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
  • C22C38/18—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 a steel plate, a part including the steel plate, and a method for manufacturing the steel plate.
• Patent Document 1 describes a high-strength cold-rolled steel sheet coil made of a dual-phase steel sheet mainly composed of ferrite and martensite, which has a structural morphology in which the volume fraction of the total structure is 10-40% ferrite and 60-90% martensite, and in which the ferrite fractions (shown in "area %) at the four corners and center of gravity of an 800 mm x 800 mm steel sheet cut out from any position on the coil are V ⁇ 1, V ⁇ 2, V ⁇ 3, V ⁇ 4, V ⁇ 5, respectively, and when the average value of these five points is V ⁇ m, all of the ferrite fractions V ⁇ 1, V ⁇ 2, V ⁇ 3, V ⁇ 4, V ⁇ 5 are within the range of V ⁇ m ⁇ 5 (area %).
• Patent Document 1 also teaches that by strictly specifying the conditions for heat treatment of the base steel sheet (cold-rolled steel sheet), a high-strength cold-rolled steel sheet coil with small variation in the ferrite fraction within the coil can be obtained, which in turn makes it possible to realize a high-strength cold-rolled steel sheet coil with small strength variation within the coil and a tensile strength of 980 MPa or more, and that such a high-strength cold-rolled steel sheet coil can be stably formed into automotive steel sheets, and specifically discloses in the examples that in addition to improving tensile strength, total elongation and hole expandability are also improved.
• the chemical composition is, in mass%, B: 0.0001 to 0.0030%, Cr: 0.001-0.70%, Mo: 0.001-0.12%, Cu: 0.001-0.40%, Ni: 0.001 to 0.30%, V: 0.001-0.300%, Sn: 0.001 to 0.040%, As: 0.001 to 0.100%, Zr: 0.001 to 0.050%, Ca: 0.0001 to 0.0010%, Mg: 0.0001 to 0.0010%, Bi: 0.001 to 0.010%, Co: 0.001 to 0.010%, W: 0.001-0.100%, Zn: 0.001 to 0.010%, and REM: 0.0001 to 0.0100%
• the steel sheet according to (1) above characterized in that it contains at least one of the following: (3)
• a hot rolling process including finish rolling the slab using a tandem rolling mill having four or more rolling stands, the hot rolling process satisfying the following conditions (a) to (c): (a) the rolling temperature in each of the rolling passes in the two stages immediately preceding the latter two stages is 960 to 1080°C, and the rolling reduction in each of the rolling passes is 30 to 50%; (b) cooling the rolled material to a cooling stop temperature of 800 to 910°C within 0.20 seconds after the rolling passes of the two rolling passes immediately preceding the latter two rolling passes; and (c) a rolling reduction rate in each rolling pass of the latter two rolling passes is 10 to 40%.
• the present invention provides a steel plate that has high strength, high uniform elongation, hole expandability, and yield ratio, and is capable of suppressing load reduction during a collision, as well as a part that includes the steel plate and a method for manufacturing the steel plate.
• FIGS. 1A and 1B are schematic diagrams for explaining characteristics related to the number density of ferrite, in which (a) shows an example that does not satisfy the characteristics related to the number density of ferrite according to the present invention, and (b) shows an example that satisfies the characteristics.
• the steel sheet according to the embodiment of the present invention has a chemical composition in mass%: C: 0.060-0.300%, Si: 0.30-1.50%, Mn: 1.00-2.70%, P: 0.100% or less, S: 0.0300% or less, sol.
• the properties such as hole expandability decrease with increasing strength of steel.
• a steel plate with excellent hole expandability while maintaining high strength for example, a tensile strength of 1180 MPa or more that enables weight reduction.
• the metal structure of the steel plate is composed mainly of martensite.
• martensitic steel has excellent strength, excessive inclusion of martensitic steel reduces properties such as uniform elongation, so that there is a problem that it is generally poor in workability.
• the inventors have found that the yield ratio can be increased and the hole expandability can be significantly improved by utilizing precipitation strengthening through the addition of Ti.
• the improvement in hole expandability due to such precipitation strengthening is due to the reduction in the hardness difference between ferrite and martensite in the metal structure.
