Effect of grain size on tensile and wire drawing behaviors in twinning-induced plasticity steel

The effect of grain size on tensile and wire drawing behaviors in twinning-induced plasticity (TWIP) steel has been investigated to understand the twinning behavior and improve the drawability. TWIP steels with the three different grain sizes were prepared, and then tensile and wire drawing tests were conducted. Twinning stress as well as yield strength (YS), tensile strength, and total elongation well complied with the Hall-Petch relationship. Especially, the dependency of twinning stress on grain size was much stronger in comparison with the slip or YS: the Hall-Petch slope for twinning stress was two times higher than that for slip. A simple model was proposed to predict twinning stress in polycrystalline metals considering grain size effect, and this model well predicted the twinning stress in TWIP steel. The large grained TWIP steel had higher twinning activity at all stain level, but it had lower strength due to the lower strain hardening rate. Accordingly, it can be thought that the coherent twin boundaries (CTBs) would be weaker barriers against dislocation movement than general incoherent high angle grain boundaries stemming from the plastic deformation mechanism of twinning shear, and dislocation generation, transmission and sliding in CTBs. Regardless of grain size, the grain orientation dependency of twinning behavior fully complied with the Schmid law. In respect of strengthening of drawn wire, grain size effect is dominant in lower drawing strain level and deformation twinning effect is dominant in higher strain level in TWIP steels. All the steels had a similar drawability although the large grained steel had higher tensile elongation than the other steels. This is due to the higher twin density and fast exhaustion of twins in the large grained steel during wire drawing process.