利用SEM观察了22MnB5钢在900 ℃不同奥氏体化时间下,热镀Al-10%Si (质量分数)镀层的微观组织变化情况,利用EDS和GD-OES分析了奥氏体化后热镀Al-10%Si镀层的元素分布。结果表明,22MnB5钢奥氏体化前,热镀Al-10%Si镀层主要由纯Al、纯Si和二者共晶反应形成的金属间化合物Fe₂SiAl₇组成,在Fe₂SiAl₇和钢基体之间存在一层薄薄的由Fe₂Al₅和FeAl₃组成的化合物层。900 ℃奥氏体化后,热镀Al-10%Si镀层中的三元共晶相Al+Si+τ₆逐渐转变为三元Al-Fe-Si或二元Fe-Al金属间化合物。奥氏体化时间为2 min时,镀层由Fe₂SiAl₇、Fe₂Al₅和FeAl₂组成;奥氏体化时间为5 min时,镀层由FeAl₂、Fe₂SiAl₂和Fe₅SiAl₄组成;奥氏体化时间为8 min时,镀层由FeAl₂和Fe₅SiAl₄组成。由于Fe₂SiAl₂和镀层/钢基体界面扩散层中Al原子的扩散系数远大于Fe原子,导致从镀层向钢基体晶界及晶粒内扩散并与之反应所消耗Al原子的量远大于从钢基体扩散到镀层中的Fe原子量,从钢基体中流入到镀层中的空位数量远大于从镀层中流入到钢基体中的空位数量。原子的不平衡扩散及镀层/钢基体界面空位数量的富余使得扩散反应层与镀层的交界区域形成了Kirkendall空洞。22MnB5钢奥氏体化时,热镀Al-10%Si镀层表面形成一层稳定的Al₂O₃氧化膜,镀层的高温氧化现象非常有限,热镀Al-10%Si镀层可以作为22MnB5钢热成形时的保护层。但热镀Al-10%Si镀层扩散过程中产生的脆性金属间化合物因高温塑性不足而导致镀层中产生大量垂直于镀层/钢基体界面并贯穿整个镀层的微裂纹,从而影响镀层的防护性能。

Hot stamping is an alternative technology to produce ultra-high strength steel (UHSS) with a tensile strength above 1 GPa for automotive bodies. At present, the hot-dip Al-10%Si (mass fraction) coating is used as a shield coating for the hot stamping steels, which protects the steels from surface oxidation and decarburization, and enhances their corrosion resistance. However, the microstructure evolution and compounds of hot-dip Al-10%Si coating during austenitization of 22MnB5 hot stamping steel are not clear yet. In this work, the thermo-mechanically induced microstructure evolution of hot-dip Al-10%Si coating is observed using SEM after austenitization of 22MnB5 hot stamping steel at 900 ℃ for different times, and the elemental depth profiles are analyzed in hot-dip Al-10%Si coating by EDS and GD-OES. The results show that before austenitization, the hot-dip Al-10%Si coating consisted of an aluminum matrix, pure silicon, and the intermetallic compound Fe₂SiAl₇, which was formed by eutectic reaction, there was a thin layer, which was composed of Fe₂Al₅ and FeAl₃ between the intermetallic compound Fe₂SiAl₇ and the steel substrate. When 22MnB5 hot stamping steel was austenitized at 900 ℃, the ternary eutectic phase Al+Si+τ₆ was transformed into an Al-Fe-Si ternary intermetallic compound or Fe-Al binary intermetallic compound gradually in the hot-dip Al-10%Si coating. When the austenitizing time was 2 min, the Al-10%Si coating was composed of the intermetallic compound Fe₂SiAl₇, Fe₂Al₅ and FeAl₂ phases; when the austenitizing time was 5 min, the Al-10%Si coating was composed of FeAl₂, Fe₂SiAl₂ and Fe₅SiAl₄ phases; when the austenitizing time was 8 min, the Al-10%Si coating was composed of FeAl₂ and Fe₅SiAl₄ phases. Because the diffusion rate of Al atoms was much larger than that of Fe atoms in the diffusion layer of intermetallic compound Fe₂SiAl₂ and coating/steel substrate, the amount of Al atoms which diffused and reacted from the coating to the grain boundaries or grain of steel substrate was much larger than that of the Fe atoms which diffused from the steel substrate to the Al-10%Si coating, also the number of vacancies which diffused from the steel substrate to the Al-10%Si coating was much larger than the other way round. Due to this imbalance, the Kirkendall void was formed in the interface between the diffusion reaction layer and the Al-10%Si the coating. The hot-dip Al-10%Si coating can be used as the protective layer, since it has a stable Al₂O₃ film formed on its surface, and its thermal oxidation was very limited, during the 22MnB5 hot stamping steel austenitizing. But the protective performances of Al-10%Si coating could be poor, because the high temperature ductility of brittle intermetallic compound was low, which induced a lot of micro cracks that were perpendicular to the interface of coating/steel substrate, and penetrated the whole coating during the diffusion process of hot-dip Al-10%Si coating.