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US20180147614A1 - Press hardened steel with increased toughness and method for production - Google Patents

Press hardened steel with increased toughness and method for production Download PDF

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Publication number
US20180147614A1
US20180147614A1 US15/824,533 US201715824533A US2018147614A1 US 20180147614 A1 US20180147614 A1 US 20180147614A1 US 201715824533 A US201715824533 A US 201715824533A US 2018147614 A1 US2018147614 A1 US 2018147614A1
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temperature
steel
rolling
processing method
coiling
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John Andrew Roubidoux
Erasmus Amoateng
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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: AMOATENG, Erasmus, ROUBIDOUX, JOHN ANDREW
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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
    • 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
    • C21D8/0294—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a localised treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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
    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • 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
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/12—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
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/26—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
    • 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
    • C22C38/48—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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21B—ROLLING OF METAL
    • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
    • B21B1/02—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling heavy work, e.g. ingots, slabs, blooms, or billets, in which the cross-sectional form is unimportant ; Rolling combined with forging or pressing
    • B21B2001/028—Slabs

Definitions

  • the present application relates to an improvement in press hardened steels, hot press forming steels, hot stamping steels, or any other steel that is heated to an austenitization temperature and formed and quenched in a stamping die to achieve desired mechanical properties in the final part.
  • These types of steels are also sometimes referred to as “heat treatable boron-containing steels.” In this application, they will all be referred to as “press hardened steels.”
  • Press hardened steels are primarily used as structural members in automobiles where high strength, low weight, and improved intrusion resistance are desired by automobile manufacturers.
  • a common structural member where press hardened steels are employed in the automobile structure is the B-pillar.
  • the as-quenched microstructure of prior art press hardened steel is fully martensitic.
  • Conventional press hardened steels have ultimate tensile strengths of approximately 1500 MPa and total elongations on the order of 6%.
  • Residual toughness refers to the toughness the material has in the press hardened condition.
  • the strength-ductility property of embodiments of the present steel alloys include ultimate tensile strengths greater than or equal to 1100 MPa and elongations of approximately 8%.
  • FIG. 1 shows a thermal profile and processing schematic for embodiments of the present alloys.
  • FIG. 2 shows another thermal profile and processing schematic for embodiments of the present alloys.
  • FIG. 3 shows a plot of stress-strain curves for composition 4310 , with results from a first pre-processing method shown in solid-line form and results from a second pre-processing method shown in dashed-line form.
  • FIG. 4 shows a plot of stress-strain curves for composition 4311 , with results from the first pre-processing method shown in solid-line form and results from the second pre-processing method shown in dashed-line form.
  • FIG. 5 shows a plot of stress-strain curves for composition 4312 , with results from the first pre-processing method shown in solid-line form and results from the second pre-processing method shown in dashed-line form.
  • FIG. 6 shows a plot of stress-strain curves for composition 4313 , with results from the first pre-processing method shown in solid-line form and results from the second pre-processing method shown in dashed-line form.
  • FIG. 7 shows the results of a double edge-notch tensile test for embodiments of the present alloys after being subjected to the second pre-processing method.
  • FIG. 8 shows the results of a double edge-notch tensile test for embodiments of the present alloys after being subjected to the first pre-processing method.
  • FIG. 9 shows the results of strain energy computations for embodiments of the present alloys plotted as a function of niobium concentration.
  • FIG. 10 shows a photomicrograph of composition 4310 after being subjected to the first pre-processing method.
  • FIG. 11 shows a photomicrograph of composition 4310 after being subject to the second pre-processing method.
  • FIG. 12 shows a photomicrograph of composition 4311 after being subjected to the first pre-processing method.
  • FIG. 13 shows a photomicrograph of composition 4311 after being subjected to the second pre-processing method.
  • FIG. 14 shows a photomicrograph of composition 4312 after being subjected to the first pre-processing method.
  • FIG. 15 shows a photomicrograph of composition 4312 after being subjected to the second pre-processing method.
  • FIG. 16 shows a photomicrograph of composition 4313 after being subjected to the first pre-processing method.
  • FIG. 17 shows a photomicrograph of composition 4313 after being subjected to the second pre-processing method.
  • Press hardened steels are generally desirable for their high strength characteristics. In practice, this permits manufacturers to produce components having greater strength and less weight relative to components produced of non-press hardened steels. These high strength characteristics are generally achieved through formation of a predominately martensitic microstructure.
  • the blank is first subjected to an austenitization heat treatment. During this heat treatment, the temperature of the blank is raised to greater than the A 3 temperature for the particular composition of the blank to thereby transform the microstructure of the blank into predominately austenite.
