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23 Common questions about graphite electrode in electric furnace steelmaking

Views: 4022     Author: Metal Technical      Publish Time: 2020-11-03      Origin: Site

Electric arc furnace steelmaking is in the ascendant in iron and steel production enterprises. At present, among the three methods of open hearth steelmaking, converter steelmaking and electric arc furnace steelmaking, the proportion of electric furnace steel reaches about 55%. Electric arc furnace steelmaking uses graphite electrode as conductive material. The consumption of graphite electrode in EAF steelmaking depends not only on the quality of electrode, but also on steelmaking operation and management level. This paper analyzes the use problems accumulated in the service of graphite electrode users in our factory over the years one by one, in order to be beneficial to the use and operation of graphite electrode in electric arc furnace steelmaking.


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01. What are the main factors affecting the consumption of graphite electrode in EAF steelmaking?



It mainly includes: (1) loading quantity and feeding method. (2) Feeding time and power-off time. (3) Smelting cycle. (4) Exhaust gas emission and dust removal system. (5) Electrode adjustment quality. (6) Load regulation quality. (7) Oxygen blowing operation. (8) Electrode connection quality. (9) Quality of electrode connector body. (10) Machining accuracy of electrode joint hole and joint.



02. What should be paid attention to in the storage of graphite electrode in steel plant?



Electrodes and joints shall be stored on clean cement floor to avoid electrode damage or soil adhesion; Do not remove the packaging of the temporarily unused electrode to prevent dust and sundries from falling on the joint thread or the end face of the electrode and the internal thread of the electrode hole. Electrodes shall be placed neatly in the warehouse. Both ends of the electrode stack shall be padded with anti-skid stacks. Generally, the stacking height of electrodes shall not exceed two meters. The stored electrodes shall be protected from rain and moisture to avoid cracks and accelerated oxidation during steelmaking. Do not store the electrode connector near the high temperature to prevent the connector from thrombolysis and overflow.



03. How to avoid electrode breaking and tripping during steelmaking?



During steelmaking, the following measures can be taken to effectively avoid electrode breaking and tripping: (1) the electrode phase sequence is correct and counterclockwise. (2) Scrap steel is evenly distributed in the steel furnace, and large scrap steel shall be placed at the bottom of the furnace as far as possible. (3) Avoid non-conductive materials in scrap. (4) The electrode column shall be aligned with the furnace top hole, and the electrode column shall be parallel. The furnace top hole wall shall be cleaned frequently to avoid the electrode breaking due to the accumulation of residual steel slag. (5) Keep the tilting system of electric furnace in good condition to keep the tilting of electric furnace stable. (6) The electrode holder shall not be clamped at the electrode connection and the electrode joint hole. (7) Select joints with high strength and high machining precision. (8) The torque applied during electrode connection shall be appropriate. (9) Before and during the electrode connection, prevent the electrode hole thread and joint thread from mechanical damage. (10) Prevent steel slag or foreign matters from being embedded in the electrode hole and joint to affect the screw connection.



04. What should be paid attention to when using graphite electrode in steel plant?



No matter the electrode is transported by forklift or crane, it should be operated carefully. In the process of electrode lifting, the damage of electrode end and thread will cause serious problems for the use of electrode, especially the thread hole and joint thread should be protected. When lifting the electrode, there shall be a protective pad to avoid damaging the electrode end face and joint thread.



05. How to connect the electrodes correctly?



When connecting, blow out the electrode hole, electrode end face and joint with compressed air. There shall be no dust and foreign matters embedded. The joint shall be kept clean and flat. When the two electrodes are screwed in to a certain extent (the gap is about 10mm), blow them with compressed air once, and then tighten the electrodes with torque pliers. The torque applied during tightening shall be appropriate, If a gap is found at the connection after tightening, it must be withdrawn and reconnected until there is no gap.



06. Correct clamping position of electrode holder



The electrode holder shall not be clamped at the connection of the electrode and the threaded hole of the electrode, but shall be clamped between the white lines at both ends of the electrode. At the same time, before the holder clamps the electrode, blow out the electrode surface and the holder with compressed air to ensure good conduction of current and heat flow between the electrode and the holder and prevent damage to the holder caused by arcing, So as to prolong the service life of the holder.



07. What are the measures to reduce electrode oxidation consumption when making steel in electric arc furnace?



The main reduction measures are: (1) reduce the oxidation consumption around the electrode, strengthen the sealing of the furnace and reduce the intrusion of air into the furnace; Minimize the exposure time of red hot electrode outside the furnace and standardize oxygen blowing operation. (2) For the smelting furnace, if conditions permit, the electrode adopts spray cooling technology, which can effectively reduce the oxidation consumption on the side of the electrode. (3) The anti-oxidation performance of the electrode body can be improved by spraying anti-oxidation substances on the electrode surface in the steel factory or by the electrode manufacturer using anti-oxidation impregnation technology before the electrode leaves the factory.



