Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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  • 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
  • C21D1/78—Combined heat-treatments not provided for above
  • C21D1/785—Thermocycling
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • 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
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  • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • 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
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  • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • C—CHEMISTRY; METALLURGY
  • 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
  • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
  • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
• • 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
  • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
  • C23C2/36—Elongated material
  • C23C2/40—Plates; 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
• • 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
  • C21D2211/008—Martensite
• • 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

Definitions

• the present invention relates to steel compositions and processing methods for production of steel using hot-dip galvanizing/galvannealing (HDG) processes such that the resulting steel exhibits high strength and cold formability.
• HDG hot-dip galvanizing/galvannealing
• the present steel is produced using a composition and a modified HDG process that together produces a resulting microstructure consisting of generally martensite and austenite (among other constituents).
• the composition includes certain alloying additions and the HDG process includes certain process modification, all of which are at least partially related to driving the transformation of austenite to martensite followed by a partial stabilization of austenite at room-temperature.
• FIGURE 1 depicts a schematic view of a HDG temperature profile with a
• partitioning step performed after galvanizing/galvannealing.
• FIGURE 2 depicts a schematic view of a HDG temperature profile with a
• FIGURE 3 depicts a plot of one embodiment with Rockwell hardness plotted against cooling rate.
• FIGURE 4 depicts a plot of another embodiment with Rockwell hardness plotted against cooling rate.
• FIGURE 5 depicts a plot of another embodiment with Rockwell hardness plotted against cooling rate.
• FIGURE 6 depicts six photo micrographs of the embodiment of FIG. 3 taken from samples being cooled at various cooling rates.
• FIGURE 7 depicts six photo micrographs of the embodiment of FIG. 4 taken from samples being cooled at various cooling rates.
• FIGURE 8 depicts six photo micrographs of the embodiment of FIG. 5 taken from samples being cooled at various cooling rates.
• FIGURE 9 depicts a plot of tensile data as a function of austenitization
• FIGURE 10 depicts a plot of tensile data as a function of austenitization temperature for several embodiments.
• FIGURE 11 depicts a plot of tensile data as a function of quench temperature for several embodiments.
• FIGURE 12 depicts a plot of tensile data as a function of quench temperature for several embodiments.
• FIG. 1 shows a schematic representation of the thermal cycle used to achieve high strength and cold formability in a steel sheet having a certain chemical composition (described in greater detail below).
• FIG. 1 shows a typical hot-dip galvanizing or galvannealing thermal profile (10) with process modifications shown with dashed lines.
• the process generally involves austenitization followed by a rapid cooling to a specified quench temperature to partially transform austenite to martensite, and the holding at an elevated temperature, a partitioning temperature, to allow carbon to diffuse out of martensite and into the remaining austenite, thus, stabilizing the austenite at room temperature.
• the thermal profile shown in FIG. 1 may be used with conventional continuous hot-dip galvanizing or galvannealing production lines, although such a production line is not required.
• the steel sheet is first heated to a peak metal temperature
• the peak metal temperature (12) in the illustrated example is shown as being at least above the austenite transformation temperature (Ay) (e.g., the dual phase, austenite + ferrite region). Thus, at the peak metal temperature (12), at least a portion of the steel will be transformed to austenite.
• FIG. 1 shows the peak metal temperature (12) as being solely above Aj, it should be understood that in some embodiments the peak metal temperature may also include temperatures above the temperature at which ferrite completely transforms to austenite (A 3 ) (e.g., the single phase, austenite region).
• the steel sheet undergoes rapid cooling.
• some embodiments may include a brief interruption in cooling for galvanizing or galvannealing.
• the steel sheet may briefly maintain a constant temperature (14) due to the heat from the molten zinc galvanizing bath.
• a galvannealing process may be used and the temperature of the steel sheet may be slightly raised to a galvannealing temperature (16) where the galvannealing process may be performed.
• the galvanizing or galvannealing process may be omitted entirely and the steel sheet may be continuously cooled.
• the rapid cooling of the steel sheet is shown to continue below the martensite start temperature (M s ) for the steel sheet to a predetermined quench temperature (18).
• M s martensite start temperature
• the cooling rate to M s may be high enough to transform at least some of the austenite formed at the peak metal temperature (12) to martensite.
• the cooling rate may be rapid enough to transform austenite to martensite instead of other non-martensitic constituents such as ferrite, pearlite, or bainite which transform at relatively lower cooling rates.
• the quench temperature (18) is below M s .
• the difference between the quench temperature (18) and M s may vary depending on the individual composition of the steel sheet being used. However, in many embodiments the difference between quench temperature (18) and M s may be sufficiently great to form an adequate amount of martensite to act as a carbon source to stabilize the austenite and avoid creating excessive "fresh" martensite upon final cooling. Additionally, quench temperature (18) may be sufficiently high to avoid consuming too much austenite during the initial quench (e.g., to avoid excessive carbon enrichment of austenite greater than that required to stabilize austenite for the given embodiment).
