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 stretchflangeability. Large-strained sheared edges are deformed and frequently cracked during stretch-flange 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.

Effects of Mean Normal Stress on Strain-Hardening, Strain-Induced Martensite Transformation, and Void-Formation Behaviors in High-Strength TRIP-Aided Steels

“…The difference in the hardness was equivalent to the previously mentioned strength ratio. Matsuno et al [41] suggested that the size and volume fraction of martensite controlled void formation in 0.036/0.067C-0.5Si-1.5Mn dual-phase ferrite-martensite steels. On the other hand, in 0.22C-0.27Si-1.89Mn-1.54Al TPF steel, the low k-value or high mechanical stability of retained austenite suppressed void formation through the effective relaxation of localized stress concentration at the matrix structure/second phase interface [6,7,32].…”

Local Yield Stress and Its Unusual Independence on Multi-axial Stress States during Lüders Deformation of Medium-Mn, High-strength Steel

“…Therefore, establishing an appropriate constitutive model for effective utilization of such materials is critical. In actual scenarios like press forming or automobile crushing processes, considering more complex yielding behaviors, such as abrupt strain path changes 32) , is necessary. Although this study provides some insights, the identification of mechanical factors that govern these complex phenomena remains a challenge for future research.…”