Steel sheet, component including same, and method for manufacturing steel sheet

Abstract

A steel sheet according to the present invention has a prescribed chemical composition, and, in particular, regarding Si and sol. Al, the steel sheet contains, in mass%, 0.010 or greater and less than 0.320% of Si and 0.010-0.400% of sol. Al; when the Si and sol. Al contents (mass%) are indicated by [Si] and [sol. Al], respectively, the formula 0.100 ≤ [Si] + [sol. Al] is satisfied; in addition, the metal structure of the steel sheet contains, in area%, less than 3.0% of retained austenite, 15.0% or greater and less than 60.0% of ferrite, and less than 5.0% of pearlite; and, furthermore, the metal structure is such that the average sphere equivalent radius of an alloy carbide in the ferrite is 0.5 nm or greater and less than 5.0 nm, the average number density thereof is 3.5 × 1016 pieces / cm3 or greater, the E value is 10.7 or greater, the I value is 1.020 or greater, and the standard deviation of the Mn concentration is 0.60 mass% or less.

Description

Steel plate, part including same, and method of manufacturing steel plate

[0001] The present invention relates to a steel sheet, a part including the same, and a method for manufacturing the steel sheet.

[0002] In recent years, efforts to reduce carbon dioxide emissions have been made in many fields from the perspective of protecting the global environment. Automobile manufacturers are also actively developing technologies to reduce the weight of vehicles in order to improve fuel efficiency. However, reducing the weight of vehicles is not easy, as emphasis is also placed on improving crashworthiness to ensure the safety of passengers.

[0003] In order to achieve both lightweight vehicle bodies and crashworthiness, the use of high-strength steel sheets to reduce the thickness of components has been considered. For this reason, steel sheets that combine high strength with excellent formability are highly desired. To meet these requirements, several technologies have been proposed. Since there are various processing methods for automotive components, the required formability varies depending on the component to be used, but ductility is considered an important indicator of formability.

[0004] Furthermore, automotive components are formed by press molding, and blank sheets for press molding are often produced by shearing, which is highly productive. Blank sheets produced by shearing require excellent edge precision after shearing. For example, if a secondary shear plane, which is a shear plane-fracture plane-shear plane configuration, occurs on the edge (sheared edge) after shearing, as shown in Figure 2, the precision of the sheared edge will be significantly reduced.

[0005] In this regard, Patent Document 1 discloses a steel sheet having a predetermined chemical composition, in which the metal structure has, in area %, less than 3.0% retained austenite, 15.0% or more and less than 60.0% ferrite, and less than 5.0% pearlite, and the average equivalent sphere radius of alloy carbides in the ferrite is 0.5 nm or more and less than 5.0 nm, and the average number density is 3.5 × 10 16 pieces / cm 3The hot-rolled steel sheet is characterized in that the E value, which indicates the periodicity of the metallographic structure, is 10.7 or more, the I value, which indicates the uniformity of the metallographic structure, is 1.020 or more, the standard deviation of the Mn concentration is 0.60 mass% or less, and the tensile strength is 980 MPa or more. Furthermore, Patent Document 1 teaches that the above configuration makes it possible to obtain a hot-rolled steel sheet having high strength, as well as excellent ductility, fatigue properties, and shear workability.

[0006] International Publication No. 2023 / 149374

[0007] It is known that increasing the strength of a steel plate generally results in a decrease in toughness, and therefore, in the development of high-strength steel plates, there is a need to improve toughness in addition to the properties such as ductility and shear workability described in Patent Document 1.

[0008] Furthermore, in order to increase the strength of steel sheets, a relatively large amount of Si is sometimes contained in the steel sheets, but since the inclusion of a relatively large amount of Si in the steel sheets may cause the formation of Si scale on the steel sheet surface, there is also a need for high-strength steel sheets with a relatively low Si content.

[0009] Therefore, an object of the present invention is to provide a steel plate that has high strength and improved ductility, shear workability, and toughness despite having a relatively low Si content, a part including the same, and a method for manufacturing the steel plate.

[0010] In order to achieve the above object, the present inventors have conducted studies focusing on the chemical composition and metallographic structure of steel sheets, particularly hot-rolled steel sheets. As a result, the present inventors have found that a relatively low Si content can be achieved by controlling the total content of Si and sol. Al in a steel sheet within a predetermined range, while the strength, ductility, and shear workability can be improved by optimizing the structure fraction of the metallographic structure of the steel sheet, and that shear workability can be further improved by controlling the standard deviation of the Mn concentration in the metallographic structure and the E value and I value, which respectively indicate the periodicity and uniformity of the metallographic structure, within predetermined ranges, and that in addition, strength and toughness can be significantly improved by appropriately controlling the size and number of alloy carbides present in ferrite, thereby completing the present invention.

[0011] The present invention has achieved the above object as follows: (1) In mass %, C: 0.030 to 0.150%, Si: 0.010 to less than 0.320%, Mn: 0.50 to 3.00%, Ti: 0.050 to 0.200%, sol. Al: 0.010 to 0.400%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Nb: 0 to 0.150%, V: 0 to 1.000%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0-2.00%, Mo: 0-1.00%, B: 0-0.0100%, Sn: 0-1.00%, Sb: 0-1.00%, Ca: 0-0.0100%, Mg: 0-0.0100%, Hf: 0-0.0100%, Bi: 0-0.010%, REM: 0-0.0100%, The alloy has a chemical composition consisting of As: 0 to 0.010%, Zr: 0 to 0.010%, Co: 0 to 2.00%, Zn: 0 to 0.010%, W: 0 to 1.00%, and the balance: Fe and impurities, satisfying 0.100≦[Si]+[sol.Al]<0.720, where [Si] and [sol.Al] are the contents (mass%) of each element, and contains, in area %, retained austenite: less than 3.0%, ferrite: 15.0% or more and less than 60.0%, and pearlite: less than 5.0%, and the alloy carbides in the ferrite have an average equivalent sphere radius of 0.5 nm or more and less than 5.0 nm, and an average number density of 3.5×10 16 pieces / cm 3(2) A steel sheet according to (1) above, characterized in that the chemical composition includes, in mass %, 0.010 to less than 0.050% Si. (3) A steel sheet according to (1) or (2) above, characterized in that the chemical composition includes, in mass %, 0.200 to 0.400% sol. Al. (4) The chemical composition is, in mass%, Nb: 0.001 to 0.150%, V: 0.001 to 1.000%, Cr: 0.001 to 2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Mo: 0.001 to 1.00%, B: 0.0001 to 0.0100%, Sn: 0.001 to 1.00%, Sb: 0.001 to 1.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.001 to 0.010%, The steel sheet according to any one of (1) to (3) above, characterized by containing at least one of REM: 0.0001 to 0.0100%, As: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Co: 0.001 to 2.00%, Zn: 0.001 to 0.010%, and W: 0.001 to 1.00%. (5) The steel sheet according to any one of (1) to (4) above, characterized by having a tensile strength of 780 MPa or more. (6) The steel sheet according to any one of (1) to (5) above, characterized by having a tensile strength of less than 980 MPa. (7) The steel sheet according to any one of (1) to (6) above, characterized by having a sheet thickness of 1.0 to 8.0 mm. (8) A part, characterized by comprising the steel sheet according to any one of (1) to (7) above.(9) A heating step of heating a slab having the chemical composition described in any one of (1) to (4) above, holding the temperature in a temperature range of 700°C or higher and 850°C or lower for 900 seconds or longer, and then further heating the slab and holding the temperature in a temperature range of 1100°C or higher for 6000 seconds or longer; a hot rolling step of hot rolling the slab, which satisfies the following conditions (a) to (c): (a) the hot rolling is performed in a temperature range of 850°C or higher and 1100°C or lower so as to reduce the plate thickness by 90% or more in total; (b) the rolling one stage before the final stage is performed at 900°C or higher and lower than 1010°C, and then a stress of 170 kPa or higher is applied to the steel plate before the final stage of rolling; and (c) the rolling reduction in the final stage is 8% or higher, and the hot rolling completion temperature Tf is 900°C or higher and lower than 1010°C. a soft reduction step, which includes soft reducing a hot-rolled steel sheet in a temperature range of 840°C or higher and lower than 900°C so as to reduce the sheet thickness by 5% or higher and less than 8%, wherein the stress applied to the steel sheet after the final stage of rolling in the hot rolling step and before the first stage of soft reduction, and the stress applied to the steel sheet after the final stage of soft reduction until cooling to 800°C, is less than 200 kPa; a first cooling step, which accelerates cooling the soft-reduced steel sheet at an average cooling rate of 50°C / sec or higher to a temperature range of 680°C or higher and lower than 720°C, and then slowly cools it in the temperature range of 680°C or higher and lower than 720°C at an average cooling rate of less than 5°C / sec for 2.0 seconds or longer; and a second cooling step, which secondarily cools the steel sheet at an average cooling rate of 50°C / sec or higher to a temperature range of 350°C or lower, and then coils it in a temperature range of 350°C or lower.

[0012] According to the present invention, it is possible to provide a steel plate, particularly a hot-rolled steel plate, which has high strength and improved ductility, shear workability and toughness despite having a relatively low Si content, a part including the same, and a method for manufacturing the steel plate.

[0013] 1A and 1B are examples of sheared end surfaces of steel plates according to an example of the present invention and a comparative example, respectively;

[0014] <Steel Sheet> A steel sheet according to an embodiment of the present invention, particularly a hot-rolled steel sheet, contains, in mass %, C: 0.030 to 0.150%, Si: 0.010 to less than 0.320%, Mn: 0.50 to 3.00%, Ti: 0.050 to 0.200%, sol. Al: 0.010 to 0.400%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Nb: 0 to 0.150%, V: 0 to 1.000%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0-2.00%, Mo: 0-1.00%, B: 0-0.0100%, Sn: 0-1.00%, Sb: 0-1.00%, Ca: 0-0.0100%, Mg: 0-0.0100%, Hf: 0-0.0100%, Bi: 0-0.010%, REM: 0-0.0100%, The alloy has a chemical composition consisting of As: 0 to 0.010%, Zr: 0 to 0.010%, Co: 0 to 2.00%, Zn: 0 to 0.010%, W: 0 to 1.00%, and the balance: Fe and impurities, satisfying 0.100≦[Si]+[sol.Al]<0.720, where [Si] and [sol.Al] are the contents (mass%) of each element, and contains, in area %, retained austenite: less than 3.0%, ferrite: 15.0% or more and less than 60.0%, and pearlite: less than 5.0%, and the alloy carbides in the ferrite have an average equivalent sphere radius of 0.5 nm or more and less than 5.0 nm, and an average number density of 3.5×10 16 pieces / cm 3 or more, an E value of 10.7 or more, an I value of 1.020 or more, and a metal structure in which the standard deviation of the Mn concentration is 0.60 mass % or less.

