Grain refinement and high precipitation hardening by combining microalloying and ultra fast cooling

摘要

Microalloying with Nb has been proved to be a very effective mean to reduce the ferrite grain size as the deformation of the austenite is cumulated during rolling under T_(nr), the non-recrystallisation temperature. Many parameters affect both the ferrite grain size and the precipitation kinetics during hot rolling Nb-bearing steel. Among these parameters the cooling rate and the coiling temperature can be easily controlled and play the most important roles in the control of the precipitation hardening. Ultra fast cooling was applied after the last pass of finishing rolling Nb and Ti-bearing steel. In all cases the Nb(C,N) precipitation was strongly retarded, increasing the precipitation hardening by the formation of finer precipitates. Moreover a non-negligible amount of Nb remained in solution. After cold rolling, precipitation hardening was still possible during annealing. During this research work, several steel grades microalloyed with Nb and Ti were hot rolled and cooled using ultra fast cooling to different coiling temperatures to precipitate different amounts of Nb(C,N). Hot rolled strips were cold rolled and annealed at different temperatures. The kinetics of precipitation and recrystallisation were studied with emphasis on the mechanical property improvements.