C、Mn配分对TRIP效应钢组织与性能影响的研究

发布时间：2018-08-16 17:21

【摘要】：发展成本低、性能好的先进高强度钢(Advanced High Strength Steel-AHSS)有助于在保证汽车安全性、舒适性的前提下,提高汽车的轻量化程度,减少尾气排放,降低环境污染。淬火-配分(Quenching and Partitioning-QP)工艺能够生产一种以马氏体+残余奥氏体为主要组织,并具有相变诱发塑性(Transformation Induced Plasticity-TRIP)效应的第3代先进高强度钢--QP钢。QP工艺中的碳配分过程能够提高奥氏体的稳定性,使奥氏体保留到室温(残余奥氏体),而残余奥氏体在应力作用下能够转变为马氏体,吸收大量的撞击能量,显著地提高汽车的安全性;同时,马氏体为QP钢提供了超高强度,用QP钢替代低强度的汽车材料时能减薄汽车零件的厚度,有效提升汽车的轻量化程度。为了进一步研究QP工艺以及残余奥氏体的稳定性,本文设计了不同参数的C、Mn配分工艺,利用光学显微镜(OM)以及扫描电镜(SEM)对试验钢中的微观组织进行了观察和分析,利用X射线能谱仪(EDS)对残余奥氏体中C、Mn元素含量以及分布进行了测试,利用万能拉伸机对试验钢的力学性能进行了测试,利用X射线衍射仪(XRD)测试了残余奥氏体的含量,本文主要研究了C、Mn元素的综合配分行为对试验钢显微组织与力学性能的影响,建立了碳配分模型,界定了碳配分的起点和终点,计算了不同碳配分温度下马氏体/奥氏体(α’/γ)界面的迁移速率,并研究了拉应力对TRIP钢中残余奥氏体机械稳定性的影响,取得了如下研究成果:(1)盐浴淬火后,马氏体相变完成时碳扩散到了α’/γ界面处,造成了α’/γ界面处Fe原子在马氏体一侧和奥氏体一侧化学势的不相等,为α’/γ界面的迁移提供了驱动力,因此,α’/γ界面迁移与碳配分同时启动,由于此时α’/γ界面处Fe原子在马氏体一侧的化学势低于奥氏体一侧,在后续的配分过程中,α’/γ界面向奥氏体一侧迁移。(2)碳配分温度对α’/γ界面的迁移速率起着决定性决定作用。碳配分温度在300℃以下时,α’/γ界面的迁移速率并不明显,当碳配分温度从300℃增加到400℃时,α’/γ界面的迁移速率迅速增大。(3)C和Mn元素在残余奥氏体中的含量远远超过了试验钢中这两种元素的原始含量,证明本试验中的Mn配分是有效的。同时,通过EDS扫描结果的分析,本文还提出了一种更有利于QP工艺中Mn元素配分的低碳中锰含铜高强度钢的成分设计。(4)强塑积的变化趋势与抗拉强度和断后伸长率上升或下降的比例有关,且断后伸长率更容易影响强塑积的变化。(5)试验钢在拉断后断口出现了铁磁性现象。由于试验钢中含有α-Fe(即铁素体和马氏体),而α-Fe在居里温度以下时就呈现出铁磁性的特征,在拉伸过程中,铁素体和马氏体中磁畴为了抵抗拉应力的作用而不被破坏,这些磁畴开始向相同的方向排列,从而在断口两端形成了两个极性不同的基元磁铁群,它们之间通过相互吸引而抵抗材料被破坏。(6)残余奥氏体的转变率和机械稳定性的变化规律与应力造成的位错增殖、塞积及残余奥氏体吸收位错(Dislocation Absorption by Retained Austenite-DARA)效应有关,随应变的增加,残余奥氏体的转变率和机械稳定性的变化过程明显地分为三个阶段,应力在残余奥氏体向马氏体转变过程中的不同阶段发挥了不同的作用。
[Abstract]:Advanced High Strength Steel-AHSS with low development cost and good performance can improve the lightweight, reduce exhaust emissions and reduce environmental pollution while ensuring the safety and comfort of automobiles. Quenching and Partitioning-QP process can produce a martensite+residual Austenite. The carbon partition process in QP process can improve the stability of austenite and keep it to room temperature (retained austenite), while retained austenite can transform into martensite under stress. Absorbing a large amount of impact energy can significantly improve the safety of automobiles. At the same time, martensite provides ultra-high strength for QP steel. When replacing low strength automotive materials with QP steel, the thickness of automotive parts can be reduced and the lightweight degree of automobiles can be effectively improved. The microstructure of the test steel was observed and analyzed by optical microscope (OM) and scanning electron microscope (SEM) with the same parameters of C and Mn partitioning process. The content and distribution of C and Mn in retained austenite were measured by X-ray energy dispersive spectrometer (EDS). The mechanical properties of the test steel were tested by universal tensile machine and X-ray irradiation. The content of retained austenite was measured by X-ray diffraction (XRD). The effect of tensile stress on the mechanical stability of retained austenite in TRIP steel was studied. The following results were obtained: (1) Carbon diffused to the interface of a'/gamma when martensite transformation was completed after salt bath quenching, resulting in the unequal chemical potential of Fe atoms at the interface of a'/gamma on the martensite side and on the austenite side, which is the migration of a'/gamma interface. As the driving force is supplied, the migration of Fe atoms on the martensite side is lower than that on the austenite side. During the subsequent partitioning process, the migration of Fe atoms to the austenite side is determined by the partition temperature. (3) The content of C and Mn in retained austenite far exceeds the original content of the two elements in the test steel, which proves that the Mn matching in this test is proved. At the same time, through the analysis of EDS scanning results, the composition design of low carbon medium manganese high strength steel containing copper which is more conducive to the composition of Mn in QP process is proposed. (4) The change trend of Strength-plasticity product is related to the tensile strength and the ratio of increase or decrease of elongation after fracture, and the elongation after fracture is more likely to affect the Strength-plasticity product. (5) Ferromagnetic phenomena appeared on the fracture surface of the test steel after tensile fracture. Because the test steel contains alpha-Fe (i.e. ferrite and martensite), and alpha-Fe shows ferromagnetic characteristics below Curie temperature. During tensile process, ferrite and martensite magnetic domains are not destroyed in order to resist tensile stress, and these magnetic domains begin to be destroyed. Arranged in the same direction, two groups of elementary magnets with different polarities are formed at both ends of the fracture surface, which resist the material being destroyed by mutual attraction. (6) The transformation rate and mechanical stability of retained austenite and the stress-induced dislocation proliferation, plugging and dislocation absorption of retained austenite (Dislocation Absorptio) The n by Retained Austenite-DARA effect is related to the transformation rate of retained austenite and the mechanical stability of retained austenite are obviously divided into three stages with the increase of strain. The stress plays a different role in the different stages of the transformation from retained austenite to martensite.
【学位授予单位】：山东建筑大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG142.1

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