What are the causes of longitudinal cracks on the surface of continuous casting billets and how to prevent them?
Longitudinal cracks on the surface of continuous casting billets will affect the quality of rolled products. For example, a longitudinal crack of 300mm long and 2.5mm deep will leave an 1125mm delamination defect on the rolled plate. Severe longitudinal cracks will cause leakage and scrap.
The study pointed out that the longitudinal cracks originated from the uneven thickness of the primary billet shell on the meniscus of the crystallizer. The tensile stress acting on the billet shell exceeds the allowable strength of the steel, and stress concentration occurs at the weak part of the billet shell, leading to fracture, which expands in the secondary cooling zone after leaving the crystallizer.
The causes of longitudinal cracks can be summarized as follows:
1) The water inlet and the crystallizer are not centred, resulting in bias flow to scour the solidified shell.
2) The melting performance of the protective slag is poor, and the liquid slag layer is too thick or too thin, resulting in uneven thickness of the slag film, making the local solidified shell too thin. When the liquid slag layer is <10mm, the longitudinal cracks increase significantly.
3) The liquid level of the crystallizer fluctuates. Liquid level fluctuation>10㎜, the probability of longitudinal cracking is 30%.
4) S+P content in steel. When S>0.02% and P>0.017% in steel, the high-temperature strength and plasticity of steel are significantly reduced, and the tendency of longitudinal cracking increases. 5) When C in steel is between 0.12 and 0.17%, the tendency of longitudinal cracking increases.
Measures to prevent longitudinal cracking are:
1) The nozzle and the copper mould tube should be aligned.
2) The fluctuation of the crystallizer liquid level is stable at ±10mm.
3) Appropriate insertion depth of the immersion nozzle.
4) Appropriate crystallizer taper.
5) The arc of the crystallizer and the upper part of the secondary cooling zone should be accurate.
6) Appropriate protective slag performance.
7) Use hot top crystallizer, that is, insert stainless steel and other materials with poor thermal conductivity into the 75mm copper plate in the meniscus area, which reduces the heat flow in the meniscus area by 50-70%, delays the shrinkage of the billet shell, reduces the depression, and thus reduces the probability of longitudinal cracks.
2. Causes and prevention methods of transverse cracks on the surface of continuous casting billets
Transverse cracks are located at the trough of the vibration mark on the inner arc surface of the billet, which is usually hidden and invisible. Pickling inspection shows that the crack depth can reach 7mm and the width is 0.2mm. The crack is located in the ferrite network area, and the network area is exactly the primary austenite grain boundary. And there are fine particles (such as AlN) precipitated on the grain boundary. Especially C-Mn-Nb(V) steel is more sensitive to cracks.
Causes of transverse cracks:
1) The vibration mark is too deep, which is the origin of transverse cracks.
2) The increase of A1 and Nb content in steel promotes the precipitation of particles (A1N) at the grain boundary, inducing transverse cracks.
3) The ingot is straightened at the brittle temperature of 900-700℃.
4) The secondary cooling is too strong.
Measures to prevent transverse cracks:
1) The crystallizer uses high frequency (200-400 times/min) and small amplitude (2-4mm) to reduce the depth of vibration marks.
2) The secondary cooling zone uses stable weak cooling to make the surface temperature of the ingot greater than 900℃ during straightening.
3) The liquid level of the crystallizer is stable, and protective slag with good lubrication and low viscosity is used.
4) Use flame to clean surface cracks.
3. What are the causes of mesh cracks on the surface of continuous casting ingots and how to prevent them?
This type of crack can only be found after the surface of the ingot is pickled, and the depth can reach 5mm.
Causes:
(1) The high-temperature ingot surface absorbs copper from the crystallizer, and the copper becomes liquid and then penetrates along the austenite grain boundary.
(2) The selective oxidation of iron on the ingot surface causes residual elements in the steel (such as Cu, Sn, etc.) to remain on the surface and penetrate along the grain boundary to form cracks. Studies have shown that the crack zone is enriched with elements such as Cu, Sn, and Sb. When the Cu content in the steel is greater than 0.1%, the cracks become more severe; when the Al content in the steel increases, the network cracks become more severe.
Prevention methods:
1) Plate Cr or Ni on the surface of the crystallizer to increase hardness.
2) Appropriate secondary cooling water volume.
3) Control residual elements in the steel such as Cu < 0.2%.
4) Control Mn/S> 40.
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4. What are the causes and preventive ‘measures for the longitudinal cracks at the corners of continuous casting billets?
The longitudinal cracks at the corners may be located near the edge of the wide face and the narrow face, some are 10-15 mm away from the edge, and some are just on the edge. In severe cases, it will cause steel leakage.
