Analysis of the causes of longitudinal cracks in the corners of chamfered mold slabs

The chamfering mold is an effective way to control the corner transverse cracks in the slab, but during the use of the chamfering mold, longitudinal corner cracks often occur in the slab. By studying the influence of factors such as the process parameters, operating conditions and equipment accuracy of the chamfering mold on the vertical corner cracks of the slab, the causes of longitudinal cracks in the corners were analyzed and effective measures were taken to reduce the incidence rate of longitudinal cracks in the corners of the chamfering mold. from 12.5% to less than 0.7%.

Keywords: crystallizer; longitudinal cracks

Introduction

In the range of solidification temperature Ts~600℃, steel has three brittle temperature zones, namely the first brittle temperature zone near the solidification temperature,

The II brittle temperature zone near 1200°C and the III brittle temperature zone between 950 and 700°C only exist under lower deformation rate conditions. The III brittleness temperature zone of steel is closely related to the transverse cracks and corner transverse cracks on the surface of the cast slab. Usually, two methods are used to avoid straightening in the brittle temperature zone: higher or lower than the III brittle temperature zone. However, since straightening using a method lower than the III brittle temperature zone requires higher equipment capabilities and also causes higher wear and tear on the equipment. At the same time, excessive cooling rate will bring more surface quality problems. Therefore, weak cooling is commonly used to make the surface temperature of the slab higher than the III brittle temperature zone during straightening. However, two-dimensional heat transfer occurs in the corners of the slab, and its temperature decreases rapidly. The temperature of the corners of the slab cannot be guaranteed to remain above the III brittle temperature zone during the straightening process. Therefore, corner transverse cracks become the continuous casting slab. One of the common surface defects. In order to solve the corner transverse crack defect that has been plaguing continuous casting production, 210 Converter Factory adopted chamfering mold technology in 2011 to change the heat transfer at the corners of the cast slab from two-dimensional heat transfer to one-dimensional heat transfer. The use of the chamfering crystallizer solves the problem of corner transverse cracks that plagued the 210 converter plant. However, during use, longitudinal corner cracks of the slab frequently occurred, and steel leakage accidents occurred due to the longitudinal corner cracks, which restricted the use of the chamfering crystallizer.

Morphology of corner longitudinal cracks

The morphology of the chamfered mold and the cross-section morphology of the chamfered billet are shown in Figures 1 and 2. It is a combination of narrow copper plate and wide copper plate

The link points are changed from right-angle connections to obtuse-angle connections with radian, which changes the heat transfer at the corners of the slab from two-dimensional to one-dimensional. This weakens the cooling intensity of the corners of the slab, thereby reducing the risk of transverse cracks at the corners of the slab. incidence.

Figure 1 Morphology of chamfered crystallizer

Figure 2 Cross-sectional morphology of chamfered billet

Figure 3 Crack morphology of corner longitudinal crack

Corner longitudinal cracks occur near the chamfers of the billet, as shown in Figure 3. There are two types of shapes: continuous and discontinuous. The distance is 5~40mm from the corner. Such defects can generally be seen with the naked eye online and are very slight. It can only be found when the corners are pickled or flame cleaned. Both inner and outer arcs may occur, but the probability of inner arcs is much higher than that of outer arcs. This is because the inner arc surface is the active side in the mold, and the taper is easy to deviate and the corner gap is easy to become larger. The quality defects of the rolled material caused by longitudinal cracks in the corners of the chamfered mold are shown in Figure 4. They are generally located 30~50mm away from the edge of the rolled material.

Figure 4 Morphology of quality defects in rolled products caused by corner longitudinal cracks

Influencing factors of longitudinal cracks in slab corners

Crystallizer taper

The taper of the narrow surface of the mold should compensate for the thermal shrinkage of the solidified billet shell. The inner cavity of the mold should be inverted taper to reduce the formation of air gaps and enable the thick bottom of the billet shell to grow evenly. The appropriate taper can compensate for the shrinkage in the width direction of the cast billet and support the billet shell. Reduce the air gap, make the heat transfer evenly, and have a good effect on inhibiting surface cracks. The taper coefficient of the narrow side of the chamfered mold should be continuously optimized and adjusted according to the composition range of different steel types. Through statistical analysis of production data, it was found that the taper coefficient of the narrow side of the chamfered mold has an obvious correspondence with the angular longitudinal cracks. Figure 5 shows the relationship between the narrow surface taper coefficient of peritectic steel, medium carbon steel, and high carbon steel and the steel type and the incidence of angular longitudinal cracks.

Figure 5 Relationship between taper and incidence rate of angular longitudinal cracks

It can be seen from Figure 5 that as the narrow surface taper coefficient gradually increases in the range of 1.1~1.35%/m, the incidence of angular longitudinal cracks decreases significantly. Therefore, the taper coefficient increases, which is beneficial to the control of angular longitudinal cracks. This is because although the chamfered surface of the chamfered mold is smaller in size, it still needs to be treated as a contact surface, and the solidification shrinkage effect of the solidified shell on the contact surface also exists. When the taper of the narrow surface is small, during the shrinkage process, the billet shell will separate from the chamfered surface, forming an air gap, resulting in uneven cooling of the billet shell. The temperature of the billet shell at the chamfered surface is higher than that of the narrow surface billet connected to it. Shell temperature, under the action of the static pressure of molten steel, the billet shell at the chamfered surface is easily squeezed repeatedly, resulting in corner longitudinal crack defects. Combined with the size of the lower opening of the casting billet, the shape of the narrow belly and the wear of the copper plate, it was finally determined that the optimal taper coefficient of peritectic steel is 1.35%/m, the optimal taper coefficient of medium carbon steel is 1.3%/m, and the optimal taper coefficient of other steel types is 1.25%/m.

