雙相鋼鋼板熱浸鍍鋅鍍層合金化與高溫氧化行為之研究

Abstract

本論文主要針對高強度鋼熱浸鍍鋅鍍層分成兩部分作探討，即高強度鋼之合金元素對熱浸鍍鋅性質的影響與經高溫熱處理後鍍層顯微結構的變化與其高溫氧化性質。探討高強度鋼合金元素對熱浸鍍鋅鍍層影響的部分，本研究使用四種雙相高強度鋼並與IF鋼做比較。而探討高溫熱處理後鍍層之結構與氧化行為部分，將使用較簡單合金元素添加之雙相鋼，進行熱浸鍍鋅處理後進行高溫熱處理之研究。研究結果顯示，在利用XPS分析下發現，即使鋼材於退火處理時使用極低的露點溫度(-60~-70℃)及還原氣氛(N2+10% H2)，鋼材表面仍然生成許多種的氧化物，然而大部份的氧化物皆在熱浸鍍鋅處理時被鋅浴內的Al發生鋁熱還原而去除。而在合金化處理後的試片中發現，大部份的Al皆固溶在Fe-Zn相內，因此本研究中初始Fe-Zn相的成長機制，是Zn向Fe-Al障蔽層擴散，且當Fe-Al相內的Zn達其固溶極限時，即會發生相變態形成Fe-Zn(Al)之介金屬化合物。雖然高Si元素添加的雙相鋼(0.44 wt%)具有良好的潤濕性與熱浸鍍性質，然而合金化的效果卻明顯地被抑制。此現象主要歸因於一個原因，即鍍層內的Si元素。當鍍層內的Si元素含量增加，Fe元素在鍍鋅層內的固溶度將降低，導致鍍層內Fe元素含量太低，至使無法形成完整的介金屬化合物層。而鍍層內Si元素的來源主要可分成兩部分，第一個是鋼材表面Si的氧化物於熱浸鍍鋅時鋁熱還原反應，導致於鍍層與鋼底材界面處附近產生元素態的Si堆積，且當鋼底材內Si元素含量增加，界面處生成的Si元素亦增加。其二是於熱浸鍍初期及合金化過程中，Si元素從鋼底材融出。由於每個Fe-Zn介金屬化合物對Si元素的固溶度不同，因此，從鋼底材擴散出來的Si元素將使得ζ相無法緊鄰著鍍層與鋼底材之界面成核成長，而是必須到較遠離界面，至沒有或較少Si元素的地方才能成核成長，亦或是ζ相成核時固溶微量的Si，而於成長階段時將Si排出ζ相外。然而不管ζ相之成長機制為何，皆將提高鍍層內Si的含量比，進而影響到Fe溶出的速率，導致高Si添加的雙相鋼材經熱浸鍍鋅處理後，鍍層內的Fe-Zn相為散亂狀而非層狀堆疊。熱浸鍍鋅鍍層於800℃以上之高溫熱處理5分鐘後，鍍層顯微結構有較明顯的改變，且在X光繞射分析下得知，當熱處理溫度低於700℃並持溫5分鐘後，鍍層結構轉變成單一Γ相結構，若於800℃以上之溫度持溫5分鐘後，鍍層結構則轉變成單一α-Fe相，且鍍層表面除了有氧化鋅的訊號外，於氧化鋅與鍍層界面處亦有氧化鋁的訊號。利用化學反應式之熱力學計算後得知，Al脫離固溶體而在表面發生氧化行為的自由能較其他合金元素低，且鋅浴內添加的Al含量較少(0.12wt.%)，因此在經高溫熱處理後，鍍層表面首先生成不具保護性的氧化鋁，接著為Zn及Mn之氧化物。於化學剝除試驗下可發現，除了可以藉由不同電位值明確辨別GI (galvanized)及GA (galvannealed)內的介金屬化合物外，當熱浸鍍鋅鍍層經高溫熱處理過後，鍍層皆呈現明顯鈍態的現象，在7.5 vol%的鹽酸測試下，至少需2000秒的剝除時間才顯露出鋼底材的電位值，甚至GA-25s的試樣必須經10000秒以上的化學剝除時間才顯露出鋼底材的電位值，顯示經高溫熱處理後之熱浸鍍鋅鍍層，具有很好的抗蝕性質，且此鍍層之起始電位值低於鋼底材之電位，顯示亦具有一定之犧牲保護作用。

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.