High-strength steel having excellent brittle crack arrestability and welding part brittle crack initiation resistance, and production method therefor

Abstract

The present invention provides a high-strength steel and a production method therefor, the high-strength steel: comprising, in wt %, C: 0.05-0.09%, Mn: 1.5-2.2%, Ni: 0.3-1.2%, Nb: 0.005-0.04%, Ti: 0.005-0.004%, Cu: 0.1-0.8%, Si: 0.05-0.03%, Al: 0.005-0.05%, P: 100 ppm or less, S: 40 ppm or less, and a remainder made up by Fe and other inevitable impurities; having a center part microstructure comprising an acicular ferrite and granular bainite mixed-phase, upper bainite, and a remainder made up by one type or more selected from the group consisting of ferrite, pearlite, and a martensite-austenite (MA) constituent; having, in a 2 mm or less subsurface region, a surface part microstructure comprising ferrite and a remainder made up by one type or more among bainite and martensite, and having a welding heat affected zone, which is formed during welding, that comprises, in area %, 5% or less of a martensite-austenite constituent.

1 . A high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance, comprising:
 by wt %, carbon (C): 0.05% to 0.09%, manganese (Mn): 1.5% to 2.2%, nickel (Ni): 0.3% to 1.2%, niobium (Nb): 0.005% to 0.04%, titanium (Ti): 0.005% to 0.04%, copper (Cu): 0.1% to 0.8%, silicon (Si): 0.05% to 0.3%, aluminum (Al): 0.005% to 0.05%, phosphorus (P): 100 ppm or less, sulfur (S): 40 ppm or less, iron (Fe) as a residual component, and inevitable impurities,   wherein a microstructure of a central portion includes, by area %, a mixed phase of acicular ferrite and granular bainite in an amount of 70% or greater, upper bainite in an amount of 20% or less, and one or more selected from a group consisting of ferrite, pearlite, and martensite-austenite (MA), as residual components; a circle-equivalent diameter of an effective grain of the upper bainite having a high angle grain boundary of 15° or greater measured using an electron backscatter diffraction (EBSD) method being 15 p.m or less; a surface portion microstructure in a region at a depth of 2 mm or less, directly below a surface, includes, by area %, ferrite in an amount of 20% or greater and one or more of bainite and martensite as residual components; and a heat affected zone (HAZ) formed during welding includes, by area %, martensite-austenite (MA) in an amount of 5% or less.   
     
     
         2 . The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of  claim 1 , comprising a thickness of 50 mm or greater. 
     
     
         3 . The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of  claim 1 , wherein, in tenns of a Cu and Ni content, a weight ratio of Cu to Ni is 0.8 or less. 
     
     
         4 . The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of  claim 1 , wherein welding heat input during welding is 0.5 kJ/mm to 10 kJ/mm. 
     
     
         5 . The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of  claim 4 , wherein a welding method during welding includes flux cored arc welding (FCAW) or submerged arc welding (SAW). 
     
     
         6 . The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of  claim 1 , comprising yield strength of 460 MPa or greater. 
     
     
         7 . The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of  claim 1 , comprising a Kca value measured at a temperature of −10° C. of 6000 or greater. 
     
     
         8 . The high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance of  claim 1 , comprising a Charpy fracture transition temperature of −40° C. or lower in a ½t position in a steel material thickness direction, where t is a steel sheet thickness. 
     
     
         9 . A method of manufacturing a high-strength steel material having excellent brittle crack arrestability and welding zone brittle crack initiation resistance, comprising:
 rough rolling a slab at a temperature of 900° C. to 1100° C. after reheating the slab at 1000° C. to 1100° C., including, by wt %, C: 0.05% to 0.09%, Mn: 1.5% to 2.2%, Ni: 0.3% to 1.2%, Nb: 0.005% to 0.04%, titanium (Ti): 0.005% to 0.04%, copper (Cu): 0.1% to 0.8%, silicon (Si): 0.05% to 0.3%, aluminum (Al): 0.005% to 0.05%, phosphorus (P): 100 ppm or less, sulfur (S): 40 ppm or less, iron (Fe) as a residual component, and inevitable impurities;   obtaining a steel sheet by finish rolling a bar obtained from the rough rolling a slab, at a temperature in a range of Ar a +60° C. to Ar 3 ° C., based on a temperature of a central portion; and   cooling the steel sheet to 500° C. or lower.   
     
     
         10 . The method of  claim 9 , wherein a thickness of the steel sheet having been finish rolled is 50 mm or greater. 
     
     
         11 . The method of  claim 9 , wherein a reduction ratio per pass of three final passes during the rough rolling a slab is 5% or greater, and a total cumulative reduction ratio is 40% or greater. 
     
     
         12 . The method of  claim 9 , wherein a strain rate of three final passes during the rough rolling a slab is 2/sec or lower. 
     
     
         13 . The method of  claim 9 , wherein a grain size of a central portion of a bar thickness before finish rolling after the rough rolling a slab is 150 μm or less. 
     
     
         14 . The method of  claim 9 , wherein a reduction ratio during the finish rolling is set such that a ratio of a slab thickness (mm) to a steel sheet thickness (mm) after the finish rolling is 3.5 or greater. 
     
     
         15 . The method of  claim 9 , wherein a cumulative reduction ratio during the finish rolling is maintained to be 40% or greater, and a reduction ratio per pass, not including skin pass rolling, is maintained to be 4% or greater. 
     
     
         16 . The method of  claim 9 , wherein the cooling the steel sheet is performed at a cooling rate of the central portion of 2° C./s or higher. 
     
     
         17 . The method of  claim 9 , wherein the cooling the steel sheet is performed at an average cooling rate of 3° C./s to 300° C./s.