Abstract

Introduction
1. Relevance of the topic
2. Review of research and development
2.1. Oxidative dephosphorization with ESR
2.2. Reductive dephosphorization using aluminum
2.3. Reductive dephosphorization using calcium
2.4. Dephosphorization of steel with barium and sodium oxides
Conclusion
Links

Introduction

The development of new technology technologies, which took place in the 50s of the twentieth century, when the rapid development of jet aviation took place, large rockets and spacecraft were created, nuclear technology developed and power plants of unique size were created, caused by the need to create alloy steels and alloys with unique properties.

The requirements for the content of harmful impurities are steadily increasing. It is known that the properties of steel and alloys largely depend on the content of such impurities as carbon, sulfur, phosphorus, nitrogen and others. The problem of reducing phosphorus in high-alloy steels is especially difficult.

Quite simple processes are required that could be relatively easy to include in the technological scheme, and which would provide a high degree of dephosphorization, low cost of materials used.

Traditional methods of steel smelting and casting fail to achieve the required depth of metal cleaning from harmful impurities. The task of radically improving the quality of alloy steels and alloys and giving them a set of unique properties was solved by cleaning these steels and alloys from harmful impurities (sulfur, phosphorus, non-ferrous metals, non-metallic inclusions, etc.) by remelting under high temperature conditions with simultaneous exposure to slag, which ensured deep cleaning of the metal from impurities and was accompanied by rapid and directional solidification of the metal purified from impurities.

Electroslag remelting (ESR) of steels and alloys is an active metallurgical process, as a result of which the remelted metal is largely purified from harmful impurities, gases and non-metallic inclusions.

1. Relevance of the topic

The known methods of dephosphorization each have their own advantages, but do not fully meet the basic requirements for the dephosphorization process. In addition, they require an increase in the number of additional operations using special equipment, which leads to an increase in steel processing costs and a decrease in productivity.

The master's work is devoted to the study of more economical and high-quality methods of dephosphorization of steel under active fluxes by the method of electroslag remelting.

2. Review of research and development

Removal of phosphorus from highly alloyed melts by traditional methods carried out under oxidizing conditions is impractical due to high losses of easily oxidized and expensive alloying components.

At present, mainly two directions of dephosphorization are considered: with the help of special fluxes (this direction is given the greatest attention) and the removal of phosphorus into the gaseous environment (a very limited number of publications). The classification of existing methods of dephosphorization of steel is shown in Picture 1.

Classification

Picture 1 - Classification of existing methods of dephosphorization of steel

In turn, dephosphorization with special fluxes, depending on the oxidizing potential of the system, are subdivided into: oxidizing, weakly oxidizing and reducing [1].

2.1. Oxidative dephosphorization with ESR

Dephosphorization during ESR is an oxidative process, i.e. it proceeds with ESR in the same way as with conventional steel melting. The distribution of phosphorus between the slag and the metal is proportional to the oxygen content in the metal (i.e., the higher the oxygen potential of the slag, the greater the degree of phosphorus removal). A high oxygen potential of the slag is achieved with a high content of iron and silicon oxides; therefore, dephosphorization and oxygen removal during ESR are incompatible.

Removal of phosphorus from steel occurs by oxidation and conversion to slag. Such oxidation occurs when there is a sufficient amount of CaO in the slag, which binds phosphorus oxides and reduces their activity. This reaction in molecular form can be written in the stoichiometric equation:

2[P]+5(FeO)+4(CaO)=(4CaO×P₂O₅)+5[Fe]

The distribution coefficient of phosphorus between the metal and the slag and, consequently, the degree of phosphorus removal increases with increasing CaO content, i.e. basicity of the slag, and the content of FeO in it. This dependence is shown graphically in Picture 2.

Graph

Picture 2 – Dependence of the distribution coefficient of phosphorus on the basicity of the slag and the content of FeO in it

There is no such mechanism for removing phosphorus from slag into the gas phase as exists for sulfur, therefore the slag is saturated with phosphorus to such a level that phosphorus removal becomes impossible. In this regard, as well as the need to have a slag with a high oxygen potential, dephosphorization during ESR is usually not performed. [2].

2.2. Reductive dephosphorization using aluminum

Due to the impossibility of removing phosphorus from stainless steel in phosphate form, it is of interest to remove it from the melt in phosphide form. Such a process would provide dephosphorization without oxidizing chromium.

To study the possibility of dephosphorization of chromium-containing iron melts, the authors [3] conducted a series of experimental melts in a Tamman furnace in an atmosphere of purified argon. The distribution of phosphorus between the metal and the slag was determined in the presence of aluminum, the content of which in the metal determines the degree of deoxidation of the bath. The results of control of experimental heats showed that an increase in the content of aluminum in the metal up to 10% does not lead to the removal of phosphorus into the slag. With a further increase in [% Al], a noticeable dephosphorization occurs.

