Classifications

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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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • 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
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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
  • C21D8/021—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
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  • 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
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  • 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
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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
  • C22C38/002—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
  • C22C38/08—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
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—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 manufacturing method thereof.
• the present invention relates to a high-strength steel sheet with excellent LME resistance that is suitable for use as a component in the automotive, electrical, and other industrial fields formed by cold pressing, and a manufacturing method thereof.
• impact angle refers to a state in which the axis of the welding electrode is not perpendicular to the surface of the steel plate.
• Patent Document 1 discloses a technology that realizes a high-strength steel sheet with excellent LME resistance, a tensile strength of 980 MPa or more, a total elongation of 20% or more, and excellent LME resistance, by controlling the frequency of corresponding grain boundaries in the steel sheet surface layer after high-temperature tensile testing and the thickness of the softened surface layer.
• Patent Document 2 discloses a technology for realizing a steel sheet having LME resistance, high strength, and excellent ductility by introducing oxygen into the surface layer of a steel slab during continuous casting to form iron oxide, which is a site for the formation of titanium nitride, and inhibiting the bonding of B in the steel with dissolved nitrogen in the subsequent annealing process and promoting the formation of (Fe, Mn) 2 B in the surface layer region of the steel sheet.
• Patent Document 3 also proposes a technique for preventing LME cracking by removing the plating layer from the area to be welded prior to spot welding.
• Patent Document 1 requires the reduction of Si in order to reduce the frequency of corresponding grain boundaries in the steel sheet surface layer, and it is presumed that it is difficult to impart good formability to steel sheets with strength levels higher than the 980 MPa class.
• Patent Document 3 requires a step of removing the plating layer in advance, which increases manufacturing costs. In addition, because the plating layer is removed, it is believed that the corrosion resistance of the welded portion decreases.
• the present invention aims to provide steel sheets that have excellent LME resistance, high strength and good formability, and a manufacturing method thereof, using a method different from that of conventional technology.
• the above-mentioned steel sheets include hot-rolled steel sheets, cold-rolled steel sheets, and plated steel sheets such as GA and GI.
• high strength refers to a TS of 980 MPa or more
• good formability refers to satisfying the relationship between tensile strength TS and elongation El shown in the following formula (7). (Formula 7) TS 1.5 ⁇ El ⁇ 390000
• the inventors conducted extensive research into the chemical composition and microstructure of steel sheets. As a result, by precisely controlling the slab heating conditions, temperature management from hot rolling to coiling, and annealing conditions, and by controlling the state of elements contained in the steel, they discovered the following:
• the dissolved elements present in the steel during welding, or the dissolved elements produced by dissolving the precipitates prevent the intrusion of zinc, improving LME resistance.
• the precipitate radius of a particular element is larger than a specified value, the precipitates will not dissolve sufficiently during welding, and not only will it be impossible to expect improvement in LME resistance due to the dissolved elements, but it is quite possible that new cracks will occur starting from the coarse precipitates not only during manufacturing but also after press working and during welding.
• the present invention has been made based on the above findings, and the gist of the present invention is as follows.
• the composition is composed of Fe and unavoidable impurities.
• the microstructure of the steel plate at the 1/4 position of the plate thickness is The total area ratio of tempered martensite and bainite is 40% or more and 85% or less, The area ratio of fresh martensite is 0% or more and 25% or less, The area ratio of retained austenite is 5% or more and 20% or less, The remainder is at least one of ferrite and pearlite in an area ratio of 0% to 20%.
• the Nb has a relationship between the amount of dissolved Nb (Nb sol ), the amount of Nb in Nb precipitates having a grain size of less than 20 nm (Nb pre ), and the total amount of Nb (Nb) contained in the steel sheet, which satisfies the following (Formula 1): A high-strength steel plate having a diffusible hydrogen content of 0.50 mass ppm or less. (Formula 1) (Nb sol /Nb) + (Nb pre /Nb) ⁇ 0.40 In formula 1, Nb sol represents the amount of dissolved Nb (mass %), and Nb pre represents the amount of Nb (mass %) in Nb precipitates having a grain size of less than 20 nm.
• V 0.500% or less by mass%, 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, Bi: 0.200% or less,
• the high-strength steel plate according to [1] comprising one or more selected from the following: [3] The high-strength steel sheet according to [1] or [2], having a plating layer on the steel sheet surface.
• [5] A method for producing a high strength steel plate according to the above [1] or [2], A slab heating process in which a steel material having the above-mentioned composition is heated at a temperature T sol ° C. or higher represented by the following (Equation 2) for 1.0 hour or more; A hot rolling process; a winding step in which the winding temperature T CT ° C. is 650 ° C. or less; An annealing process includes heating to a soaking temperature T AT ° C., holding the soaking temperature T AT ° C., which is 750 ° C. or more and 950 ° C.
• QHR is defined by the finish rolling start temperature TFET °C, the finish rolling end temperature TFDt °C, and the effective time tHR from the start of finish rolling to the end of finish rolling, as shown in (Equation 3).
• Q CT is defined by a residence time t CT from T FDT °C to 650 °C and a coiling temperature T CT °C, as shown in (Equation 4).
• the present invention it is possible to obtain high-strength steel sheets that have good formability and excellent LME resistance. Therefore, the present invention is of great value in industrial fields such as automobiles and electrical equipment, and is particularly useful for reducing the weight of automobile body frame parts.
