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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • 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
  • C21D1/26—Methods of annealing
• • 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
  • C21D8/021—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 involving particular fabrication steps or treatments of ingots or slabs
• • 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
  • C21D8/0247—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
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
  • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
  • C23C2/06—Zinc or cadmium or alloys based thereon
• • 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
  • C21D2211/001—Austenite
• • 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
  • C21D2211/002—Bainite
• • 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
• • 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

Definitions

• the present invention relates to a steel sheet and a method for manufacturing the same.
• the present invention relates to a high-strength steel sheet having excellent LME resistance, which is suitable as a member to be subjected to forming by cold pressing for use in an industrial field such as the automotive industry and the electrical machinery industry, and to a method for manufacturing the steel sheet.
• the non-self-derived LME cracking is particularly noticeable when spot welding is performed with electrodes inclined at an angle.
• the phrase "with electrodes inclined at an angle" herein refers to a state of welding electrodes in which the axis of the electrodes is not perpendicular to the surfaces of steel sheets.
• Patent Literature 1 discloses a technique which involves controlling the frequency of corresponding grain boundaries in the surface of a steel sheet after a high-temperature tensile test and the thickness of a softened surface region, thereby achieving a high-strength steel sheet having a tensile strength of 980 MPa or more and a total elongation of 20% or more, and having excellent LME resistance.
• Patent Literature 2 discloses a technique which involves introducing oxygen into the surface of a steel slab during continuous casting, thereby forming an iron oxide as a site for the formation of titanium nitride. This inhibits B in the steel from binding to dissolved nitrogen in a later annealing process and promotes the formation of (Fe,Mn) 2 B in a surface region of a steel sheet, thereby achieving a steel sheet having an LME resistance, high strength, and excellent ductility.
• Patent Literature 3 proposes a technique which involves removing, prior to spot welding, a coating layer from a portion to be welded so as to prevent LME cracking.
• Patent Literature 1 necessitates a reduction in the amount of Si in order to reduce the frequency of corresponding grain boundaries in the surface of a steel sheet. It therefore appears difficult to impart good formability to a steel sheet having a strength of 980 MPa or higher grade.
• Patent Literature 2 which involves introducing oxygen into the surface of a steel slab during continuous casting to improve the LME resistance of the steel sheet finally obtained, it is expected that the introduction of oxygen into the surface of a slab will cause the formation of scale, resulting in a reduction in yield. Further, the presence of a large amount of hard particles (Fe,Mn) 2 B in a surface region from the casting stage will cause surface cracking during casting.
• Patent Literature 3 requires a step of removing a coating layer in advance, which increases the production cost. Further, the removal of the coating layer will cause a reduction in the corrosion resistance of a weld.
• the steel sheet include a hot-rolled steel sheet, a cold-rolled steel sheet, and a coated steel sheet such as GA or GI.
• high strength refers to a TS of 980 MPa or more
• good formability means that the relationship between tensile strength TS and elongation El, shown in the following formula 7, is satisfied.
• the present inventors conducted intensive studies on the chemical compositions and microstructures of steel sheets. They have now found the following facts through precise control of slab heating conditions, temperatures from hot rolling to coiling, and annealing conditions, and through control of the states of elements existing in the steel.
• the present invention is based on the above findings, and is summarized as follows.
• the present invention it is possible to obtain a high-strength steel sheet having good formability and excellent LME resistance. Therefore, the present invention is very useful in the industrial fields of automobiles, electrical devices, etc., and is particularly useful for reducing the weight of automotive body frame parts.
• the C content is an element necessary for increasing the strengths of tempered martensite, bainite, and fresh martensite. In order to fully achieve this effect, it is necessary to make the C content at least 0.030%. Therefore, the C content is made 0.030% or more.
• the C content is preferably 0.050% or more, more preferably 0.070% or more, even more preferably 0.090% or more, and most preferably 0.100% or more.
• the C content is made 0.500% or less.
• the C content is preferably 0.400% or less, more preferably 0.300% or less, even more preferably 0.270% or less, and most preferably 0.250% or less.
• Si more than 0.01% and not more than 2.50%
• Si is an element that prevents excessive formation and growth of carbides in the steel, thereby increases the fraction of retained austenite and improving ductility. If the Si content is 0.01% or less, this effect is poor and the steel cannot have good formability. Therefore, the lower limit of the Si content is made 0.01%.
• the Si content is made more than 0.01%.
• the Si content is preferably 0.05% or more, more preferably 0.10% or more, even more preferably 0.50% or more, and most preferably 0.90% or more. However, if the Si content exceeds 2.50%, it causes a decrease in the melting point of zinc, which facilitates penetration of zinc into the steel sheet during welding, resulting in a reduction in the LME resistance of the steel sheet. Therefore, the Si content is made 2.50% or less.
• the Si content is preferably 2.30% or less, more preferably 2.00% or less, even more preferably 1.80% or less, and most preferably 1.60% or less.
