High tensile strength, refractory steel having excellent weldability and gas cuttability and method for producing same

Abstract

Accordingly to an exemplary embodiment of the present invention, a high tensile strength, refractory steel can be provided which comprises, in mass %, approximately C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and the balance consisting of iron and unavoidable impurities. For example, a weld crack sensitive composition P CM can be defined by the following equation may be about 0.25% or less: P CM =C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B. An area fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼ thick position in the plate thickness direction of the steel plate of the final rolling product is about 10% or less.

1 - 16 . (canceled) 
   
   
       7 . A high tensile strength, refractory steel having a particular weldability and a particular gas cuttability, comprising:
 at least one portion of a steel plate of a rolling product comprising approximately, in mass %, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and a balance consisting of iron and unavoidable impurities,   wherein a weld crack sensitive composition P CM  is about 0.25% or less, P CM  being defined as:
   P CM =C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B, and 
   wherein an area fraction of polygonal ferrite or pseudo polygonal ferrite in a ¼ thick position in a plate thickness direction of the steel plate is about 10% or less.   
   
   
       8 . The steel according to  claim 7 , wherein the at least one portion further comprises approximately, in mass %, Ni: 0.05 to 1.0%, Cu: 0.05 to 1.0%, and one or two or more selected from Cr: 0.05 to 1.0%, V: 0.01 to 0.06%, B: 0.0002 to 0.0030%, Ti: 0.005 to 0.025%, and Mg: 0.0002 to 0.0050%, and wherein a content of Ni is equal to or greater than half of a content of Cu. 
   
   
       9 . The steel according to  claim 7 , wherein the at least one portion further comprises approximately, in mass %, at least one of Ca: 0.0005 to 0.0040% or REM: 0.0005 to 0.0100%. 
   
   
       10 . The steel according to  claim 7 , wherein a yield strength of the steel is about 440 MPa or more. 
   
   
       11 . A method for manufacturing a high tensile strength, refractory steel having a particular weldability and a particular gas cuttability, comprising:
 heating a steel member at a temperature of about 1100 to 1300° C. in a form of at least one of a billet or a slab comprising approximately, in mass %, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and a balance consisting of iron and unavoidable impurities;   rolling the steel member at a temperature of about 800 to 950° C.;   directly quenching the steel member from a temperature greater or equal to a greater of about 750° C. or a temperature of about 150° C. lower than a temperature at a time of completing the rolling; and   tempering the steel member at a temperature lower than or equal to Ac 1 .   
   
   
       12 . A method for manufacturing a high tensile strength steel having a particular weldability and a particular gas cuttability, comprising:
 hot rolling a steel member in a form of at least one of a billet or a slab, the steel member comprising approximately, in mass %, C: 0.04 to 0.14%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Nb: 0.01 to 0.05%, Mo: 0.30% or more and less than 0.70%, Al: 0.060% or less, N: 0.0010 to 0.0060%, and a balance consisting of iron and unavoidable impurities;   self-cooling the steel member:   quenching the steel member after reheating the steel member at a temperature of about 900 to 950° C.; and   tempering the steel member at a temperature not higher than Ac 1 .