Steel plate

Abstract

A steel plate according to an aspect of the present invention has a predetermined chemical composition, an index Q obtained by Equation (1) is 0.00 or more, a carbon equivalent Ceq (%) obtained by Equation (2) is less than 0.800%, a ratio of a difference between a hardness at a surface layer portion and a hardness at a thickness center portion to the hardness at the surface layer portion at a room temperature is 15.0% or less, the hardness at the suffice layer portion at a room temperature is 400 or more in terms of Vickers hardness, and a steel thickness is 40 or more. Q =0.18−1.3(log T )+0.75(2.7×[C]+[Mn]+0.45×[Ni]+0.8×[Cr]+2×[Mo])   (1) Ceq(%)=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/4   (2)

1 . A steel plate comprising, as a chemical composition, by mass %:
 C: 0.20% to 0.35%;   Si: more than 1.00% to 2.00%;   Mn: 0.60% to 2.00%;   Cr: 0.10% to 2.00%;   Mo: 0.05% to 1.00%;   Al: 0.010% to 0.100%;   N: 0.0020% to 0.0100%;   B: 0.0003% to 0.0020%;   P: 0.0200% or less;   S: less than 0.0100%;   Cu: 0% to 0.500%;   Ni: 0% to 1.00%;   Nb: 0% to 0.050%;   V: 0% to 0.120%;   Ti: 0% to 0.025%;   Ca: 0% to 0.050%;   Mg: 0% to 0.050%;   REM: 0% to 0.100%; and   a remainder consisting of Fe and impurities, wherein an index Q obtained by Equation (1) is 0.00 or more, a carbon equivalent Ceq (%) obtained by Equation (2) is less than 0.800%, a ratio of a difference between a hardness at a surface layer portion and a hardness at a thickness center portion to the hardness at the surface layer portion at a room temperature is 15.0% or less and the hardness at the surface layer portion at a room temperature is 400 or more in terms of Vickers hardness, and   a steel thickness T is 40 mm or more,
     Q= 0.18−1.3(log T )+0.75(2.7×[C]+[Mn]+0.45×[Ni]+0.8×[Cr]+2×[Mo])   (1)
 
     Ceq (%)=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/4   (2)
 
   where the index Q of Equation (1) is calculated by substituting a numerical value of the steel thickness T (mm), a numerical value of an amount [X] of each element X in terms of mass % and 0 in a case where the element X is not contained, the carbon equivalent Ceq (%) of Equation (2) is calculated by substituting a numerical value of an amount [X] of each element X in terms of mass % and 0 in a case where the element X is not contained.   
     
     
         2 . The steel plate according to  claim 1 ,
 wherein the index Q is 0.04 or more, and   the ratio is 13.0% or less.   
     
     
         3 . The steel plate according to  claim 1  or  2 ,
 comprising, as the chemical composition, by mass %, 
 Ni is 0.05% to 1.00%. 
 
     
     
         4 . The steel plate according to  claim 1  or  2 ,
 comprising, as the chemical composition, by mass %, 
 Mn is 0.63% to 2.00%. 
 
     
     
         5 . The steel plate according to  claim 3 ,
 comprising, as the chemical composition, by mass %,   Mn is 0.63% to 2.00%.   
     
     
         6 . A steel plate comprising, as a chemical composition, by mass %:
 C: 0.20% to 0.35%;   Si: more than 1.00% to 2.00%;   Mn: 0.60% to 2.00%;   Cr: 0.10% to 2.00%;   Mo: 0.05% to 1.00%;   Al: 0.010% to 0.100%;   N: 0.0020% to 0.0100%;   B: 0.0003% to 0.0020%;   P: 0.0200% or less;   S: less than 0.0100%;   Cu: 0% to 0.500%;   Ni: 0% to 1.00%;   Nb: 0% to 0.050%;   V: 0% to 0.120%;   Ti: 0% to 0.025%;   Ca: 0% to 0.050%;   Mg: 0% to 0.050%;   REM: 0% to 0.100%; and   a remainder comprising Fe and impurities,   wherein an index Q obtained by Equation (1) is 0.00 or more,   a carbon equivalent Ceq (%) obtained by Equation (2) is less than 0.800%,   a ratio of a difference between a hardness at a surface layer portion and a hardness at a thickness center portion to the hardness at the surface layer portion at a room temperature is 15.0% or less and the hardness at the surface layer portion at a room temperature is 400 or more in terms of Vickers hardness, and   a steel thickness T is 40 mm or more,
     Q= 0.18−1.3(log T )+0.75(2.7×[C]+[Mn]+0.45×[Ni]+0.8×[Cr]+2×[Mo])   (1)
 
     Ceq (%)=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/4   (2)
 
   where the index Q of Equation (1) is calculated by substituting a numerical value of the steel thickness T (mm), a numerical value of an amount [X] of each element X in terms of mass % and 0 in a case where the element X is not contained, the carbon equivalent Ceq (%) of Equation (2) is calculated by substituting a numerical value of an amount [X] of each element X in terms of mass % and 0 in a case where the element X is not contained.