WO2017185319A1 - Low density zinc-coated trip-assisted steel and fabrication method - Google Patents

Low density zinc-coated trip-assisted steel and fabrication method Download PDF

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Publication number
WO2017185319A1
WO2017185319A1 PCT/CN2016/080625 CN2016080625W WO2017185319A1 WO 2017185319 A1 WO2017185319 A1 WO 2017185319A1 CN 2016080625 W CN2016080625 W CN 2016080625W WO 2017185319 A1 WO2017185319 A1 WO 2017185319A1
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steel alloy
zinc coated
coated steel
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PCT/CN2016/080625
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French (fr)
Inventor
Xiaochuan XIONG
Hongliang Yi
Jianfeng Wang
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GM Global Technology Operations LLC
Northeastern University
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Priority to PCT/CN2016/080625 priority Critical patent/WO2017185319A1/en
Publication of WO2017185319A1 publication Critical patent/WO2017185319A1/en

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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
    • C21D1/20—Isothermal quenching, e.g. bainitic hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D2211/002—Bainite
    • 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

Definitions

  • the present disclosure relates to low density zinc-coated TRIP-assisted steel alloys having a multiphase microstructure produced by intercritical austenitization, quenching, and zinc coating.
  • TRIP-assisted steel alloys by austenitization, quenching, and isothermal holding processes are well known for producing automotive body structural applications, having tensile strength properties on the order of greater than or equal to about 600 to less than or equal to about 1200 megapascals (MPa) .
  • MPa megapascals
  • Such processes are typically accomplished according to the following steps.
  • a steel alloy is austenitized in an austenitizing furnace. The temperature depends on the composition of the steel alloy, but typical austenizing temperatures range from greater than or equal to about 760°C up to about 900°C and the steel alloy may be held in the furnace for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes.
  • the austenitized steel alloy is then quenched to a temperature of about 400°C and held at that temperature for several minutes to elicit bainitic transformation.
  • the TRIP steel alloy is then heated and galvanized in a galvanizing bath at a temperature of about 460°C.
  • zinc coated steel is formed by cooling the galvanized steel alloy to room temperature to produce galvanized steel, or reheating the galvanized steel alloy to greater than or equal to about 500°C to less than or equal to about 565°C and held for a period of less than or equal to about 10 seconds to produce galvannealed steel followed by cooling the galvannealed steel to room temperature.
  • Conventional TRIP-assisted steel alloys for producing such a zinc coated steel alloy may, as one non-limiting example, have a composition of less than or equal to about 0.4 wt. %C; less than or equal to about 2.5 wt. %Si; less than or equal to about 1.6 wt. %Mn; less than or equal to about 1.5 wt. %Al; less than or equal to about 0.2 wt. %P; and a balance of iron and impurities including Nb, Mo, Cu, Cr, and Ni.
  • the present disclosure provides a method of producing a zinc coated steel alloy.
  • the zinc coated steel alloy has a composition of carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt. %; silicon at less than or equal to about 0.5 wt. %; manganese at greater than or equal to about 0.2 wt. %to less than or equal to about 1 wt. %; aluminum at greater than or equal to about 3 wt. %to less than or equal to about 5 wt. %; chromium at less than or equal to about 0.5 wt.
  • a steel alloy is first austenitized.
  • the austenitized steel alloy is then quenched to a temperature of greater than or equal to about 460°C to less than or equal to about 490°C.
  • the quenched steel alloy is galvanized to form a zinc coated steel alloy.
  • the zinc coated steel alloy is cooled.
  • the zinc coated steel alloy may have a multiphase microstructure of greater than or equal to about 40vol. %to less than or equal to about 60 vol. %ferrite; greater than or equal to about 15 vol. %to less than or equal to about 30 vol. %metastable retained austenite; and greater than or equal to about 10 vol.
  • the zinc coated steel alloy may be galvannealed after galvanizing the steel alloy. The quenching may occur at a rate of greater than or equal to about 15°C per second to less than or equal to about 30°C per second.
  • the zinc coated steel alloy may have an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa.
  • the zinc coated steel alloy may have an elongation of greater than or equal to about 30%to less than or equal to about 45%.
  • the zinc coated steel alloy may have a density of greater than or equal to about 7.3 g/cm 3 to less than or equal to about 7.6 g/cm 3 .
  • the present disclosure provides a method of producing a zinc coated steel alloy.
  • the zinc coated steel alloy has an elongation of greater than or equal to about 30%to less than or equal to about45%.
  • a steel alloy is first austenitized.
  • the austenitized steel alloy is then quenched to a temperature of greater than or equal to about 460°C to less than or equal to about 490°C.
  • the quenched steel alloy is galvanized to form a zinc coated steel alloy.
  • the zinc coated steel alloy is cooled.
  • the zinc coated steel alloy may have a composition of carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt.
  • the zinc coated steel alloy may have a multiphase microstructure of greater than or equal to about 40vol. %to less than or equal to about 60vol. %ferrite; greater than or equal to about 15 vol. %to less than or equal to about 30 vol.
  • the quenched steel alloy may be galvannealed after galvanizing the steel alloy. The quenching may occur at a rate of greater than or equal to about 15°C per second to less than or equal to about 30°C per second.
  • the zinc coated steel alloy may have an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa.
  • the zinc coated steel alloy may have a density of greater than or equal to about 7.3 g/cm 3 to less than or equal to about 7.6 g/cm 3 .
  • the present disclosure provides a method of producing a zinc coated steel alloy.
  • the zinc coated steel alloy has a multiphase microstructure of greater than or equal to about 40 vol. %to less than or equal to about 60 vol. %ferrite; greater than or equal to about 15 vol. %to less than or equal to about 30 vol. %metastable retained austenite; and greater than or equal to about 10vol. %to less than or equal to about 45 vol. %bainite.
  • a steel alloy is first austenitized. The austenitized steel alloy is then quenched to a temperature of greater than or equal to about 460°C to less than or equal to about490°C.
  • the zinc coated steel alloy may have a composition of carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt. %; silicon at less than or equal to about 0.5 wt. %; manganese at greater than or equal to about 0.2 wt. %to less than or equal to about 1 wt. %; aluminum at greater than or equal to about 3 wt. %to less than or equal to about 5 wt. %; chromium at less than or equal to about 0.5 wt. %; and a balance of iron and impurities.
  • the quenched steel alloy may be galvannealed after galvanizing the steel alloy.
  • the quenching may occur at a rate of greater than or equal to about 15°C per second to less than or equal to about 30°C per second.
  • the zinc coated steel alloy may have an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa.
  • the zinc coated steel alloy may have an elongation of greater than or equal to about 30%to less than or equal to about 45%.
