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
  • 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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese

Definitions

• the present invention relates to steel plates.
• Patent Document 1 discloses a precipitation-hardened martensitic steel that has high tensile strength and Charpy absorbed energy, and contains, in mass percent, C: 0.02-0.08%, Al: 0.05% or less, Cr: 8.0-13.0%, Ni: 2.0-8.0%, Co: 2.0-16.0%, Mo as an essential element, Mo + 0.5W: 3.5-8.0%, and the balance Fe and impurities.
• Patent Document 2 also discloses a high-strength steel with excellent tensile strength, ductility, and bendability, in which the microstructure has an area ratio of ferrite of 5% or more and less than 50%, and the area ratio of a mixed structure of fresh martensite and retained austenite to the total structure is more than 0% and less than 30%, and further, when analyzed with an electron beam microprobe analyzer, there are 5% or more areas where the Mn concentration is concentrated to 1.2 times the Mn concentration in the steel plate, and when the fraction of the areas where the Mn concentration is concentrated to 1.2 times the Mn concentration in the steel plate is measured in 2 ⁇ m sections, the standard deviation when measuring 100 sections is 4.0% or more.
• Patent Document 1 does not take into consideration bendability. Furthermore, automotive suspension parts such as those described above are manufactured by subjecting steel plates to multiple forming processes. Therefore, steel plates used in automotive suspension parts are required to have excellent formability even after being subjected to a certain degree of pre-strain in the previous process. When performing multiple forming processes, if the strain generated in the previous process is not sufficiently dispersed, localized deformation may progress in the subsequent process, and the steel plate may not be able to exhibit its original formability. Such localized deformation is prominent in the case of bending forming. However, Patent Document 2 does not take into consideration bendability after being pre-strained.
• the present invention was made in consideration of the above situation, and aims to provide a steel sheet that has high strength and excellent bendability after pre-straining.
• the gist of the present invention which was made based on the above findings, is as follows:
• the steel sheet according to one embodiment of the present invention has a chemical composition, in mass%, C: 0.08-0.17%, Si: 0.03-1.40%, Mn: 1.60-3.00%, Al: 0.01-0.70%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, Ti: 0.020 to 0.180%, Nb: 0.010-0.050%, Ti+Nb+(Mo/2)+V: 0.100-0.600%, The balance is Fe and impurities.
• the metal structure is, in area percent, Tempered martensite: 80.0 to 97.0%, The sum of pearlite, ferrite and bainite: 10.0% or less, and The sum of fresh martensite and retained austenite: 3.0 to 10.0%; The standard deviation of the Mn concentration of the fresh martensite and the retained austenite is 1.0 to 5.0%; The tensile strength is 1110 MPa or more.
• the steel plate described in (1) above may have a metal structure in which the area percentage of the retained austenite is 1.5% or more.
• the steel sheet according to (1) or (2) above, wherein the chemical composition is, in mass%, replacing a part of Fe, Mo: 0.600% or less, and V: 0.300% or less, may contain one or more selected from the following.
• the steel sheet according to any one of (1) to (3) above, wherein the chemical composition is, in mass%, replacing a part of Fe, B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less; and Ni: 2.0% or less; may contain one or more selected from the following.
• the steel sheet according to any one of (1) to (4) above, wherein the chemical composition is, in mass%, replacing a part of Fe, Ca: 0.020% or less, Mg: 0.020% or less, REM: 0.100% or less; and Bi: 0.020% or less; may contain one or more selected from the following.
• the chemical composition of the steel plate according to this embodiment is, in mass%, C: 0.08-0.17%, Si: 0.03-1.40%, Mn: 1.60-3.00%, Al: 0.01-0.70%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, Ti: 0.020-0.180%, Nb: 0.010-0.050%, Ti+Nb+(Mo/2)+V: 0.100-0.600%, and the balance includes Fe and impurities. Each element is described in detail below.
• C 0.08-0.17%
• C is an element necessary for obtaining a desired tensile strength of the steel plate. If the C content is less than 0.08%, the desired tensile strength cannot be obtained. Therefore, the C content is The C content is preferably 0.09% or more, 0.10% or more, or 0.11% or more.
• the C content is set to 0.17% or less. It is preferably 0.15% or less or 0.14% or less.