• the metal structure is composed mainly of ferrite and martensite, and the soft structure ferrite can be contained up to 40% by area. In this case, the hardness difference between ferrite and martensite in the metal structure increases, and the hole expandability decreases.
• the soft structure of ferrite is precipitation strengthened by Ti precipitates, thereby reducing the hardness difference between ferrite and martensite in the metal structure, and therefore it is believed that the hole expandability can be significantly improved.
• the inventors conducted a study on the assumption that in order to maintain a high and stable load even during a collision and suppress the occurrence of fracture, it is necessary to suppress the work softening after uniform elongation, which corresponds to the elongation at the maximum load point in a uniaxial tensile test. This is because by suppressing the work softening after uniform elongation, it is possible to effectively absorb the collision energy from the maximum load to fracture during a collision by the plastic deformation of the steel sheet.
• the inventors have found that the absolute value of the work softening rate after uniform elongation in a uniaxial tensile test can be reliably reduced to 250,000 MPa or less by uniformly distributing ferrite in the thickness direction of the steel plate, more specifically, by uniformly dividing a 150 ⁇ m ⁇ 150 ⁇ m region at 1/4 of the thickness position of a cross section perpendicular to the plate surface of the steel plate into nine parts, calculating the number density of ferrite in each divided region, and arranging ferrite uniformly in the metal structure so that the absolute value of the difference in the number density of ferrite in each divided region adjacent to each other in the thickness direction is N ⁇ m ⁇ 0.60 or less when the average value of the ferrite density in each divided region adjacent to each other in the thickness direction is N ⁇ m ⁇ 0.60 or less.
• the inventors have found that even when the steel plate is formed into a part, particularly a part having a complex shape such as a lower arm or a trailing arm, the load reduction during a collision can be suppressed and the occurrence of fracture can be significantly suppressed.
• Fig. 1 is a schematic diagram for explaining the characteristics of the number density of ferrite
• Fig. 1(a) shows an example that does not satisfy the characteristics of the number density of ferrite according to the present invention, that is, "When a 150 ⁇ m ⁇ 150 ⁇ m region at the 1/4 position of the plate thickness of a cross section perpendicular to the plate surface is evenly divided into nine, the number density of ferrite is calculated in each divided region, and the average value is N ⁇ m, the difference in the number density of ferrite in each divided region adjacent in the plate thickness direction is all N ⁇ m ⁇ 0.60 or less", and Fig. 1(b) shows an example that satisfies the characteristics. Referring to Fig.
• the differences in the number density of ferrite in each divided region adjacent in the plate thickness direction are calculated (i.e., the difference between N ⁇ 1 and N ⁇ 4 (0.045 pieces/ ⁇ m2 ), the difference between N ⁇ 4 and N ⁇ 7 (0.033 pieces/ ⁇ m2 ), the difference between N ⁇ 2 and N ⁇ 5 (0.017 pieces/ ⁇ m2 ), the difference between N ⁇ 5 and N ⁇ 8 (0.012 pieces/ ⁇ m2 ), the difference between N ⁇ 3 and N ⁇ 6 (0.008 pieces/ ⁇ m2 ), and the difference between N ⁇ 6 and N ⁇ 9 (0.014 pieces/ ⁇ m2 ) ( Figure 1 (a) (ii)).
• the absolute value of the work softening rate after uniform elongation in the uniaxial tensile test can be reliably reduced to 250,000 MPa or less, and therefore, even when the steel sheet is formed into a part having a complex shape such as a lower arm or a trailing arm, it becomes possible to significantly suppress the occurrence of fracture during a collision.
• the work softening rate after uniform elongation can be reduced by uniformly arranging ferrite in the plate thickness direction of a steel plate composed mainly of a structure of ferrite and martensite, and further the fact that the occurrence of fracture during a collision can be significantly suppressed thereby, was not known in the past, and was revealed for the first time by the present inventors. Therefore, according to the embodiment of the present invention, for example, despite the high strength of the tensile strength of 1180 MPa or more, it is possible to have high uniform elongation, hole expandability and yield ratio, and to significantly suppress the occurrence of fracture accompanied by load reduction during a collision, and therefore the steel plate according to the embodiment of the present invention is particularly useful for use in the automotive field.
