Bendability of Ultra-High-Strength Steel

Abstract

Utilisation of ultra-high-strength (UHS) steels is rapidly spreading from the automotive industry into many other application areas. It is necessary to know how these materials behave in common production processes such as air bending. The bendability of UHS steels is much lower compared to normal and high-strength construction steels. In this work, experimental tests were carried out using complex phase (CP) bainitic-martensitic UHS steels (YS/TS 960/1000 and 1100/1250) and S650MC HS steel in order to inspect material bendability and possible problems in the bending process. Mechanical and geometrical damages were registered and classified. The bending method used was air bending and press brake bending with an elastic lower die. The FE analysis was used to understand the stress state at different points in the material and build-up of failure. As UHS steels cannot stand large local strains, a large radius must be used in air bending. The results show that even when a large radius is used in air bending, the strain is not evenly distributed; there is a clear high strain area in the middle of the bend. It was also possible to simulate the other phenomena occurring in experimental tests, such as losing contact with the punch and ‘nut-like’ geometry, using FE analysis. Experimental test results also show that by using an elastic lower die, it is possible to avoid unwanted phenomena and obtain an almost 50% smaller punch radius, but the required force is 50% bigger than that required in air bending.