Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/001—Continuous casting of metals, i.e. casting in indefinite lengths of specific alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • 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
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
• • 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
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
  • C23C2/0224—Two or more thermal pretreatments
• • 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
  • 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
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
• • 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
  • C23C2/26—After-treatment
  • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
  • C23C2/29—Cooling or 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

• the present invention relates to methods of providing continuously cast hot rolled high strength steel sheet products, including quenching finish rolled steel sheets on a hot strip mill to produce a predominantly martensitic microstructure, followed by soaking at a temperature in an intercritical regime and holding at a lower temperature to produce a microstructure that is predominantly ferrite and retained austenite.
• the present invention provides methods of producing continuously cast hot rolled steel sheet products, including continuously casting a steel slab and then hot rolling with finish rolling on a hot strip mill, quenching on the hot strip mill to form a predominantly martensitic microstructure, and performing a thermal cycling step including soaking at an intercritical temperature followed by holding at a lower temperature.
• the thermal cycling step can take place on a continuous galvanize line or continuous annealing line.
• the resultant hot rolled steel sheet products have a microstructure comprising ferrite and retained austenite.
• Steels processed in accordance with the present invention exhibit favorable combined ultimate tensile strength and total elongation (UTS-TE) properties, e.g., of greater than 25,000 MPa-%. Steels with these properties may fall into the category of Generation 3 advanced high strength steels, and are highly desired by various industries including automobile manufacturers.
• UTS-TE ultimate tensile strength and total elongation
• An aspect of the present invention is to provide a method of producing a high strength continuously cast hot rolled steel sheet product, the method comprising:
• Another aspect of the present invention is to provide a high strength continuously cast hot rolled steel sheet product produced by the method described above. [0008]
• Fig. 1 includes plots of temperature versus time illustrating a two-step annealing process.
• Fig. 2 includes plots of temperature versus time illustrating a two-step annealing process.
• Fig. 3 is a plot of temperature versus time illustrating a two-step annealing process that combines the two-step thermal process with an optional zinc-based hot-dipped coating operation in a single production facility.
• Fig. 4 is a plot of temperature versus time for a second step of an annealing process defining soaking and holding zones in the thermal cycle.
• Figs. 5 and 6 are electron backscatter diffraction (EBSD) photomicrographs illustrating the microstructure of a high strength steel sheet product.
• EBSD electron backscatter diffraction
• Fig. 7 is an optical photomicrograph of a steel sheet product subjected to the thermal process shown in Fig. 1, showing darker ferrite grains and lighter austenite grains.
• Fig. 8 is a bar graph illustrating aspect ratios of the austenite grains shown in Fig. 7.
• Figs. 9 and 10 are graphs for a high strength steel sheet product showing austenite and ferrite grain size distributions.
• Figs. 11 is EBSD photomicrograph illustrating the microstructure of a high strength steel sheet product processed as shown in Fig. 1.
• Figs. 12 and 13 are EBSD photomicrographs showing steel sheet products processed as shown in Fig. 2.
• Fig. 14 is an EBSD photomicrograph of a steel sheet product processed as shown in Fig. 3.
• Fig. 15 is a graph of total elongation vs. ultimate tensile strength for high strength steel sheet products in comparison with other steel sheet products processed outside the scope of the present invention.
• Fig. 16 is a graph of total elongation vs. ultimate tensile strength for high strength steel products produced in mill trials.
• Fig. 17 is a plot of temperature versus time for cold rolled and hot rolled substrates processed with a thermal cycle.
• Fig. 18 is an EBSD photomicrograph of a cold rolled steel sheet substrate subjected to the thermal process shown in Fig. 17 showing darker ferrite grains and lighter retained austenite grains.
• Fig. 19 is an EBSD photomicrograph of a hot rolled steel sheet substrate subjected to the thermal process shown in Fig. 17 showing darker ferrite grains and lighter retained austenite grains.
• Fig. 20 is a plot of temperature versus time illustrating the first step of a two- step thermal treatment process in which a hot rolled sheet is quenched on the runout table to form martensite in accordance with an embodiment of the present invention.
• Fig. 21 is a plot of temperature versus time illustrating the second step of a two- step thermal treatment process in accordance with an embodiment of the present invention in which the quench annealed sheet of Fig. 17 is subjected to a thermal cycling step.
