Steel sheet in coils
| Infobox on Steel sheet in coils | |
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| Example of Steel sheet in coils |  |
| Facts | |
| Origin | Trade in steel and steel products primarily flows:
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| Stowage factor (in m3/t) |
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| Humidity / moisture | Relative humidity: < 40 - 50% |
| Ventilation | (See also text) Steel sheet in sheets requires particular humidity/moisture and possibly ventilation conditions. Steel corrosion accelerates rapidly at relative humidity > 60%. If possible, relative humidity should be reduced to below 60% by appropriate ventilation measures. |
| Risk factors | (See also text) Seawater damage, contamination and mechanical damage |
Steel sheet in coils
Description
Cold rolled steel sheeting
This material is manufactured by re-rolling and cold reduction of hot rolled steel sheeting. The first step taken in the process is when a hot rolled steel coil is unwound and passed through rolls which flex the plating in such a manner as to break and fragment the mill scale. The plating is then pickled in an acid bath for the purpose of removing the mill scale completely, as well as rust and other extraneous matter which might be present. Thereafter, the material is washed to remove any traces of acid so leaving the surface of the sheet clean, fairly smooth and dull grey in colour.
In order to prepare the steel for cold working by improving its grain structure, the plating is annealed by heating to a high temperature in a special type of furnace. It then passes on to the cold reduction mill, which consists of a number of sets of rolls, five strands in tandem where the plating is under tension between strands, progressively reduced in thickness to the required gauge.
In order to rectify the adverse effects of cold working, the coils are again annealed in sealed furnaces, into which a special oxygen free atmosphere is fed, to avoid the formation of scale upon the surface of the plating. Finally, the sheet is given a skin pass through a single strand of rolls, which is referred to as temper rolling. In this operation the sheet is further reduced, but only very slightly, and then re-coiled. The object of this is to produce the required mechanical properties and surface finish. In general, the purpose of cold rolling is to impart to hot rolled material a finer finish in the form of a fine smooth surface, accuracy of thickness and dimension and also improved tensile strength. As a rough guide, in general, plate thickness of 0,5 mm or even less are involved. Coil weights are between 5 to 15 tons per unit with a width of plating from about 900 to 1.300 mm. Coil diameters of from 1.000 to 1.400 mm are about average. Apart from being wound into coils the continuous strip, or band of steel, is also cut into separate sheets which are made up into oblong packages.
The finished coil, with a fine mirror like surface, is oiled for protection against the development of rust, secured with a steel strapping band applied around its circumference, completely wrapped in moisture-resistant paper packing, after which an outer metal cover is applied for further protection. Flat metal bands, usually not less than four, are passed transversely through the core piece of each unit and also not less than three additional bands around the circumference to secure the packing in place. The banding is also applied to ensure that the coil remains tightly wound.
Hold rolled steel sheeting
Ingots are placed in a soaking pit where they are reheated to a suitable temperature for further rolling into slabs, and then into steel sheeting in the form of a continuous strip. In some instances, slabs are taken from stock in which case the slab is placed in a reheating furnace. The ingot or slab is brought to intense white heat to ensure that the temperature is uniform throughout this initial rolling. Temperature will be in excess of 1.000ºC. During reheating, a layer of basic oxide forms on the slab, and in order to remove this, the slab passes through a scale breaker. The loosened scale is blown away by high powered jets of water. Onwards to the roughing train, where the slab passes through four to five sets of rolls in tandem which reduces the slab down to about one fourth of its original thickness, and at the same time squeezes the slab into becoming longer in length. Rolls, placed vertically, control the width of the slab as it reduces in thickness and lengthens into a continuous sheet of thin plating.
Finally, the hot plating, at a temperature of just over 800°C, passes to the finishing train where it passes through six or seven strands of rolls in tandem at great speed, and when the thickness of the plating if progressively reduced it emerges as a thin sheet of great length. Thereafter, the strip of sheeting, travelling at a speed of over 1.000 m. per minute, moves towards the coiling machine, before which it is sprayed with water to reduce it to a predetermined temperature. The entire operation is meticulously monitored by computer in order to ensure exact width and thickness of plating as well as a satisfactory cooling rate. The final product, in coil form, might consist of a plate thickness of about 1 mm to 5 mm, and 125 cm in width. Each coil usually weighs between about 7 to 15 tons depending on the length and thickness of the plating. Coils weighing up to 30 tons are not altogether uncommon.
Applications
Cold rolled steel sheeting
This type of steel sheeting is used extensively in the automobile industry for the manufacture of car bodies. It is used for household appliances such as refrigerators, bread bins, electrical home appliances, radiators, metal furniture, drums, etc. Also there is a growing demand for pre-coated cold rolled steel plating which has a diverse range of uses. To suit some customer’s requirements, coils are finished on slitting lines which cut the wide steel band down into narrow widths.
Hold rolled steel sheeting
This type of steel may eventually be sheared into short lengths of plate and made up into bundles to be sold to the fabrication industry. On the other hand, it can be destined for re-rolling in order to produce cold rolled steel sheeting.