Control of precipitation sequences during hot rolling to improve product uniformity of titanium containing high strength steels (PRETICONTROL) : final report

The PRETICONTROL project aims to understand how precipitation of Ti, in conjunction with Nb and V, influences microstructure and property development during the processing of high titanium high strength steels and hence provide recommendations for compositions which are relatively insensitive to varying processing conditions. This will be achieved by characterization of precipitation sequences during the main stages of the hot rolling process. The influence of microalloying elements on austenite recrystallization behaviour and bainite phase transformation will be investigated by thermo-mechanical techniques coupled to high resolution characterization methods. Models will be developed and applied to optimise hot strip rolling strategies. Micro-alloyed steels with Nb-V-Ti were produced to generate samples for thermo-mechanical tests. The influence of different factors on the recrystallization kinetics was investigated and quantified. While an increasing temperature, strain and strain rate accelerates recrystallization, the addition of Nb and high Ti amounts have a retarding effect. Ti has a smaller effect compared to Nb (1%Ti equivalent to ~0.4%Nb). When Nb is combined with Ti, interactions take place and some loss of Nb in solid-solution generates slight softening acceleration. No major differences in phase transformation temperatures and microstructures were observed with the additions of Ti-Nb-V. However a hardenability increase was observed with Nb and highTi additions. The presence of residual strain due to a deformation prior to transformation promotes ferrite formation. The observed bainite fraction was mainly determined by the coiling temperature. The achieved hardness levels depended as well on the coiling temperature. A hardness peak was observed for coiling temperature of ~620°C. The obtained experimental results were used to adjust austenite recrystallization, phase transformation and precipitation models. Hot torsion, pilot hot-rolling and industrial database analysis were performed to test and validate the simulation models.