高速铁路轨道用钢连铸坯MnS夹杂物弥散析出调控机理研究

The large-sized and dendritic tpye-Ⅱ MnS inclusions in the continuously cast strand of high-speed rail steel are usually elongated into long chain-like inclusions during the subsequent rolling process. The elongation of MnS causes stress concentrations and the tips of MnS inclusions easily acts as initiation sites of fatigue damage for rail in service, and seriously endanger the running safety of high-speed train. Therefore, in this study, the interaction between the dendritic growth, solute segregation, and the MnS inclusion nucleation and growth during the continuous casting process of high-speed rail steel is comprehensively investigated in oder to elucidate the growth behavior of dendrite controlled by multi-component solute diffusion, the effect of solidification structure evolution on the solute segregation, the effect of dendrite morphology on the dendritic tpye-ⅡMnS inclusions, and the quantitative relationship between the solute segregation and the large-sized spherical type MnS inclusions. Meanwhile, the dendrite fragment, grain movement, rejection and mixing of solute-enriched liquid, and the precipitation of metastable MnS inclusions in the mushy zone are studied during the continuous casting process of high-speed rail steel by applying a combination of outfield technology (final electromagnetical stirring (F-EMS) +mechanical reduction (MR)). Based on the above mentioned studies, the outfield combination technology (F-EMS+MR) is proposed to control the solidification structure and solute sgregation in order to prevent the large-sized spherical type and dendritic tpye-Ⅱ MnS inclusions formation and finally achieved the control of dispersing MnS precipitation in the continuously cast strand of high-speed rail steel.

高速铁路轨道用钢连铸坯中的大颗粒和树枝状(Ⅱ类)MnS夹杂物容易在后续轧制过程中延展形成大尺寸长条状夹杂物，成为钢轨服役过程中疲劳损伤的裂纹孕育源，严重威胁高速列车行车安全。鉴于此，本项目拟从高速轨钢连铸凝固过程枝晶生长、溶质偏析、非稳态MnS夹杂物形核及生长等方面开展深入研究，揭示高速轨钢多组元扩散控制枝晶生长行为，凝固组织演变对溶质偏析的影响规律，枝晶组织形貌对树枝状(Ⅱ类)MnS夹杂物的影响规律，以及溶质偏析与大颗粒球型MnS夹杂物之间的定量关系。同时，阐明外场协同作用下(末端电磁搅拌+机械压下)高速轨钢连铸过程枝晶断裂和晶粒迁徙行为，两相区浓缩钢液挤压排出和重新混匀行为，以及非稳态MnS夹杂物析出行为。在此基础上，研究开发外场协同作用新工艺技术，控制高速轨钢连铸坯凝固组织形貌和溶质偏析，抑制大颗粒和树枝状(Ⅱ类)MnS夹杂物形成，实现高速轨钢连铸过程MnS夹杂物弥散析出控制。

大尺寸长条形MnS夹杂物极易成为高速轨服役过程中疲劳损伤的裂纹孕育源，严重威胁高速列车行车安全。因此，研究高速铁路轨道用钢连铸坯MnS夹杂物析出规律，从而揭示连铸坯凝固组织与MnS夹杂物枝晶的定量关系，为高速轨钢连铸过程MnS夹杂物弥散析出控制提供理论指导。采用ProCAST有限元商业软件建立了重轨钢U75V连铸凝固过程传热和晶粒生长相耦合多尺度数学模型（CAFE），揭示了连铸工艺参数对铸坯凝固组织演变影响规律的研究。基于枝晶间溶质扩散定律和MnS析出热力学理论，建立了耦合钢中溶质扩散的MnS夹杂物析出模型，揭示了钢种成分和冷却速率对重轨钢U75V连铸坯中MnS夹杂物尺寸的影响规律。开发了重轨钢U75V连铸坯中MnS夹杂物三维无损检测装置和技术，揭示了连铸坯内部MnS夹杂物形态和尺寸分布规律。最终根据铸坯轧制变形理论和钢中非金属夹杂物含量的测定标准评级图显微检验法（GBT 10561-2005），建立了连铸坯中MnS夹杂物尺寸的控制标准，并系统分析了钢中S含量和连铸坯凝固组织对连铸坯MnS夹杂物尺寸的影响规律，从而开发出重轨钢U75V连铸坯MnS夹杂物弥散析出控制工艺技术并最终在攀钢重轨钢生产中成功应用。