Surface cracks in continuous casting slabs mainly include longitudinal cracks and transverse cracks. In order to improve the quality of continuous casting slabs, the causes and influencing factors of longitudinal cracks on the slab surface are tracked and analyzed, and the process operation and equipment system parameters are adjusted. , proposed measures to control longitudinal cracks on the surface of slabs such as improving the quality of molten steel and refractory materials, improving facilities and equipment conditions, and avoiding the occurrence of longitudinal cracks on the surface of continuous casting slabs.
Keywords: continuous casting slab; longitudinal cracks; influencing factors; preventive measures
Surface cracks are the main defects that affect the quality of slabs. They have a great impact on the quality of slabs and the surface quality of subsequent finished products. The causes of cracks are also complex. They will also bring corresponding defects to the rolled plates, resulting in the surface quality of many products. Cleaning and downgrading, and in severe cases, a large number of scraps, causing huge economic losses to the company. Therefore, the causes of formation must be carefully analyzed so that timely measures can be taken to prevent cracks from occurring.
Surface longitudinal crack characteristics
The cracks produced on the surface of the slab along the drawing direction are longitudinal cracks on the surface of the continuous casting slab. They are formed inside the mold and appear at the same time as the longitudinal shrinkage of the surface. They occur at the center of the wide surface of the slab or at the edges and corners. many. Generally, there are two types of surface longitudinal cracks: 1. Small longitudinal cracks, width <1 mm, depth <2 mm, length 3~25 mm, which are generally difficult to observe; 2. coarse longitudinal cracks, width 1~10mm, depth 2 ~70mm, length 30~25 m. Specific characteristics of longitudinal cracks: ① The surface of the longitudinal crack is accompanied by depressions. The larger the surface depression, the more serious the longitudinal crack, and vice versa; ② The crack will gradually expand along the direction of the columnar crystal boundary; ③ The crack is internally mixed with silicon, calcium, aluminum, etc. elements; ④ There is segregation of P, S, and Mn around the crack; ⑤ A decarburized layer is formed at the edge of the crack, indicating that high temperature causes expansion.
Usually when the length of the longitudinal crack is within 10 mm, the depth will be <0.7 mm. Before rolling the cast slab, the surface needs to be oxidized by about 1 mm during the heating process. Therefore, if the crack length is less than 10mm, there will be no plate defects. In the hot charging rolling process, before rolling, the heating time is shortened and the thickness of the surface oxide layer is reduced. Only by using the cold rolling process of the cast slab will the defects of fine longitudinal cracks on the surface not be formed. However, during hot charging rolling , surface crack defects may form.
Causes of formation of longitudinal cracks on the surface
Inside the mold, during the solidification process of molten steel, thermal stress will be formed. This force is formed by the temperature difference between the inside and outside and the top and bottom. The billet shell is affected by the thermal stress, the static pressure of the molten steel and the friction between the billet shell and the copper plate wall of the mold. As well as the stress caused by the contraction of the wide surface of the air gap slab due to the restriction of the narrow surface, the thickness of the green shell of the mold is uneven. Under the action of these forces, once the stress exceeds the high temperature allowable strength of the steel, it will Microcracks occur in the weaker areas of the shell.
After entering the secondary cooling zone, the cast slab expands when the secondary cooling water flow is strong and causes strong cooling, forming cracks inside the crystallizer, which will become larger in the secondary cooling zone. After the cast slab is solidified, the structure will undergo austenite decomposition phase transformation, the volume of the cast slab will shrink and linear shrinkage, and stress will be generated in the changes in the shape and size. In the secondary cooling zone, the liquid steel will remelt the billet shell. After the billet shell becomes thinner, it will bulge and deform under the action of the static pressure of the molten steel, aggravating stress concentration. During the bending and straightening process of the sector-shaped section, the cast slab is subjected to large mechanical stress, which causes the micro-cracks to expand and form cracks.
Influencing factors of surface longitudinal cracks
Molten steel conditions
When [C] in molten steel is between 0.10% and 0.18% and [S] > 0.015% in molten steel, the susceptibility to longitudinal cracks increases. When [C]>0.55%, the occurrence of longitudinal cracks will be reduced. Along the columnar grain boundaries, surface longitudinal cracks will extend. When the sulfur content in the molten steel is high, the solidification at the grain boundaries will be slowed down due to the segregation of sulfur at the columnar grain boundaries. After solidification, the high sulfur content will slow down the continuity at the grain boundaries, resulting in an increased possibility of cracks. If Mn/S>30, the residual elements [Cu], [As], and [Zn] are controlled to < 0.1%, and the sensitivity to crack generation will be reduced.
