【作者】 刘亚；

【导师】 苏旭平；

【作者基本信息】 湘潭大学 ， 材料学， 2008， 博士

【摘要】 热浸镀锌并经过合金化处理的高强度钢板,由于有优异的抗腐蚀性能,且成本较低,是汽车外板的首选材料。目前国内只有少数企业能生产此类钢板,也经常出现质量问题。为了改善镀层表面质量,锌池中常添加一定量的Al;同时从钢板或镀锌硬件中析出的合金元素,如Cr、Mn等,会跟熔融的Zn-Al合金发生相互作用,影响Zn-Fe反应,从而影响镀锌过程和镀层质量。因此,本工作中围绕Zn、Fe、Al、Cr四种组元的相互作用,测试了相关相图、研究了合金元素对镀层组织的影响并对界面反应过程进行模拟,为控制镀层组织、提高汽车外板的表面质量建立理论基础。本工作中,结合扩散偶法与平衡合金法,利用扫描电镜与能谱仪（SEM-EDS）、以及X射线衍射仪（XRD）等手段测定了Zn-Cr二元系、Zn-Fe-Cr三元系450℃和600℃等温截面,为Zn-Fe-Cr体系的热力学计算和建立高元体系的相图提供了可靠的数据。以基于陈氏晶格反演获得的原子间相互作用势为基础,模拟了合金元素对FeZn₁₃镀层组织的影响。首先利用X射线衍射仪法准确测试FeZn₁₃的晶体结构,并在Fullprof软件中利用Rietveld方法精修晶体结构,之后对FeZn₁₃的晶体结构以及合金元素在FeZn₁₃中的原子占位和溶解度进行了原子级模拟,为系统研究合金元素对镀层组织的影响积累了基础的数据。计算值与实验值基本吻合。利用SEM和EDS研究了纯铁在含Al和Cr的几种锌池中获得的镀层组织。在锌池中加入一定量的Cr以后,镀层的表面生成新相τ₁;含Cr锌池中加入0.1 wt%Al时,镀层组织由δ、ζ、η以及τ₁组成;含Cr锌池中Al含量为0.2 wt%时,镀层中的δ层变薄,ζ层由整齐的带状变为疏松的块状,在ζ相之间有diffuse-Δ出现;在480℃浸镀时,镀层中没有ζ层。根据扩散通道理论探讨了加Al和Cr导致镀层组织变化的原因,并建立了镀层形成的机理模型。本工作中以Al的扩散为动力学控制环节,分析了界面反应中化合物形核生长过程。分三阶段,即铁的快速溶解、抑制层的形核长大以及抑制层的固态扩散生长,建立了热浸镀锌界面反应的动力学模型。根据模型可以确定抑制层完整形成的时间;可以确定镀锌过程中消耗的总铁量、总铝量和抑制层的厚度与锌池中Al含量的关系。本工作对确定最优化的锌池铝含量、控制镀层组织,减少锌渣的生成有重要的理论意义,也是开发锌池管理工具的理论基础。更多还原

【Abstract】 Due to the excellent corrosion resistance, galvannealed coating of high-strength steels is the preferred material choice for exposed Autobody panels. There is only a couple of continuous galvanizers in China can produce galvannealed coatings; and the quality of the coatings is frequently problematic. To improve the surface quality, aluminum is frequently added to zinc bath. The alloying elements, such as Cr and Mn etc., coming from the steel will interact with the molten Zn-Al alloy, modifying the Zn-Fe reaction and deteriorating the quality of the coating. Focusing on the interaction between Zn, Fe, Al and Cr, the related phase diagrams were determined and the effect of alloying elements on coatings and simulation on the interfacial reaction were carried out, which is important to control the microstructure of coating, and improve the surface quality of Autobody panels.As a series of investigations, the Zn-Cr system and the isothermal section of the Zn-Fe-Cr system was determined using the equilibrated alloys with the aid of diffusion couple approach in the present work. The specimens were investigated by means of scanning electron microscopy （SEM） equipped with energy dispersive X-ray spectroscopy （EDS）, and X-ray diffraction （XRD）. These experimental data can be used in the thermodynamic modeling of the Zn-Fe-Cr system and the determination of phase diagrams with multiple element alloys.Based on the interatomic potentials obtained using the lattice inversion method, an atomistic study on the impact of alloy elements on the microstructure of coating was carried out. By X-ray diffraction combined with Rietveld structure refinement, the crystal structure of FeZn₁₃ was determined experimentally in this study. The structure of FeZn₁₃ and the site preferences and solubility of the alloying element T in FeZn₁₃ structure was investigated in atomistic-scale, which accumulate fundamental datum for investigation on the alloying elements’ impact. A good agreement between the experimental results and the theoretical calculations was achieved.The microstructure of hot dip galvanized coatings of the pure iron by dipping in several zinc baths containing Al and Cr was studied by means of SEM and EDS. The results show that: a quaternary phase,τ₁, was found in the coatings developed from Cr-added zinc bath. Adding 0.1 wt% Al to zinc bath, coatings are composed ofδ,ζ,ηandτ₁. Adding 0.2 wt% Al to zinc bath, the thickness ofδlayer decreases,ζlayer changes from banding to impact blocks, and diffuse-Δemergent between theζblocks . At 480℃,ζis absent from coating. Based on the diffusion path theory, the reason that the coating microstructure is changed after adding Al and Cr into the zinc bath is also discussed and a model describing the coating formation has been modeled.Considering the diffusion of aluminum in zinc bath is the rate-limiting process, a reasonable model was developed so as to simulate the kinetics of interface reaction. There are three stages included: the quick dissolution of iron, the nucleation and growth of intermetallics compound and the solid-state diffusion growth of inhibition layer. From this model, the following parameters: the time for nucleation, the time forming complete inhibition layer, total consumption of iron and aluminum and the thickness of inhibition layer, can be calculated. This study is beneficial to establishing the optimal Al level, controlling the structure of coating and depressing the amount of zinc-dross, and it accumulates fundamental theories for developing zinc-bath management tools.更多还原