US20190048447A1 - High Strength Steel Alloy and Strip and Sheet Product Made Therefrom - Google Patents

High Strength Steel Alloy and Strip and Sheet Product Made Therefrom Download PDF

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US20190048447A1
US20190048447A1 US16/161,808 US201816161808A US2019048447A1 US 20190048447 A1 US20190048447 A1 US 20190048447A1 US 201816161808 A US201816161808 A US 201816161808A US 2019048447 A1 US2019048447 A1 US 2019048447A1
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alloy
max
avg
steel alloy
strip
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Paul M. Novotny
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CRS Holdings LLC
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CRS Holdings LLC
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    • 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
    • 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
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • 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
    • C21D8/0247—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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/06—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—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
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • 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
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/58—Oils

Definitions

  • This invention relates to a steel alloy having a unique combination of high strength, high toughness, and high ductility.
  • the invention also relates to the use of the steel alloy to make thin gauge product forms such as strip and sheet that can be readily formed into parts for automotive use.
  • Steel sheet and strip that provide higher strength than conventional steels used for auto body and frame applications can be used to reduce the weight of stamped body and frame parts in this “Light Weighting” effort as long as they are sufficiently tough and formable.
  • a measure of the strength and formability of a steel is a Strip Index Number (SIN) which is the product of the ultimate tensile strength (UTS) in MPa and the Elongation in %.
  • SIN Strip Index Number
  • UTS ultimate tensile strength
  • Elongation in %.
  • steel sheet and strip having an SIN of at least about 20,000 provide sufficient weight reduction while providing adequate strength and formability.
  • an SIN of at least about 30,000 is preferred.
  • AHSS advanced high strength steels
  • the alloy according to this invention provides a unique combination of very high strength, toughness, and ductility.
  • the alloy of this invention can be characterized by the broad, intermediate and preferred weight percent compositions set forth below.
  • the balance of the alloy is iron and the usual impurities found in commercial grades of steel alloys produced for similar use and properties.
  • impurities phosphorus is preferably restricted to not more than about 0.03% max. and sulfur is preferably restricted to not more than about 0.003% max.
  • the foregoing tabulation is provided as a convenient summary and is not intended to restrict the lower and upper values of the ranges of the individual elements for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other.
  • one or more of the ranges can be used with one or more of the other ranges for the remaining elements.
  • a minimum or maximum for an element of a broad or preferred composition can be used with the minimum or maximum for the same element in another preferred or intermediate composition.
  • the alloy according to the present invention may comprise, consist essentially of, or consist of the constituent elements described above and throughout this application.
  • percent or the symbol “%” means percent by weight or mass percent, unless otherwise specified.
  • the term “about” used in connection with a weight percent value or range means the usual analytical tolerance or experimental error expected by a person skilled in the art based on known, standardized measuring techniques.
  • a thin gauge steel product such as sheet or strip product that is made from one of the steel alloys described in the table.
  • the thin gauge products can be readily formed into automotive parts because of their good ductility.
  • a thin gauge steel product in accordance with this aspect of the invention has an SIN of at least about 20,000 and better yet, and SIN of at least 25,000.
  • a preferred embodiment of the steel product has an SIN of at least about 30,000.
  • weight percent ranges described above can be further defined by the roles the elements play in the alloy of this invention.
  • the combination of the elements silicon, copper, and vanadium, and niobium when present, function as diffusion modifiers for the purposes of this invention because they have been shown to reduce the diffusion of carbon as well as deleterious tramp elements such as P and S to the grain boundaries of the alloy.
  • silicon, copper, and vanadium, and niobium when present are broadly balanced such that
  • the SIN is required to be at least about 30,000, silicon, copper, and vanadium, and optionally niobium, are preferably balanced such that
  • Molybdenum is optionally present in the alloy according to this invention and tungsten may be substituted for some or all of the molybdenum in this alloy. When present, tungsten is substituted for molybdenum on a 2:1 basis such that Mo+1 ⁇ 2W is about 0.20-0.5% and preferably about 0.15-0.3%.
  • Yttrium and magnesium may also be present in this alloy either separately or in combination. In this regard the alloy may contain about 0.001-0.025% yttrium and preferably may contain about 0.002-0.020% yttrium. The alloy may also contain about 0.001-0.01% magnesium and preferably may contain about 0.001-0.006% magnesium.
  • the magnesium and/or yttrium is added during primary melting to deoxidize the steel alloy. Magnesium and yttrium also benefit the strength and toughness of this steel by aiding in grain refinement of the alloy during processing.
  • the elements molybdenum, tungsten, and chromium combine with carbon to form M 2 C carbides (where M is Cr, Mo, and/or W) during tempering.
  • M is Cr, Mo, and/or W
  • the elements Mo, W, and Cr can be referred to as tempered carbide formers for the purposes of the alloy according to this invention.
  • chromium and molybdenum and tungsten when present, promote the formation of M 2 C carbides and can be substituted for each other in this alloy.
  • a positive addition of molybdenum and or tungsten is included as described above.
  • molybdenum, chromium, and carbon are preferably balanced such that 3.5 ⁇ (% Mo+% Cr)/(% C) ⁇ 7.5.
  • manganese and nickel are austenite stabilizers and contribute to the good hardenability of this alloy.
  • Manganese and nickel can be substituted for each other to a limited extent to stabilize austenite.
