Abstract
Bendability is an important property of ultrahigh-strength steels since the typical applications of such materials include structures manufactured by air-bending. Conventional methods to evaluate bendability, such as the bending test according to the standard VDA-238 or the conventional tensile test do not provide sufficient information to evaluate bendability of ultrahigh-strength steels due to the average nature of the material response in these tests. In this study, the mechanical properties were determined using thin tensile specimens cut from the surface of the sheet and the evaluation of bendability was carried out using frictionless bending tests. The results of the experiments and FE-modelling presented in this paper reveal that the mechanical properties of the sheet surface have a significant impact on bendability. Novel ultrahigh-strength steel with better work-hardening capacity at the surface caused by a layer of relatively soft ferrite and lower bainite has good bendability, especially when the bend line is aligned transverse to the rolling direction. Microstructural investigations reveal that in a conventional steel with a relatively hard surface microstructure, the deformation localizes into shear bands that eventually lead to fracture, but similar shear banding was not present in the novel steel surface. This can be attributed to the better work-hardening capacity which delays the onset of shear localization and fracture.
Highlights
Ultrahigh-strength strip steels (UHSS) manufactured using thermo mechanical rolling and direct quenching (TMR-DQ) are attractive structural materials due to the combination of good mechanical prop erties and weldability becoming a convenient choice in the lightweight structures
It can be seen that the conventional steel exhibits higher overall strength than the novel steel in both loading directions when the average material response measured with the full thickness samples is consid ered (Fig. 8a)
The total elongation is higher in the rolling direction, but no difference in the magnitude of the total elongation is seen for the full thickness tests in the transverse direction
Summary
Ultrahigh-strength strip steels (UHSS) manufactured using thermo mechanical rolling and direct quenching (TMR-DQ) are attractive structural materials due to the combination of good mechanical prop erties and weldability becoming a convenient choice in the lightweight structures. UHSSs are used mainly in applications that require high strength combined with good bendability such as in the lifting and transportation industry, where the use of UHSS can result in energy savings and a reduced carbon footprint. Loader crane booms are built with telescope-type structures including multiple booms and gliding rails. These structures benefit from small bending radii as it provides more space to place the booms inside each other and the gliding rails can be placed as far from each other as possible, creating a more stable and durable structure. The bend test standard ISO 7438 (2016) specifies a method for determining the deformation capability of a material in bending
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