轧制工艺乳化液的行为及作用机理的基础研究

发布时间：2018-08-01 14:57

【摘要】：乳化液以其优异的润滑及冷却性能被广泛应用于机械、金属材料加工等领域,在轧制过程中采用乳化液进行润滑及冷却能够有效降低轧制力、减小摩擦、控制磨损、改善轧材的表面质量及机械性能,对轧制生产过程节能降耗、提高产品质量具有重要意义。金属轧制工艺一般采用水包油型(O/W)乳化液作为介质,通过将其喷射在轧制入口区并随着轧制过程带入变形区起到润滑及传热作用;乳化液在金属表面上的吸附成膜过程(Plate-out)以及喷射前的稳定性能对轧制变形区润滑及传热效果的发挥至关重要,这些不仅与乳化液的组成成分有关,同时也受轧制工艺参数的影响。为了对轧制工艺润滑及传热过程进行调节和控制,从而降低轧制能耗、提高产品质量,有必要针对轧制过程中乳化液的行为及变化规律开展研究,以期为乳化液成分优化及轧制工艺控制提供指导。本文以非离子型表面活性剂制备的水包油型乳化液为研究对象,采用实验、介观/分子模拟和模型计算相结合的方法,以乳化液在轧制过程中的润滑及传热为核心,结合乳化液的稳定性及“Plate-out”性能,系统研究了乳化液在轧制过程中的行为及作用机理,为乳化液成分设计以及轧制工艺过程优化提供理论依据。具体研究结果如下:(1)采用静置实验及耗散粒子动力学方法对乳化液的稳定性能进行了研究,获得了稳定性能与乳化剂HLB值、浓度、机械搅拌强度、搅拌时间之间的关系,分析了油水界面膜特性对乳化液稳定性的作用机理。当非离子型乳化剂HLB值为13,浓度为1.6%时,制备得到的乳化液最稳定。油水界面膜厚度随着乳化剂HLB值的增大而增大,但乳化液的稳定性与界面膜厚度之间并非线性关系,当界面张力最低时乳化液最稳定;油滴粒径随着HLB值增大而增大,这与乳化剂的分子结构以及亲水基团与亲油基团的体积比有关。(2)对乳化液“Plate-out”性能进行了研究,提出了一种弹性控制乳化液“Plate-out”性能的方法,即在乳化液制备过程中添加短链醇类添加剂(丙三醇、1,2-丙二醇、乙二醇、正丙醇),作用于油滴在金属表面的“Plate-out”行为,从而影响其“Plate-out”性能。采用分子动力学方法构建添加剂分子的吸附模型以及“Plate-out”多层吸附构型,研究了添加剂分子在“Plate-out”过程中的行为及作用机理;发现添加醇类添加剂的乳化液“Plate-out”油膜量大小遵循如下规律:丙三醇1,2-丙二醇乙二醇正丙醇,但该规律与添加剂分子在金属表面的吸附能大小及制备的乳化液油滴粒径分布之间没有必然联系。通过对“Plate-out”多层吸附膜的分子动力学模拟发现:添加剂分子是通过影响油滴在吸附油膜上的再吸附过程对“Plate-out”性能产生作用,这与油相成分在添加剂体系中的均方根位移以及添加剂分子间的内聚能计算结果相一致。(3)基于微凸体扁平化理论及平均流动模型,建立了轧制变形区混合润滑模型,并通过该模型对不同轧制条件下的界面润滑特性(轧制力、流体压力分布,真实接触面积、油膜厚度变化)进行了计算分析。结果表明:表面粗糙度、轧制速度、润滑剂粘度的增大均有利于提高轧辊与轧件接触界面的油膜厚度、降低界面真实接触面积,但对轧制力分布特征没有明显影响;而压下率的提高不仅提高了真实接触面积、降低了油膜厚度,而且增大了轧制力。(4)基于限制层剪切模型从分子尺度研究了润滑剂在剪切过程中的行为以及对正压力、摩擦力和摩擦系数的影响。结果表明:限制层间压力的提高有利于将剪切动量从墙面向润滑膜中间区域传递,虽然油膜压力的提升增大了层间摩擦力,但可以有效地降低摩擦系数。稳定的层状吸附结构一般出现在距离墙面3nm内,当油膜厚度超过6nm时,中间区域将表现出润滑剂液相特性;油膜厚度与摩擦系数之间存在一个临界值厚度?(8,只有当实际油膜厚度??(8时,摩擦系数随膜厚的增大而逐渐减小。剪切速度的提高有利于增大正压力,降低摩擦系数;温度的升高不仅提高了正压力,同时还降低了吸附膜的固化程度和剪切力,从而有效降低了摩擦系数。油水混匀体系润滑膜润滑过程中,油相倾向于吸附在壁面而水则倾向于聚集在中间区域;含水油膜比纯油润滑更能降低摩擦力,当油水体积比为1:1时,摩擦系数达到最低。(5)为了考虑变形区内部任意点的微凸体扁平化及油膜厚度变化对界面传热过程的影响,将本文建立的混合润滑模型引入界面传热模型,对轧制变形区的界面传热系数及其变化特征进行了计算分析。结果表明:微凸体界面传热系数分布存在两个峰值,一个出现在入口区,另一个出现在中性面位置;越接近出口处,润滑油传热系数越高,当润滑油导热系数较高时,其传热系数甚至超过微凸体接触传热系数;在考虑金属表面氧化层热阻时,界面传热系数将以数量级程度降低;变形区内部界面传热系数分布可以划分为三个变化区间,其变化趋势主要受压力分布、真实接触面积以及油膜厚度变化的影响。
[Abstract]:The emulsion is widely used in the fields of machinery and metal material processing for its excellent lubrication and cooling properties. The use of emulsion to lubricate and cool the emulsion in the rolling process can effectively reduce the rolling force, reduce friction, control wear, improve the surface quality and mechanical energy of the rolled material, save energy and reduce consumption and improve the quality of the product. In the rolling process of metal, the O/W emulsion is usually used as a medium, and it is lubricated and heat transfer through the injection of the emulsion into the rolling entrance area and with the rolling process into the deformation zone; the adsorption forming process of the emulsion on the metal surface (Plate-out) and the stability performance before the injection are on the rolling deformation zone. The effect of lubrication and heat transfer is very important. These are not only related to the composition of the emulsion, but also influenced by the rolling process parameters. In order to adjust and control the rolling process lubrication and heat transfer process, so as to reduce the rolling energy consumption and improve the quality of the products, it is necessary to focus on the behavior and change rules of the emulsion during the rolling process. The law is carried out to provide guidance for the optimization of emulsion composition and the control of the rolling process. In this paper, the water