In order to understand the distribution patterns of large inclusions in various types of carbon steel, this paper firstly introduces the large-sample electrolysis method for extracting large inclusions from steel, and then the research progresses of large sample electrolysis analysis of large inclusions in four grades of carbon steels are summarized. Finally, the comparative analysis is conducted from the content and composition of large inclusions in the four grades of carbon steels. It is found that the particle size of large inclusions in the four grades of carbon steel is mainly distributed in the range of 50-300 μm, with a small amount of inclusions larger than 300 μm. The contents of different types of large inclusions are ranked in high content sequence as follows: mold flux>tundish flux/ladle slag>refractory material erosion>secondary oxidation/deoxidation products. The contents of large inclusions in the four grades of carbon steels in steady-state slabs rank in low sequence as follows: IF steel<medium carbon steel<high carbon steel<low carbon steel. Among them, the large inclusions in the steady-state slab of IF steel has the lowest content with about 0.035~1.227 mg/kg, and the large inclusions in low-carbon steel has the highest content reaching 0.2~4 mg/kg. The content of large inclusions in different types of slabs are ranked in high content sequence as follows: head slab > tail slab > transition slab > steady-state slab.

In view of the problems including slab pit defects, rolled material cracks, head warpage and delayed fracture existing in the billet continuous casting production of Cr12MoV high-carbon and high-chromium ledeburitic cold-work die steel, this paper systematically carries out the optimization research on continuous casting mold flux, secondary cooling system, casting speed at casting start stage, straightening pressure and hot charging annealing process. The results show that the mold flux with low basicity of 0.70 and low melting point of 884 ℃ is developed to eliminate surface pit defects caused by insufficient lubrication of mold flux. The adoption of the secondary cooling system characterized by intensive cooling in the upper section and weak cooling in the lower section, combined with a high cooling rate of 5.0 ℃/s and a high straightening temperature of casting billets above 900 ℃, reduces the crack occurrence rate from 45% to 9%. Reasonable initial casting speed ranging from 0.3 m/min to 0.75 m/min and optimized straightening pressure parameters decrease the warpage height of billet heads from 20 mm to 8 mm. The hot charging process with a temperature no less than 600 ℃ and annealing process holding at 860 ℃ for 8 h cut the billet fracture rate from 30% to less than 5%. This study establishes a full-process continuous casting process scheme for Cr12MoV steel, which provides theoretical and practical support for the industrial continuous casting production of ledeburitic steels.

To solve the frequent surface cracking of irregular beam blanks of Q420NQR1 weathering steel, the slow cooling process is studied from the viewpoint of regulating phase transformation during low-temperature cooling of cast slabs. First, continuous cooling transformation (CCT) curves of Q420NQR1 are calculated via JMatPro software, and the calculation results indicate that the bainite fraction rises progressively with the increase of cooling rate, which is consistent with the in-situ characterization results obtained by laser scanning confocal microscopy under different representative cooling rates, verifying the dominant effect of cooling rate on the phase transformation behavior of Q420NQR1. Meanwhile, comparative analysis with Q355-series steels featuring analogous base composition but reduced alloying element contents shows that Q420NQR1 weathering steel possesses a lower critical cooling rate for bainite initiation; specifically, the bainite volume fraction of Q420NQR1 reaches 38.27% at a cooling rate of 2 ℃/s, which further proves the indispensable application of slow cooling treatment for this steel grade. On this basis, when the slab is subjected to a 72 h continuous slow cooling procedure in cooling pits, the maximum in-pit cooling rate is controlled below 0.15 ℃/s, smaller than the critical cooling rate for bainite transformation, thereby avoiding the generation of excessive phase transformation stress. In addition, given the practical air-cooling temperature drop rate ranging from 1 ℃/s to 2 ℃/s, the beam blank must be charged into the slow cooling pit at a temperature higher than 567 ℃ to thoroughly eliminate bainite precipitation.

