【作者】 赵满秀；

【导师】 尹付成；

【作者基本信息】 湘潭大学 ， 材料学， 2012， 博士

【摘要】 热浸镀锌用于钢铁防护已有100多年的历史，但含硅钢的硅反应性和热浸镀锌时镀锌设备的腐蚀这两大问题一直困扰着镀锌业。目前抑制硅反应性的最有效方法是在锌池中添加合金元素，最常用的元素为镍。但镍仅对硅含量小于0.2%的钢的硅反应性有较好的抑制作用，且镍的价格昂贵，在锌池中会形成大量的锌渣。因此有必要寻求一种低成本且能抑制高硅钢硅反应性的镀锌合金。为抑制Fe-Zn界面反应，提高镀层表面质量，降低成本，或使镀层获得更优异的使用性能，所有的热浸镀锌池中均添加了不同含量的铝。钴基合金因成本低使用性能好被广泛用于热浸镀锌的沉没辊等设备用材料，但随着锌池中铝含量的增加，镀锌设备的使用寿命大大缩短。因此，有必要了解钴基合金与锌铝熔池的相互作用。增加锌池中的铝含量可提高镀层的耐蚀性和耐热性，热浸镀55%Al-Zn是应用前景最好的一种镀锌技术，但铝含量高时，铁会与铝锌池中铝发生剧烈的放热反应，快速形成脆性的铁铝化合物，严重影响钢铁制品的性能。本工作围绕上述问题开展了相关研究工作。利用平衡合金法和扩散偶相结合的方法，借助扫描电镜-能谱仪-波谱仪、电子探针和X射线衍射等分析技术研究了Zn-Al-Co、Zn-Co-Ni和Zn-Fe-Co-Si系的相平衡。通过浸镀实验研究了Co、Ni、Si对热浸镀锌界面反应的影响，并对其作用机理进行了研究。为弄清钴基合金与锌铝熔池的相互作用及锌池中Co、Ni的协同加入对Fe-Zn界面反应的影响，本工作研究了Zn-Al-Co系450、600和800°C及Zn-Co-Ni系600和450°C的相关系。在Zn-Al-Co系450、600和800°C等温截面中分别存在10个、9个和7个三相区。除Al3Co外，其它的Al-Co二元化合物均能与液相平衡，AlCo能与所有的Co-Zn二元化合物平衡。Zn在AlCo和Al13Co4中的溶解度随着温度的升高而增大，在Al5Co2中的溶解度随温度升高而减小，其在AlCo、Al5Co2和Al9Co2中的最大溶解度分别为12.0、18.1和1.6at.%。Al在Co-Zn化合物中的溶解度有限，分别不超过0.1、0.3、0.9和1.4at.%。在本工作中，实验获得了CoZn和Co5Zn21的X射线衍射数据及晶体结构。在Zn-Co-Ni系600和450°C等温截面中，均存在三个三相区，α-Co/α-Ni、γ-Co5Zn21/γ-Ni4Zn22和γ2-CoZn13/δ-NiZn8分别形成连续固溶体。Ni在β1(CoZn)和γ1(CoZn9)中的最大溶解度分别为11.8和8.1at.%，Co在β1’(NiZn)中的最大溶解度为16.5at.%。没有三元化合物存在于Zn-Co-Ni三元系中。为研究Co对Fe-Zn界面反应的作用机理及为开发新型的Zn-Co基合金提供理论基础，本工作研究了Zn-Co-Si三元系450和600°C等温截面及Zn固定在93at.%的Zn-Fe-Co-Si四元系450°C富锌角的相关系。在Zn-Co-Si三元系450和600°C等温截面中分别存在9个和8个三相区，除β1外，CoSi相能与所有的二元Co-Zn金属间化合物共存。锌在Co-Si二元化合物中的溶解度随着温度的升高而增大，其在CoSi2,CoSi和Co2Si中的最大溶解度在450°C时分别为2.0、4.3和2.7at.%，在600°C时分别为2.0、6.2和5.4at.%。在Zn-Fe-Co-Si四元系450°C等温截面中存在两个四相区。ζ-FeZn13和ζ-CoZn13形成连续固溶体。铁在CoSi2中的溶解度不超过1.2at.%，而钴在FeSi2中的溶解度可达7.8at.%，锌在(Fe,Co)Si、FeSi2和CoSi2中的最大溶解度分别为1.9、1.5和2.9at.%。硅在二元金属间化合物ζ-FeZn13中的溶解度是极其有限的，但随着钴原子溶解到这个化合物中，硅在ζ-(Fe,Co)Zn13中的溶解度可达到0.8at.%。为开发低成本且能抑制高硅钢硅反应性的锌基合金，本工作对不同硅含量的铁硅合金在含不同Co、Ni成分的锌池中进行浸镀，研究了Co或Co-Ni的协同加入对镀层组织和生长动力学的影响。研究结果表明，锌池中适量的钴使ζ相呈致密的柱状分布，避免Diffused-△相及爆发组织的形成，提高了镀层表面质量。当锌池中加入钴后，金属间化合物层的生长受扩散控制，其厚度与浸镀时间呈抛物线关系。钴对含硅量不大于0.4wt.%的高反应性钢的Fe-Zn界面反应有较好的抑制作用。锌池中加入0.1wt.%Co抑制效果最佳。当浸镀时间不大于180s时，熔池中加入0.05～0.2Cowt.%或0.05Co-0.05Ni wt.%，对硅含量不大于0.5wt.%高反应性钢的Fe-Zn界面反应均有较好的抑制作用。用所获得的Zn-Fe-Co-Si四元系的相关系解释了钴对含硅钢界面反应的作用机理。随着钴溶解到ζ-FeZn13中，硅在该化合物中的溶解度可达0.8at.%，这避免了硅在ζ-FeZn13晶界的富集及液相通道的形成，促进致密的ζ-FeZn13相层形成，从而抑制了硅的反应性。为抑制热浸镀55%Al-Zn时Fe-Al界面反应，本工作研究了锌铝池中的硅对镀层组织和生长动力学的影响。研究结果表明，熔池中的硅能改变铁与铝锌熔体界面反应形成的化合物类型和顺序。含硅的Fe2Al5趋于稳定，能有效避免Fe基与熔体的直接接触，抑制热浸镀锌铝界面反应。当熔池中的Si含量分别为0.6、1.6、2.6、3.0和3.6wt.%时，Fe2Al5的扩散激活能分别为207、274、275、260和237KJ/mol。实验获得了不同浸镀条件下的Fe2Al5生长速度常数和熔池中不同含硅量时Fe2Al5的扩散激活能。获得了熔池中的硅对镀层中金属间化合物的形成顺序和组织演变的影响规律。该研究成果有助于优化热浸镀55%Al-Zn生产工艺、控制锌渣的形成及获得高性能的镀层。更多还原

【Abstract】 Hot dip galvanizing used for steel protection has a history of over100years.However, Si reactivity and equipment corrosion have been troubling the galvanizingindustry. Galvanizing the steels in alloyed baths is one of the effective ways used tocontrol silicon reactivity. Nickel is the most used element added to zinc bath. However,The expensive nickel in zinc bath can only restrain the Si reactivity of the steel with lessthan0.2wt.