Method for processing advanced high strength steel

Abstract

A method of manufacturing an energy absorbing component for a vehicle is provided. The method includes heating a bainitic GENS steel material which has a microstructure including ferrite and bainite to a temperature above the Ac3 temperature to convert a portion of the ferrite and bainite to austenite. The method further includes forming while cooling the heated steel blank into a component in a temperature controlled steel die. During the cooling step, the steel material is cooled to a temperature below the Ms temperature to form retained austenite. A portion of the austenite transforms to martensite and bainite during the forming and cooling step. The method can further include heating the component to a temperature above the Ms temperature after the forming and cooling step to increase energy absorption characteristics. During a crash event, the strain imposed on the component converts retained austenite present in the component to martensite.

1 . A method for processing steel material, comprising the steps of:
 heating a steel material to a temperature above an upper critical temperature (Ac3) of the steel material, and the steel material having a microstructure which includes ferrite and bainite;   the heating step including converting a portion of the ferrite and bainite to austenite;   forming the steel material into a component after the steel material is heated to the temperature above the upper critical temperature (Ac3); and   cooling the steel material during the forming step, wherein a portion of the austenite transforms to martensite and bainite during the forming step.   
     
     
         2 . The method of  claim 1 , wherein the heating step includes heating the steel material to a temperature greater than 850° C. 
     
     
         3 . The method of  claim 1 , wherein the steel material includes iron in an amount of 91.95 to 98.55 wt. %, carbon in an amount of 0.15 to 0.3 wt. %, manganese in an amount of 1.5 to 2.5 wt. %, silicon in an amount of 0.6 to1.6 wt. %, chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0 wt. %, nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to 1.0 wt. %, based on the total weight of the steel material. 
     
     
         4 . The method of  claim 1 , wherein the microstructure of the steel material, prior to the heating step, includes bainite in an amount of at least 75 vol. %, based on the total volume of the steel material. 
     
     
         5 . The method of  claim 1 , wherein the forming step is conducted in a steel die while the steel die is at a temperature of 100° C. to 360° C. 
     
     
         6 . The method of  claim 1 , wherein the cooling step includes cooling the steel material from the temperature above the Ac3 temperature to a temperature below a martensite start (M s ) temperature of the material. 
     
     
         7 . The method of  claim 6 , wherein the Ac3 temperature is greater than 850° C. and the temperature below the M s  temperature is from 100° C. to 350° C. 
     
     
         8 . The method of  claim 1 , wherein the cooling step includes forming retained austenite, wherein the retained austenite is maintained in a matrix of the bainite and martensite 
     
     
         9 . The method of  claim 8 , wherein up to 15% of the austenite present in the steel material prior to the forming step is maintained as the retained austenite in the matrix of the bainite and martensite. 
     
     
         10 . The method of  claim 1 , wherein the cooling is conducted at a rate of less than 50° C./second. 
     
     
         11 . The method of  claim 1  including heating the component to a temperature above the M s  temperature of the steel material after the forming and cooling step. 
     
     
         12 . The method of  claim 1  including restriking, trimming, flanging, and/or piercing component after the forming step. 
     
     
         13 . The method of  claim 1 , wherein the forming step is conducted in a die, and the method further includes regulating the temperature of the die during and/or after the forming step to control the amount of martensite, bainite, and retained austenite in the component and thus tailor the energy absorption, weldability, and/or deformation characteristics in specific regions of the component. 
     
     
         14 . The method of  claim 1 , wherein the forming step includes shaping the steel material into the component having the shape of an energy absorbing component for a vehicle. 
     
     
         15 . A component, comprising:
 a steel material including iron in an amount of 91.95 to 98.55 wt. %, carbon in an amount of 0.15 to 0.3 wt. %, manganese in an amount of 1.5 to 2.5 wt. %, silicon in an amount of 0.6 to 1.6 wt. %, chromium in an amount of 0.55 to 0.65 wt. %, copper in an amount of 0.0 to 1.0 wt. %, nickel in an amount of 0.0 to 1.0 wt. % and aluminum in an amount of 0.0 to 1.0 wt. %, based on the total weight of the steel; and   the steel material including bainite and martensite.   
     
     
         16 . The method of  claim 1 , wherein before the heating step, the microstructure of the steel includes bainite in an amount of at least 75 vol. %, based on the total volume of the steel material, and a remainder of the microstructure includes ferrite. 
     
     
         17 . The method of  claim 1 , wherein the component is a B-pillar. 
     
     
         18 . The method of  claim 1 , wherein the cooling is conducted at a rate of greater than 10° C./second. 
     
     
         19 . The component of  claim 15 , wherein the steel material includes retained austenite in a matrix of bainite and martensite. 
     
     
         20 . The component of  claim 15 , wherein the component is a B-pillar.