Modeling of Bake Hardening Kinetics and Carbon Redistribution in Dual-Phase Steels

The bake hardening response (BHR) of a dual-phase steel with a martensite volume fraction of 0.22 is studied by means of mechanical testing, microstructural characterization, and computational simulation. After austenitization and quenching, the material is exposed to temperatures between 100 and 220 degrees C for times up to 10 000 min. The increase in yield strength during bake hardening (BH) follows the well-known two-stage characteristics. Herein, the kinetics of BH is analyzed on the basis of computational models for the formation of Cottrell atmospheres and precipitation, which represent the cause of strength increase, as well as the long-range diffusion of C, which is partitioning between the martensite and ferrite phases during aging. The simulated yield strength evolution is in good agreement with the experiments. The model clearly describes the shape of the experimentally observed BH curves as a function of temperature, time, and degree of prestrain. In contrast to previous simulation attempts, all metallurgical phenomena are treated as coupled processes and modeled in an integrated framework based on a single set of input and state parameters.