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摘要: 微合金化與熱處理工藝是提升鋼材性能最主要的兩種方法。本文以DH980高強鋼中NbC析出相為對象，研究了鈮含量分別為210 × 10–6、430 × 10–6和690 × 10–6和熱處理溫度分別為1000、1100、1200和1300 ℃的條件下，高強鋼中NbC析出相的析出行為。使用高溫硅鉬爐熔煉DH980連鑄坯并添加不同Nb含量進行鈮合金化，再將所得水冷樣置于硅鉬爐中完成不同溫度下的熱處理實驗，然后使用夾雜物自動掃描電鏡對實驗樣品進行夾雜物掃描、統計。經分析，鈮微合金化后的高強鋼中主要的夾雜物為Al2O3、MnS和NbC，其中，NbC析出相的尺寸范圍為0.7～6.0 μm，而1.0～2.0 μm尺寸的NbC居多。使用Factsage熱力學計算軟件計算NbC析出溫度及析出量，隨著鋼中鈮含量從210 × 10–6增加至690 × 10–6，NbC析出相的最高析出溫度逐漸升高，分別為1125、1200和1260 ℃，NbC析出率（NbC質量與所有夾雜物質量的比值）也逐漸從0.023%增加至0.047%、0.076%。MnS的析出溫度為1450 ℃，不隨Nb含量的變化而變化。鋼中NbC析出量隨著鈮含量的增加而增加，也隨著熱處理溫度的升高而增加。當熱處理溫度為1300 ℃時，NbC出現回溶現象，導致析出量減少。NbC尺寸主要與初始Nb含量、熱處理溫度、保溫時間有關，NbC尺寸會隨著Nb含量、熱處理溫度、保溫時間的提高而增加。本研究中建立了高強鋼中NbC析出動力學模型，預測了鋼中鈮含量、熱處理溫度、熱處理時間對NbC析出相尺寸的影響。Abstract: Microalloying and heat treatment are the most important ways to improve the steel properties . In this study, the precipitation behavior of NbC precipitates with Nb content of 210 × 10–6, 430 × 10–6, and 690 × 10–6 and heat treatment temperature of 1000, 1100, 1200, and 1300 ℃ were investigated. DH980 slab was melted in a silicon–molybdenum heating furnace with different Nb additions. The water-quenched steel samples were heated at different temperature in a furnace. The morphology and chemical composition of inclusions in steel samples were determined using an inclusion analysis system. The main inclusions in the Nb micro-alloyed high-Al high-strength steel were Al2O3, MnS, and NbC. The measured diameter of NbC precipitates ranged from 0.7 to 6.0 μm, which mainly concentrated on 1.0–2.0 μm. The precipitation temperature and amount of NbC were calculated using the thermodynamic calculation software Factsage. The initial precipitation temperature of NbC precipitates gradually increased to 1125, 1200, and 1260 ℃ as the Nb content increased from 210 × 10–6 to 690 × 10–6, respectively, and the NbC precipitation rate (the ratio of NbC mass to the mass of all inclusions) increased to 0.023%, 0.047%, and 0.076%, respectively. The precipitation temperature of MnS was 1450 ℃, which changed little with the content of Nb. Al2O3 was present at the melting temperature of steel. The amount of the precipitated NbC in steel increased with an increase in Nb content and heat treatment temperature. The NbC was dissolved in steel when the heat treatment temperature was 1300 ℃, resulting in a decrease in the precipitation of NbC. The size of the NbC precipitates was mainly influenced by the Nb content, heat treatment temperature, and heating time. With the increase in the initial Nb content, the difference in Nb content between the steel matrix and reaction boundary became larger, the diffusion driving force increased, and thus the size of NbC precipitates increased. The diffusion coefficient of Nb varied with the heat treatment temperature, which was hardly influenced by the Nb content. The diffusion coefficient increased with the increase in temperature, which promoted the diffusion of Nb. Consequently, the size of NbC increased with the temperature increased. The diffusivity of Nb increased with an increase in heating time, which also increased the size of NbC. Therefore, the size of NbC precipitates increased as the Nb content, heat treatment temperature, and heating time increased. A kinetic model of NbC precipitation in high-Al high-strength steel was developed to predict the effects of Nb content, heat treatment temperature, and heating time on the size of NbC precipitates.
