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US20180251871A1 - Hot-rolled steel with very high strength and method for production - Google Patents

Hot-rolled steel with very high strength and method for production Download PDF

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US20180251871A1
US20180251871A1 US15/908,152 US201815908152A US2018251871A1 US 20180251871 A1 US20180251871 A1 US 20180251871A1 US 201815908152 A US201815908152 A US 201815908152A US 2018251871 A1 US2018251871 A1 US 2018251871A1
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steel
hot
manganese
present
molybdenum
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Erik James Pavlina
John Andrew Roubidoux
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Cleveland Cliffs Steel Properties Inc
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AK Steel Properties Inc
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Assigned to AK STEEL PROPERTIES, INC. reassignment AK STEEL PROPERTIES, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ROUBIDOUX, JOHN ANDREW, PAVLINA, ERIK JAMES
Publication of US20180251871A1 publication Critical patent/US20180251871A1/en
Assigned to U.S. BANK NATIONAL ASSOCIATION, AS NOTES COLLATERAL AGENT reassignment U.S. BANK NATIONAL ASSOCIATION, AS NOTES COLLATERAL AGENT PATENT SECURITY AGREEMENT Assignors: AK STEEL CORPORATION, AK STEEL PROPERTIES, INC., CLEVELAND-CLIFFS INC.
Assigned to U.S. BANK NATIONAL ASSOCIATION, AS NOTES COLLATERAL AGENT reassignment U.S. BANK NATIONAL ASSOCIATION, AS NOTES COLLATERAL AGENT PATENT SECURITY AGREEMENT Assignors: AK STEEL CORPORATION, AK STEEL PROPERTIES, INC.
Assigned to BANK OF AMERICA, N.A., AS AGENT reassignment BANK OF AMERICA, N.A., AS AGENT PATENT SECURITY AGREEMENT Assignors: AK STEEL CORPORATION, AK STEEL PROPERTIES, INC., CLEVELAND-CLIFFS INC.
Assigned to U.S. BANK NATIONAL ASSOCIATION, AS NOTES COLLATERAL AGENT reassignment U.S. BANK NATIONAL ASSOCIATION, AS NOTES COLLATERAL AGENT PATENT SECURITY AGREEMENT Assignors: AK STEEL CORPORATION, AK STEEL PROPERTIES, INC., CLEVELAND-CLIFFS INC.
Assigned to AK STEEL CORPORATION, AK STEEL PROPERTIES, INC. reassignment AK STEEL CORPORATION RELEASE BY SECURED PARTY (SEE DOCUMENT FOR DETAILS). Assignors: U.S. BANK NATIONAL ASSOCIATION
Assigned to CLEVELAND-CLIFFS STEEL PROPERTIES INC. reassignment CLEVELAND-CLIFFS STEEL PROPERTIES INC. CHANGE OF NAME (SEE DOCUMENT FOR DETAILS). Assignors: AK STEEL PROPERTIES, INC.
Assigned to CLEVELAND-CLIFFS STEEL PROPERTIES reassignment CLEVELAND-CLIFFS STEEL PROPERTIES CORRECTIVE ASSIGNMENT TO CORRECT THE RECEIVING PARTY DATA PREVIOUSLY RECORDED AT REEL: 056228 FRAME: 0566. ASSIGNOR(S) HEREBY CONFIRMS THE ASSIGNMENT. Assignors: AK STEEL PROPERTIES, INC.
Assigned to CLEVELAND-CLIFFS STEEL PROPERTIES INC. reassignment CLEVELAND-CLIFFS STEEL PROPERTIES INC. CORRECTIVE ASSIGNMENT TO CORRECT THE RECEIVING PARTY DATA FROM CLEVELAND-CLIFFS STEEL PROPERTIES TO CLEVELAND-CLIFFS STEEL PROPERTIES INC. PREVIOUSLY RECORDED AT REEL: 056313 FRAME: 0443. ASSIGNOR(S) HEREBY CONFIRMS THE CHANGE OF NAME. Assignors: AK STEEL PROPERTIES, INC.
