590N/mm2 Grade High-Strength Steel Plate Supplied for Building ProjectsNKK has developed a new 590 N/mm2 grade high-tensile strength steel plate with a low yield ratio that offers both excellent weldability and earthquake-resistant characteristics, making it ideal for structural use in steel-frame buildings. The new steel plate, marketed as NKK-SA440B-U and NKK-SA440C-U, has attracted considerable attention of steel-frame builders and designers, and NKK has already received orders to supply a total of 1,700 tons of the steel for three construction projects. Of these, the largest project is a new athletic stadium being built in Niigata Prefecture, which will use about 1,000 tons of the NKK steel. The new steel represents radical improvements in weldability over the traditional 590 N/mm2 grade low-yield-ratio steels, while retaining the superior performance levels in terms of strength as well as earthquake- and impact-resistant properties. NKK succeeded in improving the steel's weldability by reducing the carbon content to about two-thirds of the traditional steels (i.e. 0.08% for 80mm-thick plate), while optimizing other elements in the steel. Due to its higher strength, conventional 590 N/mm2 grade low-yield-ratio steels incurred several restrictions during welding operations, including preheating, bead control and other requirements. The new steel was developed to meet increasing demands from steel-frame builders, designers and field operators for easy welding, better work environments and enhanced productivity. The new 590 N/mm2 grade low-yield-ratio steel offers the following advantages:
Traditionally, the majority of high-strength steels applied to steel-frame buildings were in the 490 N/mm2 grade range and the use of 590 N/mm2 grade steels has been limited to bridges, tanks and industrial machinery fields. By improving its earthquake-resistant properties, NKK first supplied its 590 N/mm2 grade low-yield-ratio steel plate for high-rise building use in 1986. In 1991, NKK received the Okochi memorial prize in technology for developing the steel. Since the Construction Ministry's general approval was granted to NKK-SA440 in 1996, its application has widely expanded in construction fields, allowing NKK to become the top domestic supplier of this type of steel, with a delivery record surpassing 10,000 tons to a total of 15 projects to date. In the latest Niigata stadium construction project, scheduled for completion in 2001, 1,000 tons of NKK's new 590 N/mm2 grade low-yield-ratio steel will be employed as structural piping (750mm in diameter, 32-45mm in thickness) for the roof's main arch.
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