抄録
In recent days, the surface contamination with flaky graphite has frequently occurred during subcritical annealing of low carbon mild steel sheets. For instance, the graphite leads to a tarnish of tin plates, which gives rise to serious troubles in many plants. The preceding experimental results revealed that the contamination was not due to the sooting from inert gas atmosphere or the carbonization of residual oil on the steel surface during annealing, but to the graphitization of carbon in the steel. This study was made on the graphite formation on low carbon mild steel surface during its annealing process. The investigation was performed by means of direct microscopic examination of polished specimens at elevated temperatures, At first, the graphite formation occurred on the surface of the overcropped carbide particles by decomposition of the carbide and the graphite flakes grew up over the ferrite surface. It was made clear that the crystal surface of steel substrate had preferential orientation for the growth of graphite flakes. X-ray and electron diffraction patterns showed that (00·1) of graphite formed on the steel surface was parallel with the surface of steel substrate. The graphite formation was observed at subcritical temperature (below A1), not above A1. The graphite flakes, which had been formed at subcritical temperature, were dissolved into the steel when heated at above A1; but they appeared again in the same position as before when the temperature was lowered to below A1. The distribution of carbide showed the diffusion of carbide particles on the surface during annealing. The diffusion was not due to the gradient of carbon concentrations, because there was a difference in carbide distribution between the product annealed in vacuum and that annealed in N2 mixed gas atmosphere.
