Super duplex stainless steel having excellent yield strength and impact toughness and menufacturing method therefor

Abstract

Provided is super duplex stainless steel having excellent yield strength and impact toughness, wherein a reduction ratio and a heat treatment temperature are controlled so as to improve mechanical properties. The super duplex stainless steel having excellent yield strength and impact toughness is thick super duplex stainless steel having a thickness of 30 mm or greater, and includes, in weight %, Cr: 24% to 26%, Ni: 6.0% to 8.0%, Mo: 3.5% to 5.0%, N: 0.24% to 0.32%, and the remainder being Fe and inevitable impurities, wherein a microstructure includes a ferrite phase, an austenite phase and a secondary austenite phase, and grain size is 25 μm or less.

1 . Super duplex stainless steel having excellent yield strength and impact toughness comprising, as thick super duplex stainless steel having a thickness of 30 mm or greater, in weight %, Cr: 24% to 26%, Ni: 6.0% to 8.0%, Mo: 3.5% to 5.0%, N: 0.24% to 0.32%, and the remainder being Fe and inevitable impurities,
 wherein a microstructure includes a ferrite phase, an austenite phase and a secondary austenite phase, and a grain size is 25 μm or less.   
     
     
         2 . The super duplex stainless steel having excellent yield strength and impact toughness of  claim 1 , wherein the yield strength of the super duplex stainless steel is 550 MPa or greater. 
     
     
         3 . The super duplex stainless steel having excellent yield strength and impact toughness of  claim 2 , wherein a sum of the yield strength and the impact toughness of the super duplex stainless steel is 750 or greater. 
     
     
         4 . A method for manufacturing super duplex stainless steel having excellent yield strength and impact toughness comprising:
 casting of preparing a slab including, in weight %, Cr: 24% to 26%, Ni: 6.0% to 8.0%, Mo: 3.5% to 5.0%, N: 0.24% to 0.32% and the remainder being Fe and inevitable impurities;   hot rolling of hot rolling the slab to produce a thick plate having a thickness of 30 mm or greater;   temperature raising of raising a temperature of the thick plate to an annealing temperature to precipitate a CrN phase inside a ferrite phase, and precipitating a sigma phase and a secondary austenite phase around the CrN phase; and   annealing of keeping the secondary austenite phase inside the ferrite phase while solid dissolving the sigma phase and the CrN phase in the ferrite phase.   
     
     
         5 . The method for manufacturing super duplex stainless steel having excellent yield strength and impact toughness of  claim 4 , wherein the temperature raising is raising a temperature from 700° C. to the annealing temperature at a rate of 0.11° C./s to 0.17° C./s. 
     
     
         6 . The method for manufacturing super duplex stainless steel having excellent yield strength and impact toughness of  claim 5 , wherein the annealing is annealing for 20 minutes to 60 minutes at a temperature of 1020° C. to 1060° C. 
     
     
         7 . The method for manufacturing super duplex stainless steel having excellent yield strength and impact toughness of  claim 4 , wherein the hot rolling is rolling with a reduction ratio of 80% or greater so that a grain size of a microstructure becomes 25 μm or less.