Steel plate and method of producing same

Abstract

A high strength steel plate having excellent ammonia SCC resistance and low-temperature toughness for use in storage tanks and the like used to contain liquefied gas in energy transport ships. The steel plate has a defined chemical composition, in particular including one or more of Cu: 0.01% to 0.5 %, Cr: 0.01% to 1.0%, Sb: 0.01% to 0.50%, or Sn: 0.01% to 0.50%, and the chemical composition satisfies a defined relationship of the Cu, Cr, Sb, and Sn content. Further, a hardness property at a 1.0 mm depth position from the surface of the steel plate is 300 HV or less, and the metallic structure at a ½ thickness position of the steel plate has a volume fraction of bainitic microstructure of 20% or more and a total volume fraction of ferritic microstructure and bainitic microstructure of 60% or more.

1 . A steel plate comprising a chemical composition containing, in mass %,
 C: 0.010% to 0.200%,   Si: 0.01% to 0.50%,   Mn: 0.50% to 2.50%,   Al: 0.060% or less,   N: 0.0010% to 0.0100%,   P: 0.020% or less,   S: 0.0100% or less, and   O: 0.0100% or less   and further, at least one selected from the group consisting of   Cu: 0.01% to 0.50%,   Cr: 0.01% to 1.00%,   Sb: 0.01% to 0.50%, and   Sn: 0.01% to 0.50%,   with the balance being Fe and inevitable impurity, wherein   a CR value obtained by the following Expression (1) is 0.70 or more,   the steel plate has a hardness property such that, at a 1.0 mm depth position from the surface of the steel plate, hardness is 300 HV or less,   the steel plate has a metallic microstructure where, at a ½ thickness position of the steel plate, a volume fraction of bainitic microstructure is 20% or more and a total volume fraction of ferritic microstructure and bainitic microstructure is 60% or more, and   
       
         
           
             
               
                 
                   
                     CR 
                     = 
                     
                       
                         2.3 
                         [ 
                         Cu 
                         ] 
                       
                       + 
                       
                         2.8 
                         [ 
                         Cr 
                         ] 
                       
                       + 
                       
                         7.3 
                         [ 
                         Sb 
                         ] 
                       
                       + 
                       
                         3.6 
                         [ 
                         Sn 
                         ] 
                       
                     
                   
                 
                 
                   
                     Expression 
                     ⁢ 
                         
                     
                       ( 
                       1 
                       ) 
                     
                   
                 
               
             
           
         
         where [X] represents content of element X in the steel, in mass %. 
       
     
     
         2 . The steel plate according to  claim 1 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of
 Ni: 0.01% to 2.00%,   Mo: 0.01% to 0.50% and,   W: 0.01% to 1.00%.   
     
     
         3 . The steel plate according to  claim 1 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of
 V: 0.01% to 1.00%,   Ti: 0.005% to 0.100%,   Co: 0.01% to 1.00%,   Nb: 0.005% to 0.100%,   B: 0.0001% to 0.0100%,   Ca: 0.0005% to 0.0200%,   Mg: 0.0005% to 0.0200%, and   REM: 0.0005% to 0.0200%.   
     
     
         4 . A method of producing a steel plate, the method applied to a steel material comprising a chemical composition containing, in mass %,
 C: 0.010% to 0.200%,   Si: 0.01% to 0.50%,   Mn: 0.50% to 2.50%,   Al: 0.060% or less,   N: 0.0010% to 0.0100%,   P: 0.020% or less,   S: 0.0100% or less, and   O: 0.0100% or less   and further, at least one selected from the group consisting of   Cu: 0.01% to 0.50%,   Cr: 0.01% to 1.00%,   Sb: 0.01% to 0.50%, and   Sn: 0.01% to 0.50%,   with the balance being Fe and inevitable impurity, wherein a CR value obtained by the following Expression (1) is 0.70 or more, the method comprising:   hot rolling with a rolling finish temperature that is Ar 3  transformation temperature or more; followed by cooling from a cooling start temperature that is the Ar 3  transformation temperature or more to a cooling stop temperature of 600° C. or less, wherein,   in the cooling, cooling rate at a 1.0 mm depth position from the steel plate surface is 150° C./s or less, and cooling rate at a ½ thickness position of the steel plate is 10° C./s or more, and   
       
         
           
             
               
                 
                   
                     CR 
                     = 
                     
                       
                         2.3 
                         [ 
                         Cu 
                         ] 
                       
                       + 
                       
                         2.8 
                         [ 
                         Cr 
                         ] 
                       
                       + 
                       
                         7.3 
                         [ 
                         Sb 
                         ] 
                       
                       + 
                       
                         3.6 
                         [ 
                         Sn 
                         ] 
                       
                     
                   
                 
                 
                   
                     Expression 
                     ⁢ 
                         
                     
                       ( 
                       1 
                       ) 
                     
                   
                 
               
             
           
         
         where [X] represents content of element X in the steel, in mass %. 
       
     
     
         5 . The method of producing a steel plate according to  claim 4 , wherein the chemical composition of the steel material further contains, in mass %, at least one selected from the group consisting of
 Ni: 0.01% to 2.00%,   Mo: 0.01% to 0.50% and,   W: 0.01% to 1.00%.   
     
     
         6 . The method of producing a steel plate according to  claim 4 , wherein the chemical composition of the steel material further contains, in mass %, at least one selected from the group consisting of
 V: 0.01% to 1.00%,   Ti: 0.005% to 0.100%,   Co: 0.01% to 1.00%,   Nb: 0.005% to 0.100%,   B: 0.0001% to 0.0100%,   Ca: 0.0005% to 0.0200%,   Mg: 0.0005% to 0.0200%, and   REM: 0.0005% to 0.0200%.   
     
     
         7 . The steel plate according to  claim 2 , wherein the chemical composition further contains, in mass %, at least one selected from the group consisting of
 V: 0.01% to 1.00%,   Ti: 0.005% to 0.100%,   Co: 0.01% to 1.00%,   Nb: 0.005% to 0.100%,   B: 0.0001% to 0.0100%,   Ca: 0.0005% to 0.0200%,   Mg: 0.0005% to 0.0200%, and   REM: 0.0005% to 0.0200%.   
     
     
         8 . The method of producing a steel plate according to  claim 5 , wherein the chemical composition of the steel material further contains, in mass %, at least one selected from the group consisting of
 V: 0.01% to 1.00%,   Ti: 0.005% to 0.100%,   Co: 0.01% to 1.00%,   Nb: 0.005% to 0.100%,   B: 0.0001% to 0.0100%,   Ca: 0.0005% to 0.0200%,   Mg: 0.0005% to 0.0200%, and   REM: 0.0005% to 0.0200%.