780N/mm2 Grade Hot-Rolled High-Strength Steel Sheet for Truck Frame Use
In response to stepped-up user demands for lightweight vehicle bodies and improved fuel efficiency, NKK has developed a new series of 780 N/mm2 (80 kg) grade hot-rolled high-strength steel sheet. Marketed as NKHA780LF, the new product excels, in particular, in fatigue strength and is ideal for truck frame use. The majority of steel materials traditionally used for truck frames are hot-rolled high-strength steel sheets ranging from 440 N/mm2 (45 kg) to 540 N/mm2 (55 kg) grades (thickness: 5 to 6 mm). Calls are increasing, however, for 780 N/mm2 grade lighter-gauge sheets (thickness: 4 to 5 mm) to achieve reduced vehicle weight. Greater fatigue strength is also being pursued to prevent fatigue fracture of steel materials, which results from body vibration during operation. However, due to abrasion problems involving the edges of press-punched holes for bolts and rivets in the truck's frame, it has been difficult to improve the fatigue strength of steel simply by increasing its tensile strength, thereby hindering the development of lighter-gauge steel. NKK has overcome these hurdles by effectively applying the optimized design and control of steel chemistries and the high-accuracy heat treatment technology in the cooling stage of hot rolling. This has resulted in improving the fatigue strength of the 780 N/mm2 steel by more than 60% compared to conventional products. In addition to fatigue strength, the new 780 N/mm2 steel also excels in press formability, weldability and paintability. The new steel has already drawn high praise from several automobile manufacturers following their sample tests. NKK intends to pursue expanded use of the new steel for under-body parts of passenger cars like suspension arms and other chassis parts requiring greater fatigue strength. |
