The Effect of Hot Deformation on the Strength of Precipitation Hardened Steels

Abstract

The chemistry of precipitation hardened micro-alloyed steels has received considerable attention over the past 20 years. In parallel with this study, the process of controlled rolling has been developed and used to optimise the properties of this type of steel. In many applications, the plate or strip product produced by controlled rolling is not suitable without further thermo-mechanical treatment. The effect that reheating and deforming micro-alloyed steels has on the structure and properties has, however, received little attention. The sensitivity of some simple steels to processing variables in the warm-forming regime, and the structure-properties relationships that determine the properties of warm-formed steels have been the subject of this thesis. The effect of warm deformation on plain carbon, vanadium and niobium steels was studied, using rolling and torsion as the mode of deformation. The microstructure was observed using the Scanning Electron Microscope in the Back Scattered Electron Mode. This relatively new technique enabled the development of sub-structure within grains to be observed. For the air-cooled materials, the proof stress increases with deformation up to a strain of 0.5 with little temperature dependence. For higher strains, the proof stress increases with a decrease in deformation temperature. The proof stress of the steels deformed by a true strain of 1.8 can be predicted with reference to the initial grain size, and sub-grain size of the deformed ferrite. Both vanadium and niobium micro-alloying additions reduce the sensitivity of the as warm-worked strength to strain for true strains greater than 0.5, although the temperature sensitivity is still quite strong.