To solve the instability of liquid steel in the continuous casting process and the inconsistent flaw detection of heavy rail steel, steel flow control was studied numerically in a tundish with a porous baffle wall by using the fluid dynamics software Fluent. The opening plan of the baffle wall was improved through orthogonal optimization of the design of the holes in the porous baffle wall. The test condition was set to a left inclination angle of α₁ = 22°, a right inclination angle of α₂ = 48°, an upward elevation angle of β = 30°, and an aperture of d = 70 mm. The simulation results of the optimization scheme showed that the uniformity of the flow and temperature fields had been significantly improved, and the flow in each strand became consistent. The maximum temperature difference was 21 K in the tundish, and the maximum temperature difference of three outlets was only 1.7 K. Dead zone volume was reduced by 10.0% compared to the original tundish, and plug flow volume was increased by 14.2%. Comparing the removal efficiency of Al₂O₃ inclusions of different size, the results showed that the removal efficiency of 10 μm and 30 μm smaller inclusions was above 87%. The removal rate of ≥50 μm larger inclusions also remained about 95%.

To study the effect of refining slag on the compositions of molten steel and inclusions, the reaction between ship plate steel and different slag systems (slag A-CaO/Al₂O₃: 1.0, SiO₂: 5 mass-% and slag B-CaO/Al₂O₃: 1.5, SiO₂: 9 mass-%) was investigated by laboratory-scale high-temperature equilibrium experiments. Results showed that the desulphurisation capacity of the two slags was very similar, and the average sulphur content for both was 0.002 mass-% in steel, but the deoxidation capacity of slag B was slightly higher than slag A. The amount of inclusions <5 μm was more in steel balanced with slag B than in that balanced with slag A. To generate inclusions smaller than 5 μm, and spherical liquid CaO–MgO–Al₂O₃–SiO₂ inclusions, and to decrease the T[O] (total oxygen) content in steel, refining slag composition should be: CaO/Al₂O₃ ∼ 1.5, and SiO₂ ∼ 9 mass-%. Slag system optimisation can reduce or even eliminate calcium treatment during production. T[O] content in the slab could be controlled to 15–21 ppm, with typical micro-inclusions of ≤10 μm, and CaO–Al₂O₃–MgO and CaO–Al₂O₃–CaS systems in molten steel.

Al₂O₃, Al₂O₃·TiN, Al₂O₃·MgO, and CaO·2Al₂O₃ are four different types of inclusions in high Al steels. To improve the steel cleanness level, the effective removal of such inclusions during secondary refining is very important, so these inclusions should be removed effectively via inclusion dissolution in the slag. The dissolution behavior of Al₂O₃, Al₂O₃·TiN, Al₂O₃·MgO, and CaO·2Al₂O₃ in CaO-SiO₂-Al₂O₃-MgO slags, as well as the steel-slag reaction, was investigated using laser scanning confocal microscopy (LSCM) and high-temperature furnace experiments, and thermodynamic calculations for the inclusion in steel were carried out by FactSage 7.1. The results showed that Al₂O₃·TiN was observed to be completely different from the other oxides. The composite oxides dissolved quickly in the slags, and the dissolution time of the inclusions increased as their melting point increased. SiO₂ and B₂O₃ in the slag were almost completely reacted with [Al] in steel, so the slags without SiO₂ showed a positive effect for avoiding the formation of Al₂O₃ system inclusions and promoting inclusions dissolution as compared with SiO₂-rich slags. The steel-slag reaction was also found to influence the inclusion types in steel significantly. Because of the rapid absorption of different inclusions in the slag, it was found that the dissolution time of inclusions mainly depends on the diffusion in the molten slag.

In order to investigate the causes of the large number of cracks and porosities formed in 33MnCrTiB fork flat steel produced by a special steel plant, scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) analysis, and large sample electrolysis of the obtained steel samples were carried out in different steps of the steelmaking processes. The main micro-inclusions in the fork flat steel samples were Al₂O₃, CaO-MgO-Al₂O₃-SiO₂, and TiN, and the macro-inclusions were mainly Al₂O₃, CaO-Al₂O₃, CaO-Al₂O₃-SiO₂-TiO₂, and CaO-MgO-Al₂O₃-SiO₂-TiO₂-(K₂O) systems which originated from the ladle slag and mold flux in the production process. In order to reduce the number of micro-inclusions effectively, the control range of components in the refining slag was confirmed by the thermodynamic calculation, where the mass ratio of CaO/Al₂O₃ should be in the range of 1.85–1.92, and the mass fraction of SiO₂ and MgO should be controlled to 7.5–20% and 6–8%, respectively. In addition, the numbers of macro-inclusions in the flat steel should be effectively reduced by optimizing the flow field of mold and preventing the secondary oxidation, and the flat steel quality problems caused by the inclusions can be improved by the optimization process above.

The evolution of MC-type primary carbonitrides (M=V, Ti, Mo; C=C, N) in terms of morphology, quantity, size and composition was systematically investigated in commercial H13 die steels with different Ti and N contents during thermal holding at 1250 °C for 5 h to 15 h. Results showed that the mean size and quantity of carbonitrides in the four samples had decreased during thermal holding. However, the mean size and quantity of MC carbonitrides had increased with increasing Ti contents when held at 1250 °C while the addition of N increased the quantity but decreased the sizes of the stable MC carbonitrides. It was concluded that the compact carbonitrides could be decomposed and changed into a fishnet structure when held at 1250 °C, especially in samples #1 and #2 containing lower Ti and N contents. The decomposition mechanism was illustrated considering the changes in Ti and Fe elements in carbonitrides. On the basis of the thermodynamic model, the thermal stability of (Ti_x,V_1−x)(C_y,N_1−y), with a larger x value, in samples #3 and #4 containing more Ti and N contents was generally higher than those in samples #1 and #2. To control the Ti-containing MC carbonitrides, the low Ti and N contents and high holding temperature should be taken into consideration.