| With the increasing speed of the train in recent years,it has a great significance to improve the high speed rail’s property,aiming at promoting the safety and cozy condition atmosphere of the train.In order to obtain the better steel with a good quality,acquisition of the good continuously casting bloom becomes a priority.Therefore,deeper understanding of the growth behavior and its evolution process of the solidified structure will play an important role in the production technology and fundamental theories of the continuously casting process.This paper is objected on the high speed rail U75V continuous casting billet from one steel plant in china combined with macroscopic examination method as its fundamental basis,and numerical simulation has been used to analyze the microstructure of the sample steel.Through this way,the growth behavior and evolution process of the microstructure of the billet can be observed,and the effect the technological parameters of the contmuous casting process has on the microstructure can be investigated.The main conclusions can be summarized as follows:(1)When the high speed rain U75V reaches the position which is 20 m below the meniscus,the temperatures in different areas of the wide face center are 1064.7,970.6,975.4,922.7,919.1,898.6 and 863.3 ℃,meanwhile the temperatures in the narrow face are 1012.9,941.7,964.1,914.3,883.1,837.3 and 797.3 ℃.Before the complete solidification,the temperature in the center of the slab drops at a slow speed due to the supplement of the latent heat of the solidification process,while a sharp drop of the temperature can be observed after the completed solidification process.(2)During the continuous process,the temperature in the cross section shows a decreasing tendency from the center to the surface with a rising trend of temperature gradient,and due to the two-dimensional heat loss in the corner of the slab,the temperature in the comer displays the lowest value.With the increasing distance below the meniscus,the liquidus and solidus of the billet will gradually move towards the center.The result shows that the liquidus will disappear at 7.2 m below the meniscus while the solidus will disappear at 15.6 m below the meniscus.(3)When the superheat is 15,25 and 35 ℃,the transformation positions of columnar dendrite to equiaxed in the axial direction of the U75V billet are 27.8,45.2 and 63.3 mm,respectively.The average areas of the columnar grains are 0.84 mm2,1.05 mm2,1.26 mm2,respectively and its average radius is 0.44,54,0.61 mm.When the casting speeds are 0.68,0.75,0.82 m/min respectively,the position of CET is moved from 45.2 mm to 44.1 mm,and then 42.7mm.The average areas of the columnar grains under this conditions are 2.59,2.45,2.35 mm and the average radius are 0.69mm,0.66 mm,0.65mm.When the specific water flow rate increases from 0.221 1/kg to 0.24 1/kg and 0.273 1/kg,the position of the CET changes from 40.7 mm to 45.2 mm and then to 47.7 mm.The average areas of the columnar grains and its average radius at this condition is 2.13,2.59,2.76 mm2 and 0.57,0.69,0.78 mm.(4)Combined with previous works,a new physical property parameter PP =D/mC0()k-1Γ has been proposed.The relation between the slab equiaxed ratio and PP bypolynomial fitting can be described as follows:rE=0.86-1.28 × 1 02 · PP-2.42 x 104 · PP2.(5)Clear dendrite structure in the slab surface can be revealed by low corrosive technology.The width in each crystalline region in the wide face can be measured as follows:43mm for columnar zone,29mm for mixed crystal zone and 118mm for equiaxed zone and the widths of columnar zone,mixed zone and equiaxed zone in the narrow face are 39 mm,32 mm and 69 mm.The equiaxed ratio of the billet is about 34.6%.The macroscopic defects of the billet can also be observed by the low corrosion technology,and the main defects of the sample steel are internal cracks,center segregation,center porosity and shrinkage cavity and so on.(6)The samples have been tested by picric acid through water bath heating for 7s at 70℃,which show the most perfect condition for experiment.With the increasing distance from the edge of the billet,the primary dendrite arm spacing keeps rising and the arm spacing of the secondary dendrite increases first and then drops.The optimal growth direction of the dendrite is not vertical but shows a certain deviation angle to the surface of the billet.This deviation angle α of the columnar dendrite in the horizontal direction may be explained by the influence of electromagnetic stirring,and the deviation angle β is probably due to the flow of the steel caused by the casting speed and electromagnetic stirring. |
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