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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to a steel sheet and a method for producing the same.
• the present invention claims priority based on Japanese Patent Application No. 2023-030697, filed in Japan on March 1, 2023, the contents of which are incorporated herein by reference.
• High-strength steel sheets are used in automobiles to reduce the weight of automobiles, improve fuel efficiency, and reduce carbon dioxide emissions, while also ensuring the safety of passengers in the event of a collision. Furthermore, high-strength steel sheets used for automobile parts are required to have not only strength, but also properties necessary for part forming, such as bendability.
• methods for improving the bendability of high-strength steel sheets include softening the surface layer of the steel sheet (Patent Document 1) and specifying the metal structure (Patent Document 2).
• the surface layer of the steel sheet is made into a ferrite main phase or a decarburized ferrite phase, improving deformability, but at the same time, hard phases that can become the starting points of cracks remain, so there is room to improve both of the bending properties described above.
• Evaluation of the bendability of thin steel plates includes a method of observing whether cracks have occurred on the surface after bending (a typical example is the 90° V-bend test) and a method of judging cracks based on the change in load during bending (a typical example is the VDA (German Association of the Automotive Industry) bend test).
• Patent Document 3 proposes a method of evaluating cracking during a collision using the VDA bend test. In evaluations using the VDA bend test, breakage is judged by the peak or drop in load, but the inventor's investigations have revealed that breakage occurs before the peak of the load.
• the present invention aims to provide a steel plate that is high in strength, has excellent bendability in terms of formability, and has excellent bendability in terms of impact resistance, and a manufacturing method thereof.
• the gist of the present invention is as follows:
• a steel sheet according to an embodiment of the present invention has, in mass%, C: 0.070 to 0.15%, Si: 0.10 to 2.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 1.500%, P: 0.0010 to 0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0 to 0.200%, B: 0 to 0.0100%, Cr: 0 to 1.000%, Mo: 0 to 1.000%, Ni: 0 to 1.000%, Cu: 0 to 1.000%, Sn: 0 to 0.500%.
• the metal structure at a 1/4 thickness position which is in a range from the surface to 1/8 to 3/8 of the plate thickness and centered at a position of 1/4 of the plate thickness of the steel plate in the plate thickness direction, contains, in terms of area ratio, 0 to 60% ferrite, 0 to 3% retained austenite, and the remainder contains one or more types selected from martensite, bainite, pearlite and cementite, the ferrite fraction in the range from the surface of the steel plate to 2 ⁇ m in the plate thickness direction of the steel plate is 95% or more and the in-plane average grain size of the ferrite
• the steel plate described in (1) above may have a tensile strength of less than 1300 MPa.
• a transition region based on the C concentration may be 150 ⁇ m or less in the plate thickness direction of the steel plate.
• a fresh martensite fraction may be 10% or less in a range of 5 to 20 ⁇ m from a surface of the steel plate in the thickness direction of the steel plate.
• a ferrite fraction may be 50% or more in a range of 5 to 20 ⁇ m from a surface of the steel plate in a thickness direction of the steel plate.
• the steel sheet has a composition, in mass%, of C: 0.070 to 0.15%, Si: 0.10 to 2.00%, Mn: 1.00 to 4.00%, sol. Al: 0.001 to 1.500%, P: 0.0010 to 0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0 to 0.200%, B: 0 to 0.0100%, Cr: 0 to 1.000%, Mo: 0 to 1.000%, Ni: 0 to 1.000%, and Cu: 0 to 1.000%.
• a cold rolling process for cold rolling a steel sheet having a chemical composition consisting of: a grinding process for grinding the surface of the steel sheet that has been subjected to the cold rolling process; an annealing process for annealing the steel sheet whose surface has been ground in the grinding process; and a quenching process for quenching the steel sheet after the annealing process.
• the surface of the steel sheet that has been subjected to the cold rolling step is ground to a depth of 0.1 ⁇ m or more; in the annealing step, the dew point is set to ⁇ 15° C. to 20° C., the annealing temperature is set to 750° C. or more; and in the quenching step, quenching is performed at an average cooling rate of 0.4° C./sec or more to a temperature of 300° C. or less.
• a dew point may be set to ⁇ 15° C. to 20° C. at least during a heating process up to the annealing temperature.
• the method for producing a steel sheet according to (6) or (7) above may further include a tempering step of tempering at 150° C. or higher after the quenching step.
• the present invention provides a steel plate with high strength, excellent bendability in terms of formability, and excellent bendability in terms of impact resistance, and a manufacturing method thereof.
• a steel sheet according to an embodiment of the present invention and a method for manufacturing the same will be described below.
• a range indicated with “to” generally includes the values at both ends as the lower and upper limits. However, values indicated as “greater than” and “less than” are not included in the range.
• each configuration of the steel plate according to this embodiment will be described.
• the steel sheet according to this embodiment contains the following elements.
• the percentage of the content of each element means mass%.
• C 0.070-0.15%
• C (carbon) is an essential element for increasing the strength of steel sheets. If the C content is less than 0.070%, sufficient tensile strength cannot be obtained. From the viewpoint of ensuring the ferrite phase and improving the elongation, the C content is preferably 0.08% or more. On the other hand, if the C content exceeds 0.15%, the ductility of the material decreases and the bendability deteriorates, so the C content is set to 0.15% or less. From the viewpoint of weldability, the C content is The content is preferably 0.14% or less.
