Transformation of Iron into Steel: 4 Processes | Metallurgy

The equipment used to transform iron into steel by removing the impurities is called a converter. The first converter was developed by Bessemer and since then many other processes have also been developed.

1. The Bessemer Process:

An Englishman, Henry Bessemer, first invented the Bessemer converter in 1856. The converter is a steel container, shaped like a concrete mixer, lined with bricks and with a perforated base. It is filled with molten pig-iron and blasts of cold air are forced through the bottom, giving off spectacular flames and sparks.

The alloying elements are added at the mouth of the con­verter, and the oxygen in the air blast, reacting in the heat of the pig-iron, oxidizes such impurities as car­bon, silicon or manganese. The colour of the flame indicates the stage of the refining.

Within 20 minutes the process is completed, the converter is tilted and the metallic contents poured into a large ladle for casting either into ingots or directly into slabs or billets. The Bessemer converter allows large quantities of steel to be quickly and cheaply produced, and its development reduced the price of steel to a fifth of its former cost.

But the original Bessemer process does not remove phosphorus from the pig-iron, and thus only iron ores, such as haematite, with a low content of phosphorus and sulphur can be used. This was because the original converter had an acid lining which removed only certain impurities. The phosphorus rich Jurassic lime­stone iron ores of Europe, e.g. in France (Lorraine ores), Luxembourg and England, were unusable in the acid converter because the phosphorus made the resulting steel brittle.

However in 1878 two English­men, Sidney Thomas and Percy Gilchrist, developed the Gilchrist-Thomas process, using a basic converter. This process makes use of a basic material, such as crushed dolomite limestone, for lining the converter and a specified quantity of burnt lime is added to the molten pig-iron. These materials effectively remove sulphur and phosphorus impurities.

In this manner basic steel as well as the original acid steel could be produced and the new technique allowed large quan­tities of phosphorus-rich iron ores around the globe to be mined and smelted into steel.

2. The Open-Hearth Process:

A few years after the development of the Bessemer converter, Martin and Siemens invented the open-hearth process, which is now used to produce either acid or basic steel. In the open-hearth furnace pig-iron lies in a shallow, saucer-shaped pool at the base of the furnace and is heated from above by a powerful flame of pre-heated air and gas at a temperature of about 1,649°C (3,000°F).

The burnt gases escape via flues to regenerators at either end of the furnace. These regenerators provide the heat required for pre-heating the blasts of hot air and gas. In an integrated steel­works the gas comes from blast-furnace wastes or from coke ovens. Thus one of the great advantages of the open-hearth process is that it employs its own waste heat and also other steelworks by-products.

It can also generate higher temperatures than the Bessemer converter so that pig-iron can be used cold and part of the furnace charge, or even the whole charge, can consist of scrap-iron. Because the process can use cold iron and scrap it is not tied to an integrated plant like a Bessemer converter which needs molten pig-iron direct from the blast furnace. Its use of scrap is particularly important today as an increasing amount of steel comes from scrap metal.

The open-hearth process is slower taking between 7 and 16 hours to complete and yielding about 10 tonnes of steel per hour; Bessemer converters take 20 minutes per charge and can yield up to 70 tonnes per hour. However, the open-hearth process enables high qua­lity alloy steel (e.g. stainless steel) to be produced un­der very precise control.

The carefully controlled quantities of the alloy metals are added to the molten pig-iron. Because of the various advantages of the open-hearth process outlined above, about 90 per cent of the world’s steel is made in this way today.

3. Electric Furnaces:

When a fuel is burnt in the furnace, a certain amount of contamination of the steel is unavoidable. This has led some steel makers to use electric furnaces to produce really good quality alloyed steels. They are particularly useful in smelting selected scrap of known chemical properties.

The process is controlled by adjusting the electrodes made of pure graphite. Electric furnaces are used mainly in the developed countries of North America, Europe and Japan where large supplies of electricity are available, e.g. in Italy, France, Switzer­land, Canada and the U.S.A. and are also used where the iron and steel industry has been recently developed, e.g. S. Korea. However, generally speaking, though useful for specialized steels, the very high cost of pro­duction in electric furnaces makes them unsuitable for general purposes.

4. Oxygen Process:

Another modern process is similar to the old ‘converter process’. The iron is placed in a converter similar to the Bessemer converter, but instead of forcing cold air from below, a high- pressure jet of oxygen is played on the molten iron from above. High quality steel can be made in this way.

The molten steel from any of the above processes is poured into moulds where it solidifies into ingots of various shapes, sizes and weights. They are later heated for rolling in steel mills into blooms, billets or slabs. They are eventually finished as steel sheets, strips, plates, bars, wires, pipes, tools, machine parts, etc.

In modern mills, however, continuous casting is often practised. This avoids making ingots which have to be re-melted and steel from the furnace is cast di­rectly into billets, blooms or slabs. This saves fuel and time and is usually practised in integrated mills.

Integrated steel mills are those which combine on one site the smelting of iron in blast furnaces, the making of steel, and the production of at least some finished products, such as steel sheets, pipes and girders. Coke-ovens are usually found on the same site.

Such integrated plants are usually very large, generally with an annual output exceeding 3 million tonnes. They have many advantages, the most important of which is that large-scale production is much cheaper than small-scale operations, as long as full capacity production can be maintained.

Integrated plants, too, do not have to continually re-heat the metal for each process. Molten pig-iron can be led directly to the steel plant and hot steel to the rolling mill. This saves the cost of re-heating at the various stages. Moreover, waste heat from one process can be used elsewhere and the by-products of some processes, e.g. the gases from coke-making, can be used in other stages of production.

Another great advantage of integration is that it ensures supplies of pig-iron for the steelworks without relying on outside sources. Steel mills which rely on pig-iron from else­where may be affected in times of shortage, or by transport difficulties.

Another method of overcoming problems of supply and demand is economic integration, where one large concern owns not only steel mills but also iron and coal mines and engineering plants. Examples of such firms with wider interests are Bethlehem Steel of the U.S.A. and Krupps of Germany.

In some cases, however, integrated steelworks are not ideal. For instance, where the market is small, there is no need for large-scale production. In the case of specialized steels for certain purposes, a small, high- quality output is also preferable. Moreover, steel- rolling mills and plants producing metal goods for other industries are best situated near their markets.

Those steelworks which depend heavily on scrap are also best situated in engineering regions, rather than close to iron ore supplies, because such regions pro­vide not only their raw scrap but also the market for their finished steel.