• quench temperature (18) may vary from about 191 °C to about 281 °C, although no such limitation is required. Additionally, quench temperature (18) may be calculated for a given steel composition. For such a calculation, quench temperature (18) corresponds to the retained austenite having an M s temperature of room temperature after partitioning. Methods for calculating quench temperature (18) are known in the art and described in J. G. Speer, A. M. Streicher, D. K. Matlock, F. Rizzo, and G. Krauss, "Quenching And Partitioning : A Fundamentally New Process to Create High Strength Trip Sheet Microstructures," Austenite Formation and Decomposition, pp. 505-522, 2003; and A. M.
• the quench temperature (18) may be sufficiently low (with respect to M s ) to form an adequate amount of martensite to act as a carbon source to stabilize the austenite and avoid creating excessive "fresh" martensite upon the final quench.
• the quench temperature (18) may be sufficiently high to avoid consuming too much austenite during the initial quench and creating a situation where the potential carbon enrichment of the retained austenite is greater than that required for austenite stabilization at room temperature.
• a suitable quench temperature (18) may correspond to the retained austenite having an M s temperature of room temperature after partitioning. Speer and Streicher et al. (above) have provided calculations that provide guidelines to explore processing options that may result in desirable microstructures. Such calculations assume idealized full partitioning, and may be performed by applying the
• the result of the calculations described by Speer et al. may indicate a quench temperature (18) which may lead to a maximum amount of retained austenite.
• quench temperatures (18) above the temperature having a maximum amount of retained austenite significant fractions of austenite are present after the initial quench; however, there is not enough martensite to act as a carbon source to stabilize this austenite. Therefore, for the higher quench temperatures, increasing amounts of fresh martensite form during the final quench.
• For quench temperatures below the temperature having a maximum amount of retained austenite an unsatisfactory amount of austenite may be consumed during the initial quench and there may be an excess amount of carbon that may partition from the martensite.
• the temperature of the steel sheet is either increased relative to the quench temperature or maintained at the quench temperature for a given period of time.
• this stage may be referred to as the partitioning stage.
• the temperature of the steel sheet is at least maintained at the quench temperature to permit carbon diffusion from martensite formed during the rapid cooling and into any remaining austenite. Such diffusion may permit the remaining austenite to be stable (or meta-stable) at room temperature, thus improving the mechanical properties of the steel sheet.
• the steel sheet may be heated above M s to a relatively high partitioning temperature (20) and thereafter held at the high partitioning temperature (20).
• a variety of methods may be utilized to heat the steel sheet during this stage.
• the steel sheet may be heated using induction heating, torch heating, and/or the like.
• the steel sheet may be heated but to a different, lower partitioning temperature (22) which is slightly below M s .
• the steel sheet may then be likewise held at the lower partitioning temperate (22) for a certain period of time.
• another alternative partitioning temperature (24) may be used where the steel sheet is merely maintained at the quench temperature.
• any other suitable partitioning temperature may be used as will be apparent to those of ordinary skill in the art in view of the teachings herein.
• FIG. 2 shows an alternative embodiment of the thermal cycle described above with respect to FIG. 1 (with a typical galvanizing/galvannealing thermal cycle shown with a solid line (40) and departures from typical shown with a dashed line).
• the steel sheet is first heated to a peak metal temperature (42).
• the peak metal temperature (42) in the illustrated embodiment is shown as being at least above A ⁇ .
• the present embodiment may also include a peak metal temperature in excess of A 3 .
• the steel sheet may be rapidly quenched (44). It should be understood that the quench (44) may be rapid enough to initiate transformation of some of the austenite formed at the peak metal temperature (42) into martensite, thus avoiding excessive transformation to non-martensitic constituents such as ferrite, pearlite, banite, and/or the like.
• the quench (44) may be then ceased at a quench temperature (46).
• a quench temperature 46.
• quench temperature (46) is below M s .
• the amount below Ms may vary depending upon the material used.
• the difference between quench temperature (46) and M s may be sufficiently great to form an adequate amount of martensite yet be sufficiently low to avoid consuming too much austenite.
• the steel sheet is then subsequently reheated (48) to a partitioning temperature
• the partitioning temperature (50, 52) in the present embodiment may be characterized by the galvanizing or galvannealing zinc bath temperature (if galvanizing or galvannealing is so used).
• the steel sheet may be re-heated to the galvanizing bath temperature (50) and subsequently held there for the duration of the galvanizing process.
• partitioning may occur similar to the partitioning described above.
• the galvanizing bath temperature (50) may also function as the partitioning temperature (50).
• the process may be substantially the same with the exception of a higher bath/partitioning temperature (52).
• the steel sheet is permitted to cool (54) to room temperature where at least some austenite may be stable (or meta-stable) from the partitioning step described above.
• the steel sheet may include certain alloying additions to improve the propensity of the steel sheet to form a primarily austenitic and martensitic microstructure and/or to improve the mechanical properties of the steel sheet.
• Suitable compositions of the steel sheet may include one or more of the following, by weight percent: 0.15-0.4% carbon, 1.5-4% manganese, 0-2% silicon or aluminum or some combination thereof, 0-0.5% molybdenum, 0-0.05%) niobium, other incidental elements, and the balance being iron.
