Mechanical properties and fracture behaviors of medium-Mn steels with and without delta-ferrite for different intercritical annealing times

The mechanical properties and fractured behaviors of 0.2C-6Mn-3Al-(0/0.6)Si medium Mn steels, denoted as 0Si and 0.6Si (0Si without delta-ferrite and 0.6Si with abundant delta-ferrite), are investigated. As the annealing time increases, delta transforms from a hard phase to a soft phase. Due to the hard delta and the weak transformation-induced plasticity (TRIP) effect in 0.6Si, the product of strength and elongation (PSE) is slightly lower in 0.6Si than in 0Si, when the annealing time is less than 7 min. However, the PSE is much higher in 0.6Si than 0Si, resulting from the soft delta and the appropriate TRIP effect in 0.6Si, when the annealing time is higher than 10 min. Microcracks of 0Si initiated at the gamma(alpha')/alpha interface, and unstable gamma could enhance the nucleation and increase the number of microcracks. Microcracks of 0.6Si propagated along the (gamma(alpha') + alpha)/delta interface, resulting in delamination on fractograph. When the hard delta transformed into soft delta, soft delta abates the plasticity mismatch of the interface, thereby leading to a decrease in the number of interfacial microcracks and delaminations.

Automated summary

The study found that when the annealing time is short, the product of strength and elongation (PSE) of 0.6Si is slightly lower than that of 0Si; however, when the annealing time is long, the PSE of 0.6Si is significantly higher than that of 0Si. Additionally, microcracks in 0Si mainly originate at the gamma(alpha')/alpha interface, while microcracks in 0.6Si propagate along the (gamma(alpha') + alpha)/delta interface, resulting in different fracture patterns.