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
  • 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
  • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
• • 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
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/22—Martempering
• • 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
  • C21D8/0221—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 characterised by the working steps
  • C21D8/0226—Hot rolling
• • 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
  • C21D8/0221—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 characterised by the working steps
  • C21D8/0236—Cold rolling
• • 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
  • C21D8/0247—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 characterised by the heat treatment
  • C21D8/0263—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 characterised by the heat treatment following hot rolling
• • 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
  • C21D8/0247—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 characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/004—Dispersions; Precipitations
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to steel plates, such as high-strength steel plates for cold press forming, which are used in automobiles and the like after undergoing cold press forming, components using such steel plates, and methods for manufacturing them.
• delayed fracture may occur due to increased residual stress within the parts or deterioration of the delayed fracture resistance properties of the steel plate itself.
• delayed fracture refers to a phenomenon in which, when a part is placed in a hydrogen intrusion environment while high stress is applied to the part, hydrogen penetrates into the steel plate that constitutes the part, reducing the interatomic bonding strength and causing localized deformation, resulting in microcracks, which then propagate and lead to fracture.
• Patent Document 1 describes a steel containing, in mass %, C: 0.13% to 0.40%, Si: 0.02% to 1.5%, Mn: 0.4% to 1.7%, P: 0.030%, S: 0.0002% to less than 0.0010%, sol.
• a high-strength steel plate having excellent resistance to delayed fracture, characterized in that it has a component composition containing Al: 0.01% or more and 0.20% or less, N: 0.0055% or less, O: 0.0025% or less, Nb: 0.002% or more and 0.035% or less, and Ti: 0.002% or more and 0.040% or less so as to satisfy formulas (1) and (2), with the balance being Fe and unavoidable impurities, and a steel structure in which the area ratio of martensite and bainite to the entire structure is 95% or more and 100% or less in total, the balance being composed of one or both of ferrite and retained austenite, the average grain size of prior austenite grains exceeding 5 ⁇ m, the following conditions are satisfied, and inclusion groups having a major axis length of 20 to 80 ⁇ m are present at 5 pieces/mm2 or less , and the tensile strength is 1320 MPa or more. [%Ti]+[%Nb]>0.007 (1) [%Ti]+
• Patent Document 2 also describes a steel containing, by mass%, C: 0.05 to 0.30%, Si: 2.0% or less (including 0%), Mn: more than 0.1% and 2.8% or less, P: 0.1% or less, S: 0.005% or less, N: 0.01% or less, Al: 0.01 to 0.50% or less, and one or more of Nb, Ti, and Zr, each of which is 0.01% or more in total, and [%C] - [%Nb] / 92.9 ⁇ 12 - [%
• the present invention discloses a high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and workability, characterized in that the steel sheet contains 50% or more (including 100%) of tempered martensite by area ratio, with the remainder being made of iron and unavoidable impurities, and has a structure with the remainder being made of ferrite, and the distribution state of precipitates in the tempered martensite is such that there are 20 or more precipitates with a circle equivalent diameter of 1 to 10 n
• the present invention has been made to solve these problems, and aims to provide steel plates and components with a tensile strength of 1470 MPa or more (TS ⁇ 1470 MPa) and excellent delayed fracture resistance, as well as methods for manufacturing the same.
• excellent delayed fracture resistance means that the material is judged to have excellent delayed fracture resistance based on the following evaluation.
• a rectangular test piece is taken from the obtained steel sheet (coil) at a position 1/4 of the coil width from the widthwise end, with a dimension of 100 mm in the direction perpendicular to the rolling and 30 mm in the rolling direction.
• the end surface on the long side having a length of 100 mm is cut out by shearing, and then while in the sheared state (without performing machining to remove burrs), it is bent so that the burrs are on the outer periphery of the bend.
• the test piece is fixed with bolts while maintaining the shape of the test piece at the time of bending.
• the clearance of the shear processing is 13%, and the rake angle is 1°.
• the bending processing is performed so that the tip bending radius is 10 mm and the angle of the inner apex of the bending is 90 degrees (V-bend).
• the punch used has a tip radius equal to the tip bending radius R and is U-shaped (the tip R portion is semicircular and the punch body has a thickness of 2R), and the die used has a corner R of 30 mm.
• the depth to which the punch pushes the steel plate is adjusted to form the tip bending angle (the angle at the inner side of the bending apex) to 90 degrees (V-shape).
• the test piece is clamped and tightened with a hydraulic jack so that the distance between the flange ends of the straight pieces during bending is the same as when they were bent (so as to cancel out the opening of the straight pieces due to springback), and the bolts are fastened in this state.
• the bolts are fixed through elliptical holes (minor axis 10 mm, major axis 15 mm) that have been provided in advance 10 mm inside from the short edge of the rectangular test piece.
• the present inventors have conducted extensive research to solve the above problems and have found that delayed fracture resistance can be significantly improved by satisfying all of the following conditions: i) The area ratio of martensite is 85% or more and less than 95%. ii) The area ratio of ferrite is 5% or more and 15% or less, and the average crystal grain size of ferrite is 10 ⁇ m or less. iii) The number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the following condition: A (pieces/ mm2 ) ⁇ 8.5 ⁇ 105 ⁇ [B] Here, [B] represents the B content (mass %).
• the present invention has been completed through further investigation based on the above findings, and the gist of the present invention is as follows. [1] In mass%, C: 0.15% or more and 0.45% or less, Si: 2.0% or less, Mn: 4.0% or less, P: 0.1% or less, S: 0.01% or less, sol. Al: 0.5% or less, N: 0.010% or less, B: Contains 0.0008% or more and 0.0100% or less, The balance is Fe and unavoidable impurities.
• the area ratio of martensite to the entire structure is 85% or more and less than 95%
• the area ratio of ferrite to the entire structure is 5% or more and 15% or less
• the steel has a steel structure in which the average grain size of ferrite is 10 ⁇ m or less
• [B] represents the B content (mass %).
• [2] The steel sheet according to [1], in which 50% or more of the ferrite is precipitated on prior austenite grain boundaries, in terms of area ratio.
• the component composition further includes, in mass%, Cu: 1.00% or less, Cr: 1.00% or less, Nb: 0.10% or less, Ti: 0.10% or less, V: 0.50% or less, Mo: 0.50% or less, Ni: 1.00% or less, Sb: 0.10% or less, Sn: 0.10% or less, As: 0.10% or less, Ta: 0.10% or less, Ca: 0.020% or less, Mg: 0.020% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, W: 0.020% or less, REM: 0.020% or less.
