Classifications

• • 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
• • 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

Definitions

• Patent Document 1 does not take into consideration internal cracks during bending.
• This disclosure has been made in consideration of the above problems, and aims to provide a steel plate having high strength, as well as excellent ductility, hole expansion property, and resistance to internal bending cracking, and a part using the same.
• the gist of the present invention is as follows.
• the chemical composition, in mass%, is C: 0.045-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, Ti: 0.020 to 0.180%, Al: 0.010-0.400%, P: 0 to 0.080%, S: 0 to 0.0100%, N: 0 to 0.0050%, O: 0 to 0.0100%, Nb: 0 to 0.100%, V: 0 to 1.000%, Cu: 0 to 1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-1.000%, B: 0 to 0.0100%, Ca: 0-0.0500%, Mg: 0 to 0.050%, REM: 0-0.1000%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0 to 0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0 to 0.500%, As
• the average aspect ratio of the prior austenite grains is 3.00 to 5.50;
• the area ratio of bainite is 70 to 95%,
• the area ratio of martensite is 5 to 30%,
• the chemical composition is, in mass%, Nb: 0.001 to 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 1.000%, 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 steel sheet according to [1] characterized in that it contains one or more of As: 0.001 to 0.050% and Sn: 0.001 to 0.050%.
• Cu 0-1.000% Cu exists in the form of fine particles in steel and has the effect of increasing the strength of the steel sheet.
• the Cu content may be 0%, but in order to obtain this effect, 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.
• the Cu content is preferably 0.800% or less, 0.600% or less, 0.400% or less, or 0.200% or less.
• Cr 0-2.000% Cr is an element effective for improving the strength of a steel sheet.
• the Cr content may be 0%, but in order to obtain this effect, 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, 1.000% or less, 0.600% or less, or 0.300% or less.
• B 0-0.0100% B has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet.
• the B content may be 0%, but in order to obtain this effect, 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 0-0.0500%
• Ca has the effect of dispersing a large number of fine oxides during deoxidation of molten steel and refining the structure of the 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 steel sheet.
• the Ca content may be 0%, but when obtaining these effects, 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, 0.0200% or less, 0.0150% or less, or 0.0100% or less.
• Mg 0-0.050% Mg has the effect of adjusting the shape of inclusions in steel to a preferred shape, thereby increasing the yield ratio of the steel sheet.
• the Mg content may be 0%, but in order to obtain this effect, the Mg content is preferably 0.001% or more.
• the Mg content is more preferably 0.005% or more, 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 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 ⁇ 0.100% Bi has the effect of increasing the yield ratio of the steel sheet by refining the solidification structure.
• the Bi content may be 0%, but in order to obtain this effect, 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 ⁇ 0.100% Ta has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel.
• the Ta content may be 0%, but in order to obtain this effect, 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.
• Zn 0-0.200% Zn has the effect of increasing the strength of the steel sheet by solid solution strengthening.
• the Zn content may be 0%, but in order to obtain this effect, the Zn content is preferably 0.001% or more.
• the Zn content is more preferably 0.005% or more, and 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 0-0.200% W has the effect of increasing the strength of the steel sheet by solid solution strengthening.
• the W content may be 0%, but in order to obtain this effect, the W content is preferably 0.001% or more.
• the W content is more preferably 0.005% or more, more preferably 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.
• the As content may be 0%, but in order to obtain this effect, the As content is preferably 0.001% or more.
• the As content is more preferably 0.005% or more, 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 0-0.050% Sn has the effect of suppressing the generation of oxides that are the starting point of fracture, thereby improving the ductility and hole expandability of the steel sheet.
• the Sn content may be 0%, but in order to obtain this effect, 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 steel plate according to this embodiment contains 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, scrap, or the manufacturing environment, and/or substances that are acceptable to the extent that they do not adversely affect the properties of the steel plate according to this embodiment.
• the chemical composition of the above-mentioned steel sheet is analyzed using a spark discharge optical emission spectrometer etc.
• C and S are analyzed using the combustion-infrared absorption method
• N is analyzed using the inert gas fusion-thermal conductivity method
• O is analyzed using the inert gas fusion-non-dispersive infrared absorption method.