• the high strength steel sheet products of the present invention have controlled compositions that, in combination with controlled annealing processes, produce desirable microstructures and favorable mechanical properties including high strengths and ultra-high formabilities.
• the steel composition may include carbon, manganese and silicon, along with any other suitable alloying additions known to those skilled in the art. Examples of steel compositions including ranges of C, Mn, Si, Al, Ti, and Nb are listed in Table 1 below.
• the steel compositions may include minor or impurity amounts of other elements, such as 0.015 max S, 0.03 max P, 0.2 max Cu, 0.02 max Ni, 0.2 max Cr, 0.2 max Mo, 0.1 max Sn, 0.015 max N, 0.1 max V, and 0.004 max B.
• minor or impurity amounts of other elements such as 0.015 max S, 0.03 max P, 0.2 max Cu, 0.02 max Ni, 0.2 max Cr, 0.2 max Mo, 0.1 max Sn, 0.015 max N, 0.1 max V, and 0.004 max B.
• substantially free when referring to the composition of the steel sheet product, means that a particular element or material is not purposefully added to the composition, and is only present as an impurity or in trace amounts.
• C provides increased strength and promotes the formation of retained austenite.
• Mn provides hardening and acts as a solid solution strengthener.
• Si inhibits iron carbide precipitation during heat treatment, and increases austenite retention.
• Al inhibits iron carbide precipitation during heat treatment, and increases austenite retention.
• Ti and Nb may act as a strength-enhancing grain refiners.
• Al may be present in an amount of at least 0.1 weight percent or at least 0.2 weight percent.
• Al may be present in an amount of from 0.5 to 1.2 weight percent, or from 0.7 to 1.1 weight percent, in certain embodiments.
• the steel sheet product may be substantially free of Al.
• the ultimate tensile strength (UTS) of the steel sheet products range from 700 to 1,100 MPa or more. In certain embodiments, the steel sheet product has an ultimate tensile strength of greater than 700 MPa, for example, from 720 to 1,100 MPa, or from 750 to 1,050 MPa.
• the steel sheet products have a total elongation (TE) typically greater than 22 percent, for example, greater than 27 percent, or greater than 33 percent.
• TE total elongation
• the steel sheet product may have a total elongation of at least 20% or at least 25% or at least 27%, e.g., from 22 to 45%, or from 25 to 40%.
• the steel sheet products may possess lambda (l) values as measured by a standard hole expansion test typically greater than 20 percent, for example, greater than 25 percent, or greater than 30 percent, or greater than 35 percent.
• the whole expansion ratio or lambda may be greater than 20%, for example, from 22 to 80%, or from 25 to 60%.
• TE total elongation
• l hole expansion
• UTS-TE Strength elongation balances
• the final microstructure of the steel sheet products may primarily comprise ferrite, e.g., at least 50% up to 80% or higher, with lesser amounts of retained austenite, e.g., from 5 to 25%, and minor amounts of fresh martensite, e.g., from 0 to 10% or 15%.
• the amounts of ferrite, austenite and martensite may be determined by standard EBSD techniques.
• retained austenite content may be determined by magnetic saturation methods. Unless otherwise specified herein, the volume percent of retained austenite is determined by the EBSD technique.
• the retained austenite comprises from 1 to 25 volume percent, for example, from 5 to 20 volume percent.
• the amount of fresh martensite may comprise less than 15 volume percent, or less than 10 volume percent, or less than 5 volume percent.
• the steel sheet product is substantially free of fresh martensite. It has been found that when fresh martensite amounts are greater than 15%, hole expansion values decrease significantly, e.g., local formability is significantly decreased.
• At least a portion of the ferrite may be formed during the soaking or heating section, as described below, by recrystallization and/or tempering of martensite, or during the cooling and holding section of the thermal cycling process by austenite decomposition.
• Some of the ferrite may be considered bainitic ferrite.
• the ferrite, austenite and martensite phases are fine grained, e.g., having average grain sizes of less than 10 microns, for example, less than 5 microns, or less than 3 microns.
• ferrite grain size may range from less than 10 microns, for example, less than 8 microns, or less than 6 microns.
• Average austenite grain size may range from less than 2 microns, for example, less than 1 micron, or less than 0.5 micron. Martensite grain size, when present, may range from less than 10 microns, for example, less than 8 microns, or less than 6 microns.
