by conducting a tensile test in accordance with JIS Z 2241.

[0052] The yield strength of the steel plate at room temperature (20°C) is preferably 295 MPa or more, more preferably 320 MPa or more, and even more preferably 350 MPa or more. In this specification, the yield strength at room temperature is measured by conducting a tensile test in accordance with JIS Z 2241.

[0053] From the viewpoint of improving corrosion resistance, the steel sheet can be made into a plated steel sheet by forming a metal plating on its surface. Note that the high-temperature strength of the steel sheet at 500°C as described above does not change whether or not it is plated. In other words, the proof stress of the plated steel sheet in this embodiment at 500°C is preferably 280 MPa or more, more preferably 290 MPa or more, and even more preferably 300 MPa or more.

[0054] The tensile strength and yield strength of the plated steel sheet at room temperature do not change whether the steel sheet is plated or not. That is, the tensile strength of the plated steel sheet in this embodiment at room temperature is preferably 400 MPa or more, more preferably 420 MPa or more, and even more preferably 440 MPa or more. The yield strength of the plated steel sheet at room temperature is preferably 295 MPa or more, more preferably 320 MPa or more, and even more preferably 350 MPa or more.

[0055] <Steel sheet manufacturing method> The steel sheet of this embodiment can be manufactured, for example, by the following procedure: The steel sheet of this embodiment can be manufactured from a steel material (base sheet) having a predetermined chemical composition by the method described in detail below.

[0056] Specifically, a method for producing the steel sheet of this embodiment includes a method in which a steel material (original sheet) having a predetermined chemical composition is heated at a temperature of 1000°C to 1300°C for 5 minutes to 5 hours, hot-rolled at a finishing rolling temperature of 850°C or higher, cooled to 300°C to 600°C and coiled, unwound from the hot-rolled steel sheet, cold-rolled at a cold-rolling reduction of 10% to 45%, heated to a maximum heating temperature of 600°C to 920°C, and annealed by holding at the maximum heating temperature for 5 seconds to 1800 seconds. The method for producing the steel sheet of this embodiment is described in detail below.

[0057] [Preparing steel material for rolling] First, a steel material such as a slab for rolling having a desired chemical composition is prepared. The chemical composition of the steel material can be explained in the same manner as the chemical composition of the steel plate described above. The steel material such as a slab can be prepared by any known method. Examples of methods for preparing a slab include a method in which steel having a desired chemical composition is melted and then the slab is prepared by ingot casting or continuous casting. If necessary, the cast material obtained by ingot casting or continuous casting may be subjected to blooming to obtain a slab.

[0058] [Hot rolling] Next, the obtained steel material such as a slab is hot-rolled to obtain a hot-rolled steel sheet. In hot rolling, the slab is soaked at a temperature of 1000°C or higher and 1300°C or lower for 5 minutes to 5 hours according to a conventional method. This is a process necessary to temporarily dissolve the microalloy. If the heating temperature is too low or the heating time is too short, the microalloy cannot be dissolved, and precipitates may remain in the steel sheet after hot rolling. On the other hand, if the heating temperature is too high or the heating time is too long, manufacturability is poor. The lower limit of the heating temperature in hot rolling is preferably 1050°C or higher, more preferably 1100°C or higher. On the other hand, the upper limit of the heating temperature in hot rolling is preferably 1280°C or lower, more preferably 1250°C or lower. In addition, the lower limit of the heating time in hot rolling is preferably 10 minutes or higher, more preferably 15 minutes or higher. On the other hand, the upper limit of the heating time in the hot rolling is preferably 4.0 hours or less, and more preferably 3.0 hours or less.

[0059] After heating the steel material as described above, hot rolling is performed so that the hot rolling finishing temperature is 850°C or higher. If the hot rolling finishing temperature is low, the strain introduced by the hot rolling may cause the microalloy to precipitate coarsely during the hot rolling stage, which may deteriorate the high-temperature strength of the final annealed sheet. Therefore, the hot rolling finishing temperature is set to 850°C or higher. The lower limit of the hot rolling finishing temperature is preferably 860°C or higher, more preferably 880°C or higher. However, if the hot rolling finishing temperature is too high, the strength of the hot-rolled steel sheet may become excessively high. Therefore, the hot rolling finishing temperature is preferably 1000°C or lower, more preferably 980°C or lower.

