Purity refers to the quantity, morphology and distribution of non-metallic inclusions in steel. In order to reduce the inclusions in the steel to the required level according to the steel type and product quality, the following 5 aspects should be started:
– Reduce the [O] content in the steel as much as possible.
– Prevent molten steel from interacting with air.
– Reduce the interaction of molten steel with refractory materials.
-Reduce slag involved in molten steel.
-Improve the flow and promote the floating of inclusions in molten steel.
From the process operation, the following measures should be taken:
(1) Slag-free tapping: The converter adopts slag blocking balls, and the electric furnace adopts eccentric bottom tapping to prevent a large amount of tapping slag from falling to the ladle.
(2) Ladle refining: select the appropriate refining method according to the steel type, to uniform temperature, fine-tune composition, reduce oxygen content, remove gas inclusions, etc.
(3) Non-oxidative pouring: After the molten steel is treated by the ladle, the total oxygen content in the steel can be reduced from 130ppm to below 20ppm. If the ladle → tundish injection is not protected or poorly protected, the total oxygen content in the molten steel in the tundish rises to the range of 60-100ppm, and returns to the level before the out-of-furnace refining.
(4) Tundish metallurgy: The large capacity of the tundish and the addition of retaining walls and dams are effective measures to promote the floating of inclusions. For example, in a 6t tundish, the slab inclusion rejection rate is 12%, and the inclusions are 0.82/m2; for a 12t tundish + retaining wall, the slab inclusion waste is 0, and the inclusions are 0.04/m2.
(5) Immersion nozzle + mold powder: mold powder should be able to fully absorb inclusions. Submerged nozzle material, nozzle shape and insertion depth should be conducive to the separation of inclusions.
