Steel plate for hot stamping

Abstract

A steel plate for hot stamping contains, in % by mass, C: 0.25% or more and 0.4% or less, Si: 1.05% or more and 1.4% or less, Mn: 0% or more and 1.4% or less, Cr: 0.6% or more and 3.0% or less, P: 0% or more and 0.03% or less, S: 0% or more and 0.02% or less, Al: 0.01% or more and 1% or less, N: 0% or more and 0.01% or less, B: 0.0005% or more and 0.005% or less, Ti: 0.005% or more and 0.1% or less, and iron and inevitable impurities as remainder. This steel plate for hot stamping exhibits excellent hardness stability in addition to a balance between strength and toughness as a relational expression of [C]+2/9[Si]+7/9[Mn]+8/9[Cr]−7/4>0 is satisfied.

1 . (canceled) 
     
     
         2 : A method for producing a steel plate, comprising a manufacturing process of a slab for steel, wherein in the process, the slab for the steel comprising:
 in % by mass,   from 0.271% to 0.4% of C,   from 1.05% to 1.4% of Si,   from 0% to 1.4% of Mn,   from 0.8% to 3.0% of Cr,   from 0% to 0.03% of P,   from 0% to 0.02% of S,   from 0.01% to 1% of Al,   from 0% to 0.01% of N,   from 0.0005% to 0.005% of B, and   from 0.005% to 0.1% of Ti,   
       and a component composition of the steel is adjusted during melting so that compositions of respective components contained in the slab satisfy the above ranges and the contents of C, Si, Mn, and Cr satisfy the following relational expressions (1) to (4): 
       
         
           
             
               
                 
                   
                     [ 
                     
                       Math 
                       . 
                           
                       1 
                     
                     ] 
                   
                 
                 
                    
                 
               
               
                 
                   
                     
                       
                         [ 
                         C 
                         ] 
                       
                       + 
                       
                         
                           2 
                           9 
                         
                         [ 
                         Si 
                         ] 
                       
                       + 
                       
                         
                           7 
                           9 
                         
                         [ 
                         Mn 
                         ] 
                       
                       + 
                       
                         
                           8 
                           9 
                         
                         [ 
                         Cr 
                         ] 
                       
                       - 
                       
                         7 
                         4 
                       
                     
                     > 
                     0 
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         
           
             
               
                 
                   
                     [ 
                     
                       Math 
                       . 
                           
                       2 
                     
                     ] 
                   
                 
                 
                    
                 
               
               
                 
                   
                     B 
                     > 
                     
                       
                         
                           - 
                           
                             4 
                             . 
                             0 
                           
                         
                         ⁢ 
                         A 
                       
                       + 
                       627 
                     
                   
                 
                 
                   
                     
                       ( 
                       2 
                       ) 
                     
                   
                 
               
             
           
         
         
           
             
               
                 
                   
                     [ 
                     
                       Math 
                       . 
                           
                       3 
                     
                     ] 
                   
                 
                 
                    
                 
               
               
                 
                   
                     B 
                     ≧ 
                     516 
                   
                 
                 
                   
                     
                       ( 
                       3 
                       ) 
                     
                   
                 
               
             
           
         
         
           
             
               
                 
                   
                     [ 
                     
                       Math 
                       . 
                           
                       4 
                     
                     ] 
                   
                 
                 
                    
                 
               
               
                 
                   
                     
                       
                         ❘ 
                         "\[LeftBracketingBar]" 
                       
                       
                         C 
                         - 
                         B 
                       
                       
                         ❘ 
                         "\[RightBracketingBar]" 
                       
                     
                     ≦ 
                     35 
                   
                 
                 
                   
                     
                       ( 
                       4 
                       ) 
                     
                   
                 
               
             
           
         
         wherein [C] represents a content of C, [Si] represents a content of Si, [Mn] represents a content of Mn, and [Cr] represents a content of Cr, A (J/cm 2 ) represents absorbed energy in a Charpy impact test at −40° C. when a flat plate is hardened using a die, B(Hv) represents a hardness when the steel plate for hot stamping is heated to an austenite region, then cooled to room temperature at a cooling velocity of 10° C./s, and hardened, and C(Hv) represents a hardness when the steel plate for hot stamping is heated to the austenite region, then cooled to room temperature at a cooling velocity of 30° C./s, and hardened. 
       
     
     
         3 : The method of  claim 2 , wherein the slab for the steel plate further comprises, in % by mass, at least one selected from the group consisting of
 Mo: from 0% to 1.0%,   Nb: from 0% to 0.1%, and   V: from 0% to 0.1%.   
     
     
         4 : The method of claim  1 , wherein the slab for the steel plate comprises, in % by mass, one or more selected from the group consisting of
 Cu: from 0% to 0.5%, and   Ni: from 0% to 0.5%.   
     
     
         5 : The method of claim  1 , wherein the manufacturing process of the slab comprises melting steel, pouring a molten steel into a mold, and performing continuous casting, thereby obtaining the slab. 
     
     
         6 : The method of  claim 5 , further comprising:
 disposing the obtained slab in a heating furnace heated to a predetermined heating temperature and holding at the heating temperature for a predetermined time, and   hot-rolling the slab into a steel plate having a predetermined thickness by allowing the slab to sequentially pass between rolls of rolling stands of a rough rolling mill and a finishing rolling mill,   cooling the steel plate after being hot-rolled to a predetermined temperature in a cooling apparatus, and   performing pickling and a cold rolling process to obtain the steel plate.