[0015] As mentioned above, in the development of high-strength steel sheets, there is a need for improved toughness in addition to properties such as ductility and shear workability. Furthermore, because a relatively high content of Si in a steel sheet can cause Si scale formation on the steel sheet surface, there is also a need for high-strength steel sheets with a relatively low Si content. In relation to this, in an embodiment of the present invention, a relatively low Si content, i.e., a Si content of less than 0.010 to 0.320 mass%, is achieved by first controlling the total Si and sol. Al content in the steel sheet to a predetermined range, i.e., less than 0.100 to 0.720 mass%. This reliably reduces or inhibits Si scale formation, and also enables the inclusion of Si and sol. Al to promote ferrite formation. Furthermore, in an embodiment of the present invention, the metallographic structure of the steel sheet is configured to contain, by area percentage, less than 3.0% retained austenite, 15.0% to less than 60.0% ferrite, and less than 5.0% pearlite, thereby improving strength, ductility, and shear workability. In addition, by controlling the standard deviation of the Mn concentration in the metal structure to 0.60 mass% or less and uniformly dispersing hard phases such as martensite in the metal structure, shear workability can be further improved. More specifically, as the strength of steel sheets increases, elements such as Mn are sometimes added in relatively large amounts to improve the hardenability of the steel sheet. However, Mn is an element that tends to segregate in a streaky manner in the steel sheet. Therefore, due to this Mn segregation, regions with high and low hardenability exist in the steel sheet, and as a result, periodic band-shaped hard phases, more specifically, hard phases containing martensite, may be formed in the metal structure of the steel sheet after quenching. Since such band-shaped hard phases reduce shear workability, it is important to suppress Mn segregation in order to improve shear workability. From this perspective, in an embodiment of the present invention, Mn segregation is suppressed to control the standard deviation of the Mn concentration in the metal structure to 0.60 mass% or less, and in connection with this, hard phases such as martensite are uniformly dispersed in the metal structure, that is, the generation of periodic band-shaped hard phases is suppressed, thereby making it possible to improve shear workability.

[0016] From the viewpoint of improving shear workability, in addition to controlling the Mn concentration in the metallographic structure, it is effective to reduce the periodicity of the metallographic structure and increase the uniformity of the metallographic structure, as will be explained in more detail later. In this regard, in an embodiment of the present invention, by appropriately controlling the E (Entropy) value, which indicates the periodicity of the metallographic structure, and the I (Inverse Difference Normalized) value, which indicates the uniformity of the metallographic structure, more specifically, by controlling the E value to 10.7 or more and the I value to 1.020 or more, in combination with the structure fraction of the metallographic structure and the Mn concentration in the metallographic structure, it becomes possible to significantly improve shear workability.

[0017] The present inventors have further investigated the steel sheet to improve not only the above-mentioned properties but also the toughness, and as a result, have discovered that alloy carbides are present in ferrite at a specific size and ratio, more specifically, the alloy carbides in ferrite have an average equivalent sphere radius of 0.5 nm or more and less than 5.0 nm, and an average number density of 3.5 × 10 16 pieces / cm 3 It has been found that the strength and toughness of a steel sheet can be significantly improved by having the alloy carbide present in ferrite as described above. While not intending to be bound by any particular theory, it is believed that the presence of alloy carbides in ferrite at the above-described average spherical equivalent radius and average number density can precipitation-strengthen the ferrite, thereby further increasing the strength of the steel sheet, and significantly improving the toughness of the steel sheet through the refinement of the structure due to the pinning effect of the alloy carbide. Therefore, according to the steel sheet according to the embodiment of the present invention, it is possible to significantly improve the ductility, shear workability, and toughness, despite the high strength, for example, of 780 MPa or more. Therefore, the steel sheet according to the embodiment of the present invention can reliably achieve the contradictory properties of high strength and excellent formability, and is therefore particularly useful in the automotive field, where both of these properties are required.

[0018] Hereinafter, the steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "mass%" unless otherwise specified. Furthermore, in this specification, "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits, unless otherwise specified.

[0019] [C: 0.030 to 0.150%] C increases the area ratio of the hard phase and, by combining with precipitation strengthening elements such as Ti, Nb, and V, increases the strength of ferrite. To fully obtain these effects, the C content is set to 0.030% or more. The C content may be 0.040% or more, 0.050% or more, 0.060% or more, or 0.070% or more. On the other hand, excessive C content may reduce the area ratio of ferrite, thereby reducing the ductility of the steel sheet. Furthermore, excessive strength may result in reduced shear workability. Therefore, the C content is set to 0.150% or less. The C content may be 0.140% or less, 0.120% or less, 0.100% or less, or 0.080% or less.

[0020] [Si: 0.010 to less than 0.320%] Si promotes the formation of ferrite to improve the ductility of the steel sheet and solid-solution strengthens ferrite to increase the strength of the steel sheet. Si also inhibits cementite precipitation. Therefore, the inclusion of Si can suppress the consumption of C in the steel to form cementite, thereby promoting the formation of alloy carbides during cooling after hot rolling. To fully achieve these effects, the Si content is set to 0.010% or more. The Si content may be 0.020% or more or 0.030% or more. On the other hand, excessive Si content may not sufficiently suppress the formation of Si scale on the steel sheet surface. Excessive Si scale formation on the steel sheet surface may deteriorate the surface quality and / or chemical conversion treatability of the steel sheet. Furthermore, excessive Si content significantly deteriorates ductility and weldability and significantly increases the A3 transformation point. This makes it difficult to perform stable hot rolling. Therefore, the Si content is set to less than 0.320%. The Si content is set to 0.310% or less, 0.300% or less, 0.290% or less, 0.280% or less, 0.270% or less, 0.260% or less, 0.250% or less, 0.240% or less, 0.230% or less, 0.220% or less, 0.210% or less, 0.200% or less, 0.180% or less, 0.150% or less, 0.120% or less, 0.100% or less, 0.080% or less, 0.060% or less. %, 0.050% or less, less than 0.050%, 0.049% or less, 0.048% or less, 0.047% or less, 0.046% or less, 0.045% or less, 0.044% or less, 0.043% or less, 0.042% or less, 0.041% or less, 0.040% or less, 0.039% or less, 0.038% or less, 0.037% or less, 0.036% or less, or 0.035% or less. In particular, by controlling the Si content to less than 0.050%, it is possible to more significantly suppress the formation of Si scale on the steel sheet surface, and as a result, it is possible to more significantly improve the surface properties of the steel sheet.

[0021] [Mn: 0.50 to 3.00%] Mn has the effect of suppressing ferrite transformation and increasing the strength of the steel sheet. To fully obtain this effect, the Mn content is set to 0.50% or more. The Mn content may be 0.70% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, excessive Mn content may result in a decrease in ductility due to excessively high strength, and / or the morphology of hard phases including martensite and the like may become periodic band-like due to Mn segregation, resulting in a decrease in shear workability. Therefore, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.

[0022] [Ti: 0.050 to 0.200%] Ti finely precipitates in steel as alloy carbides, improving the strength of the steel through precipitation strengthening. Ti also refines the structure through its pinning effect, contributing to improved toughness. To fully achieve these effects, the Ti content is set to 0.050% or more. The Ti content may be 0.080% or more, 0.100% or more, 0.120% or more, or 0.140% or more. On the other hand, excessive Ti content may cause the alloy carbides to become coarse, making it impossible to achieve the desired precipitation strengthening in ferrite. In addition, the coarsening of the alloy carbides may also reduce the average number density of the alloy carbides. Therefore, the Ti content is set to 0.200% or less. The Ti content may be 0.180% or less, 0.170% or less, 0.160% or less, or 0.150% or less.

[0023] [Sol. Al: 0.010 to 0.400%] Sol. Al has the effect of improving the quality of steel by deoxidizing. To fully achieve this effect, the sol. Al content is set to 0.010% or more. The sol. Al content may be 0.050% or more, 0.080% or more, 0.100% or more, 0.120% or more, 0.150% or more, or 0.180% or more. Sol. Al also has the effect of promoting ferrite transformation. To fully achieve this effect, the sol. Al content is preferably set to 0.200% or more. The sol. Al content may be 0.210% or more, 0.220% or more, 0.230% or more, 0.250% or more, 0.260% or more, or 0.280% or more. On the other hand, the sol. Al content may be 0.210% or more, 0.220% or more, 0.230% or more, 0.250% or more, 0.260% or more, or 0.280% or more. If Al is contained excessively, coarse oxides may form, which may reduce toughness and ductility. Therefore, the sol. Al content is set to 0.400% or less. The sol. Al content may be 0.390% or less, 0.380% or less, 0.360% or less, 0.350% or less, 0.340% or less, 0.320% or less, or 0.300% or less. Sol. Al means acid-soluble Al, which refers to solute Al present in the steel in a solid solution state.

[0024] [P: 0.100% or less] P is generally contained as an impurity, but it also has the effect of increasing the strength of steel sheet through solid solution strengthening. Excessive P content significantly reduces ductility due to grain boundary segregation. Therefore, the P content is set to 0.100% or less. The P content may be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less. The lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.

[0025] [S: 0.0100% or less] S is an element contained as an impurity and forms sulfide-based inclusions in steel, reducing the ductility of the steel sheet. Therefore, the S content is set to 0.0100% or less. The S content may be 0.0050% or less, 0.0030% or less, or 0.0020% or less. The lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0026] [N: 0.0100% or less] N is an element contained in steel as an impurity and has the effect of reducing the ductility of steel sheet. Therefore, the N content is set to 0.0100% or less. The N content may be 0.0080% or less, 0.0050% or less, or 0.0030% or less. The lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0027] [O: 0.0100% or less] When O is contained in a steel in large amounts, it forms coarse oxides that become the starting point of fracture, causing brittle fracture and hydrogen-induced cracking. Therefore, the O content is set to 0.0100% or less. The O content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less. The lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long time for refining, resulting in a decrease in productivity. Therefore, the O content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.

[0028] The basic chemical composition of the steel sheet according to the embodiment of the present invention is as described above. Furthermore, the steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe, as necessary.

[0029] [Nb: 0 to 0.150%] Nb is an element that forms carbides, nitrides, and / or carbonitrides in steel, thereby contributing to the refinement of the structure through a pinning effect and, ultimately, to the increase in strength of the steel sheet. Nb also contributes to the suppression of recrystallization. The Nb content may be 0%, but to obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.005% or more, 0.010% or more, 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, excessive Nb content may cause the formation of coarse carbides in the steel, reducing the ductility of the steel sheet. Therefore, the Nb content is preferably 0.150% or less. The Nb content may be 0.120% or less, 0.100% or less, 0.080% or less, or 0.050% or less.

