Influence of inclusions on fromation of acicular ferrite in 20MnSi steel

Phase transformations of 20MnSi steel during a continuous cooling process were examined through thermal simulation experiments. Samples with intracrystalline acicular ferrite (IAF) were obtained via deoxidization by adding CaC₂, silicon-calcium wire and heat treatment. Additionally, the microhardness of the IAF area was determined through a microhardness tester, and the IAF microstructure was observed with an optical microscope. The inclusion properties that induced IAF nucleation were analyzed by a scanning electron microscope and energy dispersive spectrometry. The results showed that the cooling rate of IAF formation in 20MnSi steel was in the range of 5-20℃/s. Furthermore, inclusions that could induce the nucleation of intracrystalline acicular ferrite were mainly composed of MnS, followed by MnO·SiO₂ and MnS·SiO₂. The size of the three types of inclusions were primarily <3 μm. The nucleation of IAF induced by MnS was determined by stress-strain energy and inert interfacial energy. Heating at high temperature and isothermal heat preservation may lead to a reduced or absent Mn-depleted zone (MDZ), which is not conducive to the formation of acicular ferrite. Inclusions with high melting temperature could contribute to the nucleation of acicular ferrite. Composite inclusions and inclusions as inlays may be able provide more suitable nucleation areas for acicular ferrite, and promoted the nucleation and subsequent growthof the additional acicular ferrite.