MnS Precipitation Behavior of High-Sulfur Microalloyed Steel Under Sub-rapid Solidification Process
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HIGH-STRENGTH medium carbon sulfur-containing microalloyed steels have been widely used in hot forging parts of automobiles, such as crankshafts and connecting rods, due to the advantage of energy-savings with elimination of traditional quenching and tempering processes.[1,2] During the continuous casting process of this type of steels, the precipitation behavior of MnS is crucial, as MnS precipitates are good lubricants for improving the cutting performance of microalloyed steels. MnS precipitates in as-cast steel slabs can be typically classified according to the morphology: globular MnS (Type I); fine rod-like MnS (Type II); and angular MnS (Type III).[3] It is well known that the mechanical properties of high-sulfur steels are closely related to the MnS precipitates’ shape and distribution.[4] In traditional continuous casting of sulfurized steels, the size of MnS precipitates is generally larger
WANLIN WANG, CHENYANG ZHU, JIE ZENG, CHENG LU, PEISHENG LYU, HAIRUI QIAN, and HUI XU are with the School of Metallurgy and Environment, Central South University, Changsha 410083, P.R. China and also with the National Center for International Research of Clean Metallurgy, Central South University, Changsha 410083, P.R. China. Contact e-mail: [email protected] Manuscript submitted August 8, 2019.
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than 10 lm.[5] In order to obtain better cutting performance, the blooms require prolonged heat treatments to decompose the MnS precipitates into finer rod-like shapes (Type II) having a mean length lower than 5 lm.[6] This extended heat treating processes would consume significant amounts of additional energy. Therefore, less energy-intensive new production methods to ensure finely dispersed MnS inclusions in high sulfur-containing microalloyed steels are necessary. Lower sulfur segregation and finer as-cast microstructure with increasing of solidification cooling rates could reduce the precipitation and growth of sulfide.[7,8] As the only industrialized sub-rapid solidification process, strip casting is an important technological revolution for the steel industry, which can produce thin strips directly from the liquid metal. Strip casting has the potential to greatly reduce operating and investment costs through the elimination of multiple rolling steps.[9,10] Strip casting has been known to provide solutions for steels with difficult casting issues including macro-segregation, precipitation of large inclusions, larger structures. Due to the rapid cooling experienced during strip casting, the morphology of as-cast microstructure could be significantly refined.[11,12] Electrical steels,[13] TRIP steels,[14] dual phase (DP) steels,[15] and other special steels with complex and non-uniform morphologies have been identified as potential products applicable for strip casting. Some past publications have
reported the formation of fine manganese sulfides during rapid solidification of low-sulfur steels, such as stainless steels and high-strength low-alloy steels.[8,16] However, there has been li
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