• Mn not less than 0.10% and not more than 5.00%
• Mn is an element that affects the area fractions of tempered martensite, bainite, and fresh martensite through enhancement of hardenability. If the Mn content is less than 0.10%, a soft phase such as ferrite will be formed excessively, and desired area fractions of tempered martensite, bainite, and fresh martensite cannot be obtained, resulting in an insufficient steel sheet strength. Therefore, the Mn content is made 0.10% or more.
• the Mn content is preferably 0.50% or more, more preferably 0.80% or more, even more preferably 1.00% or more, and most preferably 2.00% or more.
• the Mn content is made 5.00% or less.
• the Mn content is preferably 4.50% or less, more preferably 4.00% or less, even more preferably 3.70% or less, and most preferably 3.50% or less.
• the P content needs to be 0.100% or less.
• the P content is preferably made 0.080% or less, more preferably 0.070% or less, even more preferably 0.050% or less, and most preferably 0.040% or less.
• the P content is preferably made 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.
• the S content needs to be 0.0200% or less.
• the S content is made 0.0200% or less.
• the S content is preferably made 0.0180% or less, more preferably 0.0150% or less, even more preferably 0.0100% or less, and most preferably 0.0050% or less. While the lower limit of the S content is not particularly limited, in view of the restrictions of production technology, the S content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.
• Al is an element which acts as a deoxidizer and is effective in reducing inclusions in the steel, and is preferably added in a deoxidization process.
• Al increases the transformation temperature for austenitizing, resulting in inclusion of ferrite in the microstructure. Therefore, the use of Al in an amount of more than 0.100% makes it difficult to achieve a desired TS.
• the Al content is made 0.100% or less.
• the Al content is preferably made 0.080% or less, more preferably 0.070% or less, even more preferably 0.060% or less, and most preferably 0.050% or less. While the lower limit of the Al content is not particularly limited, the Al content is preferably made 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.
• N may adversely affect the LME resistance through the formation of coarse nitrides. If the N content exceeds 0.0100%, a large amount of coarse nitrides will be formed, leading to a significant deterioration of LME resistance.
• the N content is preferably as small as possible. Thus, the N content is made 0.0100% or less.
• the N content is preferably 0.0090% or less, more preferably 0.0080% or less, even more preferably 0.0070% or less, and most preferably 0.0060% or less. While the lower limit of the N content is not particularly limited, in view of the restrictions of production technology, the N content is preferably made 0.0001% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.
• Ti contributes to precipitation strengthening and, in addition, reduces the prior austenite grain size and thereby reduces the grain sizes of tempered martensite and bainite.
• Ti is effective in increasing the strength of the steel.
• the Ti content is made 0.010% or more.
• the Ti content is preferably 0.012% or more, more preferably 0.015% or more, even more preferably 0.020% or more, and most preferably 0.025% or more.
• the Ti content is made 0.200% or less.
• the Ti content is preferably 0.180% or less, more preferably 0.150% or less, even more preferably 0.100% or less, and most preferably 0.050% or less.
• Nb not less than 0.005% and not more than 0.500%
• Nb is an element that improves the LME resistance.
• the presence of dissolved Nb in the steel during welding has a great effect on the improvement of the LME resistance.
• the Nb content needs to be 0.005% or more.
• the Nb content is preferably 0.007% or more, more preferably 0.008% or more, even more preferably 0.010% or more, and most preferably 0.012% or more.
• the Nb content is made 0.500% or less.
• the Nb content is preferably made 0.400% or less, more preferably 0.350% or less, even more preferably 0.200% or less, and most preferably 0.100% or less.
• a high-strength steel sheet according to an embodiment of the present invention has a chemical composition containing the above-described components, with the balance including Fe and incidental impurities.
• a high-strength steel sheet according to an embodiment of the present invention has a chemical composition containing the above-described components, with the balance consisting of Fe and incidental impurities.
• the incidental impurities may include Zn, Pb, and As. Such impurities are allowed to be present in the steel in a total amount of up to 0.100%.
• the chemical composition may contain, in mass %, one or two or more selected from the following: V: 0.500% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Co: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.
• V 0.500% or less
• V contributes to precipitation strengthening and, in addition, reduces the prior austenite grain size and thereby reduces the grain sizes of tempered martensite and bainite.
• the steel can therefore contain V as necessary.
• the lower limit of the V content is not particularly limited, in order to achieve the above effect, the V content is preferably made 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
• the V content is made 0.500% or less.
• the V content is preferably 0.400% or less, more preferably 0.300% or less, even more preferably 0.200% or less, and most preferably 0.100% or less.
• Ta 0.10% or less
• Ta like Ti, contributes to increasing the strength of the steel through the formation of an alloy carbide and an alloy carbonitride.
• Ta partly dissolves in Nb carbide or Nb carbonitride to form a complex precipitate such as (Nb,Ta) (C,N), thereby significantly suppressing coarsening of precipitates and stabilizing the contribution of precipitation strengthening to the increase of the strength.