  • the zinc coated steel alloy may have a density of greater than or equal to about 7.3 g/cm 3 to less than or equal to about 7.6 g/cm 3 .
  • the method consists essentially of austenitizing a steel alloy and quenching the austenitized steel alloy, followed by zinc coating the quenched steel alloy.
  • the method consists of austenitizing a steel alloy and quenching the austenitized steel alloy, followed by zinc coating the quenched steel alloy.
  • FIG. 1 shows a representative automotive bumper manufactured according to an aspect of the present disclosure.
  • FIG. 2 shows a process for producing a zinc coated steel alloy using a conventional TRIP-assisted steel alloy.
  • FIG. 3 shows an exemplary process for producing a zinc coated steel alloy in accordance with certain aspects of the present disclosure.
  • FIG. 4 is a schematic chart comparing the temperatures of a conventional galvanizing process using isothermal bainitic holding to the processes according to certain aspects of the present disclosure.
  • Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific compositions, components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.
  • first, second, third, etc. may be used herein to describe various steps, elements, components, regions, layers and/or sections, these steps, elements, components, regions, layers and/or sections should not be limited by these terms, unless otherwise indicated. These terms may be only used to distinguish one step, element, component, region, layer or section from another step, element, component, region, layer or section. Terms such as “first, “ “second, “ and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first step, element, component, region, layer or section discussed below could be termed a second step, element, component, region, layer or section without departing from the teachings of the example embodiments.
  • composition refers broadly to a substance containing at least the preferred metal elements or compounds, but which optionally comprises additional substances or compounds, including additives and impurities.
  • material also broadly refers to matter containing the preferred compounds or composition.
  • disclosure of ranges includes disclosure of all values and further divided ranges within the entire range, including endpoints and sub-ranges given for the ranges.
  • High-strength, high-ductility, lightweight alloy components are particularly suitable for use in components of an automobile or other vehicle (e.g., motorcycles, boats) , but may also be used in a variety of other industries and applications, including aerospace components, industrial equipment and machinery, farm equipment, heavy machinery, by way of non-limiting example. While not limiting, the present methods and materials are particularly suitable for forming lightweight, high strength and ductility components for a vehicle, including crush cans and bumpers.
  • an exemplary automotive structure such as bumper 10, is shown that can be produced from a zinc coated steel alloy prepared according to the present disclosure.
  • the zinc coated steel alloy is formed from austenitizing, quenching, and zinc coating a steel alloy.
  • the process excludes the isothermal holding process of conventional processes (e.g., holding at a temperature of about 400°C for several minutes to elicit bainitic transformation. Instead, the bainitic transformation happens during galvanization.
  • the steel alloy may be galvannealed after galvanizing the steel alloy. It will be appreciated by those skilled in the art that numerous other components may be fabricated by the methods of the present invention, and that such additional components are deemed to be within the scope of the present invention.
  • inventive concepts in the present disclosure may also be applied to any structural component capable of being formed from austenitization, quenching, and zinc coating a steel alloy, including those used in vehicles, like automotive applications including, but not limited to, pillars, such as hinge pillars, panels, including structural panels, door panels, and door components, interior floors, floor pans, roofs, exterior surfaces, underbody shields, wheels, storage areas, including glove boxes, console boxes, trunks, trunk floors, truck beds, lamp pockets and other components, shock tower cap, control arms and other suspension, undercarriage or drive train components, and the like.
  • the present disclosure is particularly suitable for any piece of hardware subject to loads or impact (e.g., load bearing) or requiring cathodic protection.
  • Suitable steel compositions may comprise carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt. %; silicon at less than or equal to about 0.5 wt. %; manganese at greater than or equal to about 0.2 wt. %to less than or equal to about 1 wt. %; aluminum at greater than or equal to about 3 wt. %to less than or equal to about 5 wt. %; chromium at less than or equal to about 0.5 wt. %; and a balance of iron and impurities.
  • FIG. 2 a flowchart showing the steps of a process 100 for eliciting bainitic formation of a conventional TRIP-assisted alloy is shown.
  • a steel alloy is introduced into an austenitizing furnace 102 and heated to a temperature of from greater than or equal to about 760°C to less than or equal to about 900°C for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes to austenitize the steel alloy.
  • the steel alloy is quenched in a quencher 104 so that it reaches a temperature of about 400°C.
  • the steel alloy must be quenched at a rate of greater than or equal to about 20°C per second to prevent pearlite transformation.
  • the quenched steel alloy is then held in an isothermal bainitic holding unit 106 at about 400°C for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes to elicit bainitic transformation.
  • the steel alloy is then heated in a furnace 108 to about 460°C.
  • the steel alloy is then galvanized in a galvanizer 110.
  • the zinc coated steel alloy may be further galvannealed (not shown) .
  • the zinc coated steel alloy is cooled in a cooling device 112 to room temperature.
  • Conventional TRIP-assisted steel alloys for producing such a zinc coated steel alloy may, as one non-limiting example, have a composition of less than or equal to about 0.4 wt. %C; less than or equal to about 2.5 wt. %Si; less than or equal to about 1.6 wt. %Mn; less than or equal to about 1.5 wt. %Al; less than or equal to about 0.2 wt. %P; and a balance of iron and impurities including Nb, Mo, Cu, Cr, and Ni.
  • the conventional zinc coated steel alloy may have a multiphase microstructure comprising from greater than or equal to about 40 vol. %to less than or equal to about 60 vol. %ferrite; from greater than or equal to 10 vol.
  • the conventional zinc coated steel alloy may have an elongation of about 25, as a non-limiting example%.
  • the methods of producing a zinc coated steel alloy contemplated herein provide the ability to eliminate the isothermal bainitic holding step according to conventional processes (e.g., holding at a temperature of about 400°C for several minutes to elicit bainitic transformation) . Further still, the methods of producing a zinc coated steel alloy contemplated herein provide the ability to proceed directly from quenching to galvanizing. Thus, the overall process according to the present disclosure desirably reduces processing time, energy requirements, and cost. Furthermore, the zinc coated steel alloy has higher elongation and is lighter than a zinc coated steel alloy made according to a conventional process.
  • a steel alloy is introduced into an austenitizing furnace 202 and heated to a temperature of from greater than or equal to about 800°C to less than or equal to about 900°C for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes to austenize the steel alloy.
  • the steel alloy is quenched in a quencher 204 to a temperature of about 460°C. At a minimum, the steel alloy is quenched at a rate of greater than or equal to about 15°C per second to prevent pearlite formation.
  • the quenching may occur at a rate of greater than or equal to about 15°C per second to less than or equal to about 30°C per second, and even more particularly, the quenching may occur at a rate of greater than or equal to about 20°C per second to less than or equal to about 30°C per second.
