Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C18/00—Alloys based on zinc

Definitions

• the present invention relates to steel plates, components, and methods for manufacturing them.
• Patent Document 1 discloses a high-strength steel sheet having excellent workability and a manufacturing method thereof.
• Patent Document 2 discloses a high-strength cold-rolled steel sheet and a manufacturing method thereof.
• Patent Documents 1 and 2 both have a tensile strength TS of less than 1180 MPa, and stretch flangeability and toughness are not taken into consideration.
• the present invention was made in consideration of these circumstances, and aims to provide steel plates, components, and methods for manufacturing the same that have high strength with a tensile strength TS of 1180 MPa or more and excellent stretch flangeability and toughness.
• high strength means that the tensile strength TS measured in accordance with JIS Z2241 (2011) is 1180 MPa or more.
• excellent stretch flangeability means that the hole expansion ratio measured in accordance with JIS Z 2256 (2010) is 30% or more.
• excellent toughness means that the brittle-ductile transition temperature is -40°C or lower in a Charpy impact test conducted in accordance with JIS Z2242 (2016).
• martensite or bainite As the main phase, if the area ratio of ferrite is 1% or less, the total of martensite and bainite is 95% or more, and retained austenite is 5% or less, the generation of voids at the interface between the soft ferrite and the hard martensite or bainite is suppressed, improving stretch flangeability.
• Nb refines the prior austenite grain size to 10 ⁇ m or less, improving toughness.
• B segregates at the prior austenite grain boundaries, strengthening the grain boundaries and improving toughness.
• toughness is sometimes improved slightly and sometimes improved greatly, and the conditions for greatly improving toughness were investigated in more detail.
• B can segregate non-uniformly on prior austenite grain boundaries or uniformly, and it was found that toughness is greatly improved when it segregates at a uniform concentration.
• the variation in B concentration within the same prior austenite grain boundary was focused on, and toughness was greatly improved if this variation was less than 0.010% by mass.
• the inventors have investigated means for uniformly segregating B on the prior austenite grain boundaries, and have found that B can be uniformly segregated by undergoing a process of segregating B on the grain boundaries twice.
• B segregates on the austenite grain boundaries, but at this point, the diffusion of B is insufficient, and the segregated B is non-uniform.
• this steel is cooled once to form a martensite and bainite structure, and then annealed a second time, an austenite reverse transformation structure is formed.
• austenite exists after the first annealing and cooling, it is used as a nucleus to form an austenite structure with the same crystal orientation as in the first annealing. Furthermore, martensite and bainite contain a large amount of dislocations, and B that is dissolved in them diffuses rapidly to the austenite grain boundaries through the dislocations during the second annealing, so that B segregates uniformly. In order to form residual austenite as a nucleus before the second annealing, it is preferable to distribute carbon to the untransformed structure by partial quenching and distribution treatment after the first annealing to stabilize the austenite.
• the present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows. [1] In mass%, C: 0.10% or more and 0.30% or less, Si: 0.20% or more and 1.20% or less, Mn: 2.5% or more and 4.0% or less, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.01% or less, Ti: 0.100% or less, The steel sheet has a composition containing Nb: 0.002% or more and 0.050% or less, and B: 0.0015% or more and 0.0040% or less, satisfying the following formula (1), with the balance being Fe and unavoidable impurities: The sum of the area ratios of martensite and bainite is 95% or more, The area ratio of retained austenite is 5% or less, The area ratio of ferrite is 1% or less, The prior austenite grain size is 10 ⁇ m or less, The C concentration at the prior austenite grain boundary is
• the component composition further includes, in mass%, V: 0.100% or less, Mo: 0.500% or less, Cr: 1.00% or less, Cu: 1.00% or less, Ni: 0.50% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.200% or less, W: 0.400% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Co: 0.020% or less, REM: 0.0200% or less, Te: 0.020% or less,
• the steel sheet is cooled, heated to a second reheating temperature of 70 ° C. or more and 200 ° C.
• a method for producing a steel sheet [6] The method for producing a steel sheet according to [5], further comprising a plating step of plating the steel sheet after the second annealing step and before the second reheating step. [7] The method for producing a steel sheet according to [6], further comprising an alloying step of subjecting the steel sheet to an alloying treatment after the plating step. [8] A method for manufacturing a component, comprising a step of subjecting the steel plate according to any one of [1] to [3] above to at least one of forming and joining to form a component.
• the present invention can provide steel plates and components with a tensile strength TS of 1180 MPa or more, high strength, and excellent stretch flangeability and toughness, as well as methods for manufacturing the same.
• the steel plate according to the present embodiment contains, in mass%, C: 0.10% or more and 0.30% or less, Si: 0.20% or more and 1.20% or less, Mn: 2.5% or more and 4.0% or less, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.01% or less, Ti: 0.100% or less, Nb: 0.002% or more and 0.050% or less, and B: 0.0015% or more and 0.0040% or less, and satisfies the following formula (1), with the balance being Fe and unavoidable impurities.
