Numerical simulation on gradient cooling behavior of jumbo slab ingot

Shrinkage porosity defect is often found in an air cooled jumbo steel ingot, which will influence the quality of the final rolled plates. In practical production, some rolled plates are frequently rejected due to the serious shrinkage porosity of the ingot. To improve the quality of the ingot, a new cooling method, gradient cooling process (in which the upper part of the ingot is air cooled and the lower part is spray cooled) was put forward in this study. The solidification behaviors for a 60 t jumbo slab ingot under gradient cooling condition were simulated using the ProCast software, and the results were compared with those of an ingot by air cooling condition. The solidifying tendency, temperature field and distribution of shrinkage porosities in the ingot under different cooling conditions were analyzed. Simulation results show that under gradient cooling condition the solidification of the slab ingot progresses in an upward manner along the vertical z axis and in a centripetal manner along the horizontal x and y axes. Gradient cooling can efficiently reduce shrinkage porosity of the jumbo slab ingot by optimizing the solidification sequence, and making the position of shrinkage porosity move from near the middle height of the ingot (under air cooling condition) towards the head of the ingot; and the secondary shrinkage is eliminated. In addition, the solidification time of the ingot under gradient cooling is 7.3 h in this simulation, which is 2.7 h faster than that under air cooling. A 60 t jumbo slab ingot was successfully produced under gradient cooling condition. The ingot was rolled to a plate with a thickness of 100 mm and length of 18,000 mm, and ultrasonic flaw detection was performed. Some porosity was found along the axis of the plate at 4,900 - 6,000 mm from the head of the plate, indicating that the position of the defect is moved towards the head of the ingot. This distribution trend of the defect is consistent with the simulated result.