矽含量對於低碳低合金鋼之界面析出碳化物的影響

Abstract

矽是在諸多鋼鐵產品中都會添加的合金元素之一，其對於鋼鐵相變的影響主要在於能夠穩定肥粒鐵相，擴大肥粒鐵相存在的溫度區間，此一特點對於在沃斯田鐵分解相變界面上所產生的析出物應會有所影響，本實驗主要的目的便在於觀察界面析出物與矽含量之間的關聯性。 首先以熱膨脹儀進行在A1溫度附近的一系列溫度區間持溫處理，藉由相變曲線繪製與金相觀察，確認矽的添加能夠加速肥粒鐵相變，促進沃斯田鐵進行相變，並且抑制剪切性相變組織如變韌鐵的產生。之後以穿透式電子顯微鏡觀察各溫度區間的界面析出物分佈狀態，我們亦得到了與前人相同之析出物產生趨勢：當溫度越低，則析出物越細密，具有越好的析出強化效果。雖然我們所觀察到的析出物分佈狀況(sheet spacing、particle size)未能隨著矽的添加而有明顯的差異產生，但可以確認的是，矽在此扮演的角色是穩定肥粒鐵相的生成，使界面析出物能在更低的溫度區間形成，有更細密的排列。而在熱模擬計算的結果與相變曲線的比對下，我們知道了相變驅動力及界面擴散梯度在矽的添加下都有增加的趨勢，證明上述推論的正確性。此外，藉由硬度測試的結果亦發現，添加矽確實具有相當好的固溶強化能力，同時也有些許增強析出強化的效果存在。 之後利用熱膨脹儀模擬熱軋鋼板製程中最後的加速冷卻至盤捲溫度階段的連續冷卻過程，配合先前所做的持溫熱處理結果，分析在不同溫度區間下的連續冷卻製程中界面析出物的生成狀態。結果發現，矽的添加促使肥粒鐵相區擴大，得以在更大的溫度範圍生成肥粒鐵基地與界面析出物，避開變韌鐵的生成，這也符合了先前恆溫熱處理所得到的結論。

Silicon is one of the most common alloy elements in steels. Its major effect on steel phase transformation is to stabilize ferrite phase and broaden ferrite phase region. According to this feature, Si additions might influence the precipitation during austenite decomposition. The purpose of this research is to fully understand how Si performs in the interphase precipitation mechanism. The research includes two experiments: isothermal heat treatments and continuous cooling heat treatments. All heat treatments are conducted by dilatometer. The samples are first heated to austenizeand then isothermally heated or slow cooling from the temperature range which nearing TA1. The results are observed by OM and TEM, and analyzed with the condition of ferrite phase and interphase precipitation distribution. In isothermal heat treatments, with the information collected by dilatometer, the TTT diagram can be drawn. Si additions effectively accelerate ferrite transformation. Under OM obsevation, Si also inhibits bainite transformation. The ferrite phase transformation can proceed in lower temperature. However, interphase precipitation distribution seems not to be apparently influenced by Si additions. The precipitation sheet spacing and particle size mainly concerned transformation temperature. The lower temperature, the denser and smaller precipitation can be observed. Hardness test results reveal that ferrite can be strengthened by solid solution hardening of Si and precipitation hardening. The former effect provides the same hardness range in different temperature range and the latter one cause the lower transformation temperature the harder the ferrite matrix. In continuous cooling heat treatments, the OM observation also indicates that the Si addition effectively stabilize ferrite transformation. The lower starting temperature would have higher hardness and Si could maintain the ferrite matrix and prevent from forming bainite phase. Judging from the above, Si additions mainly contribute two efforts: solid solution hardening and stabilization ferrite phase. The heat treatments can be conducted in lower temperature to get denser interphase precipitation distribution in ferrite matrix which can be effectively strengthened by Si solid solution.