Thick steel plate and method for manufacturing the same

Abstract

A thick steel plate having a chemical composition containing, by mass %, C: 0.200% or more and 0.350% or less, Si: 0.05% or more and 0.45% or less, Mn: 0.50% or more and 2.00% or less, P: 0.020% or less, S: 0.005% or less, Al: 0.005% or more and 0.100% or less, at least one of Cu, Ni, Cr, Mo, V, Nb, Ti, B, REM, Ca, and Mg, and Fe and inevitable impurities. The thick steel plate being such that CI, which is defined by a particular equation, is 40 or more. Additionally, the thick steel plate having a microstructure in which the area fraction of a bainite phase is 60% or more, the area fraction of Martensite-Austenite constituent is 5% or more and less than 20%, and the remaining constituent phases are at least one of a ferrite phase, a pearlite phase, and a martensite phase.

1 . A thick steel plate having a chemical composition comprising:
 C: 0.200% or more and 0.350% or less, by mass %;   Si: 0.05% or more and 0.45% or less, by mass %;   Mn: 0.50% or more and 2.00% or less, by mass %;   P: 0.020% or less, by mass %;   S: 0.005% or less, by mass %;   Al: 0.005% or more and 0.100% or less, by mass %; and   Fe and inevitable impurities,   wherein:
 CI, which is defined by equation (1) below, is 40 or more,
   CI=60C+8Si+22Mn+10(Cu+Ni)+14Cr+21Mo+15V  (1),
 
 in equation (1), atomic symbols respectively denote the contents (mass %) of the corresponding alloy chemical elements, and a chemical element which is not contained is set to be 0, and 
 
 the thick steel plate has steel microstructure in which (i) an area fraction of a bainite phase is 60% or more and (ii) an area fraction of Martensite-Austenite constituent in the bainite phase is 5% or more and less than 20% with respect to the whole microstructure, and (iii) the remaining constituent phases are at least one of a ferrite phase, a pearlite phase, and a martensite phase. 
   
     
     
         2 . The thick steel plate according to  claim 1 , wherein the chemical composition of the thick steel plate further comprises at least one selected from:
 Cu: 0.03% or more and 1.00% or less, by mass %   Ni: 0.03% or more and 2.00% or less, by mass %;   Cr: 0.05% or more and 2.00% or less, by mass %;   Mo: 0.05% or more and 1.00% or less, by mass %;   V: 0.005% or more and 0.100% or less, by mass %   Nb: 0.005% or more and 0.100% or less, by mass %;   Ti: 0.005% or more and 0.100% or less, by mass %; and   B: 0.0003% or more and 0.0030% or less, by mass %.   
     
     
         3 . The thick steel plate according to  claim 1 , wherein the chemical composition of the thick steel plate further comprises at least one selected from:
 REM: 0.0005% or more and 0.0080% or less, by mass %;   Ca: 0.0005% or more and 0.0050% or less, by mass %; and   Mg: 0.0005% or more and 0.0050% or less, by mass %.   
     
     
         4 . The thick steel plate according to  claim 2 , wherein the chemical composition of the thick steel plate further comprises at least one selected from:
 REM: 0.0005% or more and 0.0080% or less, by mass %;   Ca: 0.0005% or more and 0.0050% or less, by mass %; and   Mg: 0.0005% or more and 0.0050% or less, by mass %.   
     
     
         5 . A method for manufacturing a thick steel plate, the method comprising:
 heating a cast piece or a steel piece to a temperature of 950° C. or higher and 1250° C. or lower,
 the cast piece or steel piece having a chemical composition comprising:
 C: 0.200% or more and 0.350% or tens, by mass %; 
 Si: 0.05% or more and 0.45% or less, by mass %; 
 Mn: 0.50% or more and 2.00% or less, by mass %; 
 P: 0.020% or less, by mass %; 
 S: 0.005% or less, by mass %; 
 Al: 0.005% or more and 0.100% or less, by mass %; and 
 Fe and inevitable impurities, 
 wherein CI, which is defined by equation (1) below, is 40 or more,
   CI=60C+8Si+22Mn+10(Cu+Ni)+14Cr+21Mo+15V  (1),
 
 in equation (1), atomic symbols respectively denote the contents (mass %) of the corresponding alloy chemical elements, and a chemical element which is not contained is set to be 0, 
 
 
   performing hot rolling with a finishing delivery temperature equal to or higher than Ar 3 , and   performing accelerated cooling, immediately after the hot rolling has been performed, at a cooling rate of 5° C./sec or more to a temperature range of 400° C. or higher and 650° C. or lower.   
     