• Si 0.10-2.00%
• Silicon (Si) is a solid solution strengthening element and is an effective element for increasing the strength of steel sheets.
• the Si content is set to 0.10% or more.
• the Si content is preferably 0.30% or more.
• the Si content is set to 2.00% or less. In addition, it may cause embrittlement of the steel parts. From the viewpoint of reducing cold formability, the Si content is preferably 1.8% or less.
• Mn 1.00-4.00%
• Mn manganese
• Mn is a strong austenite stabilizing element and is an effective element for improving the hardenability of steel sheets.
• the Mn content is set to 1.00% or more.
• the Mn content is preferably 1.50% or more.
• the Mn content is set to 4.00% or less. It promotes co-segregation with P or S, which leads to a significant deterioration of workability. From the viewpoint of preventing such deterioration, the Mn content is preferably 3.20% or less.
• sol. Al 0.001-1.500%
• Al (aluminum) is an element that has a deoxidizing effect on steel.
• the sol. Al content is set to 0.001% or more.
• the sol. Al content is preferably 0.005%. That's all.
• Al is contained in excess, the effect becomes saturated and the cost increases, and the transformation temperature of the steel increases, increasing the load during hot rolling.
• the sol. Al content is preferably 1.000% or less.
• P (phosphorus) is a solid solution strengthening element and is an element effective in increasing the strength of steel sheets.
• the P content is set to 0.0010% or more.
• the P content is preferably 0.0010% or more. .0050% or more.
• the P content is set to 0.0300% or less.
• the P content is preferably 0.0200% or less.
• S 0.0200% or less
• S sulfur
• S is an element that causes hot brittleness and also inhibits weldability and corrosion resistance. If the S content exceeds 0.0200%, the hot workability, weldability and corrosion resistance are significantly reduced, so the S content is set to 0.0200% or less.
• the S content is preferably 0.0100% or less.
• the S content is preferably low, and may be 0%, but in order to make the S content less than 0.0001%, the manufacturing cost increases significantly. Therefore, the S content may be 0.0001% or more.
• the S content may be 0.0010% or more.
• N 0.0100% or less
• N nitrogen
• the N content is preferably 0.0050% or less.
• a low N content is preferable, and 0% may be acceptable.
• the N content may be set to 0.0005% or more.
• O 0.0100% or less
• O oxygen
• the O content is preferably 0.0070% or less.
• the O content is preferably small, and may be 0%, but from the viewpoint of production costs, the O content may be 0.0001% or more.
• the O content may be 0.0010% or more.
• the steel sheet according to this embodiment may contain the above elements, with the remainder being Fe and impurities. However, for the purpose of improving various properties, it may further contain one or more elements (optional elements) selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr and REM as shown below.
• the optional elements do not have to be contained, so the lower limit is 0%.
• Ti 0-0.200%
• Ti titanium is an element that fixes N as TiN in steel, suppressing the formation of BN, which is a factor that reduces hardenability. Ti also refines the austenite grain size during heating, improving toughness. In order to obtain this effect, the Ti content is preferably 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Ti content is excessive, the ductility of the steel sheet decreases. Therefore, when Ti is contained, the Ti content is set to 0.200% or less. The Ti content is preferably set to 0.050% or less. .
• B 0-0.0100%
• B is an element that segregates at the austenite grain boundary during welding to strengthen the grain boundary and contribute to improving the resistance to molten metal embrittlement cracking.
• the B content is The B content is preferably 0.0005% or more, and more preferably 0.0008% or more.
• the B content is set to 0.0100% or less.
• the B content is preferably 0.0050% or less.
• Cr 0 ⁇ 1.000%
• Mo molybdenum
• Ni nickel
• Cu copper
• Sn 0-0.500%
• Cr chromium
• Mo molybdenum
• Ni nickel
• Cu copper
• Sn tin
• At least one selected from Mo, Ni, Cu and Sn is preferably contained in an amount of 0.001% or more, more preferably 0.010% or more, and most preferably 0.050% or more. Even more preferred. On the other hand, if these elements are contained in excess, the effect becomes saturated and the cost increases.
• the contents of Cr, Mo, Ni and Cu are all set to 1.000% or less, and the Sn content
• the contents of Cr, Mo, Ni and Cu are each preferably set to 0.600% or less, and the Sn content is preferably set to 0.300% or less.
• Nb 0-0.200%
• V 0-0.500%
• W 0 ⁇ 0.500%
• Nb (niobium), V (vanadium) and W (tungsten) are carbide forming elements and are effective in increasing the strength of steel sheets.
• Nb, V and W are selected from the group consisting of Nb, V and W.
• Each of the above elements is contained in an amount of preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
• the Nb content is set to 0.200% or less
• the V and W contents are set to any one of
• the Nb content is preferably 0.100% or less
• the V and W contents are each preferably 0.300% or less.
