摘要
伴随着汽车行业的快速崛起,汽车车身结构材料研究领域也在突飞猛进发展,而降低汽车自重,减少油耗、控制汽车尾气对环境造成的污染、安全行驶性能的提高成为汽车行业最为关心的话题之一。据相关方面调查,汽车自重每减轻1%,燃料的使用可以降低0.6～1.0%,因此,汽车的轻型化是减低油耗、节约能耗的重要途径之一。尽管近些年来其它轻质合金的大量使用,降低了汽车自重,但由于汽车用钢铁材料具有其不可替代的巨大优势,所以近些年,它在汽车行业材料中仍占主导地位,汽车用材的70～80%均为钢铁材料。汽车用板材中,为减轻自重,大量采用低合金高强度钢板已成为行业发展的必然态势,然而随着钢板强度的提高,其加工成型性能逐渐降低。实现当代汽车的轻量化是许多大型钢厂和汽车公司的重点研究内容,而研制和开发高强高塑性钢会极大地促进高强度钢板的制造及基础应用技术进步。
TWIP(Twinning Induced Plasticity)钢是一种孪晶诱导塑性钢,具有极高的塑性(60～80%)、中等的抗拉强度(600～800MPa)以及高的应变硬化能力,其能量冲击功是现阶段使用高强钢的两倍。因此,TWIP钢可以更好地减轻车体自重,增强车体抵抗撞击能力,减轻车身钢板的受力变形程度,提高汽车的行驶安全性,体现了环保、节能、安全的新时代要求。
本工作以面心立方结构的TWIP钢为研究对象,采用金相显微镜(OM)、X射线衍射仪(XRD)、扫描电子显微镜(SEM)电化学工作站等测试设备系统研究了不同处理状态TWIP钢试样的微观结构特征及其力学性能；通过盐雾腐蚀实验和应力腐蚀实验,研究了TWIP钢的腐蚀机制并对其应力腐蚀敏感性进行了评价；最后通过显微硬度分析、组织形貌观察,讨论了TWIP钢样品带状组织形成原因及其消除方法。
主要研究结论如下：
1.退火态和变形态TWIP钢在浓度为5%的NaCl盐雾腐蚀条件中表现出不同的腐蚀规律,前者腐蚀速率大,生成的腐蚀产物脱落严重,后者腐蚀程度较轻。作为汽车用钢,TWIP钢的耐蚀性较低,在NaCl盐雾情况下,需要对TWIP钢进行一定的表面处理。而变形后,孪晶密度的增大导致重位点阵晶界增多,使材料的耐蚀性提高,腐蚀速率降低。因此,在对TWIP钢进行腐蚀防护研究中,可以将表面处理和加工变形相结合,提高TWIP钢的耐蚀性。
2.TWIP钢试样的应力腐蚀敏感性随着试验介质中NaCl浓度的升高而增加。随着NaCl浓度的增大,试样的断面收缩率逐渐减小,延伸率逐渐减小。应变速率为1×10-5时,TWIP钢具有较好的应力腐蚀敏感性。TWIP钢的断口形貌为河流花样状,属于脆性断裂。为防止工件发生应力腐蚀,首先应合理选材,尽量减少使用应力腐蚀敏感性高的材料,可以采用抗应力腐蚀开裂的不锈钢品种,如高Ni奥氏体钢、高纯奥氏体钢等。其次,在对零部件进行设计的时候,应减少应力集中。另外,改善腐蚀环境,在腐蚀介质中添加缓蚀剂,也是防止应力腐蚀的良好措施。采用金属或非金属保护层,可以起到隔绝腐蚀介质的作用。此外,采用阴极保护法以及电化学保护也能够减小或防止止应力腐蚀。
3.带状组织的产生是由于Mn和S的存在,从而形成明暗相间的带状结构。含碳量较低时,奥氏体的稳定性较差。导致带状组织形成的过程是由浇注开始。
With the rapid progress in the structural materials of vehicle, how to reduce vehicle's weight, reduce fuel consumption,control pollution of the environment and improve the driving safety factor has aroused great concern in many automotive industries. According to some official statistics, after reducing vehicle's weight for 1%, the use of fuel can reduce by 0.6～1.0%. Therefore, make the car lighter will be an important way to reduce fuel consumption (about 50%). Although in recent years, many kinds of materials have been used to decrease vehicle's weight, the proportion of iron and steel has been decreased to some extent. But the steel has its unique advantages as car materials. It will dominate the automotive industry in the following years. The proportion of steel in automotive materials is up to 70～80%.
In order to reduce weight, a large number of high-strength steel has become a surely trend. As the strength of steel improved, its molding processing performance was worse. It is worthy doing research on the high-strength, high ductility steel to achieve lighter car in recent years, which greatly facilitated the manufacture of high strength steel and application of technical progress.
TWIP (Twinning Induced Plasticity) steel is a kind of steel with high plasticity index (60～80%), high strength (600～800MPa) and high strain hardening rate. Its impact energy absorption is doubled the usual high strength steel. Therefore, it can greatly reduce the body weight, increase body resistance to impact capacity, reduce the deformation of the body plate, and finally increase the safety of vehicle operation.
In this work, TWIP steel is investigated by using optical microscope (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical workstation system test equipment to investigate the characteristics of different shape and mechanical properties; by using salt spray corrosion test and stress corrosion test, the corrosion mechanisms were evaluated; Finally, micro-hardness analysis, organization observations were carried out to discussed the formation of banded structure in TWIP steel. Analysis the causes and elimination of banded structure. The main conclusions are as follows:
1. As-annealed and as-deformed TWIP steel placed in 5% NaCl salt spray corrosion conditions for different time. After this experiment, the corrosion rate of the former is large, the corrosion product picked off seriously, while the latter was corroded lighter. As automobile steel, TWIP steel shows low corrosion resistance in NaCl salt spray. It should be taken measures to improve TWIP steel's surface. After being deformed, TWIP steel change its microstructure. The increase of twin density and quantities of coincidence site lattice in grain boundary lead to increase corrosion resistance of the material. Therefore, in the corrosion protection study of TWIP steel, surface treatment and processing deformation can be combined to improve the corrosion resistance.
2. The SCC of TWIP will be accelerated with the increase concentration of NaCl. When the strain rate of TWIP steel is 1×10-5, it has more stress corrosion sensitivity. Different concentrations of NaCl solution on the TWIP steel SCC was significant. After SSRT tests, TWIP steel's tensile strength decreased with the increasing concentration of Cl-. The time required for fracture was shortened. 3. Banded structure was the result of the presence of Mn and S, MnS inclusions make the band structure more apparent. When the carbon content is low, austenite will be unstable.
引文
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