中碳合金钢高温相变的晶体学分析

发布时间：2018-09-12 18:01

【摘要】：本文主要研究50Si2Mn3钢在不同强度磁场的作用下于高温等温相变过程中的晶体学特征及扩散型相变的磁诱发作用机制。从形核长大和晶体学等角度分析了中碳硅锰铸钢在不同强度磁场作用下的晶体取向特征,并研究了不同磁场强度对磁诱发珠光体(MIP)相变开始点及可诱发出珠光体组织的最高温度,建立了相变起始点与磁场场强-相变温度-相变时间的关系曲线。具体研究内容与结果如下：12 T,973 K不同磁处理时间作用下MIP的形核与生长研究表明：低转变量的相变初期,组成相之一的珠光体铁素体相(PF)主要沿着110//ND方向生长,111//ND和100//ND等其他位向所占比例相对较低,表现出一定的择优形核与生长；随着转变量的增加,初期成核的PF维持其择优晶向长大,新生相的晶体学位向除受到界面能、体积自由能以及磁自由能等的综合影响外,还将受到母相中逐渐累积的弹性应变能的限制,空间取向趋于多向化,表现为中后期形成的MIP的择优生长趋势变弱,其他取向的MIP相对比例也有所降低,失去了110//ND的择优性；将100%转变量的试样继续延长磁处理时间,表现为无微织构特征。更高磁场下的研究重点考察：铁磁性PF在晶体生长中是否可能呈现体心立方结构(BCC)铁001易磁化方向的择优性或仍延续12T的生长规律。20 T场强是综合样品大小、等温炉炉膛尺寸、磁体有效内腔及实验操控等各因素所能采用的最高磁场强度,实际所用场强为19.8 T。19.8 T磁场作用下的MIP在组织演变过程中的晶体取向变化研究表明：MIP在形成过程中存在着与等温温度、受磁时间密切相关的晶体取向变化。在临界点(963 K)以上相对较低的等温温度(983 K)下,MIP晶体的某一特定晶向(100)在转变量范围内呈现出随受磁时间延长而增强的趋势;而在相对较高的等温温度(995 K)下,随受磁时间的延长100晶向所占的份额则表现为逐渐减弱；相同的受磁时间下,相变温度越高,初期生长的MIP晶体中的100取向的相对份额越高；因此,MIP的择优取向100与PF的生长速度相关,且主要于MIP的形成初期较强,当MIP达到一定转变量后晶体中的其他晶向(如110)转而呈现生长优势,甚至在难磁化方向也出现生长速度相对增快的趋势。根据不同磁处理条件下MIP的转变量,研究了其相变开始点(产生1%MIP)及可诱发出珠光体组织的最高温度(在所采用的等温时间范围内,意味着此温度以上将观察不到珠光体组织),建立了相变起始点与磁场场强-相变温度-相变时间的关系曲线。表明磁场对诱发珠光体相变最高温度的影响并不呈线性变化；此外,MIP增量变化及电子背散射衍射技术(EBSD)取向分析结果均表明磁场促进了易磁化100方向的晶粒于珠光体相变初期的形核与长大,存在着渗碳体以颗粒状或短杆状形态在块状铁素体内部的不连续析出方式,且后期珠光体转变量的增加主要是来自于原优先形成珠光体的长大,而非形核量的增加。
[Abstract]:In this paper, the crystallographic characteristics of 50Si2Mn3 steel during high temperature isothermal transformation and the mechanism of magnetic induction of diffusion-type transformation under the action of different magnetic fields are studied.The crystal orientation characteristics of medium carbon silicon manganese cast steel under different magnetic fields are analyzed from the viewpoints of nucleation, growth and crystallography. For the starting point of magnetic induced pearlite (MIP) phase transition and the highest temperature at which pearlite can be induced, the relationship between the starting point of phase transition and magnetic field intensity, phase transition temperature and phase transition time is established. At the beginning of transformation, the pearlite ferrite phase (PF), one of the constituent phases, grows mainly along the direction of 110 / / ND, the proportion of 111 / / ND and 100 / / / ND is relatively low, showing a certain preferred nucleation and growth; with the increase of the amount of transformation, PF maintains its preferred crystal growth at the beginning of nucleation, and the crystallographic degree of the new phase is subjected to the interface energy. Besides the influence of volume free energy and magnetic free energy, the spatial orientation tends to be multi-directional due to the gradually accumulated elastic strain energy in the parent phase. The preferential growth trend of MIP formed in the middle and late stages is weakened, and the relative proportion of MIP with other orientations is also reduced, thus losing the preference of 110//ND. The magnetic treatment time of the samples was prolonged and the samples were characterized by non-microtexture. The research focus of higher magnetic field was to investigate whether the ferromagnetic PF could show the preferred magnetization direction of BCC Fe 001 or continue the growth rule of 12T. The field strength of 20T was the comprehensive sample size, the size of the furnace, and the magnet had the same magnetism. The crystal orientation of MIP under the magnetic field of 19.8 T.19.8 T shows that there are crystal orientation changes closely related to the isothermal temperature and magnetic time during the formation of MIP. At a relatively low isothermal temperature (983 K), a specific orientation (100) of MIP crystals tends to increase with the extension of the magnetization time in the range of transformation, while at a relatively high isothermal temperature (995 K), the fraction of 100 orientations decreases with the extension of the magnetization time. Therefore, the preferred orientation 100 of MIP is related to the growth rate of PF, and is mainly stronger in the early stage of MIP formation. When the MIP reaches a certain amount of transformation, other crystal orientations (such as 110) turn to show growth advantage, even in the direction of difficult magnetization, the growth rate of MIP also appears. The starting point of phase transition (producing 1% MIP) and the highest temperature of induced pearlite structure (in the range of isothermal time, which means that no pearlite structure can be observed above this temperature) are studied according to the transformation amount of MIP under different magnetic treatment conditions. The starting point of phase transition and the magnetic field intensity-phase transition temperature are established. The results show that the effect of magnetic field on the maximum temperature of pearlite transformation is not linear. In addition, the results of MIP incremental change and electron backscatter diffraction (EBSD) orientation analysis show that magnetic field promotes the nucleation and growth of easily magnetized 100-direction grains at the initial stage of pearlite transformation, and cementite exists. In the latter stage, the increase of pearlite transformation is mainly due to the growth of the primary pearlite rather than the increase of nucleation.
【学位授予单位】：昆明理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG142.1

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