08. What is the effect of electrode phase sequence on the use of electrode?



When making steel in electric arc furnace, the positive and negative electrode phase sequence has a significant impact on the tripping and breaking of the electrode during use. If the electrode phase sequence is clockwise, the electrode will loosen after being energized for a period of time, which is easy to cause electrode loosening or joint breaking. The correct electrode phase sequence should be counterclockwise, so that the electrode will be more and more closely connected in use.



09. Why is the phase electrode required to be in parallel and aligned with the top hole of the furnace cover when making steel in the electric arc furnace?



The electrode column shall be aligned with the top hole of the furnace cover. The electrode column shall avoid friction with the furnace cover, otherwise the friction with the furnace cover during lifting and lowering will break the electrode of the furnace cover. For AC furnace, the three-phase electrode column shall be kept parallel as far as possible.



10. How to apply torque during electrode screwing?



The torque applied during electrode screwing shall be appropriate and the operation shall be inertial. Too small torque will cause thermal loosening of the joint, and too large torque will cause cracking of the electrode joint hole. Special electrode screwing tools shall be used during screwing, and do not screw too tight and loose. If there is a gap between the end faces after tightening, it must be removed and cleaned before screwing again.



11. Why is graphite sling better than metal sling?



Although the metal spreader is durable and not easy to be damaged, the thermal expansion of the metal spreader is easy to expand and crack the electrode hole after being heated during use. At the same time, it is easy to damage the thread in the electrode hole when receiving the metal spreader, resulting in large-area scraping off of the thread in the hole, making the electrode easy to trip. Because the graphite spreader has the same thermal expansion performance and hardness as the electrode, The above-mentioned bad use will not occur, but the graphite spreader has a short service life and is easy to be damaged. If serious damage is found, it shall be replaced in time.



12. How to correctly select electrodes in EAF steelmaking?



According to the design characteristics of the electric arc furnace, reasonably select the electrodes that meet the production requirements of the electric arc furnace, and select the products with the best cost performance. It is very necessary to carefully select the electrode suitable for each furnace. The special performance of the steelmaking furnace, feeding mode, maximum current intensity, the length of the electrode column under the holder, the distance between the furnace side wall and the electrode circumference, etc. are all factors that must be considered when selecting the electrode for the electric arc furnace.



13. What is the effect of resistivity on the use of electrodes in steelmaking?



The resistivity of graphite electrode is a physical index reflecting the conductivity of electrode, which is related to the manufacturing process of electrode. The state has qualitative specified values for the resistivity of graphite electrode of different specifications and varieties. Generally speaking, when selecting electrodes of certain specifications, steel mills must choose within the resistivity range specified in national Metallurgical standards, Too high resistivity will make the electrode red and hot when energized, and increase the oxidation consumption of the electrode.



14. What is the effect of bulk density performance on the use of electrode in steelmaking?



The bulk density of graphite electrode reflects the dense state of the electrode, which is closely related to the manufacturing process of the electrode. The country has specified values for the bulk density of graphite electrodes of different specifications and varieties. Products with low bulk density indicate that the overall structural porosity of the product is high, and the oxidation speed of the product is fast at high temperature, which is easy to increase the electrode consumption. Generally speaking, when selecting electrodes in steel plants, it is better for the electrode bulk density to be larger within the specified value, but the higher the bulk density is, the better, because some electrodes with high bulk density sometimes have poor thermal shock resistance, During steelmaking, surface peeling, block falling and cracks are easy to occur, which will affect steelmaking.



15. Why should steel mills prevent the mixing of multiple products when using graphite electrodes?



The graphite electrodes used in steel plants are often supplied by multiple manufacturers. When multiple products are mixed in steelmaking, it will not only make it difficult for the steel plant to count the consumption of a single product, but also due to the different raw materials and manufacturing processes adopted by various manufacturers, the physical and chemical properties and processing tolerances of electrodes and joints of various manufacturers are different. Therefore, the matching tolerance generated during mixed use is easy to cause the electrode to fall off and break. The correct way to use is to use the products of one manufacturer alone and then connect the products of another manufacturer. The number of electrode replacement from different manufacturers should be reduced, and the electrodes of the same manufacturer should use the connectors matched with the same manufacturer to prevent mixed use.