[0030] [V: 0 to 1.000%] V is an element that contributes to improving strength through precipitation strengthening and the like. The V content may be 0%, but to obtain such an effect, the V content is preferably 0.001% or more. The V content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, even if V is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the V content is preferably 1.00% or less. The V content may be 0.50% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[0031] [Cr: 0 to 2.00%] Cr has the effect of improving the hardenability of steel sheet. The Cr content may be 0%, but to obtain this effect, the Cr content is preferably 0.001% or more. The Cr content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if Cr is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the Cr content is preferably 2.00% or less. The Cr content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0032] [Ni: 0 to 2.00%] Ni has the effect of improving the hardenability of steel sheet. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing intergranular cracking of slabs caused by Cu. The Ni content may be 0%, but to obtain these effects, the Ni content is preferably 0.001% or more. The Ni content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if Ni is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the Ni content is preferably 2.00% or less. The Ni content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0033] [Cu: 0 to 2.00%] Cu has the effect of improving the hardenability of steel sheet and precipitating as carbides in steel at low temperatures to increase the strength of the steel sheet. The Cu content may be 0%, but to obtain this effect, the Cu content is preferably 0.001% or more. The Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive Cu content may saturate the effect and may result in increased manufacturing costs. Therefore, the Cu content is preferably 2.00% or less. The Cu content may be 1.50% or less, 1.00% or less, 0.50% or less, 0.30% or less, 0.15% or less, or 0.10% or less.

[0034] [Mo: 0 to 1.00%] Mo has the effect of improving the hardenability of steel sheet and precipitating as carbides in steel to increase the strength of steel sheet. The Mo content may be 0%, but to obtain such effects, the Mo content is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, excessive Mo content may increase deformation resistance during hot working and increase equipment load. Therefore, the Mo content is preferably 1.00% or less. The Mo content may be 0.80% or less, 0.50% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[0035] [B: 0 to 0.0100%] B has the effect of improving the hardenability of steel sheet. The B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more. The B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if B is contained excessively, the effect saturates and there is a risk of increasing manufacturing costs. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0050% or less, 0.0030% or less, 0.0015% or less, or 0.0010% or less.

[0036] [Sn: 0 to 1.00%] [Sb: 0 to 1.00%] Sn and Sb are elements effective in improving corrosion resistance. The Sn and Sb contents may be 0%, but to obtain such effects, the contents of these elements are preferably 0.001% or more, and may be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, excessive inclusion of these elements may result in a decrease in toughness. Therefore, the Sn and Sb contents are preferably 1.00% or less, and may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, or 0.08% or less.

[0037] [Ca: 0 to 0.0100%] [Mg: 0 to 0.0100%] [Hf: 0 to 0.0100%] Ca, Mg, and Hf are elements that can control the morphology of non-metallic inclusions. The Ca, Mg, and Hf contents may be 0%, but to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, or 0.0010% or more. On the other hand, even if these elements are contained in excess, the effects saturate, and including more than necessary in the steel sheet increases manufacturing costs. Therefore, the Ca, Mg, and Hf contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0038] [Bi: 0 to 0.010%] Bi has the effect of refining the solidification structure and thereby increasing the ductility of the steel sheet. The Bi content may be 0%, but to obtain this effect, the Bi content is preferably 0.001% or more. The Bi content may be 0.002% or more. On the other hand, even if an excessive amount of Bi is contained, the effect saturates, and adding more Bi than necessary to the steel sheet increases the manufacturing cost. Therefore, the Bi content is preferably 0.010% or less. The Bi content may be 0.005% or less or 0.003% or less.

[0039] [REM: 0 to 0.0100%] REM is an element that can control the morphology of nonmetallic inclusions. While the REM content may be 0%, to achieve this effect, the REM content is preferably 0.0001% or more. The REM content may be 0.0005% or more or 0.0010% or more. On the other hand, if REM is contained in an excessive amount, the effect saturates, and adding more REM than necessary to the steel sheet increases manufacturing costs. Therefore, the REM content is preferably 0.0100% or less. The REM content may be 0.0050% or less, 0.0030% or less, or 0.0020% or less. In this specification, REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic numbers 57 to lutetium (Lu) with atomic numbers 71. The REM content is the total content of these elements.

[0040] [As: 0 to 0.010%] As is an element effective in improving corrosion resistance. The As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more. The As content may be 0.002% or more or 0.003% or more. On the other hand, even if an excessive amount of As is contained, the effect saturates, and containing more As than necessary in the steel sheet increases the manufacturing cost. Therefore, the As content is preferably 0.010% or less. The As content may be 0.008% or less or 0.005% or less.

[0041] [Zr: 0 to 0.010%] Zr is an element that can control the morphology of non-metallic inclusions. The Zr content may be 0%, but to obtain this effect, the Zr content is preferably 0.001% or more. The Zr content may be 0.002% or more or 0.003% or more. On the other hand, even if Zr is contained in an excessive amount, the effect saturates, and adding more Zr than necessary to the steel sheet increases the manufacturing cost. Therefore, the Zr content is preferably 0.010% or less. The Zr content may be 0.008% or less or 0.005% or less.

[0042] [Co: 0 to 2.00%] Co is an element that contributes to improving hardenability and / or heat resistance. The Co content may be 0%, but to obtain these effects, the Co content is preferably 0.001% or more. The Co content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Co content may deteriorate hot workability and increase raw material costs. Therefore, the Co content is preferably 2.00% or less. The Co content may be 1.00% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0043] [Zn: 0 to 0.010%] Zn is an element that can be contained in steel sheet when scrap or the like is used as the steel raw material. Therefore, the Zn content is preferably 0.010% or less, and may be 0.008% or less, or 0.005% or less. The Zn content may be 0%, but reducing it to less than 0.001% requires a long refining time, resulting in a decrease in productivity. Therefore, the Zn content may be 0.001% or more, 0.002% or more, or 0.003% or more.

[0044] [W: 0 to 1.00%] W is an element that improves the hardenability of steel and contributes to improving strength. The W content may be 0%, but to obtain this effect, the W content is preferably 0.001% or more. The W content may be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive W content may reduce weldability. Therefore, the W content is preferably 1.00% or less. The W content may be 0.80% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0045] In the steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. The impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when industrially manufacturing the steel sheet.

[0046] [0.100≦[Si]+[sol.Al]<0.720] The chemical composition of the steel sheet according to the embodiment of the present invention must satisfy the following formula: 0.210≦[Si]+[sol.Al]<0.720, where [Si] and [sol.Al] are the contents (mass%) of each element. As described above, Si and sol.Al have the effect of promoting ferrite transformation. Therefore, by controlling the Si content to less than 0.320%, it is possible to sufficiently suppress the formation of Si scale on the steel sheet surface, while controlling the total content of Si and sol.Al to 0.100% or more, i.e., satisfying [Si]+[sol.Al]≧0.100, it is possible to promote ferrite transformation. By promoting ferrite transformation, it is possible to achieve a desired ferrite amount, more specifically, a ferrite amount of 15.0% or more and less than 60.0% by area, thereby improving the strength-ductility balance of the steel sheet. From the viewpoint of further promoting ferrite transformation, the higher the total content of Si and sol. Al, the more preferable, and may be, for example, 0.120% or more, 0.150% or more, 0.180% or more, 0.200% or more, 0.220% or more, 0.250% or more, 0.280% or more, or 0.300% or more. On the other hand, if the total content of Si and sol. Al is too high, the ferrite transformation may be excessively promoted, resulting in a decrease in strength. Therefore, the total content of Si and sol. Al is set to less than 0.720%, i.e., [Si] + [sol. Al] < 0.720. Si and sol. Al The total Al content may be 0.710% or less, 0.700% or less, 0.690% or less, 0.680% or less, 0.660% or less, 0.650% or less, 0.640% or less, 0.620% or less, 0.600% or less, 0.580% or less, 0.550% or less, 0.520% or less, or 0.500% or less.

[0047] The chemical composition of the steel sheet according to the embodiment of the present invention may be measured by a general analytical method. For example, the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. If the steel sheet has a plating layer or a coating film on its surface, the plating layer or coating film may be removed by mechanical grinding or the like, as necessary, before analyzing the chemical composition.

[0048] [Metal Structure] The metal structure of the steel sheet according to the embodiment of the present invention contains, in area percentages, less than 3.0% retained austenite, 15.0% or more but less than 60.0% ferrite, and less than 5.0% pearlite. By configuring the metal structure of the steel sheet as described above, it is possible to improve strength, ductility, and shear workability. Each structure will be described in more detail below.

[0049] [Retained austenite: less than 3.0%] Retained austenite is a structure that exists as a face-centered cubic lattice even at room temperature. Retained austenite has the effect of increasing the ductility of a steel sheet through transformation-induced plasticity (TRIP). On the other hand, retained austenite transforms into high-carbon martensite during shearing, inhibiting stable crack initiation and causing the formation of secondary shear planes. When the area fraction of retained austenite is high, the above-mentioned effects become more pronounced, resulting in deterioration of the shear workability of the steel sheet. Therefore, the area fraction of retained austenite is set to less than 3.0%. The area fraction of retained austenite may be 2.0% or less, 1.5% or less, 1.0% or less, or 0.5% or less. While the lower limit is not particularly limited, a lower area fraction of retained austenite is preferable from the viewpoint of improving shear workability; therefore, it may be 0% or 0.1% or more.

[0050] [Ferrite: 15.0% or more, less than 60.0%] Ferrite is a structure that is formed when fcc transforms to bcc at relatively high temperatures. Ferrite has a high work hardening rate and therefore acts to improve the strength-ductility balance of the steel sheet. To fully obtain this effect, the area fraction of ferrite is set to 15.0% or more. The area fraction of ferrite may be 20.0% or more, 25.0% or more, 30.0% or more, or 35.0% or more. On the other hand, because ferrite has low strength, if ferrite is contained in excess, the desired strength cannot be obtained. For this reason, the ferrite area fraction is set to less than 60.0%. The ferrite area fraction may be 55.0% or less, 50.0% or less, 45.0% or less, or 40.0% or less.

[0051] [Pearlite: Less than 5.0%] Pearlite is a lamellar structure in which cementite precipitates in layers between ferrite grains, and is softer than bainite or martensite. If pearlite is contained in excess, carbon is consumed by the cementite contained in pearlite, reducing the strength of the remaining martensite and bainite grains. This can result in the steel sheet not being able to achieve the desired strength and / or ductility being reduced. Therefore, the area fraction of pearlite is set to less than 5.0%. The area fraction of pearlite may be 4.0% or less, 3.0% or less, 2.0% or less, 1.0% or less, or 0.5% or less. While the lower limit is not particularly limited, from the viewpoint of improving the strength and / or stretch flangeability of the steel sheet, a lower area fraction of pearlite is preferable; therefore, it may be 0% or 0.1% or more.