  • the steel alloy is galvanized in a galvanizer 206. Galvanizing the steel alloy in galvanizer 206 elicits bainitic transformation while simultaneously galvanizing the steel alloy.
  • the galvanized steel alloy is then galvannealed by heating in furnace 208 the galvanized steel alloy to a temperature of from greater than or equal to about 500°C to less than or equal to about 565°C for a period of less than or equal to about 10 seconds.
  • the zinc coated steel alloy is cooled in a cooling device 210 to a temperature below 300°C.
  • the process excludes isothermal bainitic holding according to conventional processes (e.g., holding at a temperature of about 400°C for several minutes to elicit bainitic transformation) . Instead, the bainitic transformation happens during galvanization.
  • suitable steel compositions for the process described above may comprise carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt. %; silicon at less than or equal to about 0.4 wt. %; manganese at greater than or equal to about 0.2 wt. %to less than or equal to about 1 wt. %; aluminum at greater than or equal to about 3 wt. %to less than or equal to about 5 wt. %; chromium at less than or equal to about 0.5 wt. %; and a balance of iron and impurities. It is believed that the addition of aluminum offers several heretofore unrecognized benefits.
  • isothermal bainitic holding at a temperature well below e.g., 400°C the temperature required to galvanize a TRIP-assisted steel alloy (i.e., at a temperature of about 460°C) , is required to elicit meaningful bainitic transformation.
  • the time required to galvanize the TRIP-assisted steel alloy (i.e., several seconds) of the present disclosure is well below the time required by conventional isothermal bainitic holding processes (e.g., several minutes)
  • the process disclosed using the steel alloy compositions disclosed herein therefore, eliminates the need for isothermal bainitic holding according to conventional processes (e.g., holding at a temperature of about 400°C for several minutes to elicit bainitic transformation) ; rather, the galvanization is sufficient to elicit meaningful bainitic transformation.
  • the process according to the present disclosure therefore results in cost, energy, and time savings. Referring to FIG.
  • plot 306 which shows on the x-axis time 300 and on the y-axis 302 temperature, the process according to the present disclosure as a function of temperature and time is plotted as 304.
  • a conventional TRIP-assisted steel alloy undergoing a conventional process including isothermal bainitic holding as a function of temperature and time is plotted as 306.
  • plot 306 has a plateau, which relates to the isothermal bainitic holding.
  • the process omits conventional isothermal bainitic holding altogether.
  • the zinc coated steel alloy according to the present disclosure yet may result in a multiphase microstructure comprising greater than or equal to about 40 vol. %to less than or equal to about 60 vol. %ferrite; greater than or equal to about 15 vol. %to less than or equal to about 30 vol. %metastable retained austenite; and greater than or equal to about 10 vol. %to less than or equal to about 45 vol. %bainite, even without the isothermal bainitic holding required by conventional TRIP-assisted steel alloys.
  • the zinc coated steel alloy according to the present disclosure provides excellent elongation. More specifically, the zinc coated steel alloy according to the present disclosure may have an elongation of greater than or equal to about 30%to less than or equal to about45%and in some embodiments an elongation of greater than or equal to about 25%to less than or equal to about40%. It is believed that this increase of elongation may further provide cost savings as the better formability and ductility provided under the processes disclosed herein may allow products that previously had to be made in separate pieces and subsequently formed together to instead be made in one piece. Further still, the strength achieved by conventional processes using conventional TRIP-assisted steel alloys is not sacrificed. Rather, the zinc coated steel alloy according to the present disclosure may have an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa.
  • the zinc coated steel alloy according to the present disclosure is lightweight. More specifically, the zinc coated steel alloy material having compositions according to the present disclosure may be on average greater than or equal to about 3%to less than or equal to about 6%lighter than the steel alloys described above used in TRIP-assisted steel alloy bainitic transformation processes and may have a density of greater than or equal to about 7.3 g/cm 3 to less than or equal to about 7.6 g/cm 3 . Moreover, as noted above, many metal parts can be made using the compositions according to the present disclosure to form vehicle components. Vehicles having metal parts made using the compositions according to the present disclosure therefore potentially translate to weight savings. Reducing the weight of components in part is important for improving efficiency and is of great importance for fuel efficiency in mobile applications, such as in automobiles.
  • a process may consist essentially of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800°C to less than or equal to about 900°C for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of greater than or equal to about 460°C to less than or equal to about 490°C. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled.
  • the zinc coated steel alloy is comprised of carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt. %; silicon at less than or equal to about 0.5 wt.
  • the process may be further limited as further consisting essentially of any of, any combination of, or all of the following: (1) having a multiphase microstructure comprising greater than or equal to about 40 vol. %to less than or equal to about 60 vol. %ferrite; greater than or equal to about 15 vol. %to less than or equal to about 30 vol.
  • %metastable retained austenite and greater than or equal to about 10 vol. %to less than or equal to about 45 vol. %bainite; (2) galvannealing after galvanizing the steel alloy; (3) quenching at a rate of greater than or equal to about 15°C per second to less than or equal to about 30°C per second; (4) having an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa; (5) having an elongation of greater than or equal to about 30%to less than or equal to about 45%; and (6) having a density of greater than or equal to about 7.3 g/cm 3 to less than or equal to about 7.6 g/cm 3 .
  • a process may consist essentially of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800°C to less than or equal to about 900°C for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of greater than or equal to about 460°C to less than or equal to about490°C. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled. The zinc coated steel alloy has an elongation of greater than or equal to about 30%to less than or equal to about 45%.
  • the process may be further limited as further consisting essentially of any of, any combination of, or all of the following: (1) having a steel alloy comprised of carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt. %; silicon at less than or equal to about 0.5 wt. %; manganese at greater than or equal to about 0.2 wt. %to less than or equal to about 1 wt. %; aluminum at greater than or equal to about 3 wt. %to less than or equal to about 5 wt. %; chromium at less than or equal to about 0.5 wt.
  • %bainite (3) galvannealing after galvanizing the steel alloy; (4) quenching at a rate of greater than or equal to about 15°C per second to less than or equal to about 30°C per second; (5) having an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa; and (6) having a density of greater than or equal to about 7.3 g/cm 3 to less than or equal to about 7.6 g/cm 3 .
  • a process may consist essentially of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800°C to less than or equal to about 900°C for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of greater than or equal to about 460°C to less than or equal to about 490°C. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled.
  • the zinc coated steel alloy has a multiphase microstructure comprising greater than or equal to about 40 vol. %to less than or equal to about 60 vol. %ferrite; greater than or equal to about 15 vol.
