Numerical simulation of dehydrogenation annealing in bloom

Due to moisture in the ore, auxiliary material, and ladle refractory material, the hydrogen element is easily enriched in molten steel. In the metallurgy process, some hydrogen atoms form bubbles and are removed by gravity, whearas others solidify in the strand and remain in the produced steel. When the hydrogen content reaches a certain critical value, the enriched hydrogen atoms congregate to produce a white spot, which greatly reduces the strength and toughness of the steel product, and leads to brittle fracture during its service period. At present, the RH (Ruhrstahl–Heraeus) and VD (vacuum degasser) refining processes are commonly applied in steel plants, which can reduce the hydrogen content to less than 2×10⁻⁶. With the demand for high quality steel, the hydrogen content must be further decreased, so hydrogen diffusion in solid steel during the annealing process is gradually attracting increasing attention. In this study, a two-dimensional model was built to investigate the characteristic of dehydrogenation in the bloom annealing process of rail steel. Moreover, the effect of annealing temperature and annealing time on hydrogen diffusion were analyzed, and the annealing parameters were optimized. During the dehydrogenation annealing process, the hydrogen content at the corners and edges of the bloom are found to decrease rapidly, while that in the center of the strand begin to decrease in the later heating stage. As the annealing temperature increases, the starting point of dehydrogenation in the bloom center moves ahead and the maximum dehydrogenation rate increases significantly. With the extension of the soaking period, the central hydrogen content of bloom decreases significantly, but the increase rate of the dehydrogenation gradually decreases. By optimizing the bloom annealing parameters, the hydrogen content in the bloom can be steadily reduced to 0.6×10⁻⁶, which fully meets the requirement of high quality steel production.