emulsion emulsified liquid prepared by non ionic surfactants was studied by experiments, mesoscopic / molecular simulation and model calculation, with the lubrication and heat transfer of emulsion in the rolling process as the core, and the emulsification combined with emulsification. The stability and "Plate-out" properties of the liquid are studied. The behavior and mechanism of the emulsion in the rolling process are systematically studied. The theoretical basis for the design of emulsion composition and the optimization of the rolling process are provided. The results are as follows: (1) the stability of the emulsion is studied by the static experiment and the dissipative particle dynamic mechanics method. The relationship between the stability performance and the HLB value of emulsifier, the concentration, the mechanical stirring strength and the stirring time were obtained. The mechanism of the action of the oil and water interfacial film characteristics to the stability of the emulsion was analyzed. When the HLB value of the non ionic emulsifier was 13 and the concentration was 1.6%, the emulsion prepared was the most stable. The thickness of the oil and water boundary mask increased with the increase of the emulsifier HLB value. But it increases, but the stability of the emulsion has a nonlinear relationship with the thickness of the boundary film. The emulsion is most stable when the interfacial tension is the lowest; the droplet diameter increases with the increase of HLB value, which is related to the molecular structure of the emulsifier and the volume ratio of the hydrophilic group to the oil Pro Group. (2) the performance of the emulsion "Plate-out" is studied. A method to control the "Plate-out" properties of emulsion, namely, the addition of short chain alcohols (glycerol, 1,2- propanediol, ethylene glycol, propanol) in the preparation of emulsion, is used to influence the "Plate-out" behavior of the oil droplets on the metal surface, thus affecting its "Plate-out" properties. The molecular dynamics method is used to construct the addition. The behavior and mechanism of additive molecules in the process of "Plate-out" are studied by the adsorptive model of the agent and the "Plate-out" multilayer adsorption configuration. It is found that the size of the "Plate-out" oil film added to the emulsion additive is followed by the following rules: glycerol 1,2- propyl two alcohol glycol propanol, but the law and the additive molecule There is no inevitable relationship between the size of the adsorption energy on the metal surface and the distribution of the droplet size distribution of the emulsion. Through the molecular dynamics simulation of the "Plate-out" multilayer adsorption membrane, it is found that the additive molecules affect the performance of "Plate-out" by the readsorption process of the oil droplets on the adsorbed oil film, which is in the composition of the oil phase. The root mean square displacement in the additive system and the calculation results of the cohesive energy between the additives are in agreement. (3) a mixed lubrication model of the rolling deformation zone is established based on the theory of the flattening of the micro convex body and the average flow model. Through this model, the sliding characteristics of the interface under different rolling conditions (rolling force, the distribution of fluid pressure, real contact) The results show that the surface roughness, rolling speed, and the increase of the