Carbon mass fraction

The carbon mass fraction and the incidence rate of angular longitudinal cracks were compared and analyzed. The results are shown in Figure 6. Among them, the incidence rate of angular longitudinal cracks is the highest when the carbon mass fraction is in the range of 0.13~0.18%. The incidence rate of angular longitudinal cracks in high carbon steel It is also relatively high.

Figure 6 Relationship between carbon mass fraction and the incidence of angular longitudinal cracks

According to the iron-carbon phase diagram, it can be seen that for molten steel with a carbon equivalent within the range of peritectic steel, the primary billet shell formed at the meniscus will undergo a peritectic reaction of δ+L→γ. With the peritectic reaction, the cast billet will undergo a larger Large volume shrinkage and line shrinkage. At this time, the molten steel cannot be replenished well, and the surface of the billet shell is easy to form depressions and separate from the copper plate of the crystallizer. The heat transfer effect here is poor, resulting in the cooling intensity of this part being less than other parts, causing the temperature to rise and the structure to coarsen. Thick dendrites are formed, causing stress concentration and increasing crack susceptibility. Under the action of thermal stress and friction of the copper plate, the depression is more likely to induce dendrite cracking and form tiny cracks. The reason for the high incidence of angle longitudinal cracks in high carbon steel is that the high carbon steel billet shell is relatively brittle and is prone to cracking and forming micro cracks under the action of thermal stress and copper plate friction.

The crystallizer has large corner gaps

The corner gap control of chamfered molds is more stringent than that of right-angled molds. During the casting process, if the corner seams of the mold become larger, the corners of the slab will be in contact with the air, which will affect the heat transfer effect of the corners of the slab, resulting in insufficient cooling strength at the corners of the slab, and ultimately cause abnormalities in the corner structure. If it is thick, it is more likely to cause corner longitudinal crack defects. At the same time, the corner seams of the mold are large. During the pouring process, molten steel splashes into the corner seams and cools down as an external force source, which causes longitudinal corner cracks in the slab. Therefore, it is required that the corner seam value of the crystallizer before use is <0.3mm, and the dirt at the corner seam must be cleaned after each pouring stop.

 Fouling of crystallizer back plate

During the production process, there were frequent water outages in the crystallizer after the continuous casting was stopped, resulting in batches of corner longitudinal cracks and subcutaneous cracks. After the crystallizer was disassembled, it was found that the copper plate back plate was severely scaled, as shown in Figure 7; the continuous casting stop was resumed. After pouring cooling water into the crystallizer, the corner and longitudinal cracks were controlled. The reason is that the water in the crystallizer is frequently stopped after pouring is stopped, and air easily enters the pipes and oxidizes the pipes, forming scale. The back plate of the crystallizer water is also prone to scale when the water flow rate is low.

Figure 7 Picture of scaling on the back plate of the crystallizer copper plate

Design defects of chamfered mold narrow-side copper plate

The chamfered crystallizer is modified on the basis of the right-angled crystallizer. The distance between the sides of the narrow-side water tank (Figure 8) and the surface of the copper plate is also inconsistent, which easily causes uneven cooling of the shell.

Figure 8 Schematic diagram of the narrow edge of the chamfered crystallizer

Formation mechanism of corner longitudinal cracks

The peritectic steel has strong crack sensitivity and the thermoplasticity of the billet shell is poor; the unreasonable taper of the chamfered mold cannot meet the shrinkage characteristics of the cast billet; the copper plate of the mold does not have sufficient support for the cast billet, thus hindering the transfer of the billet shell. Thermal properties result in a relatively thin billet shell with poor resistance to various stresses; scaling of the copper plate of the crystallizer, uneven cooling of the billet shell, thermal stress and friction of the copper plate, resulting in stress concentration and increased crack susceptibility; The distance between the sides of the narrow-side water tank and the surface of the copper plate is also inconsistent, and design defects can easily cause uneven cooling of the billet shell. The above factors act alone or in combination, eventually causing longitudinal cracks in the corners of the chamfered mold slab.

Control measures and application effects

According to the characteristics of the chamfered crystallizer and the formation mechanism of corner longitudinal cracks, the following control measures are taken to control the corner longitudinal cracks caused by the chamfered mold.

a. Determine the taper coefficient of peritectic steel to be 1.35%/m, the taper coefficient of medium carbon steel to be 1.3%/m, and the taper coefficient of other steel types to be 1.25%/m.

b. The corner seams of the mold are required to be less than 0.3mm before the mold is put on line, and the corner seams of the mold should be cleaned and inspected after each pouring stop.

c. Strengthen the monitoring of crystallizer cooling water quality; when the crystallizer is online, ensure that the crystallizer cooling water is uninterrupted to prevent scaling of the crystallizer back plate.

By analyzing the causes of vertical corner cracks in the chamfered mold slab and taking effective measures, the corner longitudinal cracks have now been controlled, the incidence rate of corner longitudinal cracks has been reduced from 12.5% to less than 0.7%, and the chamfered mold can be used normally.

Conclusion

210 The converter plant has achieved efficient and stable use of the chamfered crystallizer through technical research and effective measures.

a. The crack sensitivity of peritectic steel is strong, the taper coefficient setting is unreasonable, the mold corner seams are large, the mold copper plate back plate is scaled, and the design defects of the chamfered mold itself are the reasons why the chamfered mold causes longitudinal corner cracks. main reason.

b. By reasonably setting the taper coefficient, keeping the mold corner gap <0.3mm, and preventing mold scaling, the vertical cracks in the corners of the chamfered mold slab are effectively controlled. Through the optimization of the above measures, the incidence rate of corner longitudinal cracks caused by chamfered molds has been reduced from 12.5% to less than 0.7%.

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