• Hot finish rolling is performed under conditions of a residence time at 900 to 1000°C of 20 seconds or more and 150 seconds or less and a finish rolling temperature of 850°C or more; Cooling is performed at an average cooling rate of 40° C./sec or more in the range from the finish rolling temperature to 650° C., Thereafter, the hot-rolled steel sheet is obtained by coiling the steel sheet at a coiling temperature of 650°C or less.
• the hot-rolled steel sheet is cold-rolled at a rolling reduction of 40% or more to obtain a cold-rolled steel sheet;
• the annealing temperature is set to 830 to 950 ° C., and the cold-rolled steel sheet is heated from 400 ° C.
• the present invention provides high-strength steel plates and components with excellent delayed fracture resistance, as well as methods for manufacturing the same.
• the steel sheet of the present invention has a composition containing, by mass%, C: 0.15% to 0.45%, Si: 2.0% or less, Mn: 4.0% or less, P: 0.10% or less, S: 0.01% or less, sol.Al: 0.50% or less, N: 0.01% or less, B: 0.0008% to 0.0100% or less, with the balance being Fe and unavoidable impurities, has a structure in which the area ratio of martensite to the entire structure is 85% to less than 95%, the area ratio of ferrite to the entire structure is 5% to 15%, the average grain size of ferrite is 10 ⁇ m or less, and the number density A of precipitates having a circle equivalent diameter of 500 nm or more satisfies the following formula (1).
• [B] represents the B content (mass %).
• C 0.15% or more and 0.45% or less C is contained to increase the strength of martensite and obtain a tensile strength of 1470 MPa or more (hereinafter also referred to as TS ⁇ 1470 MPa). Therefore, in order to obtain a desired TS, the C content is set to 0.15% or more. From the viewpoint of reducing the weight of automotive frame parts by increasing the strength, the C content is preferably 0.20% or more, more preferably 0.25% or more. On the other hand, if an excessive amount of C is added, the formation of ferrite is excessively suppressed, and the desired area ratio of ferrite cannot be obtained. Therefore, the C content is set to 0.45% or less. The C content is preferably 0.40% or less, and more preferably 0.35% or less.
• Si 2.0% or less Si suppresses the formation of film-like carbides when tempering at a temperature range of 200°C or more, and suppresses the decrease in strength and the deterioration of delayed fracture resistance.
• the Si content is preferably 0.10% or more, and more preferably 0.20% or more.
• the Si content is set to 2.0% or less (including 0%).
• the Si content is preferably 1.5% or less, and more preferably 1.0% or less.
• Mn 4.0% or less
• Mn is an element effective for improving the hardenability of steel.
• the Mn content is desirably 0.2% or more.
• the Mn content is preferably 0.5% or more.
• the Mn content is set to 4.0% or less.
• the Mn content is preferably 3.0% or less, and more preferably 1.7% or less.
• P 0.10% or less P segregates at grain boundaries and reduces grain boundary strength, which leads to deterioration of delayed fracture resistance. Therefore, the P content is set to 0.10% or less.
• the P content is preferably 0.05% or less, more preferably 0.02% or less, and further preferably 0.01% or less. There is no lower limit for the P content, but the lower limit that is currently industrially feasible is 0.002%. Therefore, the P content is preferably set to 0.002% or more.
• S 0.01% or less S forms coarse inclusions with Mn, which become the starting point of delayed fracture, leading to deterioration of delayed fracture resistance. Therefore, the S content is set to 0.01% or less.
• the S content is preferably 0.003% or less, more preferably 0.0015% or less, and further preferably 0.0008% or less. There is no lower limit, but the lower limit currently industrially feasible is 0.0002%. Therefore, the S content is preferably set to 0.0002% or more.
• Sol. Al 0.50% or less Al is contained to perform sufficient deoxidation and reduce inclusions in steel. Although there is no particular lower limit for sol. Al, in order to perform stable deoxidation, it is desirable to set the sol. Al content to 0.005% or more. The sol. Al content is more preferably 0.01% or more, and further preferably 0.02% or more. On the other hand, if the sol. Al content exceeds 0.50%, the cementite generated during coiling is difficult to dissolve in the annealing process, the number density A of the precipitates cannot be set within a desired range, and the delayed fracture resistance is significantly deteriorated. Therefore, the sol. Al content is set to 0.50% or less. The sol. Al content is preferably 0.20% or less, and more preferably 0.05% or less.
• N 0.010% or less N forms precipitates such as AlN, which become the starting point of delayed fracture, leading to deterioration of delayed fracture resistance.
• N when N exceeds 0.010%, the number density A of the precipitates cannot be set within the desired range, and delayed fracture resistance is significantly deteriorated. Therefore, the N content is set to 0.010% or less.
• the N content is preferably 0.005% or less.
• the lower limit that is currently industrially feasible is 0.0006%. Therefore, the N content is preferably set to 0.0006% or more.
• B 0.0008% or more and 0.0100% or less B is an element that improves the hardenability of steel, and has the effect of generating a predetermined area ratio of martensite even with a small Mn content.
• B segregates at grain boundaries to increase the bonding strength of the grain boundaries and suppresses the segregation of P that reduces grain boundary strength.
• the B content is set to 0.0008% or more.
• the B content is preferably 0.0015% or more, and more preferably 0.0020% or more.
• it has been found that excessive addition of B leads to the formation of Fe 23 (C, B) 6 and BN, which become the starting point of delayed fracture, and thus rather reduces the delayed fracture resistance.
• the B content is set to 0.0100% or less.
• the B content is preferably 0.0080% or less, and more preferably 0.0060% or less.
• the composition of the steel plate in the present invention contains the above-mentioned elemental elements as the basic components, with the balance being iron (Fe) and unavoidable impurities.
• the steel plate of the present invention has a composition containing the above-mentioned basic components, with the balance being iron (Fe) and unavoidable impurities.
• the component composition may contain the following. One or more selected from, by mass%, Cu: 1.00% or less, Cr: 1.00% or less, Nb: 0.10% or less, Ti: 0.10% or less, V: 0.50% or less, Mo: 0.50% or less, Ni: 1.00% or less, Sb: 0.10% or less, Sn: 0.10% or less, As: 0.10% or less, Ta: 0.10% or less, Ca: 0.020% or less, Mg: 0.020% or less, Zn: 0.020% or less, Co: 0.020% or less, Zr: 0.020% or less, W: 0.020% or less, REM: 0.020% or less.
