At the Glenbrook plant, molten steel is cast as huge, 21-centimetre-thick slabs that weigh over 10 tonnes. After they have cooled, they are reheated and passed repeatedly through hot rollers that reduce their thickness. The steel is then rolled into coils and cooled by water.
The Glenbrook company operates two modern cold mills, which have highly accurate x-ray systems to control thickness. The coils are processed to remove the iron-oxide layer that forms after hot-rolling and cooling. They are then passed through cold rollers to their final thickness.
Roofing and cladding
Some types of sheet steel are given a thin coating of metal which provides protection against rust and corrosion. Two different varieties are produced. Traditional galvanised steel (Galvsteel) is coated with zinc, whereas a newer product, Zincalume, has a coating of 55% aluminium and 45% zinc.
Pre-painted steel (Colorsteel) now accounts for a large proportion of the roofing used in New Zealand. A builder or owner can purchase sheet steel in the colour of choice, rather than having to paint the roof after a building is completed.
Pipes and beams
The agricultural industry is a major user of steel pipes. Stock-handling equipment, implement frames and agricultural trailers are some of the steel-pipe products made by New Zealand Steel. The recently completed Waikato water pipeline was made from hot-rolled coil, galvanised Zincalume, and Colorsteel.
Although no hot-rolled beams are manufactured for the New Zealand construction industry, the Glenbrook plant produces welded steel beams. These offer greater flexibility in design – for example, curved and sculptured beams.
Hot-rolled steel is produced for beams, flooring, storage tanks and flanges. Cold-rolled steel products include shelving, drums, saw blades, hinges and car parts.
Steel for export
Since 1991, New Zealand has exported more iron and steel than it imports. In 2004, 60% of Glenbrook steel, including very high-purity stainless steels, was exported.