2006 年 46 巻 1 号 p. 82-88
Tramp elements such as Cu or Sn cause a severe hot shortness. Any tramp elements have been rejected and diluted below a certain limit by the use of virgin iron source. On the other hand, the steel properties are improved if Cu precipitates are very fine and uniformly dispersed in steels. Cu is the useful alloying element for increasing hardness and improving of r-value. Recently, it has been found that MnS precipitates have a good positive effect on the heterogeneous precipitation of Cu in grains. In this study a confocal scanning laser microscope with an infrared image furnace was used to carry out the in-situ observation of precipitation behavior at high temperature. To estimate the phase diagram, alloys were analyzed by a differential scanning calorimetry (DSC) method. It is clear that the domain of γFe phase becomes smaller and the domain of the liquid phase becomes larger with the addition of Sn. As a result of observation by the confocal laser microscope, the number of precipitates increased at higher temperatures with MnS than without MnS. The observed precipitates are classified into three types. Type “I” is the Cu (Sn) precipitate at the γFe grain boundary. Type “II” is the precipitate found in the γFe grains. One is the Cu (Sn) and another complex precipitate is composed of εCu, (Mn, Cu) S and Cu7.2S4. Type “III” is the smallest εCu precipitate in the αFe grains. They nucleated after γ/α phase transformation and the size is less than 100 nm.
