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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]

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 γ to α 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. [ABSTRACT FROM AUTHOR]