金属凝固过程模拟

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Numerical simulationongradientcooling behavior of jumbo slab ingot

Abstract: Shrinkage porosity defect is often found in an air cooled jumbo steel ingot, which will influencethe quality of the final rolled plates. In practical production, some rolled plates are frequently rejected dueto the serious shrinkage porosity of the ingot. To improve the quality of the ingot, a new cooling method, gradient cooling process (in which the upper part of the ingot is air cooled and the lower part is spray

cooled) was put forward in this study. The solidification behaviors for a 60 t jumbo slab ingot under gradient cooling condition were simulated using the ProCast software, and the results were compared with those of an ingot by air cooling condition. The solidifying tendency, temperature field and distribution of shrinkage porosities in the ingot under different cooling conditions were analyzed. Simulation results show that under gradient cooling condition the solidification of the slab ingot progresses in an upward manner along the vertical z axis and in a centripetal manner along the horizontal x and y axes. Gradient cooling can efficiently reduce shrinkage porosity of the jumbo slab ingot by optimizing the solidification sequence, and making the position of shrinkage porosity move from near the middle height of the ingot (under air cooling condition) towards the head of the ingot; and the secondary shrinkage is eliminated. In addition, the solidification time of the ingot under gradient cooling is 7.3 h in this simulation, which is 2.7 h faster than that under air cooling. A 60 t jumbo slab ingot was successfully produced under gradient cooling condition. The ingot was rolled to a plate with a thickness of 100 mm and length of 18,000 mm, and ultrasonic flaw detection was performed. Some porosity was found along the axis of the plate at 4,900 - 6,000 mm from the head of the plate, indicating that the position of the defect is moved towards the head of the ingot. This distribution trend of the defect is consistent with the simulated result.

Key words: gradient cooling; slab ingot; temperature field; shrinkage; numerical simulation

1 Numerical simulation method

ProCast software was used for simulation in this study. The solidification time of the jumbo ingot is much longer than tis filling time, so the initial temperature of the ingot was set as the pouring temperature T p, which was 1,580 oC for the steel used in this study. The initial temperatures of the cast iron mould and the heat insulation plate were taken as the ambient temperature of 25 oC.

Boundary conditions were set according to Q = h eff (T hot–T cold), where Q is heat flux; h

is equivalent interface heat transfer coefficient; T hot is the hot surface eff

temperature;and T cold is the cold surface temperature. The heat transfer coefficient between mould and ingot was 1,800 W·K-1·m-2.The heat transfer coefficient between heat insulation plate and ingot was 20 W·K-1·m-2. The upper half of the mould wall was air cooled at a heat transfer coefficient of 10 W·K-1·m-2. The lower half was spray cooled at a heat transfer coefficient of

1,000 W·K-1·m-2 [10].

1 - Heat insulation plate,

2 - Ingot,

3 - Cast iron mould,

4 - Inlet

2 Experimental parameters

A 60 t rectangular steel ingot was employed in this study and its dimensions were length 2,700 mm, width 1,000 mm and height 3,600 mm. The composition of the ingot is listed in Table 1; and the finite element model of the experimental ingot is shown in Fig. 1(a). The cooling condition was gradient cooling. The upper half of the mould wall was air cooled, and the lower half was water spray cooled, as shown in Fig. 1(b). The liquidus temperature of the steel is 1,524.8 oC, and the solidus temperature is 1,494.8 oC. The latent heat of the steel is 2.7×102 kJ ·kg -1. The physical parameters of the ingot, mould and heat insulation plate for numerical simulation are listed in Table 2.For comparison, the numerical simulation for the ingot solidified under the condition of air cooling was also performed.

Table 1: Compositions of experimental steel ingot

Fig. 1: Finite element model of experimental ingot (a) and cooling location (b)

Table 2: Physical parameters of ingot, mould and heat insulation plate for numerical simulation

Mo Ni Ti Cu Fe 0.025 0.4 0.017 0.15 Bal.

