Effect of hot-rolling conditions on the tensile properties of multiphase steels exhibiting a TRIP effect

Abstract

TRIP-assisted multiphase steels have been thoroughly studied in the cold-rolled and annealed state. The effects of hot-rolling conditions on these steels are much less studied even though these are of major importance for industrial practice. This study was carried out in order to understand the effect of the hot deformation of austenite on the tensile properties of TRIP-assisted multiphase steels. Two different compositions and microstructures are investigated. The first one is a low-carbon steel (mass content of 0.15%) with a microstructure consisting of an intercritical ferritic matrix, bainite and retained austenite. The second one is a medium-carbon steel (mass content of 0.4%) that consists of bainite and retained austenite. Both steels were deformed to various strain levels below the non-recrystallisation temperature of austenite. The medium carbon steel was deformed in the fully austenitic temperature range whereas the low-carbon steel was deformed in the intercritical temperature range. In both cases, the prior hot deformation of austenite brings about a large enhancement of the work-hardening capabilities. In the case of the medium-carbon steel, this effect can be attributed to a much larger TRIP effect taking place during straining. In the case of the low-carbon steel, the improvement of the work-hardening behaviour was attributed to an interaction between the martensitic transformation and the dislocations already present within the surrounding ferrite matrix.