1. LF Ladle Furnace Refining Process Flow
LF refining is to pour the molten steel at the end of oxidation in the converter or electric furnace into the ladle furnace, remove the oxidation slag by 50-90%, and add reducing slag and deoxidizer for reduction refining. If the mixing time, slag amount and mixing power under heating are properly increased, and the slag is removed by 100% during tapping, the sulfur content in steel can be further reduced, so that [% s] < 30ppm and [% O] < 20ppm, and the liquid steel can become clean steel.
2. LF Ladle Furnace Refining Deoxidation
The solubility of oxygen in liquid and solid is very limited, and the solubility of oxygen in solid steel is much lower than that in liquid. In LF refining, the molten steel from the primary furnace often contains strong oxidizability, which constitutes a limiting factor for LF furnace to complete refining tasks such as deep deoxidation and desulfurization.
The harm of oxygen is mainly manifested in:
(1) LF furnace needs to smelt ultra-low sulfur steel, and the oxygen content in molten steel or the oxygen potential of slag will affect the balanced distribution of sulfur in steel slag. Moreover, due to the existence of oxygen, the tension between steel slag will decrease, which will affect the properties and quantity of sulfur-containing non-metallic inclusions remaining in steel. Therefore, good desulfurization must be preceded by 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 crystallization of molten steel, the segregation and aggregation of [C] and [O] in molten steel caused by selective crystallization, resulting in the reoxidation 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 porosity, porosity and rise of the billet.
(3) During the cooling and solidification of molten steel, the precipitated oxygen reacts with Si, Mn, Al and other elements in the steel to form non-metallic inclusions, which is one of the main reasons for the hairline defects of high-quality steel. In addition, the increase of the content of non-metallic inclusions also reduces various performance indexes of the steel, such as proportional limit, impact energy, elongation and magnetic conductivity.
(4) Oxygen in steel aggravates the harmful effect of sulfur, because FeO and FES can form low melting point eutectic with melting point of 1213k, which will deteriorate the plasticity of steel or destroy the body during hot processing.
Precipitation deoxidation and diffusion deoxidation are generally selected for LF furnace, such as vacuum deoxidation equipped with vacuum device.
1) Precipitation deoxidation
Precipitation deoxidation is a method of adding block deoxidizer directly to the molten steel to generate stable compounds and separate them from the molten steel into the slag. The deoxidation elements dissolved in the molten steel react with the dissolved oxygen in the molten steel to produce deoxidation products in the molten steel, which float up and remove due to low density. The general formula of deoxidation reaction of elements in molten steel is: x [M] + y [O] = MxOy.
The Compound Deoxidizer composed of A1 and alloy containing A1 and alkaline earth elements is widely used in industrial production. This is because A1 deoxidation product Al2O3 is easy to form low melting point and easy to grow composite deoxidation products (such as MCAO • n Al2O3) with other deoxidation products, which is conducive to floating and discharging liquid steel, so as to reduce the amount of such fire impurities in 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 be reduced, 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, and LF furnace refining is of great significance to the production of low sulfur steel.
Different from the desulfurization of alkaline oxidation slag, the desulfurization reaction equation of LF alkaline reduction slag is:
[FeS] + ( CaO) = ( CaS) + ( FeO) ( 1)
[MnS] +( CaO) = ( CaS) + ( MnO) ( 2)
Since most of [S] in steel 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 slag, FeO and MnO in slag and the amount of slag. At the same time, the desulfurization reaction is slag steel reaction, so the fluidity of slag has a great impact on the desulfurization reaction.
In the actual production process, within a certain alkalinity range. The sulfur distribution coefficient increases with the increase of slag alkalinity, but decreases with the increase of slag alkalinity when it reaches a certain degree. The reason is that the increase of slag (CAO) content makes the fluidity of slag worse, the kinetic conditions of the desulfurization reaction worse, and then affects the progress of the desulfurization reaction. From formula (1), it can be seen that the increase of (FeO) content in slag is not conducive to the desulfurization reaction. LF refining is a reducing atmosphere, and the reducing slag with high alkalinity is conducive to the desulfurization reaction. The amount and fluidity of refining slag have a great impact on the quality of final steel. Theoretically, refining slag with good fluidity is conducive to slag steel reaction and promote desulfurization reaction, while a large slag amount is conducive to lower sulfur removal in steel, but the increase of slag amount increases the thickness of the slag layer, which is not conducive to 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: floating up by their own buoyancy and adhering to the surface of bubbles. In the process of molten steel movement, inclusions will collide and agglomerate into large particles 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 a vital 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) Bubbles approach inclusions and collide;
(2) Liquid film is formed between bubbles and inclusions;
(3) The inclusion oscillates on the bubble surface or slides along the bubble surface;
(4) The liquid film is displaced and ruptured to form a dynamic three-phase contact nucleus (TPC);
(5) Bubble / inclusion cluster stabilization;
(6) Bubble / inclusion polymer floats up.
Among them, bubbles play an important role in the collision and adsorption of inclusions, i.e. steps (2) - (5). Therefore, controlling and formulating a reasonable bottom blowing system according to different smelting working conditions on the plant site plays an important role in improving the quality of molten steel.
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