Main causes of internal cracks in continuous casting billets and their solutions

Abstract: Taking two specifications of continuous casting billets produced by the 50t electric furnace production line of Laigang and the converter steelmaking production line of the New Second District as the research object, the causes of the formation of internal cracks were analyzed, and the types of internal cracks were analyzed using scanning electron microscopy and energy spectroscopy. . By adopting measures such as constant casting speed pouring, reasonable matching of billet casting speed and water volume, implementation of narrow temperature fluctuation control in the tundish, and improvement of the purity of molten steel, the quality of continuous casting billets has been significantly improved, and the pass rate of low-power and hot upset forging has also been improved. Significantly increased.

Keywords: continuous casting billet; internal cracks; causes; measures

Introduction

The formation of cracks in the slab is a very complex process and is the result of the interaction of heat transfer, mass transfer and stress. When the high-temperature cast slab with liquid core is running in the continuous casting machine, the action of various forces is the external cause of cracks, and the sensitivity of steel to cracks is the internal cause of cracks. Whether cracks occur in the cast slab depends on the high-temperature mechanical properties of the steel, the solidification metallurgical behavior and the operating status of the casting machine equipment. Different factories, different casting machines at different stages, and due to different conditions, the degree of influence of each factor on slab cracks is also different. The continuous casting process is a dynamic solidification process of molten steel. For the solidification of continuous casting billet, once the solidified shell is formed, elements will segregate in the solid-liquid two-phase area, and the solidified shell will undergo phase change or precipitation of compound particles. Coupled with the effect of external stress, cracks will occur.

The research object is the cast slabs produced by the 50t electric furnace production line of Laigang and the converter steelmaking production line of the New Second District (the production process is: converter/electric furnace → LF refining → VD vacuum degassing → continuous casting, the slab specifications are 180 mm × 220 mm, 260 mm ×300mm). Analyze the reasons for the formation of cracks in the continuous casting billet, use scanning electron microscopy and energy spectroscopy to analyze the types of cracks, and take corresponding measures to improve the quality of the continuous casting billet.

Causes of Internal Cracks

Causes of internal cracks

Corner cracks

Through investigation and analysis of the operation of process equipment and finished products of the 50t electric furnace production line and the new second zone production line, it was found that the water inlet and outlet volume and the temperature difference between the inlet and outlet water of the crystallizer did not change greatly. The quality of the cooling water of the crystallizer has always been good and has been carefully softened. There is slight scaling on the copper wall of the crystallizer. There are no impurities on the surface area and no water gaps blocked, thus eliminating the problem of uneven cooling caused by water quality. Through follow-up observation of the mold copper tube and the crystallizer water jacket, it was found that the water seam production accuracy was poor and the water jacket deformation resulted in uneven thickness of the billet shell inside the mold. In the later stages of production, uneven cooling caused by clogging of the nozzle, low arc imitation accuracy, unstable vibration, large amplitude deviation from the design, and a vibrating table that easily causes transverse cracks and slag inclusions in the slab are the main reasons for corner cracks.

Corner cracks generally appear in three ways: 1) Corner cracks in the cast slab occur within 250mm below the meniscus of the mold. The cracks first form at the solid-liquid interface and then expand. The formation is related to the inappropriate taper of the mold. . This form of crack rarely occurs, and when it occurs, most process conditions (superheat, drawing speed) are not very good. It should be said that the crack is caused by a combination of factors. 2) Cracks sometimes appear accompanied by depressions at a distance of more than 15mm from the thickness of the shell. The cracks are caused by uneven thickness of the shell after coming out of the crystallizer or different stresses caused by uneven cooling intensity of the foot roller and one section. 3) Cracks appear continuously in the continuous casting billet and exist throughout the same pour. The cracks are caused by uneven cooling of the crystallizer or polarization and misalignment of the crystallizer. 4) Inclusions are found in the cracks that appear after the finished product is finished, which is also the reason why the cracks cannot be welded during the rolling process.

