High modulus steels are characterized by high specific strength and specific stiffness, which can be attributed to the presence of hard reinforced particles. This paper investigates two common iron matrix composites, namely Fe/TiB₂ and Fe/TiC, prepared through in situ reaction, focusing on their structures and properties. The results show that both types of reinforced particles in the high modulus steels consist of coarse primary particles and fine eutectic particles. In comparison to Fe/TiC composites, Fe/TiB₂ composites exhibit larger elastic modulus (210 GPa). The reasons for the phenomenon that the experimentally measured values of the modulus of elasticity are lower than the calculated values at equilibrium are discussed. It was found that microporous defects left over from the casting process are often present inside the coarse primary particles, which can be the source of microcracks or fractures. In addition, matrix/particle interface stability calculations revealed that TiB₂ possesses a distinctive hexagonal structure, resulting in a smaller interfacial distance (d₀ = 1.375 Å) with the ferrite matrix phase. The high interfacial work of adhesion (W_ad = 3.992 J/m²) further confirms the stronger interfacial stability of the Fe/TiB₂ composite in comparison to the Fe/TiC composite.

The global drive toward decarbonization has spurred industry interest in the compact steel production (CSP) process for manufacturing automotive steel sheets. Understanding the hot deformation behavior, particularly the metadynamic softening mechanism occurring between passes, is essential for evaluating process feasibility under CSP process. This study investigates the metadynamic softening behavior of CP800 steel intended for CSP applications, utilizing isothermal double compression tests performed at the deformation temperatures of 1173, 1273, and 1373 K, strain rates of 0.1,1, and 5.0 s⁻¹, and the interpass times of 1, 10, and 20 s. The softening behavior was assessed through the deformation flow stress–strain curves under varying conditions, and a kinetic equation of metadynamic recrystallization was proposed and validated against experimental data. Additionally, the effect of initial austenite grain sizes of 42 μm and 92 μm on metadynamic recrystallization were analyzed. Results indicate that the final rolling pass temperature should exceed 1173 K to prevent mixed grain structures. Although grain refinement induced by the metadynamic recrystallization in CP800 steel was found to be independent of initial grain size, the final grain size itself remained sensitive to the initial grain dimensions. Adopting lower holding temperature and shorter holding durations prior to rolling is advisable for energy-efficient CSP, provided compositional homogeneity and suitable deformation temperatures are maintained. These insights contribute valuable guidance for optimization CSP process in the production of CP800 steel.

A modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) model, including the heating rates, was proposed in this study to improve the accuracy of isothermal austenite formation kinetics prediction. Since the ultrafast heating process affects the behavior of ferrite recrystallization and austenite formation before the isothermal process, which in turn influences the subsequent isothermal austenite formation kinetics, the effects of varying austenitization temperatures and heating rates on isothermal austenite formation in cold-rolled quenching and partitioning (Q&P) steel, which remain insufficiently understood, were systematically investigated. Under a constant heating rate, the austenite formation rate initially increases and subsequently decreases as the austenitization temperature rises from formation start temperature A_c1 to finish temperature A_c3, and complete austenitization is achieved more quickly at elevated temperatures. At a given austenitization temperature, an increased heating rate was found to accelerate the isothermal transformation kinetics and significantly reduce the duration required to achieve complete austenitization. The experimental results revealed that both the transformation activation energy ($Q$) and material constant ($k_0$) decreased with increasing heating rates, while the Avrami exponent ($n$) showed a progressive increase, leading to the development of the heating-rate-dependent modified JMAK model. The model accurately characterizes the effect of varying heating rates on isothermal austenite formation kinetics, enabling kinetic curves prediction under multiple heating rates and austenitization temperatures and overcoming the limitation of single heating rate prediction in existing models, with significantly broadened applicability.

Air-cooled steel is a high-strength steel widely used in automotive subframe applications. In this study, the microstructural evolution and fatigue performance of air-cooled steel welded joints subjected to a one-step heat treatment and an additional two-step heat treatment were systematically investigated. The results indicate that, after the one-step heat treatment, the microstructure of the welded joints transformed from coarse lath martensite to a mixture of tempered martensite and newly formed martensite. After the two-step heat treatment, the microstructure of the welded joints evolved from coarse lath martensite to newly formed martensite, M-A islands, bainitic ferrite, and a small amount of polygonal ferrite. Under both heat treatment conditions, the fatigue limits of the welded specimens were lower than those of the base metal, which can be attributed to the reduced overall deformation compatibility induced by the welded joints. In addition, the welded joints exhibited superior crack propagation resistance compared with the base metal after both heat treatment processes, which is likely related to the enhanced ability of dislocation structures and grain boundaries to impede crack propagation.