Austenite reconstruction tool for low-temperature toughness control in heavy gauge steels (AUSTOUGH) : final report

The expansion of oil and gas exploitation to arctic areas requires stringent material toughness requirements. Heavy plates and coils dedicated to oil and gas transport are mostly produced by thermomechanical control process, which consist of austenite conditioning during hot rolling followed by accelerated cooling. Mechanical properties of heavy skelp, critically depend on the microstructure homogeneity, which implies, adequate control of the hot rolling process to ensure sufficient strain accumulation during rolling. Pilot scale rolling drafts, designed to mirror industrial practice, were performed on 2 levels of Nb microalloyed HSLA steels. These drafts were used as input data to model material behaviour during hot rolling by means of thermomechanical simulations by dilatometry, torsion and plane strain compression. Results allowed to fine tune a FE code that ultimately was used to optimize the TMCP rolling at pilot scale of heavy wall thickness grades. In parallel, dedicated software was developed to reveal the austenite microstructure using the orientation maps of the final microstructure. The reliability of the reconstruction codes was demonstrated on a quenched austenitic FeNi alloy and on homogeneous reference specimens produced by torsion, dilatometry and plane strain compression. The applicability of the reconstruction tools on the laboratory and industrially rolled material was investigated. At the same time, the laboratory and industrially processed heavy gauge skelp were characterized from a microstructural and mechanical point of view to reveal the austenite structure and second, correlate it with the mechanical properties at multiple thickness positions and with the full thickness properties.