• the C content is more preferably 0.13% or less.
• the C content is preferably 0.09 to 0.15%, more preferably 0.10 to 0.14%, and even more preferably 0.11 to 0.13%.
• Si 0.03-1.40% Silicon is an element that improves the adhesion of zinc plating. If the silicon content is less than 0.03%, the adhesion of zinc plating during forming decreases. Therefore, the silicon content is set to 0.03% or more.
• the Si content is preferably 0.04% or more, and more preferably 0.05% or more.
• the Si content is set to 1.40% or less.
• the Si content is preferably 1.10% or less, and more preferably 0.90% or less.
• the Si content is preferably 0.04 to 1.10%, and more preferably 0.05 to 0.90%.
• Mn 1.60-3.00%
• Mn is an element necessary for improving the strength of a steel sheet. If the Mn content is less than 1.60%, the area ratio of ferrite becomes too high, making it impossible to obtain a desired tensile strength. Therefore, the Mn content is set to 1.60% or more, and preferably 1.80% or more. The Mn content reduces the standard deviation of the Mn concentration in fresh martensite and retained austenite, and is effective in preventing the deterioration of the steel. In order to further improve the bendability after strain is applied, the strain is more preferably 2.00% or more.
• the Mn content is set to 3.00% or less.
• the Mn content is preferably 2.70% or less, and more preferably 2.50% or less.
• the Mn content is preferably 1.80 to 2.70%, and more preferably 2.00 to 2.50%.
• Al 0.01 ⁇ 0.70%
• Al is an element that acts as a deoxidizer and improves the cleanliness of steel. If the Al content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained, and a large amount of Al is included in the steel sheet. Such inclusions deteriorate the surface properties of the steel sheet. Therefore, the Al content is set to 0.01% or more.
• the Al content is preferably 0.02% or less. It is preferably 0.03% or more, more preferably 0.04% or more, and even more preferably 0.05% or more.
• the Al content is set to 0.70% or less.
• the Al content is preferably 0.60% or less, 0.30% or less, and more preferably 0.10% or less.
• the Al content is preferably 0.02 to 0.60%, more preferably 0.03 to 0.30%, and even more preferably 0.04 to 0.10%.
• P 0.080% or less
• P is an element that segregates in the center of the thickness of a steel plate. If the P content exceeds 0.080%, the weldability decreases. Therefore, the P content is set to 0.080% or less.
• the P content is preferably 0.040% or less, and more preferably 0.020% or less.
• the P content may be 0.0005% or more.
• S 0.0100% or less
• S is an element that exists as sulfide. If the S content exceeds 0.0100%, the weldability decreases. Therefore, the S content is set to 0.0100% or less.
• the S content is preferably 0.0080% or less, more preferably 0.0050% or less, and further preferably 0.0030% or less.
• the S content may be 0.0005% or more.
• N 0.0050% or less
• N is an element that forms coarse nitrides in steel. If the N content exceeds 0.0050%, slab cracking may occur easily, and hot rolling may become difficult. Therefore, the N content is set to 0.0050% or less.
• the N content is preferably 0.0040% or less, and more preferably 0.0035% or less.
• the N content may be 0.0005% or more.
• Ti 0.020-0.180% Ti is an element that forms fine carbides and/or carbonitrides in steel, thereby increasing the strength of the steel plate. If the Ti content is less than 0.020%, it is difficult to obtain the desired tensile strength. Therefore, the Ti content is set to 0.020% or more, preferably 0.050% or more, and more preferably 0.080% or more.
• the Ti content is set to 0.180% or less.
• the Ti content is preferably 0.160% or less, and more preferably 0.150% or less.
• the Ti content is preferably 0.050 to 0.160%, and more preferably 0.080 to 0.150%.
• Nb 0.010-0.050%
• Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling.
• Nb is also an element that increases the strength of steel sheets by forming fine carbides and/or nitrides. If the Nb content is less than 0.010%, the desired tensile strength cannot be obtained. Therefore, the Nb content is set to 0.010% or more.
• the Nb content is preferably 0.013% or more. , and more preferably 0.015% or more.
• the Nb content is set to 0.050% or less.
• the Nb content is preferably 0.040% or less, and more preferably 0.035% or less.
