High-strength steel with excellent toughness of heat-affected zone and manufacturing method thereof

Abstract

The present invention relates to structural steel used as a material of storage tanks, pressure vessels, building structures, ship structures, etc, and, more particularly, to steel with excellent toughness of a welding heat affected zone and a manufacturing method thereof.

1 . A high-strength steel having excellent toughness in a heat-affected zone, comprising:
 in weight %, C: 0.16 to 0.20%, Mn: 1.0 to 1.5%, Si: 0.3% or less (excluding O), Al: 0.005 to 0.5%, P: 0.02% or less, S: 0.01% or less, Ti: 0.005 to 0.02%, Nb: 0.01 to 0.1%, and N: 0.006 to 0.01%; and   at least one selected from the group consisting of Ca: 0.006% or less, V: 0.03% or less, Ni: 2.0% or less, Cu: 1.0% or less, Cr: 1.0% or less, and Mo: 1.0% or less, and a balance of Fe and unavoidable impurities,   wherein a microstructure of the high-strength steel is composed of a ferrite-pearlite composite structure, and   after welding and a stress relief heat treatment, in the microstructure, 1.27×10 6  or more of precipitates of 100 nm or less are present per 1 mm 2 , and 900 or more of precipitates are distributed in a single crystal grain.   
     
     
         2 . The high-strength steel having excellent toughness in a heat-affected zone of  claim 1 , wherein the precipitates are Nb-based carbides. 
     
     
         3 . The high-strength steel having excellent toughness in a heat-affected zone of  claim 1 , wherein the steel has a surface crack sensitivity index (Cs) of 0.3 or less, defined by the following relational expression 1,
   Cs=(71.4×[C] 2 )−(30.3×[C])+3.32,  [Relational Expression 1]
   where [C] is a weight % content of a corresponding component.   
     
     
         4 . The high-strength steel having excellent toughness in a heat-affected zone of  claim 1 , wherein the steel has free-N defined by the following relational expression 2, exceeding 0,
   Free-N=[N]−{([Ti]/47.887)×14.01}−{([B]/10.81)×14.01},  [Relational Expression 2]
   where [N], [Ti] and [B] are weight % contents of corresponding components.   
     
     
         5 . The high-strength steel having excellent toughness in a heat-affected zone of  claim 1 , wherein the steel has a tensile strength of 500 MPa or higher and a Charpy impact absorption energy of 150 J or higher even after stress relief heat treatment. 
     
     
         6 . The high-strength steel having excellent toughness in a heat-affected zone of  claim 1 , wherein in the heat-affected zone of the steel, a martensite austenite constituent (MA) is 3% or less in area fraction. 
     
     
         7 . The high-strength steel having excellent toughness in a heat-affected zone of  claim 1 , wherein in the steel, the heat-affected zone has Charpy impact absorption energy of 100 J or higher at 0° C. after large heat input welding. 
     
     
         8 . A method of manufacturing a high-strength steel having excellent toughness in a heat-affected zone, comprising:
 preparing a steel slab,   the steel slab including,   in weight %, C: 0.16 to 0.20%, Mn: 1.0 to 1.5%, Si: 0.3% or less (excluding O), Al: 0.005 to 0.5%, P: 0.02% or less, S: 0.01% or less, Ti: 0.005 to 0.02%, Nb: 0.01 to 0.1%, and N: 0.006 to 0.01%, and   at least one selected from the group consisting of Ca: 0.006% or less, V: 0.03% or less, Ni: 2.0% or less, Cu: 1.0% or less, Cr: 1.0% or less and Mo: 1.0% or less, and a balance of Fe and unavoidable impurities;   heating the steel slab to a temperature in a range of 1050 to 1250° C.;   hot rolling the heated steel slab at a temperature of hot finish rolling of 910° C. or lower; and   performing cooling at a cooling rate of 20° C./Hr or less after the hot rolling.   
     
     
         9 . The method of manufacturing a high-strength steel having excellent toughness in a heat-affected zone of  claim 8 , wherein the hot finish rolling is performed at a temperature in a range of 850 to 910° C. 
     
     
         10 . The method of manufacturing a high-strength steel having excellent toughness in a heat-affected zone of  claim 8 , wherein the cooling is performed at a temperature in a range of 800 to 435° C. at a cooling rate of 20° C./Hr or less.