In ladle furnace steelmaking, the ladle used for refining outside the furnace is reborn into a “furnace” due to the addition of auxiliary heating function, and then the process of pure steel smelting is carried out.
The ladle refining (LF) furnace was successfully developed in 1971 by the Omori Special Steel Plant of Japan’s Datong Steel Company. Because of its low investment and the ability to significantly increase the output of EAF steel, it has become the main equipment matching between EAF and continuous casting. In addition, LF can improve the purity of molten steel and meet the requirements of continuous casting for molten steel composition and temperature, so the converter with the LF method has also been developed rapidly, and many steel mills are equipped with LF method.
Ladle furnace steelmaking has played a leading role in the external refining equipment, which has greatly increased the external refining ratio of converter steel and electric furnace steel in China. Not only has the metallurgical quality of China’s bearing steel caught up with the international advanced level, but also because of its advantages of complete metallurgical functions, simple structure, convenient operation and low investment, it has been widely used in domestic industrial production of special steel, ordinary steel and cast steel, and has become one of the main refining methods of pure steel in China.
LF method adopts argon stirring, graphite electrode submerged arc heating under atmospheric pressure and slag refining. A vacuum system can also be used for degassing. It is a refining method of molten steel that integrates arc heating and gas stirring. By strengthening the thermodynamic and kinetic conditions, the molten steel can be highly purified and uniform in a short time, so as to achieve various metallurgical purposes. LF furnace has a heating function, which can provide molten steel with qualified temperature for continuous casting and ensure smooth continuous casting production. It is a tough link between the primary smelting furnace and the continuous casting process.
It is characterized by a long refining time, various refining functions, and a heating device to compensate the cooling of molten steel. Including VAD furnace with arc heating under low pressure and bottom argon blowing; Asea-skf furnace with arc heating and electromagnetic stirring under normal pressure; LF furnace with three-phase submerged arc heating and bottom argon blowing under normal pressure. Among the total number of ladle furnaces in the world, the LF furnace is the most.
The process characteristics of ladle furnace steelmaking are that after the slag forming material is added to the primary molten steel in the LF furnace, the three graphite electrodes on the ladle cover are inserted into the slag layer for submerged arc heating, so as to promote the formation of reducing slag and raise the temperature of molten steel; During the whole refining process, the argon blowing and stirring system at the bottom of the ladle blows a large number of small bubbles into the steel, and the argon bubbles floating in the molten steel cause strong stirring of the molten steel, which is conducive to eliminating gas and inclusions, accelerating the reaction between steel slag, and unifying the temperature and composition of the molten steel; The argon gas escaping from the molten steel is covered on the molten pool surface to maintain the reducing atmosphere in the LF furnace and avoid secondary oxidation and gas absorption of the molten steel, so as to achieve the refining purposes of deoxidation, desulfurization, removal of inclusions and uniform temperature and composition of the molten steel.
1. Ladle Furnace Function
Ladle furnace steelmaking is an important process used to refine the molten steel melted in the primary smelting furnace (electric arc furnace, open hearth furnace and converter), and can adjust the temperature of molten steel, buffer the process and meet the requirements of continuous casting and continuous rolling. A ladle furnace is one of the main pieces of equipment for refining outside the furnace. Main functions of ladle furnace:
- Make the molten steel temperature rise and keep warm. The molten steel is heated by an electric arc to obtain new heat energy, which can not only add alloy and adjust composition during ladle furnace steelmaking, but also add slag to facilitate deep desulfurization and deoxidation of molten steel. Moreover, the opening temperature of molten steel required by continuous casting can be guaranteed, which is beneficial to the improvement of the quality of the dry cast slab.
- Argon stirring function. Argon is blown into the molten steel through the permeable brick installed at the bottom of the ladle, and the molten steel has a certain stirring function.
- Vacuum degassing function. After the ladle is hoisted into the vacuum tank, the steam jet pump is used for vacuum degassing, and argon is blown into the bottom of the ladle to stir the molten steel. The hydrogen content and nitrogen content in the molten steel can be removed, and the oxygen content and sulfur content can be further reduced. Finally, molten steel with high purity and excellent performance can be obtained. For the whole enterprise, the ladle furnace steelmaking can at least increase the following benefits: speed up the production rhythm and improve the overall metallurgical production efficiency. Application: ladle furnace is widely used in industry, steel, metallurgy and other industries.
2. Refining Process of LF Ladle Furnace
Including slag removal, heating, argon stirring, slag composition adjustment, liquid steel composition adjustment, vacuum treatment and thermal insulation. The quality of the primary steel-making liquid is related to whether the refining furnace can successfully achieve the refining purpose. Therefore, the phosphorus content of the primary steel-making liquid is required to be less than 0.015%, and the final deoxidation is carried out in different degrees when the steel is discharged from the primary smelting furnace.