• C 0.030% or more and 0.500% or less C is an element necessary for increasing the strength of tempered martensite, bainite, and fresh martensite. In order to fully obtain this effect, it is necessary to make the C content at least 0.030% or more. Therefore, the C content is set to 0.030% or more. It is preferably 0.050% or more. It is more preferably 0.070% or more. It is further preferably 0.090% or more, and most preferably 0.100% or more. On the other hand, if the C content exceeds 0.500%, the weldability and LME resistance properties, which are important when joining automobile parts, deteriorate. Therefore, the C content is set to 0.500% or less. It is preferably 0.400% or less. It is more preferably 0.300% or less. It is further 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 suppresses the excessive formation and growth of carbides in steel, increases the residual austenite fraction, and improves ductility. If the Si content is 0.01% or less, this effect becomes poor and good formability cannot be obtained, so the lower limit is set to 0.01%.
• the Si content is set to more than 0.01%. It is preferably 0.05% or more. It is more preferably 0.10% or more. It is further 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 zinc melting point, which makes it easier for zinc to penetrate into the steel sheet during welding, and as a result, the LME resistance of the steel sheet is reduced. Therefore, the Si content is set to 2.50% or less. It is preferably 2.30% or less. It is more preferably 2.00% or less. It is further preferably 1.80% or less. It is most preferably 1.60% or less.
• Mn 0.10% or more and 5.00% or less Mn is an element that affects the area ratio of tempered martensite, bainite, and fresh martensite by improving hardenability. If the Mn content is less than 0.10%, soft phases such as ferrite are excessively generated, and the desired area ratios of tempered martensite, bainite, and fresh martensite cannot be obtained, resulting in insufficient steel sheet strength. For this reason, the Mn content is set to 0.10% or more. It is preferably set to 0.50% or more. It is more preferably set to 0.80% or more. It is further preferably set to 1.00% or more, and most preferably set to 2.00% or more.
• the Mn content is set to 5.00% or less. It is preferably set to 4.50% or less. It is more preferably set to 4.00% or less. It is further preferably set to 3.70% or less, and most preferably set to 3.50% or less.
• P 0.100% or less
• P may have an adverse effect on LME resistance properties by segregating at grain boundaries and causing embrittlement, so its amount must be 0.100% or less. Therefore, the P content is set to 0.100% or less. It is preferably set to 0.080% or less. It is more preferably set to 0.070% or less. It is further preferably set to 0.050% or less. It is most preferably set to 0.040% or less. Although there is no particular lower limit, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.
• S 0.0200% or less S segregates at grain boundaries to embrittle steel during hot working, and may also adversely affect LME resistance by forming sulfides, so the amount must be 0.0200% or less. Therefore, the S content is 0.0200% or less. It is preferably 0.0180% or less. It is more preferably 0.0150% or less. It is even more preferably 0.0100% or less, and most preferably 0.0050% or less.
• the lower limit is not particularly limited, but it is preferably 0.0001% or more due to constraints on production technology. It is more preferably 0.0005% or more, and even more preferably 0.0010% or more.
• Al acts as a deoxidizer and is an effective element for reducing inclusions in steel, and is preferably contained in the deoxidation process.
• the content of Al is set to 0.100% or less. It is preferably set to 0.080% or less. It is more preferably set to 0.070% or less, even more preferably set to 0.060% or less, and most preferably set to 0.050% or less.
• the lower limit is not particularly limited, it is preferable that the content of Al is set to 0.001% or more. It is more preferably set to 0.010% or more, and even more preferably set to 0.020% or more.
• N has an adverse effect on LME resistance properties by forming coarse nitrides, and when the N content exceeds 0.0100%, a large amount of coarse nitrides is formed, and the deterioration of LME resistance properties becomes significant.
• the lower limit is not particularly limited, but it is preferably 0.0001% or more due to constraints on production technology. It is more preferably 0.0010% or more, and even more preferably 0.0020% or more.
• O 0.0100% or less
• O exists as an oxide and reduces the ductility of the steel sheet. Therefore, the content of O needs to be 0.0100% or less. Therefore, the content of O is 0.0100% or less. It is preferably 0.0075% or less. It is more preferably 0.0060% or less. It is further preferably 0.0050% or less, and most preferably 0.0045% or less.
• the lower limit of the content of O is not particularly specified, due to constraints on production technology, it is preferable that the content of O is 0.0001% or more. It is more preferably 0.0005% or more, and even more preferably 0.0010% or more.
• Ti 0.010% or more and 0.200% or less Ti contributes to precipitation strengthening, and further refines the prior austenite grain size and the accompanying tempered martensite and bainite, so that it is effective in improving the strength of the steel.
• the Ti content is set to 0.010% or more. It is preferably 0.012% or more. It is more preferably 0.015% or more. It is even more preferably 0.020% or more. It is most preferably 0.025% or more.
• the Ti content is set to 0.200% or less. It is preferably 0.180% or less. It is more preferably 0.150% or less, even more preferably 0.100% or less, and most preferably 0.050% or less.
• Nb 0.005% or more and 0.500% or less
• Nb is an element that improves the LME resistance characteristics
• the presence of solute Nb in steel during welding has a significant effect on the improvement of LME resistance characteristics.