[0052] [Remaining Structure] In the steel sheet according to the embodiment of the present invention, the remaining structure other than retained austenite, ferrite, and pearlite may be at least one of bainite, martensite, and tempered martensite. The area ratio of the remaining structure is not particularly limited, but may be, for example, 32.0% or more and 85.0% or less. In this case, more specifically, the remaining structure may be at least one of bainite, martensite, and tempered martensite in total, 32.0% or more and 85.0% or less. From the viewpoint of improving the strength of the steel sheet, it is preferable that the steel sheet contains a large amount of hard phases of bainite, martensite, and tempered martensite. Therefore, the area ratio of the remaining structure may be 33.0% or more, 35.0% or more, 40.0% or more, 45.0% or more, or 50.0% or more. On the other hand, from the viewpoint of improving the ductility of the steel sheet, it is preferable that the amount of these hard phases is small. Therefore, the area ratio of the remaining structure may be 84.0% or less, 80.0% or less, 75.0% or less, 70.0% or less, 65.0% or less, 60.0% or less, or 55.0% or less.

[0053] [Identification of Metallographic Structure and Calculation of Area Fraction] Identification of the metallographic structure and calculation of the area fraction are performed as follows. First, a sample is taken so that the metallographic structure can be observed in the cross-sectional region at 1 / 4 of the sheet thickness from the surface of the steel sheet. The sample is sized to allow observation of approximately 10 mm in a direction perpendicular to the sheet thickness direction, although this depends on the measurement device. Next, the observation cross section of the sample is polished to a mirror finish and then polished for 8 minutes at room temperature using colloidal silica that does not contain an alkaline solution to remove strain introduced into the surface layer of the sample. Crystal orientation information is obtained by measuring a region of 200 μm or more at any position in the direction perpendicular to the sheet thickness direction of the observation cross section, and a region of 200 μm or more in the sheet thickness direction centered at a 1 / 4 position from the surface, using electron backscatter diffraction at measurement intervals of 0.1 μm. For the measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 type detector) is used. At this time, the degree of vacuum inside the EBSD analyzer was 9.6×10 -5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the electron beam irradiation level is 62.

[0054] Furthermore, a backscattered electron image is taken in the same field of view. First, crystal grains in which ferrite and cementite are precipitated in layers are identified from the backscattered electron image, and the area ratio of these crystal grains is calculated to obtain the area ratio of pearlite. In the backscattered electron image, cementite exhibits a relatively high brightness value, while ferrite exhibits a low brightness value. The brightness of the backscattered electron image also varies depending on the crystal orientation. Therefore, for example, pearlite can be identified by adjusting the contrast so that cementite appears on the high brightness side and the crystal orientation difference of ferrite grains appears on the low brightness side. Next, for crystal grains other than those identified as pearlite, the obtained crystal orientation information is used to determine the region with a grain average misorientation value of 1.0° or less as ferrite using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. At this time, the grain tolerance angle is set to 15°, and the area of ​​the region determined to be ferrite is found to obtain the area ratio of ferrite.

[0055] The area fraction of retained austenite is measured by X-ray diffraction. First, a sample is taken from the cross section of the steel sheet at a position 1 / 4 of the sheet thickness from the surface so that the metal structure can be observed in a region of 1 mm or more at any position in a direction perpendicular to the sheet thickness direction, and in a region of 1 mm or more in a direction perpendicular to these two directions. For the above sample, the integrated intensities of a total of six peaks, α(110), α(200), α(211), γ(111), γ(200), and γ(220), are determined using Co-Kα radiation. Next, the volume fraction of retained austenite is obtained from the integrated intensities using the intensity averaging method, and this is considered to be the area fraction of retained austenite.

[0056] The area fraction of the remaining structure is obtained by subtracting the area fractions of retained austenite, ferrite, and pearlite from 100%. From the chemical composition of the steel plate according to the embodiment of the present invention, the remaining structure is a low-temperature transformation structure, and therefore, the remaining structure contains only a hard phase consisting of at least one of bainite, martensite, and tempered martensite. Therefore, the remaining structure of the steel plate according to the embodiment of the present invention can be estimated to be a hard phase consisting of at least one of bainite, martensite, and tempered martensite, and its area fraction can be obtained by subtracting the area fractions of retained austenite, ferrite, and pearlite from 100%.

[0057] [Average spherical equivalent radius of alloy carbides in ferrite: 0.5 nm or more and less than 5.0 nm] [Average number density of alloy carbides in ferrite: 3.5 × 10 16 pieces / cm 3 In the metal structure of the steel plate according to the embodiment of the present invention, the average equivalent sphere radius of alloy carbides in ferrite is 0.5 nm or more and less than 5.0 nm, and the average number density is 3.5 × 10 16 pieces / cm 3 That's all. Here, alloy carbide refers to a carbide containing one or more of Ti, Nb, V, and Mo, and includes, for example, TiC and a composite carbide containing Ti and at least one of Nb, V, and Nb. By having alloy carbides present in ferrite at the above-mentioned average equivalent sphere radius and average number density, the ferrite is precipitation strengthened, thereby further increasing the strength of the steel sheet, and the toughness of the steel sheet can be significantly improved by the refinement of the structure due to the pinning effect of the alloy carbides.

[0058] If the mean equivalent spherical radius of the alloy carbides in the ferrite is less than 0.5 nm, the alloy carbides cannot sufficiently precipitation strengthen the ferrite, making it impossible to achieve the desired high strength, and / or the pinning effect of the alloy carbides cannot sufficiently refine the structure, making it impossible to achieve the desired toughness. Therefore, from the viewpoint of improving strength and toughness, the mean equivalent spherical radius of the alloy carbides is preferably large, for example, 1.0 nm or more and 1.5 nm or more. On the other hand, if the mean equivalent spherical radius of the alloy carbides in the ferrite is 5.0 nm or more, the strength of the ferrite cannot be sufficiently increased, and due to the hardness difference between the crystal grains, cracks will initiate from the cutting edge of the shearing tool very early in the shearing process, forming a fracture surface, and then a shear surface will form again. As a result, secondary shear surfaces will be more likely to form, making it impossible to obtain the desired shearing workability in the steel sheet. Therefore, from the viewpoint of improving shear workability, it is preferable that the average equivalent sphere radius of the alloy carbide is small, and may be, for example, 4.0 nm or less, 3.0 nm or less, 2.5 nm or less, or 2.0 nm or less.

[0059] The average number density of alloy carbides in ferrite is 3.5 × 10 16 pieces / cm 3 If the average number density of the alloy carbides is lower than 5.0×10, the ferrite cannot be sufficiently precipitation strengthened, and the desired high strength cannot be achieved. Therefore, from the viewpoint of improving the strength, it is preferable that the average number density of the alloy carbides is high, for example, 5.0×10 16 pieces / cm 3 That's it, 8.0 x 10 16 pieces / cm 3 That's it, 10.0 x 10 16 pieces / cm 3 That's it, 12.0 x 10 16 pieces / cm 3 That's it, 15.0 x 10 16 pieces / cm 3 That's it, 18.0 x 10 16 pieces / cm 3 or more, or 20.0 x 10 16 pieces / cm 3Although the upper limit is not particularly limited, in the chemical composition and metal structure of the steel sheet according to the embodiment of the present invention, the average number density of alloy carbides in ferrite is preferably 1000.0 × 10 16 pieces / cm 3 Therefore, the average number density of alloy carbides in ferrite is, for example, 1000.0 × 10 16 pieces / cm 3 Below, 800.0 x 10 16 pieces / cm 3 or less, or 500.0 x 10 16 pieces / cm 3 If the average number density of alloy carbides in ferrite is increased, the ferrite is precipitation strengthened, but the toughness may be somewhat reduced accordingly. Therefore, in consideration of the balance between precipitation strengthening and toughness, it is preferable to control the average number density of alloy carbides within an appropriate range, for example, 100.0 × 10 16 pieces / cm 3 Below, 50.0 x 10 16 pieces / cm 3 or less, or 30.0 x 10 16 pieces / cm 3 It is preferable that:

[0060] [Measurement of the average spherical equivalent radius and average number density of alloy carbides in ferrite] The average spherical equivalent radius and average number density of alloy carbides in ferrite are measured using a three-dimensional atom probe. In the three-dimensional atom probe measurement, the laser wavelength (λ) is 355 nm, the laser power is 30 pJ, and the temperature of the needle-shaped test piece is 50 K. The device used for the three-dimensional atom probe measurement is not particularly limited. An example of the three-dimensional atom probe measurement device is the LEAP4000XHR, a product name of Ametec Co., Ltd.

[0061] For the ferrite grains within the observation field by the EBSD described above, where the area ratio of each structure was measured, a sample was collected using an FIB (focused ion beam) device. The collected sample was processed into a needle shape by a well-known method, and by using a three-dimensional atom probe, the spherical equivalent radius and number density of fine precipitates with a spherical equivalent radius ranging from less than 1 nm to several tens of nm could be accurately measured. The number density of precipitates can be obtained by dividing the number of precipitates included in the region measured by the three-dimensional atom probe by the volume of the measurement region for precipitates identified as alloy carbides by the method described below.

[0062] The total volume of precipitates in the measurement area is obtained by dividing the total number of atoms of alloy elements (Ti, Nb, V, Mo, and C) contained in all precipitates in the measurement area by the atomic density of the alloy carbide. The volume of precipitates is obtained by dividing the total volume of precipitates by the number of precipitates. From the obtained volume of the precipitates, the sphere-equivalent radius is calculated, assuming that the precipitates are spherical.

[0063] The above method was performed at 30,000 nm. 3 The average number density and average spherical equivalent radius are obtained by performing measurements on five or more samples with a volume of the measurement area. The observation area is the area where the Ga (gallium) introduced during FIB processing is less than 0.025 at%. Areas where Ga is mixed in at 0.025 at% or more are excluded from the measurement area. The Ga amount can be confirmed by using the 1D Concentration Profile function of the data analysis software IVAS 3.6.14 (manufactured by CAMECA Instruments Inc.).

[0064] Whether or not the observed precipitates are alloy carbides is determined by using the Cluster Analysis function of the analysis software IVAS 3.6.14 based on the data acquired by the three-dimensional atom probe. max =1.2nm, Order=10, N min = 10, L = 0.5 nm, and d erosion = 0.5 nm are used as analysis parameters, and precipitates recognized as clusters are identified as alloy carbides.

[0065] [E value: 10.7 or more] [I value: 1.020 or more] In the metallographic structure of the steel plate according to an embodiment of the present invention, the E value is 10.7 or more, and the I value is 1.020 or more. In order to suppress the generation of secondary shear planes, it is important to form fracture surfaces after sufficient shear planes have been formed, and it is necessary to suppress early crack generation from the cutting edge of the tool during shearing. To this end, it is important that the periodicity of the metallographic structure is low and the uniformity of the metallographic structure is high. In an embodiment of the present invention, the generation of secondary shear planes can be suppressed by controlling the E (Entropy) value, which indicates the periodicity of the metallographic structure, to 10.7 or more and the I (Inverse Difference Normalized) value, which indicates the uniformity of the metallographic structure, to 1.020 or more.