In this paper, an equiaxed-columnar crystal solidification model is adopted to conduct numerical calculation on molten steel flow, nucleation and equiaxed crystal solidification behaviors throughout the whole thin-slab continuous casting process of a domestic steel mill. The effects of intensity and installation position of secondary cooling zone electromagnetic stirring (SEMS) on the equiaxed crystal ratio (ECR) of slabs are emphatically investigated. The reliability of the numerical model is verified by the macrostructure acid etching test results of slab samples. Both experimental and numerical simulation results show that the equiaxed crystal distribution of slabs presents obvious asymmetry along the slab width under unidirectional electromagnetic stirring, and fewer equiaxed crystals form on the downstream side of the stirring flow field. The maximum equiaxed crystal ratio of 20.3% is obtained under the electromagnetic stirring current of 700 A. Restricted by the inherent flow path of molten steel, further increasing the electromagnetic stirring intensity fails to improve the equiaxed crystal ratio. In addition, compared with the original installation position of the on-site electromagnetic stirrer, moving the stirrer upward by 0.5 m increases the equiaxed crystal ratio by 7%, while continuous upward displacement will cause a decline in the equiaxed crystal ratio. For the existing thin-slab continuous casting working conditions, no matched stirring intensity and installation position of secondary cooling zone electromagnetic stirring are found to raise the slab equiaxed crystal ratio to 50%. Further research should be carried out focusing on the optimization of the secondary cooling system in future work.

Q355B steel is extensively applied in the manufacturing of ships, bridges, vehicles, and engineering structural components, which demands superior comprehensive mechanical properties such as high strength, outstanding fatigue resistance and excellent weldability. Deoxidation products and calcium aluminate inclusions in Q355B steel are prone to inducing stress concentration under external loading, exerting detrimental effects on the impact toughness and fatigue life of the steel. In addition, the coarse dendritic microstructure of continuous cast slabs facilitates the initiation and propagation of cracks, and deteriorates the structural homogeneity and mechanical performance of the steel. As rare earth elements can effectively modify inclusions and refine grains in steel, this study systematically investigates the effects of cerium (Ce) addition on inclusion modification and as-cast microstructure refinement of Q355B steel. Compared with the Ce-free group (scheme A), the addition of Ce at mass fractions of 0.001 2% (scheme B) and 0.002 0% (scheme C) reduces the total oxygen (TO) content and significantly improves the cleanliness of molten steel. Ce can transform the inherent deoxidation products and calcium aluminate inclusions in the steel into Ce-Al-O composite inclusions. Specifically, Ce-Al-O inclusions formed at the Ce mass fraction of 0.001 2% inherit the morphological characteristics of original inclusions, while a higher modification degree is achieved at the Ce mass fraction of 0.002 0%, where the Ce-Al-O inclusions present a distinct spheroidized morphology. With the addition of Ce, the number density of inclusions increases from 49 mm^-2 to 64 mm^-2 and 94 mm^-2, the maximum inclusion size decreases from 5.66 μm to 5.30 μm and 4.72 μm, and the average inclusion size drops from 2.18 μm to 2.08 μm and 1.56 μm, realizing a dispersed and refined distribution of inclusions in the steel matrix. Benefiting from the low lattice mismatch between Ce-Al-O inclusions and the steel matrix, the heterogeneous nucleation during solidification of cast slabs is promoted, which effectively refines the as-cast microstructure. The average grain size of the steel is correspondingly reduced from 68.8 μm to 48.7 μm and 41.1 μm.

During the continuous casting process, nozzle erosion leads to non-uniform expansion of the nozzle outlet cross-sectional area, which in turn induces molten steel flow deviation and steel-slag interface fluctuation in the mold. To solve this problem, a multi-area independently controllable electromagnetic braking (MAC-EMBr) technology is innovatively proposed in this study. Taking a slab mold as the research object, a three-dimensional multi-physics coupled numerical model of the internal transport behavior was established. The effects of magnetic flux density variation on the flow characteristics of molten steel and the fluctuation behavior of the steel-slag interface in the mold were numerically simulated under the erosion condition where the cross-sectional area of the right nozzle outlet increased by 7.7%, and the simulation results were compared with those of the full-width single-segment electromagnetic braking (Ruler-EMBr) technology. The results show that the optimal control effect is achieved when the magnetic flux densities of the MAC-I and MAC-II magnetic poles of the multi-area independently controllable electromagnetic braking are both set to 0.30 T. Compared with the case without electromagnetic braking, the maximum surface velocities of molten steel on the erosion side and non-erosion side are reduced to 0.166 m/s and 0.153 m/s, respectively, and the velocity difference between the two sides is narrowed from 0.014 m/s to 0.013 m/s. The maximum velocity in the jet impingement region decreases from 0.34 m/s to 0.29 m/s, with a reduction rate of 14.7%. The maximum turbulent kinetic energy on both sides is reduced from 0.036 m²/s² to 0.018 m²/s², a decrease of 50%. The maximum fluctuation height of the steel-slag interface on the erosion side is reduced from 21.5 mm to 9.3 mm, and the fluctuation difference between the two sides is narrowed from 2.8 mm to 1.5 mm. Compared with the Ruler-EMBr technology, the L-shaped magnetic field generated by the MAC-EMBr technology can effectively cover the upper recirculation zone, jet impingement zone and meniscus zone, and directly suppress the enhanced upper recirculation flow on the erosion side caused by nozzle outlet expansion. Therefore, the MAC-EMBr technology exhibits significant advantages in reducing the jet impingement velocity, improving the flow velocity symmetry and stabilizing the steel-slag interface.