Material

Specific heat Thermal conductivity Density (kg ·m -3)

(kJ ·kg -1·K -1) (W ·K -1·m -1 ) Sol Solid-li Liquid

Fig. 2: Physical parameters of ingot: specific heat (a) and thermal conductivity (b)

Fig. 3: Physical parameters of mould: specific heat (a) and thermal conductivity (b) 3 Results and analysis

3.1 Solidification process of the slab ingot under gradient cooling

The solidification process under the condition of gradient cooling on the narrow plane of the middle of the jumbo slab ingot is shown in Fig. 4. As shown in Fig. 4(a), the lower portion of the ingot shows a big temperature gradient along the transverse direction of the y axis due to the water spray cooling outside of the wall. For example, the temperature at position A decreased quickly to 1,159 oC. The upper portion of the ingot has a small temperature gradient along thetransverse and longitudinal directions due to the air cooling outside. The solidification with time is shown in Figs. 4(a) to(e). The solidification of the slab ingot progressed gradually upwards from the bottom along the longitudinal z axis at the low temperature zone, and radically from the wall towards the center along the transverse y axis. The solidification rate of the lower portion was fast. Generally, the solidification of the ingot began upwards from the bottom at a certain gradient, and finished at the casting head. This solidification method is able to ensure an open interior feeding channel and effectively reduces shrinkage porosities, therefore significantly improves the ingot quality.

id quid Ingot (steel) Fig. 2(a) Fig. 2(b) 7,400

7,200 7,000 Mould (cast iron) Fig. 3(a)

Fig. 3(b)

7,300

Heat

insulation plate 1.05 0.75 600

(a) 15 min (b) 130 min (c) 215 min(d) 297 min (e) 382 min

Fig. 4: Solidification process of slab ingot under condition of gradient cooling 3.2 Solidification time of ingot under different cooling conditions

The solidifying time is very important to ingot production because it determines the de-moulding time, production cycle, and economic profits. The solidifying time of the investigated ingot is 10 h under the condition of air cooling and 7.3 h under gradient cooling, which gives a 27% improvement of production efficiency under gradient cooling.

3.3Shrinkage porosity distribution under different cooling conditions Temperature distributions after 6 h air cooling and gradientcooling are shown in Fig.

5. Spot B at the middle and upper portion of z axis, as shown in Fig. 5(a), has a temperature of1,524 oC and has not solidified under air cooling. While the upper portion solidified earlier than did the lower portion and this leads to secondary shrinkage at the core of the slab ingot.While the solidification progress of the ingot under gradient cooling, as shown in Fig. 5(b), ensures an open feeding channel which significantly inhibits secondary shrinkage.

The distribution of the shrinkage porosities after solidification under different cooling conditions is shown in Fig. 6. The shrinkage position under the condition of air cooling is 1,900 mm from the bottom of the ingot near themiddle height along the central axis as shown in Fig. 6(a). Figure 6(b) shows that the shrinkage position is moved towards the head of the ingot, and even raised to inside the riser under gradient cooling in this study. Therefore, the secondary shrinkage is inhibited and the ingot quality is significantly improved. The simulation results show the possibility of elimination of shrinkage porosities under gradient cooling. The distribution of shrinkage porosities after solidification in production practice may not be the same as the simulation results, but the distribution trend of shrinkage porosities is

valuable and could provide theoretical guidance for industrial production.

Fig. 5: Temperature distributions of ingot after 6Fig. 6: Distribution of shrinkage porosities under the

h air cooling (a) and gradient cooling (b)conditon of air cooling (a) and gradient cooling (b)

4.Production practice for a 60 t jumbo slab ingot

In this study a 60 t jumbo slab ingot was produced by gradient cooling using the same parameters as indicated in Section 2. The melting and pouring process was 60 t EF (Electric Furnace) →LF (Ladle Furnace) →VD (Vacuum Deoxidation). The pouringtemperature was 1,580 oC. Figure 7 shows the picture of the 60 t jumbo slab ingot prepared under gradient cooling condition. It was rolled along the axial direction to a plate with a thickness of 100 mm and length of 18,000 mm. The quality of the ingot can be validated by detection results of the plate indirectly.

The TUD280 ultrasonic flaw detection results show that there are shrinkage porosities along the axis of the steel plate and at 4,900 – 6,000 mm from the head of the steel plate, as shown in Fig. 8. The position of the defect is moved towards the head of the ingot from near the middle of the ingot under air cooling condition. The size and quantity of the defects are acceptable according to the nondestructive detection standard GB2970-2004 III. This indicates that the gradient cooling process can improve the internal soundness of jumbo slab ingots by changing the distribution trend of shrinkage porosity.

Fig. 7: Picture of the 60 t slab ingot by gradient cooling

Fig. 8: Distribution of defects in plate rolled using ingot under gradient cooling

5Conclusions

(1)Under gradient cooling condition the solidification of the slab ingot progresses in an upward manner along the vertical z axis and in a centripetal manner along the horizontal x and y axes. Gradient cooling can efficiently reduce shrinkage porosity of the jumbo slab ingot by optimizing the solidification sequence, and making the position of shrinkage porosity move towards the head of the ingot or even inside the riser and the secondary shrinkage is eliminated.

