Steel plate for pressure vessel having excellent resistance to high-temperature post-welding heat treatment, and method for manufacturing same

Abstract

The purpose of the present invention is to provide a steel plate for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment, in which deterioration of strength and toughness is minimized even when a long-term post welding heat treatment (PWHT) is applied at high temperature, and a method for manufacturing same.

1 . A steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment, comprising, by weight %,
 carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, manganese (Mn): 0.4 to 0.6%, chromium (Cr): 6.5 to 7.5%, molybdenum (Mo): 0.7˜0.9%, aluminum (Al): 0.005 to 0.05%, phosphorus (P): 0.015% or less, sulfur (S): 0.020% or less, niobium (Nb): 0.002 to 0.025%, vanadium (V): 0.25 to 0.35%, and a balance of Fe and other unavoidable impurities,   wherein a microstructure is a complex structure having tempered martensite and tempered bainite.   
     
     
         2 . The steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment of  claim 1 , wherein the tempered martensite has an area fraction of 40% or more. 
     
     
         3 . The steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment of  claim 1 , wherein the steel material has tensile strength of 600 MPa or more, and a Charpy impact energy value of 100 J or more, after high-temperature post-welding heat treatment (PWHT). 
     
     
         4 . A method for manufacturing a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment, the method comprising operations of:
 preparing a steel slab including, by weight %, carbon (C): 0.10 to 0.16%, silicon (Si): 0.20 to 0.35%, manganese (Mn): 0.4 to 0.6%, chromium (Cr): 6.5 to 7.5%, molybdenum (Mo): 0.7˜0.9%, aluminum (Al): 0.005 to 0.05%, phosphorus (P): 0.015% or less, sulfur (S): 0.020% or less, niobium (Nb): 0.002 to 0.025%, vanadium (V): 0.25 to 0.35%, and a balance of Fe and other unavoidable impurities;   heating the steel slab in a temperature range of 1050 to 1250° C.;   hot rolling the heated steel slab in a temperature range of 800 to 1000° C. to manufacture a hot-rolled steel sheet;   performing heat treatment for maintaining the hot-rolled steel sheet in a temperature range of 1000 to 1050° C. for {(1.3×t)+(10 to 30)} minutes (where, t is a thickness of the steel material (mm));   cooling the heat-treated hot-rolled steel sheet at a cooling rate of 1 to 30° C./s; and   performing tempering heat-treatment for maintaining the cooled hot-rolled steel sheet in a temperature range of 800 to 825° C. for {(1.6×t)+(10 to 30)} minutes.   
     
     
         5 . The method of manufacturing a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment of  claim 4 , wherein the hot rolling is performed at a reduction ratio of 2.5 to 30% per pass. 
     
     
         6 . The method of manufacturing a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment of  claim 4 , further comprising:
 an operation of air cooling to room temperature after the hot rolling.   
     
     
         7 . The method of manufacturing a steel material for a pressure vessel having excellent resistance to high-temperature post-welding heat treatment of  claim 4 , further comprising:
 an operation of post-welding heat treatment in a temperature range of 760 to 780° C. for up to 50 hours after the tempering heat-treatment.