Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to steel plates, parts containing the same, and methods for manufacturing steel plates.
• Blank sheets produced using shearing require excellent edge precision after shearing. For example, if a secondary shear surface, consisting of a shear surface, fracture surface, and shear surface, occurs on the edge surface (sheared edge) after shearing, as shown in Figure 2, the precision of the sheared edge surface will deteriorate significantly.
• the inventors conducted research focusing on the chemical composition and metallographic structure of steel sheets, particularly hot-rolled steel sheets.
• a relatively low Si content can be achieved by controlling the total Si and sol. Al content in steel sheets within a predetermined range, while improving strength, ductility, and shear workability by optimizing the structural fraction of the steel sheet's metallographic structure.
• 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.
• strength and toughness can be significantly improved by appropriately controlling the size and number of alloy carbides present in ferrite, and thus completed the present invention.
• the present invention which has achieved the above object, is as follows. (1) in mass %, C: 0.030-0.150%, Si: 0.010 to less than 0.320%; Mn: 0.50-3.00%, Ti: 0.050-0.200%, 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: 0-1.000%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.00%, B: 0 to 0.0100%, Sn: 0-1.00%, Sb: 0 to 1.00%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Hf: 0-0.0100%, Bi: 0 to 0.010%, REM: 0-0.0100%, As: 0 to 0.010%, Zr: 0 to 0.010%, Co: 0-2.00%, Zn:
• the chemical composition is, in mass%, Nb: 0.001 to 0.150%, V: 0.001-1.000%, Cr: 0.001-2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Mo: 0.001-1.00%, B: 0.0001 to 0.0100%, Sn: 0.001 to 1.00%, Sb: 0.001 to 1.00%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Hf: 0.0001 to 0.0100%, Bi: 0.001 to 0.010%, REM: 0.0001-0.0100%, 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%
• the steel sheet according to any one of (1) to (3) above characterized in that it contains at least one of the following: (5)
• a hot rolling step including hot rolling the slab, the hot rolling step satisfying 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 carried out at 900°C or higher and lower than 1010°C, and then a stress of 170 kPa or higher is applied to the steel sheet before the final stage of rolling; and (c) the rolling 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 process includes soft reduction of the hot-rolled steel sheet in a temperature range of 840°C or higher and lower than 900°C so as to result in a thickness reduction of 5% or higher and less than 8%, and the stress applied to the steel sheet after the final stage of rolling in the hot rolling process and before the first stage of soft reduction, and the stress applied to the steel sheet after the final stage of soft reduction and until cooling to 800°C is less than 200 kPa.
• a cooling step in which the lightly reduced steel plate is acceleratedly cooled to a temperature range of 680°C or higher and lower than 720°C at an average cooling rate of 50°C/second or higher, and then slowly cooled in the temperature range of 680°C or higher and lower than 720°C at an average cooling rate of less than 5°C/second for 2.0 seconds or longer; a secondary cooling step of secondarily cooling the steel sheet at an average cooling rate of 50°C/sec or more to a temperature range of 350°C or less, and then coiling the steel sheet in a temperature range of 350°C or less.
• the present invention makes it possible to provide steel sheets, particularly hot-rolled steel sheets, that have high strength and improved ductility, shear workability, and toughness despite having a relatively low Si content, as well as parts containing the same and methods for manufacturing the steel sheets.
• 1 is an example of a sheared end surface of a steel plate according to an example of the present invention. 1 is an example of a sheared end surface of a steel plate according to a comparative example.
• the steel sheet according to the embodiment of the present invention has, in mass%, C: 0.030-0.150%, Si: 0.010 to less than 0.320%; Mn: 0.50-3.00%, Ti: 0.050-0.200%, sol.
• the chemical composition satisfies the relationship 0.100 ⁇ [Si]+[sol.
• 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%.
• the metal 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.
• 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.
• 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.
• 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, i.e., the formation of periodic band-shaped hard phases is suppressed, thereby making it possible to improve shear workability.
