Papers by Author: K.M. Banks

Abstract: 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 grain size was larger than 5µm 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 norecrystallisation temperature, which was determined by the roughing mill exit temperature.

Abstract: Various microstructure models for Nb-bearing steels were tested under industrial strip rolling conditions to establish a relationship between grain size and toughness in Ti-Nb-V microalloyed steels. For similar Nb contents, microstructure models for Nb steels were found to adequately describe recrystallisation kinetics in more complex Ti-Nb-V steels. For thick-walled linepipe (11.6mm), a minimum of 0.04%Nb is required to achieve adequate toughness. Retained strain was the dominant processing parameter factor affecting ferrite grain size. The predicted minimum amount of retained strain after the last pass required for sufficient grain refinement concurred with laboratory simulation results. For the rolling schedules investigated, metadynamic recrystallisation was predicted to occur during roughing, whilst static recrystallisation was predominant during finishing.