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EP3296416A1 - Ultrahochfestes heissgewalztes stahlblech mit hervorragender biegbarkeit sowie verfahren zur herstellung davon - Google Patents

Ultrahochfestes heissgewalztes stahlblech mit hervorragender biegbarkeit sowie verfahren zur herstellung davon Download PDF

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Publication number
EP3296416A1
EP3296416A1 EP15891914.2A EP15891914A EP3296416A1 EP 3296416 A1 EP3296416 A1 EP 3296416A1 EP 15891914 A EP15891914 A EP 15891914A EP 3296416 A1 EP3296416 A1 EP 3296416A1
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EP
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Prior art keywords
steel sheet
rolled steel
hot
high strength
ultra
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EP15891914.2A
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English (en)
French (fr)
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EP3296416A4 (de
Inventor
Yong-Woo Kim
Seok-Jong SEO
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Posco Holdings Inc
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Posco Co Ltd
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Publication of EP3296416A4 publication Critical patent/EP3296416A4/de
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties 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 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • C21D8/0284—Application of a separating or insulating coating
    • 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
    • C22C38/02—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
    • C22C38/12—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
    • C22C38/18—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • 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
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • 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
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • 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
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • 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
    • 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
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present disclosure relates to an ultra-high strength hot-rolled steel sheet which may mainly be used for components requiring high strength and excellent bending workability, such as a bumper reinforcement member and a door impact beam that are reinforcement members for the body of a car.
  • the present disclosure may provide an ultra-high strength hot-rolled steel sheet which has high strength and is thus very effective in forming a lightweight material due to thinning of the material according to the high strength thereof, and at the same time has excellent bending workability so that it is easy to ensure the shape fixability of a component through roll forming, and a method for manufacturing the same.
  • a conventional high strength hot-rolled steel sheet is generally manufactured by adding C, Si, Mn, Ti, Nb, Mo, V, and the like to high purity steel in which an amount of impurities may be minimized, to obtain a high level of strength.
  • Ultra-high strength hot-rolled steel sheets used for a bumper reinforcement member, a door impact beam, and the like that are reinforcement members for the body of a car may require high strength as well as excellent bending workability for roll forming.
  • solid solution strengthening by alloying elements such as C, Si, Mn, Cr, Mo, W, and the like, used in manufacturing the conventional high strength hot-rolled steel sheet proposed above, or high strength by precipitation hardening of alloying elements, such as Ti, Nb, Mo, and the like may degrade bending workability, and may decrease productivity during temper annealing, thus lowering price competitiveness.
  • An aspect of the present disclosure may provide an ultra-high strength hot-rolled steel sheet having excellent high strength and bending workability.
  • Another aspect of the present disclosure may provide a method for manufacturing an ultra-high strength hot-rolled steel sheet having excellent high strength and bending workability.
  • an ultra-high strength hot-rolled steel sheet having excellent bending workability may include: 0.1 to 0.25 wt % of C; 0.01 to 0.2 wt % of Si; 0.5 to 2.0 wt % of Mn; 0.005 to 0.02 wt % of P; 0.001 to 0.01 wt % of S; and a balance of Fe and other inevitable impurities, in which the ultra-high strength hot-rolled steel sheet may further include 0.001 to 0.35 wt % of at least one element selected from the group consisting of Ti, Nb, Mo, Cr, and B, and the following Relational Expression 1 may be satisfied.
  • a method for manufacturing an ultra-high strength hot-rolled steel sheet having excellent bending workability may include: preparing a slab comprising 0.1 to 0.25 wt % of C, 0.01 to 0.2 wt % of Si, 0.5 to 2.0 wt % of Mn, 0.005 to 0.02 wt % of P, 0.001 to 0.01 wt % of S, and a balance of Fe and other inevitable impurities, in which the slab may further include 0.001 to 0.35 wt % of at least one element selected from the group consisting of Ti, Nb, Mo, Cr, and B, and the following Relational Expression 1 may be satisfied; reheating the slab at a temperature of 1,100-1,300°C; manufacturing a hot-rolled steel sheet by finish hot rolling the reheated slab at a finish hot rolling temperature of 850-1,000°C; cooling the hot-rolled steel sheet at a cooling rate of 100-300 °C/s, in which the following Relational Expression 3 may
