Electric furnace magnesia-carbon brick steelmaking is a device that uses the electric arc generated between graphite electrodes as a heat source to make steel. It is divided into DC electric furnaces AC electric furnaces, of which AC electric furnaces account for the majority. According to the damage mechanism of the electric furnace, this work has developed produced the magnesia carbon brick required by the electric furnace.

1 Analysis of damage of electric furnace lining

After actual tracking investigation of various electric furnaces, it is found that the slag line is the weakest area. Generally, the slag line area after lowering the furnace has a severely corroded ring trench with a depth of 100～200Ⅻ a depth of about 100∞ in the island direction (see Figure 1). The slag line bricks near the 2" electrode of the AC electric furnace lining have the most serious erosion. According to analysis, the main cause is The slag formed by steelmaking fluctuates up down, coupled with the stirring effect of oxygen blowing, so that the saw slag will cause large slag erosion to the furnace lining. Generally, the slag contains a large amount of Fe0, which has a strong oxidizing property. The slag line here Magnesium-carbon bricks are easily oxidized. At the same time, the M90 content in most slag is low, generally below 5%. It is saturated. Generally, the Mgo in the slag can be saturated when it is controlled between 8 12%. Cause the magnesium oxide in the magnesia-carbon bricks to dissolve into the slag cause corrosion of the magnesia-carbon bricks. The slag also contains more Si02. The acidity is very strong. A large amount of c80 caF should be added during steelmaking: as a slagging agent ．This kind of slag is very dilute, the M90 in the magnesia carbon brick can easily form low-melting calcium forsterite cMS (melting point 1490 ℃) magnesia pyroxene c. Ms: (melting point 1580 ℃) very dilute viscosity. Very easy to penetrate into the pores of magnesia carbon bricks, add one yi = two two]

Fused Magnesia Carbon Brick

Increase the erosion rate. Generally, the slope of the electric furnace magnesia carbon brick is very small. In addition, the corroded surface cannot be leveled through the shaker when tapping repairing the furnace. Therefore, it is impossible to repair the severely corroded slag line part with ordinary furnace repair bricks furnace repair materials. The gunning effect is also as small as that of a converter. The life of general electric furnace linings is relatively low due to the above reasons. At the beginning of 20D, the service life of several small electric furnaces under 30 tons in Shandong Province was generally around 1,000. one of the small electric stoves. It belongs to the main equipment of the powder metallurgy process of the national "863 Plan", the smelting temperature is as high as 1,820 ℃, the furnace age has been hovering around an average of 26 furnaces. Become a major problem that plagued the enterprise project.

2 Summary of research work

The damage characteristics of the needle lining electric furnace. We focused on the scouring erosion resistance of magnesia-carbon bricks on the slag line. Due to the low degree of erosion of the molten pool by slag, the analysis believes that as long as its good thermal shock stability general corrosion resistance can be guaranteed. We have carried out research development work by analyzing the reasons that affect the shorter life of slag line magnesia-carbon bricks. Finally, the product developed-high-quality magnesia carbon bricks for electric furnaces meets the requirements of electric furnace steelmaking, as summarized below.

2.1 Selection of carbon content

Through the investigation analysis of usage. The service life of graphite content magnesia carbon bricks plays a decisive role

graphite. Especially flake graphite is a good conductor of electricity, in the slag line. According to Chen Zhaoyou's analysis. The corrosion damage of the magnesia carbon brick is essentially caused by the excessive short-circuit current of the corrosion battery. This theory is proved by the following experiments: using electric furnace slag as electrolyte, heating it to 1450 ℃ under N: protection conditions. Insert the carbon electrode the iron electrode into the molten slag at the same time, connect the electrode with thick copper wire. It can be found that there are bubbles emerging the carbon electrode in the molten pool, but the iron electrode is very calm, the graphite electrode corrodes after ten minutes. Serious, but the iron electrode did change. Under the same experimental conditions, the Fe c electrodes were respectively connected to the positive negative electrodes of an approximately 1.0V battery. There were no bubbles at the Fe c electrodes in the slag at 1,450°C. The carbon electrode also showed no corrosion.