A 1:3 physical model was established for the special-shaped tundish of a steel plant to explore the influence of key factors on the length of transition billets and clarify the action mechanism of different process parameters on transition billet length during the continuous casting of dissimilar steel grades. The results provide an important theoretical basis and practical guidance for reducing the transition billet length and optimization of tundish replacement technology in continuous casting. The continuous tracer injection method was adopted to investigate the effects of casting speed, initial casting flow rate and tundish replacement liquid level on the transition billet length in the continuous casting process of dissimilar steel grades under unsteady conditions. The results show that the increase of casting flow rate and casting speed can reduce the transition billet length. When the initial casting flow rate increased from 2.1 m³/h to 2.4 m³/h, the average transition billet length of each strand decreased by 2% for the original scheme and 0.4% for the optimized scheme. When the casting speed rose from 2.3 m/min to 3.3 m/min, the transition billet length of the optimized scheme was reduced by 2%. In addition, the liquid level during tundish replacement also exerted a remarkable effect on the tundish changing process. When the tundish replacement liquid level decreased from 240 mm to 200 mm, the average transition billet length of each strand in the original scheme and optimized scheme was reduced by 9% and 5%, respectively.

To address the issue of excessive inclusions in steel due to the mismatch of submerged entry nozzle(SEN) in a continuous casting mold with 250 mm×300 mm section at a steel plant, a 1:1 physical and mathematical model was established based on similarity principles, and then the effects of nozzle side hole inclination angle (-12°, -5°, 0°, 5°), casting speed (0.80 m/min, 0.88 m/min, 0.95 m/min), and immersion depth (160 mm, 200 mm) on the mold flow field and surface behavior were investigated in the present work. The results showed that as the side hole inclination angle increased from -12° to +5°, the surface flow velocity at each measurement point significantly increased, but the variation in the average wave height of the 1/3 large wave was relatively mild. Considering the overall flow field characteristics, under the same casting speed conditions, the nozzle inclination angle of 0°resulted in a moderate surface flow velocity and stable wave height. For scheme B, when the casting speed was increased from 0.80 m/min to 0.88 and 0.95 m/min, the surface flow velocity at each measurement point increased noticeably, but the average wave height of the 1/3 large wave remained relatively stable. Numerical simulations indicated that increasing the casting speed would expand the steel's upper circulation zone and promote the upward movement of the vortex core. Under a fixed casting speed, as the immersion depth increased, the surface flow velocity and average wave height of the 1/3 large wave showed a decreasing trend, and the low-flow velocity zone of the free liquid surface expanded. The study recommended a SEN with a 0° side hole inclination and optimized operating parameters (casting speed of 0.88 m/min, immersion depth of 160 mm) for small-section production. Under these conditions, the average wave height of the 1/3 large wave was controlled within 1 mm, the surface flow velocity reached 0.046 m/s, and both flow field stability and meniscus behavior were improved.

Owing to the unique solidification characteristics of hypo-peritectic steel, the maximum casting speed of domestically produced billets is currently limited to below 2.0 m/min, severely restricting continuous casting productivity. To enable higher casting speeds, a coupled thermo-mechanical finite element model of the billet and copper mold under high-speed casting conditions was developed to systematically investigate the influence of different mold taper designs on heat transfer behavior. By analyzing the distribution of key parameters—including air gap thickness, heat flux, and billet surface temperature—the cracking mechanism was revealed: an improper taper induces non-uniform shell growth and enlarges air gaps in the corner and off-corner regions. Based on this analysis, an innovative three-stage taper structure consisting of "high local taper at corners-parabolic body-smooth transition" is proposed for hypo-peritectic steel molds. This design effectively coordinates heat transfer between the corner and the main body, thereby preventing corner and off-corner cracking and providing a theoretical basis for optimizing the high-speed continuous casting process of hypo-peritectic steel billets.

