鋅合金保護鍍層鋼板之熱衝壓行為研究

Abstract

運用於熱衝壓製程之熱浸鍍鋅鋼材，存在著鋅氧化與液態金屬脆化(Liquid-Metal Embrittlement, LME)的疑慮，造成其應用受限。本研究中，以凝膠溶膠法製備矽烷化合物，輥塗於熱浸鍍鋅鋼板藉以形成複合保護鍍層; 鋁含量改變之鋅鋁鍍層，探討該保護鍍層於熱衝壓行為之顯微組織演化、拉伸破壞行為與鍍層之抗蝕性評估，希冀提供鋅合金保護鍍層設計之依據。 透過輥塗法，於熱浸鍍鋅鍍層披覆一層矽烷化合物鍍層，經過900℃熱處理五分鐘之後，其表面氧化獲得抑制。此外，由於鋅揮發獲得改善，合金鍍層內鋅含量也有保留，授予鍍層相當的犧牲陽極抗蝕能力。由於較薄鋅鍍層的使用，液態金屬脆化也改善。在後續電著塗裝層連續鹽霧試驗，複合合金鍍層對於電著塗裝未發現有明顯的負面影響。顯示矽烷化合物鍍層/鋅複合有效的提供障蔽保護抵抗高溫氧化，且讓合金鍍層的抗蝕能力獲得改善。 了解鋁對於高溫氧化之影響，探討5 wt.%鋁-鋅鍍層其在熱衝壓製程之顯微組織演化，與其氧化及電化學等抗蝕能力。在沃斯田體熱處理初期，原始鍍層內的初晶鋁與鋼底材反應，於鍍層/鋼底材界面形成Fe2Al5Znx相，而外層保持為純鋅。隨著溫度上升，Γ-Fe3Zn10 與α-Fe(Zn)漸成為合金鍍層組成，且於表面生成氧化層。氧化鋁混參氧化鋅成為表面氧化層的主要組成型態。沃斯田體熱處理使得鋼底材的鐵往鍍層擴散，形成鐵-鋅合金鍍層，造成合金鍍層之腐蝕電位鈍化且腐蝕電流密度抑制。 相互比較熱浸鍍鋅/鋅鐵鍍層與5 wt.%鋁-鋅鍍層，其顯微組織演化與特性改變，探討其應用性。結果顯示鍍層的 5 wt.% 鋁可有效抑制高溫氧化，然而鍍層中的來自於熱鍍浸鐵鋅合金化的鐵，卻是惡化氧化的來源。LME可以透過鋁或鐵在鍍層之合金化而獲得改善。在鹽酸測試液中，5 wt. %鋁的存在，可以有效的改善原始鍍鋅層中抗蝕性，且在氯化鈉水溶液中降低陽極溶解。然而考量到犧牲陽極保護在熱衝壓件之應用效果，沃斯田體化後之鍍鋅層提供最好的犧牲陽極保護能力。但總體表現，5 wt. %鋁鋅鍍層展現最好之抗高溫氧化、減少LME敏感性，且提供充足的犧牲陽極保護效果。 使用電子背向散射繞射技術(Electron Backscatter Diffraction, EBSD)探討鋁-鋅鍍層之相演化始終為一個開放性的題材。本研究首次使用EBSD探討55 wt.% 鋁-鋅合金鍍層相演化。初始升溫階段，FeAl3Si2相生成在鍍層/鋼底材界面，接著為鋁、鋅、FeAl3與Fe2Al5Znx相，且Fe2Al5Znx相在後續升溫中持續成長。900℃持溫結果，合金鍍層主要為鋅、 FeAl及Fe2Al5Znx相，且FeAl相隨著持溫時間增加而持續成長。然而合金鍍層與鋼底材之電位差未受到熱處理而有明顯影響，且合金鍍層的鈍化現象，源於Fe-Al相之貢獻。從EBSD 極圖結果，Fe2Al5Znx相之c-軸方向與鋼底材垂直，且朝向合金鍍層方向，顯示Fe2Al5Znx相在整個熱處理過程中，依織構方向成長。 採用高溫拉伸試驗了解其合金鍍層破壞行為，探討 55 wt.%鋁-鋅鍍層於熱衝壓時之合金鍍層破壞行為。高溫拉伸試驗後，55 wt.%鋁-鋅鍍層透過EBSD與SEM鑑定，其轉變為Zn、Fe2Al5Znx與 FeAl相。於不同拉伸破壞程度後觀察鍍層裂紋行為，其分為兩種形式-鍍層破裂與界面破裂。在初始拉伸過程中，裂紋由Fe2Al5Znx開始誘發增生，且隨著拉伸破壞量增加，Fe2Al5Znx/ FeAl之界面開始破裂且逐漸成為主導，合金鍍層最後在FeAl/鋼底材鍍層剝離，造成鋼底材暴露因而氧化。在調整升溫速率後，結果顯示高升溫速率使得合金鍍層與FeAl界面有明顯增厚，合金鍍層拉伸破壞結果顯示，高升溫速率使得具延性的FeAl增生，使得合金鍍層拉伸破壞行為因而改變。 最後討論沃斯田體化後之55 wt.%鋁-鋅鍍層，透過連續鹽霧試驗、腐蝕產物鑑定與電著塗裝層之刮痕試驗，綜合比較其腐蝕行為，佐沃斯田體化後之鋅層相互比較。沃斯田體化後，55 wt.%鋁-鋅鍍層由於高鋁含量，可以有效的抑制高溫氧化的惡化。在鹽霧試驗過程中，沃斯田體化後之55 wt.%鋁-鋅合金鍍層產生白銹，且腐蝕產物透過XRD與XPS鑑定其為鋅的氫氧化物與氫氧化鋁。在刮痕之電著塗裝，其960小時鹽霧試驗後，其刮痕拓展之程度，55 wt.%鋁-鋅合金鍍層較鋅合金鍍層成長幅度較不明顯，說明55 wt.%鋁-鋅合金鍍層之抗蝕性佳，在實際應用上具有潛力。 本研究對於相變化、高溫拉伸破壞與腐蝕行為等予以提出機制與行為探討，該結果為鋅-鋁-鐵相變化開啟新的觀點，提供熱衝壓鋼材保護鍍層設計之貢獻。

