Classifications

• • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • 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.
• 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 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+1 ⁇ 2W Up to 0.5 Up to 0.3 0.1 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.
• 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.
• 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.
• silicon, copper, and vanadium, and niobium when present are broadly balanced such that 2 ⁇ % Si + % Cu / % V + 5 / 9 x % Nb ⁇ 34.
• silicon, copper, and vanadium, and optionally niobium are preferably balanced such that 4.5 ⁇ % Si + % Cu / % V + 5 / 9 x % 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+ 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.
• 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) if desired.
• ARC arc melting in air
• ESR electroslag remelting
• 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 1835°F (1002°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.
• Example 4 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. %El. %R.A. SIN Heat ID Heat Treatment Sample ksi MPa ksi MPa
• 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.

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