Xiaotan Zuo

In the continuous casting process, the flow velocity of molten steel at the surface of the mold directly influences the melting of the flux, slag entrainment behavior, and the uniformity of heat transfer. Fast and accurate prediction of the magnitude and distribution of surface-flow velocity is crucial for defect suppression and ensuring continuous casting quality. In this study, a three-dimensional electromagnetic–fluid dynamic coupled model was established and validated to simulate the molten steel flow behavior under various casting speeds, Electromagnetic Stirring (EMS) currents, EMS frequencies, and Submerged Entry Nozzle (SEN) immersion depth. Subsequently, the maximum surface velocity in the mold was fitted to derive a predictive formula under different conditions; the median range and width of the surface high-velocity range were predicted in a similar way. As a result, a regression model derived from Computational Fluid Dynamics (CFD) was built to predict the maximum mold surface velocity and the high-velocity range within milliseconds. The reliability of the regression model was verified by newly generated simulation results, with the relative error between the predicted and simulated maximum surface velocities kept within 2%. It was shown that the maximum surface velocity increases with casting speed and EMS current but decreases with EMS frequency. The median range of the high-velocity range decreases with increasing casting speed and current. The immersion depth of SEN had a minimal impact on the maximum surface velocity in the mold and the high-velocity range. This regression model is beneficial for rapid mold design and for predicting potential slag entrainment, breakout risks, and billet defects, for the purpose of intelligent continuous casting.

To address the central cracking problem in continuous casting slabs of 38CrMoAl steel, high-temperature tensile tests were performed using a Gleeble-3800 thermal simulator to characterize the hot ductility of the steel within the temperature range of 600–1200 °C. The phase transformation behavior was computationally analyzed via the Thermo-Calc software, while the microstructure, fracture morphology, and precipitate characteristics were systematically investigated using a metallographic microscope (MM), a field-emission scanning electron microscope (FE-SEM), and transmission electron microscopy (TEM). Additionally, the effects of different holding times and cooling rates on the microstructure and precipitates of 38CrMoAl steel were also studied. The results show that the third brittle temperature region of 38CrMoAl steel is 645–1009 °C, and the fracture mechanisms can be classified into three types: (I) in the α single-phase region, the thickness of intergranular proeutectoid ferrite increases with rising temperature, leading to reduced hot ductility; (II) in the γ single-phase region, the average size of precipitates increases while the number density decreases with increasing temperature, thereby improving hot ductility; and (III) in the α + γ two-phase region, the precipitation of proeutectoid ferrite promotes crack propagation and the dense distribution of precipitates at grain boundaries causes stress concentration, further deteriorating hot ductility. Heat treatment experiments indicate that the microstructures of the specimen transformed under water cooling, air cooling, and furnace cooling conditions as follows: martensite + proeutectoid ferrite → bainite + ferrite → ferrite. The average size of precipitates first decreased, then increased, and finally decreased again with increasing holding time, while the number density exhibited the opposite trend. Therefore, when the holding time was the same, reducing the cooling rate could increase the average size of the precipitates and decrease their number density, thereby improving the hot ductility of 38CrMoAl steel.

To address the central cracking problem in continuous casting slabs of 38CrMoAl steel, high-temperature tensile tests were performed using a Gleeble-3800 thermal simulator to characterize the hot ductility of the steel within 600 – 1200 °C. Phase transformation behavior was computationally analyzed via Thermo-Calc software, while microstructure, fracture morphology, and precipitate characteristics were systematically investigated using metallographic microscope(MM), field emission scanning electron microscope(FE-SEM), and transmission electron microscopy(TEM). Additionally, the effects of different holding times and cooling rates on the microstructure and precipitates of 38CrMoAl steel were also studied. The results show that the third brittle temperature range of 38CrMoAl steel is 645 – 1009 °C, and the fracture mechanisms can be classified into three types: (I) In the α single-phase region, the thickness of intergranular proeutectoid ferrite increases with rising temperature, leading to reduced hot ductility; (II) In the γ single-phase region, the average size of precipitates increases while the number density decreases with increasing temperature, thereby improving hot ductility; (III) In the α + γ two-phase region, the precipitation of proeutectoid ferrite promotes crack propagation, and the dense distribution of precipitates at grain boundaries causes stress concentration, further deteriorating hot ductility. Heat treatment experiments indicate that under water cooling, air cooling, and furnace cooling conditions, the microstructures of the specimen transform as follows: martensite + proeutectoid ferrite → bainite + ferrite → ferrite; the average size of precipitates first decreases, then increases, and finally decreases again with increasing holding time, while the number density exhibits the opposite trend. Therefore, when the holding time is the same, reducing the cooling rate can increase the average size of precipitates and decrease their number density, thereby improving the hot ductility of 38CrMoAl steel.