[0066] As described above, the E value indicates the periodicity of the metal structure, and when the brightness is periodically arranged due to the influence of, for example, the formation of a band-shaped structure, i.e., when the periodicity of the metal structure is high, the E value decreases. In an embodiment of the present invention, a metal structure with low periodicity is required, so the E value needs to be increased. If the E value is less than 10.7, secondary shear planes are likely to occur. Starting from the periodically arranged structure, cracks are initiated from the cutting edge of the shearing tool very early in the shearing process, forming a fracture surface, and then a shear surface is formed again. This is presumably what makes secondary shear planes more likely to occur. Therefore, the E value is set to 10.7 or more. The E value is preferably 10.8 or more, more preferably 11.0 or more. A higher E value is preferable, and although there is no particular upper limit specified, it may be 13.0 or less, 12.5 or less, or 12.0 or less.

[0067] On the other hand, the I value indicates the uniformity of the metal structure, and increases as the area of ​​the region with a certain brightness increases. A high I value means that the metal structure is highly uniform. In embodiments of the present invention, a highly uniform metal structure is required, so the I value needs to be increased. If the I value is less than 1.020, cracks will initiate from the cutting edge of the shearing tool very early in the shearing process due to the influence of precipitates within the crystal grains and the hardness distribution caused by differences in element concentration, forming a fracture surface, and then a shear surface will be formed again. This is presumably likely to make secondary shear surfaces more likely to occur. Therefore, the I value is set to 1.020 or more. The I value is preferably 1.025 or more, more preferably 1.030 or more. A higher I value is preferable, and although there is no particular upper limit, it may be 1.200 or less, 1.150 or less, or 1.100 or less.

[0068] The E value and I value can be obtained by the following method. First, in an embodiment of the present invention, the photographed area of ​​the SEM image taken to calculate the E value and I value is 200 μm × 200 μm, centered at a position ¼ of the plate thickness from the surface of the steel plate in the plate thickness cross section of the steel plate, and the number of observation fields is 5. To photograph the SEM image, an SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation is used, the emitter is tungsten, and the acceleration voltage is 1.5 kV. Under the above settings, the SEM image is output at a magnification of 1000 times and a grayscale of 256 gradations.

[0069] Next, the obtained SEM image was cut into an 880 x 880 pixel region, and the image was subjected to a smoothing process with a tile grid size of 8 x 8, with a contrast enhancement limiting factor of 2.0, as described in K. Zuiderveld, Contrast Limited Adaptive Histogram Equalization, Chapter VIII. 5, Graphics Gems IV. P. S. Heckbert (Eds.), Cambridge, MA, Academic Press, 1994, pp. 474-485. The smoothed SEM image was rotated counterclockwise from 0 to 179 degrees in 1 degree increments, excluding 90 degrees, and an image was created every 1 degree, resulting in a total of 179 images. Next, for each of these 179 images, the frequency values ​​of luminance between adjacent pixels are collected in matrix form using the gray level co-occurrence matrix method (GLCM method) described in J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596.

[0070] The matrix of 179 frequency values ​​obtained by the above method is expressed as p k (k=0...89,91...179). For each image, the generated p k After summing for all k (k = 0...89, 91...179), a 256 x 256 matrix P is calculated, normalized so that the sum of each component is 1. Furthermore, the E value and I value are calculated using the following formulas (1) and (2) described in D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9, 546. The average value obtained by measuring the entire field of view is calculated.

[0071] In the following formulas (1) and (2), P(i, j) is a gray level co-occurrence matrix, and the value in the i-th row and j-th column of the matrix P is expressed as P(i, j). As described above, the calculation is performed using a 256 x 256 matrix P, so if you want to emphasize this point, the following formula (1) can be modified to the following formula (1'), and the following formula (2) can be modified to the following formula (2'). In the following formulas (1') and (2'), the value in the i-th row and j-th column of the matrix P is expressed as P(i, j). ij It is written as follows.

[0072]

[0073]

[0074] [Standard Deviation of Mn Concentration: 0.60% by Mass or Less] In the metal structure of the steel sheet according to the embodiment of the present invention, the standard deviation of the Mn concentration, more specifically, the standard deviation of the Mn concentration in the region from the surface of the steel sheet to a quarter of the sheet thickness, is 0.60% by mass or less. Such a low standard deviation of the Mn concentration is associated with suppression of Mn segregation. Therefore, due to the suppression of Mn segregation, a hard phase consisting of at least one of bainite, martensite, and tempered martensite can be uniformly dispersed, preventing cracks from forming at the cutting edge of the shearing tool very early in the shearing process. As a result, the generation of secondary shear planes can be suppressed, thereby improving shearing workability. From the viewpoint of improving shearing workability, a lower standard deviation of the Mn concentration is preferable, and may be, for example, 0.55% by mass or less, 0.50% by mass or less, 0.47% by mass or less, or 0.45% by mass or less. From the viewpoint of suppressing excessive burrs, a smaller lower limit of the standard deviation of the Mn concentration is desirable. However, due to constraints in the manufacturing process, the substantial lower limit of the standard deviation of the Mn concentration is 0.10 mass %, and the standard deviation of the Mn concentration may be, for example, 0.20 mass % or more or 0.30 mass % or more.

[0075] [Measurement of Standard Deviation of Mn Concentration] The standard deviation of Mn concentration is measured as follows. First, a sample is taken so that the metal structure can be observed in a cross-sectional region at 1 / 4 of the sheet thickness from the surface of the steel sheet. The sample is made large enough to be observed in a direction perpendicular to the sheet thickness direction, approximately 10 mm in size, depending on the measurement device. Next, the sample is mirror-polished, and the standard deviation of Mn concentration is measured using an electron probe microanalyzer (EPMA). The measurement conditions are an acceleration voltage of 15 kV and a magnification of 5000 times, and distribution images are measured at 40,000 or more locations at a measurement interval of 0.1 μm in an area of ​​20 μm in a direction perpendicular to the sheet thickness direction of the sample and 20 μm in the sheet thickness direction of the sample. Next, the standard deviation of Mn concentration is obtained by calculating the standard deviation based on the Mn concentrations obtained from all measurement points.

[0076] [Thickness] The steel sheet according to the embodiment of the present invention generally has a thickness of 1.0 to 8.0 mm, although not particularly limited thereto. For example, the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and / or 7.0 mm or less, 6.0 mm or less, 5.5 mm or less, 5.0 mm or less, 4.4 mm or less, 4.2 mm or less, or 4.0 mm or less.

[0077] [Plated Layer] The steel sheet according to the embodiment of the present invention, having the above-described chemical composition and metallographic structure, may have a plated layer on its surface for the purpose of improving corrosion resistance, etc. The plated layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized and electrolytic Zn—Ni alloy plating. Examples of hot-dip plated layers include hot-dip galvanized, alloyed hot-dip galvanized, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, and hot-dip Zn—Al—Mg—Si alloy plating. The coating weight is not particularly limited and may be a general coating weight. Furthermore, corrosion resistance can be further improved by performing an appropriate chemical conversion treatment after plating (e.g., applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0078] As described above, the steel sheet according to the embodiment of the present invention is capable of achieving excellent ductility, shear workability, and toughness despite its high strength, and therefore can reliably achieve a high level of compatibility between the contradictory properties of high strength and excellent formability. Therefore, the steel sheet according to the embodiment of the present invention is particularly useful for use in components in technical fields where compatibility between these properties is required. In a preferred embodiment, an automobile part, particularly an automobile suspension part, is provided that includes the steel sheet according to the embodiment of the present invention. Examples of automobile suspension parts include lower arms and trailing arms. These automobile parts, particularly automobile suspension parts, only need to include the steel sheet according to the embodiment of the present invention in at least a portion of these parts, and therefore at least a portion of these parts will satisfy the chemical composition and metallographic characteristics described above. In portions of the steel sheet that are not in direct contact with a mold during forming, such as press forming, and that are subjected to a relatively small degree of processing, the characteristics of the metallographic structure do not change significantly before and after forming.

[0079] [Tensile Properties] Tensile strength properties (tensile strength, total elongation) among the mechanical properties of steel sheets are evaluated in accordance with JIS Z 2241:2022. Test specimens are No. 5 test specimens of JIS Z 2241:2022. Tensile test specimens are preferably taken so that the direction perpendicular to the rolling direction is the longitudinal direction, but if the rolling direction of the steel sheet cannot be specified, they may be taken from any direction within the surface of the steel sheet.

[0080] Steel sheets according to embodiments of the present invention can achieve high tensile strength (TS), for example, 780 MPa or more. The tensile strength is preferably 800 MPa or more, 820 MPa or more, or 840 MPa or more. Despite having such extremely high tensile strength, steel sheets according to embodiments of the present invention can achieve improved ductility, shear workability, and toughness due to the specific combination of chemical composition and metallographic structure described above. The upper limit of the tensile strength is not particularly limited, and the tensile strength of the steel sheet may be, for example, 1580 MPa or less, 1470 MPa or less, 1400 MPa or less, 1300 MPa or less, 1180 MPa or less, 1100 MPa or less, less than 980 MPa, 970 MPa or less, 960 MPa or less, 950 MPa or less, 940 MPa or less, 900 MPa or less, or 860 MPa or less.

[0081] According to the steel sheet according to the embodiment of the present invention, a high total elongation (El), for example, a total elongation of 13.0% or more, can be achieved, and the product of tensile strength and total elongation (TS × El) can be 13,000 MPa·% or more. The total elongation is preferably 14.0% or more, 15.0% or more, 16.0% or more, or 18.0% or more. Furthermore, the product of tensile strength and total elongation is preferably 14,000 MPa·% or more or 15,000 MPa·% or more. By achieving a total elongation of 13.0% or more and a product of tensile strength and total elongation of 13,000 MPa·% or more, the steel sheet can significantly contribute to reducing the weight of a vehicle body without being limited to specific parts.

[0082] [Toughness] According to the steel plate according to the embodiment of the present invention, excellent toughness can be achieved, for example, a Charpy impact value at −20° C. of 50 J / cm 2 or more, preferably 60 J / cm 2 More preferably, 70 J / cm 2 More than 80 J / cm, most preferably 80 J / cm 2 The upper limit of the Charpy impact value is not particularly limited, but may be, for example, 150 J / cm 2 or 120 J / cm 2The Charpy impact value may be calculated by measuring three Charpy impact values ​​at -20°C using an impact blade with a radius of 2 mm in accordance with the provisions of JIS Z 2242:2018 based on a V-notch test piece taken from the surface of the steel plate at a position 1 / 4 of the plate thickness, and averaging the measured values. When a sub-size test piece is used, the Charpy impact value is converted to a full-size Charpy impact value according to the thickness of the test piece.

