特殊钢连铸大方坯皮下负偏析及对中心偏析作用机制研究

The center located macro-segregation of heavy sectioned special steel castings is the bottleneck of internal quality upgrading for their following forging or rolling products. In-mould electromagnetic stirring (M-EMS) is one of the key techniques to control and improve casting internal quality during continuous casting (CC) process in steel mill. Application of M-EMS, however, will bring the subsurface negative segregation band, and simultaneously increase the positive segregation degree at the columnar to equiaxed transition zone for bloom castings at a certain extent. Reduction of M-EMS stirring intensity, which can be beneficial to improving the negative segregation band, will lead to the increasing of macro-segregation degree at strand center. Accordingly, coordinated regulation for the strand subsurface negative segregation band and center macro-segregation will further improve the internal quality of special steel product. Based on the development of a coupling electromagnetic-thermal-solute model for popular CC process, the effect of macro-transport behaviors in mould region on the degree and width of the subsurface negative segregation band will be revealed. Combined with the industrial test results, the regulation mechanism on the subsurface negative segregation behavior will be elaborated based on the optimizing and design of feeding modes and M-EMS technical parameters, where a feeding mode coupling the swirling flow nozzle and M-EMS is proposed. After these above, the regulatory mechanism of mould metallurgical behavior to the degree and width of subsurface negative segregation band and center segregation will be explored based on the understanding the internal interaction rule between the subsurface negative segregation band and center segregation. Therefore, the theoretical foundation for effectively controlling the homogeneity of casting products, specially for the heavy sectioned semi-products, are expected to be provided based on the well understanding to the formation mechanism of centre located macro-segregation.

连铸中心偏析一直是特殊钢大型锻、轧件内质提升的瓶颈问题。结晶器电磁搅拌（M-EMS）是改善铸坯内质的有效手段，但其易促使铸坯皮下负偏析带形成，并加剧柱状晶/等轴晶转变区域正偏析度。若降低M-EMS搅拌强度以改善皮下负偏析，则会加重铸坯中心偏析度。为此，实现铸坯皮下负偏析与中心偏析的协同调控，将利于特殊钢产品内质进一步提高。本项目拟建立符合现代连铸工艺特征的电磁-热-溶质传输耦合模型，查明结晶器内宏观传输现象对铸坯皮下负偏析度、带宽的作用规律；基于此，提出旋流水口与M-EMS耦合浇铸方式，结合工业试验，阐明基于浇铸方式与M-EMS工艺参数设计的铸坯皮下负偏析行为调控机理；最后，明晰铸坯皮下负偏析与中心偏析之间内在作用关系，探究结晶器冶金行为对铸坯皮下负偏析度、带宽及中心偏析度的调控机制，从而有望从中心偏析形成机制理解层面上，为实现铸坯、尤其是厚大断面铸坯均质性的有效控制提供理论依据。

凝固前沿浓化钢液长程流动引起的中心偏析问题一直是铸坯及其轧材产品内部质量提升的瓶颈。结晶器电磁搅拌（M-EMS）是改善铸坯内质的有效手段，但其易促使铸坯皮下负偏析带形成，并加剧柱状晶/等轴晶转变区域正偏析度。若降低M-EMS搅拌强度以改善皮下负偏析，则会加重铸坯中心偏析度。鉴于此，本项目在建立和验证连铸电磁-热-溶质传输耦合模型基础上，明确了铸坯皮下负偏析行为与中心偏析之间内在作用关系，阐明了不同浇铸方式和M-EMS工艺参数下结晶器内宏观传输现象对铸坯皮下负偏析度、带宽大小的作用规律。结果表明：1）常规双侧孔水口+M-EMS下，由于质量守恒，随着M-EMS工作电流由100A增至600A，铸坯皮下最大偏析度由0.98降至0.85，而铸坯距表面1/4处最大偏析度由1.01增至1.11。在结晶器内过热耗散和质量守恒综合作用下，铸坯横截面上碳含量极差值随M-EMS工作电流增加呈先降低后增加趋势。2）与常规双侧孔水口相比，新型旋流水口+M-EMS可在结晶器内形成有效交替搅拌状态，利于过热耗散效果、坯壳厚度和成分周向均匀性的提高。其中，对于65Mn弹簧钢，皮下负偏析最大值由0.90增至0.92，偏析度小于0.94的负偏析带宽度由10.3mm降至3.6mm。3）旋流水口+M-EMS工况下，过强M-EMS搅拌强度（工作电流I＞600A）将在结晶器内无法形成交替搅拌模式；随着工作电流的降低（500A降至300A），交替搅拌状态的形成将促使铸坯皮下负偏析最大值由0.897逐渐增至0.976，偏析度小于0.94的负偏析带宽度约由21mm降至0mm；当工作电流进一步降低，过弱的M-EMS搅拌强度将难以有效抑制上部由旋流水口侧孔射流引起的水平旋流，进而导致铸坯皮下负偏析值增加。综上，项目研究结果从中心偏析形成机制理解层面上，揭示了铸坯皮下负偏析与中心偏析的协同调控机制，将利于特殊钢产品内质进一步提高。