  • the process may be further limited as further consisting essentially of any of, any combination of, or all of the following: (1) having a steel alloy comprised of carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt. %; silicon at less than or equal to about 0.5 wt. %; manganese at greater than or equal to about 0.2 wt. %to less than or equal to about 1 wt. %; aluminum at greater than or equal to about 3 wt.
  • a process may consist of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800°C to less than or equal to about 900°C for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of greater than or equal to about 460°C to less than or equal to about 490°C. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled.
  • the zinc coated steel alloy is comprised of carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt. %; silicon at less than or equal to about 0.5 wt.
  • the process may be further limited as further consisting of any of, any combination of, or all of the following: (1) having a multiphase microstructure comprising greater than or equal to about 40vol. %to less than or equal to about 60vol. %ferrite; greater than or equal to about 15 vol. %to less than or equal to about 30vol.
  • %metastable retained austenite and greater than or equal to about 10 vol. %to less than or equal to about 45 vol. %bainite; (2) galvannealing after galvanizing the steel alloy; (3) quenching at a rate of greater than or equal to about 15°C per second to less than or equal to about 30°C per second; (4) having an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa; (5) having an elongation of greater than or equal to about 30%to less than or equal to about 45%; and (6) having a density of greater than or equal to about 7.3 g/cm 3 to less than or equal to about 7.6 g/cm 3 .
  • a process may consist of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800°C to less than or equal to about 900°C for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of about 460°C. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled. The zinc coated steel alloy has an elongation of greater than or equal to about 30%to less than or equal to about 45%.
  • the process may be further limited as further consisting of any of, any combination of, or all of the following: (1) having a steel alloy comprised of carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt. %; silicon at less than or equal to about 0.5 wt. %; manganese at greater than or equal to about 0.2 wt. %to less than or equal to about 1 wt. %; aluminum at greater than or equal to about 3 wt. %to less than or equal to about 5 wt. %; chromium at less than or equal to about 0.5 wt.
  • %bainite (3) galvannealing after galvanizing the steel alloy; (4) quenching at a rate of greater than or equal to about 15°C per second to less than or equal to about 30°C per second; (5) having an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa; and (6) having a density of greater than or equal to about 7.3 g/cm 3 to less than or equal to about 7.6 g/cm 3 .
  • a process may consist of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800°C to less than or equal to about 900°C for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of greater than or equal to about 460°C to less than or equal to about 490°C. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled.
  • the zinc coated steel alloy has a multiphase microstructure comprising greater than or equal to about 40vol. %to less than or equal to about 60vol. %ferrite; greater than or equal to about 15 vol.
  • the process may be further limited as further consisting of any of, any combination of, or all of the following: (1) having a steel alloy comprised of carbon at greater than or equal to about 0.35 wt. %to less than or equal to about 0.45 wt. %; silicon at less than or equal to about 0.5 wt. %; manganese at greater than or equal to about 0.2 wt. %to less than or equal to about 1 wt. %; aluminum at greater than or equal to about 3 wt.

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Abstract

Methods are provided for forming low density zinc-coated TRIP-assisted steel having a multiphase bainitic microstructure produced by austenitization, quenching, and zinc coating. A conventional step of isothermal bainitic holding is eliminated and the zinc coated steel alloy may have an elongation of ≥ about 30% to ≤ 45%. The zinc coated steel alloy may have a multiphase microstructure of ≥ about 40 to ≤ about 60% ferrite; ≥ about 15 to ≤ about 30% metastable retained austenite; and ≥ about 10 to ≤ about 45% bainite. The zinc coated steel alloy preferably comprises carbon at ≥ about 0.35 to ≤ about 0.45 wt. %; silicon at ≤ about 0.5 wt. %; manganese at ≥ about 0.2 to ≤ about 1 wt. %; aluminum at ≥ about 3 to ≤ about 5 wt. %; chromium at ≤ about 0.5 wt. %; and a balance of iron and impurities.

Description

LOW DENSITY ZINC-COATED TRIP-ASSISTED STEEL AND FABRICATION METHOD FIELD
The present disclosure relates to low density zinc-coated TRIP-assisted steel alloys having a multiphase microstructure produced by intercritical austenitization, quenching, and zinc coating.
BACKGROUND
This section provides background information related to the present disclosure which is not necessarily prior art.
Producing zinc coated transformation induced plasticity-assisted (TRIP-assisted) steel alloys by austenitization, quenching, and isothermal holding processes are well known for producing automotive body structural applications, having tensile strength properties on the order of greater than or equal to about 600 to less than or equal to about 1200 megapascals (MPa) . Generally speaking, such processes are typically accomplished according to the following steps. First, a steel alloy is austenitized in an austenitizing furnace. The temperature depends on the composition of the steel alloy, but typical austenizing temperatures range from greater than or equal to about 760℃ up to about 900℃ and the steel alloy may be held in the furnace for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes. The austenitized steel alloy is then quenched to a temperature of about 400℃ and held at that temperature for several minutes to elicit bainitic transformation. The TRIP steel alloy is then heated and galvanized in a galvanizing bath at a temperature of about 460℃. Finally, zinc coated steel is formed by cooling the galvanized steel alloy to room temperature to produce  galvanized steel, or reheating the galvanized steel alloy to greater than or equal to about 500℃ to less than or equal to about 565℃ and held for a period of less than or equal to about 10 seconds to produce galvannealed steel followed by cooling the galvannealed steel to room temperature.
Conventional TRIP-assisted steel alloys for producing such a zinc coated steel alloy may, as one non-limiting example, have a composition of less than or equal to about 0.4 wt. ％C； less than or equal to about 2.5 wt. ％Si； less than or equal to about 1.6 wt. ％Mn； less than or equal to about 1.5 wt. ％Al； less than or equal to about 0.2 wt. ％P； and a balance of iron and impurities including Nb, Mo, Cu, Cr, and Ni.
While steels produced by conventional methods offer good strength and elongation for many applications, there is a continual need to improve upon the processes to reduce costs and the duration of the process associated therewith. There is a further need to reduce the mass of vehicle components for improved fuel efficiency without sacrificing requisite tensile strengths. There is yet a further need to produce zinc coated steel alloys with high elongation allowing for greater formability.
