How to choose slag retaining refractories in converter process

The quality of molten steel directly affects the properties of steel. Reducing the amount of slag in the process of converter drawing is an important link to improve the quality of molten steel. How to reduce the amount of slag in the process of converter steelmaking is a technical problem that has puzzled iron and steel enterprises for a long time, and has become a restricting factor of converter smelting special steel and high quality steel. It is necessary to deoxidize and alloying in ladle after steel drawing in converter, and to reduce the oxygen content of liquid steel by adding elements with strong affinity to oxygen. If the slag blocking measures are not taken at the end of the steel drawing, the highly oxidizing slag containing (P2O5) in the furnace will enter the ladle.

Due to the deoxidation alloying operation, the oxidation of the molten steel is reduced, the reaction balance between the right and metal is destroyed, and the phosphorus and oxygen in the slag are transferred to the molten steel, resulting in phosphorus recovery and excessive consumption of deoxidation elements. At the same time, too much slag will increase the non-metallic inclusions in the liquid steel, increase the amount of synthetic slag in the subsequent refining process and the difficulty and treatment time of the subsequent process, and also affect the service life of ladle refractory materials. Therefore, in order to prevent the phosphorus return of the liquid steel and improve the recovery rate of the alloy, it is necessary to use the slag blocking mechanism to send the slag blocking ball into the furnace before the steel is about to be finished, so as to prevent the slag in the furnace from entering the ladle.

In the process of drawing steel from the converter, because the density of the converter slag is less than that of molten steel and floats on the steel surface, the lower slag in the converter is composed of three parts: early slag. When the converter tilts to an average of 38°~ 50°, the steel slag in the converter flows out due to the slag passing through the steel outlet, that is, the early slag, as shown in Figure 1. Process slag, after the early slag begins to produce steel, can observe the vortex effect of molten steel and steel slag involved in the outflow, resulting in slag rolling, that is, process slag. Late slag, from the late stage of steel drawing to the end of steel drawing, due to slag blocking or furnace shaking is not timely, resulting in steel slag outflow, that is, late slag, as shown in Figure 2. In the slag from the converter to the ladle, the slag in the early stage accounts for 30%, the slag in the vortex effect from the surface of the molten steel is about 30%, and the slag in the late stage is about 40%.

At the end of the Bessemer blowing process, molten steel is put into the ladle and oxidizing slag is left in the furnace. When the liquid level of molten steel drops, when the depth of molten steel is lower than a certain critical value, a funnel-shaped vortex will be formed above the outlet, and part of the slag will flow out from the outlet before the completion of the molten steel, resulting in unclear separation of slag and steel. The slag and steel extraction technology is mainly developed for the confluence vortex. There are a variety of methods such as slag ball, slag stopper, high pressure gas slag stopper, fish stopper valve, slag signal detection and sliding plate slag stopper.

Slag ball: made of refractory material wrapped in the outside of the iron core, its density is between 4.3~4.4g/cm3, greater than the density of slag and less than the density of molten steel, so it can float at the interface of slag steel. When the molten steel has been poured out 3/4 to 4/5, use a specific tool to reach into the furnace and place the slag ball above the steel outlet. At the end of drawing, the vortex pushes it towards the outlet, blocking the outlet and preventing the slag from flowing out. In order to improve the thermal shock resistance of the slag ball and improve the efficiency of the slag ball, a lime slag ball is developed. The production process is to first cushion the iron core with a layer of refractory fiber. The shell of the ball is made of dolomite and lime, and synthetic resin or asphalt is used as a binder. Figure 4 shows the picture of the slag ball used on site.

Slag stopper (dart) : The slag stopper is made into an inverted cone on the top and a plug rod on the bottom. Because the shape is close to the funnel shape, it can match the flow of molten steel when the steel is produced, so the efficiency is higher than that of the slag ball. In some cases, a small circular groove is added to the upper cone and a hexagonal cone is changed to the lower cone to increase the ability to restrain the steel flow vortex. When steel is pulled out, a special mechanical device is used to hang the metal hook on the metal boom 1, reach into the converter, align it with the steel outlet, and insert the tail rod 2 into the steel outlet. Under the action of the pumping force of the steel, the slag stopper will not swim away from the steel outlet, and the buoy 3 will not be pumped away by the molten steel and block the slag, which significantly improves the fishing stop efficiency.

Because of the low cost and simple operation of the two slag retaining methods, they are widely used in steel mills. Table 1 shows the typical physicochemical indexes of slag ball and slag stopper produced by an enterprise in China.

Converter sliding plate method slag blocking: The sliding water nozzle refractory element is installed to the steel outlet of the converter, and the steel outlet is opened or closed by mechanical or hydraulic means to achieve the purpose of slag blocking. At the end of steelmaking, the automatic control process of converter sliding plate slag-blocking is to manually start the hydraulic station and open the sliding plate. When the converter tilts 35°, the instruction signal for closing the slide board is sent, and the state of the slide board is from open to closed. When the converter tilts to 75°~80°, the steel slag has all floated up. Issue a command signal to open the skateboard, and the skateboard opens again. When the converter tilts to 90°~110°, the steel out of the control system after the end of the slag to the skateboard issued a close skateboard instruction signal, at this time the state of the skateboard from open to closed. When the converter is reversed to the vertical position after the end of the steel drawing, the instruction signal of opening the slide plate is issued.

The sliding plate of converter has good slag retaining effect, but its equipment is complicated, the cost is high and the operation of the equipment is complicated. In addition, the device is installed at a specific position where the steel outlet is located, which is not convenient to install and disassemble due to the impact of splashing during blowing.