The surface of hot stamping steel is severely oxidized during heating, holding, and transfer from the heating furnace to the stamping die in the production of traditional automotive parts. Coating-free hot stamping steel with Cr and Si elements exhibits excellent oxidation resistance during hot stamping without the protection of a surface coating. This paper investigates the oxidation behavior of three types of hot stamping steel at 800–1200 °C. The results show that although Cr-Si hot stamping steel performs excellently short-term (≤7.5 min) for oxidation resistance, its long-term (≥15 min) or high-temperature (≥1100 °C) oxidation resistance is much lower than that of the conventional hot stamping steel 22MnB5, affecting the production and surface quality control of the new coating-free Cr-Si hot stamping steel. By analyzing the oxidation kinetics and characterizing the structure of oxide layers in hot stamping steel, it was found that the structural change in the Cr and Si element enrichment layer between the oxide scale and the substrate varied in oxidation performance at different temperatures. When the oxidation temperature was below 1000 °C, the solid Cr and Si enrichment layer acted as a barrier to prevent the diffusion of Fe ions. When the oxidation temperature exceeded 1100 °C, the molten Cr and Si enrichment layer effectively adapted to the substrate and avoided blistering. Meanwhile, Fe₂SiO₄ penetrated the Fe oxide layer along the grain boundary and became a rapidly diffusing channel of Fe ions, contributing to a significant increase in the oxidation rate.

In this paper, we present the effect of austenitizing temperature on the work hardening behavior of air-hardening steel LH800 by evaluating the influence of austenitizing temperature on microstructure evolution and mechanical properties, using Hollomon, Differential Crussard–Jaoul (D_C-J), and Modified C-J (M_C-J) work hardening models. The results reveal that with an increase in austenitizing temperature, there is an increase in the percentage of martensite, along with an increase in the strength and hardness of the LH800 steel; on the other hand, there is a decrease in the plasticity. Austenitized at 825 °C, LH800 steel exhibits its highest strength and good plasticity, with a tensile strength of 897 MPa and an elongation of 13.6%. The comparison between the three strain hardening models revealed that the Hollomon model was the finest fit for the experimental data utilized and could illustrate the work hardening behavior of LH800 steel most suitably. This model manifests a two-stage work hardening mechanism; the first stage is related to the plastic deformation of ferrite phase, while the second stage deals with the co-deformation of ferrite and martensite/bainite phase. As austenitizing temperature increases, the work hardening ability of LH800 steel diminishes at each stage, the transition strain decreases, and the plastic deformation of martensite starts earlier.

The aim of the present work is to evaluate the effect of ultrafast heating on the microstructure and mechanical properties of hot forming steel. The initial microstructure utilized in this study was a cold-rolled microstructure, and the test steel was heated to full austenitization at a rate of 200 °C/s, followed by water quenching. It was observed that the ultrafast heating process significantly refines both the prior austenite grains and martensite laths while inheriting high-density dislocations from the initial cold-rolled microstructure. Consequently, the coupling mechanism between dislocation strengthening and grain refinement strengthening remarkably enhanced both the yield strength and ultimate tensile strength of the test steel. Eventually, the yield strength of the hot forming steel reached 1524 MPa, along with an ultimate tensile strength of 2221 MPa and uniform elongation of 5.2%.

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.

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.