What is the role of Continuous casting slag powder?
During the pouring process, powdered or granular slag is continuously added to the surface of the molten steel in the crystallizer, which is called protection slag. The functions of protection slag are as follows: (1) Insulation and heat preservation to prevent heat dissipation; (2) Isolation of air to prevent oxygen in the air from entering the molten steel and causing secondary oxidation, which affects the quality of the steel; (3) Absorption and dissolution of inclusions that float from the molten steel to the slag interface to purify the molten steel; (4) There is a layer of slag film between the crystallizer wall and the solidified shell to act as a lubricant, reduce the resistance of the billet, and prevent the solidified shell from sticking to the copper plate; (5) Fill the air gap between the billet shell and the crystallizer to improve the heat transfer of the crystallizer. A good protection slag should be able to fully play the above five roles to achieve the purpose of improving the surface quality of the billet and ensuring the smooth continuous casting.
What are the requirements for the melting mode of slag powder?
In the continuous casting process, the protective slag added to the crystallizer must have a specified melting mode to complete the above five functions. That is, it is required to form a so-called three-layer structure of powder slag layer, sintering layer and liquid slag layer on the surface of molten steel. The slag powder with a low melting point (1100-1200℃) added to the surface of the high-temperature molten steel (about 1500℃) in the crystallizer forms a liquid slag covering layer (about 10-15mm) of a certain thickness on the surface of the molten steel by providing heat with the molten steel. The heat transfer from the molten steel to the powder slag layer slows down. The powder slag on the liquid slag layer is heated, and the slag powder is sintered together to form the so-called sintering layer (temperature is 900-600℃). On the sintering layer, the powder slag receives less heat transferred from the molten steel and the temperature is low (<500℃), so it remains in powder form and evenly covers the surface of the molten steel, preventing the molten steel from dissipating heat and preventing oxygen in the air from entering the molten steel. During the billet drawing process, due to the up and down vibration of the crystallizer and the downward movement of the solidified billet shell, the liquid slag layer on the surface of the molten steel continuously passes through the interface between the molten steel and the copper wall and squeezes into the space between the billet shell and the copper wall, forming a solid slag film on the surface of the copper wall, and a liquid slag film on the surface of the solidified shell. This layer of liquid slag film lubricates the surface of the crystallizer wall and the billet shell, just like adding lubricating oil when the motor shaft rotates. At the same time, the slag film fills the air gap between the billet shell and the copper wall, reducing thermal resistance and improving heat transfer during crystallization. As the billet drawing progresses, the liquid slag on the surface of the molten steel is continuously consumed, and the sintering layer drops to the surface of the molten steel and melts into a liquid slag layer, and the powder slag layer becomes a sintering layer. New slag powder is added to the crystallizer to keep it in a three-layer structure. This cycle continues, and the protective slag powder is continuously consumed.
How to achieve the so-called “three-layer structure” of the mold protection slag powder?
To play the five functions of the protection slag, the slag powder added to the crystallizer must form a “three-layer structure”. The key to forming a “three-layer structure” is to control the melting speed of the protection slag powder, that is, the slag powder added to the steel liquid surface should not melt into liquid all at once, but melt gradually. For this reason, carbon particles are generally added to the protection slag as a melting rate regulator. The speed of carbon particles controlling the melting rate depends on the type and amount of carbon particles added. Carbon is a high-temperature resistant material. The extremely fine carbon powder is adsorbed around the slag particles, separating the slag particles from each other, hindering the contact and fusion between the slag materials, and slowing down the melting rate. If insufficient carbon powder is added, the slag layer temperature has not reached the slag sintering temperature, and the carbon particles have been burned out, then the sintering layer is developed, the melting rate is too fast, and the liquid slag layer is too thick. If too much carbon powder is added, some carbon particles still exist after the slag is fully melted, which will cause the sintering layer to shrink and the thickness of the sintering layer to be too thin. When the amount of carbon powder added is moderate, some carbon particles in the sintering layer are burned out, and the remaining slag is still effectively controlled by the carbon particles, so that a sintering layer and liquid slag layer of appropriate thickness can be obtained. There are two types of carbon materials: graphite and carbon black. Graphite particles are coarse, with a particle size of 60-80μm. Its separation and blocking effects are poor, but the initial oxidation temperature is high (about 560℃), the oxidation rate is slow, and the ability to control the melting rate in the high temperature area is strong. Carbon black has an amorphous structure, very fine particles (0.06-0.10μm), strong separation and blocking effects, a low initial oxidation temperature (500℃), and a fast oxidation rate. Therefore, carbon black has a strong ability to control the melting rate in the low temperature area of
What are the factors that affect the absorption of inclusions in molten steel by protective slag?
