Effects of cooling intensity on MnS growth during solidification in ultra-high sulfur medium carbon steel billet
LÜ Nai-bing1,2, GAO Hang3, LIU Ke1,4, LIU Bin1, XU Shi-xin1, ZHOU Jie1
1. Research Institute of Technology, Shougang Group Co., Ltd., Beijing 100043, China; 2. Beijing Key Laboratory of Green Recyclable Process for Iron and Steel Production Technology, Beijing 100043, China; 3. Marketing Department, Guiyang Special Steel Co., Ltd., Shougang Group, Guiyang 550005, Guizhou, China; 4. Beijing Engineering Research Center of Energy Steel, Beijing 100043, China
Abstract：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 fs=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.
吕迺冰, 高航, 刘珂, 刘斌, 徐士新, 周洁. 冷却强度对超高硫中碳钢方坯凝固过程MnS生长的影响[J]. 钢铁, 2022, 57(6): 50-56.
LÜ Nai-bing, GAO Hang, LIU Ke, LIU Bin, XU Shi-xin, ZHOU Jie. Effects of cooling intensity on MnS growth during solidification in ultra-high sulfur medium carbon steel billet[J]. Iron and Steel, 2022, 57(6): 50-56.
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