高强电脉冲和磁脉冲对冷轧中碳钢组织细化的影响余文鹏硕士论文.docx

1、高强电脉冲和磁脉冲对冷轧中碳钢组织细化的影响余文鹏硕士论文学校代码：10488学 号：08102033硕 士 学 位 论 文 题 目高强电脉冲和磁脉冲对冷轧中碳钢组织细化的影响专 业材料学研究方向金属材料的相变与强韧化姓 名余文鹏导 师吴开明定稿日期： 2011年5月10日School Code: 10488Student ID: 08102033Wuhan University of Science and TechnologyMaster ThesisSubject:Effect of Electropulsing and Magneticpulsing on Grain Refineme

2、nt of Cold-Rolled Medium Carbon SteelsMajor:Materials ScienceResearch Field:Phase Transformations on SteelsMaster Candidate:Yu Wenpeng (余文鹏)Supervisor:Wu kaiming (吴开明)Date: May 10, 2011分类号：密级：UDC ：硕 士 学 位 论 文Effect of Electropulsing and Magneticpulsing on Grain Refinement of Cold-Rolled Medium Carbo

3、n Steels余文鹏指导教师姓名：吴开明教授武汉科技大学申请学位级别：硕士专业名称：材料学论文定稿日期：2011年5月10日论文答辩日期：2011年5月24日学位授予单位：武汉科技大学学位授予日期：答辩委员会主席：郭爱民 教授级高工评 阅 人：张莉芹 副教授 王红鸿 副教授Originality Statement and Copyright Authorization摘 要 高强的电脉冲分别作用于热轧态和冷轧态的中碳钢，产生了完全不同的效果。对于冷轧态的样品，电脉冲起到了明显的细化组织的效果，提高了样品的抗拉强度和伸长率。对于热轧态的样品，电脉冲对样品的作用效果不明显。这说明在电脉冲影响组

4、织的过程中，位错起到了非常重要的作用。上述现象可以用组织转变的热力学和动力学基本理论来解释。 在室温条件下，不同电流密度和不同峰值场强的高强电脉冲和磁脉冲作用在冷轧态和热轧态中碳钢65Mn和30CrMo样品上。实验结果表明：同热轧态的样品相比，冷轧态的65Mn和30CrMo在电脉冲的作用下，抗拉强度提升，塑性明显改善。从显微组织观察结果可以看出，电脉冲处理后冷轧组织中的珠光体团和铁素体尺寸明显小于没有经过电脉冲处理的样品，也小于电脉冲处理热轧态的样品。珠光体组织的局部可见珠光体片出现球化。珠光体团的细化效果随着电脉冲电流密度增加逐渐加强。这种组织细化效果的机理可能跟电脉冲作用下增加的电致形核率

5、有关。高强磁脉冲在室温下作用于冷轧态和热轧态中碳钢65Mn和30CrMo样品上，直至峰值磁场强度提高至35T，样品的显微组织也没有出现可以观察到的变化。由此可见，在本研究中未发现高强磁脉冲对组织的细化作用。 本文讨论了经典的形核理论在电脉冲作用下发生的改变和作用机理，电脉冲导致的过冷度改变继而影响形核过程也进行了讨论。电脉冲所产生的电磁力和焦耳效应也都被考虑可能会是影响形核的因素。组织细化效果的机理可以认为是电脉冲降低了形核势垒，从而增加了形核率。在室温条件下，冷轧态中碳钢的组织得到细化，不太可能是电磁力或者焦耳热效应的结果。电脉冲降低形核势垒是组织细化的主要原因，而且细化效果跟电脉冲的周期和

6、峰值电流密度有关。关键词：钢；显微组织；珠光体；电脉冲；磁脉冲；组织细化AbstractApplication of electropulsing to the hot-rolled and cold-rolled medium carbon low alloy steels has generated completely different effects. For cold-rolled steel samples, electropulsing treatment caused microstructure refinement and hence the increments of

7、tensile strength, yield strength and elongation. For hot-rolled steels, the effect of electropulsing was found to be negligibly small. This proves the importance of dislocations in the micromechanism of electropulse-induced microstructure transformation in metals. The experimental observations are e

8、xplained within the frameworks of thermodynamics and kinetics of microstructure transformation.Electropulsing and magneticpulsing were applied to hot-and cold-rolled medium carbon steel 30CrMo and 65Mn with varying peak electric voltages and currents at room temperature. The mechanical property test

9、 results of cold-rolled 30CrMo and 65Mn showed that the elongation and area reduction of the specimens treated with electropulsing increased, compared with that without the treatment of electropulsing. Microstructural observations revealed that the pearlite colonies and ferrite grain in the specimen

10、 treated with electropulsing was smaller than that without the treatment of electropulsing. It was also found that the lamellar structure of pearlite was refined. The grain refinement of pearlite was more obvious when the applied electric current density increased. The grain refinement may be attrib

11、uted to additional free energy for the nucleation of pearlite provided by electropulsing. The hot-rolled specimens had no noticable change. Magneticpulsing also had no noticable effect on microstructure in medium carbon low alloy steel. It is likely that the strength of the magneticpulsing was not s

12、o strong enough to induce the microstructure transformation. According to classical solidification theory and the mechanisms of solidification microstructure under pulsed electric current, the effect of underclooling degree on nucleation rate was discussed. The influence of electromagnetic stress ef

