High-strength steel material having excellent low-temperature strain again impact properties and method for manufacturing same

Abstract

The present invention relates to a steel material for pressure vessels, offshore structures and the like and, more specifically, to a high-strength steel material having excellent low-temperature strain aging impact properties and a method for manufacturing same.

1 . A high-strength steel material having excellent low-temperature strain aging impact properties, the steel material comprising: 0.04-0.14 wt % of carbon (C), 0.05-0.60 wt % of silicon (Si), 0.6-1.8 wt % of manganese (Mn), 0.005-0.06 wt % of soluble aluminum (sol. Al) , 0.005-0.05 wt % of niobium (Nb), 0.01 wt % or less (exclusive of 0 wt %) of vanadium (V), 0.001-0.015 wt % of titanium (Ti), 0.01-0.4 wt % of copper (Cu), 0.01-0.6 wt % of nickel (Ni), 0.01-0.2 wt % of chromium (Cr), 0.001-0.3 wt % of molybdenum (Mo), 0.0002-0.0040 wt % of calcium (Ca), 0.001-0.006 wt % of nitrogen (N), 0.02 wt % or less (exclusive of 0 wt %) of phosphorus (P), and 0.003 wt % or less (exclusive of 0 wt %) of sulfur (S), with a balance of Fe and other inevitable impurities,
 comprising a mixed structure of ferrite, pearlite, bainite and a martensite-austenite (MA) composite phase as a microstructure, wherein a fraction of the MA phase is 3.5% or less (exclusive of 0%) , wherein a sum of fractions of remaining phases, other than ferrite, is 18% or less (exclusive of 0%), and   wherein 5% strain aging DBTT temperature is −54° C. or less.   
     
     
         2 . The high-strength steel material of  claim 1 , wherein the niobium (Nb) is comprised in an amount of 0.02-0.05%. 
     
     
         3 . The high-strength steel material of  claim 1 , wherein the 5% strain aging DBTT temperature is from −54° C. to −61° C. 
     
     
         4 . The high-strength steel material of  claim 1 , wherein an average of ferrite crystal grain size is 15 μm or less. 
     
     
         5 . The high-strength steel material of  claim 1 , comprising carbonitrides having an average size of 300 nm or less at 0.01% or more by weight ratio. 
     
     
         6 . The high-strength steel material of  claim 1 , wherein a yield ratio (YS (low yield strength)/TS (tensile strength)) is 0.65-0.80. 
     
     
         7 . A method for manufacturing a high-strength steel material having excellent low-temperature strain aging impact properties, the method comprising: reheating a steel slab including 0.04-0.14 wt % of carbon (C), 0.05-0.60 wt % of silicon (Si), 0.6-1.8 wt % of manganese (Mn), 0.005-0.06 wt % of soluble aluminum (sol. Al), 0.005-0.05 wt % of niobium (Nb), 0.01 wt % or less (exclusive of 0 wt %) of vanadium (V), 0.001-0.015 wt % of titanium (Ti), 0.01-0.4 wt % of copper (Cu), 0.01-0.6 wt % of nickel (Ni), 0.01-0.2 wt % of chromium (Cr), 0.001-0.3 wt % of molybdenum (Mo), 0.0002-0 .0040 wt % of calcium (Ca), 0.001-0.006 wt % of nitrogen (N), 0.02 wt % or less (exclusive of 0 wt %) of phosphorus (P), and 0.003 wt % or less (exclusive of 0 wt %) of sulfur (S) , with a balance of Fe and other inevitable impurities in a temperature range of 1080-1250° C.;
 controlled rolling the reheated slab so that a rolling end temperature is 780° C. or more, thereby being manufactured to form a hot-rolled steel plate; 
 cooling the hot-rolled steel plate by air cooling or water cooling; and 
 after the cooling, subjecting the hot-rolled steel plate to normalizing heat treatment in a temperature range of 850-960° C. 
 
     
     
         8 . The method of  claim 7 , wherein the steel slab includes 0.02-0.05% of niobium (Nb). 
     
     
         9 . The method of  claim 7 , wherein the normalizing heat treatment is performed for {(1.3×t)+(10−60)} minutes (wherein ‘t’ refers to a steel material thickness (mm)). 
     
     
         10 . The method of  claim 7 , wherein the reheated slab is formed of 50% or more of Nb being solid-solubilized again.