1. LF Ladle Furnace Refining Process
LF refining is to pour molten steel into the ladle furnace and remove 50-90% of the oxidized slag, add reducing slag and deoxidizer for reduction refining. If the stirring time, slag amount and stirring power under heating are appropriately increased, 100% slag removal during tapping can further reduce the sulfur content in the steel, so that [%S] <30ppm and [%O] <20ppm can make the molten steel become clean steel.
2. LF Ladle Furnace Refining Deoxidation
The solubility of oxygen in liquids and solids is very limited, and the solubility of oxygen in solid steel is much lower than in liquids. In LF refining, the molten steel from the primary furnace often contains strong oxidizing properties, which makes the LF furnace complete deep deoxidation and desulfurization and other refining tasks constitute a limiting factor. The hazards of oxygen are mainly manifested in:
(1) The LF furnaces need to smelt ultra-low sulfur steel, and the oxygen content in the molten steel or the oxygen potential of the slag will affect the equilibrium distribution of sulfur in the steel slag, and due to the presence of oxygen, the tension between the steel slag will decrease, which will affect the residual properties and quantity of sulfur-containing non-metallic inclusions in steel, so good desulfurization must first have good deoxidation.
(2) The solubility of oxygen in steel decreases significantly with the decrease of temperature, and precipitates in the form of FeO. During the cooling and crystallization process of molten steel, [C] and [O] segregate and aggregate in molten steel due to selective crystallization. , Which causes the re-oxidation of carbon. The resulting CO gas will destroy the compactness or continuity of the steel, which is the main reason for the defects such as pores, looseness and rising of the steel billet.
(3) During the cooling and solidification process of molten steel, the precipitated oxygen reacts with elements such as Si, Mn, Al in the steel to form non-metallic inclusions, which are one of the main causes of hairline defects in high-quality steel. In addition, due to the increase in the content of non-metallic inclusions, various performance indicators of steel, such as proportional limit, impact energy and elongation, and magnetic permeability, are also reduced.
(4) Oxygen in steel aggravates the harmful effects of sulfur, because FeO and FeS can form a low-melting eutectic with a melting point of 1213K, which deteriorates the plasticity of the steel or damages the body during hot working.
LF furnace generally chooses precipitation deoxidation and diffusion deoxidation, such as equipped with vacuum device and vacuum deoxidation.
(1) Precipitation Deoxygenation
Precipitation deoxidation is a method of directly adding massive deoxidizers to molten steel after removing the oxidized slag to generate stable compounds and separate them from the molten steel into the slag. The deoxidizing elements dissolved in the molten steel react with the dissolved oxygen in the molten steel. The deoxidation products in the molten steel float up and are removed due to their low density. The general formula for the deoxidation reaction of elements in molten steel is x[M]+y[O]=MxOy.
A1 and the composite deoxidizer composed of alloys containing A1 and alkaline earth elements are used more in industrial production. This is because Al2O3, the deoxidation product of A1, can easily form a low-melting, easy-to-grow compound deoxidation product (such as mCaO•n Al2O3) with other deoxidation products, which is beneficial to floating up and discharging molten steel, thereby reducing the amount of such fire impurities in the steel.
(2) Diffusion Deoxidation
Diffusion deoxidation is to add deoxidizer (mainly powdered deoxidizer) to the slag surface, and the deoxidation reaction is carried out at the interface between steel and slag. When the powdered deoxidizer is added to the slag, the content of FeO in the slag is bound to decrease, and the distribution balance of oxygen in the steel slag is destroyed. In order to achieve the rebalancing, the oxygen in the molten steel diffuses or transfers to the slag, so as to continuously reduce the oxygen content in the slag, so that the oxygen in the molten steel can be continuously removed.
(3) LF Ladle Furnace Refining Desulfurization
Generally, sulfur is a harmful element in steel, which has many effects on the quality of steel. Therefore, desulfurization is one of the important metallurgical tasks in steelmaking production. For desulfurization reaction, LF furnace refining has good thermodynamic and kinetic conditions. LF furnace refining is of great significance to the production of low-sulfur steel.
Different from alkaline oxidation slag desulfurization, the desulfurization reaction equation of LF alkaline reduction slag is:
[FeS] + (CaO) = (CaS) + (FeO) (1)
[MnS] +( CaO) = (CaS) + (MnO) (2)
Since [S] in steel mostly exists in the form of [FeS], the desulfurization reaction is mainly based on formula (1). It can be seen from the above formula that the desulfurization reaction is related to the basicity of the slag, (FeO) and (MnO) in the slag, and the amount of slag. At the same time, the desulfurization reaction is a slag-steel reaction, so the fluidity of the slag has a greater influence on the desulfurization reaction.
In the actual production process, within a certain alkalinity range. The distribution coefficient of sulfur increases as the alkalinity of the slag increases, but when it reaches a certain level, it decreases with the increase in alkalinity. The reason is that the increase in the slag (CaO) content makes the fluidity of the slag worse and causes the desulfurization reaction. The kinetic conditions of the desulfurization are worsened, which in turn affects the progress of the desulfurization reaction.
It can be seen from formula (1) that the increase of the content of (FeO) in the slag is not conducive to the progress of the desulfurization reaction. LF is refined into a reducing atmosphere, and the reduced slag with high basicity is conducive to the progress of the desulfurization reaction.
The amount and fluidity of the refined slag have a greater impact on the quality of the final steel. Theoretically, the refined slag with good fluidity is beneficial to the slag-steel reaction to promote the desulfurization reaction, and a large amount of slag is beneficial to the sulfur removal in the steel. However, the increase in the amount of slag increases the thickness of the slag layer, which is not conducive to the desulfurization and the floating of inclusions. At the same time, it increases the consumption of metal materials and increases the production cost.
(4) Removal of Inclusions
Argon blowing at the bottom of ladle is the last important process before molten steel continuous casting, which is very important to the quality of liquid steel and billet. There are two main ways for inclusions to float up in liquid steel: relying on their own buoyancy and adhering to the bubble surface.
In the process of molten steel movement, inclusions will collide and agglomerate into large particle inclusions to float up by their own buoyancy (some inclusions will adhere to the bubble surface and float up by the buoyancy of bubbles).
Therefore, the ladle bottom blowing system plays an important role in the removal of inclusions in molten steel, and the size of bubbles and the flow of molten steel will affect the floating of inclusions.
The process of removing inclusions by bubbles can be divided into the following processes:
(1) The bubbles approach and collide with the inclusions;
(2) The liquid film is formed between the bubbles and the inclusions;
(3) The inclusions oscillate on the surface of the bubble or along the bubble Surface slippage;
(4) The liquid film is discharged and ruptured to form a dynamic three-phase contact nucleus (TPC);
(5) Bubbles/inclusion nuclei are stabilized;
(6) Bubbles/inclusion aggregates float up.
Among them, bubbles play an important role in the collision and adsorption of inclusions, namely steps (2)-(5). Therefore, controlling the establishment of a reasonable bottom blowing system according to different smelting conditions at the factory site plays an important role in improving the quality of molten steel.
If you are interested in LF ladle furnace, please mail at marketing2@hanrm.com for your solution.
More Articles You May Be Interested In:
Comparison of Electric Furnace, AOD furnace, LF furnace and VD Vacuum Furnace
