Microstructural control and DWTT toughness in thick-walled Nb-Ti-V microalloyed linepipe steels

Production of thick-walled low C Nb-Ti-V microalloyed linepipe steel has recently commenced in South Africa. Generally, the DWTT failures occurred when the average ferrite gain size was larger than 5 mu m and the pearlite colonies were coarse. This work examined the hot strip rolling behaviour of Nb-Ti-V microalloyed steel and quantified the most important parameters influencing final grain size and, hence, toughness. A Gleeble 1500 was used to both simulate hot rolling of relatively thick strip and also determine the no-recrystallisation temperature. For a fixed composition, final grain size was mainly controlled by the amount of strain applied below the no-recrystallisation temperature, which was determined by the roughing mill exit temperature.