Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

Abstract

One of the objectives for the development of high-strength dual-phase (DP) steel is improving the stretch-flangeability. Large-strained sheared edges are deformed and frequently cracked during stretchflange formation. Considering shearing as the first deformation, the stretch-flange deformation may be regarded as a secondary deformation. To improve the stretch-flangeability of the DP steels, many researchers have analyzed the microvoid formation. However, in these analyses, the shearing process was not considered. With this background, ex-situ mini-bending tests combined with scanning electron microscopy (SEM) monitoring of microvoid formation were conducted during the secondary deformation. Prior to the secondary deformation, several microvoids were observed on the sheared surface and fine subgrains formed in the ferrite. During secondary deformation, the preliminary microvoids present at the ferrite-martensite interface propagated into the ferrite phase. In contrast, this behavior was not observed for the reamed surface deformation, which was formed without preliminary deformation. Furthermore, microvoids were initiated on ferrite grains that were not present at the ferrite-martensite interface, and martensite islands were not cracked during secondary deformation. This result is noteworthy because martensite cracking was the main factor involved in microvoid initiation, in the absence of shearing. Electron backscattering diffraction analysis revealed that the work hardening of ferrite, prior to the secondary deformation, caused a deviation in the strain concentration sites from those found in the reamed surface deformation. Therefore, this study elucidated microvoid formation on preliminary deformed surfaces via shearing and provided insights for material development considering deformations on the sheared surfaces of materials.

Comparative Investigation of the Stretch-flange Cracking Mechanism of Ultrahigh-strength Steels with Different Microstructures and Hole-Expansion Ratios

“…The hole-expansion limit λ obtained from the hole-expansion tests in accordance with International Organization for Standardization (ISO) standards is generally used as an indicator of the stretch-angeability of materials. According to previous reports, the in uencing factors for the decrease in λ related to machining techniques were the signi cant work-hardening of the sheared edge surface [3][4][5][6][7][8][9] and the high roughness of the edge surface [7,[9][10][11]. The in uencing factors for the decrease in λ related to microstructures were the high heterogeneity due to the hardness difference between soft and hard phases in the microstructure [3,4,[12][13][14][15][16][17][18], the low fracture toughness, which represents the resistance to crack formation and propagation [19][20][21][22], and the low local ductility [9,23].…”