The ladle furnace process is the process of transferring the molten steel produced in the converter, open hearth furnace, or electric furnace to another vessel (mainly ladle) for refining, which is also called "secondary steelmaking or external refining".
The ladle furnace process divides traditional steelmaking into two steps. The first step is called primary refining, in which melting, dephosphorization, decarburization and main alloying of furnace charge are carried out under oxidizing atmosphere; The second step is refining. Deoxidation, desulfurization, removal of inclusions, denaturation of inclusions, fine adjustment of composition, control of molten steel temperature, etc. are carried out under vacuum, inert atmosphere or controllable atmosphere. Since the 1960s, various refining methods have appeared one after another. This technology has developed rapidly all over the world.
Refining outside the furnace has become an indispensable link in the modern iron and steel production process. In particular, the combination of refining outside the furnace and continuous casting is an important means to ensure the smooth production of continuous casting, expand the varieties of continuous casting, and improve the quality of billet.
In the ladle furnace process, it has become a common mode for steel plants to adopt converter (or electric furnace) - refining outside the furnace - continuous casting.
The processes of various refining methods outside the furnace are different, and their common characteristics are: there is an ideal refining atmosphere, such as vacuum, inert gas or reducing gas; Using electromagnetic force and blowing inert gas to stir molten steel; In order to compensate the temperature drop loss of molten steel during refining, arc, plasma, chemical method and other heating methods are adopted.
Ladle refining matching with continuous casting is to improve the quality of billet and ensure the stability of the continuous casting process. Selecting an appropriate refining method outside the furnace is an important means to prepare molten steel for continuous casting and provide qualified molten steel. Therefore, the ladle refining equipment selected in combination with product quality requirements shall meet the following basic requirements:
1. Adjust the temperature of molten steel to reach the pouring temperature range required by continuous casting;
2. Improve the cleanliness of molten steel, especially reduce the content of large inclusions in steel;
3. Reduce the gas content in steel (such as hydrogen and nitrogen content);
4. Reduce the content of harmful impurities (such as sulfur and phosphorus H＜2×10-6) in the steel;
5. Homogenize the temperature and composition of molten steel, and fine-tune the composition to make the chemical composition range of finished steel very narrow;
6. Change the morphology and composition of inclusions in steel to improve the fluidity of molten steel;
7. Reduce the smelting load of the steel-making furnace, shorten the smelting cycle and improve productivity;
8. The ladle refining furnace serves as a "buffer" between the steel-making furnace and the continuous casting machine to balance the production rhythm between the two, which is conducive to improving the productivity of the continuous casting machine.
Ladle Refining Method
There are dozens of refining methods outside the furnace, each of which has its own characteristics. To determine which method to choose, we should first focus on the specific situation of each plant. The requirements of the treated varieties and other relevant conditions, such as quality, cost, construction cost, etc.
Which kind of external refining method is adopted depends on the plant conditions and the requirements for product quality to establish different production processes.
For the continuous caster matched with an electric furnace or ultra-high power electric furnace, when selecting ladle refining, the product quality requirements with a high alloy ratio shall be met. For example, stainless steel adopts electric furnace AOD furnace continuous casting; Electric VOD furnace continuous casting process.
For slab, bloom and round billet casters matched with large-scale converters, it is required to provide high-quality molten steel to produce defect-free billets, such as converter RH continuous casting and converter RH + KPI continuous casting. When producing ultra-low carbon steel (carbon less than 0.0015%) or ultra-low sulfur steel (sulfur less than 0.001%), LF furnace and vacuum treatment can be used to achieve the best effect. In consideration of saving investment, the CAS-OB refining furnace process can also be adopted.
For the small billet and rectangular billet continuous caster which is mainly used to produce plain carbon steel in combination with the small-scale converter, the technology of argon blowing in the ladle or wire feeding in the ladle is generally adopted, which can basically meet the requirements of the continuous casting process and billet quality.
Ladle Furnace Process Flow
The technological advantages of the ladle refining furnace include strong refining function, suitable for the production of ultra-low sulfur and ultra-low oxygen steel; arc heating function, high thermal efficiency, large temperature rise range, and high-temperature control accuracy; with stirring and alloying functions, easy to achieve narrow The composition is controlled to improve the stability of the product; the slag steel refining process is adopted, and the refining cost is low; the equipment is simple and the investment is less. At present, ladle refining furnaces are widely used in domestic 50-150t converter steelmaking plants.
The ladle refining furnace process mainly includes two contents:
Heating and temperature control. The heating rate can reach 3～5℃/min, and the ladle refining furnace adopts the computer to dynamically control the endpoint temperature, which can ensure the control accuracy of the endpoint temperature (5℃). The use of submerged arc foam technology can reduce the radiant heat loss of the arc and improve the heating efficiency.