Coupled with the results of a 2D heat transfer model, a 3D electromagnetic stirring round billet model is developed, which is considered for the difference in the conductivity of solidified shell and molten steel. The electromagnetic field distribution features of the billet and the effect of round billet sizes on the electromagnetic field are investigated. It is found that as the solidified shell conductivity of the Φ600 mm round billet increases from 7.14 × 10⁵ S·m⁻¹ to 1.0 × 10⁶ S·m⁻¹, the magnetic induction intensity decreases and the maximum value of electromagnetic force drops from 7976.26 N·m⁻³ to 5745.32 N·m⁻³. The magnetic induction intensity on the center axis of the stirrer rarely changes in the range of Φ100–Φ200 mm. With the increase in the round billet from Φ300 mm to Φ600 mm, the magnetic induction intensity and the electromagnetic force on the center axis of the stirrer decrease slowly and then significantly. In the range of 2–8 Hz, as the current strength reaches its maximum, the electromagnetic force can be increased by increasing the current frequency for round billets of Φ100–Φ500 mm, while there is an optimal current frequency for round billets larger than Φ600 mm.

A reasonable tundish flow behavior could improve liquid steel cleanliness by promoting floating and removal of the inclusions. The flow behaviors of the tundish could be obtained by water model experiments and numerical simulations, respectively. However, the difference in density between the tracer and water in the experiment can contribute to notable errors. A new type of tracer, which is a mixture of potassium chloride (KCl) and ethanol, was proposed in this study to reduce the errors. The numerical simulation model was validated by the experimental data and its error was below 2%. By comparing the flow behaviors in seven tundishes with different inner structures obtained by simulation, it is found that the C1 can significantly reduce the dead volume ratio and C4 can improve the uniformity of liquid steel charged though each outlet. The structural strength of the baffle in C1 scheme was not considered, resulting in a crack of the baffle in the industrial trial. Industrial trials of the molten steel flow in the C4-based tundish were conducted and reported a reduction of 43.81% in the inclusions over the prototype.

Ds-type inclusions during production are an important factor affecting the performance and quality of manufactured 10B21 steel. To minimize Ds-type inclusions in steel and improve the production qualification rate of steelmaking plants, a refining slag system optimization scheme was proposed based on the analysis of current inclusion evolution during the steelmaking process, and industrial tests were conducted to verify improvements resulting from application of the proposed scheme. The results showed that the composition of Ds inclusions in 10B21 steel are mainly CaO–Al₂O₃–MgO–CaS–TiN, which exists in the form of calcium–magnesium aluminate coated with titanium nitride and calcium sulfide. The main reason for the formation of Ds inclusions is the poor fluidity of the refining slag and its low capability to absorb inclusions. The poor coverage of the refining slag on the molten steel during refining can easily cause secondary oxidation of the molten steel. Thus, the formation and growth of Ds-type inclusions are aggravated after the calcium feeding line and soft blowing operation. Here, we propose to minimize Ds inclusions using our optimized refined slag system. The mass percentage of the optimized slag system is CaO: 55–60%, Al₂O₃: 20–35%, SiO₂: 3–7%, MgO: 4–8%, (MnO + FeO) < 1%, and the basicity is controlled within the range of 7–11. We observed that our optimized refining slag system has a significantly improved ability to remove inclusions, particularly Ds inclusions, which improves the qualification rate of 10B21 steel.