• the solute elements present in the steel during welding or the solute elements generated by dissolving the precipitates prevent the intrusion of zinc, thereby improving the LME resistance characteristics.
• the Nb content needs to be 0.005% or more. It is preferably 0.007% or more. It is more preferably 0.008% or more. It is further preferably 0.010% or more, and most preferably 0.012% or more.
• the Nb content is set to 0.500% or less, preferably 0.400% or less, more preferably 0.350% or less, even more preferably 0.200% or less, and most preferably 0.100% or less.
• the high-strength steel plate according to one embodiment of the present invention has a composition containing the above-mentioned components with the balance including Fe and unavoidable impurities.
• the high-strength steel plate according to one embodiment of the present invention has a composition containing the above-mentioned components with the balance consisting of Fe and unavoidable impurities.
• unavoidable impurities include Zn, Pb, and As. It is acceptable for these impurities to be contained in a total amount of 0.100% or less.
• the alloy may contain one or more of the following elements by mass: 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, Bi: 0.200% or less.
• V 0.500% or less V contributes to precipitation strengthening, and further refines the prior austenite grain size and the accompanying tempered martensite and bainite, so that it is effective in improving the strength of the steel, and can be contained as necessary.
• the V content is preferably 0.001% or more. It is more preferably 0.005% or more, and even more preferably 0.010% or more. However, if it is contained in an amount exceeding 0.500%, V may remain in an undissolved state during heating of the steel material before hot rolling, and coarse precipitates may increase, resulting in a decrease in ductility. Therefore, when V is contained, the V content is 0.500% or less. It is preferably 0.400% or less. It is 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, produces alloy carbides and alloy carbonitrides to contribute to high strength.
• Ta can be contained as necessary to stabilize the contribution of precipitation strengthening to strength by forming a composite precipitate such as (Nb, Ta) (C, N) by partially dissolving in Nb carbides and Nb carbonitrides and significantly suppressing the coarsening of precipitates.
• the lower limit is not particularly limited, but in order to obtain the above effect, the Ta content is preferably 0.01% or more. It is more preferable to make it 0.02% or more, and even more preferable to make it 0.03% or more. However, even if Ta is contained in excess, the precipitate stabilization effect is saturated and the alloy cost also increases. Therefore, when Ta is contained, the Ta content is 0.10% or less. It is preferably 0.08% or less. It is more preferably 0.07% or less. It is even more preferably 0.06% or less, and most preferably 0.05% or less.
• W 0.10% or less W can be contained as necessary to improve the hardenability of steel and to further improve the strength of steel by refining tempered martensite and bainite.
• the content of W is preferably 0.01% or more. It is more preferable to make it 0.02% or more, and even more preferable to make it 0.03% or more.
• the content of W is 0.10% or less. It is preferably 0.08% or less. More preferably, it is 0.07% or less. It is even more preferably 0.06% or less, and most preferably 0.05% or less.
• B 0.0100% or less
• B is an element that can improve hardenability by segregating at austenite grain boundaries, and can form a structure mainly composed of tempered martensite and bainite, thereby improving the strength of the steel sheet, and can contribute to improving the LME resistance properties, so it can be contained as necessary.
• the content of B is preferably 0.0003% or more. It is more preferable to make it 0.0005% or more, and even more preferable to make it 0.0007% or more. However, if it is contained in excess of 0.0100%, coarse precipitates are generated and ductility is reduced. Therefore, when B is contained, the content of B is 0.0090% or less. It is preferably 0.0080% or less. It is more preferably 0.0070% or less. It is even more preferably 0.0050% or less, and most preferably 0.0030% or less.
• Cr 1.00% or less Cr has the effect of improving the balance between strength and ductility, so it can be contained as necessary.
• the Cr content is preferably 0.01% or more. It is more preferable to make it 0.05% or more, and even more preferable to make it 0.07% or more.
• the Cr content is 1.00% or less. It is preferably 0.80% or less. It is more preferably 0.60% or less. It is even more preferably 0.50% or less. It is most preferably 0.30% or less.
• Mo 1.00% or less Mo has the effect of improving the balance between strength and ductility, so it can be contained as necessary.
• the Mo content is preferably 0.01% or more. It is more preferable to make it 0.05% or more, and even more preferable to make it 0.07% or more.
• the Mo content is 1.00% or less. It is preferably 0.80% or less. It is more preferably 0.50% or less. It is even more preferably 0.30% or less, and most preferably 0.20% or less.
• Co 1.00% or less
• Co is an element effective in improving hardenability and is effective in strengthening steel, so it can be contained as necessary.
• the Co content is preferably 0.01% or more. It is more preferable to make it 0.05% or more, and even more preferable to make it 0.07% or more.
• the Co content is 1.00% or less. It is preferably 0.80% or less. More preferably, it is 0.60% or less. It is further preferably 0.30% or less, and most preferably 0.20% or less.
• Ni 1.00% or less Ni increases the strength of steel by solid solution strengthening, so it can be contained as necessary.
• the Ni content is preferably 0.01% or more. It is more preferable to make it 0.05% or more, and even more preferable to make it 0.07% or more.
• the Ni content is 1.00% or less. It is preferably 0.80% or less. More preferably, it is 0.60% or less. It is even more preferably 0.30% or less, and most preferably 0.20% or less.
