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。采用力学试验、显微组织观察和数字图像相关技术对超细晶（UFG）无间隙（IF）钢的屈服点现象进行了系统研究。采用累积滚焊（ARB）工艺和后续退火工艺对中频钢进行加工，制备出了从常规范围（大于几十微米）到超细范围内具有不同平均晶粒尺寸的试样。拉伸试验发现，当平均晶粒尺寸减小到2微米以下时，IF钢出现不连续屈服。屈服应力表现为“超硬化”。也就是说，它比从常规粗粒度范围外推的Hall-Petch关系所预测的要高。同时，当平均晶粒尺寸小于2微米时，材料的均匀伸长率大大降低。通过数字图像相关（DIC）技术发现，均匀伸长率的急剧下降是由于屈服不连续时发生的初始严重应变局部化所致。总之，本工作详细研究了超细晶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。如何减少屈服点现象，提高超细晶粒材料的拉伸延展性，有待进一步的研究。

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.