Quantification of Large Deformation with Punching in Dual Phase Steel and Change of its Microstructure –Part II: Local Strain Mapping of Dual Phase Steel by a Combination Technique of Electron Backscatter Diffraction and Digital Image Correlation Methods

Abstract

To evaluate heterogeneous strain distribution developed by pre-deformations in dual phase (DP) steel accurately, a combinational technique of Electron Backscatter Diffraction (EBSD) and Digital Image Correlation (DIC) methods was newly introduced in this study. A good correlation is established between kernel average misorientation calculated by EBSD and local equivalent strain measured by DIC in ferrite matrix of DP steels regardless of the difference in deformation process, which means that an EBSD orientation map can be easily converted into an applicative strain map by employing the individual correlation formula. This new technique reveals that very large strain region is locally formed within dozens of micrometer from the punched edge in a punched DP steel. On the other hand, hard martensite grains dispersed in DP steel remarkably promote the heterogeneity of strain distribution in ferrite matrix. As a result, the large strain region is also developed in the form of bands in a cold-rolled DP steel by only 60% thickness reduction at least, as if it is affected by the distribution and morphology of martensite grains. In addition, the local strain mapping demonstrates that the equivalent strain of the large strain band in cold-rolled material is comparable to that of the heavily deformed edge in punched one. The very large strain band in ferrite matrix is characterized by ultrafine grained structure, which leads to the possibility for the losing ductility in ferrite matrix and the martensite cracking.KEY WORDS: local strain mapping; dual phase steel; heterogeneous strain distribution; electron backscatter diffraction; digital image correlation.

Quantification of Large Deformation with Punching in Dual Phase Steel and Change of its Microstructure –Part III: Micro-tensile Behavior of Pre-strained Dual-phase Steel

“…After examining the results of a series of studies, 2,6) we conclude that when ferrite-martensite DP steel is subjected to the punching process, plastic deformation localizes in the soft ferritic phase, which forms an ultrafine-grained ferrite microstructure at a much smaller equivalent plastic strain than in the case of a homogeneous microstructure. In the subsequent hole expansion process, the strain localization in the ultrafine-grained microstructure facilitates void formation.…”

“…The details of the sample preparation are described elsewhere. 6) Both surfaces of these cold-rolled sheets were polished using emery papers and diamond paste, and then they were finished using colloidal silica paste for EBSD analysis. The determination of the crystallographic orientation was performed at a step size of 0.2 μm under an accelerating voltage of 20 kV using field emission scanning electron microscopy (FE-SEM).…”

“…In addition, the microstructures inhomogeneously developed in the DP steel are complicated, even though cold rolling is employed instead of the punching process. 6) These complicated microstructures hinder the understanding of the mechanical characteristics in each phase when conventional mechanical testing is employed. Meanwhile, recent advances in the micro-mechanical testing techniques have helped elucidate the contribution of the micro-constituents to the mechanical properties in several steels and alloys.…”

“…The resulting local strain mapping demonstrated that the equivalent strain of the large strain band in the cold-rolled sheet was comparable to that of the heavily deformed edge in the punched sheet. 6) In the present study, the mechanical characteristics of the cold-rolled sheets of DP steel were examined to simulate the severely deformed region of the punched edge face.…”

“…The issue that remains is to understand the relationship between the microstructures developed in the vicinity of the punched hole and the mechanical responses. Meanwhile, Nakada et al quantitatively evaluated 6) the strain distributions developed in punched and cold-rolled sheets of DP steel using electron backscatter diffraction (EBSD) analysis with a digital image correlation (DIC) method. The resulting local strain mapping demonstrated that the equivalent strain of the large strain band in the cold-rolled sheet was comparable to that of the heavily deformed edge in the punched sheet.…”