US20240344161A1 - Cold-rolled steel sheet, method for manufacturing same, and welded joint - Google Patents

Cold-rolled steel sheet, method for manufacturing same, and welded joint Download PDF

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US20240344161A1
US20240344161A1 US18/682,465 US202218682465A US2024344161A1 US 20240344161 A1 US20240344161 A1 US 20240344161A1 US 202218682465 A US202218682465 A US 202218682465A US 2024344161 A1 US2024344161 A1 US 2024344161A1
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steel sheet
cold
rolled steel
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Arisa HASEGAWA
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Nippon Steel Corp
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Nippon Steel Corp
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
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    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
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    • 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
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    • 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
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    • 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
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    • 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
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    • C21D8/0273—Final recrystallisation annealing
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    • 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
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a cold-rolled steel sheet, a method for manufacturing the same, and a welded joint.
  • steel sheets for a vehicle are required to have high strength in order to improve fuel efficiency by reducing a weight of a vehicle body in consideration of the global environment.
  • a desired strength can be imparted to the vehicle body while reducing a sheet thickness of the steel sheet and reducing the weight of the vehicle body.
  • Patent Document 1 discloses, as a high strength steel sheet used for a vehicle component or the like, a high strength steel sheet having a predetermined composition and having a predetermined steel sheet structure primarily containing martensite and bainite, in which an average number of inclusions having an average grain size of 5 ⁇ m or more in a cross section perpendicular to a rolling direction is 5.0/mm 2 or less, and the high strength steel sheet has an excellent delayed fracture resistance property, and a tensile strength of 1,470 MPa or more.
  • Patent Document 2 discloses a thin steel sheet having a steel structure in which an area ratio of ferrite is 30% or less (including 0%), an area ratio of bainite is 5% or less (including 0%), and an area ratio of martensite and tempered martensite is 70% or more (including 100%), an area ratio of retained austenite is 2.0% or less (including 0%), a ratio of a dislocation density in a range of 0 to 20 ⁇ m from a surface of the steel sheet to a dislocation density of a sheet thickness center portion is 90% or more and 110% or less, and an average of the top 10% of cementite particle sizes from the surface of the steel sheet to a depth of 100 ⁇ m is 300 nm or less, in which a maximum warpage amount of the steel sheet when sheared at a length of 1 m in a longitudinal direction of the steel sheet is 15 mm or less.
  • Patent Document 2 discloses that this thin steel sheet has a tensile strength of 980 MPa or more and can also obtain a ten
  • Patent Document 3 discloses a high strength steel sheet in which a chemical composition (C, Si, Mn, Al, P, and S) satisfies a specified range, a remainder includes iron and unavoidable impurities, martensite occupies 95 area % or more in the entire structure, a structure from a position at a depth of 10 ⁇ m from a surface the steel sheet in a sheet thickness direction to a position at a 1 ⁇ 4 thickness depth satisfies a predetermined relation, and the steel sheet has a tensile strength of 1,180 MPa or more and an excellent delayed fracture resistance property.
  • a chemical composition C, Si, Mn, Al, P, and S
  • the present inventors investigated the reason why the joint strength decreases due to the segregation of Mn and P. As a result, it was found that a difference in hardness of martensite in a welded heat-affected zone, caused by a difference in Mn content (difference in concentration), and co-segregation of Mn and P cause cracking to occur more easily. In addition, it was also found that Mn and P tend to segregate at prior ⁇ (austenite) grain boundaries.
  • the present inventors examined methods for suppressing segregation of Mn and P to prior ⁇ grain boundaries.
  • the present invention has been made in view of the above findings.
  • the gist of the present invention is as follows.
