錳硼鋼鋼板熱浸鍍鋅鍍層高溫氧化行為與合金化之研究

Abstract

本論文主要針對22MnB5鋼板熱浸鍍鋅鍍層分成兩部分作探討，第一部分為鋅浴內鋁含量0.14wt%，第二部分為提高鋅浴內鋁含量至2.5wt%與10wt%，分別探討熱浸鍍鋅性質，鍍鋅後鍍層之高溫氧化性質與合金化之差異。第一部分研究，於450℃之鋅浴熱浸鍍3秒後，已可發現有一連續的δ相於介面處生成。此外，於500℃進行合金化處理0秒時，介面已生成Γ1相，且550℃合金化25秒後，XRD鑑定下得知，鍍層開始轉變成由Γ相為主的Fe-Zn相，顯示22MnB5鋼板於熱浸鍍鋅製程上，鍍層與鋼底材具有快速之交互反應。熱浸鍍鋅試樣經600℃及700℃預熱處理後，鍍層變得非常破碎，若當鍍層內的鐵含量較高，將使鍍層內鋅元素的蒸汽化速率小於氧化速率，此時鍍層傾向生成連續氧化皮膜，免於形成破碎的鍍層形貌。此外，在700℃的預熱處理5分鐘後，可在鍍層與鋼底材介面處生成厚度約5 μm之擴散層，爾後隨著時間增加，於30分鐘的預熱處理後，擴散層厚度可達約10 μm。然而，擴散層內的Zn含量於700℃的預熱處理下皆約為20 at%；600℃的預熱處理條件下，鍍層內的鋅含量則是逐漸增加至30分鐘的約12 at%左右。計算擴散層的成長機制，皆屬於體擴散之快速擴散機制。隨後，預熱處理過後的試片再經過950℃熱處理之後的鍍層皆產生約10 μm左右的氧化物，成分為ZnO與ZnFe2O4，直接950℃熱處理的試片氧化層較薄，成分為ZnO與ZnMn2O4，此兩種不同的氧化物皆為尖晶石結構，皆無法有效抵抗高溫氧化，而增加一預熱處理並無法改善鍍層的氧化行為，反而增加鍍層高溫氧化現象的發生，對提升抗氧化性質並無幫助。接著利用Gleeble進行高溫拉伸測試，模擬現場熱衝壓製程，可發現未經預熱處理者，具有提早破壞的現象，此現象即歸咎於液態金屬脆化的發生。 第二部分研究顯示鋅浴內較高的含鋁量將造成熱浸鍍實驗時無法全面鍍覆，以10wt%Al-Zn的鋅浴而言，需要提高鋅浴溫度至470℃才可避免裸露點的產生，鍍層結構與第一年相同皆為GI－純鋅相、GA－δ相，且發現高溫氧化性質與試片表面之元素分佈相關，若試片表面Al/(Zn+Fe)的數值越高，則抵抗高溫氧化的效果越佳，氧化物越薄，也利用短時間的預熱處理；如在550℃預熱25-50秒，能夠有效再提高表面Al/(Zn+Fe)的數值，使沃斯田鐵化後試片表面幾乎無氧化物覆蓋，鍍層結構亦轉變為單一相α-Fe(Zn)之結構，因此同時解決了熱浸鍍鋅鍍層應用在熱衝壓製程所遇到的兩大問題：高溫氧化與液態金屬脆化(Liquid metal induced embrittlement)。

The research in this thesis was mainly about the high temperature oxidation behavior and alloy reaction of hot-dip galvanized 22MnB5 steel. The first part of the results and discussion was focused in low aluminum content in the zinc bath(0.14wt%). A continuous δ phase attains between coating and steel substrate after hot dip in 450℃ molten Zn bath for 3 s on 22MnB5 steel. Moreover, Γ1 phase shows up as the galvanized specimen was heated up to 500℃ and cooled down to room temperature immediately. The Γ phase, identified by the XRD, replaces the δ phase when galvannealed at 550℃ for longer than 25 s, implying that the 22MnB5 steels with faster interaction with coating. After the preheating treatment at 600℃ and 700℃, the coating layer became porous due to the vaporization of Zn, which broken the coating. As the Fe content in coating is higher, the coating tends to oxidation rather than vaporization. The period of preheating treatment at 700℃ to form a continuous diffusion layer is 5 min with thickness of 5um, while it took 30 min to form a continuous diffusion layer with thickness of 8um at 600℃. Moreover, the Zn content in diffusion layer is about 20 at.% for specimens after 700℃ preheat treatment. The results show that higher preheating treatment temperature results in higher Zn content and faster growth of diffusion layer. The specimens after preheating treatment was sent to austenize at 950℃ for 5min, main oxides were composed of zinc oxide and iron oxide, with a thicker oxide layer compared to those without preheating treatment before austenization. Hence, it can be concluded that a preheating treatment is useless for improving the high temperature oxidation resistance, but go through a preheating treatment can avoid liquid metal induced embrittlement by attain a single phase coating composed of α-Fe(Zn) due to the longer diffusion time. The aims of the second part of the results and discussion were the effect of higher aluminum content in the zinc bath on high temperature oxidation behavior and alloy reaction. The temperature of zinc bath was increased to 470℃ to avoid the bare spots after galvanizing due to the higher viscosity of Zn-Al alloy. The microstructure of hot-dip Zn-Al alloy was studied as well, composed of pure zinc for GI-specimen and δ phase for GA. Even for such a high aluminum content as 10wt%Al-Zn, the GA specimen was still covered by thicker oxides containing zinc and iron oxide after austenization at 950℃ for 5min. The XPS results show that the oxidation behavior would depends on the atomic ratio of Al/(Zn + Fe) around the surface of the coating. The preheating treatment of the first part of the research was conducted at 600℃ or 700℃ for a long time and the galvannealing process both increase the iron content in the coating, in other words, decrease the atomic ratio of Al/(Zn + Fe) around the surface of the coating. Hence, a preheating treatment at 550℃ for 25-50 seconds was chosen to increase the atomic ratio of Al/(Zn + Fe) on the top surface due to the high affinity between aluminum and oxygen, which, in turn, improves the high temperature oxidation resistance.