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 disclosure relates to a hot rolled steel sheet and a part, and more particularly to a hot rolled steel sheet having high strength, excellent hole expansion property, bendability, and through-thickness crack propagation resistance, and a part manufactured using the same.
• the steel plate used in the above-mentioned automobile suspension parts is also required to have excellent collision resistance properties.
• Patent Document 1 discloses a high-strength hot-rolled steel sheet that contains a martensite phase at an area ratio of 95% or more at the 1/4 position of the sheet thickness of the steel sheet, has a structure in which the average aspect ratio of prior austenite grains is 3.0 or more, has a 5-minute relaxation stress value of 20 MPa or less when 400 MPa is applied in a stress relaxation test, and has a tensile strength of 1180 MPa or more.
• Patent Document 1 does not take into consideration crashworthiness. The inventors believe that improving resistance to crack propagation in the plate thickness direction can be expected to improve crashworthiness.
• the present disclosure has been made in consideration of the above problems, and aims to provide a hot-rolled steel sheet having high strength, excellent hole expansion property, bendability, and resistance to crack propagation in the plate thickness direction, and a part manufactured using the same.
• the present inventors have discovered that by controlling the average aspect ratio of prior austenite grains in the inner region, high strength, as well as excellent hole expandability and bendability can be obtained in a hot rolled steel sheet. Furthermore, the inventors have discovered that by making the average aspect ratio of the prior austenite grains in the surface region smaller than the average aspect ratio of the prior austenite grains in the internal region, excellent bendability and resistance to crack propagation in the plate thickness direction can be obtained without sacrificing strength.
• the gist of the present disclosure made based on the above findings is as follows.
• the chemical composition is, in mass%, Ti: 0.001 to 0.180%, Nb: 0.001-0.100%, V: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Cr: 0.001-2.000%, Mo: 0.001-3.000%, Ni: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Ca: 0.0001-0.0500%, Mg: 0.001-0.050%, REM: 0.0001-0.1000%, Bi: 0.001-0.100%, Ta: 0.001 to 0.100%, Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001-0.200%, W: 0.001-0.200%, Sb: 0.001 to 0.500%,
• the hot-rolled steel sheet according to the above (1) characterized in that it contains one or more selected from the group consisting of As: 0.001 to 0.050% and Sn: 0.001 to 0.050%.
• the above aspects of the present disclosure make it possible to provide a hot-rolled steel sheet having high strength, excellent hole expansion properties, bendability, and resistance to crack propagation in the plate thickness direction, and a part manufactured using the same.
• hot-rolled steel sheet and parts according to one embodiment of the present disclosure (hereinafter, may be referred to as the hot-rolled steel sheet and parts according to this embodiment) will be described.
• this disclosure is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of this disclosure.
• the chemical composition of the hot-rolled steel sheet according to this embodiment is, in mass%, C: 0.050-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, Al: 0.010-0.400%, P: 0-0.080%, S: 0-0.0100%, N: 0-0.0050%, O: 0-0.0100%, and the balance: Fe and impurities.
• C 0.050-0.120%
• Si 0-3.00%
• Mn 1.20-3.00%
• Al 0.010-0.400%
• P 0-0.080%
• S 0-0.0100%
• N 0-0.0050%
• O 0-0.0100%
• C 0.050-0.120% C is an important element for improving the strength of a hot-rolled steel sheet.
• the C content is set to 0.050% or more.
• the C content is preferably 0.055% or more, 0.060% or more, or 0.070% or more.
• the C content is set to 0.120% or less.
• the C content is preferably 0.110% or less, more preferably 0.100% or less.
• Si 0-3.00% Si is an element that has the effect of suppressing the formation of carbides during ferrite transformation and improving the toughness of the hot-rolled steel sheet. Since Si may not be contained, the Si content may be 0%. In order to reliably obtain the above effect, the Si content is preferably 0.10% or more. The Si content is more preferably 0.50% or more, 0.70% or more. On the other hand, if the Si content exceeds 3.00%, the cracking sensitivity of the slab increases, making the slab difficult to handle. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.00% or less, 1.50% or less, or 1.20% or less.
• Mn 1.20-3.00%
• Mn is an element effective in improving the hardenability and strength of the hot-rolled steel sheet by solid solution strengthening.
• the Mn content is set to 1.20% or more.
• the Mn content is preferably 1.30% or more, more preferably 1.50% or more.
• MnS which adversely affects the hole expandability and bendability of the hot-rolled steel sheet, is easily generated. Therefore, the Mn content is set to 3.00% or less.
• the Mn content is preferably 2.70% or less, 2.50% or less, or 2.20% or less.