SUMMARY
This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. In various aspects, the present disclosure provides a method of producing a zinc coated steel alloy. The zinc coated steel alloy has a composition of carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5  wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities. To produce the zinc coated steel alloy, a steel alloy is first austenitized. The austenitized steel alloy is then quenched to a temperature of greater than or equal to about 460℃ to less than or equal to about 490℃. Next, the quenched steel alloy is galvanized to form a zinc coated steel alloy. Finally, the zinc coated steel alloy is cooled. The zinc coated steel alloy may have a multiphase microstructure of greater than or equal to about 40vol. ％to less than or equal to about 60 vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30 vol. ％metastable retained austenite； and greater than or equal to about 10 vol. ％to less than or equal to about 45 vol. ％bainite. The zinc coated steel alloy may be galvannealed after galvanizing the steel alloy. The quenching may occur at a rate of greater than or equal to about 15℃ per second to less than or equal to about 30℃ per second. The zinc coated steel alloy may have an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa. The zinc coated steel alloy may have an elongation of greater than or equal to about 30％to less than or equal to about 45％. The zinc coated steel alloy may have a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3 .
In yet other aspects, the present disclosure provides a method of producing a zinc coated steel alloy. The zinc coated steel alloy has an elongation of greater than or equal to about 30％to less than or equal to about45％. To produce the zinc coated steel alloy, a steel alloy is first austenitized. The austenitized steel alloy is then quenched to a temperature of greater than or equal to about 460℃ to less than or equal to about 490℃. Next, the quenched steel alloy is galvanized to form a zinc coated steel alloy. Finally, the  zinc coated steel alloy is cooled. The zinc coated steel alloy may have a composition of carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities. The zinc coated steel alloy may have a multiphase microstructure of greater than or equal to about 40vol. ％to less than or equal to about 60vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30 vol. ％metastable retained austenite； and greater than or equal to about 10vol. ％to less than or equal to about 45 vol. ％bainite. The quenched steel alloy may be galvannealed after galvanizing the steel alloy. The quenching may occur at a rate of greater than or equal to about 15℃ per second to less than or equal to about 30℃ per second. The zinc coated steel alloy may have an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa. The zinc coated steel alloy may have a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3.
In yet other aspects, the present disclosure provides a method of producing a zinc coated steel alloy. The zinc coated steel alloy has a multiphase microstructure of greater than or equal to about 40 vol. ％to less than or equal to about 60 vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30 vol. ％metastable retained austenite； and greater than or equal to about 10vol. ％to less than or equal to about 45 vol. ％bainite. To produce the zinc coated steel alloy, a steel alloy is first austenitized. The austenitized steel alloy is then quenched to a temperature of  greater than or equal to about 460℃ to less than or equal to about490℃. Next, the quenched steel alloy is galvanized to form a zinc coated steel alloy. Finally, the zinc coated steel alloy is cooled. The zinc coated steel alloy may have a composition of carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities. The quenched steel alloy may be galvannealed after galvanizing the steel alloy. The quenching may occur at a rate of greater than or equal to about 15℃ per second to less than or equal to about 30℃ per second. The zinc coated steel alloy may have an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa. The zinc coated steel alloy may have an elongation of greater than or equal to about 30％to less than or equal to about 45％. The zinc coated steel alloy may have a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3.
In certain other variations, the method consists essentially of austenitizing a steel alloy and quenching the austenitized steel alloy, followed by zinc coating the quenched steel alloy.
In yet other variations, the method consists of austenitizing a steel alloy and quenching the austenitized steel alloy, followed by zinc coating the quenched steel alloy.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for  purposes of illustration only and are not intended to limit the scope of the present disclosure.
DRAWINGS
The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.
FIG. 1 shows a representative automotive bumper manufactured according to an aspect of the present disclosure.
FIG. 2 shows a process for producing a zinc coated steel alloy using a conventional TRIP-assisted steel alloy.
FIG. 3 shows an exemplary process for producing a zinc coated steel alloy in accordance with certain aspects of the present disclosure.
FIG. 4 is a schematic chart comparing the temperatures of a conventional galvanizing process using isothermal bainitic holding to the processes according to certain aspects of the present disclosure.
Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.
DETAILED DESCRIPTION
Example embodiments will now be described more fully with reference to the accompanying drawings.
Example embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous  specific details are set forth such as examples of specific compositions, components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.
The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms"a, " "an, " and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises, " "comprising, " "including, " and "having, " are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed, unless otherwise indicated.
When a component, element, or layer is referred to as being "on, " "engaged to, " "connected to, " or "coupled to" another element or layer, it may be directly on, engaged, connected or coupled to the other component, element, or layer, or intervening elements or layers may be present. In contrast, when an element is referred  to as being "directly on, " "directly engaged to, " "directly connected to, " or "directly coupled to" another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., "between" versus "directly between, " "adjacent" versus "directly adjacent, " etc. ) . As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.
Although the terms first, second, third, etc. may be used herein to describe various steps, elements, components, regions, layers and/or sections, these steps, elements, components, regions, layers and/or sections should not be limited by these terms, unless otherwise indicated. These terms may be only used to distinguish one step, element, component, region, layer or section from another step, element, component, region, layer or section. Terms such as "first, " "second, " and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first step, element, component, region, layer or section discussed below could be termed a second step, element, component, region, layer or section without departing from the teachings of the example embodiments.
Spatially or temporally relative terms, such as "before, " "after, " "inner, " "outer, " "beneath, " "below, " "lower, " "above, " "upper, " and the like, may be used herein for ease of description to describe one element or feature's relationship to another element (s) or feature (s) as illustrated in the figures. Spatially or temporally relative terms may be intended to encompass different orientations of the device or system in use or operation in addition to the orientation depicted in the figures.
It should be understood for any recitation of a method, composition, device, or system that “comprises” certain steps, ingredients, or features, that in certain alternative variations, it is also contemplated that such a method, composition, device, or system may also "consist essentially of" or “consist of” the enumerated steps, ingredients, or features, so that any other steps, ingredients, or features. In the case of “consisting of, ” the alternative embodiment excludes any additional steps, ingredients, or features, while in the case of “consisting essentially of, ” any additional steps, ingredients, or features that materially affect the basic and novel characteristics are excluded from such an embodiment, but any steps, ingredients, or features that do not materially affect the basic and novel characteristics can be included in the embodiment.
Throughout this disclosure, the numerical values represent approximate measures or limits to ranges to encompass minor deviations from the given values and embodiments having about the value mentioned as well as those having exactly the value mentioned. Other than in the working examples provided at the end of the detailed description, all numerical values of parameters (e.g., of quantities or conditions) in this specification, including the appended claims, are to be understood as being modified in all instances by the term "about" whether or not "about" actually appears before the numerical value. "About" indicates that the stated numerical value allows some slight imprecision (with some approach to exactness in the value； approximately or reasonably close to the value； nearly) . Ifthe imprecision provided by "about" is not otherwise understood in the art with this ordinary meaning, then "about" as used herein indicates at least variations that may arise from ordinary methods of measuring and using such parameters. If, for some reason, the imprecision provided by “about” is not otherwise  understood in the art with this ordinary meaning, then “about” as used herein may indicate a possible variation of up to 5％of the indicated value or 5％variance from usual methods of measurement.
