Ultrahigh strength UOE steel pipe and a process for its manufacture

Abstract

A UOE steel pipe for use in linepipe having an ultrahigh strength in the circumferential direction of the pipe of at least 750 MPa and at most 900 MPa and having improved toughness in the base metal and weld heat affected zone along with improved joint fracture properties and good circumferential weldability is manufactured from a hot-rolled steel plate having a composition comprising, in mass percent, C: 0.03-0.08%, Mn: 1.70-2.2%, S: at most 0.0020%, Ti: 0.005-0.025%, N: at most 0.0050% and having a carbon equivalent (Ceq) as defined below of at least 0.50% and a weld cracking parameter (Pcm) as defined below of at most 0.24% wherein the temperature at the completion of water cooling after hot rolling is 350° C. or higher: Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15, Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+B.

1 . A UOE steel pipe having a base metal with a chemical composition comprising, in mass percent, C: 0.03-0.08%, Mn: 1.70-2.2%, S: at most 0.0020%, Ti: 0.005-0.025%, N: at most 0.0050%, and a remainder of iron and unavoidable impurities, wherein the below-defined carbon equivalent (Ceq) is at least 0.50% and the below-defined weld cracking parameter (Pcm) is at most 0.24%, and the strength in the circumferential direction of the pipe being at least 750 MPa and at most 900 MPa:  
       Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15,  Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+B, wherein Ceq=carbon equivalent, Pcm=weld cracking parameter, and the symbols for elements in the equations indicate the content of those elements in mass percent.    
   
   
       2 . A UOE steel pipe as set forth in  claim 1  wherein the chemical composition of the base metal further contains, in mass percent, at least one element selected from the following (i)-(iv): 
 (i) one or two of Si: 0.05-0.50% and Al: at most 0.06%,    (ii) one or more of Cu: at most 1.0%, Ni: at most 2.0%, Cr: at most 1.0%, Nb: at most 0.1%, and V: at most 0.1%,    (iii) Mo: at most 1.0%, and    (iv) Ca: at most 0.005%    
   
   
       3 . A process for manufacturing a UOE steel pipe having a carbon equivalent (Ceq) as defined below of at least 0.50% and a weld cracking parameter (Pcm) as defined below of at most 0.24% and having a strength in the circumferential direction of the pipe of at least 750 MPa and at most 900 MPa, comprising producing a steel plate having a base metal chemical composition comprising, in mass percent, C: 0.03-0.08%, Mn: 1.70-2.2%, S: at most 0.0020%, Ti: 0.005-0.025%, N: at most 0.0050%, optionally at least one element selected from the following (i)-(iv), and a remainder of iron and unavoidable impurities by hot rolling and subsequent water cooling with the temperature at the completion of water cooling being 350° C. or higher, subjecting the resulting steel plate to U-pressing and O-pressing, and then performing welding and pipe expanding to obtain a UOE steel pipe:  
       Ceq=C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15,  Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+B, wherein Ceq=carbon equivalent, Pcm=weld cracking parameter, and the symbols for elements in the equations indicate the content of those elements in mass percent,    (i) one or two of Si: 0.05-0.50% and Al: at most 0.06%,    (ii) one or more of Cu: at most 1.0%, Ni: at most 2.0%, Cr: at most 1.0%, Nb: at most 0.1%, and V: at most 0.1%,    (iii) Mo: at most 1.0%, and    (iv) Ca: at most 0.005%.