Phosphorus-containing high-strength (HS) interstitial-free (IF) steel is widely used for making complex automobile components, such as car extension support, suspension mounting beams, steering mechanism mounting support beams, reinforcing plates, and automobile cover plates, due to its high strength and suitability for extra-deep drawing. The application of HS steel reflects the development trend toward lightweight automobiles. The strength of HS IF steel can be improved by solution strengthening with phosphorus. The production of both P-containing HS IF steel and common IF steel uses titanium to remove interstitial elements by the formation of Ti(C, N). [1] However, the casting of the former is susceptible to nozzle clogging, which leads to poor cleanliness and unstable surface quality in the rolled steel, thus increasing production costs and decreasing productivity. [2][3][4] There have been few studies related to nozzle clogging with P-containing HS IF steel. However, Pande et al. investigated the effect of the addition of FeP on steel quality, and they altered the sequence of FeP addition to improve the cleanliness of the resulting steel. [5] Nozzle clogging can affect the fluid pattern in the mold, [6] increasing the probability of the mold powder becoming entrapped in the steel, which causes defects in the cold-rolled sheet.Significant research has been conducted to investigate the mechanism behind nozzle clogging. The findings can be summarized as follows. [7][8][9][10][11] 1) Clogging caused by high-melting-point deoxidation products: The deoxidation products (Al 2 O 3 ) in aluminum-killed steel attach to the nozzle inner wall under the effects of vortices in the molten steel and interfacial tension. 2) Clogging caused by reoxidation when air is sucked into the nozzle: The molten steel at the steel-refractory interface is reoxidized, thereby forming a surface-tension gradient on the nozzle inner wall, increasing the traction force toward the nozzle inner wall and causing aggregation of inclusions on the inner wall. 3) Nozzle clogging caused by refractory-molten steel reaction: The diffused oxygen from the nozzle refractory and acid-soluble aluminum from the molten steel react to produce A1 2 O 3 . 4) Nozzle clogging caused by the deposition of cold steel: Inadequate preheating of the nozzle or poor flow of the molten steel in local zones of the nozzle causes the deposition of cold steel. 5) Clogging caused by the electrical characteristics of the nozzle inner wall during casting: Recent studies have shown that the friction between the molten steel and the nozzle inner wall and between the molten steel and inclusions during casting results in mutual attraction between the inclusions and the nozzle inner wall, thereby causing clogging.

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The motion behavior of bubbles in a riser tube is studied in order to analyze the bubble evolution characteristics. Gas distribution and bubble movement in risers and vacuum chambers have important effects on liquid steel flow, mixing and refining process. It is found that the initial diameter of argon bubbles in the riser tube decreased with decreasing vacuum degree. The diameter of argon bubbles in the riser tube increased with increasing gas flow rate. The bubbles could be divided into the single bubble rising zone and the bubble breaking coalescence zone in the rising tube. After the bubbles were blown in, they changed from regular spherical shapes to flat shapes in the single bubble rising zone, and then broke apart into small bubbles in the bubble breaking coalescence zone. Variations in the gas flow rate and vacuum degree had significant effects on the regional distribution of bubble motion and bubble residence time. The critical height of the single bubble rising zone and the bubble breaking coalescence zone were stable when the bubble travel distance was greater than 280 mm.

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Aiming at the quality problems such as segregation, porosity and shrinkage cavities that are difficult to eliminate due to the size effect of large die-cast steel ingots as large forging blanks, the idea of layered casting of large steel ingots is proposed. The transient heat transfer process and cladding path of the ingot core and cladding layer under different molten steel casting temperatures, different ingot core diameters and different ingot core preheating temperatures were studied by combining numerical simulation and thermal experiments. The research results show that the cladding path has a certain functional relationship with the diameter of the ingot core and the preheating temperature of the ingot core. Obviously, the interfacial melting rate can be significantly improved. The thermal scaling experiment was carried out on the cladding path under the condition of a casting temperature of 1,560°C and no preheating of the ingot core. The microhardness of the interface is higher than that of the clad steel ingot, and the metallurgical bond of the interface is good.