The effect of mechanical behavior on bendability of ultrahigh-strength steel

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

Superior bendability of direct-quenched 960 MPa strip steels

Abstract The present paper shows the effect of microstructure on the press brake and frictionless 3-point bending of 6 mm thick ultrahigh-strength steel strips with a yield strength of 960 MPa. With a traditional press brake machine the minimum bending radii of the studied steels varied from 1.3 times the thickness to 3.0 times the thickness for the bend axis perpendicular to the rolling direction and in the range 2.0–3.5 times the thickness for the bend axis parallel to the rolling direction. The frictionless 3-point bending-equipment incorporating rotatable die-rollers has been applied to characterize the material work hardening behavior in a way relevant to the bending process, i.e. by using measured punch force vs. position data to derive the bending moment and the evolution of the flow stress and the strip curvature during the bending process. The main aim of the present paper is to establish an understanding of how bendability can be significantly improved and made more isotropic by modifying the subsurface microstructure to include a relatively soft polygonal ferrite and granular bainite layer and why the subsurface microstructure plays such a dominant role