Effect of banded morphology and grain size on the tensile behavior of acicular ferrite in HSLA steel

This paper presents a study on microscopic mechanism for the anisotropy of acicular ferrite (AF) on banded structures in HSLA steel using in situ tensile testing and digital image correlation (DIC). The different grain size of banded microstructure AF-containing formed in the processes of continuous cooling and isothermed at 450 °C respectively. Firstly, the reliability of the test system consisting of a scanning electron microscopy (SEM), a micro tensile stage and a digital image correlation (DIC) software was verified by a zero-deformation experiment. Obtained results show the mean values and standard deviations of virtual displacement are very small, and the accuracy and robustness of the test system can satisfy the requirement of tensile tests. Then, tensile tests for the steel specimen were carried out in transverse direction (TD) and rolling direction (RD). DIC was used to analyze the micro-scale von Mises strain field of the specimens. The tensile properties of the banded microstructure consisting of fine AF (FAF) and coarse AF (CAF) are obviously superior to that in AF/F banded microstructure. For the AF/F banded microstructure, the yield strength σs and ultimate tensile strength σb in RD is 15% and 8% higher than TD respectively. Moreover, σs and σb in RD is 6% and 3% higher compared to TD in FAF/CAF, while the elongation to failure is 13% lower correspondingly. The results indicate that although the AF-dominated microstructures have good mechanical properties, the effect of banded morphology on anisotropic tensile properties was still non-negligible.