This study focuses on understanding the complex interactions between the initial cold-rolled microstructure and the heating rate, and how these factors influence the post-annealed microstructure, particularly the fraction of polygonal ferrite, in the Nb-Ti microalloyed steel. Key findings include that increasing the heating rate from 3 degrees C/s to 30 degrees C/s enhances the kinetics of reverse transformation to austenite, resulting in a larger fraction of new ferrite and thus a higher proportion of polygonal ferrite in the post-annealed microstructure in most cases. Interestingly, in scenarios where recrystallization can occur by altering the initial microstructure, the combined evolution of recrystallized and new ferrite enlarges the fraction of polygonal ferrite at slower heating rates of 3 degrees C/s. It indicates that the contribution of recrystallization and reverse transformation on the evolution of polygonal ferrite depends on the interplay between initial microstructure and the heating rate. Further, the calculations of the austenite/ferrite interface velocity suggest that the accelerated reverse transformation to austenite, facilitated by increased heating rates, is little affected by kinetic transitions.

This study proposes a novel method for accelerating bainitic transformation by utilizing Mn heterogeneity. The process involves intercritical annealing to create a heterogeneous distribution of Mn prior to full austenitizing. Heterogeneous Mn distribution leads to the rapid nucleation of bainitic ferrite in the Mn-depleted austenite region, and the rapidly formed bainitic ferrite acts as a nucleation site for Mn-enriched austenite region. Consequently, the required austempering time to achieve a similar fraction of bainitic ferrite is nearly halved compared to conventional process. The effectiveness of Mn heterogeneity in accelerating the transformation is comparable to other acceleration methods, such as prior austenite grain refinement and introducing prior martensite. Moreover, the proposed method has an additional advantage in increasing the fraction of retained austenite. The retained austenite is stabilized not only by C partitioning during austempering but also by pre-partitioned Mn during intercritical annealing.

Microstructure heterogeneity has been regarded as being detrimental in obtaining reliable mechanical performance of steels. However, in the present study, we demonstrated that a proactive control of microstructure heterogeneity could deliver unprecedented tensile properties that was hardly achieved by using chemically homogeneous initial microstructure. The heterogeneity of Mn distribution generated by utilizing its solubility difference between ferrite, austenite and cementite at intercritical annealing, promoted the retention of austenite in the final microstructure subjected to the room quenching and partitioning process. The enhancement of fraction as well as the stability of austenite contributed to the simultaneous improvement of tensile strength and ductility which have been regarded as mutually exclusive properties. Furthermore, even in steel with lean Mn composition, the room temperature quenching and partitioning process combined with the chemically heterogeneous initial microstructure presented tensile properties comparable to those expected in steels with much higher Mn content, which exhibited the potential of heterogeneity-driven microstructure control for the development of advanced steel products.

We made the chemical heterogeneity in steel consisting of mostly martensite with retained austenite by applying intercritical annealing prior to full austenitization. It enables simultaneous improvement in strength and ductility, which have been regarded as mutually exclusive properties. Contribution from two types of chemical heterogeneity could be identified; micro-scale and nano-scale heterogeneity inheriting the chemistry of intercritical austenite and Mn-enriched cementite respectively. The micro-scale chemical heterogeneity creates soft and hard domains in the martensitic microstructure by dissimilar solid solution strengthening, generating strain partition and back stress which contribute to the improvement of strength. On the other hand, the nano-scale chemical heterogeneity from highly Mn-enriched cementite evolved into Mn-enriched austenite in the final microstructure. The exceptional stability of nano-scale Mn-enriched austenite renders persistent strain hardening, contributing the enhanced ductility.

We present the results of microstructural characterization of the polycrystalline yttrium iron garnet (YIG) during high temperature sintering. Three YIG samples, compacted from the calcined sol-gel powder, are annealed at 1400 degrees C for 350 min, 1050 min and 1750 min, respectively. During the annealing, the polycrystalline YIG coarsens via normal grain growth, developing self-similar steady-states in grain size distribution, grain texture, and grain boundary texture. However, the 1050-min and 1750-min samples feature x-ray peaks with narrower full width at half-maximum (FWHM), and smaller local misorientation than the 350-min one, signifying that the strain still decreases after annealing for 350 min at 1400 degrees C. EPMA analyses show that more uniform distribution of the elements is found in 1750-min sample than in 350-min sample. Notably, little segregation of metal ele-ments is observed on the grain boundary of the current YIG samples, as opposed to previously reported conventionally sintered YIGs using yttrium-oxide and iron-oxide powders, often featuring either second phases or segregation of Fe on YIG grain boundaries.

Austenite plays a key role to improve tensile properties of advanced high strength steel (AHSS). In this study, nano-sized austenite particle was produced in as-quenched martensite by using chemically heterogeneous initial microstructure consisting of Mn-enriched cementite and ferritic matrix. Mn-enriched cementite transforms into nano-sized austenite during austenitzation and considerable amount of them retain after cooling to ambient temperature. The austenite particles have an exceptional stability, which cannot be fully interpreted with the Mn enrichment. Possible contributions from the compressive stress and the C partitioning into the austenite are considered. A persistent TRIP effect by highly stable austenite contributes to remarkable improvement of tensile ductility without compromising tensile strength even in the as-quenched martensite.

We examine the microstructure and tensile properties of the quenching and partitioning (Q&P) processed medium Mn steels having different starting microstructure. Heterogeneous Mn distribution led by intercritical annealing prior to the Q&P process is more effective in retaining higher fraction of austenite; a major fraction of austenite exists as Mn-rich islands surrounded by martensite. Starting with uneven Mn distribution prior to the Q&P process enhanced the austenite stability by high Mn concentration and shielding effect by neighboring martensite. It contributed to the impressive tensile properties by operating multiple plasticity mechanisms: transformation-induced plasticity (TRIP) with twinning-induced plasticity (TWIP).