     
         6 . The method according to  claim 5 , wherein the chemical composition of the cast piece or a steel piece further comprises at least one selected from:
 Cu: 0.03% or more and 1.00% or less, by mass %;   Ni: 0.03% or more and 2.00% or less, by mass %;   Cr: 0.05% or more and 2.00% or less, by mass %;   Mo: 0.05% or more and 1.00% or less, by mass %;   V: 0.005% or more and 0.100% or less, by mass %;   Nb: 0.005% or more and 0.100% or less, by mass %;   Ti: 0.005% or more and 0.100% or less, by mass %; and   B: 0.0003% or more and 0.0030% or less, by mass %.   
     
     
         7 . The method according to  claim 5 , wherein the chemical composition of the cast piece or steel piece further comprises at least one selected from:
 REM: 0.0005% or more and 0.0080% or less, by mass %;   Ca: 0.0005% or more and 0.0050% or less, by mass %; and   Mg: 0.0005% or more and 0.0050% or less, by mass %.   
     
     
         8 . The method according to  claim 6 , wherein the chemical composition of the cast piece or steel piece further comprises at least one selected from:
 REM: 0.0005% or more and 0.0080% or less, by mass %;   Ca: 0.0005% or more and 0.0050% or less, by mass %; and   Mg: 0.0005% or more and 0.0050% or less, by mass %.   
     
     
         9 . A method for manufacturing a thick steel plate, the method comprising:
 heating a cast piece or a steel piece to a temperature of 950° C. or higher and 1250° C. or lower,
 the cast piece or steel piece having a chemical composition comprising:
 C: 0.200% or more and 0.350% or less, by mass %; 
 Si: 0.05% or more and 0.45% or less, by mass %; 
 Mn: 0.50% or more and 2.00% or less, by mass %; 
 P: 0.020% or less, by mass %; 
 S: 0.005% or less, by mass %; 
 Al: 0.005% or more and 0.100% or less, by mass %; and 
 Fe and inevitable impurities, 
 wherein CI, which is defined by equation (1) below, is 40 or more,
   CI=60C+8Si+22Mn+10(Cu+Ni)+14Cr+21Mo+15V  (1),
 
 in equation (1), atomic symbols respectively denote the contents (mass %) of the corresponding alloy chemical elements, and a chemical element which is not contained is set to be 0, 
 
 
   performing hot rolling,   performing air cooling to a temperature lower than 400° C.,   then performing reheating to a temperature equal to or higher than the Ac 3  and 950° C. or lower, and   performing cooling, immediately after the reheating has been performed, at a cooling rate of 5° C./sec or more to a temperature range of 400° C. or higher and 650° C. or lower.   
     
     
         10 . The method according to  claim 9 , wherein the chemical composition of the cast piece or a steel piece further comprises at least one selected from:
 Cu: 0.03% or more and 1.00% or less, by mass %;   Ni: 0.03% or more and 2.00% or less, by mass %;   Cr: 0.05% or more and 2.00% or less, by mass %;   Mo: 0.05% or more and 1.00% or less, by mass %;   V: 0.005% or more and 0.100% or less, by mass %;   Nb: 0.005% or more and 0.100% or less, by mass %;   Ti: 0.005% or more and 0.100% or less, by mass %; and   B: 0.0003% or more and 0.0030% or less, by mass %.   
     
     
         11 . The method according to  claim 9 , wherein the chemical composition of the cast piece or steel piece further comprises at least one selected from:
 REM: 0.0005% or more and 0.0080% or less, by mass %;   Ca: 0.0005% or more and 0.0050% or less, by mass %; and   Mg: 0.0005% or more and 0.0050% or less, by mass %.   
     
     
         12 . The method according to  claim 10 , wherein the chemical composition of the cast piece or steel piece further comprises at least one selected from:
 REM: 0.0005% or more and 0.0080% or less, by mass %;   Ca: 0.0005% or more and 0.0050% or less, by mass %; and   Mg: 0.0005% or more and 0.0050% or less, by mass %.   
     
     
         13 . The method according to  claim 5 , wherein the thick steel plate has a steel microstructure in which (i) an area fraction of a bainite phase is 60% or more and (ii) an area fraction of Martensite-Austenite constituent in the bainite phase is 5% or more and less than 20% with respect to the whole microstructure, and (iii) the remaining constituent phases are at least one of a ferrite phase, a pearlite phase, and a martensite phase. 
     
     
         14 . The method according to  claim 9 , wherein the thick steel plate has a steel microstructure in which (i) an area fraction of a bainite phase is 60% or more and (ii) an area fraction of Martensite-Austenite constituent in the bainite phase is 5% or more and less than 20% with respect to the whole microstructure, and (iii) the remaining constituent phases are at least one of a ferrite phase, a pearlite phase, and a martensite phase.