  • manganese and nickel are broadly balanced such that 3.5 ⁇ (% Mn+% Ni) ⁇ 8.0.
  • This alloy and products made therefrom are preferably prepared by vacuum melting techniques.
  • primary melting of the alloy is preferably accomplished with vacuum induction melting (VIM).
  • VIM vacuum induction melting
  • the alloy can be refined using vacuum arc remelting (VAR).
  • Primary melting may also be performed by arc melting in air (ARC) for less critical applications.
  • ARC melting the alloy may be refined by argon oxygen decarburization (AOD), electroslag remelting (ESR), or VAR.
  • the alloy of this invention is preferably processed to thin gauge forms such as strip or sheet.
  • parts made from the alloy can be austenitized for short times at a temperature of about 1400 to 1900° F. (760 and 1038° C.) and then air cooled. The parts can then be used in service.
  • annealed strip or sheet can be formed into a shaped part and then regions of the part can be selectively heat treated by induction heating to the austenitizing temperature of 1400 to 1900° F. (760 and 1038° C.) followed by cooling in air.
  • Another option is to heat the strip or sheet material to the austenitizing temperature of 1400 to 1900° F. (760 and 1038° C.) then stamp the part to form it and allow the hot stamped part to air cool.
  • the parts can be used in the air-cooled condition or after short time tempers at 400 to 700° F. (204.4-371° C.).
  • the alloy has relatively high ultimate tensile strength (UTS) in the annealed condition, i.e., at least about 150 ksi (1025 MPa) on average, combined with very high ductility (i.e., 10-25% Elongation). Therefore, parts made from annealed alloy strip or sheet may be used in some applications without any further heat treatment.
  • UTS ultimate tensile strength
  • the alloy may also be hot worked from a temperature of up to about 2100° F. (1149° C.), preferably at about 1800° F. (982° C.), to form various intermediate product forms such as billets and bars.
  • the alloy is preferably heat treated by austenitizing at about 1585° F. (863° C.) to about 1900° F. (1038° C.) for about 1-2 hours.
  • the alloy is then air cooled or oil quenched from the austenitizing temperature.
  • the alloy can be vacuum heat treated and gas quenched.
  • Parts made from the alloy in bar form are preferably deep chilled at either ⁇ 100° F. ( ⁇ 73.3° C.) or ⁇ 320° F. ( ⁇ 196° C.) for about 1-8 hours and then warmed in air.
  • the alloy is preferably tempered at about 400° F. to 600° F. (204.4-316° C.) for about 2-3 hours and then air cooled.
  • the alloy may be tempered at up to 700° F. (371° C.) when an optimum combination of strength and toughness is not required.
  • a shaped part made from a thin gauge product form of the alloy as described above.
  • the shaped part is preferably embodied as a stamped body or frame part for an automobile.
  • a thin gauge product in accordance with the present invention is a part or component made from sheet or strip having a thickness of at least about 0.0009 in. (0.0229 mm) and less than 0.25 in. (6.35 mm).
  • the examples and comparative alloys were vacuum induction melted and cast as 35 lb. (15.9 kg) heats. The heats were hot worked and rough machined into sets of duplicate standard tensile specimens. The pairs of specimens from each set were austenitized at different temperatures for 1.5 hours and then oil quenched. The specimen pairs were then tempered for 2 hours and air cooled. The combinations of austenitizing temperature and tempering temperature used for the specimen pairs of each alloy are set forth in Table 2 below.
  • Example 3 represents the alloy according to the Preferred A composition of the alloy according to the present invention and Example 4 represents the alloy according to the Preferred B composition of the alloy according to the present invention.
  • Example 4 C 0.357 0.311 Mn 4.01 4.04 Si 1.54 1.57 P 0.018 0.015 S ⁇ 0.0005 ⁇ 0.0005 Cr 2.05 1.11 Ni 4.02 0.96 Mo 0.03 0.20 Cu 0.51 0.51 V 0.36 0.24 Ti 0.0050 0.0040 Al 0.0020 0.0060 N 0.0044 0.0041 Ca 0.0015 0.0016
  • the balance of each composition in Table 4 is iron and impurities.
  • Examples 3 and 4 were melted and refined by ARC and AOD as 40-ton heats and then cast as billet on a continuous caster. The continuously cast billets were hot worked and rough machined into sets of duplicate standard tensile specimens.
  • Duplicate tensile test specimens for Example 3 were prepared from 0.150 inch thick hot rolled band.
  • Duplicate tensile test specimens for Example 4 were prepared from 0.150 inch thick hot rolled band as follows. A first set of specimens were prepared from the 0.150 in. band after grinding the band material to a final thickness of 0.110 in.
  • a second set of specimens were prepared by cold rolling the 0.150 in. band material to form strip having a thickness of 0.130 in. The strip material was ground to a final thickness of 0.087 in.
  • a third set of specimens were prepared by cold rolling the band material to form strip having a thickness of 0.110 in. and then grinding the strip material to a final thickness of 0.074 in.
  • the pairs of the tensile specimens of Example 3 were heat treated by placing the specimens into stainless steel bags which were then backfilled with argon gas and divided into subsets. Each subset was heat treated in accordance with one of the heat treatments A-H set forth in Table 2 above. Austenitizing was performed by holding the specimen subset at temperature for 1.5 hours and then oil quenching to room temperature. Tempering was performed by holding the specimen subset at the respective tempering temperature for 2 hours followed by air cooling to room temperature.
  • the pairs of tensile specimens of Example 4 were heat treated in three groups. One group was heat treated with Heat Treatment A in Table 2 above. A second group was heat treated with Heat Treatment C of Table 2 and the third group was heat treated with Heat Treatment E of Table 2. Austenitizing was performed by holding the specimens at the respective temperature for 1.5 hours and then air cooling to room temperature. Tempering was performed by holding the specimens at the respective tempering temperature for 2 hours followed by air cooling to room temperature.
  • Y.S. 0.2% offset yield strength
  • U.T.S. ultimate tensile strength
  • % El. percent elongation
  • % R.A. percent reduction in area

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Heat Treatment Of Steel (AREA)
  • Heat Treatment Of Strip Materials And Filament Materials (AREA)

Abstract

An alloy is disclosed that provides a unique combination of very high strength, toughness, and ductility. The alloy can be characterized by the following weight percent composition.
C 0.3-0.6 Mn 3.0-4.5 Si 1.0-2.0 Cr 0.6-2.5 Ni 0.6-5.0 Mo + ½W Up to 0.5 Cu 0.3-1.0 Co  0.01 max. V + 5/9Nb 0.1-0.5 Ti 0.025 max. Al 0.025 max. Ca 0.005 max. N  0.02 max.

