EP1563106B1 - Aciers ecrouis a microstructure martensitique/austenitique a lattages par paquets - Google Patents

Aciers ecrouis a microstructure martensitique/austenitique a lattages par paquets Download PDF

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EP1563106B1
EP1563106B1 EP03783653A EP03783653A EP1563106B1 EP 1563106 B1 EP1563106 B1 EP 1563106B1 EP 03783653 A EP03783653 A EP 03783653A EP 03783653 A EP03783653 A EP 03783653A EP 1563106 B1 EP1563106 B1 EP 1563106B1
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carbon steel
austenite
steel alloy
martensite
accordance
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EP1563106A1 (fr
EP1563106A4 (fr
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Grzegorz J. Kusinski
Gareth Thomas
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MMFX Technologies Corp
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MMFX Technologies Corp
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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
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/04—Modifying the physical properties of iron or steel by deformation by cold working of the surface
    • 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
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • 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
    • C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
    • 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
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/02—Modifying the physical properties of iron or steel by deformation by cold working
    • C21D7/10—Modifying the physical properties of iron or steel by deformation by cold working of the whole cross-section, e.g. of concrete reinforcing bars
    • 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
    • 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
    • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
    • 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
    • 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
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • This invention resides in the technology of low and medium carbon steel alloys, particularly those of high-strength and toughness, and the cold formability of such alloys.
  • Cold working typically consists of a series of compressions and/or expansions achieved by processes such as drawing, extruding, cold heading, or rolling.
  • Cold working causes plastic deformation of the steel which produces strain hardening while forming the steel into the shape in which it will ultimately be used.
  • Cold working which in the case of steel wire is performed by wire drawing, is typically performed in a succession of stages with intermediate heat treatments, which in the case of steel wire are termed "patenting.”
  • High-strength steel wire is an example of a high-performance steel and is useful in a variety of engineering applications including tire cord, wire rope, and strand for pre-stressed concrete reinforcements.
  • the steel most commonly used in high-strength steel wire is medium- or high-carbon steel.
  • hot-rolled rods with pearlitic microstructures are cold drawn in several stages, with intermediate patenting treatments to soften the pearlite for continued cold drawing. For example, hot rolled rods of about 5.5 mm diameter might be coarse drawn in several stages to a diameter of about 3 mm.
  • Patenting might then be performed at 850-900°C, causing austenitization of the steel, followed by transformation of the steel at 500-550°C to fine pearlitic lamellae.
  • the steel would then be pickled, in hydrochloric acid, for example, to remove the scale formed during patenting.
  • the pickling would be followed by several further drawing stages to reduce the diameter down to about 1 mm, then further patenting and pickling.
  • the final drawing would then be done in several stages to the final desired diameter, which may for example be about 0.4 mm, to achieve the desired properties, notably strength. This may be followed by further processing such as stranding, depending on the ultimate use.
  • the purpose of the initial patenting treatment is to produce a wire rod with a fine lamellar pearlite structure, which requires a low transformation temperature.
  • the process is typically performed in a molten lead bath.
  • the wire is drawn to true strains (defined below) of 6-7 to obtain high strength levels of approximately 3,000 MPa.
  • true strains defined below
  • these high strains and strengths are attainable only by applying a series of patenting treatments. Without these patenting treatments, the cold drawing will cause shear cracking of the pearlitic lamellae. Because of the need for a molten lead bath the entire process is costly and tends to raise environmental concerns.
  • Cold working is also used in the production of expandable steel tubing, i.e., tubing that is expanded on-site and in some cases below ground.
  • the microstructures are fused grains in which individual grains contain several laths of martensite separated by thin austenite films with, in some cases, an austenite shell surrounding each grain. These structures are termed “dislocated lath martensite” structures or "packet-lath” martensite/austenite” structures. Patents disclosing these microstructures are as follows:
  • the invention provides a process for manufacturing a high-strength, high-ductility alloy carbon steel according to claim 1 or claim 2 of the claims appended hereto.
