Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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
  • C21D1/18—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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold 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
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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
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
• • 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • 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
  • C23C2/02—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
  • C23C2/04—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
  • C23C2/12—Aluminium 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
  • C23C2/34—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
  • 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
  • C21D2211/008—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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling

Definitions

• the present invention relates to steel sheet.
• the present invention relates to steel sheet that can be hot formed into parts having uniform, very high tensile strength and high oidability,
• Modern vehicles contain an increasing portion of high- strength and ultra-high- strength steels in order to improve passenger safety and reduce vehicle weight.
• the configuration of many formed vehicle body parts prevents the use of cold formed advanced high-strength steels.
• hot forming followed by quenching to a martensite condition has become a popular means for producing uitra-high-strength steel parts.
• Special steels are used for hot stamping to ensure necessary hardenability to fit operational parameters. Many of these special steels are designed for quenching in water cooled dies.
• USIBOR which contains (in % by weight or wt%) 0.15-0.25%C, 0,8- 1 , 5%Mn, 0.1-0.35%Si, 0.01-0.2%Cr, less than 0.1%Ti, less than 0.1 %A1, less than 0.05%P, less than 0.03%S, and 0.0005-0.01 %B.
• This chemistry is encompassed by the steel disclosed in U.S. Patent No. 6,296,805.
• Ti and B are necessary to achieve high mechanical properties after hot pressing in a water cooled die.
• the manufacture of high-strength parts from USIBOR is described in U.S. Patent No. 6,564,604.
• the process includes heating hot rolled or cold roiled blanks above 700°C in a furnace, transferring heated blanks to dies, press forming the blanks in the die and keeping the water cooled die, with the formed blank in it, closed until the part reaches room temperature. Rapid cooling in the water cooled die, i.e. quenching, is necessary to obtain the martensite structure and hence high strength.
• the quenched steel might have been coated with Zn or Al-Si prior to heat treating for hot stamping via a continuous hot dip coating process to protect the steel substrate from oxidation during hot stamping and from subsequent corrosion attack.
• US1BOR is widely used for hot stamping and can achieve a tensile strength of 1500 MPa after quenching in a water cooled die
• USIBOR has a number of disadvantages.
• One disadvantage is that USIBOR containing 0.25 wt% C has poor voidability.
• the microstructure of USIBOR is highly sensitive to cooling rate and displays ferrite or bainite formation if cooling rates in the water cooled die are slow, hence uniform distribution of strength across hot stamped parts may not be guaranteed.
• the hot stamping process using USIBOR is generally long and the productivity of the expensive equipment used for hot stamping is relatively low.
• the ductility (e.g, elongation) of USIBOR having a tensile strength greater than 1500 MPa is relatively low.
• Air hardening steels are also well known.
• WO2006/048009 discloses air-hardenable steel containing, in tnass%, 0.07-0.15% C, 0.15-0.30% Si, 1.60-2.10% Mn, 0.5-1.0% Cr, 0.30-0.60% Mo, 0.12-0.20% V, 0.010-0.050% Ti and 0.0015-0.0040% B.
• the steel can be readily welded and galvanized. It exhibits high strength, e.g., a yield strength of 750-850 MPa and a tensile strength of 850-1000 MPa.
• the steel has the disadvantage of using large amounts of expensive elements such as Mo and V.
• Patent application publication DE 102 61 210 Al describes another air- hardenable steel alloy for the production of automobile parts in a hot pressing process.
• the alloy contains, in mass%, 0.09-0.13% C, 0.15-0.3% Si, 1.1-1.6% Mn, max 0.015% P, max 0.01 1% S, 1.0-1.6% Cr, 0.3-0.6% Mo, 0.02-0.05% Al and 0.12-0.25% V.
• the steel exhibits a yield strength of 750-1100 MPa, a tensile strength of 950-1300 MPa, and an elongation of 7-16%.
• One disadvantage of this steel is the necessity of using a large amount of expensive Mo and V.
