Effect of mold vibration parameters on surface quality of continuous casting billet

Aiming at the impact of vibration scars and grooves on the surface of 130 mm × 130 mm, 150 mm × 150 mm, and 150 mm × 210 mm billets in electric furnace steel continuous casting on the surface quality of rolled materials. Under the existing sinusoidal vibration mode, combined with industrial experiments, the mold vibration parameters such as negative slip rate, negative slip time, vibration frequency, and mold leading, which affect the surface quality of the cast slab, were optimized. Experiments show that as the negative slip rate (the ratio of negative slip time to vibration period) decreases, the surface of the cast slab tends to be flat and the groove vibration marks are eliminated. When the 150 mm×150mm billet has a drawing speed of 2.0~2.8mPmin, a negative slip rate of 33%, a vibration frequency of 130~180Pmin, a negative slip time of 0.13~0.16 s, and a vibration range of 9.6~10.2 mm, The surface quality of the cast slab is significantly improved.

Keywords: billet; surface quality; mold vibration

Improving the casting speed and tapping the potential of the casting machine are the main directions for the development of traditional continuous casting. However, as the drawing speed increases, the friction resistance in the mold increases, the probability of bonding breakouts increases and the vibration marks become irregular, resulting in a decrease in the surface quality of the cast slab. The vibration of the mold has two process effects. One is to adjust the consumption of mold slag to improve the lubrication condition of the mold; the other is to promote the healing of the billet shell, reduce the depth of vibration marks and improve the surface quality of the cast billet. The above two process effects are reflected in the control of the bonding of the shell. However, when sinusoidal vibration is used at high pulling speeds, the high vibration frequency will lead to low consumption of mold powder and poor lubrication performance. Although non-sinusoidal vibration can better achieve these two process effects, traditional continuous casting machines still use sinusoidal vibration. Therefore, in the sinusoidal vibration mode, selecting reasonable vibration parameters that match the pulling speed is an important factor in improving the surface quality of the slab.

1 Surface quality of Nangang billets

Under the current production technology and supporting mold slag conditions, the overall surface quality of the cast slab is as follows: the formation of vibration marks is imperfect, irregular, and non-standard concave-shaped vibration marks. At low pulling speeds, the vibration marks will be shallow and curved, and in severe cases, vibration scars and vibration grooves will appear. At high drawing speeds, the vibration marks are convex, the surface of the cast slab is wrinkled, slag is adhered, and there are micro cracks under the skin, as shown in Figure 1 (a) (b). Superficial concave and convex vibration marks generally do not affect the final product quality due to their shallow depth. However, defects such as slag inclusions, cracks, and subcutaneous pores caused by vibration scars, wrinkles, and vibration grooves. It cannot be burned off during hot processing, causing the thin strip coating to peel, the rod to have root scars, and in severe cases, pieces will fall off.

The formation of groove-shaped vibration marks and vibration scars causes heat transfer to slow down and uneven heat transfer in the valleys of the vibration marks, thus forming the grain structure shown in Figure 1(c). As can be seen from Figure 1(c), the grains under the vibration groove are coarser than other parts, and the segregation of phosphorus and sulfur components is more serious than other parts. Therefore, there are corrosion cracks under the groove that can be observed with the naked eye after hot pickling. Coarse grains and segregation of phosphorus and sulfur can easily cause a large number of microscopic cracks in the vibration mark valley. Under the action of external forces, such as friction and drawing force in the mold, these cracks will further expand, resulting in cracks in the lower valley of the vibration groove. The cracks occur at high temperatures, causing the cracks to be oxidized to form iron oxide. This causes the cracks to fail to weld during the rolling process, resulting in scarring and block loss on the rolled material.