The balance is iron and the usual impurities, wherein the impurities include not more than about 0.03% phosphorus and not more than about 0.003% sulfur. The alloy is further defined by containing Si, Cu, V, and optionally Nb such that 2≤(% Si+% Cu)/(% V+( 5/9)×% Nb)≤34.

Description

    CROSS REFERENCE TO RELATED APPLICATIONS
  • This application is a continuation of U.S. application Ser. No. 14/572,888, filed on Dec. 17, 2014, which claims benefit of U.S. provisional application No. 61/919,081, filed Dec. 20, 2013, the entireties of which are incorporated herein by reference.
    • BACKGROUND OF THE INVENTION Field of the Invention
  • This invention relates to a steel alloy having a unique combination of high strength, high toughness, and high ductility. The invention also relates to the use of the steel alloy to make thin gauge product forms such as strip and sheet that can be readily formed into parts for automotive use.
    • Description of the Related Art
  • The US Environmental Protection Agency requires that new cars must meet a 54.5 miles/gallon corporate average fuel economy standard by the year 2025. This requirement will likely cause automobile manufacturers to reduce the weights of their vehicles. Light-weight materials such as aluminum and composite materials that provide high strength and significant reduction in weight compared to the known steel alloys can be used for making thin-gauge auto body and frame parts. However, the use of such materials will present challenges to auto manufacturers because their production line-ups are designed for using steel alloys and a change to non-steel materials such as aluminum and composites would require substantial capital investments as well as a substantial increase in the cost of materials.
  • Steel sheet and strip that provide higher strength than conventional steels used for auto body and frame applications can be used to reduce the weight of stamped body and frame parts in this “Light Weighting” effort as long as they are sufficiently tough and formable. A measure of the strength and formability of a steel is a Strip Index Number (SIN) which is the product of the ultimate tensile strength (UTS) in MPa and the Elongation in %. For many automotive applications steel sheet and strip having an SIN of at least about 20,000 provide sufficient weight reduction while providing adequate strength and formability. However, for structural parts that require higher strength, an SIN of at least about 30,000 is preferred. Many of the steels known as advanced high strength steels (AHSS) leave something to be desired with respect to ductility because strength and ductility are inversely related properties. Good ductility is needed for strip and sheet forms of high strength steel material to provide good formability. Accordingly, it would be desirable to have a steel alloy that provides a combination of high strength and ductility that not only results in significant weight reduction in automotive body and frame parts, but also can be readily formed into such products.
    • SUMMARY OF THE INVENTION
  • The foregoing need is resolved to a large degree by a steel alloy in accordance with the present invention. The alloy according to this invention provides a unique combination of very high strength, toughness, and ductility. The alloy of this invention can be characterized by the broad, intermediate and preferred weight percent compositions set forth below.
  • Element Broad Intermediate Preferred A Preferred B
    C 0.3-0.6 0.30-0.45 0.30-0.40 0.30-0.36
    Mn 3.0-4.5 3.5-4.5 3.5-4.5 3.5-4.5
    Si 1.0-2.0 1.3-1.8 1.3-1.7 1.3-1.7
    Cr 0.6-2.5 0.75-2.35  1.6-2.35 0.75-1.5 
    Ni 0.6-5.0 0.7-4.5 3.7-4.3 0.7-2.5
    Mo + ½W Up to 0.5 Up to 0.3  0.1 max. 0.15-0.25
    Cu 0.3-1.0 0.4-0.7 0.4-0.6 0.4-0.6
    Co  0.01 max.  0.01 max.  0.01 max.  0.01 max.
    V + 5/9Nb 0.1-0.5 0.2-0.4 0.30-0.40 0.20-0.30
    Ti 0.025 max. 0.020 max. 0.020 max. 0.020 max.
    Al 0.025 max. 0.020 max. 0.020 max. 0.020 max.
    Ca 0.005 max. 0.002 max. 0.001 max. 0.001 max.
    N  0.02 max.  0.02 max. 0.020 max. 0.020 max.
  • The balance of the alloy is iron and the usual impurities found in commercial grades of steel alloys produced for similar use and properties. Among said impurities phosphorus is preferably restricted to not more than about 0.03% max. and sulfur is preferably restricted to not more than about 0.003% max.
  • The foregoing tabulation is provided as a convenient summary and is not intended to restrict the lower and upper values of the ranges of the individual elements for use in combination with each other, or to restrict the ranges of the elements for use solely in combination with each other. Thus, one or more of the ranges can be used with one or more of the other ranges for the remaining elements. In addition, a minimum or maximum for an element of a broad or preferred composition can be used with the minimum or maximum for the same element in another preferred or intermediate composition. Moreover, the alloy according to the present invention may comprise, consist essentially of, or consist of the constituent elements described above and throughout this application. Here and throughout this specification the term “percent” or the symbol “%” means percent by weight or mass percent, unless otherwise specified. Furthermore, the term “about” used in connection with a weight percent value or range means the usual analytical tolerance or experimental error expected by a person skilled in the art based on known, standardized measuring techniques.
  • In accordance with another aspect of this invention, there is provided a thin gauge steel product such as sheet or strip product that is made from one of the steel alloys described in the table. The thin gauge products can be readily formed into automotive parts because of their good ductility. A thin gauge steel product in accordance with this aspect of the invention has an SIN of at least about 20,000 and better yet, and SIN of at least 25,000. A preferred embodiment of the steel product has an SIN of at least about 30,000.
    • DETAILED DESCRIPTION
  • The weight percent ranges described above can be further defined by the roles the elements play in the alloy of this invention. The combination of the elements silicon, copper, and vanadium, and niobium when present, function as diffusion modifiers for the purposes of this invention because they have been shown to reduce the diffusion of carbon as well as deleterious tramp elements such as P and S to the grain boundaries of the alloy. Within the foregoing weight percent ranges, silicon, copper, and vanadium, and niobium when present, are broadly balanced such that

