Microstructure analysis and crack nucleation in TWIP steel tube at expansive deformation

The expansion test of twinning induced plastic (TWIP) steel tube was carried out at room temperature. Cracks appeared inside the tube edge when TWIP steel tube expands by 15.6%. The microstructure evolution, deformation mechanism and crack generation of TWIP steel after expansion were investigated by XDR OM TEM and EBSD. The results show that TWIP steel tube still austenite phase after expansion at room temperature; a great deal of dislocations are gathered around the grain boundaries and twin boundaries, the integral number of twins is high, and the mechanism of expansion deformation is the joint action of dislocations slip and deformation twins; Twins produce a large number of finer secondary or even multiple twins, intersecting each other; After expansion, the silk texture with direction of <111>∥X0 dominated by rotating brass texture {110}<111> is gradually produced through grain deformation and rotation; The proportion of small angle grain boundaries increased greatly; It is deduced that the criterion of crack nucleation is based on the difference between the dislocation pile-up energy (DPE) and the crack nucleation energy (CNE), and the expansion deformation process of TWIP steel satisfies the condition of crack nucleation.

Ultrasonic Assessment of the Influence of Cold Rolling and Recrystallization Annealing on the Elastic Constants in a TWIP Steel

The evolution of the elastic constants, , and Poisson’s ratio and acoustic birefringence of a Fe-0.5 wt% C-21.5 wt% Mn twinning-induced plasticity (TWIP) steel with reduction by cold rolling and recrystallization annealing was assessed from measurements of the times of flight of ultrasonic waves propagating along the thickness of the rolled plates. As the reduction increased, changes in the elastic constants resulted in a steadily increasing orthotropy, which was clearly shown by Poisson’s ratio and acoustic birefringence. Although optical metallography and hardness measurements showed that partial or full recrystallization is attained after annealing at 600 °C and 700 °C, the ultrasonic measurements revealed that a high level of orthotropy remains.

Unique transition of yielding mechanism and unexpected activation of deformation twinning in ultrafine grained Fe-31Mn-3Al-3Si alloy

Tensile mechanical properties of fully recrystallized TWIP steel specimens having various grain sizes (d) ranging from 0.79 μm to 85.6 μm were investigated. It was confirmed that the UFG specimens having the mean grain sizes of 1.5 μm or smaller abnormally showed discontinuous yielding characterized by a clear yield-drop while the specimens having grain sizes larger than 2.4 μm showed normal continuous yielding. In-situ synchrotron radiation XRD showed dislocation density around yield-drop in the UFG specimen quickly increased. ECCI observations revealed the nucleation of deformation twins and stacking faults from grain boundaries in the UFG specimen around yielding. Although it had been conventionally reported that the grain refinement suppresses deformation twinning in FCC metals and alloys, the number density of deformation twins in the 0.79 μm grain-sized specimen was much higher than that in the specimens with grain sizes of 4.5 μm and 15.4 μm. The unusual change of yielding behavior from continuous to discontinuous manner by grain refinement could be understood on the basis of limited number of free dislocations in each ultrafine grain. The results indicated that the scarcity of free dislocations in the recrystallized UFG specimens changed the deformation and twinning mechanisms in the TWIP steel.

Microstructure Evolution and Recrystallization Temperature Change of Cold-Rolled Fe–19Mn–0.6C Twinning-Induced Plasticity Steel during Annealing

Ultra-high twinning-induced plasticity (TWIP) steel is receiving increasing attention in the automobile industry. Self-designed Fe–19Mn–0.6C TWIP steel was subjected to reveal the relationship between microstructures, which were related to recrystallization starting/ending temperature and cold rolling. The results indicated that initial deformation twins, secondary deformation twins, and nano-twins were successively generated in rolled TWIP steel with the increase of cold rolling, deformation twins, and dislocations, as well as with the elongation of grains. The elements remained uniformly dispersed rather than agglomerated in the twin crystals and grain boundaries. The recrystallization starting temperature changes of TWIP steel were 500–525, 400–425, 400–415, and 400–410 °C at cold rolling deformations of 25%, 50%, 75%, and 88%, respectively. Furthermore, the obtained corresponding recrystallization ending temperature changes were 580–600, 530–550, 520–540, and 500–520 °C, respectively. The linear relationship between cold deformation and hardness suggests that cold rolling can increase dislocation density and thus facilitate improving the hardness of TWIP steel.