  • the blank is stamped into a predetermined shape using an internally cooled die set.
  • the stamping process also has the effect of rapidly cooling the blank below the martensite start temperature (M s ).
  • M s martensite start temperature
  • the predominately austenitic microstructure of the blank is transformed to a microstructure of predominantly martensite. Because martensite is generally characterized as a strong and hard microstructure, the stamping process generally results in a final part having high strength and high hardness.
  • hot stamping generally results in a final part with high strength and high hardness. With high levels of hardness, the final part generally has relatively low ductility and thus relatively low toughness. Thus, in some circumstances it may be desirable to have a press hardened steel having the high strength characteristics of a conventional press hardened steel, but with improved residual toughness characteristics.
  • FIG. 1 shows a conventional pre-processing method ( 10 ).
  • Pre-processing method includes subjecting a steel sheet to a plurality of pre-processing steps ( 20 , 30 , 40 , 50 ). These steps ( 20 , 30 , 40 , 50 ) are generally performed prior to hot stamping and prior to formation of press hardened steel blanks for the final hot stamping process. Generally, these steps ( 20 , 30 , 40 , 50 ) are performed on sheet material in a continuous rolling mill.
  • the press hardened steel initially begins as an as-cast slab comprising a predetermined composition. The slab then enters a re-heat furnace ( 20 ) and is subjected to a re-heat temperature of approximately 2300° F. (1260° C.).
  • the slab is subjected to rough rolling ( 30 ) and then finishing rolling ( 40 ).
  • These rolling steps progressively reduce the thickness of the slab to a final sheet thickness.
  • the temperature of the slab continuously decreases from the initial 2300° F. (1260° C.) re-heat temperature to a roughing temperature associated with rough rolling ( 30 ).
  • the roughing temperature is approximately 2000° F. (1093° C.).
  • finishing rolling ( 40 ) the slab is subject to a finishing temperature of approximately 1600° F. (871° C.).
  • the slab is subjected to rolling operations that progressively reduce the thickness of the slab by relatively large amounts during rough rolling ( 30 ) to relatively small amounts during finishing rolling ( 40 ).
  • the temperature of the slab decreases at a relatively constant rolling cooling rate ( 12 ).
  • the press hardened steel material is in a steel sheet form.
  • the steel sheet is subject to coiling ( 50 ).
  • Coiling ( 50 ) can be performed at a coiling temperature of approximately 1200° F. (649° C.).
  • coiling ( 50 ) can begin immediately after finishing ( 40 ).
  • coiling ( 50 ) may begin at temperatures above 1600° F. (871° C.) and decrease to the coiling temperature of approximately 1200° F. (649° C.).
  • the steel sheet Prior to coiling ( 50 ), the steel sheet can be cooled to the coiling temperature at one or more different cooling rates ( 14 , 16 ) as shown in FIG. 1 . For instance, at a first cooling rate ( 14 ) or second cooling rates ( 16 ), the steel sheet is cooled relatively slowly at between about 18° F./second and about 20° F./second.
  • the coiled steel sheet is permitted to cool to ambient or room temperature.
  • the coiled steel sheet is then subsequently formed into blanks of steel material for press hardening.
  • the blanks can then be subjected to the hot stamping process described above.
  • toughness can be improved by refining the grain size of the press hardened steel material by modifying certain parameters of the pre-processing steps described above.
  • FIG. 2 shows modified pre-processing method ( 100 ).
  • pre-processing method ( 100 ) of the present example includes a series of pre-processing steps ( 120 , 130 , 140 , 150 ).
  • these steps ( 120 , 130 , 140 , 150 ) are generally performed prior to hot stamping and prior to formation of press hardened steel blanks for the final hot stamping process.
  • these steps ( 120 , 130 , 140 , 150 ) are performed on sheet material in a continuous rolling mill.
  • the press hardened steel initially begins as an as-cast slab comprising a predetermined composition.
  • the slab then enters a re-heat furnace ( 120 ), where the slab is subjected to a re-heat temperature.
  • the reheat temperature in the present example is approximately 2300° F. (1260° C.).
  • the slab is subjected to rough rolling ( 130 ) and then finishing rolling ( 140 ).
  • This progressively reduces the thickness of the slab to a final sheet thickness.
  • the temperature of the slab continuously decreases from the initial 2300° F. (1260° C.) re-heat temperature of the re-heat furnace ( 120 ) to a roughing temperature of approximately 2000° F. (1093° C.) associated with rough rolling ( 130 ).