• the sum of the area ratios of martensite and bainite is 95% or more, the area ratio of retained austenite is 5% or less, the area ratio of ferrite is 1% or less, the prior austenite grain size is 10 ⁇ m or less, the C concentration at the prior austenite grain boundary is 1.5 times or more the C content in the steel, the B concentration at the prior austenite grain boundary is 0.05% or more by mass%, and the variation in the B concentration at the prior austenite grain boundary within the same grain boundary is less than 0.010% by mass. ([%N]/14)/([%Ti]/47.9) ⁇ 1.0...Formula (1) In formula (1), [%N] and [%Ti] respectively represent the N and Ti contents (mass%) in the steel.
• C 0.10% or more and 0.30% or less C has the effect of strengthening the martensite-bainite structure.
• C has the effect of segregating to the prior austenite grain boundaries and improving toughness by the reheating treatment after the second annealing process. If the C content is less than 0.10%, the area ratio of martensite and bainite decreases, and a tensile strength TS (hereinafter sometimes simply referred to as TS) of 1180 MPa or more cannot be obtained. Therefore, the C content is set to 0.10% or more.
• the C content is preferably set to 0.11% or more.
• the C content is set to 0.30% or less.
• the C content is preferably 0.28% or less.
• Si 0.20% or more and 1.20% or less Si is an element effective for solid solution strengthening, and needs to be contained in an amount of 0.20% or more. Therefore, the Si content is set to 0.20% or more. The Si content is preferably set to 0.50% or more. On the other hand, since Si is a ferrite stabilizing element, if the content exceeds 1.20%, ferrite is generated, and strength, stretch flangeability and toughness are deteriorated. Therefore, the Si content is set to 1.20% or less. The Si content is preferably set to 1.10% or less.
• Mn 2.5% or more and 4.0% or less Mn is effective for improving hardenability. If the Mn content is less than 2.5%, the area ratio of martensite and bainite decreases, resulting in a decrease in strength. Therefore, the Mn content is set to 2.5% or more. The Mn content is preferably set to 2.8% or more. On the other hand, if the Mn content exceeds 4.0%, the segregated portion becomes excessively hard and the toughness decreases. Therefore, the Mn content is set to 4.0% or less, and preferably, the Mn content is set to 3.5% or less.
• P 0.050% or less P segregates at prior austenite grain boundaries and reduces toughness, so the P content is set to 0.050% or less.
• the P content is preferably set to 0.025% or less.
• S 0.020% or less S segregates at prior austenite grain boundaries and reduces toughness, so the S content is set to 0.020% or less.
• the S content is preferably set to 0.018% or less.
• the S content is more preferably set to 0.0040% or less, and further preferably set to 0.0020% or less.
• There is no particular lower limit for the S content but since a content of less than 0.0001% increases the production costs, the S content is preferably 0.0001% or more.
• Al 0.10% or less
• Al is an element that acts as a deoxidizer, and in order to obtain such an effect, the Al content is preferably 0.005% or more.
• the Al content is set to 0.10% or less.
• the Al content is preferably set to 0.05% or less.
• N 0.01% or less N forms nitrides with Nb and B, reducing the effects of adding Nb and B. Therefore, the N content is set to 0.01% or less.
• the N content is preferably set to 0.006% or less. There is no particular lower limit, but from the viewpoint of manufacturing costs, the N content is preferably set to 0.0001% or more.
• Ti 0.100% or less Ti has the effect of fixing N in steel as TiN, suppressing the formation of BN and NbN, improving the effect of adding Nb and B, and improving toughness and stretch flangeability.
• the Ti content is preferably 0.005% or more.
• the Ti content is set to 0.100% or less, and preferably to 0.050% or less.
• Nb 0.002% or more and 0.050% or less Nb precipitates as a solid solution or fine carbides, and suppresses the growth of austenite grains during annealing. It can also refine the crystal grain size to complicate the fracture path and improve toughness. To obtain such effects, the Nb content is set to 0.002% or more. The Nb content is preferably set to 0.005% or more. On the other hand, if the Nb content exceeds 0.050%, not only does the effect saturate, but also coarse Nb carbides precipitate, lowering toughness. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably 0.040% or less.
• B 0.0015% or more and 0.0040% or less B has the effect of segregating at the prior austenite grain boundaries to increase the grain boundary strength and improve the toughness.
• the B content is set to 0.0015% or more.
• the B content is preferably set to 0.0016% or more.
• the B content is set to 0.0040% or less, and preferably to 0.0030% or less.
