Search Results (4)

Heavy reduction (HR) is an effective technique to control V segregation in continuous casting bloom, but the effect of segregation improvement is limited by the parameters such as reduction position and reduction amounts. In order to improve the macrosegregation of bloom, numerical simulation and plant experiments are adopted in this research. A heat transfer model and a reduction model with comprehensive thermo-physical parameters were established. The two models were verified by comparing the measured surface temperature and the theoretical strain at the solidification front. It is determined that the position of the HR of the bearing steel bloom is 20.82 m~24.97 m from the meniscus, and the solid fraction in the center of the bloom is 0.6~1. The total reduction of the HR is set to 30 mm, and the reduction of each roller in the reduction range is set to 4 mm, 5 mm, 9 mm, 7 mm, and 5 mm, respectively, to prevent the formation of internal cracks. Plant trials were conducted to verify the effect of the optimized HR. The results show that the carbon segregation degree on the V channel and non-channel of the bloom decreases from 1.2 to 1.16 and increases from 0.93 to 0.95, respectively, and the central carbon segregation degree decreases from 1.17 to 1.15. Meanwhile, the internal crack was not found in the bloom. Full article

Severe temperature gradients and inhomogeneous strain distribution exist in the large cross-section of GCr15 bearing steel during the hot bar rolling process, resulting in a complex microstructure evolution in the bar. To promote the performance of the bar, a thermal-mechanical coupled finite element (FE) model was developed to capture the variations in temperature and deformation strain. A subroutine, considering the dynamic recrystallization (DRX), meta-dynamic recrystallization (MDRX), static recrystallization (SRX), and grain growth (GG) of austenite grains of GCr15 steel, was developed and coupled to the FE model to predict the microstructure’s evolution during rough rolling. The simulation implies that the inner part of the bloom is deformed at high temperatures due to the heat generated by plastic deformation and slow heat conduction, while the surface temperature decreases along with the passes. The heavy reduction design with 11 passes was found to introduce higher strains at the center regions than those of the same rough rolling reduction divided into 13 passes. The higher strains at the center regions refined the grain size and promoted microstructure homogeneity. The observation of the microstructures after hot bar rolling confirmed the refinement of the heavy reduction design for rough rolling. Furthermore, the heavy rough rolling reduction was found to be beneficial for alleviating the macrosegregation of the casting bloom. Full article

Industrial experiments of mechanical soft reduction in continuous casting were conducted in the present study aiming to improve the internal quality of the bearing steel blooms. Two methods were developed to verify the solidification model for a reliable crater end in the caster, which is provided by SMS CONCAST. The verified solidification model was applied to determine the solidification status of the bloom and provides theoretical reduction region. Several trials were conducted to study the optimization of the reduction rate regarding the V-shaped and centerline segregation of the bloom. The results show an obvious improvement of internal quality in the bearing steel bloom by applying appropriate reduction during casting. Full article

Macrosegregation remains one of main defects affecting metal materials properties, which is mainly caused by interdendritic fluid flow during solidifying. However, as for controlling actual specific segregation, it is still difficult to effectively measure or simulate this kind flow instead of pure fluid flow, especially in complex casting processes of high-grade materials. Herein, a new method for obtaining velocity magnitude and direction of interdendritic fluid flow during metal solidifying is proposed from boundary layer and standard deviation obtained by measuring etched surface heights of the actual ingot and using statistical principles. Taking continuous casting bloom of GCr15 bearing steel as an example, it is indicated that the calculated velocity magnitudes under different sides and superheats can be explained by process features and, hence, solidification mechanism. The velocity magnitude and fluctuation are higher on the inner curve side and under low superheat. Meanwhile, it is found that the fluctuation extent of secondary arm spacing is more relevant with interdendritic fluid flow, although its magnitude is mainly determined by the cooling rate. Moreover, on the basis of the calculated velocity directions and magnitudes, there is a positive correlation between segregation area ratio and the effective ratio between interdendritic flow velocity and growth velocity especially in the equiaxed grain zone, which corresponds with classic macrosegregation formation theory. The above findings and comparison with other results demonstrate the validity of the new approach, which can obtain the magnitude and the direction of interdendritic fluid velocity for two or three-dimensional multiscale velocity distribution by tailoring measuring length and numbers. Full article