With the increase of the production ratio of high-quality steel in the ladle, the ladle has become one of the important equipment for refining outside the furnace , which is responsible for more and more refining tasks outside the furnace. This problem has plagued more and more steel mills. The ladle sticky slag mainly appears on the wall, bottom and full cover of the ladle. After the ladle sticks seriously, it will cause many problems. The ladle bottom sticky slag After that, it is difficult to clean the ladle air-permeable bricks during hot repair, which seriously affects the bottom blowing effect of the ladle and poses a serious threat to the quality of the molten steel [2, 3] Ladle turning slag; ladle sticky slag causes the weight of the ladle to increase and reduce the amount of molten steel after the furnace; due to the relatively strong binding of the slag and the ladle lining, it is very difficult to remove the ladle during repair, prolonging the ladle repair time, causing the ladle turnover [4, 5]. Therefore, it is necessary to study the cause of the ladle sticky slag in order to take measures to reduce the ladle sticky slag and reduce the ladle weight.
Ladle sticking problem
In the initial stage of a steel plant, the ladle sticky slag was serious, which seriously affected the normal operation of production. When the ladle sticking is serious, the average weight is 152t, and the overweight ratio is over 50%. Under the current normal use conditions of the Zhanjiang Steelmaking Plant, the average ladle weight is 142t, and it over 150t is overweight it. The current ladle overweight ratio is between 2% and 3%. From the on-site observation, it is found that its sticky slag mainly appears on the wall and bottom of it, the full lid, and the edge of it, as shown in Figure 1.
Analysis of the Causes of Weight Increase of Ladle Sticky Slag
The composition of the ladle slag has a great influence on the ladle sticky slag. The composition of the steel slag mainly affects the melting point and viscosity of the steel slag. In order to study the melting point of the steel slag, the final slag composition of the converter and the composition of the steel slag before and after refining is taken on-site, as shown in Table 1.
It can be seen from Table 1 that after the steel slag undergoes refining treatment (LATS, RH), the content of Al₂O₃ in the steel slag increases sharply. The range of Al₂O₃ in the treated slag is 35% to 50.2%; the content of CaO is about half lower than the content of the final slag of the converter, and the range is 21.5 %～37.4%; SiO₂ content fluctuates little; after RH or LATS treatment, the contents of T.Fe, MgO, and MnO are almost stable and unchanged, and have little effect on the physical properties (melting point and viscosity) of the slag. The main components of the slag are: CaO, SiO₂, Al₂O₃, so the physical properties (melting point and viscosity) of steel slag are mainly determined by it . In order to analyze the influence of steel slag composition on its melting point, the quaternary phase diagram of SiO₂-CaO-Al₂O₃-MgO needs to be drawn. Fig. 2 is the quaternary phase diagram of SiO₂-CaO-Al₂O₃-7%MgO.
The composition of the data in Table 1 is calibrated from the phase diagram of the SiO₂-CaO-Al₂O₃-7%MgO quaternary system. As shown in Fig. 2, there are six red dots. From the position of the red dots in Fig. 2, the steel slag after refining The main melting point is spinel with a high melting point (spinel whose melting point fluctuates in the range of 1550°C to 1700°C, of which four points fall in the range of 1650 to 1700°C, accounting for 66.7%. These high melting point materials are mainly CA2 (CaO ·2Al₂O₃, 1780℃), CA6 (CaO·6Al₂O₃, 1880℃) and C2AS (2CaO·Al₂O₃·SiO₂, 1590℃) and MA (MgO·Al₂O₃, 2130℃), these high-melting substances are present in solid form In the steel slag, the viscosity and melting point of the steel slag are increased, so that the steel slag is easy to adhere to the wall of it, so that it is not easy to fall when turning it, resulting in sticky slag .
In order to reduce the melting point and viscosity of the steel slag, the composition of the steel slag needs to be transferred from the red area in Figure 2 to the gray area. The gray area corresponds to the melting point of the slag below 1500°C. During the continuous casting process, the slag is always liquid, and it is easy to pour out when turning over, Ladle is not easy to stick slag. To make the slag composition fall in the gray area, two conditions need to be met. The CaO/Al₂O₃ ratio is in the range of 0.7～2, and the alkalinity (CaO/SiO₂) needs to be above 2.3.
