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摘要: 微合金鋼薄板坯連鑄過程高發邊角部裂紋，致使熱軋卷板邊部產生翹皮、爛邊等質量缺陷，是鋼鐵行業的共性技術難題。本文立足于某鋼廠QStE380TM低碳含鈮鈦微合金鋼薄板坯連鑄生產，檢測分析了鑄坯角部組織金相結構與碳氮化物析出特點、不同冷卻與變形速率條件下鋼的斷面收縮率，并數值仿真研究了不同結構結晶器和二冷區鑄坯溫度與應力的演變規律。結果表明：微合金鋼薄板坯連鑄過程存在明顯的第三脆性區，且變形速率越大，第三脆性區越顯著。傳統薄板坯連鑄工藝條件下，結晶器的中上部及其出口至液芯壓下段的二冷高溫區，鑄坯角部冷速較低，致使其組織晶界含鈮鈦微合金碳氮化物呈鏈狀析出。鑄坯在液芯壓下過程，低塑性角部因受較大變形與應力作用而引發裂紋缺陷。實施沿高度方向有效補償坯殼凝固收縮的窄面高斯凹型曲面結晶器及其足輥區超強冷工藝，可分別提升鑄坯角部冷速至10和20 ℃·s?1以上，從而促使鑄坯角部組織碳氮化物彌散析出，并促進鑄坯窄面在液芯壓下過程金屬寬展流動而降低角部壓下應力，大幅降低了微合金鋼薄板坯邊角部裂紋發生率。Abstract: Thin slab continuous casting and rolling process is an important way to produce hot-rolled strips. Recently, the process has been widely used to produce Nb/V/Ti/B bearing microalloyed steel. However, during the continuous casting of the thin slabs of the microalloyed steel, there are frequent cracks on the corners of the slabs, which would cause quality defects, such as scars and cracks at the edges of the hot-rolled coils. These defects are a common technical issue in the steel industry. In this paper, the characteristics of the microstructure and carbonitride precipitation of the thin slab corner of QStE380TM low carbon niobium–titanium bearing microalloyed steel, as well as the reduction of area of the steel under different cooling and tensile rates, were detected. Moreover, the evolutions of the temperature of the solidified shell in different structure molds and secondary cooling processes, as well as the stress of the thin slab surface during liquid core reduction, were numerically simulated. The results show that there is a significant third brittle temperature zone during continuous casting of microalloyed steel thin slabs, and the greater the deformation rate of the thin slab, the more significant the third brittle temperature zone is. Under the conventional thin slab continuous casting process, the cooling rate of the thin slab corners in the upper part of the mold and the secondary cooling zone from the mold exit to the liquid core reduction segment is lower than 5 °C·s?1, which is the key factor to lead a chain of niobium–titanium carbonitrides precipitate at the grain boundaries of the corners. As a result, the plasticity of the thin slab corners is greatly reduced. During the process of liquid core reduction, the low plasticity corners of the thin slab crack because of large deformation and stress. Applying the Gaussian concave curved surface mold, which the narrow face copper plates could efficiently compensate the shell shrinkage, the narrow face-foot roll zone hard cooling process can increase the cooling rates of the thin slab corners over 10 and 20 °C·s?1 in the mold and the narrow face-foot roller cooling zone, respectively. As a result, the carbonitrides precipitate in the thin slab corners disperses, and the stress of the thin slab corners reduces since the new mold promotes the metal flow of slab narrow surface broadsiding during the liquid core reduction. Finally, the cracking rate of the thin slab corners during the microalloyed steel thin slab casting has been reduced significantly.