Aiming at the surface defects of slabs occurring during high-speed slab continuous casting, a 1:1 full-scale physical model of mold was established to systematically investigate the effects of argon blowing flow rate, submerged entry nozzle (SEN) immersion depth, SEN bottom structure and outlet inclination angle on the flow field characteristics of molten steel in the mold. The experimental results show that the optimal parameter combination, namely a concave-bottom SEN with an outlet inclination angle of 18°, nozzle immersion depth of 150 mm and argon blowing flow rate of 4-6 L/min, can significantly optimize the molten steel flow field inside the mold. The concave bottom structure effectively buffers the kinetic energy of molten steel jet, while the relatively large outlet inclination angle prolongs the ascending flow path of molten steel; the synergistic effect of the two structural parameters reduces meniscus fluctuation and surface flow velocity. A proper immersion depth alleviates meniscus disturbance by extending the energy dissipation path of molten steel. Meanwhile, a moderate argon blowing rate improves the distribution of mold powder without inducing excessive turbulent flow. This optimized scheme achieves the coordinated improvement of meniscus stability, flow velocity distribution and steel-slag interfacial behavior. It provides a complete process solution for small-section high-speed continuous casting to balance production efficiency and slab quality.

Ultra-high strength steels feature high alloy contents, which induce severe shrinkage and high crack susceptibility during solidification owing to intense peritectic reactions. Practical production results indicate that improving the uniformity and stability of heat transfer between the mold and slab shell is an effective method to alleviate surface depressions on DP780 high-strength steel slabs. The optimization of key continuous casting process parameters was carried out as follows: increasing the mass fraction of Li₂O in mold flux to 0.6% improved the fluidity of liquid slag and inhibited the heat transfer capacity of the mold; adjusting the maximum immersion depth of the submerged entry nozzle from 180 mm to 150 mm raised the temperature at the mold corners and further optimized liquid slag fluidity; reducing the side port angle of the submerged entry nozzle from 25° to 15° increased the thickness of the liquid slag layer from 4.4 mm to 7.1 mm. Benefiting from the above optimizations of continuous casting process parameters, the surface depression index of the cast slabs was reduced by 52%.

Aiming at the typical quality defects including deep narrow-face oscillation marks and attendant trough cracks on continuous casting slabs of 780DP, 980DP and 1180DP cold-rolled dual-phase automotive steels produced by Tangshan Iron and Steel Company, this paper systematically investigates the defect formation mechanism and develops targeted control technologies. The correlation between oscillation mark depth and mark trough cracks is statistically analyzed, and the results show that crack initiation can be effectively suppressed when the oscillation mark depth is controlled within 2.0 mm. From the perspective of mold heat transfer, the influences of mold flux performance and mold narrow-face taper on the number density and depth of deep oscillation marks as well as the propagation of surface cracks are explored. The experimental results demonstrate that mold fluxes with high basicity, high melting point and high viscosity perform better in reducing oscillation mark depth. Both the quantity and depth of narrow-face deep oscillation marks decline with the decrease of mold narrow-face taper. Specifically, slab narrow-face cracks are completely eliminated at the taper of 1.22 %/m. The optimal process parameter combination is determined as Flux B matched with the mold taper of 1.22 %/m, which can remarkably restrain the occurrence of slab narrow-face surface defects. This study provides a critical technical support for high-quality continuous casting production of similar peritectic alloy steels.

The rotating continuous casting roller is partially in contact with the slab and subjected to periodic cyclic heating. Its non-contact outer surface undergoes thermal radiation from the hot slab and convective heat exchange with ambient air, while the roller inner cavity is cooled by circulating cooling water, resulting in an extremely complex internal and external heat transfer process. To fully understand and regulate the heat transfer behavior between casting rollers and slabs, a mathematical heat transfer model is established according to the actual heat transfer characteristics of the roller-slab system. The Pyviewfactor software is adopted to calculate the radiation view factors from the slab to different regional positions of the roller surface, and a corresponding calculation method for non-uniform radiative heat flux on the roller surface is proposed. Combined with dynamic mesh technology, the fully implicit alternating direction implicit (ADI) iterative algorithm is applied to solve the transient temperature field of the rotating roller. The influence of slab surface temperature on the external surface temperature of the casting roller is analyzed via numerical simulation. The results show that the maximum outer surface temperature of the roller presents an obvious linear correlation with the slab surface temperature. Moreover, this numerical calculation method can accurately characterize the temperature distribution of the water-cooled inner wall of the roller, which provides a reliable theoretical basis for the optimal determination of roller water cooling process parameters.

