Mn、Mo等合金元素对钢中奥氏体形成及分解动力学的影响

发布时间：2018-04-16 19:18

本文选题：钢铁 + 相变动力学　；参考：《清华大学》2015年博士论文

【摘要】：进一步提高钢铁材料的力学性能有赖于更加深入地理解相变动力学,而合金元素添加对相变动力学的作用仍有许多问题有待澄清。本文选取珠光体奥氏体化、铁素体相变及贝氏体相变三个过程来系统研究合金元素对相变动力学的影响,以期为钢铁材料进一步微观组织控制和力学性能优化提供实验和理论基础。首先通过理论模拟研究了Fe-0.6C-(0~2.5)M(M=Mn/Cr/Si/Ni)合金中的珠光体奥氏体化过程,重点关注合金元素配分与奥氏体生长动力学。假定初始组织为层片状珠光体,奥氏体在渗碳体/铁素体界面形核并垂直于层片方向生长,相界面条件满足局域平衡。计算结果表明Cr和Si等与C具有强烈交互作用的元素显著升高配分临界温度,而Ni则与之相反。不配分模式下,合金元素通过改变相界面C活度影响奥氏体生长速率;配分模式下,合金元素对相变驱动力的改变可能是影响奥氏体化动力学的主要因素。然后通过热处理实验研究了Fe-0.1C-1.5Mn-(0,0.5)Mo合金在650~700oC下的等温铁素体相变,重点利用EPMA测定了界面条件的演变并阐明了溶质拖曳效应的影响。铁素体相变初期受C扩散控制,随后进入准稳态阶段;Mo元素添加降低相变初期相界面处C浓度,推迟铁素体相变,而准稳态阶段开始时奥氏体C浓度十分接近PLE/NPLE线,且不受Mo元素添加影响,表明溶质拖曳效应仅减缓相变初期动力学,而至准稳态阶段时其影响可忽略。最后通过热处理实验研究了Fe-0.1C-1.5Mn-(0,0.03,0.3,0.5,1)Mo合金在500~600oC下的等温贝氏体相变,重点利用EPMA测定了不同Mo含量合金中的奥氏体C富集行为并揭示了当前各种不完全转变理论的不足。550~600oC下Mo添加量超过0.3%时观察到相变停滞现象;500oC下大量碳化物析出,相变迅速完成。相变停滞时奥氏体中的C浓度低于T0线,且随Mo含量增加和温度升高而降低,但不受原奥氏体晶粒尺寸的影响。实验测定的奥氏体C富集极限与T0’理论、溶质拖曳理论和WBs理论作了详细对比,三种理论均不能很好解释实验数据。奥氏体C富集极限相对T0线的过冷度随Mo含量增加而不断增大,并非T0’理论提出的400J?mol-1恒定值;相变停滞阶段相界面速度过低,无法产生显著溶质拖曳效应;WBs曲线也与实验数据不十分吻合。贝氏体不完全转变现象仍有待进一步研究。
[Abstract]:The further improvement of mechanical properties of iron and steel materials depends on a deeper understanding of the phase transformation kinetics, but there are still many problems to be clarified about the effect of alloying elements on the phase transformation dynamics.In this paper, three processes of pearlite austenitization, ferrite transformation and bainite transformation are selected to systematically study the effect of alloying elements on the transformation kinetics.In order to provide experimental and theoretical basis for further microstructure control and optimization of mechanical properties of iron and steel materials.In this paper, the austenitization process of pearlite in Fe-0.6C-MU / Cr / Si / Ni alloy has been studied by theoretical simulation, focusing on the alloying element partition and austenite growth kinetics.Assuming that the initial structure is lamellar pearlite, austenite nucleates at the cementite / ferrite interface and grows perpendicular to the lamellar direction, and the phase interface conditions satisfy the local equilibrium.The calculated results show that the critical temperature of partitioning is significantly increased by elements such as Cr and Si, which interact strongly with C, while Ni is the opposite.The austenitic growth rate is affected by the change of C activity at the interface between alloying elements and the change of the transformation driving force of alloying elements, which may be the main factor affecting the austenitization kinetics.Then the isothermal ferrite phase transition of Fe-0.1C-1.5Mn-0. 5Mn-0. 5Mo alloy under 650~700oC was studied by heat treatment experiment. The evolution of interface conditions and the effect of solute drag effect were mainly determined by EPMA.The ferrite phase transition is controlled by C diffusion at the beginning of the phase transformation, and then the Mo element is added to the quasi-steady phase to reduce the C concentration at the interface of the initial phase transformation, thus postponing the ferrite phase transition. At the beginning of the quasi-steady phase, the concentration of austenite C is very close to the PLE/NPLE line.It is not affected by Mo addition, which indicates that the solute drag effect only slows down the initial kinetics of phase transition, but the effect can be neglected in the quasi-steady phase.Finally, the isothermal bainitic transformation of Fe-0.1C-1.5Mn-0. 03 (0. 3 + 0. 3) and 0. 5 (1 +) Mo alloy under 500~600oC was studied by heat treatment experiment.The enrichment behavior of austenitic C in alloys with different Mo content was determined by EPMA and the deficiency of various incomplete transformation theories was revealed. A large number of carbides precipitated under 500oC were observed when Mo addition amount exceeded 0.3.The phase transition is completed quickly.The concentration of C in austenite is lower than the T0 line during the stagnation of transformation, and decreases with the increase of Mo content and temperature, but is not affected by the grain size of the original austenite.The enrichment limit of austenite C measured by experiment is compared with T0 'theory, solute towing theory and WBs theory in detail. The three theories can not explain the experimental data well.The supercooling of austenitic C enrichment limit relative to T _ 0 line increases with the increase of Mo content, and is not the 400J?mol-1 constant value proposed by T _ 0 'theory, and the interphase velocity is too low during phase transformation stagnation.The WBs curve which can not produce significant solute drag effect is not in good agreement with the experimental data.The phenomenon of incomplete transformation of bainite remains to be further studied.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1

【相似文献】