Yield behaviour associated with stacking faults in a high-temperature annealed ultra-low carbon high manganese steel

Strength and strain-hardening enhancement by generating hard delta-ferrite in twinning-induced plasticity steel

“…The delta-ferrite in L-C steel mainly exhibits small regular block morphology and uniformly distributes on the austenite matrix (Figure 1(c,d)). The experimental results show that the morphology and volume fraction of the delta-ferrite are both nearly unchanged during the present heat-treating process, which is in agreement with our previous works [8,25]. Microhardness measurement indicates that the hardness of delta-ferrite phase (312.8 HV) is about two times of austenite matrix (158.6 HV) of L-C steel.…”

“…Therefore, these ferrites are not transformed from austenite at low temperatures after hot rolling or during the cooling of heat treatment but should be delta-ferrite formed at high temperatures before hot rolling. The in-situ observations in our previous studies [25,26] have also indicated that the delta-ferrite forms during the solidification or solution treatment before hot rolling process. The austenite in both steels is severely deformed, and many deformation twins (a small number of fine plates may be -martensite) are obviously observed after tensile testing, as shown in Figure 1(b,d), indicating TWIP effect occurs during deformation.…”

“…The tensile curve of the high-temperature-annealed high-manganese dual-phase TWIP steels sometimes exhibits an upward-curved phenomenon, which is believed to be a typical characteristic that directly resulted from the deformation twinning [6,8,25]. The tensile curve for L-C steel also demonstrates an upward-curved phenomenon and an obvious hardening effect occurs in middle and later deformation stage, which is quite different from the deformation behaviour of H-C steel (Figure 3).…”

“…The tensile curve for L-C steel also demonstrates an upward-curved phenomenon and an obvious hardening effect occurs in middle and later deformation stage, which is quite different from the deformation behaviour of H-C steel (Figure 3). From the literature [8,21,22,25] in dual-phase TWIP steel, it was found that delta-ferrite plays an important role in the deformation behaviour. Therefore, it is suggested that the difference of strain-hardening behaviour between L-C and H-C steel should primarily attribute to the effect of delta-ferrite.…”

“…A special Lüders-like deformation phenomenon has been found, which is contributed to the martensite transformation induced due to the localized strain concentration [8]. The strain partitioning between delta-ferrite and austenite plays an important role in the martensite transformation and formation of stacking faults, and in turn influences the yield behaviour and strain hardening behaviour of TWIP steel [25,26]. From the literatures mentioned above, it was found that TWIP steel with dual phases shows obviously different characteristics and deformation mechanisms compared with conventional single austenite phase TWIP steels.…”