Method of producing steel strip

Abstract

Steel strips and methods for producing steel strips are provided. In an illustrated embodiment, a method includes continuously casting molten low carbon steel into a strip and hot rolling the cast strip within a temperature range such that the strip passes through the Ar 3 transformation temperature from austenite to ferrite under strain during rolling, thereby producing a strip having a yield strength above 400 Mpa and an elongation in excess of 30%. The cast steel strip has desired microstructures.

The claims defining the invention are as follows:  
     
         1 . A method of producing steel strip having a yield strength of at least 400 MPa and a total elongation of at least 30% which includes the steps of: 
 (a) continuously casting molten low carbon steel into a strip;    (b) hot rolling the cast strip within a temperature range such that the strip passes through the austenite to ferrite transformation under strain during rolling to produce a strip having a yield strength above 400 MPa and an elongation in excess of 30%.    
     
     
         2 . The method of claim l, wherein the temperature range begins above and passes through the Ar 3  transformation temperature.  
     
     
         3 . The method of  claim 1 , wherein the temperature range begins below the Ar 3  transformation temperature.  
     
     
         4 . The method of  claim 1 , wherein the steel strip cast in step (a) is less than 2 mm thick.  
     
     
         5 . The method of  claim 1 , wherein the hot rolling achieves a reduction in thickness of the steel strip of between 15 and 50%.  
     
     
         6 . The method of  claim 1 , wherein the hot rolling achieves a reduction in thickness of the steel strip of between 30 and 40%.  
     
     
         7 . A cast steel strip having a yield strength of at least 400 MPa and a total elongation of at least 30% made by the following steps: 
 (a) continuously casting molten low carbon steel into a strip;    (b) hot rolling the cast strip within a temperature range such that the strip passes through the austenite to ferrite transformation under strain during rolling to produce a strip with a microstructure providing a strip with strength above 400 MPa and an elongation in excess of 30%.    
     
     
         8 . The cast steel strip of  claim 7 , wherein the temperature range begins above and passes through the Ar 3  transformation temperature.  
     
     
         9 . The cast steel strip of  claim 7 , wherein the temperature range begins below the Ar 3  transformation temperature.  
     
     
         10 . The cast steel strip as described in  claim 7  wherein the microstructure of the cast strip has substantial fine ferrite grains having a grain size in the range of 5-20 microns.  
     
     
         11 . The cast steel strip as described in  claim 7  wherein the microstructure of the cast strip has substantial ultrafine ferrite grains having a grain size in the range of 1-4 microns.  
     
     
         12 . The cast steel strip as described in  claim 11  wherein the ultrafine ferrite grains comprise 30-40% of the strip.