A steel mill is a factory that makes steel.
So what is steel? And how is it made?
Steel is an alloy of iron and a small amount of carbon, some of it forming a compound with iron called cementite. It is produced out of iron. "Normal" iron contains quite some carbon, so in order to make steel out of iron, you need to take out some of the carbon. The carbon content of steel is usually lower than 1%. Even though this is quite a small amount, the presence of carbon is essential to obtain the specific properties of steel. Apart from carbon, steel can contain many other alloying elements, that can give many different properties. Chromium and nickel are added to make stainless steel, for instance. The simplest kinds of steel however contain iron, a small amount of carbon (less than 0.25 wt%) and some trace amounts of manganese.
Taking out some of the carbon present in normal iron is easier said than done. This is where the steel mill comes in. A steel mill can contain any number of sub-plants where different processes take place. For the sake of completeness, I will describe the full process of going from iron ore to finished steel products.
From iron ore to pig iron
The starting point for making steel is iron ore. Iron is found in nature in different minerals like hematite, magnetite, limonite or siderite. In these minerals iron is present as an oxide, that is, chemically bound to oxygen atoms. For example, the formula of hematite is Fe2O3. Apart from that, minerals containing iron are usually not found pure, but mixed with all kinds of silica based minerals.
To make iron out of iron ore, you need to break the bond between the iron and the oxygen. As anybody who has ever had a rusty car will know, iron oxide is a much more stable compound than iron, so to break it apart lots of energy is needed. Loosing the grip of the oxygen atoms on our precious iron is achieved by letting them react with carbon. Today this is usually done in a blast furnace. In a huge furnace, iron ore is combined with carbon (usually in the form of coke, a type of coal from which all volatile components have been removed) and limestone. All this is ignited and the carbon reacts with the oxygen, leaving the pure, fluid iron which exits the blast furnace at the underside at around 1500 degrees C. Limestone is added to react with the silica based minerals and other unwanted elements and forms slag, which comes floating on top of the molten iron. This slag is removed and can be used as a building material or in the cement industry.
The iron produced in the blast furnace is known as pig iron. Due to the reaction process used to produce it, pig iron still contains a rather large amount of carbon, typically around 3,5 wt%. The presence of this amount of carbon makes the material rather brittle and it is useful for only a limited amount of applications. To make a really useful material out of the pig iron, it needs to be converted into steel.
From pig iron to steel
The difference between pig iron and steel lies in the carbon content. To get from the one to the other the carbon content needs to be lowered. This is usually done in a basic oxygen furnace or in an electric arc furnace. In some cases the pig iron is first cast into ingots and left to solidify. In modern steel mills however the process is continuous and the molten iron goes directly to the steel making plant, by train in a torpedo ladle. To give you an idea of the amounts of iron we're talking about: a fully loaded torpedo ladle weighs the same as two Boeings 747 including passengers and cargo. Sixty of these ladles are filled each day.
From the torpedo ladle the iron is transferred to a Hot Metal Ladle. At this point in the process, either in the torpedo ladle or the hot metal ladle, the iron is desulphurized by adding lime and/or magnesium and/or calcium carbide. Afther this step, the iron is inserted into the convertor (usually a basic oxygen furnace).
In a basic oxygen furnace, carbon is removed from the molten iron by using a water-cooled lance to add oxygen to the furnace. The oxygen reacts with the carbon to carbon monoxide and in this way the required carbon content in the steel is achieved. The oxygen also reacts with other elements in the metal that might be present to again form a slag that is removed. During this process the temperature of the steel can reach up to 2000 degrees C. To control this temperature, steel scrap is also added to cool the mixture down. After blowing oxygen into the furnace, alloying elements can be added to the steel if desired.
An electric arc furnace is mostly used when steel is made out of solid iron or out of steel scrap. The iron or scrap is placed in a refractory-lined furnace with a lid. Carbon electrodes are lowered onto the metal and a current is made to pass between the electrodes and the metal. This forms an electric arc between electrodes and metal, the heat and radiant energy of which melts the iron. Oxygen is then again added to the furnace to remove carbon and other unwanted elements from the iron.
Once the metal has reached the required carbon content and temperature, it is poured from the furnace into a steel ladle. Here additional treatment like stirring or degassing can take place.
From steel to product
The molten metal, which has by now turned into steel, now must be cast into a shape that can be used to make products.
The steel ladle containing the molten steel is transported by crane to a casting stand. In the bottom of the steel ladle, sliding gate plates can be opened to emtpy the ladle into a tundish. The steel level in the tundish must be kept between a specific lower and upper level to maintain control over the casting speed. From the tundish, the steel is cast into the mould.
One method is to cast ingots, large steel blocks. The disadvantage of this method is that at the edges of a cast piece, the properties of the casting are not optimal. Here oxides form and small defects can occur that have a negative effect on forming techniques that will be used later. The less surface area, the better. Also casting ingots is a batch process, where a continuous process can be much more efficient.
For this reason some modern steel mills have developed a continuous casting process.
The molten steel is poured into a vertical metal mould that is open on the underside. I’m not quite sure how they start this process, probably with a temporary bottom in the mould. Once it’s started, however, it works such that fluid metal is poured into the mould at the top. At the underside, the metal is cooled down enough to be solid, but still hot enough to be quite flexible. The solidified metal is led from the mould, through an angle, onto a conveyor belt. Now you have a system that produces a continuous beam of solid steel, that can be cooled down further and cut into chunks to be processed into products.
The disadvantage of a continuous process is that if one of the components doesn't work as it should, the whole process is ruined. For this reason only the very large steel mills use the continuous method.
The first step of processing the steel is usually rolling it into smaller but longer bars. The steel ingots are passed between large press rollers that press down on them, thus decreasing the thickness of the ingots. By rolling on just one side of the ingots, sheet metal can be made. By turning the ingots/bars around their length axis between different passes through the rollers, thinner bars are made. The hotter the steel still is, the easier the rolling (hot rolling), so it is most efficient to do this just after the ingots come out of the casting process. For some applications, though, the properties of the steel improve when it is worked cold.
Steel profiles can also be made by rolling, only here the rollers are profiled as well so they press a shape into the steel. Another way to make profiles is to push the – still fairly hot and thus flexible and mouldable – steel through a die.
Steel wire is made by pulling steel bars through ever smaller dies so they eventually end up as wire.
So there you are! From iron ore to steel product. Of course there are many many more technical details to the different processes, but it would be going a bit too far to describe them all here. If you really want to know more, you should find a book on metallurgy or study materials science.
Or you can tell me the details you would like to know about and I'll add them to this w/u or put them in a w/u of their own...
Sources: www.wikipedia.nl, www.howstuffworks.com, www.corus.nl, my own memories of a visit to the Corus steel works in Ijmuiden.
Many thanks to filoraene and Professor Pi for corrections and additions.