• Cu 1.00% or less
• Cu has the effect of improving the corrosion resistance of the steel sheet, reducing hydrogen penetration into the steel sheet, and improving delayed fracture resistance.
• the Cu content is desirably 0.01% or more.
• the Cu content is preferably 0.05% or more, and more preferably 0.10% or more.
• the Cu content is set to 1.00% or less.
• the Cu content is preferably 0.50% or less, and more preferably 0.30% or less.
• Cr 1.00% or less
• Cr is an element effective in improving the hardenability of steel. Cr can be added to stably obtain a desired structure. Although the lower limit of the Cr content is not particularly specified, in order to obtain such an effect, the Cr content is desirably 0.01% or more. The Cr content is more preferably 0.05% or more, and further preferably 0.10% or more. On the other hand, if Cr is added in excess, the dissolution of cementite during annealing is delayed, and a large amount of undissolved cementite remains, making it impossible to set the number density A of precipitates within a desired range, and the delayed fracture resistance is deteriorated. Therefore, when Cr is contained, the Cr content is set to 1.00% or less. The Cr content is preferably 0.50% or less, and more preferably 0.30% or less.
• Nb 0.10% or less Nb forms fine precipitates such as NbC in steel, and has the effect of refining the prior austenite grain size through a pinning effect, thereby improving delayed fracture resistance.
• the Nb content is desirably 0.005% or more.
• the Nb content is preferably 0.01% or more.
• the Nb content is set to 0.10% or less.
• the Nb content is preferably 0.08% or less, and more preferably 0.06% or less.
• Ti forms fine precipitates such as TiC in steel, which refines the prior austenite grain size through a pinning effect, and has the effect of improving delayed fracture resistance.
• the Ti content is desirably 0.005% or more.
• the Ti content is preferably 0.01% or more.
• the Ti content is set to 0.10% or less.
• the Ti content is preferably 0.08% or less, and more preferably 0.06% or less.
• V 0.50% or less
• V has the effect of generating fine carbides containing V that become hydrogen trapping sites, thereby improving delayed fracture resistance.
• the prior austenite grain size is refined by the pinning effect, thereby improving delayed fracture resistance.
• the V content is desirably 0.003% or more.
• the V content is preferably 0.01% or more, and more preferably 0.03% or more.
• V content is set to 0.50% or less.
• the V content is preferably 0.20% or less, more preferably 0.10% or less, and further preferably 0.06% or less.
• Mo 0.50% or less Mo has the effect of generating fine carbides containing Mo that become hydrogen trapping sites, thereby improving delayed fracture resistance.
• the prior austenite grain size is refined by the pinning effect, thereby improving delayed fracture resistance.
• the Mo content is desirably 0.003% or more.
• the Mo content is preferably 0.01% or more, and more preferably 0.03% or more.
• Mo content is set to 0.50% or less.
• the Mo content is preferably 0.20% or less, and more preferably 0.10% or less.
• Ni 1.00% or less
• Ni has the effect of improving the corrosion resistance of the steel sheet, suppressing hydrogen penetration into the steel sheet, and improving delayed fracture resistance.
• Ni is also an element effective in improving the hardenability of the steel, and can be added to stably obtain a desired structure.
• the Ni content is desirably 0.01% or more.
• the Ni content is preferably 0.05% or more, and more preferably 0.10% or more.
• the Ni content is set to 1.00% or less.
• the Ni content is preferably 0.50% or less, and more preferably 0.30% or less.
• Sb 0.10% or less Sb suppresses oxidation and nitridation of the surface layer of the steel sheet, and contributes to increasing strength and improving delayed fracture resistance.
• the Sb content is desirably 0.002% or more.
• the Sb content is preferably 0.004% or more, and more preferably 0.006% or more.
• the Sb content is set to 0.10% or less.
• the Sb content is preferably 0.05% or less, and more preferably 0.02% or less.
• Sn 0.10% or less Sn suppresses oxidation and nitridation of the surface layer of the steel sheet, and contributes to increasing strength and improving delayed fracture resistance.
• the Sn content is desirably 0.002% or more.
• the Sn content is preferably 0.004% or more, and more preferably 0.006% or more.
• the Sn content is set to 0.10% or less.
• the Sn content is preferably 0.05% or less, and more preferably 0.02% or less.
• the As content is desirably 0.002% or more.
• the As content is preferably 0.004% or more, and more preferably 0.006% or more.
• the As content is set to 0.10% or less.
• the As content is preferably 0.05% or less, and more preferably 0.02% or less.
• Ta 0.10% or less Ta has the effect of increasing the strength of steel.
• the Ta content is desirably 0.002% or more.
• the Ta content is preferably 0.004% or more, and more preferably 0.006% or more.
• the Ta content is set to 0.10% or less.
• the Ta content is preferably 0.05% or less, and more preferably 0.02% or less.
• Ca 0.020% or less Ca reduces the starting points of delayed fracture by making the shape of sulfides spheroidal, thereby improving delayed fracture resistance.
• the Ca content is desirably 0.0002% or more.
• the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more.
• the Ca content is set to 0.020% or less.
• the Ca content is preferably 0.015% or less, and more preferably 0.010% or less.
• Mg 0.020% or less Mg reduces the starting points of delayed fracture by making the shape of sulfides spheroidal, thereby improving delayed fracture resistance.
• the Mg content is desirably 0.0002% or more.
• the Mg content is preferably 0.001% or more, and more preferably 0.003% or more.
• the Mg content is set to 0.020% or less.
• the Mg content is preferably 0.015% or less, and more preferably 0.010% or less.
• Zn 0.020% or less Zn improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the inclusion shape.
• the Zn content is desirably 0.001% or more.
• the Zn content is preferably 0.003% or more.
• the Zn content is set to 0.020% or less.
• the Zn content is preferably 0.015% or less, and more preferably 0.010% or less.
• Co 0.020% or less Co improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the inclusion shape.
• the Co content is desirably 0.001% or more.
• the Co content is preferably 0.003% or more.
• 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.
• Zr 0.020% or less Zr improves delayed fracture resistance by refining the prior austenite grain size and spheroidizing the inclusion shape.
• the Zr content is desirably 0.001% or more.
