Frequently Asked Questions
Carbon steel that is cast into ingots can be classified into four types: killed steel; semi-killed steel; rimmed steel and capped steel. Each type depends on the method of deoxidation during steel making, or the amount of gas evolved from the reaction of carbon with iron oxide during solidification. For continuous casting, only killed steel is used.
Killed Steel is characterised by more uniform composition and properties in comparison with the other types. The steel is fully deoxidised (killed) before casting by the addition usually of silicon, manganese and aluminium, but also sometimes vanadium, titanium and zirconium. There is virtually no gas evolution during solidification, and therefore the ingot contains very few, if any gas voids. However, this means that a shrinkage pipe forms at the top of the ingot, thus the amount of useful metal is reduced. Typically, killed steels are used for alloy steels, forging steels and steels for carburising. A typical Si-killed steel from UK steel making practices would contain more than 0.1%Si (usually 0.15%-0.35%Si); less than about 0.005%Al; 0.01 to 0.03%O 2; MnS and silicate inclusions.
Aluminium is often added to Si-killed steels to control the growth of austenite grains in reheated steels. A typical Si-killed, Al-treated steel from UK steel making practice would contain more than 0.1%Si; more than 0.01 to 0.02%Al (usually 0.03% to 0.06%); less than 0.005% O2; Al2O3 inclusions, but no silicates.
Semi-killed Steel is characterised by variable degrees of uniformity in composition, intermediate between killed and rimmed steels. Typically, more gas is evolved in semi-killed steel than in killed steel, but less than in rimmed or capped steel, and there is a pronounced tendency for chemical segregation to occur at the top of the ingot. A typical semi-killed steel from UK steel making practices would contain less than about 0.1%Si (typically 0.03%, but possibly as low as 0.01%); less than about 0.005%Al; upwards of about 0.02% O2; MnS and silicate inclusions.
Rimmed Steel is characterised by marked differences in chemical composition across the section and from top to bottom of the ingot. The steel from which the ingots are made is not fully deoxidised before casting. It is observed that the concentration of carbon, sulphur and phosphorus is lower than the average for the ingot in the outer rim, and higher in the central core. This pronounced segregation is due to greater gas evolution from the outer rim during solidification and the concentration of elements in the liquid portion increasing. Typically, rimmed steels are best suited for the production of steel sheets and plates, as the structural pattern persists through the rolling processes. After solidification of the outer rim, fine gas bubbles form in the body of the ingot and help to counteract the solidification shrinkage. Therefore no pipe forms and the yield of good quality metal is improved. The internal voids close up during rolling and forging, but the clean outer rim remains, and enhances the quality of the steel surface. Typically, rimmed steels contain 200-400ppm O2, <0.01%Si and 0.01%Al.
Capped Steel has characteristics between those of semi-killed and rimmed steel. The addition of some deoxidising elements controls the rimming action by limiting the time for gas evolution from the outer layers of the ingot. The gas evolved inside the ingot is in excess of that required to counteract solidification shrinkage, hence the ingot has a tendency to expand. The capping operation limits the formation of an excessive number of gas voids in the body of the ingot. This is achieved either mechanically or chemically. The mechanical technique involves a heavy metal top being placed on the ingot mould to restrict the expansion of the ingot. For the chemical technique, aluminium or ferrosilicon is added to the top of the ingot so that the top surface solidifies rapidly and, again restricts the expansion of the ingot. This top portion is discarded before further processing. Capped steel typically contains more than 0.15% carbon, and is used for sheet, strip, wire and bars.