Steel plate and method of producing same

Abstract

Provided is a steel plate having high strength of TS≥690 MPa, excellent low-temperature toughness, low yield ratio, and low acoustic anisotropy. A steel plate comprises a chemical composition containing certain elements in certain amounts with a balance consisting of Fe and inevitable impurities. In a microstructure at a depth position of ¼ of a plate thickness from a plate surface in a plate thickness direction, a ferrite fraction is 5% to 95% and a martensite austenite constituent fraction is 1% to 30% with residual microstructure consisting of tempered martensite and/or bainite, an average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is less than 2.0, and a number density of crystal grains having an equivalent circular diameter of more than 30 μm among the crystal grains is 250/mm2 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: 0.5% or more and less than 5.0%,   P: 0.03% or less,   S: 0.005% 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 from a surface of the steel plate in a plate thickness direction, a ferrite fraction is 5% to 95% and a martensite austenite constituent fraction is 1% to 30% with a residual microstructure consisting of tempered martensite and/or bainite, an average aspect ratio of crystal grains that are regions surrounded by large-angle grain boundaries with an orientation difference of 15 degrees or more is less than 2.0, and a number density of crystal grains having an equivalent circular diameter of more than 30 μm among the crystal grains is 250/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
 Nb: 0.05% or less,   Al: 0.100% or less,   Cr: 2.00% or less,   Mo: 1.0% or less,   Cu: 2.0% or less,   V: 0.05% or less,   Ti: 0.03% or less,   B: 0.0030% or less,   Ca: 0.007% or less,   REM: 0.010% or less, and   Mg: 0.007% or less.   
     
     
         3 . A method of producing a steel plate, the method comprising subjecting a steel material having the chemical composition according to  claim 1  to hot rolling, thereafter to first heating retention, thereafter to quenching, thereafter to second heating retention, and thereafter to cooling treatment,
 wherein in the hot rolling, a finish temperature is 900° C. or more, 
 in the first heating retention, a heating temperature is in a temperature range of Ac 3  point or more and 1000° C. or less, 
 in the quenching, an average cooling rate from 600° C. to 300° C. at a depth position of ¼ of a plate thickness from a surface of the steel plate in a plate thickness direction is 3° C./s or more, and a cooling end temperature is 300° C. or less, 
 in the second heating retention, a heating temperature is in a temperature range of Ac 1  point or more and less than the Ac 3  point, and 
 in the cooling treatment, an average cooling rate from 700° C. to 500° C. at the depth position of ¼ of the plate thickness from the surface of the steel plate in the plate thickness direction is 3° C./s or more, and a cooling end temperature is 500° C. or less and 200° C. or more. 
 
     
     
         4 . A method of producing a steel plate, the method comprising subjecting a steel material having the chemical composition according to  claim 2  to hot rolling, thereafter to first heating retention, thereafter to quenching, thereafter to second heating retention, and thereafter to cooling treatment,
 wherein in the hot rolling, a finish temperature is 900° C. or more, 
 in the first heating retention, a heating temperature is in a temperature range of Ac 3  point or more and 1000° C. or less, 
 in the quenching, an average cooling rate from 600° C. to 300° C. at a depth position of ¼ of a plate thickness from a surface of the steel plate in a plate thickness direction is 3° C./s or more, and a cooling end temperature is 300° C. or less, 
 in the second heating retention, a heating temperature is in a temperature range of Ac 1  point or more and less than the Ac 3  point, and 
 in the cooling treatment, an average cooling rate from 700° C. to 500° C. at the depth position of ¼ of the plate thickness from the surface of the steel plate in the plate thickness direction is 3° C./s or more, and a cooling end temperature is 500° C. or less and 200° C. or more.