【作者】 秦小梅；

【导师】 邸洪双； 陈礼清；

【作者基本信息】 东北大学 ， 材料加工工程， 2011， 博士

【摘要】 由于Fe-Mn-Al-C系TWIP钢具有较高的强度、延伸率、良好的低温韧性、无磁性以及较低的合金密度等优异性能,使得Fe-Mn-Al-C钢在先进的交通运输系统特别是汽车工业以及高安全标准火车工业中具有较大的应用潜力,这使得Fe-Mn-Al-C钢在汽车工业领域以及钢铁工业领域引起了人们极大的兴趣。另外,Fe-Mn-Al-C钢也可有其他方面的应用,例如在非腐蚀环境下部分替代较为昂贵的铬-镍奥氏体不锈钢；应用于需要较高强度非铁磁性材料的高速旋转的发电机带端环转子,以减少其重量；在制冷技术领域可以用于液态气体的存储与运输。因此,随着生产工艺的不断提高和完善,Fe-Mn-Al-C系TWIP钢的应用前景将更加广阔。但是,于Fe-Mn-Al-C系TWIP钢的研究刚刚起步,尚有许多问题需要解决,如：组织性能随变形条件的变化,以及塑性变形机制的形成机理还需进一步的深入研究；抗氧化行为也需要全面的探讨,使其作为替代Fe-Cr-Ni奥氏体钢的Fe-Mn-Al-C系无磁钢、低温钢,以及某些功能合金得到迅速的发展提供理论参考。本文研究了Fe-Mn-Al-C系TWIP钢的热加工行为及组织演化规律、热轧过程中组织性能的演变、冷轧薄板组织及力学性能随固溶工艺的变化规律及拉伸变形过程中的塑性变形机制及其影响因素、加工硬化机制、TWIP钢的抗高温氧化行为,取得了如下研究成果：(1)通过高温单道次压缩实验测定了两种典型成分的Fe-Mn-Al-C系TWIP钢的流变应力曲线。研究了TWIP钢在高温压缩过程中的变形行为、变形组织特点以及组织演化行为,分析了TWIP钢高温变形抗力的影响因素并建立了变形抗力模型。通过等效应变法则分析了TWIP钢的动态再结晶过程。结果表明,应变速率对30Mn20Al钢的动态再结晶激活能有明显影响,激活能的变化使TWIP钢的再结晶难易程度发生变化；试样在单轴压缩变形过程中,具内部变形是不均匀的,等效应变分布为：试样心部最大,与压头接触部分的中间位置最小。分析了等效应变确定动态再结晶的可行性。(2)研究了Fe-Mn-Al-C系TWIP钢在单轴拉伸变形过程中的加工硬化行为和显微组织特点,通过应力—应变曲线以及加工硬化速率和加工硬化指数随真应变的变化关系,研究了TWIP钢的变形行为和相关机理。TWIP钢的单轴拉伸变形过程可分为三个阶段：第一阶段,平面位错结构是主要变形组织，位错滑移为主要变形机制,因此加工硬化指数较小,加工硬化速率迅速降低,达到屈服时此阶段结束。第二阶段,位错和形变孪晶是变形组织的主要特征,形变孪晶与位错的交互作用为主要变形机制,此阶段中30Mn20A13钢低温(700,800℃)固溶处理时因为屈服平台的出现使得加工硬化速率出现极小值,在屈服平台结束点加工硬化速率开始进入平台阶段,高温(900-1000℃)固溶处理的试样则随着屈服进入平台阶段；20Mn23AlV钢变形过程中没有出现屈服平台,所以随着试样产生屈服,形变孪晶的产生,加工硬化速率下降的速率减慢；两种TWIP钢此阶段中的加工硬化指数均随应变的增加而增加。第三阶段,不同孪晶面上产生两组形变孪晶和位错是主要的变形组织,位错和形变孪晶、形变孪晶之间的交互作用为主要的变形机制,在此阶段,加工硬化速率因为形变孪晶形成速率减慢而出现平台,在试样即将断裂时又缓慢降低,加工硬化指数达到了维持在较高水平并达到峰值。正是由于TWIP钢拉伸变形过程中得到了较高的加工硬化速率和加工硬化指数,使得TWIP钢变形时产生无颈缩的大延伸,从而获得了"TWIP"效应。(3)利用单轴拉伸结合光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)对微观组织的观察,研究了不同应变速率(2.97×104～1.49×10-1s-1)和变形温度(-60～600℃)对Fe-Mn-Al-C系TWIP钢变形机制的影响。结果发现,随应变速率的增加,TWIP钢的晶粒变形程度减弱,形变孪晶数量减少,加工硬化速率由三阶段变为两阶段；随变形温度升高,TWIP钢的变形机制由以孪生为主转变为以滑移为主；通过热力学公式估算了不同温度(t)下层错能(I)的大小,分析了层错能与变形机制的依赖关系,结果表明,当-60℃≤t≤100℃,14mJ/m2≤F≤32mJ/m2时,变形机制以孪生为主；当100℃<t<300℃,32mJ/m2<F<72mJ/m2时,滑移与孪生相互竞争是主要的变形机制；当t≥300℃时,I-≥72mJ/m2时,变形机制为滑移；当变形温度升高至600℃时,TWIP钢发生软化,并在晶界产生A1N析出,容易引起微观裂纹,造成断裂。(4)利用同步差热分析仪研究了Fe-Mn-Al-C系TWIP钢的高温抗氧化行为,分析了TWIP钢的氧化动力学及氧化膜的结构及成分。结果表明,温度高于800℃, TWIP钢的抗氧化性显著减弱,氧化曲线遵循抛物线氧化规律。随着氧化温度的升高,试样表面的颜色由蓝色变为黄色,再变为黑色,氧化膜变得均匀致密,且厚度不断增厚,表面晶粒逐渐长大；随着温度的升高,因为选择性氧化使得氧化膜的形貌发生变化。更多还原