• the Nb content is preferably 0.013 to 0.040%, and more preferably 0.015 to 0.035%.
• Ti+Nb+(Mo/2)+V 0.100-0.600%
• the total amount of the Ti content, the Nb content, half the Mo content, and the V content is controlled.
• Ti + Nb + (Mo/2) + V are controlled. If the total amount of these elements is less than 0.100%, at least one of fine carbides, nitrides and carbonitrides is formed, resulting in the steel sheet being deteriorated. Therefore, the above total content is set to 0.100% or more.
• the above effect can be obtained if the content of any one of Ti, Nb, and V, or half the Mo content, is 0.100% or more.
• the above total amount is preferably 0.150% or more.
• the total amount is more preferably 0.200% or more, even more preferably 0.230% or more, and even more preferably 0.250% or more. Note that if the above total amount exceeds 0.230%, at least one of Mo and V is essentially required to be included.
• the above total amount is set to 0.600% or less.
• the above total amount is preferably 0.500% or less, 0.400% or less, or 0.300% or less.
• the above total amount is preferably 0.200 to 0.500%, more preferably 0.230 to 0.400%, and even more preferably 0.250 to 0.300%.
• the balance of the chemical composition of the steel plate according to this embodiment may be Fe and impurities.
• impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment, or substances that are acceptable to the extent that they do not adversely affect the steel plate according to this embodiment.
• the steel plate according to this embodiment may contain the following optional elements in place of a portion of Fe.
• the lower limit of the content is 0%.
• Each optional element is described below.
• Mo 0.600% or less
• Mo is an element that increases the strength of the steel plate by forming fine carbides in the steel.
• the Mo content is preferably 0.001% or more, and more preferably 0.002% or more.
• the Mo content is set to 0.600% or less.
• the Mo content is preferably 0.500% or less, 0.400% or less, 0.300% or less, 0.200% or less, or 0.100% or less.
• the Mo content is preferably 0.001 to 0.500%, 0.002 to 0.400%, 0.003 to 0.300%, 0.004 to 0.200%, or 0.005 to 0.100%.
• V 0.300% or less
• V is an element that increases the strength of the steel plate by forming fine carbides and/or nitrides in the steel.
• the V content is preferably 0.010% or more, more preferably 0.050% or more, and even more preferably 0.100% or more.
• the V content is set to 0.300% or less.
• the V content is preferably 0.270% or less, 0.240% or less, or 0.200% or less.
• the V content is preferably 0.010 to 0.270%, 0.030 to 0.240%, or 0.050 to 0.200%.
• B 0.0030% or less
• B is an element that suppresses the formation of ferrite in the cooling process and increases the strength of the steel sheet.
• the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.
• the B content is set to 0.0030% or less.
• the B content is preferably 0.0025% or less, 0.0020% or less, or 0.0015% or less.
• the B content is preferably 0.0001 to 0.0025%, 0.0005 to 0.0020%, or 0.0010 to 0.0015%.
• Cr 0.50% or less Cr is an element that exerts an effect similar to that of Mn.
• the Cr content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
• the Cr content is set to 0.50% or less.
• the Cr content is preferably 0.40% or less, 0.30% or less, or 0.20% or less.
• the Cr content is preferably 0.001 to 0.40%, 0.005 to 0.30%, or 0.010 to 0.20%.
• Cu 0.50% or less
• the Cu content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more.
• the Cu content is set to 0.50% or less.
• the Cu content is preferably 0.40% or less, 0.30% or less, or 0.20% or less.
• the Cu content is preferably 0.01 to 0.40%, 0.05 to 0.30%, or 0.10 to 0.20%.
• Ni 2.0% or less
• Ni has the effect of increasing the hardenability of the steel sheet and increasing the strength of the steel sheet.
• Ni has the effect of effectively suppressing grain boundary cracking of the slab caused by Cu.
• the Ni content is preferably 0.02% or more, more preferably 0.10% or more, and even more preferably 0.30% or more. Since Ni is an expensive element, it is economically undesirable to contain a large amount of Ni. Therefore, the Ni content is 2.0% or less.
• the Ni content is preferably 1.5% or less, 1.0% or less, or 0.50% or less.
• the Ni content is preferably 0.02 to 1.5% or less, 0.10 to 1.0% or less, or 0.30 to 0.50% or less.