• the Zr content is preferably 0.003% or more.
• the Zr content is set to 0.020% or less.
• the Zr content is preferably 0.015% or less, and more preferably 0.010% or less.
• W 0.020% or less W improves delayed fracture resistance by refining the prior austenite grain size through the formation of precipitates.
• the W content is desirably 0.001% or more.
• the W content is preferably 0.003% or more.
• the W content is set to 0.020% or less.
• the W content is preferably 0.015% or less, and more preferably 0.010% or less.
• the REM content is desirably 0.0002% or more.
• the REM content is preferably 0.001% or more, and more preferably 0.003% or more.
• the REM content is set to 0.020% or less.
• the REM content is preferably 0.015% or less, and more preferably 0.010% or less.
• REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.
• the REM concentration in the present invention refers to the total content of one or more elements selected from the above-mentioned REM.
• the REM is not particularly limited, but is preferably La and/or Ce.
• the steel structure of the steel plate of the present invention has the following configuration.
• (Configuration 1) The area ratio of martensite to the entire structure is 85% or more and less than 95%, the area ratio of ferrite to the entire structure is 5% or more and 15% or less, and the average crystal grain size of ferrite is 10 ⁇ m or less.
• (Configuration 2) The number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the following formula (1).
• [B] represents the B content (mass %).
• (Configuration 1) Area ratio of martensite: 85% or more and less than 95%, area ratio of ferrite: 5% or more and 15% or less, average grain size of ferrite: 10 ⁇ m or less
• the area ratio of martensite is 85% or more.
• the area ratio of martensite is preferably 88% or more. It has also been found that excellent delayed fracture resistance can be obtained by forming martensite as the main phase and ferrite with an area ratio of 5% to 15%.
• the area ratio of martensite is less than 95% and the area ratio of ferrite is 5% or more.
• the area ratio of martensite is preferably less than 93%, and the area ratio of ferrite is preferably 7% or more.
• the area ratio of ferrite is set to 15% or less.
• the area ratio of ferrite is preferably 12% or less.
• the morphology of ferrite is also controlled in order to obtain the effect of improving delayed fracture resistance by 5% to 15% ferrite. If the ferrite is coarse, the effect of improving delayed fracture resistance by ferrite cannot be obtained, and the delayed fracture resistance is rather deteriorated by the generation of ferrite.
• the average crystal grain size of ferrite is set to 10 ⁇ m or less.
• the average crystal grain size of ferrite is preferably 6 ⁇ m or less, and more preferably 4 ⁇ m or less.
• the average crystal grain size of ferrite is preferably 0.5 ⁇ m or more, and more preferably 1.0 ⁇ m or more.
• the balance is preferably composed of bainite, retained austenite (residual ⁇ ), and pearlite. Other than these structures, trace amounts of carbides, sulfides, nitrides, and oxides may be used.
• the area ratio of the balance structure is 10% or less, preferably 5% or less, and more preferably 3% or less.
• the balance structure may be 0%, i.e., the steel structure may consist of martensite and ferrite. Martensite also includes martensite that has undergone self-tempering during continuous cooling, and also includes martensite that has not undergone tempering by residence at approximately 150° C. or higher for a certain period of time.
• the number density A of the precipitates is preferably A (particles/mm 2 ) ⁇ 6.5 ⁇ 10 5 ⁇ [B], and more preferably A (particles/mm 2 ) ⁇ 5.0 ⁇ 10 5 ⁇ [B].
• the lower limit of A is not particularly limited, and A may be 0, or A (pieces/mm 2 ) ⁇ 0.5 ⁇ 10 5 ⁇ [B].
• (Configuration 3) In terms of area ratio, 50% or more of the ferrite is ferrite precipitated on prior austenite grain boundaries.
• the main methods for dispersing ferrite in martensite include holding the material in the two-phase region of austenite and ferrite and then quenching to transform the austenite to martensite, and holding the material in the single-phase region of austenite and then holding it in the ferrite formation temperature region to form ferrite, and then quenching to transform the remaining austenite to martensite.
• the ferrite formed by holding in the austenite single phase region and then holding in the ferrite formation temperature region is called proeutectoid ferrite, and since it is formed preferentially on the grain boundaries, it suppresses grain boundary fracture, and is particularly effective in improving delayed fracture resistance.
• proeutectoid ferrite By making ferrite precipitated on the prior austenite grain boundaries 50% or more of the total ferrite amount, a significant improvement effect on delayed fracture resistance can be obtained. Therefore, it is desirable that the area ratio of ferrite precipitated on the prior austenite grain boundaries to the total ferrite amount is 50% or more.
• the area ratio of ferrite precipitated on the prior austenite grain boundaries to the total ferrite amount is 70% or more, and more preferably, the area ratio of ferrite precipitated on the prior austenite grain boundaries to the total ferrite amount is 90% or more.
• the area ratio of ferrite precipitated on the prior austenite grain boundaries to the total ferrite amount may be 100%.
• the method for measuring each component in the above steel structure will be described below.
• the area ratios of martensite, bainite and ferrite are measured by polishing an L-section of a steel plate (a section parallel to the rolling direction and perpendicular to the steel plate surface (hereinafter also referred to as a perpendicular section parallel to the rolling direction)) and corroding it with nital, observing four fields of view in a range of 50 ⁇ m ⁇ 65 ⁇ m at a magnification of 2000 times with an SEM at a position 1 ⁇ 4 thickness from the steel plate surface, and performing image analysis on the photographed structure.
• martensite and bainite refer to structures that appear gray or white in SEM.
• ferrite is a region that appears in black contrast in SEM. Note that martensite and bainite contain trace amounts of carbides, nitrides, sulfides and oxides inside, but since it is difficult to exclude these, the area ratio of the region including these is taken as the area ratio.
• bainite has the following characteristics. That is, it has an aspect ratio of 2.5 or more, has a plate-like form, and is a slightly black structure compared to martensite.
• the width of the above plates is 0.3 to 1.7 ⁇ m.
• the distribution density of carbides with diameters of 10 to 200 nm inside bainite is 0 to 3 pieces/ ⁇ m2 .
• the surface 200 ⁇ m of the steel plate is chemically polished with oxalic acid, and the plate surface is measured using the X-ray diffraction intensity method. Calculations are made from the integrated intensities of the (200) ⁇ , (211) ⁇ , (220) ⁇ , (200) ⁇ , (220) ⁇ , and (311) ⁇ diffraction peaks measured using Mo-K ⁇ radiation.
