【作者】 周晓玲；

【导师】 周荣；

【作者基本信息】 昆明理工大学 ， 材料加工工程， 2009， 博士

【摘要】 本论文以中碳硅锰钢为对象,主要研究磁场对珠光体相变的影响规律。实验研究步骤分为三大部分：(1)弱磁场条件下(<1T)开展30Si2Mn2和50Si2Mn3钢的相关研究,涉及不同冷却速度的连续冷却相变、共析点(A1)以下的等温珠光体相变、共析点(A1)以上的等温磁循环处理等过程及特征；(2)强磁场(4T-12T)条件下,进行共析点A1以上温度等温的磁场诱发珠光体相变研究；(3)建立A1点以上磁诱发珠光体相变的理论模型及相变热力学描述。本论文的创新性成果是：利用磁场在A1点以上成功地诱发了珠光体相变；以实验研究结果为基础,研究了与珠光体相变领先相相关的理论问题,提出了结构微区成核理论模型。弱磁场下的研究表明：a)对连续冷却相变,存在一个磁场作用时间长短的问题。磁场对相变驱动力的贡献受磁场作用时间、磁场强度、合金成分、处理条件等多重因素的影响。磁场的引入对慢速冷却时高温型相变、快速冷却时低温相变的完全性具有明显的促进作用。对含碳量相对较低的钢而言,磁场热处理的实验参数可明显影响中温型相变产物贝氏体的形态。b)对A1以下的等温珠光体相变,磁场的引入使相变的孕育期缩短；珠光体相变初期具有“联发式”生长特征,在相变初期快速形成框架,较短时间内就可形成相当的珠光体量,磁场的引入明显促进了此种“联发式”生长。c)通过对A1点以上进行等温磁循环处理的研究,探讨了珠光体形核位置、领先相、核心形态及长大方式。初期形成的珠光体形态并不都呈现出两组成相的平行排列,而是受在三叉晶界部位、晶界处优先形核的先共析相铁素体(F)形态的限制,但其生长依然符合F与Cem两相协同生长特性。在珠光体形核与长大过程中,片层间距并非常数,而是随时间逐渐减小,说明相变过程为非稳态模式。珠光体中的Cem可以在F片内沉淀析出。Cem的形核与长大速度是制约珠光体形成的主要因素。强磁场下的研究表明：a)磁场诱发珠光体主要形成于奥氏体晶界,领先相为铁磁相F；施加的磁场越强,诱发的珠光体越多；等温温度越靠近A1点,磁场影响越明显；等温处理时间越长,珠光体组织的转变量越大。b)珠光体主要在已形成的F基础上成核长大,其形态特征取决于组织相F与Cem的形态。第一种为在F基体中分布有粒状或短杆状Cem的珠光体；第二种为F片与Cem呈交替形成的片层状珠光体；第三种为F呈颗粒状、Cem呈空间“网膜”状的鳞片状珠光体。c)电子衍射花样分析表明,在F基体中析出的Cem与F间尚未发现固定的晶体学位向关系。并未观察到珠光体组织的定向生长,但观察到F呈细长杆状的平行析出以及局域内相邻珠光体团有着近于平行的珠光体片层走向。在A1以上温度可通过磁场诱发珠光体相变的事实表明,在较高的温度施加磁场,尤其是强磁场,会因磁诱发铁素体或珠光体组织而降低材料的淬透性能。在理论研究方面,从磁学及热力学角度出发提出了磁诱发珠光体相变的理论模型—结构微区磁化成核模型,其理论依据为近邻原子的磁相互作用、化学相互作用、磁场附加的静磁能影响着新相的形核与长大。在A1以上,磁场诱发珠光体相变的领先相为BCC结构的铁素体,铁素体来自材料内部的“铁磁性群落FMC”。近邻原子间的交换作用及居里点以下的自发磁致伸缩效应是处于顺磁态物质的内部存在着“FMC”的主要原因。外磁场引起的铁原子磁偏转及磁致伸缩,一方面可促进现有“FMC”的长大,另一方面可在母相中感应生成新的“FMC”,提高铁磁相的形核率。相变的磁驱动力为磁场引起的两相静磁能差。在考虑总的Gibbs自由能时,局域“化学有序相”所提供的化学驱动力项也不容忽视。更多还原