• Al 0.010-0.400%
• Al has the effect of improving the soundness of steel by deoxidization and also has the effect of controlling ferrite transformation. Furthermore, if the Al content is less than 0.010%, the hole expandability, bendability, and crack propagation resistance in the thickness direction of the hot-rolled steel sheet deteriorate. Therefore, the Al content is set to 0.010% or more. The Al content is preferably 0.015% or more, 0.020% or more. On the other hand, if the Al content exceeds 0.400%, alumina precipitates in clusters, which increases the cracking sensitivity of the slab and makes the slab difficult to handle. Therefore, the Al content is set to 0.400% or less. The Al content is preferably 0.300% or less, 0.250% or less, or 0.200% or less.
• P 0-0.080%
• P is an element that affects the weldability of hot-rolled steel sheets.
• the P content is set to 0.080% or less.
• the P content is preferably 0.040% or less, 0.020% or less, or 0.010% or less.
• the P content may be 0%. From the viewpoint of refining costs, the P content may be 0.001% or more.
• S 0 ⁇ 0.0100%
• S is an element that affects the hole expandability and bendability of the hot-rolled steel sheet.
• the S content is set to 0.0100% or less.
• the S content is preferably 0.0080% or less, 0.0060% or less.
• the S content may be 0%. From the viewpoint of refining costs, the S content may be 0.0001% or more.
• N 0 to 0.0050%
• N is an element that combines with Ti to form Ti nitrides.
• the N content is set to 0.0050% or less.
• the N content is preferably 0.0040% or less, more preferably 0.0030% or less.
• the N content may be 0%. From the viewpoint of refining costs, the N content may be 0.0001% or more.
• O 0 to 0.0100%
• O is an element that, when contained in a large amount in steel, forms coarse oxides that become the starting point of fracture, causing brittle fracture and hydrogen-induced cracking. If the O content exceeds 0.0100%, brittle fracture and hydrogen-induced cracking are likely to occur. In addition, the hole expandability, bendability, and crack propagation resistance in the thickness direction of the hot-rolled steel sheet are deteriorated. Therefore, the O content is set to 0.0100% or less.
• the O content is preferably 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0035% or less. Since O may not be contained, the O content may be 0%. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be 0.0005% or more, or 0.0010% or more.
• the hot-rolled steel sheet according to this embodiment may contain the above chemical components, with the remainder being Fe and impurities.
• impurities refer to substances that are mixed in from the raw materials, such as ore and scrap, or the manufacturing environment, and/or substances that are acceptable to the extent that they do not adversely affect the properties of the hot-rolled steel sheet according to this embodiment.
• the following optional elements may be included to reduce manufacturing variations and further improve the strength of the hot-rolled steel sheet.
• the lower limit of the content of these elements is 0%.
• Ti 0.001-0.180% Ti precipitates in steel as carbides or nitrides, and has the effect of refining the metal structure through the pinning effect, and increasing the strength and yield ratio of the hot-rolled steel sheet through precipitation strengthening.
• the Ti content is preferably 0.001% or more.
• the Ti content is more preferably 0.005% or more, 0.010% or more.
• the Ti content is set to 0.180% or less.
• the Ti content is preferably 0.160% or less, more preferably 0.150% or less.
• Nb 0.001-0.100%
• Nb has the effect of increasing the strength of the hot-rolled steel sheet by refining the crystal grain size of the hot-rolled steel sheet and precipitation strengthening of NbC.
• the Nb content is preferably 0.001% or more.
• the Nb content is more preferably 0.005% or more, 0.010% or more.
• the Nb content exceeds 0.100%, the above effects are saturated.
• the hole expandability, bendability, and crack propagation resistance in the thickness direction of the hot-rolled steel sheet are deteriorated. Therefore, even if Nb is contained, the Nb content is set to 0.100% or less.
• the Nb content is preferably 0.080% or less, 0.060% or less.
• V has the effect of increasing the strength of the hot-rolled steel sheet by strengthening with precipitates, strengthening by grain refinement by inhibiting the growth of ferrite crystal grains, and strengthening by dislocation by inhibiting recrystallization.
• the V content is preferably 0.001% or more.
• the V content is more preferably 0.005% or more, 0.010% or more.
• the V content is set to 1.000% or less.
• the V content is preferably 0.800% or less, more preferably 0.600% or less.
• Cu 0.001-1.000% Cu exists in the form of fine particles in steel and has the effect of increasing the strength of the hot-rolled steel sheet.
• the Cu content is preferably 0.001% or more.
• the Cu content is more preferably 0.005% or more, and more preferably 0.010% or more.
• the Cu content is set to 1.000% or less, and preferably, the Cu content is set to 0.800% or less, and more preferably, 0.600% or less.