• Cu 1.00% or less
• the Cu content is preferably 0.01% or more. It is more preferable to make it 0.05% or more, and even more preferable to make it 0.07% or more. However, if it is contained in excess of 1.00%, the area ratio of tempered martensite, bainite, and fresh martensite becomes excessive, and the dimensional accuracy and ductility during forming are reduced. Therefore, when Cu is contained, the Cu content is 1.00% or less. It is preferably 0.80% or less. More preferably, it is 0.60% or less. It is even more preferably 0.30% or less, and most preferably 0.20% or less.
• Sn 0.200% or less
• Sb 0.200% or less
• Sn and Sb suppress decarburization of the steel sheet surface layer in a region of several tens of ⁇ m caused by nitriding or oxidation of the steel sheet surface, and prevent the area ratio of tempered martensite on the steel sheet surface from decreasing.
• they can be contained as necessary to ensure strength and material stability.
• the lower limit is not particularly limited, in order to obtain the above effect, it is preferable that the content of these elements is 0.001% or more. It is more preferable to make it 0.003% or more, and even more preferable to make it 0.005% or more.
• the content of Sn and Sb is 0.200% or less. It is preferably 0.100% or less. It is more preferably 0.070% or less. It is even more preferably 0.050% or less, and most preferably 0.030% or less.
• Ca 0.0100% or less
• Mg 0.0100% or less
• REM 0.0100% or less
• the contents of Ca, Mg and REM are set to 0.0100% or less.
• they are 0.0050% or less. More preferably, they are 0.0040% or less, even more preferably, they are 0.0035% or less, and most preferably, they are 0.0030% or less.
• the lower limits of the contents of Ca, Mg and REM are not particularly specified, but since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, it is preferable that the contents of Ca, Mg and REM are each 0.0001% or more.
• the content is more preferably 0.0005% or more, even more preferably 0.0007% or more, and most preferably 0.0010% or more.
• Zr and Te 0.100% or less
• Zr and Te are each 0.100% or less, coarse precipitates and inclusions do not increase and do not affect the precipitation of Nb, so the LME resistance characteristics do not deteriorate. Therefore, when Zr and Te are contained, the contents of Zr and Te are 0.100% or less. Preferably, they are 0.080% or less. More preferably, they are 0.070% or less, even more preferably, they are 0.060% or less, and most preferably, they are 0.050% or less.
• the contents of Zr and Te are not particularly specified, since they are elements that spheroidize the shape of nitrides and sulfides and improve the ultimate deformability of the steel sheet, it is preferable that the contents of Zr and Te are 0.001% or more. It is more preferable to set them to 0.010% or more, and even more preferable to set them to 0.020% or more.
• Hf 0.10% or less If Hf is 0.10% or less, coarse precipitates and inclusions do not increase, and Hf does not affect the precipitation of Nb, so that the LME resistance characteristics do not deteriorate. Therefore, when Hf is contained, the content of Hf is 0.10% or less. It is preferably 0.080% or less. It is more preferably 0.070% or less, even more preferably 0.060% or less, and most preferably 0.050% or less.
• the lower limit of the content of Hf is not particularly specified, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, when Hf is contained, it is preferable that the content of Hf is 0.003% or more. It is more preferable that it is 0.010% or more. It is more preferable that it is 0.020% or more, and even more preferable that it is 0.030% or more.
• Bi 0.200% or less If Bi is 0.200% or less, coarse precipitates and inclusions do not increase, and since Bi does not affect the precipitation of Nb, the LME resistance characteristics do not deteriorate. Therefore, when Bi is contained, the content of Bi is 0.200% or less. It is preferably 0.100% or less. It is more preferably 0.050% or less, even more preferably 0.030% or less, and most preferably 0.020% or less. Although the lower limit of the content of Bi is not particularly specified, since Bi is an element that reduces segregation, it is more preferable that the content of Bi is 0.001% or more. It is more preferably 0.005% or more, and even more preferably 0.010% or more.
• Area ratio of tempered martensite and bainite 40% or more and 85% or less Tempered martensite and bainite contribute to the strength of the steel sheet. Specifically, making the steel sheet mainly composed of tempered martensite and bainite is effective in maintaining high strength. In order to fully obtain such effects, the total area ratio of bainite and tempered martensite needs to be at least 40% or more. It is preferably 42% or more. It is more preferably 45% or more, even more preferably 47% or more, and most preferably 50% or more. On the other hand, if the sum of the area ratios of bainite and tempered martensite exceeds 85%, the steel structure is occupied by hard phases including tempered martensite, bainite, and fresh martensite.
• the total area ratio of bainite and tempered martensite needs to be 85% or less. It is preferably 83% or less. More preferably, it is 82% or less, even more preferably, it is 81% or less, and most preferably, it is 80% or less.
• Fresh martensite area ratio 0% or more and 25% or less
• Fresh martensite is a very hard phase, so it improves the strength of the steel.
• Fresh martensite is not necessarily required when the strength of the steel sheet is guaranteed, but if the steel sheet structure contains fresh martensite, the strength of the steel sheet is further improved and it is possible to achieve even higher strength. Therefore, the area ratio of fresh martensite needs to be 0% or more. It is preferably 2% or more. More preferably, it is 3% or more. Even more preferably, it is 4% or more, and most preferably, it is 5% or more.
