Structure and properties of TM processed HSLA steels

This paper deals with the influence of steel composition (with respect to the microalloying elements) and thermomechanical treatment on structure and mechanical behaviour of HSLA (High Strength Low Alloy) steels. Four different microalloyed steels have been TM rolled under variing conditions. The austenite structure prior to transformation could be revealed by means of a special etching technique. The precipitation behaviour has been studied by TEM, STEM and APFIM investigations. The results of mechanical testing can be explained in terms of structure (both austenite and transformed material), steel composition and TM treatment. It could be shown how precipitates in different steels influence the recrystallization behaviour of the austenite and how the austenite structure is related to the transformed structure. For example, fine strain-induced Nb carbonitrides prevent or reduce recristallization. Addition of Ti leads to relatively coarse complex carbonitrides. As a consequence, the content of solute Nb decreased and only few Nb carbonitrides precipitate during deformation. Hence the Ti-bearing steel shows a stronger tendency for recrystallization and the mechanical properties (especially the low-temperature ductility) deteriorate. (orig.)