The surface quality of continuous casting slab in medium frequency electric furnace is an important factor affecting the product qualification rate and production cost. Ensuring good surface quality of continuous casting slab is the prerequisite for slab hot delivery and direct rolling. The shape of slab surface defects is different, and the causes are complex. In addition to the common cracks and slag inclusions, argon blisters on the slab surface may also appear.
Argon blister defects often appear on the surface of wide slabs of 45 excellent carbon steel. The causes of argon bubble scar on the slab surface are as follows: argon is blown into the nozzle in production, forming a positive pressure inside the nozzle to prevent air from being drawn in. Argon bubbles are fully mixed with molten steel in the nozzle. When the argon blown into the nozzle flows out of the submerged nozzle outlet together with the liquid steel, the upper and lower raceways will be formed after touching the narrow surface of the crystallizer. The bubbles in the upper raceway are easy to discharge with the flow of liquid steel, while the bubbles in the lower raceway are not easy to discharge. If the non discharged bubbles are captured by the solidification front in the mold, blister defects will be formed on the slab, and these bubbles are unevenly distributed on the slab.
The capture of argon bubbles on the slab surface is affected by many factors, among which the size, blowing volume and drawing speed of argon bubbles are the key factors affecting the capture of argon bubbles. These factors determine the impact depth of argon bubbles and the amount of argon bubbles impacting the lower part of the mold. The research shows that the floating speed of bubbles with a diameter greater than 500 microns is greater than the drawing speed, which will be captured in the floating process, resulting in surface defects of finished products, especially under inappropriate operating conditions. For example, when the blowing volume is large and the drawing speed is relatively low, these bubbles will impact the lower part of the molten pool; When the drawing speed is large and the blowing air volume is relatively small, the downward flow speed is large after the flow strand collides with the narrow surface of the crystallizer, and the argon bubbles will also impact the lower part of the molten pool. In addition, the width of the mold, the immersion depth of the nozzle, the side hole area of the nozzle and the outlet inclination angle of the nozzle will affect the impact depth of argon bubbles in the mold.
The simulation test shows that the amount of gas entering the mold increases significantly with the increase of air blowing at the upper nozzle and stopper; When the blowing amount is the same, the gas ratio and gas amount blown into the mold by the upper nozzle are larger than that blown by the stopper; The volume of argon bubbles blown into the nozzle expands by 5 ~ 6 times after the molten steel is heated at high temperature. The size of argon bubbles increases with the increase of blowing volume and nozzle aperture, and decreases with the increase of fluid flow rate. When the flow strand is in a turbulent state in the nozzle and mold, the large bubbles may split into several small bubbles, and then the small bubbles may collide with each other and aggregate into large bubbles. The floating speed of bubbles with a diameter of 0.5mm is 17.11cm/s, and the floating speed after adsorbing inclusions is lower than the original. Under the condition of small nozzle angle, bubbles are easier to be captured by narrow surface shell; If the drawing speed increases and the maximum position of gas distribution is close to the narrow surface, it is easier to capture the narrow surface shell, that is, argon bubble scar defects are more likely to appear.
In view of the causes and influencing factors of argon blister defects on the slab surface, the following measures can be taken: Taking the production of 45 excellent carbon steel as an example, on the premise of maintaining the pressure and flow of Ar gas and O2 gas main pipe, the immersion depth of nozzle shall be maintained at 150 ~ 200mm, the pulling speed shall be maintained at 1.0 ~ 1.2m/min, and the argon flow of stopper, upper nozzle argon and argon sealing between upper and lower nozzles shall be maintained at less than 1.1l/min. At this time, the coordination between Ar gas flow, pressure and process parameters tends to be reasonable, which not only ensures the active steel flow and meniscus on the surface, but also does not cause excessive surface fluctuation, which is conducive to the floating of inclusions and bubbles, prevents the blockage of nozzle, and can effectively reduce the capture of argon bubbles, so as to avoid the occurrence of argon bubble scar defects on the surface of billet.