  • a cold-rolled steel sheet includes, as a chemical composition, by mass %: C: 0.200% or more and 0.450% or less; Si: 0.01% or more and 2.50% or less; Mn: 0.6% or more and 3.5% or less; Al: 0.001% or more and 0.100% or less; Ti: 0.001% or more and 0.100% or less; N: 0.0100% or less; P: 0.0400% or less; S: 0.0100% or less; O: 0.0060% or less; B: 0% or more and 0.0100% or less; Mo: 0% or more and 0.500% or less; Nb: 0% or more and 0.200% or less; Cr: 0% or more and 2.00% or less; V: 0% or more and 0.500% or less; Co: 0% or more and 0.500% or less; Ni: 0% or more and 1.000% or less; Cu: 0% or more and 1.000% or less; W: 0% or more and 0.100% or less; Ta:
  • a method for manufacturing a cold-rolled steel sheet according to another aspect of the present invention includes: a continuous casting process of obtaining a slab having the chemical composition according to [1] by continuous casting; a breakdown process of reducing a thickness of the slab by performing a reduction at a reduction ratio of 30% to 60% in a temperature range of 850° C. to 1,000° C.; a high-temperature heat treatment process of heating the slab after the breakdown process to 1,000° C. to 1,300° C., holding the slab for 5 to 20 hours, and cooling the slab; a hot rolling process of performing hot rolling on the slab after the high-temperature heat treatment process to obtain a hot-rolled steel sheet; a coiling process of coiling the hot-rolled steel sheet in a temperature range of 400° C.
  • a steel sheet which is an ultrahigh-strength steel sheet having a tensile strength of 1,310 MPa or more and can achieve sufficiently high joint strength after welding, and a welded joint.
  • FIG. 1 is a diagram showing a shape of a test piece for an Auger test.
  • the cold-rolled steel sheet according to the present embodiment has a predetermined chemical composition, in which a metallographic structure at a position of 1 ⁇ 4 to 3 ⁇ 4 of a sheet thickness in a sheet thickness direction from a surface contains, by volume percentage, 0% or more and 10.0% or less of retained austenite and 90.0% or more and 100% or less of one or two of martensite and tempered martensite, in the metallographic structure at the position, a P content at prior ⁇ grain boundaries is 10.0 mass % or less, and a Mn content at the prior ⁇ grain boundaries is 10.0 mass % or less, and a tensile strength of the cold-rolled steel sheet is 1,310 MPa or more.
  • % of an amount of each element means mass %.
  • C is related to a hardness of martensite and tempered martensite and is an element necessary for increasing a strength of the steel sheet and joint strength after welding.
  • a C content is set to 0.200% or more.
  • the C content is preferably 0.210% or more, and more preferably 0.220% or more.
  • the C content is set to 0.450% or less.
  • the C content is preferably 0.350% or less, and more preferably 0.300% or less.
  • Si 0.01% or More and 2.50% or Less
  • Si is a solid solution strengthening element and is an effective element for high-strengthening of the steel sheet.
  • a Si content is set to 0.01% or more.
  • the Si content is set to preferably 0.10% or more, and more preferably 0.20% or more.
  • the Si content is set to 2.50% or less.
  • the Si content is preferably 2.00% or less, and more preferably 1.80% or less.
  • Mn is an element that increases hardenability of steel by segregating at prior ⁇ grain boundaries and is an element that promotes the generation of martensite.
  • the Mn content is set to 0.6% or more.
  • the Mn content is preferably 1.0% or more.
  • the Mn content is set to 3.5% or less.
  • the Mn content is preferably 3.0% or less.
  • Al is an element having an action of deoxidizing molten steel. Therefore, an Al content is set to 0.001% or more. Al has an action of enhancing the stability of austenite like Si, and thus may be contained in order to obtain retained austenite.
  • the Al content is set to 0.100% or less.
  • the Al content is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less.
  • Ti is an element that is bonded to N to form TiN and contributes to the refinement of ⁇ .
  • the P content at ⁇ grain boundaries can be suppressed by the refinement of ⁇ .
  • a Ti content is set to 0.001% or more.
  • the Ti content is preferably 0.005% or more.
  • N is an element that is bonded to Ti to form TiN.
  • a N content is set to 0.0001% or more.
  • the N content is set to 0.0100% or less.
  • the N content is preferably 0.0080% or less, and more preferably 0.0060% or less.
  • the P content is an element contained in steel as an impurity and is an element that segregates at grain boundaries and embrittles steel. Therefore, the P content is preferably as small as possible and may be 0%. However, in consideration of a time and a cost for removing P, the P content is set to 0.0400% or less. The P content is preferably 0.0200% or less, and more preferably 0.0150% or less.
