Electric arc furnace steelmaking is gaining momentum in steel production enterprises. At present, among the three methods of open hearth, converter and electric arc furnace steelmaking, the proportion of electric furnace steelmaking has reached about 30%. Electric arc furnace steelmaking uses graphite electrodes as conductive materials. The consumption of graphite electrodes in electric arc furnace steelmaking depends not only on the quality of the electrodes, but also on the steelmaking operation and management level. Now we analyze the usage problems accumulated by our factory’s graphite electrode user service over the years one by one, in order to benefit the use and operation of graphite electrodes in electric arc furnace steelmaking.
What are the main factors affecting graphite electrode consumption in electric arc furnace steelmaking?
The main factors are: (1) Charge quantity and charging method. (2) Charging time and power-off time. (3) Smelting cycle. (4) Waste gas emission and dust removal system. (5) Electrode adjustment quality. (6) Load adjustment quality. (7) Oxygen blowing operation. (8) Electrode connection quality. (9) Electrode joint body quality. (10) Electrode joint hole and joint processing accuracy.
What should be paid attention to when storing graphite electrodes in steel plants?
Electrodes and joints should be stored on a clean cement floor to prevent them from being damaged or stuck with dirt;
For electrodes that are not in use temporarily, do not remove the packaging to prevent dust and debris from falling on the joint threads or the electrode end surface and the inner threads of the electrode hole.
Electrodes should be placed neatly in the warehouse, and both ends of the electrode stack should be padded to prevent slipping. The stacking height of electrodes should generally not exceed two meters.
Stored electrodes should be protected from rain and moisture to prevent cracks and accelerated oxidation during steelmaking. Do not store electrode joints near high temperatures to prevent the joint plugs from melting and overflowing.
How to avoid electrode breakage and tripping during steelmaking?
The following measures can be taken during the steelmaking process to effectively avoid electrode breakage and tripping:
(1) The electrode phase sequence is correct and counterclockwise.
(2) The scrap steel in the steel furnace is evenly distributed, and large pieces of scrap steel are placed at the bottom of the furnace as much as possible.
(3) Avoid the presence of non-conductive materials in the scrap steel.
(4) The electrode column is aligned with the furnace top hole and the electrode column is parallel. The furnace top hole wall should be cleaned frequently to avoid residual slag accumulation and breaking the electrode.
(5) Keep the electric furnace tilting system in good condition to keep the electric furnace tilting stable.
(6) Avoid clamping the electrode holder at the electrode connection and the electrode joint hole.
(7) Select high-strength, high-precision and high-quality joints.
(8) The torque applied when connecting the electrode should be appropriate.
(9) Before and during the electrode connection, prevent the electrode hole thread and the joint thread from mechanical damage.
(10) Prevent steel slag or foreign matter from getting embedded in the electrode holes and joints and affecting the screw connection.
When using graphite electrodes in steel mills, what should be paid attention to during crane hoisting?
Whether the electrode is transported by forklift or crane, it must be carefully operated. During the electrode lifting process, damage to the electrode end and thread will cause serious problems in the use of the electrode. In particular, the thread of the threaded hole and the joint must be protected.
When lifting the electrode, a protective pad must be used to avoid damage to the electrode end face and the joint thread.
How to connect the electrodes correctly?
When connecting, use compressed air to blow clean the electrode hole, electrode end surface and joint. There must be no dust or foreign matter embedded. The connection must be kept clean and flat. When the two electrodes are screwed in to a certain extent (the gap is about 10mm), blow them again with compressed air, and then tighten the electrodes with torque pliers. The torque applied when tightening should be appropriate. If a gap is found at the connection after tightening, it must be withdrawn and reconnected until there is no gap.
Correct clamping position of the electrode holder
The electrode holder cannot be clamped at the connection point of the electrode and the threaded hole of the electrode, but should be clamped between the white lines at both ends of the electrode. At the same time, before the holder clamps the electrode, the surface of the electrode and the holder should be blown clean with compressed air to ensure good conduction of current and heat flow between the electrode and the holder, prevent arcing from causing damage to the holder, and thus extend the service life of the holder.
What are the measures to reduce electrode oxidation consumption in electric arc furnace steelmaking?
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 the red-hot electrode outside the furnace and standardize the oxygen blowing operation.
