Hot rolled steel having improved formability

Abstract

A hot rolled steel having improved formability and minimal silver formation is made with a method comprising the addition of titanium and boron to the molten steel to combine with and remove the free nitrogen prior to rolling. Titanium is added so that the amount of nitrogen remaining after Ti addition is less than or equal to about 0.0035 wt % and boron is added to remove the balance of the nitrogen by forming BN.

1 . A hot rolled steel sheet having a composition including nitrogen, titanium and boron, said nitrogen being bound to said titanium and boron in the form of TiN and BN and wherein:  
       [N*]=[N] total −([Ti]/3.42)  
     wherein: 
 [N] total  is the total concentration (wt %) of nitrogen in the steel;  
 [Ti] is the concentration (wt %) of titanium in the steel;  
 [N*] is the concentration (wt %) of nitrogen bound to boron.  
 and where,  
   [N*]≦0.0035 wt %  
 
   
   
       2 . The steel sheet of  claim 1  wherein: 0.0005 wt %≦[N*]≦0.0035 wt %  
   
   
       3 . The steel sheet of  claim 1  wherein the concentration of BN* in the steel is ≦4.5×10 −6  wt %.  
   
   
       4 . The steel sheet of  claim 3  wherein the concentration of boron is between 5 and 25 ppm.  
   
   
       5 . The steel sheet of  claim 4  wherein the concentration of boron is between 10 and 20 ppm.  
   
   
       6 . The steel sheet of  claim 1  wherein said sheet has a yield strength less than 43 ksi.  
   
   
       7 . A hot rolled steel sheet having a composition including nitrogen, titanium and boron, said nitrogen being bound to titanium and boron in the form of TiN and BN and wherein the concentrations (in wt %) of said nitrogen, titanium and boron correspond to the following formula:  
       SR=([B]/0.77+[Ti]/3.42)/[N] total    
     wherein, SR is a stabilization ratio and wherein: 0.7≦SR≦2;  
     and wherein, 
 [N] total  is the total concentration (wt %) of nitrogen in the steel;  
 [Ti] is the concentration (wt %) of titanium in the steel; and,  
 [B] is the concentration (wt %) of boron in the steel.  
 
   
   
       8 . The steel sheet of  claim 7  wherein all nitrogen in the steel is bound to titanium and boron.  
   
   
       9 . The steel sheet of  claim 8  wherein the concentration of boron is between 5 and 25 ppm.  
   
   
       10 . The steel sheet of  claim 9  wherein the concentration of boron is between 10 and 20 ppm.  
   
   
       11 . The steel sheet of  claim 7  wherein said sheet has a yield strength less than 43 ksi.