  • the slab is further reduced to a finishing temperature of approximately 1600° F. (871° C.) associated with finishing rolling ( 140 ).
  • finishing rolling ( 140 ) in the present example is performed at a relatively lower temperature. As will be described in greater detail below, this relatively lower temperature can lead to increased grain refinement when performed in connection with a modified coiling temperature. As the temperature decreases, the slab is subjected to rolling operations that reduce the thickness of the slab by relatively large amounts during rough rolling ( 130 ) to relatively small amounts during finishing rolling ( 140 ).
  • the temperature of the slab decreases at a relatively constant rolling cooling rate ( 112 ). This cooling rate is similar to the rolling cooling rate ( 12 ) of the prior process.
  • the press hardened steel material is in a steel sheet form.
  • the steel sheet is subject to coiling ( 150 ).
  • Coiling ( 150 ) can be performed at a coiling temperature of approximately 1050° F. (566° C.).
  • coiling ( 150 ) can begin immediately after finishing ( 140 ).
  • coiling ( 150 ) may begin at approximately 1600° F. (871° C.) and decrease to the coiling temperature of approximately 1050° F. (566° C.).
  • coiling ( 150 ) can be delayed until the steel sheet reaches the coiling temperature of approximately 1050° F. (566° C.).
  • the steel sheet may be held isothermally for the entirety of coiling ( 150 ).
  • the finishing ( 140 ) is performed at the finishing temperature of about 1600° F. (871° C.)
  • the steel sheet is lowered to the coiling temperature of 1050° F. (566° C.)
  • coiling ( 150 ) is performed while the steel sheet is held at the coiling temperature.
  • the coiling temperature of approximately 1050° F. (566° C.) is generally low relative to the coiling temperatures described above with respect to conventional pre-processing method ( 10 ). As will be understood, this reduced coiling temperature can generally result in improved grain refinement of the steel sheet that can lead to increased residual toughness in a final work product after hot stamping.
  • the steel sheet Prior to coiling ( 150 ), the steel sheet can be cooled to the coiling temperature at a cooling rate ( 114 ) as shown in FIG. 2 .
  • the cooling rate ( 114 ) is between about 35° F./second and about 50° F./second.
  • cooling rate ( 114 ) in the present example is generally relatively fast. This relatively fast cooling rate can be achieved using a run-out-table accelerated cooling method. As will be understood, this relatively fast cooling rate ( 114 ) can generally lead to increased grain refinement and associated improved residual toughness in a final work product after hot stamping.
  • the coiled steel sheet is permitted to cool to ambient or room temperature.
  • the coiled steel sheet is then subsequently formed into blanks of steel material for press hardening.
  • the blanks can then be subjected to the hot stamping process described above.
  • the pre-processing methods ( 10 , 100 ) can be performed using an as-cast slab comprising a predetermined composition. It should be understood that the particular composition of the slab can be varied such that a variety of compositions can be used with the methods ( 10 , 100 ) described above. As will be described in greater detail below, various elements can be added to the slab to influence numerous metallurgical properties of the final work product.
  • 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 0.1-0.5 mass %; in other embodiments, carbon can be present in concentrations of 0.2-0.30 mass %.
  • 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 0.75-3.0 mass %; in other embodiments, manganese can be present in concentrations of 1.15-1.25 mass %.
  • Silicon is added to provide solid solution strengthening.
  • Silicon is a ferrite stabilizer.
  • silicon can be present in concentrations of 0.02-1.5 mass %; in other embodiments, silicon can be present in concentrations of 0.15-0.30 mass %.
  • Aluminum is added for deoxidation during steelmaking and to provide solid solution strengthening.
  • Aluminum is a ferrite stabilizer.
  • aluminum can be present in concentrations of 0.0-0.8 mass %; in other embodiments, aluminum can be present in concentrations of 0.02-0.15 mass %. In other embodiments, aluminum is entirely optional and can be therefore omitted or limited to an impurity element in some embodiments.
  • Titanium is added to getter nitrogen.
  • titanium can be present in concentrations of 0.0-0.060 mass %; in other embodiments, titanium can be present in concentrations of a maximum of 0.045 mass %. In other embodiments, titanium is entirely optional and can be therefore omitted or limited to an impurity element in some embodiments.
  • Molybdenum is added to provide solid solution strengthening and to increase the hardenability of the steel.
  • molybdenum can be present in concentrations of 0-0.5 mass %; in other embodiments, molybdenum can be present in concentrations of 0-0.3 mass %. In other embodiments, molybdenum is entirely optional and can be therefore omitted or limited to an impurity element in some embodiments.
