| Twin-roll strip casting has attracted the attention of world steel producers as one of the frontier technologies, because of its suitability for the steel industry developing tendency of low energy consumption, short procedure, high efficiency and low production cost. Currently, the main limiting factor of its large-scale industrialization application is that the quality of the thin strip is poor and unstable. The temperature distribution, level fluctuation of the pool and side sealing effect are the key factors for the quality of casting strip. Therefore, combining with the current technique situation and problems for twin-roll strip casting, the present study investigated the mechanisms of temperature drop and the effect of various heat preservation measures on the temperature drop of molten steel in the 80 t ladle used for twin-roll strip casting process, the effect of delivery system and operation parameters on the fluid flow and temperature distribution in the casting pool, and the effect of configurations, materials and preheating temperature of side dam on the thermal insulation property and thermal stress distribution of side dam using numerical simulation and physical simulation, and confirmed key technical parameters of ?600 mm×1000 mm twin-roll casting system. According to the research, the main results obtained are as follows:(1) The heat loss through slag surface, side wall and bottom of the 80 t ladle under traditional continuous casting were 127.04,211.32 and 36.95 kW respectively, and the temperature drop rate of the melt was 1.1 K·min-1, and the temperature drop of the molten steel was higher than 65 K in the whole casting process, which was unfavorable to the uniformity of the product quality. The heat loss through slag surface was decreased to 5.89 kW when covering flux (k=0.075 W·m-1·K-1) with thickness of 40 mm was added on the slag surface, and the heat release of the ladle was decreased from 211.32 kW in side wall to 37.29 kW, and from 36.95 kW in bottom to 3.57 kW when nanoporous materials (k=0.031 W·m-1·K-1) was replaced by the fireclay brick (k=0.2 W·m-1·K-1) with thickness of 20 mm. Furthermore, the temperature drop rate of the molten steel was decreased to 0.31 K·min-1 and the temperature drop of the molten steel was only about 18 K when using nanoporous thermal insulation materials (k=0.031 W·m-1·K-1) with thickness of 16 mm and covering flux with thickness of more than 40 mm, which is considered to meet the requirement for thermal insulation property of ladle in the twin-roll strip casting process.(2) The configuration of delivery system was the key influence for the flow field and temperature distribution, affecting the quality of the products directly. When the A3-type feeding device and B6-2-type delivery device were selected under the casting speed of 80m·min-1, the value of the level fluctuation was 1.3 mm, the variance of residence time distribution was 0.75, and the dispersion of the characteristic parameters of the fluid flow was 0.25, which is considered to be the best delivery scheme in the present study.(3) The casing speed had significant effect on the level fluctuation, fluid mixed characteristics of upper area and flow patterns along the width of strip in the casting pool, but the contact angle showed great effect only on the level fluctuation. Therefore, the suitable casing speed was the key factor for smooth production. With the casting speed increasing from 40 to 80 m·min-1, the level fluctuation was increased from 0.53 mm to 1.30 mm, the variance of residence time distribution was increased from 0.21 to 0.75, and the dispersion of the characteristic parameters of the fluid flow was decreased from 0.61 to 0.25. With the casting speed further increasing to 100 m·min-1, the level fluctuation was increased to 2.97 mm, but the variance of residence time distribution and dispersion of the characteristic parameters of the fluid flow was changed little. With the contact angle between fluid and roll increasing from 45 to 60°, the level fluctuation was decreased from 1.33 mm to 0.66 mm, the variance of residence time distribution was decreased from 0.84 to 0.67, and the dispersion of the characteristic parameters of the fluid flow was increased from 0.20 to 0.29.(4) The casting speed and the upward angle of outlet of delivery device have significant effect on the temperature distribution in the casting pool. With the casting speed increasing from 40 to 80 m·min-1, the temperature distribution uniformity of the pool surface was obviously improved, and the temperature difference of pool surface was decreased from 31 to 20 K. With the upward angle of outlet of delivery device increasing from 0 to 15°, the temperature difference of pool surface was decreased from 35 to 20 K, but with further increase of the upward angle, the temperature difference of pool surface was not changed significantly. With the contact angle between molten steel and roll increasing from 50 to 60°, the temperature difference of pool surface was increased from 20 K to the temperature that could result in local solidification. With the superheat of the molten steel increasing from 20 to 40 K, the center temperature of strip of the outlet was only increased 5 K; with the heat transfer coefficient of roll surface increasing each 1000 W·m-2·K-1, the center temperature of strip of the outlet was decreased 50 K on the average. Therefore, under the conditions of present study, the suitable casting speed is in the range of 60 to 80 m·min-1, the contact angle is the range of 50 to 55°, the upward angle of outlet of delivery device is 15° and the superheat of the molten steel is in the range of 20 to 40 K.(5) The application of thermal insulating layer (k=0.05 W·m-1·K-1) between bulk body of the side dam and steel shell could decrease the thermal stress and improve the thermal insulation property of the side dam. The external surface temperature of the side dam and the thermal stress of point A were 787.3 K and 15.66 MPa respectively with the insulating layer thickness of 6mm, but with further increase of the thickness of thermal insulating layer, this effect was not significant. The increase of thermal conductivity of the bulk body would decrease the maximum thermal stress of the side dam, but it was unfavorable to the thermal insulation property. With thermal conductivity in the range of 13.8?21.5 W·m-1·K-1, the relationship between the maximum thermal stress (a) of point A and thermal conductivity (k) was expressed by:?=0.0094k2-0.4029k+22.7535 R2= 0.99; At the initial stage of casting process, with the preheating temperature of side dam increasing, the heat loss of molten steel absorbed by the side dam was decreased, contributing to improve the heat preservation effect; with preheating temperature increasing from 673 to 1473 K, the maximum thermal stress (?) of point A was significantly decreased, and the relationship between these two factors was expressed by:?=2.3802e-5T2-0.0662T+61.3213 R2=0.99. Therefore, the suitable thickness of the insulating layer was not lower than 6 mm and the preheating temperature of side dam was not lower than 1273 K under the conditions of present study. |
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