Why is powder used in casting?

What are mold powders for continuous casting?

Continuous casting mold powder is a functional material based on silicate and containing a variety of fluxes and carbonaceous framework materials. It is a key auxiliary material in the steel metallurgical continuous casting process. It is mainly used in the continuous casting mold. When melted on the molten steel surface, a three-layer structure of liquid slag layer, sintered layer and molten slag layer is generally formed. Under normal continuous casting conditions, molten powder fills the gap between the mold and the slab at the meniscus, which has an important impact on the surface quality of the slab and the progress of the continuous casting process. Continuous casting mold powder is mainly composed of three parts: base material, flux and carbonaceous materials, containing chemical components such as CaO, SiO2, Na2O, CaF2, Al2O3, MgO, Li2O, C particles. Among them, CaO+SiO2 accounts for 60% to 70%, and CaO/SiO2 is 0.6 to 1.5.
Mold powder is an important auxiliary material in continuous casting production, in addition to pouring slabs with smaller cross-sections, most mold powder (with immersed nozzles) is used. The practice has proved that the application of continuous casting mould powder has greatly expanded and improved the types of continuous casting steel, types of continuous casting sections, quality of continuous casting billets, and continuous casting productivity. Therefore, it has long become an indispensable metallurgical auxiliary material.
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What is the function of mold powder?

The function of mold powder is to insulate and maintain heat, isolate air, prevent secondary oxidation of molten steel, purify the steel slag interface, adsorb inclusions in molten steel, lubricate the billet shell and improve solidification heat transfer.
The protective powder added to the molten steel surface of the mold gradually heats up and sinters and melts under the action of the heat released by the molten steel, forming a double-layer, three-layer or multi-layer slag layer structure on the molten steel surface of the mold. Since the billet moves downward at a certain drawing speed and the crystallizer vibrates up and down, a negative slipping motion occurs, which creates a pressure difference and sucks the slag into the space between the mold and the billet shell. Side solidification forms a solid slag film of glass or crystal, and the liquid slag state is maintained on the side of the shell. As the mold vibrates and the billet is drawn, the liquid slag and solid slag film are brought out of the lower mouth of the mold and separated from the cast billet under the impact of the secondary cooling water, thereby completing the consumption process of the mold powder. The role of mold powder from the time it is added to the time it leaves the crystallizer can be summarized as follows:

1) Thermal insulation and reduces heat loss of molten steel

The solidification of the molten steel surface and the premature solidification of the meniscus green shell will have adverse effects on the surface of the cast slab. The floating inclusions in the molten steel may be trapped by the solidified iron crystals, forming a hard shell structure composed of metal and oxides, which can cause serious defects after being rolled into the billet shell.
Mold powder is added to the molten steel surface to form a slag layer, and evenly covers the molten steel surface to isolate the molten steel from the air. When melted, the mold slag is specifically composed of five layers: solid slag layer, sintered layer, semi-melted layer, carbon-rich layer and liquid slag layer, which plays the role of thermal insulation, preventing re-oxidation and absorbing inclusions. Among them, the solid slag layer mainly plays the role of heat insulation, and the oxidation and heat generated by carbonaceous materials can also play a role in heat preservation.

2) Isolate air and prevent secondary oxidation of molten steel

When the protective powder is added to the molten steel surface in the mold, a liquid slag layer, a sintering layer and a solid slag layer are quickly formed and are evenly spread on the molten steel surface to isolate them from the air, thereby effectively preventing air from entering the molten steel. , to prevent secondary oxidation of molten steel.

3) Absorb and dissolve non-metallic inclusions

The molten steel entering the mold will inevitably bring in non-metallic inclusions. In addition, the inclusions in the liquid phase cavity of the cast slab in the mold that float to the meniscus of the molten steel may be drawn into the billet shell to form surface and subcutaneous inclusion defects. After the mold powder absorbs the inclusions, the performance is stable and the viscosity is appropriate. The inclusions can be taken away with the consumption of the mold powder.

4) Lubricating between the crystallizer wall and the billet shell

The slag film between the crystallizer and the shell is composed of a solid slag film and a liquid slag film, and the liquid slag film has a lubricating effect. The filling layer into which molten slag flows into the upper part of the mold adheres to the mold and moves with the mold, while a liquid slag lubricates the billet between the shell and the filling layer, which is actually fluid lubrication. The lubrication situation is related to the amount of liquid slag penetrating between the crystallizer and the shell and the uniformity of the penetration. The solid slag film is composed of a vitreous film and a crystalline film. Slag with low alkalinity and excellent glassy properties has good lubrication properties, while slag with high alkalinity and high crystallization rate has poor lubrication properties.

5) Improve and control the heat transfer between the slab and the mold to prevent longitudinal cracks.

The slag film fills the air gap between the shell and the crystallizer to ensure uniform heat transfer. The thickness of the crystalline layer directly determines the thermal conductivity. A thin crystalline layer means better thermal conductivity. Avoiding longitudinal cracks in medium carbon steel is the first issue to consider. The focus of the design of mold powder for medium carbon steel should be on controlling the heat flow from the slab to the mold and limiting the heat flux in the mold. It is hoped that the mold powder has a larger thermal resistance. This puts forward requirements for the composition ratio of continuous casting powder.
Therefore, in order to achieve the stability of slab quality in the continuous casting machine, it is necessary to add mold powder to the crystallizer. Manual powder adding generally uses a shovel to add powder, the powder addition is uneven and cannot melt evenly on the surface of the crystallizer. Powder needs to be added frequently and ambient temperature is too high, which is not conducive to the health of workers. The automatically adding continuous casting powder robot system produced and developed by our company, using an industrial robot (linear guide) as a platform, can add mould powder to the surface of the crystallizer without any dead ends.
casting powder, steelmaking robot, continuous casting mould powder adding

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