• fresh martensite reduces the ductility of the steel, making it difficult to achieve good formability. Therefore, the area ratio of fresh martensite needs to be 25% or less. It is preferably 23% or less. More preferably, it is 22% or less. Even more preferably, it is 21% or less, and most preferably, it is 20% or less.
• Area ratio of retained austenite 5% or more and 20% or less Retained austenite transforms into martensite due to the TRIP effect during processing, and at the same time, it improves the strength and improves the strain dispersion ability, thereby improving the ductility. Therefore, in order to ensure good formability, the area ratio of retained austenite needs to be 5% or more. It is preferably 7% or more, more preferably 8% or more. Even more preferably 9% or more, and most preferably 10% or more.
• the area ratio of retained austenite needs to be 20% or less. It is preferably 18% or less. More preferably, it is 17% or less. Even more preferably, it is 16% or less, and most preferably, it is 15% or less.
• the residual structure is deemed to include at least one of ferrite and pearlite, and specifically, the area ratio of at least one of ferrite and pearlite is 0% or more.
• the area ratio of the residual structure is preferably 20% or less. More preferably, it is 18% or less. Even more preferably, it is 15% or less. Most preferably, it is 13% or less.
• Nb sol represents the amount of dissolved Nb (mass %)
• Nb pre represents the amount of Nb (mass %) in Nb precipitates having a grain size of less than 20 nm.
• One of the important constituent items of the present invention is to determine the relationship between the amount of dissolved Nb in a steel sheet (Nb sol , unit: mass %), the amount of Nb in Nb precipitates with a grain size of less than 20 nm (Nb pre , unit: mass %), and the total amount of Nb (Nb) contained in the steel.
• LME due to transfer LME is prevented and LME resistance is improved by making Nb in the steel sheet into a solid solution state or making it possible to make it into a solid solution state during welding.
• the inventors have changed the state of Nb contained in a steel sheet by producing various steels, and have found that LME resistance is improved when (Formula 1) is satisfied.
• (Nb sol /Nb) + (Nb pre /Nb) is preferably 0.41 or more. More preferably, it is 0.42 or more.
• (Nb sol /Nb) + (Nb pre /Nb) is preferably 0.95 or less. More preferably, it is 0.90 or less.
• the lower limit of the Nb precipitates is not particularly limited, the particle size may be 0.1 nm or more.
• Amount of diffusible hydrogen in steel is 0.50 mass ppm or less. In order to ensure ductility and have good formability, the amount of diffusible hydrogen in steel is set to 0.50 mass ppm or less. It is preferable to set it to 0.30 mass ppm or less. It is more preferable to set it to 0.25 mass ppm or less. It is even more preferable to set it to 0.20 mass ppm or less, and it is most preferable to set it to 0.15 mass ppm or less. Although there is no particular lower limit for the amount of diffusible hydrogen in steel, due to constraints on production technology, it is preferable that the amount of diffusible hydrogen in steel is 0.01 mass ppm or more. It is more preferable to set it to 0.02 mass ppm or more. It is even more preferable to set it to 0.03 mass ppm or more, and it is most preferable to set it to 0.05 mass ppm or more.
• the heating temperature of the steel material needs to be Tsol ° C. or higher.
• the heating temperature of the steel material is set to Tsol ° C. or higher, which is expressed by the following formula (Formula 2). It is preferably Tsol ⁇ 1.1 ° C. or higher. More preferably, it is Tsol ⁇ 1.2 ° C.
• the start temperature of the finish rolling is preferably Tsol ° C-100 ° C or higher. More preferably, it is Tsol ° C-70 ° C or higher. Even more preferably, it is Tsol ° C-50 ° C or higher.
• the start temperature of the finish rolling of the hot rolling is preferably 1200 ° C or lower. More preferably, it is 1170 ° C or lower. Even more preferably, it is 1150 ° C or lower. The most preferred temperature is 1120° C. or lower.
• Finishing temperature of hot rolling T FDT °C
• the steel material after heating is hot-rolled to become a hot-rolled steel sheet.
• the upper and lower limits of the end temperature of the finish rolling are not particularly set, but if the finish rolling end temperature is less than 800°C, Nb dissolved during heating of the steel material will precipitate, and the amount of Nb precipitated will increase. Furthermore, the rolling load will increase, and the rolling load will become large, which may hinder cold rolling. Therefore, the finish rolling end temperature of the hot rolling is preferably 800°C or higher. More preferably, it is 820°C or higher. Even more preferably, it is 840°C or higher. Most preferably, it is 850°C or higher.
• the finish temperature exceeds 1000°C, the amount of oxide (scale) generated will increase rapidly, the interface between the base steel and the oxide will become rough, and the surface quality after pickling and cold rolling will tend to deteriorate, and the crystal grains will become excessively coarse, which may cause surface roughness of the pressed product during processing, so it is preferably 1000°C or lower. More preferably, it is 980°C or lower. Even more preferably, it is 970°C or lower. The most preferable temperature is 950° C. or lower.
• Effective time The time required from the start of finish rolling to the end of finish rolling is defined as the effective time tHR . Since Nb contained in the steel sheet starts to precipitate and grow during finish rolling, the effective time tHR is one of the parameters for controlling the form of Nb to improve the LME resistance. Although there is no particular upper or lower limit for the effective time tHR , it is preferably 3 seconds or more. More preferably, it is 4 seconds or more. Even more preferably, it is 5 seconds or more, and most preferably, it is 7 seconds or more. Moreover, the effective time tHR is preferably 15 seconds or less. More preferably, it is 12 seconds or less. Even more preferably, it is 11 seconds or less, and most preferably, it is 10 seconds or less.
