Analysis on the causes of liquid level fluctuations in the copper mould tube of peritectic steel produced by wide slab continuous casting machine

Abstract: The reasons for the fluctuation of the mold liquid level when producing peritectic steel in a wide slab continuous casting machine were analyzed. It was found that the non-uniformity of the billet shell caused by the peritectic reaction during the solidification of molten steel was the main factor causing the fluctuation of the copper mould tube liquid level. If the difference between the peritectic carbon content calculated based on the actual composition of the molten steel and the actual carbon content of the molten steel exceeds a certain range, it will aggravate the unevenness of the billet shell, which will in turn affect the fluctuation of the mold liquid level. By optimizing the composition of molten steel, the difference between the calculated peritectic point carbon content and the actual carbon content is controlled. It solves the production problem of excessive liquid level fluctuation in the crystallizer of peritectic steel produced by the ultra-wide slab continuous casting machine, and stabilizes the production of the continuous casting machine and the quality of the cast slab.

Keywords: wide slab, peritectic steel, mold level fluctuation

Preface

The wide slab continuous caster of the Second Refining and Rolling Plant of Angang was put into operation in 2005. Due to its large width-to-thickness ratio, it is difficult to control the quality defects of the slab represented by longitudinal cracks on the surface of the slab. Therefore, when designing the composition of molten steel, the composition of peritectic steel should be avoided as much as possible (that is, the carbon content in the steel is 0.09% ~ 0.14%). However, with the needs of product development, the composition design of some steel types cannot avoid the range of peritectic steel. In the process of producing peritectic steel for ultra-wide slabs, the problem of copper mould tube liquid level fluctuation is encountered, which has a great impact on the production of continuous casting machines. Safety and billet quality pose a major threat. Based on the analysis of the causes of liquid level fluctuations, corresponding measures were taken to solve this problem and ensure the smooth production of peritectic steel in the ultra-wide slab continuous casting machine.

Liquid level fluctuation problem in peritectic steel mold for ultra-wide slab production

Main parameters of Anyang Iron and Steel ultra-wide slab continuous casting machine

Anyang Iron and Steel’s ultra-wide slab continuous casting machine was put into operation in August 2005. The main equipment process parameters are shown in Table 1.

Table 1 Main technical parameters of Angang’s ultra-wide slab continuous casting machine

projectProcess parameters
modelStraight mold multi-point bending multi-point straightening arc slab continuous casting machine
Basic arc radius/mm6 670
Pulling speed range/(m·min-1)0.2~2
Metallurgical length/mm18 687
Cast slab thickness/mm150
Cast slab width/mm1 600~3 250

Introduction to ultra-wide slab mold liquid level control system

The mold liquid level control system of Anyang Iron and Steel’s ultra-wide slab continuous casting machine includes a mold liquid level detection system and a control system. It is a double closed-loop control system. The copper mould tube liquid level detection method is a radiation source, and the control execution unit uses a hydraulic cylinder. The system has been operating well since it was put into production. When automatic control is used for general steel types, the rate of controlling the fluctuation of the copper mould tube liquid level within ±3mm reaches more than 99.5%.

Liquid level fluctuation characteristics of peritectic steel mold produced by ultra-wide slab continuous casting machine

In the process of producing peritectic steel with Angang’s ultra-wide slab continuous caster, the biggest problem encountered is the fluctuation of the copper mould tube liquid level. The fluctuation situation is shown in Figure 1. The horizontal curve marked in the figure is the actual liquid level of the copper mould tube.

Figure 1 Pouring curve of wide slab peritectic steel

The characteristics of the copper mould tube liquid level fluctuation are:

(1) The start time of the copper mould tube liquid level fluctuation: starting from the middle and late stages of the second furnace, about 25 minutes after the pulling speed reaches the target pulling speed;

(2) Crystallizer liquid level fluctuation range: Starting from the normal copper mould tube liquid level fluctuation range of ±3mm, it gradually increases, reaching a maximum of ±8mm;

(3) Fluctuation frequency: The fluctuation period is 15s ~ 15.5 s, and the average pouring length of a fluctuation period is 279 mm ~ 281 mm;

(4) Adjustment measures to reduce copper mould tube liquid level fluctuations: increasing or decreasing the copper mould tube cooling water and secondary cooling water cannot suppress the copper mould tube liquid level fluctuations, reduce the pulling speed, and reduce the copper mould tube liquid level fluctuation amplitude;

(5) Surface condition of the cast slab: Corresponding to the fluctuation of the mold liquid level, there are transverse depressions on the surface of the cast slab, similar to vibration marks, as shown in Figure 2, with a spacing of about 140 mm.