• Ca 0 ⁇ 0.0100% Mg: 0-0.0100% Bi: 0 ⁇ 0.0100% Sb: 0-0.1000% Zr: 0 ⁇ 0.0100% REM: 0 ⁇ 0.1000%
• Ca (calcium), Mg (magnesium), Sb (antimony), Zr (zirconium), and REM (rare earth elements) are elements that contribute to the fine dispersion of inclusions in steel
• Bi (bismuth) is an element that contributes to the fine dispersion of Mn Mo is an element that reduces the microsegregation of substitutional alloying elements such as Si, etc. These elements each contribute to improving the bending resistance of the steel sheet, and therefore may be contained as necessary.
• one or more selected from Ca, Mg, Bi, Sb, Zr and REM are contained in an amount of 0.0001% or more, and preferably 0.0010% or more. is more preferred.
• the Ca, Mg, Bi, Sb and Zr contents are all set to 0.0100% or less.
• the REM content is set to 0.1000
• the contents of Ca, Mg, Bi, Sb and Zr are each preferably set to 0.0080% or less, and more preferably set to 0.0060% or less.
• the REM content is preferably 0.0800% or less, more preferably 0.0600% or less, and further preferably 0.0200% or less.
• REM refers to Sc, Y, and lanthanides, a total of 17 elements, and the REM content refers to the total content of these elements.
• lanthanides are added in the form of misch metal.
• the chemical composition of the steel sheet according to this embodiment can be determined by the following method.
• the chemical composition of the steel sheet may be measured by a general chemical composition measurement method.
• the chemical composition may be measured by ICP-AES (Inductively Coupled Plasma-Atomic Spectroscopy).
• Sol. Al may be measured by ICP-AES using the filtrate obtained by thermally decomposing a sample with an acid.
• C and S may be measured using a combustion-infrared absorption method
• N may be measured using an inert gas fusion-thermal conductivity method
• O may be measured using an inert gas fusion-non-dispersive infrared absorption method.
• the plating layer may be removed by mechanical grinding before the chemical composition is analyzed.
• the steel plate according to this embodiment has a chemical composition containing C, Si, Mn, sol. Al, P, S, O, and N, with the balance being Fe and impurities, or containing C, Si, Mn, sol. Al, P, S, O, and N, and further containing one or more elements selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM, with the balance being Fe and impurities.
• the metal structure in the range of 1/8 to 3/8 of the plate thickness from the surface of the steel plate in the plate thickness direction of the steel plate, centered at the position of 1/4 of the plate thickness of the steel plate, contains 0 to 60% ferrite, 0 to 3% retained austenite, and the remainder contains one or more selected from martensite, bainite, pearlite, and cementite, in terms of area ratio.
• This "range of 1/8 to 3/8 of the plate thickness from the steel plate surface” is referred to as the "1/4 thickness position" in this embodiment.
• the plate thickness direction of the steel plate in this embodiment is the direction perpendicular to the steel plate surface.
• the area ratio means the ratio of each structure to the entire metal structure in the above range.
• the remainder in the above metal structure may be one or more selected from martensite, bainite, pearlite, and cementite.
• the metal structure at the quarter thickness position more preferably contains 0 to 30% ferrite. From the viewpoint of ensuring strength, it is more preferable that the metal structure at the quarter thickness position has a total of 40 to 100% tempered martensite and fresh martensite. From the viewpoint of ensuring bendability, it is more preferable that the metal structure at the quarter thickness position has a total of 40 to 100% of tempered martensite and bainite. From the viewpoint of ensuring strength, the metal structure at the quarter thickness position preferably has a total of 0 to 5% pearlite and cementite, and more preferably has a total of 0 to 3% pearlite and cementite.
• the area ratios of ferrite, retained austenite, martensite (including tempered martensite and fresh martensite), bainite, and the like contained in the metal structure at the quarter thickness position can be measured by the method described below.
• a sample is taken with a cross section parallel to the rolling direction and thickness direction of the steel sheet as an observation surface, and the observation surface is polished and etched with nital.
• the rolling direction in this embodiment is parallel to the plane of the steel sheet and is the longitudinal direction when the steel sheet is rolled and stretched. Since highly ductile non-ferrous inclusions such as MnS contained in the steel sheet are also stretched by rolling together with the steel sheet, the rolling direction also coincides with the direction in which highly ductile non-ferrous inclusions such as MnS extend.
• the cutting direction in which the aspect ratio of highly ductile non-ferrous inclusions such as MnS is the largest when a cross section cut on a plane perpendicular to the steel sheet surface is observed may be the rolling direction.
• a method for identifying the rolling direction of a steel sheet when the rolling direction of the steel sheet is unknown, such as after the steel sheet has been processed into a part, will be described later.
• a total of five visual fields are observed with a field emission scanning electron microscope (FE-SEM) at a magnification of 5000 times in the range of 1/8 thickness to 3/8 thickness (from the position of 1/8 of the thickness of the steel plate from the surface of the steel plate to the position of 3/8 of the thickness of the steel plate from the surface of the steel plate) centered at the position of 1/4 of the thickness of the steel plate from the surface of the steel plate, with one visual field being 250 ⁇ m2 or more.
• the area ratios of ferrite, retained austenite, martensite, pearlite, cementite, and bainite are measured.
• a region that has a substructure within the grain and has multiple carbide long side directions when observed with a scanning electron microscope is judged to be tempered martensite.
• regions where cementite is precipitated in lamellar form are judged to be pearlite or cementite.