• Cr:0.001 ⁇ 2.000% Cr is an element effective in improving the strength of a hot-rolled steel sheet.
• the Cr content is preferably 0.001% or more.
• the Cr content is more preferably 0.005% or more, and more preferably 0.010% or more.
• the Cr content is set to 2.000% or less.
• the Cr content is preferably 1.500% or less, 1.200% or less, or 1.000% or less.
• Mo 0.001 ⁇ 3.000%
• Mo is an element effective in strengthening the precipitation of ferrite.
• the Mo content is preferably 0.001% or more, and more preferably 0.005% or more, and even more preferably 0.010% or more.
• the Mo content is set to 3.000% or less.
• the Mo content is preferably 2.500% or less, 2.000% or less, or 1.500% or less.
• Ni 0.001-0.500%
• Ni has the effect of suppressing phase transformation at high temperatures and increasing the strength of the hot-rolled steel sheet.
• the Ni content is preferably 0.001% or more.
• the Ni content is more preferably 0.005% or more, and more preferably 0.010% or more.
• the Ni content is set to 0.500% or less.
• the Ni content is preferably 0.300% or less, more preferably 0.150% or less.
• B 0.0001-0.0100% B has the effect of suppressing phase transformation at high temperatures and increasing the strength of the hot-rolled steel sheet.
• the B content is preferably 0.0001% or more.
• the B content is more preferably 0.0005% or more, and more preferably 0.0010% or more.
• the B content is set to 0.0100% or less.
• the B content is preferably 0.0080% or less, and more preferably 0.0050% or less.
• Ca 0.0001-0.0500%
• Ca has the effect of dispersing a large number of fine oxides during deoxidation of molten steel and refining the structure of the hot-rolled steel sheet.
• Ca also has the effect of fixing S in the steel as spherical CaS, suppressing the generation of elongated inclusions such as MnS, and improving the hole expandability of the hot-rolled steel sheet.
• the Ca content is preferably 0.0001% or more.
• the Ca content is more preferably 0.0005% or more, 0.0010% or more.
• the Ca content is set to 0.0500% or less.
• the Ca content is preferably 0.0300% or less, and more preferably 0.0200% or less.
• Mg 0.001-0.050%
• Mg has the effect of adjusting the shape of inclusions in steel to a preferred shape, thereby increasing the yield ratio of the hot-rolled steel sheet.
• the Mg content is preferably 0.001% or more.
• the Mg content is more preferably 0.005% or more, and more preferably 0.010% or more.
• the Mg content is set to 0.050% or less.
• the Mg content is preferably 0.040% or less, more preferably 0.030% or less.
• REM 0.0001 ⁇ 0.1000% REM has the effect of increasing the yield ratio of a hot-rolled steel sheet by adjusting the shape of inclusions in the steel to a preferred shape.
• the REM content is preferably 0.0001% or more.
• the REM content is more preferably 0.0005% or more, and more preferably 0.0010% or more.
• the REM content is set to 0.1000% or less.
• the REM content is preferably 0.0800% or less, more preferably 0.0600% or less.
• REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of the REM refers to the total content of these elements.
• lanthanoids they are industrially added in the form of misch metal.
• Bi:0.001 ⁇ 0.100% Bi has the effect of increasing the yield ratio of a hot-rolled steel sheet by refining the solidification structure.
• the Bi content is preferably 0.001% or more.
• the Bi content is more preferably 0.005% or more, and more preferably 0.010% or more.
• the Bi content is set to 0.100% or less.
• the Bi content is preferably 0.080% or less, 0.060% or less, or 0.040% or less.
• Ta 0.001 ⁇ 0.100% Ta, like V, has the effect of increasing the strength of the hot-rolled steel sheet by forming fine carbides in the steel.
• the Ta content is preferably 0.001% or more.
• the Ta content is more preferably 0.005% or more, more preferably 0.010% or more.
• the Ta content is set to 0.100% or less.
• the Ta content is preferably 0.080% or less, more preferably 0.050% or less.
• Zr 0.001-0.500%
• Zr has the effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening.
• the Zr content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
• the Zr content is set to 0.500% or less.
• the Zr content is preferably 0.300% or less, more preferably 0.100% or less.
• Co has the effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening.
• the Co content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
• the Co content is set to 3.000% or less.
• the Co content is preferably 1.000% or less, and more preferably 0.500% or less.
• Zn 0.001-0.200%
• Zn has the effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening.
• the Zn content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
• the Zn content is set to 0.200% or less.
• the Zn content is preferably 0.150% or less, more preferably 0.100% or less.
• W 0.001-0.200% W has the effect of increasing the strength of the hot-rolled steel sheet by solid solution strengthening.
• the W content is preferably 0.001% or more.
