Two-step hot forming of steels

Abstract

Methods for press hardening steel alloys comprised of medium-Mn are provided. The press-hardened steel alloy may have an ultimate tensile strength (UTS) of at least 1,700 MPa and a tensile elongation of at least 8%. The press-hardened steel alloy may be formed in two forming steps above the martensitic finish temperature. The press-hardened steel may have a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.

What is claimed is: 
     
         1 . A method of press-hardening a medium-Mn steel alloy, the method comprising:
 heating a blank of medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the medium-Mn steel alloy;   press hardening the blank of the medium-Mn steel alloy to form a press-hardened component at a temperature greater than or equal to about a martensitic start temperature; and   further forming the press-hardened component at a temperature greater than or equal to about a martensitic finish temperature to less than or equal to about the martensitic start temperature, wherein the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa.   
     
     
         2 . The method of  claim 1 , wherein the heating of the blank is to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy. 
     
     
         3 . The method of  claim 1 , wherein the medium-Mn steel alloy comprises carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %. 
     
     
         4 . The method of  claim 1 , wherein the medium-Mn steel alloy blank is galvanized before heating. 
     
     
         5 . The method to  claim 1 , wherein the press-hardened component has a tensile elongation of greater than or equal to about 8%. 
     
     
         6 . The method of  claim 1 , wherein the press-hardened component is air cooled to the martensitic finish temperature after further forming the press-hardened component. 
     
     
         7 . The method of  claim 1 , wherein the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. 
     
     
         8 . A method of press-hardening a medium-Mn steel alloy, the method comprising:
 heating a blank of medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the medium-Mn steel alloy;   press hardening the blank of the medium-Mn steel alloy to form a press-hardened component at a temperature greater than or equal to about a martensitic start temperature; and   further forming the press-hardened component at a temperature of greater than or equal to about the martensitic start temperature to form a press-hardened component having an ultimate tensile strength of greater than or equal to about 1,700 MPa.   
     
     
         9 . The method of  claim 8 , wherein the heating of the blank is to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy. 
     
     
         10 . The method of  claim 8 , wherein the medium-Mn steel alloy comprises carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %. 
     
     
         11 . The method of  claim 8 , wherein the medium-Mn steel alloy blank is galvanized before heating. 
     
     
         12 . The method to  claim 8 , wherein the press-hardened component has a tensile elongation of greater than or equal to about 8%. 
     
     
         13 . The method of  claim 8 , wherein the press-hardened component is air cooled to the martensitic finish temperature after further forming the press-hardened component. 
     
     
         14 . The method of  claim 8 , wherein the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%. 
     
     
         15 . A method of press-hardening a medium-Mn steel alloy, the method comprising:
 heating a blank of medium-Mn steel alloy to a temperature of less than or equal to about 850° C. to austenitize the medium-Mn steel alloy;   press hardening the blank of the medium-Mn steel alloy to form a press-hardened component;   further forming the press-hardened component at a temperature greater than or equal to a martensitic finish temperature, wherein the further forming includes at least one of trimming, punching, or re-striking the press-hardened component; and   controlling the rate at which the press-hardened component cools between a martensitic start temperature and the martensitic finish temperature, wherein the press-hardened component has a microstructure comprising martensite at greater than or equal to about 80% to less than or equal to about 98% and retained austenite at less than or equal to about 20% to greater than or equal to about 2%.   
     
     
         16 . The method of  claim 15 , wherein the heating the blank is to a temperature of less than or equal to about 800° C. to austenitize the medium-Mn steel alloy. 
     
     
         17 . The method of  claim 15 , wherein the medium-Mn steel alloy comprises carbon at greater than or equal to about 0.1 wt. % to less than or equal to about 0.4 wt. % and manganese at greater than or equal to about 5 wt. % to less than or equal to about 12 wt. %. 
     
     
         18 . The method to  claim 15 , wherein the press-hardened component has an ultimate tensile strength of greater than or equal to about 1,700 MPa. 
     
     
         19 . The method of  claim 15 , wherein the press-hardened component is air cooled to the martensitic finish temperature after further forming the press-hardened component. 
     
     
         20 . The method of  claim 15 , wherein medium-Mn steel alloy blank is galvanized before heating.