The quantity, size distribution and micromorphology of TiN inclusions in molten steel, continuous casting slabs and hot-rolled plates were systematically characterized via an Aspex automated inclusion analysis scanner, and the evolution mechanism of TiN particles in titanium microalloyed beam steel was clarified. The experimental results show that no TiN inclusions are detected in molten steel. Massive TiN particles precipitate during slab solidification, including square/aggregated elongated pure TiN and TiN inclusions formed with heterogeneous nucleation cores. During slab soaking, preformed TiN inclusions partially dissolve into the austenite matrix; subsequent in-situ reprecipitation and coarsening of TiN occur during hot rolling and post-rolling cooling, and the aggregated elongated pure TiN gradually transforms into square TiN. Continuous casting slabs and hot-rolled plates present similar distribution characteristics in terms of TiN quantity and particle size, with the maximum TiN fraction appearing at the 1/4 thickness position and slab center. The number of 5-10 μm TiN particles in hot-rolled plates is 22.3% higher than that in slabs, verifying that the dissolution-reprecipitation behavior promotes the dimensional growth of partial TiN inclusions. The quantity and size of TiN inclusions can be effectively regulated by limiting titanium content near the lower specification limit, reducing molten steel nitrogen content, strengthening the secondary cooling intensity of continuous casting, and accelerating the cooling rate after hot rolling.

To improve the thermal shock resistance and reduce the cost of slag line of submerged entry nozzle (SEN) refractories, zirconia-carbon refractories were prepared by partially replacing calcia-stabilized zirconia with zirconia hollow spheres. The effects of different additions of zirconia hollow spheres (0, 5 wt.%, 10 wt.%, and 15 wt.%) on the mechanical properties, thermal shock resistance, and slag corrosion resistance of the materials were investigated. The results show that with the increase of zirconia hollow sphere addition, the apparent porosity of the samples increases, while the bulk density and cold compressive strength decrease significantly. In contrast, the cold modulus of rupture (MOR) remains relatively stable, with a maximum reduction rate of 9.5%. In terms of high-temperature performance, as the addition of zirconia hollow spheres increases, the thermal expansion rate of the samples decreases from 0.71% to 0.61%, and the thermal shock resistance is significantly enhanced. The hot modulus of rupture first increases and then decreases, which is attributed to the fact that excessive addition of zirconia hollow spheres weakens the structural strength of the material. Regarding slag corrosion resistance, both the slag penetration resistance and slag corrosion resistance of the samples gradually decrease with the increase of zirconia hollow sphere content. When the additions of zirconia hollow spheres are 5 wt.%, 10 wt.%, and 15 wt.%, the corrosion rates increase by 4.5%, 16.6%, and 65.1%, respectively. In conclusion, an appropriate amount of zirconia hollow spheres (0-10 wt.%) partially replacing calcia-stabilized zirconia is beneficial for the high-temperature strength and thermal shock resistance of ZrO₂-C refractories, but excessive addition leads to a decline in strength and slag corrosion resistance.

The research practice of this paper indicates that there are two types of sliver defects on the surface of cold-rolled plates caused by aluminum oxide inclusions, which are respectively caused by the focused zones of two bubble inclusions at the 10 mm surface layer of casting slab and the position ranging from 1/8 to 1/4 thickness. Based on the imported domestic electromagnetic stirring equipment, the effect of mold electromagnetic stirring (M-EMS) on the flow field of the mold and the surface quality of the slab was systematically studied. The results show that when the stirring current is controlled in the range of 400-600 A, the flow field in the mold is stable, the solidification hook of the slab becomes shallow and the depth of the solidification hook at different positions is more uniform, and the number density of bubbles and inclusions is significantly reduced. When the stirring current is further increased, the fluctuation of the liquid level is obviously intensified, which is not conducive to the quality control of the slab surface. Through large-scale industrial test comparison, when the stirring current is 500 A, the overall reduction of steel defects is approximately 30%.