Thermal simulation of the chemical heating method of molten steel was carried out in a 16kW vertical molybdenum disilicide rod furnace and the influence of the alkalinity of the molten steel top slag on the quality of the molten steel and the corrosion of the ladle refractory material was studied when calcium silicate barium was used as the exothermic agent. The results showed that , when the element content in the steel remains basically unchanged, the inclusions do not increase significantly;. Increasing the alkalinity of the top slag can effectively prevent rephosphorization and resulfurization, and has the effect of dephosphorization and desulfurization, and the erosion of the refractory materials of the ladle by the top slag is not serious.
Keywords: molten steel; chemical heating method; silicon calcium barium heating agent; dephosphorization; desulfurization; refractory materials
Out-of-furnace refining and continuous casting technology are playing an increasingly important role in steelmaking production and have become an important link in modern metallurgical processes. In this process change, the control and adjustment of molten steel temperature have become a prominent issue. In recent years, various molten steel reheating technologies have emerged, among which the chemical heating method is easy to operate. The advantages of fast heating speed, low investment and low cost have been valued, and it has been widely used in major steel plants at home and abroad. However, judging from the current application of the chemical heating method of molten steel, aluminum is basically used as the exothermic agent. Since the residual acid-soluble aluminum in the molten steel is more than 0.010%), it is easy to form nodules in the nozzle of the billet continuous caster. It affects the production of continuous casting and it is also difficult to control the composition when producing low-aluminum and aluminum-free steel grades. This limits the promotion and application of the chemical heating method of molten steel. Therefore, it is very necessary to study non-aluminum heat-generating agents-calcium silicon barium heat-generating agents. The author of this article studied the effect of heating molten steel with calcium silicon barium exothermic agent on the content and inclusions of elements such as phosphorus and sulfur in the molten steel, as well as the erosion of the refractory material of the ladle. The purpose is to provide a basis for the industrial application of calcium silicon barium exothermic agent.
The experiment uses calcium silicate and barium as the exothermic agent, with a particle size in the range of 2 to 5 mm. A steel pipe with a diameter of 20 mm is used to put the exothermic agent into the slag-free area of the steel surface, and then blow oxygen.
A vertical high-temperature silicon-molybdenum rod furnace with a power of 16 kW was selected. Corundum furnace tube is used, and nitrogen gas is used to protect the crucible in the furnace. Standard Pt rh6 – Pt rh30 thermocouples and potentiometers are used for temperature calibration, and a DWT-702 temperature control cabinet and double platinum-rhodium thermocouples are used for continuous temperature measurement. The experimental device is shown in Figure 1.
1—DWT-702 control cabinet; 2—Digital temperature display; 3—Temperature control thermocouple; 4—Experimental crucible; 5—Temperature measuring thermocouple; 6—Oxygen lance; 7—Gas flow meter; 8—Gas pressure gauge
Figure 1 Schematic diagram of experimental device
Experimental raw materials
1. The steel material composition (mass fraction, %) is: C 0. 11, Mn 0. 26, Si 0. 18, S 0. 042, P 0. 037.
2. Gas: The protective gas uses nitrogen with a purity of 100%, and the top blowing gas uses oxygen with a purity of 100%.
3. Slag material: The slag used in the experiment is the top slag before argon blowing from a second steelmaking plant. Its composition (mass fraction, %) is: CaO 38.08, SiO2 17.7, MgO 7.38, FeO 12. 0, P2O5 0.75. The amount of slag is 1% of the amount of steel material.
4. Heating agent: Use silicon-calcium-barium alloy with a silicon content of 45.5%, a calcium content of 18.2%, a barium content of 15.7%, and FeO.
2 kg of steel material was used in each experiment, and the initial temperature of the molten steel in the experiment was 1580°C. The amount of exothermic agent added in the experiment is calculated to raise the temperature of 2 kg of molten steel by 30°C. Change the alkalinity of the slag and test the effect of calcium silicate and barium heating agent on the quality of molten steel and the corrosion degree of refractory materials.