【Abstract】 Fe-Mn-Al-C TWIP steel is a kind of steels which can be used in automobile industry. The steel meets the requirements for weight saving, fuel efficiency as well as the safety for its high energy absorption capacity, which is continuing to be a major concern on automobile manufacturing. And because it has the advantages of high strength and elongation, well low temperature toughness, non-magnetic and low alloy density etc, these make Fe-Mn-Al-C steel have potential to be applied in high safety train industry. Consequently, Fe-Mn-Al-C steel causes a great interest for applications in automobile and iron and steel industry. Furthermore, Fe-Mn-Al-C steel also can consider to be used in other fields, for example, partly replaces relatively expensive chromium-nickel austenitic stainless steel in the non-corrosive environment, applies to generator rotor with high strength and non-ferromagnetic in order to reduce its weight, and be used for liquid gas storage and transportation in the area of refrigeration technology. The application prospect will be wider with production technology improvement and development. However, the research of Fe-Mn-Al-C TWIP steel has just started, and there are many technical problems to be solved. The microstructure, mechanical property and deformation mechanism of strip cast should be thoroughly investigated.The deformation mechanism in tensile testing nedded to be deeply investigated. And the oxidation behavior of Fe-Mn-Al-C TWIP steel has not studied.In the thesis, deformation behavior and microstructure evaluation in hot compression, microstructure and properties evolution during hot rolling, effect of solution processing on microstructure and properties of cold rolling sheet, and deformation mechanism and influences factors in tensile deformation of Fe-Mn-Al-C steel were investigated by using OM, SEM, EBSD and TEM. Moreover, the oxidation residtance behavior at high temperature of TWIP steel was studied. The main original works of this thesis are presented as follows:(1) Flow stress curves of Fe-Mn-Al-C TWIP steel were obtained by high temperature single-pass compression tests. The TWIP steel deformation behavior and microstructure evolution during high temperature compression process were studied with a constitutive equation for TWIP steel being established. The dynamic recrystallization process of TWIP steel was analyzed through equivalent strain. The results show that, strain rate has an obvious effect on the activation energy for dynamic recrystallization of30Mn20A13steel. The internal deformation of the sample is inhomogeneous during compression tests. The maximum value of equivalent strain distributes in the center, and the middle position of the contact surface with indenter is the minimum. The feasibility of dynamic recrystallization determined by equivalent strain was analyzed.(2) The strain hardening behavior and microstructure evolution of Fe-Mn-Al-C TWIP steel during uniaxial tensile deformation were studied. TWIP steel possesses a multi stage strain hardening behavior during tensile deformation. The process can be divided into three stages. In stage I, planar dislocation structure is the main deformation structure, dislocation gliding is the main deformation mechanism, and strain hardening rate decrease and strain hardening exponent is exhibited. Primary deformation twinning occurred in stage Ⅱ, and the decreased rate of strain hardening rate becomes low and strain hardening exponent increases due to the blockage of dislocations’motion by deformation twin boundaries. But the strain hardening rate of30Mn20A13steel appears the minimum for its yield platform. In stage III, the strain hardening exponent increases to the highest value and suspends. The obstacle effect of twin boundaries to dislocation motion and twin-twin interaction has been observed by TEM, and the interactions between primary and secondary twins were found to cause the additional hardening in addition to the obstacle effect on dislocations’motion. This unique hardening behavior of TWIP steel leads to the TWIP effect in tensile deformation.(3) The mechanical properties of Fe-Mn-Al-C TWIP steel and the microstructure evolution with strain rate and temperature were investigated through tensile tests. Deformation behavior with three stages was observed as the change of strain hardening rate with low strain rate. While at high strain rate, there exist only two stages in the deformation behavior with respect to the strain hardening rate and true strain. Strain hardening exponent of this steel increases with increasing true strain. High density deformation twins can be produced during deformation with different strain rates. The stacking fault energy (SFE) of TWIP steel,（？）, at different temperatures was estimated by thermodynamic equations. The dependence of SFE on deformation mechanism was analyzed. It reveals that when-60℃≤t <100℃,14mJ/m2≤F|≤32mJ/m2, the governing deformation mode is deformation twin;when 100℃<t<300℃,32mJ/m2<F<72mJ/m2, the main deformation mechanism is silp and deformation twin; when t≥300℃, F≥72mJ/m2, the slipping is a predominant deformation mode; and when temperature is higher than600℃, dynamic recrystallizaiton occurs, and much A1N precipitate at austenite grain boundary, and these AlN can generate micro-crack.(4) The oxidation behavior of Fe-Mn-Al-C TWIP steel was studied at high temperature. Thermogravimetric analyses were conducted in isothermal conditions from400to1100℃for5h and microstructural and chemical analyses of the oxide films grown were performed by SEM. The oxide films were also analysed by X-ray diffraction. The results deduced that oxidation resistance property of TWIP steel weaked above800℃. A thick scale is formed on the substrate surface, which, regardless the oxidation temperature, is composed by different oxides arranged in the form of successive layers, so that the surface color changes from blue to khaki, and then to black with increasing temperature. The thickness of oxide film becomes thickening gradually and the upper layer seems tighter as increase the temperature. Regarding kinetics, the mathematical analysis of the collected data shows that the change of the mass of the substrate per unit area vs time is described by a parabolic law above800℃. Hence oxidation is impeded as the exposure time increases.更多还原