• the W content is more preferably 0.005% or 0.010% or more.
• the W content is set to 0.200% or less.
• the W content is preferably 0.150% or less, more preferably 0.100% or less.
• Sb 0.001-0.500%
• Sb has the effect of suppressing the generation of oxides that are the starting points of fracture, thereby improving the hole expandability of the hot-rolled steel sheet.
• the Sb content is preferably 0.001% or more.
• the Sb content is more preferably 0.005% or more, 0.010% or more.
• the Sb content is set to 0.500% or less, preferably 0.300% or less, and more preferably 0.100% or less.
• the As content is preferably 0.001% or more.
• the As content is more preferably 0.005% or more, more preferably 0.010% or more.
• the As content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.
• Sn 0.001-0.050%
• Sn has the effect of suppressing the generation of oxides that are the starting point of fracture, thereby improving the hole expandability of the hot-rolled steel sheet.
• the Sn content is preferably 0.001% or more.
• the Sn content is more preferably 0.005% or more, 0.010% or more.
• the Sn content is set to 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less.
• the chemical composition of the above-mentioned hot-rolled steel sheet may be analyzed using a spark discharge optical emission spectrometer or the like.
• Values of C and S are identified by burning the steel sheet in an oxygen stream using a gas component analyzer or the like and measuring the same by an infrared absorption method.
• Values of O and N are identified by melting a test piece taken from the steel sheet in a helium stream and measuring the same by a thermal conductivity method.
• the hot-rolled steel sheet has a plating layer or a coating film on the surface, the plating layer or the coating film is removed by mechanical grinding or the like as necessary before analyzing the chemical composition.
• the metal structure of the hot-rolled steel sheet according to this embodiment will be described.
• the average aspect ratio of prior austenite grains is 2.00 or more and less than 4.00
• the area ratio of martensite is 90% or more
• a value obtained by dividing the average aspect ratio of prior austenite grains in a surface layer region that is a region from the surface to a depth of 1/15 of the plate thickness from the surface by the average aspect ratio of the prior austenite grains in the internal region is less than 0.950.
• the internal region refers to a region from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface. In other words, it refers to a region that starts at 1/8 of the plate thickness from the surface and ends at 3/8 of the plate thickness from the surface.
• the surface layer region refers to a region from the surface to a depth of 1/15 of the plate thickness from the surface. In other words, it refers to a region that starts at the surface and ends at a depth of 1/15 of the plate thickness from the surface.
• Inner region Average aspect ratio of prior austenite grains 2.00 or more, less than 4.00 If the average aspect ratio of the prior austenite grains in the inner region is less than 2.00, the strength of the hot-rolled steel sheet decreases and the crack propagation resistance in the sheet thickness direction deteriorates. Therefore, the average aspect ratio of the prior austenite grains in the inner region is set to 2.00 or more.
• the average aspect ratio of the prior austenite grains in the inner region is preferably 2.20 or more, 2.30 or more, 2.40 or more, or 2.60 or more. On the other hand, if the average aspect ratio of the prior austenite grains in the inner region is 4.00 or more, the hole expandability and bendability of the hot-rolled steel sheet deteriorate.
• the average aspect ratio of the prior austenite grains in the inner region is set to less than 4.00.
• the average aspect ratio of the prior austenite grains in the inner region is preferably 3.80 or less, 3.60 or less, or 3.40 or less. From the viewpoint of obtaining better hole expandability and bendability in the hot-rolled steel sheet, the average aspect ratio of the prior austenite grains in the inner region is preferably less than 3.00.
• the aspect ratio of a prior austenite grain is the value obtained by dividing the major axis of the prior austenite grain by the minor axis, and is a value of 1.00 or more.
• the average aspect ratio of prior austenite grains is measured by the following method.
• a sample is taken at a quarter position from the end face in the width direction of the hot-rolled steel sheet so that the metal structure of the cross section (thickness direction x rolling direction cross section) normal to the width direction can be observed.
• the size of the sample depends on the measuring device, but may be, for example, a rectangular parallelepiped with a total thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction.
• the observation surface is mirror-polished, and then corroded by the Bechet-Beaujard method in accordance with JIS G 0551:2020 using an etching solution (a saturated aqueous solution of picric acid, an aqueous solution containing a surfactant and oxalic acid).
• etching solution a saturated aqueous solution of picric acid, an aqueous solution containing a surfactant and oxalic acid.
• Grains that appear black due to corrosion are identified as prior austenite grains.
• the observation surface where the prior austenite grains are exposed is observed by an optical microscope, and eight fields of view of 200 ⁇ m in the plate thickness direction ⁇ 600 ⁇ m in the rolling direction are photographed at a magnification of 1000 times or more for the internal region (region from the surface to 1/8 of the plate thickness depth to 3/8 of the plate thickness depth from the surface).
