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Direct quenching of thermomechanically processed low-carbon steels is a preferred production route to increase strength and toughness of ultra-high-strength steels and thus enhance the payload-to-weight ratio, e.g., of mobile cranes. However, during hot rolling, certain crystallographic textures emerge, which can generate unfavorable mechanical properties or mechanical anisotropy. In order to investigate the role of the processing route and the effect of micro-alloying elements on the texture formation and its relationship to differences between different testing directions, four different ultra-high-strength steels were subjected to various quenching procedures. It was found that despite equiaxed prior austenite grains after re-austenitization, differences in the longitudinal and transverse directions remain. The extinction of a rolling texture after re-austenitization is dependent on the austenitization condition and the addition of micro-alloying elements. In particular, Nb promotes the formation of rolling texture components and prevents the extinction thereof even through intense austenitization treatments. However, remaining preferred orientations exhibit only little influence on the anisotropy of the mechanical properties. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. This paper contains an investigation of the recrystallization behavior of low alloyed steels. Dilatometer tests and a metallographic analysis are applied to illustrate the recrystallization kinetics and the role of micro‐alloying elements. The investigations provide insight in the microstructural evolution during a hot‐rolling scenario and an easy‐to‐applicable technique for the design of thermomechanically processed steels. [ABSTRACT FROM AUTHOR]