Study of the Microstructural Refinement of a High Niobium Microalloyed Steel Under Controlled Hot Deformation

A current trend within the oil and gas field is towards the development of steel grades exhibiting higher strength and toughness; the need for linepipe components with superior mechanical properties constantly mounts pressure on steel manufacturers to innovate or develop products with such improvements. This is the case of the high-temperature processing (HTP) steel alloy developed for the API X80 steel grade used in the East-West transmission pipeline project (the largest in the world) in China within the last decade. The resulting alloy is based on a high Nb content (~0.1 wt%) which allows for controlled hot rolling due to its ability to retard the recrystallisation of the deformed austenite. The HTP steel alloy enable the possibility to achieve highly refined microstructures with superior strength and toughness over conventional steels. Grain refinement of the microstructure is a fundamental mechanism by which the steel’s strength and toughness can be increased. One of the finest microstructures observed in steel is that arising from the deformation-induced ferrite transformation (DIFT) which can produce grain sizes on the order of 1 – 2 μm. In the present research, a series of plane strain compression tests were performed in order to simulate different thermomechanical controlled processing (TMCP) routes for obtaining the finest possible microstructure following hot deformation. It was found that an adequately designed multi-pass finish rolling can lead to a remarkable grain refinement within the HTP alloy: a simulation that consisted of two finishing passes at 950°C plus two additional passes at 810°C (imparting strains of 0.22 with each pass) produced a microstructure comprising a mixture of highly deformed austenite and 30.7% DIFT. The former had an average grain size of 84 μm and an aspect ratio of around 10, whilst the latter showed a dominant polygonal morphology of size of ~1 μm. A thorough characterisation of the DIFT using various methods was undertaken to yield new findings that help elucidate on the characteristics of this phase. The results obtained indicated that the mechanism of DIFT was the same at temperatures both above and below the critical temperature Ae3; additionally, a transitory ferrite morphology was observed in microstructures corresponding to all the deformation conditions which helps to explain the evolution of DIFT phase from plates to polygonal grains.