Steel plate and method of producing same

Abstract

A steel plate comprises a certain chemical composition, wherein in a microstructure at a depth position of ¼ of a plate thickness of the steel plate, a ferrite fraction is 5% to 95% and a martensite austenite constituent fraction is 1% to 30% with a residual microstructure consisting of one or both of bainite and tempered martensite, and a number density of crystal grains having an equivalent circular diameter of more than 30 μm among crystal grains defined by large-angle grain boundaries with an orientation difference of 15 degrees or more is 200/mm 2 or less.

1 . A steel plate comprising a chemical composition containing, in mass %,
 C: 0.02% or more and 0.15% or less,   Si: 0.01% or more and 0.50% or less,   Mn: 0.05% or more and 2.50% or less,   Ni: 0.50% or more and less than 5.00%,   P: 0.03% or less,   S: 0.0050% or less, and   N: 0.0010% or more and 0.0080% or less,   with a balance consisting of Fe and inevitable impurities,   wherein in a microstructure at a depth position of ¼ of a plate thickness of the steel plate, a ferrite fraction is 5% to 95% and a martensite austenite constituent fraction is 1% to 30% with a residual microstructure consisting of one or both of bainite and tempered martensite, and a number density of crystal grains having an equivalent circular diameter of more than 30 μm among crystal grains defined by large-angle grain boundaries with an orientation difference of 15 degrees or more is 200/mm 2  or less.   
     
     
         2 . The steel plate according to  claim 1 , wherein the chemical composition further contains, in mass %, one or more selected from
 Cr: 2.00% or less,   Mo: 1.0% or less,   Al: 0.100% or less,   Cu: 2.0% or less,   Nb: 0.1% or less,   V: 0.05% or less,   Ti: 0.03% or less, and   B: 0.0030% or less.   
     
     
         3 . The steel plate according to  claim 1 , wherein the chemical composition further contains, in mass %, one or more selected from
 Ca: 0.007% or less,   REM: 0.010% or less, and   Mg: 0.007% or less.   
     
     
         4 . The steel plate according to  claim 2 , wherein the chemical composition further contains, in mass %, one or more selected from
 Ca: 0.007% or less,   REM: 0.010% or less, and   Mg: 0.007% or less.   
     
     
         5 . The steel plate according to  claim 1 , wherein the chemical composition further contains, in mass %, one or more selected from
 Cr: 2.00% or less,   Mo: 1.0% or less,   Al: 0.100% or less,   Cu: 2.0% or less,   Nb: 0.1% or less,   V: 0.05% or less,   B: 0.0030% or less,   Ca: 0.007% or less,   REM: 0.010% or less, and   Mg: 0.007% or less.   
     
     
         6 . A method of producing a steel plate, the method comprising:
 performing first heating that heats a steel material having the chemical composition according to  claim 1  in a heat treatment furnace;   thereafter hot rolling the steel material to obtain a hot-rolled steel plate;   thereafter quenching the hot-rolled steel plate;   thereafter performing second heating that heats the hot-rolled steel plate; and   thereafter cooling the hot-rolled steel plate,   wherein in the first heating, a soaking temperature in the heat treatment furnace is 900° C. or more and 1250° C. or less, and an in-furnace time in the heat treatment furnace is 600 minutes or less,   in the hot rolling, a finish temperature is 1000° C. or less and 700° C. or more at a surface of the hot-rolled steel plate,   in the quenching, an average cooling rate in a temperature range from 600° C. to 300° C. is 3° C./s or more and a cooling end temperature is 300° C. or less, at a depth position of ¼ of a plate thickness of the hot-rolled steel plate,   in the second heating, a heating temperature is Ac 1  point or more and less than Ac 3  point, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate, and   in the cooling, an average cooling rate in a temperature range from 700° C. to 500° C. is 3° C./s or more and a cooling end temperature is 500° C. or less and 200° C. or more, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate.   
     