• Unexamined Japanese Patent Application No. 2006-213959 provides a method for manufacturing hot press, high-strength, steel members with excellent productivity.
• the method uses steel sheet that contains, in mass %, 0.05 to 0.35% C, 0.005 to 1.0% Si, 0 to 4.0 Mn, 0 to 3.0% Cr, 0 to 4.0% Cu, 0 to 3.0% Ni, 0.0002 to 0.1% B, 0.001 to 3.0% Ti, ⁇ 0.1 % P, ⁇ 0.05% S, 0.005 to 0.1 % Al and ⁇ 0.01% N, with the balance Fe and inevitable impurities, where Mn+Cr/3. l+(Cu+Ni)/1.4 > 2.5%.
• the steel sheet is heated at 750-1300°C for 10-6000 seconds, and then is press-formed at 300°C or more. After pressing, ihe molded product is removed from the mold and is cooled from 1200-1 100°C down to 5-40°C at a cooling speed of 0.1°C/second or more to yield members having a martensite structure of 60% or more in area ratio.
• the step of quenching in the press mold can he eliminated.
• the members obtained have little material quality variation internally, and the shape of the members is good, with excellent uniformity.
• Unexamined Japanese Patent Application No, 2006-212663 provides a method of manufacturing hot press high-strength steel members of excellent formability.
• the method uses steel sheet that contains, in mass%, 0.05 to 0.35% C, 0,005 to 1.0% Si, 0 to 4.0% Mn, 0 to 3.0% Cr, 0 to 4.0% Cu, 0 to 3.0% Ni, 0.0002 to 0.1 % B, 0.001 to 3.0% Ti, ⁇ 0.1 % P, ⁇ 0.05% S, 0.005 to 0.1 % Al and ⁇ 0.01 % N, with a balance of Fe and inevitable impurities, where Mn+Cr 3. i+(Cu+Ni) i .4 > 2.5.
• the steel sheet is heated to 750-1300°C, is kept there for 10-6000 seconds, and then is press-formed two or more times at 300°C to yield members having a martensite structure of 60% or more in area ratio.
• the resulting members exhibit high-strength and little variability in internal material quality.
• the tensile strength of steel is known to increase with C content.
• an increase in C content decreases weldability.
• the present invention provides a high tensile strength (800- 1400 MPa) steel sheet containing (in wt%) 0.04 ⁇ C ⁇ 0.30, 0.5 ⁇ Mn ⁇ 4, 0 ⁇ Cr ⁇ 4, 2.7 ⁇ Mn+Cr ⁇ 5, 0.003 ⁇ Nb ⁇ 0.1 , 0.015 ⁇ Al ⁇ 0.1 and 0.05 ⁇ Si ⁇ 1.0.
• the steel sheet can contain one or more of Ti ⁇ 0.2, V ⁇ 0.2, Mo ⁇ 0,3 and B ⁇ 0.01 .
• the steel sheet can be hot formed in a die and can be cooled in the die, or in a cooling medium such as air, nitrogen, oil or water.
• the chemistry of the steel makes the formed sheet insensitive to cooling rate and ensures a uniform distribution of strength across parts independent of the time delay between operations and final cooling/quenching.
• a Nb content from 0.003 to 0.1 wt% makes the tensile strength less sensitive to the amount of C and reduces the amount of C needed for same tensile strength. Furthermore, since a reduction in C improves voidability, the addition of Nb achieves the same high tensile strength as C alone but with improved wekiability.
• Coating the steel sheet with a coating of Zn, Al or Al alloy can improve the corrosion resistance of the steel sheet.
