300系列沃斯田鐵不銹鋼鑄胚均質化冶金技術研究

Abstract

309L與316L均屬於300系列沃斯田鐵不銹鋼。其沃斯田鐵基地相(Austenitic matrix)內的δ-ferrite和σ相形態與後續熱抽和冷抽之製程密切相關，在高溫下δ-ferrite相易轉變成σ相，σ相為脆性金屬間相(Brittle intermetallic phase)且有高硬度，此為導致盤元(wire rod)後續抽線發生斷裂的主因。本研究旨在尋找可行的冶金路徑，避免鑄胚(cast billet)在熱抽形成8 mm盤元以及後續加工抽線成0.8 ~1.2 mm線材時發生斷裂，以供華新麗華公司後續於鹽水軋機生產高Cr/Ni 沃斯田鐵不銹鋼線材時其製程參數的優化。 本研究針對兩種連鑄鋼胚，分別取其中心處(C)、半徑處(0.5R)與近表面處(R)，於1240 °C下進行2小時空冷(快速冷卻)與6小時均質化爐冷(緩慢冷卻)熱處理，並使用場發射掃描式電子顯微鏡結合背向散射電子繞射(EBSD)功能、場發射電子微探儀(FE-EPMA)配備波長分光光譜儀(WDS)以及維克氏微硬度計(Vickers micro-hardness tester)，系統性觀察在均質化熱處理前後其硬度、分佈、形態、數量、化學成分與相變態演變機制。 研究結果顯示，均質化熱處理提供了δ-ferrite球化之驅動力，當均質化持溫時間由2小時延長至6小時，能有效促進δ-ferrite之形貌球化。此外，δ-ferrite與σ相間的相變態受溫度與冷卻速率顯著影響，於1240 °C持溫2小時後空冷可有效抑制σ相析出；於1240 °C持溫6小時爐冷，高合金沃斯田鐵不銹鋼309L與316L，δ-ferrite會相變析出σ相。 為精確控制相組成，研究進一步針對309L與316L進行不同溫度的冷卻實驗。結果顯示，309L須於600 °C至850 °C區間進行空冷以避開σ相形成區間；316L則須於較高溫之850 °C至1240 °C區間進行空冷，才能抑制σ相形成。

Both 309L and 316L are members of the 300-series austenitic stainless steels. The morphology of δ-ferrite and σ phase within the austenitic matrix is highly sensitive to subsequent hot rolling and cold drawing. At elevated temperatures, δ-ferrite is prone to transforming into the σ phase, a brittle intermetallic phase with high hardness, which is the primary cause of fracture during wire rod drawing. This study aims to identify a feasible metallurgical pathway to prevent fracture during the hot rolling of cast billets into 8 mm wire rods and the subsequent drawing into 0.8–1.2 mm wires. The findings inform the optimization of process parameters at Walsin Lihwa Corporation for the production of high-Cr/Ni austenitic stainless steel ultrafine wires. In this study, specimens were sampled from three distinct locations on the continuous-cast billets: the center (C), mid-radius (0.5R), and near-surface (R). Heat treatments were conducted at 1240 °C for 2 hours via air cooling (rapid cooling) and for 6 hours via furnace cooling (slow cooling). FESEM-EBSD (Field Emission Scanning Electron Microscopy combined with Electron Backscatter Diffraction), EPMA (Field Emission Electron Probe Microanalyzer) equipped with WDS (Wavelength Dispersive Spectroscopy), and Vickers micro-hardness tester were employed to observe the evolution of hardness, distribution, morphology, volume fraction, chemical composition, and phase transformation mechanisms before and after homogenization. The results indicate that homogenization heat treatment drives the spheroidization of δ-ferrite. Extending the homogenization time from 2 to 6 hours effectively promotes the spheroidization of δ-ferrite. Furthermore, the experimental results showed that temperature and cooling rate dominate the phase transformation mechanism between δ-ferrite and σ phase. Air cooling after a 2-hour hold at 1240 °C effectively suppresses σ phase formation. Conversely, during furnace cooling after a 6-hour hold at 1240 °C, the δ-ferrite in high-alloy 309L and 316L decomposes into σ phase. To suppress σ phase formation, cooling experiments were conducted at various temperature intervals for both 309L and 316L. The results showed that rapid cooling must be applied at a temperature between 600 °C and 850 °C in 309L, whereas 316L requires rapid cooling at high temperatures between 850 °C and 1240 °C to avoid σ formation.