Effect Of Electropulsing And Magneticpulsing On Grain Refinement Of Cold-Rolled Medium Carbon Steels

Application of electropulsing to the hot-rolled and cold-rolled medium carbon low alloy steels has generated completely different effects. For cold-rolled steel samples, electropulsing treatment caused microstructure refinement and hence the increments of tensile strength, yield strength and elongation. For hot-rolled steels, the effect of electropulsing was found to be negligibly small. This proves the importance of dislocations in the micromechanism of electropulse-induced microstructure transformation in metals. The experimental observations are explained within the frameworks of thermodynamics and kinetics of microstructure transformation.Electropulsing and magneticpulsing were applied to hot-and cold-rolled medium carbon steel 30CrMo and 65Mn with varying peak electric voltages and currents at room temperature. The mechanical property test results of cold-rolled 30CrMo and 65Mn showed that the elongation and area reduction of the specimens treated with electropulsing increased, compared with that without the treatment of electropulsing. Microstructural observations revealed that the pearlite colonies and ferrite grain in the specimen treated with electropulsing was smaller than that without the treatment of electropulsing. It was also found that the lamellar structure of pearlite was refined. The grain refinement of pearlite was more obvious when the applied electric current density increased. The grain refinement may be attributed to additional free energy for the nucleation of pearlite provided by electropulsing. The hot-rolled specimens had no noticable change. Magneticpulsing also had no noticable effect on microstructure in medium carbon low alloy steel. It is likely that the strength of the magneticpulsing was not so strong enough to induce the microstructure transformation.According to classical solidification theory and the mechanisms of solidification microstructure under pulsed electric current, the effect of underclooling degree on nucleation rate was discussed. The influence of electromagnetic stress effect and Joule heat effect on undercooling degree were investigated. The relation between electric current and undercooling degree was devived. The effect of electropulsing on nucleation rate was due to nucleation potential barrier decreased by electropulsing. The electropulsing influenced the microstructure of the cold-rolled steel at room temperature, which was not determined by the effect of electromagnetic stress effect and Joule heat effect on undercooling degree change. Electropulsing reduced the nucleation potential barrier and increased the nucleation rate and thus made microstructure refinement.