This article analyzes and explores the issue of controlling the nitrogen content of molten steel during the steelmaking process. First of all, through analysis, it is believed that the main reasons for the excessive nitrogen content during the smelting of typical steel types include the high nitrogen content in the molten steel at the end of the converter, the serious nitrogen increase in the tapping process, and the serious nitrogen increase in the tundish link between the end of refining and the tundish. On this basis, starting from the three aspects of converter low-nitrogen steel smelting technology, two-step deoxidation controlled nitrogen increase technology in the steel tapping process, and double argon seal long nozzle protective pouring technology, the nitrogen content of molten steel in the steelmaking process was analyzed. Analyze and discuss the control points and technical solutions.
Keywords: steelmaking; molten steel; nitrogen content
Past practical experience has confirmed that maintaining a high level of nitrogen content in molten steel will increase the ultimate strength, yield limit, and hardness level of the steel, while also having a considerable adverse impact on its impact toughness and plasticity properties. Increased nitrogen content will also increase the toughness-brittleness transition temperature of steel, causing low-temperature temper brittleness. Especially for low carbon steel used in deep drawing environments, maintaining a high level of nitrogen content in the steel water will have quite obvious adverse effects. Supported by the current level of technical conditions, the level of control over the nitrogen content of molten steel during steelmaking is far lower than foreign levels, and there are also significant fluctuations, which will have a considerable adverse impact on the results of steelmaking. In order to improve the control level of nitrogen content in molten steel during steelmaking, it is necessary to grasp the changing pattern of nitrogen content in the entire process, and take corresponding technical measures to reasonably control and optimize the nitrogen increase problem in the molten steel process, so as to reduce the nitrogen content in finished steelmaking products.
Converter low nitrogen steel smelting technology
During the steelmaking process, sampling was carried out at three stages: adding molten iron to the converter, drawing carbon, and the end point. The sampling results show that the changing trend of the nitrogen content of molten steel during converter smelting is shown in the figure below (see Figure 1). Combined with Figure 1, it can be seen that the nitrogen blowing effect shown by converter smelting is good. After entering the oxygen blowing stage (oxygen blowing duration is in the range of 13.0min ~ 16.0min), the nitrogen content in the molten steel shows an obvious downward trend, reaching the lowest value in the carbon pulling stage, and the molten steel has a certain level in the carbon pulling stage to the end stage. nitrogen uptake trend. At the same time, the data in Figure 1 shows that the average deoxidation rate of the converter from the mixing of molten iron to the carbon pulling reaches about 75.4%, and the denitrification rate of the molten steel drops to about 57.8% at the end of the process. From this perspective, the nitrogen content in the molten steel during the carbon pulling process is relatively low, and the trend of nitrogen absorption in the molten steel from the carbon pulling stage to the end stage is more obvious.
Figure 1 Schematic diagram of the changing trend of nitrogen content in molten steel during converter smelting
Past experience shows that as the temperature increases, the solubility of nitrogen in molten steel tends to increase to a certain extent, and there is a positive correlation between temperature and the partial pressure of nitrogen in molten steel. There is a close relationship between converter denitrification and decarbonization efficiency. The carbon monoxide generated during the decarbonization process will have a direct impact on the denitrification efficiency. Specifically, a large number of carbon monoxide bubbles will be generated during the decarburization process, and the nitrogen partial pressure of these bubbles is always maintained at a low level. Therefore, relative to the dissolved gas in the molten steel, it plays the role of a vacuum chamber. N atoms in the molten steel can enter the bubbles and generate a certain proportion of N2 molecules, which will be discharged from the steel industry along with the carbon monoxide bubbles. In the middle stage of smelting, the carbon-oxygen reaction further intensifies, the amount of carbon monoxide generated increases significantly, and a large amount of nitrogen elements are brought out of the steel. During this period, VN2 always remains at a high level.
