Mechanism of Tempered Sorbite Formation and Related Enhanced Mechanical Properties for a Typical High Carbon Steel Billet Under Strong Cooling Intensity

This article investigated the solidification structure evolution for a typical high-carbon steel billet under strong cooling intensity during the process of continuous casting. The results suggested that a regular solidification phase transformation route of ‘austenite → lamellar pearlite’ was obtained in the center surface of billet; however, an abnormal microstructure evolution route of ‘austenite → martensite → tempered sorbite (fine spheroidized cementite in the ferrite matrix)’ appeared around the corner of billet. Besides, the secondary dendrite arm spacing for the center surface was 90.92 μm, and it reduced to 49.01 μm for the corner part because of different cooling conditions. In addition, the billet corner with a major tempered sorbite phase shows higher values of tensile strength, elongation and hardness, indicating sorbite phase could improve high carbon steel mechanical properties more significantly than the phase of lamellar pearlite because of its unique homogeneously distributed spheroidized structure. Furthermore, a laboratory experiment was conducted to verify the formation mechanism of tempered sorbite, and the results suggested that martensite phase would be formed around the billet corner under strong cooling intensity during the second cooling zone, and then the quenched martensite would be transferred to tempered sorbite, as the billet was out of the second cooling zone and reheated by its inner solidification heat.