  • the packet-lath martensite/austenite microstructure is unique in its crystallographic characteristics and how these characteristics cause it to respond to cold working. Because of the high dislocation density of this microstructure and the ease with which strains in the structure can move between the martensite and austenite phases, cold working provides the microstructure with unique mechanical properties that include a high tensile strength. As a result, these alloys can be cold worked without intermediate heat treatments, while still achieving tensile strengths comparable to the tensile strengths of conventional steel alloys that have been processed by cold working with intermediate heat treatments. In the case of steel wire having the packet-lath martensite/austenite microstructure, this invention lies in the discovery that cold drawing can be performed without intermediate patenting treatments.
  • carbon steel alloys having the packet-lath martensite/austenite microstructure i.e., those whose microstructure includes laths of martensite alternating with thin films of retained austenite are cold formed, preferably without intermediate heat treatments, to a reduction sufficient to achieve a tensile strength of about 150 ksi or higher ("ksi” denotes kilo-pounds-force per square inch), equivalent to approximately 1,085 MPa or higher (“MPa” denotes megapascals, i.e., newtons per square millimeter).
  • ksi denotes kilo-pounds-force per square inch
  • MPa denotes megapascals, i.e., newtons per square millimeter
  • the benefits of this invention extend to simple packet-lath martensite/austenite microstructures containing no ferrite or insignificant amounts of ferrite, and also to microstructures that include packet-lath grains fused with ferrite grains, and to variants on these structures, including those whose packet-lath grains are encased by austenite shells, those that are free of interphase carbide precipitates, and those in which the austenite films are of a uniform orientation.
  • the discovery of the ability of packet-lath martensite/austenite microstructures to respond to cold working in this manner is surprising relative to the disclosures in patents nos.
  • FIG. 1 is a plot of tensile strength vs. true total strain for two steel alloys of dual-phase packet-lath martensite/austenite microstructure, upon cold working in accordance with this invention in the absence of intermediate heat treatments.
  • FIG. 2 is a plot of tensile strength vs. true total strain for three steel alloys of triple-phase packet-lath martensite/austenite/ferrite microstructure and one steel alloy of dual-phase packet-lath martensite/austenite microstructure, upon cold working in accordance with this invention in the absence of intermediate heat treatments.
  • Cold working in the practice of this invention can be performed by the use of techniques and equipment that have been used for cold working in the prior art on other steel alloys and microstructures.
  • cold working may consist of rolling the steel between rollers or other means of compression to reduce the thickness of and elongate the steel.
  • cold working may consist of cold-drawing or extrusion through a die.
  • cold working may consist of cold-drawing or extrusion through a die.
  • the workpiece is extruded through a series of successively smaller dies.
  • Tubing is achieved by drawing the steel through a ring-shaped die with a mandrel inside the die.
  • the tubing that has already been drawn is further drawn through a smaller ring-shaped die with a mandrel placed inside the tubing.
  • Cold working is performed at a temperature below the lowest temperature at which recrystallization occurs. Suitable temperatures are therefore those that do not induce any phase change in the steel.
  • recrystallization typically occurs at approximately 1,000°C (1,832°F), and accordingly, cold working in accordance with this invention is performed well below this temperature.
  • cold working is performed at temperatures of about 500°C (932°F) or less, more preferably about 100°C (212°F) or less, and most preferably at a temperature that is within about 25°C of ambient temperature.
  • the cold working will preferably be performed to a tensile strength within the range of from about 150 ksi to about 500 ksi.
  • the process of this invention is applicable to carbon steel alloys having packet-lath martensite/austenite microstructures such as those described in the patents cited above, as well as those described in co-pending United States Patent Applications Nos. 10/017,847, filed December 15, 2001 (entitled “Triple-Phase Nano-Composite Steels,” inventors Kusinski, G.J., Pollack, D., and Thomas, G.), and 10/017,879, filed December 14, 2001 (entitled “Nano-Composite Martensitic Steels,” inventors Kusinski, G.J., Pollack, D., and Thomas, G.).
  • the alloy composition will typically have a martensite start temperature M. of about 300°C or higher, and preferably 350°C or higher. While alloying elements in general affect the M, the alloying element that has the strongest influence on the M 5 is carbon, and achieving an alloy with M 6 above 300°C can be achieved by limiting the carbon content of the alloy to a maximum of 0.35% by Weight. In a reference example, the carbon content is within the range of from about 0.03% to about 0.35%, and in a reference example, the range is from about 0.05% to about 0.33%, all by weight.