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Key words:
- high-strength steel /
- Nb micro-alloying /
- heat treatment /
- NbC precipitate /
- kinetics
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圖 1 試驗流程圖. （a）熔煉實驗；（b）熱處理
Figure 1. Schematic of experiments: (a) smelting experiment; (b) heat treatment
圖 2 不同Nb含量的未加熱處理樣品中夾雜物三元相圖. （a）210 × 10–6；（b）430 × 10–6；（c）690 × 10–6
Figure 2. Ternary phase diagram of inclusions in steel without heating with different Nb contents: (a) 210 × 10–6; (b) 430 × 10–6; (c) 690 × 10–6
圖 3 典型夾雜物SEM圖和能譜分析. （a）Al2O3；（b）MnS；（c~d）NbC
Figure 3. SEM images and EDX analysis of typical inclusions: (a)Al2O3; (b) MnS; (c–d) NbC
圖 4 Nb質量分數為210 × 10–6時在不同溫度下處理的樣品中夾雜物三元相圖. （a）1000 ℃；（b）1100 ℃；（c）1200 ℃；（d）1300 ℃
Figure 4. Ternary phase diagram of inclusions in steel at different heating temperature with 210 × 10–6 Nb: (a) 1000 ℃; (b) 1100 ℃; (c) 1200 ℃; (d) 1300 ℃
圖 5 不同熱處理溫度下純NbC尺寸分布. （a）未經熱處理；（b）1000 ℃；（c）1100 ℃；（d）1200 ℃
Figure 5. Size distribution of pure NbC at different heating temperature: (a) before; (b) 1000 ℃; (c) 1100 ℃; (d) 1200 ℃
圖 6 NbC面積分數隨Nb含量和熱處理溫度的變化
Figure 6. Relationship between area fraction of NbC and Nb content and heating temperature
圖 7 不同Nb含量下各夾雜物、FCC、BCC、鋼液的生成情況. （a）210 × 10–6；（b）430 × 10–6；（c）690 × 10–6
Figure 7. Formation of inclusions, FCC, BCC, and liquid at different Nb content: (a)210 × 10–6; (b) 430 × 10–6; (c) 690 × 10–6
圖 8 NbC析出相動力學計算模型示意圖
Figure 8. Schematic of the kinetic calculation model of NbC precipitates
圖 9 鋼中NbC析出相的尺寸隨溫度的變化
Figure 9. Variation of NbC precipitate size in steel with heating temperature
圖 10 不同Nb含量樣品中NbC析出相尺寸隨熱處理溫度、保溫時間的變化. （a）210 × 10–6；（b）430 × 10–6；（c）690 × 10–6
Figure 10. Relationship between the size of NbC precipitates in steel and heating temperature and holding time with different Nb content: (a) 210 × 10–6; (b) 430 × 10–6; (c) 690 × 10–6
圖 11 NbC數量、尺寸隨熱處理溫度、Nb含量變化
Figure 11. Variation of amounts and NbC precipitate size in steel with heating temperature and Nb content
表 1 鋼樣主要成分（質量分數）
Table 1. Main composition of steel samples
% Sample C Mn T.Al T.S T.N Si P Cr Nb 1 0.210 2.200 0.800 0.003 0.003 0.600 0.005 0.400 0.021 2 0.043 3 0.069 
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表 2 夾雜物成分（質量分數）
Table 2. Composition of inclusion elements
% C O Al Si S Fe Mg Ca Mn P Ti N Nb Mo 3.4 16.2 48.6 0 0 0 4.6 0 0 0 0 0 0 0
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表 3 熱處理過程中C和Nb擴散系數和平衡質量分數
Table 3. Diffusion coefficients and equilibrium concentration of C and Nb during heat treatment process
T/oC wNb,eq/% DNb/(10–15 m2·s–1) DC/(10–7 m2·s–1) 900 0.00 0.828 2.05 950 0.00 0.934 2.19 1000 0.00 1.72 2.34 1050 0.00 2.35 3.44 1100 0.00 4.27 4.67 1150 0.00 5.68 7.63 1200 0.00 7.73 14.5
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表 4 熱處理過程中析出相動力學計算參數
Table 4. Calculation parameters of the kinetics of precipitates during heat treatment
L0/μm Mm/(kg·mol–1) MNbC/(kg·mol–1) ρm/(kg·m–3) ρNbC/(kg·m–3) 0.67 0.056 0.105 7.64×103 7.70×103
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