• Regions where the brightness is low and no substructure is visible are judged to be ferrite.
• Regions where the brightness is high and the substructure is not revealed by etching are judged to be fresh martensite or retained austenite. The remainder is judged to be bainite.
• the area ratio of each tissue was calculated by the point counting method.
• the measurement points were spaced 2 ⁇ m apart, and 300 or more measurement points were measured per visual field.
• the calculated values in each visual field were arithmetically averaged to obtain the area ratio of each tissue.
• the area fraction of fresh martensite can be determined by subtracting the area fraction of retained austenite determined by the EBSD method described below from the area fraction of fresh martensite or retained austenite. The sum of this and the area fraction of tempered martensite calculated by the point counting method is the area fraction of martensite. Note that if the area fraction of fresh martensite or retained austenite calculated by the point counting method is smaller than the area fraction of retained austenite determined by the EBSD method described below, the area fraction of fresh martensite is set to zero.
• the area ratio of retained austenite at the 1/4 thickness position is evaluated by performing high-resolution crystal structure analysis using the EBSD method (electron backscatter diffraction method). Specifically, a sample is taken with a cross section parallel to the rolling direction and plate thickness direction of the steel plate as the observation surface, and the observation surface is polished to a mirror finish. Furthermore, electrolytic polishing or mechanical polishing using colloidal silica is performed to remove the processed surface layer.
• EBSD method electron backscatter diffraction method
• crystal structure analysis is performed on five visual fields by the EBSD method, with a magnification of 5000 times and a size of one visual field of 250 ⁇ m2 or more.
• the step distance is 0.01 to 0.20 ⁇ m.
• the data obtained by the EBSD method is analyzed using "OIM Analysis 6.0" manufactured by TSL. From the observation results at each position, the region judged to be FCC iron is judged to be retained austenite, and the area ratio of each of the retained austenite at the 1/4 thickness position is calculated.
• the area ratio of fresh martensite or retained austenite calculated by the point counting method is smaller than the area ratio of retained austenite determined by the EBSD method, the area ratio of fresh martensite or retained austenite calculated by the point counting method is regarded as the area ratio of retained austenite.
• the ferrite fraction in outermost layer In the steel sheet according to the present embodiment, the ferrite fraction in the range from the surface of the steel sheet to 2 ⁇ m in the sheet thickness direction of the steel sheet is 95% or more. Also, in the steel sheet according to the present embodiment, the in-plane average grain size of ferrite is 2.0 ⁇ m or less. As described later, in the manufacturing method of the steel sheet according to the present embodiment, the surface of the steel sheet that has been subjected to the cold rolling process is ground and annealed, so that the crystal grains are refined during annealing due to the strain introduced by grinding.
• ferrite having an in-plane average grain size of 2.0 ⁇ m or less is generated in the surface layer of the steel sheet with a ferrite fraction of 95% or more, and soft ferrite is formed in the outermost layer of the steel sheet.
• the occurrence of microcracks on the surface of the steel sheet, particularly as seen in a 90° V-bend test, can be suppressed.
• the progression of such microcracks can also be suppressed.
• the ferrite fraction can be measured using the method shown below.
• the ferrite fraction refers to the proportion of ferrite structure in the entire metal structure within a range of 2 ⁇ m from the surface of the steel plate in the thickness direction of the steel plate.
• the ferrite fraction in the range from the surface of the steel sheet to 2 ⁇ m in the sheet thickness direction of the steel sheet can be measured by the method described below.
• a sample is taken from a cross section parallel to the rolling direction and thickness direction of the steel sheet, and the observation surface is polished and etched with nital.
• a total of five visual fields are observed with a field emission scanning electron microscope at a magnification of 5000 times, with each visual field being 250 ⁇ m2 or more.
• the area ratio of ferrite is then measured for each.
• areas with low brightness and no visible substructure are determined to be ferrite structures.
• the area ratio of the ferrite structure is calculated using the same point counting method as described above to determine the area ratio of the ferrite structure.
• the range of 2 ⁇ m from the surface of the steel plate in the thickness direction of the steel plate means a range of 2 ⁇ m from the surface of the steel plate toward the inside of the steel plate along the thickness direction of the steel plate.
• the in-plane average grain size of ferrite can be measured by the following method.
• a sample having a cross section parallel to the rolling direction and thickness direction of the steel sheet as an observation surface, and a sample having a cross section parallel to the width direction and thickness direction of the steel sheet as an observation surface are prepared.
• the observation surface of each sample is polished and etched with nital.
• the surface layer of the sample having a cross section parallel to the rolling direction and thickness direction as an observation surface is photographed at a magnification of 5000 times, and a connected image of 250 ⁇ m 2 or more obtained by moving the photographing range in the rolling direction is taken as one field of view, and a total of five connected images are obtained with a field emission scanning electron microscope.
• the surface layer is also photographed at a magnification of 5000 times, and a connected image of 250 ⁇ m 2 or more obtained by moving the photographing range in the width direction is taken as one field of view, and a total of five connected images are obtained with a field emission scanning electron microscope.
• the grain size of ferrite grains is measured at depth positions of 0.5, 1.0, 1.5, and 2.0 ⁇ m from the surface of the steel plate in the plate thickness direction for a total of 10 connected images obtained for each sample.
