稀土对微合金钢中碳化铌溶解与析出行为的影响

发布时间：2018-05-15 07:01

本文选题：稀土 + 微合金　；参考：《北京科技大学》2017年博士论文

【摘要】：通过微合金化与控轧控冷技术的有机结合,依靠碳氮化物析出与形变再结晶的交互作用使钢获得良好的强韧性,是有效提高钢铁材料性能的重要手段。在Ti、Nb、V、B、稀土等微合金元素中,Nb因具有显著抑制奥氏体晶粒长大、提高非再结晶温度、细化相变组织、提高钢的强韧性等作用,在高强低合金钢领域得到了广泛应用,稀土因其在钢中的净化、抗氧性、提高耐蚀性等作用而引起材料研究者广泛关注。我国白云鄂博矿拥有丰富的稀土资源,前期研究表明,包头钢铁公司使用白云鄂博铁矿原料生产的钢中含有一定量的残余稀土,其含量达到微合金化要求。如何充分发挥稀土的有益作用,不仅需要关注稀土本身的微合金化效果,还应着眼于稀土的存在形式及其与微合金元素Nb等的交互作用。然而,由于稀土理化性质的特殊性以及研究手段的限制,目前稀土在钢中的微合金化作用尚缺乏系统的研究。本文基于稀溶液固溶体,计算了钢中常用稀土元素La、Ce、Y在Fe基合金中形成稳定第二相的形成焓与溶解焓,获得了稀土在Fe中固溶度随温度的变化曲线。此外,Fe-RE系合金在升温与降温过程中的内耗变化规律表明,当稀土原子溶于基体时,在高温区段会出现溶质原子在晶界偏聚的阻尼峰,当稀土含量增加到超过基体最大固溶量时,会与其他合金元素反应形成化合物;进一步地,从电子结构层次深入分析了稀土原子的占位倾向及对晶界的强韧化作用机理。基于稀土与其他微合金元素的相互作用系数,计算了稀土作用下NbC的固溶度积,分析了稀土元素对高温奥氏体化状态下NbC溶解行为的影响,建立了 NbC在奥氏体区的析出动力学模型。此外,设计了奥氏体区的等温析出实验,结合等温析出后的组织观察与硬度测试结果表明,稀土元素会提高NbC在奥氏体中的固溶度积,降低其固溶温度,促进NbC在奥氏体中的溶解。通过系列等温实验与热变形实验,结合物理化学相分析与析出物统计,对添加稀土前后实验钢中NbC的析出行为进行了定量定性表征。根据差热分析曲线中第二相析出峰的变化,利用JMA模型讨论了稀土对微合金钢中NbC析出动力学的影响。结果表明,稀土添加后,NbC在奥氏体区的激活能由75.32kJ/mol升高到90.15kJ/mol,在铁素体中的激活能则由176.98kJ/mol降低为65.47kJ/mol。此外,利用第一性原理分析了稀土与Nb原子的交互作用,以及对Nb原子扩散激活能的影响,从微观角度深入解释了稀土对奥氏体与铁素体区NbC析出行为的影响机理。通过设计系列扩散偶实验,讨论了稀土 La对铁铌互扩散行为的影响。在此基础上,得出稀土 La作用下Nb在铁素体中的扩散系数。结果表明,添加稀土后,Nb在铁素体中的扩散系数增加。基于位错形核机制,建立了 NbC在铁素体中的形核与析出模型,结合NbC析出动力学实验,阐明了稀土对铁素体区NbC析出行为的影响机理。实验与计算结果均表明,稀土加入后会加快NbC在铁素体区的析出,增加铁素体中析出相密度与弥散强化效果。借助系列热变形工艺下的平均流变应力实验,确定了实验钢的未再结晶温度,从形变储能作用下回复与再结晶之间的竞争关系讨论了稀土对形变再结晶行为的影响机理。根据应变诱导析出NbC颗粒的体积分数和分布形态,在考虑钉扎和拖曳效应的情况下,预测了再结晶晶粒等温长大动力学。基于上述的理论与实验研究,对白云鄂博含稀土铁矿生产钢材的可行性和经济性进行评估,并给出了合金成分设计与热机轧制工艺设计的思路。
[Abstract]:Through the organic combination of microalloying and controlled rolling and controlled cooling technology, the interaction of carbides precipitation and deformation recrystallization makes the steel get good strength and toughness. It is an important means to effectively improve the properties of steel materials. In Ti, Nb, V, B, rare earth and other micro alloy elements, Nb has a significant inhibition of austenite grain growth and the increase of non recrystallization temperature. It has been widely used in the field of high strength and low alloy steel, which has been widely used in the field of high strength and low alloy steel. The rare earth is widely concerned because of its purification, anti oxygen and corrosion resistance in the steel. The Baiyunebo mine has rich rare earth resources. The previous research shows that the use of the Baotou Steel Corp has been used. There is a certain amount of remainder rare earth in the steel produced from Baiyunebo iron ore raw material, and its content reaches the requirement of microalloying. How to give full play to the beneficial effect of rare earth should not only pay attention to the microalloying effect of rare earth itself, but also focus on the existence of rare earth and its interaction with the microalloying element Nb, however, due to the rare earth theory. The microalloying effect of rare earth in steel is still lack of systematic study at present. Based on the solution solid solution, the formation enthalpy and dissolution enthalpy of the stable second phase of the rare earth elements La, Ce, Y in the Fe based alloy are calculated in this paper, and the solid solubility of rare earth in Fe is obtained with the temperature change. In addition, the variation of internal friction of Fe-RE alloy during heating and cooling shows that when the rare earth atoms dissolve in the matrix, the damping peak of the solute atoms will appear at the grain boundary in the high temperature zone. When the content of rare earth increases to the maximum solid solution, it will react with