Formation mechanism of oxide particles in subscale layer around surface cracks of steel

Since there are many generation factors for surface cracks on steel products manufactured by the processes of continuous casting through billet rolling, it is important for their prevention to identify the process where the surface cracks are generated. In this study, the formation mechanism of the oxide particles in the subscale layer around surface cracks of steel products was investigated towards establishing an estimation method of crack generation temperature. It was found that, the average radius leveled off corresponding with its exposed temperature after enough holding time. The average radiuses were 0.2 μm and 0.3μm for 1473K and 1573K, respectively. Those oxide particles were consisted of MnO SiO₂ and MnO Cr₂O ₃ phases. Their formation and growth mechanism was as follows: Firstly, MnO-SiO₂ precipitates at the internal oxidation reaction front. Then, MnO-Cr₂O₃ forms on the MnO- SiO₂ particles. The existence of these two phases played an important role to determine the terminal value of the average radius.