合金元素對高比強度、高延展性之低密度鐵鋁錳基不銹鋼的相變化、機械性質及腐蝕行為的影響之研究

Abstract

鋁、鎂、鈦輕金屬主要因為具有較碳鋼高的比強度，在應用上可節省 許多重量，因此近年來受到相當的重視。本研究群在過去的研究中，除了 在相變化學術研究上有多項重要發現，開發出鐵鋁錳熱軋合金鋼板八國專 利外，在近一、二年間成功開發出一種新的低密度不鏽鋼-"鐵鋁錳基不鏽 鋼"，並且具有較大部分鋁、鎂輕金屬合金高的比強度以及與沃斯田鐵不 鏽鋼相近的高延展性，鐵鋁錳基不鏽鋼的比強度與延展性分別在150~190 Mpa/(g/cm3)以及20~60%之間。 本研究群之所以能成功開發出在試驗室溫度35℃，飽和空氣桶溫度 47℃，5%鹽水濃度條件下，通過48 小時鹽霧測試後不生鏽的低密度鐵鋁 錳基不鏽鋼，在於開發出新精煉製程，大幅降低鐵鋁錳基不鏽鋼中的大量 氧化鋁介在物，顯著的改善過去因介在物的存在所造成的孔蝕現象。2005 年已成功量產YAMAHA、KATANA 等知名廠牌鑄造及鍛造型鐵鋁錳基不 鏽鋼高爾夫球頭。 與傳統的不鏽鋼ㄧ樣，鐵鋁錳基不鏽鋼中的合金元素及顯微結構對其 腐蝕行為有極大的影響。雖然在傳統不鏽鋼中，錳元素已被大部分學者認 為會降低其抗孔蝕能力，碳元素更被證實會嚴重破壞其腐蝕性質，然而， 在含有鉻的鐵鋁錳基不鏽鋼中，降低錳含量或降低碳含量時，沃斯田鐵相 會變得不穩定，造成各種析出物的形成，而影響其腐蝕機制。另外，氮元 素的添加已被認為能夠顯著的提高其不鏽鋼的抗孔蝕能力，但在不鏽鋼中 過量的氮也被發現會有Cr2N 氮化物的析出，不利金屬的抗蝕性。但是到 目前為止，氮的添加對其相變化的影響從未被研究過。因此，本計劃第一 年將深入探討錳與碳含量的變化及氮元素的添加對鐵鋁錳鉻基不鏽鋼相 變化以及腐蝕機制的影響。 鈷是一種與鎳的性質相當接近的元素，兩者的氧化電位也近似，所以 鈷的添加可增強金屬的抗蝕及抗高溫氧化能力，另外，鈷常被利用為金屬 固溶強化的主要元素之一，所以在工具鋼、超高強度麻時效鋼及超合金等中都會添加不同含量的鈷來增加金屬強度。再者，矽也常被添加至耐熱鋼 及不鏽鋼中來提高其抗高溫氧化及抗蝕能力。然而，到目前為止，鈷與矽 的添加對含鉻的鐵鋁錳基不鏽鋼相變化及腐蝕行為的研究相當少，因此延 續本計劃第一年的研究成果，我們在第二年的研究計畫中，將第一年研究 中具有較佳抗蝕及機械性質的合金成分中，添加不同含量的鈷和矽，並且 深入探討其對鐵鋁錳鉻基不鏽鋼相變化以及腐蝕機制的影響。 在傳統的不鏽鋼中，鉬元素的添加可以大幅提高金屬的抗孔蝕能力， 而且含鉬的不鏽鋼更能提升氮在抗孔蝕效果上的影響。另外，鈦元素在防 止不鏽鋼的敏化作用(sensitization)上扮演著重要的角色，主要是由於鈦能 取代鉻與碳結合，抑制了碳化鉻在晶界上的析出。然而，到目前為止，鉬 與鈦的添加對含鉻的鐵鋁錳基不鏽鋼相變化及腐蝕行為的研究相當少，因 此延續本計劃前兩年的研究成果，我們在第三年的研究計畫中，將前兩年 研究中具有較佳抗蝕及機械性質的合金成分中，添加不同含量的鉬和鈦， 並且深入探討其對鐵鋁錳鉻基不鏽鋼相變化以及腐蝕機制的影響。