  • Chromium is added to reduce the martensite start temperature, provide solid solution strengthening, and increase the hardenability of the steel.
  • Chromium is a ferrite stabilizer.
  • chromium can be present in concentrations of 0-0.5 mass %; in other embodiments, chromium can be present in concentrations of 0.15-0.25 mass %.
  • Boron is added to increase the hardenability of the steel.
  • boron can be present in concentrations of 0-0.005 mass %; in other embodiments, boron can be present in concentrations of 0.003-0.005 mass %.
  • Nickel is added to provide solid solution strengthening and reduce the martensite start temperature.
  • Nickel is an austenite stabilizer.
  • nickel can be present in concentrations of 0.0-0.6 mass %; in other embodiments, nickel can be present in concentrations of 0.02-0.3 mass %. In still other embodiments, nickel is entirely optional and can be therefore omitted or limited to an impurity element in some embodiments.
  • Niobium is added to provide improved grain refinement. Niobium can also increase hardness and strength. In certain embodiments, niobium can be present in concentrations of 0-0.090 mass %.
  • Compositions are in mass pct.
  • C B Cr Mn Nb Si 4310 0.21 0.003 0.21 1.18 0.000 0.24 4311 0.21 0.0029 0.19 1.19 0.029 0.24 4312 0.21 0.0029 0.20 1.20 0.043 0.24 4313 0.22 0.003 0.19 1.20 0.052 0.25
  • Composition 4310 of Table 1 in Example 1 was subjected to both pre-processing methods ( 10 , 100 ) described above.
  • the steel underwent simulated hot stamping. The steel was heated to approximately 930° C. for 5 min and then quenched in water-cooled copper dies. Samples subjected to each pre-processing method ( 10 , 100 ) plus simulated hot stamping were then subjected to tensile testing to generate stress-strain curves. The resulting stress-strain curves are shown in FIG. 3 with pre-processing method ( 10 ) shown in solid-line form and pre-processing method ( 100 ) shown in dashed-line form.
  • the samples subjected to pre-processing method ( 100 ) generally resulted in improved residual toughness relative to samples subjected to pre-processing method ( 10 ).
  • Photomicrographs were prepared for each sample in the hot-rolled condition prior to the simulated hot stamping and are shown in FIGS. 10 and 11 , with FIG. 10 corresponding to pre-processing method ( 10 ) and FIG. 11 corresponding to pre-processing method ( 100 ).
  • pre-processing method ( 100 ) generally resulted in a more refined grain structure relative to the grain structure produced from pre-processing method ( 10 ).
  • improved residual toughness was observed in FIG. 3 .
  • Composition 4311 of Table 1 in Example 1 was subjected to both pre-processing methods ( 10 , 100 ) described above.
  • the steel underwent simulated hot stamping. The steel was heated to approximately 930° C. for 5 min and then quenched in water-cooled copper dies. Samples subjected to each pre-processing method ( 10 , 100 ) plus simulated hot stamping were then subjected to tensile testing to generate stress-strain curves. The resulting stress-strain curves are shown in FIG. 4 with pre-processing method ( 10 ) shown in solid-line form and pre-processing method ( 100 ) shown in dashed-line form.
  • the samples subjected to pre-processing method ( 100 ) generally resulted in improved residual toughness relative to samples subjected to pre-processing method ( 10 ).
  • Photomicrographs were prepared for each sample in the hot-rolled condition prior to the simulated hot stamping and are shown in FIGS. 12 and 13 , with FIG. 12 corresponding to pre-processing method ( 10 ) and FIG. 13 corresponding to pre-processing method ( 100 ).
  • pre-processing method ( 100 ) generally resulted in a more refined grain structure relative to the grain structure produced from pre-processing method ( 10 ).
  • improved residual toughness was observed in FIG. 4 .
  • Composition 4312 of Table 1 in Example 1 was subjected to both pre-processing methods ( 10 , 100 ) described above.
  • the steel underwent simulated hot stamping. The steel was heated to approximately 930° C. for 5 min and then quenched in water-cooled copper dies. Samples subjected to each pre-processing method ( 10 , 100 ) plus simulated hot stamping were then subjected to tensile testing to generate stress-strain curves. The resulting stress-strain curves are shown in FIG. 5 with pre-processing method ( 10 ) shown in solid-line form and pre-processing method ( 100 ) shown in dashed-line form.
  • the samples subjected to pre-processing method ( 100 ) generally resulted in improved residual toughness relative to samples subjected to pre-processing method ( 10 ).