• the ratio of the long axis and the short axis obtained by measuring each prior austenite grain is calculated, and the average value is calculated by weighting the area of each prior austenite grain, thereby obtaining the average aspect ratio of the prior austenite grains.
• the average aspect ratio of the two prior austenite grains is calculated as (A1 ⁇ r1+A2 ⁇ r2)/(A1+A2).
• the prior austenite grains are identified by the reconstruction method described in “Study on High-Precision Reconstruction Method of Austenite Structure of Steel” (Hata Kengo, Wakita Masayuki, Fujiwara Tomoya, Kono Kaori, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30), and the average aspect ratio of the prior austenite grains is determined.
• the EBSD measurement data used in the reconstruction method is obtained by the following method.
• colloidal polishing or electrolytic polishing is performed on the above-mentioned observation field (field of view of 200 ⁇ m in the sheet thickness direction ⁇ 600 ⁇ m in the rolling direction), and then crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.1 ⁇ m.
• an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSD detector (EDAX Velocity (registered trademark) ultra-high speed operation EBSD detector) is used.
• the degree of vacuum in the device is 9.6 ⁇ 10 -5 Pa or less
• the acceleration voltage is 25 kV
• the irradiation current level is 16.
• version 7 or later of OIM Analysis manufactured by EDAX/TSL solution is used for the obtained crystal orientation information.
• prior austenite grains having an equivalent circle diameter of less than 2 ⁇ m are included, these are excluded from the above-mentioned measurement because prior austenite grains having an equivalent circle diameter of less than 2 ⁇ m do not adversely affect the properties of the hot-rolled steel sheet according to this embodiment.
• the rolling direction of the hot-rolled steel sheet is determined by the following method. Test pieces are taken so that the thickness cross section of the hot-rolled steel sheet can be observed. The direction perpendicular to the sheet surface is the Z direction, and a total of 12 test pieces are taken by rotating the test pieces 30° around the Z direction as an axis.
• the thickness cross section of the taken test pieces is polished, the prior austenite grain boundaries are revealed using the above-mentioned etching solution, and the average aspect ratio of the prior austenite grains is calculated by the intercept method.
• the test piece with the largest average aspect ratio of the prior austenite grains is identified, and the direction from which the test piece was taken is determined to be the rolling direction of the hot-rolled steel sheet. In other words, the direction parallel to the thickness cross section of the test piece and perpendicular to the thickness direction is determined to be the rolling direction of the hot-rolled steel sheet.
• Area ratio of martensite 90% or more Martensite is a structure that increases the strength of a hot-rolled steel sheet. If the area ratio of martensite is less than 90%, the desired strength cannot be obtained. Therefore, the area ratio of martensite is set to 90% or more.
• the area ratio of martensite is preferably 92% or more, 94% or more, or 96% or more.
• the area ratio of martensite may be 100%.
• the metal structure of the internal region of the hot-rolled steel sheet according to this embodiment may contain bainite, ferrite, pearlite and retained austenite as the remaining structure in addition to martensite.
• the area ratio of the remaining structure may be 10% or less in relation to the area ratio of martensite.
• the area ratio of the remaining structure may be 8% or less, 6% or less, or 4% or less in relation to the area ratio of martensite. Since the remaining structure may not be contained, the area ratio of the remaining structure may be 0%.
• the area ratio of martensite and the remaining structure is measured by the following method.
• a test piece is taken from the hot-rolled steel sheet so that the metal structure can be observed at the 1/4 position of the sheet thickness (in the range from the surface to the 1/8 position of the sheet thickness in the sheet thickness direction).
• the sheet thickness cross section of the test piece is mirror-polished and LePera etched, and then an area of 200 ⁇ m (sheet thickness direction) ⁇ 600 ⁇ m (direction perpendicular to the sheet thickness direction) at the 1/4 position of the sheet thickness is observed using a FE-SEM: thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL), and image analysis is performed.
• JSM-7001F thermal field emission scanning electron microscope
• the area ratio of retained austenite can be obtained by X-ray diffraction.
• a test piece taken from a hot-rolled steel sheet is ground from the sheet surface to a 1/4 position of the sheet thickness (a range from a position of 1/8 of the sheet thickness to a position of 3/8 of the sheet thickness from the surface in the sheet thickness direction), and the exposed surface is used as the observation surface.
• This observation surface is mirror-polished and then finished by electrolytic polishing.
• the integrated intensity of a total of five peaks, ⁇ (200), ⁇ (211), ⁇ (200), ⁇ (220), and ⁇ (311) is obtained using Rigaku's RINT-2500 and Mo-K ⁇ , and the volume fraction of retained austenite is calculated using the intensity averaging method. This volume fraction of retained austenite is regarded as the area fraction of retained austenite.
