The recrystallization behavior of five low alloyed steels is investigated using double hit deformation tests. It is shown, that Niobium has the biggest influence in retarding the recrystallization kinetics. Further, the microstructural evolution dependent on strain and temperature during deformation is studied with a picric acid etchant and light-optical analysis. It is shown how the microstructure of two differently alloyed ultra-high strength steels changes along with the peculiarities of the corresponding stress-strain curves including the evolution of grain size and aspect ratio of the prior austenite grain. The findings on the different recrystallization kinetics with the role of recrystallization retarding elements are further reinforced by investigations on the Zener-Hollomon parameter and the activation energy needed for dynamic recrystallization. A rolling scenario on a deformation dilatometer is simulated on a hardenable and a micro-alloyed steel to illustrate the microstructural evolution between the rolling steps. It is shown, how the two ultra-high strength steels perform different in their microstructural evolution, as the waiving of micro-alloying elements (MAE) provides finer austenite grains.

In the development of steels with increasing strength and toughness, ultra-high strength steel grades with a martensitic microstructure are gaining more importance. From the martensitic transformation product, however, strength-specific quantities derived from the former austenite grain cannot be accessed conveniently. A profound analysis of the prior austenite microstructure, its size distribution and aspect ratio is essential in order to allow conclusions on the mechanical properties. This paper presents an etchant to reveal the prior austenite grains of thermomechanical processed and press hardened martensitic steels. Furthermore, alternative detecting methods such as electron backscatter diffraction (EBSD) and high-temperature laser scanning confocal microscopy were evaluated. It was found that a picric-acid etchant in combination with a prior tempering treatment of the steel enables the visualization of prior austenite grains and their elongation in a micrometer-scale.

Li et al., [26] is as follows: The grain boundaries in UFG as well as in nanocrystalline materials will act as very effective sources and sinks for dislocations. Due to the relatively small grain size, reactions between mobile dislocations and grain boundary dislocations are very likely to occur. Thermally activated annihilation processes in the grain boundaries, which are still active at relatively low homologous temperatures, might therefore play a decisive role for the increasing ductility as the deformation rate decreases, although the homologous temperatures are rather low. Further investigations are planned in order to clarify this very important issue for understanding the paradox of strength and ductility.

The construction of mobile crane booms requires the usage of ultra‐high strength steels. Micro‐alloying elements promote grain refinement during hot rolling and result in increased toughness. The relevant strength is given through a martensitic microstructure, which is accomplished by elements retarding the γ to α transformation. Direct quenching (DQ) from the rolling heat and quenching after a preceding re‐austenitization (RQ) are two different production routes. They differ regarding their productivity, their achievable strength levels, and their resulting microstructures. In order to explore the influence of the production route in combination with prominent micro‐alloying elements, which come to application during hot‐rolling, six steels with varying content of V and Nb are investigated concerning their different properties after DQ and RQ as well as their behavior after tempering. It is found, that Nb strongly improves the strength after thermomechanical processing in the as‐rolled condition. Furthermore, Nb compensates the loss of strength during tempering. This effect is not thoroughly discussed in literature so far. Although Nb leads to grain refining during re‐austenitization, the effects on the strength of RQ steels are minimal. The effect after tempering is also weaker than after direct quenching. It is also shown, that V offers a high strength potential after tempering, however weakens the impact toughness significantly.

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For the construction of mobile crane booms, ultra-high strength steels produced via thermomechanical processing (TMP) have widely substituted steels fabricated through the conventional quenching and tempering (Q+T) route. A strong deformation of the austenite grain during hot rolling followed by direct quenching (DQ) offers benefits in terms of strength and toughness. To guarantee an optimal through-hardening, alloying elements retarding the c to a transformation are used. To explore the influence of the processing route on the critical cooling rate and the hardenability, hot deformation tests were performed on a deformation dilatometer. Different cooling rates were applied after deformation corresponding to two different rolling cycles with varying finish rolling temperatures (FRTs). The obtained hardness values were compared to those received through conventional quenching after austenitization. These investigations conducted on three steels with varying micro-alloying contents showed that Nb in combination with TMP raises strength significantly, and promotes a bainitic and ferritic transformation in solid solution. When applying low FRTs and in combination with other micro-alloying elements, NbC coarsens and reduces the effect of precipitation hardening.

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