• Ca, Mg and REM all have the effect of controlling the shape of inclusions to a preferred shape, thereby improving the formability of the steel sheet. Therefore, one or more selected from these elements may be contained. In order to more reliably obtain the effects of the above action, it is preferable that any one or more of Ca, Mg and REM is 0.0005% or more.
• the contents of Ca, Mg and REM are each more preferably 0.0010% or more, and even more preferably 0.0020% or more.
• the Ca content and/or Mg content exceeds 0.020%, or if the REM content exceeds 0.100%, excessive inclusions may be formed in the steel, reducing the ductility of the steel sheet. Therefore, the Ca content and Mg content are set to 0.020% or less, and the REM content to 0.100% or less.
• the Ca and Mg contents are preferably 0.015% or less, 0.010% or less, 0.0050% or less, or 0.0030% or less.
• the REM content is preferably 0.050% or less, 0.030% or less, 0.010% or less, 0.0050% or less, or 0.0030% or less.
• the Ca content is preferably 0.0005-0.015%, 0.0010-0.010%, 0.0015-0.0050%, or 0.0020-0.0030%.
• the Mg content is preferably 0.0005-0.015%, 0.0010-0.010%, 0.0015-0.0050%, or 0.0020-0.0030%.
• the REM content is preferably 0.0001-0.050%, 0.0005-0.030%, 0.0010-0.010%, 0.0015-0.0050%, or 0.0020-0.0030%.
• REM refers to a total of 17 elements consisting of Sc, Y and lanthanides, and the content of the above REM refers to the total content of these elements.
• lanthanides they are added industrially in the form of misch metal.
• Bi 0.020% or less
• the Bi content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0050% or more.
• the Bi content is set to 0.020% or less.
• the Bi content is preferably 0.010% or less.
• the Bi content is preferably 0.0005 to 0.016%, 0.0010 to 0.013%, or 0.0050 to 0.010%.
• Sn 0.05% or less Sn has the effect of improving plating processability during the production of plated steel sheets.
• the Sn content is preferably 0.01% or more, and more preferably 0.02% or more. However, if the Sn content exceeds 0.05%, defects may occur during hot rolling, so the Sn content is set to 0.05% or less.
• the Sn content is preferably 0.04% or less.
• the Sn content is preferably 0.01 to 0.04%, or 0.02 to 0.03%.
• the inventors have also confirmed that even if Zr, Co, Zn and W are contained as impurities, the effect of the steel plate according to this embodiment is not impaired so long as the total content is 1.00% or less. Therefore, one or more elements selected from Zr, Co, Zn and W may be contained in an amount of 1.00% or less in total.
• the chemical composition of the above-mentioned steel sheet may be analyzed using a spark discharge optical emission spectrometer or the like. Note that for C and S, values identified by burning in an oxygen stream using a gas composition analyzer or the like and measuring by infrared absorption are used. For N, values identified by melting a test piece taken from the steel sheet in a helium stream and measuring by thermal conductivity are used. In cases where the steel sheet is plated, the "chemical composition of the steel sheet" refers to the chemical composition of the base material excluding the plated layer.
• the steel plate according to this embodiment has a metal structure, in area percentages, of tempered martensite: 80.0-97.0%, pearlite, ferrite and bainite in total: 10.0% or less, and fresh martensite and retained austenite in total: 3.0-10.0%, and the standard deviation of the Mn concentration in the fresh martensite and retained austenite is 5.0% or less.
• the metal structure at the 1/4 position of the sheet thickness of the steel sheet is specified.
• the reason for this is that the metal structure at this position shows the representative metal structure of the steel sheet.
• “1/4 position of sheet thickness” means a region whose center in the sheet thickness direction is the 1/4 position of the sheet thickness.
• the "sheet thickness” refers to the sheet thickness of the base material portion excluding the plating layer. In this invention, the thickness of the plating layer is determined by observation using an optical microscope.
• Tempered martensite increases the strength of the steel sheet. If the area ratio of tempered martensite is less than 80.0%, the desired tensile strength cannot be obtained. On the other hand, if the area ratio of tempered martensite is more than 97.0%, the bendability after pre-straining is reduced. Therefore, the area ratio of tempered martensite is set to 80.0 to 97.0%.
