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A finite element model was developed to predict the deformation, temperature history, carbon diffusion, phase fraction, and hardness during the carburizing heat treatment of automotive annulus gear ring, initially made of a medium carbon steel. Carburizing gas with a constant carbon potential for entire surfaces of the gear was assumed. The temperature and pressure driven carbon diffusion was solved by the finite element simulation based on Fick's law. Both the diffusional and displacive phase transformations during the heat treatment were modeled incorporating the carbon concentration inside the gear. The constitutive equation of the transformation plasticity was incorporated into the finite element model. Strains due to the phase transformation, transformation plasticity, and thermal expansion/contraction were calculated by the finite element model. The prediction accuracy for the phase evolution, hardness distribution, and dimensional change of the gear ring was verified with the measurement data.

A 50 kg-grade cold rolled dual phase (DP) steel was developed based on the orthogonal design method. The intercritical annealing, aging and galvanizing temperatures during the heat treatment were considered as controllable factors and three levels for each of these factors were selected. The optimal experimental conditions were acquired by the analysis of variance (ANOVA) on the ultimate tensile strength (UTS) and the total elongation of the DP steel. A dilatometry analysis was performed to measure the phase transformation kinetics of the DP steel and to verify the statistically analyzed results. These metallurgical observations explained well the statistically analyzed results.