Electric arc furnace steelmaking process

1 Introduction Electric arc furnace steelmaking process limits the production of low-nitrogen steel in electric arc furnace due to its own characteristics. At present, China's special steel enterprises include some ordinary steel enterprises, which have formed the modern electric arc furnace process production of EAF LF/VD CC. Form, so how to take measures to control the nitrogen in the molten steel in the modern electric arc furnace steelmaking process, so that the nitrogen content in the electric arc furnace process product reaches the level equivalent to the converter process, in order to increase the electric furnace steel variety and improve the electric arc furnace process products. Market competitiveness is a concern of current EAF steelmaking. To this end, this paper studies some measures and methods of nitrogen control in the modern electric arc furnace steelmaking process through field experiments.

2 The change of nitrogen in the arc furnace process steel 2.1 The change of nitrogen The nitrogen increase of the molten steel in the electric arc furnace mainly includes three aspects: nitrogen increase in the arc zone, nitrogen in the atmosphere, and nitrogen in the raw material. The denitrification is mainly a bubble carrying method using a C-O reaction.

2.1.1 Nitrogen addition research in the arc region shows that the non-deoxidized steel liquid, due to the surface activity of oxygen, hinders the nitrogen absorption of the molten steel, so the bare nitrogen absorption of the molten steel does not occur during the electric arc furnace smelting. In the case where the raw materials are basically the same, the electric arc furnace smelting process is mainly to increase nitrogen in the arc zone. When the electrode is heated, the cathode and anode of the arc are alternately located on the graphite and molten steel. When working on molten steel, this part of the molten steel has a higher temperature than other parts of the molten steel. When it is a cathode, the temperature is 2400K, which is 2600K for the anode, and when the molten steel temperature exceeds 2130 °C, the effect of oxygen on nitrogen Disappearance When the partial pressure of nitrogen is constant, the solubility of nitrogen in the molten steel is related to the nitrogen dissolution reaction constant and its activity coefficient. As the temperature increases, the solubility of nitrogen in the molten steel increases. Therefore, foaming slag must be made in the process of electric arc furnace smelting to prevent the molten steel surface from being exposed in the arc zone and reduce the nitrogen increase in the arc zone.

2.1.2 Effect of electric arc furnace with molten iron on nitrogen in molten steel In the electric arc furnace, high carbon and large oxygen supply are generally used to improve the denitrification capacity of the electric arc furnace. For this reason, different amounts of molten iron are added to the steel on the 90tConsteel electric arc furnace. The effect of liquid nitrogen content, as the result, as the amount of molten iron increases, the nitrogen in the molten steel decreases almost linearly. Compared with the addition of molten iron and iron-added water greater than 40t, the nitrogen content of the tapping steel differs by nearly 20×10-6. The nitrogen content of the molten pool is closely related to the amount of decarburization or the amount of CO produced in the molten pool. The violent carbon-oxygen reaction in the molten pool can effectively reduce the nitrogen content during tapping, the decarburization amount is 0.40%-0.65%, and the tapping nitrogen content can be reduced to -6[3]. When the carbon is low, the oxygen blowing process forms bubbles. The equilibrium partial pressure of CO is low. The amount of CO increases. The amount produced is small, and the increased nitrogen of the molten pool from the electrode zone rises, so the high carbon content of the molten iron is advantageous for denitrification of the molten steel.

2.1.3 The nitrogen content of the nitrogen-increasing arc of the arc furnace tapping process is basically the same as that of the bottom-blowing argon. The change of nitrogen content in the tapping process is studied on a 90tConsteel arc furnace. According to the average value of the multi-furnace in the production test, the nitrogen content at the bottom of the electric arc furnace is 56×10-6, and the nitrogen content at the bottom of the electric arc furnace is 69.5×10-6. The average nitrogen content during argon injection is 66.1×10-6. The nitrogen content of the molten steel in the bottom blowing nitrogen and the bottom blowing argon are increased by 2.2LF, respectively. The change of nitrogen content in the molten steel is currently refined. A preferred level of nitrogen is less than. It is generally believed that the main reason for nitrogen increase is the contact between molten steel and the atmosphere, gas ionization in the arc zone, and nitrogen in raw materials, but neglecting the power supply system will also affect it. The changes in nitrogen content in the refining process were studied from the nitrogen absorption and LF power supply systems of deoxidized steel by field test.

2.2.1 Effect of deoxidation on nitrogen absorption of molten steel The change of nitrogen content in molten steel from tapping to LF/VD was studied at 60tLF. The nitrogen in the molten steel from the tapping to the feeding of aluminum increased rapidly. The average nitrogen in the steel to steel increased by 17.5×10. After the aluminum was fed, the nitrogen in the molten steel increased by an average of 16.3×10-6. After the aluminum was fed, The liquid nitrogen of the LF steel hardly changes. The main reasons for these changes are as follows.

