摘要
合金化热镀锌钢板具有良好的焊接性、涂着性、耐热性和耐腐蚀性,已广泛应用于汽车制造业。但是合金化镀锌板在冲压过程中,其镀层容易出现粉化等缺陷,严重制约了我国汽车工业的发展。研究表明,钢中存在的微量合金元素磷对镀层合金化过程有较大的影响。为了弄清磷元素对Fe-Zn合金化反应的作用机理,以及深入研究热浸镀锌过程中磷和铝对镀层组织的协同作用,本论文展开了如下几个方面的研究。
为了准确获得Zn-Fe-Al-P四元系富锌角450℃等温截面相关系,本工作利用扫描电子显微镜、能谱分析仪和X射线衍射仪等手段,采用平衡合金法重新测定了Zn–Al–P三元系在450℃的等温截面相关系,得到的实验结果与已报道的Zn–Al–P三元系的相关系完全不相同。在这一等温截面的富锌部分,存在3个三相平衡区,即η-Zn+AlP+Zn_3P_2、AlP+Zn_3P_2+ZnP_2及η-Zn+AlP+α-Al。AlP相能与所有的其他物相平衡共存,包括η-Zn、α-Al、Zn_3P_2和ZnP_2。磷在η-Zn和α-Al相中的溶解度很低,Zn在AlP相中的溶解度约为1at.%。
依据以上实验结果,采用平衡合金法测定了含93at.%Zn的Zn-Fe-Al-P四元系450℃等温截面相关系。研究结果表明:在该等温截面上出现了6个四相平衡区,即η-Zn+FeAl_3+Fe_2Al_5+AlP,η-Zn+T+Fe_2Al_5+AlP,η-Zn+FeP+Zn_3P_2+AlP,η-Zn+δ+T+AlP,η-Zn+ζ+δ+AlP和η-Zn+ζ+FeP+AlP。所有的合金样品中没有发现四元新相的存在。AlP相与除Fe_2P之外的所有其它相共存,包括L-Zn、ζ、δ、T、Fe_2Al_5、FeAl_3、FeP及Zn_3P_2。磷在ζ、T、Fe_2Al_5及FeAl_3相中的溶解度都极小,而Zn能够存在于所有的已知化合物中。
通过Zn/Fe和Zn/Fe-P固液扩散偶实验,发现钢基体中的磷能够延迟或者抑制Г相的形成,但促进镀层中ζ相的生长,且磷含量越高,ζ相的生长速度越快,类似于一般镀锌中的硅反应性。
热浸镀锌实验结果表明:在0.15wt.%Al-Zn池中浸镀纯铁片时中,由于铝和铁在铁基表面与镀层之间形成一层抑制层,使镀层中先形成δ相,随后形成一层很薄的Г相。在铁基体中添加少量的磷,能显著抑制Fe-Zn反应。
Galvannealed steel sheets are widely used in the automotive industry, due to their excellent weldability, paintability, thermostability and corrosion resistance. However, in the course of stamping, the galvannealed coating may suffer from defects, such as powdering. So it has seriously hampered the development of China's automobile industry. It has been reported that P in steel has great effects on alloying reaction between Fe and Zn during the galvannealing treatment. In order to understand the mechanism of phosphorus on the Fe-Zn alloying reaction, and intensively study the combined effect of P and Al in the hot-dip galvanizing, the following works have been carried out.
To obtain Zn-rich corner of 450℃isothermal section of the Zn-Fe-Al-P quaternary system, the 450℃isothermal section of the Zn-Al-P ternary phase diagram was investigated experimentally again using scanning electron microscopy coupled with energy dispersive x-ray spectroscopy, and x-ray diffraction. The experimental results are quite different from the reported phase equilibrium relationships of the Zn-Al-P ternary system. (L-Zn+Zn_3P_2+AlP), (L-Zn+α-Al+AlP), and (Zn_3P_2+ZnP_2+AlP) three-phase regions exist in the section. The experimental results reveal that AlP is in equilibrium with L-Zn,α-Al, Zn_3P_2, and ZnP_2, respectively. P solubility in liquid Zn and inα-Al phases is limited.
Based on the above results, the 450℃isothermal section of the Zn-Fe-Al-P quaternary system with Zn being fixed at 93at.% was determined by the equilibrated alloys approach.η-Zn+T+Fe_2Al_5+AlP,η-Zn+FeAl_3+Fe_2Al_5+AlP,η-Zn+δ+T+AlP,η-Zn+ζ+δ+AlP,η-Zn+FeP+Zn_3P_2+AlP, andη-Zn+ζ+FeP+AlP four-phase regions were found to exist in the section. No quaternary phase was found in all the equilibrium alloys. AlP was found to be in equilibrium with L-Zn,ζ,δ, T, Fe_2Al_5, FeAl_3, FeP and Zn_3P_2, but not with Fe_2P. The solubility of P inζ, T, Fe_2Al_5, and FeAl_3 is very limited; Zn is soluble in all known compounds.
The experimental results of Zn/Fe and Zn/Fe-P liquid-solid diffusion couple indicate that P in steel can delay or inhibit the formation ofГphase. But P in steel has been found to promote the growth of theζphase, and the higher phosphorus content, the faster the growth speed of theζphase. Obviously, the role of phosphorus on the alloy layer is similar to silicon reactivity of the general galvanizing.
The experimental results of pure iron hot-dip galvanizing show that total Fe-Zn alloy layer growth in the 0.15wt.%Al-Zn bath was inhibited by the initial formation of an Fe-Al intermetallic layer at the steel/coating interface in a very short time. With the increase of dipping time,δphase was found to form at first, and then a thinГphase layer formed at the steel/delta-phase interface. A small amount of phosphorus added to the substrate steel can significantly inhibit the Fe-Zn reaction by the hot-dip galvanizing.
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