The submerged nozzle injection causes convective movement of molten steel in the crystallizer. Inclusions that float to the interface of the crystallizer and slag may be drawn into the solidified shell due to the fluctuation of the crystallizer liquid level, resulting in subcutaneous inclusions or surface slag inclusions in the ingot, affecting the surface quality. Therefore, it is hoped that the inclusions that float to the interface of the slag will be quickly absorbed and dissolved by the liquid slag layer. In order to make the inclusions that float to the interface of the slag quickly transferred to the liquid slag, this process is determined by: (1) the contact area of
What are the functions of the thickness of the liquid slag layer in the crystallizer and how to measure it?
In order to achieve good use effects, the protective slag must have a liquid slag layer thickness that meets actual needs. If the liquid slag layer is too thick or too thin, it will cause surface longitudinal cracks on the slab. If the slab pulling speed is 1.2-1.5m/min, the liquid slag layer thickness is less than 5mm, the longitudinal cracks of the slab will increase significantly (from 50mm/m to 200mm/m), the liquid slag layer thickness is 6-15mm, the longitudinal cracks almost disappear, and the liquid slag layer is greater than 20mm, and the longitudinal cracks increase again. If the thickness of the liquid slag layer is less than a certain value, the slag ring formed along the periphery of the crystallizer will block the channel between the meniscus liquid slag flowing into the billet shell and the copper wall, so that the liquid slag cannot flow smoothly into the billet shell surface to form a uniform slag film, which may cause longitudinal cracks on the corresponding billet surface. So what is the required thickness of the liquid slag so that the channel for the liquid slag to flow down through the meniscus is not blocked? According to theoretical calculations, when the pulling speed is less than 1m/min, the thickness of the liquid slag layer is 5-7mm, and when the pulling speed is greater than 1m/min, the thickness of the liquid slag layer is 7-15mm. This is consistent with the critical liquid slag layer thickness measured in production practice. The method for measuring the thickness of the liquid slag layer in production is: tie a steel wire and a copper wire (or aluminum wire) together and insert them into the slag layer of the crystallizer. Since the temperature of the liquid slag is higher than the melting point of copper, the copper wire melts, and the length of the copper wire melted is the thickness of the liquid slag layer. Since the temperature of the molten steel at each point of the slab crystallizer section is different (such as the immersion nozzle area and the edge of the crystallizer), the thickness of the liquid slag layer is also different, so the thickness of the liquid slag layer at different positions can be measured.
How does slag powder play a lubricating role?
During the casting process, the crystallizer vibrates up and down, and the billet moves downward, which generates friction between the solidified shell surface and the copper wall, causing the billet shell to adhere to the copper wall, increasing the resistance to billet drawing, and causing cracks in the billet shell in the mildest case, and tearing the billet shell in the worst case. Therefore, lubrication must be performed between the billet shell and the copper wall, and this effect can only be achieved by protective slag. To ensure good lubrication, there must be a layer of liquid slag film with suitable properties and uniform thickness between the solidified shell and the copper wall. The liquid slag layer on the surface of the crystallizer steel liquid is the source of continuous supply of the liquid slag film. To this end, it is necessary to ensure that the channel where the liquid slag near the meniscus of the crystallizer flows into the billet shell and the copper wall is unobstructed and not blocked by the slag ring around the copper wall. So how is the lubricating slag film formed? When the crystallizer is filled with molten steel, the primary billet shell is formed. When protective slag powder is added to the liquid surface, the slag powder melts to form a layer of liquid slag. The liquid slag near the copper wall cools to form a slag ring. As the crystallizer moves downward, the slag is gradually squeezed into the space between the billet shell and the copper wall so that it is completely filled with slag. The temperature of the copper wall is low, and the slag shell near the copper wall remains as a solid slag skin, while the surface temperature of the solidified shell is high, and the slag near the billet shell is a liquid slag film with fluidity. In this way, the copper wall and the billet shell of the crystallizer are lubricated by the liquid slag film, which is consumed as the billet is pulled out, while the solid slag skin attached to the copper wall is basically not consumed as the crystallizer vibrates. While the slag film is continuously consumed, the liquid slag on the steel surface is continuously replenished downward through the meniscus channel to form a stable liquid slag film. The thickness of the slag film is related to factors such as slag viscosity, pulling speed, and crystallizer vibration. It is known that when the slag viscosity is constant, the pulling speed increases and the slag film thickness increases; when the pulling speed is constant and the viscosity increases, the slag film thickness decreases. Generally, the slag film thickness is 50-200μm, and the slag consumption is 0.4-0.6kg/t. Therefore, in order to make the slag film lubricate the solidified shell in the best state, the slag film thickness, slag consumption and slag viscosity should be properly matched. When the crystallizer vibration is constant, the viscosity (η) and the pulling speed (V) should be properly matched. Low viscosity and low pulling speed, or high viscosity and high pulling speed are not advisable. The product of the two, η·V, is used as an indicator to evaluate the lubrication condition. If the η·V value is too small or too large, it means that the slag film thickness and consumption are inappropriate and the lubrication condition is poor.