• the L-section of the steel plate (a vertical section parallel to the rolling direction) is polished and then corroded with nital, and 10 fields of view are observed in an area of 50 ⁇ m x 65 ⁇ m at a magnification of 2000 times using an SEM at a position 1/4 thickness from the steel plate surface, and the microstructure photographs taken are subjected to image analysis to determine the circle equivalent diameter.
• the circle equivalent diameter refers to the diameter of a perfect circle having the area of a region determined to be ferrite from the SEM photograph. The value obtained by dividing the sum of the above diameters in the 10 fields of view by the number of ferrite particles whose diameters were measured is regarded as the average grain size of ferrite.
• the method for measuring the area ratio of ferrite precipitated on the prior austenite grain boundaries is as follows. First, the L-section of the steel sheet is mirror-polished by colloidal silica vibration polishing, and then electron backscatter diffraction (EBSD) measurement is performed to obtain local crystal orientation data for 10 fields of view at a 1/4 thickness position from the steel sheet surface. At this time, the step size is 0.10 ⁇ m, and the measurement area is 50 ⁇ m ⁇ 50 ⁇ m. After cleanup processing using analysis software: OIM Analysis 7, the obtained local orientation data is analyzed to determine the prior austenite grain boundaries. The clean-up process involves replacing the orientation and CI value of pixels with a CI value of 0.2 or less with that of the neighboring pixel with the highest CI value.
• EBSD electron backscatter diffraction
• the cleanup process is performed using the Neighbor CI Correlation function of the analysis software, with the parameter Minimum Coefficient of ...
• a map of grain boundaries having a crystal orientation difference with a rotation angle of 20° or more and 50° or less is created, and these interfaces are determined to be prior austenite grain boundaries.
• the L-section of the steel sheet is corroded with nital, and the same 10 fields of view as those for EBSD are photographed with an SEM at a magnification of 3000.
• the photographed SEM image is superimposed on the grain boundary map of prior austenite in the same field of view, the ferrite present on the prior austenite grain boundaries is identified, and the area ratio is measured.
• the number density A of precipitates having a circle equivalent diameter of 500 nm or more is obtained by polishing the L cross section (vertical cross section parallel to the rolling direction) of the steel sheet, and then continuously photographing a 2 mm 2 region with an SEM in the region from the 1/5 position to the 4/5 position of the sheet thickness of the steel sheet, that is, the region from the 1/5 position of the sheet thickness from the surface of the steel sheet to the 4/5 position, sandwiching the center of the sheet thickness, and counting the number of such precipitates from the photographed SEM photographs.
• the magnification of the photograph is 2000 times. When performing a component analysis of each inclusion particle, each inclusion particle is magnified 10000 times to analyze the above precipitates.
• the precipitates having a circle equivalent diameter of 500 nm or more are precipitates containing B such as Fe 23 (C, B) 6 , and the presence or absence of a peak of B is examined by elemental analysis by energy dispersive X-ray spectroscopy (EDS) with an acceleration voltage of 3 kV, and when a peak of B is present, it is evaluated that the above precipitates are present.
• the circle equivalent diameter refers to the diameter of a perfect circle having the area of each precipitate calculated from an SEM photograph.
• the steel plate of the present invention can obtain extremely excellent delayed fracture resistance by having a tensile strength of 1800 MPa or less. Furthermore, even if the steel plate of the present invention has a tensile strength of more than 1800 MPa and not more than 1900 MPa, the area ratio of ferrite precipitated on prior austenite grain boundaries to the total ferrite is 50% or more, so that the steel plate of the present invention can obtain extremely excellent delayed fracture resistance.
• the tensile strength can be measured by cutting a JIS No. 5 tensile test piece at the 1/4 position of the coil width so that the longitudinal direction is perpendicular to the rolling direction, and conducting a tensile test in accordance with JIS Z2241 (2022).
• the steel sheet of the present invention may be a steel sheet having a plating layer on the surface.
• the plating layer may be Zn plating or plating of another metal. It may also be either a hot-dip plating layer or an electroplating layer.
• the method for producing a steel sheet of the present invention comprises holding a steel slab having the above-mentioned composition at a heating holding temperature of 1100°C or more at the slab surface temperature for 30 minutes or more, then performing hot finish rolling under conditions of a residence time at 900 to 1000°C of 20 seconds to 150 seconds and a finish rolling temperature of 850°C or more, cooling at an average cooling rate of 40°C/second or more in the range from the finish rolling temperature to 650°C, and then coiling at a coiling temperature of 650°C or less to obtain a hot rolled steel sheet, cold rolling the hot rolled steel sheet at a rolling reduction of 40% or more to obtain a cold rolled steel sheet, setting an annealing temperature to 830 to 950°C, heating the cold rolled steel sheet from 400°C to the annealing temperature at an average heating rate of 1.0°C/second or more, holding the annealing temperature for 10 seconds to 600 seconds,
• the temperature specified in each step refers to the surface temperature of the slab (steel slab) or steel plate.
• Hot rolling Heating holding temperature slab surface temperature of 1100°C or more Heating holding time (slab heating holding time): 30 minutes or more
• the steel slab is held at a heating holding temperature (slab heating holding temperature) of 1100°C or more at the slab surface temperature for 30 minutes or more to promote solid solution of precipitates such as B-based precipitates, and reduce the size and number of precipitates.
• the heating holding temperature is preferably 1150°C or more.
• the heating holding temperature is preferably 1250°C or less.
• the time for which the slab is held at the heating temperature (holding time (slab heating holding time)) is preferably 40 minutes or more.
• the holding time is preferably 50 minutes or less.
• Residence time at 900 to 1000°C 20 seconds or more and 150 seconds or less
• the slab is retained at 900 to 1000°C for 20 seconds or more and 150 seconds or less.
• Increasing the residence time in the temperature range of 900 to 1000°C generates precipitates mainly composed of BN, which coarsen the precipitates.
• the precipitates generated in these temperature ranges are difficult to dissolve by annealing heating, and the amount of dissolved B after annealing is reduced.
• the residence time exceeds 150 seconds, it is not possible to obtain an amount of dissolved B effective in suppressing delayed fracture. Therefore, the residence time is 150 seconds or less, preferably 120 seconds or less, and more preferably 100 seconds or less.
