Abstract: This steel is applicable for making seamless tubes with a high mechanical strength, high pitting resistance and a high hot-formability, by means of a process comprising meltdown in an electric-arc furnace, refining in an AOD converter, ascensional ingot casting and Gothic section billet rolling, and then transforming the billet obtained into seamless tubes by upsetting the gothic billet into a round followed by hot piercing in a vertical press, tube extrusion in a horizontal press, reduction in a stretch reduction mill and Pilger cold-rolling, and having a composition of not more than 0.02% C, 0.65 to 0.85% Mn, 0.40 to 0.60% Si, 25.2 to 25.6% Cr, 6.2 to 6.6% Ni, 3.6 to 3.8% Mo, 0.24 to 0.30% N, not more than 0.025% P, not more than 0.002% S, with no Cu and W, and with the addition of a percentage of B ranging between 0.0015 to 0.0030% and a percentage of Ca ranging between 0.0010 to 0.0050%.