• the present inventors conducted further studies to improve not only the above-mentioned properties of the steel sheet but also, primarily, toughness. As a result, the present inventors found that the strength and toughness of the steel sheet can be significantly improved by having alloy carbides present in ferrite at a specific size and ratio, more specifically, by having the alloy carbides present in ferrite so that the average spherical equivalent radius of the 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 /cm 3 or more.
• the steel sheet according to the embodiment of the present invention can significantly improve ductility, shear workability, and toughness, despite having a high tensile strength of, for example, 780 MPa or more. Therefore, the steel sheet according to the embodiment of the present invention 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.
• the Si content is set to less than 0.320%.
• Si content is 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, 0.23 0% 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 %, 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.
• Mn has the effect of suppressing ferrite transformation and increasing the strength of the steel sheet.
• 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.
• 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, thereby decreasing 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.
• 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.
• Sol. Al has the effect of improving the soundness of steel by deoxidization. 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, excessive sol.
• Al content may form coarse oxides, reducing 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, and indicates solute Al present in the steel in a solid solution state.
• P 0.100% or less
• P is an element generally contained as an impurity, but it also has the effect of increasing the strength of steel sheets 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.
• 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.
• 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.
• 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.
• the basic chemical composition of the steel sheet according to an embodiment of the present invention is as described above. Furthermore, if necessary, the steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe.
• 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, thereby increasing the 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Ni has the effect of improving the hardenability of the steel sheet. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing intergranular cracking of the slab 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.
• excessive Ni content may saturate the effect and increase 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.
• 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 these effects, 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• Ca, Mg, and Hf are elements that can control the morphology of nonmetallic 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.
• 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.
• 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.
• the Bi content is preferably 0.010% or less.
• the Bi content may be 0.005% or less or 0.003% or less.
• REM is an element that can control the morphology of nonmetallic inclusions.
• the REM content may be 0%, but 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.
• 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.
• 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.
• 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.
• the As content is preferably 0.010% or less.
• the As content may be 0.008% or less or 0.005% or less.
• Zr is an element that can control the morphology of nonmetallic 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.
• the Zr content is preferably 0.010% or less.
• the Zr content may be 0.008% or less or 0.005% or less.
• 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.
• 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.
• Zn is an element that can be contained in steel sheets when scrap or the like is used as a 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.
• W is an element that improves the hardenability of steel and contributes to improving its strength.
• the W content may be 0%, but to achieve 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.
• 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.
• the remainder other than the above elements consists of Fe and impurities.
• Impurities are components that are mixed in during the industrial production of steel plate due to various factors in the manufacturing process, including raw materials such as ore and scrap.
• [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
• [Si] and [sol. Al] are the contents (mass%) of each element.
• Si and sol. Al have the effect of promoting ferrite transformation. Therefore, by controlling the Si content to less than 0.320%, the formation of Si scale on the steel sheet surface is sufficiently suppressed, while the total content of Si and sol. Al is controlled to 0.100% or more, i.e., [Si] + [sol. Al] ⁇ 0.100, thereby promoting ferrite transformation.
• a desired ferrite amount more specifically, a ferrite amount of 15.0% or more and less than 60.0% by area %, can be achieved, thereby improving the strength-ductility balance of the steel sheet.
• Si and sol. Al are preferably used.
• the total content of Si and sol. Al is 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.
• the chemical composition of the steel sheet according to the embodiment of the present invention may be measured using a common analytical method.
• 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 the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method. If the steel sheet has a plating layer or paint film on its surface, the plating layer or paint film may be removed by mechanical grinding or the like, as necessary, before analyzing the chemical composition.
• the metallographic 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.
• 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 steel sheets through transformation-induced plasticity (TRIP).
• TRIP transformation-induced plasticity
• retained austenite transforms into high-carbon martensite during shearing, inhibiting stable crack initiation and causing secondary shear plane formation.
• the area fraction of retained austenite is set to less than 3.0%.
• 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 steel sheet. To fully obtain this effect, the ferrite area fraction is set to 15.0% or more. The ferrite area fraction 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.