The anode reaction is: C+O2. One Co＋2e

｜Ilj extremely anti-house is: Fe2＋＋2e—Fe

The overall response is: C + (Feo, one co + Fe

The battery is: cI slag fFe

It can be seen the corrosion battery that in order to reduce the corrosion of magnesia-carbon bricks at the three-phase junction of molten steel-slag-magnesia-carbon bricks, it can be solved the following four aspects: ①Increase the internal resistance of the corrosion battery, the specific measures are to reduce the graphite content accordingly. (Because graphite is a good conductor of electricity): ②Add metals with lower standard potential than graphite electrodes in the magnesia-carbon bricks, such as si, Al, Mg, ca other metals to prevent corrosion of the battery. These metals can neutralize the positive charge of graphite in a large amount, cause internal short circuit reduce corrosion current: ③Connect the magnesia carbon brick directly to the negative electrode of the DC power supply for cathodic protection; ④Add a slagging agent that can increase the internal resistance of the slag. Since the two items ③ ④ need to be done by the steel mill, we can only make suggestions for them. In many cases, we can't do anything. Therefore, we focus on the two measures ① ②.

At the beginning of 1998, we used 18A magnesia-carbon bricks at a certain 5-ton electric furnace slag line, the service life was only 79 furnaces. According to the "corrosion" battery theory, the carbon content is reduced, the internal resistance is increased, the service life of 14A magnesia carbon bricks reaches 90 times.

In our production, through the adjustment of different process ratios, we determined that the carbon content of the slag line magnesia-carbon bricks should be controlled between 12 14% according to different furnace temperatures other operating conditions. As for the magnesia-carbon bricks in the molten pool the magnesia-carbon bricks that are immersed in molten steel molten slag, the carbon content is generally controlled between 6-10%, low-grade products are selected for graphite. The fixed carbon content is more than 92%; the graphite used in the slag line uses products with a fixed carbon content of more than 96%.

Graphite particle size is an important factor, the finer the particle size, the easier it is to oxidize. For example, the relationship between the particle size of a certain flake graphite made by a certain factory the oxidation temperature is as follows:

l Graphite particle size mm 0.28～0.20. J25～0.1540.06S～o. 0760.04S～0.054<o. 0385

l Significant chlorination temperature ℃ 672565863768l

I Oxidation peak temperature ℃ 7797678745 3l699

Through many years of practice, considering several factors such as oxidation resistance, molding performance product raw material cost, we selected 100 mesh flake graphite as the graphite particle size, passed the implementation of the above measures. It has achieved good results in the use of magnesia carbon bricks,

2.2 Selection of additives (analysis of corrosion resistance oxidation resistance of printing additives)

In order to prevent the oxidation of carbon corrosion of the magnesia-carbon bricks, some metal powders such as Al, Mg, si, ca are added as antioxidants. Because electric furnace slag has strong penetration, erosion, erosion oxidation, various additives are used in magnesia carbon

The metal AJ forms A14cj in the magnesia carbon brick, which is in the form of whiskers, the generation temperature is generally 900 ℃; AI+N2-AIN, the generation temperature 264? The role of the bricks in the process of corrosion resistance is very different.

Generally 800°C: while metallic silicon forms B_-sic in magnesia-carbon bricks, the general formation temperature is above 1100°C. When AI Si are used as antioxidants together, the formation temperature of A] 4c3 is 800 ℃, the formation temperature of Ibusjc is 7 ℃. These three reactions can greatly increase the strength of magnesia-carbon bricks play an anti-oxidation effect. Especially at low temperature, when Si AI are used as antioxidants, Si Al form a eutectic at 577°C. Therefore, the formation temperature of A14cj B-sic can be reduced. AI|c3 gradually disappeared above 1400°C most of them formed A12 MA, while fbu sic was oxidized to SiO2 by c0. At 1600°C, Abc3 B-sjc had all formed A12qb, SiO2, MA so on. this analysis, if purely used as an antioxidant, the composite form of AI+Si in magnesia-carbon bricks is better than adding alone. However, considering that Aba forms MA A at high temperature, sic forms sjq, SiO2 will interact with M90. Cao (present in the slag) forms low-melting forsterite cMs (melting point 1490°C) forsterite c3Ms3 (melting point 1580°C). Therefore, the corrosion resistance of magnesia-carbon bricks with Al as an antioxidant is higher than that with si antioxidant, especially in the slag line there is a large scene slag.

The phases produced at various temperatures after the addition of metal aluminum were made by P Wilas. The relationship between the generation sequence of ideal reactant phases thermal gradients in magnesia-carbon bricks containing metal AL" (P Wilas (Wakiams) et al. 1991) It can be seen that Al powder exists in the form of MA at high temperature, its melting point is 2130℃, which is a high temperature phase.

