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 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 230HV0.1 or less and hardness variation is 30HV0.1 or less, a maximum value of hardness in the thickness direction is at a position 1.0 mm or more and ¼ or less of the thickness of the steel plate from the surface of the steel plate, and hardness variation in the thickness direction is 70HV1 or less. Further, 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.

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: 0.010% to 0.060%,   N: 0.0010% or more and 0.0100% or less,   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,   at a 0.5 mm depth position from the surface of the steel plate, average hardness is 230HV0.1 or less and hardness variation is 30HV0.1 or less, a maximum value of hardness in the thickness direction is at a position 1.0 mm or more and ¼ or less of the thickness of the steel plate from the surface of the steel plate, and hardness variation in the thickness direction is 70HV1 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.   
     
     
         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%,   Al: 0.010% to 0.060%,   N: 0.0010% or more and 0.0100% or less,   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 accelerated cooling from a cooling start temperature that is the Ar 3  transformation temperature or more; followed by reheating, wherein,   in the accelerated cooling, cooling stop temperature is in a range from 200° C. to 600° C. and cooling rate at a ¼ plate thickness position is 20° C./s to 120° C./s, and   the reheating is performed until end-point temperature at a 0.5 mm depth position from the surface of the steel plate is in a range from 400° C. to 680° C., with end-point temperature at a ¼ plate thickness position being 500° C. or less.   
     
     
         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 2.000%   Cr: 0.01% to 1.000%   Sn: 0.01% to 0.500%   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 0.1000%   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%.