The Formation Mechanism of Proeutectoid Ferrite on Medium-Carbon Sulfur-Containing Bloom

Proeutectoid ferrite is harmful to the toughness of slab or billet during continuous casting since it can provide crack propagation path at low temperature preferentially. Single variable controlling method is used in this paper and the results show that the proeutectoid ferrite of medium-carbon sulfur-containing bloom is more developed than that of sulfur-free bloom, especially on the surface. Thermodynamic calculations show that the formation area of ferrite increased with the increase of sulfur content, suggesting that sulfur is beneficial to the formation of ferrite. MnS inclusions are the core of proeutectoid ferrite and the number of precipitated MnS inclusion is positively correlated to the area fraction of precipitated proeutectoid ferrite. FIB–TEM technique is further conducted to investigate the orientation relationship and crystal structure of MnS inclusion and proeutectoid ferrite. The results show that MnS inclusion are not directly related to the formation of proeutectoid ferrite. Mn-depleted zone around MnS inclusion observed in EDS and EPMA results is the direct inducing factor for the formation of proeutectoid ferrite in medium-carbon sulfur-containing bloom.