Figure 1: Surface scars (a) cracks (b) and vibration marks on the surface of the continuous casting billet (c)

2 Analysis of causes of surface quality defects of Nangang billets

So far, Takeuchi E and Brimacombo JK have proposed a relatively mature vibration mark formation mechanism through the analysis of mathematical models. It is believed that during the vibration of the crystallizer, the meniscus was impacted by the pressure generated in the mold slag, making it difficult for the partially solidified meniscus to return evenly to the crystallizer wall, resulting in vibration marks. It is determined by the vibration parameters of the mold and the behavior of the mold powder at the meniscus. Normal vibration marks are flat and concave straight vibration marks. The overall manifestations of vibration marks in Nanjing Iron & Steel Co., Ltd.’s continuous casting billets are shallow and curved at low drawing speeds. In severe cases, vibration scars and vibration grooves appear. At high drawing speeds, vibration marks are convex, the surface of the cast slab is wrinkled, slag is adhered, and there are tiny cracks under the skin. The causes can be summarized as follows:

(1) Poor lubrication and frictional resistance cause the vibration marks to bend along the drawing direction, resulting in curved vibration marks.

(2) Poor lubrication, the casting billet and the copper plate of the mold are bonded, and the regular vibration mark troughs become metal peaks. This forms convex vibration marks.

(3) When the vibration parameters of the mold are too large, the mechanical force between the mold and the slab will increase, which will easily cause molten steel to overflow on the bent meniscus, forming groove-shaped vibration marks. When the thermal insulation of the mold powder is not good, the liquid level fluctuates greatly, and the strength of the steel is high, it will also lead to the formation of groove vibration marks.

(4) When the lubrication between the casting billet and the mold is severe, the casting billet and the copper plate of the mold will be bonded. The bonding increases the heat transfer there, intensifies the solidification of the meniscus, increases the thickness of the meniscus shell, and increases the strength. This causes the molten steel to overflow and solidify on the wall of the vessel, forming a knot phenomenon on the surface of the cast slab, that is, a scar with roots.

Therefore, the current main causes of surface defects in Nangang’s cast slabs are: poor lubrication, large mechanical force between the cast slab and the mold, resulting in high billet drawing resistance.

The quality of the lubrication between the mold and the billet depends on the performance of the mold slag and the vibration parameters of the mold. At high drawing speeds, it is often measured by the consumption of mold powder and the friction force of the billet. The relationship between the friction force of billet drawing, the consumption of mold powder and the vibration parameters is as shown in Equations (1) and (2), and the relationship between the sinusoidal vibration parameters is as shown in Equations (3) ~ (6).

Parameters in the formula: f – vibration frequency P times.min-1; vc- pulling speed Pm.min- 1; vm- crystallizer operating speed Pm.min- 1; dL- liquid slag film thickness Pm; G- mold slag viscosity PPs; Q-mold powder consumption Pkg.m-2; negative slip rate (ratio of negative slip time and vibration period) P%; S-vibration range Pm; tn-negative slip time Ps.

It can be seen from formulas (1) and (2) that the consumption of mold powder is inversely proportional to the vibration frequency of the mold and inversely proportional to the 0.5 power of the product of the viscosity of the mold powder and the pulling speed. The frictional resistance of casting is proportional to the difference between the operating speed of the mold and the casting speed, that is, it is proportional to the mold lead, inversely proportional to the thickness of the liquid slag film, and proportional to the viscosity of the mold powder. Therefore, when the performance of the mold powder is constant, the lubrication and mechanical force between the mold and the slab are mainly determined by the vibration parameters of the mold, that is, the vibration frequency and the mold lead. A large vibration frequency and mold lead will cause poor lubrication between the mold and the billet, increase the mechanical force, and increase the billet pulling resistance.

According to equations (3) and (4), when the mold vibration range is constant, the vibration frequency and mold lead are related to the setting of the negative slip rate. Therefore, in view of the causes of surface defects in Nanjing Steel’s continuous casting slabs, it is necessary to improve lubrication and reduce the mechanical force between the slab and the mold. When the performance of the mold powder is certain, there must be reasonable matching of the mold vibration parameters, such as vibration frequency, mold lead, negative slip rate, etc.

