Evaluation of cleanliness and distribution of inclusions in the thickness direction of interstitial free (IF) steel slabs
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摘要: 為了減少IF鋼生產過程中冷軋板缺陷以及降低夾雜物對鋼的成材性能的影響，重要的是明確IF鋼鑄坯厚度方向夾雜物分布規律，本文采用氧氮分析、掃描電鏡和能譜分析、夾雜物自動掃描以及原貌分析等手段進行了系統的分析。結果表明，鑄坯厚度方向T.O（總氧）和N的平均質量分數值分別為1.6×10?5和1.7×10?5，T.O在內弧表面1/8處最高，為2.0×10?5，內弧1/8～3/8區間N質量分數較高，為1.8×10?5；共統計1177個夾雜物，70%以上夾雜物的尺寸都在5 μm以內，平均尺寸為2.8 μm，內、外弧3/8處夾雜物平均尺寸較大，分別為4.0 μm、4.4 μm；鑄坯中心TiN析出量較多，內外弧表面以Al2O3和Al2O3?TiOx為主，尺寸在5～10 μm之間，Al2O3?TiN在內外弧1/4處呈不規則狀，尺寸在3～5 μm；當凝固率0.646 < f ≤ 0.680時，凝固前沿液相以及δ相中開始有TiN析出，尺寸在3～6 μm之間波動。Abstract: During the production of Al-killed titanium-alloyed interstitial free steel, to reduce defects in cold rolled sheets and decrease the influence of inclusions on the properties of the steel, it is important to clarify the distribution of inclusions in the thickness direction of IF (interstitial free) steel along the slab. In this study, standard metallographic techniques were employed to analyze the total oxygen and nitrogen by performing scanning electron microscopy, energy spectroscopy, automatic scanning electron microscopy, and original morphology analysis. The results show that the average mass fractions of T.O and N are 1.6 × 10?5 and 1.7 × 10?5, respectively, and the T.O for the 1/8 thickness from the inner arc is 2.0 × 10?5, while the content of N for between the 1/4 and 3/8 thickness from the inner arc is 1.8 × 10?5. A total of 1177 inclusions were counted. More than 70% of inclusions are within 5 μm in size, and the average size of inclusions in the thickness direction is 2.8 μm. The sizes of inclusions for the 3/8 thickness from both the inner and outer arcs are larger at 4.0 μm and 4.4 μm, respectively. The amount of precipitation of TiN is large in the slab center, and there are mainly Al2O3 and Al2O3–TiOx near the inner and outer arc surfaces with sizes between 5 and 10 μm. Al2O3–TiN distributes irregularly in the 1/4 thickness from the inner and outer arcs, and the size fluctuates between 3 and 5 μm. The size of TiN during solidification fluctuates between 3 and 6 μm. TiN precipitates in the liquid and δ phase of the solidification front when the solidification rate is between 0.646 and 0.680, and the size fluctuates between 3 and 6 μm.
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Key words:
- slabs /
- thickness /
- inclusions /
- distribution /
- titanium nitride /
- precipitation
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圖 2 IF鋼厚度方向全氧和氮含量變化
Figure 2. Total oxygen and nitrogen changes in the thickness direction of the IF slab
圖 3 IF鋼鑄坯厚度方向夾雜物尺寸統計
Figure 3. Sizes of inclusions in the thickness direction of the IF slab
圖 4 IF鑄坯厚度方向氧化物夾雜物分布. （a） 氧化夾雜物數量密度；（b） 氧化夾雜物面積分數
Figure 4. Distribution of oxide inclusions in the thickness direction of the IF slab: (a) number density changes of oxide inclusions; (b) areal density changes of oxide inclusions