[0060] [Coiling of hot-rolled steel sheets] After hot rolling, the hot-rolled steel sheet is cooled to 300°C or higher and 600°C or lower and then coiled. If the temperature at which the hot-rolled steel sheet is cooled and then coiled into a coil is high, there is a concern that precipitates may precipitate and become coarse during coiling, so coiling is preferably performed at a low temperature. Therefore, the coiling temperature for the hot-rolled steel sheet is set to 600°C or lower, preferably 570°C or lower, and more preferably 550°C or lower. On the other hand, if the temperature is too low, the strength of the steel material may become too high, making coiling difficult and causing a problem in the manufacturing process. Therefore, the coiling temperature for the hot-rolled steel sheet is set to 300°C or higher, preferably 340°C or higher, and more preferably 380°C or higher.

[0061] Furthermore, the cooling of the hot-rolled steel sheet obtained by hot rolling from finish rolling to coiling is preferably rapid cooling immediately after finish rolling. Therefore, it is preferable to rapidly cool to 600°C or less within 20 seconds after finish rolling. The average cooling rate of the hot-rolled steel sheet obtained by hot rolling from finish rolling to coiling is preferably 10°C / s or more, more preferably 20°C / s or more, in consideration of productivity. On the other hand, if the average cooling rate is too high, the equipment costs will increase. Therefore, the average cooling rate is preferably 200°C / s or less, more preferably 150°C / s or less. The coiled hot-rolled steel sheet after coiling may be naturally cooled to room temperature.

[0062] Furthermore, the coiled hot-rolled steel sheet may be pickled before cold rolling, which will be described later. The pickling method is not particularly limited, and any known method may be applied. For example, the steel sheet may be immersed in hydrochloric acid or the like to remove scale.

[0063] [Cold rolling] Next, the coiled hot-rolled steel sheet is unwound and cold-rolled at a cold-rolling reduction of 10% to 45%. This results in a cold-rolled steel sheet of a predetermined thickness. The rolling reduction in this embodiment is synonymous with the "rolling reduction." Specifically, when the thickness of the steel sheet before rolling is h1 and the thickness of the steel sheet after rolling is h2, the rolling reduction (%) is "(h1-h2) / h1×100."

[0064] If the cold rolling reduction is too high, recrystallization during heating is promoted in the annealing process described below, and as the recrystallization progresses, the precipitates become coarse. This may result in a deterioration of the high-temperature strength of the final product. Therefore, the cold rolling reduction is set to 45% or less, preferably 42% or less, and more preferably 40% or less. On the other hand, in order to uniformly and finely precipitate Ti that remains in solid solution during hot rolling and the subsequent cooling process without precipitating, it is necessary to introduce dislocations into the steel that serve as preferential nucleation sites for precipitates. Therefore, it is necessary to ensure a cold rolling reduction of at least a certain level. Therefore, the cold rolling reduction is set to 10% or more, preferably 15% or more, and more preferably 20% or more.

[0065] The thickness of the steel sheet after cold rolling is preferably 0.4 mm or more and 2.0 mm or less. By being in this range, the steel sheet is likely to exhibit the fire-resistant structural function required when applied to building components.

[0066] The cold rolling finishing temperature is preferably -60°C or higher and 150°C or lower. If the cold rolling finishing temperature is too low, low-temperature embrittlement may occur, which may result in cracks during cold rolling. Therefore, the cold rolling finishing temperature is preferably -60°C or higher. The lower limit of the cold rolling finishing temperature is more preferably -50°C or higher, and even more preferably -40°C or higher. However, if the cold rolling finishing temperature is too high, strain aging may occur during processing, which may result in embrittlement and cracks during rolling. Therefore, the cold rolling finishing temperature is preferably 150°C or lower, more preferably 130°C or lower, and even more preferably 120°C or lower.

[0067] [Annealing] Annealing usually involves heating the cold-rolled steel sheet obtained by the above-mentioned cold rolling, soaking the heated steel sheet, and cooling the steel sheet after soaking. Annealing is necessary because cold-rolled steel sheets lack ductility and cannot be formed. On the other hand, depending on the annealing conditions, recrystallization or reverse transformation may occur, causing the precipitates to coarsen during this process, making it impossible to ensure the desired properties. Therefore, the maximum heating temperature when heating cold-rolled steel sheets is set to 600°C or higher and 920°C or lower. It is preferably 650°C or higher and 900°C or lower, and more preferably 680°C or higher and 880°C or lower.