Middle crack

The Mn/S of high-quality special steel produced by Laigang Special Steel and Xin Second District can basically reach more than 100. Segregation caused by high sulfur rarely occurs, and slag entrainment caused by operation rarely occurs. This type of crack is caused by the purity of molten steel, secondary oxidation, and foreign inclusions, but the occurrence rate is less.

There are two main types of middle cracks:. 1) Slender cracks that grow along columnar crystals. The cracks are caused by high superheat of molten steel or the temperature return of the continuous casting billet. There are no inclusions in the middle of the cracks. The cracks can be welded by a larger compression ratio during the rolling process. 2) Cracks grow along columnar grain boundaries, and inclusions are distributed inside the cracks in a chain shape. According to the electron microscope analysis results of the inclusions in the aforementioned cracks, the inclusions contain elements such as Si, Mn, Al, Ca, S, Mg, K, and Na. Na and K elements are mainly caused by the involvement of mold powder into the molten steel. In addition, the segregation of S and Mn elements generates sulfides, as well as composite inclusions such as Al₂O and calcium magnesium aluminate. Because the cracks contain a large number of inclusions, this type of crack is difficult to eliminate in steel.

Central crack

The proportion of center cracks is relatively small. Due to unstable processes and large changes in pulling speed, straightening with liquid core or center shrinkage holes and center looseness are prone to occur, resulting in center cracks. The cooling in the secondary cooling zone is too strong, and then the temperature returns to generate tensile stress, forming a star-shaped or radial central crack). At the end of solidification of the steel billet, the solidification front bridge is built to seal in the molten steel at intervals. The upper molten steel cannot be filled, and shrinkage holes are generated during continuous solidification, and a central crack appears on the cross section.

Through on-site tracking, it was found that the occurrence rate of central cracks is closely related to the casting speed and its changes. Therefore, the casting speed must be strictly controlled during production to stabilize the production rhythm.

Microscopic analysis of internal crack defects in steel

Metallographic inspection

As can be seen from Figure 1, the cracks in the optical microscope photo of the cross-section cracks of the uneroded rolled material have no obvious direction, and the cracks mostly appear in the shape of curved bifurcations with a length exceeding 1mm.

Figure 1 Cross-sectional cracks in uneroded rolled material

A in Figure 2 is an optical microscope photo after being etched by 4.5% HNO₃ ethanol solution. The structure of the rolled material is pearlite + ferrite. B is the morphology when magnified 500 times. The cracks are in the form of intergranular and transgranular composites, and the crack edges are jagged.

Figure 2 Morphology of cracks

Figure 3 shows the longitudinal morphology of the crack, a) shows the morphology of the uneroded crack. b) is the structure and crack morphology after erosion. The structure of the rolled material is pearlite + ferrite structure and is distributed in strips along the rolling direction. The cracks are obviously split along the rolling direction, and the pearlite and ferrite are unevenly distributed.

Figure 3 Morphology of cracks in continuous casting billet

Figure 4 shows the morphology of the crack. The cracked sample was subjected to metallographic observation. The matrix structure is pearlite + ferrite. Reticular ferrite is distributed along the grain boundaries, and there is a curved ferrite strip with inclusions on the grain boundary, which contains a large number of plastic inclusions. The cracks are ferrite-based bands with accumulation of inclusions. Figure 5 shows cracks caused by inclusions.

Figure 4 Morphology of cracks in continuous casting billet

Figure 5 Cracks caused by inclusions

Scanning electron microscope and energy spectrum analysis

Figure 6 shows the shape and distribution of typical inclusions in steel. It can be seen that the inclusions in steel are smaller in size but larger in number.

Through inclusion energy spectrum analysis, typical inclusions in steel include: Al₂O₃ inclusions, composite inclusions, TiN inclusions, white spots, etc.

Figure 6 SEM and energy spectrum analysis of inclusions

1) Al₂O₃ inclusions: Figure 7 shows the scanning electron microscope and energy spectrum analysis of Al₂O₃ inclusions. Table 1 shows the elemental composition of Al₂O₃ inclusions.

2) Composite inclusions: Figure 8 shows the scanning electron microscope and energy spectrum analysis of composite inclusions. Table 2 shows the elemental composition of composite inclusions.