• C is an element effective in increasing the strength of the steel plate.
• C forms carbides and/or carbonitrides with Nb in the steel, and also contributes to refining the structure due to the pinning effect of the precipitates formed.
• the C content is set to 0.060% or more.
• the C content may be 0.070% or more, 0.080% or more, 0.100% or more, 0.120% or more, or 0.150% or more.
• the C content is set to 0.300% or less.
• the C content may be 0.280% or less, 0.250% or less, 0.200% or less, 0.180% or less, or 0.160% or less.
• Si is an element that suppresses the formation of iron carbides and contributes to improving strength and formability.
• the Si content is set to 0.30% or more.
• the Si content may be 0.40% or more, 0.50% or more, 0.60% or more, 0.70% or more, or 0.80% or more.
• the Si content is set to 1.50% or less.
• the Si content may be 1.40% or less, 1.20% or less, 1.10% or less, 1.00% or less, or 0.90% or less.
• the P content is set to 0.100% or less.
• the P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less.
• the lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will lead to an increase in costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.
• Sol. Al is an element that acts as a deoxidizer for molten steel. In order to obtain such an effect, the sol. Al content is set to 0.001% or more. The sol. Al content may be 0.010% or more, 0.020% or more, 0.030% or more, 0.050% or more, or 0.100% or more. On the other hand, if sol. Al is contained excessively, the ferrite fraction becomes high, which may cause a large difference in hardness between ferrite and martensite, resulting in a decrease in hole expandability. Therefore, the sol. Al content is set to 0.500% or less. The sol. Al content may be 0.400% or less, 0.300% or less, or 0.200% or less. Sol. Al means acid-soluble Al, and refers to solid-solution Al present in the steel in a solid solution state.
• N 0.0070% or less
• the N content may be 0.0050% or less, 0.0040% 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 lead to an increase in costs. Therefore, the N content may be 0.0001% or more, or 0.0005% or more.
• Ti is an element that precipitates in steel as Ti carbides such as TiC, strengthens soft structures such as ferrite by precipitation strengthening, and contributes to improving strength and yield ratio. Furthermore, Ti can reduce the hardness difference between ferrite and martensite in the metal structure due to precipitation strengthening, so it is also effective in improving hole expandability. In order to fully obtain these effects, the Ti content is 0.070% or more. The Ti content may be 0.080% or more, 0.090% or more, 0.100% or more, or 0.120% or more. On the other hand, if Ti is contained excessively, coarse carbides and the like are generated in the steel, which may cause slab cracks during hot rolling or reduce the workability of the steel sheet. Therefore, the Ti content is 0.170% or less. The Ti content may be 0.160% or less, 0.150% or less, 0.140% or less, or 0.130% or less.
• Nb is an element that forms carbides, nitrides and/or carbonitrides in steel and contributes to the refinement of prior austenite grains and thus to the high strength of steel sheet by the pinning effect.
• the Nb content is set to 0.001% or more.
• the Nb content may be 0.005% or more, 0.010% or more, 0.030% or more, 0.050% or more, 0.080% or more, or 0.100% or more.
• the Nb content is set to 1.000% or less.
• the Nb content may be 0.800% or less, 0.600% or less, 0.500% or less, 0.400% or less, 0.300% or less, or 0.200% or less.
• the basic chemical composition of the steel plate according to the embodiment of the present invention is as described above. Furthermore, the steel plate may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.
• Cr is an element that enhances the hardenability of steel and contributes to improving strength and/or corrosion resistance.
• the Cr content may be 0%, but in order to obtain these effects, the Cr content is preferably 0.001% or more, and may be 0.01% or more, 0.05% or more, or 0.10% or more.
• the Cr content is preferably 0.70% or less, and may be 0.60% or less, 0.50% or less, 0.40% or less, or 0.30% or less.
• Mo is an element that enhances the hardenability of steel and contributes to improving strength.
• the Mo content may be 0%, but in order to obtain such an effect, the Mo content is preferably 0.001% or more.
• the Mo content may be 0.01% or more, 0.02% or more, or 0.03% or more.
• the Mo content is preferably 0.12% or less.
• the Mo content may be 0.10% or less, 0.08% or less, 0.06% or less, or 0.05% or less.