  • the P content may be set to 0.0001% or more.
  • a S content is preferably as small as possible and may be 0%. However, in consideration of a time and a cost for removing S, the S content is set to 0.0100% or less.
  • the S content is preferably 0.0050% or less, more preferably 0.0040% or less, and even more preferably 0.0030% or less.
  • the S content may be set to 0.0001% or more.
  • O is an element that is contained as an impurity.
  • the O content is set to 0.0060% or less.
  • the O content is preferably 0.0050% or less, and more preferably 0.0030% or less.
  • the O content may be 0%. However, from the viewpoint of a refining cost or the like, the O content may be set to 0.0005% or more or 0.0010% or more.
  • the remainder excluding the above elements basically includes Fe and impurities.
  • the impurities are elements that are incorporated from raw materials and/or in a steelmaking process and are allowed to be contained in a range in which the characteristics of the cold-rolled steel sheet according to the present embodiment are not clearly deteriorated.
  • the chemical composition of the cold-rolled steel sheet according to the present embodiment may contain one or two or more selected from the group consisting of B, Mo, Nb, Cr, V, Co, Ni, Cu, W, Ta, Sn, Sb, As, Mg, Ca, Y, Zr, La, and Ce in the following ranges. Since these elements may not be contained, lower limits thereof are 0%. In addition, even if these elements are contained as impurities, the effects of the cold-rolled steel sheet according to the present embodiment are not impaired as long as the amounts of the elements are within the ranges described below.
  • B, Mo, Cr, Ni, and As are elements that improve the hardenability and contribute to the high-strengthening of the steel sheet. Therefore, these elements may be contained.
  • a B content is set to 0.0001% or more, a Mo content, a Cr content, and a Ni content are each set to 0.010% or more, and an As content is set to 0.001% or more. More preferably, the B content is 0.0010% or more, the Mo content and the Cr content are each 0.100% or more, and the As content is 0.005% or more. It is not essential to obtain the above effects. Therefore, it is not necessary to particularly limit lower limits of the B content, the Mo content, the Cr content, the Ni content, and the As content, and the lower limits thereof are 0%.
  • the B content is set to 0.0100% or less, the Mo content is set to 0.500% or less, the Cr content is set to 2.000% or less, the Ni content is set to 1.000% or less, and the As content is set to 0.050% or less.
  • the B content is preferably 0.0030% or less, the Mo content is preferably 0.300% or less, the Cr content is preferably 1.000% or less, the Ni content is 0.500% or less, and the As content is preferably 0.030% or less.
  • Nb, V, Cu, W, and Ta are elements having an action of improving the strength of the steel sheet by precipitation hardening. Therefore, Nb, V, Cu, W, and Ta may be contained. In order to sufficiently obtain the above effect, each of a Nb content, a V content, a Cu content, a W content, and/or a Ta content is preferably 0.001% or more.
  • the Nb content is set to 0.200% or less
  • the V content is set to 0.500% or less
  • the Cu content is set to 1.000% or less
  • the W content and the Ta content are each set to 0.100% or less.
  • Co is an element effective in improving the strength of the steel sheet.
  • a Co content may be 0%. However, in order to obtain the above effect, the Co content is preferably 0.010% or more, and more preferably 0.100% or more.
  • the Co content is set to 0.500% or less.
  • Ca, Mg, La, Ce, Y, Zr, and Sb are elements that contribute to the fine dispersion of inclusions in steel, and are elements that contribute to the improvement in the formability of the steel sheet by this fine dispersion. Therefore, these elements may be contained. In order to obtain the above effects, it is preferable that one or more of Ca, Mg, La, Ce, Y, Zr, and Sb are contained and the amount of each element is set to 0.001% or more.
  • a Ca content is set to 0.040% or less, and each of a Mg content, a La content, a Ce content, a Y content, a Zr content, and a Sb content is set to 0.050% or less.
  • Sn is an element that suppresses the coarsening of grains and contributes to the improvement in the strength of the steel sheet. Therefore, Sn may be contained.
