Solidified end characteristics and center quality control of bearing steel bloom
It is presently a dominant process to produce quality bearing steel by continuous casting and rolling instead of traditional ingot casting and forging.To improve the frequently observed severe center shrinkage and center seg-regation in the bloom continuous casting process of GCr15 bearing steel,the numerical simulation has been carried out to predict the solidification process of the continuous casting,and the industrial trials of adjusting the casting speed have been made to explore the influence mechanism and effect of external-field control technology such as F-EMS and soft reduction.The center shrinkage and cracks of the cross-sectional and longitudinal sections of the casting bloom under different processes were observed by the thermal acid etching tests.The distribution of C segre-gation on the cross-section of the casting bloom was detected by drilling cuttings sampling The results showed that the solidification endpoint at a casting speed of 0.95 m/min was only 13.0 m.Accordingly,although the center shrinkage could be improved by increasing the F-EMS strength and using soft reduction.But F-EMS also stirred more high-concentration molten steel into the center of the bloom.Due to the small width of the molten pool in the billet center,the dilution effect on the high-concentration solute was small,and the molten pool was difficult to di-lute these molten steel,so that the segregation of the bloom center was worsened.However,under the process of F-EMS current intensity of 540 A and 7 mm under soft reduction,when the casting speed increased to 1.2 and 1.4 m/min,internal cracks were generated on the inner arc side of the strand,and due to the wide mushy zone of the GCr15 bearing steel,when the casting speed was 1.4 m/min,the crack sensitive interval of the casting blooms was closer to the surface of the casting bloom than the white bright band at the press-down roller where the crack was genera-ted,and finally the white bright band is closer to the center of the billet than the depressed crack.At a casting speed of 1.1 m/min,although the central shrinkage was controlled,the negative segregation occurred in the central area,resulting in the homogeneity of the strand could not be controlled to a high level.Under the casting speed of 1.0 m/min,the central solid-phase rate at F-EMS was around 0.1,the central solid-phase rate of the strand at the No.2 and No.3 reduction rollers were within 0.30-0.75,the center shrinkage rating of the cross-section for strand was below 0.5,the center segregation degree was 1.003,and the carbon range was 0.125％,which was the best quality.
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