[0083] <Method for Manufacturing Steel Sheet> Next, a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below. More specifically, although the following specifically describes the manufacture of a hot-rolled steel sheet, the steel sheet according to an embodiment of the present invention encompasses any steel sheet having the chemical composition and metallographic structure described above, i.e., not only a hot-rolled steel sheet, but also a cold-rolled steel sheet, a plated steel sheet, and the like. Therefore, the following description merely describes a preferred method for manufacturing a steel sheet according to an embodiment of the present invention when the steel sheet is a hot-rolled steel sheet.

[0084] A method for producing a steel plate according to an embodiment of the present invention includes: a heating step of heating a slab having the chemical composition described above in relation to the steel plate and holding the slab in a temperature range of 700°C or higher and 850°C or lower for 900 seconds or longer, and then further heating the slab and holding the slab in a temperature range of 1100°C or higher for 6000 seconds or longer; and a hot rolling step of hot rolling the slab, the hot rolling satisfying the following conditions (a) to (c): (a) the hot rolling is performed in a temperature range of 850°C or higher and 1100°C or lower so as to reduce the plate thickness by 90% or more in total; (b) the penultimate rolling stage is performed at a temperature of 900°C or higher and lower than 1010°C, and then a stress of 170 kPa or higher is applied to the steel plate before the final rolling stage; and (c) the reduction rate in the final stage is 8% or higher, and the hot rolling completion temperature Tf is 900°C or higher and lower than 1010°C. The method is characterized by comprising: a soft reduction step, which includes soft reduction of a hot-rolled steel sheet in a temperature range of 840°C or higher and lower than 900°C so as to reduce the sheet thickness by 5% or higher and less than 8%, in which the stress applied to the steel sheet after the final stage of rolling in the hot rolling step and before the first stage of soft reduction, and the stress applied to the steel sheet after the final stage of soft reduction until cooling to 800°C, is less than 200 kPa; a first cooling step, which accelerates cooling the soft-reduced steel sheet at an average cooling rate of 50°C / sec or higher to a temperature range of 680°C or higher and lower than 720°C, and then slowly cools it in the temperature range of 680°C or higher and lower than 720°C at an average cooling rate of less than 5°C / sec for 2.0 seconds or longer; and a second cooling step, which secondarily cools the steel sheet at an average cooling rate of 50°C / sec or higher to a temperature range of 350°C or lower, and then coils it in the temperature range of 350°C or lower.

[0085] In this manufacturing method, the temperature of the slab and the temperature of the steel sheet refer to the surface temperature of the slab and the surface temperature of the steel sheet, respectively. Furthermore, the stress refers to the tension applied to the steel sheet in the rolling direction. Each step will be explained in detail below.

[0086] [Heating Process] First, a slab having the chemical composition described above in relation to the steel sheet is heated and held in a temperature range of 700°C to 850°C for 900 seconds or more, and then further heated and held in a temperature range of 1100°C or more for 6000 seconds or more. From the viewpoint of productivity, the slab used is preferably cast by a continuous casting method, but may also be produced by an ingot casting method or a thin slab casting method. When holding in a temperature range of 700 to 850°C, the steel sheet temperature may be varied or constant within this temperature range. Similarly, when holding in a temperature range of 1100°C or more, the steel sheet temperature may be varied or constant within a temperature range of 1100°C or more. The upper limit of the heating temperature in the temperature range of 1100°C or more is not particularly limited, but it is preferably 1350°C or less from the viewpoint of thermal efficiency.

[0087] During austenite transformation in the temperature range of 700°C to 850°C, Mn is distributed between ferrite and austenite. Therefore, extending the transformation time allows Mn to diffuse within the ferrite region. This eliminates the microsegregation of Mn unevenly distributed in the slab, resulting in a significant reduction in the standard deviation of Mn concentration in the final metal structure. To achieve a desired standard deviation of Mn concentration, the holding time in the temperature range of 700°C to 850°C must be 900 seconds or more, preferably 1000 seconds or more. The upper limit is not particularly limited, but for example, the holding time in the temperature range of 700°C to 850°C may be 1800 seconds or less. Similarly, to reduce the standard deviation of Mn concentration within the desired range, the holding time in the temperature range of 1100°C or higher is also important. Specifically, it must be 6000 seconds or more, preferably 7000 seconds or more. The upper limit is not particularly limited, but for example, the holding time in the temperature range of 1100°C or higher may be 10000 seconds or less.

[0088] [Hot Rolling Step] In the hot rolling step, the slab is hot rolled so as to satisfy the following conditions (a) to (c): (a) hot rolling is performed in a temperature range of 850°C or higher and 1100°C or lower, so as to reduce the plate thickness by 90% or more in total, (b) the rolling one stage before the final stage is performed at 900°C or higher and lower than 1010°C, and then a stress of 170 kPa or higher is applied to the steel plate before the final stage of rolling, and (c) the rolling reduction in the final stage is 8% or higher, and the hot rolling completion temperature Tf is 900°C or higher and lower than 1010°C.

[0089] The hot rolling is preferably performed using a reverse mill or a tandem mill as multi-pass rolling. In particular, from the viewpoint of industrial productivity and the stress load on the steel sheet during rolling, it is more preferable to perform hot rolling using a tandem mill for at least the final two stages.

[0090] [Condition (a)] By performing hot rolling to reduce the thickness by 90% or more in total in the temperature range of 850°C or higher and 1100°C or lower, the recrystallized austenite grains are refined and the accumulation of strain energy in the unrecrystallized austenite grains is promoted. In relation to this, the recrystallization of austenite is promoted and the atomic diffusion of Mn is promoted, making it possible to reduce the standard deviation of the Mn concentration. If the total thickness reduction in the temperature range of 850°C or higher and 1100°C or lower is less than 90%, it may be impossible to achieve the desired standard deviation of the Mn concentration. The upper limit of the thickness reduction is not particularly limited, but for example, the total thickness reduction in the temperature range of 850°C or higher and 1100°C or lower may be 98% or less.

[0091] The total thickness reduction (thickness reduction rate) in the temperature range of 850°C or higher and 1100°C or lower can be expressed as {(t0-t1) / t0} x 100(%), where t0 is the entrance thickness before the first rolling in rolling in this temperature range, and t1 is the exit thickness after the final stage of rolling in this temperature range.

[0092] [Condition (b)] In the hot rolling process, the rolling one stage before the final stage is performed at a temperature of 900°C or higher but lower than 1010°C, and then a stress of 170 kPa or higher is applied to the steel sheet before the final rolling stage. This reduces the number of crystal grains having the {110}<001> crystal orientation in the recrystallized austenite after the rolling one stage before the final rolling stage. Because {110}<001> is a crystal orientation that is difficult to recrystallize, suppressing the formation of this crystal orientation can effectively promote recrystallization during the final rolling stage. As a result, the band-shaped structure of the steel sheet is improved, the periodicity of the metallographic structure is reduced, and the E value can be increased. If the stress applied to the steel sheet is less than 170 kPa, the desired E value may not be obtained. From the viewpoint of further increasing the E value, the stress applied to the steel sheet is preferably 190 kPa or higher. The upper limit of the stress applied to the steel sheet is not particularly limited, but for example, the stress applied to the steel sheet may be 350 kPa or less or 300 kPa or less. The stress applied to the steel sheet is a tension in the rolling direction, and can be controlled by adjusting the roll rotation speed during tandem rolling. In addition, the stress can be determined by dividing the measured load in the rolling direction by the cross-sectional area of ​​the sheet being passed through.

[0093] [Condition (c)] In the hot rolling process, the reduction rate in the final stage is controlled to 8% or more, and the hot rolling completion temperature Tf is controlled to 900°C or more and less than 1010°C. By controlling the reduction rate in the final stage of hot rolling to 8% or more, recrystallization due to the reduction in the final stage can be promoted. As a result, the band-shaped structure of the steel sheet is improved, the periodicity of the metallographic structure is reduced, and the E value can be increased. Furthermore, by controlling the hot rolling completion temperature Tf to 900°C or more, an excessive increase in the number of ferrite nucleation sites in austenite can be suppressed. As a result, excessive generation of ferrite in the finally obtained metallographic structure can be suppressed, and the desired strength can be obtained. The upper limit of the reduction rate in the final stage is not particularly limited, and may be, for example, 30% or less, 20% or less, or 15% or less. Furthermore, by controlling Tf to less than 1010°C, coarsening of the austenite grain size can be suppressed, the periodicity of the metallographic structure can be reduced, and the desired E value can be obtained.

[0094] [Soft Reduction Process] [Thickness Reduction in the Temperature Range of 840 to Less than 900°C: Total of 5 to Less than 8%] In the soft reduction process, the hot-rolled steel sheet is soft-reduced in a temperature range of 840°C or higher and lower than 900°C to achieve a total thickness reduction of 5% or higher and less than 8%. This allows the average equivalent sphere radius and average number density of alloy carbides in ferrite to be controlled within the desired range. If the total thickness reduction in the soft reduction is less than 5% and / or 8% or higher, the average equivalent sphere radius and / or average number density of alloy carbides in ferrite cannot be controlled within the desired range. For example, the average equivalent sphere radius of the alloy carbides may become smaller, or the average number density of the alloy carbides may become smaller. Soft reduction may be performed, for example, in the final stage of a rolling mill, particularly a finishing mill, or by installing new reduction equipment between the finishing mill and the cooling bed. Soft reduction may be performed in multiple stages using multiple rolls.

[0095] The total thickness reduction during soft reduction can be expressed as {(t0-t1) / t0} x 100(%), where t0 is the entrance thickness before the first rolling of soft reduction, and t1 is the exit thickness after the final stage of soft reduction.

[0096] [Stress applied to the steel sheet after the final stage of rolling in the hot rolling process and before the first stage of rolling under soft reduction, and stress applied to the steel sheet after the final stage of rolling under soft reduction and until cooling to 800°C: less than 200 kPa] In the soft reduction process, the stress applied to the steel sheet after the final stage of rolling in the previous hot rolling process and before the first stage of rolling under soft reduction, and the stress applied to the steel sheet after the final stage of rolling under soft reduction and until cooling to 800°C are each controlled to less than 200 kPa. By setting the stress applied to the steel sheet at these locations to less than 200 kPa, austenite recrystallization proceeds preferentially in the rolling direction, and an increase in the periodicity of the metallographic structure can be suppressed. As a result, the E value can be increased. If the stress applied to the steel sheet at these locations is 200 kPa or more, the desired E value may not be obtained. From the viewpoint of further increasing the E value, it is preferable that the stress applied to the steel sheet at these locations be 180 kPa or less. There is no particular limitation on the lower limit of the stress applied to the steel sheet, but, for example, the stress applied to the steel sheet at the above locations may be 30 kPa or more. The stress applied to the steel sheet is a tension in the rolling direction, and can be controlled by adjusting the roll rotation speed during rolling. In addition, the stress can be determined by dividing the measured load in the rolling direction by the cross-sectional area of ​​the sheet being passed through.

