基于滲碳體調控低合金鋼中塊狀逆變奧氏體與奧氏體晶粒尺寸

張獻光; 劉歡; 張健; 王洪利; 任英杰; 楊文超; 陳佳俊; 石鵬

doi:10.13374/j.issn2095-9389.2022.09.27.002

基于滲碳體調控低合金鋼中塊狀逆變奧氏體與奧氏體晶粒尺寸

doi: 10.13374/j.issn2095-9389.2022.09.27.002

1.
北京科技大學冶金與生態工程學院，北京 100083
2.
攀鋼集團研究院有限公司，攀枝花 617000

基金項目: 國家自然科學基金資助項目（51804232）；北京市自然科學基金資助項目（2212041）；北京科技大學青年教師學科交叉研究項目（中央高校基本科研業務費專項資金）資助項目（FRF-IDRY-20-020）

詳細信息

通訊作者:
E-mail: xgzhang@ustb.edu.cn

中圖分類號: TG156.5
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出版歷程
- 收稿日期: 2022-09-27
- 網絡出版日期: 2022-12-16
- 刊出日期: 2023-05-31

Controlling the formation of reverted globular austenite and the as-transformed austenite grain size in low-alloy steel based on cementite

1.
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China
2.
Pangang Group Research Insitute Co., Ltd., Panzhihua 617000, China

More Information

Corresponding author: E-mail: xgzhang@ustb.edu.cn

摘要

摘要: 逆變奧氏體微觀組織顯著影響鋼鐵材料的最終組織性能，闡明塊狀奧氏體的形成規律對于精準掌握逆相變至關重要。本文以Fe–2.5Mn–1.5Si–0.35C合金為研究對象，通過OM、SEM和EBSD等手段研究了不同預回火條件下晶內塊狀奧氏體與最終奧氏體晶粒尺寸的演變規律。研究結果表明，隨預回火溫度自350 ℃升高至650 ℃，晶內塊狀奧氏體體積分數呈現出先增加后迅速降低的趨勢；400 ℃預回火條件下，隨預回火時間的延長，晶內塊狀奧氏體體積分數先增加后趨于穩定；預回火促使晶內塊狀奧氏體形成，導致最終奧氏體晶粒顯著細化。隨著預回火溫度的升高，逆相變前滲碳體發生粗化，增加了晶內塊狀奧氏體的有效形核位點，此促進了晶內塊狀奧氏體的形成。此外，晶內塊狀奧氏體具有多重取向，晶內塊狀奧氏體的增加，使得逆相變后奧氏體晶粒顯著細化。本研究提供了一種在不改變鋼化學成分的條件下，通過控制滲碳體實現對逆相變晶內塊狀奧氏體形成和最終奧氏體晶粒尺寸調控的新方法。
- 逆相變 /
- 預回火 /
- 塊狀奧氏體 /
- 奧氏體晶粒尺寸 /
- 滲碳體
Abstract: Austenite reversion has been widely used in the traditional heat treatment of steels, and recently, it has been used in the fabrication of advanced high-strength steels. The microstructure of reverted austenite significantly influences the final microstructure and properties of steel; thus, it is crucial to understand the formation of globular austenite to accurately grasp its reversion behavior. In this paper, an Fe–2.5Mn–1.5Si–0.35C alloy was chosen as the research object, and the evolution of intragranular globular austenite and finally transformed austenite grain size were studied under different pre-tempering conditions using a metallographic optical microscope, scanning electron microscope, and electron backscatter diffraction. It was found that as the pre-tempering temperature was increased from 350 ℃ to 650 ℃, the volume fraction of intragranular globular austenite first increased and then rapidly decreased. At the pre-tempering temperature of 400 ℃, the volume fraction of intragranular globular austenite initially increased and remained stable thereafter, when the pre-tempering duration was increased from 1 to 10 h. Fine cementite particles were primarily formed immediately before the reversion in the non-tempered or low-temperature pre-tempered initial structures. This provided less effective nucleation sites for the formation of intragranular globular austenite. Therefore, lesser intragranular globular austenite grains were formed, thereby resulting in relatively coarse finally transformed austenite grains after reversion. The cementite particles were gradually coarsened as the pre-tempering temperature was increased to 550 ℃, thereby increasing the number of effective nucleation sites for the formation of intragranular globular austenite. Conversely, when the martensite samples were pre-tempered at a high temperature of 650 ℃, Mn is seriously enriched into the cementite particles before the reversion, largely reducing the driving force for reversion. This resulted in the growth of intragranular globular austenite under the partitioning local equilibrium mode, with a slow growth rate, resulting in a low volume fraction. Therefore pre-tempering can effectively promote the formation of intragranular globular austenite. Owing to its multiple orientations, increased intragranular globular austenite formation resulted in significantly refined austenite grains after reversion. This study provided a new strategy to regulate the formation of intragranular globular austenite and finally transformed austenite grain size by controlling the size and composition of cementite particles through pre-tempering without changing the chemical composition of the steel.
- reversion /
- pre-tempering /
- globular austenite /
- austenite grain size /
- cementite