Assigned to CLEVELAND-CLIFFS STEEL PROPERTIES, INC. (F/K/A AK STEEL PROPERTIES, INC.), CLEVELAND-CLIFFS STEEL CORPORATION (F/K/A AK STEEL CORPORATION),, IRONUNITS LLC, CLEVELAND-CLIFFS INC. reassignment CLEVELAND-CLIFFS STEEL PROPERTIES, INC. (F/K/A AK STEEL PROPERTIES, INC.) RELEASE BY SECURED PARTY (SEE DOCUMENT FOR DETAILS). Assignors: U.S. BANK TRUST COMPANY, NATIONAL ASSOCIATION, SUCCESSOR IN INTEREST TO U.S. BANK NATIONAL ASSOCIATION
Assigned to CLEVELAND-CLIFFS INC., CLEVELAND-CLIFFS STEEL PROPERTIES INC. (F/K/A AK STEEL PROPERTIES, INC.), CLEVELAND-CLIFFS STEEL CORPORATION (F/K/A AK STEEL CORPORATION) reassignment CLEVELAND-CLIFFS INC. RELEASE BY SECURED PARTY (SEE DOCUMENT FOR DETAILS). Assignors: U.S. BANK NATIONAL ASSOCIATION
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    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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    • 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
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    • 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
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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    • 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
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    • 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
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • 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
    • C23C2/04—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
    • C23C2/12—Aluminium 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
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • 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

  • Hot-rolled steels are produced by subjecting an ingot of a predetermined thickness to a series of rollers to progressively decrease the thickness of the ingot. Throughout the rolling process, the steel is maintained at a very high temperature that is generally above the recrystallization temperature; final reduction passes may occur at temperatures below the recrystallization temperature of austenite. Once the rolling process is complete, the steel is coiled as it is cooling. The final steel coil is then cooled to ambient temperature.
  • hot-rolled steels can be used in the context of automotive frames.
  • the automotive industry continually seeks more cost-effective materials that are lighter for more fuel-efficient vehicles. While thinner steel materials can meet this need, higher strength is necessary to accommodate these thickness reductions. Thus, it is desirable to increase the strength of steel materials used in hot-rolling processes.
  • the steels of the present application solve the problem of poor weldability and low elongation in hot-rolled steels by a novel alloying strategy that incorporates transition metal elements that increase the propensity of martensite formation after hot-rolling processes despite relatively low cooling rates encountered during the hot-rolling processes.
  • FIG. 1 depicts a photomicrograph corresponding to composition reference 4339-1 listed in Table 1.
  • FIG. 2 depicts a photomicrograph corresponding to composition reference 4339-2 listed in Table 1.
  • FIG. 3 depicts a photomicrograph corresponding to composition reference 4340-1 listed in Table 1.
  • FIG. 4 depicts a photomicrograph corresponding to composition reference 4340-2 listed in Table 1.
  • FIG. 5 depicts a photomicrograph corresponding to composition reference 4341-1 listed in Table 1.
  • FIG. 6 depicts a photomicrograph corresponding to composition reference 4341-2 listed in Table 1.
  • FIG. 7 depicts a photomicrograph corresponding to composition reference 4342-1 listed in Table 1.
  • FIG. 8 depicts a photomicrograph corresponding to composition reference 4342-2 listed in Table 1.
  • the present embodiment involves a high strength, hot-rolled steel that exhibits an ultimate tensile strength of approximately 1500 MPa.
  • the steel of the present example is produced in a relatively heavy gauge, or high thickness, of greater than 3 mm, it should be understood that in other embodiments various other suitable thicknesses may be used.
  • the present embodiment exhibits generally high strength.
  • the steel of the present example includes a predominately martensitic microstructure after hot-rolling, coiling, and cooling to ambient temperature.
  • the steel of the present embodiment has sufficient hardenability or susceptibility to thermal heat treatment.
  • the term “sufficient hardenability” is defined by the formation of martensite during coiling and after hot rolling.
  • martensite is generally more likely to form in response to relatively fast cooling rates.
  • the hardenability of the steel is sufficiently high such that martensite forms even with the relatively slow cooling rates that are present in commercial hot-rolling and coiling operations.
  • Carbon is generally understood to have a direct relationship with hardenability.
  • increasing carbon additions to a steel can likewise increase hardenability.