As used herein, the term “composition” refers broadly to a substance containing at least the preferred metal elements or compounds, but which optionally comprises additional substances or compounds, including additives and impurities. The term “material” also broadly refers to matter containing the preferred compounds or composition.
In addition, disclosure of ranges includes disclosure of all values and further divided ranges within the entire range, including endpoints and sub-ranges given for the ranges.
The present disclosure provides methods of producing lightweight, high strength and ductility components. High-strength, high-ductility, lightweight alloy components are particularly suitable for use in components of an automobile or other vehicle (e.g., motorcycles, boats) , but may also be used in a variety of other industries and applications, including aerospace components, industrial equipment and machinery, farm equipment, heavy machinery, by way of non-limiting example. While not limiting, the present methods and materials are particularly suitable for forming lightweight, high strength and ductility components for a vehicle, including crush cans and bumpers.
Referring first to FIG. 1, an exemplary automotive structure, such as bumper 10, is shown that can be produced from a zinc coated steel alloy prepared according to the present disclosure. In certain variations, the zinc coated steel alloy is formed from austenitizing, quenching, and zinc coating a steel alloy. Notably, the  process excludes the isothermal holding process of conventional processes (e.g., holding at a temperature of about 400℃ for several minutes to elicit bainitic transformation. Instead, the bainitic transformation happens during galvanization. In other aspects, the steel alloy may be galvannealed after galvanizing the steel alloy. It will be appreciated by those skilled in the art that numerous other components may be fabricated by the methods of the present invention, and that such additional components are deemed to be within the scope of the present invention. Thus, while exemplary components are illustrated and described throughout the specification, it is understood that the inventive concepts in the present disclosure may also be applied to any structural component capable of being formed from austenitization, quenching, and zinc coating a steel alloy, including those used in vehicles, like automotive applications including, but not limited to, pillars, such as hinge pillars, panels, including structural panels, door panels, and door components, interior floors, floor pans, roofs, exterior surfaces, underbody shields, wheels, storage areas, including glove boxes, console boxes, trunks, trunk floors, truck beds, lamp pockets and other components, shock tower cap, control arms and other suspension, undercarriage or drive train components, and the like. Specifically, the present disclosure is particularly suitable for any piece of hardware subject to loads or impact (e.g., load bearing) or requiring cathodic protection.
Suitable steel compositions may comprise carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or  equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities.
Referring to FIG. 2, a flowchart showing the steps of a process 100 for eliciting bainitic formation of a conventional TRIP-assisted alloy is shown. A steel alloy is introduced into an austenitizing furnace 102 and heated to a temperature of from greater than or equal to about 760℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes to austenitize the steel alloy. After austenitization, the steel alloy is quenched in a quencher 104 so that it reaches a temperature of about 400℃. At a minimum, the steel alloy must be quenched at a rate of greater than or equal to about 20℃ per second to prevent pearlite transformation. The quenched steel alloy is then held in an isothermal bainitic holding unit 106 at about 400℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes to elicit bainitic transformation. The steel alloy is then heated in a furnace 108 to about 460℃. The steel alloy is then galvanized in a galvanizer 110. Optionally, the zinc coated steel alloy may be further galvannealed (not shown) . Finally, the zinc coated steel alloy is cooled in a cooling device 112 to room temperature.
Conventional TRIP-assisted steel alloys for producing such a zinc coated steel alloy may, as one non-limiting example, have a composition of less than or equal to about 0.4 wt. ％C； less than or equal to about 2.5 wt. ％Si； less than or equal to about 1.6 wt. ％Mn； less than or equal to about 1.5 wt. ％Al； less than or equal to about 0.2 wt. ％P； and a balance of iron and impurities including Nb, Mo, Cu, Cr, and Ni. The conventional zinc coated steel alloy may have a multiphase microstructure comprising  from greater than or equal to about 40 vol. ％to less than or equal to about 60 vol. ％ferrite； from greater than or equal to 10 vol. ％to less than or equal to 20 vol. ％metastable retained austenite； and from greater than or equal to about 20 vol. ％to less than or equal to about 50 vol. ％bainite. The conventional zinc coated steel alloy may have an elongation of about 25, as a non-limiting example％.
In accordance with certain aspects of the present disclosure, the methods of producing a zinc coated steel alloy contemplated herein provide the ability to eliminate the isothermal bainitic holding step according to conventional processes (e.g., holding at a temperature of about 400℃ for several minutes to elicit bainitic transformation) . Further still, the methods of producing a zinc coated steel alloy contemplated herein provide the ability to proceed directly from quenching to galvanizing. Thus, the overall process according to the present disclosure desirably reduces processing time, energy requirements, and cost. Furthermore, the zinc coated steel alloy has higher elongation and is lighter than a zinc coated steel alloy made according to a conventional process.
Referring to FIG. 3, a flowchart showing the steps of the process 200 of austenitizing, quench, and zinc coating the steel alloy disclosed herein is shown. A steel alloy is introduced into an austenitizing furnace 202 and heated to a temperature of from greater than or equal to about 800℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes to austenize the steel alloy. After austenitization, the steel alloy is quenched in a quencher 204 to a temperature of about 460℃. At a minimum, the steel alloy is quenched at a rate of greater than or equal to about 15℃ per second to prevent pearlite formation. More particularly, the quenching may occur at a rate of greater than or equal to about 15℃ per  second to less than or equal to about 30℃ per second, and even more particularly, the quenching may occur at a rate of greater than or equal to about 20℃ per second to less than or equal to about 30℃ per second. After quenching, the steel alloy is galvanized in a galvanizer 206. Galvanizing the steel alloy in galvanizer 206 elicits bainitic transformation while simultaneously galvanizing the steel alloy. Optionally, the galvanized steel alloy is then galvannealed by heating in furnace 208 the galvanized steel alloy to a temperature of from greater than or equal to about 500℃ to less than or equal to about 565℃ for a period of less than or equal to about 10 seconds. Finally, the zinc coated steel alloy is cooled in a cooling device 210 to a temperature below 300℃. Notably, the process excludes isothermal bainitic holding according to conventional processes (e.g., holding at a temperature of about 400℃ for several minutes to elicit bainitic transformation) . Instead, the bainitic transformation happens during galvanization.
