The chromium drainage sand for ladles made of chromite as the main raw material by adding lubricants, sintering aids and reducing agents is safe and reliable to use, with an average automatic pouring rate of 92.5%.
Keywords: chromite; drainage sand; ladle
Bengang Special Steel Plant has a relatively large proportion of vacuum heats, so the self-opening rate of the ladle nozzle is correspondingly low. The self-opening rate is less than 70% due to various reasons such as: problems with material selection, particle gradation, and adding operations. The low self-opening rate makes it difficult to achieve oxidation-free pouring during continuous casting, pollutes the molten steel, burns the nozzle, and even affects the smooth flow of production. To solve this problem, the formula has been adjusted many times. Finally, a self-opening rate of 90%-92% was achieved, providing the necessary conditions for smooth production and improvement of billet quality.
Refractory materials for ladles
The walls and bottom of the cladding are made of magnesia carbon bricks, while the seat bricks and upper nozzle bricks are made of corundum. The upper surface of the seat brick is 30 mm lower than the upper surface of the bottom brick. The shape of the inner cavity of the nozzle is cylindrical with a diameter of 55 mm.
Analysis of operating environment of diversion sand
Selection of drainage sand raw materials
According to the principle of diversion and pouring and the requirements for diversion sand, the diversion sand should have good fluidity and chemical stability, moderate particle size, good compaction and reasonable high temperature performance. To this end, a material with reasonable particle gradation, moderate melting point, the ability to produce extremely high viscosity liquid phase under long-term use at high temperatures, and good thermal insulation should be selected.
Residence time of molten steel in the ladle
According to statistics, when the lifting time from the completion of tapping to the start of pouring is less than 120 minutes, the self-opening rate is the highest; when it is greater than 120 minutes, the self-opening rate decreases as time increases. The average self-opening rate of heats without vacuum treatment is 4.64 percentage points higher than that of VD vacuum treatment heats, mainly because the time is longer. Caused by thickening of the sintered layer.
Influence of steel type
The influence of steel type is mainly reflected in the liquidus temperature. Bearing steel GCr15 has the lowest self-opening rate, which is 12 percentage points lower than the average self-opening rate. The main reason is that the temperature in the early stage of smelting is almost the same as that of other steel types, and the amount of liquid phase formed by the drainage sand is also similar, but the temperature at the start of pouring is 30-50°C different. At this time, the liquid phase is reduced by a part compared with other steel types. This part of the liquid phase solidifies, the strength increases, and the self-opening rate decreases.
Nozzle burning and cleaning operation
The nozzle burning and cleaning operation includes: burning and cleaning the inner cavity of the nozzle, the surface of the nozzle seat brick, and the residual steel, residue, fire mud and other foreign matter around the nozzle seat brick. If the burning and washing are not clean, the molten steel will penetrate into the drainage sand, causing the drainage sand to float on the slag layer, thickening the sintered layer, and eventually causing the pouring to stop automatically.
Sand adding operation
The drainage sand is added using a funnel. The location at the bottom of the funnel, the amount of sand added, and the lifting speed of the funnel are the key factors that determine the distribution and density of the drainage sand. When using, use a long tube funnel to add it vertically, and slowly and evenly lift the funnel up. The amount added per furnace is 30-50kg (the standard is to cover the four corners of the nozzle brick and present a spherical crown with an arch height of 100-150mm).
Additives before tapping
Before tapping, add alloy, aluminum ingots, lime and other materials into the ladle. They must be operated according to the regulations to prevent them from falling on the drainage sand above the nozzle seat brick and affecting the drainage effect.
Preheating of the nozzle and slide plate before adding sand
If the nozzle and slide plate are not sufficiently preheated, adverse consequences such as poor drainage of the drainage sand and thickening of the sintered layer will occur.
Analysis of drainage sand material
Drainage sand is used as the filling material for the nozzle at the bottom of the ladle. On the one hand, it is required to be sintered at a lower temperature to avoid accidents caused by the floating sand when adding molten steel. On the other hand, it is required that the steel facing surface cannot be sintered too thickly under high temperature and long-term refining conditions, otherwise the automatic pouring rate will be affected. Thus affecting normal smelting operations. However, traditional drainage sand such as siliceous or magnesia is obviously difficult to meet the needs of automatic drainage under high temperature or long-term refining conditions. For this reason, chromium drainage sand was developed. Chromium diversion sand refers to ladle diversion materials made mainly of chromite and adding an appropriate amount of performance modifiers.