White residue refining process. The ladle refining furnace uses white slag to refine molten steel, realize desulfurization and deoxidation of molten steel, and produce ultra-low sulfur steel and low oxygen steel. Therefore, white slag refining is the core of the ladle refining furnace process operation, and it is also an important guarantee for improving the cleanliness of molten steel. The main points of the white slag refining process are:
- Tapping slag, control the amount of slag less than 5kg/t;
- Modification of slag, control slag R 2.5, (TFe+MnO) 3.0%;
- White slag refining, generally use Al2O3-CaO-SiO2 slag, control the basicity of the slag R 4, (TFe+MnO) 1%, to ensure the desulfurization and deoxidation effect;
- Control the atmosphere in the ladle refining furnace to be weakly oxidizing to avoid reoxidation of the slag;
- Proper stirring to avoid exposure of the molten steel surface and ensure a high mass transfer rate in the molten pool.
Alloy fine-tuning with narrow composition control. Alloy fine-tuning and narrow composition control technology is one of the key technologies to ensure the stability of steel composition performance, and it is also an important metallurgical function of the ladle refining furnace.
Argon Blowing in Ladle and CAS method
Argon blowing technology in the ladle is the most common and simplest treatment process outside the furnace. Its main metallurgical function is to uniform the composition and temperature of molten steel and promote the floating of inclusions. Generally, the gas stirring intensity of argon blowing in the ladle is 0.003 ~ 0.01m3 / (t · min) (standard state). Improving the argon blowing intensity in the ladle is conducive to the mixing of molten pools and the floating of inclusions. If the argon blowing intensity is too high, the steel surface will be exposed, resulting in secondary oxidation.
In order to solve this problem, the strong argon blowing process is adopted to blow the slag liquid surface open, and then the argon protective atmosphere is rapidly formed in the closed immersion bell to avoid the oxidation of molten steel. This argon-blowing process is usually called the CAS method. CAS method not only improves the argon blowing intensity, but also the argon atmosphere in the bell jar improves the alloy yield, and adds the function of alloy fine adjustment to the ladle argon blowing process.
In order to solve the problem of temperature rise of molten steel, top blowing oxygen gun and aluminum shot adding equipment are added to CAS method to realize temperature rise of molten steel through oxidation and heating of aluminum dissolved in molten steel, which is generally called CAS-OB process.
RH method is a vacuum circulation degassing method
RH method is one of the vacuum circulation degassing methods. The basic process principle is to continuously lift the molten steel into the vacuum chamber by using bubbles for degassing, decarburization and other reactions, and then return to the ladle. Therefore, the RH process does not require a specific ladle clearance height, and the reaction speed is not limited by the ladle clearance height. Compared with other vacuum treatment processes, the disadvantages are complex equipment and a large investment. The advantages of RH method technology are:
- Fast reaction speed, short treatment cycle, high production efficiency, often used with a converter;
- High reaction efficiency. Molten steel reacts directly in the vacuum chamber to produce ultra-low carbon clean steel with H ≤ 0.00005% and N ≤ 0.0025%;
- Oxygen blowing decarbonization and secondary combustion can be conducted for thermal compensation to reduce the treatment temperature drop;
- Powder injection desulfurization can be carried out to produce ultra-low sulfur steel with S ≤ 0.0005%.
VD furnace and VOD furnace
As a vacuum degassing equipment, the VD furnace is usually combined with a ladle refining furnace to produce various alloy structural sheets of steel, high-quality carbon steel and low-alloy high-strength steels. The top blowing oxygen supply system is added to the VD furnace to form the VOD furnace, which can complete the vacuum oxygen blowing decarbonization function and is suitable for smelting low-carbon stainless steel. Compared with the RH vacuum treatment process, the refining strength of the VD (VOD, VHD or VAD) furnace is strictly limited by ladle clearance.
Generally, the clearance height of the ladle is required to be 600mm to complete the degassing treatment of the molten steel; In order to carry out the carbon deoxidation process of molten steel, the clearance height of the ladle shall be 900mm. To realize the oxygen blowing decarburization process, the clearance height of the ladle is required to be 1000 ~ 1200mm. Since the decarburization reaction strength of molten steel is strictly limited by the clearance height of the ladle, the decarburization cycle of the VD furnace is long (generally 40 ~ 45min), and the whole treatment cycle is 75 ~ 90min.
In addition, the VD furnace can realize steel slag reaction under vacuum conditions, which is conducive to desulfurization and deoxidation of the molten pool. During VD furnace treatment, the oxygen content in steel can be reduced from 0.01% to 0.002%; The sulfur content can also be from 0.01% to below 0.00%, and the average desulfurization rate can reach 84%.
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