• 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 pearlite area fraction is set to less than 5.0%.
• the pearlite area fraction 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 pearlite area fraction is preferable; therefore, it may be 0% or 0.1% or more.
• the remaining structure other than the 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.
• 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.
• 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.
• the metallographic structure is identified and the area ratio is calculated 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 thickness from the surface of the steel sheet. The sample is large enough to observe approximately 10 mm in a direction perpendicular to the thickness direction, depending 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 alkaline solution to remove strain introduced into the surface layer of the sample.
• Crystal orientation information is obtained by measuring a region of 200 ⁇ m or more at any position in the direction perpendicular to the thickness direction of the observation cross section, and a region of 200 ⁇ m or more in the thickness direction centered at the 1/4 position from the surface, using electron backscatter diffraction at measurement intervals of 0.1 ⁇ m.
• 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.
• the degree of vacuum in the EBSD analyzer is set to 9.6 ⁇ 10 ⁇ 5 Pa or less, the acceleration voltage is set to 15 kV, the irradiation current level is set to 13, and the electron beam irradiation level is set to 62.
• a backscattered electron image is taken in the same field of view.
• 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.
• cementite shows relatively high brightness values
• ferrite shows low brightness values.
• the brightness of the backscattered electron image also varies depending on the crystal orientation. For this reason, it is possible to identify pearlite, for example, 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.
• the obtained crystal orientation information is used with the "Grain Average Misorientation" function included in the "OIM Analysis (registered trademark)" software provided with the EBSD analyzer, and regions with a Grain Average Misorientation value of 1.0° or less are determined to be ferrite.
• the Grain Tolerance Angle is set to 15°, and the area of the region determined to be ferrite is calculated to obtain the ferrite area ratio.
• the area fraction of retained austenite is measured using X-ray diffraction.
• a sample is taken from the cross section of the steel plate at a position 1/4 of the plate thickness from the surface so that the metal structure can be observed in an area of 1 mm or more at any position perpendicular to the plate thickness direction, and also in an area of 1 mm or more in a direction perpendicular to these two directions.
• Co-K ⁇ radiation is used to determine the integrated intensity of a total of six peaks: ⁇ (110), ⁇ (200), ⁇ (211), ⁇ (111), ⁇ (200), and ⁇ (220).
• the volume fraction of retained austenite is obtained from the integrated intensity using the intensity averaging method, and this is considered to be the area fraction of retained austenite.
• the area fraction of the remaining structure is obtained by subtracting the area fractions of retained austenite, ferrite, and pearlite from 100%. Because the chemical composition of the steel plate according to an embodiment of the present invention indicates that the remaining structure is a low-temperature transformation structure, 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 an 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%.
• 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 particles/cm 3 or more.
• the 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.
• the mean spherical equivalent radius of the alloy carbides in the ferrite is less than 0.5 nm, the alloy carbides will not be able to sufficiently strengthen the ferrite through precipitation, making it impossible to achieve the desired high strength, and/or the pinning effect of the alloy carbides will not be able to sufficiently refine the structure, making it impossible to achieve the desired toughness. Therefore, from the perspective of improving strength and toughness, a large mean spherical equivalent radius of the alloy carbides is preferable, and may be, for example, 1.0 nm or more and 1.5 nm or more.
• the mean spherical equivalent radius of the alloy carbides in the ferrite is 5.0 nm or more, the strength of the ferrite will not be sufficiently increased, and due to the difference in hardness between 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 achieve the desired shearing workability in the steel plate.
• the average spherical equivalent 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.
• the average number density of alloy carbides in ferrite is lower than 3.5 ⁇ 10 16 /cm 3 , the ferrite cannot be sufficiently precipitation strengthened, making it impossible to achieve the desired high strength. Therefore, from the viewpoint of improving strength, the average number density of alloy carbides is preferably high, and may be, for example, 5.0 ⁇ 10 16 /cm 3 or more, 8.0 ⁇ 10 16 /cm 3 or more, 10.0 ⁇ 10 16 /cm 3 or more, 12.0 ⁇ 10 16 /cm 3 or more, 15.0 ⁇ 10 16 /cm 3 or more, 18.0 ⁇ 10 16 /cm 3 or more, or 20.0 ⁇ 10 16 /cm 3 or more.
