Rimmed steel

Boiling steel refers to steel that is not well deoxidized during steelmaking. When steelmaking relies on oxygen to remove excess carbon, and excess oxygen will generate a variety of oxides as inclusions, so it must be deoxidized.

Molten steel contains a lot of oxygen, during the solidification process and the carbon in the steel produces a fierce reaction, the release of CO bubbles, so that the molten steel in the ingot mold "boiling" phenomenon named. The CO gas pressure pCO during the solidification of molten steel is 140kPa±25kPa. Boiling steel is limited to the production of carbon content 0.02% ~ 0.30% of low carbon steel, carbon content is too high or too low molten steel, in the solidification mold boiling is very weak, it is difficult to get a good boiling steel ingot. Boiling steel can only be produced by die casting and not by continuous casting.

Boiling steel can be divided into 5 zones:

(1) Hard shell belt. After contact with the mold wall, the molten steel is strongly cooled, forming a dense and solid shell band composed of fine equiaxed crystals without bubbles. The general thickness is 12 ~ 25mm. 

(2) Honeycomb bubble belt. When the cylindrical crystals in the lower part of the ingot grow towards the ingot center, they gradually grow into elliptical bubbles parallel to the growth direction of the cylindrical crystals.

(3) Central rigid belt. In order to prevent excessive segregation, after capping the liquid steel, the carbon-oxygen reaction is inhibited, the gas stops precipitation, the growth of honeycomb bubbles is terminated, and crystallization continues to form a central solid zone composed of columnar crystals without bubbles.

(4) Secondary bubble zone. With the growth of columnar crystals, when the carbon and oxygen are enriched to a certain extent due to segregation, the carbon and oxygen reaction occurs again, and the resulting bubbles are distributed in the entire height of the ingot, becoming the secondary bubble zone.

(5) Ingot core belt. When the difference between the mold wall temperature and the ingot center temperature is very small, the core of the ingot forms an equiaxial crystal, which becomes the ingot core belt. At this time, the boiling has largely stopped, but a small number of pores are still formed, and some bubbles can gradually move up and aggregate to grow into large bubbles on the head. Constitute the loose area of the head.

Classification of boiling steel

According to the different degree of deoxidation during smelting, steel can be divided into boiling steel, killed steel and semi-killed steel (the new standard cancelling semi-killed steel). Boiling steel is an incomplete deoxidized steel. Steel in the late smelting without adding deoxidizer (such as silicon, aluminum, etc.), pouring molten steel in the ingot mold boiling phenomenon (gas escape), after the ingot solidified, honeycomb bubbles distributed in the ingot, in the rolling process of this bubble cavity will be bonded. This type of steel is characterized by a very low silicon content in the steel, the standard provisions are trace or no more than 0.07%, usually injected into a small steel ingot without a thermal cap.

Kill steel is completely deoxidized steel. Usually cast into a large top with a small thermal cap at the bottom of the ingot type, the liquid steel is calm and not boiling during pouring. Because there is a thermal cap on the upper part of the ingot mold (which is used to supplement the liquid steel when the liquid steel is solidified), the cap head needs to be cut off after rolling, so the yield of the steel is low, but the structure is dense, the segregation is small, and the quality is uniform. High quality steels and alloys are generally killed steels.

Semi-killed steel is a more complete deoxidation steel. The degree of deoxidation is between boiling steel and killed steel, and there is boiling phenomenon when pouring, but it is weaker than boiling steel. This kind of steel has some advantages of boiling steel and killed steel, and it is difficult to master in smelting operation.