• a straight line parallel to the steel plate surface is assumed, and for the ferrite grains intersecting with this straight line, (length of the straight line)/(number of ferrite grains intersecting with the straight line-1) is defined as the grain size of the ferrite grains at that depth position.
• the length of the straight line i.e., the observation distance, is set to 50 ⁇ m or more.
• the in-plane average grain size of ferrite is obtained by arithmetically averaging all of the grain sizes of these ferrite grains, i.e., the grain sizes obtained from the depth positions of four points in each of the 10 fields of view, a total of 40 straight lines.
• the above measurement can be performed by preparing a sample in which an arbitrary direction in the sheet plane of the part is set as the reference direction and a cross section parallel to the reference direction and the sheet thickness direction is used as the observation surface, and a sample in which a cross section perpendicular to the reference direction in the sheet plane and parallel to the sheet thickness direction is used as the observation surface.
• the average grain size (in-plane average grain size) of the part of ferrite in the measurement range that intersects with a straight line parallel to the steel sheet surface can be evaluated.
• the ferrite grains have a small cross-sectional area, and that the ferrite grain size on a straight line parallel to the steel sheet surface is within a specified range (2.0 ⁇ m or less).
• the steel plate in this embodiment has such a configuration, and therefore it is possible to suppress the occurrence and growth of microcracks on the surface of the steel plate, which are particularly observed in a 90° V-bend test.
• the steel sheet according to this embodiment has a tensile strength of 950 MPa or more. Since the steel sheet has a tensile strength of 950 MPa or more, it can be preferably used as a steel sheet for automobiles. In consideration of the contribution to weight reduction of automobiles, the tensile strength is preferably 980 MPa or more, more preferably 1050 MPa or more, and even more preferably 1100 MPa or more.
• the steel plate according to this embodiment more preferably has a tensile strength of less than 1300 MPa.
• the tensile strength of less than 1300 MPa has the advantage that elongation can be easily ensured.
• the tensile strength exceeds 1400 MPa, the weldability deteriorates, so the tensile strength is set to 1400 MPa or less.
• the steel plate according to this embodiment preferably has a transition region based on the C concentration of 150 ⁇ m or less in the thickness direction of the steel plate.
• the transition region in this embodiment is a region where the C concentration is 20% to 90% of the C concentration in the steady state portion described below. If the transition region based on the C concentration is 150 ⁇ m or less, the change in C concentration from the low carbon region to the high carbon region from the surface layer to the interior becomes steeper compared to a case where the transition region based on the C concentration is greater than 150 ⁇ m with the same amount of decarburization. Therefore, a low carbon region of sufficient thickness can be secured in the surface layer, and the occurrence and progression of microcracks in the surface layer can be suppressed. If the transition region is 100 ⁇ m or less, bendability can be further improved, which is more preferable.
• C concentration is the carbon concentration in the steel plate.
• C concentration can be measured using a method using a GDS (glow discharge optical emission spectrometer) as shown below.
• the C concentration at each depth can be obtained.
• the measurement time is set so that the measured depth of the C concentration in the steady state is 50 ⁇ m or more.
• the steady state is set to a range that is within ⁇ 5% of the arithmetic mean of the C concentration in the steady state, which is the average C concentration of the base material before decarburization.
• the steady state portion measured by this measurement method is defined as a C concentration of 100%.
• the transition region is the region in the thickness direction of the steel plate where the C concentration is 20% to 90%.
• the fresh martensite fraction is 10% or less in the range of 5 to 20 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
• the metal structure in this range is important in suppressing the growth of microcracks. By making the fresh martensite fraction in this range 10% or less, the effect of suppressing the growth of microcracks is improved.
• the fresh martensite fraction in this range is more preferably 5% or less.
• the fresh martensite fraction means the ratio of the fresh martensite structure to the entire metal structure in a range of 5 to 20 ⁇ m from the surface of the steel plate in the plate thickness direction of the steel plate.
• the fresh martensite fraction is determined by calculating the proportion of fresh martensite in the metal structure within the above range based on the area fraction obtained by the method for measuring the area fraction of the metal structure described above.
• the range of 5 to 20 ⁇ m from the surface of the steel plate in the thickness direction of the steel plate means a range of 5 ⁇ m or more and 20 ⁇ m or less from the surface of the steel plate toward the inside of the steel plate along the thickness direction of the steel plate.
• the ferrite fraction is 50% or more in a range of 5 to 20 ⁇ m from the surface of the steel plate in the plate thickness direction of the steel plate.
• the ferrite fraction in this range is more preferably 70% or more.
• the ferrite fraction means the ratio of the ferrite structure to the entire metal structure in the range of 5 to 20 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
• the formation and propagation of microcracks is suppressed by appropriately arranging each structure having the characteristics described above in the plate thickness direction.
• the 90° V-bend test can be used as a method for observing the occurrence of microcracks on the surface.
• the 90° V-bend test is a method that makes it possible to observe microcracks on the surface of a steel plate, i.e., initial microcracks, and can evaluate the bending performance that corresponds to the formability of the steel plate. Suppressing initial microcracks means that the characteristics in the 90° V-bend test are improved, which means that the formability of the steel plate is improved.
• the VDA bend test can also be used as a method for evaluating the progression of microcracks.
