Application of niobium-molybdenum strengthening mechanisms in high strength linepipe steels

The synergistic effects of niobium and molybdenum in lowering austenite-to-ferrite transformation temperatures have been known for approximately 45 years. The benefits have been widely exploited in linepipe steels since 1971 when 485 MPa (X70) linepipe produced by IPSCO [1] was installed in Canada in the TransCanada and Novacorp gas transmission systems. At that time the steels were cast as semi-killed ingots and had inferior transverse Charpy properties due to the presence of MnS and silicate inclusions. Other applications have been found in hot-rolled long products [2] and Nippon Steel’s HT80 quenched and tempered plate [3,4]. As linepipe yield strengths have increased to X80, X100 and above, and carbon contents have been reduced to 0.03-0.06 percent, the Nb-Mo combination has become indispensable for producing economical steels when used in combination with chromium, copper and nickel. This paper provides a brief chronology of the adoption of Nb-Mo and Nb-Mo-B alloying since the mid 1960s. (AU) Copyright © 2014 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved.