Study on the application of magnesia carbon brick MT-14A in the bottom of GOR converter

The GOR converter smelting stainless steel process developed by the National Institute of Metallurgy of Ukraine has similar equipment and smelting processes to Western Europe and Japan’s K-BOP, STP and other processes. It has low investment, low cost, high productivity, and is suitable for smelting a variety of high-quality special steels. The advantages are applied in steel and foundry enterprises. China introduced GOR converter steelmaking in early 2006, and the main steel grades produced are J series, 200 series and 300 series non-ultra-low carbon stainless steels. In the early stage of production, many factors affected the lower furnace age. After continuous research and transformation, a higher furnace age has been achieved, but there are still some problems to be solved.

This article introduces the use of MT-14A in the bottom of the GOR converter, and samples the bottom bricks with a relatively low life, analyzes the cause of damage and corrosion mechanism, and provides a reference for the material selection of the bottom of the GOR converter.

1 Experiment

The first furnace bottom of 65t GOR furnace uses 1000mm long magnesia carbon brick MT-14A, and the replacement furnace bottom brick length is 800mm coal loaded MT-14A. The physical properties and chemical composition of magnesia carbon brick MT-14A are shown in Table 1. The smelting steel grades are 304 and J4, the average smelting time is about 120min, and the temperature is generally not higher than 1710℃.

Take the remaining bricks after use and cut 5 bricks (labeled 1#~5#) from the working face inward and parallel to the working face for physical and chemical analysis; measure its bulk density and apparent porosity according to GB2997-82, and measure the medium as kerosene. Fluorescence analysis was performed with the S4PIONEER wavelength dispersive X-ray fluorescence instrument produced by Bruker, Germany.

2 Results and discussion

For the first time, 1000mm long magnesia-carbon bricks were used on the GOR furnace bottom, and the maximum furnace age was 62 times, and the average furnace age was about 50 times; the second time, 800mm long magnesia-carbon bricks were used on the GOR furnace bottom, and the maximum furnace life was For 41 times, the average furnace age is less than 30 times. During the production process, the bricks were found to be broken during the first use of the furnace bottom for about 30 times, and the bricks were found to be broken after the second furnace bottom was used about 6 times, and the service life of the magnesia-calcium-carbon bricks was less than half of that. Figure 1 is a photo of the used magnesia-carbon brick furnace bottom. It can be seen that the used GOR furnace bottom magnesia-carbon bricks have been eroded seriously, and the entire furnace bottom along the wind eye line on both sides of the 900mm wide strip area is seriously damaged, showing obvious deep pits, and the shortest remaining bricks are less than 80mm. The residual bricks on the steel side are longer, with the longest being nearly 700mm (the original furnace bottom brick is 800mm long); the residual bricks on the tapping side are relatively short, with a residual length of about 350mm. Along the line of the wind eye bricks, the third brick on the steel side and the fourth brick on the tapping side began to find that the length of the remaining bricks was uneven, and there was a phenomenon of broken bricks. The fracture length was about 100mm, and the section was flat. There is no obvious "mushroom head" on the top of the air gun.

Fig 1. The morphology of the magnesia carbon brick at the bottom of the GOR furnace after use


Fig 2. Macro photo of magnesia carbon brick MT-14A residual brick

The photo of the working face of the remaining bricks after use is shown in Figure 2. It can be seen from Figure 2 that the eroded working face of residual bricks has a large slope without obvious slag layer. The working face surface is uneven, with large particles protruding. From the color, there is a decarburized metamorphic layer of about 3-10mm. It can be seen from the section of the magnesia carbon brick MT-14A residual brick in Figure 3 that the MT-14A residual brick is obviously divided into a slag layer, a metamorphic layer, a transition layer and an original layer. It can be seen from the SEM picture of the surface of the MT-14A residual brick in Figure 4 that the large particles of fused magnesia in the magnesia carbon brick MT-14A residual brick have many cracks, which are very serious, and there are many small cracks in the matrix; MT-14A residual bricks There are serious voids between the large particles of fused magnesia in the bricks. The fracture of the magnesia carbon brick, the cracks of the large particles of fused magnesia and the cracks in the matrix may be caused by thermal shock damage. Magnesia carbon bricks are mainly composed of large and small particles of fused magnesia and graphite. The expansion coefficient of fused magnesia is 0.6~5×10-5K-1, and the thermal expansion coefficient in the flake graphite layer is 1~2×10 -6K-1, the absolute expansion of large magnesia particles is much larger than that of small particles. The temperature in the furnace is maintained at about 1710°C during steelmaking, and the temperature in the furnace drops sharply during tapping. Therefore, the interface between large magnesia particles and graphite in magnesia carbon bricks generates greater stress than the interface between small magnesia particles and graphite. Therefore, the large particles of fused magnesia in the magnesia-carbon bricks are prone to cracks under the conditions of rapid cooling and heating. After multiple high-temperature expansion and low-temperature shrinkage, huge stresses will be generated in the magnesia-carbon brick MT-14A. The cracks in the sand developed into large cracks and even fractures after instability and propagation. Therefore, it was found that the magnesia carbon brick MT-14A broke bricks at the bottom of the furnace.