Basis of ingredients
The drainage sand is often in a high temperature state for a long time in the ladle nozzle, so the refractoriness of the sintered layer should be high. Since the drainage sand is filled into the nozzle and directly receives steel, the initial sintering temperature should not be too high. The drainage sand is subject to a large static pressure of molten steel during use. In order to prevent the shed material from being shed, a lubricant needs to be added to reduce the friction between the drainage sand particles. At high temperatures, chromite can form secondary spinels under the action of reducing agents to expand in volume after being desolvated, which is conducive to improving the automatic pouring rate, so reducing agents need to be added.
The starting sintering temperature, refractoriness, angle of repose and volume stability are the main indicators to measure the quality of drainage sand. The size of the angle of repose directly reflects the flow performance of the sample. The study found that when the iron content in the drainage sand is constant, the volume increase caused by reduction increases with the increase of Cr2O3 content and decreases with the increase of Al2O3 content. After testing and analyzing the drainage sand, it was determined that the formula in Table 1 can meet the usage conditions.
Table 1 Composition of drainage sand
Mechanism of use of drainage sand
In the drainage sand, the iron in chromite mostly exists in the form of FeO. At high temperatures, part of Fe2+ is rapidly oxidized and becomes Fe3+. The increased Fe3+ exists in the form of R2O3 or Fe(Fe2O4), which will be dissolved out when cooled. In the early stage of use, due to sufficient oxygen supply, desolvation is more likely to occur in cracks and holes. Since an easily oxidizable reducing agent is added to the drainage sand, the oxygen partial pressure between the drainage sand rapidly decreases at high temperatures and causes the Fe2O3 solid solution to undergo a reduction reaction, which causes a large volume expansion and makes the sintered layer brittle and porous. Chromite often contains a certain amount of gangue, that is, various silicate inclusions. During the heating process, the gangue in chromite not only undergoes physical and chemical changes itself, but also reacts with chromite. The original composition was changed to form secondary spinel. Its changes can be expressed by the following reactions:
n(Fe, Mg) R2O4+ O2→(n-4)(Fe, Mg) R2O4+2Fe2O3+4R2O3 (1)
2Fe2O3+C→4FeO + CO2 (2)
4FeO+4R2O3→4FeR2O4 (secondary spinel) (3)
In the formula: R represents a trivalent cation, C represents a reducing agent, n≥4.
(1) is a desolvation reaction, and (2) is a reduction reaction.
Since additives such as sintering aids are added to the drainage sand, the effect of secondary spinel is more obvious at high temperatures. The sintering aid has a low melting point and its main function is to react with raw materials such as chromite ore. Due to the small amount added, it reacts with chromite at high temperatures to form a brittle new phase with higher refractoriness, which plays a bonding role between chromite particles. When the slide is opened, due to the presence of lubricant, there is no sintered drainage sand in the lower part of the nozzle, and it flows out quickly by its own gravity, allowing a large amount of cold air to enter, causing great thermal stress on the upper and lower sides of the sintered layer of the drainage sand. Thermal stress causes cracks in the brittle sintered layer to expand rapidly, and the sintered layer is completely destroyed under the static pressure of the molten steel, thus achieving the purpose of automatic diversion.
5.1. The chromium drainage sand, which is mainly made of chromite and added with lubricants, sintering aids and reducing agents, is safe and reliable to use, with an automatic pouring rate of 92%.
5.2. Under the conditions of use, FeO in chromite reacts to dissolve and form secondary spinel, which changes the volume of the sintered layer and causes cracks. This is the main reason for the high automatic pouring rate of the drainage sand.
5.3. The material selection of the drainage sand should comply with the refining process requirements of Taiwan. The critical particle diameter of the drainage sand should match the diameter of the nozzle. The diversion sand has an appropriate amount of liquid phase and a thin sintered solid phase layer at high temperatures, which is the prerequisite for the nozzle to open automatically, and the sintered layer should be as far away from the slide plate as possible. Correct sand adding operation and related operations are necessary conditions to improve the self-opening rate of the nozzle.