• the steel can therefore contain Ta as necessary. While the lower limit of the Ta content is not particularly limited, in order to achieve the above effect, the Ta content is preferably made 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more. However, a too-high Ta content will lead to saturation of the precipitate stabilizing effect and increased alloying cost. Therefore, when the steel contains Ta, the Ta content is made 0.10% or less.
• the Ta content is preferably made 0.08% or less, more preferably 0.07% or less, even more preferably 0.06% or less, and most preferably 0.05% or less.
• the steel can contain W as necessary to improve the hardenability of the steel and to further increase the strength of the steel through a reduction in the grain sizes of tempered martensite and bainite.
• the W content is preferably made 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.
• a W content of more than 0.10% may increase the amount of coarse precipitates, such as WN and WS, which remain undissolved during heating of the slab in a hot rolling process, resulting in a reduction in ductility. Therefore, when the steel contains W, the W content is made 0.10% or less.
• the W content is preferably made 0.08% or less, more preferably 0.07% or less, even more preferably 0.06% or less, and most preferably 0.05% or less.
• B is an element which, through its segregation at austenite grain boundaries, can improve hardenability.
• B can form a microstructure composed mainly of tempered martensite and bainite, thereby increasing the strength of the steel sheet, and can contribute to improving the LME resistance. Therefore, the steel can contain B as necessary.
• the lower limit of the B content is not particularly limited, in order to achieve the above effects, the B content is preferably made 0.0003% or more, more preferably 0.0005% or more, and even more preferably 0.0007% or more. However, if the B content exceeds 0.0100%, coarse precipitates will be formed, resulting in a reduction in ductility. Therefore, when the steel contains B, the B content is made 0.0090% or less.
• the B content is preferably made 0.0080% or less, more preferably 0.0070% or less, even more preferably 0.0050% or less, and most preferably 0.0030% or less.
• the steel can contain Cr as necessary. While the lower limit of the Cr content is not particularly limited, in order to achieve the above effect, the Cr content is preferably made 0.01% or more, more preferably 0.05% or more, and even more preferably 0.07% or more. However, if the steel contains Cr in an amount in excess of 1.00%, the area fraction of fresh martensite will be too high, resulting in a reduction in the dimensional accuracy and the ductility of the steel sheet during forming. Therefore, when the steel contains Cr, the Cr content is made 1.00% or less. The Cr content is preferably made 0.80% or less, more preferably 0.60% or less, even more preferably 0.50% or less, and most preferably 0.30% or less.
• the steel can contain Mo as necessary. While the lower limit of the Mo content is not particularly limited, in order to achieve the above effect, the Mo content is preferably made 0.01% or more, more preferably 0.05% or more, and even more preferably 0.07% or more. However, if the steel contains Mo in an amount in excess of 1.00%, the area fraction of fresh martensite will be too high, resulting in a reduction in the dimensional accuracy and the ductility of the steel sheet during forming. Therefore, when the steel contains Mo, the Mo content is made 1.00% or less. The Mo content is preferably made 0.80% or less, more preferably 0.50% or less, even more preferably 0.30% or less, and most preferably 0.20% or less.
• the steel can contain Co as necessary. While the lower limit of the Co content is not particularly limited, in order to achieve the above effect, the Co content is preferably made 0.01% or more, more preferably 0.05% or more, and even more preferably 0.07% or more. However, if the steel contains Co in an amount in excess of 1.00%, the area fraction of fresh martensite will be too high, resulting in a reduction in the dimensional accuracy and the ductility of the steel sheet during forming. Therefore, when the steel contains Co, the Co content is made 1.00% or less. The Co content is preferably made 0.80% or less, more preferably 0.60% or less, even more preferably 0.30% or less, and most preferably 0.20% or less.
• Ni increases the strength of the steel through solid solution strengthening. Therefore, the steel can contain Ni as necessary. While the lower limit of the Ni content is not particularly limited, in order to achieve the above effect, the Ni content is preferably made 0.01% or more, more preferably 0.05% or more, and even more preferably 0.07% or more. However, if the steel contains Ni in an amount in excess of 1.00%, the area fraction of fresh martensite will be too high, resulting in a reduction in the dimensional accuracy and the ductility of the steel sheet during forming. Therefore, when the steel contains Ni, the Ni content is made 1.00% or less. The Ni content is preferably made 0.80% or less, more preferably 0.60% or less, even more preferably 0.30% or less, and most preferably 0.20% or less.
• the steel can contain Cu as necessary. While the lower limit of the Cu content is not particularly limited, in order to achieve the above effect, the Cu content is preferably made 0.01% or more, more preferably 0.05% or more, and even more preferably 0.07% or more. However, if the steel contains Cu in an amount in excess of 1.00%, the area fractions of tempered martensite, bainite, and fresh martensite will be too high, resulting in a reduction in the dimensional accuracy and the ductility of the steel sheet during forming. Therefore, when the steel contains Cu, the Cu content is made 1.00% or less. The Cu content is preferably made 0.80% or less, more preferably 0.60% or less, even more preferably 0.30% or less, and most preferably 0.20% or less.
