Effect of austenite grain size on the acicular ferrite transformation in Ti–Zr treated steel
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摘要: Ti、Zr的復合氧化物可以有效誘導針狀鐵素體形核，從而細化晶粒。為了研究Ti–Zr處理鋼中針狀鐵素體轉變機理，使用25 kg真空感應爐中熔煉試驗所需鋼種，向低合金鋼中添加了質量分數為0.038%鈦和0.008%鋯。利用高溫激光共聚焦顯微鏡原位觀察了奧氏體化溫度對針狀鐵素體轉變行為的變化，使用掃描電鏡觀察了Ti–Zr處理鋼種的夾雜物成分和針狀鐵素體在夾雜物表面形核，使用光學顯微鏡觀察不同奧氏體化溫度下的微觀組織變化差異。結果表明，隨著奧氏體化溫度從1250 ℃增加至1400 ℃，奧氏體晶粒尺寸從125.6 μm 增加至279.8 μm，針狀鐵素體開始轉變溫度和側板條鐵素體開始轉變溫度先增加，在1350 ℃條件下達到最大值，后又降低，針狀鐵素體的體積分數由39.6%增加至83.6%；Ti–Zr處理鋼中核心為Zr–Ti–O，外部為Al–Ti–Zr–O的氧化物為核心表面析出MnS的復合氧化物主要集中在1.5～3 μm，可以有效促進針狀鐵素體形核，貧Mn區和夾雜物與鐵素體之間的良好晶格關系為該型夾雜物能夠促進針狀鐵素體形核機理。奧氏體晶粒尺寸的增加導致多邊形鐵素形核位點的減少和針狀鐵素體的形核空間的增加，鈦鋯復合處理形成大量的有效誘發針狀鐵素體形核的夾雜物，這共同導致了針狀鐵素體體積分數增加。Abstract: The composite oxides of Ti and Zr can effectively induce acicular ferrite nucleation and refine austenite grain size. To study the transformation mechanism of acicular ferrite in Ti–Zr treated steel, the mass fraction of 0.038% titanium and 0.008% zirconium were added to low alloy steel by melting in a 25 kg vacuum induction furnace. The effect of austenitizing temperature on acicular ferrite transformation behavior was observed in-situ using a high-temperature laser confocal microscope: the samples were heated to 1250, 1300, 1350, and 1400 ℃ at a heating rate of 5 ℃·s?1 and then cooled to 400 ℃ at a cooling rate of 3 ℃·s?1 after holding for 300 s. The ferrite transformation behavior of samples during the above process was observed using a high-temperature confocal microscope. The inclusion composition of Ti–Zr treated steel and the nucleation of acicular ferrite on the inclusion surface were observed using a scanning electron microscope. The variation in microstructure at different austenitizing temperatures was observed using an optical microscope. The austenite grain size was found to increase from 125.6 to 279.8 μm with increasing austenitizing temperature from 1250 to 1400 ℃. The initial transformation temperature of acicular ferrite and side-plate ferrite increased, reached a maximum at 1350 ℃, and then decreased. The volume fraction of acicular ferrite increased from 39.6% to 83.6%. In Ti–Zr treated steel, the size of complex inclusion with Zr–Ti–O in core and Al–Ti–Zr–O in exterior and MnS precipitated on the surface was mainly concentrated in 1–3 μm. It could effectively promote acicular ferrite nucleation. The Mn-poor region and the good lattice relationship between complex inclusions and ferrite were the mechanisms by which the type of inclusions in the steel could promote acicular ferrite nucleation. Using classical nucleation theory, the nucleation potential of acicular ferrite under different conditions was calculated. The results showed that when the austenitizing temperature was 1300 ℃, the nucleation potential of acicular ferrite was the strongest, reaching 191.7 mm?2. The calculation results were consistent with the variated law of acicular ferrite volume fraction. An increase in austenite grain size led to a decrease in polygonal ferrite nucleation sites, an increase in acicular ferrite nucleation space, and the formation of many inclusions that effectively induced nucleation of acicular ferrite treated by titanium and zirconium, which increased the acicular ferrite volume fraction.
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圖 2 共聚焦顯微鏡原位觀察奧氏體晶粒長大過程. (a) 600 ℃; (b) 1000 ℃; (c) 1350 ℃; (d) 1350 ℃保溫300 s
Figure 2. In-situ observation of austenite grain growth by confocal microscope: (a) 600 ℃; (b) 1000 ℃; (c) 1350 ℃; (d) 1350 ℃ holding for 300 s
圖 3 不同奧氏體化溫度處理后的光學顯微圖. (a) 1400 ℃; (b) 1350 ℃; (c) 1300 ℃; (d) 1250 ℃
Figure 3. Optical micrographs after different austenitizing temperatures: (a) 1400 ℃; (b) 1350 ℃; (c) 1300 ℃; (d) 1250 ℃
圖 5 原位觀察不同奧氏體化溫度下的鐵素體轉變. (a～b) 1400 ℃; (c～d) 1350 ℃; (e～f) 1300 ℃; (g～h) 1250 ℃
Figure 5. In-situ observation of ferrite transformation at different austenitizing temperatures: (a–b) 1400 ℃; (c–d) 1350 ℃; (e–f) 1300 ℃; (g–h) 1250 ℃
圖 6 不同奧氏體化溫度下的鐵素體轉變溫度
Figure 6. Ferrite transformation temperature at different austenitizing temperatures
圖 7 不同奧氏體化溫度下的微觀組織圖像. (a) 1400 ℃; (b) 1350 ℃; (c) 1300 ℃; (d) 1250 ℃
Figure 7. Microstructure at different austenite temperatures: (a) 1400 ℃; (b) 1350 ℃; (c) 1300 ℃; (d) 1250 ℃
圖 8 不同奧氏體化溫度下不同鐵素體的體積分數
Figure 8. Volume fraction of different ferrites at different austenitizing temperatures
圖 9 夾雜物成分和數量隨尺寸的變化. (a)夾雜物質量分數; (b)夾雜物數量分數
Figure 9. Variation in composition and number of inclusions with inclusions diameter: (a) mass fraction of inclusions; (b) number fraction of inclusions
圖 10 不同奧氏體化溫度下夾雜物數密度和氧化物類型夾雜物的平均直徑
Figure 10. Number density of inclusions and average diameter of oxides inclusions at different austenitizing temperatures
圖 11 針狀鐵素體在夾雜物表面形核過程. (a) 1400 ℃; (b) 1350 ℃; (c) 1300 ℃; (d) 1250 ℃
Figure 11. Acicular ferrite nucleated on the surface of inclusions: (a) 1400 ℃; (b) 1350 ℃; (c) 1300 ℃; (d) 1250 ℃
圖 12 不同奧氏體晶粒尺寸下的針狀鐵素體形核能力
Figure 12. Nucleation potential of acicular ferrites at different austenite grain sizes
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