Ladle turnover process: tapping from converter/electric furnace – secondary refining – continuous casting – ladle preparation – waiting for tapping.
The normal turnover time is 100~140min depending on the type of steel and continuous casting machine. The tapping temperature of the ladle is 1680-1700℃, the steel filling time is 100~120min, and the typical ladle slag composition (%) of the full continuous casting operation is: Al2O317%~26% SiO28%~10%, CaO42%~47%, MgO5%~11%, FeO18%~22%.
If the process of smelting ultra-low carbon steel such as silicon steel, bridge steel, and automobile plate steel must undergo vacuum treatment, and at the same time, use argon blowing to the bottom of the ladle for stirring, and the LF furnace uses arc heating, reducing atmosphere in the furnace, white slag refining, gas stirring and other means to strengthen the comprehensive refining effects such as thermodynamic and kinetic conditions, desulfurization, alloying, and heating. Therefore, the slag basicity range is large, the temperature of the molten steel and slag is higher, the residence time of the molten steel in the ladle is prolonged, the thermal shock resistance is strong, the stirring force is large, and the damage to the lining of the ladle is aggravated.
Causes of damage to ladle refractory materials
- The ladle is used to transport high-temperature molten steel. During transportation, the high-temperature molten steel and slag at about 1680℃ scour and erode it, especially the slag line, which is seriously eroded and is an important factor in determining the service life of a ladle.
- LF and other off-furnace refining treatments seriously damage unfired bricks.
- When the converter taps steel and flows out molten steel, the lining is subjected to drastic temperature changes, which causes cracks and peeling of the lining material.
- When the ladle is loaded with molten steel during the converter tapping, the high-temperature molten steel has a strong mechanical scouring on its bottom, which makes the lining material in this part prone to damage due to thermal shock.
Damage mechanism of ladle refractory
The damage mechanism of ladle refractory materials is mainly caused by high-temperature slag erosion and penetration.
The slag line of the ladle is mainly melted, and the side wall is cracked and thermally peeled due to the penetration of slag.
The melting rate is related to the slag temperature, viscosity and reaction rate of the material. The high temperature of molten steel, the long retention time in the ladle, the low viscosity of slag, and the pore penetration, liquid phase penetration and diffusion in the solid phase of the matrix material cause qualitative changes in the composition and structure of the material surface, forming a metamorphic layer with a high degree of solubility, which is prone to peeling and accelerates the damage of the lining bricks.
The lining of the ladle with the same or different chemical composition of the refractory material has different damage rates due to its different organizational structure and performance. The ladle cannot operate continuously, resulting in a decrease in the lining temperature or even a cold ladle, which is also prone to lining structure peeling, reducing the service life of the ladle.
The erosion of slag on refractory materials is not limited to the dissolution of the surface, but the slag can also invade (penetrate) the interior of the refractory material, expand its reaction area and depth, and cause qualitative changes in its composition and structure near the surface of the material, forming a highly soluble metamorphic layer, accelerating damage, and the proportion of this intrusion is roughly proportional to the porosity.
Therefore, even if the chemical composition of the refractory is the same, its melting loss rate is significantly different due to its different organizational structure.
The higher the open porosity of the refractory material, the faster the slag intrusion rate is, and the intrusion rate is approximately proportional to the porosity. Even if the apparent porosity of the refractory material is the same, the erosion rate will also change if the shape, size and distribution of the pores are different.
Characteristics of ladle lining refractory materials
According to the above analysis, the refractory material for ladle lining should have the following characteristics: dense and uniform organizational structure; micro-expansion at high temperature and good volume stability; high strength and a small ratio of medium-temperature strength to high-temperature strength.
During the use of refractory materials, slag can easily penetrate from the heating surface into the deep interior, which significantly reduces the porosity near the working surface and densifies it, forming a very thick metamorphic layer.
When the temperature changes drastically, cracks parallel to the working surface will appear at the junction between the metamorphic layer and the original brick layer, causing the bricks to peel off and be damaged. The way to reduce the structural spalling of refractory materials is to reduce the depth of slag penetration, which can be done from the following aspects:
(1) Improve the slag permeability resistance of refractory materials;
(2) Reduce the porosity of refractory materials and reduce the erosion channel of slag;
(3) Slag reacts with refractory materials to form a high melting point compound barrier to prevent slag penetration;
(4) Increase the viscosity of slag. The greater the viscosity of slag, the worse its erosion to refractory materials.