• Sn and Sb suppress decarburization in a surface region having a thickness of about a few tens of ⁇ m, caused by nitridation or oxidation of the surface of the steel sheet, thereby preventing a reduction in the area fraction of tempered martensite in the steel sheet surface.
• Sn and Sb have the effect of ensuring the strength and the material stability. Therefore, the steel can contain these elements as necessary. While the lower limit of the content of each element is not particularly limited, in order to achieve the above effects, the content of each element is preferably made 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.
• each element exceeds 0.200%, the ductility of the steel sheet may be reduced due to embrittlement of the steel sheet. Therefore, when the steel contains Sn and Sb, the content of each element is made 0.200% or less.
• the content of each element is preferably 0.100% or less, more preferably 0.070% or less, even more preferably 0.050% or less, and most preferably 0.030% or less.
• the content of each element is preferably made 0.0050% or less, more preferably 0.0040% or less, even more preferably 0.0035% or less, and most preferably 0.0030% or less.
• each element makes the shape of a nitride or sulfide spherical and improves the limiting deformability of the steel sheet
• the content of each of Ca, Mg and REM is preferably made 0.0001% or more, more preferably 0.0005% or more, even more preferably 0.0007% or more, and most preferably 0.0010% or more.
• each in an amount of 0.100% or less causes no increase in the amount of coarse precipitates and inclusions and has no effect on the precipitation of Nb, and thus does not deteriorate the LME resistance. Therefore, when the steel contains Zr and Te, the content of each element is made 0.100% or less.
• the content of each element is preferably made 0.080% or less, more preferably 0.070% or less, even more preferably 0.060% or less, and most preferably 0.050% or less.
• each element makes the shape of a nitride or sulfide spherical and improves the limiting deformability of the steel sheet
• the content of each of Zr and Te is preferably made 0.001% or more, more preferably 0.010% or more, and even more preferably 0.020% or more.
• Hf in the steel in an amount of 0.10% or less causes no increase in the amount of coarse precipitates and inclusions and has no effect on the precipitation of Nb, and thus does not deteriorate the LME resistance. Therefore, when the steel contains Hf, the Hf content is made 0.10% or less.
• the Hf content is preferably made 0.080% or less, more preferably 0.070% or less, even more preferably 0.060% or less, and most preferably 0.050% or less.
• the Hf content is preferably made 0.003% or more, more preferably 0.010% or more, even more preferably 0.020% or more, and still more preferably 0.030% or more when the steel contains Hf.
• the inclusion of Bi in the steel in an amount of 0.200% or less causes no increase in the amount of coarse precipitates and inclusions and has no effect on the precipitation of Nb, and thus does not deteriorate the LME resistance. Therefore, when the steel contains Bi, the Bi content is made 0.200% or less.
• the Bi content is preferably made 0.100% or less, more preferably 0.050% or less, even more preferably 0.030% or less, and most preferably 0.020% or less. While the lower limit of the Bi content is not particularly specified, in view of the fact that Bi reduces segregation, the Bi content is preferably made 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more.
• Tempered martensite and bainite contribute to the strength of the steel sheet.
• the total area fraction of bainite and tempered martensite needs to be 40% or more.
• the total area fraction is preferably 42% or more, more preferably 45% or more, even more preferably 47% or more, and most preferably 50% or more.
• the steel microstructure is occupied by hard phases including tempered martensite, bainite, and fresh martensite.
• the total area fraction of bainite and tempered martensite needs to be 85% or less.
• the total area fraction is preferably 83% or less, more preferably 82% or less, even more preferably 81% or less, and most preferably 80% or less.
• Fresh martensite is a very hard phase, and therefore increases the strength of the steel.
• Fresh martensite is not necessarily required if the strength of the steel sheet is secured; however, the inclusion of fresh martensite in the steel sheet microstructure can further increase the strength of the steel sheet, achieving further strengthening. Therefore, the area fraction of fresh martensite needs to be 0% or more.
• the area fraction is preferably 2% or more, more preferably 3% or more, even more preferably 4% or more, and most preferably 5% or more.
• fresh martensite reduces the ductility of the steel, making it difficult to achieve good formability. Therefore, the area fraction of fresh martensite needs to be 25% or less.
• the area fraction is preferably 23% or less, more preferably 22% or less, even more preferably 21% or less, and most preferably 20% or less.
• the area fraction of retained austenite needs to be 5% or more.
• the area fraction is preferably 7% or more, more preferably 8% or more, even more preferably 9% or more, and most preferably 10% or more.