  • Photomicrographs were prepared for each sample in the hot-rolled condition prior to the simulated hot stamping and are shown in FIGS. 14 and 15 , with FIG. 14 corresponding to pre-processing method ( 10 ) and FIG. 15 corresponding to pre-processing method ( 100 ).
  • pre-processing method ( 100 ) generally resulted in a more refined grain structure relative to the grain structure produced from pre-processing method ( 10 ). As a consequence of this, improved residual toughness was observed in FIG. 5 .
  • Composition 4313 of Table 1 in Example 1 was subjected to both pre-processing methods ( 10 , 100 ) described above.
  • the steel underwent simulated hot stamping. The steel was heated to approximately 930° C. for 5 min and then quenched in water-cooled copper dies. Samples subjected to each pre-processing method ( 10 , 100 ) plus simulated hot stamping were then subjected to tensile testing to generate stress-strain curves. The resulting stress-strain curves are shown in FIG. 6 with pre-processing method ( 10 ) shown in solid-line form and pre-processing method ( 100 ) shown in dashed-line form.
  • the samples subjected to pre-processing method ( 100 ) generally resulted in improved residual toughness relative to samples subjected to pre-processing method ( 10 ).
  • Photomicrographs were prepared for each sample in the hot-rolled condition prior to the simulated hot stamping and are shown in FIGS. 16 and 17 , with FIG. 16 corresponding to pre-processing method ( 10 ) and FIG. 17 corresponding to pre-processing method ( 100 ).
  • pre-processing method ( 100 ) generally resulted in a more refined grain structure relative to the grain structure produced from pre-processing method ( 10 ).
  • improved residual toughness or retained ductility was observed in FIG. 6 .
  • a sample for each composition (e.g., 4310 , 4311 , 4312 , 4313 ) was subject to each pre-processing method ( 10 , 100 ) described above. Steels then underwent a simulated press hardening procedure in which they were austenitized at approximately 930° C. for 300 s and then quenched in flat, water-cooled dies. Double-edge notched tensile tests were then performed. Plots were then prepared of the resulting data for each composition as shown in FIGS. 7 and 8 . For instance, FIG. 7 shows the results for each sample subjected to pre-processing method ( 100 ).
  • FIG. 7 shows the results for each sample subjected to pre-processing method ( 100 ).
  • FIGS. 7 and 8 shows the results for each sample subjected to pre-processing method ( 10 ).
  • the data for each composition is identifiable by symbols. For instance, circles correspond to composition 4310 , triangles correspond to composition 4311 , stars correspond to composition 4312 , and crosses correspond to composition 4313 .
  • FIGS. 7 and 8 materials subjected to pre-processing method ( 100 ) exhibited a higher peak load/force prior to fracture in compassion to the materials subject to pre-processing method ( 10 ).
  • FIGS. 7 and 8 are indicative of pre-processing method ( 100 ) resulting in increased toughness or retained ductility.
  • Example 6 The data discussed above with respect to Example 6 was analyzed further. In particular, integration of the area under the force-displacement curves shown in FIGS. 7 and 8 can be used to obtain a value of strain energy. Strain energy is considered a measure of material toughness. Accordingly, a measure of material toughness for each sample discussed above with respect to Example 6 was generated.
  • FIG. 9 utilizes a different symbolic scheme to identify the correspondence between specific data points and composition. For instance, in FIG. 9 , circles correspond to composition 4310 , crosses correspond to composition 4311 , triangles correspond to composition 4312 , and squares correspond to composition 4313 .
  • FIG. 9 depicts a comparison of samples subjected to pre-processing method ( 10 ) and samples subjected to pre-processing method ( 100 ). In each case, the steels underwent simulated hot stamping prior to testing.
  • solid symbols represent processing method ( 10 ) and open symbols represent processing method ( 100 ).
  • samples subjected to pre-processing method ( 100 ) generally resulted in increased strain energy and therefore increased toughness.
  • some increase in toughness was observed in response to a composition with increased niobium.
  • composition 4313 included the highest niobium concentrations and also included the highest strain energy or toughness measurements.
  • a press hardenable steel comprising by total mass percentage of the steel:
  • a press hardenable steel of any one of Examples 8 through 23 or any one of the following Examples further comprising the step of cooling the press hardenable steel from the re-heat furnace temperature to the rolling temperature at a first cooling rate, and cooling the press hardenable steel from the rolling temperature to the coiling temperature at a second cooling rate, wherein the second cooling rate is greater than the first cooling rate.

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