13、fect and Joule heat effect on undercooling degree were investigated. The relation between electric current and undercooling degree was devived. The effect of electropulsing on nucleation rate was due to nucleation potential barrier decreased by electropulsing. The electropulsing influenced the micro

14、structure of the cold-rolled steel at room temperature, which was not determined by the effect of electromagnetic stress effect and Joule heat effect on undercooling degree change. Electropulsing reduced the nucleation potential barrier and increased the nucleation rate and thus made microstructure

15、refinement.Keywords: Steel; Microstructure; Pearlite; Electropulsing; Magneticpulsing; Grain refinementContentsOriginality Statement and Copyright Authorization IAbstract IIIKeywords: IVContents VNomenclature IXChapter 1 Literature review 11.1 Overview 11.2 Effects of magnetic field on solidificatio

16、n 21.3 Effects of magnetic field heat treatment 31.4 Effects of electropulsing on solidification 71.5 Effects of electropulsing heat treatment 121.6 Effects of electropulsing on Fe-base alloy at room temperature 15Chapter 2 Materials and experiment procedures 192.1 Materials 192.2 Equipment of elect

17、ropulsing generator 192.3 Equipment of magneticpulsing generator 212.4 Procedure of electropulsing treatment 222.5 Simulation of conductivity of ferrite and cementite 232.6 Pearlite colony size measurements 242.7 Optical microscopy and scanning electron microscopy (SEM) 252.8 Mechanical properties t

18、ext 252.9 Hardness test 26Chapter 3 Effects of electropulsing 283.1 Introduction 283.2 Experiments 293.3 The microstructure observations and analysis 303.4 Mechanical properties 343.5 Micromechanism 373.6 Effect of electropulsing on dislocation 38Chapter 4 Effects of magneticpulsing 404.1 Introducti

19、on 404.2 Experimental 404.3 Results and discussion 42Chapter 5 Conclusions 47References 48Acknowledgements 55Publication list 56NomenclatureACAlternating current AArea of cross section BxBroadening in diffraction peak BBurger vector CCapacitance CiCapacitance of capacitor i CCarbon in ferrite in at

20、% TCrystallite size GemChange in electromagnetic free energy of the system JCurrent density in a small element DCDirect current vdDrift velocity Electric conductivity DElectric displacement field EElectric field Electric potential EMElectromagnetic FElectromagnetic free energy FdvElectromagnetic fre

21、e energy of volume element dV IECPElectropulsing current dWElemental work doneCeqEquivalent capacitance eqEquivalent conductivity JeEquivalent current density FeForce due to electric field JfFree current density aLattice parameter of ferrite LLength BMagnetic field Magnetic permeability AMagnetic ve

22、ctor potential HMagnetizing field NAOptical aperture 0Permeability of space RPosition vector iPotential at point indexed i rsResidual stress RiResistance of conducting element indexed i QResistive heat generated rResistivity SEMScanning electron microscope GShear modulus dVSmall volume element SFSta

23、cking fault Total charge density JTotal current density Vector differential operator VoscVoltage in the oscilloscopeWavelength jx, jy, jzX, y, z component of current rx, ry, rzX, y, z coordinate of position vector Chapter 1 Literature review1.1 Overview Improving the properties of steels by grain re

24、finement is one kind of traditional but an effective method. Grain refinement strengthening has been utilized for many kinds of steels. Elctropulsing and magneticpulsing treatment is a rapid and nonequilibrium process which has been applied widely in material treatment for over 30 years. With the ra

25、pid development of electric pulse generator manufacturing technology, it is not difficult to obtain a powerful electropulsing generator. This promotes the research activities on electropulsing and magneticpulsing effects on materials heat treatment.In recent years, it is reported that electropulsing

26、 has special effects such as electricplasticity 1, crack healing 2, casting structure refinement 3, 5, enhancement of corrosion resistance 4, extending of fatigue lives 6. Since its discovery, many works have been done on the application of electropulsing in the grain refinement of liquid steel or s

27、olidified structure 7-12. Burstein 13 utilized elctropulsing to treat stainless steel. It was found that elctropulsing caused the amount of martensite greatly reduced or even disappeared. This phenomenon is so called electrochemical induced annealing. Conrad 13, 14 et al. also reported that the grai

28、n refinement was observed in cold-worked Cu by electropulsing treatment during annealing. By means of this treatment some perfect fine-grains were obtained, as later called spark annealing 15. The improvement of physical and mechanical properties of some hard-to-deform metals under the influence of

29、an external magnetic field has been investigated for the last 30 years. Until now the magnetic filed treatment technology has not been applied widely in the field of engineering. It has been thought that it is successful in relaxing stresses 16-19, and improving fatigue properties 20, 21 and some ot

30、her physical properties 22 of metals in virtue of changing dislocation substructures. Meanwhile, static magnetic field and pulsed magnetic field have being applied in molten metal solidification and heat treatment process. Most of these studies on electropulsing and magneticpulsing were done in the

31、course of heat treatment at higher temperatures or in molten metal. The effect of magneticpulsing on grain refinement has received less attention in the research literature than is warranted. The present work aims to investigate the effect of electropulsing on the grain refinement of a medium carbon low alloy steel at room temperature and to provide new insights into elctropulsing effect and to explore its furthe