A Study Of Theory And Technology On Soft Reduction For 350mm×470mm Bloom Casting

Since 21th century,a number of bloom casters in the domestic steel mills have been newly built.Comparing with the traditional medium and small billets,the quality of the bloom is more concentrated in internal quality.By soft reduction technology for continuous casting,the inner defects of bloom such as center porosity and central segregation,can be effectively solved.Therefore,in order to improve the internal quality of the bloom,soft reduction technology becomes particularly important.The technologies related to soft reduction include casting solidification model,strain model of high temperature casting slab,the model of slab segregation,and the soft reduction parameters.Aiming at the bloom caster in the steelmaking plant of Benxi Steel,the solidification model and soft reduction strain model are developed,and the solidification and reduction process are studied to optimize the operation parameters of the soft reduction process.In addition,a large number of industrial tests were implemented in order to verify and determine optimum soft reduction parameters.The following conclusions are obtained.(1)The solidification process for 350mm×470mm bloom casting is numerically simulated,and the results are verified by nail-shooting test.The appropriate instantaneous heat flux of cooling in the bloom mold is determined as:q-2680-270(?)for bearing steel casting.(2)The strain evolution during the soft reduction of bloom strand is analyzed.Besides the reduction strain,there are also bulging strain,straightening strain and thermal strain.The formula to calculate the soft reduction strain suitable to bloom caster is modified as follows:?M=300?abs?M/l2fs(3)Based on the strain model,the strain evolution during reduction of bearing steel bloom is analyzed.The results show that both the bulging strain and straightening strain increase with the increase of casting speed.The soft reduction strain is proportional to the casting speed.The largest soft reduction quantity at a single roller is suggested as not more than 4mm.(4)As reduction quantity on bloom is less than 3 mm,the minimum central solid fraction in soft reduction zone should be greater than 0.40 to avoid internal crack defect.(5)The suitable soft reduction rate range is 0.84?0.90 mm/min under the casting condition of Benxi Steel.(6)With the optimized operation parameters of soft reduction,the inner quality of cast bloom is improved significantly.The ratio of bloom macrostructure without shrinkage cavity defects for bearing steel is more than 95%.The rolling bar of diameter below 130 mm for bearing steel can meet level B ultra-sonic detection.The rolling bar of diameter below 250 mm for medium carbon steel can meet level B ultra-sonic detection,and that of diameter below 200 mm can meet level A ultra-sonic detection.