How Do Light-Burned Magnesia Balls Protect Against Slag Splashing?

Campaign life is a comprehensive technical index for converter steelmaking. Extending campaign life can not only cut refractory consumption, but also streamline production scheduling and facilitate sound operational cycles. Therefore, substantially raising converter campaign life by using light-burned magnesia balls has long been a key goal for iron and steel enterprises.

Converter linings operate under high temperature and strongly oxidizing conditions, and are normally eroded at a rate of 0.2–0.8 mm per heat. Slag splashing for furnace protection is a newly developed technology to extend campaign life in recent years. This process utilizes slag with saturated or supersaturated MgO content at the final stage of steelmaking. High-pressure nitrogen is injected to cool and solidify the slag on the lining surface, forming a well-bonded layer with a high melting point. The splashed slag layer features excellent corrosion resistance. It inhibits oxidation and decarburization on the surface of lining bricks, and reduces scouring and erosion by high-temperature molten slag. As a result, the lining bricks are well protected, the loss rate of refractories and the usage of gunning materials are reduced. Meanwhile, the service life of furnace linings is prolonged, the converter operating rate is increased, and production costs are lowered.

Medium-temperature tempering is performed at 350–500℃. The resultant microstructure consists of tempered troostite and spheroidal graphite, which is suitable for thick components requiring good wear resistance, dimensional stability and elasticity. High-temperature tempering takes place at 500–600℃. Its microstructure is composed of tempered sorbite and spheroidal graphite, delivering an optimal combination of toughness and strength. Hence this process is widely adopted in practical production.

Light-burned magnesia balls are produced with high-activity magnesia as the main raw material, plus appropriate additives. The materials are mixed, briquetted or pelletized, and finally dried to finish the products.

Light-burned magnesia balls are mainly applied for slag formation in converter steelmaking. When added into converter slag, they react rapidly and adjust slag composition effectively. The main mineral phases in the slag are converted into high-melting-point MgO, C₃S and C₂S, which increases the viscosity and melting point of the slag and greatly optimizes the effect of slag splashing protection. Accordingly, light-burned magnesia balls serve as an efficient regulator for slag control in converter slag splashing operations.

With the rapid development of the metallurgical industry, open-hearth steelmaking has been phased out, and large-capacity converters have achieved remarkable progress. In the slag splashing protection process, nitrogen is blown through the oxygen lance after tapping. The residual slag containing a certain amount of MgO inside the furnace is evenly coated onto the furnace wall, forming a uniform slag layer to protect the furnace lining.

To improve the mechanical properties of ductile iron, castings are generally heated to 30–50℃ above the Ac₁ temperature (the final temperature of austenite formation during heating), held at the target temperature and then oil-quenched to obtain martensite. Tempering is usually conducted after quenching to relieve residual stress.

Low-temperature tempering produces a microstructure of tempered martensite, residual bainite and spheroidal graphite. This structure boasts outstanding wear resistance and is used for parts requiring high wear resistance and mechanical strength. Medium-temperature tempering at 350–500℃ yields tempered troostite and spheroidal graphite, ideal for thick workpieces with requirements for wear resistance, stability and elasticity. High-temperature tempering at 500–600℃ forms tempered sorbite and spheroidal graphite. Thanks to its balanced toughness and strength, this treatment is widely used in industrial production.