• the remainder other than the above-mentioned components is Fe and unavoidable impurities. Note that for the optional components described below, if the content is below the lower limit, the effect of the present invention is not impaired, so if these optional elements are contained below the lower limit, they are treated as unavoidable impurities.
• the steel plate according to this embodiment may further contain, by mass%, at least one element selected from V: 0.100% or less, Mo: 0.500% or less, Cr: 1.00% or less, Cu: 1.00% or less, Ni: 0.50% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.200% or less, W: 0.400% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Mg: 0.0200% or less, Co: 0.020% or less, REM: 0.0200% or less, Te: 0.020% or less, Hf: 0.10% or less, and Bi: 0.200% or less.
• V 0.100% or less V has the effect of forming fine carbides and increasing strength. If the V content exceeds 0.100%, coarse V carbides may precipitate, decreasing toughness. Therefore, when V is contained, the V content is set to 0.100% or less.
• the V content is preferably 0.080% or less, and more preferably 0.060% or less.
• the lower limit of the V content is not particularly limited and may be 0.000%, but since V has the effect of forming fine carbides and increasing strength, it is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.010% or more.
• Mo 0.500% or less Mo has the effect of improving hardenability and increasing the area ratio of bainite and martensite. If the Mo content exceeds 0.500%, the effect is saturated. Therefore, if Mo is contained, the Mo content is set to 0.500% or less.
• the Mo content is preferably 0.200% or less, and more preferably 0.150% or less.
• the lower limit of the Mo content is not particularly limited and may be 0.000%, but since Mo has the effect of improving hardenability and increasing the area ratio of bainite and martensite, it is preferably 0.010% or more, more preferably 0.020% or more, and further preferably 0.030% or more.
• Cr 1.00% or less Cr has the effect of improving hardenability and increasing the area ratio of bainite and martensite. If the Cr content exceeds 1.00%, the effect is saturated. Therefore, if Cr is contained, the Cr content is set to 1.00% or less.
• the Cr content is preferably 0.300% or less, and more preferably 0.250% or less.
• the lower limit of the Cr content is not particularly limited and may be 0.000%, but since it has the effect of improving hardenability and increasing the area ratio of bainite and martensite, it is preferably 0.01% or more, more preferably 0.015% or more, and further preferably 0.030% or more.
• Cu 1.00% or less
• Cu has the effect of increasing strength by solid solution.
• Cu also has the effect of improving delayed fracture resistance. If the Cu content exceeds 1.00%, grain boundary cracking is likely to occur. Therefore, when Cu is contained, the Cu content is set to 1.00% or less.
• the Cu content is preferably 0.60% or less, and more preferably 0.30% or less.
• the lower limit of the Cu content is not particularly limited and may be 0.000%, but since it has the effect of increasing strength by solid solution, it is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.05% or more.
• Ni 0.50% or less
• Ni has the effect of improving hardenability, but if the Ni content exceeds 0.50%, the effect is saturated. Therefore, when Ni is contained, the Ni content is set to 0.50% or less.
• the Ni content is preferably 0.20% or less, and more preferably 0.15% or less.
• the lower limit of the Ni content is not particularly limited and may be 0.00%, but since Ni has an effect of improving hardenability, it is preferably 0.01% or more, more preferably 0.02% or more, and further preferably 0.03% or more.
• Sb 0.200% or less
• Sb has the effect of suppressing surface oxidation, nitridation, and decarburization of the steel sheet, but if the Sb content exceeds 0.200%, the effect is saturated. Therefore, if Sb is contained, the Sb content is set to 0.200% or less.
• the Sb content is preferably 0.050% or less, and more preferably 0.020% or less.
• the lower limit of the Sb content is not particularly limited and may be 0.000%, but since Sb has the effect of suppressing surface oxidation, nitridation, and decarburization of the steel sheet, it is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more.
• Sn 0.200% or less Like Sb, Sn has the effect of suppressing surface oxidation, nitridation, and decarburization of the steel sheet. If the Sn content exceeds 0.200%, the effect is saturated. Therefore, if Sn is contained, the Sn content is set to 0.200% or less.
• the Sn content is preferably 0.050% or less, and more preferably 0.020% or less.
• the lower limit of the Sn content is not particularly limited and may be 0.000%, but since it has the effect of suppressing surface oxidation, nitridation, and decarburization of the steel sheet, it is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more.
• Ta 0.200% or less Ta has the effect of forming fine carbides and increasing strength. If the Ta content exceeds 0.200%, coarse Ta carbides may precipitate and the toughness may decrease. Therefore, if Ta is contained, the Ta content is set to 0.200% or less.
• the Ta content is preferably 0.100% or less, and more preferably 0.070% or less.
• the lower limit of the Ta content is not particularly limited and may be 0.000%, but since it has the effect of forming fine carbides and increasing strength, it is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.010% or more.
