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Steel Vacuum Degassing
1. What is steel? Why do we need to make steel?
From the perspective of chemical composition, steel is mainly divided into carbon steel (including low carbon steel, medium carbon steel and high carbon steel) and alloy steel.
- In addition to iron and carbon as the main components, carbon steel also contains a small amount of impurities such as Mn, Si, S, P, etc. It has certain mechanical properties and process properties and is cheap.
- Based on carbon steel, alloy steel has certain elements added purposefully, and its performance is significantly improved.
The task of steelmaking: According to the requirements of steelmaking, adjust the range of carbon and alloy elements in steel, and reduce the content of impurities such as P, S, H, O, N to below the allowable range.
2. The influence of alloying elements on steel properties
- C and Si can increase the strength and hardness of steel
- Mn improves hardenability and toughness, and reduces the harmfulness of S
- P causes cold brittleness
- S causes hot brittleness, fatigue fracture, and poor mechanical properties
- Other trace metals: Cr, Ni, Mo, Ti, etc.
- Other non-metallic inclusions: hydrogen, oxygen, nitrogen, etc.
3. Steel smelting process
- Decarburization, deoxidation, dephosphorization, desulfurization
- Remove inclusion gas (H2 N2), remove non-metallic inclusions
- Adjust the temperature of molten steel and adjust the composition of molten steel
4. Vacuum refining process outside the furnace
- Refining outside the furnace – the process of moving the molten steel from the converter to another reactor for refining.
- Primary refining – melting, dephosphorization, decarbonization and alloying of the charge in an oxidizing atmosphere
- Refining – deep decarbonization, degassing, deoxidation, removal of inclusions, adjustment of composition, optimization of process and product structure, development of high value-added products, energy saving and consumption reduction, cost reduction and economic benefits increase under vacuum, inert gas or controlled atmosphere.
5. Development history of refining outside the furnace
- In the 1940s: Degassing treatment was carried out under low vacuum.
- In the 1950s: The steam jet pump with high pumping speed was introduced, and the DH and RH vacuum methods were used to degas molten steel, reducing the amount of impurities.
- In the 1960s and 1970s: On the basis of vacuum degassing, VOD, VAD, ASEA, LFV, and RH-OB flourished.
- In the 1980s: The alloy wire feeding method and the combined use of refining equipment with different functions appeared.
- In the 1990s: Circulation vacuum treatment (RH, RH-OB), lifting vacuum treatment (DH), alloy fine-tuning and temperature treatment (CAS.CAS-OB), LF ladle refining furnace, vacuum argon deoxidation (VD), vacuum oxygen decarburization (VOD), ladle powder spraying (KIP, TN, SL), wire feeding method, argon oxygen refining (AOD) were widely used.
6. Classification of refining outside the furnace
6.1. Classification by refining method
Treatment method under normal pressure:
- Slag washing method: synthetic slag washing, same furnace slag washing;
- Ar refining method: Gazal, CAB, AOD, CLU
Vacuum refining method:
- Degassing: ladle degassing method, DH, RH
- Degassing C, O gas mainly: VD, RH-OB, RH-PI
Vacuum and heating method: ASEA-SKF、VAD、LF
6.2. Classification by main purpose of refining
- AOD, CLU, VD, VOD, RHOB and VODC are suitable for smelting low carbon and ultra-low carbon steel and alloy steel;
- Barrel furnace, VAD (VHD) and LF have heating and stirring functions and flexible temperature control, and can be used for degassing, deoxidation, removal of inclusions and alloying;
- DH, RH, different forms of drip degassing and vacuum Ar blowing methods can remove gas, oxygen and inclusions in steel, but there is no heating equipment, suitable for vacuum degassing of ordinary steel and medium and low alloy steel;
- The same slag washing method is mainly used for desulfurization, deoxygenation and removal of inclusions in molten steel during the process of electric furnace steelmaking.
7. Types of steel degassing processes can be split into three main options:
7.1. RH refining furnace -- This process focuses on decarburization and degassing, and is an excellent system for producing ultra-low carbon steel grades.