【Abstract】 The main purpose of this paper has concerned with the influence of magnetic field on pearlite transformation in medium carbon Si-Mn casting steel. Experimental study has been divided into three main steps:(1) Carry out the related studies on the influences of low magnetic field (<1T) on continuous cooling phase transformation with different cooling rates, isothermal pearlite phase transformation at temperature below the eutectoid temperature (A1), and magnetic circling isothermal treatment at temperature above A1, in 30Si2Mn2 and 50Si2Mn3 steels. (2) Explore the possibility of applied high magnetic field (up to 12T) on inducing pearlite transformation at temperatures above the A1 point; (3) Build up the theoretical model of magnetic-inducing pearlite phase transformation (MIPT) above eutectoid temperature and put forward the related thermodynamic description of phase transformation. In this paper, the innovation lies in the fact that external magnetic field has been utilized to induce and has successfully induced pearlite transformation at temperatures above the eutectoid point; On the basis of experimental results, some theoretical issues related to the preceding phase of pearlite transformation have been discussed; Theoretically, a model of magnetic-induced pearlite transformation has been put forward.Studies carried out under weak magnetic field have indicated that:a) For continuous cooling phase transformation, there exists a problem of how long the sample expose to a magnetic field for. The contribution of magnetic field supplying to the driving force of phase transformation depends on multiple factors, such as the time length of applying a magnetic field, the intensity of the field, the chemical composition, and the heat-treating conditions. Applying a magnetic field can promote the completion of the high temperature-type phase transformation under slow cooling condition, the low temperature-type one under rapid cooling condition. For the steel with lower carbon content, the experimental parameters of heat treatment under the field can significantly affect the morphologies of bainite, which is the product of bainite phase transformation. b) For the isothermal pearlite transformation at temperature below A1, the applied magnetic field can shorten the incubation period. In the early stage of transformation, the formation of pearlite colonies shows the feature of " an evoked growing mode". A frame of pearlite colonies can quickly form at the very beginning, and a relative high transformed fraction can be reached within short time. Applied magnetic filed can enhance the " evoked growing-mode" happening in the early stage of transformation. c) Through the exploration of magnetic circling treatment at temperature above A1, the nucleation positions, preceding phase, nucleus morphology and growth mode of induced pearlites have been studied. Two phases of earlier induced pearlites do not have to parallel to each other, the morphology of earlier pearlite is confined by the configurations of pro-eutectoid ferrite that has nucleated at the grain corners and grain boundaries of austenite, but the growth of pearlite is still consistent with the cooperation-growth mode of two phases. During the nucleation and growth of pearlite, the interlamellar spacing does not hold constant but decreases with time, which indicates that the isothermal transformation shows non-steady-state mode. The eutectoid cementite can precipitate within ferrite plate. The nucleation and growth rate of cementite phase is one of main constraining factors in the formation of pearlite.Studies carried out under high magnetic field have indicated that:a) magnetic-induced pearlite mainly formed at austenite grain boundaries, the preceding phase of pearlite is ferromagnetic phase—ferrite; the stronger the applied field, the more the induced pearlite; the closer to Al point the isothermal temperature, the more apparent the influence of magnetic field on the transformation; the longer the isothermal holding time, the larger the volume fraction of induced pearlite. b) Pearlite mainly nucleate and grow on the basis of the formed ferrite grains, the morphology depends on the configurations of both ferrite and cementite:First kind is the one with bar-like cementite phase distributed within ferrite matrix; The second is the lamellar pearlite; The third is the scale-like pearlite in which cementite phase takes the form of scale. c) Through the diffraction patterns’analysis, so far, it has not been found that there is fixed crystallographic orientations between the precipitates (cementite phase) and the ferrite matrix. The phenomenon that pearlite colonies arranged along the direction of applied magnetic filed has not been observed. But, we observed the phenomena that several long bar-like ferrite phases parallel to each other within austenite, and some adjacent pearlitic colonies show nearly the same direction of lamellae. The fact that magnetic field can induce the pearlitic transformation at temperatures above A1 indicates:applying a magnetic filed, esp. high magnetic field, at higher temperature range, can degrade the hardenability of materials due to the formation of magnetic-induced ferrite or pearlite. In the theoretical research, from the view of magnetism and thermodynamics, a theoretic model of magnetic-induced pearlite transformation has been put forward, that is the magnetizing and nucleating model of fluctuating clusters or structural micro ranges, its theoretic basis is that:there are magnetic interactions and chemical interactions among adjacent atoms, the additional magnetostatic energy contribution can set significant influences on the nucleation and growth of new phases. At temperatures above A1, the preceding phase of magnetic-induced transformation is ferrite with body-centric cubic structure, ferrite phase formed on the basis of " ferromagnetic cluster, FMC" existing in the interior of austenite. Both the interactions between adjacent atoms and the spontaneous magnetostrictive effect below Curie temperature are the main causes of existing "FMCs" within paramagnetic phase. External magnetic field can cause the magnetic deflection of iron atoms and the magnetostrictive effect, hence, can promote the growth of existing "FMCs" and induce more new "FMCs" within austenite, the latter factor may increase the nucleation rate of ferromagnetic phase. The magnetic driving force of phase transformation is the magnetostatic energy difference between the transforming phases caused by the introduction of magnetic field. The chemical driving force item supplied by local "chemical order phases" cannot be neglected when considering the total Gibbs free energy.更多还原