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 steel plate comprising, by weight:
 0.05 to 0.20% of C,   0.3 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,   0.0002% or less of B, and   a balance of Fe, with inevitable impurities,   the steel plate 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   the steel plate consists of a combined structure of tempered martensite of 40 to 58 area % and tempered bainite of 42 to 60 area % as a microstructure,   wherein a grain aspect ratio, which is a ratio of long axis to short axis, of the microstructure is 1.1 to 2.5,   wherein Charpy impact toughness of the steel plate is 300J or more at −60° C., and   wherein a nil-ductility transition temperature of the steel plate is −50° C. or lower.   
     
     
         2 . The steel plate according to  claim 1 , wherein tensile strength of the steel plate is 600 MPa or more. 
     
     
         3 . The steel plate according to  claim 1 , comprising, by weight: 0.12 to 0.20% of C. 
     
     
         4 . The steel plate according to  claim 1 , comprising, by weight: 0.09 to 0.20% of C. 
     
     
         5 . Nb. The steel plate according to  claim 1 , comprising, by weight: 0.005 to 0.03% of 
     
     
         6 . The steel plate according to  claim 1 , wherein a nil-ductility transition temperature of the steel plate is  31  85° C. to −50° C.,
 wherein tensile strength of the steel plate is 600 MPa to 660 MPa, and 
 wherein, at −60° C., the Charpy impact toughness of the steel plate is 300J to 334J. 
 
     
     
         7 . The steel plate according to  claim 1 , comprising, by weight: 0.09 to 0.12% of C, and 0.01 to 0.03% of Nb.