【作者】 赵四新；

【导师】 毛大立； 王巍；

【作者基本信息】 上海交通大学 ， 材料物理与化学， 2007， 博士

【摘要】 钢中贝氏体相变的温度范围较宽,可形成广泛形貌的贝氏体组织。深入了解贝氏体相变机制对钢种研制有指导性意义。本文采用了具有较广泛适用性的微观组织定义,研究符合该定义的微观组织的形成机制。基于Boltzmann分布,应用Cottrell基本位错理论,推导出更为精确的计算刃型位错处可富集碳原子个数的公式。计算了奥氏体中单个和多个刃型位错处富集的碳原子个数,其随等温温度降低和奥氏体中碳浓度增加而增加。建立了描述刃型位错处富集碳原子个数及其周围贫碳区半径随时间变化关系的模型。计算了贝氏体预相变期间,碳原子向位错处富集过程和贝氏体的孕育期,理论值与实验值吻合较好。通过比较刃型位错附近的贫碳区半径和切变形核的临界半径,并分析了等温温度、界面能、位错形态、应变能等影响因素,发现温度较低时,下贝氏体可能切变形核,温度较高时,切变形核几乎不可能。分析了热力学和动力学参数,用Zener-Hillert和Bosze-Trivedi公式,计算了合金含量不同的钢的贝氏体铁素体长大速度。Fe-C合金的贝氏体铁素体长大速度符合Zener-Hillert公式,合金含量较低的钢的实验值略低于理论值,Fe-C-8.7Ni wt.%的贝氏体铁素体长大速度比理论值约慢2个数量级。扩散控制动力学方程可描述部分而非全部钢种的贝氏体铁素体长大速度。通过对界面条件和溶质拖曳效应等因素的分析,提出贝氏体相变机制可能与成分相关。基于贝氏体相变的切变机制,改进了最大形核驱动力和自催化因子的计算方法,建立了描述贝氏体体积分数随时间变化的模型。理论值和实验结果吻合较好。根据切变理论获得的最大贝氏体体积分数在较高温度时与实验值有较大偏差。提出贝氏体相变可能与温度相关。研究了Nb含量不同对贝氏体相变的影响。在30℃/s和20℃/s冷却时,Nb含量从0.01 wt.%增加到0.2 wt.%,会使Bs点升高约20℃,但是对B f的影响不明显;连续冷却过程中,先形成粗大的贝氏体铁素体片层,后继形成的贝氏体铁素体片层逐渐变细;Nb的碳化物的形成可能与Nb的扩散相关。实验用钢在30℃/s和20℃/s连续冷却时,贝氏体的形成可用扩散机制较好地解释。通过分析成分和温度对贝氏体相变的影响,提出一个可能的相变机制:如果钢中的合金元素含量较低,在较高温度等温,贝氏体可能以扩散方式形成,随着温度的降低,贝氏体以切变方式形成;如果钢中的合金元素含量很高,温度较高时,由于合金元素的溶质拖曳效应,可能难以通过扩散方式形成贝氏体组织,等温温度较低时,贝氏体可能以切变方式形成。更多还原

【Abstract】 Bainite can be formed at wide range of temperature in steels with various microstructures. A deep understanding of the bainite transformation mechanism is necessary for the development of novel steels. Based on the analysis of the existed definition about bainite, the paper focuses on the transformation mechanism of the products according with the general microstructure definition of bainite.Based on Boltzmann distribution and dislocation theory proposed by Cottrell, an exact formula was derived for calculating the carbon atoms number aggregating at edge dislocations, which increases with decreasing isothermal temperature and increasing carbon concentration.A model was developed to describe the relationship between carbon atoms drifting to edge dislocations and isothermal holding time. Furthermore, the radius of carbon depleted zone around edge dislocation was obtained. The theoretical results of aggregation process of carbon atoms to edge dislocations and bainite incubation time are consistent with the experimental data. By comparing the radius of carbon depleted zone with critical radius of displacive nucleus and considering the isothermal holding temperature, interfacial energy, dislocation morphology and stored energy etc., it is found that lower bainite displacive nucleation probably occurs at lower temperature rather than at higher temperature.The growth rates of bainitic ferrite in steels with varying alloy concentration were calculated by Zener-Hillert and Bosze-Trivedi equations after reevaluating the thermodynamic and kinetic parameters. A good agreement between experimental and theoretical results was found in Fe-C alloy. A slight overestimate of the theoretical results was observed in medium alloyed steels. However, the experimental data are two orders lower than theoretical ones in Fe-C-8.7Ni wt.% alloys. Taking the interface condition, supersaturation and solute drag effect into account, it is suggested that the bainite transformation mechanism may be related to steel composition.A kinetic model based on displacive mechanism was proposed after modifying the maximum nucleation driving force and autocatalysis nucleation factor. A satisfactory agreement with experimental data indicates that the model is capable of representing the relationship between the formation of bainite and isothermal holding time. It was found that the deviation of the theoretical maximum bainite fraction and the experimental ones is large at higher temperature. It is implied that the bainite transformation mechanism may be associated with transformation temperature.The effects of Nb content on the bainite transformation were studied. It is shown that Bs is increased about 20℃with cooling rates of 30℃/s and 20℃/s when the Nb content is raised from 0.01 to 0.2 wt.%. No significant effect of Nb content on Bf was observed. Upon continuous cooling the coarser bainitic ferrite plates are formed, followed by the formation of thinned bainitic ferrite plates. The process of the formation of NbC may concern with the diffusional process of Nb. The experimental results indicate the bainite formation is concerned with diffusional process.Coupling the obtained results, a possible mechanism of bainite transformation was suggested. If the carbon and alloy content are low, bainite may be transformed by diffusional processes at higher temperature. With decreasing the temperature, bainite transformation may proceed by displacive mechanism. In the case of higher carbon and alloy concentration, bainite transformation would not be expected by diffusional process due to the solute drag. However, bainite may be transformed displacively at lower temperature resulting from sufficient driving force.更多还原