脉冲强磁场对低合金钢贝氏体转变的影响及机理研究

Magnetic field has been reported to have favorable effects on solid-state phase transformation and improve its sturcture and properties by utilizing the response of different structure on the magnetic field. Little is known on the effects of pulsed intense magnetic on bainite transformation in low alloy steels. Comparing with intense magnetic field with the same power, the pulsed intense magnetic field can obtain higher magnetic density. This proposed research will study the effects of pulsed intense magnetic field on bainite transfrmation in low alloy steels under continuous cooling. Samples will be treated with pulsed intense magentic during the incubation of bainite, then undergo bainite transformation in continous cooling, which will be observed under confocal scanning laser microscope. Of particular interests are the effects of applied direction and parameters of pulsed intense magentic on the nucleation of bainite and precipitation of carbides, their morphologies, orientations and distributions characteristics. Principles of electromagnetics, ferromagnetics and crystallography will be applied to analyze the thermodynamics, kinetics and mechanisms of bainite transformation, yielding a theoretical base for the application of pulsed intense magnetic field in the heat treatment of steels. A physics-based mathematic model of bainite transformation will be developed considering the effects of pulsed magnetic field. Coupled with numerical simulations of heat treatment, the bainite transformation model developed from this research can be applied to predict microstructure evolution and properties in heat treatment of steels. Consequently, this proposed research impose both scientific significance and practical values.

利用钢中不同组织对磁场的响应，将磁场作用到钢的固态相变过程，进而改善其组织与性能的研究日益受到广泛关注。但将脉冲强磁场施加到低合金钢的贝氏体转变过程中还鲜有报道。与相同功率的强磁场相比，脉冲强磁场可以得到瞬间更高的磁场强度。本项目在低合金钢的连续冷却过程中施加不同方向、不同参数的脉冲强磁场，研究脉冲强磁场对低合金钢贝氏体转变过程中铁素体和碳化物的形核特点、两相形态、分布及位向关系的影响规律；在低合金钢的贝氏体孕育期施加脉冲强磁场，借助高温共聚焦激光扫描显微镜，对连续冷却过程中的贝氏体转变进行原位观察，研究脉冲强磁场预处理对低合金钢贝氏体转变过程的影响规律；利用电磁学、铁磁学及贝氏体相变原理，结合数值模拟方法，对脉冲强磁场作用下贝氏体转变热力学、动力学和相变机理进行深入研究，为低合金钢的热处理过程中应用脉冲强磁场技术，进而改善贝氏体组织与性能提供理论依据，故研究具有重要的科学意义和实际意义。

脉冲强磁场作用下钢的贝氏体相变的研究可旁证贝氏体相变机制。本项目首先选用SA508-3钢作为研究对象，在SA508-3钢连续冷却过程中施加不同参数的脉冲磁场，并与相同工艺的正常热处理比较，研究脉冲磁场处理、脉冲电流处理和脉冲电流+脉冲磁场复合处理对粒状贝氏体组织形态、M/A岛的形态与分布的影响。结果发现，在SA508-3钢的连续冷却过程中施加脉冲磁场，粒状贝氏体中的M/A岛数量增加，尺寸变小，分布更均匀。与正常热处理相比，脉冲电流加热处理通过降低奥氏体中贫碳区的数量抑制贝氏体转变。与脉冲电流处理相比，脉冲电流+脉冲磁场复合处理抑制了贝氏体转变，促进了马氏体转变，粒状贝氏体中M/A岛的数量增多，尺寸减小，分布更均匀。.采用高温共聚焦激光扫描显微镜原位观察SA508-3钢的连续冷却贝氏体转变过程，研究脉冲电流+脉冲磁场复合预处理对SA508-3钢连续冷却过程中贝氏体的形核、生长方式、生长速度的影响。结果发现，脉冲电流+脉冲磁场复合预处理通过促进SA508-3钢连续冷却过程中Fe(N,C)的形核与长大提高BS温度，改变贝氏体的初始形核位置，增加贝氏体的生长速度。.选用Cr5钢为研究对象，在Cr5钢的等温贝氏体转变过程中施加不同参数的脉冲强磁场，并与相同工艺的正常热处理比较，研究脉冲磁场处理对Cr5钢中下贝氏体的组织形态、碳化物和铁素体的分布与位向关系的影响。结果发现，与正常处理相比，在等温过程施加脉冲磁场促进了Cr5钢等温贝氏体转变。与正常处理相比，脉冲电流加热处理抑制了Cr5钢的等温贝氏体转变。与脉冲电流处理相比，脉冲电流+脉冲磁场复合处理对Cr5钢等温贝氏体形核和长大的抑制作用更加明显。脉冲磁场处理后Cr5钢中贝氏体铁素体和母相奥氏体具有K-S取向关系。.本项目的实验结果中，脉冲电流加热处理抑制了贝氏体转变以及脉冲电流+脉冲磁场复合预处理对贝氏体转变动力学的影响均旁证了贝氏体转变的切变机制。同时，本项目的实施，对完善贝氏体钢的热处理工艺，具有重要的科学意义和实际意义。