【作者】 李智；

【导师】 贺跃辉； 苏旭平；

【作者基本信息】 中南大学 ， 材料学， 2008， 博士

【摘要】 热浸镀锌是一种用以保护钢件免受腐蚀的、在工业上广泛应用的技术。但是,含硅量高于0.07 wt%的钢在一般镀锌时,会出现镀层外观质量差、厚度较大、易于从基体上剥落等镀锌工业中所指的硅反应性现象。含磷的钢热浸镀锌时也会出现与含硅钢相似的现象。到目前为止,关于硅反应性的形成机理,还存在着很大的争议;在控制硅反应性方面,也没有一种完全有效的办法。因此,对于含硅(或磷)钢的热浸镀锌,还有许多基础工作要做。本研究就是围绕热浸镀锌的这些基础问题而开展的。在本研究工作中,进行了固态Zn/Fe、Zn/Fe-Si扩散偶实验,分析了硅对锌-铁金属间化合物形成的影响及其生长动力学规律。在Zn/Fe、Zn/Fe-Si扩散偶中,总扩散层的生长均为抛物线形式,但是,硅的加入使得Zn/Fe-Si扩散层组织中δ相的生长受到严重抑制,并且,δ层与ζ层间的界面变得极不规则,δ相呈不连续小岛状突起伸入到ζ中。硅的加入量对扩散层组织及合金层总厚度影响不大。测试了与硅反应性相关的Zn-Fe-P和Zn-Al-P两个三元系的450℃等温截面的富锌部分,得到了这两个体系在热浸镀锌温度下的相平衡关系。在Zn-Fe-P三元系450℃等温截面的富锌-铁部分,存在8个三相平衡;在Zn-Al-P三元系450℃等温截面的富锌部分,存在3个三相平衡;磷在液锌、α-Fe以及4个Zn-Fe化合物中的溶解度都非常低。根据所得到的相图,分析了磷对热浸镀锌组织的影响。含磷钢基体热浸镀锌时,镀层将出现硅反应性的现象。为了解锌池中铋对锌铁反应的作用,实验测试了Zn-Fe-Bi三元系的450℃等温截面。该截面中存在5个三相平衡和6个两相平衡,没有发现三元新相形成。铋在四种Zn-Fe化合物和α-Fe中溶解度极低,而铁在富锌相中也几乎不溶。根据热浸镀锌温度下该体系的相平衡信息,分析了锌池中的铋对热浸镀锌组织的影响。铋对控制硅反应性没有作用。用工业纯铁和含硅钢进行热浸镀锌,系统地研究了浸镀温度、钢基中硅含量以及硅磷协同作用对热浸镀锌镀层组织的影响,分析了工业纯铁和含硅钢镀层生长动力学规律。工业纯铁和含硅钢随温度升高,镀层厚度的变化规律相似,但是其镀层生长动力学不同。在450℃的锌池中进行热浸镀锌时,工业纯铁镀层总厚度呈抛物线增加,镀层生长由扩散控制,含硅钢镀层总厚度呈线性增加,其生长由界面反应控制。硅促进了ζ相的生长而阻碍δ相的生长,与固态扩散偶中硅对ζ相和δ相的作用一致。在同时含硅和磷的钢基体中,当有效硅含量达到0.07 wt%时,镀层组织呈现出硅反应性组织的特征,镀层较厚。基于对Zn-Fe-Si三元系450℃等温截面扩散通道的分析,提出了热浸镀锌中硅反应性的模型。当硅浓度较低的钢镀锌时,开始生成正常层状组织。由于硅在晶界和相界富集,导致扩散通道超越ζ相,并切割ζ相和液相的共轭线,这时液相出现在ζ相晶界形成液体通道,液锌可以通过δ相侵蚀基体,整个镀层生长由液相与δ相反应速度决定,生长速度保持不变,即镀层生长为线性规律,反应性组织的形成存在孕育期。当钢中硅浓度较高时,扩散通道开始就超越ζ相,并切割ζ相和液相的共轭线,反应性组织的形成不需要孕育期。更多还原

