This paper presents an overview of our recent works on the effects of hydrogen on γ–ε martensitic transformations in steels. The study first discusses how hydrogen impacts these transformations. While hydrogen suppresses thermally-induced γ–ε martensitic transformation, it increases the fraction and number density of deformation-induced ε–martensite and decreases its thickness. Secondly, we discuss the effects of γ–ε martensitic transformations on hydrogen kinetics. The study also highlights the significance of low hydrogen diffusivity in the hexagonal-close-packed (HCP) lattice of pure iron, demonstrating the effectiveness of ε–martensite in resisting hydrogen. Moreover, the characteristic behavior of the HCP phase-related diffusionless transformation from a hydride is discussed. We believe that this overview will assist in developing hydrogen-resistant steels and in exploring new microstructural control concepts using hydrogen.

Fe–9 mass%Ni alloy is widely used as a cryogenic steel owing to its excellent low-temperature strength and toughness. However, Ni is an expensive element, with medium-Mn steel considered an inexpensive alternative. Considering the Fe–10%Mn–0.1%C alloy is brittle at low temperatures, the application of intercritical annealing with two-step hot rolling could lead to toughening. Herein, the effect of intercritical annealing on the toughness of a Fe–10%Mn–0.1%C alloy with elongated prior-austenite grains (PAGs) formed via a two-step hot-rolling process was investigated. Intercritical annealing was performed on the specimens with and without two-step hot rolling. For both specimens, intercritical annealing resulted in softening of α’-martensite and an increase in the amount of retained austenite. In the specimen not subjected to the two-step hot rolling process, the fracture morphology transitioned from ductile to intergranular with a decrease in the temperature. Intercritical annealing improved the toughness when ductile fracture occurred. In the case of intergranular fracture, the effect of intercritical annealing on the toughness was negligible. In the two-step hot-rolled specimen with elongated PAGs, the fracture morphology transitioned from ductile to separation fracture with ductile fracture, and intercritical annealing improved the toughness at all temperature ranges. The improvement in toughness during separation fracture is attributed to the expansion of the plastic zone owing to ductile crack progression and the formation of sub-cracks, which promote the strain-induced transformation of retained austenite and ε-martensite.

Two orientations ({110}<001> and {110}<112>) evolve as secondary grains in heavily cold rolled reduction of 91.5% in grain-oriented silicon steel. We investigated the secondary recrystallization mechanism of these two grains by temperature gradient batch annealing method. This method induces the continuous growth of secondary grains along the temperature gradient direction. Consequently, selective growth behavior can be easily evaluated from macrostructure. Furthermore, the orientation relationships between secondary recrystallized grains and primary recrystallized grains at the interface of them can be investigated by interrupting the temperature gradient batch annealing process during the secondary recrystallization.

It was clarified that secondary grains which have higher frequency of CSL (Coincidence Site Lattice) boundaries grow more preferentially and the effective CSL boundaries tolerance angle was 10 degrees from precise CSL orientations. Both {110}<001> and {110}<112> grains statistically had a high frequency of effective CSL boundaries (more than 14.5%) and CSL boundaries corresponding to each orientation of secondary grains disappeared preferentially at growing fronts of each secondary grain.

It can be deduced that CSL boundaries dominate the selective growth behavior of {110}<001> and {110}<112> grains, which have two or more neighboring CSL boundaries to the matrix and thus successively grow as secondary grains. CSL boundaries are supposed to have lower grain boundary energy and higher mobility. Therefore, CSL boundaries suffer lower pinning forces from inhibitors and start to migrate from higher inhibition level (lower temperature). From these results, CSL boundaries play a dominant role in the secondary recrystallization of heavily cold rolled grain-oriented silicon steel.

The microstructures developed by inhomogeneous plastic flow are known to play a crucial role in the cracking in sheet metal bending. Observations on the cross-section perpendicular to the bending axis show that cracks propagate along banded microstructures, known as shear bands, which have already developed almost diagonally to the depth direction. However, the relationship between the shear-band formation and the initial microstructures has not been clarified for dual-phase (DP) steel with hard martensite and soft ferrite. In this study, the evolution of shear bands under three-point bending was investigated for a coarse-grained DP steel. It was shown that shear bands initially develop in ferrite at the boundaries with martensite. They are originally planar and tend to stretch diagonally to the depth direction. When further extension of a shear band is obstructed by hard martensite, it changes direction to circumvent it. Furthermore, these shear bands already develop at the early stages of bending, whereas the formation of surface grooves and voids requires larger bending angles, and their shapes and positions are strongly affected by the shear bands. It is, thus, suggested that the shear-band development is essential to the formation of grooves and voids, which eventually leads to cracking.