• W 0.400% or less W has the effect of forming fine carbides and increasing strength. If the W content exceeds 0.400%, coarse W carbides may precipitate and the toughness may decrease. Therefore, when W is contained, the W content is set to 0.400% or less.
• the W content is preferably 0.300% or less, and more preferably 0.250% or less.
• the lower limit of the W content is not particularly limited and may be 0.000%, but since it has the effect of forming fine carbides and increasing strength, it is preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.010% or more.
• Zr 0.0200% or less
• Zr has the effect of making the shape of inclusions spherical and suppressing stress concentration, thereby improving toughness. If the Zr content exceeds 0.0200%, a large amount of inclusions may be formed, resulting in a decrease in toughness. Therefore, when Zr is contained, the Zr content is set to 0.0200% or less.
• the Zr content is preferably 0.0150% or less, and more preferably 0.0100% or less.
• the lower limit of the Zr content is not particularly limited and may be 0.0000%, but since it has the effect of making the shape of inclusions spherical, suppressing stress concentration, and improving toughness, it is preferably 0.0001% or more.
• the Zr content is more preferably 0.0010% or more, and further preferably 0.0020% or more.
• Ca 0.0200% or less Ca can be used as a deoxidizer. If the Ca content exceeds 0.0200%, a large amount of Ca-based inclusions may be generated, and the toughness may decrease. Therefore, when Ca is contained, the Ca content is set to 0.0200% or less.
• the Ca content is preferably 0.0100% or less, and more preferably 0.0080% or less.
• the lower limit of the Ca content is not particularly limited and may be 0.0000%, but since it can be used as a deoxidizer, it is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.
• Mg 0.0200% or less Mg can be used as a deoxidizer. If the Mg content exceeds 0.0200%, a large amount of Mg-based inclusions may be generated, and the toughness may decrease. Therefore, if Mg is contained, the Mg content is set to 0.0200% or less.
• the Mg content is preferably 0.0100% or less, and more preferably 0.0080% or less.
• the lower limit of the Mg content is not particularly limited and may be 0.0000%, but since Mg can be used as a deoxidizer, it is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.
• Co 0.020% or less Co has the effect of increasing strength by solid solution strengthening. If the Co content exceeds 0.020%, the effect is saturated. Therefore, if Co is contained, the Co content is set to 0.020% or less.
• the Co content is preferably 0.015% or less, and more preferably 0.010% or less.
• the lower limit of the Co content is not particularly limited and may be 0.000%, but since Co has the effect of increasing strength by solid solution strengthening, it is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.005% or more.
• REM 0.0200% or less REM has the effect of making the shape of inclusions spherical and suppressing stress concentration, thereby improving toughness. If the REM content exceeds 0.0200%, a large amount of inclusions may be formed, resulting in a decrease in toughness. Therefore, when REM is contained, the REM content is set to 0.0200% or less.
• the REM content is preferably 0.0100% or less, and more preferably 0.0050% or less.
• the lower limit of the REM content is not particularly limited and may be 0.0000%, but since it has the effect of making the shape of inclusions spherical, suppressing stress concentration, and improving toughness, it is preferably 0.0001% or more, more preferably 0.0005% or more, and further preferably 0.0010% or more.
• REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoid elements with atomic numbers from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
• the REM content is the total content of one or more elements selected from the above REM.
• La, Ce, and Nd are preferably contained.
• Te 0.020% or less Te has the effect of making the shape of inclusions spherical and suppressing stress concentration, thereby improving toughness. If the Te content exceeds 0.020%, a large amount of inclusions may be formed, and toughness may decrease. Therefore, when Te is contained, the Te content is set to 0.020% or less.
• the Te content is preferably 0.015% or less, and more preferably 0.010% or less.
• the lower limit of the Te content is not particularly limited and may be 0.000%, but since it has the effect of making the shape of inclusions spherical, suppressing stress concentration, and improving toughness, it is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.004% or more.
• Hf 0.10% or less Hf has the effect of making the shape of inclusions spherical and suppressing stress concentration, thereby improving toughness. If the Hf content exceeds 0.10%, a large amount of inclusions is formed and the toughness decreases. Therefore, when Hf is contained, the Hf content is set to 0.10% or less.
• the Hf content is preferably 0.08% or less, and more preferably 0.05% or less.
• the lower limit of the Hf content is not particularly limited and may be 0.000%. However, since Hf has the effect of making the shape of inclusions spherical, suppressing stress concentration, and improving toughness, it is preferable to make it 0.01% or more.
• Bi 0.200% or less Bi has the effect of reducing segregation and improving bendability. If the Bi content exceeds 0.200%, a large amount of inclusions may be formed, resulting in a decrease in bendability. Therefore, when Bi is contained, the Bi content is set to 0.200% or less.
• the Bi content is preferably 0.100% or less, more preferably 0.050% or less.