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Key words:
- microalloyed steel /
- thin slab continuous casting /
- corner cracks /
- mold /
- liquid core reduction /
- hard cooling
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圖 1 QStE380TM鋼薄板坯角部金相組織形貌. (a) 皮下5 mm; (b) 皮下10 mm
Figure 1. Morphologies of the microstructure of thin slab corner of QStE380TM: (a) 5 mm beneath the surface; (b) 10 mm beneath the surface
圖 2 QStE380TM鋼薄板坯角部皮下5 mm處析出物的形貌與組成. (a) 析出物掃描; (b) 析出物能譜
Figure 2. Morphology of the precipitate of thin slab corner of QStE380TM at 5 mm beneath the surface: (a) precipitate graph of SEM; (b) EDS of the precipitate
圖 3 不同變形速率條件下QStE380TM鋼斷面收縮率隨溫度變化. (a) 冷卻速率3.5 ℃·s?1; (b) 冷卻速率15.0 ℃·s?1
Figure 3. Variation of the reduction in the area of QStE380TM under different tensile rates: (a) 3.5 °C·s?1 cooling rate; (b) 15.0 °C·s?1 cooling rate
圖 4 強冷卻控冷條件下的試樣金相與析出物透射形貌. (a) 金相組織; (b) 析出物透射形貌
Figure 4. Morphologies of the microstructure and distribution of precipitate of the thermal simulation sample under the hard cooling conditions: (a) microstructure; (b) precipitate
圖 5 鑄坯角部區域的保護渣膜厚度分布. (a) 寬面; (b) 窄面
Figure 5. Distribution of mold flux film around slab corner: (a) wide face; (b) narrow face
圖 6 鑄坯角部區域的氣隙分布. (a) 寬面; (b) 窄面
Figure 6. Distribution of air gap around slab corner: (a) wide face; (b) narrow face
圖 8 QStE380TM鋼薄板坯連鑄過程表面溫度演變
Figure 8. Evolution of surface temperatures during thin slab continuous casting of QStE380TM
圖 9 QStE380TM鋼薄板坯液芯壓下過程應力演變
Figure 9. Evolution of QStE380TM thin slab stress during liquid core reduction
圖 10 高斯凹型曲面結晶器與窄面足輥強冷卻噴淋. (a) 高斯曲面結晶器示意圖; (b) 高斯曲面結晶器實物圖; (c)窄面足輥強冷卻噴淋
Figure 10. Gaussian concave surface mold and cooling structure for mold narrow face rollers: (a) schematic for Gaussian concave surface mold; (b) practical Gaussian concave surface mold; (c) practical cooling structure for mold narrow face rollers
圖 11 新結晶器內鑄坯角部區域保護渣膜厚度分布. (a) 寬面; (b) 窄面
Figure 11. Distribution of mold flux film around slab corner in the new mold: (a) wide face; (b) narrow face
圖 12 新結晶器內鑄坯角部氣隙分布. (a) 寬面; (b) 窄面
Figure 12. Distribution of air gap of slab corner in the new mold: (a) wide face; (b) narrow face
圖 13 新晶器內鑄坯角部溫度與冷卻速度演變
Figure 13. Evolutions of temperature and cooling rate of slab corner in the new mold
圖 14 薄板坯窄面足輥強冷新工藝下鑄坯二冷溫度場（a）及冷速（b）演變
Figure 14. Evolutions of temperature (a) and cooling rate (b) of slab corner during secondary cooling
圖 15 新工藝下鑄坯寬面應力演變與鑄坯窄面形貌. (a) 應力演變; (b) 鑄坯形貌
Figure 15. Stress evolution of slab wide face under new casting conditions: (a) stress evolution; (b) slab morphology
圖 16 不同工藝下QStE380TM鋼鑄坯角部不同位置處碳氮化物析出形貌. (a) 傳統工藝距鑄坯窄面5 mm; (b) 傳統工藝距鑄坯窄面10 mm; (c) 新工藝距鑄坯窄面5 mm; (d) 新工藝距鑄坯窄面10 mm
Figure 16. Distribution of precipitate in different positions of slab corner under different casting conditions of QStE380TM: (a) 5 mm away from the slab narrow face under conventional casting condition; (b) 10 mm away from the slab narrow face under conventional casting condition; (c) 5 mm away from the slab narrow face under new casting condition; (d) 10 mm away from the slab narrow face under new casting condition
表 1 某鋼廠QStE380TM鋼薄板坯連鑄工藝參數
Table 1. Thin slab continuous casting process of QStE380TM steel in a plant
Casting speed m/min Slab size/
(mm×
mm)Casting temper-
ature /
℃Mold wide
face cool-
ing water/
（L·min?1）Mold nar-
row face cooling
water/
（L·min?1）Temper-
ature difference of mold/
℃Liquidus
temper-
ature /
℃4.0 1250×90 1550 6677 310 4.5 1525 
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表 2 4.0 m·min?1拉速連鑄QStE380TM鋼二冷區各區水量
Table 2. Secondary cooling water flow at the speed of 4.0 m·min?1 for casting QStE380TM thin slab
Segment Cooling zone Water flow/(L·min?1) Foot roller Segment 1 330.0 Grid Segment 2 1720.5 Segment 1 3.0 1128.4 3.1 386.5 3.2 254.2 Segment 2 4.0 990.5 4.1 340.6 4.2 215.5 Segment 3 5.0 665.5 5.1 244.3 5.2 149.5 Segment 4 6.0 345.5 6.1 100.5 6.2 145.0
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