Alloy reaction and high temperature oxidation behaviors of hot-dip galvanized dual phase steels
Date Issued
2014
Date
2014
Author(s)
Lin, Ko-Chun
Abstract
The aims of the present dissertation were to understand the hot-dip galvanized advanced high strength steel (AHSS) on alloy element reaction and high temperature oxidation behaviors. Four kinds of CMnSiCr dual-phase steels using a hot-dip simulator to investigate the effect of the alloying elements on the microstructure of the galvanizing (GI) and galvannealing (GA) coatings in first part. The results show that the dual-phase steels had good galvanizability because no bare spot was observed and the Fe-Zn phases were readily formed at the interface in GI specimens. However, the alloy reaction during the GA process was significantly hindered with increasing Si content in the steel substrate. XPS results show that external selective oxidation took place under an extremely low dew point (-60~-70℃) atmosphere during the annealing prior to hot dipping. However, most of the oxides were reduced during the hot-dipping process by the aluminothermic reduction. After the hot-dipping process, the Al was solid-soluted in the Fe-Zn phase, suggesting that the Fe-Zn phase was formed from the transformation of the Fe-Al inhibition alloy. Meanwhile, the Si enriched at the interface owing to the aluminothermic reduction and dissolued from the steel substrate during the GI and GA reaction. Owing to the solubility of Si in the ζ phase is extremely low, hence, the ζ phase cannot homogeneously nucleate at the steel substrate/Zn coating interface, but can be found at the area away from the interface. Moreover, the coating enriched with Si leads to decreas the solubility of Fe in the Zn coating, and results in lower Fe content than general GA steels does. Therefore, the Fe-Zn phases on the high Si content dual-phase steels were relatively non-uniform compared to those on interstitial-free (IF) steel. High temperature oxidation behavior on hot-dip galvanizied coating was thoroughly discussed in second part. The results of second part show that the morphologies of galvanized coating layers on GI and GA-25s specimens were obviously different from as-received to 800 and 900 ℃ heat-treated specimens. The specific microstructure of coating layers were thoroughly identified by X-ray diffraction, which show that the microstructure of coating layers were mainly with Γ phase when heat treatment temperature was below 700 ℃, whereas the coating layers were mainly with α-Fe(Zn) and ZnO when heat treatment temperature was above 800 ℃. Moreover, two oxide layers were observed by EPMA mapping and EDS analyses, where the upper part was identified as an oxide layer with the composition of Zn and Mn, and the lower part was an incompact alumina oxide layer beacuse lower Al content in the Zn bath. The chemical stripping tests were used to analyze the anticorrosion properties of heat-treated coating layers. The results show that heat-treated GI and GA-25s specimens at 800 and 900 ℃ presented in nobler properties in 7.5 vol% HCl solution than those of as-received specimens, which heat-treated GI specimen took at least 2000s of stripping to present in final stable steel substrate potential. More than 10000s of stripping time is needed for the 900℃ heat-treated GA-25s specimen. Otherwise, compared to the heat-treated steel substrates, lower start potential of heat-treated coating suggests the coating with sacrificial properties.
Subjects
高強度鋼
熱浸鍍鋅
露點溫度
合金化
障蔽層
化學剝除
Type
thesis
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