• the VDA bend test is a method for determining cracks based on changes in the load applied to the test object, and can evaluate bending performance that corresponds to crash performance. Being able to suppress the progression of microcracks means that the properties in the VDA bend test are improved, which means that the crash performance of the steel sheet for automobiles is improved.
• the steel sheet according to the present embodiment may have a plating layer such as zinc plating on the surface of the steel sheet as the base material.
• the zinc plating layer is, for example, a hot-dip galvanized layer.
• the zinc plating layer means a plating layer containing 80 mass % or more of Zn.
• the presence of the hot-dip galvanized layer on the surface improves corrosion resistance.
• the coating weight of the zinc-coated layer is preferably 150 g/ m2 or less, and more preferably 100 g/ m2 or less.
• the coating weight is preferably 20 g/ m2 or more.
• the chemical composition of the zinc plating layer is not limited, but it is preferable that it contains, for example, Al: 0.1-2.0%, Fe: 5.0% or less, with the remainder being Zn and impurities.
• the adhesion weight and chemical composition of the zinc plating layer are determined using the following method.
• the plating layer is melted using hydrochloric acid containing an inhibitor, and the adhesion weight is determined by comparing the weight before and after melting.
• the solution obtained after melting is quantitatively analyzed using ICP to measure the chemical composition of the plating layer.
• the position in the sheet thickness direction in this embodiment is the depth based on the surface of the base steel sheet (the interface between the Fe phase and the plating layer).
• the method for manufacturing steel sheet in this embodiment includes a cold rolling process in which a steel sheet having a predetermined chemical composition is cold rolled, a grinding process in which the surface of the steel sheet that has undergone the cold rolling process is ground, and an annealing process in which the steel sheet whose surface has been ground in the grinding process is annealed.
• ⁇ Cold rolling process> a steel sheet having the following chemical composition is cold rolled.
• C 0.070-0.15%
• Si 0.10-2.00%
• Mn 1.00-4.00%
• sol. Al 0.001-1.500%
• P 0.0010-0.0300%
• S 0.0200% or less
• N 0.0100% or less
• O 0.0100% or less
• B 0 to 0.0100%
• Ni: 0-1.000% Cu: 0-1.000%
• Sn: 0-0.500% Nb: 0 to 0.200%
• V 0 to 0.500%
• W 0-0.500%
• Ca 0-0.0100%
• Mg 0 to 0.0100%
• Bi 0 to 0.0100%
• Sb 0 to 0.1000%
• Zr 0 to 0.0100%
• REM 0 to 0.1000%
• the cold rolling conditions are not particularly limited, and the above-mentioned hot-rolled steel sheet can be subjected to cold rolling under normal conditions to produce a cold-rolled steel sheet.
• the manufacturing conditions of the steel sheet to be subjected to the cold rolling process There are no limitations on the manufacturing conditions of the steel sheet to be subjected to the cold rolling process.
• molten steel having the above-mentioned chemical composition is cast under normal conditions to form a steel billet, which is then hot-rolled under normal conditions to produce a hot-rolled steel sheet, which can then be subjected to cold rolling.
• ⁇ Grinding process In the grinding process, the surface of the steel sheet that has been subjected to the cold rolling process is ground to an average of 0.1 ⁇ m or more in the sheet thickness direction. By grinding the surface of the steel sheet to 0.1 ⁇ m or more, strong strain is introduced into the steel sheet. Therefore, recrystallization is promoted in the surface layer of the steel sheet structure during heating in the subsequent annealing process, and fine ferrite grains can be obtained in the outermost layer of the steel sheet. With regard to the amount of grinding of the steel sheet, it is more preferable to grind the surface of the steel sheet by 0.15 ⁇ m or more, from the viewpoint of the amount of strain introduced into the interior of the steel sheet.
• the amount of grinding of the steel plate ( ⁇ m) is calculated based on the change in the weight of the steel plate before and after grinding.
• the weight loss of the steel plate before and after grinding is divided by the area of the ground steel plate to obtain the weight loss (g) per m2 .
• the amount of grinding ( ⁇ m) is then calculated based on the value of an offline test previously performed under the same conditions (a value representing the relationship between the weight loss per m2 and the amount of grinding ( ⁇ m)).
• An example of the grinding means is grinding with a grinding brush at a predetermined rotation speed, reduction amount, and grinding speed.
• an example is grinding with a Hotani D-100 grinding brush at a rotation speed of 1000 to 1500 rpm, a reduction amount of 2.0 mm, and a grinding speed of about 100 mpm.
• the amount of grinding may be adjusted by performing grinding multiple times, about 2 to 10 times. It is preferable to grind the entire surface of the steel plate.
• the annealing process includes a heating process in which a steel sheet having a predetermined chemical composition (the same chemical composition as the steel sheet according to the present embodiment to be obtained) is heated to a predetermined annealing temperature (maximum heating temperature), a holding process in which the heated steel sheet is held at the annealing temperature for a certain period of time, and a cooling process in which the steel sheet is cooled from the annealing temperature to the predetermined temperature. From the viewpoint of productivity, it is preferable to anneal the steel sheet by passing it through a continuous annealing line.
• Heating process In the heating process, the steel sheet is heated to an annealing temperature.
• the heating rate is not particularly limited, and it is important to control the atmosphere described below.