Calculate the amount of ladle slag according to the content of Al₂O₃ in steel slag before and after refining in Table 1, the change of acid-soluble aluminum in molten steel, and the amount of aluminum alloy added during refining. The weight of ladle slag is shown in Table 2, and from Table 2, the weight of steel slag Up to 5800kg, the minimum is 1700kg.
In order to increase the ratio of alkalinity (CAO/SiO₂) to CaO/Al₂O₃, so that the slag component falls in the gray area, the CaO content in the ladle slag needs to be increased. It is now designed that 800kg, 600kg, 400kg of lime is added during the converter tapping process and the main components (SiO₂, CaO, MgO, Al₂O₃) in the SiO₂-CaO-Al₂O₃-7%MgO quaternary phase diagram after the refining of sample numbers an～f The position changes as shown in Figure 3.
Sample No. a, b, d, e, f The position of the steel slag composition in the quaternary phase diagram is all the red dots in the gray area and the two green dots outside the gray area. The two green dots are after adding 400 kg of lime. The position of the steel slag component in the SiO₂-CaO-Al₂O₃-7%MgO quaternary phase diagram falls between 1500 and 1600°C. During the continuous casting process, as the temperature decreases, the high-melting substance MA will precipitate. The pink point in Figure 3 is the position of the composition of the steel c slag of the sample c (the total steel slag is 1700kg) after adding lime to the quaternary phase diagram. The melting point of the steel slag is all greater than 1500℃ and the highest is greater than 1800℃, mainly because of the amount of lime Too much will cause the CaO content in the steel slag to reach a saturated state, so that CaO solids precipitate out, increasing the melting point of the steel slag.
Based on the above analysis, it can be concluded that in order to reduce ladle sticky slag, 400-800 kg of lime is added during tapping to increase the ratio of the basicity of the steel slag to CaO/Al₂O₃, thereby reducing the melting point of the steel slag to below 1500°C.
In addition to the reasons mentioned above, the main factors affecting the ladle sticky slag are refractory material, converter slag composition, ladle insulation, smelting steel type, ladle turnover speed, ladle maintenance and insulation methods.
Countermeasures and measures to reduce the weight of ladle
In order to reduce the weight of the ladle, considering the temperature drop caused by adding lime during the tapping process, lime should not be added too much. 400-800kg lime is added during the tapping process of the converter unit, and the corresponding amount of lime added after the furnace is formulated according to different steel types.
- For automobile sheet steel, 800kg lime is added after the furnace
- Ordinary low-carbon steel grades require 800kg of lime after the furnace
- Add 400～600kg lime to other steel grade furnaces.
After taking measures, the distribution of slag components in the quaternary phase diagram after refining treatment is shown in Figure 4. It can be seen from Figure 4 that after adding 400~800kg lime in the converter tapping process, the melting point of the refining slag is mainly concentrated in the low melting point range There is only a small part in the high melting point region, which is consistent with the theoretical calculation results.
After the converter adopts measures, the weight of it is significantly reduced. The change in the weight of it is shown in Figure 5. From Figure 5, it can be seen that after taking the steel process and adding lime, its weight decreases almost linearly; afterward, the average ladle weight is within a small range of 142t Fluctuation, the maximum is 144t, maintained within the ideal ladle weight range.
In order to improve the serious situation of ladle sticky slag, the following measures can also be taken:
- It is necessary to strictly control the number of turnover ladles and increase the turnover rate of red hot ladles.
- Strengthen the maintenance operation of the ladle to clean the slag along with the ladle in time to prevent the ladle slag from falling down after the rim of it is looped. Repair the obvious melting loss and peeling parts of the wall in time to avoid the penetration of slag and molten steel and intensify the sticky slag;
- Pour slag as soon as possible after pouring steel. Strengthen the production organization of the driving behind the furnace, reduce the time from it pouring to can turning, and avoid the phenomenon of ladle sticking.
- Improve the operation level of converter steel tapping slag, reduce the converter slag entering it, add lime after the furnace and during refining, and fully melt its slag.
- Control the number of ladles in use; reduce the waiting time of it; heat the ladle before the steel; use it cover during use; use the heat insulation layer for the permanent layer of the ladle, and use the low thermal conductivity refractory material for the wall;
- Use effective ladle covering agent: improve the spreadability of the ladle covering agent and improve its thermal insulation performance; reduce the content of SiO2 in the covering agent, reduce its viscosity and reduce the ladle sticky slag.