Quality Standard for Internal Control of RP, HP and UHP Graphite Electrodes
The main indexes reflecting the quality and performance of graphite electrode are bulk density db, resistivity ρ, flexural strength σ, elastic modulus E, thermal expansion coefficient α and ash A%. According to the differences in these indicators and the differences in raw materials and manufacturing processes, According to the national standard (YB/T 4088 89-2000), graphite electrodes are divided into three types: ordinary power graphite electrode (RP), high power graphite electrode (HP) and ultra-high power graphite electrode (UHP). According to the needs of users, the post-graphite electrode production plant has added two kinds of high bulk density graphite electrode (HD) and quasi-ultra-high graphite electrode (SHP).
L the bulk density is the ratio of the mass of the graphite electrode to its volume. The larger the volume density is, the denser the electrode is, which is positively correlated with the strength and oxidation resistance. The higher the bulk density of the same kind of electrode is, the lower the resistivity is.
The ways to improve the bulk density are as follows: adjusting the formula, increasing the amount of small particle size material and powder, using coke with high true density, using asphalt with high coking value and increasing the number of impregnation, etc.
L resistivity is a parameter used to measure the conductivity of the electrode. It refers to a property of the resistance of the conductor to the current when the current passes through the conductor, which is numerically equal to the resistance of the conductor with a length of 1m and a cross-sectional area of 1m2 at a certain temperature. Unit μ Ω m. The lower the resistivity, the better the conductivity and the lower the consumption of the electrode.
The ways to reduce the resistivity are: using high quality raw materials, increasing the volume density of products, increasing graphitization temperature and so on.
Flexural strength is a parameter that characterizes the mechanical properties of graphite materials, also known as flexural strength. It refers to the ultimate resistance of the body to the moment of bending to the moment of fracture when the external force is perpendicular to the axis of the object, in MPa. The strength of graphite is significantly different from that of other metals and nonmetals. Its strength increases with the increase of temperature and reaches the highest at 2000-2500 ℃, which is 1.8-2 times that of normal temperature, and then decreases. The more difficult it is to break the electrode and joint with high strength in use.
The ways to improve the flexural strength are to reduce the particle size of coke in the formula, improve the strength of carbonaceous raw materials, improve the bulk density of products, reduce the internal defects of products and so on.
L elastic modulus is an important aspect of mechanical properties, which is an index to measure the elastic deformation ability of materials. It refers to the ratio of stress to strain of materials in the range of elastic deformation, in GPa. The greater the elastic modulus, the greater the stress needed to produce a certain elastic deformation. In short, the greater the elastic modulus, the more brittle the material is, and the smaller the elastic modulus is, the softer the material is.
The higher the bulk density of the product is, the greater the elastic modulus is, but the worse the thermal vibration resistance of the product is, the more likely it is to crack off the block. In production, it is often through the adjustment of formula particle size and the volume density of products to master an elastic modulus value which is more suitable for use.
The coefficient of thermal expansion refers to the measurement of the degree of expansion of a material after heating, that is, when the temperature rises by 1 ℃, the expansion ratio constant of a unit of solid material in a particular direction is called the coefficient of linear expansion along that direction. Unit 1 × 10-6 / ℃. Where there is no special note, the coefficient of thermal expansion refers to the coefficient of linear expansion, and the coefficient of axial and radial linear expansion of graphite electrode varies greatly, which is 0.8-1 times larger in the radial direction than in the axial direction. The coefficient of thermal expansion in the quality index of graphite electrode refers to the coefficient of axial thermal expansion.
The thermal expansion coefficient of graphite electrode is a very important thermal parameter, the lower the value, the stronger the thermal stability of the product, the higher the oxidation resistance, reflecting that the less the break and the lower the consumption.
The way to reduce the coefficient of thermal expansion is mainly determined by the inherent properties of raw materials to use good quality raw materials, the formula uses a larger particle size formula or increase the amount of large particles (but will reduce the density and strength of the products).