Function
① Dehydrogenation: The dehydrogenation efficiency of completely deoxidized steel liquid is ≮60%, and for incompletely deoxidized steel liquid, due to the violent C-O reaction, it is >70%. Degassing time is 5~20min, [H] <2ppm.
② Remove N: N is easy to form N-compounds, and the denitrification efficiency is 0~10%;
③ Deoxygenation: ∑ [O] 0.002~0.005%;
④ Decarburization: There are requirements for the initial [C], and the treatment is 15min, [C] can be reduced to 0.002%;
⑤ Desulfurization: efficiency 50~75%;
⑥ Reduce non-metallic inclusions: improve the purity of molten steel;
⑦ Fine-tuning of composition: the control accuracy of alloy elements is ±0.003~0.010%
Process parameters
① Processing capacity: weight of molten steel in ladle, ≮30t;
② Degassing time: the degassing time of molten steel under vacuum is 20~30min;
③ Circulation factor: the number of times the molten steel in the ladle passes through the vacuum chamber, equal to 5;
④ Circulation flow: the amount of molten steel passing through the vacuum chamber per unit time;
⑤ Vacuum degree: the pressure in the vacuum chamber. RH treatment 67~134Pa, RH-OB treatment, 20000~33000Pa
7.2. VD/VOD -- With the growing demand for high-quality steel, vacuum treatment has become an essential step in secondary metallurgical processes. Generally speaking, VD is more used for alloy steel, and VOD is more used for stainless steel.
VD refining furnace
It was first used in Germany in the 1950s. The primary steel liquid in the electric furnace and converter is placed in a sealed tank to evacuate, and Ar is blown at the bottom of the ladle for stirring. It is used to produce low-carbon steel, and the composition and temperature can be strictly controlled. Function:
① Effective degassing, reducing [H] and [N];
② Deoxidation, removing [0] through C+[0]=C0;
③ Removing [S] through alkaline top slag:
④ Controlling chemical composition and temperature through alloy fine-tuning and blowing Ar;
⑤ Blowing Ar makes inclusions float without aggregation.
VD is mainly used in conjunction with electric arc furnace and LF to produce pipeline steel, [S]<10ppm; [H]<1.5-2ppm; [N]< 40ppm; [0]< 20ppm.
The target working vacuum requirement range for VD is about 1.33 mbar (1 Torr) to 0.67 mbar (0.5 Torr). Similar to RH refining furnace, the maximum pumping speed requirement for VD can reach 1 million cubic meters per hour.
VOD refining furnace
VOD refining furnaces are used to smelt stainless steel with oxygen lances installed on VD furnaces, providing the most suitable conditions for large-scale decarburization and achieving high purity and cleanliness. They are usually used for deep decarburization of high-alloy steels and to remove carbon without affecting the chromium content when producing stainless steels.
The commonly used target working vacuum requirements for VOD are approximately between 50-200 mbar. The most common pumping speed requirements are between 20,000 and 120,000 m3/h.
8. Types of Vacuum Systems in Steel Degassing Process
The vacuum system is a key component of the steel degassing process because it creates and maintains the required vacuum in the process chamber. In steel degassing plants, there are generally two vacuum solutions. Previously, steam ejector vacuum systems (usually with liquid ring vacuum pumps) were used, but nowadays, mechanical vacuum systems consisting of Roots pumps (supported by dry screw vacuum pumps) have become a cost-saving and environmentally friendly alternative. Below we will make a comparative analysis of these two vacuum solutions:
8.1.1 What is steam jet equipment? What are its advantages?
The steam jet pump is a working steam with a certain pressure that passes through a Laval nozzle, decompresses and speeds up (the potential energy of the steam is converted into kinetic energy) and sprays into the mixing chamber at a supersonic speed, mixes with the pumped medium, and exchanges energy. The mixed gas enters the diffuser, decelerates and pressurizes (the kinetic energy is converted into pressure energy). In order to reduce the exhaust load of the subsequent pump, a condenser is configured to exchange heat through convection between two media with a certain temperature difference, so as to achieve the purpose of condensing the high-temperature medium and discharge it to atmospheric pressure.