• the total area fraction of martensite is obtained by subtracting the area fraction of retained austenite obtained by X-ray diffraction from the sum of the area fractions of martensite and retained austenite obtained by observation using the Fe-SEM described above. If the total area fraction of martensite is calculated to be a negative value, the total area fraction of martensite is set to 0%.
• the area ratio of pearlite is obtained by the following method. For the same region (200 ⁇ m ⁇ 600 ⁇ m) as that used to determine the area ratio of martensite and retained austenite, only the corroded layer is removed by polishing and the specimen is mirror-finished. The specimen is then etched using a nital solution, and observed using an FE-SEM to perform image analysis. The area where cementite and ferrite are arranged in a lamellar shape is determined as pearlite, and the area ratio of this area is calculated to obtain the area ratio of pearlite.
• the area ratio of ferrite is obtained by the following method. Note that the following operation is performed on the regions other than the regions determined to be pearlite by the above-mentioned method.
• the same region (200 ⁇ m ⁇ 600 ⁇ m) as that used to determine the area ratio of martensite and retained austenite is subjected to colloidal polishing or electrolytic polishing, and then crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.2 ⁇ m.
• an EBSD analysis device consisting of a thermal field emission scanning electron microscope (JSM-7200F manufactured by JEOL) and an EBSD detector (EDAX Velocity (registered trademark) ultra-high speed EBSD detector) is used. At this time, the degree of vacuum in the device is 9.6 ⁇ 10-5 Pa or less, the acceleration voltage is 25 kV, and the irradiation current level is 16.
• the following analysis is performed on the obtained crystal orientation information using version 7 or later of OIM Analysis (registered trademark) manufactured by EDAX/TSL solution.
• OIM Analysis registered trademark
• the measurement points between which the crystal orientation difference is 15° or more are regarded as crystal grain boundaries, and the area surrounded by the crystal grain boundaries is regarded as a crystal grain.
• the difference in crystal orientation between all measurement points present within the crystal grain is calculated, and the average value of these differences is calculated to obtain the GAM value (Grain Average Misorientation value) of the crystal grain.
• Crystal grains with a GAM value of 0.5° or less are regarded as ferrite, and the area ratio of ferrite is obtained by calculating the area ratio of ferrite.
• the area fraction of bainite is obtained by subtracting the area fractions of martensite, retained austenite, pearlite, and ferrite obtained above from 100%. In the calculation, when the area fraction of bainite is a negative value, the area fraction of bainite is set to 0%.
• the area ratio of the metal structure is calculated by image analysis using FE-SEM, X-ray diffraction, and EBSD analysis, so the total of each structure may not be 100%. In that case, the area ratio of each structure is corrected so that the total is 100%. For example, if the total of the area ratios of each structure is 103%, the area ratio of each structure is corrected by multiplying it by "100/103".
• Electron gun type Thermal emission type
• Current irradiation number 9 WD (Working Distance): 10mm
• Acceleration voltage 20 kV
• Objective aperture number 4 Number of pixels: 5120 x 3840
• the value obtained by dividing the average aspect ratio of the prior austenite grains in the surface region by the average aspect ratio of the prior austenite grains in the internal region is 0.950 or more, the desired bendability and crack propagation resistance in the thickness direction cannot be obtained in the hot-rolled steel sheet. Therefore, the value obtained by dividing the average aspect ratio of the prior austenite grains in the surface region by the average aspect ratio of the prior austenite grains in the internal region is less than 0.950.
• the value obtained by dividing the average aspect ratio of the prior austenite grains in the surface region by the average aspect ratio of the prior austenite grains in the internal region is preferably 0.930 or less, 0.900 or less.
• the lower limit of the value obtained by dividing the average aspect ratio of the prior austenite grains in the surface region by the average aspect ratio of the prior austenite grains in the inner region is not particularly limited, but may be 0.800 or more, or 0.850 or more.
• the average aspect ratio of the prior austenite grains in the surface region is obtained by measuring the surface region (the region from the surface to a depth of 1/15 of the plate thickness from the surface) using the same method as that used to measure the average aspect ratio of the prior austenite grains in the internal region.
• the hot-rolled steel sheet according to this embodiment may have a tensile strength of 1180 MPa or more.
• the tensile strength is more preferably 1200 MPa or more.
• the upper limit of the tensile strength does not need to be particularly limited, but from the viewpoint of suppressing die wear, it may be set to 1500 MPa or less or 1300 MPa or less.
• the hot-rolled steel sheet according to the present embodiment may have a hole expansion ratio of 50% or more.
