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Iron and Steel Manufacturing

Реферат

№ K-15313

Content

1. Crucible and high-frequency methods
2. Acid and basic steels
3. Bessemer steel
4. Open-hearth processes
5. Electric arc process
6. Oxygen processes
7. Vacuum melting and ESR
Annotation
Dictionary

(Реферат має схематичні рисунки)

Annotation

Steel is made by the Bessemer, Siemens Open Hearth, basic oxygen furnace, electric arc, electric high-frequency and crucible processes. In both the Acid Bessemer and Basic Bessemer (or Thomas) processes molten pig iron is refined by blowing air through it in an egg-shaped vessel, known as a converter, of 15-25 tonnes capacity. In the Siemens process, both acid and basic, the necessary heat for melting and working the charge is supplied by oil or gas.
Both the gas and air are preheated by regenerators, two on each side of the furnace, alternatively heated by the waste gases. The regenerators are chambers filled with checker brickwork, brick and space alternating. The high nitrogen content of Bessemer steel is a disadvantage for certain cold forming applications and continental works have, in recent years, developed modified processes in which oxygen replaces air.
The Huntsman crucible process has been superseded by the high frequency induction furnace in which the heat is generated in the metal itself by eddy currents induced by a magnetic field set up by an alternating current, which passes round water-cooled coils surrounding the crucible. The eddy currents increase with the square of the frequency, and an input current which alternates from 500 to 2000 hertz is necessary. As the frequency increases, the eddy currents tend to travel nearer and nearer the surface of a charge (i.e. shallow penetration). The heat developed in the charge depends on the cross-sectional area which carries current, and large furnaces use frequencies low enough to get adequate current penetration.
The remaining methods for making steel do so by removing impurities from pig iron or a mixture of pig iron and steel scrap. The impurities removed, however, depend on whether an acid (siliceous) or basic (limey) slag is used. An acid slag necessitates the use of an acid furnace lining (silica); a basic slag, a basic lining of magnesite or dolomite, with line in the charge. With an acid slag silicon, manganese and carbon only are removed by oxidation, consequently the raw material must not contain phosphorus and sulphur in amounts exceeding those permissible in the finished steel.
In both the Acid Bessemer and Basic Bessemer (or Thomas) processes molten pig iron is refined by blowing air through it in an egg-shaped vessel, known as a converter, of 15-25 tonnes capacity (Fig. 1). The oxidation of the impurities raises the charge to a suitable temperature; which is therefore dependent on the composition of the raw material for its heat: 2% silicon in the acid and 1,5-2% phosphorus in the basic process is normally necessary to supply the heat. The "blowing" of the charge, which causes an intense flame at the mouth of the converter, takes about 25 minutes and such a short interval makes exact control of the process a little difficult.
In the Siemens process, both acid and basic, the necessary heat for melting and working the charge is supplied by oil or gas. But the gas and air are preheated by regenerators, two on each side of the furnace, alternatively heated by the waste gases. The regenerators are chambers filled with checker brickwork, brick and space alternating.
The furnaces have a saucer-like hearth, with a capacity which varies from 600 tonnes for fixed, to 200 tonnes for tilting furnaces (Fig. 1). The raw materials consist essentially of pig iron (cold or molten) and scrap, together with lime in the basic process. To promote the oxidation of the impurities iron ore is charged into the melt although increasing use is being made of oxygen lancing. The time for working a charge varies from about 6 to 14 hours, and control is therefore much easier than in the case of the Bessemer process.
The heat required in this process is generated by electric arcs struck between carbon electrodes and the metal bath (Fig. 1). Usually, a charge of graded steel scrap is melted under an oxidising basic slag to remove the phosphorus. The impure slag is removed by tilting the furnace. A second limey slag is used to remove sulphur and to deoxidise the metal in the furnace. This results in a high degree of purification and high quality steel can be made, so long as gas absorption due to excessively high temperatures is avoided. This process is used extensively for making highly alloyed steel such as stainless, heat-resisting and high-speed steels.
The high nitrogen content of Bessemer steel is a disadvantage for certain cold forming applications and continental works have, in recent years, developed modified processes in which oxygen replaces air. In Austria the LID process (Linz-Donawitz) converts low phosphorus pig iron into steel by top blowing with an oxygen lance using a basic lined vessel (Fig. 2b). To avoid excessive heat scrap or ore is added. High quality steel is produced with low hydrogen and nitrogen (0,002%). A further modification of the process is to add lime powder to the oxygen jet (OLP process) when higher phosphorus pig is used.
The aircraft designer has continually called for new alloy steels of greater uniformity and reproducibility of properties with lower oxygen and sulphur contents. Complex alloy steels have a greater tendency to macro-segregation, and considerable difficulty exists in minimising the non-metallic inclusions and in accurately controlling the analysis of reactive elements such as Ti, Al, B. This problem led to the use of three processes of melting.

Ціна

40

грн.

Ключові слова: And, Steel, Iron, Manufacturing

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