309不銹鋼鋼胚均質化處理之研究

Abstract

本研究主要目的是針對S309沃斯田鐵系不銹鋼鋼胚量測連鑄鋼胚內所形成δ-ferrite和Sigma之數量、分佈、型態、位置及化學組成等數據，與後續進行2/6小時均質化熱處理之關聯性，並釐清影響均質化熱處理製程中，達成δ-ferrite形貌鈍(球)化或使δ-ferrite含量減少之關鍵因素，並探討Sigma相析出的機制，以應用於後續華新麗華公司鹽水軋機生產高Ni/Cr合金沃斯田鐵不銹線材之製程參數優化。 實驗中分析S309不銹鋼鋼胚組織經過三種高溫均質化處理製成，分別為未熱處理、在1240℃做2/6小時熱處理，各別取鑄胚三種位置(近表面、1/2半徑處、心部)共九個試片，比較之間形貌、含量、硬度、各元素定量濃度、不同相中的分佈及S309經高溫均質化處理後，不同顯微組織演化之機構，並且用謝夫列爾組織圖驗證不同相的合理性。從實驗結果可知，Sigma為一硬且脆的介屬相，其存在將對於S309盤元於後續抽線製程中易出現斷線問題。而Sigma相結構在EBSD下易與δ-ferrite混淆且此二者化學組成相近。然而，在硬度方面有著顯著的差異。此外，鑄胚內外(心部、R)由於冷卻速率有顯著差異，在半徑R處因冷卻速率快，其分部形貌均勻且細小，推論應有有更好的加工性。 由實驗結果可知δ-ferrite常伴隨著Sigma相的生成，將造成後續盤元加工斷線的原因。為了找出未來改善製程的路徑，藉由Thermal-Calc模擬軟體中相圖的Isothermal Sections，發現在原先的均質化溫度1240℃為兩相區(BCC+FCC)，然而當溫度降至1090℃時變成FCC單相區，可為未來改善製程的方向。

The primary purpose of this study is to analyze the quantity, distribution, morphology, location, and chemical composition of δ-ferrite and Sigma phase formed in the S309 austenitic stainless steel cast billet. The correlation between the microstructural data and the subsequent 2/6-hour homogenization heat treatment will be examined. The key factors affecting the achievement of δ-ferrite morphology blunt/spheroidized or reducing δ-ferrite amount in the homogenization heat treatment will be clarified. Additionally, the mechanism of Sigma precipitation will be explored for optimizing process parameters in order to manufacture high-alloy austenite stainless wire in Walsin Company. Three types of samples were performed on the S309 stainless steel cast billet, including no heat treatment, 2 and 6 hours heat treatment at 1240℃, and three specimens were taken from three locations of the cast billet, i.e., near surface, 1/2 radius, and center for the purpose of comparison, including morphology, volume fraction, hardness, the quantitative concentrations of elements distributed in different phases, and mechanisms of microstructural evolution after high-temperature homogenization treatment. The rationality of different phases was verified using the Schaeffler diagram. The experimental results showed that the Sigma is a hard and brittle phase that can cause cracking during subsequent wire drawing processes. The structure of the Sigma phase was confused with δ-ferrite in EBSD analyses, and their chemical compositions were similar. They were only significant different in hardness. Moreover, the cooling rate of the center and outside of the cast billet were quite different. The microstructure at the radius R was much uniform and fine expecting with a better performance in subsequent wire drawing. The experimental results showed that δ-ferrite is often accompanied by the generation of Sigma. According to the isothermal sections using Thermo-Calc simulation demonstrates the original forging temperature of 1240℃with a two-phase region (BCC+FCC). Still, when the temperature decreased to 1090℃, an FCC single-phase region was achieved providing a better approach for future process improvement.