Ladle Furnace Steelmaking Process

ladle furnace steelmaking process

1) LF Ladle Furnace Steelmaking Process

LF Ladle furnace steelmaking process is to put the molten steel in the final stage of oxidation in the converter or electric furnace, pour it into the ladle furnace, remove 50-90% of the oxidized slag, and add reducing slag and deoxidizer for reduction refining. If the stirring time, slag amount, and stirring power under heating are appropriately increased, and the slag removal is 100% during tapping, the sulfur content in the steel can be further reduced, so that [%S] <30ppm and [%O] <20ppm, so that the molten steel can be reduced. become clean steel.

2) LF refining and deoxygenation

The solubility of oxygen in liquids and solids is very limited, and the solubility of oxygen in solid steel is much lower than that in liquids. In LF ladle furnace steelmaking refining process, the molten steel from the primary smelting furnace often contains strong oxidizing properties, which makes the LF furnace to complete the refining tasks such as deep deoxidation and desulfurization, which constitute a limiting factor. The hazards of oxygen are mainly manifested in:

(1) The LF furnace needs to smelt ultra-low sulfur steel, and the oxygen content in the molten steel or the oxygen potential of the slag will affect the balanced 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 The character 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 of molten steel, [C] and [O] in molten steel are segregated and aggregated due to selective crystallization. , resulting in carbon reoxidation. The resulting CO gas will destroy the compactness or continuity of the steel and is the main cause of defects such as pores, looseness, and rise in 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 are one of the main reasons for hairline defects in high-quality steel. In addition, due to the increased 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 effect 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.

In the ladle furnace steelmaking process, the LF furnace generally chooses precipitation deoxidation and diffusion deoxidation, such as vacuum device and vacuum deoxidation.

(1) Precipitation deoxidation

Precipitation deoxidation is a method of directly adding a bulk deoxidizer to molten steel after cleaning the oxidized slag to generate stable compounds and separate them from molten steel and enter the slag. The deoxidizing elements dissolved in molten steel react with dissolved oxygen in molten steel. The deoxidized products in the molten steel float up and remove due to their low density. The general formula of element deoxidation reaction in molten steel is x[M]+y[O]=MxOy.

Composite deoxidizers composed of A1 and alloys containing A1 and alkaline earth elements are widely used in industrial production. This is because the Al2O3 deoxidation product of A1 is easy to form a composite deoxidation product (such as mCaO•n Al2O3) with a low melting point and easy growth with other deoxidation products, which is beneficial to float and discharge the molten steel, thereby reducing the number of such impurities in the steel.

(2) Diffusion deoxidation

Diffusion deoxidation is adding deoxidizer (generally mainly powder deoxidizer) to the slag surface, and the deoxidation reaction is carried out at the interface of steel and slag. When the powdery deoxidizer is added to the slag, the (FeO) content in the slag is bound to decrease, and the distribution balance of oxygen in the steel slag is destroyed. In order to achieve a rebalance, the oxygen in the molten steel diffuses or transfers into the slag. By reducing the oxygen content in the slag, the oxygen in the molten steel can be removed successively.

3) LF refining desulfurization

Usually, 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 the desulfurization of alkaline oxidizing slag, the desulfurization reaction equation of LF alkaline reducing 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 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 great influence on the desulfurization reaction.

In the actual Ladle furnace refining steelmaking production process, within a certain range of alkalinity. The distribution coefficient of sulfur increases with the increase of the basicity of the slag, but when it reaches a certain level, it decreases with the increase of the basicity. The kinetic conditions deteriorated, which in turn affected the desulfurization reaction. It can be seen from formula (1) that the increase of (FeO) content in the slag is not conducive to the progress of the desulfurization reaction, and the LF refining is 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 refining slag have a great influence on the quality of the final steel. In theory, the refining slag with good fluidity is conducive to the slag-steel reaction to promote the desulfurization reaction, while a large amount of slag is conducive to lowering the sulfur in the steel. However, the increase in the amount of slag increases the thickness of the slag layer, which is not conducive to desulfurization and the floating of inclusions, and at the same time increases the consumption of metal materials and thus increases the production cost.

4) Removal of inclusions

Argon blowing at the bottom of the ladle is the last important process before the continuous casting of molten steel, and it is very important to the quality of molten steel and slab. Float on the surface. During the movement of the molten steel, the inclusions will collide and condense into large particles to float up by their own buoyancy (some inclusions will adhere to the surface of the bubbles and float up by the buoyancy of the bubbles. Therefore, the ladle bottom blowing system is suitable for The removal of inclusions in molten steel and plays a vital role. The size of bubbles and the flow of molten steel will affect the floating of inclusions.

The process of removing inclusions from bubbles can be divided into the following processes:

(1) The bubbles approach the inclusions and collide;

(2) A liquid film is formed between bubbles and inclusions;

(3) The inclusions oscillate on the surface of the bubble or slip along the surface of the bubble;

(4) The liquid film is expelled and ruptured to form a dynamic three-phase contact nucleus (TPC);

(5) Stabilization of bubble/inclusion nuclei;

(6) The bubble/inclusion polymer floats up.

Among them, bubbles play an important role in the collision and adsorption of inclusions, that is, steps (2)-(5). Therefore, in the ladle furnace refining steelmaking process, a reasonable bottom blowing system should be formulated according to different smelting conditions at the factory site. It is very important to improve the quality of molten steel.

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