• 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 steel plate according to this embodiment will be described.
• the average aspect ratio of prior austenite grains is 3.00 to 5.50
• the area fraction of bainite is 70 to 95%
• the area fraction of martensite is 5 to 30%
• the ratio of the sum of the amount of Ti and the amount of Nb analyzed as electrolytic extraction residue at a depth of 1/60 of the plate thickness from the surface to the sum of the amount of Ti and the amount of Nb analyzed as electrolytic extraction residue at a depth of 1/4 of the plate thickness from the surface is 1.5 or more.
• the region from 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface is, in other words, the 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 metal structure in this region is specified because the metal structure in this region represents a typical metal structure of a steel plate.
• the surface referred to here means the interface between the steel sheet and the plating layer, the coating film, or the like.
• the interface between the steel sheet and the plating layer or coating film, etc. is identified by a BSE image (specifically, a COMPO image (BSE compositional image)) obtained by the following method.
• a sample is cut out so that the cross section of the steel sheet through the sheet thickness can be observed.
• the cut out sample is mechanically polished and then mirror-finished.
• a scanning microscope is used to observe an area of 40,000 ⁇ m2 or more at a magnification of 400 times.
• the cross section is observed using a BSE image (specifically, a COMPO image), a clear difference in contrast is confirmed between the plating layer or coating film, etc. and the base steel (steel sheet). Therefore, the position where the contrast changes from the outermost surface can be identified as the interface between the steel sheet and the plating layer or coating film, etc.
• the interface is identified by a similar method.
• 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 value of the aspect ratio of the prior austenite grains.
• the average value of the aspect ratios of the two prior austenite grains is calculated as "(A1 ⁇ r1+A2 ⁇ r2)/(A1+A2)".
• the general formula can be expressed by the following formula: where ri is the "major axis/minor axis" of the i-th prior austenite grain, and Ai is the area of the i-th prior austenite grain.
• the rolling direction of the steel sheet is determined by the following method.
• a test piece is taken from an arbitrary position 50 mm or more away from the end of the steel plate so that the plate thickness cross section can be observed.
• the plate thickness cross section is observed at each magnification of 100x, 200x, 500x, and 1000x using an optical microscope.
• an observation result at an appropriate magnification at which the inclusion size can be measured is selected.
• the observation range is 500 ⁇ m or more in width and the range of the entire plate thickness, and an area with dark brightness is judged to be an inclusion. When observing, observation may be performed in multiple fields of view.
• Bainite is a structure consisting of fine crystal grains and carbides. If the area ratio of bainite is less than 70%, the desired ductility cannot be obtained in the steel plate. Therefore, the area ratio of bainite is set to 70% or more. The area ratio of bainite is preferably 75% or more, more preferably 80% or more. On the other hand, if the area fraction of bainite exceeds 95%, the steel plate cannot obtain the desired strength. Therefore, the area fraction of bainite is set to 95% or less. The area fraction of bainite is preferably 93% or less, 90% or less, or 87% or less.
• Area ratio of martensite 5 to 30% Martensite is a structure that increases the strength of a steel sheet. If the area ratio of martensite is less than 5%, the desired strength cannot be obtained. Therefore, the area ratio of martensite is set to 5% or more. The area ratio of martensite is preferably 7% or more, 10% or more, or 15% or more. On the other hand, if the area ratio of martensite exceeds 30%, the desired ductility cannot be obtained. Therefore, the area ratio of martensite is set to 30% or less. The area ratio of martensite is preferably 25% or less, more preferably 20% or less.
• the metal structure of the internal region of the steel plate according to the present embodiment may contain ferrite and pearlite as a remaining structure in addition to bainite and martensite.
• the area ratio of the remaining structure may be 0 to 5%.
• the area ratio of the remaining structure may be 3% or less, 2% or less, or 1% or less, or there may be no remaining structure, i.e., the area ratio of the remaining structure may be 0%.
• the total area ratio of bainite and martensite may be 95 to 100%.
• the total area ratio of bainite and martensite may be 97% or more, 98% or more, 99% or more, or 100%.
