1. Feature of Ladle Metallurgical Furnace
A ladle metallurgical furnace is used for refining equipment outside the furnace. The purpose of refining is achieved by submerged arc heating, bottom blowing argon stirring, deoxidizing and desulfurizing molten steel with high basicity synthetic slag, removing non-metallic inclusions and alloying.
LF furnace can accurately control the composition of molten steel and smelting temperature. It can be matched with the electric furnace to replace the reduction period of an electric furnace, can be matched with the converter to produce high-quality alloy steel, and can be combined with RH or VD furnace; If the LF furnace is equipped with a vacuum system for degassing the molten steel, this is called LFV.
At present, a ladle metallurgical furnace has become indispensable buffer equipment for preserving molten steel between steelmaking and continuous casting. Because of its simple equipment, low investment cost, flexible operation, and good refining effect, it has been widely used in the metallurgical industry.
2. Equipment Composition of LF Refining Furnace
Ladle metallurgical furnace equipment is mainly composed of furnace body, arc heating system, bottom blowing argon stirring system, alloy, slag feeding system, dust removal system, temperature measuring, and sampling device, ladle car control system, etc.
At present, the ladle with a sliding nozzle, which can be used for refining molten steel and directly for pouring, is mostly used for refining outside the furnace to avoid the temperature drop and secondary oxidation of molten steel during re-tapping, which is often referred to as ladle furnace.
2.1 Furnace Body of Ladle Metallurgical Furnace
LF furnace body is a ladle furnace. It is slightly different from the ordinary sliding nozzle ladle. The upper mouth of this ladle has a water-cooled flange, which is sealed with the furnace cover through a sealing rubber ring to prevent the intrusion of cold air; In addition to the sliding nozzle for pouring, the bottom of the ladle is also equipped with permeable bricks capable of blowing inert gas to achieve the function of stirring molten steel.
2.2 LF Refining Furnace Cover
The LF melting furnace cover is of a water-cooled structure, and the inner layer is lined with refractory materials. In order to prevent the adhesion between the furnace cover and the ladle caused by splashing, an anti-splash baffle is hung under the furnace cover. In order to protect the reducing atmosphere in the furnace, prevent the ladle from radiating heat and improve the thermal efficiency, the furnace cover and the ladle mouth should be well sealed.
2.3 Arc Heating System
The arc heating system used in the ladle metallurgical furnace is the same as that of the steel-making electric furnace. It is divided into a three-phase AC ladle furnace and a single-phase DC ladle furnace with the arc heat generated between the graphite electrode and the melt as the heat source. However, there is no solid material melting process in the LF refining furnace, so the submerged arc heating method should be used. Therefore, compared with the steel-making electric furnace, the lower secondary voltage can be used.
2.4 Other Related Equipment
The LF melting furnace is equipped with a dust removal system like the steel-making electric furnace; An alloy and slag hopper is arranged above the furnace cover, and the weighed alloy or slag is sent into the furnace through the chute and feeding port through the electronic scale. In addition, there are 1-2 wire feeders beside the furnace to facilitate the final deoxidation by feeding aluminum or silicon calcium into the molten steel after refining.
A deep pit, commonly known as an accident safety pit, shall be dug directly under the ladle metallurgical furnace station. It is better that the pit can hold the molten slag and steel of one or two furnaces, so as to prevent the slag and steel from flowing all over the ground or burning the ladle car and other equipment in case of ladle leakage, which will affect the continuation of subsequent production.
The temperature measuring and sampling device can be composed of an industrial manipulator, probe disassembly device, measuring gun, and positioning device. Through data communication and signal handover and control, the device transmits the instructions from the operator to the manipulator and gun measuring device through PLC (programmable logic controller), so as to realize unmanned temperature measurement and sampling.
3. Refining Process of Ladle Metallurgical Furnace
Oxidation slag is a major hazard of molten steel refining outside the furnace. Therefore, the LF furnace should be matched with an eccentric bottom electric furnace without slag tapping.
3.1 Operation of Reducing Slag Tapping Method
In front of the furnace, the slag shall be fully removed in the oxidation period to produce the reduced dilute slag, that is, lime and fluorite shall be added, with a volume ratio of 3:5:1. The amount is 2% ~ 5% of the water content of steel, and firebricks can be used to replace part of fluorite. The initial adjustment of alloy composition is carried out in front of the furnace, and the fine adjustment of alloy composition is carried out in the refining ladle. After the formation of thin slag, the melting time of the alloy shall be ≥ 10min. Carbon powder shall be added in the middle to maintain the color and reduce the atmosphere of the slag. When the temperature meets the process requirements, the steel can be tapped and the molten steel can be directly poured into the refining ladle.
3.2 Operation of Ladle Slag Tapping Method
The operation of this method is to use two ladle, i.e. running ladle and LF furnace refining ladle. The running ladle is used for tapping, and then the molten steel is injected into the LF furnace refining ladle through the nozzle, while all the oxide slag is left in the running ladle. If the sulfur and phosphorus content is not high, the oxide slag can be returned to the furnace.
The alloys with primary adjusted components such as Ni, Mo and Fe are added to the primary smelting furnace, and Cr Fe is made into small pieces ≤ 20mm, which are added to the refining ladle and baked together with the ladle. The baking temperature of the refining ladle is ≥ 800 ℃. This method has large heat dissipation and serious gettering of molten steel.
3.3 Operation of Stripping out Oxide Slag from Refining Ladle
This method needs to set a slag stripping station. After tapping, the refining ladle is transferred to this station to strip out the oxide slag, make slag again, and then heat and refine.
Before tapping, the permeable bricks of the refining ladle shall have good air permeability, and argon blowing shall be started before tapping, with a pressure of 0.2 ~ 0.3MPa. Only in this way can the permeable bricks be prevented from being drowned by molten steel and ensure air permeability.
4. Refining Effect of Ladle Metallurgical Furnace
LF refining furnace has a high ability of deoxidation, desulfurization, degassing, and removal of non-metallic inclusions in steel, so it can be used to smelt high purity steel. If the refining time of molten steel treated by LF furnace is more than 40min, the total amount of inclusions on the steel can be reduced by 50%, more than 10% μ The inclusions above M (μ m) can be completely removed, (o) can be reduced to (20-30) x10-4% (s) can be reduced to 20x10-4%. For deep desulfurization, the LF furnace can reduce (s) to 5x10-4% if synthetic slag with high alkalinity is used for refining, and with the continuous addition of deoxidizer and slag materials, the fluidity of the slag line can be continuously improved without affecting the service life of the slag line, and sufficient stirring time is given or multiple slag changing operations are adopted.
The refining of the LF furnace is carried out under the bottom blowing argon stirring and strong reducing atmosphere. The water vapor, argon and nitrogen content of furnace gas are very low and will not be transferred to the molten steel through the slag layer. As long as the argon blowing time is sufficient, the hydrogen and nitrogen content in the molten steel will be reduced in general. Generally, (H) can be reduced to 5-6x10-4% and (n) can be reduced to 55x10-4%.
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