  • Further alloying elements such as molybdenum, titanium, niobium, and aluminum, can also be present in amounts sufficient to serve as nucleation sites for fine grain formation yet low enough in concentration to avoid affecting the properties of the finished alloy by their presence.
  • concentration should also be low enough to avoid the formation of inclusions and other large precipitates, which may render the steel susceptible to early fracture.
  • austenite stabilizing elements examples of which are nitrogen, manganese, nickel, copper, and zinc. Particularly preferred among these are manganese and nickel.
  • the nickel concentration is preferably within the range of about 0.25% to about 5%, and when manganese is present, the manganese concentration is preferably within the range of from about 0.25% to about 6%.
  • Chromium is also included in many embodiments of the invention. All concentrations herein are by weight.
  • Certain embodiments of the invention involve alloys that include a ferrite phase in addition to the packet-lath martensite/austenite grains (triple-phase alloys) while others contain only the packet-lath martensite/austenite grains and do not include a ferrite phase (dual-phase alloys).
  • the presence or absence of the ferrite phase is determined by the type of heat treatment in the initial austenitization stage. By appropriate selection of the temperature, the steel can be transformed into a single austenite phase or into a two-phase structure containing both austenite and ferrite.
  • the alloy composition can be selected or adjusted to either cause ferrite formation during the initial cooling of the alloy from the austenite phase or to avoid ferrite formation during the cooling, i.e., to avoid the formation of ferrite grains prior to the further cooling of the austenite to form the packet-lath microstructure.
  • the austenite films in a single packet-lath grain are all of approximately the same orientation, although the crystallographic orientation may vary, or those in which the austenite films in a single packet-lath grain are all of the same crystal plane orientation.
  • the latter can be achieved by limiting the grain size to ten microns or less.
  • the grain size in these cases is within the range of about 1 ⁇ m (micron) to about 10 ⁇ m (microns), and most preferably from about 5 ⁇ m (microns) to about 9 ⁇ m (microns).
  • the preparation of -phase packet-lath martensite/austenite microstructures that do not contain ferrite begins with the selection of the alloy components and the combining of these components in the appropriate portions as indicated above.
  • the combined components are then homogenized ("soaked") for a sufficient period of time and at a sufficient temperature to achieve a uniform austenitic structure with all elements and components in solid solution.
  • the temperature will be above the austenite recrystallization temperature but preferably at a level that will cause very fine grains to form.
  • the austenite recrystallization temperature typically varies with the alloy composition, but in general will be readily apparent to those skilled in the art. In most cases, best results will be achieved by soaking at a temperature within the range of 800°C to 1150°C. Rolling, forging or both are optionally performed on the alloy at this temperature.
  • the alloy is subjected to a combination of cooling and grain refinement to the desired grain size, which as noted above may vary. Grain refinement may be performed in stages, but the final grain refinement is generally achieved at an intermediate temperature that is above, yet close to, the austenite recrystallization temperature.
  • the alloy may first be rolled at the homogenization temperature to achieve dynamic recrystallization, then cooled to an intermediate temperature and rolled again for further dynamic recrystallization.
  • the intermediate temperature is between the austenite recrystallization temperature and a temperature that is about 50 degrees Celsius above the austenite recrystallization temperature.
  • the intermediate temperature to which the alloy is cooled is preferably between about 900° to about 950°C, and most preferably between about 900° to about 925°C.
  • the preferred intermediate temperature is about 850°C.
  • Dynamic recrystallization can also be achieved by forging or by other means known to those skilled in the art. Dynamic recrystallization produces a grain size reduction of 10% or greater, and in many cases a grain size reduction of from about 30% to about 90%.
  • the alloy is quenched by cooling from a temperature above the austenite recrystallization temperature down to the martensite start temperature M s , then through the martensite transition range to convert the austenite crystals to the packet-lath martensite/austenite microstructure.
  • the conversion occurs only in the austenite crystals.
  • the optimal cooling rate varies with the chemical composition, and hence the hardenability, of the alloy.
