High strength pipeline steels with optimized HAZ properties

Large diameter linepipes for long-distance transport of natural gas are produced using either heavy gauge coil or heavy plate material. The combination of high strength and excellent lowtemperature toughness of modern linepipe steels is a direct result of the thermomechanically controlled processing (TMCP), which leads to a fine grain size after rolling. Because TMCP allows attainment of high levels of strength by reducing the grain size, it was possible to lower the C content and to improve the weldability significantly. Addition of Nb is an effective measure to inhibit recrystallization during finish rolling, which is the key to obtaining a finegrained microstructure. The temperature threshold below which recrystallization is severely retarded between rolling passes depends on the level of the Nb addition and accumulated strain. Thus, the level of the Nb content can be adjusted to the limitations of the rolling mill. Double submerged arc welding during the production of large-diameter linepipes leads to severe changes in the microstructure of the heat-affected zone (HAZ), including grain coarsening by more than one order of magnitude, as well as different transformation products. Because of the large austenite grain size close to the fusion line, the phase transformation during cooling is retarded and C-rich particles can form. This can have a negative effect on the toughness in the HAZ. An experimental investigation was carried out at Salzgitter Mannesmann Forschung GmbH (SZMF), in which the Nb content of laboratory heats was varied between 0.02% and 0.10%. These heats were thermomechanically rolled to a wall thickness of 25 mm and subsequently used for doublelayer submerged arc welding trials. The processing parameters during rolling and welding were held constant in order to ensure that the effect of the alloying elements could be isolated. The fusion line toughness was determined at -20°C as well as -40°C and the microstructure was investigated by high-resolution scanning electron microscopy. It was found that high levels of toughness in the heat-affected zone could be achieved across the full range of Nb studied. (AU) © 2015 Companhia Brasileira de Metalurgia e Mineração (CBMM)