  • these detrimental characteristics are avoided while also increasing hardenability of the steel through use of substitutional or transition metal elements in lieu of increasing carbon substantially.
  • substitutional or transition metal elements can include manganese, molybdenum, niobium, vanadium, chromium, or some combination thereof.
  • manganese is the primary alloying addition used to increase hardenability of the steel while avoiding other detrimental conditions such as reduced weldability and reduced elongation to fracture.
  • Other elements such as molybdenum, niobium, chromium, and/or vanadium can also be similarly used to increase hardenability.
  • carbon is held at a relatively low level that will be described in greater detail below.
  • certain substitutional or transition metal elements are added to increase hardenability.
  • the particular amount of increased hardenability is determined by the increase required to promote the formation of martensite despite the relatively slow cooling rates encountered during coiling and subsequent ambient air cooling.
  • the cooling rate can be approximately 0.05 to 2° C./s.
  • different cooling rates can be used while still promoting the formation of martensite.
  • the embodiments of the present alloys include manganese, silicon, chromium, molybdenum, niobium, vanadium, and carbon additions in concentrations sufficient to obtain one or more of the above benefits.
  • the effects of these and other alloying elements are summarized as:
  • Carbon is added to reduce the martensite start temperature, provide solid solution strengthening, and to increase the hardenability of the steel.
  • Carbon is an austenite stabilizer.
  • carbon can be present in concentrations of 0.1-0.50 weight %; in other embodiments, carbon can be present in concentrations of 0.1-0.35 weight %. In still other embodiments, carbon can be present in concentrations of about 0.22-0.25 weight %.
  • Manganese is added to reduce the martensite start temperature, provide solid solution strengthening, and to increase the hardenability of the steel.
  • Manganese is an austenite stabilizer.
  • manganese can be present in concentrations of 3.0-8.0 weight %; in other embodiments, manganese can be present in concentrations of 2.0-5.0 weight %; in still other embodiments, manganese can be present in concentrations greater than 3.0 weight %-8.0 weight %; and in still other embodiments, manganese can be present in concentrations greater than 3.0 weight %-5.0 weight %.
  • Silicon is added to provide solid solution strengthening.
  • Silicon is a ferrite stabilizer.
  • silicon can be present in concentrations of 0.1-0.5 weight %; in other embodiments, silicon can be present in concentrations of 0.2-0.3 weight %.
  • Molybdenum is added to provide solid solution strengthening, to increase the hardenability of the steel, and to protect against embrittlement.
  • molybdenum can be present in concentrations of 0-2.0 weight %; in other embodiments, molybdenum can be present in concentrations of 0-0.6 weight %; in still other embodiments, molybdenum can be present in concentrations of 0.1-2.0 weight %; in other embodiments, molybdenum can be present in concentrations of 0.1-0.6 weight %; in yet other embodiments molybdenum can be present in concentrations of 0.4-0.5 weight %; and in yet other embodiments molybdenum can be present in concentrations of 0.3-0.5 weight %.
  • Chromium can be added to reduce the martensite start temperature, provide solid solution strengthening, and increase the hardenability of the steel. Chromium is a ferrite stabilizer. In certain embodiments, chromium can be present in concentrations of 0-6.0 weight %; in other embodiments, chromium can be present in concentrations of 2.0-6.0 weight %; in other embodiments, chromium can be present in concentrations of 0.2-6.0 weight %; and in other embodiments chromium can be present in concentrations of 0.2-3.0 weight %.
  • Niobium can be added to increase strength and improve hardenability of the steel. In some embodiments niobium can also be added to provide improved grain refinement. In certain embodiments, niobium can be present in concentrations of 0-0.1 weight %; in other embodiments, niobium can be present in concentrations of 0.01-0.1 weight %; and in other embodiments, niobium can be present in concentrations of 0.001-0.055 weight %.
  • Vanadium can be added to increase strength and improve hardenability of the steel.
  • vanadium can be present in concentrations of 0-0.15 weight %; and in other embodiments, vanadium can be present in concentrations of 0.01-0.15 weight %.
  • Boron can be added to increase the hardenability of the steel.
  • boron can be present in concentrations of 0-0.005 weight %.
  • the hot-rolled steels can be processed using conventional steel making, roughing, and finishing processes.
  • the steels can be continuously cast to produce slabs of approximately 12-15 cm in thickness. Slabs are then reheated at temperatures of 1200-1320° C., and hot-rolled to a final gauge of ⁇ 2.5 mm, with the final reduction pass occurring at a temperature of approximately 950° C. Scale on the hot-rolled steel coil can be removed by pickling and/or abrasive blasting using processes that are known in the art.
  • the alloys of the present application can be as-hot-rolled (that is, bare or uncoated) or they can also be coated with an aluminum-based coating, a zinc-based coating (either galvanized or galvannealed), after hot-rolling and scale removal.
  • Such coating can be applied to the steel sheet using processes known in the art, including hot dip coating or electrolytic coating.
  • Composition range Compositions are in weight percent.
  • Ingots were formed for each composition described above in Table 1.
  • the ingots were formed by vacuum melting each composition in an induction furnace to cast 11-kg ingots.
  • the as-cast ingots had an initial thickness of 45 mm. Once formed, the ingots were reheated to 1316° C. and rolled to a final thickness of approximately 3.6 mm.
  • the rolling of each ingot was completed in eight passes. On the final rolling pass, a temperature measurement was taken and it was observed that the temperature of each ingot was ⁇ 955° C.
  • coiling was simulated by subjecting each ingot to furnace equilibration at approximately 566° C. with a range of 450 to 650° C. and subsequent cooling to ambient temperature.
  • FIG. 1 shows a micrograph of an ingot with the composition of reference 4339-1 in Table 1.
  • FIG. 2 shows a micrograph of an ingot with the composition of reference 4339-2 in Table 1.
  • FIG. 3 shows a micrograph of an ingot with the composition of reference 4340-1 in Table 1.
  • FIG. 4 shows a micrograph of an ingot with the composition of reference 4340-2 in Table 1.
  • FIG. 5 shows a micrograph of an ingot with the composition of reference 4341-1 in Table 1.
  • FIG. 6 shows a micrograph of an ingot with the composition of reference 4341-2 in Table 1.
  • FIG. 7 shows a micrograph of an ingot with the composition of reference 4342-1 in Table 1.
  • FIG. 8 shows a micrograph of an ingot with the composition of reference 4342-2 in Table 1.
  • Ingots made with compositions of references 4339-1, 4339-2, and 4340-1 were observed to include varying amounts of ferrite, pearlite, and bainite.
  • a martensitic microstructure was observed in ingots made with compositions of references 4340-2, 4341-1, 4341-2, 4342-1, and 4342-2.
  • the presence of martensite in these samples was unexpected when considering the cooling rates applied to each ingot. As described above, relatively slow cooling rates generally favor the formation of ferrite, pearlite, and bainite over the formation of martensite. However, martensite formation was observed even though the expectation was ferrite, pearlite, bainite, and/or other non-martensitic constituents.
  • a martensitic microstructure can be formed when manganese is at least 5 wt. % while other substitutional elements are minimal and the carbon content is approximately 0.23 weight %. Less manganese can be present while still forming a martensitic microstructure if other substitutional elements are included. For instance, for steels containing approximately 4 wt. % manganese, additions of molybdenum, niobium, and/or vanadium can still promote the formation of a martensitic microstructure. Similarly, for steels containing approximately 3 wt. % manganese, an addition of 3 wt % chromium can still promote the formation of a martensitic microstructure.
  • Example 4 As can be seen in Table 2, the compositions noted above in Example 4 as being susceptible to formation of martensitic microstructure after hot-rolling and relatively slow cooling also exhibited tensile strengths of approximately 1500 MPa. Ultimate tensile strengths in excess of 1400 MPa were achieved using several alloy strategies that produced martensitic microstructure in the as-hot-rolled condition.
  • manganese e.g., reference 4340-2
  • alloying with a combination of manganese, molybdenum, and niobium e.g., reference 4341-1
  • alloying with a combination of manganese, molybdenum, niobium, and vanadium e.g., reference 4341-2
  • a high strength steel comprising by total weight percentage of the steel:

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