• the area ratios of bainite, martensite and the remaining structure are measured by the following method.
• a test piece is taken from the steel plate so that the metal structure can be observed at the 1/4 position of the plate thickness (in the range from the surface to the 1/8 position of the plate thickness in the plate thickness direction).
• the plate thickness cross section of the test piece is mirror-polished and LePera etched, and then an area of 200 ⁇ m (plate thickness direction) ⁇ 600 ⁇ m (direction perpendicular to the plate thickness direction) at the 1/4 position of the plate thickness is observed using a FE-SEM: thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL), and image analysis is performed.
• the total area ratio of martensite and retained austenite is obtained by calculating the area ratio of the uncorroded areas (i.e., the white areas).
• the area ratio of retained austenite can be obtained by X-ray diffraction.
• a test piece taken from a steel plate is ground from the plate surface to a 1/4 position of the plate thickness (a range from a position of 1/8 of the plate thickness to a position of 3/8 of the plate thickness from the surface in the plate 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 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 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
• ER 1/60 /ER 1/4 is set to 1.5 or more.
• ER 1/60 /ER 1/4 is preferably 1.8 or more or 2.0 or more.
• the upper limit of ER 1/60 /ER 1/4 is not particularly limited, but ER 1/60 /ER 1/4 may be 5.0 or less, 4.0 or less, or 3.5 or less.
• the steel sheet according to this embodiment may have a total elongation (total elongation at break) of 10.0% or more, and a hole expansion ratio of 50% or more.
• the total elongation is preferably 12.0% or more or 13.0% or more.
• the hole expansion ratio is preferably 55% or more, 60% or more, or 70% or more.
• a small rectangular piece having a parallel part of any width may be taken and a tensile test may be performed, and the tensile strength may be calculated from the maximum test force and the original cross-sectional area of the parallel part.
• the hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.
• the length of the crack is measured at the center of the plate width, on the inner side of the bend at the center of the bent part in a plane perpendicular to the bending axis and perpendicular to the plate surface.
• the test piece after bending is cut at the above plane, the cross section is mirror-polished, and the cracks are observed under an optical microscope, and the length of the crack observed on the inner side of the bend of the test piece is measured.
• the thickness of the steel plate according to the present embodiment is not particularly limited, but may be 1.2 to 8.0 mm. If the thickness of the steel plate 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 steel plate 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, 4.8 mm or less, or 3.6 mm or less.
• the 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 steel plate according to the present embodiment has high strength, excellent ductility and hole expandability, and is considered to have excellent crash resistance properties by suppressing the occurrence of internal bending cracks. Therefore, it can be suitably used for parts, particularly automobile parts. Among automobile parts, it can be suitably used for automobile suspension parts such as lower arms, trail links, and knuckles.
• the steel sheet according to this embodiment may be a hot-rolled steel sheet.
• a part manufactured using the steel plate according to this embodiment has the same chemical composition as the above-mentioned steel plate.
• the part may have a mixture of processed and unprocessed parts.
• the unprocessed part has the same metal structure as the above-mentioned steel plate.
• the processed part basically has the same metal structure as the above-mentioned steel plate, but if it has been heavily processed, it may not have the above-mentioned metal structure. Therefore, when measuring the metal structure of a part, the measurement is performed on the unprocessed part. If there is no unprocessed part, the measurement is performed on the part that has not been heavily processed.
• the steel plate according to this embodiment can be stably manufactured.
• the temperature of the slab and the temperature of the steel plate in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel plate, and are measured with a radiation thermometer.
• a preferred method for producing a steel sheet according to this embodiment is as follows: Rough rolling is started in a temperature range of 1050 to 1200 ° C., and the surface temperature rise width ⁇ T from 1 second to 10 seconds after the end of rough rolling is set to 150 ° C. or more, In the finish rolling, the total rolling reduction rate below 1050°C is 30 to 50%, After the finish rolling is completed, the steel sheet is cooled to a temperature range of 500 to 680 ° C.
• the slab to be subjected to rough rolling 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.