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圖 1 Fe–2.5Mn–1.5Si–C系合金相圖(α: 鐵素體, γ: 奧氏體, θ: 滲碳體)

Figure 1. Phase diagram of the Fe–2.5Mn–1.5Si–C alloy system(α: ferrite, γ: austenite, θ: cementite)

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圖 2 預回火與逆相變熱處理示意圖(As Q: 淬火馬氏體, TM: 預回火馬氏體). (a) 不同預回火溫度 (b) 不同預回火時間

Figure 2. Schematics of the pre-tempering and reversion heat treatments at various pre-tempering (a) temperatures and (b) periods (As Q: as-quenched martensite, TM: pre-tempered martensite)

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圖 3 初始淬火馬氏體及780 ℃保溫60 s后的逆變奧氏體微觀組織圖(OM: 光學顯微鏡, SEM: 掃描電鏡, PAGB: 原奧氏體晶界, TM: 回火馬氏體, γ_G(M): 塊狀奧氏體(淬火后為馬氏體), γ_A(M)：針狀奧氏體(淬火后為馬氏體)). (a) 初始淬火馬氏體, OM; (b) 初始淬火馬氏體, SEM; (c) 逆變奧氏體, OM; (d) 逆變奧氏體, SEM

Figure 3. As-quenched martensite and partially reverted austenite microstructures after being heated at 780 ℃ for 60 s(OM: optical microscope, SEM: scanning electron microscope, PAGB: prior γ grain boundary, TM: tempered martensite, γ_G(M): globular γ (martensite at room temperature), γ_A(M): acicular γ (martensite at room temperature)): (a) as-quenched martensite, OM; (b) as-quenched martensite, SEM; (c) partially reverted austenite, OM; (d) partially reverted austenite, SEM

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圖 4 淬火馬氏體790 ℃保溫60 s后逆變奧氏體的晶體學分析. (a) SEM顯微組織; (b) EBSD分析得到的BC+IPF圖; (c) (001)_α極圖; (d) Δβ圖

Figure 4. Crystallographic analyses of reverted austenite for As Q after reversion at 790 ℃ for 60 s: (a) SEM image; (b) EBSD-analyzed BC + IPF figure; (c) (001)_α pole figure; (d) Δβ figure

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圖 5 不同溫度預回火馬氏體經780 ℃保溫5 s逆相變處理后的光學顯微組織圖: (a) TM350; (b) TM400; (c) TM450; (d) TM500; (e) TM550; (f) TM600; (g) TM650; (h)晶內塊狀奧氏體體積分數隨預回火溫度的變化

Figure 5. OM images after reversion (780 ℃ for 5 s) of martensite pre-tempered at various temperatures: (a) TM350; (b) TM400; (c) TM450; (d) TM500; (e) TM550; (f) TM600; (g) TM650; (h) change in the volume fraction of intragranular globular austenite against the pre-tempering temperature

