Steel plate having excellent pwht resistance for low-temperature pressure vessel and method for manufacturing same

Abstract

The present disclosure relates to a steel plate having excellent PWHT resistance and low-temperature toughness for a low-temperature pressure vessel and a method for manufacturing the same, wherein the steel plate comprises, in terms of wt %, 0.07-0.17% of C, 0.15-0.40% of Si, 0.3-0.7% of Mn, 0.012% or less of P, 0.015% or less of S, 3.0-4.0% of Ni, 0.03-0.25% of W, and a balance of Fe and inevitable impurities, has a micro-structure comprising 25-80 area % of tempered bainite and the balance of tempered martensite, and has a tensile strength of 600 MPa or more.

1 . A steel plate having excellent PWHT resistance for a low-temperature pressure vessel comprising:
 in terms of wt %, 0.07-0.17% of C, 0.15-0.40% of Si, 0.3-0.7% of Mn, 0.012% or less of P, 0.015% or less of S, 3.0-4.0% of Ni, 0.03-0.25% of W, and a balance of Fe and inevitable impurities,   wherein the steel plate has a steel microstructure including 25-80 area % of tempered bainite and a balance of tempered martensite.   
     
     
         2 . The steel plate of  claim 1 , wherein the steel plate maintains tensile strength at 600 MPa or more even when the steel plate is subjected to PWHT for a maximum of 20 hours at 580-640° C. 
     
     
         3 . The steel plate of  claim 1 , wherein the steel microstructure includes 30-70 area % of tempered bainite and a balance of tempered martensite. 
     
     
         4 . The steel plate of  claim 1 , wherein the steel plate has a Charpy impact energy value of 200 J or more at −110° C. even when the steel plate is subjected to PWHT for a maximum of 20 hours at 580-640° C. 
     
     
         5 . A method for manufacturing a steel plate having excellent PWHT resistance for a low-temperature pressure vessel, the method comprising:
 reheating a steel slab at a temperature of 1050-1250° C., the steel slab including, in terms of wt %, 0.07-0.17% of C, 0.15-0.40% of Si, 0.3-0.7% of Mn, 0.012% or less of P, 0.015% or less of S, 3.0-4.0% of Ni, 0.03-0.25% of W, and a balance of Fe and inevitable impurities;   hot rolling the reheated steel slab and terminating the rolling at a temperature of 800° C. or more to obtain a hot rolled steel plate;   heating the hot rolled steel plate to 800-950° C., followed by water cooling; and   tempering the water cooled steel material at 550-660° C. for {1.5×t+(10−30)} minutes [wherein t is a thickness (mm) of the steel material].   
     
     
         6 . The method of  claim 5 , further comprising:
 a PWHT process for a maximum of 20 hours at 580-640° C. after the tempering.   
     
     
         7 . The method of  claim 5 , wherein a steel microstructure obtained by the tempering has 25-80 area % of tempered bainite and a balance of tempered martensite. 
     
     
         8 . The method of  claim 5 , wherein a steel microstructure obtained by the tempering has 30-70 area % of tempered bainite and a balance of tempered martensite. 
     
     
         9 . The method of  claim 5 , wherein a reduction rate per pass in the hot rolling is 5-30%. 
     
     
         10 . The method of  claim 5 , wherein a cooling rate in the water cooling is 2.5 to 30° C./sec.