  • Sn is an element that may cause a decrease in cold formability of the steel sheet attributed to the embrittlement of ferrite.
  • Sn content is set to 0.050% or less.
  • the Sn content is preferably 0.040% or less.
  • the chemical composition of the cold-rolled steel sheet according to the present embodiment can be obtained by the following method.
  • the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES) for chips according to JIS G 1201 (2014).
  • ICP-AES inductively coupled plasma-atomic emission spectrometry
  • the chemical composition is an average content in the entire sheet thickness.
  • C and S may be measured using a combustion-infrared absorption method
  • N may be measured using an inert gas fusion-thermal conductivity method
  • O may be measured using an inert gas fusion-non-dispersive infrared absorption method.
  • the metallographic structure at the position of 1 ⁇ 4 to 3 ⁇ 4 of the sheet thickness in the sheet thickness direction from the surface contains, by volume percentage, 0% or more and 10.0% or less of retained austenite and 90.0% or more and 100% or less of one or two of martensite and tempered martensite.
  • Retained austenite contributes to the improvement in the formability of the steel sheet by improving uniform elongation of the steel sheet through a TRIP effect. Therefore, retained austenite (retained 7) may be contained.
  • the volume percentage of retained austenite is preferably set to 1.0% or more.
  • the volume percentage of retained austenite is more preferably 2.0% or more, and even more preferably 3.0% or more.
  • the volume percentage of retained austenite is set to 10.0% or less.
  • the volume percentage of retained austenite is preferably 8.0% or less, and more preferably 7.0% or less.
  • one or two of martensite and tempered martensite are contained.
  • Martensite (so-called fresh martensite) and tempered martensite are aggregates of lath-shaped grains and greatly contribute to the improvement in strength. Therefore, the cold-rolled steel sheet according to the present embodiment contains martensite and tempered martensite in a total volume percentage of 90.0% to 100%.
  • the microstructure may contain ferrite and bainite in addition to retained austenite, martensite, and tempered martensite.
  • Pearlite is a structure having strength intermediate between martensite and ferrite, but is a structure that has poor deformability and deteriorates workability. Therefore, it is preferable that pearlite is substantially not contained.
  • the reason for specifying the metallographic structure at the position of 1 ⁇ 4 to 3 ⁇ 4 of the sheet thickness from the surface centered at a position of 1 ⁇ 2 of the sheet thickness in the sheet thickness direction from the surface is that, in the cold-rolled steel sheet according to the present embodiment, the metallographic structure at this position is a representative structure of the steel sheet and has a strong correlation with the characteristics.
  • the volume percentage of each structure in the metallographic structure (microstructure) at the position of 1 ⁇ 4 to 3 ⁇ 4 of the sheet thickness in the sheet thickness direction from the surface of the cold-rolled steel sheet according to the present embodiment is measured as follows.
  • the volume percentages of ferrite, bainite, martensite, tempered martensite, and pearlite are obtained by collecting a test piece from a random position in a rolling direction and in a width direction of the steel sheet, polishing a longitudinal section parallel to the rolling direction, and observing a structure revealed by inital etching in a range of 1 ⁇ 4 to 3 ⁇ 4 of the sheet thickness in the sheet thickness direction from the surface using a scanning electron microscope (SEM).
  • SEM scanning electron microscope
  • a region with no substructure revealed and a low luminance is defined as ferrite.
  • a region with no substructure revealed and a high luminance is defined as martensite or retained austenite.
  • a region in which a substructure is revealed is defined as tempered martensite or bainite.
  • Bainite and tempered martensite can be distinguished from each other by further carefully observing intragranular carbides.
  • tempered martensite includes martensite laths and cementite generated within the laths.
  • cementite included in the tempered martensite has a plurality of variants.
  • bainite is classified into upper bainite and lower bainite.
  • Upper bainite includes lath-shaped bainitic ferrite and cementite generated at the interface between the laths and can be easily distinguished from tempered martensite.
  • Lower bainite includes lath-shaped bainitic ferrite and cementite generated within the laths.
