Processing and Structure of High Strength Steels

There is currently an extensive effort to develop new steels with a better combination of strength and ductility, the so-called Ultra-High Strength Steels (UHSS) and Advanced High Strength Steels (AHSS). The new steels must be able to be manufactured using existing plant and at the same time meet ever decreasing cost demands. This study looks at developing AHSS, with a focus primarily on the microalloying precipitation behaviour and microstructural evolution during thermomechanical processing of transformation induced plasticity (TRIP) assisted steels. This approach combines two mechanisms that are known to enhance properties, namely the TRIP effect and precipitation hardening arising from strain induced precipitation. In respect of the TRIP effect, a detailed investigation was undertaken to look at the effect of the intercritical anneal on strength and ductility. For the strain induced precipitation, the effect of combined microalloy additions was investigated. Initially, the effect of intercritical annealing was studied using a trial as-cold rolled multiphase V microalloyed TRIP assisted steel with a polygonal ferrite matrix (1.5Mn C+Si+Al+N≈1.0 (C 0.2) and V˂0.2 wt%) focusing on the isothermal bainite transformation at 460°C. The V(C,N) precipitates were extensively observed in all phases (ferrite, retained austenite, martensite and bainitic ferrite) after isothermal bainite transformation in a random manner. The tensile testing suggested considerable improvements in yield, ultimate tensile strengths and elongation after 180s isothermal bainite holding time. According to SEM/TEM observations, these effects were attributed to the evolution of retained austenite from block to film morphology, and possible interaction between microalloying precipitates and newly formed dislocations as a result of bainite transformation. The average density of V(C,N) slightly increased after intercritical annealing, though it did not show a significant variation during the isothermal bainite transformation. Nevertheless, the size distribution of V(C,N) precipitates did not change significantly. The effect of a combined addition of Nb, Mo and V was investigated in TRIP multiphase steel (0.12C 1.50Mn 1.50Si and NbVTiMo≤0.20 wt%), which was compared to a V single addition steel with otherwise matched composition. A comparative study was undertaken using intercritical annealing after ~20% cold-rolled deformation of the rough rolled specimens. The resulting microstructure in both alloys comprised acicular/bainitic ferrite with an uneven proportion of allotriomorphic ferrite and retained austenite and martensite. An average density of precipitates increased in both alloys after intercritical annealing, with an average of 137 and 219 precipitate/µm2 in NbVMo and V steels, respectively. NbV(C,N), NbVMo(C,N) and V(C,N) were observed in the NbVMo steel. In both alloys, precipitates with different morphologies were observed, located in the matrix, on dislocations and at grain boundaries. The results suggested that the NbMo addition retarded the growth/coarsening of precipitates with a size of lower than 15nm. Also, much greater precipitation strengthening was observed in the NbVMo steel after intercrital annealing compared to the V steel. The last part of the project was to systematically study the effect of Nb and Mo on the V steel as a function of thermomechanical processing. Laboratory simulations were developed by plane strain compression testing which accurately replicate the whole thermomechanical process route from the hot rolling through intercritical annealing, followed by the bainite transformation. After hot and controlled rolling at intercritical annealing range the resulting microstructure was acicular/bainitic ferrite, retained austenite and martensite surrounded by allotriomorphic ferrite in both alloys. The TEM observations suggested that a noticeable number of precipitates were formed in the NbVMo steel up to the finishing stage (i.e. average of 112 precipitate/µm2 in NbVMo containing steels). It was also found that the V(C,N) precipitation occurred in austenite and ferrite below the finishing stage (i.e. ≤830˚C) with an average cooling rate of ~12˚C/s. The overall findings suggest that the high dislocation density in the ~20% cold rolled acicular/bainitic ferrite could lead to an intense precipitation and coarsening of V(C,N) during the intercritical annealing.