1. Introduction
Steel, because of its versatile properties and its recycling possibilities, is the basic material for sustained development in modern industrial society. It enjoys a broad range of uses in almost all important sectors of industry, such as apparatus and machinery manufacture, bridge building, steel-framed building construction, power and environmental engineering, transportation, and the packaging industry, to name just a few.
The level of steel making and steel utilisation of a country or region depends on the demographic evolution and on the technical and economic state of development. In the period from 1900 to 2005 , world crude steel production rose from 40 mill. t to more than 1,1 bill. t. Germany ranks in sixth place with an annual output of 44,7 mill. t in 2005, while the EU economic area was the world's second largest steel producer until 2002 . China took over first that position in 2002. Steel is, and will continue to be, the no. 1 material in this century with the best price/performance ratio.
Process routes for the production of crude steel
- Reduction of iron ore, mainly via the blast furnace - BOF converter process route,
- Melting of steel scrap in electric arc furnaces.
Iron ores are primary raw materials and are imported predominantly from Brazil, Canada, Australia and Sweden. The use of steel scrap to make steel is one of the oldest methods of recycling.
2. Process routes
2.1 Iron ore - blast furnace - converter process route
With this process route, iron ores, fluxes and coke as well as other reductants such as coal, oil, gas or processed waste plastics are firstly reduced in blast furnaces into hot metal, which is then converted into crude steel in downstream basic oxygen furnace (BOF) steel plants.
Principle of the blast furnace process
A blast furnace is a shaft-like unit that operates according to the countercurrent principle. The coarse-grained charge materials (coke and burden, i.e. iron ore + fluxes) are charged via the furnace top, while reducing gas flows upward, counter to the descending burden.
2.1.1 Preparation of the blast furnace charge materials
An important aspect of the blast furnace process is the preparation of the charge materials, in order to ensure the gas flow through the burden column. Iron ores are charged in the form of lump ores, sinter and pellets. Lump ores are naturally mined ores that are crushed and screened to a certain grain size before their use. However, as a result of preparation and enrichment processes in the iron ore mines to increase the Fe content, very fine-grained ores increasingly accumulate which have to undergo agglomeration. This is done by means of pelletising and sintering.
Pelletising involves the forming of ore fines (pellet feed) and concentrates with grain sizes of well under 1 mm into pellets measuring around 10 to 15 mm in diameter. To do this, the ore mix is moistened and a binding agent added. The "green" pellets are then formed in rotating drums or on rotary discs . These green pellets are dried and indurated at temperatures of more than 1000 °C. This can take place in shaft or rotary furnaces or on a travelling grate. Pellet plants are generally located at the iron ore producers.
The sintering (= agglomeration) is performed at strand sintering plants, where the strands can measure more than 4 m in width and over 100 m in length. Sintering involves charging a mix of ore fines together with coke breeze, fluxes, in-plant returns and return fines onto a circulating grate, or sinter strand, and igniting the coke breeze contents in the surface by means of gas flames in an ignition furnace. A stream of gas or air is drawn from top to bottom through the mix. A flame front thus passes through the roughly 500 mm thick layer over the strand length and agglomerates the mix into coarse lumps of ore. Sintering plants are located in proximity of the blast furnaces on the works sites of the steel producers. Pelletising and sintering plants can produce around 6 million tonnes of pellets and sinter, respectively, per year.
All the iron ore carriers contain oxygen, which has to be removed through reduction in the blast furnace process. To do this, carbon is used.
The most important carbon carrier is blast furnace coke, which nowadays is produced in modern, environmentally friendly coking plants. Understood by coking is the heating of coal in coking chambers, closed off from the outside air, in the course of which the volatile constituents such as coke oven gas, tar, benzol, hydrogen sulphide and ammonia are expelled, collected and recycled for other uses.
2.1.2 Hot metal production
Hot air or "blast" with a temperature of 1200 °C is blown via tuyeres into the lower part of the blast furnace to produce reducing gas. The coke carbon present in this region gasifies with the oxygen contained in the blast to form reducing gas (carbon monoxide), where it generates temperatures as high as 2200 °C. The formed gas rises, binds the oxygen, and thereby brings about the reduction of the ores. The ascending gases heat the burden. Small amounts of carbon become detached in the iron, which lowers the melting temperature of the hot metal. The tramp elements in the charge materials form a molten slag and can thus be separated off. Hot metal and slag collect in the lower region of the blast furnace (hearth) and leave that lower furnace region at a temperature of around 1500 °C via a tap hole, which has to be opened. The hot metal and slag are separated by way of a refractory-lined trough and runner system and conducted to hot metal torpedo ladles and slag ladles, respectively. To optimise the process and lower the production costs, other carbon carriers such as coal, oil, gas or processed waste plastics are injected as a coke substitute via the tuyeres. Operation of a blast furnace without coke is not possible, however. Coke retains its solid structure in regions of the blast furnace where the ores soften and melt, thereby guaranteeing the gas flow and serving as a supporting structure for the overlying, solid burden column.
