先進高強度鋼之延遲破壞暨低溫脆性研究

Abstract

從工業革命至今，鋼鐵一直是最廣泛使用的合金。 其發展甚至可以追溯到1850年代。 近幾十年，在商用非鐵合金的競爭、全球氣候變遷、生產成本提高、先進研究技術的引入等等…綜合因素下， 鋼鐵發展開始有革命性的突破。 這些新開發的鋼材被稱作先進高強度鋼（AHSS）。 至今，已成功開發兩個世代的先進高強度鋼。 同時，次世代的先進高強度鋼概念也已被提出，並同時逐漸投入實際應用。 然而，在已開發的先進高強度中，仍然有些問題值得討論。 第一部份係針對超高強度麻田散鐵係鋼材之延遲破壞進行研究。 本研究主要目的係希望建立用以評估延遲破壞之方法。 同時透過此檢測方式，探討能否利用添加微合金鈮、釩元素、熱處理、碳化物控制，達到抑制硼添加麻田散鐵係鋼材(B-MART)之延遲破壞行為。 第一部份研究結果指出，B-MART鋼材具有優異的機械性質。 淬火狀態下，母材15B30的抗拉強度達到1900MPa。 添加釩元素的15B30V鋼材的抗拉強度，則提升到了2100MPa。 鈮元素添加的效益，主要反映在降伏強度及延展性上。 延遲破壞之檢測結果顯示，利用氫脆方法可以評估熱處理製程對硼系麻田散鐵鋼延遲破壞現象之影響，另一方面定應變測試法則可評估微合金添加對淬火之硼系麻田散鐵鋼延遲破壞之影響。 在氫脆測試結果中顯示，於低溫回火後之15B30/M鋼鐵有著最佳的抗氫脆能力。 進一步分析可知此性能的提升，係由於$epsilon$碳化物的析出。 而定應變測試法結果顯示，微合金元素Nb或V添加皆能提昇15B30超高強度鋼鐵之抗延遲破壞能力，同時也顯示B元素添加則對此鋼材之抗延遲破壞能力有負面的影響。 第二部分的重點為開發符合Norsok MDS-Y70標準之次世代海洋用鋼材。 主要針對顯微結構對於海洋用鋼低溫衝擊韌性的影響進行探討， 研究結果發現，淬火鋼板的表面區域主要為麻田散鐵所組成； 而淬火鋼板心部，則是由變韌鐵、麻田散鐵以及少量的肥粒鐵所組成。 回火麻田散鐵的延脆轉換溫度(DBTT)，明顯的較變韌鐵組織來的低。 變韌鐵的出現會導致鋼材低溫衝擊韌性大幅降低。其原因在於變韌鐵單胞內的介面角度為低角度介面， 由於高角度介面在變韌鐵單胞內的消失，造成海洋用鋼板的低溫韌性不佳，而利用沃斯回火製程生成之變韌鐵試片，亦呈現相同結果。

From industrial revolution to the present, steel is the most popular applying alloy in the world. It has been developed for centuries, which could trace back to 1850s. However, the break-through development of steel just occurred in recent decades. In these decades, the newly developed steels are called advance high strength steels (AHSS). Many causes were involved in the linebreak development of AHSS such as the competition of non-ferrous commercial alloys, global climate, raising cost of production, advanced research technique, etc. Until now, two generation of AHSSs have already been developed. linebreak Furthermore, the scheme of the next generation steel was proposed, and was also placed into practice. However, there are still several issues in the develop-ed AHSSs, which are worth discussing. The first part of this research aims at investigating the delayed fracture(DF) of ultrahigh-strength martensitic steels. The main purposes of this study are to building up the evaluation methods of DF and searching for the effective ways to suppress the DF on 15B30 boron added martenstic (B-MART) steels, including slight addition of niobium and vanadium, heat treatments and precise control of carbides. The results of the first part show that these B-MART steels exhibit the most excellent properties. The tensile strength (TS) of as-quenched 15B30 reaches 1900MPa. With addition of vanadium, the TS of 15B30V reaches about 2100MPa. The effect of niobium addition contributes to the yield strength and ductility. In the meantime, the DF of tempered B-MART steels was examined by hydrogen embrittlement test, which showed that, comparing to high temperature, the low temperature tempered 15B30/M steel exhibits a better hydrogen resistance. Further analysis inferred that, this improvement was contributed by the precipitating of epsilon-carbide. On the other hand, the DF of quenched B-MART steels was evaluated through constrain loading technique, that indicated boron had the negative effect on DF. On the contrary, niobium and vanadium had the positive effect on DF. The purpose of second part is to develop the next generation offshore steel, which meets the Norsok MDS-Y70 standard. This investigation is mainly concentrated on the effect of microstructure on low-temperature impact linebreak toughness of the direct water quenched offshore steel. Martensite dominates the microstructure of quenched surface. In contrast, major bainite, martensite and a few ferrite are observed from the central region of quenched specimen. Ductile to brittle transition temperature of tempered martensite is significantly lower than that of bainite dominated microstructure. The presence of bainite greatly impairs low-temperature impact toughness of the steel due to linebreak the presence of low angle interfaces within the bainite packet. The disappearance of high angle interfaces in the bainite packet results in significantly deteriorated low-temperature impact energy of the offshore steel. Similar results are con-firmed in austempered specimens, which are dominated by bainite.