  • an ultra-high strength hot-rolled steel sheet having excellent bending workability while having excellent strength may be provided.
  • FIG. 1 is a graph illustrating values derived from Relational Expression 1 indicating TS ⁇ T-EL and bending workability of an Inventive Example and a Comparative Example.
  • the present disclosure relates to an ultra-high strength hot-rolled steel sheet which may have high strength and may thus be very effective in forming a lightweight material due to thinning of the material according to the high strength thereof, as well as having excellent bending workability so that it may be easy to ensure the shape fixability of a component through roll forming, and a method for manufacturing the same.
  • the present inventor derived a Relational Expression indicating bending workability through bending testing measurements of steels having various elements. Based on this Relational Expression, an ultra-high strength hot-rolled steel sheet having excellent bending workability while having a tensile strength of 1 GPa or higher and a tensile strength ⁇ elongation at rupture (TS ⁇ T-EL) of 10,000 or greater may be provided.
  • T-EL tensile strength ⁇ elongation at rupture
  • the ultra-high strength hot-rolled steel sheet having excellent bending workability may include: 0.1 to 0.25 wt % of C; 0.01 to 0.2 wt % of Si; 0.5 to 2.0 wt % of Mn; 0.005 to 0.02 wt % of P; 0.001 to 0.01 wt % of S; and a balance of Fe and other inevitable impurities, in which the ultra-high strength hot-rolled steel sheet may further include 0.001 to 0.35 wt % of at least one element selected from the group consisting of Ti, Nb, Mo, Cr, and B.
  • Carbon (C) 0.1 to 0.25 wt %
  • Carbon (C) may be the most economical and effective element in reinforcing steel.
  • the content of C is less than 0.1 wt %, it may be difficult to secure a desired level of strength.
  • the content of C exceeds 0.25 wt %, a problem may occur in which bending workability may be degraded due to an excessive increase in strength.
  • Silicon (Si) may deoxidize molten steel, and may be effective in solid solution strengthening.
  • the content of Si is less than 0.01 wt %, the deoxidizing effect and the strength increasing effect may be insufficient.
  • the content of Si exceeds 0.2 wt %, red scale may be formed on a surface of a steel sheet due to Si, so that a problem may occur in which the quality of the surface of the steel sheet may be greatly degraded and weldability may also be deteriorated.
  • Manganese (Mn) 0.5 to 2.0 wt %
  • Manganese (Mn) may be an element effective in solid solution strengthening of steel as in Si.
  • the content of Mn be included in an amount of 0.5 wt % or more in order to exhibit such an effect.
  • the content of Mn exceeds 2.0 wt %, segregation may develop excessively in a thickness center portion of a slab at the time of casting the slab in a continuous casting process, so that a problem may occur in which weldability and formability of a finished product may be degraded.
  • the content of Mn be included in an amount of 0.5 to 2.0 wt %.
  • Phosphorus (P) may be effective in solid solution strengthening and promoting of ferrite transformation as in Si.
  • the content of P may be insufficient to obtain a desired level of strength in an exemplary embodiment in the present disclosure.
  • the content of P exceeds 0.02 wt %, bending workability may be degraded due to band organization by microsegregation.
  • S Sulfur
  • Mn or the like may bond with Mn or the like to form a nonmetallic inclusion. Accordingly, toughness of steel may be greatly degraded.
  • At least one element selected from the group consisting of titanium (Ti), niobium (Nb), molybdenum (Mo), chromium (Cr), and boron (B) be added, in addition to the above-mentioned advantageous composition.
  • the addition of the above elements may allow a high level of tensile strength and excellent bending workability to be obtained, thus further improving the effect of the present disclosure.
  • the at least one element selected from the group may be included in an amount of 0.001 to 0.35 wt %.
  • Ti may be effective in suppressing growth of crystal grains in a heating process for hot rolling as Ti is present in steel as TiN. Further, the Ti remaining after reacting with nitrogen (N) may be an element useful to improve strength of steel through solid solution strengthening thereof.
  • Nb as a precipitate formation element may allow for formation of a Nb-based precipitate, such as Nb (C, N).
  • a Nb-based precipitate such as Nb (C, N).
  • Nb is solid solutioned in a heating furnace having a temperature of about 1,200°C, a fine precipitate may be formed during hot rolling to effectively increase strength of steel.
  • Mo may be an element useful to improve impact toughness and bending workability by strengthening yield strength and grain boundary through solid solution strengthening.
  • Cr may perform solid solution strengthening of steel, and may delay bainite phase transformation during cooling to help form martensite.
  • B may be contained as an alternative of Si, may improve temperability in an extremely minute amount thereof, and may strengthen grain boundary to improve strength.
  • the remainder thereof may be Fe.
  • unintended impurities may be inevitably incorporated from raw materials or steel manufacturing environments, so that they may not be excluded. These impurities are commonly known to a person skilled in the art, and are thus not specifically mentioned in this specification.
