Patent ID: 7959709

Claim:
A method of producing steel for a steel pipe excellent in sour-resistance performance, wherein the steel comprises, in % by mass, C: 0.03 to 0.4%, Mn: 0.1 to 2%, Si: 0.01 to 1%, P: 0.015% or less, S: 0.002% or less, Ti: 0.2% or less, Al: 0.005 to 0.1%, Ca: 0.0005 to 0.0035%, N: 0.01% or less, and O (oxygen): 0.002% or less, the balance being Fe and impurities, the method comprising controlling the amount of Ca addition charged into the steel that is molten in a ladle according to a N content in the molten steel prior to Ca addition, wherein non-metallic inclusions in the steel are mainly composed of Ca, Al, O, and S, and, as a result of the controlling step, a CaO content in the inclusions is in the range of 30 to 80%, the ratio of the N content in the steel to the CaO content in the inclusions satisfies the relation expressed by equation (1), and a CaS content in the inclusions satisfies the relation expressed by equation (2), 0.28≦[N]/(% CaO)≦2.0 (1) (% CaS)≦25% (2) where [N] represents the mass content (ppm) of N in the steel, (% CaO) represents the mass content (%) of CaO in the inclusions, and (% CaS) represents the mass content (%) of CaS in the inclusions wherein in order to control a presence of inclusions such that carbonitrides such as Ti, Nb, and the like are formed on surfaces of CaO—CaS—Al 2 O 3 inclusions, Ca is added such that in controlling the amount of Ca addition into the molten steel in the ladle, the ratio of the N content in molten steel to the amount of Ca addition to the molten steel satisfies the relation expressed by equation (3) below according to the N content in the molten steel prior to the Ca addition: 200≦[N]/WCA≦857 (3) where [N] represents the mass content (ppm) of N in the molten steel prior to the Ca addition and WCA represents the amount of Ca addition (kg/t-molten steel) to the molten steel.