  • the resulting packets are of approximately the same small size as the austenite grains produced during the rolling stages, but the only austenite remaining in these grains is in the thin films and in some cases in the shell surrounding each packet-lath grain.
  • the thin austenite films are to be of a single variant in crystal orientation, this is achieved by controlling the process to achieve a grain size of less than 50 ⁇ m (microns).
  • grain refinement to the desired grain size can be accomplished by heat treatment alone.
  • the alloy is quenched as described in the preceding paragraph, then reheated to a temperature that is approximately equal to the austenite recrystallization temperature or slightly below, then quenched once again to achieve, or to return to, the packet-lath martensite/austenite microstructure.
  • the reheating temperature is preferably within about 50 degrees Celsius of the austenite recrystallization temperature, for example about 870°C.
  • Processing steps such as heating the alloy composition to the austenite phase, cooling the alloy with controlled rolling or forging to achieve the desired reduction and grain size, and quenching the austenite grains through the martensite transition region to achieve the packet-lath structure are performed by methods known in the art. These methods include castings, heat treatment, and hot working of the alloy such as by forging or rolling, followed by finishing at the controlled temperature for optimum grain refinement.
  • Controlled rolling serves various functions, including aiding in the diffusion of the alloying elements to form a homogeneous austenite crystalline phase and in the storage of strain energy in the grains.
  • controlled rolling guides the newly forming martensite phase into a packet-lath arrangement of martensite laths separated by thin films of retained austenite.
  • the degree of rolling reduction can vary and will be readily apparent to those of skill in the art. Quenching is preferably done fast enough to avoid formation of detrimental microstructures including pearlite, bainite, and particles or precipitates, particularly interphase precipitation and particle formation, including the formation of undesirable carbides and carbonitrides.
  • the retained austenite films will constitute from about 0.5% to about 15% by volume of the microstructure, preferably from about 3% to about 10%, and most preferably a maximum of about 5%.
  • Triple-phase alloys have a microstructure consisting of two types of grains, ferrite grains and packet-lath martensite/austenite grains, fused together as a continuous mass.
  • the individual grain size is not critical and can vary widely.
  • the amount of ferrite phase relative to the martensite-austenite phase may vary. In most cases, however, best results will be obtained when the martensite/austenite grains constitute from about 5% to about 95% of the triple-phase structure, preferably from about 15% to about 60%, and most preferably from about 20% to about 40%, all by weight.
  • Triple-phase alloys can be prepared by first combining the appropriate components needed to form an alloy of the desired composition, then soaking to achieve a uniform austenitic structure with all elements and components in solid solution, as in the preparation of the dual-phase alloys described above.
  • a preferred soaking temperature range is from about 900°C to about 1,170°C.
  • the relative amounts of each of the two phases at equilibrium varies with the temperature to which the composition is cooled in this stage, and also with the levels of the alloying elements.
  • the distribution of the carbon between the two phases (again at equilibrium) also varies with the temperature.
  • the relative amounts of the two phases are not critical to the invention and can vary.
  • the temperature to which the composition is cooled in order to achieve the dual-phase ferrite-austenite structure is preferably within the range of from about 800°C to about 1,000°C.
  • the alloy is rapidly quenched by cooling through the martensite transition range to convert the austenite crystals to the packet-lath martensite/austenite microstructure.
  • the cooling rate used during this transition is great enough to substantially avoid any changes to the ferrite phase and to avoid undesirable austenite decomposition.
  • water cooling may be required to achieve the desired cooling rate, although for certain alloys air cooling will suffice.
  • the desired cooling rate is slow enough that air cooling can be used.
  • Dual-phase alloy compositions are those that contain from about 0.04% to about 0.12% carbon, from zero to about 11.0% chromium, from zero to about 2.0% manganese, and from zero to about 2.0% silicon, all by weight, the remainder being iron.
  • Triple-phase alloy compositions are those that contain from about 0.02% to about 0.14% carbon, from zero to about 3.0% silicon, from zero to about 1.5% manganese, and from zero to about 1.5% aluminum, all by weight, the remainder being iron.
  • autotempering The formation of precipitates or other small particles within the microstructure upon cooling is collectively referred to as "autotempering.”