The slag line magnesia-carbon bricks for a 20-ton electric furnace in a steel plant use two antioxidants, A] 2% Al+s% (half each). The specific indicators are:

Volume density,

M20, %C. % Apparent porosity,% pressure resistance at room temperature, MPI high temperature bending resistance. MPI remarks

g, cm3

12～15A12%

76~S213~152.96~3.021.5~3.040~60

】0～12Al＋Si2%

The erosion rate is obviously different, the service life of Al+2% products is between 55 75 furnaces. A-core% products can be used for more than 100 furnaces. The analysis believes that in addition to the above analysis, the main reason is that Ab slag form CA6 (melting point ls50°C), so it shows better corrosion resistance. The sj02 formed by si the slag form a eutectic cMs (1490℃), c3M& (1580℃). Therefore, the comprehensive conclusion is that the corrosion resistance of magnesia carbon bricks with AI alone is better than A]+si composite with SI alone. Therefore, we use AI as an additive in the production of slag line magnesia carbon bricks.

2.3 Strictly control the lower apparent porosity higher bulk density

According to Tao Youxiang’s research, the penetration rate of molten slag in the refractory material is proportional to the fourth power of the pore diameter of the refractory material. The electric furnace slag line cannot always be covered by the high degree high refractoriness slag like the slag splashing of the converter. Protection is always immersed in very dilute highly permeable slag. Therefore, the density apparent porosity of the magnesia-carbon brick of the electric furnace slag line play a vital role in the erosion resistance of the brick. Observed the residual bricks of the electric furnace slag line, it is found that the corrosion behavior mainly forms the reaction zone the decarburization zone the slag layer is very thin. The reaction zone is the erosion area the molten slag penetrates into the magnesia-carbon bricks after decarburization. In this area, Feon in the slag is reduced to Fe. The depth of slag penetration into the brick is mainly determined by the thickness of the decarburized layer. To reduce the thickness of the decarburized layer, one is to use antioxidants. The second is to increase the density, reduce the apparent porosity, increase the slag penetration resistance.

At the beginning of 2000, the two sets of slag-line magnesia-carbon bricks produced for a certain factory were made of the same material, but the porosity was slightly higher due to certain reasons when the set of bricks were produced, but the service life was much different. The indicators are:

"M pit o, %C. % Volume density, g, cm3 apparent porosity,% normal temperature resistance to anus. MPa high temperature flexural resistance, MPa grid note 1I78.914.52.974. The service life of the 4ST3.2 first set of bricks 1I77.714.62.977.3.4431.31 is reduced by l/10 compared with the second set of bricks, only the apparent porosity exceeds l in the index. 3%. To achieve the goal of reducing the apparent porosity, many f's now treat the formed magnesia-carbon bricks with 400-600°C maturation treatment chips then immerse them with medium-temperature asphalt. The effect is better. Long service life. But the production process is cumbersome, but it can 【;】 reduce the cost of raw materials. In 2000, the author specifically studied the influencing factors of the apparent porosity of magnesia-carbon bricks formulated many corresponding measures to reduce the apparent porosity of the produced slag line bricks to less than 2%.

Therefore, in order to ensure the performance of the product, we generally control the bulk density of the slag line magnesia carbon brick at 3. Between oo～3.08g, cm3, the apparent porosity is generally controlled at 0. 5～2. s%. In order to ensure the stability of this reference, we mainly use three measures to solve it:

2.3.1 Adjust tree gift performance

The current binding agent for magnesia-carbon bricks is mainly thermoplastic thermo-homogeneous resins, I use self-produced thermal-period resins. Generally, thermoplastic resins are required to have higher Zhou Hanyi in order to ensure higher bonding strength. The addition of solvent is small, its viscosity varies greatly with the influence of ambient temperature, especially in winter. Therefore, it is generally necessary to heat in a water bath before use, also to heat magnesia. Mountain is limited by T art conditions, in the storage of mud. The temperature is difficult to control when the processes are connected, resulting in a decrease in the temperature of the mud. Each magnesia particle forms a very hard ball with fine powder. The fluidity becomes poor the molding is very difficult. Therefore, the product also shows an increase in porosity a decrease in bulk density, the product quality cannot be guaranteed. Equipment performance is also severely damaged.

In response to this problem, under the guidance of experts, the effect of the viscosity of the thermoplastic resin was found:

q=, (x-y. T)

n-wax viscosity

x——molar ratio of phenol formaldehyde

y——resin solid content

T-temperature