After entering the final stage of smelting, the carbon-oxygen reaction slows down relatively, and the Vc level decreases during this period. A large amount of air enters the furnace due to the factor that the partial pressure outside the furnace is higher than the partial pressure inside the furnace, causing the nitrogen partial pressure level to increase. Against this background, affected by factors such as the high temperature in the furnace and the fact that the rate of nitrogen addition is significantly higher than the rate of denitrification, the nitrogen solubility reached the highest level during this period. Moreover, since there is a positive correlation between the oxygen absorption of molten steel and the solubility of nitrogen in the steel, there is a negative correlation with the carbon content in the molten steel. Therefore, the later stage of smelting is affected by the oxygen absorption factor of deep blowing air into the molten steel after the carbon pulling step, resulting in a significant increase in nitrogen in the molten steel. Define the nitrogen dissolution reaction equilibrium constant as KN, define the nitrogen mass fraction in the molten steel as N, define the carbon monoxide partial pressure in the bubbles as PCO, define the denitrification rate as VN2, and define the decarburization rate as VC. Then the corresponding relationship between the decarbonization rate and the carbon removal rate can be described as shown in the following formula (1):
From this perspective, in order to control the nitrogen content in key molten steel in the converter, the core issue is to maximize the control of nitrogen content in the molten steel from carbon pulling to the end point without affecting the smelting effect. Technical measures that can be taken include the following aspects:
① Improve the low blowing argon supply intensity in the later stage of converter smelting, and provide a method to generate a certain proportion of argon bubbles to accelerate the denitrification efficiency in the later stage of smelting. It is recommended to increase the standard from 0.02m3/(min·t steel) in the original plan to 0.04m3/(min·t steel);
② Strictly control scrap steel consumption. Taking T steel as the standard, the scrap steel consumption is controlled within the range of 40.0kg, and the low gun position is used to operate in the later stage of blowing to avoid deep blowing, supplementary blowing and other quality problems caused by improper operating methods or excessive addition of scrap steel. question. Oxygen lance must not be used to sweep the furnace mouth before tapping to prevent air from entering the molten steel and causing a significant increase in the nitrogen content in the molten steel;
③ Taking the temperature of molten steel as the reference standard, add the slag foaming agent according to the T steel standard after entering the later stage of smelting, and the dosage should be controlled according to the 1.0kg~3.0kg standard. This measure can fully mobilize the activity of the slag, generate a large amount of carbon monoxide and accelerate the denitrification reaction. The slag has a certain covering effect on the surface of the molten steel after emulsification treatment. It can avoid direct contact between air and molten steel, and achieve the purpose of controlling the end-point nitrogen content level of molten steel in this way.
Two-step deoxidation control and nitrogen addition technology in the tapping process
The mass transfer from the gas-liquid interface to the liquid phase in the tapping process has certain restrictive effects on the nitrogen addition of molten steel. The nitrogen absorption efficiency of molten steel is largely affected by the number of adsorbable nitrogen vacancies at the gas-liquid interface, and the key factor that determines this indicator is the occupation of adsorbable nitrogen vacancies by active elements on the surface of the molten steel. Under the background of increasing oxygen content in molten steel, the pores occupied by nitrogen atomic speeds under surface nitrogen molecules are occupied by oxygen, causing the formation process of nitrogen molecules to be hindered.
From this perspective, due to the existence of oxygen atoms, the combination of air and molten steel has a certain degree of isolation, which has a very direct impact on reducing the secondary oxidation reaction of molten steel and the efficiency of nitrogen absorption. The following table (see Table 1) gives the data on the impact of deoxidation methods on the amount of nitrogen added during the tapping process. Combining Table 1, it can be seen that there is no nitrogen addition reaction during the tapping process of non-deoxidized steel (such as electrical steel). On the contrary, for girder steel, wheel steel, etc. that use materials such as aluminum and iron that undergo mandatory deoxidation treatment during the tapping process, the amount of nitrogen added during the tapping process reaches more than 12.0*10-6. The above data reflects that the amount of nitrogen added during the tapping process increases. The nitrogen situation will be directly affected to a large extent by the oxygen activity of the molten steel after tapping. This also suggests that in order to reduce the increase of nitrogen during the tapping process, the key issue is to reasonably optimize the tapping deoxidation operation to ensure that the oxygen activity of the molten steel after tapping is maintained above 50.0*10-6.
Based on the above analysis, in order to solve the problem of nitrogen addition in the tapping process of low-nitrogen steel, the key is to improve and optimize the tapping deoxidation process and form a technical solution based on two-step deoxidation. The key points in the specific implementation process are as follows: During the tapping process of low-nitrogen steel, the molten steel is partially deoxidized to ensure that the oxygen activity of the molten steel reaches the standard of 50.0*10-6 or above after deoxidation treatment, and the remaining oxygen is fed through a small platform. During the wire feeding process, special attention should be paid to the strict control of the argon blowing intensity at the bottom of the ladle and the wire feeding rate, so as to avoid the problem of nitrogen increase in exposed steel. Through reasonable improvement of the tapping process, the purpose of controlling the level of nitrogen addition during tapping of aluminum-killed steel can be achieved.
