超微細粒鉄鋼材料の降伏現象

本研究2015-2016年的目标已经实现。 采用累积叠轧焊（ARB）工艺制备了从常规晶粒尺寸（大于几十μ m）到超细晶粒尺寸的超细晶（UFG）无间隙原子（IF）钢试样，通过力学性能测试、显微组织观察和数字图像相关（DIC）技术，系统地研究了超细晶（UFG）IF钢的屈服点现象。 拉伸试验发现，当平均晶粒尺寸小于2 μ m时，IF钢出现不连续屈服。屈服应力表现为“超硬化”。也就是说，它变得高于从传统的粗晶粒尺寸范围外推的Hall-Petch关系所预测的。同时，当平均晶粒尺寸小于2 μ m时，材料的均匀延伸率大大降低。通过数字图像相关分析（DIC）发现，均匀延伸率的急剧下降是由于非连续屈服时发生的严重应变局部化引起的，从而深入研究了超细晶IF钢的屈服点现象，加深了对晶粒尺寸对材料强度和塑性影响的认识。如何减少超细晶材料的屈服点现象，提高其拉伸塑性，有待于进一步的研究。

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.