Cu-containing low-alloy steel having excellent balance between strength and low-temperature toughness and method for producing same

Abstract

Provided is a Cu-containing low alloy steel having excellent balance between strength and low-temperature toughness. The Cu-containing low alloy steel has a chemical composition comprising, by mass %, C: 0.01 to 0.08%, Si: 0.10 to 0.40%, Mn: 0.80 to 1.80%, Ni: 0.80 to 2.50%, Cr: 0.50 to 1.00%, Cu: 0.80 to 1.50%, Mo: 0.20 to 0.60%, Al: 0.010 to 0.050%, Nb: 0.030 to 0.080%, and N: 0.005 to 0.020%, and further comprising Ca: 0.010% or less as needed, and consisting of Fe and inevitable impurities as the balance; has a 0.2% yield strength of 525 MPa or higher. The Cu-containing low alloy steel has a ductile-brittle fracture appearance transition temperature (FATT) as measured by the 2 mm V-notch Charpy impact test of −70° C. or less.

1 . A Cu-containing low-alloy steel having an excellent balance between strength and low-temperature toughness, having a chemical composition comprising, in terms of % by mass, 0.01-0.08% C, 0.10-0.40% Si, 0.80-1.80% Mn, 0.80-2.50% Ni, 0.50-1.00% Cr, 0.80-1.50% Cu, 0.20-0.60% Mo, 0.010-0.050% Al, 0.030-0.080% Nb, and 0.005-0.020% N, with the balance being Fe and unavoidable impurities,
 wherein a 0.2% proof stress is 525 MPa or higher and a ductile/brittle fracture appearance transition temperature (FATT), as measured through a 2-mm V-notched Charpy impact test, is −70° C. or lower.   
     
     
         2 . The Cu-containing low-alloy steel having an excellent balance between strength and low-temperature toughness according to  claim 1 , wherein the chemical composition further comprises up to 0.010% by mass Ca. 
     
     
         3 . The Cu-containing low-alloy steel having an excellent balance between strength and low-temperature toughness according to  claim 1 , wherein the Cu-containing low-alloy steel has an absorbed energy of 130 J or higher in a 2-mm V-notched Charpy impact test at −80° C. 
     
     
         4 . The Cu-containing low-alloy steel having an excellent balance between strength and low-temperature toughness according to  claim 1 , wherein, after thermal refining, an average EBSD grain diameter is 10 μm or less and a maximum EBSD grain diameter is 120 μm or less in cases when boundaries having a misorientation of 15° or larger are taken as grain boundaries. 
     
     
         5 . A process for producing the Cu-containing low-alloy steel having an excellent balance between strength and low-temperature toughness according to  claim 1 , comprising thermal refining which comprises
 heating a steel to a temperature in a range of 850-950° C. to conduct quenching,   thereafter heating the steel to a temperature in a range of [(A C3  transformation point)−80° C.] to [(A C3  transformation point)−10° C.] to conduct intercritical quenching, and   further conducting tempering at 560-660° C.   
     
     
         6 . The process for producing the Cu-containing low-alloy steel having an excellent balance between strength and low-temperature toughness according to  claim 5 , wherein the thermal refining is applied to a steel for a large structure, the steel having a thick portion with a thickness of 150-500 mm. 
     
     
         7 . The process for producing the Cu-containing low-alloy steel having an excellent balance between strength and low-temperature toughness according to  claim 5 , wherein the steel is produced by hot forging and then subjected to the thermal refining.