Electric Resistance Welded Steel Pipe

Abstract

An electric resistance welded steel pipe excellent in terms of fatigue strength. The steel pipe including a chemical composition containing, by mass %, C: 0.35% or more and 0.55% or less, Si: 0.01% or more and 1.0% or less, Mn: 1.0% or more and 3.0% or less, P: 0.02% or less, S: 0.01% or less, Al: 0.005% or less, N: 0.0050% or less, Cr: 0.1% or more and 0.5% or less, and the balance being Fe and inevitable impurities and a metallic microstructure including pearlite, ferrite, and bainite, in which the area ratio of the pearlite is 85% or more, the total of the area ratios (including 0) of the ferrite and the bainite is 15% or less, and in which a prior austenite grain size is 25 μm or more.

1 . An electric resistance welded steel pipe, the steel pipe having a metallic microstructure including pearlite, ferrite, and bainite, and a chemical composition comprising:
 C: 0.35% or more and 0.55% or less, by mass %;   Si: 0.01% or more and 1.0% or less, by mass %;   Mn: 1.0% or more and 3.0% or less, by mass %;   P: 0.02% or less, S: 0.01% or less, by mass %;   Al: 0.005% or less, by mass %;   N: 0.0050% or less, by mass %;   Cr: 0.1% or more and 0.5% or less, by mass %; and   Fe and unavoidable impurities as a balance,   wherein an area ratio of the pearlite is in the range of 85% or more,   the total of area ratios (including 0) of the ferrite and the bainite is in the range of 15% or less, and   a prior austenite grain size is in the range of 25 μm or more.   
     
     
         2 . The electric resistance welded steel pipe according to  claim 1 , the steel pipe having the chemical composition further comprising at least one of Ti: 0.005% or more and 0.1% or less, by mass %, B: 0.0003% or more and 0.0050% or less, by mass %, Mo: 2% or less, by mass %, W: 2% or less, by mass %, Nb: 0.1% or less, by mass %, V: 0.1% or less, by mass %, Ni: 2% or less, by mass %, Cu: 2% or less, by mass %, Ca: 0.02% or less, by mass %, and REM: 0.02% or less, by mass %. 
     
     
         3 . The electric resistance welded steel pipe according to  claim 1 , wherein the tensile strength of the steel pipe is in the range of 825 MPa to 855 MPa. 
     
     
         4 . The electric resistance welded steel pipe according to  claim 1 , wherein the torsion fatigue strength of the steel pipe is in the range of 175 MPa to 180 MPa. 
     
     
         5 . The electric resistance welded steel pipe according to  claim 1 , wherein the prior austenite grain size is in the range of 29 nm to 36 nm.