Abstract: Heavy reduction is a important technology that can effectively improve the internal quality of the billets. However, effective prevention of cracks was very essential during the implementation of this mechanical reduction technology. In this study, formation mechanisms of cracks in the 72A steel ascast billets (180 mm×180 mm) with heavy reduction of 5～20 mm by a single pair of rollers were discussed. It was found that tendency of cracks was weaker during heavy reduction compared to soft reduction. With heavy reduction by 16 mm at a solid fraction of larger than 0.3by the single pair of rollers, no cracks were observed in the 72A steel billets. Based on the experimental result, formation mechanisms of cracks were proposed as following: compared to soft reduction, stress can be much more effectively transferred to the center of the billets. With the initiations of cracks during reduction, they tended to develop towards the center of the billets by single pair of rollers, nevertheless, the initiated cracks preferred to grow towards the shells of the billets during soft reduction by multi-pairs of rollers.

Key words: billet, heavy reduction, soft reduction, crack, mechanics, segregation