微細還原鐵粉之製程與特性

Abstract

鐵系粉末射出成形零件已廣泛地應用於汽車和3C產業中，目前原料主要是以粒徑小、球形和燒結活性高的羰基鐵粉為主，但是由於其價格昂貴，因此，本實驗之目的是發展可應用於射出成形之還原鐵粉，傳統之還原鐵粉主要用於粉末冶金，由於粒徑太大(約75 μm)，因此無法用於高密度的金屬射出成形製程。本實驗所用原料為熱軋後鐵銹皮、磁鐵礦粉和純赤鐵礦粉，第一階段是將氧化鐵球磨至10 μm以下，結果發現磁鐵礦粉球磨速率較鐵銹皮粉快，但是粒徑分佈較窄，而化學成分分析的結果為純赤鐵礦粉純度最高，而磁鐵礦粉最低。 第二階段是將氧化鐵粉在氫氣中還原，三種還原鐵粉還原程度達99 %以上所需的還原溫度分別為：磁鐵礦粉和鐵銹皮粉還原溫度需高達850℃，而較細的純赤鐵礦粉還原溫度則為730℃，但是三種粉末還原後燒結現象皆已發生，以至於無法獲得細粉。 第三階段是藉由添加氧化鋁粉在氧化鐵粉中，抑制還原時發生燒結的現象，並於還原後利用沈降法分離，實驗結果顯示燒結確實受到抑制，但分離成效不彰，經SEM觀察發現氧化鋁顆粒黏附在鐵粉表面上，推測氧化鋁粉和鐵粉中的不純物間產生反應，或是因鐵粉表面擴散將氧化鋁粉鑲埋在鐵粉表面上。此外，本實驗利用空氣分級氣流粉碎機將還原後的燒結體進行粉碎，實驗結果顯示粉碎後的粉末形貌可以分為四種，不規則形（樹枝形）、等軸形、近球形和片形，形貌上的差異是由於還原時鐵粉燒結程度的不同，燒結程度越高，粉末間鍵結強度越強，因此粉碎過程中會產生高程度的塑性變形，以至於粉末形貌越來越接近球形，最後因粉末表面大量加工硬化產生破裂，形成片狀碎屑。 在第四階段中，分析還原鐵粉的燒結性質，以磁鐵礦粉製備的還原鐵粉，由於純度較低，燒結密度相當低；而以純赤鐵礦粉製備的還原鐵粉具有高於羰基鐵粉的燒結密度。將還原鐵粉應用於粉末射出成形製程中，實驗過程中並無任何缺陷生成，因此本研究所開發的還原鐵粉確實具有取代羰基鐵粉的可能性。

Ferrous metal injection molded (MIM) components have been widely used in automobile and electronic industries. One of the raw materials for MIM is the carbonyl iron powder (CIP), which has fine particle size, spherically shaped, and high sinterability , but it is expensive. Therefore, the objective of this study was to develop a new reduced iron powder for MIM applications. The typical reduced iron powder was used in conventional press and sinter parts. However, it is not suitable for MIM because of its large particle size (~75 μm). The raw materials used in this study for making fine reduced iron powder included hot rolling millscale, coarse magnetite powder, and fine hematite powder. In the first phase, the iron oxides were ground to fine powders, smaller than 10 μm. The analysis showed that the grinding rate of magnetite was faster and its particle size distribution was narrower than those of millscale. As of purity, pure hematite was the best followed by magnetite. In the second phase, the iron oxide powders were reduced in hydrogen. The reduction temperatures required for each oxide to reach 99 % reduction rate were: 850℃ for magnetite and millscale, and 730℃ for hematite. However, the reduced powders were sintered into iron cakes that were difficult to break into powder form. In the third phase, the alumina powder was added into iron oxide powders in order to suppress sintering phenomena during reduction. After reduction, alumina powder was separated by sedimentation method. Results indicated that sintering was inhabited but the separation of alumina and iron powders was poor. Scanning electron microscopy observations showed that there were large amounts of alumina particles adhered onto iron particles. It was believed that either alumina reacted with impurities in the iron powder or the surface diffusion caved the alumina particles to stick to the iron powder surfaces. Another approach was to use the air classifier mill to break up the sintered iron cakes into powders. The morphology of such particles was classified into 4 types in this study: irregular (dendritic), equalaxed, near spherical, and flaky. The morphological differences were affected by the extent of sintering. The higher the extent of sintering, the stronger the inter-particle bonding. The sinterablility of reduced iron powders was evaluated in the final phase. Low purity of the magnetite powder caused poor sintered density of reduced iron powder. On the contrary, the sintered density of reduced iron powder made from pure hematite powder was even higher than that of the carbonyl iron powder and was suitable for the MIM process. The results indicated that reduced iron powder can be an alternative for carbonyl iron powder.