Processes have also been developed, though, for reducing ores without the use of coke. These are grouped under the terms 'direct reduction' and 'smelting reduction'.
Direct reduction does not produce any molten hot metal, as it operates at lower temperatures than the blast furnace process. Only the oxygen is extracted from the ores, and the gangue constituents of the ores remain in the sponge iron product (DRI= Direct Reduced Iron). In most direct reduction processes the reducing gas is generated by transforming natural gas into hydrogen and carbon monoxide. DRI is charged mainly in electric arc furnaces.
The smelting reduction process operates in two stages. First of all the ores are reduced to sponge iron, and this is then transformed into hot metal, similar to that from blast furnaces, through the input of coal and oxygen. Of the smelting reduction processes, only the Corex technique has so far been applied industrially.
For reasons of cost efficiency, both processes are restricted to certain regions and plant configurations and are far from attaining the output of a large-capacity blast furnace.
Large-capacity blast furnaces (hearth diameter around 15 m; total volume approx. 6000 m³) produce some 12000 t hot metal per day or 4 mill. t hot metal per year. This means moving and supplying large quantities of materials on a daily basis, e.g. 19200 t iron ore carriers, 4000 t coke, 1750 t injected coal, and 11 mill. m³ blast, which is heated in hot blast stoves to over 1200 °C. There is also the accumulation of 3300 t slag daily, which is utilised mainly as a construction material in the cement industry or in road building, as well as of 17 mill. m³ blast furnace gas which, after cleaning, is used for its energy content. In 2002 , some 608 mill. t hot metal were produced in blast furnaces around the world.
The life of a blast furnace, i.e. the duration until its refractory lining needs to be replaced completely, nowadays ranges from 15 to 20 years.
2.1.3 Crude steel production
The hot metal contains spurious tramp elements such as carbon, silicon, sulphur and phosphorus. These constituents are removed in BOF steel plant converters.
Illustration of an oxygen top-blowing converter
From hot metal comes crude steel. The impurities are oxidised in converters by top-blowing oxygen through a water-cooled lance. Certain quantities of scrap, accounting for as much as 25% of the total charge, are added as cooling agents, since the oxidation process generates a strong amount of heat.
Scrap charging in a converter shop
A converter holds up to 400 t crude steel. Added along with hot metal and scrap are lime, for slag forming purposes, and alloying agents. The blowing process takes some 20 minutes. Also practised nowadays besides pure top-blowing with oxygen is combined blowing, in which inert stirring gases or oxygen is additionally injected through the converter bottom.
2.2 Steel scrap - electric arc furnace process route
Increasing importance is being attached to scrap recycling for reasons of optimum raw materials utilisation and environmental protection. Steel offers everything needed in this respect, making it a particularly eco-friendly material. Used as the melting unit these days is the electric arc furnace, whose arc makes it possible to transform electrical energy into melting heat with very good efficiency and a high energy density.
The electrical current cannot simply be taken from the public supply system. Using a transformer, it is necessary to turn a current of high voltage into a current of lower voltage (600 to 1000 v) and high amperage (55 to 78 kA). The most important parameter for the performance of an arc furnace is the specific apparent power of the transformer in relation to one tonne of charge material, in which respect values of up to 1000 kVA/t are achieved. Graphite electrodes conduct the electrical current and create the arc to the metallic charge.
A.C. electric arc furnace with eccentric bottom tap hole
The main structural elements of an arc furnace are the furnace shell with eccentric bottom tap hole system and working door, the removable roof with graphite electrodes, and the tilting mechanism. The furnace shell has a refractory lining. Arc furnace tap weights nowadays range as high as 200 t, the annual output of such furnaces being around 1.5 mill. t.
To charge the furnace, the roof is lifted and swung aside. The scrap is conveyed in large buckets over the furnace and then charged into the furnace. The roof is moved back into place and the electrodes are lowered, igniting an arc on the cold scrap. During the melt-down process, temperatures in the arc reach as high as 3500 °C, and in the steel bath as high as 1800 °C. The high temperatures also enable the dissolution of difficult-to-melt scrap alloy constituents. Additional injection of oxygen or of other fuel-gas mixtures accelerates the melt-down process. Once the required chemical composition and temperature of the steel have been attained, the furnace is emptied into a ladle by tilting.