3) TiN inclusions: Figure 9 shows the scanning electron microscope and energy spectrum analysis of TiN inclusions. Table 3 shows the elemental composition of TiN inclusions.

Figure 7 Scanning electron microscope and energy spectrum analysis of Al₂O₃ inclusions

Table 1 Elemental composition % of Al₂O₃ inclusions

C0AlFe
3.0639.7855.291.87

Figure 8 Scanning electron microscope and energy spectrum analysis of composite inclusions

Table 2 Elemental composition of composite inclusions %

COMgAlSiSCaFe
0.9129.063.6720.581.779.6427.836.55

Figure 9 Scanning electron microscope and energy spectrum analysis of TiN inclusions

Table 3 Elemental composition of TiN inclusions

CN    TiCrFe
1.2417.9172.122.85.92

4) White spots: Figure 10 shows the scanning electron microscope and energy spectrum analysis of the white spots. The figure shows that the cracks and the cavities next to them are very clean, and no inclusions are found. Figure 11 Scanning electron microscope and energy spectrum analysis of white spots. Grind away 0.2mm from the cracked sample, polish it, and observe it under an electron microscope. After energy spectrum analysis of the elliptical shape part, the composition is as follows. It is suspected to be a hydrogen-induced crack. Table 4 shows the components after energy spectrum analysis of the elliptical morphological parts.

Figure 10 White spot scanning electron microscope and energy spectrum analysis

Figure 11 White spot scanning electron microscope and energy spectrum analysis

Table 4 Composition % after energy spectrum analysis of elliptical morphological parts

CCrFe
1.311.1897.51

Summary

The main manifestations of cracks after finished products are as follows:

1) Inclusion type cracks. The cracks caused by inclusions are often deep (>1.0mm, even as deep as ten millimeters), with sharp ends, and some even produce secondary cracks. There are often more complex inclusions in or near the cracks. Energy spectrum analysis of these inclusions shows that in addition to containing more FeO, there are also enrichments in elements such as Si, Mn, Al, Ca, S, and sometimes Mg, K, Na and other elements. .

2) White spots. The white spots are round or oval silver-white spots on the fracture, sometimes like a duckbill. The white spots on the transverse acid-leached test pieces of steel are relatively straight or jagged radiating fine cracks of varying lengths. On longitudinal acid-leached specimens, slight white spots are generally jagged, slightly curved fine cracks at a certain angle parallel to the rolling direction or perpendicular to the rolling direction. When observed under a microscope, the white spots appear to be jagged, slightly curved fine cracks, and the distribution of the cracks is irregular. The cracks are both along the grain boundaries and transgranular. There is no plastic deformation around the crack, no oxidative decarburization, and no segregated non-metallic inclusions. The size is generally a few millimeters to one or two centimeters.

3) Bubbles-inclusion symbiotic cracks. The root of the crack is not sharp and has a similar shape to the root of bubble-type cracks, but the depth is mostly around 2 to 3 mm, and all contain complex inclusions.

Measures to prevent cracks in cast slabs

In view of the causes and formation mechanisms of the above cracks, corresponding measures have been taken:

1) Implementing constant casting speed casting improves the homogenization of the internal quality of the slab. Constant-speed pouring not only ensures the stability of the production organization and process, but also ensures that the amount of secondary cooling water will not change too much, and the liquid phase cavity will not change too much, reducing internal cracks in the slab.

2) The selection of casting speed and water volume must be reasonably matched to ensure that the casting billet is cooled evenly during operation. Currently, the maximum casting speed for 260mm×300 is 0.8m/min, and the maximum casting speed for 165mm×200mm section is 1.3m/min. The maximum drawing speed for the 180mm×220mm section is 1.2 m/min.

3) Adopt high-quality steel weak cooling process. Reduce the cooling water volume of the crystallizer, strictly control the temperature difference between the inlet and outlet water at 7 to 9°C, and then reduce the secondary cooling water volume to 0.3 to 0.4L/kg. It satisfies the weak cooling of high-quality steel billets and avoids the brittle temperature range during straightening.