  • the volume percentage of retained austenite is obtained as described below: a test piece is collected from a random position in the steel sheet, a rolled surface is chemically polished from the surface of the steel sheet to a 1 ⁇ 4 thickness position, and the volume percentage of retained austenite is quantified from integrated intensities of ( 200 ) and ( 210 ) planes of ferrite and integrated intensities of ( 200 ), ( 220 ), and ( 311 ) planes of austenite by MoKa radiation.
  • the P content at the prior ⁇ grain boundaries is 10.0 mass % or less
  • the Mn content at the prior ⁇ grain boundaries is 10.0 mass % or less.
  • Segregation of Mn and P usually occurs when elements are distributed between a solid phase and a liquid phase during dendrite growth during solidification in a continuous casting step.
  • a difference in hardness of martensite partially occurs (partial hardening) due to a difference in Mn content (difference in concentration) in a heat-affected zone of a welded part, resulting in a difference in joint strength after welding. It is presumed that this is because an Ms point changes depending on the difference in Mn content.
  • cracking is likely to occur due to the co-segregation of Mn and P. Therefore, in order to increase the joint strength after welding, it is necessary to reduce the segregation of Mn and P.
  • the segregation of Mn and P is suppressed. More specifically, the P content at the prior ⁇ grain boundaries is set to 10.0 mass % or less, and the Mn content at the prior ⁇ grain boundaries is set to 10.0 mass % or less.
  • a test piece for an Auger test having a size shown in FIG. 1 is cut out from a position of 1 ⁇ 4 to 3 ⁇ 4 of the sheet thickness from the surface, centered at the position of 1 ⁇ 2 of the sheet thickness from the surface of the steel sheet.
  • This test piece is immersed in an aqueous solution of ammonium thiocyanate having a concentration of 20 mass % for 48 hours.
  • An impact test is conducted on the test piece after the immersion to obtain a fracture surface. In the impact test, the test piece is cooled with liquid nitrogen and is then fractured by being hit with a hammer in a vacuum.
  • the fracture surface becomes a grain boundary (prior ⁇ grain boundary) fracture surface, so that the P content and the Mn content are measured by conducting Auger electron spectroscopy on this fracture surface. Accordingly, the P content and the Mn content at the prior ⁇ grain boundaries are obtained.
  • a measuring device is not particularly limited, but the measurement is performed using, for example, JAMP-9500F manufactured by JEOL, Ltd.
  • JAMP-9500F manufactured by JEOL, Ltd.
  • a portion on the grain boundary fracture surface in which no precipitate is present is measured at least three times, and AES peaks of P and Mn are measured.
  • the AES peak intensities are subjected to sensitivity correction by corresponding relative sensitivity factors (RSF) to obtain grain boundary segregation concentrations with reference to Non-Patent Document (Analytic Chemistry, vol. 35 (1986)).
  • a prior ⁇ grain size (average grain size) is preferably 15 ⁇ m or less.
  • the prior ⁇ grain size can be measured by the following method.
  • a test piece is collected from any position in a rolling direction and a width direction of the steel sheet, a longitudinal section parallel to the rolling direction is polished, and a structure revealed using a saturated aqueous solution of picric acid from a range of 1 ⁇ 4 to 3 ⁇ 4 of the sheet thickness in the sheet thickness direction from the surface is observed using an optical microscope.
  • a mesh-like black line is determined to be the prior ⁇ grain boundary.
  • a surfactant is added or a temperature of immersion in the saturated aqueous solution of picric acid is changed to about 20° C. to 80° C.
  • any magnification of 200 to 1000-fold is selected, and an image of the structure is acquired.
  • Three images including at least 200 or more grains are photographed, and an average grain size of the prior ⁇ (austenite) grain sizes is measured in the photographed images using a point calculation method.
  • the measurement method of the prior ⁇ grain size is not limited to the above method, and the prior ⁇ grain size can also be measured by using an inverse analysis of prior austenite using scanning electron microscope-electron back scattering diffraction pattern (SEM-EBSD).
  • SEM-EBSD scanning electron microscope-electron back scattering diffraction pattern
  • the cold-rolled steel sheet according to the present embodiment described above may have a coating layer containing zinc, aluminum, magnesium, or an alloy of these metals on the surface.
