The two most important raw materials for the production of steel are iron ore and scrap metal.
In the blast furnace of the ironworks – with a ceramic tile lining – the reduction of iron ore to iron takes place by removing the oxygen from the iron oxide minerals through reduction with carbon and coke. The so-called pig iron from the blast furnace, apart from iron, also consists of 4–5 per cent carbon, as well as smaller quantities of other substances. The pig iron, in liquid form, then normally goes to the steel plant for further processing.
In Sweden there are blast furnaces at the SSAB plants in Luleå and Oxelösund.
In Sweden there are two different ways of producing steel. The processes are differentiated according to the raw material used in the process – pig iron (made from iron ore) or steel scrap.
With ore-based production, steel is mainly produced from pig iron. The carbon content is reduced through decarburisation with oxygen in a BOF (Basic Oxygen Furnace) converter. The necessary energy for the temperature increase is obtained from the oxidisation reactions, C to CO2 and Si to SiO2, which generate a lot of heat. In order to cool the process, steel scrap is added to the converter, comprising about 20 per cent of the content in the converter. During the process, a slag of the nongaseous reaction products and of added slag formers, e.g. lime, is formed.
The ore-based steelworks (also called integrated steel mills) in Luleå and Oxelösund together account for about two thirds of the Swedish crude steel production.
Scrap-based steel production
With scrap-based production, electric arc furnaces are mainly used for the melting of steel scrap, a process which demands electric energy. The specific energy utilisation (calculated as kWh per tonne of produced steel) when using scrap metal as a raw material is only one fifth, in comparison with ore-based steel production.
In Sweden, scrap-based production of steel takes place at ten locations comprising about one third of the Swedish crude steel production.
Read more about companies and plants
In many parts of the world, sponge iron serves to complement metal scrap as a base material in steel production. Sponge iron is produced through removing, at lower temperatures, the oxygen in the iron ore with the aid of carbon monoxide and hydrogen gas produced from natural gas.
In Sweden, the sponge iron is produced by Höganäs AB in accordance with the Höganäs Sponge Iron Process. The production serves exclusively the company’s own production of high class iron powder.
The slag has several important functions in the production process. Slag is one of the active components in the metallurgical processes. It is a desulfarisation agente and it have impart to the desired properties of the steel. The slag’s composition, function and quantity depend on the raw materials that are used, the process in which it is used and which type of iron or steel is being produced.
Within the scrap-based steel industry the slag also functions as an insulating layer on the steel melt in the furnace; this protects the melt from contact with the air and prevents energy losses.
Slag is also used to protect the lining (the heat-resistant ceramic tile) in BOF converters and in electric arc furnaces.
Steel, strictly speaking, is an alloy suited for metal forming when in a solid state, having iron as the base metal and where carbon is the most common alloying element.
The carbon content is of fundamental importance for the properties of steel. With higher carbon content the strength of the steel increases, whereas its toughness (ability to deform without fracturing) and weldability decrease. For the steel to be properly formable the carbon content may not, however, be more than 2 per cent.
In the production process, alloying elements are added in order to give the steel the desired properties.
Read more about alloying elements in the section on Raw Materials.
Alloy steel refers to steel with fixed minimum limits for different alloying elements. Alloy steels are broken down into high-alloy steel and low-alloy steel. The dividing line between these two types is to be found more or less where the total alloy content exceeds about 5 per cent.
Examples of alloy steel grades are stainless steel, high-speed steel, tool steels and bearing steels. Some 60 per cent of Swedish steel production comprises alloy steels. This is a substantially higher share than that found in the rest of the world. In the rest of the EU, as in the USA and Japan, the alloy steels comprise only 10–15 per cent of the total production. This share is even lower in other parts of the world.
Non-alloy steel normally refers to steel with a lower content of alloying elements than that required for alloy steel.
Ladle metallurgy is a designation for those processes that take place in connection with the steel furnace when refining of the liquid steel takes place. In the ladle or ladle furnace there takes place:
- Deoxidation (sealing)
- Further cleaning of the steel, e.g. vacuum treatment.
- Adjustment of the alloy composition
- Setting of temperatures suitable for casting.
Continuous casting or ingot casting
Casting normally takes place in the form of a continuous advancing strand which is cut into predetermined lengths after solidifying, so-called continuous casting. The products from continuous casting are slabs, blooms or billets, refering to their cross-sectional dimentions.
Sweden has a relatively high share of ingot casting, about 10 per cent. Where certain alloy steels, for quality reasons, cannot undergo continuous casting, or when large dimensions are required, the older casting method is still used. This takes place in an ingot mould. The product is called ingot which must then be rolled or forged into slabs, blooms or billets.
The largest share of the steel plant production comprises products that must be further processed and finished. The processing of ingots and slabs is carried out to achieve the desired shape and properties. This normally takes place through heat treatment, i.e. rolling or forging and possible cold working. The products can also undergo finishing in different ways e.g. various heat treatments, straightening and grinding.
Hot rolling from ingots most often takes place in two stages. Firstly, the ingots are rolled into slabs which undergo checking and surface treatment (removing surface defects). The corresponding surface treatment takes place with the continuous cast slabs also. The slabs are heated and rolled into different product types/forms such as plate, strip, bar, profiles, wire or tube.
Sometimes additional cold rolling (at room temperature) must take place, for example in the production of thin plate. When wire, rod or bar is rolled, rolling mills are used with grooved rollers.
The forging process is often used to form large component parts, but also parts with irregular thickness and shape, for example crankshafts for motor vehicles and ships.
Thin wire is produced through the cold drawing of hot rolled wire rod. Drawing is also used in bar and tube production.
Casting process for finished parts
The method is used in the first place to form large, irregular parts, such as engine blocks, but also small parts (investment casting).
Various thermal treatments can also change the steel’s properties. Hardening is carried out, for example, to make the steel harder; this takes place through the steel, under highly controlled, being first heated and then quenched (rapidly cooled). Tempering, annealing, quenching and cooling are examples of other treatments.
Read more about heat treatments (energihandbok.se)
Straightening, grinding, polishing and galvanising are examples of mechanical finishing processes.