Cu-CONTAINING LOW ALLOY STEEL EXCELLENT IN TOUGHNESS OF WELD HEAT AFFECTED ZONE, AND METHOD FOR MANUFACTURING THE SAME

Abstract

A Cu-containing low alloy steel includes a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 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% and Nb: 0.020 to 0.080% and N: 0.005 to 0.020%, and as required, Ca: 0.010% or less, the balance consisting of Fe and unavoidable impurities.

1 . A Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, wherein the steel comprises a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 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: 0.020 to 0.080% and N: 0.005 to 0.020%, the balance consisting of Fe and unavoidable impurities. 
     
     
         2 . The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 1 , wherein the chemical composition further contains, in % by mass, Ca: 0.010% or less. 
     
     
         3 . The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 1 , wherein a structure of the weld heat affected zone (HAZ) after affected by weld heat at a heat input of 3.5 kJ/mm has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 4%. 
     
     
         4 . The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 1 , wherein a structure of an two phase region coarse grain HAZ (ICCGHAZ) present in the weld heat affected zone (HAZ) has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 5%. 
     
     
         5 . The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 1 , wherein the Cu-containing low alloy steel is a thermally refined Cu-containing low alloy steel, and the thermally refined Cu-containing low alloy steel has a 0.2% yield strength of 525 MPa or higher and a ductile-brittle fracture appearance transition temperature (FATT) measured by a V-notch Charpy impact test of −70° C. or lower. 
     
     
         6 . A method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 1 , wherein the method comprises thermal refining conditions for manufacturing the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, and the thermal refining conditions comprises quenching a Cu-containing low alloy steel by heating in a temperature range of 850 to 950° C., thereafter two phase region quenching the resultant by heating in a temperature range of the (Ac 3  transformation temperature−50° C.) or higher to the (Ac 3  transformation temperature—10° C.) or lower, and further tempering the resultant at 500 to 600° C. 
     
     
         7 . A method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 1 , wherein by hot forging the Cu-containing low alloy steel having a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 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% and Nb: 0.020 to 0.080%, and one or two of N: 0.005 to 0.020% and Ca: 0.010% or less, the balance consisting of Fe and unavoidable impurities, the Cu-containing low alloy steel is applied as a steel for a large structure having a thick-wall part of 150 mm to 450 mm in plate thickness. 
     
     
         8 . The method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 7 , the method comprising a step of a thermal refining conditions, the thermal refining conditions comprises quenching a Cu-containing low alloy steel by heating in a temperature range of 850 to 950° C., thereafter two phase region quenching the resultant by heating in a temperature range of the (Ac 3  transformation temperature−50° C.) or higher to the (Ac 3  transformation temperature−10° C.) or lower, and further tempering the resultant at 500 to 600° C. 
     
     
         9 . The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 2 , wherein a structure of the weld heat affected zone (HAZ) after affected by weld heat at a heat input of 3.5 kJ/mm has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 4%. 
     
     
         10 . The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 2 , wherein a structure of an two phase region coarse grain HAZ (ICCGHAZ) present in the weld heat affected zone (HAZ) has island martensite (Martensite-Austenite constituent: MA) with area ratio of less than 5%. 
     
     
         11 . The Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 2 , wherein the Cu-containing low alloy steel is a thermally refined Cu-containing low alloy steel, and the thermally refined Cu-containing low alloy steel has a 0.2% yield strength of 525 MPa or higher and a ductile-brittle fracture appearance transition temperature (FATT) measured by a V-notch Charpy impact test of −70° C. or lower. 
     
     
         12 . A method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 2 , wherein the method comprises thermal refining conditions for manufacturing the Cu-containing low alloy steel excellent in toughness of a weld heat affected zone, and the thermal refining conditions comprises quenching a Cu-containing low alloy steel by heating in a temperature range of 850 to 950° C., thereafter two phase region quenching the resultant by heating in a temperature range of the (Ac 3  transformation temperature−50° C.) or higher to the (Ac 3  transformation temperature−10° C.) or lower, and further tempering the resultant at 500 to 600° C. 
     
     
         13 . A method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 2 , wherein by hot forging the Cu-containing low alloy steel having a chemical composition containing, in % by mass, C: 0.01 to 0.06%, Si: 0.05 to 0.40%, Mn: 0.20 to 0.70%, Ni: 1.20 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% and Nb: 0.020 to 0.080%, and one or two of N: 0.005 to 0.020% and Ca: 0.010% or less, the balance consisting of Fe and unavoidable impurities, the Cu-containing low alloy steel is applied as a steel for a large structure having a thick-wall part of 150 mm to 450 mm in plate thickness. 
     
     
         14 . The method for manufacturing a Cu-containing low alloy steel excellent in toughness of a weld heat affected zone according to  claim 13 , the method comprising a step of a thermal refining conditions, the thermal refining conditions comprises quenching a Cu-containing low alloy steel by heating in a temperature range of 850 to 950° C., thereafter two phase region quenching the resultant by heating in a temperature range of the (Ac 3  transformation temperature−50° C.) or higher to the (Ac 3  transformation temperature−10° C.) or lower, and further tempering the resultant at 500 to 600° C.