• the austenite grains may be substantially equiaxed, e.g., having average aspect ratios less than 3: 1 or less than 2: 1, for example, about 1 : 1. It has been found that amounts of retained austenite below about 5% results in significantly decreased total elongations (TE). It has further been found that amounts of retained austenite above 25% can only be obtained at very high carbon levels, which results in poor weldability.
• a two-step thermal treatment process is used to produce advanced high strength steel products with favorable mechanical properties, such as those described above.
• the first step is performed on a hot strip mill after a steel slab has been continuously cast and hot rolled including a finish rolling step on the hot strip mill.
• the finish rolled steel sheet is quenched on the hot strip mill to form a
• the second step includes soaking the steel sheet in an intercritical regime followed by holding at a lower temperature.
• the second step can be performed on a continuous annealing line or continuous galvanizing line.
• the final steel sheet product comprises ferrite and retained austenite, i.e., having a microstructure that is predominantly ferrite and retained austenite.
• the steel sheet products may be produced on a combined continuous casting and hot rolling line in which molten steel is continuously cast to form a slab of steel that may be subjected to initial rough hot rolling to reduce thickness, followed by final hot rolling to further reduce thickness.
• the continuous casting/hot rolling line may comprise an endless strip production (ESP) process or the like.
• ESP endless strip production
• the sheet may be fed to a runout cooling table where it may be subjected to a quenching process to a temperature below MF, e.g., using a thermomechanical schedule as shown in Fig. 20.
• the resultant quenched hot rolled sheet may then be subjected to the second step of the process, e.g., by adopting a two-stage thermal cycle as shown in Fig. 21.
• Step 1 of the two-step process may be performed using continuous casting and hot rolling equipment and processes as disclosed in Arvedi U.S. Patent Nos. 5,329,688; 5,497,821; 6,125,916; 7,343,961; 7,832,460; 7,967,056; 8,025,092; 8,162,032; 8,257,647; and 9,186,721, which are incorporated herein by reference.
• Figs. 1 represents a continuous annealing line (CAL) followed by a continuous annealing line (CAL) production route.
• Fig. 2 represents a CAL plus continuous galvanizing line (CGL) production route.
• Fig. 3 represents a specially designed line allowing for both CAL + CAL or CAL + CGL steps to take place in a single facility. While a direct-fired furnace (DFF) followed by a radiant tube (RT) furnace embodiment is shown in Fig. 3, other embodiments such as all radiant tube, electric radiant heating, and the like may be used to achieve the desired thermal cycles.
• DFF direct-fired furnace
• RT radiant tube
• the goal of the first step of the process is to achieve a predominantly martensitic microstructure in the continuously cast and hot rolled steel sheet product.
• the hot rolled steel sheet has a finish rolling temperature above the A3, for example, a finish rolling temperature of at least 820°C may be provided on the hot strip mill prior to quenching.
• the quench annealing temperature may typically range from 830 to 980°C, for example, from 830 to 940°C, or from 840 to 930°C, or from 860 to 925°C.
• the continuously cast and finish rolled steel sheet is quenched to room temperature, or to a controlled temperature above room temperature, as more fully described below.
• the quench temperature may not necessarily be room temperature but should be below the martensite start temperature (M s ), and preferably below the martensite finish temperature (MF), to form a microstructure of predominantly martensite.
• Quenching to below MF may be accomplished by conventional techniques such as water quenching, submerged knife/nozzle water quenching, gas cooling, rapid cooling using a combination of cold, warm or hot water and gas, water solution cooling, other liquid or gas fluid cooling, chilled roll quench, water mist spray, wet flash cooling, non-oxidizing wet flash cooling, and the like.
• water quenching may be used in Step 1 as is typically used to cool the strip after finish rolling on the runout table of a conventional hot strip mill, CSP mill, or ESP mill.
• a quench rate of from 30 to l,000°C/sec may typically be used.
• cooling and quenching systems and processes may be adapted for use in the processes of the present invention.
• Suitable cooling/quenching systems and processes conventionally used on a commercial basis may include water quench, water mist cooling, dry flash and wet flash, oxidizing and non oxidizing cooling, alkane fluid to gas phase change cooling, hot water quenching, including two-step water quenching, roll quenching, high percentage hydrogen or helium gas jet cooling, and the like.
