Effects of cooling intensity on MnS growth during solidification in ultra-high sulfur medium carbon steel billet

Medium-carbon ultra-high sulfur free-cutting steel SAE144 is a structural steel with both mechanical properties and machinability,thus used as automotive engine sealing valves, etc. The production process is mostly converter/electric furnace→LF refining→continuous casting→wire rod hot rolling→cold drawing and machining,the market popularity has steadily increased in recent years. If the MnS inclusions in the steel are not properly controlled, problems such as failure in flaw detection, poor machinability, severe band structure, significant dissimilarity in mechanical properties, and even drawing fractures are likely to occur. MnS inclusions are mostly formed in the late stage of billet solidification and are deformed synchronously with the steel matrix during rolling. Controlling the original size of MnS in the billet has become the most critical step to control the morphology and size of MnS inclusions in hot-rolled products.In order to control MnS inclusions in hot-rolled ultra-high sulfur medium-carbon steel wire rod, the segregation of S element and generation, growth and ripening of MnS was analyzed by the billet solidification numerical simulation calculation, second phase precipitation theory and Ostwald ripening theory in 160 mm² billet. The calculation results show that MnS is generated at the end of the solidification in cast billet when solid phase fraction f_s=0.446 with the segregation ratio of S reaches 2.19. The growth process of MnS during solidification determines the diameter of MnS particles in the billet. The theoretical calculation shows that when the secondary cooling water volume is fixed to 0.6 L/kg, the MnS in the 160 mm² billet center grows up to 30.6, 32.2 and 34.6 μm at the casting speed of 1.6, 2.1 and 2.6 m/min respectively, which is consistent with the actual test results. The key to controll the size of MnS in wire rod is to increase the cooling intensity of the secondary cooling zone and reduce the casting speed of the billet. Based on the calculation method in this paper, control range of continuous casting process parameters matching the MnS diameter in 160 mm² billet is presented.