As noted above, suitable steel compositions for the process described above may comprise carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.4 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities. It is believed that the addition of aluminum offers several heretofore unrecognized benefits. In typical bainitic formation processes using conventional TRIP-assisted steel alloys, isothermal bainitic holding at a temperature well below (e.g., 400℃) the temperature required to galvanize a TRIP-assisted steel alloy (i.e., at a temperature of about 460℃) ,  is required to elicit meaningful bainitic transformation. Further still, the time required to galvanize the TRIP-assisted steel alloy (i.e., several seconds) of the present disclosure is well below the time required by conventional isothermal bainitic holding processes (e.g., several minutes) The process disclosed using the steel alloy compositions disclosed herein, therefore, eliminates the need for isothermal bainitic holding according to conventional processes (e.g., holding at a temperature of about 400℃ for several minutes to elicit bainitic transformation) ； rather, the galvanization is sufficient to elicit meaningful bainitic transformation. The process according to the present disclosure therefore results in cost, energy, and time savings. Referring to FIG. 4, which shows on the x-axis time 300 and on the y-axis 302 temperature, the process according to the present disclosure as a function of temperature and time is plotted as 304. A conventional TRIP-assisted steel alloy undergoing a conventional process including isothermal bainitic holding as a function of temperature and time is plotted as 306. As can be seen, plot 306 has a plateau, which relates to the isothermal bainitic holding. Notably, however, the process omits conventional isothermal bainitic holding altogether.
As noted above, the zinc coated steel alloy according to the present disclosure yet may result in a multiphase microstructure comprising greater than or equal to about 40 vol. ％to less than or equal to about 60 vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30 vol. ％metastable retained austenite； and greater than or equal to about 10 vol. ％to less than or equal to about 45 vol. ％bainite, even without the isothermal bainitic holding required by conventional TRIP-assisted steel alloys.
The zinc coated steel alloy according to the present disclosure provides excellent elongation. More specifically, the zinc coated steel alloy according to the present disclosure may have an elongation of greater than or equal to about 30％to less than or equal to about45％and in some embodiments an elongation of greater than or equal to about 25％to less than or equal to about40％. It is believed that this increase of elongation may further provide cost savings as the better formability and ductility provided under the processes disclosed herein may allow products that previously had to be made in separate pieces and subsequently formed together to instead be made in one piece. Further still, the strength achieved by conventional processes using conventional TRIP-assisted steel alloys is not sacrificed. Rather, the zinc coated steel alloy according to the present disclosure may have an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa.
As mentioned above, the zinc coated steel alloy according to the present disclosure is lightweight. More specifically, the zinc coated steel alloy material having compositions according to the present disclosure may be on average greater than or equal to about 3％to less than or equal to about 6％lighter than the steel alloys described above used in TRIP-assisted steel alloy bainitic transformation processes and may have a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3. Moreover, as noted above, many metal parts can be made using the compositions according to the present disclosure to form vehicle components. Vehicles having metal parts made using the compositions according to the present disclosure therefore potentially translate to weight savings. Reducing the weight of components in part is  important for improving efficiency and is of great importance for fuel efficiency in mobile applications, such as in automobiles.
In another example, a process may consist essentially of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of greater than or equal to about 460℃ to less than or equal to about 490℃. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled. The zinc coated steel alloy is comprised of carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities. In certain other variations, the process may be further limited as further consisting essentially of any of, any combination of, or all of the following: (1) having a multiphase microstructure comprising greater than or equal to about 40 vol. ％to less than or equal to about 60 vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30 vol. ％metastable retained austenite； and greater than or equal to about 10 vol. ％to less than or equal to about 45 vol. ％bainite； (2) galvannealing after galvanizing the steel alloy； (3) quenching at a rate of greater than or equal to about 15℃ per second to less than or equal to about 30℃ per second； (4) having an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa； (5) having  an elongation of greater than or equal to about 30％to less than or equal to about 45％； and (6) having a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3.
In another example, a process may consist essentially of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of greater than or equal to about 460℃ to less than or equal to about490℃. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled. The zinc coated steel alloy has an elongation of greater than or equal to about 30％to less than or equal to about 45％. In certain other variations, the process may be further limited as further consisting essentially of any of, any combination of, or all of the following: (1) having a steel alloy comprised of carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities； (2) having a multiphase microstructure comprising greater than or equal to about 40 vol. ％to less than or equal to about 60 vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30 vol. ％metastable retained austenite； and greater than or equal to about 10vol. ％to less than or equal to about 45 vol. ％bainite； (3) galvannealing after galvanizing the steel alloy； (4) quenching at a rate of greater than or equal to about  15℃ per second to less than or equal to about 30℃ per second； (5) having an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa； and (6) having a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3.
In another example, a process may consist essentially of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of greater than or equal to about 460℃ to less than or equal to about 490℃. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled. The zinc coated steel alloy has a multiphase microstructure comprising greater than or equal to about 40 vol. ％to less than or equal to about 60 vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30 vol. ％metastable retained austenite； and greater than or equal to about 10vol. ％to less than or equal to about 45 vol. ％bainite. In certain other variations, the process may be further limited as further consisting essentially of any of, any combination of, or all of the following: (1) having a steel alloy comprised of carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities； (2) having an elongation of greater than or equal to about 30％to less than or equal to about 45％； (3) galvannealing after  galvanizing the steel alloy； (4) quenching at a rate of greater than or equal to about 15℃per second to less than or equal to about 30℃ per second； (5) having an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa； and (6) having a density of greater than or equal to about 7.4 g/cm3 to less than or equal to about 7.6 g/cm3.
In another example, a process may consist of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of greater than or equal to about 460℃ to less than or equal to about 490℃. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled. The zinc coated steel alloy is comprised of carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities. In certain other variations, the process may be further limited as further consisting of any of, any combination of, or all of the following: (1) having a multiphase microstructure comprising greater than or equal to about 40vol. ％to less than or equal to about 60vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30vol. ％metastable retained austenite； and greater than or equal to about 10 vol. ％to less than or equal to about 45 vol. ％bainite； (2) galvannealing after galvanizing the steel alloy； (3) quenching at a rate of greater than  or equal to about 15℃ per second to less than or equal to about 30℃ per second； (4) having an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa； (5) having an elongation of greater than or equal to about 30％to less than or equal to about 45％； and (6) having a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3.
In another example, a process may consist of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of about 460℃. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled. The zinc coated steel alloy has an elongation of greater than or equal to about 30％to less than or equal to about 45％. In certain other variations, the process may be further limited as further consisting of any of, any combination of, or all of the following: (1) having a steel alloy comprised of carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities； (2) having a multiphase microstructure comprising greater than or equal to about 40vol. ％to less than or equal to about 60vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30vol. ％metastable retained austenite； and greater than or equal to about 10 vol. ％to less than or equal to about 45 vol. ％bainite； (3) galvannealing after galvanizing the  steel alloy； (4) quenching at a rate of greater than or equal to about 15℃ per second to less than or equal to about 30℃ per second； (5) having an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa； and (6) having a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3.