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圖 6 不同溫度預回火馬氏體經780 ℃保溫5 s逆相變處理后的顯微組織圖. (a) TM350, OM; (b) TM350, SEM; (c) TM550, OM; (d) TM550, SEM; (e) TM650, OM; (f) TM650, SEM

Figure 6. OM and SEM images after reversion (780 ℃ for 5 s) of martensite pre-tempered at various temperatures: (a) TM350, OM; (b) TM350, SEM; (c) TM550, OM; (d) TM550, SEM; (e) TM650, OM; (f) TM650, SEM

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圖 7 不同預回火時間下逆變奧氏體(780 ℃保溫5 s)光學微觀組織圖. (a) TM1h; (b) TM2h; (c) TM5h; (d) TM7h; (e) TM10h；(f)晶內塊狀奧氏體體積分數隨預回火時間的變化

Figure 7. OM images after reversion (780 ℃ for 5 s) of austenite pre-tempered for various periods: (a) TM1h; (b)TM2h; (c)TM5h; (d) TM7h; (e) TM10h; (f) change in the volume fraction of intragranular globular austenite against the pre-tempering duration

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圖 8 不同初始組織逆相變結束(825 ℃保溫15 s)后EBSD分析得到的淬火馬氏體(a, c, e)及相同區域重構獲得的原奧氏體晶粒(b, d, f)的IPF+BC圖: (a, b) As Q; (c, d) TM550; (e, f) TM650；(g)定量統計獲得的不同起始組織相變后奧氏體晶粒尺寸

Figure 8. IPF + BC figures of the (a, c, e) as-quenched martensite analyzed using EBSD immediately after reversion (825 ℃ for 15 s) and the (b, d, f) reconstructed prior austenite grains of the same area for various initial structures: (a,b) As Q; (c,d) TM550; (e,f) TM650; (g) quantified austenite grain size immediately after reversion for various initial structures

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圖 9 不同程度預回火馬氏體OM與SEM微觀組織圖. (a) TM350, OM; (b) TM350, SEM; (c) TM550, OM; (d) TM550, SEM; (e) TM650, OM; (f) TM650, SEM

Figure 9. OM and SEM images of various degrees of pre-tempered martensite: (a) TM350, OM; (b) TM350, SEM; (c) TM550, OM; (d) TM550, SEM; (e) TM650, OM; (f) TM650, SEM

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圖 10 不同起始組織逆相變前滲碳體微觀組織及尺寸分布圖. (a) TM350; (b) TM550; (c) TM650; (d) 尺寸分布 ($d_{\text{θ}} ^{{\rm{ave}}}$: 平均滲碳體顆粒尺寸)

Figure 10. Microstructure and size distribution of cementite particles immediately before reversion for various initial structures: (a) TM350; (b) TM550; (c) TM650; (d) size distribution diagram ($d_{\text{θ}} ^{{\rm{ave}}} $: average θ particle size)

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圖 11 不同預回火條件下晶內塊狀奧氏體形成和相變后奧氏體晶粒尺寸演變示意圖

Figure 11. Schematic of intragranular globular austenite formation and the evolution in austenite grain size after reversion under various initial pre-tempered conditions

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表 1 實驗用鋼的主要化學成分（質量分數）

Table 1. Nominal composition of the alloy used in this study %

C Mn Si S P Al Fe

0.34 2.51 1.47 0.002 0.0048 0.028 Bal.

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表 2 TM350和TM550逆相變前的滲碳體數密度分布

Table 2. Cementite number density distributions for TM350 and TM550 immediately before reversion

Type Number density
($d_{\text{θ}} ^{{\rm{ave}}} $≥100 nm)/
(10¹² m^?2) Number density ratio of
TM550 to TM350

TM350 2.454 1.855
TM550 4.552

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