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.

可弯曲性是超高强度钢的重要性能，因为此类材料的典型应用包括通过空气弯曲制造的结构。评估弯曲性的常规方法，例如根据标准VDA-238的弯曲试验或常规拉伸试验，由于这些测试中材料响应的平均性质，因此无法提供足够的信息来评估超高强度钢的弯曲性。在这项研究中，使用从薄板表面切下的薄拉伸试样确定机械性能，并使用无摩擦弯曲试验进行弯曲性评估。本文提出的实验和有限元建模结果表明，板材表面的机械性能对弯曲性有重要影响。由较软的铁素体层和较低的贝氏体层组成的新型超高强度钢，在表面具有更好的加工硬化能力，具有良好的可弯曲性，特别是当弯曲线横向于轧制方向排列时。显微组织研究表明，在具有相对较硬的表面微观结构的常规钢中，变形局部化为剪切带，最终导致断裂，但是新型钢表面没有类似的剪切带。这可以归因于更好的加工硬化能力，它可以延迟剪切局部化和断裂的开始。显微组织研究表明，在具有相对较硬的表面微观结构的常规钢中，变形局部化为剪切带，最终导致断裂，但是新型钢表面没有类似的剪切带。这可以归因于更好的加工硬化能力，它可以延迟剪切局部化和断裂的开始。显微组织研究表明，在具有相对较硬的表面微观结构的常规钢中，变形局部化为剪切带，最终导致断裂，但是新型钢表面没有类似的剪切带。这可以归因于更好的加工硬化能力，它可以延迟剪切局部化和断裂的开始。