20MnSi中夹杂物对针状铁素体形成的影响

通过热模拟试验对20MnSi连续冷却过程中的相变规律进行了测定,通过电石、硅钙线脱氧及热处理得到了含有晶内针状铁素体试样,利用显微硬度仪对针状铁素体聚集区进行了显微硬度的测定,利用光学显微镜对晶内针状铁素体进行了形貌观察,利用扫描电镜和能谱仪对诱导针状铁素体生成的夹杂物的性质进行了分析。结果表明,20MnSi中可以形成晶内针状铁素体的冷却速度范围为5~20℃/s;能够诱发针状铁素体组织形核的夹杂物主要为Mn S夹杂,其次为MnO·SiO_2和MnS·SiO_2夹杂,并且3类夹杂物的尺寸主要在小于3μm的区间内;MnS夹杂促进针状铁素体形核是由应力-应变能和惰性界面能等原因共同造成的;高温加热和等温保温有利于使贫锰区减弱或消失,不利于针状铁素体的形成;高熔点夹杂物有利于诱导针状铁素体的形核,复合夹杂物和镶嵌存在的夹杂物可以为针状铁素体的形核提供多个合适的形核区,有利于促进多个针状铁素体的同时形核、长大。

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,inclu-sions 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<3μm. The nucleation of IAF in-duced by MnS was determined by stress-strain energy and inert interfacial energy. Heating at high temperature and iso-thermal heat preservation may lead to a reduced or absent Mn-depleted zone(MDZ),which is not conducive to the for-mation 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.