Superheat of molten steel
When high-temperature molten steel flows in the crystallizer, approximately 2mm of the solidified shell will be eaten away every 10°C as the superheat of the molten steel increases. Therefore, the normal growth of the primary solidified shell will be affected by the superheat of the molten steel. If the molten steel is overheated too high, the primary green shell becomes thinner and is subject to greater thermal stress, easily forming longitudinal cracks on the surface; if the molten steel is overheated too low and the protective slag melts poorly, the cooling of the meniscus will be uneven and longitudinal cracks will occur on the surface. possibility increases. Therefore, during pouring, the superheat of molten steel must be strictly controlled and maintained within a reasonable range, thereby effectively reducing the possibility of longitudinal cracks on the surface.
The occurrence of surface longitudinal cracks is also affected by fluctuations in pulling speed. During the production process, the fluctuation of the drawing speed will be affected by factors such as the composition of the molten steel, temperature, and the change of the nozzle, which will affect the thickness of the billet shell and lead to the possibility of cracks. Practice has proved that when the pulling speed fluctuation is controlled at 0.1 m/min, it will not have much impact on the surface longitudinal cracks; but when it exceeds 0.1 m/min, the fluctuation range will become larger and the incidence of cracks will increase. Therefore, keeping the pulling speed stable will help reduce the possibility of cracks.
Mold powder melts inside the mold and helps lubricate and improve heat transfer in the filled air gap. If the thickness of the liquid slag layer is too thin, it will cause insufficient melting and poor fluidity, hinder lubrication and heat transfer, and make the thickness of the primary green shell uneven. Therefore, it is necessary to select a mold slag with good performance and a certain liquid slag layer thickness, preferably above 10mm, with sufficient melting and good fluidity. For low carbon steel, it is necessary to choose a molding powder with low melting point, low carbon content, low crystallization temperature, moderate viscosity and fast melting speed; for medium and high carbon steel series, the molding powder requires low solidification temperature, low crystallization temperature, moderate viscosity and fast melting speed. ; For peritectic steel series, the protective powder requires high crystallization temperature, high alkalinity and moderate viscosity; for medium-carbon low alloy steel, the protective powder mainly ensures lubrication and appropriately reduces heat transfer intensity.
Effective Strategies to Prevent Longitudinal Cracks
In the production process, effective strategies to prevent longitudinal cracks mainly include the following aspects: ① [S] <0.02% in molten steel, and maintain [Mn]/[S] > 20; ② Promote coordinated production and pass the molten steel in the tundish The heat is reasonably controlled at 15~25℃; ③ Try to keep the pulling speed stable; ④ Try to use automatic control of the crystallizer liquid level to reduce the fluctuation of the crystallizer liquid level. If the automatic liquid level control is unstable, use manual steel pressing in time to reduce the fluctuation range. Maintain it within ±3mm; ⑤ Use mold powder with good performance; ⑥ Adjust the appropriate argon blowing system in the middle package to improve the purity of molten steel and promote the floating of non-metallic inclusions; ⑦ Keep the outlet inclination and insertion depth of the immersed nozzle appropriate. Centering installation; ⑧ Strengthen monitoring of the mist cooling system in the secondary cooling area, adopt a weak cooling system, and select an appropriate secondary cooling water system according to different steel types and casting machine speeds to facilitate control of the cooling water volume and allow the casting to The surface of the billet is cooled evenly. When the surface temperature rises less than 100℃/min, the thermal stress is reduced, and the surface temperature of the billet is >900℃; ⑨ Strengthen the operation, especially when opening the pouring and changing the water port, and prevent the plug rod from shaking seriously.
As a result of the combined action of many reasons, longitudinal cracks on the surface of the slab are caused. To improve slab quality and control the occurrence of longitudinal cracks, we should ensure appropriate molten steel composition, drawing speed, mold surface temperature, etc. By taking scientific preventive measures, the occurrence of longitudinal cracks on the slab surface will be greatly reduced.