【作者】 武会宾；

【导师】 贺信莱； 杨善武；

【作者基本信息】 北京科技大学 ， 材料物理与化学， 2005， 博士

【摘要】 微合金钢中采用组织细化技术得到的中温转变组织具备优良的综合力学性能。但这类组织属于非平衡组织,具有向平衡组织演化的自发趋势。当这类钢在受热(如回火或其它过程中被加热)时,若中温转变组织一旦粗化(演化为平衡组织),则其优良的性能将不复存在。因而研究细化的中温转变组织的热稳定性和影响热稳定性的因素具有重大的实用价值。本工作运用光学金相技术、透射电子显微术、硬度测量方法并结合热模拟实验,研究了弛豫-析出控制相变(RPC)技术参数对所得中温转变组织热稳定性的影响规律。具体内容包括:RPC技术参数(特别是弛豫时间)对工业生产的超细化低碳贝氏体钢回火组织与性能的影响,同时与控轧后空冷(AC)以及传统的再加热淬火(RQ)工艺生产的钢板回火组织及性能进行了比较。另外,分析了奥氏体区变形量和变形后等温弛豫时间对一种简单成分钢在重加热过程中组织演化行为的影响,并通过与四种作为奥氏体模型钢的铁镍合金对比,揭示了中温转变组织在受热过程中的演化机制。结果表明:不同RPC技术参数得到的钢板随回火温度升高均呈现软化-硬化-再软化的变化规律,只是各种钢的变化幅度及具体硬度值有所不同。经过AC工艺得到的钢板在回火后硬度和强度变化不明显,而经过RQ处理后的钢板随回火温度升高强度和硬度单调下降。回火前RPC和RQ两种工艺得到的钢板均为板条状贝氏体和少量粒状贝氏体的复合组织。随回火温度的升高或时间延长过程中,RPC工艺钢组织类型变化不明显,只是经过不同弛豫时间的试样组织粗化速度有所不同,而RQ工艺钢板条结构很快消失,并迅速演变成多边形铁素体。实验结果表明,利用RPC工艺得到的高强韧性钢板具有良好的组织热稳定性。这种稳定性起源于组织内特定的位错与析出相互作用状态。简单成分含Nb微合金钢在奥氏体区变形后等温弛豫不同时间再水冷,均可得到以贝氏铁素体为主的复合组织。但在各样品的贝氏铁素体内部,位错组态与应变诱导析出颗粒的分布状态明显不同。未经弛豫的样品中不存在应变诱导析出颗粒,弛豫很长时间的样品中析出颗粒基本分布在位错线之外,而在经过适当时间弛豫的样品中,析出颗粒基本分布在位错线上并对位错形成钉扎。在随后的650℃或700℃重加热等温过程中,这些非平衡组织呈现向平衡组织演化的趋势。弛豫60s的样品重加热等温时热稳定性最高,而弛豫1000s的样品,虽然重加热前硬度最低,重加热等温时组织演化却进行得最快。重加热等温过程中发生的组织演化是按板条内位错摆脱钉扎发生多边形化,板条间小角倾转晶界通过位错攀移而逐渐消失和发生再结晶形成多边形铁素体的次序进行的。更多还原

【Abstract】 The intermediate transformation products in microalloyed steels obtained by refiningtechnique possess excellent synthetic properties. However, as non-equilibrious microstructures,intermediate transformation products always exhibit a tendency to evolving into equilibriousones during reheated (tempered or reheated in other process). Their excellent properties will belost once they become equilibrious microstructures. For great practical signification of theproblem, it deserves to carry out an investigation on thermo-stability of intermediatetransformation products and the factors determining the property. In the present study,metallographic examination, transmission electron microscopy, hardness measurement alongwith thermal simulation test are employed to investigate the influence of Relaxation-Precipitation Controlling transformation (RPC) technique on the thermo-stability ofintermediate transformation products. Following work is undertaken in this study: Effect of theparameter of RPC technique on the microstructures and mechanical properties of an ultra finelow carbon bainitc steel tempered at different temperature have been investigated, andcompared with those of same composition steels which processed by AC (controlled rolling andair cooling) and RQ (reheating and quenching) technique. The effect of deformation andrelaxation time on microstructure evolution in a simple component steel, which was reheated atdefinite temperatures, were investigated. By comparison with four Fe-Ni alloys, which serve asaustenitic model steels, the evolution mechanism of intermediate transformation products wasdiscussed. It was found that:With the tempering temperature rising, the hardness and strength of all the RPC platesdemonstrate a fluctuation phenomenon, but the amplitude of hardness change and the effect ofhardening are different in samples relaxed for different time. The hardness and strength of ACplate do not obviously changed and those of RQ plate decrease faster as the temperatureincreased. Both of the main microstructures of RPC steel and RQ steel are lath-like bainite andlittle granular bainite before tempering. The microstructure of RPC steel does not changeobviously after tempering, except for the rate of microstructures coarsening are different insamples relaxed for different time. Otherwise, the boundaries of bainitic laths in RQ steeldisappear quickly and finally a polygonal ferrite occurred. These results indicate that the steelswith high strength and toughness manufactured by RPC technique possess high thermo-stability.It is derived from the different states of dislocation and precipitates interaction in steelsintroduced by various techniques.Cooled in water after isothermal relaxation of deformed austenite for different time, a Nb-bearing microalloyed steel always exhibits synthetic microstructures, among which bainiticferrite dominates. Dislocation configurations and distribution of strain induced precipitatesinside bainitic ferrite of samples relaxed for different time are distinct. The strain inducedprecipitates do not occur in no relaxed sample while they distribute outside dislocations insample relaxed for 1000s. The most of strain induced precipitates distribute along dislocationsand pin dislocations in sample relaxed for proper time. When these samples are reheated to andheld at 650℃ or 700℃, the non-equilibrious microstructures tend to evolve into equilibriousones. The sample relaxed for 60s displays the highest thermo-stability, while microstructureevolution is quickest in the sample relaxed for 1000s even though it is softest before reheating.Dislocations inside laths getting rid of pinning of precipitates and their polygonization play theprecursor to the evolution of microstructures during reheating and holding, followed by gradualdisappearance of lath boundaries caused by dislocation climb. Finally, recrystallization happensand polygonal ferrite appears. Pre-strain before reheating accelerates the evolution. By hardnessmeasurement, it is found that softening is not single process occurring during reheating, inwhich hardness fluctuates with time. There are two peaks, which correspond to further growingand coarsening of Nb(C, N) precipitates forming in austenite before intermediate transformationand re-nucleating of the precipitates in intermediate transformation products respectively, inhardness-time curve of each sample having undergone relaxation during reheating at definitetemperature, while single peak occurs in the curve of the sample without relaxed. These resultsindicate that: Thermal stability of intermediate transformation products in Nb-bearing steels isdetermined by interaction state between dislocations and precipitates, rather than by dislocationdensity. The interaction state mainly forms in deformed austenite and maintains afterintermediate transformation.Fe-Ni alloys as austenite model steels can simulate the interaction between the dislocationconfiguration and the precipitates. This interaction is accordant to that of the simple steelsundergone the same deformation and relaxation. These results give further evidence thatthermo-stability of intermediate transformation products in bainitic steels is determined byinteraction state between dislocations and precipitates.更多还原