• Cu is an element that contributes to improving strength by precipitation strengthening or solid solution strengthening.
• the Cu content may be 0%, but in order to obtain such an effect, the Cu content is preferably 0.001% or more.
• the Cu content may be 0.01% or more, 0.03% or more, or 0.05% or more.
• the Cu content is preferably 0.40% or less.
• the Cu content may be 0.30% or less, 0.20% or less, 0.10% or less, or 0.08% or less.
• Ni is an element that contributes to improving strength by precipitation strengthening or solid solution strengthening.
• the Ni content may be 0%, but in order to obtain such an effect, 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.
• the Ni content is preferably 0.30% or less.
• the Ni content may be 0.20% or less, 0.15% or less, 0.10% or less, or 0.08% or less.
• Sn 0 to 0.040%, As: 0 to 0.100%, Zr: 0 to 0.050%, Ca: 0 to 0.0010%, Mg: 0 to 0.0010%, Bi: 0-0.010%, Co: 0-0.010%, W: 0-0.100%, Zn: 0-0.010%, and REM: 0-0.0100%]
• Sn, As, Zr, Ca, Mg, Bi, Co, W, Zn, and REM may be contained in the steel sheet as optional elements or may be present in the steel sheet as tramp elements.
• the contents of these elements may be as follows: Sn: 0-0.040% or 0.020%, As: 0-0.100% or 0.050%, Zr: 0-0.050% or 0.030%, Ca: 0-0.0010% or 0.0008%, Mg: 0-0.0010% or 0.0008%, Bi: 0-0.010%, Co: 0-0.010%, W: 0-0.100% or 0.050%, Zn: 0-0.010%, and REM: 0-0.0100% or 0.0050%.
• the Sn, As, Zr, Bi, Co, W and Zn contents may be 0.001% or more, 0.005% or more, or 0.008% or more, respectively.
• the Ca, Mg and REM contents may be 0.0001% or more, 0.0002% or more, or 0.0005% or more.
• the remainder other than the above elements consists of Fe and impurities.
• Impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when the steel plate is industrially manufactured.
• the chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analytical method.
• the chemical composition of the steel plate 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 may be measured using the inert gas fusion-thermal conductivity method
• O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
• the metal structure of the steel plate according to the embodiment of the present invention contains ferrite: 10 to 40% in area%.
• the desired uniform elongation can be achieved by containing 10% or more of ferrite, which is a soft structure, in area%. From the viewpoint of further improving the uniform elongation, the higher the area ratio of ferrite, the more preferable, for example, 12% or more, 15% or more, 18% or more, 20% or more, 22% or more, or 25% or more.
• the area ratio of ferrite is set to 40% or less. From the viewpoint of further increasing the strength, yield ratio, and/or hole expandability, the lower the area ratio of ferrite, the more preferable, for example, 38% or less, 35% or less, 32% or less, 30% or less, 28% or less, or 26% or less.
• the metal structure of the steel plate according to the embodiment of the present invention contains, in terms of area%, 60 to 90% martensite.
• high strength for example, high strength with a tensile strength of 1180 MPa or more
• the higher the area ratio of martensite the more preferable it is, and for example, it may be 65% or more, 68% or more, 70% or more, 72% or more, or 75% or more.
• the area ratio of martensite becomes too high, uniform elongation may decrease.
• the metal structure of the steel plate according to the embodiment of the present invention may contain bainite.
• the area ratio of bainite is set to 10% or less, and may be, for example, 9% or less, 8% or less, 6% or less, 5% or less, or 3% or less.
• the lower limit is not particularly limited, and the area ratio of bainite may be 0%, and may be, for example, 0.5% or more, 1% or more, or 2% or more.
• the remaining structure other than ferrite, martensite, and bainite may be 0% in terms of area percent, but if a remaining structure exists, the remaining structure may be at least one of pearlite and retained austenite. If the area ratio of at least one of pearlite and retained austenite exceeds 5% in total, it may lead to a decrease in uniform elongation, or it may become impossible to control ferrite and/or martensite within a desired range. Therefore, the total area ratio of at least one of pearlite and retained austenite is 5% or less, and may be, for example, 4% or less, 3% or less, or 2% or less. On the other hand, the lower limit is not particularly limited, and the total area ratio of at least one of pearlite and retained austenite may be 0%, for example, 0.1% or more, 0.5% or more, or 1% or more.
