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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.

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 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.

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.

Hot rolled lower bainitic steels are suitable candidate materials for applications in which very high strength has to be combined with a certain degree of ductility. Typical applications include parts for trucks and trailers, heavy duty equipment and general mechanical engineering. To obtain suitable properties, an optimised combination of chemical composition and processing is required. For low alloyed C–Mn steels, Cr, Mo and B are typically added to suppress ferrite formation and increase bainitic hardenability during continuous cooling.1–5) In bainitic steels, Cr additions are typically less than 1 wt%5,6) to optimize the hardenability without compromising the welding behavior too much. B slows down the ferrite nucleation as it segregates to the austenite grain boundaries.7) Therefore, B is only effective as a hardening element when it is in solid solution, hence without formation of BN. This can be done by adding an element with a higher affinity for N, e.g. Ti.8) Mo is especially effective in increasing the hardenability when it is used together with B.9) Next to the chemical composition, the main processing parameters that determine the properties of the final hot rolled bainitic steel are the finish rolling temperature, the cooling rate and the coiling temperature.6) When rolling below the temperature of non-recrystallization (Tnr), deformation energy is stored in the austenite prior to cooling. This leads to a finer microstructure after transformation, which increases the strength.10) The stored energy, Effect of the Cr Content and Coiling Temperature on the Properties of Hot Rolled High Strength Lower Bainitic Steel

The effect of finish rolling temperature on the austenite-(γ) to-bainite (α) phase transformation is quantitatively investigated in high-strength C-Mn steels using an alternative crystallographic γ reconstruction procedure, which can be directly applied to experimental electron backscatter diffraction mappings. In particular, the current study aims to clarify the respective contributions of the γ conditioning during the hot rolling and the variant selection during the phase transformation to the inherited texture. The results confirm that the sample finish rolled at the lowest temperature [1102 K (829 °C)] exhibits the sharpest transformation texture. It is shown that this sharp texture is exclusively due to a strong variant selection from parent brass {110}\( \left\langle {1\bar{1}2} \right\rangle \), S {213}\( \left\langle {\bar{3}\bar{6}4} \right\rangle \) and Goss {110}〈001〉 grains, whereas the variant selection from the copper {112}\( \left\langle {\bar{1}\bar{1}1} \right\rangle \) grains is insensitive to the finish rolling temperature. In addition, a statistical variant selection analysis proves that the habit planes of the selected variants do not systematically correspond to the predicted active γ slip planes using the Taylor model. In contrast, a correlation between the Bain group to which the selected variants belong and the finish rolling temperature is clearly revealed, regardless of the parent orientation. These results are discussed in terms of polygranular accommodation mechanisms, especially in view of the observed development in the hot-rolled samples of high-angle grain boundaries with misorientation axes between 〈111〉γ and 〈110〉γ.

An alternative crystallographic austenite reconstruction programme written in Matlab is developed by combining the best features of the existing models: the orientation relationship refinement, the local pixel-by-pixel analysis and the nuclei identification and spreading strategy. This programme can be directly applied to experimental electron backscatter diffraction mappings. Its applicability is demonstrated on both quenching and partitioning and as-quenched lath-martensite steels.

Recrystallization of an Fe–22Mn–0.6C TWIP steel occurs by site-saturated nucleation and causes a retained rolling texture, as opposed to some literature data, in which texture randomization is reported. The present results indicate that the annealing heating rate did not significantly influence the results. Randomization might be induced by a large hot-rolled grain size and annealing twins. TWIP steels appear to have higher r-values than typical Cube-dominated face-centred cubic metals, confirming their higher deep drawing potential.

The static recrystallisation behaviour of cold rolled and annealed TWinning Induced Plasticity (TWIP) steels is important for its industrial production. The recrystallisation kinetics have been determined for an Fe-Mn-C-Si-Al TWIP steel using hardness measurements and microstructure analysis: it has been shown that recrystallisation progresses rapidly with increased annealing temperature. Recrystallisation was faster at higher cold reductions, and a smaller final grain size was observed at lower annealing temperatures. This indicates that the mechanism is nucleation dominated at lower temperatures; grain growth at higher temperatures appears similar for all reductions. The recrystallisation results in a crystallographic texture where the main components of the cold rolling texture are preserved in the final texture after annealing, although some randomisation was observed.

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.

A fully austenitic TWIP steel was cold rolled. During cold rolling, the evolution of the crystallographic texture was monitored. The development of a brass type of texture was found, which is typical for low stacking fault energy (SFE) materials. Intensive electron microscopic observations, with TEM and SEM, revealed four active deformation mechanisms: micro twinning, dislocation slip, the formation of stacking faults and microscopic shear banding. The effect of macroscopic shear banding was expected to be minimal. Putting together all the results showed that both micro twinning and slip play a major role in the development of the observed brass texture.

Gold nanoparticles (Au(NP)) with carboxyl groups on their surface were used in combination with PAH for the layer-by-layer coating of CaCO(3) microparticles, followed by the dissolution of the CaCO(3) core. SEM, TEM, and confocal microscopy are used to characterize the hybrid nanoparticles/polyelectrolyte capsules. As the Au(NP) have carboxyl groups on their surface, their charge density is pH pH swelling dependent; therefore, the capsules exhibit a pH-dependent swelling and can be deconstructed both at low and high pH. By covalent cross-linking of the carboxyl groups of the Au(NP) and the amino groups of the PAH, it is possible to suppress the laser pH-responsive behavior. Au(NP) are used as activation centers using IR light and this ability is used to release encapsulated material from the nanoparticles/polyelectrolyte capsules as well as for the enhancement of detection and imaging of such capsules by Raman microspectrosopy.

The influence of Cr and N additions on the mechanical properties of austenitic Fe-Mn-Cr-C-N alloys was studied. The ductility and the strain-hardening behavior were investigated in detail, because these alloys may potentially be used for crash-relevant automotive body parts. It was found that Cr and low N additions to a Fe-18Mn-0.25C alloy resulted in a higher ductility and a reduced strain hardening. Increasing the N content up to 0.22 mass pct resulted in a further increase of ductility and a more favorable strain-hardening behavior. X-ray diffraction and transmission electron microscopy studies revealed that the strain-hardening behavior was linked to the presence of strain-induced martensite and mechanical twinning. The analysis of the mechanical properties and the microstructure clearly demonstrates that, in the Fe-Mn-Cr-C-N system, both N additions and combined N and Cr additions increase the stacking fault energy.

The influence of Cr and N additions on the mechanical properties of a Fe-Mn-C steel was investigated. The chemical composition was found to have a pronounced effect on the strain-hardening behavior, due to the strain-induced sequence of the γ → ▓ → α′ martensitic transformations. It was found that Cr and N suppress this transformation sequence. At Cr levels higher than 7.5 mass pct, no α′ martensite was formed, which led to a pronounced improvement of the ductility. The differences in transformation behavior can be attributed to the change in the intrinsic stacking-facult energy (ISFE): in the compositional range studied, Cr and N additions cause an increase of the ISFE.