The effect of low levels of molybdenum in high strength linepipe steels

Heavy plates for large-diameter linepipes are produced using low-carbon microalloyed steels by thermomechanical rolling. In combination with accelerated cooling, this processing strategy has made it possible to achieve yield strength levels of 690 MPa and above in combination with attractive technological properties. While the focus in the case of high strength grades above the 485 MPa yield strength level was initially on heavy plate with moderate wall thickness, improved low-temperature toughness and weldability at higher wall thicknesses have gained importance in recent years. This has made it necessary to adapt the steel composition and the processing parameters in order to maintain the balance of properties. The microstructure that is obtained after accelerated cooling from the homogeneous austenite depends on the steel composition and the cooling conditions. Alloying additions that retard ferrite formation are used in order to obtain a predominantly bainitic microstructure. Molybdenum is well known to be especially effective in this respect and is, therefore, used frequently in high strength linepipe grades at a level of 0.2% or higher. However, this leads to an increase of the carbon equivalent which impairs weldability. In an experimental investigation that was carried out at Salzgitter Mannesmann Forschung GmbH, three laboratory heats were cast with a variation of the molybdenum content up to 0.2%. Coupons were rolled down to a wall thickness of 25 mm followed by accelerated cooling. The plates were characterized with regard to their microstructure, tensile properties and low-temperature toughness. In addition, welding trials were carried out in order to assess the toughness in the heat affected zone and the influence of the mol ybdenum content. It was found that an addition of 0.1% molybdenum led to a strength increase while excellent levels of low-temperature toughness were maintained in the base material and in the heat affected zone. (AU) © 2015 Companhia Brasileira de Metalurgia e Mineração (CBMM)