• the plate thickness cross section is preferably parallel to the rolling direction, in cases where the rolling direction of the steel plate cannot be specified, the plate thickness cross section does not necessarily need to be parallel to the rolling direction.
• a test piece is taken from the steel plate, and the cross section of the test piece is polished using silicon carbide paper of #600 to #1500, and then finished to a mirror surface using a dilution solution such as alcohol or a liquid in which diamond powder with a grain size of 1 to 6 ⁇ m is dispersed in pure water.
• this cross section is polished for 8 minutes at room temperature using colloidal silica with a grain size of 0.25 ⁇ m that does not contain an alkaline solution, to remove the strain introduced into the surface layer of the test piece.
• a rectangular area of 150 ⁇ m in the plate thickness direction and 150 ⁇ m in the direction perpendicular to the plate thickness direction, centered at 1/4 of the plate thickness position from the steel surface, is measured by electron backscatter diffraction at measurement intervals of 0.1 ⁇ m to obtain crystal orientation information.
• an EBSD device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL).
• JSM-7001F thermal field emission scanning electron microscope
• DVC5 type detector DVC5 type detector manufactured by TSL
• the degree of vacuum in the EBSD device is 9.6 ⁇ 10 ⁇ 5 Pa or less
• the acceleration voltage is 15 kV
• the irradiation current level is 13
• the electron beam irradiation level is 62
• other observation conditions are preferably as follows.
• Objective aperture number 4 Number of pixels: 4096 x 5120 pixels
• the regions with a bcc crystal structure are judged to be "ferrite, martensite, bainite, and pearlite".
• the grain average misorientation (GAM value: Grain Average Misorientation) is calculated using the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer.
• the regions with a GAM value of 0.5° or less are identified as ferrite, and their area ratio is calculated.
• the "GAM value” is the average misorientation between adjacent pixels in a region surrounded by grain boundaries with a misorientation of 15° or more.
• the pearlite area ratio is calculated by performing image analysis on a structure photograph obtained by using the FE-SEM in the same region as the EBSD measurement region, that is, a rectangular region 150 ⁇ m in the plate thickness direction and 150 ⁇ m in the direction perpendicular to the plate thickness direction, centered at a 1/4 position of the plate thickness from the steel plate surface.
• a structure in which plate-shaped ferrite and Fe-based carbides are layered is regarded as pearlite.
• the area ratio of martensite is calculated by subtracting the area ratios of the retained austenite, ferrite, bainite, and pearlite from 100%. Metal structures other than martensite are sequentially identified, and the last remaining metal structure is regarded as martensite.
• the absolute value of the work softening rate after uniform elongation in the uniaxial tensile test can be reliably reduced to 250,000 MPa or less, thereby making it possible to suppress the load reduction during a collision.
• an automobile suspension part such as a lower arm or a trailing arm having a complex shape, it is possible to significantly suppress the occurrence of fracture during a collision.
• the difference in number density of ferrite in each divided region adjacent in the plate thickness direction is as small as possible.
• the difference in number density of ferrite in each divided region adjacent in the plate thickness direction may all be N ⁇ m ⁇ 0.55 or less, N ⁇ m ⁇ 0.50 or less, N ⁇ m ⁇ 0.50 or less, N ⁇ m ⁇ 0.45 or less, N ⁇ m ⁇ 0.40 or less, N ⁇ m ⁇ 0.35 or less, or N ⁇ m ⁇ 0.30 or less.
• N ⁇ m is 0.050 pieces/ ⁇ m 2 or more.
• N ⁇ m is 0.050 pieces/ ⁇ m 2 or more.
• the ferrite grains are made finer, the number density of ferrite naturally increases. Therefore, by increasing the number density of ferrite, the difference in the number density of ferrite in each divided region adjacent to each other in the plate thickness direction is reduced, and the load reduction at the time of collision is suppressed.
• the region with a GAM value of 0.5° or less is identified as ferrite, and the number of ferrite particles in each of the divided regions into which the 150 ⁇ m ⁇ 150 ⁇ m region is equally divided into 9 is counted to calculate the number density N ⁇ 1 to N ⁇ 9, and the average value thereof is determined as N ⁇ m (FIGS. 1(a)(i) and (b)(i)).
• the number density of ferrite is calculated by counting all the number of ferrite particles detected and identified under the above measurement conditions. When counting, if ferrite exists on the boundary between multiple divided regions or in multiple divided regions across the boundary, the ferrite is counted in each of the multiple divided regions.
• the difference in number density of ferrite in each divided region adjacent in the plate thickness direction is calculated (FIGS. 1(a)(ii) and (b)(ii)), and it is determined whether the value obtained by dividing the difference in number density by N ⁇ m satisfies the requirement of 0.60 or less (FIGS. 1(a)(iii) and (b)(iii)).
• the steel sheet according to the embodiment of the present invention generally has a sheet thickness of 1.0 to 8.0 mm, although it is not particularly limited thereto.
• the sheet 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 plate according to the embodiment of the present invention has high uniform elongation, hole expandability and yield ratio despite its high strength, and can significantly suppress the occurrence of fracture accompanied by load reduction during collision, and in particular, it is possible to suppress the occurrence of fracture accompanied by load reduction during collision even when forming a part having a complex shape. Therefore, the steel plate according to the embodiment of the present invention can reliably achieve a high level of the contradictory properties of high strength and excellent workability, and can achieve excellent impact resistance. Therefore, the steel plate according to the embodiment of the present invention is useful for use in parts in technical fields where these properties are required, and is particularly useful in the automotive field.
• an automotive part particularly an automotive suspension part
• automotive suspension parts include lower arms and trailing arms.
• These automotive parts, particularly automotive suspension parts only need to include the steel plate according to the embodiment of the present invention in at least a part of these parts, and therefore at least a part of these parts satisfy the above-mentioned chemical composition and metal structure characteristics. In areas of steel sheets that do not come into direct contact with the die during press forming or other forming processes and that are relatively lightly processed, the characteristics of the metal structure do not change significantly before and after forming.
• the upper limit of the tensile strength is not particularly limited, but for example, the tensile strength of the steel plate may be 1780 MPa or less, 1470 MPa or less, 1400 MPa or less, or 1300 MPa or less.
• a high uniform elongation can be achieved, specifically a uniform elongation of 5.0% or more can be achieved.
• the uniform elongation is preferably 5.2% or more, 5.5% or more, 5.8% or more, or 6.0% or more.
• the upper limit of the uniform elongation is not particularly limited, but for example, the uniform elongation of the steel sheet may be 15.0% or less, 10.0% or less, 8.0% or less, or 7.0% or less.
• the tensile strength and uniform elongation are measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is preferably parallel to the rolling direction perpendicular to the rolling direction of the steel sheet (C direction) and performing a tensile test in accordance with JIS Z 2241:2022.
• the JIS No. 5 test piece may be taken from any direction within the surface of the steel sheet.
• the initial hole is expanded with a conical punch with an apex angle of 60° until a crack penetrating the plate thickness occurs, and the hole diameter d1 mm at the time of crack occurrence is measured, and the hole expansion ratio ⁇ (%) of each test piece is calculated by the following formula.
• yield ratio (YR) According to the steel plate having the above chemical composition and metal structure, in addition to high tensile strength, the yield ratio can be increased, and more specifically, a yield ratio of 75% or more can be achieved.
• the yield ratio is preferably 78% or more or 80% or more, more preferably 82% or more or 84% or more.
• the upper limit is not particularly limited, but for example, the yield ratio may be 95% or less, 92% or less, 90% or less, or 88% or less.
• the yield ratio is determined by the following formula based on the tensile strength and 0.2% proof stress measured by taking a JIS No.
• the method for producing a steel sheet according to an embodiment of the present invention comprises: A heating step comprising heating a slab having the chemical composition described above in relation to the steel plate and holding it at a temperature of 1180-1350°C for at least 6000 seconds; A hot rolling process including finish rolling the slab using a tandem rolling mill having four or more rolling stands, the hot rolling process satisfying the following conditions (a) to (c): (a) the rolling temperature in each of the rolling passes in the two stages immediately preceding the latter two stages is 960 to 1080°C, and the rolling reduction in each of the rolling passes is 30 to 50%; (b) cooling the rolled material to a cooling stop temperature of 800 to 910 ° C.