• the area fraction of retained austenite exceeds 20%, elements concentrated in the retained austenite, especially Si, will diffuse during welding, causing a decrease in the melting point of zinc. This may facilitate the penetration of zinc into the steel sheet during welding, resulting in a deterioration of the LME resistance of the steel sheet.
• the area fraction of retained austenite needs to be 20% or less.
• the area fraction is preferably 18% or less, more preferably 17% or less, even more preferably 16% or less, and most preferably 15% or less.
• the inclusion of a remaining microstructure, such as ferrite or pearlite, in the steel does not impair the effects of the present invention.
• a remaining microstructure it may include at least one of ferrite and pearlite, i.e., the area fraction of at least one of ferrite and pearlite may be 0% or more.
• the area fraction of the remaining microstructure is preferably 20% or less, more preferably 18% or less, even more preferably 15% or less, and most preferably 13% or less.
• Nb sol represents the amount (mass %) of dissolved Nb
• Nb pre represents the amount (mass %) of Nb in Nb precipitates having a particle size of less than 20 nm.
• Nb sol the amount of Nb in Nb precipitates having a particle size of less than 20 nm
• Nb pre the amount of Nb in Nb precipitates having a particle size of less than 20 nm
• Nb pre the amount of Nb in Nb precipitates having a particle size of less than 20 nm
• Nb, unit: mass %) the total amount of Nb contained in the steel sheet
• Nb in the steel sheet is brought to a dissolved state, or to a state which can be converted into a dissolved state during welding, thereby preventing non-self-derived LME and improving the LME resistance.
• the present inventors through experiments conducted using various steel sheets with varying states of Nb present therein, have found that the LME resistance is improved when Formula 1 is satisfied.
• Nb sol /Nb) + (Nb pre /Nb) is preferably 0.41 or more, more preferably 0.42 or more. While the upper limit is not particularly limited, "(Nb sol /Nb) + (Nb pre /Nb)” is preferably 0.95 or less, more preferably 0.90 or less.
• the lower limit of the particle size of the Nb precipitates is not particularly limited; for example, the Nb precipitates may have a particle size of 0.1 nm or more.
• Amount of Diffusible Hydrogen in Steel 0.50 mass ppm or less
• the amount of diffusible hydrogen in the steel is made 0.50 mass ppm or less.
• the amount of diffusible hydrogen is preferably made 0.30 mass ppm or less, more preferably 0.25 mass ppm or less, even more preferably 0.20 mass ppm or less, and most preferably 0.15 mass ppm or less. While the lower limit of the amount of diffusible hydrogen in the steel is not particularly specified, in view of the restrictions of production technology, the amount of diffusible hydrogen is preferably made 0.01 mass ppm or more, more preferably 0.02 mass ppm or more, even more preferably set to 0.03 mass ppm or more, and most preferably 0.05 mass ppm or more.
• the steel material heating temperature needs to be equal to or higher than T sol °C. Therefore, the steel material heating temperature is made equal to or higher than T sol °C represented by the following Formula 2.
• the steel material heating temperature is preferably equal to or higher than "T sol ⁇ 1.1"°C, more preferably equal to or higher than "T sol ⁇ 1.2"°C, even more preferably equal to or higher than "T sol ⁇ 1.3"°C, and most preferably equal to or higher than "T sol ⁇ 1.5"°C.
• the heating time at the temperature is made 1.0 hours or more.
• the finish rolling start temperature is preferably made "T sol °C - 100°C” or higher, more preferably “T sol °C - 70°C” or higher, even more preferably “T sol °C - 50°C” or higher, and most preferably "T sol °C - 30°C” or higher.
• a finish rolling start temperature exceeding 1200°C may increase a scale loss during pre-heating of the slab, causing breakage of the sheet during hot rolling. Therefore, the finish rolling start temperature in hot rolling is preferably made 1200°C or lower, more preferably 1170°C or lower, even more preferably 1150°C or lower, and most preferably 1120°C or lower.
• the steel material after the heating is hot-rolled into a hot-rolled steel sheet. While the upper and lower limits of the delivery temperature in the finish rolling are not particularly specified, a finish rolling delivery temperature of less than 800°C will cause precipitation of Nb which has been dissolved during heating of the steel material, resulting in an increase in the amount of Nb precipitated. Further, such a delivery temperature increases a rolling load, and thus a rolling burden, which may impede a cold rolling process. Therefore, the finish rolling delivery temperature in hot rolling is preferably made 800°C or higher, more preferably 820°C or higher, even more preferably 840°C or higher, and most preferably 850°C or higher.
• the finish rolling delivery temperature is preferably made 1000°C or lower, more preferably 980°C or lower, even more preferably 970°C or lower, and most preferably 950°C or lower.
