鈮微合金鋼高溫應變析出行為之研究

Abstract

本研究使用鈮微合金鋼進行高溫應變析出行為研究，針對不同鈮含量與鋁氮比進行析出時間、析出物尺寸、再結晶狀況以及後續淬火麻田散組織之分析。材料沃斯田鐵化並回溶析出物後，於各實驗溫度下進行25%壓縮應變並持溫，藉由不同持溫時間後之第二次壓縮可以了解材料之再結晶狀況並推測各溫度之開始析出時間，獲得各材料之析出-開始時間-溫度曲線(PSTT curve)，結果發現本次材料之鈮含量與鋁氮比差異對於開始析出時間影響不大，析出曲線之鼻端皆約位於875~900℃、2~4秒左右，而900℃時各材料之抗軟化效果佳、再結晶發生較慢以及析出時間較早，因此選擇900℃做後續析出物尺寸之分析。後續分析發現，鈮含量較高會使析出物平均尺寸較大，且於析出初期便有粗化的趨勢發生，金相中亦反映其持溫後期之再結晶程度較大，顯示添加較多鈮可能加速析出物粗化；而鋁氮比差異於本次設計之時間內對於抗軟化無明顯影響，唯析出後期亦呈現粗化之趨勢，顯示氮元素較多，與鈮形成碳氮化合物後，可能對於長期維持析出物細小不利。 本次材料淬火後生成全板條麻田散鐵結構，其微結構中除了一般之麻田散鐵板條外，亦可觀察到自回火麻田散鐵與相互穿透雙晶。自回火麻田散鐵之形成源自於較高溫形成之子塊體中，屬相同變體之板條間原本即具有相近方位關係，因此熱能足夠調適小角度晶界使其消失而粗化。而由於熱處理中應不含高碳區域，故雙晶非細板麻田散鐵而是相互穿透雙晶，相互穿透雙晶之形成源自於一些板條之間天生即具有雙晶之方位關係，當板條於較高溫度生成時，相間生長之塊體交界處可能使二者接觸，便有機會互相穿透形成雙晶結構。

The strain-induced precipitation behavior of niobium micro-alloyed steels at high temperature has been studied in this research. Based on the difference of niobium content and aluminum/nitrogen ratio, the precipitation time, precipitates size, recrystallization and structure of martensite after quench have been analyzed. After austenization treatment, and redissolving precipitates, materials were subjected to 25% compression deformation, and then held at the temperature we designed. According to the second compression deformation after different holding time, the degree of recrystallization of materials can be known and used to predict the time that precipitation started, and the precipitation-start time-temperature(PSTT) curve was obtained. Results reveal that different niobium content and aluminum/nitrogen ratio show little effect on the start time of precipitation. The nose tip of PSTT curve is approximately in the range of 875~900℃ and 1~3 second. 900℃ was chosen to be the best work temperature and the temperature for analysis because materials showed good softening resistance, slow recrystallization and early precipitation time at 900℃. Follow-up analysis shows that when niobium content is high, the size of precipitates become larger, and indication of coarsening can be observed at early stage of precipitation. Metallography also reveals that recrystallization happens more obviously when niobium content is high, indicating that more niobium may cause faster coarsening of precipitates. Aluminum/nitrogen ratio shows little effects on softening resistance in the time we designed; however, coarsening which occurred at late stage indicates that it may be detrimental for keeping precipitates small if the excess nitrogen form carbonitride with niobium. Fully lath martensite was obtained after quench from austenite in experimental material, not only general martensite lath but auto-tempered martensite and interpenetrating twin were able to be observed in the microstructure. Auto-tempered martensite forms from the sub-blocks which formed at high temperature. The lathes which belong to the same variant possess similar orientation relationship between each other, so when thermal energy is enough to adapt the small angle boundary, martensite will be coarsening. Because there was no high carbon region after heat treatment, the twin structure in the microstructure is interpenetrating twin instead of plate martensite. Interpenetrating twin forms from some lathes which naturally possess twin orientation relationship between each other. If the twin-relation lathes contact at the boundary of blocks, these lathes which formed at high temperature may penetrate each other to form interpenetrating twin.