High tensile strength steel and marine structure having excellent weld toughness

Abstract

In order to provide a high tensile strength steel having excellent low temperature toughness and which can withstand large heat input welding, a steel comprises, in mass percent, C: 0.01-0.10%, Si: at most 0.5%, Mn: 0.8-1.8%, P: at most 0.020%, S: at most 0.01%, Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al: 0.001-0.05%, N: 0.0030-0.0080%, O: 0.0005-0.0035%, if necessary at least one of Ti: 0.005-0.03%, Nb: 0.003-0.03%, and Mo: 0.1-0.8%, and a remainder of Fe and impurities, and the N/Al ratio is 0.3-3.0.

1 . A high tensile strength steel consisting essentially, in mass percent, of C: 0.01-0.10%, Si: at most 0.5%, Mn: 0.8-1.8%, P: at most 0.020%, S: at most 0.01%, Cu: 0.8-1.5%, Ni: 0.2-1.5%, Al: 0.001-0.05%, N: 0.003-0.008%, O: 0.0005-0.0035%, Ti: 0-0.03%, Nb: 0-0.03%, Mo: 0-0.8%, Cr: 0-0.80%, B: 0-0.002%, V: 0-0.05%, Ca: 0-0.005%, Mg: 0-0.005%, REM: 0-0.01%, and a reminder of Fe and impurities, wherein the N/Al ratio is 0.3-3.0, wherein the value of Pcm represented by the following equation (I) is at most 0.25, and for Cu particles having a major axis with a length of at least 1 nm dispersed in the steel, the average equivalent circle diameter of the Cu particles is in the range of 4-25 nm and the plane-converted area fraction thereof is in the range of 3-20%.
     Pcm =C+(Si/30)+(Mn/20)+(Cu/20)+(Ni/60)+(Cr/20)+(Mo/15)+(V/10)+5B  (I)   
   
   
       2 . A high tensile strength steel as set forth in  claim 1  which contains, in mass percent, Ti: 0.005-0.03%. 
   
   
       3 . A high tensile strength steel as set forth in  claim 1  which contains, in mass percent, Nb: 0.003-0.03%. 
   
   
       4 . A high tensile strength steel as set forth in  claim 1  which contains, in mass percent, Mo: 0.1-0.8%. 
   
   
       5 . A high tensile strength steel as set forth in  claim 1  which contains, in mass percent, at least one of Cr: 0.03-0.80% and B: 0.0002-0.002%. 
   
   
       6 . A high tensile strength steel as set forth in  claim 1  which contains, in mass percent, V: 0.001-0.05%. 
   
   
       7 . A high tensile strength steel as set forth in  claim 1  which contains, in mass percent, at least one of Ca: 0.0005-0.005%, Mg: 0.0001-0.01%, and REM: 0.0001-0.01%. 
   
   
       8 . A high tensile strength steel as set forth in  claim 1  wherein the steel is manufactured by heating a billet at a temperature of 950° C. or less, hot rolling the heated billet, cooling the resulting hot rolled steel, and tempering the cooled steel. 
   
   
       9 . A steel plate comprising a high tensile strength steel as set forth in  claim 1 , wherein the ferrite fraction of the plate is at least 70%, the average diameter is 30 micro-meters or less, and the flatness is 1.5 or less at a depth of ½t in the thickness direction of the plate. 
   
   
       10 . A marine structure using a high tensile strength steel as set forth in  claim 1 .