【Abstract】 Hot-dip galvanizing is a widespread used industrial technology in protecting steel from corrosion.But when the content of Si in steel is higher than 0.07wt%,the coating become thicker with poor appearance, and is easily flaked from the base steel in batch galvanizing.This is commonly known as silicon reactivity.Phosphorus-containing steel behaves in an analogous manner with silicon-containing steel when galvanizing.Up to now,there is still lack of convincing knowledge about the mechanism about silicon reactivity;no methods were completely effective in avoiding silicon reactivity.Therefore,there are a lot of fundamental work to do in galvanizing steels contained silicon or phosphorous.The researches are carried out about the basic problems occurred during hot-dip galvanizing.Diffusion experiments of solid Zn/Fe,Zn/Fe-Si diffusion couples were carried,and the effect of silicon on the formation and the growth kinetics of diffusion layer were analyzed.The thickening kinetics of total layer for each couple were seen to be parabolic,but growth of the phase was hindered heavily,the interface between theζphase and the phase became irregular,δlayer grew intoζlayer in island shape in Zn/Fe-Si couples because of the adding of silicon.The amount of silicon had little influence to the structure and the thickness of total layer.The zinc corner of 450℃isothermal section of the Zn-Fe-P and Zn-Al-P system had been determined experimentally,and the phase relation of these systems at the temperature related galvanizing were obtained.Experimental results indicated that,there were eight three-phase fields with no indication of a ternary compound in the portion of the Zn-Fe-P phase diagram lying below 50at.%P;there were three three-phase fields in the portion of the Zn-Al-P phase diagram lying below 50 at.%P;P solubility in liquid zinc,α-Fe and all four Zn-Fe compounds,includingζ,δ,Γ₁,andΓ,is limited at 450℃.According the phase diagram,the effect of phosphorus on the microstructure of coating was analyzed.Silicon reactivity arose in the phosphorus-containing steel while galvanizing.In order to understand the effect of Bi to the Zn-Fe reaction,the 450℃isothermal section of the Zn-Fe-Bi ternary phase diagram was determined experimentally.There were five three-phase fields and six two-phase fields with no indication of a ternary compound in this section. Bi solubility inα-Fe and four Zn-Fe compounds and Fe solubility in the Bi-rich phase were limited at this temperature.The effect of Bi in the zinc bath on the structure of coating was analyzed according to the information of ternary phase equilibria.Bi had no effect on controlling silicon reactivity.The effect of bath temperature,the content of Si in steel and the synergy of Si and P on the structure of coatings were studied systematically by hot-dip galvanizing pure iron and silicon-containing steel,and the rule of growth kinetics of the coating were analyzed.The rule of coating thickness changed with bath temperature in silicon-containing steel is similar to that in pure iron,but the growth kinetics of two steels was different.When pure iron was hot-dip galvanized in the zinc bath at 450℃,the growth of the coating was diffusion-controlled,in which the coating thickness increased parabolically with immersion time,but the kinetics of the coating in silicon-containing steel seemed to be linear suggesting interfacial reaction control.Silicon hindered the growth of the 5 phase and provoked the growth of theζphase.When the steel contained Si and P synchronously, and the effective Si reached the amount of 0.07 wt%,the coating became thicker,and the microstructure had the characteristic of silicon reactivity.The model of silicon reactivity in hot-dip galvanizing was proposed on the basis of diffusion path in the 450℃isothermal section of Zn-Fe-Si ternary system.When the steel with lower silicon was hot-dip galvanized, normal intermetallic layers formed at first.Since Si in coating trended to gather in the grain boundaries and phase boundaries,the diffusion path in coating overstepped theζphase,cut the tie-line ofζand liquid phase equilibrium.The liquid phase appeared at theζphase boundaries,in which liquid channels formed.Therefore,the liquid phase eroded the substrate through theδphase,the growth of the coating was controlled by the reaction speed between the liquid phase and theδphase,the growth speed was invariable,and the thickness of coatings increased linearly.It can be concluded that there was an incubation period in forming the reactive microstructure.When the content of Si in the steel was higher, the diffusion path overstepped theζphase from the very beginning,and cut the tie-line ofζand liquid phase equilibrium.Therefore it needn’t incubation period in forming the reactive microstructure.更多还原