• the average number density of alloy carbides in ferrite may be, for example, 1000.0 ⁇ 10 16 /cm 3 or less, 800.0 ⁇ 10 16 /cm 3 or less, or 500.0 ⁇ 10 16 /cm 3 or less. If the average number density of alloy carbides in ferrite is increased, the ferrite is precipitation strengthened, but the toughness may correspondingly decrease somewhat.
• the average number density of alloy carbides within an appropriate range, for example, 100.0 ⁇ 10 16 /cm 3 or less, 50.0 ⁇ 10 16 /cm 3 or less, or 30.0 ⁇ 10 16 /cm 3 or less.
• a FIB (focused ion beam) device is used to collect samples of ferrite grains within the observation field using the EBSD described above, after measuring the area ratio of each structure.
• the collected samples are then processed into needle shapes using well-known methods, and a three-dimensional atom probe is used to accurately measure the spherical equivalent radius and number density of fine precipitates with spherical equivalent radii ranging from less than 1 nm to several tens of nm.
• the number density of precipitates can be obtained by dividing the number of precipitates contained in the area measured with the three-dimensional atom probe by the volume of the measurement area, for precipitates identified as alloy carbides using the method described below.
• the total volume of precipitates within the measurement area is obtained by dividing the total number of atoms of alloying elements (Ti, Nb, V, Mo, and C) contained in all precipitates within the measurement area by the atomic density of the alloy carbides.
• the volume of the precipitates is obtained by dividing the total volume of the precipitates by the number of precipitates. From the obtained volume of the precipitates, the spherical equivalent radius is calculated, assuming that the precipitates are spherical.
• the above-described method is performed on five or more measurement data sets with a measurement volume of 30,000 nm3 or more to obtain the average number density and average spherical equivalent radius.
• the observation area is the area where the Ga (gallium) introduced during FIB processing is less than 0.025 at%. Areas containing 0.025 at% or more of Ga are excluded from the measurement area.
• the Ga content can be confirmed by using the 1D Concentration Profile function of the data analysis software IVAS 3.6.14 (manufactured by CAMECA Instruments Inc.).
• 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 for data acquired by a three-dimensional atom probe.
• E value: 10.7 or more [I value: 1.020 or more]
• the E value is 10.7 or more
• the I value is 1.020 or more.
• the E (Entropy) value which indicates the periodicity of the metallographic structure, is controlled to 10.7 or more
• the I (Inverse Difference Normalized) value which indicates the uniformity of the metallographic structure, is controlled to 1.020 or more, thereby making it possible to suppress the generation of secondary shear planes.
• the E value indicates the periodicity of the metal structure, and when brightness is periodically arranged due to the influence of band-shaped structure, etc., i.e., when the periodicity of the metal structure is high, the E value decreases. In embodiments 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 initiate from the cutting edge of the shearing tool very early in the shearing process, forming a fracture surface, and then a shear plane is formed again. This is thought to make 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, and more preferably 11.0 or more. The higher the E value, the better. There is no particular upper limit, but it may be 13.0 or less, 12.5 or less, or 12.0 or less.
• the I value indicates the uniformity of the metal structure, and increases as the area of a region with a certain brightness increases.
• a high I value indicates a high uniformity of the metal structure.
• a highly uniform metal structure is required, so the I value must 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 another shear surface will form. This is thought to make it easier for secondary shear surfaces to form. Therefore, the I value is set to 1.020 or more.
• the I value is preferably 1.025 or more, and more preferably 1.030 or more. A higher I value is preferable, and there is no particular upper limit, but it may be 1.200 or less, 1.150 or less, or 1.100 or less.
• the E and I values can be obtained by the following method.