%Si, and a mass of zinc dross would form as nickel element is added to zincbath. Therefore, it is necessary to develop a zinc-based alloy with lower cost to suppressthe Si reactivity of the steel with higher content of Si. All zinc baths contain differentlelves of aluminum for different purpose, such as suppressing Fe-Zn interface reaction,improving surface quality, or obtaining coating with better performance. Cobalt-basedsuper-alloys materials are used for both sink roll and stabilizer roll in the galvanizingindustry due to their relatively low cost and reasonably good performance. However, theservice life of Cobalt-based super-alloys are shorten as Al concentration in the molten zincbath increased. Therefore, it is necessary to understand the interaction of the Co-basedalloy and Zn-Al bath. To satisfy the demands for better corrosion-resistance andoxidation-resistance at elevated temperatures, higher levels of aluminum are added to Znbath. Hot dip55%Al-Zn is a very promising technique used for steel protection. However,the higher levels of aluminum would result in the strong and rapid exothermic reactionbetween the steel matrix and the A1-Zn bath. The brittle Fe-Al compounds whichseriously undermine the performance of the iron and steel products form quickly. Therelated researches are carried out about the above problems in the present work. The phaseequilibria of the Zn-Al-Co, Zn-Co-Ni and Zn-Fe-Co-Si systems have been determinedusing equilibrated alloys with the aid of a diffusion couple approach. The specimens wereinvestigated by means of scanning electron microscopy equipped with energy dispersiveX-ray spectroscopy, electron probe microanalysis and X-ray diffraction. The effect of Co,Ni, Si on interface reaction during the hot-dip galvanizing have been studied by hot-dipgalvanizing and hot-dip55%Al-Zn.To understand interaction between cobalt-based alloys and the molten Al-Zn and theinfluence of contemporary use of Co and Ni in zinc bath on Fe/Zn interface reaction, thephase relations of the Zn-Al-Co system at450,600, and800°C and the Zn-Co-Ni systemat450and600°C have been investigated. Ten three-phase regions exist in the isothermalsection at450°C, nine at600°C and seven at800°C. The liquid phase is in equilibrium with all Al-Co compounds except Al3Co. AlCo phase can coexist with all Co-Zn binaryphases. The Zn solubility in AlCo and Al13Co4increases with temperature increases, andthat in Al5Co2decreases. The maximum solubility of Zn in AlCo, Al5Co2, Al9Co2is12.0,18.1and1.6at.%, respectively. The Al solubility in the Co-Zn compounds is extremelylimited, no more than0.1at.%,0.3at.%,0.9at.%and1.4at.%The X-ray powderdiffraction patterns and crystal structures of the CoZn and Co5Zn21have been obtainedexperimentally. Three three-phase regions have been confirmed in the isothermal sectionsof the Zn-Co-Ni ternary system at450°C and600°C, respectively. Three continuous solidsolutions regions, i.e., α-Co/α-Ni, γ-Co5Zn21/γ-Ni4Zn22and γ2-CoZn13/δ-NiZn8, exist in thesections The maximum solubility of Ni in β1(CoZn) and γ1(CoZn9) is11.8and8.1at.%,and that of Co in β1’(NiZn) is16.5at.%,. No true ternary compound was found inZn-Co-Ni ternary systems.To understand the mechanism of action of Co on Fe/Zn interface reaction and providethe theory foundation for development of zinc-based alloys, the phase relations of theZn-Co-Si ternary system at450and600°C and the450°C isothermal section ofZn–Fe–Co–Si quaternary system with the Zn content being fixed at93at.