viscosity of the lubricant can improve the oil film thickness of the contact interface of the roller and the rolling parts, reduce the actual contact area of the interface, but have no obvious influence on the rolling force distribution, and the increase of the pressing rate not only improves the true connection of the rolling force. The contact area reduces the thickness of the oil film and increases the rolling force. (4) the effect of the lubricant on the shear process and the effect on the positive pressure, friction force and friction coefficient are studied from the molecular scale based on the limiting layer shear model. The results show that the increase of the confining interlayer pressure is beneficial to the shear momentum from the wall to the middle area of the lubricating film. Transfer, although the increase of the oil film pressure increases the interlayer friction, it can effectively reduce the friction coefficient. The stable layered adsorption structure usually appears in the distance wall 3nm. When the thickness of the oil film is over 6nm, the intermediate region will show the liquid properties of the lubricant; there is a critical thickness between the oil film thickness and the friction coefficient (8, only) When the actual oil film thickness is 8, the friction coefficient decreases with the increase of the thickness of the film. The increase of the shear speed is beneficial to increase the positive pressure and reduce the friction coefficient; the increase of the temperature not only increases the positive pressure, but also reduces the curing degree and shear force of the adsorption film, thus effectively reducing the friction coefficient. The lubricating film of the oil and water mixing system is effectively reduced. In the process of lubrication, the oil phase tends to adsorb on the wall and water tends to gather in the middle area; the water bearing oil film can lower the friction force more than the pure oil lubrication. When the oil and water volume ratio is 1:1, the friction coefficient reaches the lowest. (5) in order to consider the influence of the flat flatting of the micro convex body at any point inside the deformation zone and the change of the oil film thickness to the interface heat transfer process The mixed lubrication model established in this paper is introduced into the interface heat transfer model and the interfacial heat transfer coefficient and its change characteristics of the rolling deformation zone are calculated and analyzed. The results show that there are two peaks in the distribution of the heat transfer coefficient at the interface of the micro convex body, one appears in the entrance area and the other is now on the neutral surface; the closer to the outlet, the heat transfer heat transfer. The higher coefficient is, when the coefficient of thermal conductivity is high, the heat transfer coefficient of the lubricating oil is even higher than that of the micro convex body. When the thermal resistance of the oxide layer on the metal surface is considered, the heat transfer coefficient of the interface will be reduced in order of magnitude. The distribution of heat transfer coefficient in the internal interface of the deformation zone can be divided into three changing intervals, and the trend of the change is mainly affected by the pressure distribution. The influence of real contact area and oil film thickness changes.
【学位授予单位】：重庆大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG335

【参考文献】