3 Optimization of crystallizer vibration parameters

3.1 Characteristics of current vibration parameters

The production process of Nangang electric furnace steel is: electric arc furnace yLF (VD) y5 stream billet continuous casting. There are many types of castings, mainly including low, medium and high carbon steel, structural carbon steel (65Mn), structural alloy steel (40Cr), spring steel (60Si2Mn), etc. The crystallizer vibrates in a sinusoidal manner. The vibration frequency changes with the pulling speed. The maximum vibration frequency is 220 times Pmin. The amplitude is fixed. It is currently 4.6~5.1 mm and the negative slip rate is 32%~41%. The characteristics of the crystallizer vibration parameters under the current casting machine conditions are shown in Table 1. The control of vibration parameters is based on the premise that the vibration range is constant, the negative slip rate or the negative slip time is selected according to the pulling speed range (requirements of the slab section), and then other vibration parameters (vibration) are determined according to the mathematical relationship between sinusoidal vibration forms. frequency, negative slip time, mold lead, vibration mark spacing).

3.2 Vibration parameter optimization

Since the vibration range is not easy to change, and other vibration parameters are adjusted through the negative slip rate, according to equations (1) ~ (6), under the premise of a certain vibration range, the negative slip rate and each vibration parameter, tension can be obtained The relationship between billet friction and mold powder consumption is shown in Table 2.

Table 1 Vibration parameters of the crystallizer before and after optimization (vibration range 10mm)

parametersSectionPulling speed Pm.min- 1 Negative slip rate P %Negative slip time P sVibration frequency P times.min- 1Crystallizer lead P mmVibration mark spacing P mm
Before optimization  150x 2101. 0~  1. 740、410. 19~ 0. 13131~  1956<  10
150x 1502. 0~  2. 8350. 15~ 0. 11154~  1933. 914~ 16
130x 1302. 8~  3. 2330. 11~ 0. 10174~  1923. 315~ 16
After optimization150x 2101. 0~  1. 837~  380. 15~ 0. 18120~  1554. 6~  4. 911. 6~  12. 5
150x 1501. 8~  2. 833~  340. 11~ 0. 13156~  1753. 2~  3. 615~ 16
130x 1302. 8~  3. 231~  330. 10~ 0. 11175~  1812. 8~  3. 516~  17. 7

Table 2 The relationship between vibration parameters, billet friction force, mold slag amount and negative slip rate at a given vibration range

To ensure that each vibration parameter can meet the billet quality requirements during the negative slip rate adjustment, set each vibration parameter as follows:

(1) Negative slip time: set to 0. 10 ~ 0. 25s. The negative slip time is proportional to the depth of vibration marks. Deep vibration marks will bring about a series of surface quality problems, so the negative slip time cannot be too long. However, in order to ensure the consumption of mold powder, the negative slip time cannot be too small. At present, the pouring condition of Nangang is mainly characterized by poor lubrication. In order to ensure sufficient lubrication, it is hoped that the negative slip time will be closer to the upper limit, preferably 0. 12 ~ 0.15 s.

(2) Crystallizer lead: set to 3~4 mm. The mold lead is too small to prevent the shell from sticking. If the mold lead is too large, the mechanical interaction between the mold and the billet will be too great, resulting in uneven vibration marks and uneven depths. In severe cases, vibration grooves and scars will appear. Therefore, the maximum lead cannot exceed 5 mm.

(3) Vibration mark spacing: set to 10~16 mm. In order to ensure the uniformity of the shell thickness and prevent transverse cracks, the spacing between vibration marks should not be too large, and the spacing between vibration marks here is controlled to be less than 16 mm. However, the spacing between vibration marks cannot be too small, otherwise the vibration marks will be too dense and the sources of cracks will increase.

(4) Crystallizer vibration frequency. The vibration frequency of the crystallizer is a function of the pulling speed, negative slip rate and vibration range. Under high pulling speed, in order to obtain a shallow vibration mark depth, a high vibration frequency method is generally used. However, according to the equipment conditions of Nangang, the vibration frequency of the crystallizer must not be greater than 180 times Pmin. Otherwise, the crystallizer will swing too much, which will cause a series of surface quality problems. Therefore, the maximum vibration frequency is set to be less than 180 times Pmin. High vibration frequency will lead to low consumption of mold powder and poor lubrication. Therefore, it is required that the vibration frequency cannot be set too high.

Because after the crystallizer vibration range and negative slip rate are set, the vibration frequency decreases as the pulling speed decreases. Therefore, in order to ensure negative slipping vibration and effectively control bonding, there should be a minimum frequency limit under low casting speed conditions. According to the literature, the value of frequency ∫ should be at least equal to 2V (V is represented by inPmin, 1 in=2. 54 cm).

