Warm rolling of steels containing metastable austenite

Abstract

Warming a metastable steel before or during cold rolling suppresses the transformation of austenite to martensite, resulting in lower mill loads and higher amounts of reduction at similar loads. As-warm rolled steel has enhanced mechanical properties when compared to steel reduced the same amount by cold rolling. Warm rolling followed by subsequent annealing also results in better mechanical properties than those achieved in material cold rolled the same amount and then annealed. Metastable steel that has been warm rolled, on subsequent room temperature rolling (cold rolling), also shows enhancement in both strength and ductility

What is claimed is: 
     
         1 . A method of rolling a metastable steel comprising the steps of:
 a. Selecting a metastable steel having an instability factor (IF) greater than or equal to 2.9, wherein IF is calculated by the following equation:     IF= 37.193-51.248(% C)−0.4677(% Cr)−1.0174(% Mn)−34.396 (% N)−2.5884(% Ni)   b. Prior to rolling, warming said metastable steel to a warming temperature greater than 80° F.; and   c. Rolling said metastable steel.   
     
     
         2 . The method of  claim 1 , wherein the warming temperature is near or above the M d  temperature for the particular metastable steel composition. 
     
     
         3 . The method of  claim 1 , wherein the warming temperature is near or above the M d 30 temperature for the particular metastable steel composition. 
     
     
         4 . The method of  claim 1 , wherein the warming temperature is less than or equal to 250° F. 
     
     
         5 . The method of  claim 3 , wherein the M d 30 temperature for the metastable steel is calculated according to the following equation:
     M   d 30=551−462(% C+% N)−68*% Cb−13.7*Cr−29(% Cu+% Ni)−8.1*% Mn−18.5*% Mo−9.2*% Si
   
     
     
         6 . The method of  claim 3 , wherein the M d 30 temperature for the metastable steel is calculated according to the following equation:
     M   d 30=413-462*(% C+% N)−13.7*% Cr−8.1*% Mn−18.5*% Mo−9.5*% N %−9.2*% Si
   
     
     
         7 . The method of  claim 1 ,  2 ,  3 ,  4 ,  5 , or  6 , further comprising the step of wherein after rolling, the metastable steel is rolled at room temperature. 
     
     
         8 . The method of  claim 7 , further comprising the step of wherein the metastable steel is annealed prior to being rolled at room temperature. 
     
     
         9 . The method of  claim 1 ,  2 ,  3 ,  4 ,  5 , or  6  further comprising the step of wherein after rolling, the metastable steel is rolled in a second rolling step and prior to the second rolling step, the metastable steel is warmed to a warming temperature greater than 80° F.