Austenite in the process of thermomechanical treatment of microalloyed spring steels

On a classical CrV-bearing spring steel 50CrV4 (1.8159) the effects of thermomechanical treatment (TMT) and additional microalloying with titanium and niobium on the processes in austenite were investigated. The aim of this study was to achieve an austenite state, that promises -after subsequent quench and tempering- mechanical properties of spring steels superior to those after conventional treatment. For laboratory tests the hot deformation simulator Wumsi was employed. It was found that after reheating to usual austenitization temperatures the austenite microstructure of steel 50CrV4 is widely uninfluenced by the initial as-delivered microstructure. Despite the finer austenite grain size occurring after austenitization, additional microalloying with Ti and Nb raises the recrystallization temperature considerably. The finding that the incubation time of austenite recrystallization can be prolonged up to several minutes by microalloying is decisive for the practical applicability of TMT. In this way the substructured state of polygonized austenite beneficial for martensite formation can be preserved over the time needed for additional production steps between hot rolling and hardening of leaf springs. Moreover, the deformation of a not recrystallizing austenite may favourably influence the distribution of undesirable tramp elements in spring steels.