The advantages of multi-stage steam jet pump are:
- Large suction capacity: can meet the maximum processing capacity and exhaust volume requirements of vacuum refining (usually from a few kg/h to hundreds of kg/h)
- Fast suction rate: usually 3 to 6 minutes to reach the rated vacuum degree requirements
- Strong adaptability to the pumped medium: can effectively pump out the high-temperature and dusty gas generated by steelmaking
- Long equipment life and reliable operation: because the steam jet pump has almost no moving parts
8.1.2 What are the shortcomings of the steam jet pump vacuum system?
The RH method is an important off-furnace refining method with functions such as degassing, decarburization, temperature compensation, uniform temperature composition of molten steel, and removal of inclusions in steel. Due to its good refining effect and short processing cycle, it has been widely used in steelmaking production. In order to further improve the product structure and steel quality, Chongqing Iron and Steel New District adopted RH vacuum refining technology.
RH vacuum refining technology has been developed since 1959, and the equipment and process have been mature. The vacuum equipment used in the traditional RH process used to be a multi-stage steam jet pump vacuum system. From the current application status at home and abroad, it is found that although the multi-stage steam pump system equipment is mature and the supporting facilities are complete, it is significantly affected by steam pressure and temperature fluctuations in use, sludge recovery is difficult, the equipment maintenance is large, and the air extraction capacity decreases with the extension of use time.
- Water consumption: Steam and cooling water consumption is high, resulting in high operating costs
- Wastewater generation: Steam ejectors produce a large amount of wastewater rich in dust that needs to be treated
- Wastewater treatment: Dust needs to be precipitated in hot water, and the recycled water still contains a small amount of dust
- Equipment wear: Pipes and gate valves exposed to water rich in corrosive particles will wear out prematurely
8.2.1 What is a dry vacuum pump system?
An oil-free dry mechanical vacuum pump (also referred to as a dry mechanical vacuum pump) is a vacuum pump that can start pumping air from atmospheric pressure and discharge the pumped gas directly into the atmosphere, with no oil or other working media in the pump chamber. With the continuous development of the steel vacuum degassing industry, a mechanical vacuum system composed of Roots vacuum pumps (use dry screw vacuum pumps as backing pump) has become a cost-saving and environmentally friendly option.
8.2.2 What are the advantages of a dry mechanical vacuum pump system over a steam jet pump system?
Pump type | Investment cost | Power consumption | Operating system | Pollution | Construction |
Steam Jet Pump | Smaller initial investment | Higher energy and steam consumption | Steam supply systems, evacuation systems, thermal storage systems, and related water treatment systems | Wastewater pollution, high operating costs | Large area |
Dry Mechanical Vacuum Pump | Larger initial investment | Energy consumption is only 10% of steam jet pumps | Vacuum pumping system | No wastewater pollution, low operating costs | Simple,small area |
8.2.3 Concrete analysis of the advantages of dry mechanical vacuum pump system compared to steam jet pump system
No. | Items | Dry vacuum pump | Steam jet pump | Remarks | |
1 | Operating Media | Electricity | Steam | Electricity is stable, steam pressure and temperature fluctuate | |
2 | Pumping capacity configuration | Adjust the operating module as needed | Cannot be changed after being fixed |
| |
3 | Production | Start and stop at any time as required | Affected by steam pressure and temperature |
| |
4 | Dust removal | Dry dust removal for easy recycling | Wet dust removal for difficult recycling |
| |
5 | Running cost | Power consumption | 6.18KWH/ton steel | 0.101 ton/ton steel |
|
unit price | 0.6 RMB/KWH | 280 RMB/ton | Steam is calculated according to the low price of outsourcing | ||
Running cost per ton steel | 3.71 RMB/ton steel | 28.28 RMB/ton steel | Excluding wastewater treatment costs of jet pump system | ||
6 | Initial investment | Relatively large | Relatively small investment in a single pump | If the initial investment in the steam system and water treatment system is taken into account, it is not low | |
9.What does the steel vacuum degassing solution of VACCULEX's dry mechanical vacuum pump (Roots vacuum pump, screw vacuum pump) system include?