• the retention time is less than 20 seconds, the texture may become non-uniform. Therefore, the retention time is 20 seconds or more.
• the retention time is preferably 30 seconds or more.
• Finish rolling temperature 850° C. or higher
• the finish rolling temperature (FT) is set to 850° C. or higher in order to suppress non-uniformity of the hot rolled texture.
• the finish rolling temperature is preferably 870° C. or higher.
• the finish rolling temperature is preferably 930° C. or lower.
• Average cooling rate (first average cooling rate) in the range from the finish rolling temperature to 650°C: 40°C/sec or more In cooling after hot finish rolling, cooling is performed with an average cooling rate of 40°C/sec or more in the range from the finish rolling temperature to 650°C. In the temperature range from the finish rolling temperature to 650°C, B segregates to grain boundaries and Fe23 (C,B) 6 precipitates as austenite recrystallizes. In order to suppress the precipitation of Fe23 (C,B) 6 as much as possible, the average cooling rate (first average cooling rate) is set to 40°C/sec or more. The average cooling rate is preferably 60°C/sec or more.
• the average cooling rate is preferably 500° C./sec or less, and more preferably 300° C./sec or less.
• the average cooling rate in the hot rolling process is "(temperature at the start of cooling (finish rolling temperature) (°C) - temperature at the end of cooling (°C) (650°C)) / cooling time from the start of cooling to the end of cooling (seconds)".
• Winding temperature 650°C or less After cooling to 650°C as described above, the steel sheet is further cooled as necessary before winding. At this time, if the winding temperature exceeds 650°C, the precipitation of Fe23 (C,B) 6 is promoted, and the delayed fracture resistance property is deteriorated. Therefore, the winding temperature is set to 650°C or less. Preferably, the winding temperature is 600°C or less. Also, the winding temperature is preferably 500°C or more.
• Cold rolling Reduction 40% or more If the reduction (cumulative reduction (cold rolling)) in cold rolling is 40% or more, the recrystallization behavior and texture orientation in the subsequent continuous annealing can be stabilized. If the reduction is less than 40%, some of the austenite grains during annealing may become coarse, resulting in a decrease in strength. In addition, the reduction is preferably 80% or less.
• Continuous annealing Average heating rate from 400° C. to the annealing temperature 1.0° C./sec or more
• the steel sheet is annealed and tempered in a continuous annealing line (CAL), and further subjected to temper rolling as necessary.
• CAL continuous annealing line
• Fe 23 (C, B) 6 is generated in the ferrite region during annealing and coarsens, it is necessary to increase the average heating rate from 400° C. to the annealing temperature in order to reduce Fe 23 (C, B) 6 and fully obtain the effect of grain boundary strengthening by B.
• the average heating rate at 400° C. or higher is 1.0° C./sec or higher.
• the annealing temperature is preferably 1.5° C./sec or higher, more preferably 3.0° C./sec or higher.
• the above average heating rate is preferably 10° C./sec or lower.
• the average heating rate here is "annealing temperature (° C.) - 400 (° C.) (described below) / heating time (minutes) from 400° C. to the annealing temperature".
• Annealing temperature 830 to 950°C Soaking time (holding time at annealing temperature): 10 seconds or more and 600 seconds or less
• annealing is performed at a high temperature for a long time.
• the annealing temperature needs to be 830°C or higher.
• the annealing temperature is preferably 840°C or higher, and more preferably 850°C or higher.
• annealing at a temperature higher than 950° C. causes the prior austenite grain size to become coarse, and the grain size of the ferrite formed thereafter also becomes coarse.
• the annealing temperature is set to 950° C. or less.
• the annealing temperature is preferably 900° C. or less.
• an excessively long soaking time also leads to coarsening of the prior austenite grain size, and coarsening of the average grain size of ferrite. Therefore, the soaking time is set to 600 seconds or less.
• the soaking time is preferably 540 seconds or less, and more preferably 480 seconds or less.
• the soaking time is set to 10 seconds or more, preferably 30 seconds or more, and more preferably 60 seconds or more.
• Average cooling rate from annealing temperature to Ar 3 point (second average cooling rate): 10° C./sec or more Average cooling rate from Ar 3 point to (Ar 3 point - 80° C.) (third average cooling rate): 1 to 10° C./sec Average cooling rate from (Ar 3 point - 80° C.) to a cooling stop temperature of 260° C.
• cooling is performed from the annealing temperature to Ar 3 point (° C.) at an average cooling rate of 10° C./sec or more (second average cooling rate), cooling from Ar 3 point (° C.) to (Ar 3 point - 80° C.) at an average cooling rate of 1 to 10° C./sec (third average cooling rate), and cooling from (Ar The material is cooled from the third point (-80°C) to a cooling stop temperature of 260°C or lower at an average cooling rate of 10°C/sec or higher (fourth average cooling rate).
• the second average cooling rate is "(annealing temperature (° C.)-Ar 3 point (° C.))/cooling time from the annealing temperature to Ar 3 point (seconds)".
• the third average cooling rate is "(Ar 3 point (° C.)-(Ar 3 point-80) (° C.))/cooling time (seconds) from Ar 3 point (° C.) to (Ar 3 point-80) (° C.)”.
• the fourth average cooling rate is "((Ar 3 point - 80) (°C) - cooling stop temperature (°C) below 260°C) / cooling time (seconds) from (Ar 3 point - 80°C) to a cooling stop temperature below 260°C”.
• the element symbols represent the content (mass%) of each element. Elements that are not contained are calculated as 0 (zero).
• the average cooling rate from the annealing temperature to the Ar 3 point is set to 10° C./sec or more.
• the second average cooling rate is preferably 20° C./sec or more, and more preferably 30° C./sec or more.
• the second average cooling rate is preferably 150° C./sec or less, and more preferably 100° C./sec or less.
• the average cooling rate from the Ar 3 point to (Ar 3 point - 80°C) (third average cooling rate) exceeds 10°C/sec, ferrite is not sufficiently generated and the delayed fracture resistance deteriorates. Therefore, the average cooling rate from the Ar 3 point to (Ar 3 point - 80°C) (third average cooling rate) is set to 10°C/sec or less. On the other hand, if the average cooling rate from the Ar 3 point to (Ar 3 point - 80°C) (third average cooling rate) is less than 1°C/sec, ferrite will be excessively generated, so the average cooling rate from the Ar 3 point to (Ar 3 point - 80°C) (third average cooling rate) is set to 1°C/sec or more.
