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Steel is an alloy of iron mixed with a relatively small amount of carbon content ranging from 0.2 to 2 percent by weight. Other additional alloying elements such as manganese and tungsten might be present in steel in accordance with the grade (Salmon et al., 2009). Alloying materials like carbon serve to harden steel, making it tough, hard and corrosion-resistant as compared to iron. These properties result in the wide application of steel in modern construction and automobile industries. A steel production plant is integrated with three major processing units which include a blast furnace, steel furnaces and rolling mills. A blast furnace is a structure designed like a chimney that contains various iron ores which are FeO, Fe2O3 and Fe3O4. These ores are transformed to iron metal. Coke and limestone are combusted in the presence of air fed from below (Hassan & MacGinley, 2005). Coke reacts with air to form carbon monoxide that becomes a reducing agent as indicated in the following chemical equations.

            2C + O2                       2CO

            3CO + Fe2O3                  2Fe + 3CO2

A steel furnace involves the addition of manganese and various alloys, and extracting impurities, such as sulfur and phosphorus. There has been advancement in the techniques of steel production process. The Bessemer process, which involved large pear-shaped converters, was used in the 19th century used. In the early 20th century an open-hearth furnace was used. As a result, the basic oxygen usage was involved, cutting the time used to produce steel from a whole day to few hours. The most modern electrified furnaces referred to as electric-arc furnace are used in the 21st century. These electric furnaces are cheaper to build, operate and sidestep the energy needed for blast furnaces. Coke has been replaced by hydrogen and carbon monoxide that act as a reducing agent. The last part of steel production entails forming various shapes by the rolling mills. Steel is rolled into sheets, strips and bars among others.

Steel takes different forms depending on the alloy involved in its production. Carbon steel is the most common type with 2 percent and below carbon content which often consists of manganese. Stainless steel contains approximately 12 percent chromium, though nickel might be present, it is rust-resistant steel. Galvanized steel has a zinc coating for rust protection. Electroplated steel is comprised of tin coating obtained through application of electric current. Tool steel is another form of steel that is manufactured by high temperature heating and then rapidly cooled.

Structure of Steel

The addition of carbon to iron influences the characteristics of iron depending on the amount of carbon present in iron. According to Galvery & Marlow (2001), carbon steel has a freezing temperature which makes it mushy once within this range. This property is of significance during welding by facilitating its control in a pasty condition easily than in liquid state. The freezing temperature range increases with an increase in carbon content; however, this trend reverses and narrows beyond 2 percent weight of carbon. The range finally disappears when carbon portion is raised to 4.3 percent. Steel containing 95.7 percent iron and 4.3 percent carbon freezes at 1130 °C and then it is referred to as an eutectic alloy.

The four structures of carbon steel include austenite, ferrite, cementite and martensite. Austenite is used to describe face-centered grain structure when carbon is in solid form and exists when the temperature exceeds 7230 C, which is referred to as transition temperature. This structure cannot be viewed microscopically due to the high temperature; but few steel alloys are able to retain this structure even after cooling (Hassan & MacGinley, 2005). Ferrite structures relate to pure body-centered iron because of slow cooling of iron containing 0.8 percent carbon and below. Cementite steel is characterized by its hardness because 6.7 percent carbon content. Its hardness can be reduced by the addition of soft ferrite. The degree of cooling a steel cementite structure affects the products; slow cooling results in course-grained pearlite, while faster cooling produces ferrite that is harder and tougher. Pearlite structure is comprised of both cementite and ferrite. The pure form of Pearlite is observed when 0.8 percent of carbon is used.

Properties of Steel

Steel alloys posses different properties depending on the materials employed in its manufacture. Hardness property refers to the ability to resist pressure, stress and force. Hardness in steel is associated with the presence of manganese in the alloy. Steel has high thermal conductivity due to its metallic nature. Steel can withstand high temperatures due to availability of tungsten in the compound. Other properties of steel include elasticity, which is the capability to regain the original form after stretch. Steel is also characterized by plasticity, which is the capacity to change shape permanently due to stress and ductility, ability to be stretched (Salmon et al, 2009). Steel is malleable and magnetic, implying that it can be compressed and attracted by magnets. Stainless steel is rust-resistant, which is called corrosion resistance.

Treatment of Steel

Steel treatment is necessary to enhance its properties, such as to boost hardness or strength and to perfect it in its application. Steel can be subjected to heat treatment to change its properties using either heating and faster cooling, or heating and slow cooling. The temperature range used to make steel hard is determined by the content of carbon. Plain steel under 0.4 percent cannot be hardened using heart treatment. With an increase in carbon content from 0.4 to 0.8 percent, the temperature reduces from 820 °C to 780 °C. The temperature is steady at 780 °C when the carbon content is 0.8 percent. The following diagram shows the heat treatment process for steels.

 Heat treatment processes of steel include normalizing, annealing, hardening, tempering, spheroidising and stress relieving. Under normalizing, steel is heated to about 40 °C above the Higher Critical Limit and then cooled slowly in air. The resulting structure is fine pearlite due to the cooling in air. The objectives of normalizing are to generate uniform austenite, desegregate grains in steel castings, and create a reasonable hardening (Salmon et al., 2009). Normalizing is primarily used for the restoration of ductility of hot or cold worked steels and retention of other properties (Totten, 2006).

Annealing refers to reheating of steel and then cooling it slowly. Steels are heated at 25 °C above the Upper Critical Temperature, whereby the structure of steel at this point is Austenite. The steel structure obtained after annealing is a large grained Pearlite. Annealing is primarily deployed to enhance the properties of cast and forged steels before machining. This is achieved through eliminating internal stress and purifying the crystalline structure.

Spheroidising involves heating the steel at a temperature below the Lower Critical Temperature, followed by a slow cooling in the furnace. The attained structure after spheroidising is spheroidite. Spheroidising is primarily deployed to enhance the properties of medium and high carbon steels before cold work or machining (Salmon et al., 2009).

Stress relieving involves heating the steel to below the Upper Critical Temperature in order to result in re-crystallization, after which it is cooled slowly to attain a fine Pearlite structure. Stress relieving entails heating the metal to a temperature range of 550 °C to 650 °C and maintaining before cooling at a regulated rate. It is used to avoid distortion of steel constituents. Stress relieving is primarily deployed to increase the ductility of low carbon steels to maximum after cold working (Hassan & MacGinley, 2005).

Hardening treatment refers to heating and cooling rapidly the steel in appropriate liquid, such as oil. Heating steel at a temperature of about 760 °C transforms the iron crystals to face-centered cubic structure resulting in relocation of carbon atoms to the central position initially possessed by iron. Immersing the hot steel in a cold fluid cools it faster leading to trapping of carbon atoms. The product is a very hard and brittle and is referred to as martensite.

Tempering involves reheating of the steel to lower temperatures in order to reduce the level of hardness and increase its hardness. Hardened steels are extremely brittle and therefore they are inappropriate for a wide range of applications. Tempering converts the structure of steel from Martensite to Bainite. The following figure shows the tempering temperatures and the respective application of steels obtained.

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