• the residence time t CT is one of the parameters for controlling the form of Nb to improve the LME resistance characteristics.
• the residence time t CT is 20 seconds or less. More preferably, it is 18 seconds or less. Even more preferably, it is 17 seconds or less, and most preferably, it is 15 seconds or less.
• Coiling temperature T CT ° C. after hot rolling 650 ° C. or less If the coiling temperature after hot rolling is higher than 650 ° C., the precipitation of Nb and the growth of precipitated Nb proceed excessively, and (Formula 1) is no longer satisfied, which may cause deterioration of LME resistance characteristics, and may cause an oxide film that is difficult to remove by pickling to form on the surface of the hot-rolled sheet, which may cause a deterioration of the surface appearance after cold rolling. Therefore, the coiling temperature after hot rolling is set to 650 ° C. or less. It is preferably 630 ° C. or less. More preferably, it is 620 ° C. or less. It is further preferably 610 ° C.
• the lower limit of the coiling temperature is preferably 300 ° C. or more. More preferably, it is 320 ° C. or more. More preferably, the temperature is 350° C. or higher, and most preferably, the temperature is 400° C. or higher.
• the obtained 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 during subsequent cold rolling.
• a temperature of 150°C or higher it is preferable to use a temperature of 150°C or higher.
• heat treatment may be performed in a box annealing furnace with a soaking temperature of 500°C and a soaking time of 4 hours.
• the obtained hot-rolled steel sheet (hot-rolled coil) may be subjected to treatment such as pickling, if necessary.
• the pickling method for the hot-rolled coil may follow a conventional method.
• Skin-pass rolling may also be performed to correct the shape of the hot-rolled coil and improve its pickling properties.
• the steel may be subjected to the annealing process (heat treatment) described below, or may be subjected to cold rolling and then heat treatment.
• the cold reduction is preferably 25% or more, more preferably 30% or more, even more preferably 32% or more, and most preferably 35% or more.
• the upper limit of the cold rolling reduction is preferably 75% or less, more preferably 70% or less, even more preferably 67% or less, and most preferably 65% or less.
• the temperature rise time t is one of the parameters for controlling the form of Nb to improve the LME resistance characteristics.
• the temperature rise time t is preferably 300 seconds or more. More preferably, it is 400 seconds or more. Even more preferably, it is 450 seconds or more, and most preferably, it is 490 seconds or more.
• the temperature rise time t is preferably 700 seconds or less. More preferably, it is 650 seconds or less. Even more preferably, it is 600 seconds or less, and most preferably, it is 590 seconds or less.
• T AT is 750°C or more and 950°C or less
• the soaking temperature T AT °C is set to 750°C or more.
• it is set to 770°C or more. More preferably, it is set to 800°C or more, and further preferably, it is set to 840°C or more.
• the soaking temperature T AT °C is set to 950°C or less.
• it is set to 940°C or less. More preferably, it is set to 930°C or less, and further preferably, it is set to 920°C or less.
• the holding time t AT at the soaking temperature T AT ° C. is one of the control parameters for promoting austenitization of the steel sheet during soaking, and at the same time, since it is also related to the precipitation of Nb and the growth of Nb precipitates, it is one of the parameters for controlling the LME resistance characteristics.
• QHR defined by the finish rolling start temperature TFET °C, the finish rolling end temperature TFDT °C, and the effective time tHR from the start of finish rolling to the end of finish rolling in the following (formula 3)
• QCT defined by the residence time tCT from TFDT to 650°C in the coiling process
• QCT defined by the coiling temperature TCT °C in the following (formula 4)
• QAT defined by the soaking temperature TAT °C, the heating time t from 650°C to the soaking temperature TAT °C, and the holding time tAT at the soaking temperature TAT °C in the annealing process in the following (formula 5) satisfy the following (formula 6), that is, QHR , QCT, and QAT satisfy the following (formula 3)
• QCT defined by the residence time tCT from TFDT to 650°C in the coiling process
• QCT defined by the coiling temperature TCT °C in the following (formul
• the sum of QHR , QCT , and QAT is set to 6000 or less.
• it is set to 5990 or less. More preferably, it is set to 5980 or less. Even more preferably, it is set to 5970 or less, and most preferably, it is set to 5950 or less.
• There is no particular lower limit for the sum of QHR , QCT, and QAT but considering the range that can be implemented in actual operation, it is preferably set to 3500 or more. More preferably, it is set to 3600 or more. Even more preferably, it is set to 3700 or more. Most preferably, it is set to 3800 or more, and even more preferably, it is set to 3900 or more.
• Q AT value is 4700 or less (optimal condition)
• the evaluation is performed by the sum of QHR , QCT , and QAT . Therefore, the upper and lower limits of each value are not particularly set, but from the viewpoint of suppressing excessive growth of Nb during heating in the annealing process that directly leads to the final structure of the steel sheet, QAT is preferably 4700 or less. More preferably, it is 4650 or less, and even more preferably, it is 4630 or less. Most preferably, it is 4600 or less.
• QAT is preferably 3500 or more. More preferably, it is 3800 or more. Even more preferably, it is 4100 or more. Most preferably, it is 4120 or more.