[0068] The heating rate when heating the cold-rolled steel sheet is not particularly limited, but is preferably 1°C / sec or more and 100°C / sec or less. By setting the heating rate in this range, Ti precipitates can be appropriately distributed during heating. The heating rate when heating the cold-rolled steel sheet is more preferably 2°C / sec or more and 50°C / sec or less.

[0069] The soaking temperature when the heated cold-rolled steel sheet is soaked is preferably 600°C or higher and 920°C or lower. By setting the temperature in this range, the strain introduced by cold rolling can be appropriately recovered and Ti precipitates can be finely precipitated in the matrix. The soaking temperature when the cold-rolled steel sheet is soaked is more preferably 650°C or higher and 900°C or lower.

[0070] The soaking time when the heated cold-rolled steel sheet is soaked is 5 seconds or more and 1800 seconds or less. By setting the soaking time in this range, it is possible to finely disperse Ti precipitates while appropriately recovering the strain introduced by cold rolling, and it is possible to suppress coarsening of the Ti precipitates due to recrystallization. The soaking time when the heated cold-rolled steel sheet is soaked is preferably 10 seconds or more and 1500 seconds or less, and more preferably 15 seconds or more and 1200 seconds or less.

[0071] When cooling the steel sheet after soaking, the cooling rate and cooling pattern are not particularly limited, and any cooling may be performed, including reheating to a temperature below the heating temperature. The cooling rate is not particularly limited, but is preferably 1°C / sec or more and 200°C / sec or less, and more preferably 2°C / sec or more and 100°C / sec or less.

[0072] When such annealing is carried out, there are no limitations on the type of annealing furnace or the atmosphere composition, but for example, annealing can be carried out using a batch annealing furnace in an atmosphere of hydrogen, nitrogen or a mixture thereof.

[0073] Although the present invention is directed to steel sheets, the product form is not particularly limited. After the above-mentioned hot rolling and cold rolling, the steel sheets can be annealed and then subjected to plating treatments such as chemical conversion coating, hot-dip galvanizing, electrogalvanizing, galvannealed hot-dip galvanizing, and vapor deposition, as well as various painting processes, paint primer treatments, and organic coating treatments. For example, the plating treatment may involve applying galvanization to a steel sheet that has been heat-treated in a process such as a continuous annealing line (CAL or CAPL) in a hot-dip galvanizing line or electrogalvanizing line, or applying heat treatment and hot-dip galvanization in a continuous hot-dip galvanizing line (CGL). More specifically, when performing electrogalvanizing, hot-dip galvanizing, and galvannealed hot-dip galvanizing, it is preferable to carry out the following processes.

[0074] [Electrogalvanized treatment] The electrogalvanizing treatment may be carried out by cooling the steel sheet to room temperature after the annealing treatment, followed by electrogalvanizing in a conventional manner. For example, the electrogalvanizing treatment may be carried out by passing a current through the steel sheet while immersing it in a zinc solution at 50°C to 60°C. The electrogalvanizing treatment improves the corrosion resistance of the steel sheet.

[0075] [Hot-dip galvanizing treatment] The hot-dip galvanizing treatment may be carried out by conventionally galvanizing the steel sheet obtained by cooling to room temperature after the annealing treatment. For example, the hot-dip galvanizing treatment may be carried out by immersing the steel sheet in a hot-dip galvanizing bath. This immersion treatment applies hot-dip galvanizing to the base steel sheet, thereby obtaining a hot-dip galvanized steel sheet. The hot-dip galvanizing treatment improves the corrosion resistance of the steel sheet.

[0076] [Galvannealed hot-dip plating treatment] The galvannealing treatment may be carried out, for example, by forming a galvannealed layer on the surface of the steel sheet according to a conventional method after the above-mentioned hot-dip galvanizing treatment. The alloying treatment may be carried out, for example, by maintaining the steel sheet at a predetermined temperature after the above-mentioned hot-dip galvanizing treatment so that the desired alloying is achieved. The alloying temperature is not particularly limited, but if the alloying temperature is too low, the alloying does not proceed sufficiently, so the alloying temperature is preferably 450°C or higher, more preferably 460°C or higher, and even more preferably 480°C or higher. The alloying treatment time is not particularly limited, and may be adjusted so that the desired alloying is achieved. For example, the alloying treatment time is preferably 10 seconds or longer and 60 seconds or shorter. The corrosion resistance of the steel sheet is improved by carrying out the galvannealing treatment.