• the method is characterized by including a cooling step including water-cooling the finish-rolled steel plate, cooling it to a temperature range of 600 to 750 ° C. within 4.0 seconds from the start of water cooling, then air-cooling in the temperature range for 2.0 to 8.0 seconds, and water-cooling the steel plate to 50 ° C. or less within 13 seconds after air-cooling.
• a cooling step including water-cooling the finish-rolled steel plate, cooling it to a temperature range of 600 to 750 ° C. within 4.0 seconds from the start of water cooling, then air-cooling in the temperature range for 2.0 to 8.0 seconds, and water-cooling the steel plate to 50 ° C. or less within 13 seconds after air-cooling.
• the temperatures described for the slab and steel plate refer to the surface temperature of the slab and the surface temperature of the steel plate, respectively.
• the coarse carbides present in the structure can be completely solid-dissolved, and the starting point of cracks can be eliminated. If the holding temperature is less than 1180 ° C or the holding time is less than 6000 seconds, the solid-dissolution of the coarse carbides is incomplete. If the solid solution of the coarse carbides is incomplete, the area ratio of martensite may become less than 60% due to the occurrence of ferrite or bainite transformation originating from such carbides in the cooling process described below, and as a result, the desired strength may not be obtained.
• the upper limit of the heating temperature of the slab is set to 1350°C or less from the viewpoint of the capacity and productivity of the heating equipment.
• the upper limit of the holding time in the temperature range of 1180 to 1350°C is preferably 10000 seconds or less.
• the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
• the conditions of the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
• the rolling temperature in each rolling pass of the two stages immediately preceding the last two stages is controlled to 960 to 1080°C, and similarly, the reduction ratio in each rolling pass of the two stages immediately preceding the last two stages is controlled to 30 to 50%.
• the sixth and seventh rolling passes correspond to the "rolling passes of the last two stages”. Therefore, in this case, the "rolling passes of the two stages immediately preceding the last two stages" refers to the fourth and fifth rolling passes.
• the rolling temperature in each of the rolling passes immediately before the last two stages exceeds 1080°C, the austenite grains after recrystallization become coarse, the number of austenite grain boundaries decreases, and the ferrite nucleation sites decrease. As a result, the number density of ferrite in the final metal structure cannot be sufficiently uniform in the thickness direction, and similarly, work softening after uniform elongation becomes significant.
• the rolling temperature in each of the rolling passes in the two stages immediately preceding the last two stages is 1000 to 1060°C.
• the rolled material is cooled to a cooling stop temperature of 800 to 910 ° C. within 0.20 seconds after the rolling pass of the two stages immediately before the latter two stages.
• the time to cool to a cooling stop temperature of 800 to 910 ° C. after the rolling pass of the two stages immediately before the latter two stages exceeds 0.20 seconds or the cooling stop temperature is higher than 910 ° C., the grain growth of the austenite grains after recrystallization cannot be sufficiently suppressed, and even if appropriate cooling is performed in the subsequent cooling process, the number density of ferrite cannot be controlled within a desired range in the plate thickness direction.
• the cooling stop temperature is lower than 800 ° C., excessive generation of ferrite may occur in the finally obtained metal structure. If excessive ferrite is formed, the strength may decrease, or the difference in hardness between ferrite and martensite may not be sufficiently reduced even by precipitation strengthening due to Ti precipitates, resulting in a decrease in hole expandability.
• the reduction rate of each rolling pass of the latter two stages exceeds 40%, the driving force of ferrite transformation becomes too large, which may lead to excessive generation of ferrite in the finally obtained metal structure. If ferrite is excessively generated, the strength may decrease, or the hardness difference between ferrite and martensite may not be sufficiently reduced even by precipitation strengthening due to Ti precipitates, and the hole expansion property may decrease.
• the reduction rate in each rolling pass of the latter two stages of finish rolling is 15 to 38%.
