热镀锌TRIP钢还原铁/基板界面表征

 预氧化还原工艺是改善热镀锌先进高强钢(AHSS)可镀性的有效手段之一。当它用于生产热镀锌先进高强钢时,有时也会出现镀层附着性不良的问题。为了明确还原铁/基板界面对热镀锌高强钢镀层附着性的影响,以预氧化还原工艺生产的、镀层附着性不同的热镀锌TRIP钢为研究对象,采用GDOES、SEM、FIB、TEM等手段研究了还原铁/基板界面位置的微观特征。结果表明,镀层附着性不良表现为150~300 nm厚的还原铁层与镀层一起从基板表面脱落,失效发生在还原铁/基板界面,而非镀层/还原铁界面,与常见的镀层/基板界面Fe₂Al₅抑制层缺失引起的镀层附着性不良明显不同。镀层附着性不同的试样在还原铁/基板界面位置均存在硅、锰元素富集,但硅、锰富集程度有差异,镀层附着性不良试样的界面硅、锰富集更高。还原铁/基板界面氧化物形态显著影响了还原铁/基板界面的结合力,当还原铁与基板之间形成连续的氧化膜时,界面结合力较差,易发生界面分离;当还原铁/基板界面形成细小的、不连续的氧化物颗粒时,界面结合力较好。在对热镀锌先进高强钢实施预氧化还原工艺时,为了获得良好的还原铁/基板界面结合力,可以通过提高退火气氛露点促进合金元素内氧化,从而减少和细化还原铁/基板界面氧化物的数量和尺寸,避免形成连续的界面氧化膜。

Characterization of reduced-iron/substrate interface of hot-dip galvanized TRIP steel sheet

Oxidation-reduction process is an effective way to improve the galvanizability of advanced high strength steel (AHSS). When it is used to produce hot-dip galvanized AHSS,poor coating adhesion occurs occasionally. To figure out the influence of reduced-iron/substrate interface on coating adhesion of hot-dip galvanized AHSS,two TRIP steel sheets with different coating adhesion,which were industrially produced by oxidation-reduction process,were characterized by means of GDOES,SEM,FIB and TEM. It was found that a 150-300 nm thick reduced iron layer was peeled off together with the zinc coating from the steel substrate,indicating that the coating adhesion failure occurred at the reduced-iron/substrate interface instead of the coating/reduced-iron interface. It was totally different from the common coating adhesion failure caused by the absence of Fe₂Al₅ inhibition layer at the coating/substrate interface. Different degree of Si and Mn enrichment was found at the reduced-iron/substrate interface for the samples with different coating adhesion,in which the samples with poor coating adhesion showed higher Si and Mn enrichment. The bonding strength of the reduced-iron/substrate interface was significantly affected by the morphology of the interface oxides. When a continuous oxidation film was formed between the reduced-iron and substrate,poor interface bonding strength might cause coating adhesion failure easily. When a small amount of fine and discontinuous oxide particles was formed between the reduced-iron and substrate,the interface bonding strength was improved. To achieve good reduced iron/substrate interface bonding strength when oxidation-reduction process is performed on hot-dip galvanized AHSS,it is still necessary to promote the formation of internal oxidation by increasing the dew point of the annealing atmosphere. Therefore,the amount and the size of the oxides at the reduced-iron/substrate interface can be reduced,and the formation of continuous oxidation film can be prevented.