Tetsu-to-Hagane
Online ISSN : 1883-2954
Print ISSN : 0021-1575
ISSN-L : 0021-1575
Regular Article
Quantification of Large Deformation with Punching in Dual Phase Steel and Change of its’ Microstructure – Part I: Proposal of the Quantification Technique of the Punching Damage of the Dual Phase Steel
Tatsuo YokoiHiroshi ShutoKen-ichi IkedaNobuo NakadaToshihiro TsuchiyamaTakahito OhmuraYoji MineKazuki Takashima
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2016 Volume 102 Issue 5 Pages 244-252

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Abstract

Dual Phase (DP) steel is used in automotive body parts for weight saving and crashworthiness, however there is an issue of DP steel in low stretch flange ability evaluated by hole expanding tests. In order to improve stretch flange ability of DP steel, it is important to estimate the damage of punching quantitatively and to clarify the change of microstructure before and after punching because the hole expansion ratio is decided in the ductility remained after pre-strain equivalent to punching. Therefore we tried to measure the damage of punching by unique techniques of Electron Backscatter Diffraction (EBSD), nano-indentation and micro-tensile testing and to observe fracture surface by Scanning Transmission Electron Microscope (STEM). Average EBSD-Kernel Average Misorientation (KAM) value and pre-strain damage have strong correlation, thus average KAM value can become the index of the damage. The nanohardness and tensile strength using micrometer-sized specimens increased with increasing average KAM value in the ferritic phase as approaching the punching edge. A shear type fracture occurred without necking in the specimen cut out in the area of the edge. The ultrafine-grained ferritic microstructure was observed in the sample cut out in the same area with STEM. It seems that the ductility loss of the punched DP steel was probably attributed to localized strain into the ultrafine-grained ferritic microstructure.

Relationship between Ave.KAM value and Nano hardness, Micro-tensile strength. Fullsize Image
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© 2016 The Iron and Steel Institute of Japan
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