Tapping and pouring the steel

The finished steel is tapped from the furnace into a well heated steel ladle of the required capacity. The size of the ladle is usually selected so that it accommodates the whole heat from the furnace and part of the slag which is required as thermal insulation of the metal surface. From the ladle the steel is poured into metal moulds to produce ingots or into sand moulds to obtain steel castings.

Depending upon the kind of ingots to be produced, the moulds may have a square, round, rectangular or other cross section. A definite height-to-cross section ratio has been established in practice for ingot moulds; the height should be 5 or 6 times more than the diameter or size of the cross section. The mould walls are tapered to facilitate stripping from the solidified ingots.

Ingots are produced in a wide size range - from 100 kg to 100 tons. Ingots weighing from 6 to 8 tons are the most common size; they are used in rolling departments. Heavier ingots, designed for special purposes, are seldom poured.

In the top pouring of steel each mould is filled with liquid steel separately. Bottom pouring consists in teeming the steel from the ladle into the funnel of a vertical runner (called the fountain) from which the steel enters several moulds (2 to 6 pieces) through narrower runners. The advantages of top pouring over bottom pouring lie in the greater opportunities for nonmetallic inclusions to become separated from the metal and in that the hotter metal is in the top of the ingot. Bottom pouring provides better surface quality of the ingot and hotter metal at the bottom.

As the liquid metal poured into the mould cools, its volume is reduced thereby leading to shrinkage of the ingot. First to solidify is the outside of the ingot adjacent to the mould walls; the interior part of the ingot remains in a liquid state for some time after the mould is teemed. A shrinkage cavity, or pipe, is formed in the upper central portion of the ingot where the metal continues to be in the molten state for a long time.

The extent of the pipe in the ingot can be reduced somewhat if solidification in the upper part of the mould is retarded. This is done by applying a hot top. Hot tops are extensions of the moulds and are lined with a refractory material. Due to the poor heat conductivity of the hot top walls the metal in this part of the mould is retained in the liquid state for a comparatively long time and a considerable part of the pipe is transferred to the ingot top which is subsequently cut off together with the pipe. In some cases the metal in the hot top is additionally heated with a gas torch or by introducing a thermit mixture. Such measures reduce the volume of the pipe.

A large economy is achieved in the continuous casting of steel, a new method that is being applied at a number of plants. Liquid metal runs in a continuous stream from the ladle into the intermediate device and then further, into the water-cooled solidifiers. A steel seeder is placed in the bottom of each solidifier. This seeder prevents the first portion of the molten metal from dropping straight through the solidifier. Upon contact with the seeders and water-cooled walls the liquid metal begins to solidify rapidly on the ingot surface. It welds to the seeders with which it is pulled downwards, out of the solidifier, by rollers. Solidification of the continuous ingot produced in the solidifier is intensified when it passes through the zone of secondary cooling by water.

The completely solidified ingot next passes to the gas-cutting trucks. Continuing to travel downwards together with the trucks the ingots are cut into billets of definite length which are carried by conveyers over the inspection table and are delivered to the store house. The billets are subsequently rolled into sections of the required shape and size.

Vacuum casting of steel has proved to be a very effective and efficient method. In this procedure steel, made in any unit, is held in a closed chamber from which air and gases are continuously evacuated. As a result of this treatment the steel has a minimum content of gases and nonmetallic inclusions.

Vacuum treatment is usually carried out in the ladle before pouring the moulds. This method of producing high-quality metal is not as widely applied as it deserves.