Causes and Solutions of Corner Crack Defects in Continuous Casting Billets

Abstract: This article studies the corner crack defects of cast slabs and analyzes the process, operating factors and equipment reasons for corner crack defects in cast slabs. Through a series of improvement measures such as improving the quality of molten steel, adjusting the cooling water volume of the copper mould tube and secondary cooling water, improving the performance of online equipment, and chamfering the copper mould tube. It improves the uneven thermal stress and mechanical stress acting on the solidified billet shell, reduces the incidence of cracks at the corners of the cast billet, and improves the quality of the continuous casting slab.

Keywords: continuous casting; corner cracks; copper mould tube; secondary cooling

Baotou Steel Rare Earth Steel Plate Factory’s two dual-flow slab continuous casters, the 1650 mm continuous caster and the 2150 mm continuous caster, were tested under thermal load on October 19, 2013 and April 28, 2014 respectively. It took only one year to successfully develop and produce more than 80 steel types, and reached production in May 2015. After the mid-term maintenance in September 2015, a large number of small cracks began to appear in the corners of continuous casting billets such as peritectic steel, sub-peritectic steel, and microalloy steel, especially in peritectic steel. Edge crack defects occur when rolling thick specifications, causing the hot plate to be degraded or scrapped, which directly affects the production organization of steelmaking and steel rolling. To this end, the Rare Earth Steel Plate Factory established a corner crack research team to analyze the causes of corner cracks in the continuous casting billet. A series of control measures have been adopted to significantly reduce corner cracks in peritectic steel, sub-peritectic steel, and microalloy steel.

Basic performance parameters of continuous casting machine

Baotou Steel Rare Earth Steel Plate Factory has two continuous casting machines. The 1650 mm continuous casting machine is designed by MCC Jingcheng Company, and the 2150 mm continuous casting machine is designed by Siemens VAI. The core technology and key equipment are provided by Siemens VAI. Both continuous casting machines adopt advanced technologies such as hydraulic vibration, dynamic light reduction, electromagnetic stirring of the cast stream, and hydrogen-oxygen cutting. The designed production capacity of the two continuous casting machines is 5.36 million t/a of qualified slabs.

The basic parameters of continuous casting equipment are shown in Table 1.

Table 1 Basic performance parameters of continuous casting machine

nameparameter
Continuous casting machine modelStraight arc
Arc radius/m10.0
Metallurgical length/m36.9
Crystallizer length/mm900
Number of streams2
Pouring slab thickness/mm230, 250 (reserved)
Pouring slab width/mm900 ~ 2 150
Fixed length/m8~11
Number of sectorsCurved section, sections 1 – 15
Pulling speed range/(m·min – 1)0 . 8 ~ 1 . 8

Corner cracks in cast slabs

Current status of medium carbon steel continuous casting billets

A large number of corner crack defects occurred during the production of medium carbon Q345B, peritectic ship plate B, micro-alloy steel and other steel types, with a defect rate of ≥ 4. 0%. The defect of corner cracks in the billet affects the hot rolling production schedule, leads to an increase in the inventory of continuous casting billets, and affects normal production. Figure 1 shows the corner cracks of the cast slab.

Figure 1 Diagram of narrow side corner crack of slab

Influence of corner cracks in continuous casting billet

The impact of corner cracks on the continuous casting billet mainly occurs within 30 mm from the hot rolled edge, appearing in three forms: “tongue-shaped/hook-shaped” cracks, bubbles or warped skin, as shown in Figure 2. On thick gauge steel coils, “tongue-shaped/hook-shaped” cracks are dominant; on thin gauge steel coils, bubbles or peeling cracks are dominant.

Figure 2 Edge crack morphology

Mechanism of crack generation in continuous casting billet

When the molten steel with a C content of 0.09% to 0.15% in the continuous casting billet solidifies, large volume shrinkage and linear shrinkage may occur, and corner transverse cracks may easily occur. In addition, N in steel reacts with elements containing Nb, V, Ti, etc. to form nitrides that precipitate at the grain boundaries, thereby reducing the thermoplasticity of the steel and promoting the occurrence of cracks. The C content of molten steel has the highest probability of cracks in the peritectic area. Therefore, strictly controlling the C content of molten steel and trying to avoid pouring with a C content of 0. 09% ~ 0. 15% can effectively reduce the occurrence of cracks.

It can be seen from the Fe-C phase diagram that when the high-temperature molten steel is cooled to 1493°C, a peritectic reaction of δ (ferrite) + L (liquid phase) → γ (austenite) transformation will occur. Along with the peritectic reaction, large volume changes and linear shrinkage will occur. The uneven effects of solidification shrinkage and hydrostatic pressure of molten steel make the surface of the thin shell rough, wrinkled, and even dented. The molten steel solidifies and cools slowly in the depressed area, and the structure is coarse. Under the action of thermal stress and the static pressure of the molten steel, stress concentration will occur, leading to the occurrence of microcracks. After the continuous casting billet comes out of the mold, excessive secondary cooling or uneven cooling, as well as mechanical stress, can cause cracks to expand and produce macroscopic longitudinal, transverse or corner cracks.

Influencing factors of corner cracks in continuous casting slabs

Superheat of molten steel

The higher the superheat of the molten steel in the tundish, the thinner the billet shell generated in the crystallizer, and the poorer its ability to withstand stress, and the easier it is for dents and cracks to occur in the weak areas of the billet shell. When the tundish temperature is too low, the fluidity of the mold slag is reduced, and the heat transfer and inclusion adsorption capabilities become poor, which can easily lead to uneven cooling of the continuous casting billet and crack defects.