• Effective time t HR is herein defined as the time required from the start of finish rolling to the end of finish rolling. Since Nb contained in the steel sheet begins to precipitate and grow during finish rolling, the effective time t HR is one of the parameters for controlling the form of Nb to improve the LME resistance. While the upper and lower limits of the effective time t HR are not particularly specified, the effective time t HR is preferably made 3 seconds or more, more preferably 4 seconds or more, even more preferably 5 seconds or more, and most preferably 7 seconds or more. The effective time t HR is preferably made 15 seconds or less, more preferably 12 seconds or less, even more preferably 11 seconds or less, and most preferably 10 seconds or less.
• Residence time t CT is herein defined as the time from the finish rolling delivery temperature to 650°C. Since the precipitation of Nb and the growth of precipitated Nb are likely to proceed during the period from the completion of finish rolling to the sheet temperature reaching 650°C, the residence time t CT is one of the parameters for controlling the form of Nb to improve the LME resistance. While the upper and lower limits of the residence time t CT are not particularly specified, the residence time t CT is preferably made 5 seconds or more, more preferably 7 seconds or more, even more preferably 8 seconds or more, and most preferably 9 seconds or more. The residence time t CT is made 20 seconds or less, more preferably 18 seconds or less, even more preferably 17 seconds or less, and most preferably 15 seconds or less.
• the coiling temperature after hot rolling is made 650°C or lower.
• the coiling temperature is preferably 630°C or lower, more preferably 620°C or lower, even more preferably 610°C or lower, and most preferably 600°C or lower.
• the lower limit of the coiling temperature is not particularly limited, if the coiling temperature is lower than 300°C, the strength of the hot-rolled sheet increases, which may cause an increase in the rolling burden in cold rolling and a defective sheet shape, resulting in a reduction in productivity. Therefore, the lower limit of the coiling temperature is preferably made 300°C or higher, more preferably 320°C or higher, even more preferably 350°C or higher, and most preferably 400°C or higher.
• the resulting hot-rolled steel sheet may be subjected to intermediate heat treatment at a temperature below 650°C, as necessary, to prevent an increase in load in later cold rolling.
• the heat treatment is preferably performed at a temperature of 150°C or higher.
• the heat treatment may be carried out, for example, in a box annealing furnace at a soaking temperature of 500°C for a soaking time of 4 hours.
• the resulting hot-rolled steel sheet may be subjected to a treatment, such as pickling, as necessary. Pickling of the hot-rolled coil may be performed by a common method.
• the hot-rolled coil may also be subjected to skin pass rolling to correct its shape and enhance its pickling properties.
• the steel sheet may be subjected directly to the below-described annealing step (heat treatment), or may be subjected to cold rolling prior to the heat treatment.
• the cold rolling reduction is preferably made 25% or more, more preferably 30% or more, even more preferably 32% or more, and most preferably 35% or more.
• an excessive reduction causes a too-high rolling load, leading to an increase in the burden on a cold rolling mill. Therefore, the cold rolling reduction is preferably made 75% or less, more preferably 70% or less, even more preferably 67% or less, and most preferably 65% or less.
• the heating time t is one of the parameters for controlling the form of Nb to improve the LME resistance. While the upper and lower limits of the heating time t are not particularly specified, the heating time t is preferably made 300 seconds or more, more preferably 400 seconds or more, even more preferably 450 seconds or more, and most preferably 490 seconds or more. The heating time t is preferably made 700 seconds or less, more preferably 650 seconds or less, even more preferably 600 seconds or less, and most preferably 590 seconds or less.
• Holding time t AT at the soaking temperature T AT °C is one of the control parameters for promoting austenitizing of the steel sheet during soaking and, since it is related to the precipitation of Nb and the growth of Nb precipitates, is also one of the parameters for controlling the LME resistance. While the upper and lower limits of the holding time t AT are not particularly specified, the holding time t AT is preferably made 15 seconds or more in order to sufficiently promote austenitizing. The holding time t AT is more preferably made 30 seconds or more, even more preferably 50 seconds or more, and most preferably 100 seconds or more.
• Q HR 0.5 T FET + T FDT ⁇ log 10 t HR
• Q CT 0.5 T CT + 650 ⁇ log 10 t CT
• Q AT 0.5 650 + T AT ⁇ log 10 t + T AT ⁇ log 10 t AT Q HR + Q CT + Q AT ⁇ 6000
• Q HR , Q CT , and Q AT are each defined by the temperatures at the start and end of each of the effective time t HR , the residence time t CT , the heating time t, and the holding time t AT , during which heat of 650°C to T sol is applied.
• the sum of Q HR , Q CT , and Q AT is made 6000 or less, preferably 5990 or less, more preferably 5980 or less, even more preferably 5970 or less, and most preferably 5950 or less.
• Q AT is preferably made 4700 or less, more preferably 4650 or less, even more preferably 4630 or less, and most preferably 4600 or less.
• the steel sheet may be reheated.
• C is concentrated in untransformed austenite. This enhances the stability of austenite and increases the area fraction of retained austenite contained in the steel sheet after cooling, making it possible to further enhance the ductility.