  • autotempering will purposely be avoided by using a relatively fast cooling rate.
  • the minimum cooling rates that will avoid autotempering are evident from the transformation-temperature-time diagram for the alloy.
  • the vertical axis represents temperature and the horizontal axis represents time, while curves on the diagram indicate the regions where each phase exists either by itself or in combination with another phase(s).
  • a typical such diagram is shown in Thomas, U.S. Patent No. 6,273,968 B1 , referenced above.
  • the minimum cooling rate is a line of descending temperature over time which abuts the left side of a C-shaped curve.
  • the region to the right of the curve represents the presence of carbides, and cooling rates that avoid carbide formation are therefore those represented by lines that remain to the left of the curve.
  • the line that is tangential to the curve has the smallest slope and is therefore the slowest rate that can be used while still avoiding carbide formation.
  • interphase precipitation and “interphase precipitates” are used herein to denote the formation of small alloy particles at locations between the martensite and austenite phases, i.e., between the laths and the thin films separating the laths. "Interphase precipitates” does not refer to the austenite films themselves. Interphase precipitates are to be distinguished from “intraphase precipitates,” which are precipitates located within the martensite laths rather than along the interfaces between the martensite laths and the austenite films. Intraphase precipitates that are about 500 ⁇ or less in diameter are not detrimental to toughness and may in fact enhance toughness. Thus, autotempering is not necessarily detrimental provided that the autotempering is limited to intraphase precipitation and does not result in interphase precipitation.
  • substantially no carbides is used herein to indicate that if any carbides are present, their distribution and amount are such that they have a negligible effect on the performance characteristics, and particularly the corrosion characteristics, of the finished alloy.
  • a cooling rate that is sufficiently high to prevent carbide formation or autotempering in general may be one that can be achieved with air cooling or one that requires water cooling.
  • air cooling can still be done when the levels of certain alloying elements are reduced provided that the levels of other alloying elements are raised. For example, a reduction in the amount of carbon, chromium, or silicon can be compensated for by raising the level of manganese.
  • Rolling is performed in a controlled manner at one or more stages during the austenitization and first-stage cooling procedures, for example, to aid in the diffusion of the alloying elements to form a homogeneous austenite crystalline phase and then to deform the crystal grains and store strain energy in the grains, while in the second-stage cooling, rolling can serve to guide the newly forming martensite phase into the packet-lath arrangement of martensite laths separated by thin films of retained austenite.
  • the degree of rolling reductions can vary, and will be readily apparent to those skilled in the art.
  • the retained austenite films will constitute from about 0.5% to about 15% by volume of the microstructure, preferably from about 3% to about 10%, and most preferably a maximum of about 5%.
  • the proportion of austenite relative to the entire triple-phase microstructure will be a maximum of about 5%.
  • the actual width of a single retained austenite film is preferably within the range of about 50 ⁇ to about 250 ⁇ , and preferably about 100 ⁇ .
  • the proportion of austenite relative to the entire triple-phase microstructure will in general be a maximum of about 5%.
  • This example illustrates the deformation of a carbon steel rod with a packet-lath martensite/austenite microstructure, by a cold drawing process according to the present invention to an area reduction of 99%.
  • the experiment reported in this example was performed on a steel rod measuring 6 mm in diameter and having an alloy composition of 0.1% carbon, 2.0% silicon, 0.5% chromium, 0.5% manganese, all by weight, and the balance iron, with a microstructure consisting of grains measuring approximately 50 ⁇ m (microns) in diameter, each grain consisting of laths of martensite measuring approximately 100 nm in thickness alternating with thin films of austenite measuring approximately 10 nm in thickness, with no ferrite phases and each grain surrounded by an austenite shell measuring approximately 10 nm in thickness.
  • the rod was prepared by the method described in co-pending United States patent application serial no. 10/017,879, filed December 14, 2001 , referenced above.
  • the uncoated steel rod was surface cleaned and lubricated, then cold drawn through lubricated dies in 15 passes at a temperature of 25°C to a diameter of 0.0095 inch (0.024 cm). At a final wire diameter of 0.0105 inch (0.027 cm), representing a total area reduction of 99%, the wire had a tensile strength of 390 ksi (2,690 MPa).
