High-strength ultra-thick steel with excellent cryogenic strain aging impact toughness at center zone thereof, and method for manufacturing same

Abstract

An aspect of the present invention is to provide high-strength ultra-thick steel with excellent cryogenic strain aging impact toughness at the center thereof, and a method for manufacturing same. An embodiment of the present invention provides high-strength ultra-thick steel with excellent cryogenic strain aging impact toughness at the center thereof, and a method for manufacturing same, the steel comprising, by wt %, 0.02-0.06% of C, 1.8-2.2% of Mn, 0.7-1.1% of Ni, 0.2-0.5% of Mo, 0.005-0.03% of Nb, 0.005-0.018% of Ti, 80 ppm or less of P, 20 ppm or less of S, and the remainder of Fe and other evitable impurities, wherein the average grain size of grains having a high boundary angle of 15 degrees or greater is 15 μm or less as measured in a range of ⅜t-⅝t in the thickness (t) direction by EBSD.

1 . A high-strength ultra-thick steel material having excellent cryogenic strain aging impact toughness in a center zone thereof, the steel material comprising:
 by wt %, 0.02-0.06% of C, 1.8-2.2% of Mn, 0.7-1.1% of Ni, 0.2-0.5% of Mo, 0.005-0.03% of Nb, 0.005-0.018% of Ti, 80 ppm or less of P, 20 ppm or less of S, and a balance of Fe and inevitable impurities,   wherein an average grain size of grains having a high boundary angle of 15 degrees or greater, measured by EBSD, is 15 μm or less in a ⅜t-⅝t zone in a thickness (t) direction.   
     
     
         2 . The steel material of  claim 1 , wherein the steel material has a microstructure including acicular ferrite, granular bainite, and upper bainite. 
     
     
         3 . The steel material of  claim 1 , wherein the steel material has a thickness of 5-90 mm. 
     
     
         4 . The steel material of  claim 1 , wherein the steel material has yield strength of 500 MPa or more. 
     
     
         5 . The steel material of  claim 1 , wherein, after a heat treatment is performed on the steel material at 250° C. for 1 hour after deformation of 5%, a transition temperature is −60° C. or less in a strain aging impact test. 
     
     
         6 . A method for manufacturing a high-strength ultra-thick steel material having excellent cryogenic strain aging impact toughness in a center zone thereof, the method comprising:
 reheating a steel slab including, by wt %, 0.02-0.06% of C, 1.8-2.2% of Mn, 0.7-1.1% of Ni, 0.2-0.5% of Mo, 0.005-0.03% of Nb, 0.005-0.018% of Ti, 80 ppm or less of P, 20 ppm or less of S, and a balance of Fe and inevitable impurities to a temperature of 1000-1080° C.;   obtaining a bar by rough-rolling the reheated steel slab at a temperature of 850-1050° C.;   obtaining a hot-rolled steel material by finish-rolling the bar at a temperature of 700-800° C. at a total reduction ratio of more than 60%; and   cooling the hot-rolled steel material to a temperature of 500° C. or less at a cooling rate of 3° C./s or more.   
     
     
         7 . The method of  claim 6 , wherein a total reduction ratio during the rough-rolling is 40% or more.