The making of iron and steel: Reduction: lronmaking, Refining: Steelmaking

The making of iron and steel

Overview

Before undertaking a detailed study of the processes involved in steelmaking, it is helpful to have an overview of the whole operation, so that the inter-relation of the various steps can be seen in perspective.

Location

The nature and location of steelworks depend on many factors:

the availability of raw material;

supply;

transport (particularly deep-water port facilities);

human resources and services availability;

markets; and often

government factors.

In early times when coal was consumed in far greater proportions in steel production, the trend was to site integrated plants either near the coal source, or near low-grade, but cheap ore.

However, with the improvements in bulk transport and the relatively greater cost of handling finished products, the trend is towards locating integrated plants on deep water ports, perhaps thousands of kilometres from ore and/ or coal but close to markets and services. 

They may supply semi-finished products to more specialised finishing mills still closer to the market. The other major steelmaking method involves the melting and refining of steel scrap in an Electric Arc Furnace. 

Pig iron and refined iron ore pellets (briquetted iron) can also be used, and because this method is economic at lower volumes these operations are called mini mills. Plants that use Electric Arc Furnaces can be sited closer to the market.

Where market size permits, some degree of product specialisation is practised.

Raw Materials

The basic raw materials of large-scale steelmaking are:

iron ore, treated in some way after it comes from the mine;

coal, which must be converted to coke;

limestone;

steel scrap – important both to the integrated steelworks as secondary feed to the steelmaking furnace, and to the smaller scale “mini-mill” operator and special

steel producer;

fluxing materials;

refractory materials; and

alloys, which are elements added to steel to give it special properties for a variety of uses.

The output, or capacity, of a steelworks is generally expressed in terms of tonnes per annum of raw steel (the gross output from steelmaking furnaces). However, the marketed tonnage is only about 90% of the raw steel because of process losses and recycled scrap arising from rejects and trimming to size. Nowadays integrated steelworks are rarely viable at less than 3 million tonnes p.a. Whereas mini mills operate successfully at 100,000 tonnes p.a. and upwards to 1.5 to 2.0 million tonnes p.a.

The Steelmaking Process

Integrated steelmaking operations fall into three main phases:

Reduction: lronmaking

Iron ore, as mined, is a combination of iron with oxygen and various other unwanted substances, generally known as “gangue”. The first metallurgical step is to reduce iron ore to metallic iron, a process which is mostly carried out in a blast furnace, using coke as both a fuel and reducing agent. The metallic iron produced by such a furnace contains a relatively high proportion of carbon (4%) and is passed to the steelmaking process as a liquid at approximately 1450C, called “hot metal”.

Refining: Steelmaking

The refining of iron to make steel is where the carbon content of hot metal is lowered, usually to less than 1 % by an oxidation process in a steelmaking furnace. At the same time, alloying materials are added to the furnace to achieve the required chemical composition of the final product. The chemical content is controlled very closely during this stage. Originally most steel was produced by the “Bessemer” and “open hearth” processes but these have been replaced by the more modern “basic oxygen steelmaking” (BOS) and “electric arc furnace” (EAF) processes.

The BOS process uses pure oxygen, injected by a lance, for refining the relatively impure hot metal (and scrap is used for temperature control).

The electric arc furnace uses primarily electrical energy to supply heat to melt scrap steel, sponge iron, or mixtures of scrap and other iron units. Compared to the BOS process, the EAF requires less chemical reaction for refining.

Shaping & Coating

The liquid steel can then be cast or formed into a variety of solid shapes via the ‘continuous casting’ process. The cast steel can then be forged or rolled in successive steps to produce anyone of the many required shapes. Rolling is the most common method of shaping. The modern rolling mill is a huge installation, costing millions of dollars and incorporating highly complex electronic control systems. The amount of work to which the steel is subjected, and the schedule on which this work is carried out, have significant effects on its physical characteristics – it dictates whether the steel can be subsequently bent, machined, cut, or subjected to any other engineering operation, or formed into tubes, pipes or wire.

Once shaped, steel may be coated with other metals such as zinc or tin, or with organic coatings like paint or PVC.

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