Abstract
The refinement of ferrite grain of ultra low carbon steel was observed in cyclic laser irradiation. The high heating and cooling rates were considered to cause the promoted nucleation and the suppressed grain growth. The process of refinement of grain during the laser irradiation was investigated by Monte Carlo simulation. In order to obtain the accurate simulation, the apparent activation energy of austenite grain growth was measured by isothermal heating experiments and used in the simulation of austenite grain growth. The results of the austenite grain growth simulations had a good agreement with the experimental results. Consequently, α → ← γ phase transformations which involved temporal and spatial heterogeneous nucleation were modeled with Monte Carlo method in order to simulate the process of the cyclic laser irradiation. The phase transformed fraction of ferrite and austenite with high heating and cooling rates were also obtained by simulation, and the curves of transformed fraction of ferrite and austenite showed sigmoidal variations with temperature. The results of the simulations showed the decrease of ferrite and austenite grain size with increase of times of the cyclic laser irradiation, which could explain the refinement of ferrite grain by the cyclic laser irradiation.