  • This example is another illustration of the cold working of carbon steel rods with packet-lath martensite/austenite microstructures in accordance with the present invention.
  • two different alloys were used, Fe/8Cr/0.05C and Fe/2Si/0.1C, with a microstructure consisting of grains measuring approximately 50 ⁇ m (microns) in diameter, each grain consisting of laths of martensite measuring approximately 150 nm in thickness alternating with thin films of austenite measuring approximately 10 nm in thickness, with no significant ferrite phases, each grain surrounded by an austenite shell measuring approximately 10 nm in thickness.
  • the steel rods were 6 mm in diameter, and were surface cleaned and lubricated, then cold drawn through lubricated dies in a series of passes at a temperature of 25°C.
  • the drawing schedule shown in Table I was used for the Fe/8Cr/0.05C alloy, and a similar drawing schedule was used for the Fe/2Si/0.1C alloy.
  • a o represents the initial rod diameter and A is the rod diameter after the particular pass.
  • This example illustrates cold working in accordance with the present invention, using carbon steel rods with packet-lath martensite/austenite microstructures that contain ferrite crystals as a third phase (in addition to the laths of martensite and the thin films of austenite, i.e., a triple-phase microstructure).
  • the alloy was Fe/2Si/0.1C, with a microstructure consisting of ferrite fused with packet-lath grains similar to those described above in Examples 1 and 2, containing martensite laths alternating with thin films of austenite and encased in an austenite shell.
  • the rods were prepared by the method described in United States patent application no. 10/017,847, filed December 14, 2001 , referenced above, using a reheat temperature of 950°C to achieve a ferrite content of 70 volume percent of the microstructure.
  • the initial rod diameter was 0.220 inch (5.59 mm), and the cold working consisted of drawing the rods through lubricated conical dies at a temperature of 25°C in 15 passes with approximately 36% reduction per pass to a final diameter of 0.037 inch (0.94 mm).
  • the drawing schedule is shown in Table II, where A o represents the initial rod diameter and A is the rod diameter after the particular pass.
  • the tensile strength of the final wire was 2760 MPa (400 ksi).
  • This example is a further illustration of the cold work of carbon steel rods whose microstructure consists of packet-lath martensite/austenite and ferrite crystals, in accordance with the present invention.
  • the alloy was Fe/2Si/0.1C as in Example 3, with a microstructure consisting of ferrite fused with packet-lath grains similar to those described above in Examples 1 and 2, containing martensite laths alternating with thin films of austenite and encased in an austenite shell.
  • a rod of this composition was prepared by the general method described in United States patent application no. 10/017,847, filed December 14, 2001 , referenced above.
  • the rod was initially hot rolled to a diameter of 0.25 inch (6.35 mm), then heated to 1,150°C for about 30 minutes to austenitize the composition, then quenched in iced brine to transform the austenite to substantially 100% martensite, then rapidly reheated to convert the structure to approximately 70% ferrite and 30% austenite. The rod was then quenched in iced brine to convert the austenite to the packet-lath martensite/austenite structure. The rod was then cold drawn in 7 passes at a reduction of 35% per pass to a final diameter of 0.055 inch (1.40 mm), resulting in a tensile strength of 1,875 MPa (272 ksi).
  • This example is a still further illustration of the cold working of carbon steel rods whose microstructure consists of packet-lath martensite/austenite and ferrite crystals, in accordance with the present invention, demonstrating the effect of varying the relative amounts of packet-lath martensite/austenite and ferrite.
  • the steel alloy was Fe/2Si/0.1C as in Examples 3 and 4, and the rods were prepared as described in Example 4, using different reheat temperatures to achieve ferrite contents of 0%, 56%, 66%, and 75%, corresponding to contents of packet-lath martensite/austenite contents of 100%, 44%, 35%, and 25%, respectively, all by volume.
  • Drawing schedules similar to that shown in Table II were used on all four microstructures, and the resulting tensile strengths are plotted against the true total strain in FIG. 2 , in which the squares represent the 100% packet-lath alloy, the triangles represent the 44% packet-lath alloy, the circles represent the 34% packet-lath alloy, and the diamonds represent the 25% packet-lath alloy.
