High Strength Thick-Gauge Electric-Resistance Welded Steel Pipe Excellent in Hardenability, Hot Workability and Fatigue Strength and Method of Production of the Same

Abstract

The present invention provides high strength thick-gauge electric-resistance welded steel pipe excellent in hardenability, hot workability, and fatigue strength and a method of production of the same, that is, thick-gauge electric-resistance welded steel pipe containing, by mass %, C: 0.25 to 0.4%, Si: 0.01 to 0.50%, Mn: 0.8 to 1.5%, P: 0.05% or less, S: 0.05% or less, Al: 0.05% or less, Ti: 0.005 to 0.05%, B: 0.0005 to 0.01%, N: 0.001 to 0.05%, and a balance of Fe and unavoidable impurities, having a critical cooling rate V c expressed by equation <1> of less than 30° C./s, and having a ratio of thickness t and outside diameter D, t/D, of over 0.15 to 0.30 in range: log Vc =2.94−0.75β  <1> where, β=2.7C+0.4Si+Mn

1 . High strength thick-gauge electric-resistance welded steel pipe excellent in hardenability, hot workability, and fatigue strength containing, by mass %,
 C: 0.25 to 0.4%,   Si: 0.01 to 0.50%,   Mn: 0.8 to 1.5%,   P: 0.05% or less,   S: 0.05% or less,   Al: 0.05% or less,   Ti: 0.005 to 0.05%,   B: 0.0005 to 0.01%,   N: 0.001 to less than 0.004%, and   
     a balance of Fe and unavoidable impurities, having a critical cooling rate Vc expressed by equation <1> of less than 30° C./s, and having a ratio of thickness t and outside diameter D, t/D, of over 0.15 to 0.30 in range:
   Log  Vc= 2.94−0.75β  <1> 
 Where, β=2.7C+0.4Si+Mn 
 
   
   
       2 . High strength thick-gauge electric-resistance welded steel pipe excellent in hardenability, hot workability, and fatigue strength as set forth in  claim 1 , characterized by further containing, by mass %, one or more of
 Cr: 0.1 to 1%,   Mo: 0.05 to 1%,   V: 0.01 to 0.5%, and   Ni: 0.1 to 1%:   where, in equation <1>,
   β=2.7C+0.4Si+Mn+0.45Ni+0.8Cr+2Mo 
   
   
   
       3 . High strength thick-gauge electric-resistance welded steel pipe excellent in hardenability, hot workability, and fatigue strength as set forth in  claim 1 , characterized by further containing, by mass %, Nb: 0.01 to 0.1%. 
   
   
       4 . High strength thick-gauge electric-resistance welded steel pipe excellent in hardenability, hot workability, and fatigue strength as set forth in  claim 1 , characterized by further containing, by mass %, Ca: 0.0002 to 0.005%. 
   
   
       5 . A method of production of high strength thick-gauge electric-resistance welded steel pipe excellent in hardenability, hot workability, and fatigue strength characterized by heating electric-resistance welded steel pipe having the ingredients as set forth in  claim 1  to 800 to 1200° C. and stretch reducing rolling it by a cross-section reduction rate of 40 to 80% in range.