• the area of the SEM image taken to calculate the E and I values is 200 ⁇ m x 200 ⁇ m, centered at a position 1/4 of the way down the thickness from the surface of the steel plate in the cross section of the steel plate's thickness, and the number of fields of view observed is 5.
• an SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation is used, with a tungsten emitter and an acceleration voltage of 1.5 kV. With the above settings, the SEM image is output at a magnification of 1000x and a grayscale of 256 gradations.
• the resulting SEM image was cropped to an 880 x 880 pixel region and subjected to smoothing processing with a tile grid size of 8 x 8 and 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 then rotated counterclockwise in 1-degree increments from 0 to 179 degrees, excluding 90 degrees, and an image was created for each degree, resulting in a total of 179 images.
• 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, respectively. The average value obtained by measuring the entire field of view is calculated.
• P(i, j) is a gray level co-occurrence matrix
• the value in the i-th row and j-th column of the matrix P is expressed as P(i, j).
• 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').
• the value in the i-th row and j-th column of the matrix P is expressed as P ij .
• the standard deviation of the Mn concentration 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 initiating from the cutting edge of the shearing tool very early in the shearing process.
• 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.
• 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.
• 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 ⁇ m in an area of 20 ⁇ m in a direction perpendicular to the sheet thickness direction of the sample and 20 ⁇ m in the sheet thickness direction of the sample.
• the standard deviation of the Mn concentration is obtained by calculating the standard deviation based on the Mn concentrations obtained from all measurement points.
• 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.
• 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.
• the steel sheet according to the embodiment of the present invention may have a plating layer on its surface for the purpose of improving corrosion resistance, etc.
• the plating layer may be an electroplated layer or a hot-dip plated layer.
• electroplated layers include electrogalvanized galvanized and electrolytic Zn—Ni alloy plating.
• 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.
• 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).
• the steel sheet according to the embodiments 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 embodiments of the present invention is particularly useful for use in parts in technical fields where compatibility between these properties is required.
• an automobile part particularly an automobile suspension part, is provided that includes the steel sheet according to the embodiments of the present invention. Examples of automobile suspension parts include lower arms and trailing arms.
• the tensile strength characteristics are evaluated in accordance with JIS Z 2241:2022.
• the test specimen is a No. 5 test specimen of JIS Z 2241:2022.
• the tensile test specimen is 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, the specimen may be taken from any direction within the surface of the steel sheet.
• Steel plates 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.
• steel plates according to embodiments of the present invention can achieve improved ductility, shear workability, and toughness due to the specific combination of chemical composition and metal structure described above.
• the tensile strength of the steel plate 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.
• Steel sheets according to embodiments of the present invention can achieve high total elongation (El), for example, a total elongation of 13.0% or more, and a product of tensile strength and total elongation (TS x El) of 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.
• the product of tensile strength and total elongation is preferably 14,000 MPa ⁇ % or more or 15,000 MPa ⁇ % or more.
• excellent toughness can be achieved, for example, a Charpy impact value at -20°C of 50 J/cm2 or more , preferably 60 J/ cm2 or more, more preferably 70 J/ cm2 or more, and most preferably 80 J/ cm2 or more.
• the upper limit of the Charpy impact value is not particularly limited, but may be, for example, 150 J/ cm2 or 120 J/ cm2 .
• the Charpy impact value is 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 specimen taken from a position 1/4 of the plate thickness from the surface of the steel plate, and averaging the results. When a sub-size test specimen is used, the Charpy impact value is converted to a full-size Charpy impact value according to the thickness of the test specimen.
• ⁇ Steel sheet manufacturing method> 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 manufacturing method when the steel sheet according to an embodiment of the present invention is a hot-rolled steel sheet.
