Thisarticle describes the research and use of magnesium-aluminum-carbon skateboards.
Key words: steel ladle; magnesium-aluminum-carbon slide plate
Background
With the promotion of new steelmaking technology and continuous casting technology, the competition in the steel market is becoming more and more fierce. Many steel mills have begun to produce high-quality clean steel, special steel and alloy steel, which makes the skateboard in a very harsh environment. In the process, especially when the molten steel is treated with calcium, the erosion of the refractory material is intensified, which seriously affects the service life and effect of the slide. Ordinary Al2O3-C and Al2O3-ZrO2-C slides have severe melting loss and reduced durability, which cannot meet the requirements of continuous use in steel mills. Therefore, in ordinary alkaline slides, it is necessary to study to improve its thermal shock resistance, oxidation resistance and high temperature strength, so as to increase the service life of the slides.
Skateboard damage mechanism
The melting point of calcium (850°C) is much lower than that of iron, and it has a high vapor pressure at a pouring temperature of about 1600°C. The calcium vapor volatilized from the molten steel forms active CaO in the negative pressure zone, which intensifies the melting loss of the slide. When the ordinary aluminum-carbon slide plate is casted with high-calcium steel, the periphery of the hole is severely eroded by Ca and CaO, and a special form of melting loss is formed on the surface of the upper slide plate. It looks like a horseshoe in shape, so it is called horseshoe melting loss. Calcium corrosion will be aggravated if the slide plate is used for long-term flow control when pouring steel (Figure 1). The [Ca] reaction mechanism is as follows:
The generated CaO continues to react with Al2O3 to generate 12CaO·7Al2O3 (melting point 1392°C).
Development and Research of Magnesium Aluminum Carbon Skateboard
Skateboard performance requirements and raw material selection
According to the erosion mechanism, the slide plate should have the following characteristics:
(1) Reduce the components that can react with CaO, FeO, MnO, reduce the formation of low melting point compounds, and enhance their corrosion resistance;
(2) Reduce the size of the pores, make the pores finer, and reduce the penetration of CaO, FeO, MnO, and oxygen;
(3) Reduce the carbon content and reduce the oxidation of carbon on the sliding working face;
(4) It has high wear resistance, so that it can be used for many times, and the high temperature strength of the material is required;
(5) High thermal shock resistance, reducing the occurrence of cracks in the use and production of skateboards.
Figure 1 Schematic diagram of melting loss of aluminum-carbon slide plate and photos after use
According to the characteristics of the above skateboards, high-quality fused magnesia particles with well-developed crystals are selected as aggregates, and spinel aggregates with relatively small particle sizes are introduced as the main raw material to alleviate the shortcomings of relatively large thermal expansion coefficients of fused magnesia; in the matrix Introduce a small amount of metal additives, appropriate amount of carbon, low expansion raw materials, etc., use different treatment processes, compare the performance, and develop high-grade alkaline skateboards. Using the excellent slag resistance and chemical corrosion resistance of magnesia and spinel raw materials, according to the market demand, combined with the characteristics of the production process of our factory, we develop a slide plate with high strength, low expansion and high thermal shock resistance. The magnesium-aluminum-carbon slide plate developed by our company is mainly aimed at calcium-treated steel to meet the production needs of steel mills.
Comparative analysis of tests and indicators
The chemical composition of the main raw materials used in the skateboard, the test formulas of the four samples and the comparison of various indicators are listed in Table 1, Table 2 and Table 3, respectively. The comparison of the flexural strength and linear expansion of the four samples is shown in Fig. 2 and 3.