The need for a high aluminum content to obtain a very low oxidation potential makes this method unacceptable for steelmaking. Therefore, other possibilities of dephosphorization under reducing conditions were studied.

2.3. Reductive dephosphorization using calcium

In research [4] calcium was chosen as a deoxidizer, which is a strong phosphide-forming element and practically does not transform into metal. The fact that calcium is unlimitedly soluble in its fluoride was used, and a slag containing 20% Ca and 80% CaF₂ was used for dephosphorization. The experiments were carried out in a Tamman furnace, where, in a magnesite crucible, the metal was treated with slag previously alloyed with metallic calcium in an amount of 19% by weight of the slag.

Experiments have shown that when the iron-chromium melt is treated with the indicated slag, the metal is dephosphorized. With the initial phosphorus content of 0.1%, as a result of treatment, it decreased to 0.065–0.091%. The degree of dephosphorization increased with increasing temperature.

The possibility of steel dephosphorization by metallic calcium under reducing conditions at elevated pressure was confirmed in the work [5]. The experiments were carried out with various combinations of calcium with CaO, CaCl and CaF₂. The studies described have shown the fundamental possibility of dephosphorization under reducing conditions. However, in real conditions of steel production, the application of their results is not possible.

In work [6] the authors studied the dephosphorization of steel during treatment with metallic calcium and calcium carbide. It was determined that with the addition of 1% calcium, the degree of dephosphorization is more than 65%, with 2% - more than 80%. Dephosphorization proceeded more efficiently at lower process temperatures. The carbon content, which determines the melting point of steel, has a great influence on the dephosphorization process. The process proceeds more fully at low temperatures due to the intense evaporation of calcium.

As a result of laboratory and industrial experiments to study the degree of use of calcium carbide, it was determined that the lower the carbon content in the metal, the more completely CaC₂ decomposes and the higher the efficiency of its use during dephosphorization. It has been established that the maximum amount of calcium carbide in metal refining should not exceed 10%. However, high losses of calcium for evaporation increase the cost of the process.

2.4. Dephosphorization of steel with barium and sodium oxides

In work [7] Studies of slags based on BaO showed that barium oxide has a strong effect on the coefficient of activity of P₂O₅ in the slag. The most effective are fluxes based on BaO - BaF₂, but the high cost of BaO flux does not allow using more than 10-20% of BaO in steel production.

Researchers at work [8] it is noted that barium compounds of the BaCO₃-BaCl₂, BCO₃-BaO-BaCl₂, BaCO₃-BaCl₂-Fe₂O₃, BaO-BaF₂-Cr₂O₃ systems contribute to a decrease in the phosphorus content in the melt to 70%.

Thus, BaO-based fluxes are more preferable from the point of view of dephosphorization of high-alloy steels in contrast to CaO-based fluxes, but the high cost of fluxes does not allow their use in mass production [9].

In research [10] Guanziang Li studied the effect of K₂O and Na₂O additions in slags of the SiO₂-FeO-P₂O₅ system and showed that the maximum dephosphorizing ability of the slag is achieved at the ratio (m(CaO) + m(Na₂O)) / m(FeO) equal to 1.3-1.5, and with an increase in temperature, the dephosphorizing ability decreases. The distribution coefficient of phosphorus was 1.6-2.8. But this method requires the use of additions of K₂O and Na₂O in the amount of 8-10%, which significantly increases the cost of the alloy.

Conclusion

Studies show that lime-based fluxes with various additives have a low distribution coefficient of phosphorus and a sufficiently high oxidation potential of the system, and, therefore, increase the likelihood of alloying losses. Fluxes based on barium and soda oxides have high values of the distribution coefficient of phosphorus and lower values of the oxidation potential, but do not exclude possible losses of alloying agents. Barium compounds are quite expensive, which leads to a significant increase in the cost of steel. Fluxes using calcium and calcium carbide, in addition to a high distribution coefficient of phosphorus, provide the lowest oxidation values of the melt, which excludes oxidation and loss of alloying elements. A significant drawback of such fluxes is the intense evaporation of calcium at the temperatures of metallurgical processes and, as a result, a very low coefficient of its use.

Generalized data on the dephosphorizing ability of fluxes are presented in Picture 3.

Grafic

Picture 3 – Comparison of dephosphorizing properties of various fluxes [1]

When writing this abstract, the master's work has not yet been completed. Final completion: November 2021. The full text of the work and materials on the topic can be obtained from the author or his manager after that date.

Alyona Popova

Faculty of Metallurgy and Thermal Engineering

Department steel metallurgy and alloys

Speciality Steel metallurgy

Investigation of the mechanism of phosphorus removal during electroslag remelting of steel under active fluxes

Scientific adviser: Ph.D., Professor Alexander Trojanskiy