| 研究実績の概要 |
The objectives of the present research for year 2015-2016 have been achieved. The yield point phenomenon in ultrafine grained (UFG) interstitial free (IF)steel has been systematically studied by mechanical tests, microstructure observation and Digital Image Correlation (DIC) technique.
Accumulative roll-bonding (ARB) process and subsequent annealing were applied to the IF steel to fabricate the specimens having various mean grain sizes from conventional range (larger than tens micron-meters) to ultrafine range. It was found by the tensile test that discontinuous yielding appeared in the IF steel when the mean grain size was decreased to smaller than 2 micron meters. The yield stress showed an “extra-hardening”. Namely, it became higher than that predicted by the Hall-Petch relationship extrapolated from the conventionally coarse grain size range. Meanwhile, the uniform elongation of the material greatly decreased as the mean grain size became smaller than 2 micron. By the digital image correlation (DIC) technique, it was found that the dramatic decreasing in the uniform elongation is attributed to the initial severe strain localization occurred with the discontinuous yielding.
In conclusion, the present work investigated the yield point phenomena in ultrafine grained IF steel in details and deepened the understanding of the grain size effect on the strength and ductility of the materials. Further research works are expected on how to diminish the yield point phenomena and enhance the tensile ductility of the materials having ultra-fine grain sizes.
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