• Q HR is preferably 1200 or less. More preferably, it is 1170 or less, and even more preferably, it is 1150 or less. Also, Q CT is preferably 850 or less. More preferably, it is 840 or less, and even more preferably, it is 835 or less. On the other hand, although there is no particular lower limit, Q HR is preferably 250 or more. Q HR is more preferably 600 or more, and even more preferably, it is 700 or more. Also, Q CT is preferably 150 or more. Q CT is more preferably 170 or more, and even more preferably, it is 180 or more.
• the steel sheet may be cooled as it is after being held at the soaking temperature TAT for the holding time tAT , or may be cooled to an arbitrary holding temperature and then held and cooled, or may be cooled to an arbitrary cooling stop temperature, and then reheated to the holding temperature, held, and then cooled.
• reheating may be performed.
• C is concentrated in the untransformed austenite, and the area ratio of the retained austenite contained in the steel sheet after cooling is increased by increasing the stability of the austenite, and ductility can be further improved. Therefore, when reheating is performed, it is preferable to cool the steel sheet to a cooling stop temperature of 100°C to 350°C, reheat it to a holding temperature of 200°C to 450°C, hold it, and then cool it to room temperature.
• the reheating temperature is preferably 200°C or higher. It is more preferable to set it to 210°C or higher.
• the reheating temperature is preferably 450°C or lower. It is more preferable to set it to 430°C or lower. It is even more preferable to set it to 410°C or lower, and most preferable to set it to 400°C or lower.
• reheating it is preferable to hold the material at a temperature of 200°C or higher and 450°C or lower for 10 seconds or more, and then cool it to room temperature. It is more preferable to hold the material at a temperature of 200°C or higher and 450°C or lower for 20 seconds or more, and even more preferable to hold the material at 25 seconds or more. There is no particular upper limit to the holding time, but it is preferable to hold the material at 500 seconds or less. It is more preferable to hold the material at 400 seconds or less.
• the steel sheet that has been subjected to the annealing is immersed in a galvanizing bath at 440°C or more and 500°C or less to perform hot-dip galvanizing, and then the coating weight is adjusted by gas wiping or the like.
• the temperature of the plating bath is preferably 440°C or more, more preferably 450°C or more, and even more preferably 455°C or more.
• the temperature of the plating bath is preferably 500°C or less, more preferably 490°C or less, and even more preferably 485°C or less.
• the plating weight (amount of plating per side) is preferably 20 g/m2 or more from the viewpoints of corrosion resistance and plating weight control.
• the plating weight is more preferably 25 g/m2 or more , and even more preferably 30 g/m2 or more . It is most preferably 32 g/m2 or more . Also, from the viewpoint of adhesion, it is preferably 120 g/m2 or less.
• the plating weight is more preferably 100 g/ m2 or less , and even more preferably 70 g/m2 or less . It is most preferably 65 g/m2 or less .
• the Al content in the hot-dip galvanizing is preferably 0.08% or more. More preferably, it is 0.09% or more. Even more preferably, it is 0.10% or more, and most preferably, it is 0.12% or more.
• the Al content in the hot-dip galvanizing is preferably 0.30% or less. More preferably, it is 0.25% or less. Even more preferably, it is 0.22% or less, and most preferably, it is 0.20% or less.
• the effect of the present invention will not change even if the plating bath contains elements other than Al, Mg, and Si, such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn.
• the alloying treatment of hot-dip galvanizing is performed in a temperature range of 450°C or more and 600°C or less after the hot-dip galvanizing treatment. If alloying treatment is performed at a temperature exceeding 600°C, untransformed austenite may be transformed into pearlite, the area ratio of retained austenite may become less than 5%, and ductility may decrease. Therefore, when alloying treatment of hot-dip galvanizing is performed, it is preferable to perform the alloying treatment of hot-dip galvanizing in a temperature range of 450°C or more. It is more preferable to set it to 460°C or more.
• the Fe concentration of the plating layer of the alloyed hot-dip galvanized steel sheet is preferably 8 to 17%. That is, the Fe concentration of the plating layer of the alloyed hot-dip galvanized steel sheet is preferably 8% or more.
• the Fe concentration of the plating layer of the alloyed hot-dip galvanized steel sheet is preferably 17% or less. More preferably, it is 16% or less. Even more preferably, it is 15% or less.
• Some steel sheets were subjected to the coiling process, and the steel was subjected to a heating process without cold rolling to obtain a hot-rolled steel sheet (HR). Further, some of the steel sheets were subjected to a hot-dip galvanizing treatment to obtain hot-dip galvanized steel sheets (GI) and galvannealed steel sheets (GA).
• a hot-dip galvanizing bath a zinc bath containing 0.19 mass% Al was used for the hot-dip galvanized steel sheets (GI), and a zinc bath containing 0.14 mass% Al was used for the galvannealed steel sheets (GA), and the bath temperature was 465° C.
• the coating weight was 45 g/m 2 per side (double-sided plating), and for the GA, the Fe concentration in the plating layer was adjusted to be within the range of 9 mass% to 12 mass%.
• the area ratios of fresh martensite, tempered martensite and bainite were determined by polishing a thickness cross section (L cross section) parallel to the rolling direction of the steel plate, etching it with nital, and observing 10 fields of view at 1/4 of the plate thickness position (a position equivalent to 1/4 of the plate thickness in the depth direction from the steel plate surface) at 2000x magnification using a scanning electron microscope (SEM).