Causes: For square billets, it may be that the thickness of the water seam along the height of the crystallizer is uneven, resulting in poor cooling of the corners of the crystallizer; the crystallizer taper is too small, and the crystallizer fillet radius is too small.
For slabs, it may be due to
(1) Improper support of the narrow face causes bulging of the narrow face. The narrow face has a bulge of 6-12 mm accompanied by longitudinal cracks at the corners, resulting in steel leakage.
(2) Improper taper.
(3) Insufficient cooling water on the narrow face.
Improvement methods:
For square billets 1) Control the geometry of the crystallizer to prevent deformation. 2) Appropriate fillet radius. 3) When assembling the crystallizer, keep the thickness of the cooling water seam consistent to ensure uniform cooling.
For slabs 1) Adjust the gap between the narrow face rollers to limit the bulging by 1~2mm. 2) Appropriate taper (1.0%/m). 3) Appropriate cooling water volume. 4) Do not deviate the water inlet and the crystallizer.
5. What are the causes and preventive measures for transverse cracks at the corners of continuous casting billets?
This is a fine transverse crack located at the corners of the billet. The possible causes are:
1) The taper of the crystallizer is too large. 2) The surface of the crystallizer is scratched. 3) The arc of the crystallizer outlet and the zero section are not accurate.
Improvement method: Adjust the taper of the crystallizer, strictly align the arc, and adjust the secondary cooling so that the temperature of the corner of the billet during straightening cannot be less than 800℃.
6. How are subcutaneous bubbles of continuous casting billets formed?
Located below the surface of the billet, there are large bubbles with a diameter and length of more than 1 mm and 10 mm, respectively, growing in the direction of columnar crystals. These bubbles are called surface bubbles if they are exposed to the outside, subcutaneous bubbles if they are not exposed, and subcutaneous pinholes if they are smaller than bubbles and dense.
In the heating furnace, the surface bubbles of the ingot or the inner surface of the subcutaneous bubbles are oxidized to form a decarburized layer, which cannot be welded after rolling and forms surface defects. Shallow bubbles can be removed by grinding wheels, wind shovels and flame cleaning. Deeply buried bubbles are difficult to find and will cause cracks in the product.
Insufficient deoxidation of molten steel is the main reason for the formation of bubbles. If enhanced deoxidation is used to reduce the oxygen content in steel, the aluminium content in the molten steel will reach 0.01-0.015%, thereby eliminating bubbles. In addition, the gas content (especially hydrogen) in the molten steel is also an important reason for the formation of bubbles.
Therefore, all materials added to the molten steel (such as ferroalloys, slag powder, etc.) should be dried, the ladle and the tundish should be baked, the amount of lubricating oil should be appropriate, and the injection flow should be protected. The effect of reducing bubbles is obvious.
7. What is the folding defect on the surface of the continuous casting billet?
There are transverse folding marks on the surface of the billet, accompanied by transverse cracks in severe cases. Causes:
(1) The suspension inside the crystallizer causes the solidified shell to tear. Due to the strong cooling of the crystallizer, the molten steel leaking from the tear immediately solidifies on the surface to form folding marks;
(2) Improper adjustment of the crystallizer vibration parameters;
(3) The arc alignment between the crystallizer outlet and the secondary cooling section is poor;
(4) The crystallizer is poorly lubricated, and the billet shell and the copper wall are bonded.
8. What is the cause of the “cold mole” on the surface of the billet?
The metal block or slag block embedded under the surface of the billet is called a “cold mole”. The causes are:
(1) During open pouring, the splashing of the steel flow sticks to the cold steel on the surface of the crystallizer and embeds into the solidified shell;
(2) The crystallizer liquid level fluctuates too much, and the insoluble matter in the slag is drawn into the solidified shell.
9. What is the heavy skin defect on the surface of the continuous casting billet?
The horizontal discontinuity on the surface of the billet and the obvious traces of incomplete welding are called heavy skin. Causes:
(1) The pouring of the crystallizer stops suddenly, or the billet drawing stops instantly. If the pouring is stopped for too long, obvious reconnection will be formed on the surface of the billet;
(2) The molten steel is too viscous, the temperature is too low, the nozzle is blocked, the injection flow is deviated, etc. may cause heavy skin.
10. Why is the surface of the continuous casting billet sometimes concave?
This defect is common in the narrow side of square billets or slabs. Causes:
(1) The taper of the crystallizer is too large;
(2) The secondary cooling zone is unevenly cooled.
Using appropriate crystallizer taper and uniform secondary cooling can prevent it.
11. Why is the surface of the continuous casting billet sometimes convex?
This defect is common in the narrow side of rectangular billets and slabs. The reason is that the foot roll or support roll of the crystallizer is not well aligned and the cooling is too weak, causing the solidified shell to bulge.
One Response
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