Steel sheet in coils
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.
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.
Cold rolled steel sheeting
The effects of contact with moisture
Cold rolled steel sheeting can in a certain sense be considered a finished product. Any rust on the plating is not acceptable. In consideration of these facts, during loading or transport and discharge of the material, it must be kept completely dry at all times. Damage, resulting in heavy claims, can arise through contact with rain during loading and/or discharge from the ocean carrier. On voyage, ship and especially cargo sweat have been responsible fro heavy losses. Likewise, penetration of sea water into cargo compartments can be particularly devastating. Free moisture flowing across the outer metal envelope can pass beneath the overlaps of the packing and eventually penetrate the contents. As explained in similar circumstances with hot rolled coils, but with more serious results, through capillary action, moisture, which has penetrated and contacted the goods, is drawn between the turns of plating. So called protection or strengthening rings, which consist of an angle iron placed around the outer and inner circumference edges of the packing, have proved to be an innovation which promotes rather than reduces the incidence of damage. Strengthening of the edge packing does provide greater protection against handling damage, but the benefits to be derived from this are far outweighed when the packing comes into contact with free moisture. The moisture trickles or runs across the surface of the packing to be caught up and accumulated in the edge protections. This water seeps through, penetrating the inner packing and eventually contacts the plating of the coil.
In the event of the plating developing a rust condition, whereby part of the coil so affected will be rejected and considered unusable for its originally intended purpose, loss in value will result. Depending upon the degree and overall extent of the rust, a decision will have to be made as to whether the plating qualifies as second choice material or whether it should be degraded to the value of scrap. One thing is certain, such material affected by rust cannot, in most instances, be reconditioned.
Depending upon the extent of this damage, claims can be forthcoming. A type of handling damage often encountered with cold rolled steel coils is often referred to as ovalisation, which can be caused by lowering a coil at such a speed that it lands heavily. This can result in the unit being forced out of the round so that it becomes oval in shape. In such a condition the coil will not fit into the decoiling machines.
To suit customer’s requirements cold rolled coils are often cut into short sections, in general measuring roughly 1.000 x 2.000 mm. The stack of sheets is wrapped in moisture-resistant paper and finally fitted with an outer metal envelope. Each package is then secured with flat metal strapping bands placed longitudinally and transversely around each unit. Beneath each package there is usually a framework of strong wooden bearers, which impart to the package rigidity and facilitates manipulation by fork-lift trucks.
Hold rolled steel sheeting
Hot rolled steel coils are not usually wrapped for protection against contact with moisture and the development of rust. They are secured with a number of flat metal strapping bands transversely through the eye or core of the coil, and securing bands are fixed around the outer circumference of the coil. There are instances when hot rolled coils are pickled and then oiled for protection, after which they are wrapped in moisture-proofed kraft paper and completely enfolded in a metal envelope, all secured with flat metal strapping bands.
Unwrapped hot rolled steel coils are often stored in the open, uncovered and exposed to the elements. Therefore, it is not unusual for such material to be partly or completely rusty in appearance at the time when it is shipped. Wrapped hot rolled steel coils pickled and oiled (H.R.P.O.) should be kept dry as any rust on the plating is inadmissible. H.R.P.O. coils should be given the same consideration as cold rolled coil.
Often referred to as mechanical damage, this consists of physical defects to the plate edges caused during such manipulations as loading and unloading, when the side of the coil is permitted to strike some object. Heavy score marks across the plate edges may or may not be of importance depending upon the intended purpose for the material. If the plating is for re-rolling and the edges have been sheared to an ordered width, then deep incisions are not admissible; likewise, where tearage of the edges of the plating is concerned and depending on the depth of tearing. Buckling or bending of plate edges is of less importance unless the affected plating is actually turned over to form a fold or bent beyond the elasticity of the metal. Usually there are rolls forward of the uncoiling machine which flatten out any bent edges. Another undesirable defect is telescoping and this applies especially to the centre core turns of plating. Each unit should be coiled in such a manner that all plate edges are in line. If edges are projecting too far (telescoped), the possibility of damage developing during handling and in stow are considerably enhanced. In lifting coils, the use of chains and wires should be avoided. Only gear such as broad braided wire slings and ‘C’ hooks should be used. The ideal type of lift truck should be fitted with a circular bar prong if damage is to be avoided. Single steel plates are usually handled with chain or wire bridle type slings, the ends of the legs of which are equipped with a special type of lifting hook. Incorrect placing of dunnage in stacks ashore, before shipment, can cause permanent deformation.