• the dew point is set to -15°C to 20°C.
• the strain introduced in the grinding process promotes recrystallization, resulting in fine crystal grains in the outermost layer of the steel plate. This promotes the decarburization reaction, and the ferrite fraction in the outermost layer of the steel plate can be increased to 95% or more.
• the dew point in the furnace is less than ⁇ 15° C., a sufficient ferrite fraction cannot be obtained, so the dew point is set to ⁇ 15° C. or higher. From the viewpoint of obtaining a high ferrite fraction, the dew point is more preferably ⁇ 10° C. or higher. On the other hand, if the dew point exceeds 20°C, decarburization proceeds excessively, the hard phase in the surface layer of the base steel sheet is significantly reduced, and coarse oxides are formed in the refined layer, resulting in deterioration of coating adhesion and powdering properties. For this reason, the dew point is set to 20°C or less.
• the steel sheet After heating to the annealing temperature under the above conditions, the steel sheet is held at the specified maximum heating temperature for 5 seconds or more. If the holding time is less than 5 seconds, it is not possible to sufficiently secure austenite, which will later become a hard phase. There is no particular upper limit on the holding time. However, if the holding time is too long, the manufacturability of the steel sheet is hindered, so from the viewpoint of cost, the holding time is preferably less than 500 seconds. From the viewpoint of sufficiently securing austenite at low cost, the holding time is more preferably about 10 to 120 seconds.
• the annealing temperature is set to 750°C or higher to ensure sufficient austenite, which later becomes the hard phase.
• the annealing temperature is set to 1000°C or less.
• the annealing temperature is preferably 900°C or less.
• the dew point may be set to -15°C to 20°C during the holding process.
• annealing may be performed without setting the dew point at -15°C to 20°C, and a process of heating the steel sheet at a dew point of -15°C to 20°C may be provided separately from the annealing process before or after the annealing process.
• a quenching step and a tempering step may be performed after the holding step in the annealing step. This can reduce the fresh martensite fraction and further suppress the development of microcracks.
• the steel plate heated and held in the annealing process is cooled and quenched so that the temperature of the steel plate is 300° C. or less.
• the temperature of the steel plate is 300° C. or less.
• the average cooling rate during quenching is 0.4°C/s or more to obtain a hard layer.
• the ferrite fraction of the surface layer (5 to 20 ⁇ m) can be made less than 90%. Excessive softening of the surface layer not only leads to a decrease in bending strength, but also makes it difficult to ensure the strength of the steel plate.
• the steel plate that has been cooled and held in the quenching process is tempered so that the temperature of the steel plate is 150°C or higher.
• the conditions for tempering are a tempering holding time of 2 seconds or more. There is no particular upper limit, but since the effect saturates, it is preferable to set the tempering holding time to 500 seconds or less. In order to obtain sufficiently soft tempered martensite, it is preferable to temper the steel plate so that the temperature is 200°C or higher. Also, in order to ensure strength, it is more preferable to temper the steel plate so that the temperature is 500°C or higher.
• the quenching process is carried out following the annealing process, but it is also possible to cool the steel plate to a predetermined temperature after the holding period of the annealing process, and then heat the steel plate again to carry out the quenching process.
• a plating process may be performed between the annealing process and the quenching process, between the quenching process and the tempering process, or after the tempering process.
• the plating process may be performed as a part of the quenching process.
• a plating process may be performed after the tempering process. If a plating process is performed after the tempering process, the above-mentioned plating layer may be formed by electroplating.
• the surfaces of these cold-rolled steel sheets were ground (grinding step).
• the amounts of the steel sheet surfaces ground are shown in Tables 3A and 3B.
• the steel sheet was heated to the annealing temperature at the dew point shown in Tables 3A and 3B and held at that temperature (annealing step).
• the conditions controlled during annealing are shown in Tables 3A and 3B.
• hot-dip plating was performed under the conditions shown in Tables 3A and 3B (type of plating (GA: alloyed hot-dip galvanizing, GI: hot-dip galvanizing), steel sheet temperature before plating, alloying temperature), and a plating layer was formed on the cold-rolled steel sheet after annealing.
• type of plating GA: alloyed hot-dip galvanizing
• GI hot-dip galvanizing
• the ferrite fraction and average in-plane grain size of ferrite were measured for the obtained steel sheets within 2 ⁇ m from the surface of the steel sheets using the following method.
• the structure at the quarter thickness position was observed by the following method. A sample was taken with a cross section parallel to the rolling direction and thickness direction of the steel sheet as the observation surface, and the observation surface was polished and etched with nital. Next, a total of five visual fields were observed with a field emission scanning electron microscope at a magnification of 5000 times, with each visual field being 250 ⁇ m2 or more, in a range of 1/8 to 3/8 thickness centered at a position of 1/4 of the thickness of the steel sheet from the surface of the steel sheet (a position of 1/8 of the thickness of the steel sheet from the surface of the steel sheet to a position of 3/8 of the thickness of the steel sheet from the surface of the steel sheet).
• the area ratios of ferrite, retained austenite, martensite, bainite, pearlite, and cementite were measured.
