Steel having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, and method for manufacturing same

Abstract

The present invention relates to steel suitable for a pressure vessel that can be used as petrochemical manufacturing equipment, a storage tank, and the like, and, more specifically, to steel having excellent hydrogen-induced cracking (HIC) resistance and low-temperature impact toughness and a method for manufacturing same.

1 . A steel material having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, comprising:
 by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025%, Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004%, and a remainder of Fe and inevitable impurities,   wherein the number of one or more oxidizing inclusion among an Al—O-based oxidizing inclusion, a Ca—O-based oxidizing inclusion, and an Al—Ca—O-based oxidizing inclusion, having a size of 10 μm or more in the steel material, is 50 or less per 1 mm 2 , and   the steel material satisfies the following [Relationship 1] and [Relationship 2]:   
       
         
           
             
               
                 
                   
                     Ceq 
                     ⁢ 
                     
                       ≤ 
                       
                         
                           0 
                           . 
                           4 
                         
                         ⁢ 
                         5 
                       
                     
                   
                 
                 
                   
                     [ 
                     
                       Relationship 
                       ⁢ 
                           
                       1 
                     
                     ] 
                   
                 
               
             
           
         
         where Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, and C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components, 
       
       
         
           
             
               
                 
                   
                     1.2 
                     ≤ 
                     
                       Ca 
                       / 
                       S 
                     
                     ≤ 
                     
                       4 
                       . 
                       0 
                     
                   
                 
                 
                   
                     [ 
                     
                       Relationship 
                       ⁢ 
                           
                       2 
                     
                     ] 
                   
                 
               
             
           
         
         where Ca and S are amounts (% by weight) of respective components. 
       
     
     
         2 . The steel material of  claim 1 , further comprising at least one of Cu: 0.5% or less or Cr: 0.35% or less. 
     
     
         3 . The steel material of  claim 1 , wherein a fraction of ferrite is 70% or more, a fraction of pearlite is 20 to 30%, and the remainder is bainite (including 0%). 
     
     
         4 . The steel material of  claim 3 , wherein an average grain size of the ferrite is 25 μm or less. 
     
     
         5 . The steel material of  claim 1 , wherein, when the steel material is evaluated perpendicular to a rolling direction at a t/4 point in a thickness direction (where t is a thickness (mm) of the steel material), after welding and heat treatment (PWHT) of the steel material, a yield strength is 260 MPa or more, a tensile strength is 485 MPa or more, and an average Charpy impact absorption energy (CVN, −46° C.) value at a temperature of −46° C. is 150J or more. 
     
     
         6 . A method of manufacturing a steel material having excellent hydrogen-induced cracking resistance and low-temperature impact toughness, comprising:
 heating a steel slab including, by weight, C: 0.12 to 0.18%, Si: 0.2 to 0.5%, Mn: 0.8 to 1.5%, P: 0.015% or less, S: 0.003% or less, Al: 0.015 to 0.045%, Nb: 0.005 to 0.025%, Ni: 0.01 to 0.5%, Mo: 0.01 to 0.12%, V: 0.005 to 0.03%, Ti: 0.003% or less (excluding 0), N: 0.002 to 0.01%, Ca: 0.0005 to 0.004%, and a remainder of Fe and inevitable impurities, and satisfying the following Relationship 1 and Relationship 2 to a temperature range of 1100 to 1200° C.;   rough-rolling the heated steel slab at a temperature of 1050° C. or higher and finishing hot-rolling at a temperature of Ar3 or higher after the rough-rolling, to produce a hot-rolled steel sheet;   air-cooling the hot-rolled steel sheet;   reheating the air-cooled hot-rolled steel sheet to a temperature of Ac3 or higher and maintaining the reheating for (2.3t+30) minutes or more (where t is a thickness of the steel sheet (mm));   quenching the reheated hot-rolled steel sheet to room temperature at a cooling rate of 0.4° C./s or more; and   tempering the cooled hot-rolled steel sheet for (3.4t+30) minutes or more (where t is a thickness of the steel sheet (mm)) in a temperature range of 600 to 700° C.:   
       
         
           
             
               
                 
                   
                     Ceq 
                     ⁢ 
                     
                       ≤ 
                       
                         
                           0 
                           . 
                           4 
                         
                         ⁢ 
                         5 
                       
                     
                   
                 
                 
                   
                     [ 
                     
                       Relationship 
                       ⁢ 
                           
                       1 
                     
                     ] 
                   
                 
               
             
           
         
         where Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, and C, Mn, Cr, Mo, V, Cu, and Ni are amounts (% by weight) of respective components, 
       
       
         
           
             
               
                 
                   
                     1.2 
                     ≤ 
                     
                       Ca 
                       / 
                       S 
                     
                     ≤ 
                     
                       4 
                       . 
                       0 
                     
                   
                 
                 
                   
                     [ 
                     
                       Relationship 
                       ⁢ 
                           
                       2 
                     
                     ] 
                   
                 
               
             
           
         
         where Ca and S are amounts (% by weight) of respective components. 
       
     
     
         7 . The method of  claim 6 , wherein the steel slab further comprises at least one of Cu: 0.5% or less or Cr: 0.35% or less. 
     
     
         8 . The method of  claim 6 , comprising performing post-weld heat treatment (PWHT) for more than 1 hour per inch of steel thickness in a temperature range of 550 to 650° C., after welding the steel material. 
     
     
         9 . The method of  claim 7 , comprising performing post-weld heat treatment (PWHT) for more than 1 hour per inch of steel thickness in a temperature range of 550 to 650° C., after welding the steel material.