• the area ratio of tempered martensite is preferably 85.0% or more and 95.0% or less.
• the area ratio of tempered martensite is preferably 85.0 to 95.0%.
• Sum of area ratios of pearlite, ferrite and bainite 10.0% or less If the sum of the area ratios of pearlite, ferrite and bainite is high, the desired tensile strength cannot be obtained. Therefore, the sum of the area ratios of these structures is set to 10.0% or less.
• the sum of the area ratios of these structures is preferably 7.0% or less, 5.0% or less, or 3.0% or less. The lower the sum of the area ratios of these structures, the better, so it may be set to 0.0%.
• Total area ratio of fresh martensite and retained austenite 3.0 to 10.0%
• the total area ratio of fresh martensite and retained austenite is set to 3.0% or more. It is preferably 4.0% or more or 5.0% or more.
• the total area ratio of fresh martensite and retained austenite is set to 10.0% or less. Preferably, it is 9.0% or less or 8.0% or less.
• the total area ratio of fresh martensite and retained austenite is preferably 4.0 to 9.0%, and more preferably 5.0 to 8.0%.
• the total area fraction of fresh martensite and retained austenite may be set to 3.0-10.0%, and the area fraction of retained austenite may be set to 1.5% or more.
• the area fraction of retained austenite in the second phase may be set to 1.5% or more.
• the bendability after pre-straining can be further improved. It is more preferable that the area fraction of retained austenite is 2.0% or more, 3.0% or more, or 4.0% or more.
• the total area ratio of the above-mentioned structures is 100%.
• the total of pearlite, ferrite, and bainite is 10.0% or less, and the total of fresh martensite and retained austenite is 3.0 to 10.0%, with the remainder being tempered martensite.
• a test piece is taken from the steel plate in a cross section parallel to the rolling direction, at a depth of 1/4 of the plate thickness from the surface and at the center position in the plate width direction, so that the metal structure can be observed.
• the cross section of the above test piece is polished using #600 to #1500 silicon carbide paper, and then finished to a mirror finish using a liquid in which diamond powder with a grain size of 1 to 6 ⁇ m is dispersed in a diluted solution such as alcohol or pure water.
• the sample is polished for 30 minutes at room temperature using colloidal silica that does not contain an alkaline solution to remove the distortion introduced into the surface layer of the sample.
• an area with a length of 60 ⁇ m in the rolling direction and a length of 160 ⁇ m in the plate thickness direction centered at a position 1/4 of the plate thickness from the surface is measured using electron backscatter diffraction at measurement intervals of 0.1 ⁇ m to obtain crystal orientation information.
• an EBSD device consisting of a field emission scanning electron microscope (FE-SEM: JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD device is 9.6 ⁇ 10 ⁇ 5 Pa or less, the acceleration voltage is 15 kV, and the irradiation current level is 13.
• a diffraction pattern database of Iron Alpha (bcc structure) and Iron Gamma (fcc structure) is used.
• a region having an fcc crystal structure is identified using the “Phase Map” function installed in the software “OIM Analysis (registered trademark) manufactured by TSL” attached to the EBSD analysis device, and the area ratio of this region is calculated. This gives the area ratio of retained austenite.
• the extracted "pearlite, fresh martensite, and tempered martensite” are differentiated into pearlite, fresh martensite, and tempered martensite using the following method.
• test piece is taken from the steel plate to be measured and heat treated to generate ferrite. Specifically, the test piece is isothermally held in the range of 630-750°C for one hour, and then quenched under conditions where the average cooling rate from that temperature to 300°C is 50°C/s. The I ⁇ of the generated ferrite is then measured, and this value is used to distinguish between other structures.
• Standard deviation of Mn concentration in fresh martensite and retained austenite 1.0 to 5.0 mass%
• the standard deviation of the Mn concentration of fresh martensite and retained austenite is set to 5.0 mass% or less. Preferably, it is 4.0 mass% or less, 3.0 mass% or less.
• the Mn concentration in fresh martensite and retained austenite is measured using the following method.
• the Mn concentration is measured in an area 100 ⁇ m in the thickness direction and 200 ⁇ m in the rolling direction at a position 1/4 of the thickness from the surface of the steel plate using a field emission type electron probe microanalyzer (FE-EPMA: JEOL JXA-8530F).