• the reheating temperature is preferably made 200°C or higher, more preferably 210°C or higher, even more preferably 230°C or higher, and most preferably 250°C or higher.
• the reheating temperature is preferably made 450°C or lower, more preferably 430°C or lower, even more preferably 410°C or lower, and most preferably 400°C or lower.
• the coating weight (per one surface) is preferably 20 g/m 2 or more from the viewpoint of corrosion resistance and control of the coating weight.
• the coating weight is more preferably 25 g/m 2 or more, even more preferably 30 g/m 2 or more, and most preferably 32 g/m 2 or more.
• the coating weight is preferably 120 g/m 2 or less, more preferably 100 g/m 2 or less, even more preferably 70 g/m 2 or less, and most preferably 65 g/m 2 or less.
• the hot-dip galvanizing is preferably performed using a galvanizing bath containing 0.08% to 0.30% of Al.
• the amount of Al in the hot-dip galvanizing is preferably made 0.08% or more, more preferably 0.09% or more, even more preferably 0.10% or more, and most preferably 0.12% or more.
• the amount of Al in the hot-dip galvanizing is preferably made 0.30% or less, more preferably 0.25% or less, even more preferably 0.22% or less, and most preferably 0.20% or less.
• the treatment is performed in a temperature range of 450°C to 600°C. If the alloying treatment is performed at a temperature exceeding 600°C, untransformed austenite will be transformed into pearlite, and the area fraction of retained austenite will be less than 5%, which may result in a reduction in ductility. Therefore, when an alloying treatment of a galvanized coating is performed, the alloying treatment is preferably performed in a temperature range of not less than 450°C, more preferably not less than 460°C, even more preferably not less than 465°C, and most preferable not less than 470°C.
• the alloying treatment is preferably performed in a temperature range of not more than 600°C, more preferably not more than 570°C, even more preferably not more than 550°C, and most preferably not more than 530°C.
• the alloyed coating layer of the hot-dip galvanized steel sheet preferably has an Fe concentration of 8% to 17%.
• the Fe concentration of the alloyed coating layer of the hot-dip galvanized steel sheet is preferably 8% or more, more preferably 9% or more, and even more preferably 10% or more.
• the Fe concentration of the alloyed coating layer of the hot-dip galvanized steel sheet is preferably 17% or less, more preferably 16% or less, and even more preferably 15% or less.
• the alloyed coating layer is thus formed by performing the alloying treatment on the hot-dip galvanized steel sheet.
• the steel sheet after annealing was held at a holding temperature of 200°C to 450°C for 10 seconds or more, and then cooled to room temperature. In either case, the cooled steel sheet was cold-rolled at a rolling reduction of 50% to obtain a high-strength cold-rolled steel sheet (CR).
• CR high-strength cold-rolled steel sheet
• HR hot-rolled steel sheets
• G hot-dip galvanized steel sheets
• GA galvannealed steel sheets
• a galvanizing bath containing 0.19 mass % Al was used for the hot-dip galvanized steel sheets (GI), and a galvanizing bath containing 0.14 mass % Al was used for the galvannealed steel sheets (GA).
• the temperature of each bath was 465°C.
• the coating weight was 45 g/m 2 per one surface (double-sided coating).
• the Fe concentration in the coating layer was adjusted to be within the range of 9 mass % to 12 mass %.
• TM tempered martensite
• B bainite
• FM fresh martensite
• yR retained austenite
• the area fractions of fresh martensite, tempered martensite, and bainite were determined by the following method.
• the cross-section was observed by a scanning electron microscope (SEM) at 2000-fold magnification in 10 fields of view at a 1/4 thickness position (a position at a distance of 1/4 of the sheet thickness from the steel sheet surface).
• SEM scanning electron microscope
• the area fractions of constituent microstructures (the total fraction of tempered martensite and bainite, and the fraction of fresh martensite) were calculated.
• fresh martensite was determined by a light gray microstructural region
• tempered martensite and bainite were determined by a dark gray region in which carbides were precipitated.
• the area fraction of retained austenite was determined by the following method. Each steel sheet was polished from the 1/4 thickness position to a plane by a thickness of 0.1 mm, and then further chemically polished to a plane by a thickness of 0.1 mm. The plane (surface) was subjected to X-ray diffraction analysis using CoK ⁇ rays to measure the integrated intensity ratios of the diffraction peaks of the ⁇ 200 ⁇ , ⁇ 220 ⁇ and ⁇ 311 ⁇ planes of fcc iron, and the ⁇ 200 ⁇ , ⁇ 211 ⁇ and ⁇ 220 ⁇ planes of bcc iron. The nine integrated intensity ratios obtained were averaged to determine the area fraction of retained austenite.
