| High-quality steel is the development direction of China's iron and steel industry.The products of thin slab continuous casting and rolling(TSCR)have the characteristics of fast solidification,fine grain and thin thickness.They have obvious advantages in some high value-added products and manufacturing costs.Therefore,using TSCR process to produce high-quality steel is a new way to realize green steel products.CSP of WISCO with its unique advantages in low cost and process technology,has transformed its products from ordinary steel to high-quality.Silicon steel,high strength automobile steel,weathering steel,medium and high carbon steel and other high value-added steels have gradually become the main varieties of production lines.However,crack defects and banded structure defects caused by segregation of alloying elements are particularly prominent in the process of continuous casting of high-quality steel,which have become the main restrictive link in the structural adjustment and stable running of product lines.Aiming at the key problems affecting the production of thin slab continuous casting steel,relying on the TSCR production line of WISCO,the project adopted the method of numerical simulation and physical simulation to transfer the temperature and cooling structure of funnel-shaped mould copper plate under the typical working conditions measured by seven-row thermocouples.The heat and flow behavior were systematically analyzed.The detailed data and evolution rules of the whole process from molten steel flow in mould,heat transfer and solidification,structure formation and elemental segregation formation mechanism of slab in secondary cooling section were obtained.On this basis,the slab segregation control technology and low stress non-uniform cooling funnel-shaped mould technology were developed and successfully applied to the production of high-quality steel,which solved the problem of hot rolled banded structure caused by high crack rate and element column crystal segregation of high-quality steel.The main conclusions are as follows:(1)The temperature measurement results,temperature variation curves and heat flux density of the mould copper plate were analyzed by actual measurement.The results showed that:The transverse temperature and heat flux distribution of the broadside of the copper plate are"M"type as a whole.The temperature near the central line of the copper plate is relatively low,and there is a high temperature zone at one fourth of the width of both sides.The temperature of the first row of thermocouples near the meniscus is relatively high,and the heat flux reaches5.35MW/m~2,which fluctuates sharply with a maximum difference of 1.3MW/m~2.This is the main reason for the thermal corrosion of the copper plate in the mould and the longitudinal crack in the center of the slab.In the direction of slab drawing,the temperature of copper plate decreases with the increase of the distance from the meniscus,and the higher the cast speed,the higher the temperature and the higher the heat flux.(2)The numerical simulation of flow field and temperature field under electromagnetic braking and the hydraulic model test of the mould show that for the narrow section mould(1020mm),the reflux below flow field is dominant.With the increase of the cross-section width,the upward reflux will gradually increase and the fluctuation of liquid level will gradually increase.When the width of the mould is larger than 1250 mm,the impact angle changes,the flow field turns to downward flow,the surface velocity decreases,and the liquid level fluctuation decreases.In the range of 200 mm to 400 mm from the center(the junction of convex arc and concave arc of funnel-shaped mould,and the area of excessive arc and plane),the liquid level fluctuates greatly.In addition,the overall test results of the water model are basically the same as those of the numerical simulation,which further proves the correctness of the numerical simulation results.(3)The effect of different cooling rates on segregation were studied.The results show that:The greater the cooling strength of 50CrV4 steel,the faster the solidification rate,and the better the segregation control.When the cooling rate is greater than 5K/s,the column crystal segregation can be effectively controlled.(3)Aiming at the problem of uneven distribution of elements and banded structure in hot rolled plate,mechanism analysis and segregation control technology were studied.The results show that:Column crystal segregation of slab is inherited into hot rolled sheet after elongation along rolling direction during rolling,which is the main reason for formation of banded structure.The larger the cooling intensity and the faster the solidification speed,the more conducive to segregation control.When the cooling rate is greater than 5K/s,the intergranular segregation can be effectively controlled.The continuous casting process of medium and high carbon steel was optimized by means of low superheat,high casting speed,the second cooling strength of the front second cooling section is strengthened and the shell is strengthened.soft cooling at the back second cooling section is used for inhibiting the transgranulation.The industrial test results show that:The liquid phase end position of slab was extended from 5.85m to 7.9m,the columnar crystal proportion was significantly reduced,and about 15%equiaxed crystals appeared in the center.The center[C]segregation ratio of slab decreases to 1.04,the column crystal segregation at 1/4thickness of slab decreases significantly,the band structure grade of product is less than grade 1,and the problem of band structure disappears basically.(4)Aiming at the surface longitudinal crack of hot rolled sheet and surface crack of mould copper plate in CSP production line of WISCO,the design of funnel surface and cooling water gap of mould were studied,and low stress and non-uniform cooling mould technology were put forward.The results show that:The uneven temperature of the shell at the junction of the two curved arcs of the funnel-shaped mould results in stress concentration,the erosion of nozzle jet formed at the liquid level cause the shell thinning,which is the root cause of longitudinal cracks in thin slab casting.According to the solidification shrinkage characteristics of steel grades and the non-uniform arrangement principle of copper plate perforated cooling water channel,980-54 mould was designed.Compared with the original funnel mold,the stress of the shell in the new mould was significantly reduced.After the technology was popularized and applied,the crack rate of medium and high carbon steel was reduced by 65.8%compared with that of the original mold.The average steel-passing capacity of copper plate is 127,000 tons,the service life of copper plate is 33.26%higher than that of common copper plate,and the overall crack correction rate of the product is reduced from 0.68%to 0.14%. |
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