Large fluctuations in pulling speed

The casting speed has a great influence on the quality of the continuous casting billet. Many cracks in the continuous casting billet are caused by the lifting and lowering of the casting speed. When the pulling speed increases, the flow of molten steel in the copper mould tube accelerates, which easily causes liquid level fluctuations in the copper mould tube. At the same time, the solidification time in the copper mould tube becomes shorter, and the thermal stress of the molten steel on the shell coming out of the copper mould tube increases. At low casting speeds, the cooling rate of the continuous casting billet is too fast, which is also prone to cracks.

Continuous casting machine equipment

copper mould tube copper plate. When the copper plate passing capacity in the mold reached 400 furnaces (55,000 tons), the lower corners of the narrow-sided copper plates were severely worn, resulting in excessive cooling of the corners of the slab. In the later stages of the use of the mold, the deformation of the wide-side copper plates causes large corner gaps in the mold, which increases the stress at the corners of the continuous casting billet and easily causes cracks.

copper mould tube taper and vibration. If the taper of the mold is too small, the shell will easily detach from the copper plate in the mold, resulting in weak cooling and too thin shell, causing the corner grains to grow rapidly and form coarse grains, which are prone to corner cracks. When the mold taper is too large, the drawing resistance increases, resulting in increased stress in the continuous casting billet and easy to induce corner cracks. At the same time, the polarization of the copper mould tube will also increase the stress of the solidified shell of the continuous casting billet and the uneven cooling of the continuous casting billet, which can easily cause corner cracks.

Sector segment arc deviation and roll gap deviation. Due to the small number of spare parts for spare sector sections and the long online service life of the sector sections, the deviation of the outer arc line and middle roll gap value of some sector sections is greater than ±1 mm, resulting in large deformation of the continuous casting billet (more obvious for small-section slabs), resulting in cracks .

copper mould tube cooling and secondary cooling

Unreasonable copper mould tube cooling and secondary cooling water quality, water quantity, water pressure, etc. cause uneven cooling of the continuous casting shell and easily form various cracks on the surface of the continuous casting billet.

Mold powder performance

Among the various properties of mold flux, viscosity has the greatest impact on the occurrence of surface cracks. When the viscosity is high, the cracks will become more severe with the increase of the drawing speed. When the viscosity is low, the probability of cracks is smaller.

Solutions and effects

Based on the analysis of the causes of the above cracks, corresponding corrective measures were formulated and achieved relatively good results.

Process improvement measures

(1) Strictly control the pouring temperature of molten steel, especially the superheat of molten steel for peritectic steel and medium carbon steel within 18 ~ 25°C.

(2) Use standardized operations to reasonably control the working time between molten iron pretreatment, converter, refining, and continuous casting to ensure constant casting speed (see Table 2). The constant drawing rate of continuous casting reaches more than 95%, and the probability of corner cracks caused by the fluctuation of drawing speed is reduced.

Table 2 Typical speed chart

Steel typeLow carbon steellmedium carbon steelperitectic steelpipeline steel
Pulling speed/(m ·min – 1)1 . 51 . 41 . 31 . 3

Note: Constant pulling speed refers to the typical pulling speed within ±0. 05 m/min.

(3) According to different continuous casting steel types, set different copper mould tube water volumes and secondary cooling water volumes, appropriately reduce the cooling intensity of the copper mould tube water, and adjust the specific water volume from 0. 8L/kg to 0. 56 L/kg. , especially the weak cooling system is implemented on the edge of the continuous casting billet to increase the temperature of the narrow side of the continuous casting billet.

(4) Select special mold powder according to different carbon contents of steel types.

Equipment improvement measures

(1) Realize the life management of the copper plate of the copper mould tube. The newly repaired copper plate will be inspected offline when the steel passing capacity reaches 300 furnaces (40,000 tons) for the first time. When the copper plate is subsequently put on line and the amount of steel passed reaches 150 furnaces (20,000 tons), it will be inspected and repaired offline to reduce corner cracks in the continuous casting billet caused by the copper plate of the mold.

(2) Check the vibration of the continuous casting mold vibration table regularly every week, and control the polarization of the continuous casting mold vibration table within ±0. 3 mm to ensure the equipment accuracy of the continuous casting mold vibration table. Adjust different mold tapers for different steel types, and establish a matching relationship between the taper of the mold, continuous casting steel type and casting speed.

(3) Improve the quality of sector section inspection and maintenance, strictly implement the sector section replacement cycle system, strictly prohibit the sector section from serving beyond the service period, and regularly check the arc alignment, roll gap, nozzle blockage, roller rotation and lubrication of the sector section. In particular, the arc alignment accuracy of the continuous casting mold, bending section, and sector section is controlled within ±0. 5 mm to improve the roll gap control accuracy of the sector section. Control the roll gap deviation of the sector section within ±0. 5 mm, and effectively reduce the occurrence of corner cracks in the cast slab by improving the arc alignment and roll gap accuracy of the sector section.

(4) Use a chamfered mold to slow down the heat transfer at the corners, improve the uniformity of the corner temperature and cooling, thereby reducing the corner stress of the continuous casting billet, which can significantly reduce the occurrence of corner cracks.

Conclusion

By controlling the superheat of continuous casting molten steel, adopting constant casting speed and using special mold powder, the cooling intensity of continuous casting mold water and secondary cooling water is reduced. The arc alignment and roll gap accuracy of the sector-shaped segments of the continuous casting machine are controlled within ±0. 5 mm, the polarization of the continuous casting mold vibration table is controlled within ± 0. 3 mm, and measures such as chamfering the continuous casting mold are adopted. The corner cracks of the continuous casting billet have improved significantly. Especially when producing medium carbon Q345B, peritectic ship plate B, and micro-alloy steel, the corner cracks of the continuous casting billet are greatly reduced, and the corner crack rate is reduced to less than 1%.

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