Solidification structure and hardenability optimization of 20CrMnTiH gear steel considering dendrite sedimentation

Production practices have shown that the morphology and dimensions of the solidification structure in blooms are pivotal in determining solute distribution, thereby influencing the consistency of hardenability in high-quality gear steels. To address the issue of asymmetric lengths between the inner and outer arc columnar crystal zones caused by dendrite sedimentation in continuously cast blooms, a treatment considering dendrite sedimentation was proposed using the solidification structure simulation model (CAFE model) in the commercial software ProCAST. The effects of superheat, casting speed, specific water amount, and current intensity of mold electromagnetic stirring (M-EMS) on the equiaxed crystal ratio and the symmetry of the solidification structure of 20CrMnTiH gear steel blooms were simulated and analysed. The results showed that increasing superheat from 30°C to 50°C reduced the equiaxed crystal ratio from 25.77% to 15.73% and decreased the length difference between the inner and outer arc columnar crystal zones from 20.6 mm to 11.7 mm. A higher casting speed from 0.85 m·min−1 to 1.15 m·min−1 also reduced the equiaxed crystal ratio from 24.60% to 20.10%, but increased the length difference from 16.5 mm to 18.0 mm. Additionally, as the specific water amount raised from 0.264 L·kg−1 to 0.462 L·kg−1, the equiaxed crystal ratio dropped from 24.88% to 17.23%, while the length difference decreased from 18.6 mm to 15.1 mm. Lowering the M-EMS current intensity from 300 A to 100 A further reduced the equiaxed crystal ratio from 21.63% to 10.05% and decreased the length difference from 18.1 mm to 8.0 mm. These findings suggest that increasing superheat and decreasing M-EMS current intensity are effective strategies for enhancing the symmetry of the solidification structure in 20CrMnTiH gear steel blooms during continuous casting. Based on these results, an optimized process was proposed: with an M-EMS current of 100 A, superheat of 40°C, casting speed of 1.05 m·min−1, and a specific water amount of 0.33 L·kg−1, the resulting columnar crystal length was extended, and the symmetry was significantly improved. Consequently, the hardenability differences of the J9 and J15 of rolled products were minimized to 0.3 and 1.0 HRC, respectively, thereby meeting the numerical simulation control strategy and customer requirements.