The amount of nitrogen added in the tapping process before and after process optimization is shown in the table below (see Table 2). Combined with the data in Table 2, it can be seen that after the application of the two-step deoxidation control nitrogen increase technology during the tapping process, the amount of nitrogen increase during the tapping of the molten steel has been significantly controlled (basically controllable within the range of 5.0*10-6), and the amount of nitrogen added during the tapping period has been increased. Nitrogen fluctuations are maintained within a small range and are controllable.
| Table 1 Data representation of the impact of deoxidation methods on the amount of nitrogen added during the tapping process | ||||||
| Steel type | Oxygen activity of molten steel on small platform | Nitrogen increase during tapping | Deoxidation method | Number of samples | ||
| average value | interval | average value | interval | |||
| Electrical steel, IF steel | 420.0*10-6 | 300.0*10-6~500.0*10-6 | 2.0*10-6 | 0.0*10-6 ~4.0*10-6 | Not deoxygenated | 30furnace |
| Beam steel, wheel steel | 11.0*10-6 | 3.0*10-6~ 22.0*10-6 | 12.0*10-6 | 5.0*10-6 ~22.0*10-6 | Deoxidation of aluminum and iron | 25furnace |
Table 2 Schematic representation of the amount of nitrogen added in the tapping process before and after process optimization
| Steel type | Before application | After application | Number of samples | ||
| Beam steel | 11.3*10-6 | 6.0*10-6 ～ 19.0*10-6 | 3.0*10-6 | 1.0*10-6 ～ 5.0*10-6 | 14 furnaces |
| axle steel | 16.5*10-6 | 11.0*10-6 ～ 23.0*10-6 | 4.6*10-6 | 1.0*10-6 ～ 6.0*10-6 | 12 furnaces |
| wheel steel | 13.8*10-6 | 8.0*10-6 ～ 23.0*10-6 | 3.1*10-6 | 1.0*10-6 ～ 6.0*10-6 | 15 furnaces |
Double argon seal long nozzle protection pouring technology
First, the long nozzle structure should be reasonably optimized. Before structural optimization, the contact area between the bowl of the shroud and the ladle drain was in line contact mode; after structural optimization, the contact area between the wrist of the shroud and the ladle drain was adjusted to a surface contact mode. By optimizing the structure of the shroud, the sealing stability of the shroud and the ladle shroud during contact is significantly improved.
Moreover, after the implementation of this structural transformation measure, in addition to the argon blowing protection function of the long nozzle at the bowl mouth, an annular joint argon gas chamber is also installed at a distance of 30.0mm below the bowl mouth. After the argon blowing action is performed during smelting, the A new layer of argon gas chamber will be generated at the location, so as to exert the protective effect of double layer argon gas on pouring.
Secondly, reasonable improvements should be made to the sealing ring material. The material selected for the sealing ring and its transportation will have a great impact on the use effect. Before the double argon seal long nozzle protection pouring was implemented, the material selected for the sealing ring was relatively hard and had a high risk of fragmentation during transportation. Not only was it difficult to ensure structural integrity during use, but it also had a relatively adverse impact on the sealing effect.
It is difficult to fall off from the long nozzle in the later stage of smelting and must be removed by burning oxygen through an oxygen tube. It also has a considerable adverse effect on the control of nitrogen content in molten steel. Due to the above problems, in order to achieve reasonable control of the nitrogen content of molten steel during steelmaking, the material of the sealing ring should also be improved. The basic performance indicators of the sealing ring material before and after improvement are shown in the table below (see Table 3). Combined with the data in Table 3, it can be seen that after the material improvement, the overall performance of the sealing ring is better, the texture is relatively soft, and it is not prone to chipping during use, and it is easier to remove from the long nozzle bowl after smelting is completed. On the one hand, it reduces The operation difficulty for smelting personnel, on the other hand, is to optimize the sealing performance.
| Table 3 Basic performance indicators of sealing ring materials before and after improvement | ||||
| Sealing coil material | Al₂O₃ | Refractory | Linear shrinkage rate (1100.0℃ *24.0h) | Moisture content |
| Before improvement | 30.0 ～ 40.0% | >1350.0℃ | ＜ 3.0% | <1.0% |
| After improvement | ≥40.0% | >1450.0℃ | ＜ 2.0% | <1.0% |
Conclusion
Combining relevant practical work experience and the accumulation of operational data during steelmaking, it is not difficult to find that the main reasons for excessive nitrogen content during the smelting of typical steel types include high nitrogen content in the molten steel at the end of the converter, severe nitrogen increase in the tapping process, and the end of refining ~ Nitrogen increase in the tundish link is serious in these aspects. In response to the above problems, it is necessary to carry out systematic research and analysis, scientifically control the nitrogen content through technological improvements, converter low-nitrogen steel smelting technology, two-step deoxidation control tapping process nitrogen increasing technology, and double argon sealing long nozzle The comprehensive application of protective pouring technology strictly controls the nitrogen content level of key molten steel in the converter within the range of 13.0*10-6. At the same time, the nitrogen level from the end of tapping and refining to the tundish link is strictly controlled within the range of 5.0*10-6 , in order to achieve the purpose of reasonably controlling the nitrogen increase problem in the molten steel link.