Figure 2 Surface conditions of cast slabs produced during peritectic steel mold level fluctuations

Hazards of copper mould tube liquid level fluctuations

Excessive fluctuations in the copper mould tube liquid level will affect the surface quality of the slab and increase the probability of defects such as slag inclusions, pits, and longitudinal cracks on the slab surface. When the fluctuation of the copper mould tube liquid level exceeds the vibration amplitude of the copper mould tube, it will cause the liquid slag to flow into the gap between the shell and the copper plate of the copper mould tube unevenly, causing the shell and the copper plate of the copper mould tube to stick. And the resulting steel leakage accident threatens the smooth operation of continuous casting production.

When lower casting speeds are used for production, the continuous casting pouring cycle will increase, which not only reduces the production efficiency of the continuous casting machine, but also causes difficulties in matching the furnace and machine, and also makes it difficult to control the superheat of the molten steel in the tundish. And for some steel types, when the casting speed is low, the third high-temperature brittle zone cannot be avoided during straightening, causing batch straightening cracks.

Analysis of the causes of liquid level fluctuations in peritectic steel molds

If the crystallizer foot roller or sector support roller is bent, the roll gap will change periodically during the rotation of the roller. The degree of squeezing of the billet shell by the roller also changes periodically, causing the molten steel in the billet shell to move upward periodically, causing periodic changes in the liquid level of the mold. As mentioned before, the pouring length of the copper mould tube in one cycle is 279 mm ~ 281 mm, so the diameter of the roller in question should be about 89 mm. However, the continuous casting machine does not have a roller with this diameter, so the mechanical reasons are ruled out.

Peritectic steel can easily cause liquid level fluctuations in the copper mould tube. If the copper mould tube is cooled unevenly, the primary green shell at the same height will have inconsistent transition times into peritectic transition. That is, when the shell has not yet entered peritectic transformation at the weak cooling location, the peritectic transformation has begun to occur in adjacent locations due to strong cooling. At the point where peritectic transformation has begun, the billet shell separates from the copper mould tube wall due to phase transformation shrinkage, the air gap increases, heat transfer slows down, and the billet shell becomes thinner. However, due to the small air gap between the green shell and the copper mould tube wall near the green shell where the peritectic transformation has not started, the green shell still grows rapidly, eventually causing uneven solidification thickness of the primary green shell. In addition, in production practice, it has been found that when pouring peritectic steel, the copper mould tube has periodic liquid level fluctuations that exceed the allowable value, which easily promotes the generation of surface cracks. This fluctuation is related to the dynamic bulging of the billet caused by uneven solidification of the billet shell in the meniscus area. After the billet leaves the mold, it runs in the guide section. The static pressure of the molten steel causes the billet to bulge between two adjacent rollers. The bulging billet is compressed when it passes the next pair of rollers, causing the liquid level of the mold to fluctuate up and down. Due to the inhomogeneity of the peritectic steel billet shell, the shell bulges of thicker and thinner billet shells are different. The mold level control system will raise the stopper rod position to fill the mold with molten steel. At the same time, as the billet drawing progresses, the bulging area is compressed to the middle of the two rollers, and the molten steel in the liquid phase cavity is also filled with molten steel into the copper mould tube, causing the liquid level to rise rapidly and making the growth of the billet shell in the copper mould tube more uneven. Repeatedly, the copper mould tube liquid level fluctuation rapidly intensifies. In fact, other elements in steel also affect the peritectic point. Someone has studied the influence of other elements in steel on the solidification mode and the inhomogeneity of the billet shell. They used the formula to calculate the influence of other elements on the carbon content C B of the peritectic point. The formula is:

C B = 0. 1967 + 0. 0036 [Al] – 0. 0316 [Mn] -0.0103[Si] + 0. 1411[Al] 2+ 0. 05 [Al][Si] – 0. 0401[Ni] + 0. 03255 [Mo] + 0. 0603 [V] – 0. 0024 [Cr] +0.00142[Cr]2 -0.00059[Cr] [Ni]+0.0266[W]

  For peritectic steel, the greater the difference between C B and the actual composition C of molten steel, the greater the inhomogeneity of the billet shell during the solidification process of molten steel, and the greater the fluctuation range of the copper mould tube liquid level.