An arc furnace can produce any steel grade, completely regardless of the charge (scrap, DRI , hot metal, as well as any combinations). Today, crude steel is produced not only in A.C. electric arc furnaces, which operate with three graphite electrodes, but also in direct-current arc furnaces fitted with only one electrode.
Direct-current electric arc furnace
This unit, besides offering more favourable conditions for the melt-down of scrap, consumes somewhat less electrical energy and electrode and refractory material.
2.3 Secondary metallurgy
The high quality demands which the properties of the steels produced by either the blast furnace/converter or electric arc furnace route have to meet make post-treatment necessary. This is done in the secondary metallurgy process, i.e. ladle or vacuum treatment of the liquid crude steel.
Vacuum treatment facility in the secondary metallurgy stage
This production step primarily pursues the aim of reducing the carbon, nitrogen, hydrogen, phosphorus and several incidental elements in the steel to ultra-low levels, in addition to homogenising the liquid steel and keeping the temperature within tight and precise limits.
3. Casting of the steel
The liquid steel, which is produced in large quantities, has to undergo downstream processing. For this purpose it is given certain shapes, dimensions and weights by means of casting. In an integrated iron and steel mill, the capacious casting shop lies, in terms of material flow, downstream of the steel plant and upstream of the rolling mills. Steel is cast according to the ingot or continuous casting method. Ingot casting, which involves pouring the steel portion by portion into permanent (ingot) moulds, is gradually decreasing in importance and used only for high-weight pieces that are to be processed further by forging.
The liquid steel intended for hot rolling reduction is generally cast by the continuous method nowadays, which in Germany accounts for a share of about 97 %, and world-wide for around 90%.
In the continuous casting process, the liquid steel passes from the casting ladle via a tundish, in closed-stream mode, into a short, water-cooled copper mould. The shape of the mould determines the shape of the strand. Before the start of casting, the bottom of the mould is closed-off by means of a link-type chain or so-called dummy bar. As soon as the required metal level has been reached, the mould is subjected to vertical oscillations so that the strand does not adhere to the mould wall. The incandescent strand, once solidified in its surface zone, is withdrawn from the mould, firstly with the aid of the dummy bar, and then by pinch rolls, while the mould is continuously replenished with liquid steel from the top.
Because of its liquid core, the strand has to be carefully sprayed and cooled with water and supported on all sides by rollers until it has solidified completely, thereby avoiding any breakout through the still thin surface zone.
When it has solidified completely, the strand can be cut to certain lengths by travelling cutting torches or shears. The accelerated cooling produces a uniform solidification structure with favourable technological properties. High casting speeds are achieved these days. Depending on the section and the number of strands to be cast at the same time, speeds range from 0.6 to 6 m/min, the latter for cast sections from 1500 to 2000 mm in width and around 250 mm in thickness.
To be able to cast several heats one after the other without interruption, the follow-on ladle containing the liquid steel has to be brought quickly into casting position. Such sequence casting takes place with the aid of turrets, which can accommodate two ladles.
Continuous casting technology supersedes not only conventional ingot casting but also the blooming-slabbing and billet mills in the downstream rolling stage. The yield of rolled products per tonne of liquid steel can be increased with continuous casting by 10 to 12 % compared with the 85 % for ingot casting, leading to considerable savings in energy and raw materials. The cleanness possible in continuous casting is also better than in ingot casting. The rapid solidification produces an homogeneous structure with less segregation.
Conventional continuous casting of slabs (discharge roller table)
The sections continuously cast for long products, such as beams, rails or wire rod, range from 100 x 100 mm to 450 x 650 mm. Slab casters for flat products produce sections measuring 300 x 2000 mm. So-called jumbo casters can achieve sections up to 2700 mm in width.
A currently revolutionary development is near-net-shape casting or casting-rolling, as it saves appreciable rolling work when producing flat steel products (see also: Research & Technology / Production Technology / Shaping & Coating). It is designed to achieve cast thicknesses ranging from 50 to 90 mm for thin slab casting, from 10 to 15 mm for direct strip casting, and from 1 to 5 mm for strip casting. Thin slab technology-based casting rolling has meanwhile become an established process world-wide.
4. Outlook
The routes for producing iron and steel, as well as the product developments and fields of use for the steel grades have reached a very advanced state, yet still offer diverse potentials. The steel industry continues to face challenges with regard to innovations in plant and process engineering, product development, and product application, particularly in the use of steel as a resource-conserving lightweight material.
Source:www.stahl-online.de