• dry flash and/or wet flash oxidizing and non-oxidizing cooling/quenching such as disclosed in published PCT Application No. W02015/083047 to Fives Stein may be used.
• the martensite can be optionally tempered to soften the steel somewhat to make further processing more feasible. Tempering may take place in a continuous annealing process by raising the temperature of the steel in the range of room temperature to about 500°C, e.g., from 30 to 500°C, and holding for up to 600 seconds, e.g., from 1 to 600 seconds.
• Tempering may also take place using a batch annealing process, where the same temperature could be reached over a period of several days.
• the intermediate batch tempering may be performed at a temperature of from 125 to 500°C for up to seven days. If tempering is utilized, the tempering temperature may be held constant, or may be varied within this preferred range.
• the temperature may be ramped down to room temperature.
• the rate of such ramp-down may typically range up to 40°C/sec, for example, from 0.1 to 20°C/sec.
• the second step of the thermal treatment process may include a first soak stage that is conducted at relatively high soaking temperature, and a second hold stage that is conducted at relatively low temperature. These stages may be defined as the“soaking” and “holding” zones, as described in Figs. 4 and 21.
• the temperatures are controlled in order to promote the formation of the desired microstructure in the final product.
• a soaking zone temperature in an intercritical regime between Ai and A3 may be used, for example, a soak temperature of at least 720°C may be used.
• the soak temperature may typically range from 720 to 850°C, for example, from 760 to 825°C.
• the peak annealing temperature may be typically held for at least 15 seconds, for example, from 20 to 300 seconds, or from 30 to 150 seconds.
• the soaking zone temperature may be achieved by heating the steel from a relatively low temperature below M s , e.g., room temperature, at an average rate of from 0.5 to 50°C/sec, for example, from about 2 to 20°C/sec.
• the ramp-up may take from 25 to 800 seconds, for example, from 100 to 500 seconds.
• the first stage heating of the second step may be accomplished by any suitable heating system or process, such as using radiant heating, induction heating, direct fired furnace heating and the like.
• the steel may be cooled to a controlled temperature above room temperature to the holding zone.
• the steel sheet product is maintained at a temperature above 300°C between the second step soaking process and the second step holding process.
• Cooling from the soaking to holding zone may be accomplished by conventional techniques such as water cooling, gas cooling and the like. An average cooling rate of from 5 to 400°C/sec may typically be used. Any suitable types of cooling and quenching systems may be adapted for use in cooling from the soaking temperature to the holding temperature, including those described above.
• the holding zone step is carried out at a typical temperature of from 360 to 445°C, for example, from 370 to 440°C.
• the holding zone may be held for up to 800 seconds, for example, from 30 to 600 seconds.
• the holding zone temperature may be held constant, or may be varied somewhat within the preferred temperature range.
• the steel may be reheated, such as by induction or other heating method, to enter a hot-dip coating pot at the proper temperature for good coating results, if the steel is to be hot-dip coated.
• the temperature may be ramped down to room temperature.
• a ramp-down may typically take from 10 to 1,000 seconds, for example, from about 20 to 500 seconds.
• the rate of such ramp-down may typically range from 1 to l,000°C/sec, for example, from 2 to 20°C/sec.
• heating systems that may be adapted for use in the soak and hold thermal cycling step of the present invention are disclosed in U.S. Patent Nos. 5,798,007; 7,368,689; 8,425,225; and 8,845,324, U.S. Patent Application No. 2009/0158975, and Published PCT Application No. WO/2015083047, assigned to Fives Stein. Additional examples of heating systems that may be adapted for use in the soak and hold thermal cycling step of the present invention include U.S. Patent No. 7,384,489 assigned to Drever
• the second-step thermal cycling processes may be performed on a continuous annealing line (CAL). After going through a CAL process, the steel may be electrogalvanized to produce a zinc based coated product.
• CAL continuous annealing line
• the annealed steel sheet is hot-dip galvanized at the end of the holding zone.
• Galvanizing temperatures may typically range from 440 to 480°C, for example, from 450 to 470°C.
• the galvanizing step may be performed as part of the second-step holding zone process on a continuous galvanizing line (CGL), e.g., as shown in Fig. 2.
• CGL continuous galvanizing line
• This CGL process can be used to produce both a zinc-based or zinc alloy -based hot-dip galvanized product or reheated after coating to produce an iron- zinc galvanneal type coated product.
