Production of thin steel strip

Abstract

A cast carbon steel strip is prepared by continuously casting in a twin roll caster and cooling to transform the strip from austenite to ferrite at a temperature range between 400° C. and 850° C. at a cooling rate of not more than about 100° C./sec, such that the strip has a yield strength of greater than 450 MPa. The cast strip before cooling is passed through a hot rolling mill to reduce the thickness of strip by at least 15% and up to 50%.

1 . A cast steel strip prepared by a process comprising the steps of: 
 supporting a casting pool of molten low carbon steel on a pair of chilled casting rolls forming a nip between them and continuously casting solidified strip of no more than 5 mm in thickness and including austenite grains by rotating the rolls in mutually opposite directions such that the solidified strip moves downwardly from the nip;    passing the strip through a rolling mill in which it is hot rolled to produce a reduction in the strip thickness of at least 15%, and    cooling the strip to transform the austenite to ferrite within a temperature range between 850° C. and 400° C. and at a cooling rate of more than about 100° C./sec.    
     
     
         2 . The cast steel strip of  claim 1 , wherein the low carbon steel is a silicon/manganese killed steel, and the strip is hot rolled in the temperature range of 900° C. to 1100° C. and then is cooled at a cooling rate in the range of 100° C./sec to 300° C./sec to produce a final strip having a yield strength of at least 450 MPa.  
     
     
         3 . The cast steel strip of  claim 1 , wherein the low carbon steel is a silicon/manganese killed steel, and the strip is cooled at a cooling rate in the range of 100° C./sec to 300° C./sec to produce a strip with a final yield strength of at least 450 MPa.  
     
     
         4 . The cast steel strip of  claim 3 , wherein the final yield strength is between 450 MPa and 700 MPa.  
     
     
         5 . The steel strip of  claim 1 , wherein the cooling rate is in the range of 100° C./sec to 300° C./sec and the strip has a yield strength of at least 450 MPa.  
     
     
         6 . The cast steel strip of  claim 5 , wherein the yield strength is between 450 MPa and 700 MPa.  
     
     
         7 . The cast steel strip of  claim 1 , wherein the low carbon steel is a silicon/manganese killed steel having the following composition by weight:  
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Carbon 
                    0.02-0.08% 
                 
                     
                   Manganese 
                    0.30-0.80% 
                 
                     
                   Silicon 
                    0.10-0.40% 
                 
                     
                   Sulfur 
                   0.002-0.05% 
                 
                     
                   Aluminum 
                   less than 0.01% 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         8 . The cast steel strip of  claim 1 , wherein the low carbon steel is aluminum killed steel.  
     
     
         9 . The cast steel strip of  claim 1 , wherein the aluminum killed steel has the following composition by weight:  
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Carbon 
                    0.02-0.08% 
                 
                     
                   Manganese 
                   0.40% max 
                 
                     
                   Silicon 
                   0.05% max 
                 
                     
                   Sulfur 
                   0.002-0.05% 
                 
                     
                   Aluminum 
                   0.05% max 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         10 . The cast steel strip of  claim 1 , wherein the cooling rate is in the range of 100° C./sec to 300° C./sec.  
     
     
         11 . The cast steel strip of  claim 1 , wherein the finished cast steel strip has a yield strength of greater than 450 MPa.  
     
     
         12 . The cast steel strip of  claim 2 , wherein the final cast steel strip has a yield strength in the range of 450 MPa to 700 MPa.  
     
     
         13 . The cast steel strip of  claim 12 , wherein the cast steel has the following composition by weight:  
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Carbon 
                    0.02-0.08% 
                 
                     
                   Manganese 
                    0.30-0.80% 
                 
                     
                   Silicon 
                    0.10-0.40% 
                 
                     
                   Sulfur 
                   0.002-0.05% 
                 
                     
                   Aluminum 
                   less than 0.01% 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         14 . A cast steel strip prepared by a process comprising the steps of: 
 supporting a casting pool of molten low carbon steel on a pair of chilled casting rolls forming a nip between them and continuously casting solidified strip of no more than 5 mm in thickness and including austenite grains by rotating the rolls in mutually opposite directions such that the solidified strip moves downwardly from the nip;    passing the strip through a rolling mill in which the strip is hot rolled to produce a reduction in the strip thickness of at least 15%; and    continuously cooling the strip to transform the austenite to ferrite within a temperature range between 400° C. and 850° C. at a cooling rate of not less than 90° C./sec without inhibiting the cooling rate.    
     
     
         15 . The cast steel strip of  claim 14 , wherein said cooling rate is in the range 100° C./sec to 300° C./sec.  
     
     
         16 . The cast steel strip of  claim 14 , wherein the low carbon steel is a silicon/manganese killed steel having the following composition by weight:  
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Carbon 
                   0.02-0.08% 
                 
                     
                   Manganese 
                   0.30-0.80% 
                 
                     
                   Silicon 
                   0.10-0.40% 
                 
                     
                   Sulfur 
                   0.002-0.05%  
                 
                     
                   Aluminum 
                   less than 0.01% 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         17 . The cast steel strip of  claim 14 , wherein the low carbon steel is aluminum killed steel.  
     
     
         18 . The cast steel strip of  claim 17 , wherein the aluminum killed steel has the following composition by weight:  
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Carbon 
                    0.02-0.08% 
                 
                     
                   Manganese 
                   0.40% max 
                 
                     
                   Silicon 
                   0.05% max 
                 
                     
                   Sulfur 
                   0.002-0.05% 
                 
                     
                   Aluminum 
                   0.05% max 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         19 . The cast steel strip of  claim 14 , wherein the finished strip has a yield strength of greater than 450 MPa.  
     
     
         20 . The cast steel strip of  claim 14 , wherein said cooling rate is in the range 100° C./sec to 300° C./sec and the strip has a yield strength of at least 450 MPa.  
     
     
         21 . The cast steel strip of  claim 20 , wherein the strip has a yield strength in the range of 450 MPa to 700 MPa.  
     
     
         22 . The cast steel strip of  claim 14 , wherein the low carbon steel is a silicon/manganese killed steel, and the strip is cooled at a cooling rate in the range of 100° C./sec to 300° C./sec to produce a strip having a yield strength of at least 450 MPa.  
     
     
         23 . The cast steel strip of  claim 22 , wherein the final strip has a yield strength in the range of 450 MPa to 700 MPa.  
     
     
         24 . The cast steel strip of  claim 14 , wherein the low carbon steel is a silicon/manganese killed steel, and the strip is hot rolled in the temperature range of 900° C. to 1100° C. and then is cooled at a cooling rate in the range of 100° C./sec to 300° C./sec to produce a final strip having a yield strength of at least 450 MPa.  
     
     
         25 . The cast steel strip of  claim 24 , wherein the final strip has a yield strength in the range of 450 MPa to 700 MPa.  
     
     
         26 . The cast steel strip of  claim 24 , wherein the steel has the following composition by weight:  
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Carbon 
                    0.02-0.08% 
                 
                     
                   Manganese 
                    0.30-0.80% 
                 
                     
                   Silicon 
                    0.10-0.40% 
                 
                     
                   Sulfur 
                   0.002-0.05% 
                 
                     
                   Aluminum 
                   less than 0.01%.