LF Ladle Refining Furnace

1. The Characteristics of LF Ladle Refining Furnace

LF ladle refining furnace is used for refining equipment outside the furnace, through submerged arc heating, bottom blowing argon stirring, using high basicity synthetic slag to deoxidize, desulfurize, remove non-metallic inclusions and alloying to achieve the purpose of refining.

LF ladle furnace can precisely control molten steel composition and melting 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 for duplexing; if The LF furnace has its own vacuum system to degas the molten steel, which is LFV.

At present, the LF ladle furnace has become indispensable buffer equipment for the preservation of molten steel between steelmaking and continuous casting. And 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 Ladle Furnace

LF furnace equipment is mainly composed of furnace body, arc heating system, bottom blowing argon mixing system, alloy and slag feeding system, dust removal system, temperature measurement and sampling device, ladle car control system, etc.

At present, the ladle with a sliding nozzle, which can be used not only for refining liquid steel, but also for pouring directly, is mostly used for out of furnace refining, so as to avoid the temperature drop during re-tapping and the secondary oxidation of liquid steel, which is often referred to as ladle furnace.

2.1 Furnace Body of LF Ladle Furnace

LF furnace body is a ladle furnace. It is slightly different from the ordinary sliding nozzle ladle. The upper mouth of this ladle is provided with a water-cooled flange, which is sealed with the furnace roof through a sealing rubber ring to prevent the invasion of cold air; In addition to the sliding nozzle for pouring, the bottom of the ladle is also equipped with breathable bricks that can blow inert gas to achieve the function of stirring liquid steel.

2.2 Furnace Roof of LF Ladle Refining Furnace

The LF furnace cover is a water-cooled structure, and the inner layer is lined with refractory. 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 heat dissipation of the ladle and improve the thermal efficiency, the furnace cover and the ladle mouth shall be well sealed.

2.3 Arc Heating System

The arc heating system used in the LF furnace is the same as that of the steel-making electric furnace. Taking the arc heat generated between graphite electrode and melt as the heat source, it is divided into a three-phase AC ladle furnace and a  single-phase DC ladle furnace. However, there is no solid material melting process in LF furnace, so the submerged arc heating method should be adopted, so lower secondary voltage can be used compared with steelmaking electric furnace.

2.4 Other Related Equipment

LF furnace, like an electric steelmaking furnace, is equipped with a dust removal system; 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 also 1-2 wire feeders beside the furnace, so as to feed aluminum or silicon calcium into the molten steel for final deoxidation after refining.

A deep pit, commonly known as an accident safety pit, shall be dug directly below the LF station. It is better that the pit can contain one or two furnaces of slag steel molten liquid, so as to avoid that once the ladle is leaked, the slag steel flows all over the ground or burns out the ladle car and other equipment, 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 LF Ladle Furnace

Oxidation slag is great harm to liquid steel refining outside the furnace. Therefore, the LF ladle furnace should be matched with an eccentric bottom electric furnace without slag tapping.

3.1 Reduction Slag Tapping Operation

In front of the furnace, full slag raking is carried out at the end of oxidation to produce reduced dilute slag, that is, lime and fluorite are added, and the volume ratio is 3:5:1. The amount is 2% ~ 5% of the amount of steel water, and firebricks can be used to replace some 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 whole melting time of the alloy (≥ 10min). Carbon powder should be added in the middle to maintain the slag color and reduce the atmosphere. When the temperature meets the process requirements, the steel can be tapped and the liquid steel can be directly turned into the refining ladle.

3.2 Operation of Ladle Slag Discharging Method

The operation of this method is to use two ladles, i.e. operation ladle and the LF furnace refining ladle. The running ladle is used for tapping, and then the liquid 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 content of sulfur and phosphorus is not high, the oxide slag can be returned to the furnace.

The primary components such as Ni, Mo, Fe, and other alloys are added to the primary refining furnace. CR Fe is made into small pieces ≤ 20mm, added to the refining ladle and baked with the ladle. The baking temperature of the refining ladle is ≥ 800 ℃. This method has large heat dissipation and serious suction of molten steel.

3.3 Operation of Stripping out Oxide Slag from Refining Ladle

This method requires setting up a slag raking station. After tapping, transfer the refining ladle to this station to pick up the oxidized slag, make slag again, and then heat and refine.

Before tapping, the permeable brick of the refining ladle shall maintain 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 brick be prevented from drowning by liquid steel and ensure air permeability.

4. Refining Effect of LF Ladle Refining Furnace

LF furnace has a high ability for deoxidation, desulfurization, degassing and removing 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% and more than 10% μ All inclusions above M (micron) can be 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 material, its fluidity is continuously improved without affecting the service life of slag line, sufficient mixing time is given or multiple slag replacement operations are taken.

The refining of the LF furnace is carried out under the bottom blowing argon stirring and strong reduction atmosphere. The water vapor, argon and nitrogen content of furnace gas is very low and will not be transmitted to liquid steel through the slag layer. As long as the argon blowing time is sufficient, the hydrogen and nitrogen content in liquid steel is reduced in general. Generally, (H) can be reduced to 5-6x10-4% and (n) can be reduced to 55x10-4%.

LF furnace uses arc heating and argon blowing stirring to accurately control the temperature of liquid steel, with an accuracy of ± (3-5 ℃), and can also accurately control the composition of liquid steel in a very narrow range.

If you are interested in LF ladle refining furnace, please mail at [email protected], or [email protected].