In another example, a process may consist of introducing a steel alloy into an austenitizing furnace and heating the steel alloy to a temperature of from greater than or equal to about 800℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes, followed by quenching the steel alloy to a temperature of greater than or equal to about 460℃ to less than or equal to about 490℃. The quenched steel alloy is then galvanized. The zinc coated steel alloy is then cooled. The zinc coated steel alloy has a multiphase microstructure comprising greater than or equal to about 40vol. ％to less than or equal to about 60vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30vol. ％metastable retained austenite； and greater than or equal to about 10 vol. ％to less than or equal to about 45 vol. ％bainite. In certain other variations, the process may be further limited as further consisting of any of, any combination of, or all of the following: (1) having a steel alloy comprised of carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities； (2) having an elongation of greater than or equal to about 30％to less than  or equal to about 45％； (3) galvannealing after galvanizing the steel alloy； (4) quenching at a rate of greater than or equal to about 15℃ per second to less than or equal to about 30℃ per second； (5) having an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa； and (6) having a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3.
The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims (20)

  1. A method of producing a zinc coated steel alloy comprising:
    austenitizing a steel alloy at a temperature of greater than or equal to about 800℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes；
    quenching the steel alloy； and
    galvanizing the steel alloy to form a zinc coated steel alloy； wherein the steel alloy comprises carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities.
  2. The method of claim 1, wherein the zinc coated steel alloy comprises greater than or equal to about 40 vol. ％to less than or equal to about 60 vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30 vol. ％metastable retained austenite； and greater than or equal to about 10 vol. ％to less than or equal to about 45 vol. ％bainite.
  3. The method of claim 1, further comprising heating the zinc coated steel alloy to a temperature of from greater than or equal to about 500℃ to less than or equal to about 565℃ for a period of less than or equal to about 10 seconds.
  4. The method of claim 1, wherein the quenching occurs at a rate of greater than or equal to about 15℃ per second to less than or equal to about 30℃ per second.
  5. The method of claim 1, wherein the zinc coated steel alloy has an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa.
  6. The method of claim 1, wherein the zinc coated steel alloy has an elongation of greater than or equal to about 30％to less than or equal to about 45％.
  7. The method of claim 1, wherein the zinc coated steel alloy has a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3.
  8. A method of producing a zinc coated steel alloy consisting essentially of:
    austenitizing a steel alloy at a temperature of greater than or equal to about 800℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes；
    quenching the steel alloy；
    galvanizing the steel alloy to form a zinc coated steel alloy； and
    cooling the zinc coated steel alloy, wherein the zinc coated steel alloy has an elongation of greater than or equal to about 30％to less than or equal to about 45％.
  9. The method of claim 8, wherein the zinc coated steel alloy comprises greater than or equal to about 40 vol. ％to less than or equal to about 60 vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30 vol. ％metastable retained austenite； and greater than or equal to about 10 vol. ％to less than or equal to about 45 vol. ％bainite.
  10. The method of claim 8, further comprising heating the zinc coated steel alloy to a temperature of from greater than or equal to about 500℃ to less than or equal to about 565℃ for a period of less than or equal to about 10 seconds.
  11. The method of claim 8, wherein the quenching occurs at a rate of greater than or equal to about 15℃ per second to less than or equal to about 30℃ per second.
  12. The method of claim 8, wherein the zinc coated steel alloy comprises carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities.
  13. The method of claim 8, wherein the zinc coated steel alloy has an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa.
  14. The method of claim 8, wherein the zinc coated steel alloy has a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3.
  15. A method of producing a zinc coated steel alloy comprising:
    austenitizing a steel alloy at a temperature of greater than or equal to about 800℃ to less than or equal to about 900℃ for a period of greater than or equal to about 1 minute to less than or equal to about 10 minutes；
    quenching the steel alloy；
    galvanizing the steel alloy to form a zinc coated steel alloy, wherein the galvanized steel alloy comprises greater than or equal to about 40 vol. ％to less than or equal to about 60 vol. ％ferrite； greater than or equal to about 15 vol. ％to less than or equal to about 30 vol. ％metastable retained austenite； and greater than or equal to about 10vol. ％to less than or equal to about 45 vol. ％bainite.
  16. The method of claim 15, wherein the zinc coated steel alloy has an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa.
  17. The method of claim 15, further comprising heating the galvanized steel alloy to a temperature of from greater than or equal to about 500℃ to less than or equal to about 565℃ for a period of less than or equal to about 10 seconds.
  18. The method of claim 15, wherein the quenching occurs at a rate of greater than or equal to about 15℃ per second to less than or equal to about 30℃ per second.
  19. The method of claim 15, wherein the zinc coated steel alloy comprises carbon at greater than or equal to about 0.35 wt. ％to less than or equal to about 0.45 wt. ％； silicon at less than or equal to about 0.5 wt. ％； manganese at greater than or equal to about 0.2 wt. ％to less than or equal to about 1 wt. ％； aluminum at greater than or equal to about 3 wt. ％to less than or equal to about 5 wt. ％； chromium at less than or equal to about 0.5 wt. ％； and a balance of iron and impurities.
  20. The method of claim 15, wherein the zinc coated steel alloy has an ultimate tensile strength of greater than or equal to about 700 MPa to less than or equal to about 900 MPa, an elongation of greater than or equal to about 30％to less than or equal to about 45％, and a density of greater than or equal to about 7.3 g/cm3 to less than or equal to about 7.6 g/cm3.
PCT/CN2016/080625 2016-04-29 2016-04-29 Low density zinc-coated trip-assisted steel and fabrication method WO2017185319A1 (en)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN101353761A (en) * 2008-09-11 2009-01-28 北京科技大学 TRIP steel plate for high strength cold rolling hot dip galvanizing and preparation thereof
CN103361547A (en) * 2012-03-30 2013-10-23 鞍钢股份有限公司 Production method of ultrahigh-strength steel plate for cold forming and steel plate
WO2015115112A1 (en) * 2014-02-03 2015-08-06 Ｊｆｅスチール株式会社 Alloyed hot-dip galvanized steel sheet and method for producing same

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN101353761A (en) * 2008-09-11 2009-01-28 北京科技大学 TRIP steel plate for high strength cold rolling hot dip galvanizing and preparation thereof
CN103361547A (en) * 2012-03-30 2013-10-23 鞍钢股份有限公司 Production method of ultrahigh-strength steel plate for cold forming and steel plate
WO2015115112A1 (en) * 2014-02-03 2015-08-06 Ｊｆｅスチール株式会社 Alloyed hot-dip galvanized steel sheet and method for producing same

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