• a method for manufacturing a steel sheet according to an embodiment of the present invention includes: a heating step in which a slab having the chemical composition described above in relation to the steel plate is heated to a temperature range of 700°C or more and 850°C or less and held for 900 seconds or more, and then further heated to a temperature range of 1100°C or more and held for 6000 seconds or more; a hot rolling step including hot rolling the slab, the hot rolling step satisfying 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 carried out at 900°C or higher and lower than 1010°C, and then a stress of 170 kPa or higher is applied to the steel sheet before the final stage of rolling; and (c) the rolling 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
• a soft reduction process includes soft reduction of the hot-rolled steel sheet in a temperature range of 840°C or higher and lower than 900°C so as to result in a thickness reduction of 5% or higher and less than 8%, and the stress applied to the steel sheet after the final stage of rolling in the hot rolling process and before the first stage of soft reduction, and the stress applied to the steel sheet after the final stage of soft reduction and until cooling to 800°C is less than 200 kPa.
• a cooling step in which the lightly reduced steel plate is acceleratedly cooled to a temperature range of 680°C or higher and lower than 720°C at an average cooling rate of 50°C/second or higher, and then slowly cooled in the temperature range of 680°C or higher and lower than 720°C at an average cooling rate of less than 5°C/second for 2.0 seconds or longer;
• the method is characterized by including a secondary cooling step in which the steel sheet is secondarily cooled to a temperature range of 350°C or less at an average cooling rate of 50°C/sec or more, and then coiled in a temperature range of 350°C or less.
• the temperature of the slab and the temperature of the steel plate refer to the surface temperature of the slab and the surface temperature of the steel plate, respectively. Furthermore, stress refers to the tension applied to the steel plate in the rolling direction.
• 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.
• 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.
• the steel sheet temperature When held in a temperature range of 700 to 850°C, the steel sheet temperature may be varied or constant within this temperature range.
• the steel sheet temperature may be varied or constant within the 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.
• the holding time in the temperature range of 700°C to 850°C must be 900 seconds or more, preferably 1000 seconds or more. There is no particular upper limit, but for example, the holding time in the temperature range of 700°C to 850°C may be 1800 seconds or less.
• 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. There is no particular upper limit, but for example, the holding time in the temperature range of 1100°C or higher may be 10,000 seconds or less.
• Hot rolling process In the hot rolling step, the slab is hot rolled so as to satisfy the following conditions (a) to (c).
• 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;
• the rolling one stage before the final stage is carried out at 900°C or higher and lower than 1010°C, and then a stress of 170 kPa or higher is applied to the steel sheet before the final stage of rolling; and
• 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 reverse mill or tandem mill for multi-pass rolling.
• a tandem mill for at least the final two stages of hot rolling.
• Hot rolling to achieve a total thickness reduction of 90% or more in the temperature range of 850°C or higher and 1100°C or lower primarily refines recrystallized austenite grains and promotes the accumulation of strain energy within unrecrystallized austenite grains. In relation to this, austenite recrystallization is promoted, and Mn atomic diffusion is also 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.
• 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 this temperature range and t1 is the exit thickness after the final rolling in this temperature range.
• 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.
• ⁇ 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.
• 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.
• the stress applied to the steel sheet is less than 170 kPa, the desired E value may not be obtained.
• 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.
• 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.
• the hot rolling completion temperature Tf is controlled to 900°C or more and less than 1010°C.
• 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.
• 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 desired ranges.
• the average equivalent sphere radius and/or average number density of alloy carbides in ferrite cannot be controlled within the desired ranges, and for example, the average equivalent sphere radius of alloy carbides may become smaller or the average number density of alloy carbides may become lower.
• 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.
• the 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 light reduction, and stress applied to the steel sheet after the final stage of rolling under light reduction and until cooling to 800°C are each controlled to less than 200 kPa.
• austenite recrystallization preferentially proceeds in the rolling direction, and an increase in the periodicity of the metallographic structure can be suppressed.
• the average cooling rate here refers to 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 end of accelerated cooling (when the steel plate is removed from the cooling equipment) divided by the time required from the start of accelerated cooling to the end of accelerated cooling.
• the average cooling rate is less than 50°C/sec, relatively large amounts of ferrite and pearlite may be produced.