• the amount of hydrogen in steel was determined by the following method. Test specimens of about 5 ⁇ 30 mm were cut out from a hot-rolled steel sheet, a cold-rolled steel sheet, and a galvanized steel sheet. For the test specimen of galvanized steel sheet, the coating on the surface of the test specimen was removed in advance using a router (precision grinder). Each test specimen was placed in a quartz tube, and the internal atmosphere of the tube was replaced with Ar, and then heated at 200°C/hr. Hydrogen generated until the temperature reached 400°C was subjected to gas chromatography to measure the amount of released hydrogen by a temperature rise analysis method. The cumulative value of the amount of hydrogen detected in the temperature range from room temperature (25°C) to less than 250°C was taken as the amount of diffusible hydrogen.
• the total amount of Nb contained in the steel (Nb total ), the amount of dissolved Nb in the steel sheet (Nb sol ), and the amount of Nb in Nb precipitates having a particle size of less than 20 nm (Nb pre ) were measured in the following manner.
• the total amount of Nb contained in the steel (Nb) was measured by wet chemical analysis.
• the amount of Nb in Nb precipitates having a particle size of 20 nm or less was measured by the following method. Precipitates in the steel material were captured as residues, and the amount of Nb in all residues was determined. Thereafter, the amount of Nb present in residues having a particle size of 20 nm or more was determined.
• the amount of Nb in Nb precipitates having a particle size of 20 nm or less was determined as the difference between the amount of Nb in all residues and the amount of Nb present in the residues having a particle size of 20 nm or more. Specific procedures are as follows. A plurality of test specimens of about 20 ⁇ 50 mm were cut out from a hot-rolled steel sheet, a cold-rolled steel sheet, or a galvanized steel sheet. For a test specimen of a galvanized steel sheet, the coating on the surface of the test specimen was removed in advance using a router (precision grinder). The surface of each test specimen was polished about 50 ⁇ m by preliminary electrolytic polishing to obtain a fresh surface.
• the resulting test specimen was subjected to electrolysis using 10 vol % acetylacetone - 1 mass % tetramethylammonium chloride - methanol as an electrolytic solution for extracting precipitates.
• the resulting electrolytic solution after electrolysis was passed through a filter having a pore size of 0.2 ⁇ m to capture residues.
• the residues were then decomposed with an acid, and the Nb concentration was quantified in mass % unit using ICP emission spectrometry. The value obtained was used as the amount of Nb in all residues.
• a remaining test specimen was subjected to electrolysis using 10 vol % acetylacetone - 1 mass % tetramethylammonium chloride - methanol.
• the residues captured on the alumina filter had a particle size of 20 nm or more.
• the residues were decomposed with an acid, and the Nb concentration was quantified in mass % unit using ICP emission spectrometry.
• the value obtained was used as the amount of Nb present in the residues having a particle size of 20 nm or more.
• the difference between the amount of Nb in all residues and the amount of Nb present in the residues having a particle size of 20 nm or more was taken as the amount of Nb in Nb precipitates having a particle size of 20 nm or less (Nb pre ).
• the amount of dissolved Nb in the steel sheet (Nb sol ) was determined as the difference between the total amount of Nb contained in the steel (Nb) and the amount of Nb in all residues determined by the above method.
• Nb precipitates are composed mainly of NbC
• an NbN precipitate or other Nb precipitates may be contained in the steel sheet.
• a tensile test was performed according to JIS Z 2241 (2011) using a JIS No. 5 test specimen which had been taken from a steel sheet such that the tensile direction was perpendicular to the rolling direction of the steel sheet, and the TS (tensile strength) and EL (total elongation) of the test specimen were measured.
• steel sheets having a TS of 980 MPa or more were evaluated as acceptable, judging that the intended strength was achieved.
• Steel sheets having a "TS 1.5 ⁇ El" of 390,000 or more were evaluated as acceptable, judging that the intended formability was achieved.
• the LME resistance was evaluated in the following manner.
• a sample having the dimensions of: 100 mm in a direction perpendicular to the rolling direction, and 30 mm in the rolling direction, was taken from a steel sheet.
• the sample and a 980GA sample having the same size were stacked to prepare an evaluation sample.
• Resistance spot welding was performed on the evaluation sample under the conditions of an electrode inclination angle of ⁇ and a welding pressure of 3.5 kN.
• the electrode inclination angle in spot welding is herein defined as the angle ⁇ between a line passing through the major axis of a nugget and a line parallel to the surface of a steel sheet in a cross-section of a spot-welded member.
• the welding current pattern was controlled so that the diameter of a nugget formed would fall within the range of 3.5 ⁇ t to 5.5 ⁇ t, t being the thickness (1.2 mm) of one steel sheet.
• Dr6-type CuCr electrodes were used, and the clearance between the evaluation sample and an electrode was 2.0 mm.
• the electrode inclination angles and holding times in the LME resistance evaluation tests are shown in Tables 3-1 and 3-2.
• the high-strength steel sheets of the Inventive Examples all have a high TS and a good TS ⁇ El balance, and are excellent in LME resistance, whereas the steel sheets of the Comparative Examples are poor in at least one of TS, TS ⁇ El balance, and LME resistance.

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