1 Mechanism of slag entrainment in high speed continuous casting copper mould tube
Japanese scholars used physical simulation to study the mechanism of slag entrainment in high speed continuous casting tubes in detail, among which the research results of Yoshida J et al. and Watanabe J et al. are the most representative. There are five main ways of slag entrainment:
(1) The stream hits the narrow surface and moves upward along the narrow surface, pushing the protective slag to move near the nozzle, resulting in shear slag entrainment;
(2) The asymmetric flow field on the left and right sides of the nozzle causes vortex slag entrainment near the nozzle;
(3) The argon bubbles blown into the crystallizer burst at the steel/slag interface, entraining the protective slag into the molten steel;
(4) Unsteady slag entrainment caused by unsteady flow on the surface of the molten steel;
(5) The negative pressure area formed by the high-speed steel flow at the nozzle outlet sucks the liquid protective slag along the outer wall of the nozzle, causing slag entrainment.
Japanese scholar Teshima T used the JFE Fukuyama No.5 high-speed casting machine as the background, used a 1:3 water model and industrial experiments to study the flow behaviour of molten steel in the slab high-speed continuous casting crystallizer, and proposed to use the liquid surface fluctuation index “F-number” to evaluate the liquid surface fluctuation. The author studied the relationship between the F-number and the incidence of cold-rolled coil defects and believed that when the F-number is controlled between 1.7 and 3.0, the incidence of cold-rolled coil surface defects is the lowest. The calculation formula of the F-number is shown in formula (1).
F=ρQLve（1-sinα）/4D
Where: ρ is the density of molten steel, kg/m3; QL is the volume flow rate of molten steel, m3/s; ve is the speed of the molten steel stream hitting the narrow surface of the crystallizer, m/s; α is the angle of the molten steel stream hitting the narrow side, (゜); D is the distance between the stream impact point and the meniscus, m.
Although the F-number was proposed, it is still an indirect indicator for evaluating slag rolls. In the practice of high-speed continuous casting, liquid surface fluctuation and surface flow rate are mainly used for evaluation. Tsai H T et al. proposed that the defect rate is low when the liquid level fluctuation is controlled below ±3 mm. There is much controversy about the critical surface velocity for slag entrainment. The critical surface velocity obtained by different scholars using water models is shown in Table 3. In the practice of high-speed continuous casting, the more direct method to control liquid level fluctuation and surface velocity is to optimize the structure of the submerged nozzle and apply electromagnetic metallurgical technology.
2 Research on the submerged nozzle for high-speed continuous casting
The submerged nozzle structure is one of the few parameters in the continuous casting process that is easy to change and has a profound impact on the flow field of the crystallizer. The submerged nozzle structure mainly includes the number of nozzle outlet holes (double holes or multiple holes), outlet angle, bottom shape, and outlet shape. The current mainstream of conventional thick slab continuous casting is to use a double-hole nozzle. Literature reports that the submerged nozzle for conventional slab high-speed continuous casting has the following characteristics:
(1) The use of a large-angle nozzle is conducive to reducing the liquid level fluctuation and slag entrainment risk of the high-speed continuous casting tube.
Cao Na et al. used a 1:0.4 water model to study the steel/slag interface behaviour of the tube of a high-speed continuous casting (maximum 2.2 m/min) of a slab with a thickness of 220 mm at Meishan Steel. They believed that when the nozzle inclination angle increased from 10° to 20°, the liquid level fluctuation at the meniscus decreased from 13.7 to 6.2 mm. Deng Xiaoxuan and LI L et al. used a 1:1 water model and numerical simulation to study the liquid level fluctuation and surface velocity of the tube of Shougang Jingtang No.3 CCM high-speed continuous casting (maximum 2.5 m/min). They believed that increasing the nozzle angle from 15° to 20° could effectively reduce the liquid level fluctuation and was successfully applied to the high-speed continuous casting industrial practice. Teshima T et al. reported that JFE Fukuyama No.5 CCM used an immersion nozzle with an inclination of 45° in high-speed continuous casting (pulling speed of 1.9 m/min and width of 1,600 mm). They believed that the use of a large inclination nozzle would help control the “F-number” of high-speed continuous casting within a reasonable range and reduce the incidence of cold-rolled sheet defects.
(2) The concave bottom nozzle is beneficial in reducing the liquid level fluctuation and slag entanglement probability.
The literature used a 1:1 water model and industrial experiments to study the effects of concave bottom, convex bottom and flat bottom on the liquid surface characteristics of Shougang Jingtang No.3 CCM high-speed continuous casting. The water model results showed that the flow field symmetry of the concave bottom and flat bottom nozzle was better than that of the convex bottom. Both the water model and industrial experiments showed that the liquid level fluctuation and surface flow velocity of the convex bottom nozzle were significantly greater than those of the concave bottom. Therefore, from the perspective of reducing slag entanglement, it is recommended to use a concave bottom nozzle for high-speed continuous casting. The literature reported the flow characteristics of nozzles with five different bottom shapes under high-speed continuous casting conditions. It is believed that the kinetic energy of the flow at the outlet of the convex bottom nozzle is large and the turbulent kinetic energy is small, but the kinetic energy of the concave bottom nozzle is weak due to the interaction with the bottom of the nozzle, but the turbulent kinetic energy is large, which is beneficial to eliminate the negative flow area at the nozzle outlet and alleviate nozzle blockage.
(3) There is no unified understanding of the nozzle outlet shape.
Xiong Xiao et al. used a 1:1 water model to study Shougang Jingtang No.3 CCM high-speed continuous casting and found that the elliptical nozzle has the largest flow expansion angle and inclination angle during high-speed continuous casting, and the flow velocity is evenly distributed at the outlet, so it is recommended for use in high-speed continuous casting. Bai H et al. believe that the square outlet has the largest flow velocity and negative flow area, and the rectangular outlet is the optimal outlet shape.
