High-strength hot rolled steel sheet and method for producing the same

Abstract

A high-strength hot rolled steel sheet has a chemical composition containing, in % by mass, C: 0.04 to 0.18%, Si: 0.1 to 3.0%, Mn: 0.5 to 3.5%, P: more than 0% and 0.050% or less, S: more than 0% and 0.010% or less, Al: more than 0% and 1.5% or less, N: more than 0% and 0.010% or less, O: more than 0% and 0.003% or less, and Nb: 0.020 to 0.100%, with the balance being Fe and incidental impurities. The steel sheet has a steel microstructure in which bainite is a main phase and in which the volume fraction of retained austenite is less than 3%. (The amount of solute Nb/the total amount of Nb) is 0.300 or more and less than 0.800, and the amount of Nb present as precipitates having a diameter of 100 nm or more is 0.010 to 0.030% by mass.

1 . A high-strength hot rolled steel sheet having a chemical composition containing, in % by mass,
 C: 0.04 to 0.18%,   Si: 0.1 to 3.0%,   Mn: 0.5 to 3.5%,   P: more than 0% and 0.050% or less,   S: more than 0% and 0.010% or less,   Al: more than 0% and 1.5% or less,   N: more than 0% and 0.010% or less,   O: more than 0% and 0.003% or less, and   Nb: 0.020 to 0.100%,   with the balance being Fe and incidental impurities,   wherein the high-strength hot rolled steel sheet has a steel microstructure in which bainite is a main phase and in which a volume fraction of retained austenite is less than 3%,   wherein a ratio of an amount of solute Nb to an amount of Nb contained, which is given by (the amount of solute Nb/a total amount of Nb), is 0.300 or more and less than 0.800, and   wherein an amount of Nb present as precipitates having a diameter of 100 nm or more is 0.010 to 0.030% by mass.   
     
     
         2 . The high-strength hot rolled steel sheet according to  claim 1 , wherein the chemical composition further contains, in % by mass, at least one or two elements selected from
 Cr: 0.005 to 2.0%,   Cu: 0.005 to 0.5%,   Ni: 0.005 to 2.0%,   Mo: 0.005 to 1.0%,   V: 0.005 to 0.5%,   Ti: 0.005 to 0.15%,   B: 0.0002 to 0.0050%,   Ca: 0.0001 to 0.0050%,   REM: 0.0001 to 0.0050%,   Sb: 0.0010 to 0.10%, and   Sn: 0.0010 to 0.50%.   
     
     
         3 . A method for producing the high-strength hot rolled steel sheet according to  claim 1 , the method comprising:
 heating a slab having the chemical composition in a temperature range of 1150 to 1300° C.; holding the slab in the temperature range for 0.2 to 3.5 hours;   then hot-rolling the slab to obtain a steel sheet,   the hot-rolling including rough rolling in a temperature range of 1080° C. or higher at a total rolling reduction of 80 to 90%, cooling of the resulting steel sheet to finish rolling start temperature at an average cooling rate of 1 to 20° C./s, and finish rolling, the finish rolling being performed under the condition that a rolling reduction per pass at lower than or equal to T (° C.) determined from a formula below is 25% or less; after completion of the finish rolling, allowing the resulting steel sheet to naturally cool for 2.0 s or longer;   then cooling the resulting steel sheet in a temperature range down to 550° C. at an average cooling rate of 50° C./s or more; and then coiling the resulting steel sheet at a coiling temperature higher than or equal to Ms temperature (° C.) and 550° C. or lower:   
       
         
           
             
               
                 
                   T 
                   ⁡ 
                   ( 
                   
                     ° 
                     ⁢ 
                         
                     
                       C 
                       . 
                     
                   
                   ) 
                 
                 = 
                 
                   800 
                   + 
                   
                     2500 
                        
                     [ 
                     Nb 
                     ] 
                   
                   + 
                   
                     1000 
                        
                     [ 
                     Ti 
                     ] 
                   
                 
               
               , 
             
           
         
         where [Nb] and [Ti] are a content (% by mass) of Nb and a content (% by mass) of Ti, respectively, and are each 0 when a corresponding element is not contained. 
       
     
     
         4 . A method for producing the high-strength hot rolled steel sheet according to  claim 2 , the method comprising:
 heating a slab having the chemical composition in a temperature range of 1150 to 1300° C.; holding the slab in the temperature range for 0.2 to 3.5 hours;   then hot-rolling the slab to obtain a steel sheet,   the hot-rolling including rough rolling in a temperature range of 1080° C. or higher at a total rolling reduction of 80 to 90%, cooling of the resulting steel sheet to finish rolling start temperature at an average cooling rate of 1 to 20° C./s, and finish rolling, the finish rolling being performed under the condition that a rolling reduction per pass at lower than or equal to T (° C.) determined from a formula below is 25% or less; after completion of the finish rolling, allowing the resulting steel sheet to naturally cool for 2.0 s or longer;   then cooling the resulting steel sheet in a temperature range down to 550° C. at an average cooling rate of 50° C./s or more; and then coiling the resulting steel sheet at a coiling temperature higher than or equal to Ms temperature (° C.) and 550° C. or lower:   
       
         
           
             
               
                 
                   T 
                   ⁡ 
                   ( 
                   
                     ° 
                     ⁢ 
                         
                     
                       C 
                       . 
                     
                   
                   ) 
                 
                 = 
                 
                   800 
                   + 
                   
                     2500 
                        
                     [ 
                     Nb 
                     ] 
                   
                   + 
                   
                     1000 
                        
                     [ 
                     Ti 
                     ] 
                   
                 
               
               , 
             
           
         
         where [Nb] and [Ti] are a content (% by mass) of Nb and a content (% by mass) of Ti, respectively, and are each 0 when a corresponding element is not contained.