(2)The significantly accelerated cooling rate under the gradient cooling process improves the production efficiency.The solidifying time under gradient cooling condition is 2.7 h less than that under air cooling conditon for a 60 t steel ingot in this study.

(3) A 60 t ingot was prepared under gradient cooling, and the ultrasonic flaw detection shows that after the ingot is rolled to a plate, some porosity can be found along the axis of the plate, but the position of the defect is moved towards to the head of the ingot. This distribution trend of the defect is consistent with the simulated result. References

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for heavy plate. Steel Rolling, 2003, 20(06): 1-4. (in Chinese)

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Grades and the Study of New Technique. Shanghai Metals, 2011, 33(6): 57-60. (in Chinese)

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Research on Unidirectional Solidification Technology. Steekmaking, 1998 (01): 52-55.

(in Chinese)

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Re-melting Process. Journal of Iron and Steel Research International, 2007, 14(5): 7-12.

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Heat Transfer, and Mass Transfer in Electro-slag Re-melting Process. Ironmaking and Steelmaking, 1999, 26(6): 423-438.

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Different Cooling Conditions. China Foundry Machinery and Technology, 2008 (4): 39-41. (in

Chinese)

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Distance and High-temperature Plate’s Surface Temperature on Spray Cooling Heat Transfer Coefficient. Journal of University of Science and Technology Beijing, 2007(1): 67-69. (in Chinese)

大型扁锭梯度冷却特性的数值模拟

摘要：缩松缺陷往往被发现在一个空气冷却的巨型钢锭，这将影响最终轧制板的质量。在实际生产中，由于一些冷轧板频率因此拒绝锭的严重缩孔。提高钢锭质量，一个新的冷却方法，梯度冷却过程（其中钢锭上部空气COO在这项研究中，导致了较低的部分是喷雾冷却）。利用ProCAST软件模拟了60 t大型钢锭温度梯度冷却条件下的凝固行为对空气冷却条件下的锭坯进行了比较。凝固的趋势，温度场分布在钢锭缩孔在不同的条件进行了分析。仿真结果表明，在梯度冷却条件下，平板锭的凝固沿垂直轴向上向上，并且在一个百分方式沿X轴和Y轴。梯度冷却可以通过优化凝固顺序，有效地降低大型板坯锭的收缩率缩松缩松从靠近锭中间高度的（在空气冷却条件下）向锭头方向移动，消除二次收缩。此外，凝固的在这一模拟中，梯度冷却锭的离子时间是7.3小时，比空气冷却快2.7小时。在梯度冷却条件下成功地制备了一种60吨巨型平板锭条件。该锭被卷到一个板的厚度为100毫米，长度为18000毫米，并进行超声波探伤。一些孔隙度被发现沿板的轴线在4900—6000毫米，从该盘的头部，表明该位置的缺陷是向的锭头。这种缺陷的分布趋势与模拟结果是一致的。

关键词：梯度冷却；扁锭；温度场；收缩；数值模拟

1数值模拟方法

ProCAST软件进行本研究模拟。对大型钢锭凝固时间比TIS充盈时间更长，所以锭的初始温度为浇注温度TP，这是1580oC对于本研究中所用的钢。的铸铁模具初始温度和保温板为25oC.环境温度

根据Q＝Heff边界条件（是–Rcold），其中Q是热通量；Heff等效界面传热系数；特热面温度；和Tcold是山坳三维表面温度。钢锭之间的传热系数为1800 W·K-1·m-2.the传热系数保温板锭之间为20 W·K-1·M-2。上半部分模具的壁是空气冷却，在10 W M-2·K-1·传热系数。下半部分喷雾冷却的传热系数1000 W·K-1·M-2 [ 10 ]。

1 -隔热板2锭3铸铁模具4口

2实验参数

在这项研究中，它的尺寸为60吨矩形钢锭，长度为2700毫米，宽1000毫米和高度3600毫米。在表1中列出了锭的成分；实验锭的单元模型如图1所示（一）。冷却条件为梯度冷却。模具壁上半部分空气冷却，下半部为水喷雾冷却，如图1（乙）。钢的液相线温度1524.8oC，和固相线温度是1494.8oC.潜热的钢是2.7×102 kJ··。我的物理参数ngot，数值模拟模具隔热板列于表2。相比之下，对钢锭的空气冷却条件下凝固数值模拟还高Rmed。

表1实验用钢锭的组成

图1实验锭的有限元模型（一）和冷却位置（二） 表2：用于数值模拟的锭、模、隔热板的物理参数

图2：锭的物理参数：比热（一）和导热率（二）

图3：模具的物理参数：比热（一）和导热率（二） 3.结果与分析

3.1梯度冷却下的板坯凝固过程

Mo Ni Ti Cu Fe 0.025 0.4 0.017 0.15 Bal. Material

Specific heat

Thermal

conductivity

Density (kg ·m -3) (kJ ·kg -1·K -1) (W ·K -1·m -1 )

Sol id Solid-li quid Liquid Ingot (steel) Fig.