(2) For smelting furnaces, if conditions permit, the electrode adopts spray cooling technology, which can effectively reduce the oxidation consumption on the electrode side.
(3) Steel mills spray antioxidants on the electrode surface, or electrode manufacturers use antioxidant impregnation treatment technology before the electrode leaves the factory to improve the antioxidant performance of the electrode body.
What is the influence of electrode phase sequence on the use of electrodes?
During steelmaking in an electric arc furnace, the tripping and breaking of the positive and negative electrodes in the electrode phase sequence during use have a significant impact. If the electrode phase sequence is clockwise, the electrode will become loose after being energized for a period of time, which can easily cause the electrode to loosen or the joint to break. The correct electrode phase sequence should be counterclockwise, so that the electrodes will become more and more tightly connected during use.
Why is it required that the phase electrodes be parallel and aligned with the top hole of the furnace cover during steelmaking in an electric arc furnace?
The electrode column should be aligned with the top hole of the furnace cover. The electrode column should avoid friction with the furnace cover, otherwise the friction with the furnace cover during lifting and lowering will cause the furnace cover to squeeze and break the electrode. For AC furnaces, the three-phase electrode columns should be kept as parallel as possible.
How to apply torque when rotating the electrode?
The torque applied when the electrode is screwed should be appropriate and the operation should be smooth. Too little torque will cause the joint to loosen due to heat, and too much torque will cause the electrode joint hole to crack. When screwing, use a special electrode screwing tool. Do not screw too tight or too loose. If there is a gap in the end face contact after tightening, it must be removed and cleaned before re-screwing.
Why are graphite slings better than metal slings?
Although metal hangers are durable and not easy to damage, when heated during use, the thermal expansion of metal hangers can easily cause the electrode holes to burst. At the same time, when the metal hangers are taken off, the threads in the electrode holes can be easily damaged, causing large areas of the threads in the holes to be scraped off, making the electrode easy to disengage. Graphite hangers have the same thermal expansion properties and hardness as electrodes, so the above-mentioned poor use conditions will not occur. However, graphite hangers have a short service life and are easily damaged. If serious damage is found, they must be replaced in time.
How to choose the right electrode when making steel in electric arc furnace?
According to the design characteristics of the electric arc furnace, choose the electrode that is suitable for the production of the electric arc furnace reasonably, and choose the product 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, the charging method, the maximum current intensity, the length of the electrode column under the clamp, the distance between the furnace side wall and the electrode circumference, etc. are all factors that must be considered when choosing the electrode for the electric arc furnace.
What effect does resistivity have on the use of electrodes in steelmaking?
The resistivity of graphite electrode is a physical indicator that reflects the conductive performance of the electrode. It is related to the manufacturing process of the electrode. The country has qualitatively specified values for the resistivity of graphite electrodes of different specifications. Generally speaking, when steel mills select electrodes of certain specifications, they must choose the resistivity range specified by the national metallurgical standards. Too high a resistivity will cause the electrode to turn red and hot when powered on, increasing the oxidation consumption of the electrode.
What impact does bulk density performance have on the use of electrodes in steelmaking?
The volume density of graphite electrode reflects the compact state of the electrode and is closely related to the manufacturing process of the electrode. The country has specified values for the volume density of graphite electrodes of different specifications and varieties. Products with low volume density indicate that the overall structure of the product has a high porosity, and the product oxidizes faster at high temperatures, which can easily lead to increased electrode consumption. Generally speaking, when steel mills select electrodes, the larger the volume density of the electrode is within the specified value, the better. However, the higher the volume density, the better. Because some electrodes with too high volume density are sometimes prone to surface peeling, block falling and cracking during steelmaking due to poor thermal shock resistance, which will affect steelmaking.
Why should steel mills prevent mixing of multiple products when using graphite electrodes?
The graphite electrodes used in steel mills are often supplied by multiple manufacturers. When multiple products are mixed during steelmaking, it will not only make it difficult for steel mills to count the consumption of a single product, but also because of the different raw materials and manufacturing processes used by each manufacturer, the physical and chemical properties and processing tolerances of the electrodes and connectors of each manufacturer are somewhat different.
