Influence of hot working parameters on microstructure evolution,tensile behavior and strain aging potential of bainitic pipeline steel

The microstructure evolution, tensile properties and aging behavior were studied in a low-carbon pipeline steel by performing a number of physical simulations in a thermo-mechanical simulator. The austenite status, before entering the finishing stage, was varied by varying parameters in the austenitization and the roughing stages. The finishing parameters and the subsequent cooling strategy were kept fixed throughout all applied simulations. It was found that starting the finish rolling stage with pancaked austenite is more effective in motivating the formation of acicular ferrite and refining martensite/austenite phase than refining the prior austenite grains. The former effect resulted in improving both of ultimate tensile strength and proof stress without significant ductility decrease. Increasing the deformation during roughing stage resulted in stimulating the transformation kinetics during subsequent processing and consequently decreasing the untransformed austenite that forms the martensite/austenite microconstituent. Furthermore, increasing the austenitization temperature is found to enhance both of strength and ductility when starting the finishing stage with pancaked austenite. On the other hand, attaining aging effect in the studied steel was only possible after creating new dislocations by pre-straining. An increase up to 63 MPa in the yield strength is recorded due to aging after 2% pre-straining.