%have beeninvestigated. Nine three-phase regions exist in the isothermal section of Zn-Co-Si systemat450°C, and eight at600°C. The CoSi phase can coexist with all compounds in Zn-Cobinary system except the β1-CoZn phase. The Zn solubility in Co-Si binary compoundsincreases with temperature increases. The maximum solubility of Zn in CoSi2, CoSi andCo2Si is about2.0at.%,4.3at.%and2.7at.%at450°C, and2.0at.%,6.2at.%, and5.4at.%at600°C, respectively. It was found that2four-phase regions exist in the isothermalsection of Zn–Fe–Co–Si quaternary system at450°C. No true ternary or quaternarycompound was found in the present study. The ζ-FeZn13and ζ-CoZn13form a continuoussolid solution. The Fe solubility in CoSi2is no more than1.21at.%, whereas the Cosolubility in FeSi2is high up to7.8at.%. The maximum solubility of Zn in (Fe,Co)Si,FeSi2, and CoSi2is1.9,1.5,2.9at.%, respectively. The Si solubility in binary ζ-FeZn13phase is rather limited, with Co dissolving in this compound, the solubility of Si is up to0.8at.%.To develop a low cost zinc-based alloy used for suppressing the Si reactivity of thesteel with higher content of Si, the interface reactions between the iron-silicon alloys anddifferent baths containing Co or Co and Ni have been studied. An appropriate amount ofCo in Zinc bath make the ζ phase be columnar, and avoid the formation of the diffused-△phase and outbreak microstructure. When Co element was added into zinc bath, thegrowth of the intermetallic layers was diffusion-controlled, in which its thickness increased parabolically with immersion time. The suppressing effect of Co in the zinc bathon silicon reactivity of iron–silicon alloys with no more than0.4wt.%Si is better, and thebest effet is achieved when Co content increased to0.1wt.%. Adding0.05～0.2wt.%Coor0.05Co+0.05Ni wt.%to zinc bath can effectively restrain Fe/Zn interface reaction ofhigher reactivity steel with no more than0.5wt.%Si when immersion time is on morethan180s. The mechanism of action of Co on Fe/Zn interface reaction is explained usingthe phase relations of the Zn-Fe-Co-Si quaternary system. The Si solubility in the ζ-FeZn13phase is up to0.8at.%with Co dissolving in this compound, which avoids the enrichingof silicon at the boundary of ζ-FeZn13phase and the forming of the liquid channel. Thecompact ζ-FeZn13layer could form, and the silicon reactivity during the hot-dipgalvanizing of the Si-containing steel is restrained.To supress Fe/Al interface reaction, the effect of silicon in the55%Al-Zn bath on themicrostructure and the growth kinetics of the coating has been investigated. The typiesand forming sequence of intermetallic compounds in the55%Al-Zn coating were changedas silicon is added to the bath. The containing-Si Fe2Al5tends to stable. The interfacereaction can be effectively holded back by Fe2Al5with Si. The activation energy of Fe2Al5was evaluated to be207,274,275,260, and237kJ/mol, when the content of silicon in thebath is0.6,1.6,2.6,3.0, and3.6(wt.%) respectively. The growth rate constant of Fe2Al5was obtained under different conditions. The forming sequence of intermetalliccompounds and the evolving regulars of the microstructure in the coating have beeninvestigated. The results obtained by present work could help to optimize technologicalparameters, control zinc dross forming and obtain high performance coating.更多还原