Micromechanical origins of remarkable elongation-to-fracture in AHSS TRIP steels via continuous bending under tension
Micromechanical origins of remarkable elongation-to-fracture in AHSS TRIP steels via continuous bending under tension
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DOI：
10.1016/j.msea.2021.141876
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发表时间：
2021-09
期刊：
影响因子：
6.4
通讯作者：
Rishabh Sharma;C. Poulin;M. Knezevic;M. Miles;D. Fullwood
中科院分区：
文献类型：
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作者：
Rishabh Sharma;C. Poulin;M. Knezevic;M. Miles;D. Fullwood
已知连续拉伸弯曲（CBT）实现的从延伸到破坏的效果远高于传统的单轴简单拉伸（ST）应变路径。然而，提供这种增加延展性的详细机制尚未完全了解。很明显，在CBT过程中，通过不断移动塑性变形区域，可以避免典型ST试件中的颈缩现象。流动应力最大的材料体积被限制在一条移动的线上，在这条线上，辊子与板材接触，并在施加的拉伸载荷上叠加弯曲应力。因此，避免了大体积材料承受应力大于材料流动应力而导致ST过程中应变局部化的情况。然而，这种现象对伸长率整体增加的贡献程度尚不清楚。在当前的实验中，通过微调Q&P 1180和TBF 1180的CBT成形参数，分别获得了比ST高4.56倍和3.7倍的断裂伸长率（ETF）。通过数字图像相关对ST和CBT钢板在最终断口附近的最大局部应变进行比较，结果显示，对于两种含有不同数量残余奥氏体（RA）的钢，CBT期间避免塑性应变局部化占CBT试样伸长率增加的不到一半。利用高分辨率EBSD （HREBSD）对两种应变路径进行几何上必要的位错演变监测，表明板材中大部分CBT样品的硬化率较低，这可能与板材外层应力的周期性有关。有趣的是，板材中心的GND演化，没有经历相同的循环应力振幅，比板材边缘更接近于ST行为。这似乎有助于在部分CBT处理后在ST中拉动的试件的残余延展性急剧下降。RA的转化速率也在钢中被跟踪，与st相比，CBT期间的转化速率明显较低。这表明，在CBT下实现的较慢的转化速率也导致了更高的应变-失效水平。
Continuous bending under tension (CBT) is known to achieve elongation-to-failure well above that achieved under a conventional uniaxial simple tension (ST) strain path. However, the detailed mechanism for supplying this increased ductility has not been fully understood. It is clear that the necking that occurs in a typical ST specimen is avoided by constantly moving the region of plastic deformation during the CBT process. The volume of material in which the flow stress is greatest is limited to a moving line where the rollers contact the sheet and superimpose bending stress on the applied tensile load. Hence the condition of a large volume of material experiencing stress greater than the material flow stress, leading to strain localization during ST, is avoided. However, the magnitude of the contribution of this phenomenon to the overall increase in elongation is unclear. In the current set of experiments, an elongation to fracture (ETF) of 4.56x and 3.7x higher than ST was achieved by fine-tuning CBT forming parameters for Q&P 1180 and TBF 1180, respectively. A comparison of maximum local strains near the final point of fracture in ST and CBT sheets via digital image correlation revealed that avoidance of localization of plastic strain during CBT accounts for less than half of the increased elongation in the CBT specimens for two steels containing different amounts of retained austenite (RA). Geometrically necessary dislocation evolution is monitored using high-resolution EBSD (HREBSD) for both strain paths, indicating a lower hardening rate in the CBT samples in the bulk of the sheet, potentially relating to the cyclical nature of the stress in the outer layers of the sheet. Interestingly, the GND evolution in the center of the sheet, which does not experience the same amplitude of cyclic stress, follows the ST behavior more closely than the sheet edges. This appears to contribute to a precipitous drop in residual ductility for the specimens that are pulled in ST after partial CBT processing. The rate of transformation of RA is also tracked in the steels, with a significantly lower rate of transformation during CBT, compared to ST. This suggests that a slower transformation rate achieved under CBT also contributed to higher strain-to-failure levels.