1998 Volume 38 Issue 9 Pages 1022-1031
The effect of vanadium and niobium on the restoration behavior after hot deformation in medium carbon spring steels was investigated by use of a thermomechanical processing simulator. It was clarified that vanadium is effective to inhibit restoration after hot deformation, because vanadium carbides and/or nitrides finely precipitate during and after hot deformation. On the other hand, niobium is not effective to inhibit restoration. This is because the amount of niobium soluble in austenite is essentially very small in medium carbon steels and that niobium exists as coarse carbides before hot-deformation.
The degree of strengthening in martensite was not so large within the present deformation condition equivalent to Controlled Tapered Leaf Rolling, because the enhanced dynamic recovery and recrystallization caused the steady-state deformation(small work-hardening). Further, the increase in the amount of retained austenite and the decrease in solute carbon resisted to increase the strength. However the martensite structure became fine owing to the refinement of the austenite grains by hot deformation. As a result, the toughness and the fatigue strength increased by hot deformation and direct quenching. The above results indicate that the combination of the present thermomechanical processing and the Controlled Tapered Leaf Rolling would be effective to decrease the weight of the springs. The present investigations have also clarified that the vanadium addition and the low temperature deformation is preferable to maximize the effect of the thermomechanical processing.