• An optional nickel-based coating step can take place prior to the CGL step in the process to improve zinc coating properties.
• the use of a continuous galvanizing line in the second step may increase the efficiency of producing coated GEN3 products.
• the microstructure of the resultant product is shown in Figs. 5 and 6.
• EBSD techniques using commercial ED AX orientation imaging microscopy software show the dark ferrite grains and light austenite grains in Fig. 5.
• the microstructure of the resultant product is shown in Fig. 11.
• Mechanical properties of Sample No. 2 are listed in Table 2.
• Grain size distributions of austenite and ferrite are shown in Figs. 9 and 10, respectively.
• the average austenite grain size is less than 1 micron and the average ferrite grain size is less than 10 microns.
• the microstructure includes about 80 volume percent ferrite with an average grain size of about 5 microns, about 10 volume percent retained austenite having substantially equiaxed grains and an average grain size of about 0.5 micron, and about 10 volume percent fresh martensite with an average grain size of about 5 microns.
• Mechanical properties of Sample No. 1 are listed in Table 2 below.
• Fig. 7 is an optical image showing the microstructure of the steel shown in Fig. 2, Sample No. 6, which was subjected to the two- step annealing process shown in Fig. 1.
• the dark regions of the photomicrograph are ferrite grains, while the light regions are austenite grains.
• Fig. 8 is a graph illustrating the aspect ratios of the austenite grains shown in Fig. 7.
• the optical image of Fig. 7 was used to determine the aspect ratios of the austenite grains using image analysis with commercially available software. Fig. 7 shows that the average aspect ratio is less than 3: 1 for the austenite grains.
• Comparative Examples 1-4 exhibited UTS levels less than 700 MPa.
• Comparative Examples 5-8 exhibited UTS levels greater than 1,100 MPa.
• a cold rolled sheet having a composition listed in Table 2 Sample No. C12, was subjected to a two-step annealing process similar to that illustrated in Fig. 2, except the holding zone temperature in the second anneal was outside the preferred range of the invention.
• Mechanical properties of Sample No. C12 are listed in Table 2.
• Fig. 15 plots the total elongation (TE) and ultimate tensile strength (UTS) of Samples 1-8 of Examples 1-8, as well as Samples C1-C12 of Comparative Examples C1-C12. A line corresponding to UTS'TE of 25,000 is roughly drawn in Fig. 15.
• the high strength steel sheet samples produced in accordance with the present invention possess superior combinations of strength and elongation versus the comparative samples, i.e., high total elongation properties at high UTS levels are observed for the inventive examples.
• the Sample 1 through Sample 8 steels fall into the category of Generation 3 advanced high strength steels, which are highly desirable for automotive and other industries.
• Mill trials were conducted for samples labeled M1-M5 in Table 3 below using either the CAL+CAL or CAL+CGL process.
• Samples Ml, M2 and M5 the CAL+CAL processing times and temperatures shown in Fig. 1 were used.
• Samples M3 and M4 the CAL+CGL processing times and temperatures shown in Fig. 2 were used.
• Fig. 16 shows the strength-elongation balance of the mill trial materials, all meeting a minimum UTS-TE of 25,000.
• the trial materials exhibited lambda values greater than 20%.
• Figs. 18 and 19 show similar austenite contents, distributions and morphologies.
• Fig. 18 shows the austenite content of cold rolled sample 11 A
• Fig. 19 shows the austenite content of hot rolled sample 12A.
• a fine, predominantly equiaxed distribution of austenite is observed in both microstructures.
• Step 1 martensite is formed on a hot strip mill by water quenching after finish rolling using the thermomechanical schedule shown in Fig. 20.
• An overageing step (not shown) could optionally be added at the end of this thermal cycle to soften and toughen the hot band for subsequent processing.
• Step 2 a thermal cycle as shown in Fig. 21 is then performed.
• any numerical range recited herein is intended to include all sub-ranges subsumed therein.
• a range of“1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.
• the use of the singular includes the plural and plural encompasses singular, unless specifically stated otherwise.
• the use of“or” means“and/or” unless specifically stated otherwise, even though“and/or” may be explicitly used in certain instances.
• the articles“a,” “an,” and“the” include plural referents unless expressly and unequivocally limited to one referent.

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