连铸控流模式对大方坯及棒材组织结构与宏观偏析影响

Macrostructure and macrosegregation behavior of bloom products under various flow control modes of the casting process

以中碳结构钢大方坯及其热轧棒材为研究对象,通过对铸坯和轧材进行低倍侵蚀和成分分析,揭示了连铸控流模式对大方坯凝固组织与宏观偏析分布特征的影响及其铸轧遗传性.研究表明:常规直通水口浇注模式下,结晶器电磁搅拌(Mold electromagnetic stirring,M-EMS)电流由0增加到800 A,铸坯等轴晶率由6.06%仅可增加到11.71%,难以有效避免大方坯常见的中心缩孔缺陷与突出的中心线偏析.采用新型五孔水口浇注模式,即使不开启M-EMS,铸坯中心等轴晶率仍可达23.1%,大方坯中心缩孔级别可降至1.0级以下,满足后续热轧大棒材探伤要求.同时发现,五孔水口浇注模式下,大方坯铸态组织中往往会出现较为明显的柱状晶到等轴晶转变(Columnar to equiaxed transition,CET)区,铸坯断面碳偏析指数呈M型分布,表现为断面1/4位置CET区域碳偏析指数最高.大棒材轧制基本改变不了铸坯断面宏观偏析的分布形态,且可能导致中心线偏析指数增加.同时指出,基于连铸控流模式的作用规律和铸‒轧遗传性特征,以及特殊钢长材热加工对中心致密度和偏析分布与程度的要求,实际生产中应从连铸工艺源头合理地控制铸态组织与宏观偏析分布形态.

Owing to the large cross-section and wide solidification-temperature zone,bloom castings of medium-and high-carbon steels are prone to severe central shrinkage and macrosegregation defects.Flow control technologies such as nozzle injection mode and electromagnetic stirring,together with the casting speed,play a key role in the as-cast macrostructure and macrosegregation distribution in bloom castings achieving soundness and compositional homogeneity of the final as-rolled products.Based on the production process of a medium-carbon-steel bloom casting and its heavy section bars,various flow control modes have been adopted in the casting production to study their effects on the semiproduct solidification structure and the carbon distribution across the bloom section and the following rolled bars.The conventional nozzle with a single straight port shows that the equiaxed crystal ratio in the casting process can only increase from 6.06%to 11.71%with the M-EMS intensity changes from 0 to 800 A,in which a shrinkage cavity and macrosegregation exist along the centerline on the strand casting.While the novel five-port nozzle has been adopted,the equiaxed crystal ratio can reach 23.1%even with the M-EMS power off,and the center cavity index drops down to grade 1.0 or below,meeting the requirement of microvoid flaw detection for the bar products.Additionally,the carbon segregation across the bloom cross-section is observed to resemble an M-shaped curve with a maximum carbon segregation index in the columnar to equiaxed transition zone instead of the popular center region.For the heavy section bars rolled from bloom casting,a similar carbon distribution pattern as the cast is observed after hot working but with an even higher centerline segregation index.Therefore,considering the special quality requirements for the subsequent hot processing,the macrostructure and pattern and intensity of macro-segregation should be regulated from the beginning of casting with a reasonable flow control mode as mentioned in the study.