Since the silicon content in the calcium silicate barium heating agent is higher than that of calcium and barium, the amount of SiO2 formed after oxygen blowing is larger, resulting in a decrease in the alkalinity of the slag and a decrease in the dephosphorization and desulfurization capabilities of the slag. To improve this situation, lime is added to the slag. The alkalinity in Experimental Schemes 2 and 3 in Table 1 is the alkalinity calculated assuming that the calcium silicate and barium heating agent is completely oxidized (regardless of the oxidation of elements in the steel) and calculated together with the top slag and added lime.
Table 1 programe of experiment
|serial number||Experimental conditions|
|1||Add top slag, add heating agent, and directly blow oxygen to raise the temperature|
|2||Add top slag, add heating agent, add lime to make the alkalinity of top slag reach 2.46, blow oxygen to raise the temperature|
|3||Add top slag, add heating agent, add lime to make the alkalinity of top slag reach 3.0, blow oxygen to raise the temperature|
Experimental results and analysis
Effect on phosphorus, sulfur and other components in molten steel
Phosphorus and sulfur
Phosphorus and sulfur in molten steel are the main factors affecting the quality of steel. Changes in phosphorus and sulfur components have a great impact on the hot and cold brittleness of steel. Therefore, when studying the effect of calcium silicon barium exothermic agent on steel quality, the control of phosphorus and sulfur content in molten steel, especially the control of phosphorus and sulfur return, is the focus of this experiment.
After the heating agent is added to the molten steel, oxygen is blown to raise the temperature. After the temperature rise is completed, samples are taken for component analysis. The changes in phosphorus content and sulfur content in molten steel after adding calcium silicon and barium and heating it up are shown in Figure 2. It can be seen that using calcium silicate and barium as the exothermic agent, and adding a small amount of alkaline substances, such as lime, during the heating process can reduce phosphorus return. When the alkalinity of the slag is 2.46, the phosphorus content in the steel increases slightly, but when the alkalinity is 3.0, the phosphorus content basically does not increase. Experiments have been conducted on using calcium silicate and barium as the exothermic agent without adding lime, and found that there is more phosphorus back (generally 0. 005% ~ 0. 010%). It can be seen from the changes in sulfur content in Figure 2 that when lime is added to increase the alkalinity of the slag to 2.46 and 3.0, the sulfur content is lower after heating, which is significantly lower than that without adding lime.
Figure 2 Changes of phosphorus content and sulfur content in molten steel with slag alkalinity
Silicon, carbon and manganese
The contents of silicon, carbon and iron in the molten steel before and after oxygen blowing and heating are shown in Table 2. It can be seen from Table 2 that the silicon, carbon and manganese content in the molten steel did not change much before and after adding silicon-calcium-barium heating agent and oxygen blowing, and the composition of the steel ingot basically met the target value requirements. This shows that the silicon-calcium-barium exothermic agent has basically no effect on the silicon, carbon and manganese content in the molten steel during the chemical heating process.
Table 2 Contents of silicon, carbon and manganese in molten steel before and after adding heating agent and blowing oxygen
|serial number||Before oxygen||After oxygen|
Impact on inclusions in steel
Use XQF2000 image analyzer to analyze inclusions in steel. The analysis process is as follows:
Place the sample on the stage → XJZ-6 upright microscope → CCD camera → scanner → AS1000 platform controller → computer analysis → frozen image → shadow correction → feature extraction → calculate the area fraction of particle inclusions.
The calculated inclusion content in steel is listed in Table 3, and the photos of inclusions processed by the camera and scanner are shown in Figure 3. The change of inclusion content in steel with slag basicity is shown in Figure 4. When calcium silicon barium exothermic agent is used, the oxide inclusions, silicate inclusions and alumina inclusions in the steel decrease as the slag basicity increases, especially the changes in oxide inclusions are more obvious. Because the original product of calcium silicon barium alloy reacts with oxygen is barium silicate, which is easy to eliminate, and a small amount of Al₂O₃ can be dissolved in it, so when using calcium silicon barium heat generating agent, the inclusion content in the steel is less.