[0077] The steel sheet and plated steel sheet in this embodiment utilize microalloys, and have excellent high-temperature strength while keeping the amount of alloy added low. Therefore, they can be suitably used as structural steel sheets that are fire-resistant and may be exposed to high temperatures, mainly as steel substrates used in buildings.

[0078] As described above, this specification discloses various aspects of the technology, but the main technologies among them are summarized below.

[0079] A steel sheet according to a first aspect contains, by mass%, 0.010% to 0.050% C, 0% to 1.00% Si, 0% to 0.80% Mn, 0.01% to 0.10% Al, 0.04% to 0.20% Ti, 1 ppm to 50 ppm B, and 0 ppm to 80 ppm N, with the balance being iron and unavoidable impurities. The mass ratio of Ti to C (Ti) / (C) is 3.6 or greater. The area fraction of recrystallized structures in a plane perpendicular to the rolling direction is 25% or less. When observed with a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM) at a position 1 / 4 of the sheet thickness in the plane perpendicular to the rolling direction, the circle equivalent diameter of precipitates is 26 nm or less. This configuration makes it possible to provide a steel sheet that utilizes microalloys and has excellent high-temperature strength while reducing the amount of alloy added.

[0080] The steel sheet of the second aspect is the steel sheet of the first aspect, and further contains, by mass%, one or more selected from the group consisting of V: ​​0.05% or less, Nb: 0.05% or less, W: 0.05% or less, Ta: 0.05% or less, Zr: 0.05% or less, and Hf: 0.05% or less.

[0081] The steel sheet of the third aspect is the steel sheet of the first or second aspect, further containing, in mass %, one or more elements selected from the group consisting of Cr: 0.20% or less, Mo: 0.20% or less, Cu: 0.20% or less, Ni: 0.20% or less, and Co: 0.20% or less.

[0082] The steel sheet in a fourth aspect is the steel sheet in any one of the first to third aspects, further containing, as the inevitable impurities, one or more selected from the group consisting of, by mass%, P: 0.03% or less, S: 0.01% or less, and O: 0.002% or less.

[0083] The steel sheet in a fifth aspect is the steel sheet in any one of the first to fourth aspects, further containing one or more elements selected from the group consisting of Mg: 30 ppm or less, Li: 30 ppm or less, Na: 30 ppm or less, Ca: 30 ppm or less, and REM: 30 ppm or less.

[0084] A plated steel sheet according to a sixth aspect has the steel sheet according to any one of the first to fifth aspects and a metal plating formed on the surface of the steel sheet.

[0085] A seventh aspect of the present invention relates to a method for producing a steel sheet, comprising the steps of: heating a steel material having the chemical composition described in any one of the first to fifth aspects at a temperature of 1000°C to 1300°C for 5 minutes to 5 hours, and hot-rolling the hot-rolled steel sheet obtained by the hot-rolling at a finishing rolling temperature of 850°C or higher; cooling the hot-rolled steel sheet obtained by the hot-rolling to a temperature of 300°C to 600°C and coiling it; unwinding the coiled hot-rolled steel sheet and cold-rolling the hot-rolled steel sheet at a cold-rolling reduction of 10% to 45%; and heating the cold-rolled steel sheet obtained by the cold-rolling to a maximum heating temperature of 600°C to 920°C and annealing it by holding it at the maximum heating temperature for 5 seconds to 1800 seconds. This configuration makes it possible to provide a steel sheet that is excellent in high-temperature strength while utilizing microalloys and reducing the amount of alloy added.

[0086] The present invention will be explained in more detail below with reference to examples, but the scope of the present invention is not limited to these examples. [Example]

[0087] [Test No. 1] First, an approximately 50 kg ingot (steel type A with the chemical composition shown in Table 1 below) was produced using a vacuum melting furnace. In Table 1, values ​​marked with "<" indicate that the value was below the measurement limit. The remainder of the chemical composition shown in Table 1 is iron and unavoidable impurities. As mentioned above, P and S are unavoidable impurities, and the values ​​shown in the P and S columns indicate the amounts that were unavoidably contained.

[0088] The produced ingot was once hot-worked to a steel plate with a thickness of 25 mm. It was then held at a heating temperature of 1150°C for 30 minutes or more, and hot-rolled to a thickness of 3.2 mm at a hot-rolling finish temperature of 900°C. After hot rolling, it was immediately quenched to a temperature of 500°C by laminar water cooling, then inserted into a holding furnace at 500°C, held there, and then furnace-cooled. This simulated the coil winding of a hot-rolled steel plate (hot-rolled steel plate).

