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Application of magnesia-chromium refractory materials (magnesia-chromium bricks) in RH/RH-OB furnaces

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RH/RH-OB lining is used under vacuum sealing conditions, where the breather pipe, pipe mouth, bottom and oxygen blowing outlet are all impacted by the maximum molten steel circulation speed. Therefore, its initial erosion method may be the cracking of the refractory material close to the hot surface, causing corrosion. Estimated based on the usage conditions of RH/RH-OB, it is believed that the lining will cause structural fracture or weakening in all processes, which includes the following points:

(1) Thermal shock resistance damage caused by rapid heating or cooling of the hot surface between preheating and molten steel treatment (the greater the temperature difference, the greater the damage);

(2) Due to changes in oxygen pressure and/or temperature, Fe+2/Fe+3 oxides alternately cycle (the refractory chromite and chromite spinel phases as well as the zones that absorb iron oxide will be affected by this ) causes damage to the lining;

(3) With the melting of the usual refractory bonding phase, a channel is provided for the invasion of slag, and the slag is penetrated (silicate bonding is particularly sensitive). This leads to erosion of the direct hot surface portion where iron oxide is enriched in the iron oxide absorption zone. When all relevant temperatures are considered, a substantial liquid phase will form wherever the iron oxide content exceeds 30%. Therefore, in this case, even if the circulating molten steel contacts the lining parts that are less eroded, such as the upper part of the lower cylinder side wall (excluding the oxygen blowing tuyere area), it will be partially liquefied and the liquid surface area will be corroded. ;

(4) Because the slag is soaked, the leading edge of hot surface erosion is close to the inside and cracks are formed, so some damage may be the result of peeling loss of part of the hot surface. Usually this damage to the lining is discontinuous (discontinuous type). Due to the relatively fast advancement of the erosion front and the peeling ability of the flowing molten steel, the peeling loss rate of severely corroded parts is larger (forward damage).

According to the above-mentioned corrosion mechanism of the working lining of the RH/RH-OB device, the refractory material for the working lining is usually directly combined with magnesia-chrome bricks or alkaline rammed integral lining. And adopt partitioned lining (comprehensive lining) according to the different use conditions and product quality of different parts.

Under this condition, the selected magnesia-chrome bricks should have the following requirements:

(1) When subjected to thermal shock during use, there is little deterioration in strength and structure;

(2) The slag is difficult to penetrate, and even if it penetrates, it can maintain the bonding between particles and the due strength;

(3) High peeling resistance.

The magnesia-chromium bricks with these properties are related to the development degree of the secondary spinel generated on the grain boundaries, and are also affected by the chemical composition of the magnesia-chrome bricks on the secondary spinel formed.

Magnesia chrome bricks for RH/RH-OB installations include traditional direct bonded magnesia chrome bricks, rebonded magnesia chrome bricks, semi-rebonded magnesia chrome bricks and special composite magnesia chrome bricks. Among them, the best amount of Cr2O3 in magnesia-chrome bricks is the special composite magnesia-chrome brick with a Cr2O3/MgO ratio equal to 0.2~0.4. Typical characteristics of these products are shown in Table 1.

Product CategoryBrick A (traditional)Brick B (composite) Brick C (special)Brick D (recombined)
chemical composition,%    
MgO63.761.760.059.8
Cr₂O15.218.021.319.2
CaO0.70.50.60.6
SiO₂1.61.11.01.8
Bulk density, kg/m3.13.23.33.3
Apparent porosity,%16161313
Normal temperature compressive strength, MP37626346
Flexural strength, MPa    
normal temperature491011
1260℃12121416
1480℃3345
Flexural strength before cycling, MPa49914
Flexural strength after cycling at 1200℃, MPa2232
loss,%50786786
Bending temperature after cycling at 1300℃, MPa1121
loss,%75897893
Erosion rate①,mm/n0.80.70.50.4
Loss index, %100886250

Magnesia-chromium brick C in Table 1 is a special composite directly bonded magnesia-chromium brick with sintered magnesia-chromium sand as particles and periclase and chromium as binding matrix. Magnesia-chrome bricks B and D are semi-rebonded magnesia-chrome bricks and rebonded magnesia-chrome bricks respectively; magnesia-chrome brick A is a traditional and ideal direct-bonded magnesia-chrome brick. As shown in Table 1, brick A has the best thermal shock resistance but the worst erosion resistance; combined brick D has the best erosion resistance, but has the greatest thermal shock damage. Semi-rebonded brick B and special composite brick C have moderate erosion resistance and better thermal shock resistance than brick D. Among them, the special composite high-chromium brick C is the best combination of various properties, with corrosion resistance and thermal shock resistance ranking second, low porosity and high strength.