Ash refers to the content of solid elements other than carbon and graphite in the product. The ash content in the graphite electrode is mainly affected by the ash content of the raw material used, and the ash content of the needle coke of petroleum coke is relatively low, so the ash content of the graphite electrode generally does not exceed 0.5%, and the ash content less than 1% has no obvious effect on steelmaking. However, the impurity elements in ash will reduce the antioxidant performance of the electrode.
L the thermal vibration factor is a parameter that characterizes the thermal vibration resistance, and the thermal vibration resistance is a property of the material itself, which describes a property that withstands rapid cooling and heat, in other words, It is a kind of anti-fragmentation property of the material under a certain temperature gradient, and it is a very important comprehensive factor affecting the use of the electrode.
K-thermal shock resistance factor, w, σ-tensile strength, MPa;E-- elastic modulus, MPa; λ-thermal conductivity, w / m ·k; α-thermal expansion coefficient, 1 K is a relative value, the larger the value, the stronger the thermal vibration resistance. The K value of graphite electrode has a high correlation with its performance in electric arc furnace, that is, the higher K value corresponds to the lower breaking and fracture of graphite electrode.
Brief introduction of EAF steelmaking.
The modern steelmaking methods mainly include converter steelmaking method, open furnace steelmaking method and electric furnace steelmaking method. The open hearth furnace steelmaking method has been basically eliminated. The most fundamental difference between the electric furnace steelmaking method and the converter steelmaking method is that the electric furnace steelmaking method takes the electric energy as the heat source, and the electric arc furnace steelmaking is the most widely used electric furnace steelmaking method. What we usually say about electric furnace steelmaking mainly refers to electric arc furnace steelmaking, because other types of electric furnaces, such as induction furnace, electroslag furnace, etc., produce less steel.
Electric arc furnace steelmaking is an arc produced by discharge between electrode and charge, which converts electric energy into heat energy in arc light, and heats and melts metal and slag by the direct action of radiation and arc. A steelmaking method for smelting steel and alloys of various components.
Consumption mechanism of graphite electrode.
The consumption of graphite electrode in EAF steelmaking is mainly related to the quality of the electrode itself. It is also closely related to the furnace condition of steelmaking (such as new and old furnace, mechanical failure, continuous production, etc.) and steelmaking operation (such as smelting steel, oxygen blowing time, furnace charge, etc.). This paper only discusses the consumption of graphite electrode itself, and its consumption mechanism has the following aspects:
The consumption of n end includes the sublimation of graphite material caused by arc high temperature and the loss of chemical reaction between the end of electrode and molten steel and slag. The high temperature sublimation rate at the end mainly depends on the current density passing through the electrode, the second is related to the diameter of the oxidized side of the electrode, and the end consumption is also related to whether the electrode is inserted into the molten steel to increase the carbon.
The chemical composition of n side oxidation electrode is carbon. Under certain conditions, carbon will be oxidized with air, water vapor and carbon dioxide. The amount of oxidation on the side of the electrode is related to the unit oxidation rate and the exposed area. In general, the amount of oxygen on the side of the electrode accounts for about 50% of the total consumption of the electrode. In recent years, in order to improve the smelting speed of electric furnace, the frequency of oxygen blowing operation has been increased, resulting in an increase in the oxidation loss of the electrode. In the process of steelmaking, it is often observed that the redness of the electrode torso and the taper of the lower end is an intuitive method to measure the anti-oxidation ability of the electrode.
When the stump loss electrode is continuously used at the junction of the upper and lower electrodes, a small section of the electrode or joint (that is, the residue) is detached due to the oxidation thinning of the body or the penetration of the crack. The magnitude of the stump loss is related to the shape of the joint, the internal structure of the electrode, the vibration of the electrode column and the impact.
N the results of rapid cooling and heating in the smelting process and poor thermal vibration resistance of the electrode itself.
N electrode fracture includes electrode trunk fracture and joint fracture. Electrode breaking is related to the quality and processing cooperation of graphite electrode and joint, as well as steelmaking operation. The cause is often the focus of controversy between steel mills and electrode manufacturers.
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