According to the vibration parameter control characteristics of Nangang casting machine, after setting the vibration parameter target, this study optimized the negative slippage rate of its different cross-section cast slabs. The optimized negative slip rate and corresponding vibration parameters of each section were calculated, analyzed and organized according to formulas (1) ~ (6), as shown in Table 1.

3.3 Vibration parameter optimization industrial test and result analysis

Based on the above analysis and optimization of vibration parameters, according to Table 1, a vibration parameter optimization experiment was carried out on the second strand casting machine of Nangang Iron and Steel Co., Ltd. (vibration range 10 mm). Table 3 shows the surface quality characteristics of the experimental slabs. It can be seen from Table 3 that as the negative slip rate decreases, the surface of the cast slab tends to be flat, and the vibration marks develop from curved convex fold-shaped vibration marks to flat straight concave vibration marks, and the groove-shaped vibration marks are eliminated.

Through the statistics of the drawing motor current in the test, it was found that the negative slip rate decreased within the set range, and when the mold lead decreased accordingly, the motor current value showed a downward trend. This shows that the resistance to billet drawing is reduced, that is, the mechanical force and frictional resistance between the billet shell and the crystallizer in the mold are reduced. This is advantageous in suppressing bending of vibration marks and suppressing folded groove-shaped vibration marks. Therefore, reducing the negative slip rate within a certain range, reducing the mold lead, and optimizing various vibration parameters are the keys to solving the surface quality problem of Nangang’s cast slabs. Combining basic laboratory research and industrial tests, the reasonable vibration parameters shown in Table 4 are obtained.

Table 3 Effects of different negative slip rates on the surface quality of the cast slab

Steel typePulling speed Pm.min-1Negative slip rate P %Cast slab surface characteristics
65Mn1. 5141The vibration marks are perfect, curved, and folded, with a depth of 0. 38 mm.
65Mn1. 5440The vibration marks are shallow, curved, and shallowly folded.
65Mn1. 5439There are no obvious vibration marks, and the vibration marks on the edges are curved and shallowly folded.
65Mn1. 5138The vibration marks are shallow, straight, concave and rounded, and the depth of the vibration marks is 0. 36 mm.
451. 6241There are obvious bending and folding vibration marks with a depth of 0. 4 mm.
451. 6540Small bends and obvious folding vibration marks.
451. 6439Straight, shallow curved concave vibration marks, slightly shallow folded shape, vibration mark depth 0. 3mm.
451. 6538The vibration marks are straight, shallow and perfect, and the vibration marks are flat and concave.
72A2. 834Small bends and obvious folding vibration marks.
72A2. 832The surface is smooth and the vibration marks are shallow and straight.

Table 4 Optimized vibration parameters of each slab section

Section PmmPulling speed Pm.min-1Negative slippage rate P%Vibration frequency P times.min-1Negative slippage time PsVibration range setting Pmm
150*2200. 9~  1. 73890~  165   0. 13~  0. 259.6~10.2
1. 8~  1. 937141~  179  0. 12~  0. 16
150* 1502. 0~  2. 833130~  180  0. 13~  0. 16 
130*1302. 8~  3. 232163~  1900. 10~  0. 1210~10.3

4 Conclusion

(1) The surface quality problems of Nangang’s cast slabs are mainly caused by low consumption of mold powder, poor lubrication, and high drawing resistance. This is mainly caused by unreasonable mold vibration parameters, which are mainly manifested in high negative slip rate of the mold, high vibration frequency, low mold powder consumption, and high friction resistance.

(2) Research shows that because the negative slip rate of the Nangang mold is set too high, in order to ensure the surface quality of the cast slab, the relationship between the vibration parameters must be met. Within a certain vibration range, the negative slip rate must be appropriately reduced to ensure the consumption and lubrication of the mold slag at high pulling speeds, and to reduce the mechanical interaction between the mold and the slab at low pulling speeds.

(3) Combining industrial experiments and laboratory research, reasonable crystallizer vibration parameters were proposed and good surface quality of the slab was obtained.

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