Quality Analysis And Finite Element Simulation Of Large Size GCr15Bearing Steels During Continuous Casting And Rolling

In recent years, the developments of steel industry promote the steel materials toward to the higher performance, and requirements of specifications and quality have become more sophisticated in order to satisfy different using conditions. Domestic production of bearing steels has developed rapidly and the gas content is close to international advanced level. However, the production of large size GCrl5bearing steels is limited, and there is still some instability in internal quality such as internal crack, center shrinkage, center porosity, segregation. With the developments of computer technology and finite element method, numerical simulation of continuous casting and rolling process can provide the basis for optimal design of the production process. In this paper, the production process and defects of large size bearing steel has been studied by experimental analysis and finite element simulation method, and the following results have been achieved:1. The thermal field simulation of in-mold continuous casting of GCr15rectangular blooms.(1) The temperature field simulation results of the right angle and rounded geometric model have been compared. When the bloom corner is the right angle, the corner temperature in the mold exit is negative by average and Flint heat flux, which does not meet the actual situation. The geometric model of the bloom corner should be the fillet, not the right angle.(2) The thickness of the air gap is different at different stages, and the mold vertically can be divided into close contact area, air gap formed area, and air gap stable area. Different correction factors should be taken into account different areas. The formation of air gap in the cross-section of bloom is uniform, and heat flux is linear from the center to corner, and the surface temperature curve is smooth and corner temperature is slower decline in this case, and the region which is from the corner25-65mm become a hot spot area where it is easy for the depression, subsurface crack and other defects to appear.(3) The simulation results is realistic when the round corner geometric model, Savage heat flux boundary and modified corner air gap have been adopted. The simulation results of the shell thickness at the wide face center, narrow face center, corner (diagonal), wide face hot spot area and narrow face hot spot area are21.9mm,17.8mm,54.5mm,18.2mm,16.2mm respectively.(4) A mathematical model in which the surface temperature is changing with time and with solid zone, mushy zone and liquid zone for the unidirectional in-mold solidification of GCr15bloom has been established. The results of mathematical model agree well with the finite element simulation ones.2. The internal quality analysis of GCr15bearing steel bloom(1) Without soft reduction, the occurrence of internal cracks of bearing steel is mainly due to foreign inclusions (mainly Al2O3, MgO and aluminum silicate) by slag entrapment in unsteady casting process, and using the slag system of high alkalinity and Al2O3content in the refining process may also result in some inclusions in the bloom. Based on the control of unsteady casting process, reducing the alkalinity of slag system appropriately and adjusting the time and flow of argon, and using flow control devices and electromagnetic stirring can improve the removal of inclusions.(2) The internal cracks will appear during soft reduction process when the equivalent plastic strain is greater than the critical strain (0.4～1.5%) and equivalent stress exceeds the critical fracture stress (3.9～7.2MPa) if the pressure of soft reduction is excessive and the control of gas content in the continuous casting bloom is not ideal. When the single-roller rolling reduction decreases from3mm to2mm, the equivalent strain and stress are reduced by21.7%and18.8%, and the ratio of internal cracks during soft reduction process will be reduced.(3) Industrial tests have shown that the internal bloom quality can be significantly improved, and the levels of center porosity and shrinkage are low if the production process is controlled appropriately, especially by adjusting the dynamic soft reduction.3. The deformation behavior of shrinkage during bloom blooming process(1) When9pass rolling schedule is utilized and the shrinkage is in the center, the change rate of shrinkage area increases with the increasing diameter of shrinkage (the compression ratio can reach82%when the diameter of shrinkage is40mm, and the compression ratio is only71.4%when the diameter is10mm), and the shrinkage will be smaller and it is more difficult for the shrinkage to close with the extension of time.(2) For the Φ20mm shrinkage in the center, the amount of Y compression is the maximum and the compression ratio is41.4%if9pass rolling schedule is adopted, and Z compression is the maximum and the compression ratio is77.6%if11pass rolling schedule is adopted, and there is not much difference in the change rates of shrinkage area with different rolling pass and all rates have reached the value higher than80%.(3) When the shrinkage is located at the position of1/4narrow face and1/2wide face, the deformation at the highest point of shrinkage is smaller than the one at the lowest point if9pass rolling schedule is adopted, which causes shrinkage change into similar ellipse that the upper half is slightly larger than the lower half and symmetrical on the left and right. The change rate of shrinkage area can reach85.8%which is higher than the center shrinkage, and the bonding is easier when the shrinkage is closer to the roll.(4) When the steel is turned during the blooming process, large reduction in one direction results in a large spread in the other direction. After turning the steel, the effect of reduction in the other direction on the shrinkage would be weakened and the compression ratio of the shrinkage along the other direction will be decreased, which is not benefitial to the overall bonding, and both pass reduction and the spread have to be considered.