The chemical compositions of the steel types with mold liquid level fluctuations in wide slab continuous casters are shown in Table 2. The actual composition of molten steel, the calculated value of C B and the fluctuation of the copper mould tube liquid level are shown in Table 3.

Table 2 Chemical composition of a certain peritectic steel / wt%

elementCSiMn        PSNbAlMo
determination0. 1 ~ 0. 130. 15 ~ 0.351 .40~ 1 .50   ≤0.015≤0.0080.018-0.03    0.02-0.0450. 1 -0. 15
control0. 1 ~ 0. 130.20~ 0.301 .42 ~ 1 .50   ≤0.010≤0.0030. 02- 0. 025        0.02-0.0450. 1 – 0. 013

Table 3 Actual chemical composition of molten steel, C B value and copper mould tube liquid level

project Actual chemical composition of molten steel/wt%CB rangeCB ~ C rangecopper mould tube liquid level fluctuation amplitude
CSiMn       Al         Mo
Case 10.10.231 .46         0.0360. 1090. 15280.0528    Violent fluctuations, amplitude greater than ±8 mm
Case 20. 120.231 .46         0.0250. 1140. 15280.0328     There is fluctuation, the amplitude is ±5 ~ 7 mm
Case 30. 130.31 .43        0.0290. 1130. 15310.0231     Normal, amplitude less than ±3 mm

It can be seen from Table 3 that when the difference between the peritectic point carbon content C B and the actual carbon content is larger, the copper mould tube liquid level fluctuates larger, and vice versa. The actual situation on site also confirmed the above research conclusion. From this point of view, the reason for the liquid level fluctuation in the wide slab mold is the uneven growth of the billet shell during the solidification process of peritectic steel. The “pump” effect caused by the bulging of the billet shell between the guide rollers in the sector-shaped section causes fluctuations in the copper mould tube liquid level. When the pulling speed is reduced, the thickness of the billet shell increases, and the bulge of the billet shell between the sector-shaped guide rollers decreases. Its influence on the copper mould tube liquid level is weakened, and the copper mould tube liquid level tends to be stable.

Improvement measures and effects

According to the analyzed reasons, it was decided to optimize the composition of the molten steel to reduce the fluctuation of the copper mould tube liquid level. Taking into account factors such as the control level of steelmaking components, continuous casting process, and steel rolling performance guarantee, the optimized chemical composition of this steel type is shown in Table 4.

Table 4 Optimized chemical composition of peritectic steel / wt%

elementCSiMnAlMoCB rangeCB ~ C range
determination0. 12 ~ 0. 140. 15 ~ 0.351 .40~ 1 .500.02 ~ 0.0450. 1 ~ 0. 150. 151 ~ 0. 1510.011 ~ 0.031
control0. 12 ~ 0. 130.20~ 0.301 .42 ~ 1 .500.02 ~ 0.0450. 1 ~ 0. 130. 151 ~ 0. 1520.021 ~ 0.032

After the composition is optimized, the copper mould tube liquid level is stable, as shown in Figure 3. The mold liquid level is stable, the copper mould tube liquid level fluctuation is less than ±3 mm, and the continuous casting speed remains stable. Not only the furnace-machine matching is achieved, but the surface quality and internal quality of the cast slab are improved, providing high-quality blanks for the steel rolling process.

Figure 3 Pouring curve after optimization of molten steel composition

Conclusion

Through analysis, we found out the reason why the mold liquid level fluctuated severely when the ultra-wide slab continuous casting machine of Angang produced peritectic steel. The uneven shell growth caused by peritectic reaction and the bulging of the shell between the rollers in the sector section are the reasons for the fluctuation of the copper mould tube liquid level. The greater the difference between the peritectic carbon content calculated based on the alloy composition of the molten steel and the actual carbon content of the molten steel, the greater the fluctuation of the copper mould tube liquid level. By optimizing the composition of molten steel and controlling the above difference below 0.032 percentage points, the liquid level of the mold can be stabilized, ensuring smooth continuous casting production and stable slab quality.

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