• suitable compositions of the steel sheet may include one or more of the following, by weight percent: 0.15-0.5%) carbon, 1-3% manganese, 0-2%> silicon or aluminum or some combination thereof, 0-0.5% molybdenum, 0-0.05% niobium, other incidental elements, and the balance being iron.
• other embodiments may include additions of vanadium and/or titanium in addition to, or in lieu of niobium, although such additions are entirely optional.
• carbon may be used to stabilize austenite. For instance, increasing carbon may lower the Ms temperature, lower transformation temperatures for other non-martensitic constituents (e.g., bainite, ferrite, pearlite), and increase the time required for non-martensitic products to form. Additionally, carbon additions may improve the hardenability of the material thus retaining formation of non-martensitic constituents near the core of the material where cooling rates may be locally depressed. However, it should be understood that carbon additions may be limited as significant carbon additions may lead to detrimental effects on weldability. [0037] In some embodiments manganese may provide additional stabilization of austenite by lowering transformation temperatures of other non-martensitic constituents, as described above. Manganese may further improve the propensity of the steel sheet to form a primarily austenitic and martensitic microstructure by increasing hardenability.
• non-martensitic constituents e.g., bainite, ferrite, pearlite
• molybdenum may be used to increase hardenability.
• silicon and/or aluminum may be provided to reduce the formation of carbides. It should be understood that a reduction in carbide formation may be desirable in some embodiments because the presence of carbides may decrease the levels of carbon available for diffusion into austenite. Thus, silicon and/or aluminum additions may be used to further stabilize austenite at room temperature.
• nickel, copper, and chromium may be used to stabilize austenite. For instance, such elements may lead to a reduction in the M s temperature. Additionally, nickel, copper, and chromium may further increase the hardenability of the steel sheet.
• niobium or other micro-alloying elements, such as
• titanium, vanadium, and/or the like may be used to increase the mechanical properties of the steel sheet.
• niobium may increase the strength of the steel sheet through grain boundary pinning resulting from carbide formation.
• EXAMPLE 2 The Rockwell hardness of each of the steel compositions described in Example 1 and Table 1 above was taken on the surface of each sample. The results of the tests are plotted in FIGS. 3-5 with Rockwell hardness plotted as a function of cooling rate. The average of at least seven measurements is shown for each data point.
• the compositions V4037, V4038 and V4039 correspond to FIGS. 3, 4, and 5, respectively.
• Example 1 The results of these tests are shown in FIGS. 6-8.
• the compositions V4037, V4038, and V4039 correspond to FIGS. 6, 7, and 8, respectively.
• FIGS. 6-8 each contain six micrographs for each composition with each micrograph representing a sample subjected to a different cooling rate.
• a critical cooling rate for each of the compositions of Example 1 was estimated using the data of Examples 2 and 3 in accordance with the procedure described herein.
• the critical cooling rate herein refers to the cooling rate required to form martensite and minimize the formation of non-martensitic transformation products. The results of these tests are as follows:
• Embodiments of the steel sheet were made with the compositions set forth in
• the materials were processed by melting, hot rolling, and cold rolling.
• the materials were processed by melting, hot rolling, and cold rolling.
• Example 5 The compositions of Example 5 were subjected to Gleeble dilatomety. Gleeble dilatomety.
• dilatomety was performed in vacuum using a 101.6x25.4x1 mm samples with a c- strain gauge measuring dilation in the 25.4 mm direction. Plots were generated of the resulting dilation vs. temperature. Line segments were fit to the dilatometric data and the point at which the dilatometric data deviated from linear behavior was taken as the transformation temperature of interest (e.g., Aj, A 3 , M s ). The resulting transformation temperatures are tabulated in Table 5.
• Gleeble methods were also used to measure a critical cooling rate for each of the compositions of Example 5.
• the first method utilized Gleeble dilatomety, as described above.
• the second method utilized measurements of Rockwell hardness.
• Rockwell hardness measurements were taken.
• Rockwell hardness measurements were taken for each material composition with a measurement of hardness for a range of cooling rates.
• a comparison was then made between the Rockwell hardness measurements of a given composition at each cooling rate.
• Example 5 The compositions of Example 5 were used to calculate quench temperature and a theoretical maximum of retained austenite. The calculations were performed using the methods of Speer et al. , described above. The results of the calculations are tabulated below in Table 6 for some of the compositions listed in Example 5.
• EXAMPLE 9 The samples of the compositions of Example 5 were subjected to the thermal profiles shown in FIGS. 1 and 2 with peak metal temperature and quench temperature varied between samples of a given composition. As described above, only composition V4039 was subjected to the thermal profile shown in FIG. 1, while all other compositions were subjected to the thermal cycle shown in FIG. 2. For each sample, tensile strength measurements were taken. The resulting tensile measurements are plotted in FIGS. 9-12. In particular, FIGS. 9-10 show tensile strength data plotted against austenitization temperature and FIGS. 11-12 show tensile strength data plotted against quench temperature. Additionally, where the thermal cycles were performed using Gleeble methods, such data points are denoted with "Gleeble.” Similarly, where thermal cycles were performed using a salt bath, such data points are denoted with "salt.”

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