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- Published: Thursday, 20 June 2013 18:43
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Steel production involves several processing stages including iron making, primary and secondary steelmaking, casting and hot rolling. These are followed by some of the following fabrication processes: cold rolling, forming, forging, joining, machining, coating and/or heat treatment.
Steels can be made either from raw materials (e.g. iron ore, coal and limestone) or by recycling steel scrap.
In response to the requirements of society, steel processing is subject to significant innovation in order to reduce costs, improve quality and to minimise its environmental impacts.
These complex processes produce a wide variety of steel compositions, in many different shapes and sizes, each tailored closely to the requirements of the use of the steel.
In the Blast Furnace iron oxide is reduced by carbon, in the form of coke, in a blast of hot air. Limestone is added to flux the impurities into a slag. The resulting molten iron, called “hot metal”, has too high a carbon content to be useful and further processing is required to produce steel from this hot metal.
The purpose of the Basic Oxygen Steelmaking (BOS) process is to refine the hot metal produced in the blast furnace into raw liquid steel, which may be subsequently refined in the secondary steelmaking shop.
The main functions of the Basic Oxygen Furnace (BOF) are to decarburize and remove phosphorus from the hot metal, and to optimize the steel temperature so that any further treatments prior to casting can be performed with minimal reheating or cooling of the steel.
The exothermic oxidation reactions that occur during BOS generate a lot of heat energy - more than is necessary to attain the target steel temperature. This extra heat is used to melt scrap and/or iron ore additions.
In the Electric Arc Furnace (EAF), recycled steel scrap is melted and converted into high quality steel by using high-power electric arcs. The main task of most modern EAFs is to convert the solid raw materials to liquid crude steel as fast as possible and then refine further in subsequent secondary steelmaking processes. Nevertheless, if time is available, almost any metallurgical operation may be performed during flat bath operation period (after melting), which is usually performed as a pre-treatment to the secondary steelmaking operations.
Secondary steelmaking is a critical step in the steel production process between the primary processes (Basic Oxygen Furnace or Electric Arc Furnace) and casting. Some elements are added and some have to be removed during secondary steelmaking in order to fine-tune the composition of the steel to meet the specification and the customer’s requirements. The temperature, internal quality and the inclusion content of the steel also have to be carefully controlled during secondary steelmaking. Several types of furnace and other equipment are used for these purposes. Each has a specific role to play and their use is selected depending upon the particular requirements of the type of steel being made. After secondary steelmaking, the ladle of liquid steel is taken, at the required composition, quality, cleanness, time, temperature and at least cost to the casting process.
Continuous Casting: After secondary steelmaking, the molten steel is usually continuously cast via a tundish into a water-cooled copper mold causing a thin shell to solidify. This ‘strand’ is then withdrawn through a set of guiding rolls and further cooled by spraying with a fine water mist. The solidified shell continues to thicken until the strand is fully solidified. Finally, the strand is cut into desired lengths and these are either discharged to a storage area or to the hot rolling mill. A wide range of strand dimensions can be cast depending on final application: ‘slabs’ for flat products such as plate and strip, ‘blooms’ for sections such as beams, and ‘billets’ for long products such as wire. Techniques have also been developed to cast steel directly to thin strip (<3 mm thick) and cast strip is now available commercially.
Hot rolling is the most efficient process of primary forming used for the mass production of steel. The principal effects of hot rolling are the elimination of the cast ingot structure defects and obtaining the required shape, dimensions and surface quality of a product. The main parameter which determines the ability of a rolling process to eliminate the cast structure defects is called the rolling ratio. It is calculated as the ratio of the cross-sections of the initial stock and the final product. The value of the rolling ratio required for obtaining steel structure of a good quality depends on the steel type, e.g. it is about 2 for the structural steels but it can be as high as 12 for tool steels.
The first operation of any hot rolling process is heating of the stock to the proper deformation temperature. During heating and hot forming a scale (oxide layer) forms on the stock surface which must be systematically removed. Descaling can be performed mechanically (by crushing during a hot forming operation) or by spraying with water under high pressure.
During rolling, deformation of material occurs between dies in the form of rotating, driven rolls. It is a stationary process - this means that a deformation zone does not change its position in time. The transporting force during rolling is the friction between rolls and a processed material.
Hot rolling is carried out using various types of rolling mills. The selection of the appropriate rolling technique is made according to the particular hot-rolled product.
For the required geometric product features (such as a shape, dimensions and surface quality) to be obtained, some finishing and (eventual) heat treatment operations are necessary after hot rolling. Typical finishing operations are: cooling, straightening, sizing and surface cleaning. The further stages of the production process are: quality control (it can also be performed during hot forming), marking, piling and preparation for storage and transportation. Some hot-rolled products are coated for protection and/or decoration.
Hot rolling comprises manufacturing technologies for both semi-finished and finished products. Semi-finished hot-rolled steel products are the starting materials for further hot metal forming processes. The following principal hot-rolled product groups can be distinguished:
• flat products
• long products
• seamless tubes
• specialty products, such as wheels, rings, bars with the periodically varying profile, etc.
Hot-rolled products are often subjected to further processing, such as cold-rolling, forming, machining and joining, in order to achieve a variety of steel products offered for sale.
Heat treatment of steels may be carried out for a number of reasons, for example:
• Inter process thermal treatment to allow further processing, e.g. softening, H2 removal
• To produce specific final properties in the bulk material
• To produce specific surface properties in the final component, e.g. hardness
Different steel microstructures have very different typical mechanical properties; therefore, different types of heat treatment may be used depending on the grade of steel and its desired microstructure and properties. Heat treatments (i.e. defined heating and cooling regimes) are tailored to promote the formation of a specific microstructure, thereby developing the steel’s properties to meet application requirements.
The alloying content of a steel also affects its microstructure and properties, therefore in each case the specific heat treatment parameters will be dependent on many variables such as the composition, size, processing history of the steel (or steel component) being treated. Examples of heat treatments include:
• Annealing (to soften/increase ductility) e.g. inter process
• Normalizing (to soften, give uniform microstructure)
• Quenching (Hardening) & Tempering (to increase strength, while maintaining toughness & ductility)
• Stress-relieving (to remove residual stresses after a forming operation)
• Surface treatments (to produce specific properties at the component’s surface)
The heat treatment of steels may involve several processing stages, e.g. austenitizing or hardening followed by tempering or surface treatments. Heat treatment processes may be carried out in various types of furnace, which differ in characteristics such as pressure, atmosphere, number of chambers, continuous or batch etc.
With modern applications becoming increasingly challenging, heat treatment, which can lead to improved mechanical properties or allow additional processing of steels, is an important part of the production process. This module will give you examples of different types of heat treatment and where they might be used. It will also introduce some of the many types of steel which rely on specific heat treatments to give them very specific microstructures and properties to meet the requirements of demanding applications.