• continuous casting 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 present inventors have found that in order to favorably control the ratio (ER 1/60 /ER 1/4 ) between the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1/60 of the plate thickness from the surface and the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1/4 of the plate thickness from the surface, it is effective to strictly control the temperature history from the start of rough rolling to the start of finish rolling. Specifically, the present inventors have found that it is effective to increase the temperature difference between the surface layer and the interior after the start of rough rolling.
• the surface temperature rise width ⁇ T from 1 second to 10 seconds after the end of rough rolling is set to 150 ° C. or more.
• a person skilled in the art can make ⁇ T 150 ° C.
• the temperature 10 seconds after the end of rough rolling is 150 ° C. or more higher than the temperature 1 second after the end of rough rolling indicates that the temperature is sufficiently raised by reheating from the inside, and that the temperature difference between the surface layer and the inside is increased by cooling.
• ER 1/60 /ER 1/4 can be preferably controlled.
• the temperature 1 second and 10 seconds after the end of rough rolling are measured using a radiation thermometer installed between the rough rolling mill and the finishing rolling mill.
• the total reduction rate below 1050°C is set to 30-50%.
• the total reduction rate in the temperature range below 1050°C to 30-50% it is possible to effectively control the average aspect ratio of the prior austenite grains in the region from 1/8 of the plate thickness depth from the surface to 3/8 of the plate thickness depth from the surface.
• the total rolling reduction in the temperature range below 1050°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 1050° C and t1 is the outlet thickness of the last rolling in the temperature range below 1050°C.
• the average cooling rate is the temperature difference between the start and end points of the set range divided by the elapsed time from the start point to the end point.
• the steel sheet After the slow cooling is complete, it is preferable to cool the steel sheet so that the average cooling rate in the temperature range from the slow cooling completion temperature to 200°C is 30°C/s or more. By cooling under these conditions, the desired amount of martensite can be obtained. Cooling should be performed to a temperature range of 200°C or less, and may be, for example, 100°C or less. After cooling, the steel sheet is wound into a coil.
• 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.
• ⁇ T Surface temperature rise from 1 second to 10 seconds after the end of rough rolling ("surface temperature 10 seconds after the end of rough rolling” - “surface temperature 1 second after the end of rough rolling")
• Total rolling reduction total rolling reduction in finish rolling below 1050°C
• Average cooling rate average cooling rate in the temperature range from the end of slow cooling temperature to 200°C
• the average aspect ratio and metal structure of the prior austenite grains in the region from the surface to a depth of 1/8 of the plate thickness to a depth of 3/8 of the plate thickness from the surface the ratio (ER 1/60 /ER 1/4 ) of the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1/60 of the plate thickness from the surface to the total amount of Ti and Nb analyzed as electrolytic extraction residue at a depth of 1/4 of the plate thickness from the surface
• tensile strength (TS), total elongation (EL), hole expansion ratio ( ⁇ ), and crack length (L) within bending were determined by the above- mentioned method.
• TS tensile strength
• EL total elongation
• ⁇ hole expansion ratio
• L crack length
• the steel plate was deemed to have high strength and passed the test. On the other hand, if the tensile strength was less than 980 MPa, the steel plate was deemed to not have high strength and failed the test.
• the steel plate was deemed to have excellent ductility and passed the test. On the other hand, if the total elongation was less than 10.0%, the steel plate was deemed to not have excellent ductility and failed the test.
• the steel plate 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 steel plate was deemed to have poor hole expansion properties and was judged to have failed.
• the steel plate was deemed to have excellent resistance to internal cracking within bending and was judged to have passed. On the other hand, if the crack length within the bend was more than 50 ⁇ m, the steel plate was deemed to have poor resistance to internal cracking within bending and was judged to have failed.
• Tables 1 to 4B show that the steel plates according to the present invention have high strength, as well as excellent ductility, hole expansion property, and resistance to internal bending cracks.
• the lower arm (part) was manufactured by press processing.
• 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 Tables 4A and 4B.
• the above aspects of the present disclosure make it possible to provide a steel plate having high strength, as well as excellent ductility, hole expansion property, and resistance to internal bending cracking, and a part using the same.

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