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 and has hardness properties such that, at a 0.5 mm depth position from the surface of the steel plate, average hardness is 210 HV or less and variation of the average hardness is 50 HV or less, and a metallic microstructure where, at a 0.5 mm depth position from the surface of the steel plate, a volume fraction of bainitic microstructure is 90% or more and, 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.

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,   with the balance being Fe and inevitable impurity, wherein   the steel plate has hardness properties such that, at a 0.5 mm depth position from the surface of the steel plate, average hardness is 210 HV or less and variation of the average hardness is 50 HV or less, and   the steel plate has a metallic microstructure where, at a 0.5 mm depth position from the surface of the steel plate, a volume fraction of bainitic microstructure is 90% or more and, 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.   
     
     
         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
 Cu: 0.01% to 0.50%,   Ni: 0.01% to 2.00%,   Cr: 0.01% to 1.00%,   Sn: 0.01% to 0.50%,   Sb: 0.01% to 0.50%,   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%,   A1:0.060% or less,   N: 0.0010% to 0.0100%,   P: 0.020% or less,   S: 0.0100% or less, and   O: 0.0100% or less,   with the balance being Fe and inevitable impurity, the method comprising:   hot rolling with a rolling finish temperature that is Ar 3  transformation temperature or more; followed by primary cooling from a cooling start temperature that is the Ar 3  transformation temperature or more; followed by surface heating by recuperation; and followed by secondary cooling, wherein,   in the primary cooling, cooling rate from 600° C. to 400° C. at a 0.5 mm depth position from the surface of the steel plate is 30° C./s to 100° C./s,   the surface heating by recuperation occurs until end-point temperature at a 0.5 mm depth position from the steel plate surface is 500° C. or more, and   in the secondary cooling, at a ½ thickness position of the steel plate, cooling rate to a cooling stop temperature of 600° C. or less is 10° C./s or more.   
     
     
         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
 Cu: 0.01% to 0.50%,   Ni: 0.01% to 2.00%,   Cr: 0.01% to 1.00%,   Sn: 0.01% to 0.50%,   Sb: 0.01% to 0.50%,   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%.