• the measurement conditions are an acceleration voltage of 15 kV, and the distribution image of the Mn concentration is measured. More specifically, the measurement interval of the FE-EPMA is 0.2 ⁇ m, and the number of measurement points is 500,000.
• Tensile strength 1110 MPa or more
• the steel plate according to this embodiment has a tensile strength of 1110 MPa or more.
• the tensile strength may be 1180 MPa or more, or 1300 MPa or more. The higher the tensile strength, the more preferable it is, but it may be 1500 MPa or less.
• the tensile strength is measured in accordance with JIS Z 2241:2022.
• a JIS No. 5 test piece (thickness: original thickness of steel plate) specified in JIS Z 2241:2022 is used, the longitudinal direction of which is taken to coincide with the rolling direction of the steel plate, the gauge length is 50 mm, and the tensile speed is a crosshead displacement speed of 3 mm/min.
• the tensile strength is measured using the test piece in the plated state, and "thickness: original thickness of steel plate” is the thickness of the base material excluding the plating layer.
• the original cross-sectional area of the test piece used to calculate the tensile strength is the original cross-sectional area of the base material excluding the plating layer.
• a preferred method for producing a steel sheet according to this embodiment is as follows: A slab heating process in which a slab having the above-mentioned chemical composition is heated to a temperature range of 1200° C. or more and maintained at the temperature range for 30 minutes or more; A rough rolling process in which rough rolling is performed in a temperature range of 1000 to 1300 ° C.
• a finish rolling step in which rolling is performed twice or more at a rolling reduction rate of 24% or more, the final pass rolling reduction rate is 24 to 60%, and the finish rolling completion temperature is in the temperature range of 960 to 1060°C;
• a cooling step in which the average cooling rate in the temperature range of 900 to 400 ° C. is 30 ° C./s or more;
• a heat treatment process in which the temperature is maintained in the range of 450 to 600 ° C.
• a heat treatment step of holding the resultant product at a temperature in the range of 650 to 750° C. for 10 to 3010 seconds is then carried out.
• a cold rolling step of performing cold rolling with a cumulative reduction of 15% or less may be further carried out after the coiling step and before the heat treatment step.
• the heating temperature of the slab is 1200°C or higher.
• the holding time in the temperature range of 1200°C or higher is 30 minutes or more. If the heating temperature of the slab is less than 1200°C or the holding time in the temperature range of 1200°C or higher is less than 30 minutes, the coarse precipitates cannot be sufficiently dissolved, resulting in significant fluctuation in the tensile strength of the steel sheet.
• the upper limit of the heating temperature and the upper limit of the holding time in the temperature range of 1200°C or higher are not particularly limited, but may be 1300°C or lower and 300 minutes or less, respectively.
• the steel sheet temperature may be varied or may be constant.
• the slab to be heated is not particularly limited except that it has the above-mentioned chemical composition.
• a slab produced by melting molten steel of the above-mentioned chemical composition using a converter or electric furnace, etc. and then using a continuous casting method can be used.
• a continuous casting method an ingot casting method, a thin slab casting method, etc. may also be used.
• Rough rolling process In the rough rolling process, rough rolling is performed in a temperature range of 1000 to 1300 ° C., with the reduction ratios of the first to third passes being 10 to 30%, and the reduction ratios of the fourth and subsequent passes being 15 to 50%. If the temperature at which rough rolling is performed is less than 1000 ° C., the precipitation of alloy carbides will progress, resulting in significant fluctuations in the tensile strength of the steel sheet. For this reason, rough rolling is performed in a temperature range of 1000 ° C. or higher. On the other hand, if rough rolling is performed at a temperature of 1300 ° C. or higher, it will cause an increase in fuel costs, so rough rolling is performed in a temperature range of 1300 ° C. or lower.
• the reduction rates for each of the first to third passes should be 10% or more, and for each of the fourth passes and onwards, 15% or more.
• the reduction rates for each of the first to third passes are 15% or more or 20% or more, and for each of the fourth passes and onwards, 20% or more or 25% or more.
• the reduction ratios for each of the first to third passes should be 30% or less, and for each of the fourth passes and beyond, 50% or less.
• the reduction ratios for each of the first to third passes should be 25% or less, and for each of the fourth passes and beyond, 40% or less.
