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FINISHIl~GFig. A.l The fundamentals of Steelmaking
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-Since steel is required more frequently in strips, sheets and bars, the solid chunk of steel must be physically changed into longer, thinner forms. This cannot be achieved in a single operation and the first stages of reduction take place in a Rolling Mill. Basically, a rol~ing mill consists of two cylin-ders of steel called Rolls, wi~h a· gap between them which, rather like the old fashioned clothes mangle, can be set as required. The cylinders counter rotate and the steel, reheated to a red-hot state, is driven through the rotating rolls. Out from the other side the steel emerges a little thinner and,
correspondingly, a little longer. This rolling process'continues in many forms; some mills are said to be reversing mills because the steel is passed backwards and forwards betvleen' the rolls with a smaller gap at each pass. Other mills are set to reduce
from one given thickness to another and the steel passes through only once, usually through a series of such mills.
Some later rolling stages involve the use of shaped rolls so that steel of different cross-sectional shapes can be produced,
e.g. railway lines. /
The remaining stage in this simplified description of making steel, is to cut the steel into the dimensions required by customers and to put the pieces through various Finishing processes. In general, these finishing processes either im-prove the quality, e.g. make sure railway lines are particular-ly straight, or appparticular-ly some protective coating. The most usual coatings are tin and zinc, both of which prevent the steel form rusting, but plastic on top of the zinc is becoming in-creasingly popular since i t can be coloured and textured and is pleasant to touch.
Steel products are further processed. For example, steel wire may be woven into cables, steel sheets may be formed into tubes and boilers, steel sections may be fabricated into bridges, etc ..
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-A closer look will now be taken at some steel works processes and, in particular, some of the problems of planning and control-ling a works will be mentioned.
The Blast Furnace
A blast furnace is physically a giant process which can be 75 metres (240 feet) in height (see Fig. A.2). It is basically a lined steel vessel which is charged from the top with coke, iron ore and limestone. Suitable proportions of these are taken by a "skip", or hopper, up the skip hoist and dropped in at the top.
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The coke burns and provides both heat '(up to about 1.600 C) and is the source of carbon. Hot air is blown (or blasted) up through the bottom of the furnace to maintain a high tempera-ture. The iron ore is, naturally, the source of iron while the limestone acts like a sort of scavanger, i t soaks up and com-bines with unwanted impurities to form slag. Fortunately, slag is lighter than molten iron and so i t floats on tdp.
Every 3 or 4 hours the furnace is tapped, that is to say the molten iron which has collected at the bottom is drawn off
into a ladle or ladles for transportation to steelmaking. Iron continues to be tapped until the level of the floating slag is reached. At this point, the slag is drawn off separately and can be used, after processing, for road making or ferti-lizers.
Controlling a blast furnace is not an easy task especially since i t must be fed with heterogeneous raw materials and the normal objective is to produce iron with constant characteris-tics.
Steelmaking
The most popular method of making steel today involves the use of an LD, or oxygen converter (see Fig. A.3). It is nothing
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STEEL~lAKIi~GPROCESSES
ADDITIVES
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-OXYGEN
Fig. A.3 Basic Oxygen Furnace
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-more than a steel vessel, lined with refractory material, which can be tilted in a vertical plane.
Molten iron from the blast furnace is poured in and followed by steel scrap and other sources of particular elements called for in the final analysis. Limestone is again used as a scavanger just as in the blast furnace. A n~dern furnace will make some 200-300 tons of steel at a time and will take 40 minutes typi-cally.
There is no external form of heating, rather oxygen is blow~
in through a lance in order to "encourage" the required (exo-thermic) reactions. Great turbulence c~n be experienced, the quantities of input materials and oxygen blown are carefully calculated, usually by computer, to arrive at the expected rosult.
Although more controllable than some predecessors, i t remains a fact that not all attempts result in precisely the intended wlalysis. This causes problems of what to do with the mismade steel and how to speedily remake, or supply in some other way, the type of steel called for.
The other main problems arc organizational, e.g. ensuring that all the materials arrive when needed and having suitable cranes available at the right time.
After the prescribed 40 minutes the steel is drawn off, or tapped, into a ladle which takes the steel to a teeming bay.
The other modern steel making process is the Electric Furnace (see Fig. A.4). The basic idea is still to produce steel of a specified analysis but the approach is quite different. No molten iron is used and so a works using the electric process does not need blast furnaces. Steel scrap is used as the main iflput material and i t is melted by the heat caused when electro-des are lowered into the pile of metal.
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