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.

先共析铁素体可以优先提供低温裂纹扩展路径，在连铸过程中对板坯或钢坯的韧性有害。本文采用单变量控制方法，结果表明，中碳含硫大花坯的先共析铁素体比无硫大花坯更发达，尤其是在表面。热力学计算表明，铁素体的形成面积随着硫含量的增加而增加，说明硫有利于铁素体的形成。MnS夹杂物是先共析铁素体的核心，析出MnS夹杂物的数量与析出先共析铁素体的面积分数呈正相关。进一步使用 FIB-TEM 技术研究了 MnS 夹杂物和先共析铁素体的取向关系和晶体结构。结果表明，MnS夹杂与先共析铁素体的形成没有直接关系。在 EDS 和 EPMA 结果中观察到的 MnS 夹杂物周围的 Mn 贫化区是中碳含硫大华中先共析铁素体形成的直接诱导因素。