• FIG. 1 shows the change in tensile strength (MPa) with C for various steel sheet compositions when the amount of C ranges from 0,06 to 0.12 wt%, with and without Nb addition;
• FIG. 2 shows the change in tensile strength (MPa) with C for various steel sheet compositions when the amount of C ranges from 0.06 to 0.18 wt%, with and without Nb;
• FIG. 3 depicts a Continuous Cooling Transformation (CCT) diagram for a steel according to the present invention, plotting cooling curves as temperature in degrees C vs log of time in seconds;
• CCT Continuous Cooling Transformation
• FIGS. 4a-4d are photomicrographs, taken at varying magnifications, of a steel of the present invention cooled at different cooling rates;
• FIG. 5 is a plot of weld current vs sample number for steels of the present invention, the plot specifically shows the non-scatter of expulsion of the steel in spot welding.
• FIG. 6 is a collection of four (4) photomicrographs showing, from top to bottom and left to right, a complete spot weld of a steel of the present invention, a higher magnification of the base metal, heat affected zone, and the welded zone of the spot weld.
• the present invention provides a steel sheet that can be hot formed into a part having a uniform distribution of strength and improved weldability.
• the steel sheet is a low alloy steel composition and contains, in wt %, 0.04 ⁇ C ⁇ 0.30, 0.5 ⁇ Mn ⁇ 4, 0 ⁇ Cr ⁇ 4, 2.7 ⁇ Mn+Cr ⁇ 5, 0.003 ⁇ Nb - 0.10, 0.015 ⁇ Al ⁇ 0.1 and 0.05 ⁇ Si ⁇ 1.0.
• the steel sheet can contain one or more of Ti ⁇ 0.2, V ⁇ 0.5, Mo ⁇ 0.6 and B ⁇ 0.015. This chemistry makes a sheet that after hot forming is insensitive to cooling rate and ensures a uniform distribution of strength across parts independent of the time delay between operations and final cooling/quenching.
• the guaranteed uniformity of tensile properties regardless of cooling rate in specific locations of a formed part can substantially increase the productivity of hot forming.
• tensile strength increases with increasing C, the increase in C decreases weldahility.
• Nb the tensile strength increase can be maintained and weldability improved.
• concentrations of the various component elements of the steel sheets of the present invention are limited for the followings reasons.
• concentrations are given in weight % (i.e., wt%).
• the amount of C is limited to the range of from 0.04 to 0,30 wt %.
• the lower limit for the amount of C is 0.06 wt%, more preferably 0.08 wt%.
• the upper limit for the amount of C is 0.18 wt%, more preferably 0.16 wt%.
• the amount of Mn is limited to the range of from 0.5 to 4 wt%.
• the lower limit for the amount of Mn is 1 wt%, more preferably, 1.5 wt%.
• the upper limit for the amount of Mn is 3.5 wt%, more preferably 3.0 wt%.
• Chromium is important for improving quenchability. However, too much Cr will adversely affect manufacturability during manufacturing. Thus, the amount of Cr is limited to the range of from 0 to 4 wt%. Preferably, the lower limit for the amount of Cr is 0.2, more preferably, 0.5 wt%. Preferably, the upper limit for the amount of Cr is 3.5 wt%, more preferably 3.0 wt%.
• the combined amount of Mn and Cr is limited to the range of from 2.7 to 5 wt% in order to make the steel insensitive to cooling rate after forming and to ensure a uniform distribution of strength across parts independent of the time delay between operations and final cooling/quenching.
• the lower limit for Mn+Cr is 3.0, more preferably, 3.3 wt%.
• the upper limit for Mn+Cr is 4.7 wt%, more preferably 4.4 wt%.
• the amount of Nb is limited to the range of from 0.003 to 0.1 wt%.
• the lower limit for the amount of Nb is 0.005, more preferably, 0.010 wt%.
• the upper limit for the amount of Nb is 0.09 wt%, more preferably 0.085 wt%.
• the amount of Al is limited to the range of from 0.015 to 0.1 wt%.
• the lower limit for the amount of Al is 0.02, more preferably, 0.03 wt%.
• the upper limit for the amount of Al is 0.09 wt%, more preferably 0.08 wt%.
• the amount of Si is effective for increasing the strength of steel sheet. However, too much Si creates a problem of surface scale.
