超高強度海洋用鋼熱處理性質之研究

Abstract

超高強度海洋用鋼屬於超高強度低合金鋼 (HSLA)，經由TMCP製程可達到降伏強度大於690 MPa之強度，但必須滿足Norsok S690Q之低溫韌性規範，於低溫 (-40 °C) 衝擊測試平均大於100 J，且任一測試試片不得低於70 J。本研究使用四種微合金成分，分別進行兩種軋延量 (OS1及OS2試片為67%，OS3與OS4試片為80%) 之TMCP製程後再經由回火後探討其低溫衝擊韌性，實驗結果顯示軋延量提升有助於改善低溫衝擊韌性。此外，利用表層 (回火麻田散鐵) 與心部 (回火麻田散鐵與變韌鐵) 位置之回火試片進行低溫衝擊試驗，其結果顯示回火麻田散鐵具有較佳之低溫韌性及較低之延性轉脆溫度。由EBSD結晶方位間角度關係圖顯示回火麻田散鐵除了晶界為高角度界面，其晶粒內次結構之界面亦為高角度界面，因此具有較佳之低溫韌性。 80 %軋延量之試片進行沃斯回火熱處理 (430 °C與490 °C) 探討上/下變韌鐵對於低溫衝擊韌性之影響，將可提供銲接之參考。沃斯回火後之衝擊測試結果顯示，下變韌鐵具有較佳之衝擊韌性，而上變韌鐵之韌性不佳。由EBSD結晶方位間角度關係分析得知其韌性不佳之原因為上變韌鐵次結構內低角度之界面數量增加，且冷卻過程中殘留沃斯田鐵不完全變態，形成麻田散鐵與沃斯田鐵共同組成之MA組織。MA組織生成於晶界以及界面間之位置，由於MA組織硬且脆易於裂縫成核裂化韌性，其破斷形貌呈現脆性劈裂破壞。然而，微合金元素中添加Mo及V將促使MA組織生成，由顯微結構分析顯示OS4試片於430 °C沃斯回火已觀察到MA組織，而490 °C沃斯回火MA組織數量明顯增加，導致其衝擊測試結果皆較相同沃斯回火熱處理之OS3試片差。 此外，亦配合實驗使用Simufact模擬TMCP製程中冷卻過程之相變態比例與殘留應力，其依據JMatPro計算之材料冶金性質進行TMCP製程參數之模擬。模擬參數分別為理想狀態與非理想狀態，其差異為上/下方之冷卻速率是否一致，其結果顯示非理想狀態冷卻將導致鋼板Z方向有明顯變形。

Ultra high strength offshore steel is one of high strength low alloy steel (HSLA) which possessed high Y.S (≥ 690 MPa) by TMCP process. According to Norsok S690Q standard, the average of low temperature (-40 °C) impact energy is higher than 100 J and one of other is must higher than 70 J. There are four chemical compositions and two rolled ratio (OS1 & OS2 are 67 %, OS3 & OS4 are 80 %) in this study. The investigation of low temperature impact toughness used tempered offshore steels which were manufactured by TMCP and then carried on temper heat treatment. The result showed that offshore steel with 80 % thickness reduction possessed excellent low temperature impact toughness. In addition, the structure of surface (tempered martensite) and center (tempered martensite and bainite) were studied. It is tempered martensite that revealed good low temperature toughness and lower ductile to brittle transition temperature (DBTT). EBSD crystal orientation map of tempered martensite was examined which showed its grain boundaries and sub-structure were high-angle boundaries. Two 80 % thickness reduction specimens were conducted by austempered heat treatment (430 °C and 490 °C ) to investigate low temperature impact toughness of upper and lower bainite which could be provided for welding process. The impact energy of austempered treatment showed that the toughness of lower bainite is higher than upper bainite. Because of the increasing amount of lower-angle boundaries and uncompleted transformation of retained austenite which formed MA constituent, toughness of upper bainite was degenerated which was examined by EBSD crystal orientation map. MA constituent located at grain boundaries and interface of sub-structure due to its hard and brittle which degenerated low temperature toughness of upper bainite. Fractagraphy of upper bainite revealed brittle quasi-cleavage. However, it results that adding Mo and V in steel promoted formation of MA constituent. According to microscopic examination, there is MA constituent in OS4 specimen which was carried out 430 °C austempered treatment. After 490 °C austempered treatment, the amount of MA constituent were increased. Due to element effect which promoted formation of MA constituent, the impact toughness of OS4 specimens were lower than OS3 specimens for identical austempered treatment. In addition to experiment, the cooling process of TMCP was simulated by Simufact software. Simufact software which calculated metallurgical properties according to database of JMatPro simulated the ratio of phase and retained stress. The parameters of simulation were divided into ideal state and nonideal state that is the cooling system of top and bottom whether identity or not. The result of simulation showed non-ideal state caused steel deformation by TMCP.