Definition and Appearance
Surface pores of billet refer to the hole defects formed on the surface of the billet during the solidification process. These pores are usually caused by the fact that the gas fails to escape in time during the solidification of liquid metal and is wrapped inside the billet. Some of the gas near the surface floats to the surface of the billet due to various reasons in the later stage of solidification, thus forming surface pores. They have irregular shapes and sizes, and may appear alone or in groups. The pores on the surface of the billet will affect the appearance quality of the billet, reduce its surface finish, and may become the source of crack propagation in the subsequent processing process, affecting the mechanical properties and service life of the product.
Causes of surface pores
Raw materials
Insufficient deoxidation of molten steel will leave too much oxygen in the molten steel. During the solidification of molten steel, oxygen reacts with carbon to generate carbon monoxide gas. If the gas is not discharged in time, pores will form on the surface of the billet.
Raw materials contain more impurities such as moisture and oil. These impurities will decompose and produce gas during the steelmaking process, increasing the gas content in the molten steel, which will lead to the formation of pores.
Smelting process
Smelting temperature that is too high or too low will affect the quality of molten steel. If the temperature is too high, the molten steel will increase the gas absorption; if the temperature is too low, the fluidity of the molten steel will deteriorate, and the gas will be difficult to discharge, which will easily lead to pore defects.
Uneven stirring will also make the composition and temperature of the molten steel uneven, and the gas content in the local area will be too high, thus forming pores.
Casting process
If the casting speed is too fast, the molten steel will rise too fast in the crystallizer, and the gas will not have time to escape, and it is easy to be wrapped on the surface of the billet to form pores.
Poor performance of protective slag, such as poor air permeability, will hinder the discharge of gas on the surface of molten steel, resulting in pores.
Excessive fluctuation of the crystallizer liquid level will cause the slag layer on the surface of the molten steel to be drawn into the molten steel, which will also affect the discharge of gas and increase the probability of pore formation.
Excessive flow of argon blowing from the plug rod will cause a large number of argon bubbles to be drawn into the molten steel, part of which will be discharged during the solidification process of the crystallizer shell, and part of which will appear on the surface of the ingot.
Preventive measures for surface pores
Strictly control the quality of raw materials, ensure that the raw materials entering the furnace are dry and clean, and reduce the content of impurities and moisture. At the same time, select a suitable deoxidizer to ensure that the molten steel is fully deoxidized.
Accurately control the smelting temperature and time to ensure uniform composition and temperature of the molten steel. Strengthen the stirring in the furnace to promote the floating discharge of gas and inclusions.
Reasonably control the casting speed to avoid being too fast or too slow. Keep the crystallizer liquid level stable, use protective slag with good performance, improve the solidification conditions of the molten steel, and promote gas discharge.
The mechanism of surface pores is similar to that of subcutaneous bubbles, but they are usually not so concentrated, are irregularly distributed, and are more likely to occur sporadically.