  • the coating layer may be substantially made of zinc, aluminum, magnesium, or an alloy of these metals.
  • the presence of the coating layer on the surface of the steel sheet improves corrosion resistance.
  • the coating layer may be a known coating layer.
  • the sheet thickness cannot be reduced to a certain sheet thickness or less even though high-strengthening is achieved because of concerns about perforation and the like.
  • One of the purposes of the high-strengthening of the steel sheet is to reduce the weight by thinning. Therefore, even if a high strength steel sheet is developed, an application range of a steel sheet with low corrosion resistance is limited. In a case where coating layer containing zinc, aluminum, magnesium, or an alloy of these metals is provided on the surface, the corrosion resistance is improved, and an applicable range is widened, which is preferable.
  • the “surface” in the “position of 1 ⁇ 4 to 3 ⁇ 4 of the thickness from the surface of the steel sheet” means a surface of the base metal excluding the coating layer.
  • the tensile strength (TS) is set to 1,310 MPa or more. From the viewpoint of an impact absorption property, the tensile strength of the steel sheet is preferably 1,400 MPa or more and more preferably 1,470 MPa or more.
  • a welded joint according to the present embodiment is obtained by joining the cold-rolled steel sheet according to the present embodiment to another steel sheet (which may be the cold-rolled steel sheet according to the present embodiment) by welding. Therefore, the welded joint according to the present embodiment is a welded joint in which a plurality of steel sheets are joined together, and at least one steel sheet is the cold-rolled steel sheet according to the present embodiment described above.
  • the steel sheets are joined through a welded part, and in a case where the welding is spot welding, the steel sheets are joined through a spot-welded part.
  • the cold-rolled steel sheet according to the present embodiment can be stably manufactured according to the following manufacturing method, although the effects can be obtained as long as the cold-rolled steel sheet has the above-described characteristics regardless of the manufacturing method.
  • the cold-rolled steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps (I) to (VIII):
  • the slab after the breakdown step is heated to 1,000° C. to 1,300° C., is held at the temperature for 5 to 20 hours (SP treatment), and is then cooled.
  • the slab after the BD and the SP treatment is heated and hot-rolled to obtain a hot-rolled steel sheet.
  • a heating temperature prior to the hot rolling is not limited. However, when the temperature is lower than 1,100° C., there is a concern that carbides and sulfides generated between the casting and the SP treatment step are not solubilized and become coarse and a grain size during annealing becomes coarse. Therefore, the heating temperature is preferably 1,100° C. or higher. An upper limit of the heating temperature is not particularly specified, but is generally 1,300° C. or lower.
  • recrystallization is utilized to refine ⁇ and suppress P segregation to grain boundaries.
  • a sheet thickness reduction ratio in each stand from a (n ⁇ 3)th stand to the nth stand is set to 30% or more, and a rolling temperature in the last stand (nth stand) is set to 900° C. or lower. That is, for example, in a rolling mill having seven stands, sheet thickness reduction ratios in a fourth stand, a fifth stand, a sixth stand, and a seventh stand are each set to 30% or more, and a rolling temperature in the seventh stand is set to 900° C. or lower.
  • an austenite grain size is refined by recrystallization during the rolling, and this refined grain boundary is used as a diffusion path to promote the diffusion of Mn, P, and the like and the segregation is reduced.
  • the sheet thickness reduction ratio in any of the stands is less than 30%, or when the rolling temperature in the nth stand is higher than 900° C., a hot-rolled structure becomes coarse and a duplex-grain structure, and a structure after the annealing step described later also becomes coarse.
  • a hot rolling completion temperature is lower than 830° C., a rolling reaction force increases and it becomes difficult to stably obtain a target sheet thickness. Therefore, a rolling temperature in the last stand is preferably 830° C. or higher.
  • the sheet thickness reduction ratio in each of the (n ⁇ 3)th stand to the nth stand is preferably set to 50% or less.
  • the finish rolling is performed using the rolling mill having four or more stands so that continuous rolling with a short interpass time for the last four passes of the rolling is performed. This is because, when the interpass time is long, even if the reduction is performed at a large sheet thickness reduction ratio, strain tends to recover between passes and does not sufficiently accumulate.
