含鈦之大入熱量銲接鋼板粗晶熱影響區顯微結構分析暨針狀肥粒鐵相變機制之研究

Abstract

為了提升銲接效益、減少能源消耗並節省施工成本，新一代460 MPa等級之大入熱量銲接鋼板取代了傳統以肥粒鐵–波來鐵為主的鋼板結構，並廣泛地應用在造船業以及建築業上。新開發的鋼板添加微量之Ti, Nb, B等元素，經大入熱量銲接後，粗晶熱影響區僅有數個沃斯田鐵晶粒的寬度；其主要結構為包覆著先前沃斯田鐵晶界的高溫肥粒鐵與佈滿晶粒內部之粗大針狀肥粒鐵為主。這種複合的顯微結構大幅地改善了熱影響區的韌性，使大入熱量銲接實際地應用在厚鋼板的接合。事實上，傳統的研究認為要改善熱影響區的韌性必須透過細化顯微結構來達成；然而，經大入熱量銲接後，熱影響區形成粗大的針狀肥粒鐵組織亦能夠提供所需的機械性質。 本研究目的在於探究經大入熱量銲接後粗大的針狀肥粒鐵形成機制，輔以動態熱膨脹儀設計焊接熱模擬以及恆溫熱處理製程，控制(1)先前沃斯田鐵晶粒尺寸、(2)相變態溫度、(3)相變態時間與(4)析出物成核位置，各製程所得到之顯微結構與奈米級的碳氮複合型析出物將利用穿透式電子顯微鏡觀察，並討論沃斯田鐵晶粒尺寸與高溫肥粒鐵的生成與否對於針狀肥粒鐵相變態之影響。其次，透過掃描式電子顯微鏡配以電子背向散射繞射技術可得到顯微結構與晶體學方位關係。上述實驗結果將與真實銲接後的粗晶熱影響區結構作比較討論，透過探討針狀肥粒鐵相變態之機制來控制焊接後粗晶熱影響區之顯微結構，將有助於工業上更高品級之大入熱量焊接鋼板的開發。

To improve the welding efficiency and to reduce economic cost, the advanced steel plates for the high-heat-input welding have been developed from the conventional ferrite–pearlite steel plates with a yield strength of about 460 MPa for shipbuilding and construction industries. The newly developed steel plates, after high-heat-input welding, have a novel microstructure in HAZ, where great amounts of coarse acicular ferrite are enveloped by the networks of allotrimorphic ferrite forming around the prior austenite boundaries. Such a novel microstructure improves the toughness in HAZ and enables the high heat input welding to be a pratical application for join steel thick plates. In fact, different from the conventional idea that the refinement of microstructure could improve the toughness in HAZ, the coarse acicular ferrite formed in HAZ during the high-heat-input welding in the present steel plates also provides required mechanical properties. The present study attempts to explore the mechanism to form coarse acicular ferrite in steels during the high-heat-input welding. The transformation of coarse acicular ferrite was studied via the simulated heat treatments using a dilatometer to control several parameters: prior austenite grain size, transformation temperature, transformation interval, and nucleation sites. The corresponding microstructure of acicular ferrite and the nanoscaled carbonitrides (as the nucleation sites for acicular ferrite) were investigated using transmission electron microscopy (TEM). More attention has been paid to the morphology of acicular ferrite. The effect of the grain size of prior austenite on acicular ferrite transformation has also been studied. Moreover, the macrostructure and the corresponding crystallography were analyzed by electron back scattering pattern (EBSD) using scanning electron microscopy (SEM). The results have been compared with the microstructure in the real HAZ of weld plates. It is believed that understanding the transformation of coarse acicular ferrite facilitates the development of higher grades of steel plates for the high-heat-input welding.