• the area fraction of ferrite was (V ⁇ )
• the area fraction of retained austenite was (V ⁇ )
• the area fraction of bainite was (VB)
• the area fraction of fresh martensite was (VfM)
• the area fraction of tempered martensite was (VtM).
• Pearlite and cementite were treated as other structures, and their total values were recorded.
• each phase was carried out as follows: Areas that had a substructure within the grains and had multiple carbide long side directions when observed with a scanning electron microscope were judged to be tempered martensite. Areas where cementite precipitated in a lamellar form were judged to be pearlite or cementite. Areas with low brightness and no visible substructure were judged to be ferrite. Areas with high brightness and no visible substructure due to etching were judged to be fresh martensite or retained austenite. The remainder was judged to be bainite. The area ratio of each tissue was calculated by the point counting method. In the point counting method, measurements were made at 300 or more measurement points per visual field. The calculated values in each visual field were arithmetically averaged to obtain the area ratio of each tissue.
• the area fraction of fresh martensite was calculated by subtracting the area fraction of retained austenite calculated by the EBSD method described above from the area fraction of fresh martensite or retained austenite. The sum of this and the area fraction of tempered martensite calculated by the point counting method was taken as the area fraction of martensite.
• the area ratio of the retained austenite at the 1/4 thickness position was evaluated by performing high-resolution crystal structure analysis by the EBSD method.
• a sample was taken from a cross section parallel to the rolling direction and thickness direction of the steel sheet as an observation surface, and the observation surface was polished to a mirror finish, and electrolytic polishing or mechanical polishing using colloidal silica was performed to remove the processed surface layer.
• crystal structure analysis was performed on five visual fields by the EBSD method, with a magnification of 5000 times and a size of one visual field of 250 ⁇ m2 or more. The step distance was 0.01 to 0.20 ⁇ m.
• the data obtained by the EBSD method was analyzed using "OIM Analysis 6.0" manufactured by TSL Co., Ltd. From the observation results at each position, the regions judged to be FCC iron were judged to be retained austenite, and the area ratio of each of the retained austenite at the 1/4 thickness position was calculated.
• V ⁇ ferrite fraction
• VfM fresh martensite fraction
• the carbon concentration of the steel sheets was measured using a glow discharge optical emission analyzer (GD-Profiler 2 manufactured by HORIBA Mfg. Co., Ltd.). After the sample surface was degreased and cleaned, continuous measurement of the carbon concentration was performed from the sample surface. After the measurement, the amount of thinning was measured with a micrometer, and the carbon concentration at each depth was obtained by assuming that the thinning occurred at a constant rate. The measurement time was set so that a steady portion with a carbon concentration of 50 ⁇ m or more was obtained. The steady portion was defined as a region with a fluctuation range of ⁇ 5% after noise removal. Based on the results of the above measurements, the region in the thickness direction of the steel plate where the C concentration is 20% to 90% was defined as the transition region.
• GD-Profiler 2 manufactured by HORIBA Mfg. Co., Ltd.
• the tensile strength of the steel sheets was measured by the following method.
• Yield stress (YS) A JIS No. 5 test piece was taken from the steel plate in a direction perpendicular to the rolling direction, and the yield stress was measured in accordance with JIS Z 2241:2011.
• Tensile strength (TS) A JIS No. 5 tensile test piece was taken from the rolling direction and the direction perpendicular to the thickness direction (width direction) of the steel plate, and a tensile test was performed in accordance with JIS Z 2241:2011 to measure the tensile strength (TS).
• Elongation (EL)) A JIS No. 5 test piece was taken from the steel plate in a direction perpendicular to the rolling direction, and the elongation of the steel plate was measured in accordance with JIS Z 2241:2011.
• ⁇ VDA bending test> A bending test was carried out in accordance with VDA 238-100, and bending performance was evaluated by assigning a score according to the bending angle of the VDA standard as follows: The test specimens were taken in a direction in which the bending ridge was parallel to the rolling direction. 0 point: bending angle ⁇ 120-TS x 0.05 1 point: bending angle ⁇ 120-TS ⁇ 0.05 2 points: bending angle ⁇ 120-TS ⁇ 0.04
• ⁇ 90° V-bending test> A 90° V-bending test was carried out in accordance with JIS Z 2248. The test piece had a rectangular shape of 30 mm x 150 mm. The bending performance was evaluated by assigning a score according to the limit r/t obtained in a 90° V-bend as follows, where t in the limit r/t is the plate thickness and r is the minimum bending radius at which no cracks occurred. 0 points: Limit r/t > 2.0 x (TS/1000) 1 point: limit r/t ⁇ 2.0 ⁇ (TS/1000) 2 points: limit r/t ⁇ 1.5 ⁇ (TS/1000)
• the steel plate disclosed herein has high strength, excellent bendability in terms of formability, and excellent bendability in terms of impact properties, making it highly applicable in industry.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)

Priority Applications (1)

Applications Claiming Priority (2)

Publications (1)

Family

ID=92590667

Family Applications (1)

Country Status (2)

Citations (9)

• 2024
  • 2024-03-01 WO PCT/JP2024/007819 patent/WO2024181566A1/ja active Application Filing
  • 2024-03-01 JP JP2025504010A patent/JPWO2024181566A1/ja active Pending

Patent Citations (9)

Also Published As

Similar Documents

Legal Events