  • 2≤(% Si+% Cu)/(% V+( 5/9)×% Nb)≤34.
  • For strip applications where the SIN is required to be at least about 30,000, silicon, copper, and vanadium, and optionally niobium, are preferably balanced such that

  • 4.5≤(% Si+% Cu)/(% V+( 5/9)×% Nb)≤10.
  • Molybdenum is optionally present in the alloy according to this invention and tungsten may be substituted for some or all of the molybdenum in this alloy. When present, tungsten is substituted for molybdenum on a 2:1 basis such that Mo+½W is about 0.20-0.5% and preferably about 0.15-0.3%. Yttrium and magnesium may also be present in this alloy either separately or in combination. In this regard the alloy may contain about 0.001-0.025% yttrium and preferably may contain about 0.002-0.020% yttrium. The alloy may also contain about 0.001-0.01% magnesium and preferably may contain about 0.001-0.006% magnesium. The magnesium and/or yttrium is added during primary melting to deoxidize the steel alloy. Magnesium and yttrium also benefit the strength and toughness of this steel by aiding in grain refinement of the alloy during processing.
  • The elements molybdenum, tungsten, and chromium combine with carbon to form M2C carbides (where M is Cr, Mo, and/or W) during tempering. The elements Mo, W, and Cr can be referred to as tempered carbide formers for the purposes of the alloy according to this invention.
  • Thus, chromium and molybdenum and tungsten when present, promote the formation of M2C carbides and can be substituted for each other in this alloy. For thin gauge product forms such as strip and sheet where an SIN of at least 30,000 is desired, a positive addition of molybdenum and or tungsten is included as described above. Further, within the foregoing weight percent ranges, molybdenum, chromium, and carbon are preferably balanced such that 3.5≤(% Mo+% Cr)/(% C)≤7.5.
  • The elements manganese and nickel are austenite stabilizers and contribute to the good hardenability of this alloy. Manganese and nickel can be substituted for each other to a limited extent to stabilize austenite. For the thin gauge product applications where an SIN of at least about 30,000 is desired, within the foregoing weight percent ranges, manganese and nickel are broadly balanced such that 3.5≤(% Mn+% Ni)≤8.0.
  • This alloy and products made therefrom are preferably prepared by vacuum melting techniques. In this regard, primary melting of the alloy is preferably accomplished with vacuum induction melting (VIM). When desired, as for more critical applications, the alloy can be refined using vacuum arc remelting (VAR). Primary melting may also be performed by arc melting in air (ARC) for less critical applications. After ARC melting, the alloy may be refined by argon oxygen decarburization (AOD), electroslag remelting (ESR), or VAR.
  • The alloy of this invention is preferably processed to thin gauge forms such as strip or sheet. In strip or sheet form, parts made from the alloy can be austenitized for short times at a temperature of about 1400 to 1900° F. (760 and 1038° C.) and then air cooled. The parts can then be used in service. Alternatively, annealed strip or sheet can be formed into a shaped part and then regions of the part can be selectively heat treated by induction heating to the austenitizing temperature of 1400 to 1900° F. (760 and 1038° C.) followed by cooling in air. Another option is to heat the strip or sheet material to the austenitizing temperature of 1400 to 1900° F. (760 and 1038° C.) then stamp the part to form it and allow the hot stamped part to air cool. The parts can be used in the air-cooled condition or after short time tempers at 400 to 700° F. (204.4-371° C.). The alloy has relatively high ultimate tensile strength (UTS) in the annealed condition, i.e., at least about 150 ksi (1025 MPa) on average, combined with very high ductility (i.e., 10-25% Elongation). Therefore, parts made from annealed alloy strip or sheet may be used in some applications without any further heat treatment.
  • The alloy may also be hot worked from a temperature of up to about 2100° F. (1149° C.), preferably at about 1800° F. (982° C.), to form various intermediate product forms such as billets and bars. The alloy is preferably heat treated by austenitizing at about 1585° F. (863° C.) to about 1900° F. (1038° C.) for about 1-2 hours. The alloy is then air cooled or oil quenched from the austenitizing temperature. When desired, the alloy can be vacuum heat treated and gas quenched. Parts made from the alloy in bar form are preferably deep chilled at either −100° F. (−73.3° C.) or −320° F. (−196° C.) for about 1-8 hours and then warmed in air. If lower strength is acceptable the refrigeration step may be eliminated for parts made from bar products. The alloy is preferably tempered at about 400° F. to 600° F. (204.4-316° C.) for about 2-3 hours and then air cooled. The alloy may be tempered at up to 700° F. (371° C.) when an optimum combination of strength and toughness is not required.
  • In accordance with a further aspect of the present invention there is provided a shaped part made from a thin gauge product form of the alloy as described above. The shaped part is preferably embodied as a stamped body or frame part for an automobile. A thin gauge product in accordance with the present invention is a part or component made from sheet or strip having a thickness of at least about 0.0009 in. (0.0229 mm) and less than 0.25 in. (6.35 mm).
    • WORKING EXAMPLES Example I
  • To demonstrate the unique combination of properties provided by the alloy according to the present invention, representative examples of the alloy and examples of comparative alloys were melted, processed, and tested. The weight percent compositions of the tested alloys are set forth in Table 1 below. Examples 1 and 2 represent the alloy according to the present invention. Alloys A and B are comparative alloys.
  • TABLE 1
    Element Example 1 Example 2 Alloy A Alloy B
    C 0.51 0.36 0.48 0.38
    Mn 0.68 4.02 6.88 4.71
    Si 1.45 1.44 1.43 1.70
    Cr 1.98 1.98 1.99 1.60
    Ni 6.85 3.96 0.70 3.88
    Mo — — — 0.12
    Cu 0.64 0.52 0.64 0.66
    V 0.22 0.36 0.21 0.30