Occasionally the situation does arise, where the side edges of the plating are continuously jagged in appearance over a considerable distance. This type of defect is in no way connected with transport or handling. It is a defect which develops during the rolling of the plating in the mill.
Unwrapped hot rolled steel sheeting may spend a considerable period of time in open storage. The goods are exposed to rain and possibly a polluted atmosphere. Therefore, free moisture trickles down and across the edges of the plating, and seeps between the turns of sheeting to penetrate a limited distance whilst the coil is static. As rust is a natural phenomenon of steel, it is not surprising that most hot rolled products appear to be either rusty or partly rusty. A thin even film of surface rust on the plating, originally through contact with fresh water, is usually of no consequence. Before the plating can be used, the mill scale, or remnants thereof, and rust and extraneous matters which might be present, must be removed before the goods are cold rolled or provided with a protective coating. This cleaning of the material is accomplished by subjecting the sheet to a pickling process, whereby the plating is passed through a hydrochloric acid bath.
Loading during rain
If it is decided to load during rainy weather, care should be taken that not too much free moisture may collect on the tanktop and bank up at the aft end of the compartment as otherwise the coils might actually be partially submerged in water. This water could become contaminated by chlorides from salt crystals left behind from previous sea water washings or rests of previous cargoes and/or impurities brought in with the cargo. If this occurs, those coils affected would sustain damage.
Contact with sea water
Sea water has a devastating effect upon steel products and rapidly causes a serious rust condition to develop whereby the material may become pitted and eroded.
The importance of strapping bands
Strapping bands on the coils should be completely and sufficiently tight in order to afford maximum possibilities of delivering the goods still in a tightly wound condition. Coils which have slackened off, owing to broken securing bands, can upset the stability of the stow and cause problems in handling during discharge and afterwards.
To suit customers’ requirements, coils are often unwound, cut into plate lengths of about 2 m and stacked into bundles. Each bundle is secured with a number of criss-crossed flat metal strapping bands. Bundles of hot rolled steel sheets are usually unwrapped and unprotected against the development of rust. The goods will therefore probably have a rusty appearance at the time of shipment. For transportation by sea, all unpacked hot rolled mild steel plates and/or steel sheets, whether in bundle form or loose, should be given the same consideration as hot rolled steel sheeting in coils.
Storage and transport
The degree of rusting of steel consignments should be recorded in the shipping documents before acceptance of the consignment, possibly using the following definitions:
- Wet before shipment
- Partly rust stained to rusty
- Gear marked
- Contaminated by foreign substance
- Contaminated by saltwater
- Chafed in places
- Packing torn exposing contents
Steel products require 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. However, the following should be noted:
Steel exhibits a lower temperature than the external temperature anticipated during transit If the temperature of the ambient air outside the ship rises, this has only a minimal effect on the temperature of the cargo. Ventilation with “warm” external air may result in cargo sweat on the “cold” steel, if the temperature of the latter is below the dew point of the ambient air. In such a case, ventilation may encourage corrosion.
The steel is warmer than the external temperatures anticipated in transit Ventilation may be performed without any risk of cargo sweat formation. However, cooling of the ship’s sides may cause their temperature to drop below the dew point of the hold air, resulting in ship sweat inside the hold. In this case, the temperature of the hold air should be adjusted by ventilation to match that of the external air.