Based on the Maxwell’s equations, a finite element model is established to study the characteristics of electromagnetic field in the mold of billet and bloom continuous casting with electromagnetic stirring (M-EMS). A novel integral method for calculating electromagnetic torque is proposed to evaluate the stirring intensity of stirrer. In order to verify the accuracy of the model, a well-designed electromagnetic torque detecting device is fabricated. The predicted value of electromagnetic torque and magnetic flux density are consistent with the measured data. The optimum frequency is determined by the maximal electromagnetic torque of the strand. The effect of stirring current intensity and different stirrer positions along the length of mold on the electromagnetic field has been studied numerically. The results show that the optimum frequency is smaller when the copper tube of the mold is thicker and the section size is bigger. Besides, the electromagnetic torque of the strand is a quadratic function of the running current intensity. Moreover, the installation position of stirrer strongly affects the prediction of electromagnetic field distribution, further influencing the optimum frequency and the electromagnetic torque of strand.

C110 oil well casing tubes should have high strength and corrosion resistance which is commonly used for deep wells operation containing corrosive media. In this paper, the microstructure evolution of a kind of semi-macro segregation originated banded structure in casing tubes is studied under different heat treatments. It is shown that the characteristics of the banded structure will change significantly in subsequent hot working and heat treatment processes. For the hot-rolled ones, the banded structure is composed of pearlite plus bainite. After quenching, it evolves into martensite band with high concentration solute elements. Finally, the banded structure will change into a carbide banding under the following tempering process. The temperature and cooling rate of the tempering practice show an obvious effect on the final band structure. To improve anti-SSC (sulfide stress corrosion cracking) performance, the favorable QT (quenching and tempering) practice for C110 steel should be a higher tempering temperature and a quicker cooling rate, from which the banded structure defects can be decreased together with an obvious improvement of the tube wall hardness uniformity.

AlN precipitates are frequently adopted to pin the austenite grain boundaries for the high-temperature carburization of special gear steels. For these steels, the grain coarsening criterion in the carburizing process is required when encountering the composition optimization for the crack-sensitive steels. In this work, the quantitative influence of the Al and N content on the grain size after carburization is studied through pseudocarburizing experiments based on 20Cr steel. According to the grain structure feature and the kinetic theory, the abnormal grain growth is demonstrated as the mode of austenite grain coarsening in carburization. The AlN precipitate, which provides the dominant pinning force, is ripened in this process and the particle size can be estimated by the Lifshitz−Slyosov−Wagner theory. Both the mass fraction and the pinning strength of AlN precipitate show significant influence on the grain growth behavior with the critical values indicating the grain coarsening. These criteria correspond to the conditions of abnormal grain growth when bearing the Zener pinning, which has been analyzed by the multiple phase-field simulation. Accordingly, the models to predict the austenite grain coarsening in carburization were constructed. The prediction is validated by the additional experiments, resulting in accuracies of 92% and 75% for the two models, respectively. Finally, one of the models is applied to optimize the Al and N contents of commercial steel.

In modern industries, gears function as pivotal transmission elements whose operational performance is directly dependent on the microstructural characteristics of gear steels. While high-temperature carburizing (950–1050 °C) substantially improves process efficiency through accelerated carbon diffusion, it inevitably promotes austenite grain coarsening. This study investigates the effect of Nb microalloying on grain stability in SAE8620H gear steel during high-temperature carburizing. Experimental steels with varying Nb contents were prepared via vacuum induction suspension melting, followed by hot rolling, solution treatment, and pseudo-carburizing. Thermodynamic calculations, optical microscopy, transmission electron microscopy, and energy-dispersive spectroscopy were employed to analyze the mechanisms. Thermodynamic results revealed that higher Nb content retains more Nb(C, N) phases at elevated temperatures, effectively suppressing grain coarsening. Without preheating, increased Nb content refined grains but exhibited limited inhibition at high temperatures. Preheating (1330 °C × 10 min + water quenching) promoted uniform and fine Nb(C, N) precipitates, significantly enhancing grain refinement. When Nb content exceeded 0.053 wt.%, grain coarsening was fully inhibited under 1050 °C × 2 h carburizing. This study establishes the optimal Nb content range, elucidates the micro-mechanisms, and proposes a preheating process to improve high-temperature carburizing performance in gear steels.

To elucidate austenite grain coarsening and mixed-grain formation in continuously cast Nb–Ti microalloyed steel during high-temperature holding, high-temperature holding experiments, Clyne–Kurz microsegregation calculations, Thermo-Calc equilibrium precipitation analysis, Gladman pinning calculations, and multi-order-parameter phase-field simulations were combined to investigate the effect of precipitation differences between dendritic and interdendritic regions on heterogeneous austenite grain growth. The experimental results show that mixed-grain structures of varying severity are present after holding for 1 h at 1050–1300 °C. The austenite grains coarsen overall with increasing temperature, while pronounced coarsening of the initially fine-grained regions occurs above approximately 1200 °C. Microsegregation calculations suggest that Nb, Ti, and C are enriched in the interdendritic region, which may promote higher Nb(C,N) and TiN precipitation compared with the dendritic region. As temperature increases, grain-boundary mobility increases continuously. At 1150 °C, a considerable amount of precipitates remains in the interdendritic region, whereas pinning in the dendritic region is weaker, causing the difference in effective grain-boundary mobility between the two regions to increase. At higher temperatures, extensive dissolution of interdendritic Nb(C,N) reduces the regional difference in pinning and, consequently, the difference in effective grain-boundary mobility. The phase-field simulations capture the general characteristics of heterogeneous austenite grain-growth behavior and provide mechanistic insight into the influence of spatially varying effective grain-boundary mobility: at 1050 °C, low grain-boundary mobility and strong pinning result in slow grain coarsening; near 1150 °C, preferential growth in the dendritic region while the interdendritic region remains pinned produces the highest mixed-grain severity; at 1300 °C, extensive precipitate dissolution causes pronounced overall grain coarsening while reducing the regional grain-size contrast. The experimental observations were further interpreted using microsegregation and precipitation calculations to reveal the possible relationship between solidification-induced compositional heterogeneity and heterogeneous grain growth. These results indicate that local variations in effective grain-boundary mobility associated with solidification microsegregation are an important factor influencing the formation of mixed-grain structures.