2(a) Fig. 2(b) 7,4

00 7,200 7,000

Mould (cast iron) Fig. 3(a) Fig. 3(b) 7,300

Heat

insulation plate 1.05 0.75

600

在大型板坯连铸中间，在梯度冷却条件下凝固过程如图4所示。如图4（1），该锭的下一部分沿Y轴由于水喷淋冷却壁外横向温度梯度过大。例如，在位置1159o温度迅速下降 C.对钢锭的上部有一个小的温度梯度沿横向和纵向方向由于空气冷却外。凝固的时间是在无花果显示。4（一）（电子）。铸坯的凝固过程从底部沿纵向轴线在低温区逐渐向上，并从根本上沿壁向中心方向发展横向轴。低分的凝固速度快。一般来说，在钢锭凝固开始向上在一定的梯度从底部，并在浇注完成克头。这凝固的方法能够保证一个开放的室内饲养通道，有效减少缩孔，因此显著提高钢锭质量。

图4：在梯度冷却条件下，板坯凝固过程

3.2在不同冷却条件下的凝固时间

凝固时间对锭坯的生产是非常重要的，因为它决定了成型时间，生产周期，经济效益。所研究的锭的凝固时间为10小时空气冷却和梯度冷却7.3小时的情况下，梯度冷却条件下的生产效率提高了27%。

不同冷却条件下的缩松分布

6小时后，温度分布梯度冷却空气冷却，如图5所示。点B在Z轴的中部和上部，如图5（a），具有温度of1524oC和N空气冷却凝固。而上部凝固比下部，这导致继发性收缩在扁锭的核心。而固化程序RESS的钢锭在梯度冷却，如图5（b），确保一个开放的进料通道显著抑制二次收缩。

在不同的冷却条件下凝固，如图6所示的缩孔分布。在空气冷却条件下的收缩位置为1900毫米锭的中间高度附近沿中心轴底部，如图6所示（一）。图6（二）显示，该收缩位置被移向锭头，甚至提升到内部在梯度降温下的立管。因此，二

次收缩是抑制和锭质量显着改善。仿真结果表明，消除的可能性在梯度冷却的缩松离子。在生产实践中凝固后缩孔的分布不可能像仿真结果相同，但分布缩松化趋势是有价值的，可以为工业生产提供理论指导。

图6：5小时空气冷却（一）和梯度冷却后锭的温度分布

图6：分布的缩孔在保证空气冷却（a）和（b）梯度冷却

4巨型平板锭的生产实践

在这项研究中，使用相同的参数，在第2节所示的梯度冷却产生的60吨巨型板坯锭。熔炼和浇注工艺为60 T EF（电炉）→LF（钢包F炉）→VD（真空脱氧）。的效果是1580oC.如图7所示的60吨大型钢锭温度梯度冷却条件下制备的图片。它被卷沿轴向方向板的厚度为100毫米和18000毫米的长度。通过间接检测板的检测结果，验证了该方法的有效性。

tud280的超声波探伤结果表明有缩孔沿钢板的轴和4900–6000毫米钢板的头部，如图8所示。TH缺陷的电子位置向在空气冷却条件下的锭中间的锭头移动。根据无损检测方法的缺陷的大小和数量是可以接受的无损检测标准gb2970-2004 III，这表明梯度冷却过程可以通过改变收缩分布趋势提高大型板坯钢锭内部质量孔隙度。

图7：60吨平板锭的梯度冷却

图8：在梯度冷却作用下，用锭轧制的缺陷分布

5结论

（1）梯度冷却条件下板坯钢锭凝固过程中沿垂直轴上的方式和一种向心的方式沿X轴和Y轴。梯度通过优化凝固顺序，有效地降低大型板坯锭的缩松，使缩松位置朝着锭头方向移动甚至内部的立管和二次收缩被淘汰。

(2）显着加速冷却速度下的梯度冷却工艺提高了生产效率，在梯度冷却条件下的凝固时间小于2.7小时。一个60吨钢锭在研究空气冷却条件。

（3）在梯度冷却条件下制备了60锭锭，并将超声波探伤结果显示，当锭卷到一个板上时，可以在板的轴线上找到一些孔隙率缺陷的位置移动向上的锭头。这种缺陷的分布趋势与模拟结果是一致的。

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