Continuous Casting Powder

1.What is the function of continuous casting mold flux?
During the casting process, powder or granular slag, called mold slag, should be continuously added to the molten steel surface of the mold. The role of the protective slag has the following aspects:
(1)Thermal insulation to prevent heat dissipation;
(2)Separate the air to prevent the oxygen in the air from entering the molten steel for secondary oxidation, which affects the quality of the steel;
(3)Absorb and dissolve the inclusions floating from molten steel to the interface of steel slag to purify molten steel;
(4)There is a layer of slag film between the mold wall and the solidification shell to lubricate, reduce the drawing resistance, and prevent the bonding between the solidification shell and the copper plate;
(5)Fill the air gap between the shell and the mold to improve the heat transfer of the mold. A good mold slag should be able to give full play to the above five aspects, so as to improve the surface quality of the slab and ensure the smooth running of continuous casting.
2. How does the mold slag play a lubricating role?
During the casting process, the mold vibrates up and down, the billet moves downward, and friction occurs between the surface of the solidified shell and the copper wall, so that the billet shell and the copper wall are bonded, which increases the resistance of billet drawing, and the lighter one causes cracks in the billet shell. In severe cases, the shell will be cracked. Therefore, lubrication must be carried out between the billet shell and the copper wall, which can only be achieved by mold flux. To ensure good lubrication, there must be a liquid slag film with suitable properties and uniform thickness between the solidified shell and the copper wall. The liquid slag layer on the molten steel surface of the mold is the source of continuous supply of the liquid slag film. To this end, it is necessary to ensure that the channel between the liquid slag near the meniscus of the mold flowing into the blank shell and the copper wall is unobstructed, and is not blocked by the slag ring around the copper wall.
3. What is the design principle of mold flux composition?
The mold slag slag commonly used in continuous casting is based on the slag system composed of CaO-SiO2-Al2O3 ternary compounds, and contains an appropriate amount of Na2O, CaF2, K20 and other compounds. This kind of slag material is weakly acidic or neutral liquid slag after melting, and has good wettability to molten steel, and the slag viscosity changes smoothly with temperature.
4. What are the main raw materials used to prepare mold powder?
The raw materials used for the preparation of mold powder are: natural minerals, industrial wastes and industrial products. The raw materials that have been used as basic slag materials include: cement, cement clinker, wollastonite, feldspar, quartz, power plant flue ash, blast furnace slag, electric furnace white slag, etc. The auxiliary materials for the flux include: caustic soda, fluorite, barite, cryolite, borax, lithium carbonate, etc. Melting rate regulators include natural graphite, carbon black, lamp black, etc.
5. What is the effect of mold flux on the quality of continuous casting slabs?
The mold flux is added to the molten steel surface of the mold. The quality of the mold flux mainly affects the surface quality of the slab:
(1) Longitudinal cracks on the slab surface: Longitudinal cracks originate from the inhomogeneity of the thickness of the primary slab shell in the meniscus region of the mold. The liquid slag on the molten steel surface cannot flow into and distribute evenly around the casting billet, resulting in uneven thickness of the solidified shell. Stress concentration is likely to occur in the thin shell of the billet. When the stress exceeds the high temperature strength of the solidified shell, cracks occur.
(2) Slag inclusion: Slag inclusion in the slab can be divided into surface slag inclusion and subcutaneous slag inclusion. Slag inclusions vary in size. From a few millimeters to more than ten millimeters, the slag inclusions have different depths on the surface. Slag inclusions seriously impair the surface quality of the product and must therefore be removed prior to hot working. The mold shell is involved in slag, which is an important source of slag inclusion.
6. What types of mold flux are there in continuous casting?
According to the composition of the designed mold powder, the mold powder is made by selecting suitable raw materials through crushing, ball milling, mixing and other production processes. There are four types.
(1)Powdered mold powder: It is a mechanical mixture of various powdery materials. In the process of long-distance transportation, due to the long-term vibration, the materials of different specific gravity are segregated, and the uniform state of the slag is destroyed, which affects the stability of the use effect. At the same time, when adding slag powder to the crystallizer, the dust is flying, which pollutes the environment.
(2)Granular mold powder: In order to overcome the disadvantage of polluting the environment, an appropriate amount of binder is added to the powdery slag to make a granular mold powder similar to millet grains. The production process is complicated and the cost has increased.
(3)Pre-melting mold slag: Mix the slag-forming materials and put them into a pre-melting furnace to melt them into one. After cooling, they are crushed and ground, and an appropriate melting rate regulator is added to obtain a pre-melting powder mold slag. The premelted mold powder can also be further processed into granular mold powder. The production process of premelting mold flux is complicated and the cost is high. But the advantage is to improve the uniformity of mold slag formation.
(4)Heat-generating mold slag: add a heat-generating agent (such as aluminum powder) to the slag powder to make it oxidize to release heat and quickly form a liquid slag layer. However, the slag-forming speed of this kind of slag is not easy to control, and the cost is high, so it is less used.
7. What are the main physical and chemical properties of continuous casting mold flux?
After the mold slag is prepared, the physical and chemical properties of the slag should be measured. The main physical and chemical indicators are as follows:
(1) Chemical composition: The chemical composition of each brand of mold flux should be analyzed, and the content of each oxide should be within the specified range, which is the minimum index.
(2) Melting temperature, the slag powder is made into a sample of Φ3×5mm, and the sample is heated on a special instrument to the temperature at which the cylinder turns into a hemisphere, and the temperature reaching the hemispherical point is defined as the melting temperature.
(3) Viscosity: It indicates the flow properties of slag powder melted into liquid. The fluidity of slag has an important influence on the lubricating effect of slag absorbing inclusions and shell.
(4) Melting speed: The melting speed is a measure of the speed of the slag melting process, which is related to whether a stable three-layer structure can be formed on the molten steel surface of the mold and the required thickness of the slag layer.
(5) Spreadability: It indicates the covering ability and uniformity of the slag added to the molten steel surface. It can be measured by the area of mold powder in a certain volume that flows down from the specified height to the spreading area on the plate.
(6) Moisture: Mold powder is easy to absorb moisture. If the amount of adsorbed moisture exceeds the specified requirement (such as 0.5%), the slag powder will agglomerate and endanger the use effect.
8. How to control the moisture of mold powder?
The moisture content of mold slag is divided into two categories: adsorbed water and crystal water. Moisture can make mold powder agglomerate and deteriorate its quality, so the moisture should be limited to less than 0.5%. The water absorption of mold slag is mainly determined by the type and particle size of raw materials. The finer the particle size, the greater the water absorption. Moisture control method: the baking temperature of raw materials is not lower than 110 ℃. Properly extend the baking time. The raw materials after baking should be batched and mixed in time, and the prepared slag powder should be sealed and packaged in time. For steel grades with higher quality requirements, it is best to bake the mold flux raw materials to above 800 °C to remove crystal water, or to use pre-melted mold flux.