High Strength Seamless Steel Pipe for Machine Structure Use Superior in Toughness and Weldability, and Method of Production of The Same

Abstract

A high strength seamless steel pipe for machine structure use superior in toughness and weldability characterized by containing, by mass %, C: 0.03 to less than 0.1%, Mn: 0.8 to 2.5%, Ti: 0.005 to 0.035%, Nb: 0.003 to 0.04%, and B: 0.0003 to 0.003%, limiting Si: 0.5% or less, Al: 0.05% or less, P: 0.015% or less, S: 0.008% or less, and N: 0.008% or less, further containing one or more of Ni: 0.1 to 1.5%, Cr: 0.1 to 1.5%, Cu: 0.1 to 1.0%, and Mo: 0.05 to 0.5%, and having a balance of Fe and unavoidable impurities, the metallurgical structure being a single phase structure of self-tempered martensite or a mixed phase structure of self-tempered martensite and lower bainite.

1 . A high strength seamless steel pipe for machine structure use superior in toughness and weldability characterized by containing, by mass %, C: 0.03 to less than 0.1%, Mn: 0.8 to 2.5%, Ti: 0.005 to 0.035%, Nb: 0.003 to 0.04%, and B: 0.0003 to 0.003%, limiting Si: 0.5% or less, Al: 0.05% or less, P: 0.015% or less, S: 0.008% or less, and N: 0.008% or less, further having one or more of Ni: 0.1 to 1.5%, Cr: 0.1 to 1.5%, Cu: 0.1 to 1.0%, and Mo: 0.05 to 0.5%, and having a balance of Fe and unavoidable impurities, the metallurgical structure being a single phase structure of self-tempered martensite or a mixed phase structure of self-tempered martensite and lower bainite. 
   
   
       2 . A high strength seamless steel pipe for machine structure use superior in toughness and weldability as set forth in  claim 1 , characterized in that in said metallurgical structure, the average size of areas surrounded by high angle boundaries with an orientation difference of 15° or more is 30 μm or less. 
   
   
       3 . A high strength seamless steel pipe for machine structure use superior in toughness and weldability as set forth in  claim 1  or  2 , characterized in that in said metallurgical structure, the average particle size of the cementite is 400 nm or less and the density is 2×10 5 /mm 2  or more. 
   
   
       4 . A method of production of a high strength seamless steel pipe for machine structure use superior in toughness and weldability as set forth in  claim 1 , said method of production of a high strength seamless steel pipe for machine structure use superior in toughness and weldability, characterized by forming a steel billet comprised of steel having the chemical ingredients as set forth in  claim 1  into a pipe by hot piercing, rolling, and elongation steps and cooling by acceleratedly cooling the obtained steel pipe from the outer surface of the steel pipe from a temperature of 750° C. or more until a temperature T (° C.) satisfying the following equation (1) by a cooling rate V (° C./s) of 5 to 50° C./s while rotating the pipe in the circumferential direction, and subsequently by air cooling
   150< T< 821.34× V   −0.3112   (1).