What Is Killed Steel?

The killed steel refers to completely deoxidized steel, that is, the mass fraction of oxygen does not exceed 0.01% (usually often in the range of 0.002% to 0.003%). According to the degree of deoxidation during smelting, steel can be divided into boiling steel, semi-killed steel, killed steel and special killed steel. The sedated steel is usually cast into a large ingot shape with a thermal insulation cap, and the molten steel is sedated and does not boil during casting. Because the upper part of the ingot mold has a thermal insulation cap (used to supplement the molten steel when the molten steel is solidified), this head of the cap needs to be cut off after rolling and blooming, so the shrinkage of the steel is low, but the structure is dense, the segregation is small, and the quality is uniform. High-quality steels and alloy steels are generally calm steels.

The killed steel refers to completely deoxidized steel, that is, the mass fraction of oxygen does not exceed 0.01% (usually often in the range of 0.002% to 0.003%). According to the degree of deoxidation during smelting, steel can be divided into boiling steel, semi-killed steel, killed steel and special killed steel. The sedated steel is usually cast into a large ingot shape with a thermal insulation cap, and the molten steel is sedated and does not boil during casting. Because the upper part of the ingot mold has a thermal insulation cap (used to supplement the molten steel when the molten steel is solidified), this head of the cap needs to be cut off after rolling and blooming, so the shrinkage of the steel is low, but the structure is dense, the segregation is small, and the quality is uniform.

1. Boiling steel

During the steelmaking, only ferromanganese was added for deoxidation, and the deoxidation was incomplete. When this molten steel ingot is cast, a large amount of carbon monoxide gas escapes, and the molten steel is boiling, so it is called boiling steel, and its code is "F".

The boiling steel structure is not dense enough, the composition is not uniform, and the segregation of impurities such as sulfur and phosphorus is serious, so the quality is poor. But because of its low cost and high output, it is widely used in general engineering.

2. Calm steel

Ferromanganese, ferrosilicon, and aluminum ingots are used as deoxidants during steelmaking, and the deoxidation is complete. This type of molten steel can fill the ingot mold calmly and solidify when it is cooled, so it is called as a calm steel, and its code is "Z".

Although the cost of killed steel is higher, the structure is dense, the composition is uniform, the sulfur content is less, the performance is stable, so the quality is good. Suitable for

Low-carbon aluminum killed steel (LCAK steel) is often used as a raw material for refrigerators and other home appliance panels. With the advancement of society, users have increasingly higher quality requirements for low-carbon aluminum killed steel cold-rolled sheets. High cleanliness. As a typical non-steady-state casting, due to the extremely unstable casting state in the continuous casting start-up phase, it is easy to cause the mold slag to be involved and the molten steel to be severely oxidized.

For low carbon aluminum killed steel, the main factor that deteriorates the cleanliness of the slab is the tundish secondary oxidation, and in addition to the tundish secondary oxidation caused by air, a greater degree of secondary oxidation is caused by the tundish refractory Materials and covering slag.

Therefore, if you want to improve the cleanliness of low carbon aluminum killed steel (LCAK steel), the following five aspects can be performed.

(1) To ensure the quality of refractory materials, use low-silicon refractory materials, and increase the level of masonry tundish refractory linings to make them smoother;

(2) Ensure the baking time of the tundish and the spout to prevent the inner water from contaminating the molten steel

(3) Strengthen the protective pouring, pass a large amount of argon gas into the tundish and crystallizer for argon washing, and use a closed tundish as much as possible to reduce the suction of molten steel. Some studies have concluded that when the tundish capacity is 60t, blowing argon at a flow rate of 4500 L / min for 6 minutes can reduce the oxygen concentration in the atmosphere in the tundish to less than 0.2%;

(4) Cover the slag with a low oxidation tundish. Some studies have found that, because the activity of SiO2 in the slag can be reduced, the oxygen content of the tundish molten steel can be reduced better when the high-basicity slag is used;

(5) Wait for the tundish to reach a certain level before pouring.

(6) Reduce the increase rate of the drawing speed during pouring and reduce the drastic fluctuation of the liquid surface to reduce the involvement of the mold slag ^[3] .