  • the cold-rolled steel sheet obtained in the cold rolling step is heated to an annealing temperature of higher than Ac3° C. at an average temperature rising rate of 2° C./sec or faster, is held at this annealing temperature for 60 to 300 seconds, and is, after being held, cooled to 250° C. or lower at an average cooling rate of 10° C./sec or faster.
  • the productivity decreases.
  • a temperature (° C.) at the Ac3 point can be obtained by the following method.
  • a coating layer containing zinc, aluminum, magnesium, or an alloy of these metals may be formed on the surface of the steel sheet from the viewpoint of increasing the corrosion resistance of the steel sheet.
  • the steel sheet may be immersed in a plating bath to form a hot-dip plating within a range in which the above average cooling rate can be satisfied.
  • the hot-dip plating may be heated to a predetermined temperature and alloyed to obtain an alloyed hot-dip plating.
  • the plating layer may further contain Fe, Al, Mg, Mn, Si, Cr, Ni, Cu, or the like. Any of the above methods may be used for the plating layer for the purpose of increasing corrosion resistance.
  • plating conditions and alloying conditions known conditions may be applied depending on a composition of the plating.
  • the cold-rolled steel sheet after the annealing step is held at 150° C. to 400° C. for 500 seconds or shorter.
  • a portion or the entirety of martensite is tempered and becomes tempered martensite.
  • a holding temperature is lower than 150° C., martensite is not sufficiently tempered, and the effect cannot be sufficiently obtained.
  • the holding temperature is higher than 400° C.
  • a dislocation density in tempered martensite decreases, which may lead to a decrease in tensile strength.
  • the holding time is longer than 500 seconds, the tensile strength decreases, and the productivity decreases.
  • a lower limit of the holding time is not limited, but the holding time is preferably set to 100 seconds or longer in a case where the metallographic structure primarily contains tempered martensite.
  • heating may be performed as necessary.
  • the cold-rolled steel sheet after the holding step is welded to other steel sheets.
  • Other steel sheets are not limited, and may be or may not be the cold-rolled steel sheet according to the present embodiment.
  • the welding may be performed to join three or more steel sheets by performing the welding a plurality of times.
  • a welding method is not limited, but spot welding is preferable in a case where an application to vehicle components is considered.
  • the obtained cold-rolled steel sheets were annealed under the conditions of Table 2-3 and then held under the conditions of Table 2-3.
  • some of the cold-rolled steel sheets were heated or cooled to (galvanizing bath temperature—40°) C to (galvanizing bath temperature+50°) C in the middle of the annealing (cooling stage) and immersed in the galvanizing bath to be galvanized (examples with Present in the field of Presence of absence of plating in the tables).
  • some of the galvanized cold-rolled steel sheets were further heated to a temperature range of 470° C. to 550° C. to be alloyed (examples of Present in the field of Presence or absence of alloying in the tables).
  • JIS No. 5 test piece was collected from the obtained cold-rolled steel sheets at a right angle to a rolling direction, and a tensile strength was measured according to JIS Z 2241: 2011.
  • a servo motor pressure type single-phase AC welder (power supply frequency: 50 Hz) was used, and as an electrode, a Cr—Cu DR type electrode having a radius of curvature of 40 mm at a tip and a diameter of 6 mm at the tip was used.
  • Welding conditions were a weld force of 440 kgf, an energization time of 0.28 sec, and a hold time of 0.1 sec.
  • a welding current was set under a condition in which a nugget diameter of 5-t could be obtained.
  • Example Nos. 1 to 30 of the present invention present invention examples
  • the chemical composition, the metallographic structure, the Mn content and the P content (segregation degree) at the prior ⁇ grain boundaries were within the ranges of the present invention, and as a result, a strength as high as 1,310 MPa or more and sufficient joint strength were provided.
  • a steel sheet which is an ultrahigh-strength steel sheet having a tensile strength of 1,310 MPa or more and can achieve sufficiently high joint strength after welding, and a welded joint.
  • the steel sheet and the welded joint contribute to a reduction in weight of a vehicle body or the like and thus have high industrial applicability.

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