Impact on corrosion of ladle refractory materials
Erosion of magnesium carbon cylinders
During the experiment, a magnesium carbon rod with a diameter of 24 mm was put into the molten steel after adding the top slag, and was taken out during tapping and pouring to simulate the erosion of the magnesium carbon crucible lining by the molten steel. Figure 5 is a photo of the magnesium carbon rod before erosion and after erosion. It can be seen from Figure 5(b) that the corrosion on the surface of the magnesium carbon rod is relatively serious. The surface of the intact magnesium carbon rod is relatively smooth. After being corroded by four furnaces of molten steel (mainly steel slag), the surface of the magnesium carbon rod appears very obviously uneven, indicating that the surface has been severely damaged.
Observing the cut section of the magnesium carbon rod, we found that there were no major changes inside the magnesium carbon rod, and basically only the surface was eroded. The radius of the intact magnesium carbon rod is 12 mm, while the radius of the magnesium carbon rod after four furnaces of steel slag erosion is about 9mm, a decrease of 3mm. The average steel slag erosion per furnace of the lining is about 0.75 mm thick. Therefore, the erosion of the magnesium carbon furnace lining by the silicon-calcium-barium exothermic agent is not serious. The main reason is: in order to prevent phosphorus and sulfur reversion, the alkalinity of the slag is increased, and a large amount of calcium oxide is added to the slag to make the slag alkaline. The erosion of the alkaline magnesium carbon crucible by the alkaline slag is very small. Experiments have proven that magnesium carbon crucible has strong corrosion resistance.
Table 3 Area fraction of inclusion content in copper
|serial number||oxide||silicate||aluminum oxide||total inclusions|
Fig.3 Distributlon offaclusion in steel with Si-Ca-Ba bealng
Figure 4 Changes of inclusions in steel with slag basicity
Figure 5 Magnesium carbon rod before erosion (a) and after erosion (b)
Erosion of magnesium-aluminum spinel crucible
The castable material of the magnesium-aluminum spinel crucible was taken from a certain steelmaking plant. The erosion of the inner wall of the crucible during the laboratory melting of molten steel is shown in Figure 6. It has been determined that the average erosion rate is about 2 mm/furnace. The erosion is more serious than the magnesium carbon furnace lining, but it does not exceed the average erosion rate of the refractory material lining the ladle of a second steelmaking plant (1. 88mm/furnace).
There may be several reasons for this situation:
① The added slag contains FeO. FeO easily reacts with the acidic oxides and basic oxides in the crucible to form low melting point compounds. This is the main cause of crucible erosion;
②The main components of the magnesium-aluminum spinel crucible are MgO and Al2O3, and it is an alkaline crucible. The acidic oxides (SiO2 ~ P2O5, etc.) in the slag react with the alkaline oxides in the crucible, causing erosion of the inner wall of the crucible. This is the secondary cause of crucible erosion;
③The slag contains some alkaline oxides (CaO ~ MgO). These alkaline oxides react with the Al2O3 in the crucible and cause slight erosion of the crucible.
Fig. 6 Inside wall shape of mg-Al crucible after erosion
(1) When using silicon calcium barium heating agent, the method of adding alkaline substances to increase the alkalinity of the slag should be used to prevent phosphorus reversion, and sulfur reversion can improve the dephosphorization and desulfurization effects of the slag.
(2) When using silicon calcium barium exothermic agent, the erosion of magnesium carbon refractory materials is not serious.
(3) When using calcium silicon barium exothermic agent, changing the properties of the top slag by adding CaO to the top slag can basically ensure that the inclusion content in the steel is not increased.
(4) When using calcium silicon barium heating agent, the content of silicon, carbon and manganese in molten steel can be well controlled.