    The balance of each composition in Table 1 is iron and impurities.
  • The examples and comparative alloys were vacuum induction melted and cast as 35 lb. (15.9 kg) heats. The heats were hot worked and rough machined into sets of duplicate standard tensile specimens. The pairs of specimens from each set were austenitized at different temperatures for 1.5 hours and then oil quenched. The specimen pairs were then tempered for 2 hours and air cooled. The combinations of austenitizing temperature and tempering temperature used for the specimen pairs of each alloy are set forth in Table 2 below.
  • TABLE 2
    Austenitizing Tempering
    Heat Treatment ID Temperature Temperature
    A 1635° F. 400° F.
    B 1635° F. 600° F.
    C 1685° F. 400° F.
    D 1685° F. 500° F.
    E 1735° F. 400° F.
    F 1755° F. 535° F.
    G 1800° F. 400° F.
    H 1730° F. 445° F.

    After heat treatment, the test specimens were finish machined to final dimension and tested. The results of room temperature tensile tests for each example are presented in Tables 3A to 3D below including the 0.2% offset yield strength (Y.S.), the ultimate tensile strength (U.T.S.), the percent elongation (% El.), and the percent reduction in area (% R.A.). Also included in the tables are calculations of the SIN for each specimen (SIN=UTS in MPa×% El.). Average values for each pair of the tested specimens are also presented in the tables.
  • TABLE 3A
    Y.S. U.T.S.
    Heat ID Heat Treatment Sample ksi MPa ksi MPa % El. % R.A. SIN
    Example 1 A A1 186.7 1287.5 300.7 2072.9 17.7 32.7 36,691
    A2 184.8 1274.2 301.6 2079.3 14.4 23.2 29,942
    Avg. 185.8 1280.8 301.1 2076.1 16.1 27.9 33,317
    B B1 192.0 1323.7 255.7 1762.7 15.3 31.4 26,969
    B2 196.0 1351.5 256.7 1770.1 18.5 39.6 32,747
    Avg. 194.0 1337.6 256.2 1766.4 16.9 35.5 29,858
    C C1 164.4 1133.4 304.5 2099.5 14.4 18.2 30,233
    C2 165.0 1137.4 304.0 2096.0 15.4 18.3 32,278
    Avg. 164.7 1135.4 304.3 2097.7 14.9 18.3 31,255
    D D1 173.3 1194.8 274.1 1890.1 16.1 21.8 30,431
    D2 177.0 1220.3 274.5 1892.4 19.3 39.6 36,524
    Avg. 175.1 1207.6 274.3 1891.3 17.7 30.7 33,477
    E E1 151.5 1044.4 304.0 2095.7 15.4 21.1 32,273
    E2 162.3 1119.2 304.3 2097.7 18.0 35.8 37,759
    Avg. 156.9 1081.8 304.1 2096.7 16.7 28.5 35,016
  • TABLE 3B
    Y.S. U.T.S.
    Heat ID Heat Treatment Sample ksi MPa (ksi) (MPa) % EL. % R.A. SIN
    Example 2 A A1 121.3 836.4 284.4 1960.9 15.8 19.0 30,903
    A2 125.3 863.6 285.0 1965.2 17.1 28.9 33,546
    Avg. 123.3 850.0 284.7 1963.1 16.4 24.0 32,225
    B B1 139.6 962.5 240.8 1660.3 19.4 21.8 32,211
    B2 143.6 990.4 239.1 1648.8 21.4 18.3 35,285
    Avg. 141.6 976.4 240.0 1654.6 20.4 20.0 33,748
    C C1 121.7 839.2 285.3 1966.7 15.7 23.9 30,878
    C21
    Avg. 121.7 839.2 285.3 1966.7 15.7 23.9 30,878
    D D1 124.6 859.4 264.6 1824.1 17.3 25.4 31,557
    D2 128.9 888.5 263.1 1814.2 21.4 28.1 38,824
    Avg. 126.8 874.0 263.8 1819.2 19.4 26.7 35,191
    E E1 111.8 770.9 289.5 1996.2 13.7 13.2 27,348
    E2 115.2 794.0 289.3 1994.9 14.8 17.6 29,525
    Avg. 113.5 782.5 289.4 1995.6 14.3 15.4 28,436
    1Sample not tested because of forging defect.
  • TABLE 3C
    Y.S. U.T.S.
    Heat ID Heat Treatment Sample ksi MPa ksi MPa % El. % R.A. SIN
    Alloy A A A1 61.9 426.7 100.6 693.7 8.8 8.6 6,104
    A2 60.8 418.9 115.7 797.5 9.5 8.5 7,568
    Avg. 61.3 422.8 108.1 745.6 9.1 8.5 6,836
    B B1 67.0 461.8 132.2 911.6 9.7 8.0 8,843
    B2 65.2 449.4 154.9 1067.7 11.8 9.4 12,598
    Avg. 66.1 455.6 143.5 989.6 10.8 8.7 10,721
    C C1 62.8 433.1 146.3 1008.5 11.6 11.5 11,699
    C2 63.0 434.1 132.3 912.4 12.1 11.5 11,040
    Avg. 62.9 433.6 139.3 960.4 11.9 11.5 11,369
    D D1 65.0 448.4 140.0 965.5 12.5 10.7 12,068
    D2 62.9 433.5 127.6 879.7 10.1 10.8 8,885
    Avg. 64.0 441.0 133.8 922.6 11.3 10.7 10,477
    E E1 59.6 411.1 132.8 915.5 12.0 12.4 10,986
    E2 57.3 395.1 117.4 809.7 10.7 11.5 8,663
    Avg. 58.5 403.1 125.1 862.6 11.4 12.0 9,825
  • TABLE 3D
    Y.S. U.T.S.
    Heat ID Heat Treatment Sample ksi MPa ksi MPa % El. % R.A. SIN
    Alloy B A A1 55.9 385.1 221.2 1525.2 6.7 6.5 10,265
    A2 64.5 444.6 212.4 1464.1 6.7 5.9 9,853
    Avg. 60.2 414.8 216.8 1494.7 6.7 6.2 10,059
    B B1 91.7 632.2 220.5 1520.0 5.5 9.3 8,360
    B2 79.5 547.9 208.6 1438.4 5.8 13.2 8,343
    Avg. 85.6 590.1 214.5 1479.2 5.7 11.2 8,351
    C C1 70.4 485.4 188.6 1300.4 4.9 4.1 6,372
    C2 68.5 472.5 210.0 1447.8 5.5 4.0 7,963
    Avg. 69.5 478.9 199.3 1374.1 5.2 4.1 7,168
    D D1 68.3 471.1 195.6 1348.3 5.5 4.9 7,416
    D2 66.9 461.5 219.3 1512.0 6.7 8.4 10,130
    Avg. 67.6 466.3 207.4 1430.2 6.1 6.7 8,773
    E E1 53.2 366.9 162.1 1117.6 3.7 3.3 4,135
    E2 49.8 343.4 139.4 961.1 3.8 3.3 3,652
    Avg. 51.5 355.2 150.7 1039.4 3.8 3.3 3,894
    • Example II
  • In order to demonstrate that the alloy of the present invention is capable of providing the desired combination of properties when scaled up to commercial production-size heats, two additional heats were melted, processed, and tested. The weight percent compositions of the tested alloys are set forth in Table 4 below. Example 3 represents the alloy according to the Preferred A composition of the alloy according to the present invention and Example 4 represents the alloy according to the Preferred B composition of the alloy according to the present invention.
  • TABLE 4
    Element Example 3 Example 4
    C 0.357 0.311
    Mn 4.01 4.04
    Si 1.54 1.57
    P 0.018 0.015
    S <0.0005 <0.0005
    Cr 2.05 1.11
    Ni 4.02 0.96
    Mo 0.03 0.20
    Cu 0.51 0.51
    V 0.36 0.24
    Ti 0.0050 0.0040
    Al 0.0020 0.0060
    N 0.0044 0.0041
    Ca 0.0015 0.0016

