Formation of Acicular Ferrite in Mg Treated Ti-bearing C-Mn Steel

The effects of Mg content, inclusion size and austenite grain size on the acicular ferrite nucleation in Mg treated Ti-containing C-Mn steel were studied by high temperature experiment and metallurgical analysis. The composition analysis and size distribution counting of the inclusions and nucleuses were carried out on scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) and image process software, separately. The effectiveness of inclusions with different size and the influence of heat treatment on acicular ferrite formation were statistically investigated. The results obtained are as fellows: the optimal Mg content for acicular ferrite nucleation is 0.0015-0.0026 mass%. The best austenite grain size for acicular ferrite nucleation is about 120 mu m. The inclusions for nucleating acicular ferrite mainly consist of spherical Ti-Mg oxide covered by MnS, and there is a Mn-depleted zone (MDZ) around them. The optimized size of inclusions for acicular ferrite nucleation is between 1-4 mu m in this study. The effectiveness of inclusions inducing acicular ferrite would be increased after heat treatment.