his alloy element to form a compound; further, from the alloy element, the compound will be formed. The occupying tendency of rare earth atom and the mechanism of strengthening and toughening of grain boundary are analyzed in depth by electronic structure. Based on the interaction coefficient of rare earth and other microalloying elements, the solid solubility product of NbC under the action of rare earth is calculated. The effect of rare earth elements on the dissolution behavior of NbC under austenitizing condition is analyzed, and NbC is established in austenite. In addition, the isothermal precipitation experiment of austenite area is designed. The results of microstructure observation and hardness test after isothermal precipitation show that the rare earth elements can increase the solid solubility product of NbC in austenite, reduce the solution temperature and promote the dissolution of NbC in austenite. The precipitation behavior of NbC in the experimental steel before and after the addition of rare earth was qualitatively characterized by physical and chemical phase analysis and precipitation. According to the change of the precipitation peak in the second phase of the differential heat analysis curve, the effect of rare earth on the precipitation kinetics of NbC in microalloy steel was discussed by JMA model. The results showed that after the addition of rare earth, the NbC was in austenite. The activation energy of the region is increased from 75.32kJ/mol to 90.15kJ/mol, and the activation energy in the ferrite is reduced from 176.98kJ/mol to 65.47kJ/mol.. The interaction between rare earth and Nb atoms and the effect on the activation energy of Nb atom diffusion are analyzed by the first principle, and the NbC analysis of the austenite and ferrite region is explained from the microscopic angle. By designing series of diffusion couple experiments, the effect of rare earth La on the interdiffusion of iron and niobium is discussed. On this basis, the diffusion coefficient of Nb in ferrite under the action of rare earth La is obtained. The result shows that the diffusion coefficient of Nb in ferrite is added after adding rare earth. Based on the dislocation nucleation mechanism, the NbC in ferrite is established. The nucleation and precipitation model in the body, combined with the NbC precipitation kinetic experiment, clarifies the influence mechanism of rare earth on the precipitation behavior of NbC in ferrite region. The experiment and calculation results show that the addition of rare earth will accelerate the precipitation of NbC in the ferrite region, increase the precipitation density and dispersion strengthening effect in the ferrite. The non recrystallization temperature of the experimental steel was determined by mean rheological stress experiment. The influence mechanism of re on the deformation recrystallization behavior was discussed from the relationship between the recovery and recrystallization under the action of deformation energy storage. The volume fraction and distribution of NbC particles precipitated by strain induced precipitation and the effect of nailing and towing were predicted. The kinetics of isothermal growth of crystalline grains. Based on the theoretical and experimental studies mentioned above, the feasibility and economy of the steel produced by rare earth iron ore in Baiyunebo are evaluated, and the design of the alloy composition and the design of the hot machine rolling process are given.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG142.1

【参考文献】