    The balance of each composition in Table 4 is iron and impurities.
  • Examples 3 and 4 were melted and refined by ARC and AOD as 40-ton heats and then cast as billet on a continuous caster. The continuously cast billets were hot worked and rough machined into sets of duplicate standard tensile specimens. Duplicate tensile test specimens for Example 3 were prepared from 0.150 inch thick hot rolled band. Duplicate tensile test specimens for Example 4 were prepared from 0.150 inch thick hot rolled band as follows. A first set of specimens were prepared from the 0.150 in. band after grinding the band material to a final thickness of 0.110 in. A second set of specimens were prepared by cold rolling the 0.150 in. band material to form strip having a thickness of 0.130 in. The strip material was ground to a final thickness of 0.087 in. A third set of specimens were prepared by cold rolling the band material to form strip having a thickness of 0.110 in. and then grinding the strip material to a final thickness of 0.074 in.
  • The pairs of the tensile specimens of Example 3 were heat treated by placing the specimens into stainless steel bags which were then backfilled with argon gas and divided into subsets. Each subset was heat treated in accordance with one of the heat treatments A-H set forth in Table 2 above. Austenitizing was performed by holding the specimen subset at temperature for 1.5 hours and then oil quenching to room temperature. Tempering was performed by holding the specimen subset at the respective tempering temperature for 2 hours followed by air cooling to room temperature.
  • The pairs of tensile specimens of Example 4 were heat treated in three groups. One group was heat treated with Heat Treatment A in Table 2 above. A second group was heat treated with Heat Treatment C of Table 2 and the third group was heat treated with Heat Treatment E of Table 2. Austenitizing was performed by holding the specimens at the respective temperature for 1.5 hours and then air cooling to room temperature. Tempering was performed by holding the specimens at the respective tempering temperature for 2 hours followed by air cooling to room temperature.
  • The results of room temperature tensile tests for Example 3 are presented in Table 5 below including the 0.2% offset yield strength (Y.S.), the ultimate tensile strength (U.T.S.), the percent elongation (% El.), and the percent reduction in area (% R.A.). Also included in the tables are calculations of the SIN for each specimen (SIN=UTS in MPa×% El.). Average values for each pair of the tested specimens are also presented in the tables.
  • TABLE 5
    Y.S. U.T.S.
    Heat ID Heat Treatment Sample ksi MPa ksi MPa % El. % R.A. SIN
    Example 3 A A1 150.2 1035.5 267.9 1846.8 17.0 38.8 31,395
    A2 141.9 978.3 271.6 1872.6 18.0 21.3 33,707
    Avg. 146.0 1006.9 269.7 1859.7 17.5 30.0 32,551
    B B1 161.2 111.2 230.7 1590.5 21.3 42.7 33,877
    B2 159.9 1102.5 228.9 1578.2 20.2 31.3 31,879
    Avg. 160.5 1106.9 229.8 1584.6 20.8 37.0 32,878
    C C1 143.3 987.7 269.2 1856.4 17.9 31.3 33,229
    C2 139.7 963.0 267.7 1845.8 20.1 39.2 37,101
    Avg. 141.5 975.4 268.5 1851.1 19.0 35.2 35,165
    D D1 146.6 1010.8 250.1 1724.4 20.6 36.6 35,522
    D2 156.5 1079.0 245.4 1691.8 20.0 39.1 33,837
    Avg. 151.6 1044.9 247.7 1708.1 20.3 37.8 34,680
    E E1 141.9 978.6 265.6 1831.1 17.6 35.7 32,264
    E2 137.5 948.0 275.1 1896.8 * 36.7 **
    Avg. 139.7 963.3 270.3 1864.0 17.6 36.2 32,264
    F F1 146.0 1006.5 243.0 1675.4 22.4 43.9 37,528
    F2 147.2 1014.8 245.0 1689.2 * 40.4 **