  • An ultra-high strength steel having excellent bending workability may be obtained by satisfying the following Relational Expression 1 obtained by evaluating bending workability in various compositions by the present inventor, while satisfying the alloy composition range described above. 85.3 ⁇ 311.5 C ⁇ 0.1 Si ⁇ 4.0 Mn ⁇ 5.3 Cr ⁇ 2.6 Ni ⁇ 6.6 Ti ⁇ 660.6 B ⁇ 39 P ⁇ 6.9 cooling rate ⁇ 0
  • [C], [Si], [Mn], [Cr], [Ni], [Ti], [B], and [P] may refer to wt % of the content of each element.
  • the Relational Expression 1 maybe a relational expression obtained from measurements of bending workability of steels having various elements, and a sufficient amount of a martensite microstructure may be secured by satisfying the Relational Expression 1.
  • the Relational Expression 2 may be satisfied to enable smooth formation of the ultra-high strength steel having a tensile strength of 1 Gpa or higher into components. That is, as a value of bending workability (R/t) decreases, it may be possible to enable smooth formation of components, and when the value of bending workability (R/t) is less than or equal to a value of (tensile strength ⁇ 0.00517 - 2.60345), it may be possible to enable the formation of components through smooth roll forming.
  • the hot-rolled steel sheet provided in an exemplary embodiment in the present disclosure may preferably satisfy the above element conditions while having the microstructure thereof including ferrite of 95 area% or more and a second phase of 5% or less that includes at least one selected from the group consisting of bainite, martensite, and a carbide, such as cementite, and may secure a sufficient level of ductility by having the above microstructure.
  • bainite and a coarse carbonitride may be formed around a ferritic grain boundary, so that a desired level of strength may not be obtained or an interphase hardness difference ( ) may occur.
  • the ultra-high strength hot-rolled steel sheet may have a tensile strength of 1 Gpa or higher. This is the reason that, when the tensile strength is lower than 1 Gpa, a problem may occur in which thinning of the material may be limited and the effect of lightening components may be lowered.
  • the ultra-high strength hot-rolled steel sheet may have a tensile strength ⁇ elongation at rupture (TS ⁇ T-EL) of 10,000 or greater. This is the reason that, when this value is less than 10, 000, a problem may occur in which formability or shape fixability may be degraded at the time of processing components.
  • T ⁇ T-EL tensile strength ⁇ elongation at rupture
  • a slab having a composition satisfying the alloy composition range, according to an exemplary embodiment in the present disclosure, and the Relational Expression 1 may first be prepared, in order to manufacture the ultra-high strength hot-rolled steel sheet having excellent strength and excellent bending workability, according to an exemplary embodiment in the present disclosure. Then, the prepared slab may be heated at a temperature of 1,100-1,300°C, and the heated slab may undergo hot rolling at a finish hot rolling temperature of 850-1,000°C, may be cooled, and may be coiled after termination of the cooling at 350°C or lower, so that the ultra-high strength hot-rolled steel sheet having excellent bending workability, according to an exemplary embodiment in the present disclosure, may be completed.
  • a reheating temperature of the slab may be 1,100°C or higher, which may secure a temperature of the slab to reduce rolling load.
  • the reheating temperature may be 1,300°C or lower.
  • Hot rolling may be performed on the reheated slab. At this time, it may be preferable to perform finish rolling at 850-1,000°C.
  • finish hot rolling temperature is lower than 850°C, rolling load may greatly increase.
  • finish hot rolling temperature exceeds 1,000°C, the microstructure of the steel sheet may be coarsened, so that the steel may be weakened, scales thereof may be thickened, and a deterioration in surface quality, such as scale defects or the like, due to high temperature rolling, may occur.
  • the finish hot rolling temperature be limited to 850-1,000°C.
  • Cooling Rate 100-300 °C/s
  • the hot-rolled steel sheet may be preferable to cool the hot-rolled steel sheet as described above. Further, it may be preferable to cool the hot-rolled steel sheet at a cooling rate of 100-300 °C/s from the finish hot rolling temperature to a cooling termination temperature and then coil the hot-rolled steel sheet.
  • a cooling rate is less than 100 °C/s, a fraction of the second phase except for martensite may exceed 5%, so that it may be difficult to secure a desired level of strength in an exemplary embodiment in the present disclosure.
  • the cooling rate exceeds 300 °C/s, a problem may occur in which elongation and toughness may be reduced.
  • the cooling of the hot-rolled steel sheet may be performed within a cooling rate range obtained by the following Relational Expression 3. 85.3 ⁇ 311.5 C ⁇ 0.1 Si ⁇ 4.0 Mn ⁇ 5.3 Cr ⁇ 2.6 Ni ⁇ 6.6 Ti ⁇ 660.6 B ⁇ 39 P ⁇ 6.9 cooling rate ⁇ 0
  • [C], [Si], [Mn], [Cr], [Ni], [Ti], [B], and [P] may refer to wt % of the content of each element, and units of the cooling rate may be °C/s, which may refer to cooling rate from finish hot rolling temperature to coiling temperature.
