<Original Paper>'In-situ' Observation of the Behavior of Precipitation in the Steel Containing Cu, Mn and S

&quot;In-situ&quot; observation of precipitation in the steel containing Cu, Mn and S was made on heating and cooling using a confocal scanning laser microscope. In low Cu steel (less than 0.5mass%Cu), MnS precipitates appeared but Cu precipitates did not appear on cooling. MnS precipitation occurred in two steps. In first step, some large precipitates appeared with the δ/γ transformation. During this transformation, S was concentrated in δ phase, so MnS precipitation occurred. In second step, some small precipitates appeared at lower temperature. The temperature became low, the equilibrium solubility products of MnS decreased and MnS precipitation occurred. In Fe-10 mass%Cu steel, some holes appeared on heating and MnS and Cu precipitates appeared on cooling. The solid Cu precipitates appeared on heating and they melted and sank into grain boundaries at 1094℃, so some holes were left. The behavior of Cu precipitation on cooling changed with the difference of the MnS content. In the case that MnS content was very low, Cu precipitates were mainly at the γ grain boundary. But in the case that MnS content was high, Cu precipitates were not only at the grain boundary but also in the grain. It was because MnS precipitates in the grain acted as the nucleation site of Cu precipitates.