The tundish is a container located between the ladle and the crystallizer in the continuous casting process. It is responsible for many tasks such as receiving and storing molten steel, smoothing the flow, purifying the molten steel, and diverting the molten steel into the crystallizer. As the last refractory container before the molten steel solidifies, it has a great influence on the final steel quality. Therefore, the structure of the tundish and the refractory materials used are crucial. Judging from the current research work done by people, magnesium-calcium refractory materials have been widely used in clean steel smelting because of their ability to purify molten steel. For example, tundish coating.
Around 1975, WISCO Refractories Factory and Luoyang Refractories Research Institute cooperated and developed magnesium-chromium intermediate package coating, but due to the use of magnesium-chromium coating in the process of hexavalent chromium generation, this material is gradually eliminated.
Around 1990, Cr-free and excellent performance of magnesium intermediate ladle coating and magnesium-calcium coating applicable to clean just smelting were developed. This material is widely used in continuous casting production, to meet the needs of China’s large-scale intermediate package and clean steel continuous casting production.
Magnesium intermediate ladle coating takes magnesium sand as the main raw material, and according to the use conditions, there are electrofused magnesium sand, medium grade magnesium sand and ordinary sintered magnesium sand for selection. The main binding agent is silica micropowder, water glass powder and polyphosphate. According to the construction method, the intermediate package coating can be divided into artificial coating and mechanical spraying two kinds, of which the mechanical spraying is mainly used in large-scale intermediate package with high construction efficiency. The thermal conductivity of the intermediate package coating is small, which can meet the needs of the intermediate package for heat preservation and ensure the smooth progress of multi-furnace continuous casting. Coating construction is easy to apply, baking and use of non-cracking, non-flaking, with good integrity and adhesion. However, the intermediate package coating due to the addition of water in the construction process will make the calcium oxide hydration, baking process of hydration product decomposition caused by cracks on the surface of the material, and at the same time, the evaporation of water will have an impact on the permanent layer, reducing the service life of the permanent layer. If the water is not discharged during baking, it will even cause the liquid steel to oxidize and absorb hydrogen.
The study shows that the addition of SiO2 micropowder can improve the compressive strength of magnesium coatings at room and intermediate temperatures, and the material also has good performance at high temperatures, and the appropriate addition amount of SiO2 micropowder is 4 wt%.
By adjusting the matrix composition of the magnesium intermediate package coating, it was used to improve the medium temperature strength of the intermediate package coating. It was found that the addition of SiO2 micropowder increased the mid-temperature strength of the material, and the room-temperature flexural and compressive strengths, line change rate and slag resistance were stabilized when the addition amount exceeded 4 wt%. Boron glass was also introduced and found to improve the mid-temperature strength, but the amount added should not be too much.
By adding admixtures such as sodium tripolyphosphate, sodium tripolyphosphate and carboxymethyl salts, carboxymethyl salts, and inorganic salts to the coatings, it was found that the addition of sodium tripolyphosphate had a significant improvement on the properties of the coatings, such as bulk density and flexural and compressive strength.
Magnesium olivine was used to replace magnesium sand to study its effect on coating properties. The results show that: with the increase of magnesium olivine addition, the bulk density, room temperature flexural and compressive strength of the material are gradually reduced, the rate of change gradually increases, and the material slag resistance deteriorates, but when the magnesium olivine addition is less than 30wt%, the above changes are not significant.
The lightweighting of magnesium intermediate package coatings was studied by adding fiber and organic blowing agent, and it was found that the addition of fiber could effectively reduce the bulk density of the intermediate package coatings, but it would make the coating drying shrinkage increase, and the addition of organic blowing agent could further reduce the bulk density of the coatings and reduce the drying shrinkage.
What is the protective coating for magnesium?
Magnesium can be coated by electrochemical plating in which the part is covered with a metal that possess the desired characteristics of the end material. The metal is present as a salt in solution and is reduced into its metallic form onto the part’s surface.
What is the protective layer of magnesium?
Magnesium starts off instantly forming an oxide layer. But, after a while, in the presence of moisture, it will absorb carbon dioxide from the air and form a carbonate. Magnesium is basic and so will react with the CO2 in the air.
What is the black coating on magnesium?
Black magnesium coating
In addition to low reflectivity, black magnesium coatings provide excellent thermal, corrosive and wear resistance; allowing engineers to extend the lifespan of components in space equipment where maintenance is mostly impossible.
Does magnesium form a protective coating naturally?
When exposed to air and water, magnesium forms a natural oxide layer. While this layer offers some protection, it is not robust enough to withstand acidic or neutral environments. The oxide layer can easily break down, exposing the underlying metal to further corrosion.