  • the Relational Expression 3 may be applied to the method for manufacturing an ultra-high strength hot-rolled steel sheet having a sufficient amount of martensite by adding a factor of cooling rate capable of securing the sufficient amount of martensite to the Relational Expression 1 obtained from measurements of bending workability of steels having various elements.
  • the hot-rolled steel sheet may be cooled at a cooling rate of 100-300 °C/s from the finish hot rolling temperature to a temperature of 350°C or lower and then coil the hot-rolled steel sheet.
  • the coiling temperature may be a temperature at which the cooling may be terminated, and as long as the coiling temperature is 350°C or lower, the cooling may be terminated and the coiling may be performed at any temperature.
  • a separate device may be required to set the cooling termination temperature to 20°C or lower, room temperature. Thus, it may be preferable to terminate the cooling and perform the coiling at a temperature of 20°C or higher.
  • the coiled hot-rolled steel sheet may be further subjected to an operation of natural cooling at room temperature, pickling, removing of scales from a surface layer thereof, and anointing to manufacture a pickled steel sheet.
  • the steel sheet may be reheated at 450-480°C and hot-dip galvanized to manufacture a hot-dip galvanized steel sheet.
  • the reheating temperature is lower than 450°C, there may be a disadvantage that the hot-dip galvanizing may not be performed due to a deterioration in plating adhesion.
  • the reheating temperature exceeds 480°C, the precipitate may be coarsened due to a heat treatment effect, so that there may be the risk of decreasing strength due to a reduction in a precipitation hardening effect. Further, an environmental problem caused by vaporization of molten zinc and a problem of plating quality degradation may occur.
  • a steel slab satisfying the composition shown in Table 1 below was heated to 1, 150°C, and underwent finish hot rolling at the temperature (FDT) shown in Table 2 below. Then, the steel slab was cooled to the coiling temperature (CT) shown in Table 2 at a cooling rate of 200 °C/s, and coiled at the CT shown in Table 2.
  • CT coiling temperature
  • Inventive Examples 1 to 6 displayed in Table 1 illustrate the composition of slabs satisfying the alloy composition range, according to an exemplary embodiment in the present disclosure
  • Comparative Examples 1 to 9 illustrate the composition of slabs having the composition beyond the alloy composition range, according to an exemplary embodiment in the present disclosure, in units of wt %. Further, a material test was performed on the manufactured hot-rolled steel sheet described above, and results thereof are shown in Table 2.
  • FDT and CT may refer to finish hot rolling temperature and coiling temperature
  • YS, TS, T-El, and TS ⁇ T-EL may refer to yield strength, tensile strength, elongation, and tensile strength ⁇ elongation, respectively.
  • YS may refer to 0.2% offset yield strength or lower yield point
  • yield ratio may be a ratio of yield strength to tensile strength. The tensile test was performed with a specimen collected, according to JIS 5, based on a 90° direction with respect to a rolling direction of a rolled sheet.
  • R/t (actual measurement) shown in Table 2 may be a value obtained by collecting a specimen based on the 90° direction with respect to the rolling direction of the rolled sheet, performing a 90° bending test thereon, and dividing the minimum bending radius R, at which cracking may not occur, by a thickness t of the material, and R/t (limit) may represent a value of (Tensile strength ⁇ 0.00517 - 2.60345).
  • R/t (actual measurement) exceeds R/t (limit) bending workability was evaluated as being degraded.
  • Comparative Examples 3 to 5 illustrate a tensile strength lower than 1 Gpa due to formation of a bainite structure, rather than a martensite structure of 95% or more, at the CT outside the range of the present disclosure.
  • FIG. 1 illustrates a graph of TS ⁇ T-EL and the values derived from the Relational Expression 3, according to the Comparative Example and the Inventive Example.
  • the parts indicated by the square points are the Comparative Example, and the parts indicated by the round points are the Inventive Example. It can be seen that all of the round points corresponding to the Inventive Example, according to an exemplary embodiment in the present disclosure, are located within the part indicated by the deviant crease lines.

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EP15891914.2A 2015-05-12 2015-05-12 Ultrahochfestes heissgewalztes stahlblech mit hervorragender biegbarkeit sowie verfahren zur herstellung davon Withdrawn EP3296416A1 (de)

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EP3964592A1 (de) * 2020-09-07 2022-03-09 ThyssenKrupp Steel Europe AG Warmgewalztes stahlflachprodukt und verfahren zur herstellung eines warmgewalzten stahlflachprodukts

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WO2020058244A1 (de) * 2018-09-18 2020-03-26 Voestalpine Stahl Gmbh Verfahren zur herstellung ultrahochfester stahlbleche und stahlblech hierfür
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WO2022049282A1 (de) * 2020-09-07 2022-03-10 Thyssenkrupp Steel Europe Ag Warmgewalztes stahlflachprodukt und verfahren zur herstellung eines warmgewalzten stahlflachprodukts

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JP2018518596A (ja) 2018-07-12
WO2016182098A1 (ko) 2016-11-17

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