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V、N微合金化低碳贝氏体钢的成分设计与组织特征
【作者】 高飞；
【作者基本信息】 昆明理工大学 ， 材料学， 2011， 硕士
【摘要】 本文研究了V、N微合金化低碳贝氏体钢的合金成分设计及其连续冷却相变特性,设计并冶炼了七炉低碳贝氏体钢,利用Formastor-FⅡ全自动相变仪上测定其连续冷却转变曲线；并且为了提高V、N微合金化低碳贝氏体钢的综合力学性能,细化相变组织,运用Gleeble-1500D热模拟试验设备进行一系列的热模拟实验,利用金相显微镜、扫描电镜、透射电子显微镜进行组织结构及析出物的观察；最后对V、N微合金化低碳贝氏体钢进行轧制,测定实验钢的力学性能,得到下述主要研究结果：在含V的低碳贝氏体钢中,加入微量的B元素,可使实验钢与Nb-B系低碳贝氏体钢类似,即在空冷的冷速下便可以得到完全的贝氏体组织；而去掉元素B后,含V低碳贝氏体钢的贝氏体淬透性降低,生成贝氏体组织的临界冷却速度升高。而向钢中加入合金元素Mo、Cr可以提高实验钢的贝氏体淬透性,降低生成贝氏体组织的临界冷速和贝氏体转变温度,提高钢的硬度,但相比之下,Mo的作用要优于Cr。实验钢经过热模拟变形处理后发现,随着变形量的增大,板条尺寸减小,组织明显细化；而实验钢经变形后,保温时间的变化也将影响相变组织的尺寸,保温时间在120s时,板条束的细化效果较好；另外在不同温度下变形后直接水冷试样的板条组织均较粗大,但经保温弛豫120s处理后,贝氏体束的尺寸明显变小。变形后经保温弛豫处理的试样中分布着许多V(C,N)析出粒子,这些析出物有的在原奥氏体晶界处形成,在钢中主要起到限制高温奥氏体长大的作用,还有些析出粒子在晶粒内部形成,在冷却过程中可以促进贝氏体的形核,细化板条组织。在低碳贝氏体钢中加入V、N元素,冷却后得到的贝氏体板条尺寸明显减小,且屈服强度要比未加V、N的实验钢高出90MPa,维氏硬度也达到了300HV。对于V-N微合金化低碳贝氏体钢,终轧后空冷弛豫25s处理的实验钢,其贝氏体板条尺寸要明显小于未弛豫的试样,且其轧后及回火后的强韧性均要优于未弛豫的实验钢。经终轧弛豫后的实验钢中有较多的V(C,N)析出粒子形成,促进了针状铁素体的形成；通过再结晶细化奥氏体晶粒以及针状铁素体对原奥氏体晶粒的分割作用,细化了组织,改善钢的综合性能。
【Abstract】 In this paper, the composition design and transformation characterstics during continuous cooling of vanadium and nitrogen microalloyed low carbon bainitic steels were researched. Seven experimental steels were prepared and CCT curves were determined on Formastor-F II automatic transformation measuring apparatus. For increasing comprehensive performance and refining microstructures of vanadium and nitrogen microalloyed low carbon bainitic steels, thermal simulation experiments were tested by Gleeble-1500. And microstructures and precipitates were observed by Microscope, SEM and TEM. Finally experimental steels were hot rolled and mechanical properties were tested. Results as follows:Similar to Nb-B steel, the steel containing V and B can get fully bainitic structures at the air cooling rate. Removing B from low carbon bainitic steel containing V leads to lower bainite hardenability. And adding Mo and Cr into these steels can improve bainite hardenability, lower critical cooling rate VC-95%B, decrease the bainite transformation temperature and increase hardness.But compared to Cr, Mo is the more effective element to promote bainite transformation.It is found from the results of thermal simulation that the size of laths is reduced and microstructures are refined with increasing deformation. The holding time after deformation will affect the size of microstructures and the size of lath is better refined when the holding time is 120s. The microstructures were thick in samples with different deformation temperature, but after holding 120s after deformation the size of bainite laths is reduced. In addition, these are V(C,N) precipitates in samples with holding, and the precipitates formed in grain boundries can inhibit grain growing and precipitates formed in the grain can improve strength and promote the nucleation of bainite, refine microstructures.The size of bainite laths is smaller after cooling in low carbon steel contain vanadium and nitrogen, and yield strength is 90MPa higher than the steel without vanadium and nitrogen. Also the size of bainite laths is smaller in low carbon steel contain vanadium and nitrogen with relaxation 25s after hot rolled than the steel without relaxation, and comprehensive performance is superior to the steel without relaxation. There are some V(C,N) precipitates which can promote the formation of acicular ferrite in the steel with relaxation 25s. The effect of recystallization and acicular ferrite partitioning on austenite grain can refine microstructure and improve mechanical properties.