[0097] [Primary Cooling Step] [Accelerated Cooling of the Lightly Reduced Steel Sheet to a Temperature Range of 680°C or Higher and Less than 720°C at an Average Cooling Rate of 50°C / second or Higher] The lightly reduced steel sheet is first accelerated cooled to a temperature range of 680°C or higher and less than 720°C at an average cooling rate of 50°C / second or higher. In the primary cooling step, alloy carbides are mainly finely precipitated in the steel during subsequent slow cooling in the temperature range of 680°C or higher and less than 720°C. In connection with this, ferrite is precipitation strengthened, thereby increasing the strength of the steel sheet, and the microstructure is refined due to the pinning effect of the alloy carbides, thereby significantly improving the toughness of the steel sheet. Therefore, when accelerated cooling to a temperature range of 680°C or higher and less than 720°C is performed, it is preferable to suppress the formation of ferrite and pearlite by cooling at a relatively fast average cooling rate, i.e., an average cooling rate of 50°C / second. This makes it possible to suppress the generation of ferrite and the like, which have a small amount of precipitation strengthening during accelerated cooling, and to effectively exhibit the precipitation strengthening and pinning effects caused by alloy carbides during the subsequent slow cooling, thereby significantly improving the strength and toughness of the steel plate that is finally obtained.

[0098] The average cooling rate referred to here means the value obtained by dividing the temperature drop of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to the completion of accelerated cooling (when the steel plate is removed from the cooling equipment) by the time required from the start of accelerated cooling to the completion of accelerated cooling.

[0099] If the average cooling rate is less than 50°C / s, relatively large amounts of ferrite and pearlite may be produced. The average cooling rate is preferably 70°C / s or more. While there is no particular upper limit to the average cooling rate, increasing the average cooling rate requires large-scale cooling equipment, which increases equipment costs. Therefore, considering equipment costs, the average cooling rate is preferably 300°C / s or less. To achieve the above-mentioned average cooling rate, for example, cooling water may be appropriately sprayed onto the steel sheet surface after completion of soft reduction. On the other hand, if the cooling stop temperature of accelerated cooling is less than 680°C or more than 720°C, the desired amount of ferrite may not be obtained, or the average spherical equivalent radius and / or average number density of alloy carbides precipitated during subsequent slow cooling may not be controlled within the desired range.

[0100] [Slow Cooling at an Average Cooling Rate of Less than 5°C / Second for 2.0 Seconds or More in a Temperature Range of 680°C or More and Less than 720°C] Next, by slowly cooling the steel sheet at an average cooling rate of less than 5°C / second for 2.0 seconds or more in a temperature range of 680°C or more and less than 720°C, ferrite can be formed and alloy carbides can be precipitated. In addition, the formed ferrite can be precipitation strengthened by the alloy carbides, and the structure can be refined by the pinning effect of the alloy carbides. This makes it possible to achieve both strength and toughness of the steel sheet. The average cooling rate here refers to the value obtained by dividing the temperature drop of the steel sheet from the cooling stop temperature of accelerated cooling to the cooling stop temperature of slow cooling by the time required from the stop of accelerated cooling to the stop of slow cooling.

[0101] If the slow cooling time is less than 2.0 seconds and / or the average cooling rate is 5°C / second or more, the desired amount of ferrite may not be obtained, or the average spherical equivalent radius and / or average number density of alloy carbides may not be controlled within the desired range. The slow cooling time is preferably 3.0 seconds or more. The upper limit of the slow cooling time is determined by the equipment layout, but can generally be less than 10.0 seconds. Furthermore, there is no particular limitation on the lower limit of the average cooling rate of slow cooling, and for example, the average cooling rate of slow cooling may be 0.1°C / second or more.

[0102] [Secondary Cooling Step] [Average Cooling Rate to Temperature Range of 350°C or Less: 50°C / second or More] The steel sheet after the primary cooling step is first secondarily cooled to a temperature range of 350°C or less (coiling temperature) at an average cooling rate of 50°C / second or more. This suppresses excessive pearlite formation and hardens the matrix structure, making it possible to achieve the desired strength. The average cooling rate here refers to the value obtained by dividing the temperature drop of the steel sheet from the cooling stop temperature of slow cooling, where the average cooling rate is less than 5°C / second, to the coiling temperature, by the time required from the stop of slow cooling, where the average cooling rate is less than 5°C / second, to the coiling temperature.

[0103] [Coiling temperature: 350°C or less] The secondarily cooled steel sheet is coiled in a temperature range of 350°C or less. By setting the coiling temperature to 350°C or less, it is possible to reduce the amount of iron carbide precipitation and also reduce the variation in hardness distribution within the hard phase. As a result, it is possible to obtain a desired I value. There is no particular restriction on the lower limit of the coiling temperature, but the coiling temperature may be, for example, room temperature (25°C) or higher.

[0104] According to the steel sheet manufactured by the above manufacturing method, the metal structure is configured to contain, in area %, retained austenite: less than 3.0%, ferrite: 15.0% or more but less than 60.0%, and pearlite: less than 5.0%, and the standard deviation of the Mn concentration in the metal structure can be controlled to 0.60 mass% or less, and further, the E value can be controlled to 10.7 or more and the I value can be controlled to 1.020 or more. In connection with this, it is possible to significantly improve the strength, ductility, and shear workability. In addition, by appropriately controlling the soft reduction step and the primary cooling step in particular as described above, it is possible to make the average equivalent sphere radius of alloy carbides in ferrite 0.5 nm or more but less than 10.0 nm, and the average number density of the alloy carbides 0.10 × 10 16 pieces / cm 3 That's it, 1.45 x 10 16 pieces / cm 3In this regard, it is possible to control the ferrite content to less than 100%. In addition, in connection with this, it is possible to further increase the strength of the steel sheet by precipitation strengthening the ferrite, and to significantly improve the toughness of the steel sheet by refining the structure due to the pinning effect of the alloy carbides. Therefore, the steel sheet manufactured by the above manufacturing method can reliably achieve both the contradictory properties of high strength and excellent formability, and is therefore particularly useful in the automotive field where both properties are required to be achieved.

[0105] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0106] In the following examples, steel sheets according to the embodiments of the present invention were produced under various conditions, and the tensile strength (TS), total elongation (El), toughness, and shear workability of the obtained steel sheets were examined.

[0107] First, molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Tables 1 and 2. These slabs were heated and held under the conditions shown in Table 3, further heated to a temperature of 1100°C or higher and held for the time shown in Table 3, and then hot-rolled. Hot-rolling was performed under the conditions shown in Table 3, with the rolling one stage before the final stage of hot rolling being performed at 1000°C, and then a stress shown in Table 3 was applied to the steel sheet before the final stage of rolling. Next, the hot-rolled steel sheet was subjected to soft reduction, primary cooling, and secondary cooling under the conditions shown in Table 4 to obtain a hot-rolled steel sheet having a plate thickness shown in Table 6. The average cooling rate in the slow cooling in the primary cooling step was 4°C / sec.

[0108]

[0109]

[0110]

[0111]

[0112] The properties of the obtained steel sheets were measured and evaluated by the following methods.

[0113] [Tensile strength (TS) and total elongation (El)] The tensile strength (TS) and total elongation (El) were measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the direction perpendicular to the rolling direction of the steel sheet (C direction) and conducting a tensile test in accordance with JIS Z 2241: 2022. More specifically, the test was conducted at room temperature in the range of 10 to 35°C, and a tensile test force was applied to the test piece, and strain was applied until fracture occurred.

[0114] [Charpy Impact Value] The Charpy impact value was calculated by measuring three Charpy impact values ​​at −20° C. using an impact blade with a radius of 2 mm in accordance with the provisions of JIS Z 2242: 2018 based on a V-notch test piece taken from the surface of the steel plate at a position ¼ of the plate thickness, and averaging the measured values. When a sub-size test piece was used, the value was converted into a full-size Charpy impact value according to the thickness of the test piece.

[0115] [Shear workability (presence or absence of secondary shear surface)] The shear workability of the hot-rolled steel sheets was evaluated by a punching test. Three punched holes were made in each hot-rolled steel sheet with a hole diameter of 10 mm, a clearance of 10%, and a punching speed of 3 m / s. Next, the thickness cross sections of the punched holes perpendicular to the rolling direction and parallel to the rolling direction were embedded in resin, and the cross-sectional shapes were photographed using a scanning electron microscope. In the obtained observation photographs, sheared edges as shown in Figure 1 or Figure 2 can be observed. Figure 1 shows an example of a sheared edge of a hot-rolled steel sheet according to an example of the present invention, and Figure 2 shows an example of a sheared edge of a hot-rolled steel sheet according to a comparative example. Figure 1 shows a sheared edge with sag, sheared edge, fractured edge, and burr. On the other hand, Figure 2 shows a sheared edge with sag, sheared edge, fractured edge, sheared edge, fractured edge, and burr. Here, the sag refers to a smooth, rounded surface region, the shear surface refers to a region of the punched end surface that has been separated by shear deformation, the fracture surface refers to a region of the punched end surface that has been separated by a crack that has developed near the cutting edge, and the burr refers to a surface with a protrusion that protrudes from the underside of the hot-rolled steel plate.

[0116] Of the obtained sheared end surfaces, if a shear surface-fracture surface-shear surface, as shown in Figure 2, was observed on two surfaces perpendicular to the rolling direction and two surfaces parallel to the rolling direction, it was determined that a secondary shear surface had been formed. Four surfaces for each punched hole, a total of 12 surfaces, were observed, and if no secondary shear surface was observed on any surface, the hot-rolled steel sheet was judged to have excellent shear workability and passed, and this was recorded as "Good" in the table. On the other hand, if even one secondary shear surface was formed, the hot-rolled steel sheet was judged to have poor shear workability and failed, and this was recorded as "NG" in the table.

[0117] [Surface Texture] The surface texture of the steel sheet was evaluated by measuring the arithmetic mean roughness Ra in accordance with the provisions of JIS B 0601:2013. Specifically, 10 locations were randomly selected on the steel sheet surface, and the arithmetic mean roughness was measured at each location using a contact surface roughness meter. Finally, the average value of all the obtained arithmetic mean roughnesses was determined as the arithmetic mean roughness Ra of the steel sheet surface. When Ra was more than 2.0 μm, it was evaluated as A, and when Ra was 2.0 μm or less, it was evaluated as AA.

[0118] The steel plate has a tensile strength (TS) of 780 MPa or more, a total elongation (El) of 13.0% or more, TS × El of 13,000 or more, acceptable shear workability, and a Charpy impact value of 50 J / cm 2 Steel plates with the above properties were evaluated as having high strength and improved ductility, shear workability and toughness. The results are shown in Tables 5 and 6.

