What are the common defects of large ingot and the improvement measures?

Large steel ingots are needed to produce extra-thick steel plates, and the internal quality problems of large steel ingots are mainly defects such as loose center and delamination at the tail. Reflected on the steel plate, the head point-like dense defects, head area defects, tail area defects and shrinkage defects were characterized by ultrasonic flaw detection. The reasons need to be analyzed and controlled. 1, spot-like dense head, area defect 1) Reason: From the results of anatomy and macro-erosion of the defect samples, it can be seen that the spot-like dense defects in the center of the head should be pores and segregation eventually formed during the solidification of molten steel. For large flat ingot, it mainly comes from the selective crystallization in the early stage of molten steel, which leads to the aggregation of low melting point substances in the later stage of solidification and centralized solidification under the condition of low superheat. Because of the loose dendrite bridging, a V-shaped segregation zone is formed under the riser, and there is no seepage deformation during rolling, forming point-like dense defects. 2) Measures: including reducing superheat, increasing pouring speed, prolonging ingot holding time and increasing single pass reduction. 2. Tail defect 1) Reason: The anatomical results of the tail defect mainly show that there are external inclusions in the center, which are composed of Al2O3 or inclusions containing Al2O3 and SiO2. The results of flaw detection in other places are good, which shows that the molten steel itself is pure. Al2O3 in steel plate may be related to the fact that deoxidized products did not float to the interface of steel slag in the later refining process, or it may be accumulated in the solidification process of molten steel in ingot mould and failed to float in time. At the same time, molten steel enters the ingot mould through the runner brick. First of all, the performance of runner brick is unstable. If the superheat of molten steel is high, the runner brick will be eroded and melted into molten steel. Second, the channel brick has cracks when it is put into use, and after being crushed by external force, it enters the molten steel with the steel flow. 2) Measures: The main process improvement measures adopted include extending soft blowing time after vacuum, increasing steel allowance and using good and stable raw and auxiliary materials. 3. Causes of shrinkage cavity defect 1): The main causes of shrinkage cavity are high pouring superheat, fast pouring speed and poor riser insulation effect. 2) Measures: It mainly includes controlling the superheat of ladle, controlling the flow during pouring, sufficient riser pouring height and good insulation effect.