圖 5 IF鋼鑄坯厚度方向TiN分布統計. （a） 數量密度; （b） 面積分數
Figure 5. Distribution of TiN in the thickness direction of the IF slab: (a) number density; (b) areal density
圖 6 鑄坯中不同形貌的夾雜物. （a~d） Al2O3；（e~h） Al2O3-TiOx；（i）~（l） Al2O3-TiN；（m~p） TiN
Figure 6. Morphologies of different inclusions in the slab: (a?d) Al2O3; (e?h) Al2O3-TiOx; (i?l) Al2O3-TiN; (m?p) TiN
圖 7 IF鋼鑄坯厚度方向夾雜物類型和尺寸變化分布圖. （a） 不同類型夾雜物尺寸分布；（b） 夾雜物尺寸分布圖；（c） 氧化夾雜物的面積占比分布圖；（d） TiN的面積占比分布圖
Figure 7. Type and size changes and distribution of inclusions in the thickness direction of the IF slab: (a) the type and size changes and distribution of different inclusions; (b) size changes of the inclusion; (c) density distribution of oxide inclusions; (d) density distribution of TiN
圖 8 液相和δ相中TiN和MnS析出曲線. （a） lgQ?f, lgK?f; （b） ΔGTiN?f
Figure 8. Beginning precipitation point of TiN and MnS in the liquid and δ phases: (a) lgQ?f, lgK?f; (b) ΔGTiN?f
表 1 試驗鑄坯化學成分（質量分數）
Table 1. Chemical composition of the test steel slab
% C Si Mn P S Als Ti T.O N 0.0015 0.0050 0.1300 0.0110 0.0050 0.0450 0.0600 0.0022 0.0025 
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表 2 夾雜物成分（質量分數）
Table 2. Composition of inclusions
% 位置 O Al Ti N Mg S Fe 圖6(a) 38.47 43.37 1.27 — — — 16.89 圖6(b) 56.84 43.16 — — — — — 圖6(c) 43.45 56.55 — — — — — 圖6(d) 50.03 49.72 0.25 — — — — 圖6(e) 34.00 42.78 5.57 — — 1.13 16.33 圖6(f) 38.69 50.84 8.43 2.04 — — — 圖6(g) 36.42 42.78 19.52 — 1.29 — — 圖6(h) 39.98 47.05 12.97 — — — — 圖6(i) 13.15 1.47 35.15 28.13 — — 22.09 圖6(j) 11.19 14.34 46.05 28.42 — — — 圖6(k) 8.29 9.21 47.30 35.20 — — — 圖6(l) 20.96 13.54 35.30 30.20 — — — 圖6(m) 5.87 — 56.33 14.62 — — 23.17 圖6(n) — — 70.08 29.29 — — — 圖6(o) — — 79.29 17.77 — — — 圖6(p) — — 65.16 34.84 — — —
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反應式 標準吉布斯自由能/(J·mol?1) 3[Ti]+5[O]=Ti3O5(s) $\Delta {G^{\ominus}}$=?1772320+569.32T 2[Al]+3[O]=Al2O3(s) $\Delta {G^{\ominus}}$=?1208271+390.91T Ti3O5(s)+10/3[Al]=5/3Al2O3(s)+3[Ti] $\Delta {G^{\ominus}}$=?241466+82.22T 注：T為反應溫度。
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元素 Al Ti O C Si Mn P S Al 0.045 0.004 ?1.4 0.091 0.056 — 0.029 0.03 O ?0.83 ?0.6 ?0.12 ?0.45 ?0.131 ?0.021 0.07 ?3.9 Ti 0.004 0.013 ?1.8 — 2.1 ?0.043 ?0.06 ?0.27
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表 5 不同析出反應的標準吉布斯自由能
Table 5. Standard Gibbs free energies for different precipitates
序號 反應式 標準吉布斯自由能/(J·mol?1) ① [Ti]+[N]=TiN(s) $\Delta {G^{\ominus}}$=?291000+107.91T ② [Ti]+[S]=TiS(s) $\Delta {G^{\ominus}}$=?153000+77T ③ [Mn]+[S]=MnS(s) $\Delta {G^{\ominus}}$=?177650+99.45T
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表 6 鋼中各元素的相互作用系數
Table 6. Interaction coefficients of the elements
元素 Al Ti O C Si Mn P S N Ti 0.004 0.013 ?1.8 — 2.1 ?0.043 ?0.06 ?0.27 ?2.06 N ?0.01 ?0.6 ?0.12 0.13 0.048 ?0.02 0.059 0.007 0 Mn — ?0.05 ?0.083 ?0.054 ?0.0327 0 ?0.06 ?0.048 ?0.091 S 0.041 ?0.18 ?0.27 0.111 0.075 ?0.029 0.035 ?0.046 0.01
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