In continuous casting production, network cracks sometimes appear on the surface of round billets. The cracks are small and difficult to find when finishing molds. This causes surface cracks when users are rolling and brings huge losses to the enterprise.
It is analyzed that the network cracks are caused by uneven cooling of the primary blank shell, which causes the austenite grains to be coarse. The copper element in the matrix is enriched in the austenite grain boundaries, which reduces the grain boundary bonding force and induces intergranular cracks; in addition, Due to the instability of the liquid level during the casting process, the mold slag is involved and concentrated in the austenite grain boundaries, resulting in stress concentration points and forming the origin of cracks.
Control measures:
1. Ensure good arc and centering accuracy.
Good arc and centering accuracy can ensure that the blank shell runs accurately along the arc radius, and it can also prevent the primary blank shell from being unevenly cooled in the mold. The distance between the mold foot roller and the counter-arc template should be controlled to be less than 0.1mm, the distance between the fixed section support roller and the counter-arc template should be less than 0.3mm, and the mold flow distance should be kept within the standard value of ±2mm. At the same time, ensure that the level of the mold is ± Within 0.01mm.
2. Control liquid level fluctuations
When the liquid level fluctuates, the composition of the mold slag will cause the plasticity of the grain boundary to decrease after entering the austenite grain boundary. In addition, the matrix grain boundary itself is rich in low melting point elements, which will become the origin of cracks. Ensure that the liquid level fluctuation is within ±2mm. within,
3. Use suitable mold powder
Compared with the low-basicity slag, the high-basicity slag casting process is more likely to form slag sticks, which prevents the liquid slag from flowing into the channel. Therefore, for mold flux, low alkalinity should be used.
4. Ensure the insertion depth of the mold nozzle and the nozzle centering.
When the mold nozzle is not centered during the casting process, it will cause uneven cooling around. Part of the green shell has coarse grains, which can easily cause microcracks. When the nozzle is inserted too deep, the temperature around the mold is low, the mold powder cannot be fully melted, and the powder slag layer directly enters between the casting billet and the copper tube, which will increase the friction and aggravate the formation of cracks. The insertion depth of the nozzle of the mold and the centering of the nozzle should be ensured.
5. Prevent the coating on the inner wall of the copper pipe from falling off
The shedding of the coating on the copper tube of the mold will cause the penetration of Cu and cause surface micro-cracks. To prevent the coating on the inner wall of the copper tube from falling off, the mold copper tube should be replaced when the coating is falling off within 150mm from the meniscus.
6. Ensure the uniformity of the second cooling water spray
Ensuring the uniformity of the secondary cold water helps to reduce surface cracks.
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