Processing and properties of high carbon microalloyed steels

Five steels were used in the present work to investigate the dynamic, static and metadynamic recrystallization behavior of hypereutectoid steels of 1% carbon, alloyed with high silicon and microalloying levels of vanadium. Compression tests were performed using single and double hit schedules at temperatures between 875--1100°C, strain rates of 0.01s-1 -1s-1, and inter-pass times of 0.1--500 seconds and 0.1--30 seconds for static recrystallization and metadynamic recrystallization, respectively. For dynamic recrystallization, it was found that an increase in carbon and vanadium content led to smaller grain sizes. Equations were generated that can be used to predict the critical strain for dynamic recrystallization. Of interest is the finding that there is an activation energy for deformation specifically associated with dynamic recrystallization (i.e. peak strain). This activation energy associated with the peak strain is lower than that associated with the steady state stress. This is contrary to Sellar's original observation that the peak strain is a function of the activation energy for deformation according to the Zener-Hollomon relationship. The static recrystallization kinetics were measured, and the appearance of 'plateau' regions where softening is arrested for a period of time was used to quantify the kinetics of strain-induced precipitation and generate the precipitation temperature time diagrams for the three steels. Despite the wide range of V levels, the steels exhibited very similar precipitation kinetics at all temperatures tested, being particularly close below 950°C. A kinetic equation for static recrystallization is proposed which takes the V and Si concentrations into account. The metadynamic recrystallization results show that there is a transition strain region in which both static and metadynamic recrystallization take place during the inter-pass time. The results also revealed that V and Si have a strong solute drag effect, on the kinetics of metadynamic recrystallization. A kinetic equation for metadynamic recrystallization is proposed which takes the V and Si concentrations into account. The influence of chemistry on the microstructural characteristics and the mechanical properties in these steels was also investigated. The results show that a grain boundary cementite network exists in hypereutectoid steel without vanadium, with the thickness of this network increasing with increasing reheat temperature. Vanadium additions result in the formation of discrete grain boundary cementite particles rather than a continuous network along the grain boundaries. The mechanical properties of the resulting microstructures were evaluated by shear punch tests at room temperature. The results show that alloying and hot working increase the strength and ductility of these steels.