【作者】 王超；

【导师】 余黎明；

【作者基本信息】 天津大学 ， 材料科学与工程， 2021， 硕士

【摘要】 中锰钢因在汽车工业应用中具有较低的经济成本和出色的综合性能而备受关注。本研究设计了一种Cr、Mo微合金化的中锰钢,研究了不同淬火温度和等温时间条件下的微观组织和机械性能变化规律,分析了微合金化对于中锰钢残余奥氏体含量及稳定性的影响。主要的研究内容和结果如下:含Cr、Mo实验钢钢经双相区淬火及低温回火处理后,获得了以板条铁素体和奥氏体为主的基体组织。随淬火温度升高及等温时间延长,残余奥氏体含量均先增加后降低,稳定性呈下降趋势。Cr、Mo的加入减小了高温淬火后板条组织的宽度,并降低了残余奥氏体含量及其稳定性。Cr、Mo的加入通过沉淀硬化与固溶强化有效地提高了中锰钢的强度,并维持与参考钢相当的塑性,因而表现出更高的强塑积。在750℃退火1 h并在200℃回火20 min后,含Cr、Mo实验钢获得了最佳的综合力学性能,抗拉强度达1272MPa,伸长率达50%,强塑积高达63 GPa%。随等温热处理时间的延长,实验钢强塑积先升高后降低,在30 min时达到最大。进一步对其强韧化机理分析发现,含Cr、Mo实验钢优异的力学性能主要与残余奥氏体含量及稳定性有关。即使Cr、Mo的加入降低了组织中的残余奥氏体含量,但由于其形成的碳化物主要分布在铁素体基体上,形成沉淀硬化,导致变形过程中残余奥氏体承担更多的应变配分,从而加速残余奥氏体向马氏体转变,形成更强的相变诱发塑性(Transformation induced plasticity,TRIP)效应。淬火温度比等温时间对奥氏体稳定性及TRIP效应的影响程度更大。在750℃经30min～6 h等温处理的样品在其加工硬化行为的第二阶段均表现出了持续的TRIP效应,而在其他温度淬火样品的TRIP效应明显不足。更多还原

【Abstract】 Medium-Mn steels have attracted a lot of attention for their low economic cost and superior comprehensive mechanical properties in automotive industrial applications.In present study,a Cr and Mo microalloying medium Mn steel was designed to study the microstructure and mechanical properties under different quenching temperature and isothermal time conditions.And the influence of microalloying on the amount and stability of retained austenite was analyzed.The main research contents and results are as follows:After the experimental steel is intercritically heat-treated in the dual phase zone,a matrix structure mainly consisted by lath ferrite and austenite is obtained.The amount and stability of retained austenite show the same changing trend with the increase of quenching temperature and the extension of isothermal time.The addition of Cr and Mo effectively reduced the grain size of the lath structure,and reduced the retained austenite amount and its stability.The addition of Cr and Mo effectively improves the strength of medium-Mn steel through precipitation hardening and solid solution strengthening,and maintains the ductility equivalent to that of the reference steel,thus letting the Cr-Mo microalloyed steel exhibiting a higher strong product of tensile strength and elongation(PSE).After Annealing at 750 ℃ for 1 h and tempering at 200 ℃ for 20 min,the Cr-Mo microalloyed experimental steel obtained the best comprehensive mechanical properties,with a tensile strength of 1272 MPa,an elongation of 50%,and a PSE of up to 63 GPa%.With the extension of the isothermal time,the PSE of the experimental steel increased first and then decreased,and reached the maximum at 30 min.With further analysis of its strengthening and toughening mechanism,it is found that the excellent mechanical properties of Cr-Mo microalloyed steel are mainly related to the amount and stability of retained austenite.Even though the addition of Cr and Mo reduced the amount of retained austenite in the structure,the carbides formed by them are mainly distributed on the ferrite matrix,providing a precipitation hardening effect and causing the retained austenite to bear more strain partitioning during the deformation process,thereby accelerating the transformation to martensite and causing a stronger Transformation-induced-plasticity(TRIP)effect.The quenching temperature has a greater influence on the TRIP effect than the isothermal time.The samples quenched at 750 ℃ for 30 min～6 h all showed persistent TRIP effect in the second stage of their work hardening behavior,while the TRIP effect of samples quenched at other temperatures was obviously insufficient.更多还原