    Avg. 146.6 1010.6 244.0 1682.3 22.4 42.2 37,528
    G G1 130.0 896.1 266.9 1840.5 17.9 29.1 32,945
    G2 131.1 904.0 268.5 1851.4 17.8 24.4 32,584
    Avg. 130.5 900.0 267.7 1845.9 17.8 26.8 32,765
    H H1 137.3 946.7 262.4 1808.9 20.6 39.7 37,264
    H2 136.8 942.9 267.8 1846.4 * 32.9 **
    Avg. 137.0 944.8 265.1 1827.7 20.6 36.3 37,264
    * = Invalid measurement - Specimen broke outside gage section
    ** = No value could be calculated.
  • The results of room temperature tensile tests for Example 4 are presented in Tables 6A, 6B, and 6C below.
  • TABLE 6A
    Y.S. U.T.S.
    Heat ID Heat Treatment Sample ksi MPa ksi MPa % El. % R.A. SIN
    Example 4 A A1 174.0 1199.4 266.6 1837.9 * * **
    Group 1 A2 173.9 1199.1 257.1 1772.3 13.9 41.3 24,635
    Avg. 173.9 1199.2 261.8 1805.1 13.9 41.3 24,635
    C C1 164.8 1136.3 253.4 1746.9 14.0 32.7 24,457
    C2 169.0 1165.4 257.7 1776.9 14.6 23.2 25,924
    Avg. 166.9 1150.8 255.5 1761.9 14.3 27.9 25,191
    E E1 164.4 1133.4 257.3 1774.0 13.1 39.5 23,240
    E2 172.5 1189.3 262.9 1812.6 13.0 35.5 23,492
    Avg. 168.4 1161.4 260.1 1793.3 13.0 37.5 23,366
    * = Invalid measurement - Specimen broke outside gage section
    ** = No value could be calculated.
  • TABLE 6B
    Y.S. U.T.S.
    Heat ID Heat Treatment Sample ksi MPa ksi MPa % El. % R.A. SIN
    Example 4 A A1 179.4 1237.2 262.3 1808.4 * 33.2 ***
    Group 2 A2 177.3 1222.4 253.3 1746.2 * 41.9 ***
    Avg. 178.4 1229.3 257.8 1777.3 37.5
    C C1**
    C2 175.1 1207.4 263.2 1814.6 * 43.2 ***
    Avg. 175.1 1207.4 263.2 1814.6 43.2
    E E1 171.0 1178.7 266.3 1836.2 * 36.9 ***
    E2 176.4 1215.9 265.2 1828.4 13.1 36.8 23,934
    Avg. 173.7 1197.3 265.8 1832.3 13.1 36.8 23,934
    * = Invalid measurement - Specimen broke outside gage section.
    **= No test because sample was damaged during processing.
    *** = No value could be calculated.
  • TABLE 6C
    Y.S. U.T.S.
    Heat ID Heat Treatment Sample ksi MPa ksi MPa % El. % R.A. SIN
    Example 4 A A1 191.5 1320.1 270.6 1865.6 * 40.7 **
    Group 3 A2 185.1 1276.3 263.2 1814.6 11.9 43.1 21,666
    Avg. 188.3 1298.2 266.9 1840.1 11.9 41.9 21,666
    C C1 167.8 1156.9 258.7 1783.8 12.8 46.5 22,797
    C2 171.4 1181.6 263.3 1815.1 11.6 17.3 21,109
    Avg. 169.6 1169.2 261.0 1799.4 12.2 46.9 21,953
    E E1 171.1 1179.7 260.7 1797.1 12.6 44.5 22,554
    E2 174.7 1204.2 260.5 1796.4 13.6 46.1 24,377
    Avg. 172.9 1191.9 260.6 1796.8 13.1 45.3 23,465
    * = Invalid measurement - Specimen broke outside gage section
    ** = No value could be calculated.
  • The data presented in Tables 3A-3D, 5, and 6A-6C show that the preferred alloys according to the present invention provide a desirable combination of strength and ductility that makes them uniquely suitable for use in automotive parts made from thin gauge product forms such as strip and sheet. Although one of the specimens was too damaged to be tested and the elongation measurements for some of the specimens were not valid, considered as a whole the data show that the preferred embodiments of the alloy of this invention provide the combination of properties for which the alloy was designed. The unique combination of high strength and higher than expected ductility, provides a novel solution to the automotive industry for making shaped body and frame parts with reduced weight without sacrificing strength and toughness.
  • The terms and expressions which are employed herein are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. It is recognized that various modifications are possible within the invention described and claimed herein.