• SEM scanning electron microscope
• the area ratios of each structure were calculated using the obtained structural images.
• fresh martensite was defined as the light gray structure region
• tempered martensite and bainite were defined as the dark gray structure region in which carbides were precipitated.
• the area ratio of retained austenite was determined by polishing the steel plate from the 1/4 position to 0.1 mm down, then chemically polishing the surface for a further 0.1 mm, measuring 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 using CoK ⁇ radiation in an X-ray diffractometer, and averaging the nine integrated intensity ratios obtained.
• the amount of hydrogen in steel was measured by cutting test pieces of approximately 5 x 30 mm from hot-rolled, cold-rolled or galvanized steel sheets. For galvanized steel sheets, a router (precision grinder) was used to remove the plating from the surface of the test pieces, which were then placed in a quartz tube. The inside of the quartz tube was replaced with Ar, and the temperature was raised at 200°C/hr. The amount of hydrogen released up to 400°C was measured by gas chromatography using temperature rise analysis. The cumulative 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 with a grain size of less than 20 nm (Nb pre ) were measured by the following procedure. First, the total amount of Nb (Nb) contained in the steel was measured by wet chemical analysis. Next, a method for measuring the amount of Nb with a grain size of 20 nm or less will be described.
• the precipitates in the steel material were captured as residuals, the amount of Nb in the total residuals was obtained, and then the amount of Nb present in the residuals with a grain size of 20 nm or more was obtained, and the difference between the amount of Nb in the total residuals and the amount of Nb present in the residuals with a grain size of 20 nm or more was obtained.
• the specific procedure is as follows. A plurality of test pieces of hot-rolled steel sheet, cold-rolled steel sheet, or galvanized steel sheet cut to about 20 x 50 mm were prepared, and for the galvanized steel sheet, the plating on the surface of the test piece was removed using a router (precision grinder).
• the surface of the collected test piece was polished to about 50 ⁇ m by preliminary electrolytic polishing to obtain a new surface.
• the obtained test piece was electrolyzed using 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol as an electrolyte for extracting precipitates.
• the obtained electrolytic solution after electrolysis was passed through a filter with a pore size of 0.2 ⁇ m to capture the residue, and then the residue was acid-decomposed and the Nb concentration was quantified in mass% units using ICP emission analysis. This was taken as the amount of Nb in the total residue.
• the remaining test piece was electrolyzed using the same 10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol.
• the residues attached to the remaining part of the metal sample after electrolysis were immersed in methanol prepared separately, and the residues attached to the remaining part of the metal sample were collected in a container using ultrasonic vibration. Then, the electrolyte after electrolysis and the methanol containing the residues attached to the remaining part of the metal sample were used to capture the residues using an alumina filter with a pore size of 20 nm. These residuals were immersed in hexametaphosphoric acid and dispersed in the hexametaphosphoric acid using ultrasonic vibration.
• the residuals in the hexametaphosphoric acid in which the residuals were dispersed were captured using a new alumina filter with a pore size of 20 nm.
• the residuals captured on the alumina filter have a particle size of 20 nm or more.
• These captured residuals were decomposed with acid, and the Nb concentration was quantified in mass% units using ICP emission spectrometry. This was taken as the amount of Nb present in the residuals having a particle size of 20 nm or more.
• the difference between the amount of Nb in the total residuals and the amount of Nb present in the residuals having a particle size of 20 nm or more was obtained, and this was taken as the amount of Nb (Nb pre ) having a particle size of 20 nm or less.
• the amount of dissolved Nb in the steel was determined by calculating the difference between the amount of Nb in the total residuals obtained by the above method from the amount of all Nb (Nb) contained in the steel, and the amount of dissolved Nb in the steel sheet (Nb sol ) was determined.
• the above-mentioned Nb precipitates are mainly NbC, but may contain NbN or other Nb precipitates.
• the tensile test was performed in accordance with JIS Z 2241 (2011) using a JIS No. 5 test piece sampled so that the tensile direction was perpendicular to the rolling direction of the steel plate, and TS (tensile strength) and EL (total elongation) were measured.
• TS tensile strength
• EL total elongation
• the LME resistance properties were evaluated by taking a sample from the steel plate with a dimension of 100 mm in the direction perpendicular to the rolling direction and 30 mm in the rolling direction, and overlapping it with a 980GA sample cut to the same size.
• the stroke angle in spot welding is defined as the angle ⁇ between a line passing through the major axis of the nugget and a line parallel to the surface of the steel plate in the cross section of the spot-welded component.
• the welding current pattern was controlled so that the resulting nugget diameter was in the range of 3.5 ⁇ t to 5.5 ⁇ t.
• t is the thickness of one steel plate (1.2 mm).
• a Dr6 type CuCr electrode was used, and the clearance between the overlapped evaluation sample and the electrode was 2.0 mm.
• the welding stroke angle and hold time in the evaluation of LME resistance properties are as shown in Tables 3-1 and 3-2.
• the high-strength steel sheets of the invention examples all have high TS, TS ⁇ El balance, and excellent LME resistance, whereas the comparative examples are inferior in at least one of TS, TS ⁇ El balance, and LME resistance.

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