[0089] The coiled hot-rolled steel sheet was unwound, and the 3.2 mm hot-rolled steel sheet was ground to 1.2 mm. The ground hot-rolled steel sheet was cold-rolled to 0.8 mm at a cold-rolling finishing temperature of 20°C. That is, cold rolling was performed at a cold-rolling reduction ratio of 33%. Next, the cold-rolled steel sheet (cold-rolled steel sheet) was immersed in a fluidized bed heat treatment furnace heated to 750°C, and after reaching the target temperature of -10°C, was held there for 90 seconds. Then, it was immersed in a fluidized bed heat treatment furnace heated to 460°C, held there for 120 seconds, and then cooled. This simulated annealing in a continuous annealing line or continuous hot-dip galvanizing line, and steel sheet No. 1 was produced.

[0090] [Test No. 2-7] In the manufacturing method of the steel sheet of Test No. 1 described above, steel sheets were manufactured under various conditions by changing the type of steel used (steel type A or B having the chemical composition shown in Table 1 below), the cold rolling conditions, and the annealing conditions. The various conditions are shown in Table 2. In Test No. 4, before cold rolling, a 3.2 mm hot-rolled steel sheet was ground to 2.0 mm, and the ground steel sheet was cold-rolled to 0.8 mm so that the cold rolling reduction was 60%.

[0091] [Table 1]

[0092] [Table 2]

[0093] (area ratio of recrystallized structure) For the steel sheets of Test Nos. 1 to 7, microstructural observation was performed to confirm the area ratio of the recrystallized structure (recrystallization ratio). When calculating the recrystallization ratio, the measurement samples were prepared and mirror-polished so that the microstructure perpendicular to the rolling direction could be observed. Then, starting from 0.1 mm from the surface, micro-Vickers hardness tests were performed at 120 points: six points at 0.1 mm pitch in the thickness direction and 20 points at 0.1 mm pitch in the direction perpendicular to the thickness direction, with a load of 200 gf. Points with a hardness of less than 160 Hv were defined as recrystallized regions, and the proportion of recrystallized regions to the total number of measurement points (120 points) was calculated as the recrystallization ratio (%). The results are shown in Table 3.

[0094] [Table 3]

[0095] (precipitate) In order to confirm the state of Ti-containing precipitates, the steel sheets of Test Nos. 3, 4, and 7 were subjected to the following structural observations. Samples were taken at a position 1 / 4 of the steel sheet thickness on a surface perpendicular to the rolling direction, and after preparing the samples using the thin film method, STEM observations were performed to measure the Ti-containing precipitates. 2 Two or more fields of view were observed in the above regions. STEM observations were performed using a Talos F200X field-emission transmission electron microscope (manufactured by FI Japan). Four types of images were acquired for each field: BF image (takeoff angle: ~9 mrad), DF2 image (takeoff angle: 12-20 mrad), DF4 image (takeoff angle: 23-55 mrad), and HAADF image (takeoff angle: 59-200 mrad). Image analysis was performed using the image in which precipitates were most easily observed. Furthermore, during STEM observation, regions where no dislocations were observed were defined as recrystallized regions, and regions where dislocations were clearly observed were defined as non-recrystallized regions. If both recrystallized and non-recrystallized regions were present, STEM observations were performed for each region in one or more fields of view. The circle-equivalent diameters of the observed precipitates were calculated, and the average of the 10 largest to 100 largest precipitates was calculated. This was used as the circle-equivalent diameter of the precipitates in the steel sheet. The results are shown in Table 4.

[0096] [Table 4]

