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The size effect of α′ martensite constituents before annealing on the microstructure and tensile properties of intercritically annealed medium Mn steel was systematically investigated. The cold-rolled Fe–9Mn–0.05C (wt%) steel was austenized at three different temperatures, water-quenched to room temperature to vary the sizes of the martensite constituents, and then annealed at 640 °C for 10 min. When the austenizing temperature increased, the sizes of the prior austenite (γ) grains and of the packets and blocks of α′ martensite increased; however, the width of the laths changed only insignificantly. The annealed specimens had a dual-phase microstructure with lath-shaped ferrite (α) and retained γ (γR) phases. The volume fraction of γR decreased with increasing austenizing temperature because the specimen austenized at the higher temperature underwent a slower reverse transformation in the early stage of intercritical annealing. The slowed kinetics of the reverse transformation with increasing austenizing temperature was attributed to the reduction in area of block boundaries which provide nucleation sites for the reverted γ. The widths of the α and γR laths were almost independent of the austenizing temperature. The partitioning of Mn and C atoms from α into γR laths became active with increasing austenizing temperature, resulting in a more stable γR. The specimen austenized at higher temperature exhibited a lower strain hardening rate (SHR) due to the less active transformation-induced plasticity (TRIP) in γR with the higher phase stability. The ultimate tensile strength decreased with increasing austenizing temperature because the SHR was lowered by the less active TRIP with increasing austenizing temperature. The uniform elongation increased with increasing austenizing temperature due to delayed necking caused primarily by the flow stress, which dropped with increasing austenizing temperature.

The effect of grain size on tensile properties, particularly elongation (El), was systematically investigated using cold-rolled and annealed Fe–18Mn–0.6C–1.5Si (wt.%) twinning-induced plasticity (TWIP) steels. Whereas both the yield (YS) and ultimate tensile strengths (UTS) decreased with grain coarsening, the El increased up to a grain size of approximately 18 μm, and then sharply decreased beyond the critical grain size. This unusual reduction in El in coarse-grained specimens was caused by the transition of the fracture mode from ductile to brittle. The transition in fracture stemmed from a change in the deformation mechanism from mechanical twinning to strain-induced e-martensitic transformation. The e plates provided nucleation sites and propagation paths for cracks, causing quasi-cleavage brittle fracture in coarse-grained specimens. The cracks growing along the { 1 0 1 ¯ 0 } or (0 0 0 1) plane were bent by e plates with different plane orientations and were blocked by γ-austenite plates. When the local stress in the γ plate near the crack tip was high, new e-martensitic plates formed within the γ plate, enabling the crack to pass through the γ plate, and changed the crack path. The quasi-cleavage fractured surface consisted of ( 1 0 1 ¯ 1 ), ( 1 0 1 ¯ 2 ), and ( 2 1 ¯ 1 ¯ 1 ) planes. Ridges of bamboo-like structures observed on the fractured surface formed by the variation in crack path occurring at the e plates with different orientations. The Si-added TWIP steel exhibited a superior combination of UTS and El of more than 70,000 MPa %.

The effect of Ti precipitates on hydrogen embrittlement (HE) was investigated using Ti-free and Ti-bearing TWIP steels through slow tensile tests and thermal desorption analysis. Whereas Ti-free steel did not show the reduction in elongation by hydrogen charging, Ti-bearing steel revealed a great reduction in elongation, indicative of the low resistance to HE. During plastic deformation, hydrogen atoms migrated from grain boundaries and dislocations to twin boundaries for Ti-free steel and to the interfaces of Ti particles for Ti-bearing steel. The low resistance to HE of Ti-bearing steel was due to a ductile to brittle transition near hydrogen-trapped TiN particles.

The effect of pre-strain before hydrogen charging on the resistance to hydrogen embrittlement (HE) in the 316L austenitic stainless steel was investigated through the slow strain rate tensile test (SSRT), transmission electron microscopy, and thermal desorption analysis (TDA). The pre-strain suppressed mechanical twinning during the SSRT, regardless of hydrogen charging. However, it accelerated the ¾-martensitic transformation in hydrogen-charged specimens. The TDA revealed that whereas hydrogen atoms migrated from grain boundaries and dislocations mainly to the austenite (£)/¾ interfaces in pre-strained specimens during the SSRTs, they moved to the boundaries of fresh mechanical twins, which newly formed during the SSRTs, in the annealed specimen. The elongation loss by hydrogen charging became greater with increasing the pre-strain, indicating that pre-straining deteriorated the resistance to HE. This elongation loss by pre-strain resulted from both the increase in fraction of ¾-martensite with pre-strain and the segregation of hydrogen atoms to the £/¾ interfaces.