  • the plot shows that all four microstructures achieved a tensile strength well in excess of 2,000 MPa, and those in which the packet-lath martensite/austenite portions exceeded 25% produced higher tensile strengths than the microstructure in which the packet-lath portion was 25%.

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Abstract

La présente invention concerne des alliages d'acier écrouis ayant une résistance à la traction élevée, qui sont produits par écrouissage d'alliages dont la microstructure comprend des grains dans lesquels des lattages de martensite s'alternent avec de fins films d'austénite stabilisé. En raison d'une densité de dislocation élevée de cette microstructure et de la tendance des contraintes à se déplacer entre les phases martensitiques et austénitiques, les contraintes produites par l'écrouissage confèrent à la microstructure des propriétés mécaniques uniques comprenant une résistance à la traction élevée. De manière surprenante, ce résultat peut être obtenu sans impliquer de traitements thermiques intermédiaires (patentage dans le cas de fils d'acier) de l'acier entre les étapes de réduction de l'écrouissage.

Claims (15)

  1. Processus de fabrication d'un acier au carbone allié à ductilité élevée et à haute résistance, ledit processus comprenant le fait :
    (a) de former un alliage d'acier au carbone ayant une microstructure comprenant en alternance de lattes de martensite et de films d'austénite résiduelle, et
    (b) de travailler ledit alliage d'acier au carbone à froid dans une série de passages sans traitement thermique intermédiaire entre les passages jusqu'à une réduction suffisante pour obtenir une résistance à la traction d'au moins 1034 MPa (150 ksi),
    dans lequel l'étape (a) comprend :
    (i) la formation d'une composition d'alliage d'acier au carbone présentant une température de début de martensite d'au moins 300°C,
    (ii) le chauffage de ladite composition d'alliage d'acier au carbone à une température suffisamment élevée pour provoquer son austénitisation, afin de produire une phase austénitique homogène avec tous les éléments d'alliage en solution, et
    (iii) le refroidissement de ladite phase austénitique homogène à travers ladite plage de transition martensitique à une vitesse de refroidissement suffisamment rapide pour obtenir ladite microstructure évitant la formation de carbures aux interfaces entre lesdites lattes de martensite et lesdits films d'austénite résiduelle,
    où ladite composition d'alliage d'acier au carbone contient 0,04 % jusqu'à 0,12 % de carbone, 0 % jusqu'à 11% de chrome, 0 % jusqu'à 2,0 % de manganèse, et 0% jusqu'à 2,0 % de silicium, tous les pourcentages étant en poids, le reste étant du fer présent en même temps que des impuretés inévitables.
  2. Processus de fabrication d'un acier au carbone allié à ductilité élevée et à haute résistance, ledit processus comprenant le fait :
    (a) de former un alliage d'acier au carbone, ayant une microstructure comprenant une alternance de lattes de martensite alternant et de films d'austénite résiduelle, et
    (b) de travailler l'alliage d'acier au carbone à froid dans une série de passages sans traitement thermique intermédiaire entre les passages jusqu'à une réduction suffisante pour obtenir une résistance à la traction d'au moins 1034 MPa (150 ksi),
    dans lequel l'étape (a) comprend :
    (i) la formation d'une composition d'alliage d'acier au carbone présentant une température de début de martensite d'au moins 300°C,
    (ii) le chauffage de ladite composition d'alliage d'acier au carbone à une température suffisamment élevée pour provoquer son austénitisation, afin de produire une phase austénitique homogène avec tous les éléments d'alliage en solution,
    (iii) le refroidissement de ladite phase austénitique homogène pour transformer une partie de ladite phase austénitique en cristaux de ferrite, formant ainsi une microstructure à deux phases comprenant des cristaux de ferrite fusionnés avec des cristaux d'austénite, et
    (iv) le refroidissement de ladite microstructure à deux phases à travers ladite plage de transition martensitique sous des conditions provoquant la conversion desdits cristaux d'austénite en une microstructure, contenant une alternance de lattes de martensite et de films d'austénite résiduelle,
    où ladite composition d'alliage d'acier au carbone contient 0,02 % jusqu'à 0,14 % de carbone, 0 % jusqu'à 3,0 % de silicium, 0 % jusqu'à 1,5 % de manganèse, et 0 % jusqu'à 1,5 % d'aluminium, tous les pourcentages étant en poids, le reste étant du fer présent en même temps que des impuretés inévitables.