• the Bi content is further preferably 0.010% or less, and even more preferably 0.005% or less.
• the lower limit of the Bi content is not particularly limited and may be 0.000%, but since Bi has the effect of reducing segregation and improving bendability, it is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.003% or more.
• Sum of area ratio of martensite and bainite 95% or more Both martensite and bainite are hard phases, and are necessary to achieve a TS of 1180 MPa or more. Therefore, the sum of the area ratios of martensite and bainite is set to 95% or more. The sum of the area ratios of martensite and bainite is preferably 96% or more. The upper limit of the sum of the area ratios of martensite and bainite is not particularly limited, and may be 100%.
• Area ratio of retained austenite 5% or less
• Retained austenite may be contained as a residual structure other than martensite and bainite. Therefore, the area ratio of retained austenite is set to 5% or less.
• the area ratio of retained austenite is preferably 4% or less.
• the area ratio of retained austenite may be 0% or more than 0%.
• Ferrite area ratio 1% or less If soft ferrite is present, voids are generated at the interface with the martensite and bainite of the parent phase, and stretch flangeability is reduced, so the ferrite area ratio is set to 1% or less.
• the ferrite area ratio may be 0%.
• the area ratio of each structure is measured as follows.
• the area ratio of the retained austenite is determined by chemically polishing the rolled surface of a test piece taken from each steel plate up to the t/4 position of the steel plate thickness, measuring the X-ray diffraction intensity and diffraction peak position of the polished surface with an X-ray diffraction (XRD) device, calculating the volume ratio, and using this value as the area ratio of the retained austenite.
• XRD X-ray diffraction
• SEM images of the observation surface are taken at a magnification of 2000 times with a field of view of 57.1 ⁇ m ⁇ 42.9 ⁇ m in three fields of view.
• the obtained SEM images are analyzed by image analysis to determine the total area ratio of martensite, bainite, and retained austenite, as well as the area ratio of structures other than martensite, bainite, and retained austenite (ferrite).
• the area ratios of martensite and bainite are calculated by subtracting the area ratio of retained austenite obtained by XRD from the area ratios of martensite, bainite, and retained austenite obtained by image analysis.
• the average value of the three fields of view is taken as the area ratio of the structure.
• Prior austenite grain size 10 ⁇ m or less
• the prior austenite grain size is set to 10 ⁇ m or less.
• Prior austenite is preferably 9 ⁇ m or less.
• the prior austenite grain size is preferably 1 ⁇ m or more.
• the prior austenite grain size is more preferably 2 ⁇ m or more, and even more preferably 3 ⁇ m or more.
• the grain size of the prior austenite grains is measured as follows. After polishing the plate thickness cross section parallel to the rolling direction of each steel plate, it is etched with picral to prepare the observation surface. On the observation surface, the microstructure at the plate thickness position t/4 is photographed with an SEM at a magnification of 2000 times with a field of view of 57.1 ⁇ m x 42.9 ⁇ m, and three fields of view are obtained to obtain SEM images. The grain size of each prior austenite grain is determined from the obtained structure image by image analysis, and the average value of the three fields of view is taken as the grain size of the prior austenite grains (average crystal grain size).
• C concentration at prior austenite grain boundaries 1.5 times or more the C content in steel
• C segregates at prior austenite grain boundaries to strengthen the grain boundaries and improve toughness. If the C concentration at the prior austenite grain boundaries is 1.5 times or more the C content in steel, the above effect can be obtained. Therefore, the C concentration at the prior austenite grain boundaries is set to 1.5 times or more the C content in steel. In other words, the C concentration at the prior austenite grain boundaries satisfies the following formula. C concentration (mass%) of prior austenite grain boundary/C content (mass%) in steel ⁇ 1.5
• the C concentration at the prior austenite grain boundaries is preferably 2.0 times or more, and more preferably 2.5 times or more, the C content in the steel.
• the C concentration is preferably less than 7% by mass, and more preferably 2% by mass or less.
• B concentration at prior austenite grain boundaries 0.05% or more by mass B can strengthen the grain boundaries and improve toughness by segregating to the prior austenite grain boundaries. If the B concentration at the prior austenite grain boundaries is 0.05% or more by mass, the above effect can be obtained. Therefore, the B concentration at the prior austenite grain boundaries is 0.05% or more by mass.
• the B concentration at the prior austenite grain boundaries is preferably 0.07% or more by mass, and more preferably 0.10% or more. There is no upper limit for the B concentration at the prior austenite grain boundaries, but in order to suitably prevent hard carboborides from precipitating on the grain boundaries and further improve toughness, it is preferably less than 6% by mass. More preferably, it is 2% or less by mass.
• Variation in B concentration of prior austenite grain boundaries within the same grain boundary less than 0.010% by mass
• the variation in B concentration of prior austenite grain boundaries within the same grain boundary is set to less than 0.010% by mass.
