Influence of the soft reduction process on the sensitivity of the inner crack in heavy rail steel bloom

Continuous casting technology has greatly improved production efficiency; however, in the continuous casting process of heavy rail steel, problems of center segregation, center porosity, and shrinkage cavity of the bloom occur, which notably affect the billet quality of heavy rail steel. The soft reduction technique can effectively improve these problems, but internal cracks will appear, and the quality of the bloom will deteriorate if the parameters are not properly set. To examine the inner crack induced by the soft reduction of continuous casting bloom, this study established a mathematical model for the soft reduction of a 230 mm×280 mm section bloom by the ABAQUS finite-element software. The thermal-stress coupling model was used to predict the inner crack of the heavy rail steel bloom, using the soft reduction model during the soft reduction process. First, the temperature at different central solidification fractions of 0.3–0.7 in the bloom was calculated. The equivalent plastic strain that was located at the solidification front at different central solidification fractions was then calculated by the soft reduction model. Results indicated that the equivalent plastic strain at the solidification front in the bloom did not exceed the critical strain of 0.4% at the central solidification fractions with a reduction amount of 7 mm. Moreover, the inner crack did not occur at the solidification front. Simultaneously, the soft reduction model calculation with a different reduction amount was conducted at the central solidification fraction of 0.6. Results showed that the equivalent plastic strain at the solidification front exceeded the critical plastic strain of 0.4% when the reduction amount was more than 7 mm, which resulted in an inner crack. The greater the reduction amount, the more serious is the inner crack. Concurrently, industrial experimental results had a good agreement with the model calculation results.