合金元素矽鉻銅對低碳鋼高溫氧化的影響

Abstract

鋼鐵材料因為優異機械性質與較低製造成本是最常用的金屬材料，熱軋延是將鋼錠輥軋成鋼板的唯一加工製程，在此嚴苛環境下(1050 ~ 1280 ºC持溫1~2小時均質化處理)會導致鋼板嚴重氧化，除了影響材料利用效率外，後續冷軋的氧化層去除也是製程重點。低碳鋼中的Si、Cr作為去氧化劑、抗腐蝕元素而刻意添加，Cu則固溶於基材作為後續奈米Cu析出所需，然而在高溫環境下液體Cu傾向於表面、晶界析出，熱軋過程中可能造成熱脆(Hot Shortness)問題。 本研究探討低碳鋼中Si、Cr和Cu對熱軋氧化層形成的影響，取六種Si、Cr、Cu含量不同的熱軋試片進行微結構分析後，對同樣六種試片做高溫氧化實驗。熱軋試片從再熱(1230 ºC持溫2小時)開始，緊接著為粗軋、精軋與盤捲步驟，取最後盤捲鋼板做OM、SEM、EDS、EBSD、EPMA等為結構分析；高溫氧化則取精軋前的溫度1150 ºC持溫1分鐘並爐冷降溫，升降溫過程皆保持於氬氣環境防止升降溫過程的氧化層生成。接著以OM、SEM、EDS、EBSD、EPMA、TEM等儀器探討氧化層微結構及高溫狀態下的析出機制。結果顯示，Si、Cr、Cu於界面處析出會降低氧化層厚度，Si、Cr與Fe0.95O形成的Fayalite、Chromite會增加氧化層於底材的附著性。含Cu量0.1 wt%的試片，Cu會在底材／氧化層界面處連續分佈，含Cu量0.3 wt%的試片則不止於界面處析出，在底材晶界也有以奈米Cu析出的現象。在同時含Si、Cr、Cu的試片中，Cu則作顆粒狀散亂分佈於Fayalite、Chromite所形成的次氧化層中，推測Si、Cr在高溫氧化中可以改善液體Cu所造成的熱脆現象。

Carbon steel is the most commonly used material nowadays because of its superior mechanical properties and low cost. Hot rolling is the only way to process a billet steel to steel plate. Under harsh conditions, such as homogenization at 1050 ºC ~ 1280 ºC for 1 ~ 2 h, the steel plate tends to suffer severe oxidation, which not only reduces the utilization efficiency of material, but also impacts on the following deoxidizing process before cold rolling. For low carbon steel, Si and Cr are added as deoxidizing agent and anti-corrosion elements. Cu, on the other hand, is added for the subsequent precipitation of Cu nano particles. However, under high temperature conditions, liquid Cu tends to precipitate at the surface and grain boundaries, which may lead to hot shortness during the hot rolling process. This research studied the effect of Si, Cr and Cu on oxide scale formation during hot rolling. Six as-received hot rolling specimens with different concentrations of Si, Cr and Cu were employed through high temperature oxidation experiment. The hot rolling process begins with reheating process, which is held at 1230 ºC for 2 h, followed by rough rolling, finish rolling, and coiling. The final coiling steel plates were characterized using OM, SEM, EDS, EBSD, EPMA. The high temperature oxidation experiment is conducted at 1150 ºC for 1 min, followed by cooling to room temperature in furnace. The atmosphere of oxidation experiment is kept in Ar to prevent from forming scale during heating and cooling processes. The various steel samples after high temperature oxidation were characterized using OM, SEM/EDS, EBSD, EPMA, and TEM to gain better understanding on the scale microstructure and precipitation mechanism. Experimental results show that Si, Cr, Cu precipitating at the interface tend to reduce the scale thickness. Moreover, Si and Cr react with FeO to form Fayalite and Chromite structure, which strengthens the adhesion between the scale and the steel substrate. For the sample with 0.1 wt% Cu, Cu precipitates at the interface as a continuous layer. For the sample with 0.3 wt% Cu, Cu precipitates not only at the interface but also at the grain boundaries of steel as Cu nano particles. In the presence of Si, Cr and Cu, Cu particles form and disperses into the subscale composed of Fayalite and Chromite. It is thus inferred that during high temperature oxidation, Si and Cr ameliorate hot shortness caused by liquid Cu.