- Combination of vacuum pump modules
- Calculation of vacuum system suction capacity
- Vacuum pump selection
- Safety guarantee during vacuum system operation
- Vacuum system layout
- Process gas cooling method
- Vacuum system filtering method
- Dust removal and recovery method of vacuum system
10. Typical cases of dry mechanical vacuum pumps (Roots vacuum pumps, screw vacuum pumps) in refining furnaces in the steel degassing industry
10.1.Typical case 1
Construction scale: A dry mechanical vacuum system with a pumping capacity of 800kg/h (20℃ dry air) under 67PaA conditions and 210tRH. Main technical transformation content: Compared with the traditional wet vacuum system, the new dry mechanical vacuum system removes the steam supply system (fast-burning boiler or gas boiler for saturated steam superheating), interstage condenser, sewage, sludge treatment system, and huge circulating cooling water system. The main equipment is 2 sets of air coolers, 4 sets of dust filters, 18 sets of vacuum pump groups, 1 nitrogen storage tank, and 1 nitrogen storage tank. The investment in energy-saving technical transformation is 17.5 million yuan more than that of the traditional steam jet vacuum system, and the construction period is 8 months. It can save 20,539tce of energy per year, with an annual energy-saving economic benefit of 17.03 million yuan and an investment payback period of 1 year.
10.2.Typical Case 2
Construction scale: A dry mechanical vacuum system with a pumping capacity of 507kg/h (20℃ dry air) under 67PaA and 150tRH. Main technical transformation content: A new dry mechanical vacuum system with main equipment including 1 air cooler; 3 dust filters; 14 vacuum pump sets (13 for use and 1 for standby); 2 gas storage tanks; 56 frequency converters; 1 DCS control system. The investment in energy-saving technical transformation is 15.62 million yuan more than that of the traditional steam jet vacuum system, and the construction period is 1 year. It can save 14,207tce of energy per year, with an annual energy saving economic benefit of 10.37 million yuan and an investment payback period of 1.5 years.
10.3.Typical Case 3
The production practice of Chongqing Iron and Steel’s 210tRH running for 7 months and processing 420,000 tons of molten steel shows that the metallurgical effect of the dry (mechanical pump) vacuum system applied to RH can reach or exceed that of the multi-stage steam jet pump vacuum system, with a dehydrogenation rate of 63.5% and a maximum decarburization capacity of 10@10-6. The dry vacuum pump uses electricity as the driving energy source, and its operation is not restricted by steam pressure and temperature. The pump can be started and stopped at any time according to the actual production situation, and the production organization is flexible.
After 11 months of operation, about 10,000 tons of molten steel have been processed, and direct economic benefits of 19.58 million yuan have been obtained.
It has played a good demonstration and application role for RH and other vacuum refining equipment and optimization of domestic and foreign steel enterprises.
10.4.More typical cases
60T VD furnace mechanical vacuum pump system
RH vacuum furnace vacuum system modification
65T Dual-station VOD refining furnace upgrade
11.Advantages of VACCULEX's dry mechanical vacuum pumps (Roots vacuum pumps, screw vacuum pumps).
11.1. Full coverage of models and types of dry mechanical vacuum pumps (Roots vacuum pumps, screw vacuum pumps) for steel degassing(the only one in China). The maximum pumping speed of Roots vacuum pumps is 110,000 m3/h, and the maximum pumping speed of screw vacuum pumps is 2700 m3/h.。
11.2. Mature and professional solution provider, many years of supporting experience accumulation insteel vacuum degassing industry, perfect pre-sale and after-sale service, one-stop solution for customers’ vacuum problems in steel degassing
11.3. Excellent vacuum calculation ability, reliable and stable dry mechanical vacuum pump product quality, competitive price model, truly win-win cooperation.