• the variation is preferably 0.009% or less by mass, and more preferably 0.008% or less by mass. The smaller the variation, the more preferable, but from the viewpoint of production technology, the variation should be 0.001% or more.
• the C concentration, B concentration and variation of the prior austenite grain boundary are measured as follows.
• a needle-shaped sample is prepared from a region including the prior austenite grain boundary by the SEM-FIB (Focused Ion Beam) method.
• the needle-shaped sample obtained is subjected to 3DAP analysis using a 3DAP device (LEAP4000XSi, manufactured by AMETEK). The measurement is performed in laser mode.
• the sample temperature is 80K or less.
• the C and B concentrations of the prior austenite grain boundary are obtained from the number of C ions, the number of B ions and the number of other ions detected from the prior austenite grain boundary.
• the C and B concentrations are the average values of the two samples.
• the prior austenite grain size is very large compared to the area sampled by the SEM-FIB method, so the grain boundaries targeted by one sampled sample are all the same grain boundaries.
• the prior austenite grain size is about 9 ⁇ m, while the sampled sample area is about 0.1 ⁇ m in diameter. Therefore, the variation in the obtained B concentration is the variation within the same grain boundary.
• a steel plate having a tensile strength TS of 1180 MPa or more.
• the tensile strength TS of the steel plate is preferably 1250 MPa or more.
• the above-mentioned steel sheet may have a plating layer on at least one side.
• the plating layer is preferably any one of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrolytic galvanized layer.
• the composition of the plating layer is not particularly limited and may be a known composition.
• the composition of the hot-dip galvanized layer is not particularly limited and may be any common one.
• the coating layer contains Fe: 20 mass% or less, Al: 0.001 mass% to 1.0 mass% and further contains one or more selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0 mass% to 3.5 mass%, with the balance being Zn and unavoidable impurities.
• the Fe content in the plating layer is less than 7 mass%, and when the plating layer is an alloyed hot-dip galvanized layer, in one example, the Fe content in the plating layer is 7 mass% or more and 15 mass% or less, more preferably 8 mass% or more and 13 mass% or less.
• the coating weight of the plating is not particularly limited, it is preferable that the coating weight per one side of the steel sheet is 20 to 80 g/m 2.
• the plating layer is formed on both the front and back sides of the steel sheet (high-strength cold-rolled steel sheet).
• the method for producing a steel sheet according to this embodiment includes a hot rolling process in which a steel slab having the above-mentioned composition is hot-rolled to obtain a hot-rolled sheet, a pickling process in which the hot-rolled sheet is pickled, a cold rolling process in which the hot-rolled sheet after the pickling process is cold-rolled to obtain a cold-rolled sheet, a first annealing process in which the cold-rolled sheet is heated to a first heating temperature of Ac3 point or higher, and a cooling rate of 50°C/s or more for the cold-rolled sheet after the first annealing process to a cooling stop temperature of 100°C or higher and lower than the Ms point.
• the slab heating temperature is preferably 1100°C or higher, and preferably 1300°C or lower, taking into consideration the rolling load and the generation of scale.
• the slab heating method is not particularly limited, and for example, it can be heated in a heating furnace according to a conventional method.
• the above-mentioned steel slab is hot-rolled to obtain a hot-rolled sheet.
• the hot rolling may be performed according to a conventional method.
• the cooling after hot rolling and it is cooled to the coiling temperature.
• the hot-rolled sheet is wound into a coil.
• the coiling temperature is preferably 400°C or higher. If the coiling temperature is 400°C or higher, the strength of the hot-rolled sheet does not increase and the coiling becomes easy. It is more preferable that the coiling temperature is 550°C or higher.
• the coiling temperature is preferably 750°C or lower. Note that the hot-rolled sheet may be heat-treated for the purpose of softening before pickling.
• the hot-rolled sheet is pickled in a pickling process.
• the scale of the hot-rolled sheet wound around the coil can be removed.
• the method for removing the scale is not particularly limited, but in order to completely remove the scale, it is preferable to perform pickling while uncoiling the hot-rolled coil.
• the pickling method is not particularly limited, and may be a conventional method.
• the hot rolled sheet is cold rolled to obtain a cold rolled sheet.
• the hot rolled sheet from which the scale has been removed is appropriately washed and then cold rolled to obtain a cold rolled sheet.
• the method of cold rolling is not particularly limited and may be a conventional method.
• First annealing step heating to a first heating temperature of Ac3 point or higher
• the cold-rolled sheet is heated to a first heating temperature of Ac3 point or higher, and annealed in the austenite single phase region. If the first heating temperature is lower than Ac3 point, ferrite is generated. This ferrite has reduced dislocations by annealing, and there are no dislocations that serve as a diffusion path for B (boron) during the second annealing, making it difficult to uniformly segregate boron.
