Consumption of graphite electrode in electric arc furnace smelting

With the rapid development of China's economy, the demand for special steels from all walks of life has become more and more extensive, which has led to the rapid development of electric arc furnace steelmaking technology. Large capacity and ultra high power electric arc furnace and large furnace refining technology have become the symbol of modern steel industry. These electric furnaces all use ultra-high power graphite electrodes of different specifications.

With the continuous improvement of arc power and the continuous optimization of smelting technology, the physical and chemical indexes of graphite electrode are required to be higher and higher, and the consumption index is lower and lower. As the graphite electrode occupies a certain proportion of the cost in smelting and has a large consumption, all manufacturers pay great attention to the consumption and use effect of the graphite electrode, which is also included in the production as an important assessment index.

Consumption mechanism of graphite electrode during EAF smelting

As a conductive material in electric arc furnace smelting, the consumption of graphite electrode is proportional to the consumption of electric work. Modern electric arc furnace steelmaking uses electric energy and chemical energy as thermal energy to realize the purpose of "four removal" (P, C, O, S), "two removal" (gas and impurities) and "two adjustment" (temperature and composition) in the steelmaking process. The use performance of graphite electrode is mainly reflected in whether it is applicable and how much it is consumed, while the consumption of electrode is directly related to its own quality. The consumption of graphite electrode in electric arc furnace smelting mainly consists of the following parts.

1. Consumption of graphite electrode end and cylindrical surface in electric furnace

The electric arc generated by the graphite electrode in the electric arc furnace can be divided into long, medium and short arcs, while the melting charge and temperature rise depend on the electric arc power. The arc length is proportional to the secondary voltage and inversely proportional to the secondary current and the heating rate. In order to improve smelting speed and greatly shorten smelting time, forced oxygen blowing with high chemical energy operation is adopted, which puts forward higher requirements for oxidation resistance and thermal shock resistance of graphite electrode. The end consumption of graphite electrode in smelting includes sublimation generated in high temperature of arc and chemical reaction generated in contact with molten steel and steel slag. The oxidation loss of graphite electrode accounts for about 2/3 of the total consumption. Its oxidation loss is the product of unit oxidation speed and area and is proportional to time. The longer the heating time is in smelting, the greater the consumption. Therefore, it is necessary to install a water-cooled spray system on the electrode of electric arc furnace. In normal smelting, the carbon content of the graphite electrode entering the molten steel is generally about 0.01%, and it is normal that the end consumption switch is in a non tapered shape.

2. Residual consumption of graphite electrode generated in smelting

Residue consumption refers to the non productive consumption part that the lowest electrode falls into the furnace for some reason and becomes the final waste and leaves the production process. The generation of residue is not only related to the internal quality of the joint and electrode, but also directly related to the distribution of distribution in the furnace, the atmosphere in the furnace and the power transmission operation. The main appearance phenomena are: there are herringbone cracks and large longitudinal cracks or splits at the bottom end of the residue; The joint is not tight, which causes the joint to fall off or break due to oxidation; The connection is not in place or the fitting is not good, resulting in falling off or breaking; The electrode is broken at the bottom of the joint or hole due to external force; The electrode is broken inside the joint or hole due to external force; The electrode can be seriously broken due to the unreasonable distribution in the furnace, which leads to large area of material collapse after drilling or the unreasonable operation of power transmission curve; The electrode itself has poor quality. On the premise of ensuring the electrode quality, this part of loss is not large in normal production, but the direct users attach great importance to it.

3. The electrode surface is oxidized and peeled, accompanied by the consumption of cracking and chipping

In normal smelting production, if the surface of graphite electrode is uneven or accompanied by peeling and chipping, there is a problem of carburization in molten steel. On the one hand, this phenomenon reflects the poor oxidation resistance and thermal shock resistance of the electrode; On the other hand, too long horizontal oxygen blowing time or too large oxygen blowing volume in smelting will cause serious oxygen enrichment in furnace and on furnace, which will increase the electrode peroxidation loss; Second, if there is serious falling off, the problem of electrode must also be considered. This abnormal consumption is a test of the internal quality of products and the level of technical services.

4. Direct loss caused by broken graphite electrode during smelting

It is a common phenomenon that graphite electrode breaks in all electric furnace smelting, and it is also the main factor affecting consumption. It is normal for continuous consumption and use in complex environments to break occasionally, but it is not normal for continuous breaking. The reason is related to many factors. In general, it can be divided into: artificial breaking and mechanical breaking. Artificial breaking mainly includes: bump and scratch during lifting, improper connection or method, improper sliding in the holder, hard collision or poor transmission control sensitivity, etc. In addition to mechanical failure in mechanical fracture, electrode quality problems and operation problems often exist at the same time and are difficult to distinguish. There are mainly the following phenomena:

(1) Broken electrode body

First, the electrode may have structural defects and may have low strength; Second, there is still short arc operation after penetration during smelting, and there is greater lateral impact force of material collapse; Third, the three-phase electrodes on the furnace are seriously not vertical and there are phenomena such as hanging furnace slag or scraping furnace cover. The sound is crisp and loud when it breaks.

(2) Broken electrode body hole bottom

First, the electrode end structure is loose or has dark lines, the joint and hole are improperly matched, or the linear expansion coefficient of material difference is not matched; Second, the whole phase electrode is not concentric, the electrode travel is too long or the rise and fall are not sensitive; Third, the lining in the furnace is unreasonable, and there are non-conductive objects under the electrode. When breaking, the sound is not loud but the inclination is heavy.

(3) The connector breaks irregularly

First, the taper of joint processing is different or the ellipse of joint hole is too large; Second, the excessive contact resistance caused by the excessive dust in the hole during connection makes the local oxidation of the joint thread too fast; Third, the electrode connection is not in place and does not meet the torque requirements, resulting in looseness; Fourth, the holder is inclined, and the electrode is not concentric with the furnace cover hole. The sound is crisp and small when it breaks.

(4) The joint breaks regularly

First, the quality of the joint itself varies greatly, and the strength of the joint cannot meet the requirements of the smelting furnace; Second, the electrode hole and the joint tolerance are not properly matched or the coupling torque cannot meet the requirements, resulting in buckling; Third, the fluctuation range of secondary current in power supply is too large or there is sudden increase, and the maximum instantaneous current is more than 1.2 times of the rated value; Fourth, when the input power is too large, the thermal vibration is too large, and the electrode connection is easy to redden, indicating excessive resistance. The sound is dull when it is broken.