• the amount of Si is limited to the range of from 0.05 to 0.35 wt%.
• the lower limit for the amount of Si is 0.07, more preferably, 0.1 wt%.
• the upper limit for the amount of Si is 0.3 wt%, more preferably 0,25 wt%.
• Ti can be optionally added to the steel with B in an amount of ⁇ 0.1 wt% to improve quenchability.
• Ti combines with N at very high temperature, hence preventing BN formation.
• B in solution improves quenchability.
• Ti beyond the stoichiometric ratio to nitrogen is a carbide forming element. It strengthen steel by forming very fine carbides. It's effect is similar to Nb.
• V can be optionally added to the steel in an amount of ⁇ 0.2 wt% to increase the strength of the steel via fine precipitation. It also adds to hardenability of steel.
• Mo can be optionally added to the steel in an amount of ⁇ 0.3 wt% to increase strength and improve quenehabiiity.
• B can be optionally added to the steel in an amount of ⁇ 0.005 wt% to increase hardenability and hence strength of the steel.
• the steel also contains Fe and can contain unavoidable impurities.
• the steel sheet of the present invention has a martensitic microstructure that can include up to 10% lower bainite phase.
• the microstructure is predominantly martesnite.
• the amount of bainite can be up to 10%, preferably less than 5% and more preferably less than 1 %.
• the steel sheet of the present invention has a tensile strength in the range of
• the lower limit of the tensile strength is preferably 900 MPa, more preferably 1000 MPa.
• the final strength depends mostly on carbon content in martensite.
• the steel sheet of the present invention can exhibit an elongation in the range of from 4 to 9%, preferably 5 to 9%, more preferably 6 to 9%.
• the steel sheet of the present invention can be made by processes that begin with conventional steelmaking and casting processes and then follow with hot roiling.
• the cast slabs may be charged directly to a reheating furnace before hot rolling or cooled before doing so. There is no restriction on the finishing temperature in the hot rolling process other than that it should be above Ar 3 .
• the coiling temperature after hot rolling depends on the processing after hot rolling. If cold rolling is required to obtain the final thickness, then a coiling temperature between 700°C and 600°C is preferred. If the final required thickness can be obtained directly by hot roiling, then a coiling temperature between 600°C and 500°C is recommended.
• the hot rolled sheet can be pickled.
• the hot rolled sheet can be pickled before cold rolling to the required thickness.
• the hot rolled or cold roiled steel sheet can be protected from oxidation and/or corrosion by coating one or both sides of the steel sheet with Zn, Ai or an Al alloy, such as Al-Si.
• the coating can be performed by continuously hot dip coating the steel sheet.
• Steel sheets with or without coatings are heated to the temperature of full austenitization, i.e., to at least Ac 3 + 5°C, before being formed, e.g., by stamping, in one or several dies to the shape desired.
• the hot formed part is then cooled in a die or in a cooling medium such as air, nitrogen, oil or water. Different cooling media provide different cooling rates.
• the formed parts exhibit uniform martensite structure across the parts regardless of cooling rate.
• the final strength can be controlled by the chemistry (in particular, the amounts of C and Nb) and/or by heating below or above the temperature of full austemtization.
• FIG. 3 An example of such diagram is shown in Figure 3, As it seen from this figure, ferrite transformation does not occur at cooling rates higher than l °C/sec. Microstructures at 3°C/sec and higher cooling rates shown in Error! Reference source not fonn L-A & C show a martensitic microstructure. However, there is high degree of tempering at the lower cooling rates, Error! Reference source not found.- B & D. Despite tempering martensite, high hardness of 350IIV was obtained at 3°C/sec cooling rate and it increases as the cooling rate increase. Cooling a steel of the present invention in any medium (air, oil, die, nitrogen) which results in cooling rates higher than l °C/sec or preferably higher than 3°C/sec will produce a fully martensitic - high strength steel.
• any medium air, oil, die, nitrogen

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