Description / Shipment / Storage
Sheet metal is metal by an industrial process into thin, flat pieces. It is one of the fundamental forms used in metalworking and it can be cut and bent into a variety of shapes. Countless everyday objects are constructed with sheet metal. Thicknesses can vary significantly, extremely thin thicknesses are considered foil or leaf, and pieces thicker than 6 mm (0.25 inch) are considered plate.
Sheet metal is available in flat pieces or coiled strips. The coils are formed by running a continuous sheet of metal through a roll slitter.
The thickness of sheet metal is commonly specified by a traditional, non-linear measure known as its gauge. The larger the gauge number, the thinner the metal. Commonly used steel sheet metal ranges from 30 gauge to about 8 gauge. Gauge differs between ferrous (iron based) metals and nonferrous metals such as aluminium or copper; copper thickness, for example is measured in ounces (and represents the thickness of 1 ounce of copper rolled out to an area of 1 square foot).
There are many different metals that can be made into sheet metal, such as aluminium, brass, copper, steel, tin, nickel and titanium. For decorative uses, important sheet metals include silver, gold and platinum (platinum sheet metal is also utilized as a catalyst).
Sheet metal is used for car bodies, airplane wings, medical tables, roofs for buildings (architecture) and many other applications. Sheet metal of iron and other materials with high magnetic permeability, also know as laminated steel cores, has applications in transformers and electric machines. Historically, an important use of sheet metal was in plate armor worn by cavalry, and sheet metal continues to have many decorative uses, including in horse tack.
Steel sheets are flat products with a rectangular cross-section, the width of which is much larger than the tickness. A distinction is drawn between hot- and cold-rolled sheet.
Hot-rolled sheet in sheets is produced from semifinished products (slabs or billets), which are reduced to certain thicknesses by rolling and annealling and cut mechanically or by burning into rectangular or also other shaped sheets.
Cold-rolled sheet in sheets is produced by removing rust from hot-rolled sheet by “pickling” it in a weak acid solution, then washing, brushing, drying, oiling and unrolling the sheet and finally performing cold-rolling by passing the sheet through a reducing mill and cutting into rectangular or also other shaped sheets. Cold-rolled steel is a more highly finished product and has a smoother surface, greater dimensional (thickness, width, length) and greater strength.
Mild hot rolled steel plates of various sizes are used extensively in tank construction, ship building and in the fabrication industry. The larges sizes are shipped in single pieces and in most instances stored outside, unprotected against the elements, prior to shipment. Therefore, they are generally loaded in a rusty condition. Stacks of these plates when viewed from the side often look wavy owing to wrong placing of dunnage. Prior to shipment the stacks are not under pressure, as they might be in the hold of a ship, so that the apparent deformations will probably only be temporary. Sometimes in handling, a plate will develop a transverse kink which must be considered as damage as the defect is permanent and will required a re-rolling operation to re-flatten it. In handling large plates special gear is required in the form of hooks coupled to chains.
Long plates are usually stowed athwartships in the hold of the ship, but care must be taken to use plenty of suitable dunnage placed in line through the stow; this being particularly required when other cargo is to be loaded on top. One incident involved a coil stowage on top of plates, which is not to be recommended – through lack of and improper placing of dunnage, the entire shipment of plates was deformed to a serious extent.
Small dimensional hot rolled steel plates are shipped in unprotected bundles which are secured with metal strapping bands. The sizes of these bundles vary, but dimensions of plates around 1 to 2 m. are not uncommon. Such goods usually appear to be in a rusty (i.e. rust stained) condition when shipped.
Hot-rolled sheets are held together in packages with steel strapping. In order to prevent movement of the sheets in transit and during storage, at least two steel straps are used in both the transverse and longitudinal direction. An oxidation layer (scale), which provides limited corrosion protection, is formed on the surface of steel during hot-rolling or annealing. This layer is, however, highly sensitive and may easily flake off.
Pickled and oiled hot-rolled sheet and cold-rolled sheet, together with hot-dip galvanized fine sheet (i.e. electrolytically) coated sheet, for example plated with zinc etc.) electric steel sheet and tinplate are provided with multilayer packaging in accordance with their quality, the route and duration of transport and frequency of handling.
In the case of hot-rolled steel, it is usual to store it in the open and to transport it without protection, such that no protection is provided against rain etc. Such sheet therefore generally exhibits a layer of surface rust (rust film). Since the rust is removed from the steel (by pickling) prior to further processing, the quality of the steel is not impaired. Hot-rolled sheets must also be protected from chloride solutions (e.g. seawater or fertilizer), since pickling cannot remove uneven local corrosion or pitting corrosion. Especially in the case of damage by salt water, the sheets should be rinsed off with fresh water as soon as possible after arrival with the receiver and then pickled because significant delay prior to pickling may have the above-stated consequences. For reasons of quality maintenance, the aim should always be to store, handle and transport the sheets in the dry.
Cold-rolled sheets are more sensitive to corrosion than hot-rolled sheets, such that not only cold-rolled steel, but also surface-treated hot-rolled steel is additionally packed, for example, in fiber-reinforced packing or plastic-coated kraft paper and plastic films. It is therefore important to keep moisture away at all times; unprotected storage in the open or unprotected cargo handling in wet weather should be avoided.
Galvanized or tinned sheet and electric steel sheet should be treated similarly to cold-rolled sheet, but no corrosion protection agents, such as VCl paper, should be used for galvanized and tinned sheet as such agents may react with the surface coatings. Moisture may, for example, give rise to a white bloom on the zinc coating. If rain or condensation water penetrates between the closely sheets of metal, the thin, protective zinc oxide layer does not form, but rather a thicker layer of pure zinc oxide. At the contact points between the sheets, this layer has the appearance of scurf.
Hot-rolled steel is used e.g. for the manufacture of pipes, steel doors and tanks or is further processed into cold-rolled steel.
Much cold-rolled steel is processed in the automotive industry, but some is also used in the manufacture of household goods (e.g. fridges).
Galvanized sheet is used, e.g. in the automative industry or in the production of roofing materials (e.g. flashing, guttering).
Tin plate is used in the packaging industry for the production of cans, for household goods and toys and for similar products.
Electric steel sheet is used, for example, in transformers, electrical machinery and equipment.
Stainless steel sheet is used, for example, in the manufacture of machinery, tools and containers.
- Wet damage (corrosion)
- Contamination / defilement
- Mechanical influence