Low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness

Abstract

An aspect of the present invention relates to a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness, the steel comprising, by weight, 0.02-0.10% of carbon (C), 0.5-2.0% of manganese (Mn), 0.05-0.5% of silicon (Si), 0.05-1.0% of nickel (Ni), 0.005-0.1% of titanium (Ti), 0.005-0.5% of aluminum (Al), 0.005% of less of niobium (Nb), 0.015% or less of phosphorus (P), 0.015% or less of sulfur (S), and the balanced amount of Fe and inevitable impurities, the microstructure of which comprises: by area, 60% or more of acicular ferrite and a balanced amount of one or more phases of bainite, polygonal ferrite and martensite-austenite constituent (MA).

1 . A low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness, comprising:
 by weight percent, 0.02 to 0.10% of carbon (C), 0.5 to 2.0% of manganese (Mn), 0.05 to 0.5% of silicon (Si), 0.05 to 1.0% of nickel (Ni), 0.005 to 0.1% of titanium (Ti), 0.005 to 0.5% of aluminum (Al), 0.005% or less of niobium (Nb), 0.015% or less of phosphorus (P), 0.015% or less of sulfur (S), a balance of Fe and other inevitable impurities,   wherein a microstructure includes, in area percent (%), 60% or more of acicular ferrite and the balance including at least one phase of bainite, polygonal ferrite and martensite-austenite constituent (MA).   
     
     
         2 . The steel of  claim 1 , wherein
 a size of the acicular ferrite measured in terms of the equivalent of a circle diameter is 30 μm or less.   
     
     
         3 . The steel of  claim 1 , wherein
 the bainite has a 30 area % or less.   
     
     
         4 . The steel of  claim 1 , wherein
 the MA phase is 10 area % or less, and a size of the MA phase measured in terms of the equivalent of a circle diameter is 5 μm or less.   
     
     
         5 . The steel of  claim 1 , wherein
 a yield ratio of the steel is 0.85 or less and tensile strength of the steel is 490 MPa or greater.   
     
     
         6 . The steel of  claim 1 , wherein
 yield strength of the steel is 440 MPa or less.   
     
     
         7 . The steel of  claim 1 , wherein
 an impact transition temperature of the steel is −60° C. or lower.   
     
     
         8 . A method of manufacturing a low yield ratio and high-strength steel having excellent stress corrosion cracking resistance and low temperature toughness, the method comprising:
 heating a slab including, by weight percent, 0.02 to 0.10% of carbon (C), 0.5 to 2.0% of manganese (Mn), 0.05 to 0.5% of silicon (Si), 0.05 to 1.0% of nickel (Ni), 0.005 to 0.1% of titanium (Ti), 0.005 to 0.5% of aluminum (Al), 0.005% or less of niobium (Nb), 0.015% or less of phosphorus (P), 0.015% or less of sulfur (S), a balance of Fe and other inevitable impurities, to 1000 to 1200° C.;   rough-rolling the heated slab at a temperature of 1100 to 900° C.;   finishing-rolling at a temperature between Ar 3 +100° C. and Ar 3 +30° C. on the basis of a center temperature after the rough rolling; and   cooling to a temperature of 300° C. or lower after the finishing-rolling.   
     
     
         9 . The method of  claim 8 , wherein
 in performing cooling, first cooling is performed such that a cooling rate at the central portion is 15° C./s or greater up to Bs−10° C. to Bs+10° C., and   second cooling is performed up to 300° C. or lower such that a cooling rate at the central portion is 10 to 50° C./s.   
     
     
         10 . The method of  claim 8 , wherein
 a cooling start temperature is Ar 3 +30° C. to Ar 3 .   
     
     
         11 . The method of  claim 8 , wherein
 the rough rolling is performed so that the last three rolling passes have a reduction ratio of 10% or greater per pass.   
     
     
         12 . The method of  claim 8 , wherein
 the finishing rolling is performed such that a reduction ratio per pass is 10% or greater and a cumulative reduction ratio is 60% or greater.