• the hole expansion ratio is preferably 55% or more, more preferably 60% or more.
• the tensile strength is evaluated by conducting a tensile test in accordance with JIS Z 2241: 2022.
• the test piece is a No. 5 test piece of JIS Z 2241: 2022.
• the tensile test piece is taken from a quarter portion from the end in the plate width direction, and the direction perpendicular to the rolling direction is the longitudinal direction.
• a small rectangular piece having a parallel portion of any width may be taken and the tensile strength may be determined by performing a tensile test using the small piece.
• the direction of the small rectangular piece is perpendicular to the rolling direction.
• the hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.
• the maximum bending angle obtained by a bending test based on the VDA standard described later may be 55° or more.
• the maximum bending angle is preferably 60° or more.
• the test piece for the bending test is a 60 mm (rolling direction) x 30 mm (width direction) test piece taken from a hot-rolled steel sheet. Using this test piece, a bending test is performed under the following conditions based on the VDA standard (VDA238-100:2017-04) specified by the German Association of the Automotive Industry. If the thickness of the test piece exceeds 2.5 mm, the surface on the punch side is ground to reduce the thickness to 2.5 mm before the bending test. When the thickness of the test piece is 2.5 mm or less, the maximum bending angle obtained by the following formula is used.
• ⁇ t represents the maximum bending angle obtained by the bending test
• t represents the plate thickness
• uEL represents the uniform elongation (total elongation at the maximum test force).
• the uniform elongation is a value obtained by performing a tensile test using the method described above.
• Maximum bending angle when the plate thickness is 2.5 mm or less ⁇ t -13.852 ⁇ (1-t/2.5) ⁇ (uEL + 0.22) 0.292
• Test piece dimensions 60 mm (rolling direction) x 30 mm (width direction) Bending ridge: Parallel to the width direction
• Test method Roll support, punch pressing Roll diameter: ⁇ 30 mm
• the crash resistance of the hot-rolled steel sheet is evaluated by the crack propagation resistance in the sheet thickness direction.
• the crack propagation resistance in the sheet thickness direction is obtained by calculating the energy W from the bending angle-load curve during the VDA bending test.
• the hot-rolled steel sheet according to this embodiment may have W exceeding 5.0 ⁇ 10 5 (° ⁇ N).
• W is the value ( ⁇ Fd ⁇ ) obtained by integrating the load with the bending angle.
• F is the load
• ⁇ is the bending angle obtained by the bending test performed based on the above-mentioned VDA standard.
• the bending test is performed with a test piece thickness of 2.5 mm, and the other conditions are as described above.
• the punch side surface is ground to make the thickness 2.5 mm before the bending test.
• W the thickness of the test piece is 2.5 mm or less
• W Wt ⁇ (2.5/t) 2.02
• the thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the hot-rolled steel sheet is less than 1.2 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the thickness of the hot-rolled steel sheet according to the present embodiment may be 1.2 mm or more. It is preferably 1.4 mm or more. On the other hand, if the sheet thickness exceeds 8.0 mm, it may be difficult to obtain the above-mentioned metal structure after hot rolling. Therefore, the sheet thickness may be 8.0 mm or less, and is preferably 6.0 mm or less.
• the hot-rolled steel sheet according to the present embodiment having the above-mentioned chemical composition and metal structure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance, etc., to form a surface-treated steel sheet.
• the plating layer may be an electroplating layer or a hot-dip plating layer.
• the electroplating layer include electrogalvanizing and electrolytic Zn-Ni alloy plating.
• the hot-dip plating layer include hot-dip galvanizing, alloyed hot-dip galvanizing, 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 the same as in the past.
• it is possible to further improve corrosion resistance by carrying out an appropriate chemical conversion treatment after plating for example, coating with a silicate-based chromium-free chemical conversion treatment solution and drying it).
• the hot-rolled steel sheet according to the present embodiment has high strength, excellent hole expansion property, bendability, and resistance to crack propagation in the thickness direction, and therefore can be suitably used for parts, particularly automobile parts.
• the hot-rolled steel sheet according to the present embodiment can be suitably used for automobile suspension parts such as lower arms, trail links, and knuckles.
• These automobile parts may be made of only the hot-rolled steel sheet according to the present embodiment, or may be formed by joining the hot-rolled steel sheet according to the present embodiment and other steel sheets.
• the parts manufactured using the hot-rolled steel sheet according to this embodiment have the same chemical composition as the above-mentioned steel sheet.
• the parts may have a mixture of processed and unprocessed parts.
• the unprocessed parts have the same metal structure as the above-mentioned steel sheet.
