Optimization Of Soft And Heavy Reduction Process At Solidification End Of Bearing Steel Bloom And Internal Quality Control Of Bloom

With the rapid development of advanced manufacturing industry,the demand for large-section and high-quality steel is increasing,and the production technology of large-scale continuous casting billet has become one of the mainstream trends.Central segregation,shrinkage cavity and porosity are common internal quality problems of bearing steel bloom,which will have a great impact on the performance of subsequent rolled products.The soft and heavy reduction process at the end of solidification can not only reduce the enrichment degree of solidification front of molten steel,but also compensate the solidification shrinkage of the core of the bloom,which is an effective means to improve the internal quality of the bloom.In this paper,the solidification heat transfer model and the thermal/mechanical coupling model are established based on the process parameters.The numerical simulation of the solidification heat transfer process and the reduction process of bearing steel blooms with different sections is carried out respectively.A new soft and heavy reduction process system is developed,and industrial tests are carried out to determine the reasonable reduction system of blooms with different sections.The main conclusions are as follows:（1）The highest and average carbon segregation ratios on the V segregation line of 380mm×450mm bearing steel bloom under the reduction process first were 1.33and 1.22,respectively.The center porosity was 1.5 grade and the center carbon segregation ratio was 1.17.Under the condition of reduction process 2,the central shrinkage cavity of the bloom is grade 1,the central carbon segregation ratio is 1.145,and the highest and average carbon segregation ratios of the V segregation channel are1.3 and 1.18,respectively.The center shrinkage of 250 mm×250 mm bearing steel bloom is 1.5 grade,the center porosity is 2 grade,the highest and average carbon segregation ratio in V segregation channel is 1.43 and 1.09,and the center carbon segregation ratio is 1.1.（2）Based on the mold corner air gap heat flux model and the actual process parameters,the temperature drop trend of the bloom surface and center is calculated by numerical simulation.The accuracy of the model is verified by field temperature measurement.It is clear that the distance from the solidification end point to the meniscus is 21.92m and 24.15m when the casting speed of 250mm×250mm bearing steel bloom is 1.08 m/min and 1.2 m/min respectively.When the casting speed of380mm×450mm bloom is 0.45m/min,the distance from the solidification end point to the meniscus is 23.13m.The effects of casting speed and superheat on the solidification end point of the bloom under typical working conditions were quantitatively analyzed.The fluctuation range of the solidification end point and the position of the corresponding reduction roller were determined,which provided reference and initial temperature field for the selection of the reduction interval and the simulation of the reduction process.（3）A three-dimensional single roll reduction model of bearing steel bloom with macro shrinkage cavity after full solidification was established.It was analyzed that the shrinkage cavity size had no effect on the closure of shrinkage cavity.When the reduction of single roller increases from 5mm to 13mm,the maximum transmission efficiency of the reduction of 380mm×450mm bearing steel bloom core in the reduction direction is 0.0216,and the extreme values of deviation ratio()and volume change(_（1）in the thickness direction are 0.067 and 0.021,respectively.The maximum transfer efficiency of the core reduction of 250mm×250mm bearing steel bloom is 0.0332,and the extreme values of the deviation ratio()and volume change(_（1）in the thickness direction are 0.1067 and 0.024,respectively.The closure of the shrinkage cavity of the bloom decreases with the decrease of the temperature difference between the inside and outside of the bloom.Under the conditions of this study,the macroscopic shrinkage cavity formed by the full solidification of the bearing steel bloom cannot be completely pressed under solid phase reduction.（4）Based on the single-roll reduction model of non-shrinkage bloom,the main reason for the cracks in the ZDT～ZST region at the solidification front of the bloom is that the excessive reduction makes the tensile stress in the extension direction or the broadening direction and the shear stress in different directions produce stress concentration.With the increase of reduction,the equivalent stress at the corner of the bloom gradually increases,and extends along the corner to the center of the wide surface,and extends along the direction of reduction to the center of the bloom,and the equivalent stress value at the corner and its vicinity is the largest.The theoretical single roller reduction should be less than 8.16 mm when the 1～5#roller position of380 mm×450 mm bearing steel bloom is pressed without introducing internal cracks.For the 250 mm×250 mm bloom,with the increase of the central solid fraction,the theoretical single-roller reduction gradually increases and the maximum reduction before the solidification end point is less than 7.2 mm.Combined with industrial tests,it is shown that the process of large reduction should not be adopted before the solidification end point of bearing steel bloom.（5）Based on the single-roll reduction model with spherical solute enrichment zone,the influence of bearing steel bloom on solute enrichment zone during rolling process was analyzed.The results show that at the same position,the feeding rate of the reduction effect increases with the increase of the diameter of the solute enrichment zone and the reduction amount,and decreases with the decrease of the temperature difference between the inside and outside of the bloom at the reduction position.When the solid fraction in the center of the billet is>0.67,under the conditions of this study,when the reduction feeding rate is>4.84%,the reduction effect can meet the basic requirements of eliminating the shrinkage cavity introduced by the solidification volume shrinkage in the solute enrichment zone.For 380mm×450mm bearing steel bloom,the reduction effect of 4#～7#roller position can completely feed the solidification process of the condensed solute enrichment zone.for250 mm×250 mm bearing steel bloom,the reduction of 5#～8#roller position can completely feed the solidification process of solute enrichment zone.The reasonable starting position of the reduction interval is before the 5#roll position.The multi-roller combined liquid core reduction process can effectively improve segregation and eliminate shrinkage cavity of bearing steel bloom.（6）Based on the numerical simulation results,the soft and heavy reduction system of 380mm×450mm bearing steel bloom with a casting speed of 0.45m/min was optimized.The total reduction was 28.4mm,and the solid phase ratio in the center of the bloom was 0.51～1,and the average reduction rate was 6.68mm/m.The industrial test results show that the maximum carbon segregation degree on the V segregation line of the bloom under this process is 1.16,the average segregation degree is 1.08,and the average central segregation degree is 1.023.This process basically eliminates the central shrinkage of the bloom,and the V-shaped segregation becomes blurred from obvious sharpness.When the casting speed is 1.2m/min,the reasonable reduction range of 250mm×250mm bearing steel bloom is 0.45～1,the total reduction is 27mm,and the average reduction rate is 5mm/m.The results show that the highest carbon segregation degree on the V-shaped segregation line is 1.17,and the central segregation degree ranges from 0.994 to 1.002.The process basically eliminates the central shrinkage cavity and weakens the trace of V segregation.