• B boron
• the austenite grain size will become larger than 10 ⁇ m. Therefore, the first heating temperature is set to Ac3 point or higher.
• the first heating temperature is preferably Ac3 point + 10 ° C. or higher, more preferably Ac3 point + 20 ° C. or higher. Since the austenite structure formed in the second annealing has the same crystal structure as the austenite structure formed in the first annealing, the first heating temperature is preferably 980° C. or less so that the austenite grain size in the first annealing is 10 ⁇ m or less. The first heating temperature is more preferably 950° C. or less.
• the Ac3 point is calculated using the following formula.
• the cooling stop temperature of the partial quenching is less than 100°C, martensite transformation occurs before C distribution occurs, and a sufficient amount of residual austenite cannot be obtained before the second annealing. If the amount of residual austenite is insufficient, austenite with a different orientation from that in the first annealing is generated, and it is difficult to uniformly segregate B (boron) at the grain boundaries of such austenite. Therefore, the cooling stop temperature in the cooling step is set to 100°C or higher.
• the cooling stop temperature is preferably 120°C or higher, more preferably 150°C or higher.
• the cooling stop temperature in the cooling step is set to be lower than the Ms point, preferably Ms point -20°C or lower, more preferably Ms point -30°C or lower.
• the Ms point is calculated using the following formula.
• Ms (°C) 499-308 ⁇ [C]-10.8 ⁇ [Si]-32.4 ⁇ [Mn]-16.2 ⁇ [Ni]-27 ⁇ [Cr]-10.8 ⁇ [Mo] (In the above formula, [M] is the content (mass%) of element M in the steel sheet, and the value of an element that is not contained is 0 (zero).)
• the average cooling rate is set to 50°C/s or more.
• the average cooling rate is preferably 60°C/s or more, and more preferably 70°C/s or more.
• the upper limit of the average cooling rate is not particularly limited, but if the cooling rate is too high, it becomes difficult to control the cooling stop temperature, so the average cooling rate is preferably 1000° C./s or less, and more preferably 200° C./s or less.
• the average cooling rate (°C/s) in the cooling step is "(first heating temperature (°C))-(cooling stop temperature (°C))/(cooling time (seconds) from the first heating temperature (°C) to the cooling stop temperature (°C))".
• the first reheating temperature is set to 300° C. or higher.
• the first reheating temperature is preferably 310° C. or higher, and more preferably 320° C. or higher.
• the first reheating temperature exceeds 400°C, the untransformed austenite decomposes into cementite, and no residual austenite is formed before the second annealing. If the amount of residual austenite is insufficient, austenite with a different orientation from that in the first annealing is formed, and it is difficult to uniformly segregate B (boron) at the grain boundaries of such austenite. Therefore, the first reheating temperature is set to 400°C or less. The first reheating temperature is preferably 390°C or less, and more preferably 380°C or less.
• the holding time at the first reheating temperature (reheating holding time) is less than 60 seconds, carbon distribution is insufficient and no retained austenite is formed before the second annealing. If the amount of retained austenite is insufficient, austenite with a different orientation from that in the first annealing is formed, and it is difficult to uniformly segregate B (boron) at the grain boundaries of such austenite. Therefore, the holding time at the first reheating temperature is set to 60 seconds or more, preferably 80 seconds or more, and more preferably 100 seconds or more.
• the holding time at the first reheating temperature is preferably less than 900 seconds, more preferably 600 seconds or less.
• the mixture After being held at the first reheating temperature, the mixture is cooled to room temperature.
• the room temperature at which the cooling is stopped in the first reheating step is not particularly limited, but can be 5 to 50°C.
• the second heating temperature is set to the Ac3 point or higher.
• the second heating temperature is preferably the Ac3 point + 10°C or higher, more preferably the Ac3 point + 20°C or higher.
• the second heating temperature is preferably 980° C. or less, and more preferably 950° C. or less.
• the average cooling rate (°C/s) in this step is "(second heating temperature (°C))-(cooling stop temperature (°C))/(cooling time (seconds) from the second heating temperature (°C) to the cooling stop temperature (°C))".
• the steel is heated (reheated) to a second reheating temperature.
• C segregates to the prior ⁇ grain boundaries by reheating, improving toughness. If the second reheating temperature is less than 70°C, the diffusion of C is slow and C segregation is insufficient. Therefore, the second reheating temperature is set to 70°C or higher.
• the second reheating temperature is preferably 90°C or higher.
• the second reheating temperature is set to 200° C. or less.
• the second reheating temperature is preferably 190° C. or less. If the holding time at the second reheating temperature (reheating holding time) is less than 600 seconds, the diffusion of C is slow and C segregation is insufficient. Therefore, the holding time at the second reheating temperature is set to 600 seconds or more.