The light metals of aluminum, magnesium, titanium alloys have received considerable attention in recent years because of their much higher specific strength (strength to weight ratio) than carbon steels, which can save much weight in applications. In our previous studies, some important academic results concerning phase transformations in Fe-Al-Mn and Cu-Al-Mn alloys have been achieved and we has obtained licenses of Fe-Al-Mn. hot-rolled alloy steel plates patent from eight countries. Besides, in the past one to two years, we have also successfully developed a new low density stainless steel- "Fe-Al-Mn based stainless steel", which has higher specific strength (150-190 Mpa/(g/cm3)) than most aluminum and magnesium light metals and possesses excellent elongation (20-60％) comparable to austenitic stainless steels The present workers have invent a refining technic to effectively reduce so many aluminum-oxide inclusions that the low density Fe-Al-Mn based stainless steel didn』t get rusty after 48 hours salt spray test (the temperature of testing room is 35℃, the temperature of saturated air barrel is 47℃, the concentration of salt solution is 5 %).Therefore, in 2005, the commercial productions of Fe-Al-Mn based stainless steel forged and precision casting type golf club heads of YAMAHA and KATANA companies have been manufactured by mass and sold over the world. Like the conventional stainless steels, alloying elements and microstructures of the Fe-Al-Mn based stainless steel have significant influences on the corrosion behavior. For the conventional stainless steels, increasing Manganese content has been considered to reduce the pitting resistance and higher carbon content also has been concluded to lead to serious damages on corrosion resistance. However, in the Fe-Al-Mn based stainless steel, with decreasing manganese and carbon contents, the austenite phase would change into unstable state and then might induce the formations of many kinds of precipitates, which could result in the harmful influences on the resistance to corrosion and pitting. On the other hand, the addition of nitrogen to the conventional stainless steels has been found to significantly increase the pitting resistance, but the excess nitrogen more than its solubility in matrix would lead to the precipitation of Cr2N and then the decreasing in corrosion resistance would be followed. However, up to date, the effects of nitrogen addition on phase transformations in the Fe-Al-Mn based stainless steels have never been studied. Therefore, the purposes of the present project in the first year are to investigate the influences of manganese, carbon and nitrogen contents on phase transformations and corrosion mechanisms in the Fe-Al-Mn based stainless steels. The addition of cobalt could increase the corrosion and high temperature oxidation resistance because its similar physic properties and oxidation potential to nickel. Besides, cobalt is mainly considered as one of solid solution strength elements; therefore, cobalt is found in high speed tool steels, high strength maraging steels and superalloys. Moreover, silicon is often added to heat-resisting and stainless steels to improve the corrosion and high temperature oxidation resistance. However, to date, there are few studies in the influences of cobalt and silicon additions on the phase transformations and corrosion behaviors of the Fe-Al-Mn based stainless steels. Therefore, extending the results of this project in the first year, we are going to select some appropriate compositions having higher corrosion resistance and better mechanical properties summarized in the first year』s results, and then add cobalt and silicon to the selected compositions. Accordingly, the purposes of the present project in the second year are to investigate the influences of cobalt and silicon contents on phase transformations and corrosion mechanisms in the Fe-Al-Mn based stainless steels. For conventional stainless steels, molybdenum addition could greatly improve the pitting resistance and magnify the beneficial effects of nitrogen on the resistance to pitting corrosion. Furthermore, titanium plays an important role in the prevention of sensitization of stainless steel since titanium could replace chromium to combine with carbon, which inhibits the precipitations of chromium carbides at grain boundaries. However, so far, there are few studies concering the influences of molybdenum and titanium additions on the phase transformations and corrosion behaviors of Fe-Al-Mn based stainless steels. Therefore, extending the results of this project in the past two year, we are going to select some appropriate compositions having higher corrosion resistance and better mechanical properties summarized in the past two year』s results, and then add molybdenum and titanium to the selected compositions. Accordingly, the aims of the present project in the third year are to investigate the effects of molybdenum and titanium contents on phase transformations and corrosion mechanisms in the Fe-Al-Mn based stainless steels.