Common defects and improvement measures in large ingot production
It is necessary to use large ingot in the production of extra-thick steel plate, and the internal quality problems of large ingot are mainly defects such as loose center and stratification of tail. Reflected on the steel plate, using ultrasonic detection qualitative, respectively head spot dense defects, head area defects, tail area defects, shrinkage defects. Need to analyze the cause and control.
Head spot dense, area defect
1) Cause: According to the anatomy of the defect sample and the results of low-power erosion, the dense point-like defect in the center of the head should be the final porosity and segregation formed during the solidification process of the liquid steel. For large flat ingot, the main source is the separation crystallization of liquid steel in the early stage, which leads to the accumulation of low melting point substances in the late solidification period, and concentrated solidification under the condition of low superheat. Due to the loose formation of dendrite bridging, the shape is like the "V" type segregation zone under the head, and there is no penetration deformation during the rolling process, forming point-like intensive defects.
2) Measures: including reducing the superheat, increasing the pouring speed, extending the holding time of the ingot, and increasing the reduction amount of single pass rolling.
Tail area defect
1) Cause: The anatomical results of the defects in the tail area are mainly manifested as the presence of external inclusions in the center, the composition of which is Al2O3 or inclusions containing Al2O3 and SiO2. The inspection results in other places are good, indicating that the liquid steel itself is pure. Al2O3 in the steel plate may be related to the deoxidation products in the late refining process not sufficiently floating into the steel slag interface, or it may be caused by the accumulation of molten steel during the solidification process in the ingot mold and the failure to float in time.
At the same time, the molten steel enters the ingot mold through the soup brick. First, the performance of the soup brick is unstable. If the superheat of the molten steel is high, the soup brick will be eroded and melted and enter the molten steel. The second is that when the soup brick is put into use, there is a crack in itself, which is broken by external force and enters the liquid steel with the steel flow.
2) Measures: The main process improvement measures are to extend the soft blowing time after vacuum, increase the amount of steel, and use good and stable quality raw and auxiliary materials.
Shrinkage porosity defect
1) Causes: The causes of shrinkage hole mainly include high pouring superheat, fast pouring speed, poor thermal insulation effect of riser, etc.
2) Measures: mainly include control of the superheat of the hanging bag, flow control operation during pouring, sufficient pouring height of the riser and good insulation effect.
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