[0097] (strength) The steel sheets of Test Nos. 1 to 7 were subjected to room temperature tensile tests in accordance with JIS Z 2241. In addition, high temperature tensile tests were carried out in accordance with JIS G 0567. Specifically, tensile test specimens with a parallel portion length of 50 mm and a parallel portion width of 12.5 mm were prepared from the obtained heat-treated steel sheets, and strain rates of 0.015 min -1 Tensile tests equivalent to those described above were conducted using a Shimadzu Corporation precision universal testing machine (model number: AG-100kNE / XR) to measure the yield strength and tensile strength at room temperature (room temperature characteristics) and the yield strength at 500°C (high temperature characteristics). The standard values ​​for yield strength at room temperature were 295 MPa, for tensile strength 400 MPa, and for yield strength at 500°C 280 MPa. Results above these standard values ​​were judged to be "pass." Furthermore, as a comparative example, SGC400 ([steel type] XH0316H, [chemical composition] C: 0.16% by mass, Si: 0.03% by mass, Mn: 0.95% by mass, P: 0.015% by mass, S: 0.005% by mass, Al: 0.025% by mass, Ti: 0% by mass, B: 0% by mass, N: 0.002% by mass) was prepared, and a tensile test was conducted in the same manner as for the steel sheets of Test Nos. 1 to 7 (Test No. 8). The results are shown in Table 5.

[0098] [Table 5]

[0099] (Consideration) As shown in Table 5 above, in Tests Nos. 1 to 3, in which the steel sheet composition, recrystallization rate, and precipitate size satisfied the conditions of the present invention, the yield strength at 500°C was 280 MPa or more, and it was found that each steel sheet utilized microalloys to reduce the amount of alloy addition while providing excellent high-temperature strength.

[0100] On the other hand, Test Nos. 4 and 7 did not satisfy the conditions of the present invention with respect to the chemical composition of the steel sheet, the recrystallization rate, and the size of the precipitates. Test Nos. 5 and 8 also did not satisfy the conditions of the present invention with respect to the chemical composition of the steel sheet. Furthermore, Test No. 6 did not satisfy the conditions of the present invention with respect to the chemical composition of the steel sheet and the recrystallization rate. In Test Nos. 4 to 8, the yield strength at 500°C was less than 280 MPa, resulting in poor high-temperature strength.

Claims

1. In mass%, C: 0.010% or more and 0.050% or less, Si: 0% or more and 1.00% or less, Mn: 0% or more and 0.80% or less, Al: 0.01% or more and 0.10% or less, Ti: 0.04% or more and 0.20% or less, B: 1 ppm or more and 50 ppm or less, and N: 0 ppm or more and 80 ppm or less, the remainder being iron and inevitable impurities; The mass ratio of Ti to C (Ti) / (C) is 3.6 or more, The area ratio of the recrystallized structure in the plane perpendicular to the rolling direction is 25% or less, A steel sheet in which, when observed with a transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM) at a position 1 / 4 of the sheet thickness in a plane perpendicular to the rolling direction, the circle equivalent diameter of precipitates is 26 nm or less.

2. Furthermore, in mass%, V: 0.05% or less, Nb: 0.05% or less, W: 0.05% or less, Ta: 0.05% or less, Zr: 0.05% or less, and The steel sheet according to claim 1, further comprising one or more elements selected from the group consisting of Hf: 0.05% or less.

3. Furthermore, in mass%, Cr: 0.20% or less, Mo: 0.20% or less, Cu: 0.20% or less, Ni: 0.20% or less, and The steel sheet according to claim 1, further comprising one or more selected from the group consisting of Co: 0.20% or less.

4. Furthermore, the inevitable impurities include, in mass %, P: 0.03% or less, S: 0.01% or less, and 2. The steel sheet according to claim 1, further comprising one or more elements selected from the group consisting of: O: 0.002% or less.

5. moreover, Mg: 30 ppm or less, Li: 30 ppm or less, Na: 30 ppm or less, Ca: 30 ppm or less, and The steel sheet according to claim 1, further comprising one or more selected from the group consisting of REM: 30 ppm or less.

6. A plated steel sheet comprising the steel sheet according to any one of claims 1 to 5 and a metal plating formed on the surface of the steel sheet.

7. A method for producing the steel sheet according to any one of claims 1 to 5, A steel material satisfying the chemical composition of the steel plate, Heating at a temperature of 1000°C or higher and 1300°C or lower for 5 minutes to 5 hours, and hot rolling at a rolling finish temperature of 850°C or higher; cooling the hot-rolled steel sheet obtained by the hot rolling to 300°C or higher and 600°C or lower and coiling it; Unwinding the wound hot-rolled steel sheet and cold-rolling the hot-rolled steel sheet at a cold rolling reduction rate of 10% or more and 45% or less; and heating the cold-rolled steel sheet obtained by the cold rolling to a maximum heating temperature of 600°C or higher and 920°C or lower, and annealing the cold-rolled steel sheet by holding it at the maximum heating temperature for 5 seconds or higher and 1800 seconds or lower.

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