• the reduction rate of each pass can be expressed as ⁇ 1-(t1/t0) ⁇ x 100(%), where t0 is the inlet thickness of each pass and t1 is the outlet thickness of each pass.
• Finish Rolling Step rolling is performed at least twice with a reduction of 24% or more, with the final pass reduction being 24 to 60% and the finish rolling completion temperature being in the temperature range of 960 to 1060°C.
• the term "two times" here includes the final pass. That is, in this embodiment, the reduction rate of the final pass is set to 24% or more, and one or more passes of rolling with a reduction rate of 24% or more are performed. There is no particular upper limit to the reduction rate in the finish rolling process, but the reduction rate in each pass may be 60% or less.
• the reduction ratio of the final pass is 24% or more. Preferably, it is 28% or more or 30% or more. From the viewpoint of suppressing an increase in the load on the equipment, the reduction ratio of the final pass is 60% or less. Preferably, it is 50% or less or 40% or less.
• the finish rolling completion temperature should be 960°C or higher. It is preferably 980°C or higher or 1000°C or higher.
• the finish rolling completion temperature should be 1060°C or less. It is preferably 1040°C or less.
• the finish rolling completion temperature refers to the exit temperature of the final pass of finish rolling.
• the average cooling rate in the temperature range of 900 to 400 ° C. is 30 ° C./s or more. If the average cooling rate in the temperature range of 900 to 400 ° C. is less than 30 ° C./s, a sufficient amount of martensite cannot be generated, and a desired amount of tempered martensite cannot be obtained after the heat treatment step. Therefore, the average cooling rate in the temperature range of 900 to 400 ° C. is 30 ° C./s or more. It is preferably 50 ° C./s or more or 70 ° C./s or more. There is no particular upper limit, but it may be 200 ° C./s or less from the viewpoint of preventing an increase in cooling equipment.
• the average cooling rate is the temperature difference between the start and end points of the set range divided by the elapsed time from the start point to the end point.
• the steel sheet is coiled at a temperature range of 200° C. or less. If the coiling temperature exceeds 200° C., bainite is generated, the timing of austenite generation becomes non-uniform, and the standard deviation of the Mn concentration of fresh martensite and retained austenite increases. Therefore, the coiling temperature is set to 200° C. or less. It is preferably set to 150° C. or less or 100° C. or less.
• Cold rolling process After uncoiling the coiled steel sheet, cold rolling with a cumulative reduction of 15% or less may be performed. This cold rolling process is not essential and may not be performed. By performing cold rolling with a cumulative reduction of 15% or less, fine precipitates are generated, and the strength of the steel sheet can be further increased.
• the cumulative reduction of the cold rolling is preferably 10% or less. On the other hand, if the cumulative reduction exceeds 15%, recrystallized ferrite is generated, and the standard deviation of the Mn concentration of fresh martensite and retained austenite becomes large. In addition, pickling may be performed before cold rolling.
• the cumulative reduction rate of cold rolling can be expressed as (1-t/t 0 ) ⁇ 100(%), where t is the sheet thickness after cold rolling and t 0 is the sheet thickness before cold rolling.
• the heat treatment step includes a first heat treatment in which the wire is held at a temperature range of 450 to 600°C for 10 to 200 seconds, and a second heat treatment in which the wire is held at a temperature range of 650 to 750°C for 10 to 3,010 seconds.
• the heat treatment temperature of the first heat treatment is less than 450°C or the heat treatment time is less than 10 seconds, the precipitation of carbides will be insufficient, and in the second heat treatment, the growth of austenite will be significant, and the standard deviation of the Mn concentration of fresh martensite and retained austenite will be large. Furthermore, if the heat treatment temperature of the first heat treatment is more than 600°C or more than 200 seconds, the Mn concentration in cementite will be significant, the timing of austenite formation will be uneven, and in the second heat treatment, the growth of austenite will be significant, and the standard deviation of the Mn concentration of fresh martensite and retained austenite will be large.
• the first heat treatment temperature is 450°C or more and 600°C or less, and the heat treatment time is 10 seconds or more.
• the heat treatment temperature is 500°C or more or 550°C or more, and the heat treatment time is 15 seconds or more.
