Comprehensively understand the material properties of casting powder

Casting powder is an important auxiliary material in continuous casting. Along with the development of steel-making and continuous casting technology in China, the technology of slag protection has been continuously improved.Under the background of fierce competition in the steel market at present, continuous casting is making comprehensive technical improvement with the characteristics of specialization, constant drawing speed, high quality and multiple varieties.

The application of continuous casting slag has greatly expanded and improved the variety of continuous casting steel, the type of continuous casting section, the quality of continuous casting billet and the productivity of continuous casting, so it has already become an indispensable auxiliary material for metallurgy.The Casting powder on the molten steel surface of the crystallizer is added, and under the action of heat released by molten steel, it gradually heats up and sinter and melt, forming a double-layer, three-layer or multi-layer slag layer structure on the molten steel surface of the crystallizer. As the casting billet moves downward at a certain speed and the mould vibrates up and down, negative slipage movement is generated and the pressure difference is generated to suck the molten slag between the mould and the mould shell. Under the cooling of the mould wall, the solid slag film of vitreous body or crystal is formed by solidification on the side of the mould wall, and the liquid slag state is maintained on the side of the mould shell.With the process of mold vibration and billet drawing, the liquid slag and solid slag film are taken out of the lower opening of the mold and separated from the billet under the impact of secondary cold water, thus completing the consumption process of Casting powder.The effect of the Casting powder from joining to leaving the mold can be summarized as follows:

(1) To prevent the secondary oxidation of molten steel.

The tundish injection flows into the mould and the metal surface in the mould is constantly renewed due to the impact of the injection flow.When the Casting powder is added to the molten steel surface in the crystallizer, the liquid slag layer, the sintering layer and the solid slag layer are formed rapidly and spread evenly on the molten steel surface to isolate it from the air, thus effectively preventing the air from entering the molten steel and preventing the secondary oxidation of the molten steel.

(2) Thermal insulation to reduce heat loss of molten steel.

The solidification of molten steel surface and the advance solidification of the primary billet shell on the meniscus will have a bad effect on the surface of the billet.Because the floating inclusions in the molten steel may be caught by the solidified iron crystals, forming a hard shell structure composed of metals and oxides, it can cause serious defects when it is involved in the billet shell.The thermal insulation effect of slag is realized by the solid slag layer with low temperature and low volume density covering the molten steel surface.Therefore, an appropriate increase in the thickness of the slag layer can improve the thermal insulation performance of the slag and increase the temperature of the liquid slag layer.However, too thick solid slag layer will prolong the sintering time of Casting powder at high temperature, leading to the risk of agglomeration and slag strip seriously endangering continuous casting production.

(3) Absorption and dissolution of non-metallic inclusions into.

Non-metallic inclusions are inevitably brought into the molten steel in the mould. In addition, inclusions floating on the meniscus of molten steel in the liquid cavity of the mould may be involved into the billet shell to form surface and subsurface inclusions.From the point of view of thermodynamics, silicate slag system can absorb and dissolve such nonmetallic inclusions, but its dissolution rate is affected by many factors.Under the economic and clean steel production mode, although the inclusion in molten steel is reduced, it will affect the properties of molten slag after entering the slag. It is hoped that the property change of molten slag should be controlled within the permissible range of continuous casting process.

(4) It plays a lubrication role between the mold wall and the billet shell.

A layer of liquid slag with appropriate properties and uniform thickness is needed between solidified billet shell and copper wall of mold to reduce solid-solid friction.The slag layer on the steel surface continuously provides lubricant between the billet shell and the mold wall.In order to ensure the continuous supply of liquid and slag, the meniscus must be kept smooth, and in order to give full play to the lubrication effect, the liquid and slag must have the properties of glass state, and there should be no precipitation of high melting point crystals in the liquid and slag.In order to reduce the friction force, a certain amount of slag consumption must be maintained.

(5) Improve and control the heat transfer between the casting billet and the mold.

The results show that the thermal conductivity of air is about 0.09 W/(m·K), while that of slag film is about 1.2 W/(m·K).During the solidification process, the volume shrinkage of the billet shell causes an air gap between it and the mold, which increases the thermal resistance and slows down the heat transfer in the lower part of the mold, which is not conducive to the rapid increase of the thickness of the billet shell and the increase of the drawing speed to improve the productivity.But on the other hand, in the meniscus region air gap is very small, the heat transfer ability of the flux film is too strong, the green shell by uneven cooling effect at the beginning of the big and the thickness becomes uneven, and as a result of the non-uniform flow field and temperature field in the early green shell thickness nonuniformity is stronger, and the stress caused by peritectic reaction, such as groups, may cause cracks on slab surface and subcutaneous.Especially for the slabs of medium and thick plates, even the subcutaneous cracks of the unexposed slabs are easy to remain on the steel plates when rolling the thick specification plates, leading to a large amount of cleaning and even a high scrap rate.In the production of thick slab, low temperature zone is easy to appear because of the large meniscus of section, and it is easy to bond. CH Refractory produced high quality Casting powder plays a role in effectively coordinating and controlling the heat transfer in the meniscus area and ensuring the lubrication of the billet.