In forming operations the ingots, slabs, billets, and blooms obtained after casting are further processed to produce strip, sheets, plate, bar, rod, or other structural shapes through various hot forming and sometimes cold forming operations, depending on the final product. In hot forming operations, preheated (typically in the range of 1800°F), solidified steel is reduced in cross-section through a series of forming steps by applying mechanical pressure through work rolls to produce semifinished shapes for further hot or cold rolling, or finished shapes. The hot forming mills can be grouped into one of the following four types18:
1. Primary mills
2. Section mills
3. Flat mills (plate, hot strip, and sheet)
4. Pipe and tube mills (seamless and butt-weld)
Preheating of steel helps the slabs to undergo a surface preparation step called "scarfing," which removes defects prior to entering the rolling mill by removing a thin layer of the steel surface through localized melting and oxidation. Surface scale is removed from the heated slab by a scale breaker and water sprays prior to its entry into this mill.
Finishing processes clean the surface of the semifinished, hot-rolled steel products before forming or cold rolling or coating operations. Mill scale, rust, oxides, oil, grease, and soil are chemically removed from the surface of the steel using solvent cleaners, pressurized water, air blasting, abrasives, alkaline agents, salt baths, or acid pickling. Salt bath descaling is a finishing process that uses the physical and chemical properties of molten salt baths to remove heavy scale from the surface of selected stainless and high-alloy steels in subsequent water quenching steps. The two salt bath descaling operations are1:
1. Oxidizing (or Kolene®). This removes scale using molten salt baths other than those containing sodium hydride.
2. Reducing (or Hydride®). This removes scale using molten salt baths containing sodium hydride.
These two salt bath descaling processes may be either batch or continuous and are conducted prior to combination acid pickling (hydrofluoric and nitric acids). Descaling may also be performed using an electrolytic solution of sodium sulfate. The other mechanical descaling operation known as blast cleaning uses abrasives such as sand, steel, or iron grit to clean the steel surface. A compressed air blast cleaning apparatus or rotary-type blasting cleaning machine is used to bring the abrasives in contact with the steel.18
The acid pickling process chemically removes oxides and scale from the surface of the steel by the action of water solutions of inorganic acids. It is widely used because of its comparatively low operating costs and ease of operation. Carbon steel is usually pickled with hydrochloric acid; stainless steels are pickled with sulfuric, hydrochloric, nitric, and hydrofluoric acids.18 The pickling process uses various organic chemicals to inhibit the acid from attacking the base metal. Wetting agents may be used for effective contact of the acid solution with the metal surface. The pickling bath ends with the steel being passed through one or more rinse operations. Alkaline cleaners may also be used to remove mineral oils, grease, and animal fats and oil (used in some rolling solutions) from the steel surface prior to cold rolling. Common alkaline cleaning agents include caustic soda, soda ash, alkaline silicates, and phosphates.18
Steel that has been hot-rolled and pickled may be cold-rolled immediately at ambient temperatures before further oxidation can occur. This is done to impart the desired mechanical and surface properties in the steel, and for cold working of the pipe and tube. The two main types of cold mill products are cold-rolled sheets/coils for sale or for further processing in galvanizing and coatings lines, and cold-rolled coils for subsequent tinning.1
Cold rolling hardens the steel, which must then be heated in an annealing furnace to make it more ductile. The annealing process involves heating the strip to about 1300°F in an inert atmosphere to prevent oxidation, and then allowing it to cool such that the crystal structure of the steel changes.1 In batch annealing, gas burners are used to indirectly heat stacked coils, whereas in continuous annealing the coils are unwound and passed through an extended furnace. After the steel has undergone the annealing process, it is run through a temper mill to produce the desired flatness, metallurgical properties, and surface finish.
Steel-coating operations, such as hot coating and electroplating, improve resistance to corrosion and improve appearance. Hot coating operations involve immersing precleaned steel into molten baths of zinc, zinc/aluminum alloy, aluminum, chromium, lead, antimony, tin/lead alloy, and zinc/nickel alloy. Electroplated steel production uses electrodes to deposit a metal coating (zinc, chromium, tin, nickel, brass, cobalt, copper, nickel/tin alloy, zinc/nickel alloy, and zinc/iron/aluminum alloy) onto the surface of the steel. Figure 2.7 illustrates the forming and finishing processes and Table 2.14
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