Hot-dip Zn galvanized steel for hot stamping process still has main challenges, e.g., oxidation and liquid metal embrittlement(LME), which restrain the practices. In present study, sol-gel silane coating/hot-dip galvanized composite coating, and Al-Zn coating on steel were systematically investigated for the hot stamping process. The galvanized steel with the silane coating restrains high-temperature oxidation after austenitization at 900℃ for 5 min. LME is no-sensitive while a thinner Zn coating is applied. Moreover, the silane composite coating preserves more Zn in the alloy coating, conferring better corrosion resistance, yet imparted plenitudinous corrosion resistance to the austenitized silane/galvanized steel with a ED coating with artificial scratches after SST. The silane composite coating effectively inhibits oxidation and confirms enhanced corrosion resistance for hot-stamped galvanized steel parts. A 5 wt.% Al-Zn coated steel is employed to detail the microstructural evolution, oxidation, and the related corrosion behavior changes during the austenitization heat treatment. During the initial stages of austenitization, the primary Al in the coating reacted with the steel substrate, and Fe2Al5Znx phase forms in the interface between coating/steel substrate. With continued austenitization, the inner part of the coating transforms to the Γ-Fe3Zn10 phase and the outer part of the coating to the FeAl phase, accompanying with the formation of a thin surface oxide layer mainly composed of Al2O3 with minor ZnO. The austenitization treatment enriches the coating with Fe, which ennobles the corrosion potential and reduces the corrosion current density of the coating. Microstructural evolution of a 5 wt. % Al-Zn coating on press hardening steel was studied in comparison to hot-dip galvanized (GI) and galvannealed (GA) coatings. The results show that the presence of 5 wt. % Al effectively suppresses oxidation during austenitization; meanwhile, the presence of Fe resulting from galvannealing deteriorates oxidation. Alloying with Al or Fe in the coating prior to austenitization reduces the susceptibility to LME. The presence of Al in the as-coated Zn coating enhances the corrosion resistance in HCl solution and reduces the cathodic kinetics in NaCl solution. However, for sacrificial anode protection, the austenitized GI steel outperforms the other austenitized-coated steels. Nevertheless, the 5 wt .% Al-Zn coating exhibits better overall performance including high-temperature oxidation resistance, less LME susceptibility, and adequate sacrificial anode protection. Employment of EBSD to identify the phase transformation in Al-Zn coating is still absent. EBSD is employed to identify the comprehensive microstructural evolutions of the 55 wt.% Al-Zn coated steel during austenitization heat treatment. As the heat treatment from 500℃ to 800℃, the coating transforms into Al, Zn, FeAl and Fe2Al5Znx phases in the alloy coating. Initially the FeAl3Si2 phase forms in the interface between alloy coating/steel substrate, as well as then the Fe2Al5Znx phase forms and keeps growing with the temperature rising. During austenitization at 900℃ for different time, the alloy coating is primarily composed of Zn, FeAl and Fe2Al5Znx phases. Moreover, the FeAl phase continues coarse with the soaking time increasing. The potential gap within coating and steel substrate is not evidently affected during the heat treatment. The passivation behaviors take place contributing to the formation of Fe-Al phases. The EBSD pole figures show that the Fe2Al5Znx phase keeps c-axis toward the steel substrate in the heat treatment process. A galvanized 55 wt.% Al-Zn coated steel is used in this work to investigate the high temperature tensile test properties. After high temperature tensile test, the 55 wt.% Al-Zn coating transforms to an alloy layer composed of Zn, Fe2Al5Znx, and FeAl. During various degrees of tensile deformation, cracks in the alloy layer are classified as two types-coating and interface cracking. In the initial deformation process, primary cracks mainly resulted from the fracture of the brittle Fe2Al5Znx phases. As the deformation process continues, interface crack is observed predominantly at the Fe2Al5Znx/ FeAl interface. Further deformation resulted in decohension between the FeAl phase/ steel substrate, leading to the exposure and subsequent oxidation of the steel substrate. With higher heating rate prior to isothermal soaking(within the same total heating time), the alloy coating and interface layer grow coarser. The cracking behaviors in the alloy layer is affected while the high heating rate makes the ductile FeAl phase grow. Corrosion of an austenitized 55 wt.% Al-Zn coating is detailed via SST, identification of corrosion products and scratched painting in comparison with austenitized Zn coating. high temperature oxidation is effectively suppressed due to high Al content of 55 wt.% Al-Zn coating. The white rusts forms in austenitized 55 wt.% Al-Zn coating during SST exposures, and the corrosion products are identified as typical Zn-hydroxides and Al(OH)3 via XRD and XPS. The 55 wt.% Al-Zn coating demonstrates lighter growing-factor of width expand of scratches on ED coating on Zn-coated steel after long SST exposures, implying 55 wt.% Al-Zn coating is a potential protective coating practice. The sensible mechanism is detailed to describe the phase transformation path, cracking behaviors and corrosion resistance, bringing about a novel perspective of the Zn-Al-Fe phase in this present study.