兩種S690Q高強度海洋用鋼回火效應之研究

Abstract

本實驗分別使用兩種符合Norsok S690Q規範之海洋用鋼，兩者主要的差距在於釩元素的有無。藉由調整回火溫度參數，測量其室溫及低溫之衝擊韌性、硬度變化，並且藉後續之金相觀察、破斷面觀察、電子背向散射繞射分析，調查不同回火溫度所造成的影響。研究結果顯示，未添加釩元素的直接淬火試片在低溫時的衝擊韌性不佳，在500 °C進行回火30分鐘則可以改善此狀況，但回火至660 °C時相較580 °C則不會使衝擊性質再有提升，而其硬度從580 °C開始隨著回火溫度提升也不會有明顯的下降。添加微量釩元素的直接淬火試片在低溫也能有良好的衝擊韌性，但回火至相同溫度對於低溫衝擊韌性所提升的量則較未添加釩元素的鋼材要少，而回火至660 °C時相較580 °C也沒有對性質再有提升。 此外，由顯微結構與電子背向散射繞射之結果可以發現，原本密集的麻田散鐵介面在回火後會變得稀疏，造成高角度介面的減少與整體介面角度的下降。與衝擊試驗結果做比較可以發現，回火後介面角度較低，但低溫衝擊韌性較好，顯示高角度介面對於衝擊性質的助益並不如性質較脆的麻田散鐵組織轉變成回火麻田散鐵組織明顯。此研究建立較完整的海洋用鋼之低溫衝擊韌性資訊以及金相、破斷面觀察等資料，可提供日後海洋用鋼的應用與發展所需。

In this research, two kinds of S690Q offshore steel specimens which can be characterized by the alloying of vanadium were tempered at several temperatures between 500~660 °C. Mechanical properties including hardness and low-temperature toughness were tested, and they were also investigated by several techniques like SEM and EBSD. The results show that the direct-quenched specimen without vanadium condition was brittle at low temperature, and its toughness could be improved when tempered at 500 °C for 30 minutes. However, increasing the tempering temperature from 580 °C to 660 °C doesn’t improve the low-temperature toughness anymore. For the vanadium-alloyed specimen, the direct-quenched specimen was ductile at low temperature, and it could become more ductile after tempering. However, the benefits from tempering were less effective to the vanadium-alloyed steels. Tempering at the temperatures higher than 580 °C has little improvement in their low-temperature toughness properties. It can be observed from the results of SEM and EBSD that the substructure of lath martensite generated in the water-quenched specimens loosened during tempering. It is accompanied by fewer high-angle boundaries and the decreased average boundary angle. Comparing with the results of fracture toughness tests, it can be discovered that the high-angle boundaries enhancement to fracture toughness is less effective than the benefits from the brittle martensite structure transforming to tempered martensite during tempering. This research includes detailed data of low-temperature toughness, SEM microstructure observations, fractured surface observations, and EBSD analyses, which may be useful in the future offshore steel studies.