[0119]

[0120]

[0121] With reference to Tables 1 to 6, it is believed that in Comparative Examples 3 and 33, the holding time in the temperature range of 700°C or higher and 850°C or lower in the heating step was short, which prevented the Mn microsegregation from being sufficiently eliminated. As a result, the standard deviation of the Mn concentration in the finally obtained metallographic structure could not be sufficiently reduced, and shear workability was reduced. In Comparative Example 4, the sheet thickness reduction rate in the temperature range of 850°C or higher and 1100°C or lower in the hot rolling step was low, which prevented the atomic diffusion of Mn from being promoted. As a result, the standard deviation of the Mn concentration in the finally obtained metallographic structure could not be sufficiently reduced, and shear workability was reduced. In Comparative Examples 5 and 36, the holding time in the temperature range of 1100°C or higher in the heating step was short, which similarly prevented the standard deviation of the Mn concentration from being sufficiently reduced, and shear workability was reduced. In Comparative Example 6, it is believed that the load stress on the steel sheet before the final stage of rolling in the hot rolling step was low, which prevented the promotion of recrystallization by the final stage of reduction. As a result, the E value, which indicates the periodicity of the metal structure, decreased, and the shear workability decreased. It is believed that in Comparative Example 7, the coarsening of the austenite grain size could not be measured sufficiently because the hot rolling completion temperature Tf was high. As a result, the E value, which indicates the periodicity of the metal structure, decreased, and the shear workability decreased. It is believed that in Comparative Example 8, the reduction rate in the final stage of the hot rolling process was low, and therefore recrystallization due to the reduction in the final stage could not be promoted. As a result, the E value, which indicates the periodicity of the metal structure, decreased, and the shear workability decreased.

[0122] In Comparative Example 9, the sheet thickness reduction rate in the temperature range of 840°C or higher and lower than 900°C in the soft reduction step was low, which reduced the average spherical equivalent radius of alloy carbides in ferrite, and it is believed that the pinning effect of the alloy carbides prevented sufficient refinement of the microstructure. As a result, the desired Charpy impact value could not be achieved, and toughness was reduced. In Comparative Example 11, the slow cooling time in the primary cooling step was short, so the desired ferrite area ratio could not be obtained, and the average number density of alloy carbides in the ferrite also decreased. As a result, ductility and toughness were reduced. In Comparative Examples 12 and 44, it is believed that the stress applied to the steel sheet before the first-stage rolling in the soft reduction step was high, which prevented austenite recrystallization from being promoted. As a result, the E value, which indicates the periodicity of the metal structure, decreased, and shear workability was reduced. In addition, in Comparative Example 12, the average cooling rate of the accelerated cooling in the primary cooling step was low, so a relatively large amount of pearlite was formed, resulting in reduced TS and ductility. In Comparative Example 13, the stress applied to the steel sheet after the final stage of rolling in the soft reduction step and before cooling to 800°C was high, which is thought to have prevented austenite recrystallization from being promoted. As a result, the E value, which indicates the periodicity of the metallographic structure, decreased, and shear workability deteriorated. In addition, in Comparative Example 13, the cooling stop temperature of the accelerated cooling in the primary cooling step was high, so the average number density of alloy carbides precipitated during the subsequent slow cooling could not be controlled within the desired range, resulting in a decrease in toughness. In Comparative Examples 14 and 15, the average cooling rate to the coiling temperature of 350°C or less was low, so a relatively large amount of pearlite was formed, resulting in a decrease in TS, ductility, etc.

[0123] In Comparative Example 29, TS decreased due to a low C content. In Comparative Example 30, the desired amount of ferrite was not obtained due to a high C content, resulting in reduced ductility. Furthermore, shear workability decreased due to excessive strength enhancement. In Comparative Example 31, ferrite was generated relatively abundantly due to a low Mn content, resulting in reduced TS. In Comparative Example 32, the average number density of alloy carbides in ferrite was reduced due to a low Ti content, and the improvement in strength due to precipitation strengthening and the refinement of the structure due to the pinning effect were not fully achieved, resulting in reduced TS and toughness. In Comparative Example 39, austenite recrystallization was not promoted due to the high stress applied to the steel sheet after the final rolling stage in the soft reduction process and before cooling to 800°C. As a result, the E value, which indicates the periodicity of the metal structure, decreased, and shear workability decreased. In Comparative Example 40, the plate thickness reduction rate was high in the temperature range of 840°C or higher and lower than 900°C during the soft reduction step, making it impossible to control the average sphere-equivalent radius and average number density of alloy carbides in ferrite within the desired range, and it is believed that the pinning effect of the alloy carbides prevented sufficient refinement of the microstructure. As a result, the desired Charpy impact value could not be achieved, and toughness was reduced. In Comparative Example 41, the slow cooling time in the primary cooling step was too short, resulting in a decrease in the average number density of alloy carbides in ferrite, and consequently, toughness was reduced. In Comparative Example 46, the cooling stop temperature of the accelerated cooling in the primary cooling step was too low, making it impossible to control the average sphere-equivalent radius and average number density of alloy carbides precipitated during the subsequent slow cooling within the desired range, resulting in a decrease in toughness. In Comparative Example 49, the total content of Si and sol. Al was high, which excessively accelerated ferrite transformation, resulting in a decrease in TS. In addition, it is believed that the high sol. Al content in Comparative Example 49 led to the formation of coarse oxides. On the other hand, in Comparative Example 50, the total content of Si and sol. Al was low, so that the desired amount of ferrite was not obtained, and the ductility was reduced.

[0124] In contrast to this, all of the steel sheets according to the examples of the invention have a predetermined chemical composition, and by appropriately controlling the conditions in the manufacturing method, they contain, in area %, less than 3.0% retained austenite, 15.0% or more but less than 60.0% ferrite, and less than 5.0% pearlite, and the average equivalent sphere radius of alloy carbides in the ferrite is 0.5 nm or more but less than 5.0 nm, and the average number density is 3.5 × 10 16 pieces / cm 3 It was possible to obtain steel sheets having a metallographic structure in which the E value was 10.7 or more, the I value was 1.020 or more, and the standard deviation of the Mn concentration was 0.60 mass% or less. As a result, despite the high tensile strength of 780 MPa or more, ductility, shear workability, and toughness were significantly improved. In particular, in Examples 1, 2, 10, 16, 17, 19 to 28, 34, 35, 37, 38, 42, 43, 47, and 48 of the present invention, in which the Si content was controlled to less than 0.050%, the surface texture was evaluated as AA, and the surface texture was more significantly improved.

Claims

1. In mass%, C: 0.030 to 0.150%, Si: 0.010 to less than 0.320%, Mn: 0.50 to 3.00%, Ti: 0.050 to 0.200%, sol. Al: 0.010 to 0.400%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, O: 0.0100% or less, Nb: 0 to 0.150%, V: 0 to 1.000%, Cr: 0 to 2.00%, Ni: 0 to 2.00%, Cu: 0-2.00%, Mo: 0-1.00%, B: 0-0.0100%, Sn: 0-1.00%, Sb: 0-1.00%, Ca: 0-0.0100%, Mg: 0-0.0100%, Hf: 0-0.0100%, Bi: 0-0.010%, REM: 0-0.0100%, The alloy has a chemical composition consisting of As: 0 to 0.010%, Zr: 0 to 0.010%, Co: 0 to 2.00%, Zn: 0 to 0.010%, W: 0 to 1.00%, and the balance: Fe and impurities, satisfying 0.100≦[Si]+[sol.Al]<0.720, where [Si] and [sol.Al] are the contents (mass%) of each element, and contains, in area %, retained austenite: less than 3.0%, ferrite: 15.0% or more and less than 60.0%, and pearlite: less than 5.0%, and the alloy carbides in the ferrite have an average equivalent sphere radius of 0.5 nm or more and less than 5.0 nm, and an average number density of 3.5×10 16 pieces / cm 3 or more, an E value of 10.7 or more, an I value of 1.020 or more, and a standard deviation of Mn concentration of 0.60 mass% or less.

2. The steel sheet according to claim 1, characterized in that the chemical composition contains, in mass %, Si: 0.010 to less than 0.050%.

3. The steel sheet according to claim 1 or 2, characterized in that the chemical composition contains, in mass %, 0.200 to 0.400% sol. Al.

4. The chemical composition is, in mass%, Nb: 0.001 to 0.150%, V: 0.001 to 1.000%, Cr: 0.001 to 2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Mo: 0.001 to 1.00%, B: 0.0001 to 0.0100%, Sn: 0.001 to 1.00%, Sb: 0.001 to 1.00%, Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.001 to 0.010%, The steel sheet according to any one of claims 1 to 3, characterized in that it contains at least one of REM: 0.0001 to 0.0100%, As: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Co: 0.001 to 2.00%, Zn: 0.001 to 0.010%, and W: 0.001 to 1.00%.

5. The steel sheet according to any one of claims 1 to 4, characterized in that it has a tensile strength of 780 MPa or more.

6. Steel sheet according to any one of claims 1 to 5, characterized in that it has a tensile strength of less than 980 MPa.

7. The steel sheet according to any one of claims 1 to 6, characterized in that it has a thickness of 1.0 to 8.0 mm.

8. A part, characterized in that it comprises a steel sheet according to any one of claims 1 to 7.

9. A hot rolling process comprising a heating step of heating a slab having the chemical composition defined in any one of claims 1 to 4, holding the slab in a temperature range of 700°C or higher and 850°C or lower for 900 seconds or longer, and then further heating the slab and holding the temperature in a temperature range of 1100°C or higher for 6000 seconds or longer; and hot rolling the slab, the hot rolling process satisfying the following conditions (a) to (c): (a) the hot rolling is carried out in a temperature range of 850°C or higher and 1100°C or lower so as to reduce the plate thickness by 90% or more in total; (b) the penultimate rolling stage is carried out at a temperature of 900°C or higher and lower than 1010°C, and then a stress of 170 kPa or higher is applied to the steel plate before the final rolling stage; and (c) the reduction rate in the final stage is 8% or higher, and the hot rolling completion temperature Tf is 900°C or higher and lower than 1010°C. a soft reduction step, which includes soft reducing a hot-rolled steel sheet in a temperature range of 840°C or higher and lower than 900°C so as to reduce the sheet thickness by 5% or higher and less than 8%, wherein the stress applied to the steel sheet after the final stage of rolling in the hot rolling step and before the first stage of soft reduction, and the stress applied to the steel sheet after the final stage of soft reduction until cooling to 800°C, is less than 200 kPa; a first cooling step, which accelerates cooling the soft-reduced steel sheet at an average cooling rate of 50°C / sec or higher to a temperature range of 680°C or higher and lower than 720°C, and then slowly cools it in the temperature range of 680°C or higher and lower than 720°C at an average cooling rate of less than 5°C / sec for 2.0 seconds or longer; and a second cooling step, which secondarily cools the steel sheet at an average cooling rate of 50°C / sec or higher to a temperature range of 350°C or lower, and then coils it in a temperature range of 350°C or lower.

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