• the third average cooling rate is preferably 3°C/sec or more, more preferably 5°C/sec or more.
• the average cooling rate (fourth average cooling rate) from (Ar 3 point - 80°C) to the cooling stop temperature of 260°C or less is less than 10°C/sec, a large amount of bainite, ferrite and pearlite are generated, and sufficient strength and delayed fracture resistance cannot be obtained. Therefore, the average cooling rate from (Ar 3 point - 80°C) to 260°C or less is set to 10°C/sec or more.
• the fourth average cooling rate is preferably 70°C/sec or more.
• the fourth average cooling rate is more preferably 100°C/sec or more, and further preferably 200°C/sec or more.
• the fourth average cooling rate is preferably 1000° C./sec or less, and more preferably 750° C./sec or less.
• the cooling stop temperature should be 260°C or less.
• the cooling stop temperature is preferably 250°C or less, and more preferably 240°C or less.
• Holding temperature 150-260°C Holding time: 20 to 1500 seconds
• the carbides distributed inside the martensite are carbides that are formed during holding in the low temperature range after quenching. Excellent delayed fracture resistance and tensile strength of 1470 MPa or more (TS ⁇ 1470 MPa) are ensured. To achieve this, it is necessary to control the holding temperature to 150 to 260° C. and the holding time to 20 to 1500 seconds. If the holding temperature is lower than the lower limit of 150° C. or the holding time is short, the distribution density of carbides within the transformed phase becomes insufficient, resulting in a deterioration in delayed fracture resistance.
• the holding temperature is higher than the upper limit of 260° C., the coarsening of carbides within grains and at block grain boundaries becomes significant, and there is a risk of the delayed fracture resistance being deteriorated. If the holding time exceeds 150 to 260° C., the coarsening of carbides within the grains and at the block grain boundaries becomes significant, and the delayed fracture resistance may deteriorate.
• the holding temperature is preferably 250° C. or less, and more preferably 240° C. or less.
• the retention time is preferably 50 seconds or more, and more preferably 100 seconds or more.
• the retention time is preferably 1300 seconds or less, more preferably 1000 seconds or less.
• the steel sheet obtained in this manner can be subjected to skin pass rolling in order to stabilize press formability, such as by adjusting the surface roughness and flattening the sheet shape.
• the skin pass elongation is preferably 0.1% or more.
• the skin pass elongation is preferably 1.0% or less.
• the obtained steel sheet may also be subjected to a plating treatment. That is, after continuous annealing, a plating treatment may be performed on the surface of the steel sheet. By performing the plating treatment, a steel sheet having a plating layer on the surface is obtained.
• the delayed fracture resistance of high-strength cold-rolled steel sheets is significantly improved, and the use of high-strength steel sheets contributes to improving component strength and reducing weight.
• the thickness of the steel sheets of the present invention is preferably 0.5 mm or more. In addition, the thickness is preferably 2.0 mm or less.
• the member of the present invention is obtained by subjecting the steel plate of the present invention to at least one of forming and joining processes.
• the manufacturing method of the member of the present invention also includes a step of subjecting the steel plate of the present invention to at least one of forming and joining processes to form the member.
• the steel plate of the present invention has a tensile strength of 1470 MPa or more and has excellent delayed fracture resistance. Therefore, members obtained using the steel plate of the present invention also have high strength and are superior in delayed fracture resistance compared to conventional high-strength members. Furthermore, the use of the members of the present invention makes it possible to reduce weight. Therefore, the members of the present invention can be suitably used, for example, for vehicle body frame parts.
• general processing methods such as pressing can be used without restrictions.
• general welding methods such as spot welding and arc welding, riveting, crimping, etc. can be used without restrictions.
• a slab having each component composition was held at a heating holding temperature shown in Table 2 for a heating holding time shown in Table 2, then subjected to a residence time at 900 to 1000°C shown in Table 2, hot finish rolling at a finish rolling temperature shown in Table 2, cooling at a first average cooling rate shown in Table 2, and coiling at a coiling temperature shown in Table 2 to obtain a hot rolled steel sheet. Thereafter, the hot-rolled steel sheets were cold-rolled at the reduction ratios (cold rolling reduction ratios) shown in Table 2 to obtain cold-rolled steel sheets.
• the cold-rolled steel sheet was heated to the annealing temperature shown in Table 2 at the average heating rate shown in Table 2, held for the soaking time shown in Table 2, and then cooled to 260°C or less under the cooling conditions shown in Table 2 (second average cooling rate, third average cooling rate, fourth average cooling rate, cooling stop temperature), reheated as necessary, and subjected to continuous annealing at the holding temperature shown in Table 2 for the holding time shown in Table 2.
• the obtained steel sheet was electroplated to obtain a steel sheet with a Zn plating layer formed.
• the metal structure of the obtained steel sheets was quantified by the method described above, and further, a tensile test and a delayed fracture resistance evaluation test were performed. Specifically, the tissue measurements were performed as follows. The area ratios of martensite, bainite, and ferrite were measured by polishing the L-section (vertical section parallel to the rolling direction) of the steel sheet, corroding it with nital, observing four fields of view in a range of 50 ⁇ m ⁇ 65 ⁇ m at a magnification of 2000 times with an SEM at a position of 1/4 thickness from the steel sheet surface, and performing image analysis on the photographed structure.
• martensite and bainite refer to structures that are gray or white in SEM. Bainite has the following characteristics.
• it has an aspect ratio of 2.5 or more, has a plate-like form, and is a slightly black structure compared to martensite.
• the width of the above plates is 0.3 to 1.7 ⁇ m.
• the distribution density of carbides with diameters of 10 to 200 nm inside bainite is 0 to 3 pieces/ ⁇ m2 .
• ferrite is a region that exhibits black contrast in SEM. Martensite and bainite contain small amounts of carbides, nitrides, sulfides, and oxides inside, but since it is difficult to exclude these, the area ratio of the region including these was used as the area ratio.
• the surface 200 ⁇ m of the steel plate was chemically polished with oxalic acid, and the plate surface was subjected to X-ray diffraction intensity measurement. Calculations were made from the integrated intensity of the (200) ⁇ , (211) ⁇ , (220) ⁇ , (200) ⁇ , (220) ⁇ , and (311) ⁇ diffraction peaks measured using Mo-K ⁇ radiation.