Table 1 Chemical composition of main raw materials
| Raw material name | MgO | CaO | SiO2 | Al2O3 | SiC | Fe2O3 | C |
| Fused magnesia particles | 97.2 | 0.85 | 0.91 | 0.59 | |||
| Fused magnesia fine powder | 97.5 | 0.79 | 0.89 | 0.65 | |||
| Spinel particles | 22.5 | 75.8 | |||||
| Silicon carbide | 98.09 | 0.12 | |||||
| Carbon | 95.19 |
Table 2 Experimental comparison formula
| Raw material name | Specification | 1#sample | 2#sample | 3#sample | 4#sample |
| Fused magnesia particles | 3~1mm | 65 | 65 | 65 | 65 |
| Fused magnesia fine powder | -200head | 13 | 12 | 12 | 12 |
| Spinel particles | -18head | 10 | 10 | 10 | 10 |
| Metal additives A | -325head | 3 | 3 | 5 | 3 |
| Metal additives B | -325head | 1 | 0 | 2 | 1 |
| Carbon | -100head | 3 | 3 | 3 | 3 |
| SiC | -18head | 5 | 5 | 5 | 3 |
| B4C | -325head | 0.5 | 0.5 | 0.5 | 0.5 |
| Modified resin (additional) | 3.5 | 3.5 | 3.5 | 3.5 |
Table 3 Index comparison
| Sample No | Compressive strength/MPa | Porosity/% | Bulk density/(g.cm3) | ||||||
| 200℃ | 800℃ | 1400℃ | 200℃ | 800℃ | 1400℃ | 200℃ | 800℃ | 1400℃ | |
| 1# | 101 | 78 | 81 | 6.2 | 5.7 | 5.9 | 2.96 | 2.97 | 2.97 |
| 2# | 98 | 82 | 85 | 6.4 | 5.8 | 5.6 | 2.97 | 2.99 | 2.98 |
| 3# | 97 | 75 | 79 | 6.8 | 6.1 | 6.3 | 2.95 | 2.97 | 2.97 |
| 4# | 100 | 79 | 77 | 6.7 | 5.9 | 5.5 | 2.95 | 2.93 | 2.97 |
According to the analysis results of the above indicators, the following conclusions can be drawn:
(1) The increase of metal additive A is conducive to the increase of the strength of the skateboard.
(2) The increase of metal additive B can cause cracks in the fired slide plate, which is caused by excessive volume expansion during the firing process.
(3) After high-temperature firing, the sliding plates of the four samples all had cracks in different degrees, and the volume expansion was large.
(4) There is little difference in the compressive strength, slag resistance and oxidation resistance of the four samples.
(5) The flexural strength of the 4# sample has obvious advantages, and the linear expansion is relatively low.
Fig.2 Comparison of flexural strength of four samples
Fig.3 Comparison of linear expansion of four samples
In view of the above conclusions, it was decided to select the 4# sample slide plate for field test.
Field use of magnesium aluminum carbon skateboard
In a domestic steel factory, the magnesium-aluminum-carbon slide plate of the 4# sample was used for comparison. The pouring time was 30-50 minutes. The steel types were mainly calcium-treated steel, high-oxygen steel, and high-manganese steel. Through on-site use, it can basically meet the needs of steel mills for 4 consecutive slides, and some skateboards can be used for 5 consecutive slides. After use, there is no phenomenon of falling or peeling off the surface of the board, and the surface of the board is good without obvious oxidation and calcium erosion. The hole of the slide plate is about 5 mm wide. The status photos of the magnesium-aluminum-carbon slide plate in the field test after being used for 2, 3 and 4 times are shown in Fig. 4, Fig. 5 and Fig. 6.
Figure 4 The condition of the magnesium-aluminum-carbon skateboard after being used twice
Figure 5 The condition of the magnesium-aluminum-carbon skateboard after being used for 3 times
Figure 6 The condition of the magnesium-aluminum-carbon skateboard after being used for 4 times
In conclusion
(1) The flexural strength of magnesium-aluminum-carbon materials increases with the increase of silicon carbide content in the composition. The reason is that silicon carbide is a refractory non-oxide with strong covalent bonds, and the bond energy between atoms is relatively large. Compared with MgO, it has a higher elastic modulus and strength; the addition of silicon carbide is beneficial to the strength of the material. Improvement; with the increase of silicon carbide content, the thermal expansion coefficient decreases, although the addition of silicon carbide can increase the elastic modulus of the material, but the degree of increase in the elastic modulus is less than the decrease in the thermal expansion coefficient, so the whole effect is the resistance of the material Thermal shock performance is improved.
(2) Adding graphite is easier to improve the strength, density and oxidation resistance of the slide than carbon black.
(3) Appropriate addition of metal additive B is beneficial to improve the thermal shock resistance of the skateboard.