• the processed parts basically have the same metal structure as the above-mentioned steel sheet, but in the case of strong processing or at the end of the part, the metal structure may not be present or may be difficult to determine. Therefore, when measuring the metal structure of the part, the end is avoided and the measurement is performed on the part that has not been processed. If there is no unprocessed part, the measurement is performed on the part that has not been heavily processed.
• the part that has not been processed or has not been heavily processed refers to, for example, a flat part of the part, a part where the thickness increase or decrease due to processing is small, and a part that has not been subjected to punching, hole expanding, bending, or the like.
• a test piece is taken from the part that is flat and has the largest area, and is examined near the center of gravity.
• the hot-rolled steel sheet according to this embodiment can be stably manufactured.
• the temperature of the slab and the temperature of the steel sheet in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.
• a preferred method for producing a hot-rolled steel sheet according to this embodiment is as follows: (1) before rough rolling, a step of applying strain to a slab having the above-mentioned chemical composition one or more times so that the width direction strain is 3 to 15% in total; (2) A process of performing finish rolling on the strained slab so that the total reduction in the temperature range below 1040 ° C is more than 30% and less than 46%, and the reduction in the temperature range from 1020 to 1040 ° C is 20 to 30%; (3) After the completion of the finish rolling, a step of accelerated cooling is performed at an average cooling rate of 30°C/s or more down to 200°C. Each step will be described below.
• the total strain imparted to the slab in the width direction can be expressed as (1-w1/ w0 ) x 100 (%), where w0 is the width direction length of the slab before the first strain is imparted, and w1 is the width direction length of the slab after the final strain is imparted.
• a method for imparting strain in the width direction of the slab for example, there can be mentioned a method of imparting strain using a roll installed so that the rotation axis is perpendicular to the plate surface of the slab.
• the slab to which strain is applied is not particularly limited except for the chemical composition described above.
• a slab produced by melting molten steel of the above chemical composition using a converter or electric furnace, etc. and then by continuous casting can be used.
• an ingot casting method, thin slab casting method, etc. may be used.
• the heating temperature may be in the range of 1100 to 1300°C.
• the strained slab is preferably rolled at a total reduction rate of more than 30% and less than 46% in a temperature range below 1040° C., and at a reduction rate of 20 to 30% in a temperature range of 1020 to 1040° C.
• the total reduction rate in a temperature range below 1040° C. to more than 30% and less than 46%, the average aspect ratio of prior austenite grains in the inner region can be preferably controlled.
• the total rolling reduction in the temperature range below 1040°C can be expressed as (1-t1/ t0 ) x 100(%), where t0 is the inlet thickness of the first rolling in the temperature range below 1040° C and t1 is the outlet thickness of the last rolling in the temperature range below 1040°C.
• the ratio between the average aspect ratio of the prior austenite grains in the inner region and the average aspect ratio of the prior austenite grains in the surface region can be preferably controlled.
• the rolling reduction can be expressed by (1- t3 / t2 ) x 100(%), where t2 is the entry thickness and t3 is the delivery thickness.
• 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 conditions in the example are an example of conditions adopted to confirm the feasibility and effect of the present disclosure, and the present disclosure is not limited to this example of conditions.
• the present disclosure may adopt various conditions as long as they do not deviate from the gist of the present disclosure and achieve the purpose of the present disclosure.
• the average aspect ratio and metal structure of the prior austenite grains in the internal region were determined using the methods described above.
• tensile strength when the thickness of the hot-rolled steel plate was 2.5 mm or less, values corrected by the above formula were used.
• the metal structure of the internal region of the hot-rolled steel sheet contained, in addition to martensite, bainite, ferrite, pearlite, and retained austenite as residual structures. The measurement results are shown in Table 4.
• the hot-rolled steel sheet was deemed to have excellent hole expansion properties and was judged to have passed. On the other hand, if the hole expansion ratio was less than 50%, the hot-rolled steel sheet was deemed to have poor hole expansion properties and was judged to have failed.
• the hot-rolled steel sheet was deemed to have excellent bendability and passed the test. On the other hand, if the maximum bending angle was less than 55%, the hot-rolled steel sheet was deemed to not have excellent bendability and passed the test.
• the hot-rolled steel sheets according to the examples of the present invention have high strength, as well as excellent hole expandability, bendability, and resistance to crack propagation in the sheet thickness direction.
• the hot-rolled steel sheets according to the comparative examples are inferior in one or more of the above properties.
• lower arms (components) were manufactured by press working. The flat portion of the lower arm was evaluated in the same manner as described above. The measurement results and evaluation results were the same as those shown in Table 4.
• the above aspects of the present disclosure make it possible to provide a hot-rolled steel sheet having high strength, excellent hole expansion properties, bendability, and resistance to crack propagation in the plate thickness direction, and a part manufactured using the same.

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