4) In the secondary cooling system, change the water and gas pipelines to stainless steel pipelines.

5) Implement narrow temperature fluctuation control of the tundish. High-temperature molten steel casting makes the thickness of the shell thinner, the strength decreases, and the columnar crystals are developed and coarse. The high temperature of molten steel is one of the important reasons for inducing internal cracks. The temperature of molten steel directly affects the smoothness of the continuous casting operation and the quality of the slab. Pouring temperatures that are too high or too low will cause many slab defects and production accidents. Currently, the temperature fluctuation range of the tundish is ±5°C.

6) Strictly align the tundish to ensure that the centering deviation of the nozzle is <5mm.

7) Optimize the ratio of each section of secondary cold water. The cooling intensity of the secondary cooling section zero, section I, section II, and section III is appropriately adjusted to achieve a gradual weakening of cooling. In particular, the zero-stage water should not be too strong to prevent the surface temperature from rising due to strong cooling and causing stress and cracks at the solidification front inside the billet.

8) Improve the purity of molten steel. The requirements for inclusion elements and non-metallic inclusions in continuous casting billets are: firstly, the number of non-metallic inclusions should be small and the total oxygen content T[O] should be low; secondly, the size of non-metallic inclusions should be small.

Laiwu Iron and Steel has developed a synthetic slag with high alkalinity, low melting point, good fluidity and strong ability to adsorb inclusions. It has fast slag change and good fluidity, shortens the initial slag formation time, extends the high alkalinity operation time, and fully deoxidizes and desulfurizes. . It absorbs floating inclusions in molten steel and improves the cleanliness of molten steel.

Ensure the refining soft blowing time and promote the floating of added impurities. Narrow composition control is implemented, with the deviation of carbon within 0.03%, manganese deviation within 0.05%, [P]<0.015%, [S]<0.010%.

A double-layer slag system is used in the tundish. The top layer of slag is insulated with rice husk, and the bottom layer of slag is made with alkaline covering agent to fully insulate and cover the molten steel in the tundish to prevent contact between the molten steel and the air. At the same time, it fully absorbs the floating silicate and aluminate inclusions, reduces the inclusion content in the molten steel, and further purifies the molten steel. The insertion depth of the immersed nozzle is 80 to 120 mm to avoid slag entrapment in molten steel.

9) Use light pressing technology. Larger reduction is beneficial to improving center porosity, but will aggravate billet cracks. The reduction amount is generally 6 to 7 mm. After exceeding this value, the reduction amount continues to increase. There will be no obvious improvement in the central segregation, but the central cracks will increase.

10) Reduce the hydrogen content in steel and prevent white spots. Raw materials and steelmaking equipment must be dry or baked red hot and free of moisture. Use less or no scrap steel that is severely rusted, and try to use newly roasted lime; check each water-cooling part before smelting to prevent water-cooling parts from leaking into the furnace. Try to avoid smelting steel types that are prone to white spots under rainy and humid conditions. Use effective degassing processes such as argon blowing and vacuum treatment, and the degassing or slow cooling time should be as sufficient as possible to ensure that the hydrogen in the steel can be fully removed and diffused.

11) Make sure the equipment is in good condition. In order to effectively control the occurrence of slab cracks, improve the maintenance level under existing equipment conditions, and formulate measures to regularly check the working condition of the equipment, including checking whether the guide roller is deformed, rotated, loose and misaligned. Whether each fan shape is misaligned or misaligned; whether the spray ring is deformed or rotated; whether the cooling nozzle is clogged; through statistical analysis, an upper limit for the amount of steel passed is established, and extended service of the crystallizer is strictly prohibited.

Effects

Through the above measures, the quality of Laigang’s high-quality steel has shown a stable and upward trend. The low-power pass rate of the continuous casting billet of the 50t electric furnace production line is above 99.7%, and the hot upsetting pass rate is above 99.5%. The pass rate of low-power continuous casting billets of the steelmaking production line in the New Second District is above 99.7%, and the pass rate of hot upsetting is above 99.4%, both of which have met the target requirements.

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