High strength steel plate and manufacturing method therefor

Abstract

A high strength steel plate according to an aspect of the present disclosure includes, by weight: 0.05 to 0.20% of C, 0.15 to 0.55% of Si, 0.9 to 1.75% of Mn, 0.001 to 0.05% of Al, 0.03% or less of P, 0.03% or less of S, 0.05 to 0.3% of Cr, 0.05 to 0.6% of Ni, 0.005 to 0.35% of Cu, 0.05 to 0.2% of Mo, 0.005 to 0.07% of V, 0.005 to 0.04% of Nb, 0.0005 to 0.005% of Ca, 0.005 to 0.025% of Ti, 0.002 to 0.006% of N, less than 0.0005% of B, and a balance of Fe, with inevitable impurities, satisfies relationships of Cu+Ni+Cr+Mo: 1.5% or less, Cr+Mo: 0.4% or less, V+Nb: 0.1% or less, and Ca/S: 1.0 or higher, and includes a combined structure of tempered martensite and tempered bainite as a microstructure.

1 . A high strength steel plate comprising, by weight: 0.05 to 0.20% of C, 0.15 to 0.55% of Si, 0.9 to 1.75% of Mn, 0.001 to 0.05% of Al, 0.03% or less of P, 0.03% or less of S, 0.05 to 0.3% of Cr, 0.05 to 0.6% of Ni, 0.005 to 0.35% of Cu, 0.05 to 0.2% of Mo, 0.005 to 0.07% of V, 0.005 to 0.04% of Nb, 0.0005 to 0.005% of Ca, 0.005 to 0.025% of Ti, 0.002 to 0.006% of N, less than 0.0005% of B, and a balance of Fe, with inevitable impurities, satisfying relationships of Cu+Ni+Cr+Mo: 1.5% or less, Cr+Mo: 0.4% or less, V+Nb: 0.1% or less, and Ca/S: 1.0 or higher, and comprising a combined structure of tempered martensite and tempered bainite as a microstructure. 
     
     
         2 . The high strength steel plate according to  claim 1 , wherein the tempered martensite comprises 30 to 60 area % in the microstructure, the tempered bainite comprises 40 to 70 area % in the microstructure, and the sum of the tempered martensite and the tempered bainite is 100 area %. 
     
     
         3 . The high strength steel plate according to  claim 2 , wherein the tempered martensite comprises 40 to 60 area % in the microstructure, and the tempered bainite comprises 40 to 60 area % in the microstructure. 
     
     
         4 . The high strength steel plate according to  claim 1 , wherein a nil-ductility transition temperature of the steel plate is −50° C. or lower. 
     
     
         5 . The high strength steel plate according to  claim 1 , wherein tensile strength of the steel plate is 600 MPa or more. 
     
     
         6 . The high strength steel plate according to  claim 1 , wherein Charpy impact toughness of the steel plate is 300J or more at −60° C. 
     
     
         7 . The high strength steel plate according to  claim 1 , wherein a grain aspect ratio (a long axis/short axis ratio) of the microstructure is 1.1 to 2.5. 
     
     
         8 . A method of manufacturing a high strength steel plate, comprising:
 reheating a steel slab at 1050 to 1250° C., the steel slab comprising, by weight: 0.05 to 0.20% of C, 0.15 to 0.55% of Si, 0.9 to 1.75% of Mn, 0.001 to 0.05% of Al, 0.03% or less of P, 0.03% or less of S, 0.05 to 0.3% of Cr, 0.05 to 0.6% of Ni, 0.005 to 0.35% of Cu, 0.05 to 0.2% of Mo, 0.005 to 0.07% of V, 0.005 to 0.04% of Nb, 0.0005 to 0.005% of Ca, 0.005 to 0.025% of Ti, 0.002 to 0.006% of N, less than 0.0005% of B, and a balance of Fe, with inevitable impurities, satisfying relationships of Cu+Ni+Cr+Mo: 1.5% or less, Cr+Mo: 0.4% or less, V+Nb: 0.1% or less, and Ca/S: 1.0 or higher,   rolling the slab in a temperature range of Tnr to Tnr+100° C. to provide a steel plate,   austenizing the steel plate in a temperature range of 870 to 950° C.,   quenching the austenized steel plate to a temperature range of 300° C. or lower, and   tempering the quenched steel plate in a temperature range of 595 to 700° C.   
     
     
         9 . The method according to  claim 8 , wherein an accumulated reduction amount of the rolling is 50 to 90%. 
     
     
         10 . The method according to  claim 8 , wherein a grain aspect ratio (a long axis/short axis ratio) of a microstructure of the steel plate by the rolling is controlled to have a range of 1.1 to 2.5. 
     
     
         11 . The method according to  claim 8 , wherein the austenizing is performed for a time period of 1.6*t (where, t denotes a thickness (mm) of the steel plate)+(10 to 30 minutes). 
     
     
         12 . The method according to  claim 8 , wherein the tempering is performed for a time period of 2.4*t (where, t denotes a thickness (mm) of the steel plate)+(10 to 30 minutes).