Brief Discussion on Improving Automatic Casting Rate of Ladle

There are many factors that affect the ladle pouring rate, among which the most important are production conditions, ladle volume, ladle filler sand quality and different steel grades.

The following introduces the four main factors and the aspects we should focus on to improve the automatic pouring rate of the ladle.

1 Ladle filler sand quality

Chromium ore sand, quartz sand and a certain amount of additives should be used (the composition requirements are shown in Table (1):

After contact with molten steel, the ladle filler sand in the ladle nozzle typically forms a three-layer structure (molten layer, sintered layer, and loose layer). ① The molten layer is formed when the ladle filler sand is melted by the high-temperature molten steel. It should not be excessive, and ideally, it should be absent. Therefore, the chromium sand must have high high-temperature resistance. ② The sintered layer, located below the molten layer, is also formed by sintering at high temperatures. It prevents the molten steel from continuously penetrating downward. Since automatic pouring relies on the pressure generated by the molten steel in the ladle to break the sintered layer, it should not be too thick. Therefore, the sand must have a high sintering temperature to prevent over-sintering at a certain molten steel temperature. ③ The particle ratio must be appropriate to prevent molten steel from penetrating the ladle filler sand particles and causing material shedding. Molten steel that penetrates between the ladle filler sand particles gradually solidifies as the molten steel temperature drops, forming a strong sand-steel mixed layer that can easily prevent automatic pouring. ④ To ensure smooth flow of ladle filler sand from the nozzle, the sand must have low thermal conductivity, minimal expansion upon heating, resistance to cracking, and good fluidity. ⑤) Appropriate additives must be added. Adding a suitable lubricant can reduce friction between ladle filler sand particles. After melting, chromium sand forms secondary spinel under the action of carbon materials, causing volume expansion and facilitating automatic pouring.

Thus, in general, the requirements for ladle filler sand include: good fluidity (particle size requirements are as follows: ladle filler sand particle size and shape requirements are: approximately round is preferred; polygonal shapes are acceptable, but flaky, pointed, or angular shapes must be avoided. The particle size of chromium ore sand should ideally be between 1.60 and 0.15 mm, while the particle size of quartz sand should ideally be between 2.50 and 0.15 mm. The prepared particle size must be free of other impurities and foreign matter.), good chemical stability, excellent thermal insulation, moderate sinterability, and high refractoriness.

2. ladle filler sand Addition Method and Amount

In practice, the duct-type method of adding ladle filler sand is significantly superior to the traditional throwing method. The advantages of this method include: ① quantitative and targeted sand addition; ② reduced risk of waste slag from the nozzle entering the nozzle; ③ easy detection of uncleaned nozzles or foreign matter; ④ a tendency to form a "cone" shape, thereby increasing the static pressure of the molten steel; and ⑤ a relatively high automatic pouring rate. Disadvantages include: ① relatively complex operation; ② relatively slow sand addition speed; ③ the need for sand addition pipes and lifting devices.

Generally speaking, the method and amount of ladle filler sand added to the upper nozzle are to form a "cone" shape. This "cone" shape increases the contact area between the molten steel and the sintering layer, which in turn increases the static pressure of the molten steel on the sintering layer.

3. Molten Steel Retention Time in the Ladle

Research has found that the automatic pouring rate is highest when the steel resides in the ladle for ≤2 hours. Between 2 and 4 hours, the automatic pouring rate decreases with increasing retention time. Between 4 and 6 hours, the automatic pouring rate rebounds. After 6 hours, the automatic pouring rate decreases again. The automatic pouring rate decreases by approximately 6% after VOD treatment compared to without VOD treatment. The automatic pouring rate decreases significantly when the time from the end of argon blowing to the start of pouring exceeds 20 minutes.

Therefore, to improve the ladle pouring rate, it is necessary to: ① maintain a steel residency time of ≤2 hours; ② avoid VOD treatment; and ③ control the time from the end of argon blowing to the start of pouring to within 20 minutes.

4. Dryness of ladle filler sand

When moisture-rich or damp ladle filler sand is added to the ladle nozzle, the water vapor generated reacts with the quartz at high temperatures. SiO2 reacts to form a silica gel-like product, which easily binds other ladle filler sand together, preventing the molten steel from being crushed by its own weight and causing drainage. Therefore, baking and drying the ladle filler sand can also improve the automatic pouring rate of the ladle.

5. Nozzle Installation and Cleaning

For automatic ladle pouring, ladle repairers must carefully clean the base bricks, cold steel on the upper nozzle, and any residual mud left during the installation of the slide. Failure to do so will negatively impact automatic pouring. Factors that can affect pouring performance during repairs include: ① Failure to clean residual cold steel in the nozzle reduces the nozzle diameter, thereby affecting the amount of ladle filler sand added and the speed at which the sand flows when the slide is opened. ② Failure to clean the scrap around the nozzle base bricks not only reduces the amount of ladle filler sand added, but also causes the scrap to adhere to the ladle filler sand due to the increased temperature during online baking, hindering its smooth flow. ③ Waste slag from the bottom of the ladle falls into the nozzle after the ladle is properly filled, which is not detected during sand filling and can cause blockage. ④ After the slide is installed, excess slurry is not cleaned thoroughly, leading to clogging of the nozzle. ⑤ After the nozzle is installed, sand is added directly without being baked (this is equivalent to dampening the ladle filler sand).

Based on the above factors, ladle repair workers should pay attention to the following points when hot repairing a ladle: Scrap steel around the nozzle and its base bricks must be completely cleaned. If molten steel is severely stuck to the bottom of the ladle, clean it from the front of the ladle to ensure that the trumpet-shaped sand funnel of the nozzle base brick is completely cleaned. ② After the slide is installed, excess slurry must be cleaned before being blown out with a duct. If ducting is used alone or is insufficient, the slurry may be moved forward by the wind and may become trapped between the nozzle and base brick joint (due to its own weight and adhesiveness), thus clogging the nozzle after ladle filling. It is best to use a curved duct to extend from the rear nozzle to ensure that the trumpet-shaped sand funnel of the nozzle base brick is completely cleaned out. ③ It's best not to sand-fill a newly installed nozzle immediately. Instead, heat it over a fire (to preheat the slide and nozzle) for 10 minutes before sand-filling. ④ Before sand-filling, the sand-filler should inspect the nozzle for foreign matter. If foreign matter is found blocking the nozzle or the nozzle is red, the bag should be turned over before sand-filling.

6. Upper Limit Temperature of the Ladle

Research has shown that a ladle bottom upper limit temperature greater than 1000°C is more conducive to the start of molten steel pouring.

7. Ladle Tapping Temperature

A ladle tapping temperature of approximately 1650°C has a higher self-opening rate than one at 1700°C.

8. Steel Flow Direction and Position Relative to the Ladle During Tapping

If the ladle is tapping from the furnace mouth, meaning the ladle nozzle is located directly at the edge of the converter tapping flow, it is necessary to employ the method of adding a 10mm thick steel plate cap to the ladle filler sand before tapping, tapping along the ladle wall to a depth of 14°C, and then rapidly tapping.

To sum up, the automatic pouring rate of the ladle is not only related to the quality of the ladle filler sand, but also to the user's process, usage method, ladle retention time, the cleaning degree of the nozzle and its seat bricks, etc. In order to achieve a 100% automatic pouring rate, there must be close cooperation between the supply and demand parties, and between the various processes involving the ladle on the user side.