Therefore, the matching tolerances generated when mixed use can easily cause the electrodes to fall off and break. The correct way to use it is to use a product from a certain manufacturer alone, and then connect it to another manufacturer’s product after it is finished. To reduce the number of replacements of electrodes from different manufacturers, the electrodes of the same manufacturer should use the connectors that match the same manufacturer to prevent mixing.
What are the characteristics of needle coke?
Needle coke is a high-quality carbon raw material, which is divided into coal-based and oil-based. Its surface has obvious strip-like lines. When broken, most of them are long needle-like fragments. Under a microscope, a fibrous structure can be observed, so it is called needle coke. Needle coke is easy to graphitize at high temperatures above 2000 degrees. The graphite electrode made from it not only has a low resistivity and high volume density, but also has a small thermal expansion coefficient. It is an essential raw material for the production of ultra-high power electrodes and high-power electrodes. The price of needle coke is much more expensive than that of ordinary coke, currently about 5-8 times higher.
Does the dust collection system on the electric arc furnace have any impact on the consumption of electrodes?
The fan used in the dust collection system generates a certain negative pressure when working, which increases the air flow rate around the red-hot electrode during steelmaking, thereby increasing the oxidation consumption of the electrode. During steelmaking, a well-adjusted dust collection system can maintain a good working environment and stabilize the consumption of the electrode.
How to avoid increased electrode consumption during steelmaking?
To avoid increased electrode consumption during steelmaking, the following should be done: (1) Maintain a good power supply state and supply power within the allowable current intensity range of the electrode according to the design requirements of the electric furnace. (2) Prevent the arc starting 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, use spray cooling technology for the electrode. (5) Set up a correct exhaust gas emission system. (6) Use a correct oxygen blowing system.
How long is the production cycle of graphite electrodes?
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, quality inspection (2 days) – finished product packaging and delivery (1 day), that is, from feeding to product delivery, the fastest production cycle without interruption is 60 days. The production of electrode joints requires two more impregnation and three more roasting processes than electrodes, so the fastest production cycle is 90 days.
What are the characteristics of electrodes produced by tandem graphitization furnaces?
The development direction of graphitization furnace is internal heat series graphitization furnace. Since the current density of the series column is the same, the difference in electrode resistivity is very small; secondly, the resistivity of the two ends of the internal series graphitization product is slightly lower than that of the middle (the resistivity of the two ends of the Acheson furnace graphitization product is higher than that of the middle), which is beneficial to reduce the resistance of the connection when the user uses it and alleviate the overheating and reddening phenomenon of the joint connection. Therefore, the electrode quality uniformity produced by the series graphitization furnace is better than that of the Acheson furnace, and is more suitable for the production requirements of electric arc furnace steelmaking.
Why is the quality of electrode joints important in electric arc furnace steelmaking?
The joint plays a key role in connecting the electrodes in steelmaking. The quality of the joint is directly related to the use of the electrodes in electric furnace steelmaking. No matter how good the electrode quality is, if there is no high-quality joint to match it reasonably, problems will occur in steelmaking. According to relevant data, in electric furnace steelmaking, more than 80% of electrode use accidents are caused by joint breakage and loosening. Therefore, the selection of high-quality electrode joints is a guarantee for the normal use of arc furnace steelmaking electrodes.
Which quality indicators of graphite electrode (joint) products have an impact on electric furnace steelmaking?
(1) The quality indicators of the electrode, such as volume density, resistivity, strength, elastic modulus and thermal expansion coefficient. (2) The quality indicators of the joint, such as volume density, resistivity, strength, elastic modulus and thermal expansion coefficient. (3) The processing accuracy of the electrode and the joint. No matter how good the quality of the electrode and the joint is, without good processing accuracy (mainly refers to the matching of the electrode and the joint), the use effect will not be good. (4) The internal structure quality of the electrode and the joint requires that there are no cracks inside that cause hidden dangers in use.
What are the consequences of severe oxidation of the electrode end surface at the upper end of the electrode holder?
When steel is made in a smelting furnace, scrap steel burns in the furnace. At the same time, due to the 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 surface. When a new electrode is connected to the upper end, the oxidation and deformation of the lower electrode end face cannot contact the new electrode well, and the electrode gap is large, which is easy to cause oxidation and breakage of the internal joint. Without changing the steelmaking conditions, the best preventive measure is to add a protective cover to the electrode end face at the upper end of the holder to block the flame and air to achieve the purpose of protecting the electrode end face.
