Causes Of Damage And Methods For The Improvement Of Magnesium-chromium And Magnesium-calcium Bricks Used in VOD Furnaces.

Furnace body:

Double water-cooled structure lined with refractory materials (e.g. magnesium-chromium bricks, aluminum-magnesium-chromium bricks, etc.).

Vacuum system:

Top vacuum hood, piping and vacuum pump for vacuum decarburization.

Oxygen lance system:

Located above the furnace opening for blowing in oxygen for decarburization reactions.

Tilting mechanism:

Supports tilting of the furnace body for charging or dumping of slag.

Temperature measurement and sampling device:

Monitoring of liquid steel temperature and composition.

Liquid steel transfer:

Pour the liquid steel from the primary furnace (e.g. AOD, electric furnace) into the VOD furnace.

Vacuum decarburization:

close the furnace cover and vacuum, blowing oxygen decarburization, control CO partial pressure to avoid excessive oxidation.

Alloying adjustment:

Adding alloying elements (e.g. Cr, Ni) to adjust the steel composition.

Desulfurization/degassing:

Remove impurities such as sulfur, hydrogen and nitrogen through vacuum environment.

Steel discharge:

Tilting the furnace after completing the refining, the steel is poured into the ladle.

 In the configuration of the refractory materials for VOD furnace lining, the bottom and steel parts of the direct combination of magnesium-chromium bricks;

 Slag line parts of the semi-recombined magnesium-chromium bricks, electrofusion re-combined with aluminum-magnesium-chromium bricks; lining patching material using MgO-MgO·Al2O3 quality castables.

 Mainly due to slag infiltration makes magnesium sand become loose, chemical erosion, structural spalling, pulverization; at the same time due to slag infiltration caused by high temperature spalling, steel is strongly agitated or slag abrasion. Magnesium-chromium brick after heating and cooling, the strength and toughness of the material is reduced, the organization cracking, promote the slag to the brick infiltration, making the particles easy to lose, and accelerate the erosion of the brick. In order to improve the magnesium-chromium bricks at high temperatures greater than 1700 ℃ resistance to VOD slag erosion, improve its performance, should be improved from the magnesium-chromium brick production process.

(1)Strictly control the chemical composition in the bricks.

Content of Cr2O3

For VOD furnace slag line parts with magnesium-chromium bricks, generally its erosion rate are reduced with the increase of Cr2O3 content in the bricks, so increase the content of Cr2O3, can increase the magnesium-chromium brick erosion resistance.

w(Cr2O3)/w(MgO) ratio

w(Cr2O3)/w(MgO)=2/3 is optimal. When the content of Cr2O3 exceeds 25%, the material is saturated with secondary magnesium-chromium spinel, which prevents the penetration of low alkalinity slag, and can improve the resistance of bricks to erosion.

SiO2 content

SiO2 is the main factor in reducing the erosion resistance of high-temperature fired magnesium-chromium bricks, and its content should be as low as possible, so 'to make bricks should be strictly controlled its content to less than 1% is appropriate.

Fe2O3 and Al2O3 content

Spinel in magnesium-chromium bricks increases with the decrease of Fe2O3 content and the increase of Al2O3 content, so the adjustment of Fe2O3 and Al2O3 content in bricks can promote the content of spinel and improve the erosion resistance of bricks.

Strict control of impurity CaO

CaO can generate low melting point silicate phase with SiO2 in the molten Badger and MgO in the bricks, reducing the anti-erosion effect of the material, so its content should be strictly controlled.

(2) Brick making process.

Particle size and its distribution.

Multi-stage batching of coarse, medium and fine interrupted particles is used to fill the gaps of coarse particles to obtain products with low porosity.

High pressure molding.

Semi-finished product body density is the key to obtain low porosity products, so high-pressure molding is also seen to be beneficial to improve the material said body density, reduce the porosity of the material, reduce the slag erosion channel, improve the material's anti-erosion effect.

Ultra-high temperature firing.

Control the cooling rate, improve the partial pressure of oxygen in the firing atmosphere and other process conditions is to improve the performance of magnesium-chromium brick is an important aspect.

 VOD furnace slag line parts using asphalt or tar combined with dense dolomite products or magnesium carbon bricks (w(C)=10%); the bottom and steel parts using asphalt or tar combined with dolomite products.

 The damage mechanism of magnesium calcium bricks used in VOD furnace and magnesium chrome bricks have similarities, still chemical erosion, structural spalling, high temperature spalling, wear and tear and so on.

 The difference is that when using magnesium dolomite bricks or dolomite bricks, SiO2 in the low alkalinity slag and CaO in the bricks generate 2CaO-SiO2 or 3CaO-SiO2, which can make the slag become high melting point and high viscosity slag. However, when the solubility of FeO or Al2O3 in the slag is high, it will generate low melting point with CaO in the bricks and aggravate its destruction.

(1) Improve the fusion resistance and inhibit cracking.

The main method is to increase the content of MgO in dolomite or increase the content of CaO in magnesium dolomite; in order to improve the thermal shock resistance of magnesium dolomite bricks, zirconium oxide can be added to the material.

(2) Enhance the anti-hydration property of raw materials.

1) Additives.

Add all kinds of additives (Fe2O3, CuO, CaCl2, 2CaO-Fe2O3 or CaO-TiO2) in raw materials to promote the crystallization of CaO particles to increase, and CaO and completely covered, to improve the microstructure of magnesium-calcium sand.

2) Special treatment of raw materials.

In the production process of raw materials, adding rare earths (La2O3, CeO2, Y2O3) and other special additions to the raw materials can improve the anti-hydration effect of magnesium-calcium sand; magnesium-calcium sand or calcium sand is carbonated and passivated; soaking treatment (using an aqueous solution of phosphoric acid or boric acid, etc. to soak the raw materials, generating a layer of inorganic calcium salts on the surface of the particles to encapsulate the surface of the material); spraying treatment (spraying organic solvents such as styrene, butadiene, 2% borax solution to coat the surface and isolate the air).

(3) Products waterproofing measures.

1) Improve the brick-making process.

Using oil or paraffin impregnation can block the pores in the bricks, prevent air from entering and making the free CaO hydration in the products; using vacuum and aluminum foil packaging can isolate the air and prevent the hydration of the products.

2) Using anhydrous resin.

Use anhydrous resin to make magnesium and calcium carbon to reduce the hydration in the mixing and milling process.

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