基于二次枝晶间距变化特征的连铸方坯CET位置判断新方法

A New Method for CET Position Determination of Continuous Casting Billet Based on the Variation Characteristics of Secondary Dendrite Arm Spacing

从碳钢连铸坯实际凝固组织入手对典型枝晶二次枝晶间距(secondary dendrite arm spacing,SDAS)进行测量分析,并发现了铸坯表面向中心凝固过程中的SDAS突增现象。结合铸坯横断面二维温度场数值模型分析可知,柱状晶向等轴晶转变(columnar to equiaxed transition,CET)的过程会影响铸坯内部的传热过程,这种影响最终以典型枝晶SDAS突增的形式体现出来。基于典型枝晶SDAS突增现象,确立了铸坯内部CET定量判定的新方法,即将典型枝晶SDAS最大增加率的起始位置确定为铸坯内部CET起始位置。计算所得CET位置与铸坯内部温度梯度变化拐点的最大相对误差仅为8.3%,且与生长速率变化区间相对应,同时也与实际凝固组织形貌转变位置吻合,证明了该方法的有效性。

High-end steel products play an essential role in economic development and infrastructure projects.Nowadays,continuous casting is an important production process of high-end steel products due to higher efficiency and lower energy consumption.However,the quality and properties of end products,such as steel billets,bloom,and bar,will be affected by internal segregation defects,which are closely related to solidification structure characteristics.In the research on the solidification structure characteristics of billets,the columnar to equiaxed transition(CET) determination is of great significance to determine equiaxed crystal zones and the quality control of continuous casting billets.In this work,the secondary dendrite arm spacing(SDAS) of typical dendrites was measured and analyzed using the actual solidification structure of continuous casting billets and the mutation of SDAS during the solidification process from the billet surface to the center was found.Combined with the two-dimensional temperature field numerical model of the billet cross section,it can be seen that the CET will affect the heat transfer process in the billet and this will be reflected as the mutation of SDAS in typical dendrites.This work proposed a new method for the quantitative determination of CET in the continuous casting billets based on this mutation,and the starting position of the maximum SDAS increase rate is determined as the starting position of the CET.The CET positions calculated using the new method correspond to changes in the thermal gradient and growth rate in the billet,and are consistent with the positions of the actual solidification structure morphology transformations,which prove the effectiveness of this method.