The major strengthening mechanisms in bainitic steels arise from the bainitic ferrite plate thickness rather than the length, which primarily determines the mean free slip distance. Both the strength of the austenite from where the bainite grows and the driving force of the transformation, are the two factors controlling the final scale of the bainitic microstructure. Usually, those two parameters can be tailored by means of selection of chemical composition and transformation temperature. However, there is also the possibility of introducing plastic deformation on austenite and prior to the bainitic transformation as a way to enhance both the austenite strength and the driving force for the transformation; the latter by introducing a mechanical component to the free energy change. This process, known as ausforming, has awoken a great deal of interest and it is the object of ongoing research with two clear aims. First, an acceleration of the sluggish bainitic transformation observed typically in high C steels (0.7–1 wt. %) transformed at relatively low temperatures. Second, to extend the concept of nanostructured bainite from those of high C steels to much lower C contents, 0.4–0.5 wt. %, keeping a wider range of applications in view.

The mechanical properties of direct quenched low alloy martensite steel can be altered considerably through control of the austenite condition prior to quenching. This can be done by varying the finish rolling temperature during hot rolling. The two main aspects to be taken into account are the prior austenite grain size and the accumulated strain due to rolling in the non-recrystallization regime. It is shown that rolling in the non-recrystallization regime increases the strength, but reduces the impact toughness. Increasing the accumulated strain, however, results in increasing the impact toughness for the same strength. Strain accumulation increases the forces during hot rolling and as such imposes a practical limit to the maximum strain that can be accumulated in the austenite. The current work illustrates the optimized combination between strength, impact toughness and processability of a direct quenched low alloy martensitic steel.

A metallurgical route to produce S700MC/Grade100 steel on a hot strip mill has been developed at ArcelorMittal Europe, based on laboratory simulations and industrial trials. The importance of ThermoMechanically Controlled Processing (TMCP) has been illustrated, mainly to optimize the low temperature toughness. Furthermore, it has been shown that the optimum combination of strength and low temperature toughness can be found through optimization of the transformation and the precipitation. The concept developed shows a good robustness against typical industrial process variations.The steel shows a good weldability using typical MAG welding parameters, without loss in strength and a good toughness. The high cycle fatigue performance of the steel is very good.

Metallurgical concepts for new ultra-high strength martensitic steels have been developed through direct quenching after hot rolling. In addition to the chemical composition, the hot rolling, quenching, and annealing parameters need to be optimized to fulfill the requirements for the demanding applications for which these steels are used. It is also shown that the welding behavior is influenced by the choice of alloying concept. Typical applications also require a high fatigue resistance, especially of formed components. For that reason, a dedicated set-up was developed that allows differentiation between materials, which is illustrated through the effect of inclusions on the fatigue performance of a bent test piece.

The basic characterization of the austenite grain size and shape prior to quenching to martensite was already used in the past to optimize the mechanical properties and impact toughness of low Carbon martensitic steel. This basic characterization can typically be done by optical microscopy. To better understand the mechanisms that generate the different properties, however, a more detailed analysis is required. An algorithm has been developed to calculate the austenite orientation starting from the martensite orientation measured by EBSD. The method has been applied to explain the different properties of martensite after austenitization and quenching and of direct quenched martensite with different levels of accumulated strain. It is shown that a small austenite size is needed to improve the impact toughness, regardless of the process route. The strength is strongly depending on the dislocation density in the austenite.