Claims (31)

1. A steel alloy consisting essentially of, in weight percent:
C 0.30-0.45 Mn 3.5-4.5 Si 1.0-1.8 Cr 1.6-2.5 Ni 3.7-4.5 Mo + ½W  0.3 max. Cu 0.3-0.7 Co  0.01 max. V + 5/9Nb 0.30-0.50 Ti 0.020 max. Al 0.020 max. Ca 0.002 max. N  0.02 max.
and the balance is iron and the usual impurities, wherein said impurities include not more than about 0.025% phosphorus and not more than about 0.0025% sulfur; and wherein

4.5≤(% Si+% Cu)/(% V+( 5/9)×% Nb)≤10;  (a)

3.5≤(% Mo+% Cr)/(% C)≤7.5; and  (b)

3.5≤% Mn+% Ni≤8.0.  (c)
2. The alloy as claimed in claim 1 which contains 1.3-1.8% silicon.
3. The alloy as claimed in claim 1 which contains 1.3-1.7% silicon.
4. The alloy as claimed in claim 1 which contains 1.6-2.35% chromium.
5. The alloy as claimed in claim 1 which contains 3.7-4.3% nickel.
6. The alloy as claimed in claim 1 which contains not more than about 0.1% Mo+½W.
7. The alloy as claimed in claim 1 which contains 0.4-0.7% copper.
8. The alloy as claimed in claim 1 which contains 0.4-0.6% copper.
9. The alloy as claimed in claim 1 which contains 0.30-0.40% V+ 5/9Nb.
10. The alloy as claimed in claim 1 which also contains 0.001-0.025% yttrium.
11. The alloy as claimed in claim 1 which also contains 0.001-0.01% magnesium.
12. A thin gauge article made from the alloy claimed in claim 1.
13. A shaped part made from the thin gauge article claimed in claim 12.
14. A steel alloy consisting essentially of, in weight percent:
C 0.30-0.40 Mn 3.5-4.5 Si 1.0-1.7 Cr  1.6-2.35 Ni 3.7-4.3 Mo + ½W  0.1 max. Cu 0.4-0.6 Co  0.01 max. V + 5/9Nb 0.30-0.40 Ti 0.020 max. Al 0.020 max. Ca 0.002 max. N  0.02 max.
and the balance is iron and the usual impurities, wherein said impurities include not more than about 0.025% phosphorus and not more than about 0.0025% sulfur; and wherein

4.5≤(% Si+% Cu)/(% V+( 5/9)×% Nb)≤10;  (a)

3.5≤(% Mo+% Cr)/(% C)≤7.5; and  (b)

3.5≤% Mn+% Ni≤8.  (c)
15. The alloy as claimed in claim 14 which also contains 0.001-0.025% yttrium.
16. The alloy as claimed in claim 14 which also contains 0.001-0.01% magnesium.
17. A thin gauge article made from the alloy claimed in claim 14.
18. A shaped part made from the thin gauge article claimed in claim 17.
19. A steel alloy consisting essentially of, in weight percent
C 0.30-0.36 Mn 3.5-4.5 Si 1.3-1.7 Cr 0.75-1.5  Ni 0.7-2.5 Mo + ½W 0.15-0.25 Cu 0.4-0.6 Co  0.01 max. V + 5/9Nb 0.20-0.30 Ti 0.020 max. Al 0.020 max. Ca 0.002 max. N  0.02 max.
and the balance is iron and the usual impurities, wherein said impurities include not more than about 0.025% phosphorus and not more than about 0.0025% sulfur; and wherein

2≤(% Si+% Cu)/(% V+( 5/9)×% Nb)≤34.
20. The steel alloy as claimed in claim 19 which also contains 0.001-0.025% yttrium.
21. The steel alloy as claimed in claim 19 which also contains 0.001-0.01% magnesium.
22. The steel alloy as claimed in claim 19 which contains at least 1.6% chromium.
23. The steel alloy as claimed in claim 19 which contains at least 3.7% nickel.
24. The steel alloy as claimed in claim 23 which contains not more than 4.3% nickel.
25. The steel alloy as claimed in claim 19 which contains at least 3.5% manganese.
26. The steel alloy as claimed in claim 19 which contains at least 1.3% silicon.
27. The steel alloy as claimed in claim 26 which contains not more than 1.7% silicon.
28. The steel alloy as claimed in claim 19 which contains at least 0.4% copper.
29. The steel alloy as claimed in claim 28 which contains not more than 0.6% copper.
30. The steel alloy as claimed in claim 19 wherein Mo+½W is not more than 0.1%.
31. The steel alloy as claimed in claim 19 wherein V+ 5/9Nb is not more than 0.4%.
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