• the average cooling rate is preferably 70°C/sec or higher. There is no particular upper limit to the average cooling rate, but if the average cooling rate is increased, the cooling equipment will become larger and the equipment costs will increase. Therefore, considering the equipment costs, it is preferable to set the average cooling rate to 300°C/sec or lower. To achieve the above average cooling rate, for example, it is sufficient to appropriately spray cooling water onto the steel sheet surface after the completion of soft reduction.
• 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 able to be controlled within the desired range.
• the average cooling rate 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.
• 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 the 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.
• [Secondary cooling process] [Average cooling rate to a temperature range of 350°C or less: 50°C/sec 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/sec 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/sec, to the coiling temperature, by the time required from the stop of slow cooling, where the average cooling rate is less than 5°C/sec, to the coiling temperature.
• the steel sheet produced by the above-described manufacturing method has a metallographic structure containing, by area percentage, less than 3.0% retained austenite, at least 15.0% but less than 60.0% ferrite, and less than 5.0% pearlite.
• the standard deviation of the Mn concentration in the metallographic structure can be controlled to 0.60 mass% or less, and the E value can be controlled to 10.7 or more and the I value can be controlled to 1.020 or more. This also enables significant improvements in strength, ductility, and shear workability.
• the average equivalent-sphere radius of alloy carbides in the ferrite can be controlled to 0.5 nm or more but less than 10.0 nm, and the average number density of the alloy carbides can be controlled to 0.10 ⁇ 10 16 particles/cm 3 or more but less than 1.45 ⁇ 10 16 particles/cm 3 .
• steel plates according to embodiments of the present invention were manufactured under various conditions, and the tensile strength (TS), total elongation (El), toughness, and shear workability of the resulting steel plates were examined.
• molten steel was cast using a continuous casting method to form slabs with the various chemical compositions shown in Tables 1 and 2. These slabs were heated and held under the conditions shown in Table 3, then 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 carried out under the conditions shown in Table 3, with the rolling one stage before the final stage of hot rolling being carried out at 1000°C, and then the stress shown in Table 3 was applied to the steel plate before the final stage of rolling. Next, the hot-rolled steel plate was subjected to soft reduction, primary cooling, and secondary cooling under the conditions shown in Table 4, resulting in hot-rolled steel plate with the plate thickness shown in Table 6. The average cooling rate during slow cooling in the primary cooling process was 4°C/second.
• 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 1 ⁇ 4 of the plate thickness, and averaging the measured values. When a sub-size test piece was used, it was converted into a full-size Charpy impact value according to the thickness of the test piece.
• 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
• the burr refers to a surface with a protrusion that protrudes from the underside of the hot-rolled steel plate.
• Comparative Examples 5 and 36 the holding time in the temperature range of 1100°C or higher during the heating process was short, which similarly prevented the standard deviation of the Mn concentration from being fully reduced, resulting in reduced shear workability.
• Comparative Example 6 it is believed that the load stress on the steel sheet before the final rolling stage in the hot rolling process was low, which prevented the recrystallization due to the final reduction in the rolling stage. As a result, the E value, which indicates the periodicity of the metal structure, decreased, and shear workability deteriorated.
• Comparative Example 7 it is believed that the high hot rolling completion temperature Tf prevented the coarsening of austenite grain size from being adequately measured.
• Comparative Examples 12 and 44 the stress applied to the steel sheet before the first stage of rolling in the soft reduction process 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 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 process was low, resulting in the formation of a relatively large amount of pearlite, 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.
• TS decreased due to the low C content.
• the desired amount of ferrite was not obtained due to the high C content, resulting in reduced ductility. Furthermore, shear workability decreased due to excessively high strength.
• TS decreased due to the low Mn content, resulting in the formation of a relatively large amount of ferrite.
• the low Ti content reduced the average number density of alloy carbides in the ferrite, preventing sufficient improvement in strength through precipitation strengthening and refinement of the structure through the pinning effect, resulting in reduced TS and toughness.
• 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.
• 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.
• 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. As a result, toughness and ductility decreased.
• Comparative Example 50 the total content of Si and sol. Al was low, so the desired amount of ferrite was not obtained, and ductility decreased.

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