     
         7 . A method of producing a steel plate, the method comprising:
 performing first heating that heats a steel material having the chemical composition according to  claim 2  in a heat treatment furnace;   thereafter hot rolling the steel material to obtain a hot-rolled steel plate;   thereafter quenching the hot-rolled steel plate;   thereafter performing second heating that heats the hot-rolled steel plate; and   thereafter cooling the hot-rolled steel plate,   wherein in the first heating, a soaking temperature in the heat treatment furnace is 900° C. or more and 1250° C. or less, and an in-furnace time in the heat treatment furnace is 600 minutes or less,   in the hot rolling, a finish temperature is 1000° C. or less and 700° C. or more at a surface of the hot-rolled steel plate,   in the quenching, an average cooling rate in a temperature range from 600° C. to 300° C. is 3° C./s or more and a cooling end temperature is 300° C. or less, at a depth position of ¼ of a plate thickness of the hot-rolled steel plate,   in the second heating, a heating temperature is Ac 1  point or more and less than Ac 3  point, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate, and   in the cooling, an average cooling rate in a temperature range from 700° C. to 500° C. is 3° C./s or more and a cooling end temperature is 500° C. or less and 200° C. or more, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate.   
     
     
         8 . A method of producing a steel plate, the method comprising:
 performing first heating that heats a steel material having the chemical composition according to  claim 3  in a heat treatment furnace;   thereafter hot rolling the steel material to obtain a hot-rolled steel plate;   thereafter quenching the hot-rolled steel plate;   thereafter performing second heating that heats the hot-rolled steel plate; and   thereafter cooling the hot-rolled steel plate,   wherein in the first heating, a soaking temperature in the heat treatment furnace is 900° C. or more and 1250° C. or less, and an in-furnace time in the heat treatment furnace is 600 minutes or less,   in the hot rolling, a finish temperature is 1000° C. or less and 700° C. or more at a surface of the hot-rolled steel plate,   in the quenching, an average cooling rate in a temperature range from 600° C. to 300° C. is 3° C./s or more and a cooling end temperature is 300° C. or less, at a depth position of ¼ of a plate thickness of the hot-rolled steel plate,   in the second heating, a heating temperature is Ac 1  point or more and less than Ac 3  point, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate, and   in the cooling, an average cooling rate in a temperature range from 700° C. to 500° C. is 3° C./s or more and a cooling end temperature is 500° C. or less and 200° C. or more, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate.   
     
     
         9 . A method of producing a steel plate, the method comprising:
 performing first heating that heats a steel material having the chemical composition according to  claim 4  in a heat treatment furnace;   thereafter hot rolling the steel material to obtain a hot-rolled steel plate;   thereafter quenching the hot-rolled steel plate;   thereafter performing second heating that heats the hot-rolled steel plate; and   thereafter cooling the hot-rolled steel plate,   wherein in the first heating, a soaking temperature in the heat treatment furnace is 900° C. or more and 1250° C. or less, and an in-furnace time in the heat treatment furnace is 600 minutes or less,   in the hot rolling, a finish temperature is 1000° C. or less and 700° C. or more at a surface of the hot-rolled steel plate,   in the quenching, an average cooling rate in a temperature range from 600° C. to 300° C. is 3° C./s or more and a cooling end temperature is 300° C. or less, at a depth position of ¼ of a plate thickness of the hot-rolled steel plate,   in the second heating, a heating temperature is Ac 1  point or more and less than Ac 3  point, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate, and   in the cooling, an average cooling rate in a temperature range from 700° C. to 500° C. is 3° C./s or more and a cooling end temperature is 500° C. or less and 200° C. or more, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate.   
     
     
         10 . A method of producing a steel plate, the method comprising:
 performing first heating that heats a steel material having the chemical composition according to claim  5  in a heat treatment furnace;   thereafter hot rolling the steel material to obtain a hot-rolled steel plate;   thereafter quenching the hot-rolled steel plate;   thereafter performing second heating that heats the hot-rolled steel plate; and   thereafter cooling the hot-rolled steel plate,   wherein in the first heating, a soaking temperature in the heat treatment furnace is 900° C. or more and 1250° C. or less, and an in-furnace time in the heat treatment furnace is 600 minutes or less,   in the hot rolling, a finish temperature is 1000° C. or less and 700° C. or more at a surface of the hot-rolled steel plate,   in the quenching, an average cooling rate in a temperature range from 600° C. to 300° C. is 3° C./s or more and a cooling end temperature is 300° C. or less, at a depth position of ¼ of a plate thickness of the hot-rolled steel plate,   in the second heating, a heating temperature is Ac 1  point or more and less than Ac 3  point, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate, and   in the cooling, an average cooling rate in a temperature range from 700° C. to 500° C. is 3° C./s or more and a cooling end temperature is 500° C. or less and 200° C. or more, at the depth position of ¼ of the plate thickness of the hot-rolled steel plate.