Chundong Hu

This study investigates the hot deformation behavior, dynamic recrystallization (DRX), and process optimization of a low-alloy ultrahigh-strength steel through isothermal hot compression tests conducted over a temperature range of 850~1200 °C and strain rates of 0.01~10 s−1.A constitutive equation was established, and processing maps incorporating power dissipation efficiency and plastic instability were constructed at different true strains. The results show that deformation above 1050 °C promotes flow stabilization, while nearly complete DRX is achieved at temperatures exceeding 1150 °C. At 1050 °C, increasing strain rate enhances the recrystallized fraction, which is attributed to the accelerated accumulation of deformation-stored energy and more active grain-boundary bulging associated with discontinuous DRX. TEM observations further reveal characteristic dislocation lath structures and localized dislocation rearrangement near grain boundaries, providing direct microstructural evidence for DRX nucleation and growth. Based on the microstructural characteristics at a true strain of 0.7, four characteristic deformation domains are identified, and a uniform equiaxed grain structure can be obtained under the optimal hot-working parameters of 1150~1200 °C and 0.1~10 s−1.

Austenitic Fe-Mn-Al-C lightweight steels have attracted considerable interest for automotive applications due to their exceptional specific strength, where κ-carbides precipitation critically influences mechanical properties. This review systematically examines the crystal structure, classification, and precipitation kinetics of κ-carbides, emphasizing their spatial distribution-dependent effects: coarse κ-carbides at austenite grain boundaries induce embrittlement and degrade toughness, while nanoscale κ’-carbides within grains enhance strength and ductility through dislocation interactions (e.g., Orowan bypassing and shearing), activating deformation mechanisms such as Dynamic Slip Band Refinement (DSBR), Shear Band-Induced Plasticity (SIP), and Microband-Induced Plasticity (MBIP). Thermodynamic calculations guide alloy design to ensure a single-phase austenite structure at the typical hot-rolling finishing temperature (~900 °C), avoiding harmful phases while promoting beneficial precipitates. Mn suppresses κ-carbide formation, whereas Al and C act as promoters, with intragranular κ’-carbides favoring higher Al/C concentrations (e.g., >6.2% Al and >1.0% C). Heat treatment parameters critically influence κ-carbide distribution, where rapid cooling (e.g., water quenching) suppresses κ-carbides, and subsequent aging (500–700 °C) enables homogeneous precipitation of κ’-carbides. Pre-deformation prior to annealing further accelerates κ-carbide nucleation by introducing crystal defects. Optimal performance requires integrated composition-processing-microstructure optimization to achieve a nnanoscaleκ’-carbide-strengthened austenite matrix through controlled composition and thermo-mechanical processing to achieve an optimal strength-ductility balance.

A novel ultrahigh-strength steel with Co and strengthened through nanoscale precipitation was developed. We found that the Co element had a synergistic effect on the precipitation process. The simulation results indicate that adding Co to steel can suppress the tracer diffusion coefficients of all the elements in the steel, hindering the atomic self-diffusion rate and long-range diffusion effect. A decrease in the atomic diffusion rate of precipitations will affect the nucleation, distribution, and growth of precipitations. The Atom probe tomography (APT) results indicate that the Co element not only dispersed uniformly in the matrix itself but also induced the uniform distribution of the precipitation phases. During the nucleation process of the precipitation, the rejected Co atoms formed small regions of high Co concentrations around the precipitation, inhibiting the coarsening of the precipitation. Under the synergistic effect of Co, the high number density of nanoscale NiAl and M₂C enhanced the strength of the steel.

Precipitation strengthening is one of the fundamental factors occurring at high temperatures in medium-alloy structural steels, which offer greater durability under service conditions. This research employed transmission electron microscopy (TEM) via carbon replicas combined with scanning electron microscopy (SEM) to analyze carbide evolution and its influence on both mechanical properties and high-temperature strength. During the tempering process, ε-carbides precipitate at 200 °C and subsequently transform into M₃C at 400 °C and coarser M₇C₃ at 600 °C. Coarser carbides (M₇C₃ and M₃C) and metastable carbides (ε-carbides) are not sufficient to make steel strong at high temperatures. Moreover, nucleating and growing at interfaces, rod-shaped M₃C diminishes the toughness of the steel. Under tempering at 600 °C, a substantial amount of nanoscale M₂C carbides precipitate. This improvement not only elevate the material’s toughness but also leads to an enhancement of yield strength (from 1237 ± 12 MPa to 1340 ± 8 MPa) along with a rise in high-temperature strength (from 388 ± 8 MPa to 421 ± 4 MPa). Combined with high toughness, nanoscale M₂C with high thermal stability promoted both yield strength at room temperature and high-temperature strength. The type and size of carbides serve as key determinants for yield strength while being decisive parameters for high-temperature strength.