• the second heat treatment temperature of the second heat treatment is 650°C or more and 750°C or less, and the heat treatment time is 10 seconds or more.
• the heat treatment temperature is 500°C or more or 550°C or more, and the heat treatment time is 15 seconds or more and 3010 seconds or less.
• the second heat treatment temperature is 680°C or more and 720°C or less, and the second heat treatment time is 100 seconds or less or 500 seconds or less.
• the steel sheet that has been heat-treated in the above-mentioned temperature range may be left to cool to room temperature, or may be gas-cooled or water-cooled. It may also be plated during the gas-cooling process.
• the steel plate according to this embodiment can be manufactured using a manufacturing method including the steps described above.
• the conditions in the example are an example of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to this example of conditions.
• Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.
• a slab having the chemical composition shown in Table 1 was manufactured by continuous casting.
• the obtained slab was used to manufacture steel plates with a thickness of 3.0 mm under the conditions shown in Tables 2 and 3. Note that pickling was performed before cold rolling. After the second heat treatment, the plate was air-cooled to 500°C, plated, and air-cooled to room temperature.
• the blank spaces in Table 1 indicate that the element was not intentionally included.
• the "average cooling rate" in the “cooling process” in Table 3 refers to the average cooling rate in the temperature range of 900 to 400°C.
• the obtained steel sheets were investigated using the above-mentioned methods for the area ratio of each structure, the standard deviation of the Mn concentration of fresh martensite and retained austenite, the tensile strength, and the bendability after applying a 2% tensile pre-strain. The results are shown in Table 4.
• TM tempered martensite
• P pearlite
• F ferrite
• FM fresh martensite
• Mn standard deviation standard deviation of Mn concentration in fresh martensite and retained austenite
• the tensile strength was 1110 MPa or more, it was judged to have high strength and passed, and if the tensile strength was less than 1110 MPa, it was judged to not have high strength and failed.
• JIS No. 5 test piece thickness: original thickness of steel plate specified in JIS Z 2241:2022 was taken so that the longitudinal direction coincided with the rolling direction of the steel plate. Then, a tensile deformation of 2% was applied to the test piece with a gauge length of 50 mm, a tensile speed of 3 mm/min in crosshead displacement speed, and other conditions in accordance with JIS Z 2241:2022.
• both ends of the test piece after tensile deformation were cut, and a bending test piece 60 mm long and 25 mm wide was prepared from the center of the parallel part of the test piece.
• a bending test was then performed on the obtained bending test piece using the V-block method specified in JIS Z 2248:2022, so that the bending axis passed through the center of the bending test piece in the longitudinal direction and was parallel to the width direction of the bending test piece.
• V-shaped pressing metal fittings with various tip radii R and an angle of 90° were used, and the angle of the tapered surface of the V-block was 90°.
• the surface of the bending test piece after the bending test was then visually inspected to determine the limit bending radius at which no cracks occurred.
• the limit bending deformation is the tip radius R divided by the plate thickness t (R/t). If the limit bending deformation is 2.0 or less, the specimen is deemed to have excellent bendability after pre-straining and passed the test. If the limit bending deformation is more than 2.0, the specimen is deemed to have poor bendability after pre-straining and failed the test. Note that the bending test was performed using test pieces in a plated state, but the above "plate thickness t" is the thickness of the base material excluding the plated layer.
• Test Nos. 1, 2, 18, 20, and 25-31 which satisfy all the requirements of the present invention, have a strength of 1110 MPa or more and show excellent bendability after pre-straining.
• Test Nos. 3-5, 8, 9, 13, 15, 16, 22, 24, and 32 have an excessive standard deviation in Mn concentration, which resulted in a decrease in bendability after pre-straining.
• Test No. 33 did not undergo two heat treatments and was unable to obtain the desired metal structure, resulting in a decrease in strength and a decrease in bendability after pre-straining, despite the extremely low standard deviation in Mn concentration.
• Test Nos. 6 and 10-12 have metal structures that do not meet the requirements of the present invention, which resulted in a decrease in bendability after pre-straining.
• Test No. 14 the metal structure does not meet the requirements of the present invention, which resulted in a decrease in strength. Furthermore, in Test No. In No. 7, 17, 19, and 21-23, the chemical composition was outside the scope of the present invention, resulting in reduced strength.

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