Function of Mold Flux in Continuous Casting Machine

In order to adapt to the development of continuous casting technology, continuous casting mold flux was developed in the 1960s. Continuous casting mold flux plays an important role in the continuous casting process. In the process of exerting its function, there are many critical stages. In each stage, the continuous casting mold flux must play a good role, otherwise it will reduce the surface and internal quality of the continuous casting billet. Selecting and applying suitable continuous casting mold flux can prevent quality defects such as longitudinal cracks on the surface of continuous casting billets. Also, it can prevent accidents such as bonding breakouts during continuous casting production.

Mold flux has the following five functions in the continuous casting machine: heat insulation, anti-oxidation, absorption of non-metallic inclusions, lubrication and improvement of heat transfer.

1. Heat insulation and heat preservation to prevent crusting on the liquid steel surface

Add low-melting-point mold flux on the high-temperature molten steel surface when casting. Generally, it requires the mold flux to form a three-layer structure of liquid flux layer, sintered layer and raw flux layer. The temperature of the three layers of mold flux is different. The temperature of the liquid flux layer is higher, which is similar to the temperature of molten steel. The sintering layer is about 800~900°C, and the original flux layer is 400~500°C. After adding mold flux, the heat dissipation of molten steel is 10 times smaller than that of bare molten steel, which can reduce a large amount of heat loss and prevent crusting.

2. Isolate the air to prevent secondary oxidation

Mold flux will quickly form a liquid flux layer under the high temperature of molten steel. It can hinder the diffusion of air to the molten steel surface. Utilizing the three-layer structure of mold flux to isolate oxygen and nitrogen, the molten steel is prevented from secondary oxidation and nitrogen absorption. It not only prevents the secondary oxidation of molten steel, avoids the oxidation of alloy elements in molten steel, but also plays a role to prevent molten steel from absorbing nitrogen from the air. It is beneficial to improve the cleanliness of molten steel.

3. Absorb non-metallic inclusions in molten steel

The mold flux is required to have a good ability to absorb molten inclusions. It is considered that the melting speed of inclusions with low viscosity increases, and the high alkalinity, low SiO₂, Al₂O₃, high Na₂O, and CaF₂ of liquid flux are beneficial to increase the solubility of inclusions.

4. Lubricating effect of flux film

The surface tension of molten steel in the crystallizer and the cooling effect of the copper wall form an inwardly curved solidification shell, coupled with the vibration of the mold and the capillary action of the air gap, the liquid flux on the meniscus surface can be sucked into the gap between the billet shell and the copper wall The air gap forms a flux film and acts as a lubricant.

5. Improve the heat transfer of the crystallizer

The shrinkage of the billet shell in the crystallizer creates an air gap, which increases the thermal resistance and reduces the heat flow. If the air gap is filled with a uniform flux film, the reduction of the air gap thermal resistance will improve heat transfer, and the solidified billet shell will grow uniformly.