In addition, among the traditional directly bonded magnesia-chromium bricks, magnesia-chromium bricks fired at high temperature are better. When producing magnesia-chrome bricks, regarding the reaction between chromium ore and periclase, research has been conducted on high-grade chromium ore and nearly single-crystal fused MgO as raw materials, as shown in Figure 1. And came to the following conclusions:

(1) The Cr2O3 component in chromium ore is solid solution in fused MgO. Spinel containing (Mg, Fe, Al, cr)2O4 is dissolved out (the content of each R2O3 in the spinel varies depending on the chemical composition of the chromium ore used). R2O3 is easy to solidify inside MgO in the order of Fe2O3>Al2O3>Cr2O3. However, the solid solubility of each R2O3 content increases in the order of Cr2O3>Fe2O3>Al2O3 near the contact surface with chromium ore.

(2) SiO2 and MgO in chromium ore react to form a liquid phase with SiO2 and MgO as the main components. When the SiO2 content is high, the amount of liquid phase generated is also high. It was also confirmed that SiO2 promotes the formation of liquid phase. In addition, like the erosion of electrofused MgO, a liquid phase is generated all the way to the inside, as shown in Figure 2.

It can be concluded that even high-grade (high Cr2O3 content) chromium ore with high SiO2 content is not suitable as a raw material for high-temperature fired directly bonded magnesia-chromium bricks used under harsh conditions.

Since iron oxides are greatly affected by oxygen pressure, the Fe2O3 content in the magnesia-chromium bricks used for lining the RH/RH-OB device should also be controlled.

By adding a certain amount (for example, 13%) of sintered chromium spinel sand (5~0.5mm) to magnesia-chromium bricks to produce composite magnesia-chromium bricks, magnesia-chromium refractory materials composed of heterogeneous multi-phase materials can be obtained. It was also found that there is a direct bond between sintered chromium spinel particles and fused periclase fine powder, but there is less direct bonding between fused granular magnesia chromium material and periclase fine powder. It is not difficult to deduce from this that the sintered chromium spinel refractory material combined with electrofused MgO is used under vacuum and conditions of severe temperature fluctuations, atmosphere changes and slag erosion. It will have higher volume stability and corrosion resistance than refractory materials based on fused magnesium chromium material or sintered chromium spinel.

To improve its performance, the following technical measures can be adopted

(1) Use high-purity magnesia and high-purity (very low SiO2) chromium ore as raw materials and increase the proportion of chromium ore to produce high-quality magnesia-chrome bricks with a high Cr2O3/MgO ratio;

(2) Add a certain amount of Cr2O3 powder or ultrafine chromium ore powder to promote the sintering of magnesia-chromium series materials and obtain high-quality magnesia-chromium bricks with developed secondary spinels;

(3) Add an appropriate amount of Fe-Cr and other metal powders to reduce the porosity of magnesia-chromium bricks and form a microporous structure in the matrix through their oxidation during firing;

(4) The magnesia-chromium bricks are fired in an oxidizing atmosphere under ultra-high temperature conditions, and slowly cooled after firing to obtain a well-developed secondary spinel crystal structure;

(5) Add a certain amount of special additives with a smaller thermal expansion rate than magnesia-chrome bricks or additives such as CaCO3 (0.1~2.0mm) and ZrO2 to improve the thermal stability of magnesia-chrome bricks.

By adopting the above measures, high-quality magnesia-chrome bricks with high high-temperature strength, excellent corrosion resistance and high thermal stability can be made.

LMM COTINUOUS CASTING

LMM ROLLS

LMM GROUP

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

Our Product have been supplied to world’s top steel manufacturer Arcelormittal, TATA Steel, EZZ steel etc. We do OEM for Concast and Danieli for a long time

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