The present study investigates the morphological evolution of carbonitrides and the effect of these precipitates on grain boundary pinning during pseudo-carburizing a Nb-Ti-Al microalloyed steel. The result indicated that three kinds of complex precipitates with different morphologies containing Nb, Ti, and Al respectively were observed in samples austenitized at different temperatures and times. The NbC and TiN precipitates played an important role in pinning grain boundaries and suppressing the growth of austenite grains, relying on the high thermal stability of TiN precipitates and small size of NbC precipitates. The precipitate characteristics affected the size of austenite grain. Based on the Zener pinning model, the effect of precipitate characteristic on austenite grain size was quantitatively analyzed. It is found that the existence of NbC and TiN precipitated at high temperature makes austenite grain growth difficult when austenite grain boundaries were pinned by fine and diffused precipitates.

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Suitable MnS inclusions in gear steel can significantly improve the steel machinability and reduce the manufacturing costs. Two gear steel samples with different sulphur contents were prepared via aluminium deoxidation followed by calcium treatment. The shape, size, composition and percentage distribution of the inclusions present in the steel samples were analyzed using an electron probe micro-analysis (EPMA) technique. The average diameter of MnS precipitated on an oxide inclusion is less than 5 µm. It was found that the steel with high sulphur content contains a greater number of elongated MnS precipitates than low sulphur steel. Moreover, there are more oxide inclusions such as calcium-aluminates and spinels with a small amount of solid solution of (Ca,Mn)S in low content sulphur steel after calcium treatment, which indicates the modification of solid alumina inclusions into liquid aluminates. The typical inclusions generated in high sulphur steel are sulphide encapsulating oxide inclusions and some core oxides were observed as spinel. The formation mechanisms of complex inclusions with different sulphur and calcium contents are discussed. The results are in good agreement with thermodynamic calculations.

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