• the holding time at the second reheating temperature is preferably 800 seconds or more.
• the upper limit of the holding time at the second reheating temperature is not particularly limited, but in order to prevent precipitation of carboborides, the second reheating temperature is preferably 43,200 s or less (0.5 day) or less.
• a plating treatment may be performed on at least one side of the steel sheet in a plating step to obtain a steel sheet (high-strength plated steel sheet).
• the steel sheet high-strength plated steel sheet
• the steel sheet may be subjected to a heat treatment to alloy the plating layer of the steel sheet to obtain an alloyed plated steel sheet.
• the manufacturing conditions other than those mentioned above can be determined by conventional methods.
• the steel plate according to this embodiment obtained as described above preferably has a thickness of 0.5 mm or more. Also, it is preferable that the thickness is 2.0 mm or less.
• a member using at least a part of the above-mentioned steel plate can be provided.
• the above-mentioned steel plate can be formed into a desired shape by press working to form an automobile part.
• the automobile part may contain a steel plate other than the steel plate according to this embodiment as a material.
• a high-strength steel plate having a TS of 1180 MPa or more and excellent stretch flangeability and toughness can be provided, so that a member having a TS of 1180 MPa or more and excellent stretch flangeability and toughness can be provided.
• the steel plate according to this embodiment can be suitably used as an automobile part that contributes to weight reduction of the vehicle body.
• the steel plate according to this embodiment can be suitably used in general members used as frame structural parts or reinforcing parts among automobile parts.
• the method for manufacturing the above-mentioned component includes a step of subjecting the above-mentioned steel plate to at least one of forming and joining to form the component.
• the forming process may be performed by a general processing method such as pressing, without any restrictions.
• the joining process may be performed by a general welding method such as spot welding or arc welding, riveting, or crimping, without any restrictions.
• the obtained slab was reheated and hot-rolled, and then wound to obtain a hot-rolled coil (hot-rolled sheet).
• the hot-rolled coil was pickled while being unwound, and cold-rolled to obtain a cold-rolled sheet.
• the thickness of the hot-rolled sheet was 3.0 mm, and the thickness of the cold-rolled sheet was 1.2 mm.
• Annealing was performed under the conditions shown in Table 2 in a continuous hot-dip galvanizing line to obtain steel sheets (cold-rolled steel sheet (CR), hot-dip galvanized steel sheet (GI), and alloyed hot-dip galvanized steel sheet (GA)).
• the hot-dip galvanized steel sheet was immersed in a 460 ° C. plating bath, and the plating coverage was 35 g / m 2 per side.
• the galvannealed steel sheets were produced by adjusting the coating weight to 45 g/ m2 per side, and then performing an alloying treatment by holding at 520°C for 40 s.
• the obtained steel sheets, except for Steel Sheet No. 9, were subjected to a reheating treatment (second reheating step) under the conditions shown in Table 2.
• the steel sheets obtained were evaluated for the total area ratio of martensite and bainite, the area ratio of retained austenite, the area ratio of ferrite, the prior austenite grain size, the carbon concentration at the prior austenite grain boundaries, the boron concentration at the prior austenite grain boundaries, and the variation in the boron concentration at the prior austenite grain boundaries within the same grain boundaries, according to the methods described above.
• the tensile strength TS, stretch flangeability, and toughness were evaluated according to the methods described below. The results are shown in Table 3.
• the Charpy impact test was performed in accordance with JIS Z 2242 (2016). From the obtained steel plate, a test piece was taken with a width of 10 mm, a length of 55 mm, and a 90° V-notch with a notch depth of 2 mm at the center of the length so that the direction perpendicular to the rolling direction of the steel plate was the V-notch imparting direction. Then, a Charpy impact test was performed in a test temperature range of -120 to +120 ° C. A transition curve was obtained from the obtained brittle fracture surface ratio, and the temperature at which the brittle fracture surface ratio was 50% was determined as the brittle-ductile transition temperature.
• test piece measuring 100 mm wide x 100 mL was taken from a steel sheet (cold-rolled steel sheet or plated steel sheet) and a hole expansion test was conducted in accordance with JIS Z 2256 (2010).
• a 10 mm diameter hole was punched into the test piece with a clearance of 12 ⁇ 1%, and a conical punch with an apex angle of 60° was raised to expand the hole.
• the examples of the present invention have a tensile strength TS of 1180 MPa or more, and are excellent in stretch flangeability and toughness.
• the comparative examples are inferior in one or more of tensile strength TS, stretch flangeability, and toughness.
• the components obtained by forming the steel plate of the present invention, by joining the steel plate, and by further forming and joining the steel plate had high strength and excellent stretch flange formability and toughness, just like the steel plate of the present invention, because the steel plate of the present invention has high strength and excellent stretch flange formability and toughness.

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