Revealing the small post-necking elongation in twinning-induced plasticity steels

The mechanism of small post-necking elongation (PE) in twinning-induced plasticity (TWIP) steels has been investigated from a metallurgical and mechanical engineering perspective to understand and increase the PE. To achieve the above aims, seven TWIP steels with systematic variations in chemical composition, grain size, and pre-strain were prepared. The steels were then analyzed using tensile curves, electron backscatter diffraction techniques, numerical simulation, and a comparison study with other metals. The region of diffuse necking was not observed in C-added TWIP steels due to the negative strain rate sensitivity (m). The negative m value led to strain concentration at the narrow necking region, leading to localized necking and then shear cracks, resulting in the small PE. The dynamic strain aging (DSA) effect was closely related to the negative m value in TWIP steels. Therefore, the PE had a strong linear relationship with the onset strain of serration in the tensile curve. The texture development with an increased dislocation density by pre-strain can suppress the localized necking in TWIP steels, leading to an improvement in the PE. TWIP steel with a larger grain size had higher PE because the DSA was suppressed with increasing grain size.