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The morphology of MnS formed by solidification in steels and its modification by alloying of Al, Si, C and Ti have been investigated by means of optical and scanning electron microscopy. The morphology of MnS formed after the primary crystallization of Fe phase can be classified into (i) droplet MnS formed by monotectic reaction, (ii) rod-like MnS formed by eutectic reaction and (iii) fish-born type MnS formed by irregular eutectic reaction. The morphology of primarily formed MnS in high sulfur concentration has been observed to be (iv) spherical, (v) dendritic and (vi) angular depending on the additional elements and melting atmosphere. The mechanism of MnS formation has been discussed on the basis of the phase diagram information. It has been shown that the spherical shape of the primary and secondary solidified MnS is formed through the metastable reactions. The eutectic, dendritic and angular MnS are formed by the stable reactions, where the dispersed particles like TiN and Al_2O_3 with high melting temperature act as nucleants of MnS crystals. The addition of C and Si promotes the stable reactions by enlarging the temperature difference between the eutectic and monotectic points.