Take 2#~3# residual bricks from the first line of the wind eye, and cut 5 bricks (label 1#~5#) from the working face inward and parallel to the working face for physical and chemical analysis. The sampling thickness and analysis results are shown in Table 1.

Table 1.Analysis results of residual magnesia-carbon bricks at the bottom of the furnace

It can be seen from Table 1 that the bulk density of sample 1# is 3.30g·cm-3, which is far greater than the bulk density of sample 2#~5# (2.70~2.96g·cm-3), while the bulk density of the original brick The bulk density is 3.17g·cm-3; the apparent porosity of the 1# sample is 9.7%, which is significantly higher than the 7.5% and 8.6% of the 2# and 3#, but less than the 12.0% of the 4# and 5# samples. 11.1%, the apparent porosity of the original brick is 4.30%. From the point of view of chemical composition, the contents of CaO, SiO2, Fe2O3, Al2O3, Cr2O3, and MnO in sample 1# are much higher than those in samples 2#~5#, and the contents of CaO and SiO2 in all samples are much higher than the original samples. brick. Except 1# sample, the Fe2O3 content of other samples is far lower than the original brick. Only 1# sample contains oily Cr2O3. The MnO content of 1# sample is much higher than that of 2#~5# sample, but the original brick The C content in MnO.1# sample is only 3.7%, which is far lower than that in 2#~5# samples (C content is 15.32%~15.56%). The C and MgO content in all samples are low. In the original brick. Combined with Figures 1 to 4, it can be considered that in addition to the aforementioned thermal shock damage, the decarburization reaction and the melting loss of the magnesia carbon bricks are also the reasons for the damage of the magnesia carbon bricks.

(1) Decarburization reaction: The decarburization reaction is the contact between the working surface and the slag during tapping, and the slag hangs on the surface of the magnesia carbon brick. The graphite in the magnesia carbon brick is covered by the CaO, SiO2, Fe2O3, Al2O3, Various oxides such as Cr2O3 and MnO are reduced to form CO2 or CO, resulting in a sharp decrease in the C content near the working surface. At the same time, under the condition of high temperature and low vapor pressure, the C in the magnesia carbon brick will also interact with the C in the MgO and the brick. Impurities such as SiO2, Fe2O3, Al2O3 react, but C reacts preferentially with the impurities in the brick, and the GOR furnace bottom temperature decreases in the order of 2#~5# samples; in addition, the graphite and resin in the magnesia carbon brick are not formed after carbonization. Shaped carbon reacts with O2 under high temperature conditions and will also consume C. Therefore, the C content of all samples is lower than that of the original brick. The C content of samples 2#~5# increases from 15.32% to 15.56%, and at the same time The content of MgO decreases from 2#~4# samples, and the 1# sample is in direct contact with slag and molten steel, so the loss of C is the most serious.

(2) Melting loss of magnesia-carbon bricks: After the working face is in contact with the slag during tapping, part of the slag hangs on the surface of the working layer due to a sudden drop in temperature. When molten steel is smelted, CaO hangs in the slag on the working face under high temperature conditions. SiO2, Fe2O3, MnO, Al2O3, etc. react with the fused magnesia in the magnesia carbon brick to form low-melting forsterite, forsterite, magnesia rhodonite and other mineral phases to corrode fused magnesia. Some low-melting mineral phases and molten steel infiltrate into the magnesia-carbon brick along the voids left by the decarburization reaction and the fused magnesia (periclase) grain boundaries, eroding the magnesia-carbon bricks, resulting in the 1# closest to the working surface. The composition of the slag in the sample is higher, and the composition of the 2#~5# sample is higher than that of the original brick. Among them, Cr2O3 only appears in 1# bricks, and the content of Fe2O3 in 2#~4# bricks is less than that of the original bricks. This may be because Cr2O3 and Fe2O3 in the stainless steel slag are absorbed by the periclase phase in the working layer, which prevents it from further increasing to magnesium. Diffusion in carbon bricks.

Fig 3. Sectional photo of magnesia carbon brick MT-14A residual brick



Fig 4. SEM image of MT-14A residual brick surface

3 Conclusion

(1) 1000mm long magnesia carbon brick MT-14A can be used for 62 times in GOR furnace with an average furnace age of about 50 times; 800mm long magnesia carbon brick can be used for 41 times with an average furnace age of less than 30 times.

(2) The magnesia carbon brick MT-14A has broken bricks during use. It is not an ideal GOR furnace bottom brick, but it is easy to maintain. Some intermittent casting companies can choose MT-14A as the GOR furnace bottom according to their own production characteristics. brick.

(3) The damage mechanism of magnesia carbon brick MT-14A is thermal shock damage, fused magnesia melting loss and decarburization reaction.