MARTENSITIC CHROMIUM-CONTAINING STEEL AND PIPES USED IN THE OIL INDUSTRY

FIELD : metallurgy. SUBSTANCE : invention relates to metallurgy, namely to martensitic chromium-containing steel. Steel has a chemical composition containing, mass% : C : not more than 0.10, Si : 0.05 to 1.00, Mn : 0.1 to 1.0, Cr : from more than 8 to 12, V : 0.01 to 1.0, dissolved Al : from 0.005 to 0.10, N : not more than 0.100, Nb : 0 to 1, Ti : 0 to 1, Zr : 0 to 1, B : 0 to 0.01, Ca : 0 to 0.01, Mg : 0 to 0.01, rare earth metal (REM) : 0 to 0.50, additional Mo : 0 to 2 and/or W : from 0 to 4, the rest is Fe and impurities. As impurities, the chemical composition contains, mass% : P : not more than 0.03, S : not more than 0.01, Ni : not more than 0.5 and O : not more than 0.01. Effective amount of Cr, defined by expression : effective amount of Cr = Cr-16.6×C, is not less than 8 % by weight, and molybdenum equivalent, defined by the expression : molybdenum equivalent = Mo + 0.5×W, is from 0.03 to 2 % by weight. Steel has a microstructure containing 0 to 5 vol% ferrite, 0 to 5 vol% austenite, the rest is temper martensite in which the grain size of the preceding austenite is at least 8.0 in accordance with ASTM E112. Steel has a yield strength of from 379 to less than 551 MPa, and the degree of grain-boundary segregation, defined as the ratio of the maximum content at the grain boundaries to the average content within the Mo or W grains with the content of any one of Mo and W or defined as the mean of the ratio of the maximum content at the grain boundaries to the average content within the grain of each element, with the content of both elements Mo and W, is not less than 1.5. EFFECT : steel has high corrosion resistance, resistance to sulfide stress cracking and the resistance to intergranular cracking caused by hydrogen. 9 cl, 2 tbl