16. What are the characteristics of needle coke?



Needle coke is a kind of high-quality carbon raw material, which is divided into coal series and oil series. Its surface shows obvious strip lines. Most of them are long needle fragments when broken. The fibrous structure can be observed under the microscope, so it is called needle coke. Needle coke is easy to graphitize at a high temperature of more than 2000 degrees. The graphite electrode made of needle coke not only has low resistivity, high bulk density, but also has low thermal expansion coefficient. It is a necessary raw material for the production of ultra-high power electrode and high power electrode. The price of needle coke is much more expensive than that of ordinary coke, which is about 5-8 times higher at present.



17. Will the dust collection system on the electric arc furnace affect the use and consumption of electrodes?



The fan used in the dust suction system generates a certain negative pressure during operation. During steelmaking, the air flow rate around the red hot electrode increases, which also increases the oxidation consumption of the electrode. During steelmaking, a well adjusted dust suction system not only maintains a good working environment, but also makes the consumption of the electrode stable.



18. How to avoid increasing electrode consumption during steelmaking?



To avoid the increase of electrode consumption during steelmaking, we should: (1) maintain a good power supply state and transmit power within the allowable current intensity of the electrode according to the design requirements of the electric furnace. (2) Prevent the arcing point from being immersed in the molten pool. (3) Prevent the electrode from being immersed in molten steel to increase carbon. (4) If conditions permit, spray cooling technology shall be adopted for the electrode. (5) Set up the correct exhaust emission system. (6) Adopt correct oxygen blowing system.



19. How long does the production cycle of graphite electrode take?



The production process and corresponding time of a batch of ultra-high power or high-power graphite electrodes are as follows: electrode pressing (3 days) - roasting (25 days) - impregnation (4 days) - re roasting (15 days) - graphitization (10 days) - machining and quality inspection (2 days) - packaging and delivery of finished products (1 day), that is, from feeding to product delivery, The fastest production cycle without stopping is 60 days, and the production cycle of electrode joint is 90 days because it needs more two dipping and three baking treatment than the electrode.



20. What are the characteristics of electrodes produced by series graphitization furnace?



The development direction of graphitization furnace is internal heat series graphitization furnace. Because the current density of series connection column is the same, the difference of electrode resistivity is very small; Secondly, the resistivity at both ends of the inner series graphitized product is slightly lower than that in the middle (the resistivity at both ends of the graphitized product of Acheson furnace is higher than that in the middle), which is conducive to reducing the resistance at the connection and alleviating the overheating and redness at the connection. Therefore, the electrode quality uniformity produced by the series graphitized furnace is better than that of Acheson furnace, It is more suitable for the production requirements of electric arc furnace steelmaking.



21. Why does the quality of electrode joint play an important role in EAF steelmaking?



The joint plays a key role in electrode steelmaking. The quality of the joint is directly related to the use of the electrode in electric furnace steelmaking. No matter how good the quality of the electrode is, if there is no high-quality joint to match it reasonably, there will be problems in steelmaking. According to relevant data, in electric furnace steelmaking, more than 80% of electrode use accidents are caused by joint breaking, loosening and tripping. Therefore, the selection of high-quality electrode joint is the guarantee condition for the normal use of electrode in electric arc furnace steelmaking.



22. What quality indexes of graphite electrode (joint) products have an impact on electric furnace steelmaking?



(1) The bulk density, resistivity, strength, elastic modulus and thermal expansion coefficient of the electrode. (2) Quality indexes such as bulk density, resistivity, strength, elastic modulus and thermal expansion coefficient of the joint. (3) The machining accuracy of electrodes and joints. No matter how good the quality of electrodes and joints is, without good machining accuracy (mainly referring to the cooperation between electrodes and joints), the use effect is not good. (4) The internal structure quality of electrode and joint requires that there are no cracks causing hidden dangers.



23. What are the consequences of serious oxidation of the electrode end face at the upper end of the electrode holder?



During steelmaking in the smelting furnace, the scrap is burned in the furnace. At the same time, due to oxygen blowing in the furnace, the height of the flame column is often higher than the electrode end face at the upper end of the holder, which is easy to oxidize the electrode end face. If the oxidation is serious, the electrode end face will be deformed from a plane to an inclined plane. When a new electrode is connected at the upper end, the oxidation deformation of the lower electrode end face can not be in good contact with the new electrode, The electrode gap is large, which is easy to cause oxidation and fracture of the internal joint. Without changing the steelmaking conditions, the best preventive measure is to add a protective cover on the electrode end face at the upper end of the holder to block the flame and air, so as to achieve the purpose of protecting the electrode end face.


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