Method of producing steel strip

Abstract

Steel strips and methods for producing steel strips are provided. In an illustrated embodiment, a method for producing steel strips includes continuously casting molten steel into a strip, said molten steel comprising a concentration of residuals of 2.0 wt % or less is selected with regard to the microstructure of the finished strip to provide a desired yield strength; and cooling the strip to transform the strip from austenite to ferrite in the temperature range of 850° C. to 400° C. Cast steel with improved yield strength properties is produced by such method.

The claims defining the invention are as follows:  
     
         1 . A method of producing steel strip comprising the steps of: 
 (a) continuously casting molten steel into a strip including austenite grains, said molten steel comprising a concentration of residuals selected with regard to the microstructure of the finished strip to provide a desired yield strength; and    (b) cooling the cast strip to transform austenite grains in the strip to ferrite in a temperature range between 850° C. and 400° C.    
     
     
         2 . The method of  claim 1  wherein the total amount of the residuals is 2.0 wt % or less.  
     
     
         3 . The method of  claim 1  wherein the total amount of the residuals is 1.2 wt % or less.  
     
     
         4 . The method of  claim 1  wherein the cast strip produced in step (a) has a thickness of no more than 2 mm.  
     
     
         5 . The method of  claim 1  wherein the cast strip produced in step (a) includes austenite grains that are columnar.  
     
     
         6 . The method of  claim 1  wherein the steel is low carbon steel.  
     
     
         7 . The method of  claim 6  wherein the low carbon steel is a silicon/manganese killed steel.  
     
     
         8 . The method in  claim 7  wherein the silicon/manganese killed steel includes, by wt %:  
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Carbon 
                    0.02-0.08% 
                 
                     
                   Manganese 
                    0.30-0.80% 
                 
                     
                   Silicon 
                    0.10-0.40% 
                 
                     
                   Sulphur 
                   0.002-0.05% 
                 
                     
                   Aluminium 
                   less than 0.01% 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         9 . The method of  claim 1  wherein the low carbon steel is an aluminum killed steel.  
     
     
         10 . The method described in  claim 9  wherein the aluminum killed low carbon steel has the following composition by weight:  
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Carbon 
                   0.02-0.08% 
                 
                     
                   Manganese 
                    0.40% max 
                 
                     
                   Silicon 
                   0.05% max 
                 
                     
                   Sulphur 
                   0.002-0.05% 
                 
                     
                   Aluminum 
                   0.05% max 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         11 . The method of  claim 1  wherein the continuous caster is a twin roll caster.  
     
     
         12 . The method of  claim 1  further comprising the step of in-line hot rolling the casted strip of step (a) prior to step (b).  
     
     
         13 . The method of  claim 1  wherein step (b) includes cooling the casted strip to transform the strip from austenite to ferrite in a temperature range between 850° C. and 400° C. at a selected cooling rate of at least 0.01° C./sec to produce a microstructure that provides required yield strength of the casted strip, the microstructure being selected from a group consisting of: 
 (i) predominantly polygonal ferrite;  
 (ii) a mixture of polygonal ferrite and low temperature transformation products; and  
 (iii) predominantly low temperature transformation products.  
 
     
     
         14 . The method of  claim 13  wherein the cooling rate is selected so that the microstructure is either (ii) a mixture of polygonal ferrite and low temperature transformation products; or (iii) predominantly low temperature transformation products.  
     
     
         15 . A cast steel strip produced by the steps of: 
 (a) continuously casting molten steel into a strip including austenite grains, said molten steel comprising a concentration of residuals selected with regard to the microstructure of the finished strip to provide a desired yield strength; and    (b) cooling the cast strip to transform the austenite grains in the strip to ferrite in a temperature range between 850° C. and 400° C.    
     
     
         16 . The cast steel strip of  claim 15  wherein the total amount of the residuals is 2.0 wt % or less.  
     
     
         17 . The cast steel strip of  claim 15  wherein the total amount of the residuals is 1.2 wt % or less.  
     
     
         18 . The cast steel strip of  claim 15  wherein the cast strip produced in step (a) includes austenite grains that are columnar.  
     
     
         19 . The cast steel strip of  claim 15  wherein the steel is low carbon steel.  
     
     
         20 . The cast steel strip of  claim 19  wherein the low carbon steel is a silicon/manganese killed steel.  
     
     
         21 . The cast steel strip of  claim 20  wherein the silicon/manganese killed steel includes, by wt %:  
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Carbon 
                    0.02-0.08% 
                 
                     
                   Manganese 
                    0.30-0.80% 
                 
                     
                   Silicon 
                    0.10-0.40% 
                 
                     
                   Sulphur 
                   0.002-0.05% 
                 
                     
                   Aluminum 
                   less than 0.01% 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         22 . The cast steel strip of  claim 19  wherein the low carbon steel is an aluminum killed steel.  
     
     
         23 . The cast steel strip described in  claim 22  wherein the aluminum killed low carbon steel has the following composition by weight:  
       
         
           
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   Carbon 
                    0.02-0.08% 
                 
                     
                   Manganese 
                   0.40% max 
                 
                     
                   Silicon 
                   0.05% max 
                 
                     
                   Sulphur 
                   0.002-0.05% 
                 
                     
                   Aluminum 
                   0.05% max 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
                
                
                
                
               
            
           
         
       
     
     
         24 . The cast steel strip of  claim 15  further comprising the step of in-line hot rolling the casted strip of step (a) prior to step (b).  
     
     
         25 . The cast steel strip of  claim 15  wherein step (b) includes cooling the casted strip to transform the strip from austenite to ferrite at a selected cooling rate of at least 0.01° C./sec to produce a microstructure that provides required yield strength of the casted strip, the microstructure being selected from a group consisting of: 
 (i) predominantly polygonal ferrite;  
 (ii) a mixture of polygonal ferrite and low temperature transformation products; and  
 (iii) predominantly low temperature transformation products.  
 
     
     
         26 . The cast strip of  claim 25  wherein the cooling rate is selected so that the microstructure is either (ii) a mixture of polygonal ferrite and low temperature transformation products; or (iii) predominantly low temperature transformation products.