(1) Slag Removal
The slag of the primary smelting furnace has a great influence on the refining process. First, it corrodes the refining ladle lining; second, the high content of (FeO), (SiO2), (P2O5) in the slag will seriously affect the refining process. Therefore, it is necessary to remove the slag of the primary smelting furnace. The removal methods are as follows: (1) remove the slag before the tapping of the electric furnace, and cast the slag retaining ball to stop the slag during the tapping of the converter and the flat furnace. (2) When the refining furnace automatically tilts 20 ° ~ 30 °, the slag covered on the primary steel-making liquid shall be scraped off by the slag scraper. (3) The intermediate ladle avoids the slag from the primary smelting furnace from entering the refining furnace. (4) Use the pressure skimmer to remove the slag on the surface of the primary steel-making liquid.
The reduced atmosphere and high basicity slag are produced by arc heating and argon stirring. When the furnace cover is lowered to cover the ladle mouth, the electrode is lowered and inserted into the slag for submerged arc heating. Supplement the temperature drop of molten steel from the primary smelting furnace into the refining furnace, heat absorption of slag forming agent and alloy, heat loss of molten steel in the ladle furnace caused by bottom blowing argon and vacuum pumping during the refining process, so as to ensure that the molten steel can be refined smoothly and finally meet the temperature required for tapping. The electrode-heating power of the LF furnace can be determined by the following formula, and then corrected by the heat balance calculation of the furnace and the heating rate of molten steel.
(3) Argon Stirring
Argon blowing and stirring at the bottom of the ladle have great influence on desulfurization, deoxidation, dehydrogenation, denitrification and inclusion floating. In fact, argon blowing is started when the ladle furnace receives steel. The bottom blowing argon flow rate during heating is generally 40% ~ 45% of that during vacuum operation (the bottom blowing argon flow rate is the maximum during vacuum operation).
(4) Alkaline Refining Slag
Slagging is an important link in ladle furnace steelmaking process. Lime and fluorite are added to the liquid steel twice in different proportions (e.g. 5:1 or 4:1). The amount of lime and fluorite is 1% ~ 2% of the amount of liquid steel to make high alkalinity refining slag for desulfurization; Then, the ferrosilicon powder, calcium silicon powder and aluminum powder or carbon powder are mixed in a certain proportion and directly added to the steel liquid surface or injected into the steel liquid by the injection method to form a slag system with a slag composition of cao60% ± 5%, sio210% ± 5% and Al2O3 330% ± 5%, or a slag system with a slag composition of cao60%, Al2O3 330% and caf210%. The main factors affecting the desulfurization reaction are not only the slag with high basicity in reducing atmosphere, but also the stirring energy of argon blowing. Argon blowing and stirring at the bottom of the ladle can accelerate the diffusion of sulfur in the molten steel to the slag steel interface, and the desulfurization rate can be increased by 1.5-2.0 times to 90%.
(5) Composition Adjustment of Molten Steel
When the heating temperature reaches ≥ 1600 ℃, the alloy adjustment composition can be added to the molten steel. The allowable alloy composition adjustment range of ladle furnace refining is large, easy to be uniform and high yield. All alloys that are easy to be oxidized during alloying in the primary smelting furnace can be moved to the ladle furnace for refining.
(6) Vacuum Treatment
The ladle furnace performs degassing and de-inclusion refining under vacuum. According to the requirements of steel grade, it is divided into high vacuum treatment (below 133pa) and low vacuum treatment (below 2666pa). The steels for high vacuum treatment include steels for pressure vessels, nuclear power plants, hot wall hydrogenation reactors, etc. The steels for low vacuum treatment include steam turbine blades, gun steel, hot rolling rolls and other steels.
In vacuum treatment, the argon blowing flow rate at the bottom of the ladle varies with the vacuum degree. The argon flow rate of high vacuum treatment reaches the maximum, and the argon flow rate of low vacuum treatment is about 35% of the maximum flow rate. Under the stirring action of vacuum and bottom blowing argon, the deoxidation and removal of non-metallic inclusions are completed, and the floating separation of non-metallic inclusions in ladle furnace refining is relatively complete. Finally, after the target value (mainly the gas content) is reached through temperature measurement, sampling and testing, the task of vacuum treatment is completed. Generally, vacuum refining is 15-20min.
(7) Heat Preservation
Complete insulation and narrow range alloy trimming. At this time, it is still necessary to blow argon at an appropriate flow rate to uniform the temperature and composition. Finally, temperature measurement (different pouring methods, different tapping temperatures), sampling for full analysis (including steel and slag samples), etc. all meet the tapping requirements, and tapping can be conducted.