  3. Processus selon la revendication 1 ou 2 dans lequel l'étape (b) comprend le travail à froid dudit alliage d'acier au carbone jusqu'à une réduction suffisante pour obtenir une résistance à la traction allant de 1034 MPa (150 ksi) à 3447 MPa (500 ksi).
  4. Processus selon la revendication 1 ou 2 dans lequel l'étape (b) comprend le travail à froid dudit alliage d'acier au carbone jusqu'à une réduction de la section transversale d'au moins 20 % par passage.
  5. Processus selon la revendication 1 ou 2 dans lequel l'étape (b) comprend le travail à froid dudit alliage d'acier jusqu'à une réduction de la section transversale d'au moins 25 % par passage.
  6. Processus selon la revendication 1 ou 2 dans lequel l'étape (b) comprend le travail à froid dudit alliage d'acier au carbone jusqu'à une réduction de la section transversale de 25 % à 50 % par passage.
  7. Processus selon la revendication 1 ou 2 dans lequel l'étape (b) est exécutée à une température de 100°C ou moins.
  8. Processus selon la revendication 1 ou 2 dans lequel l'étape (b) est exécutée à une température ambiante de 25°C.
  9. Processus selon la revendication 1 ou 2 dans lequel ledit alliage d'acier au carbone se présente sous forme de tige ou de câble, et l'étape (b) comprend l'étirage dudit alliage d'acier au carbone à travers une filière.
  10. Processus selon la revendication 1 ou 2 dans lequel ledit alliage d'acier au carbone se présente sous forme d'une feuille, et l'étape (b) comprend le laminage dudit alliage d'acier au carbone.
  11. Processus selon la revendication 1 dans lequel ladite composition d'alliage d'acier au carbone présentant une température de début de martensite d'au moins 350°C.
  12. Processus selon la revendication 1 dans lequel lesdits films d'austénite résiduelle ont une orientation uniforme.
  13. Processus selon la revendication 1 dans lequel ladite température de l'étape (ii) est de 800°C à 1150°C.
  14. Processus selon la revendication 2 dans lequel l'étape (iii) comprend le refroidissement de ladite phase austénitique homogène à une température allant de 800°C à 1000°C.
  15. Processus selon la revendication 2 dans lequel l'étape (ii) comprend le chauffage de ladite composition d'alliage d'acier au carbone à une température allant de 1050°C à 1170°C, et l'étape (iii) comprend le refroidissement de ladite phase austénitique homogène à une température allant de 800°C à 1000°C.
EP03783653A 2002-11-19 2003-11-18 Aciers ecrouis a microstructure martensitique/austenitique a lattages par paquets Expired - Lifetime EP1563106B1 (fr)

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US645833 1984-08-31
US42783002P 2002-11-19 2002-11-19
US427830P 2002-11-19
US10/645,833 US20040149362A1 (en) 2002-11-19 2003-08-20 Cold-worked steels with packet-lath martensite/austenite microstructure
PCT/US2003/036875 WO2004046400A1 (fr) 2002-11-19 2003-11-18 Aciers ecrouis a microstructure martensitique/austenitique a lattages par paquets

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RU2005119192A (ru) 2006-01-20
US20080236709A1 (en) 2008-10-02
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TR200501633T2 (tr) 2005-06-21
CA2502114C (fr) 2012-07-24
MXPA05005104A (es) 2005-07-01
BR0316361A (pt) 2005-09-27
NO20053021L (no) 2005-08-18
BR0316361B1 (pt) 2011-12-27
NO20053021D0 (no) 2005-06-20
ES2386425T3 (es) 2012-08-20
US20040149362A1 (en) 2004-08-05
EP1563106A4 (fr) 2006-08-16
AU2003291066B2 (en) 2008-08-28
JP2006506534A (ja) 2006-02-23
CA2502114A1 (fr) 2004-06-03
PT1563106E (pt) 2012-08-02

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