2015 Volume 55 Issue 9 Pages 2018-2026
The effect of REM (Ce,La,Nd) addition on austenite grain refinement in the heat affected zone in Fe-0.07/C-0.05/Si-1.5/Mn-0.003/S (mass%) steel was investigated by microstructural observation by SEM/EDS and thermodynamic analysis. It was found that the 30 ppm of REM addition is most effective for reducing the austenite grain size in annealing at 1450°C for 10 sec, but the grain size increases with increasing REM contents of more than 30 ppm. Since the total numbers of precipitates of oxides, sulfides and oxysulfides increase with increasing REM contents, this austenite grain growth cannot be explained by Zener’s pinning model. A thermodynamic calculation showed that REM oxides and MnS in oxysulfides are formed by the miscibility gap in 30 ppm REM steel, which results in the formation of liquid MnS-rich precipitates due to eutectic reaction. On the other hand, the melting temperature of MnS-rich precipitates increases with increasing REM in oxysulfides, and the solid precipitate particles are not effective for inhibiting grain growth. It is suggested that austenite grain growth is suppressed by the liquid phase pinning effect of MnS-rich precipitates.