The figure below shows eight typical states of industrial steel ingots., which have been cast in identical, tapering molds. They are numbered and numbered according to the degree of suppression of gas evolution during solidification.. The dashed lines show the level, to which steel was originally cast in each mold. Depending on the carbon content, and even more - from the oxygen content, the structure of the ingots is different. Under the number 1 the steel is completely calmed down, calm steel, and under number 8 – steel, to which deoxidation operations were not applied, strong boiling steel.
When casting into ingots, steels are classified into three main types, depending on degree of deoxidation or, which is the same, by the amount of gases, released during the solidification of the ingot.
These four types are:
– calm steel;
– half-dead steel;
– boiling steel.
In English, calm steel is called slightly "intimidating" – killed steel. Calm steel is steel, in which there is practically no gas evolution during the solidification of the ingot after its casting. This is ensured by the complete deoxidation of steel. – complete removal of oxygen from it and the formation of a shrinkage cavity in the upper part of the ingot. This part of the ingot is then cut and scrapped..
All alloy steels, most low alloy steels and many carbon steels are usually used as quiet steels. With continuous casting, the steel is also completely "quieted". Calm steel is characterized by a homogeneous structure and a uniform distribution of chemical composition and properties.
To obtain a calm steel, it is deoxidized with aluminum., as well as manganese or silicon ferroalloys. Besides, sometimes calcium silicide and other special deoxidizers are used.
In semi-calm steel, the release of gases during its deoxidation is not completely suppressed., since steel is only partially deoxidized. In English they call it semikilled steel. The degree of gas evolution in these steels is greater, than in calm steels, but less than boiling. Before the start of gas evolution, an ingot crust of considerable thickness is formed in the ingot.. Correctly "semi-oxidized" steel ingot has no shrinkage cavity, but there are bubbles widely dispersed in thickness in the central zone of the upper part of the ingot. These bubbles, However, welded when rolling the ingot. Semi-quiescent steels usually have a carbon content of 0,15 to 0,30 %. They are widely used in the production of long products, strip and pipes.
The main differences between semi-calm steels are:
1) varying degrees of inhomogeneity of the chemical composition - average between the degrees of calm and boiling steels;
2) less segregation of chemical elements, than in calm steel;
3) pronounced tendency of positive chemical segregation in the center of the upper part of the ingot (picture).
Boiling steel is characterized by:
– high degree of gas evolution during solidification of steel in the mold;
– noticeable difference in chemical composition over the cross section of the ingot and between the upper and lower parts of the ingot (cm. picture).
This leads to the formation of relatively pure iron in the outer shell of the ingot and the inner core of the ingot with a high concentration of alloying and impurity elements., special, carbon, nitrogen, sulfur and phosphorus, which have a low melting point. The cleaner outer part of the ingot is used for rolling. Boiling steel ingots are well suited for the production of many products, such as slabs, sheets, wire, pipes, as well as long products with requirements for surface cleanliness and viscous properties.
Boiling steel production technology is limited by the maximum carbon and manganese content. This steel does not contain any appreciable amount of strong deoxidizers, such as aluminum, silicon or titanium. Boiling steel is cheaper, than calm and semi-calm, so only a small amount of a deoxidizer is used for it, and the top of the ingot is not scrapped.
In English, boiling steel is usually rimmed steel. From the English word rim - rim, repair, bandage. This term reflects the main feature of the boiling steel ingot - the outer shell (repair, rim, bandage) pure steel. Another type of boiling steel is described in the English-language technical literature - capped steel.
FROMapped steel – it is a type of steel with characteristics similar to boiling steel, but according to the degree of suppression of gas evolution during solidification, it is located between semi-calm and boiling steels (cm. picture). The name of this type of steel comes from the English word cap in the Russian meaning "cover", so these ingots after casting are mechanically or chemically closed, "Clog" from above. Therefore, their Russian term could be "sealed steels".
Fewer deoxidizers are used to produce sealed steel ingots, than for semi-calm ingots. This makes it possible to a certain extent to control the formation of the outer layer of the ingot during its solidification..
Ingot plugging technology is a variation of boiling steel production technology. The ingot plugging operation limits the time for gas evolution and prevents excessive gas bubbles from forming inside the ingot. Clogged ingots are usually applied to steels with a carbon content greater than 0,15 %, from which sheets are made, ribbons, thin slabs, strips, wire and rods.
For mechanical clogging of steel ingots, heavy cast iron covers are used., to seal the mold from above and stop the formation of the outer shell, how it happens with boiling ingots. Chemical clogging of ingots is carried out in open molds. Clogging is done by adding aluminum or ferrosilicon on top of the mold, which leads to rapid solidification of the upper surface of the ingot. The top of the ingot is then cut off and scrapped.
Source: B.P. Bhardwaj, Steel and Iron Handbook