(1) The process of tapping strengthens the deoxidation of deoxidizer and desulfurization of desulfurizer, and the dissolved oxygen decreases rapidly. The dissolved oxygen in the LF is less than 15×10, and the argon is stirred during the tapping process, causing the molten steel surface to be exposed, so the nitrogen rises quickly.

(2) After entering LF, in order to slag and further uniform composition and temperature, it is still necessary to carry out higher power stirring, and at the same time, in order to reduce the dissolved oxygen in the molten steel to 3 as early as possible, and to ensure sufficient time for the inclusions to float up. After the slag is formed, the aluminum wire is operated. After the wire is fed, the dissolved oxygen is less than 5×10-6. The feeding process, the operation after the LF to the feeding line, and the low dissolved oxygen all create conditions for the molten steel to absorb nitrogen. Therefore, this process has a large amount of nitrogen uptake.

(3) After feeding the line to the LF stage, on the one hand, at this time, due to tapping and LF addition of slag, the amount of slag is large and the molten steel is well covered. On the other hand, at this time, there is no large agitation necessary for carbon addition and a large amount of alloying, so that the molten steel is not exposed, so the amount of nitrogen increase is small. The deoxidized steel liquid has a serious nitrogen absorption, and it should be avoided from contact with the atmosphere during the LF refining process.

2.2.2 The influence of the power supply system on the nitrogen content of molten steel is more than 0.035% in the case of high sulfur content. Due to the surface activity of sulfur, the contact between molten steel and the atmosphere is hindered. In this case, different studies have been studied. The situation of nitrogen increase in molten steel under the power supply position. The lower the power supply position, the more serious the nitrogen increase is. The maximum nitrogen increase in the test is; the higher the gear position, the smaller the difference in nitrogen increase, such as 4-8 gear (ie between 4 and 8) The 8th gear is only 0.3×10. Under the condition of no electricity, the molten steel is covered by the slag and has no contact with the atmosphere. The thermodynamics and kinetic conditions of the molten steel are insufficient. At this time, the nitrogen absorption is very weak. The liquid has a certain protective effect.

If the LF heating is powered by high power, the molten steel will rapidly heat up in a short time, which can reduce the chance of arc ionization and nitrogen increase, and reduce the time of high temperature slag, which is beneficial to prevent nitrogen from entering the molten steel through the slag. In addition, if the high-power supply time is equivalent to the duration of the foam slag, the foam slag surrounds the arc, which can effectively improve the utilization of electric energy, reduce the radiation to the lining, and prevent arc ionization from increasing nitrogen. Therefore, the shorter the power supply time, the better the prevention of nitrogen increase in molten steel. However, if the foamed slag is not well made, the high-temperature slag at the time of high-power supply will be more likely to absorb nitrogen. In the field test, due to the lack of foaming slag, the arc exposure is severe, resulting in a lower power supply position, the higher the arc power, and the greater the nitrogen increase in the molten steel.

2.3 Changes in nitrogen content in the molten steel during continuous casting The continuous casting molten steel deoxidizes well and absorbs nitrogen as long as it is in contact with the atmosphere. Continuous casting tundish covering agent and mold protection slag have little or no effect on the nitrogen absorption of molten steel. In order to study the mechanism of molten steel nitrogen increase in continuous casting process, tests were carried out on 150t ladle and 28t tundish. The number of head blank inclusions is the highest, 1.3 to 1.76 times higher than the normal billet. There is a phenomenon of microscopic inclusion aggregation in the inner arc, quarter of the slab and the central region of the slab.

3.3.3 Source of inclusions in the slab The tracer is found in the typical inclusion probe. It is proved that there are ladle slag, intermediate slag and intermediate lining refractory into the crystallizer and solidified in the slab. . KO was also found in the inclusions, which proves that the casting process is fluctuated by the liquid level of the crystallizer, and the mold flux is entangled in the liquid phase of the molten steel and remains in the slab. It is indicated that the entrapment of ladle slag, tundish covering agent and mold flux slag and tundish refractory material is the main source of foreign inclusions in the slab.

3.4 Metallurgical effect after the implementation of the improvement measures According to the results of this test, corresponding measures have been taken in the production, such as optimizing the refining process, using the long nozzle to pour the ladle to the tundish, improving the immersion nozzle structure and insertion depth, and developing the high-efficiency ladle The drainage agent improves the self-opening rate of the ladle and prevents the slag from being poured into the tundish. After the improvement, the amount of inclusions in the steel is reduced, the cleanliness of the slab is improved, and the total oxygen content is reduced from 0.0069% to 0.0038%, and a significant effect is obtained.

4 Conclusions (1) The quantity, composition and evolution of inclusions in No. 45 steel of BOF-LF-CC process were found out, which provided a basis for further improving the process and improving the cleanliness of steel.

(2) Tracer results show that BaO and CeOO are contained in the micro-inclusions and large inclusions in the slab, indicating that the ladle slag, the tundish and the mold slag, and the intermediate lining refractory are in the slab. The main source of foreign inclusions.

(3) After the improvement measures are taken according to the test results, the total oxygen content and the number of inclusions in the slab are significantly reduced.