• the average grain size of ferrite was obtained by polishing the L-section of the steel plate (a vertical section parallel to the rolling direction) and then etching it with nital, and observing 10 fields of view in an area of 50 ⁇ m x 65 ⁇ m at a magnification of 2000 times using an SEM at a position 1/4 of the thickness from the steel plate surface, and performing image analysis of the photographed structure to determine the circle equivalent diameter.
• the method for measuring the area ratio of ferrite precipitated on the prior austenite grain boundaries is as follows. First, the L-section of the steel sheet was mirror-polished by colloidal silica vibration polishing, and then electron backscatter diffraction (EBSD) measurement was performed to obtain local crystal orientation data for 10 fields of view at a 1/4 thickness position from the steel sheet surface. At this time, the step size was 0.10 ⁇ m, and the measurement area was 50 ⁇ m ⁇ 50 ⁇ m. After cleanup processing using analysis software: OIM Analysis 7, the obtained local orientation data was analyzed to determine the prior austenite grain boundaries. The clean-up process involved replacing the orientation and CI value of pixels with a CI value of 0.2 or less with that of the neighboring pixel with the highest CI value.
• EBSD electron backscatter diffraction
• cleanup processing was performed using the Neighbor CI Correlation function of the analysis software under the condition that the parameter Minimum Coefficient Index was set to 0.2.
• a map of grain boundaries having a crystal orientation difference with a rotation angle of 20° or more and 50° or less was created, and these interfaces were determined to be prior austenite grain boundaries.
• the L-section of the steel sheet was corroded with nital, and the same 10 fields of view as those for EBSD were photographed with an SEM at a magnification of 3000.
• the photographed SEM image was superimposed on the grain boundary map of prior austenite in the same field of view, the ferrite present on the prior austenite grain boundaries was identified, and the area ratio was measured.
• an indentation was made in advance with a Vickers hardness tester at the observation position on the steel plate to serve as a mark for the observation position.
• the number density A of precipitates having a circle equivalent diameter of 500 nm or more was obtained by polishing the L cross section (vertical cross section parallel to the rolling direction) of the steel sheet, and then continuously photographing a 2 mm 2 region with an SEM in the region from the 1/5 position to the 4/5 position of the sheet thickness of the steel sheet, that is, the region from the 1/5 position of the sheet thickness from the surface of the steel sheet to the 4/5 position, sandwiching the center of the sheet thickness, and counting the number of such precipitates from the photographed SEM photograph.
• the magnification of the photograph was 2000 times.
• each inclusion particle was magnified 10000 times to analyze the above precipitates.
• the precipitates having a circle equivalent diameter of 500 nm or more are precipitates containing B such as Fe 23 (C, B) 6 , and the presence or absence of a peak of B was examined by elemental analysis by energy dispersive X-ray spectroscopy (EDS) with an acceleration voltage of 3 kV, and when a peak of B was present, it was evaluated that the above precipitates were present.
• B such as Fe 23 (C, B) 6
• EDS energy dispersive X-ray spectroscopy
• tensile test For the tensile test, a JIS No. 5 tensile test piece was cut out at 1/4 of the coil width so that the direction perpendicular to the rolling was the longitudinal direction, and a tensile test (in accordance with JIS Z2241 (2022)) was carried out to evaluate the tensile strength TS. A tensile strength TS of 1470 MPa or more was deemed to have passed.
• the delayed fracture resistance was evaluated as follows. A strip test piece was taken from the width direction of the obtained steel plate (coil) at a 1/4 position of the coil width, with a rolling transverse direction of 100 mm and a rolling direction of 30 mm. The cut-out of the end face on the long side with a length of 100 mm was sheared, and the sheared state (without machining to remove burrs) was bent so that the burrs were on the bending outer periphery side, and the test piece shape at the time of bending was maintained and fixed with a bolt. The shearing clearance was 13%, and the rake angle was 1°.
• the bending was performed with a tip bending radius of 10 mm and an angle of 90 degrees (V bending) on the inside of the bending apex.
• the punch had a tip radius of 10 mm and a U-shape (the tip R part was semicircular and the punch body had a thickness of 2R) that was the same as the tip bending radius R, and the die had a corner R of 30 mm.
• the punch was then adjusted to a depth into which the steel plate was pressed, and the bending angle at the tip (the angle at the inner side of the bending apex) was 90 degrees (V-shaped).
• the test piece was clamped and tightened with a hydraulic jack so that the distance between the flange ends of the straight piece during bending was the same as when the straight piece was bent (so as to cancel the opening of the straight piece due to springback), and the bolts were fastened in that state.
• the bolts were passed through an elliptical hole (minor axis 10 mm, major axis 15 mm) that was previously provided 10 mm inside from the short edge of the strip test piece and fixed.
• the obtained bolted test piece was immersed in a solution in which a 0.1% by mass aqueous solution of ammonium thiocyanate and a McIlvaine buffer solution were mixed in a mass ratio of 1:1 and the pH was adjusted to 8.0 and a solution in which the pH was adjusted to 7.4, respectively, to carry out a delayed fracture resistance evaluation test.
• the temperature of the solution was 20°C, and the amount of liquid per 1 cm3 of the surface area of the test piece was 20 ml.
• the presence or absence of visually visible cracks (length 1 mm or more) was confirmed, and those in which no cracks were observed in the solution adjusted to pH 8.0 were judged to have excellent delayed fracture resistance.
• the steel plates within the scope of the present invention had high strength and excellent delayed fracture resistance.
• the steel sheets within the scope of the present invention the steel sheets having a tensile strength of 1800 MPa or less were evaluated as "A (pass)" and were particularly excellent in terms of delayed fracture resistance.
• the steel plate in which the area ratio of ferrite precipitated on prior austenite grain boundaries to the total ferrite was 50% or more was evaluated as " ⁇ (pass)" in terms of delayed fracture resistance, and was therefore particularly excellent.
• at least one of the tensile strength and the delayed fracture resistance was insufficient.
• the components obtained by forming and joining the steel plates of the present invention have high strength and excellent resistance to delayed fracture, similar to the steel plates of the present invention, because the steel plates of the present invention have high strength and excellent resistance to delayed fracture.

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• 2024
  • 2024-01-25 EP EP24750124.0A patent/EP4632098A4/en active Pending
  • 2024-01-25 KR KR1020257024406A patent/KR20250126110A/ko active Pending
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