| Bearing is an essential basic component in high-speed train,aerospace,metallurgical machinery,wind power generation and other fields.In this paper,a two-dimensional solidification heat transfer model was established for a 240 mm×240 mm bloom of GCr15 bearing steel in a domestic steel mill,and the model was verified measured temperature according to the field.Then,the variation of temperature field and shell thickness of GCr15 bearing steel bloom with continuous casting process conditions was explored,and the average cooling rate of mushy zone at different positions below the bloom surface was obtained.Samples were taken at the positions 20 mm,60 mm and 100 mm below bloom surface,respectively.The solidification processes of bearing steel at different cooling rates were observed dynamically in situ through high temperature laser confocal microscope(HT-CLSM).And the growth kinetics of austenite grains on the melt surface was revealed.A threedimensional solidification structure evolution model for bearing steel bloom was established with CAFE method.The model was verified throuth comparing with macrostructure in transverse section of bloom obtained with etching method.The variation mechanisms of equiaxed grain ratio,CET position,total number of grains and average radius of grains with continuous casting processing conditions were revealed.The research contents and main results are as follows:(1)A two-dimensional solidification heat transfer model for bearing steel bloom was established according to the steel grade composition and continuous casting processing parameters.The surface temperature of the bloom was continuously measured in time and space via infrared thermal imager.The reliability of the model was evaluated through comparing the calculated surface temperature of the bloom with the measured results.then the influences of different processing parameters(superheat,casting speed and specific water flowrate in secondary cooling)on the temperature field and shell thickness of bloom were investigated.The results show that as the casting speed and secondary cooling water ratio remain unchanged,the superheat changes from 19 ℃ to 29℃ and 39℃,and the bloom surface temperature increases,but the increment is small,with an average increment of 5℃.The center temperature of the bloom changes greatly before 5.5 m below the meniscus,and the influence gradually weakens after 5.5 m from the meniscus.The average difference between the center temperature of the bloom at 12 m from the meniscus is 10℃.Also it is found that the center temperature of bloom at the superheat of 19℃ is the lowest.The vanishing positions of the solid and liquid lines move 0.22 m and 0.33 m along the drawing direction,respectively.At a casting speed of 0.85 m/min,the reheating time at the foot roll becomes short,and the surface temperature of the bloom after entering the secondary cooling zone is lower than other.In the subsequent secondary cooling zone,the surface temperature is higher than that at 0.65 m/min and 0.75 m/min due to the high casting speed,short residual time and less heat dissipation.With the increase of casting speed from 0.65 m/min to 0.75 m/min and 0.85 m/min,the center temperature of the bloom changes little below 8 m from meniscus.At 8.9 m,10.36 m and 11.5 m below the meniscus,the center temperature of the bloom decreases more rapidly.The locations where the liquidus line disappeares moved 1.65 m and 0.97 m on average along the casting direction.Keeping the superheat and casting speed unchanged,with the increase of secondary cooling water ratio from 0.8 L/kg to 0.9 L/kg and 1.0 L/kg,when the cooling intensity is enhanced,the surface heat dissipation of the bloom becomes more singnificant.After the bloom enters the secondary cooling zone,the surface temperature of the bloom is relatively lowest at the specificflow rate.The specificflow rate has little influence on the center temperature of bloom.At the position 16 m away from meniscus,the center temperature of bloom under three cooling conditions differs by about 6℃.The locations where the liquidus and solidus lines disappear move 0.25 m and 0.1 m on average in the opposite direction of casting.Compared with superheat and speaficwanter flowrate,casting speed has greater influence on bloom temperature field and shell thickness.(2)According to the calculation results of solidification heat transfer,it can be derived that the cooling rates of mushy zone at 20 mm,60 mm and 100 mm away from the bloom surface are 57.28℃/min,18.44℃/min and 17.54℃/min,respectively.On this basis,HT-CLSM was used to observe and measure the growth process of austenite grains on the melt surface.The results show that with the decrease of cooling rate,the austenite grain size increases and density decreases at the end of solidification.The number of grains in the measured area of 484μm×484μm are 27,17 and 14,respectively.According to the statistics of the average growth velocity of austenite grains in the areas 20 mm,60 mm and 100 mm away from the bloom surface,the growth stage can be roughly divided into three periods,that is the initial slow growth stage,them idle fast grow stage,and the final slow growth stage.Through comparing the relationship between the liquid phase fraction on the melt surface and the melt undercooling at different cooling rates,it is found that the higher the cooling rate,the shorter the solidification initial stage with slow growth.When the cooling rate is 57.28℃/min,the innifial solidification stage locates with in the undercooling rang of 19-43℃.When the cooling rate is 18.44℃/min,the innifial solidification stage locates with in the undercooling rang of 3-60℃.When the cooling rate is 17.54℃/min,there is no obvious difference between the early solidification stage and the middle growth stage.In the solidification process,when the cooling rates are 57.28℃/min,18.44℃/min and 17.54℃/min,the maximum growth rates of austenite grains are 11.9 μm/s,3.34 μm/s and 2.2 μm/s,respectively.(3)The solidification structure evolution model for bearing steel bloom were developed with CAFE method.The simulated results of solidification structure was compared with the macrostructure of bloom to evaluate the reliability of the model.And the influence of processing parameters(superheat,casting speed and specific water flow rate in secondary cooling zone)on the solidification structure of bearing steel bloom was analyzed.The results show that as the superheat changes from 19℃ to 29℃ and 39℃,and the inner equiaxed grain ratio decreases by 3.33%and 1.74%,respectively.The distance between CET and bloom surface is 48.07 mm,49.80 mm and 55.65 mm,respectively.And and the average grain radius is 1.334 mm,1.356 mm and 1.356 mm,respectively.When the superheat and specific water flow rate is constant,the casting speed changes from 0.65 m/min to 0.75 m/min and 0.85 m/min,and the inner equiaxed grain ration decreases by 2.79%and 4.5%,respectively.The distance between CET and bloom surface is 47.58 mm,49.80 mm and 59.43 mm,respectively.And the average grain radius is 1.367 mm,1.356 mm and 1.351 mm,respectively.As casting speed changes from 0.8 L/kg to 0.9 L/kg and 1.0 L/kg,and the inner equiaxed crystal ratia decreases by 2.32%and 1.31%,respectively.The CET locations are 49.76 mm,49.80 mm and 55.89 mm,respectively.And the average grain radius is 1.368 mm 1.356 mm and 1.365 mm,respectively.It can be concluded that the effect of casting speed on CET position and average grain radius is more significant than superheat and specific water flow rate. |
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