Stretch-flangeability as a parameter of formability is measured on dual-phase (DP) and complex-phase (CP) steels by the hole-expansion ratio (HER), and nanoindentation is introduced to assess the hardness of constituent phases before and after HER testing. Hole-expansion ratios of two dual-phase (DP1, DP2) and one complex-phase (CP1) steels are measured as 51%, 126% and 136%, respectively. The primary site of void formation is found to be the interfacial boundary for DP1, the ferrite phase close to the martensite phase where numerous geometrically necessary dislocations (GNDs) are formed for DP2, and the martensite phase for CP1. The hardness ratio of the hard to soft phase is a key indicator of formability, and this introduces the stress concentration from strain disparity at the interfacial boundaries between the hard and soft phases. Here, it is founded that strain-hardenability of constituent phases depends on the hardness of the GND layer, and the strain disparity under deformation is determined by the GND layer hardness as well as the hardness ratio of the hard to soft phase. This study suggests the GND layer as a stress-dispersion layer and the hardness of GND layers in soft phases as a critical role in formability.

通过扩孔率（HER）在双相（DP）和复杂相（CP）钢上测量拉伸凸缘性作为可成形性的参数，并引入纳米压痕以评估HER前后的组成相的硬度测试。两种双相（DP1，DP2）和一种复相（CP1）钢的扩孔率分别为51％，126％和136％。发现空隙形成的主要部位是DP1的界面边界，接近马氏体相的铁素体相，其中DP2形成了许多几何上必要的位错（GND），而CP1则形成了马氏体相。硬相和软相的硬度比是可成形性的关键指标，这会引入在硬相和软相之间的界面边界处的应变差异引起的应力集中。这里，已经发现，组成相的应变可淬性取决于GND层的硬度，变形时的应变差异由GND层的硬度以及软硬相的硬度比决定。这项研究表明，GND层是应力分散层，而GND层在软相中的硬度是可成型性的关键作用。