High strength steel sheet having excellent ductility and stretch flangeability

Abstract

A high strength steel sheet as hot rolled and cold rolled products useful for frame components for vehicles and automobiles such as frames for trucks, and to a method of producing the steel sheet, as well as a use thereof.

1 . A high strength steel strip having a cementite-free microstructure comprising:
 0.005-0.08 wt. % C;   1.30-2.30 wt. % Mn;   2-35 ppm B;   5-65 ppm N;   0.005-0.1 wt. % Al_ tot;   0.03 to 0.20 wt. % Ti;   0-1.5 wt. % Cu;   0-0.75 wt. % Cr;   0-0.05 wt. % Mo;   0-0.50 wt. % Ni;   0-0.30 wt. % V;   0-0.6 wt. % Si;   0-0.03 wt. % P;   0-0.01 wt. % S;   
       
         
           
             
               
                 C 
                 
                   
                     Ti 
                     sol 
                   
                   + 
                   V 
                 
               
               ≤ 
               0.25 
             
           
         
         wherein Ti_sol=Ti-((48/14)·N) remainder iron and inevitable impurities, the steel strip having a yield strength of at least 570 MPa, a tensile strength of at least 760 MPa, a total elongation (A50) of at least 10.3% and a hole expansion ratio (X) value of at least 70%. 
       
     
     
         2 . The steel according to  claim 1  containing at least one of
 0-1.5 wt. % Cu; 
 0-0.75 wt. % Cr; 
 
     
     
         3 . The steel according to  claim 1  wherein the microstructure comprises bainitic and ferritic grains. 
     
     
         4 . The steel according to  claim 1  wherein C is at most 0.045 wt. %. 
     
     
         5 . A process for producing a high strength steel strip having a cementite-free microstructure, a yield strength of at least 570 MPa, a tensile strength of at least 760 MPa, a total elongation (A50) of at least 10.3% and a hole expansion ratio (λ) value of at least 70%, said process comprising:
 casting a melt into a slab or strip having the following composition; 
 0.005-0.08 wt. % C; 
 1.30-2.30 wt. % Mn; 
 2-35 ppm B; 
 5-65 ppm N; 
 0.005-0.1 wt. % Al_tot; 
 0.03 to 0.20 wt. % Ti; 
 0-1.5 wt. % Cu; 
 0-0.75 wt. % Cr; 
 0-0.05 wt. % Mo; 
 0-0.50 wt. % Ni; 
 0-0.30 wt. % V; 
 0-0.6 wt. % Si; 
 0-0.03 wt. % P; 
 0-0.01 wt. % S; 
 
       
         
           
             
               
                 C 
                 
                   
                     Ti 
                     sol 
                   
                   + 
                   V 
                 
               
               ≤ 
               0.25 
             
           
         
         wherein Ti_sol=Ti-((48/14)·N); 
         remainder iron and inevitable impurities, 
         reheating the slab to a slab reheating temperature of at least 1200° C., hot rolling the slab or strip to a hot-rolled strip wherein the hot-rolling finishing temperature is above Ar3, 
         cooling the hot-rolled strip at an average cooling rate of 15 to 100° C./s on the run-out table within a time period of 2 seconds between finish rolling and start of cooling to a coiling temperature below 500° C. and then coil cooling by natural cooling to ambient temperature. 
       
     
     
         6 . The process according to  claim 5  wherein the slab or strip contains at least one of
 0-1.5 wt. % Cu; 
 0-0.75 wt. % Cr; 
 
     
     
         7 . The process according to  claim 5 , wherein the carbon content of the steel is at least 0.03% and wherein the coiling temperature is at least 420° C. 
     
     
         8 . The process according to  claim 5  wherein C is at most 0.045 wt. %. 
     
     
         9 . The process according to  claim 5  wherein the coiling temperature is at least 440° C. and/or at most 480° C. 
     
     
         10 . The process according to  claim 5  wherein hot-rolled strip is subsequently cold-rolled to obtain a cold-rolled strip. 
     
     
         11 . The process according to  claim 10 , wherein the cold-rolled strip is annealed by reheating the strip to a temperature above Ar 3 , holding it and subsequently cooling it to ambient temperatures. 
     
     
         12 . The process according to  claim 10 , wherein the total cold rolling reduction is between 50 and 90%. 
     
     
         13 . The process according to  claim 10 , wherein the cold-rolled full-hard strip is reheated to a solution temperature above Ac3 in the range of 850-1000° C., held at the solution temperature for 2 to 8 minutes, cooled with a cooling rate in the range of 15 to 50° C./s to a holding temperature between 440 and 480° C., held at the holding temperature for 0 to 30 min to allow the bainitic transformation to take place, and then cooled to room temperature. 
     
     
         14 . A car or truck component selected from an automotive chassis component, a component of the body in white, a component of the frame or the subframe, said component having been produced from the steel sheet according to  claim 1 . 
     
     
         15 . A car or truck component selected from an automotive chassis component, a component of the body in white, a component of the frame or the subframe, said component having been produced by means of the process according to  claim 5 . 
     
     
         16 . The process according to  claim 6 , wherein the carbon content of the steel is at least 0.03% and wherein the coiling temperature is at least 420° C. 
     
     
         17 . The process according to  claim 11 , wherein the cold-rolled full-hard strip is reheated to a solution temperature above Ac3 in the range of 850-1000° C., held at the solution temperature for 2 to 8 minutes, cooled with a cooling rate in the range of 15 to 50° C./s to a holding temperature between 440 and 480° C., held at the holding temperature for 0 to 30 min to allow the bainitic transformation to take place, and then cooled to room temperature