• the transformation to ferrite can be promoted and Ti precipitates can be properly precipitated. Therefore, the air-cooling operation for 2.0 to 8.0 seconds in the temperature range of 600 to 750°C after water cooling is important not only for the proper generation of ferrite, but also from the viewpoint of the improvement effect of hole expandability due to precipitation strengthening caused by Ti precipitates. For example, if the air-cooling temperature is less than 600°C, the transformation to ferrite cannot be sufficiently promoted, while a relatively large amount of bainite may be generated. In such a case, the generation of a large amount of bainite reduces the uniform elongation, and further, the generation of martensite associated with the generation of bainite reduces the generation of sufficient strength, which may result in insufficient strength being obtained.
• the air cooling temperature exceeds 750°C or the air cooling time is less than 2.0 seconds, the transformation to ferrite cannot be sufficiently promoted, and uniform elongation decreases.
• the air cooling time exceeds 8.0 seconds, a relatively large amount of ferrite may be generated. In such cases, the strength decreases, and the precipitation strengthening caused by Ti precipitates cannot sufficiently reduce the hardness difference between ferrite and martensite, and the hole expansion ability may decrease.
• the air cooling temperature is preferably 620 to 730°C, and the air cooling time is preferably 3.0 to 6.0 seconds.
• the metal structure is composed of a structure containing, in terms of area percentage, 10-40% ferrite and 60-90% martensite, so that high strength, for example, high strength with a tensile strength of 1180 MPa or more, can be achieved while significantly improving uniform elongation. Furthermore, by controlling the Ti content in the steel to 0.070 mass% or more, the soft structure of ferrite is precipitation strengthened by Ti precipitates, thereby increasing the yield ratio and reducing the hardness difference between ferrite and martensite in the metal structure, and therefore it is possible to significantly improve the hole expandability.
• the number density of ferrite in the thickness direction of the steel plate is made uniform within a predetermined range, so that the load reduction during collision can be suppressed. Therefore, the steel plate manufactured by the above manufacturing method has high uniform elongation, hole expandability, and yield ratio despite its high strength, and can significantly suppress the occurrence of fracture accompanied by a load reduction during collision. Therefore, steel sheets manufactured by the above manufacturing method reliably achieve a high level of both the opposing properties of high strength and excellent workability, while also achieving excellent impact resistance, making them particularly useful in the automotive field where these properties are required.
• steel sheets according to the embodiments of the present invention were manufactured under various conditions, and the tensile strength (TS), yield ratio (YR), uniform elongation (uEl), hole expansion ratio ( ⁇ ), and work softening rate after uniform elongation of the obtained steel sheets were investigated.
• molten steel was cast by continuous casting to form slabs with various chemical compositions shown in Tables 1 and 2. These slabs were heated to a temperature of 1180 to 1350°C and held for a time of 6000 to 10000 seconds, and then hot-rolled. Hot rolling was performed by rough rolling and finish rolling. More specifically, the rough rolling conditions were the same in all examples and comparative examples, and the finish rolling was performed under the conditions shown in Table 3 using a tandem rolling mill consisting of five rolling stands. Next, the finish-rolled steel plate was water-cooled, air-cooled, and water-cooled under the conditions shown in Table 3, and then coiled to obtain a steel plate having a thickness of 2.4 to 3.4 mm.
• the burr was placed on the die side, and the initial hole was pushed out with a conical punch having an apex angle of 60 ° until a crack penetrating the plate thickness occurred, and the hole diameter d1 mm at the time of the crack occurrence was measured, and the hole expansion ratio ⁇ (%) of each test piece was calculated using the following formula.
• Maximum difference in number density of ferrite/N ⁇ m in Table 4 means the maximum value of the six number density differences shown in Figure 1, i.e., the difference between N ⁇ 1 and N ⁇ 4, the difference between N ⁇ 4 and N ⁇ 7, the difference between N ⁇ 2 and N ⁇ 5, the difference between N ⁇ 5 and N ⁇ 8, the difference between N ⁇ 3 and N ⁇ 6, and the difference between N ⁇ 6 and N ⁇ 9, divided by N ⁇ m. Therefore, if this value is 0.60 or less, the requirement that "the difference in number density of ferrite in each divided region adjacent in the plate thickness direction is all N ⁇ m ⁇ 0.60 or less" is met. Also, in the metal structure shown in Table 4, the remaining structure was at least one of pearlite and retained austenite.

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