How Steels Are Made?
The overall process of steel production is shown in Figure 3.3. This process consists of the following three phases:
1. reducing iron ore to pig iron
2. refining pig iron (and scrap steel from recycling) to steel
3. forming the steel into products
|Steel Production Process|
Limestone is used to help remove impurities. Prior to reduction, the concentration of iron in the ore is increased by crushing and soaking the ore.
The iron is magnetically extracted from the waste, and the extracted material is formed into pellets and fired. The processed ore contains about 65% iron.
Reduction of the ore to pig iron is accomplished in a blast furnace. The ore is heated in the presence of carbon. Oxygen in the ore reacts with carbon to form gases.
A flux is used to help remove impurities. The molten iron, with an excess of carbonin solution, collects at the bottom of the furnace. The impurities, slag, float on top of the molten pig iron.
The excess carbon, along with other impurities, must be removed to produce high-quality steel. Using the same refining process, scrap steel can be recycled. Two types of furnaces are used for refining pig iron to steel:
1. basic oxygen
2. electric arc
The basic oxygen furnaces remove excess carbon by reacting the carbon with oxygen to form gases. Lances circulate oxygen through the molten material. The process is continued until all impurities are removed and the desired carbon content is achieved.
Electric furnaces use an electric arc between carbon electrodes to melt and refine the steel. These plants require a tremendous amount of energy, and are used primarily to recycle scrap steel.
Electric furnaces are frequently used in minimills, which produce a limited range of products. In this process, molten steel is transferred to the ladle.
Alloying elements and additional agents can be added either in the furnace or the ladle. During the steel production process, oxygen may become dissolved in the liquid metal.
As the steel solidifies, the oxygen can combine with carbon to form carbon monoxide bubbles that are trapped in the steel and can act as initiation points for failure. Deoxidizing agents, such as aluminum, ferrosilicon and manganese, can eliminate the formation of the carbon monoxide bubbles.
Completely deoxidized steels are known as killed steels. Steels that are generally killed include:
■ Those with a carbon content greater than 0.25%
■ All forging grades of steels
■ Structural steels with carbon content between 0.15 and 0.25 percent
■ Some special steel in the lower carbon ranges
Regardless of the refining process, the molten steel, with the desired chemical composition, is then either cast into ingots (large blocks of steel) or cast continuously into a desired shape. Continuous casting with hot rolling is becoming the standard production method, since it is more energy efficient than casting ingots, as the ingots
must be reheated prior to shaping the steel into the final product.
Cold-formed steel is produced from sheets or coils of hot rolled steel which is formed into shape either through press-braking blanks sheared from sheets or coils, or more commonly, by rollforming the steel through a series of dies. No heat is required to form the shapes (unlike hot-rolled steel), and thus the name cold-formed steel.
Cold-formed steel members and other products are thinner, lighter, and easier to produce, and typically
cost less than their hot-rolled counterparts (Elhajj, 2001).