Patent ID: 8168010

Claim:
A low alloy steel for oil well pipes consisting of, by mass %, C: 0.25 to 0.35%, Si: 0.05 to 0.3%, Mn: 0.05 to 1.0%, P: 0.025% or less, S: 0.01% or less, Al: 0.005 to 0.10%, Cr: 0.1 to 1.0%, Mo: 0.6 to 1.0%, Ti: 0.002 to 0.05%, V: 0.05 to 0.3%, B: 0.0001 to 0.005%, N: 0.01% or less. O (oxygen): 0.01% or less, Ca: 0 to 0.01%, Mg: 0 to 0.01% and Zr: 0 to 0.1%, and the balance Fe and impurities, which has a stress intensity factor K ISSC value of 25 or more measured in accordance with the NACE TM 0177D method, and has a half-value width H for the peak of the [211] crystal face and a hydrogen diffusion coefficient D (10 −6 cm 2 /s) that satisfy the following equation (1): 30 H+D≦ 19.5 equation (1).