金属凝固过程模拟

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Numerical simulationongradientcooling behavior of jumbo slab ingot

Abstract: Shrinkage porosity defect is often found in an air cooled jumbo steel ingot, which will influencethe quality of the final rolled plates. In practical production, some rolled plates are frequently rejected dueto 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.

Key words: gradient cooling; slab ingot; temperature field; shrinkage; numerical simulation

1 Numerical simulation method

ProCast software was used for simulation in this study. The solidification time of the jumbo ingot is much longer than tis filling time, so the initial temperature of the ingot was set as the pouring temperature T p, which was 1,580 ºC for the steel used in this study. The initial temperatures of the cast iron mould and the heat insulation plate were taken as the ambient temperature of 25 ºC.

Boundary conditions were set according to Q = h eff (T hot–T cold), where Q is heat flux; h

is equivalent interface heat transfer coefficient; T hot is the hot surface eff

temperature;and T cold is the cold surface temperature. The heat transfer coefficient between mould and ingot was 1,800 W·K-1·m-2.The heat transfer coefficient between heat insulation plate and ingot was 20 W·K-1·m-2. The upper half of the mould wall was air cooled at a heat transfer coefficient of 10 W·K-1·m-2. The lower half was spray cooled at a heat transfer coefficient of

1,000 W·K-1·m-2 [10].