Melting Steel without Damaging the Furnace

Introduction

In the process of steel smelting, a seemingly paradoxical phenomenon is often perplexing: steel at temperatures up to 1,500°C or more can easily melt ordinary steel, but the furnace, which is the carrier of this fiery steel, remains unharmed. Behind this phenomenon lies the scientific mystery of refractory materials in modern industry. In this article, we will take you to understand the internal structure of the furnace, reveal how refractory materials create this industrial miracle, and introduce the most commonly used types of refractory materials in the steel industry and their working principle.

Basic Structure and Working Principle of Steelmaking Furnace

Steelmaking furnace is the core equipment of steel production, and its design takes full consideration of durability and safety under high-temperature environment. Modern steelmaking furnace is mainly divided into three types: converter, electric furnace, and flat furnace, although they are different in smelting process, but the basic structure principle is similar.

A typical furnace consists of a multi-layer structure: the outermost layer is a strong steel shell that provides structural support; the middle layer is an insulating layer that reduces heat transfer to the outside; and the innermost layer is a refractory layer that comes into direct contact with the hot molten steel, which is the key line of defense for the furnace against high temperatures. The furnace is also equipped with auxiliary facilities such as charging system, slag discharge port, steel outlet and exhaust gas treatment system.

During the steelmaking process, the temperature inside the furnace is usually maintained between 1600-1700°C, which is much higher than the melting point of ordinary steel (about 1500°C). It is the special refractory lining that allows the furnace to work for a long time under such extreme conditions without being melted down.

Definition and key properties of refractory materials

Refractory materials are non-metallic materials that are capable of maintaining their physical and chemical stability at high environmental temperatures. According to international standards, refractory materials must have a refractoriness rating of more than 1580°C. The ability of these materials to resist extreme temperatures stems from their unique physical and chemical properties.

Key performance indicators for refractory materials include:

• Refractoriness: the ability of the material to resist melting at high temperatures

• High-temperature strength: the ability to maintain structural integrity at high temperatures

• Thermal shock stability: the ability to resist rapid temperature changes without rupture

• Slag resistance: the ability to resist erosion by molten metal and slag

• Volumetric stability: the ability to maintain shape and dimensional stability at high temperatures.

Together, these properties determine the performance and service life of a refractory in a steelmaking furnace. A good quality refractory will remain stable in the extreme environment of a steelmaking furnace, ensuring that the furnace structure is not damaged by the hot molten steel.

Types of Refractories Commonly Used in Steelmaking Furnaces

There is a wide variety of refractories used in the steel industry, which can be categorized into three main types: acidic, neutral and alkaline according to their chemical composition. The following are some of the most commonly used refractories in steelmaking furnaces:

• Magnesia carbon bricks: composed of magnesia sand and graphite, they have excellent slag resistance and thermal shock stability, and are widely used in the slag line parts of converters and electric furnaces.

• High alumina bricks: containing Al₂O₃ in more than 48%, with good high-temperature strength and erosion resistance, commonly used in steelmaking furnace non-direct contact with molten steel area.

• Dolomite bricks: the main components are CaO and MgO, with excellent resistance to alkaline slag, mainly used in the smelting of some special steel grades.

• Zirconium materials: ZrO₂-containing materials with extremely high melting point and chemical inertness, used in the critical high-temperature parts of the steelmaking furnace.

• Unshaped refractory materials: including castables, coroplasts, etc., easy to construct, used for repair and complex shape parts.

These materials are selected and combined according to the temperature, chemical environment and mechanical stress requirements of different parts of the steelmaking furnace, and together constitute the protective system of the steelmaking furnace.

How refractory materials work

Refractory materials protect steel furnaces from molten steel by several mechanisms:

1. Thermal protection mechanism:

   Refractory materials have extremely high melting point and thermal stability. Refractory materials have very high melting point and thermal stability. Taking magnesium carbon bricks as an example, the melting point of its main component magnesia (MgO) is as high as 2800℃, much higher than the temperature of molten steel. At the same time, the multilayer structure of the refractory material is designed to form an effective thermal barrier, significantly reducing the rate of heat conduction.

2. Physical protection mechanism:

   Refractory materials are characterized by low thermal conductivity and high heat capacity. When molten steel comes into contact with the refractory lining, the heat is not rapidly transferred to the steel structure of the furnace. A "sinter layer" is formed on the surface of the refractory material and this dense surface further prevents penetration of the molten steel and slag.

3. Chemical protection mechanism:

   Refractory materials are designed to react with slag of specific composition to form high melting point compounds. For example, MgO in magnesium carbon bricks reacts with SiO₂ in the slag to form a high melting point magnesium olivine (2MgO-SiO₂, melting point 1890°C), which forms a protective layer on the surface of the brick.

4. Structural design protection:

   Modern steelmaking furnace adopts composite refractory lining design, different parts use different materials

Auxiliary role of insulation materials

In addition to refractory materials that are in direct contact with high temperatures, a wide range of insulation materials are widely used in steelmaking furnaces, which together form a complete thermal protection system:

• Ceramic fiber: lightweight and porous, with extremely low thermal conductivity, used as the insulation layer for the furnace roof and side walls.If you want to learn more about this material, here is a guide to ceramic fiber for you.

• Lightweight refractory bricks: contain a large number of closed pores, which can withstand a certain temperature and have a good thermal insulation effect.

• Microporous heat-insulating board: nano-scale pore structure, is currently one of the best artificial materials for heat-insulating performance.

• Thermal insulating castables: easy to construct and used to fill the heat insulation needs of complex shaped parts.

These thermal insulating materials assist the refractory in its work by

• Significantly lowering the surface temperature of the furnace, improving the working environment and reducing heat loss

• Reducing the temperature difference between the inside and outside of the refractory layer, minimizing thermal stress and extending service life

• Acting as a buffer layer to absorb the stresses caused by the expansion of the refractory material

• Provides additional safety in the event of refractory layer damage

The synergistic design of insulation and refractory allows modern steel furnaces to achieve higher energy efficiency while maintaining safety.

Maintenance and replacement of refractory materials

Despite their excellent resistance to high temperatures, refractory materials wear out over long periods of time. Maintenance of the refractory lining of steel furnaces is an important part of steel production:

Common wear and tear mechanisms:

• Cracking and spalling due to thermal stress

• Chemical attack by slag and molten steel

• Mechanical wear during charging and discharging

• Thermal shock damage due to rapid temperature changes

Maintenance techniques:

• Spray Patching Technology: Spraying refractory slurries onto damaged areas using specialized equipment to achieve rapid repair.

• Hot state repair: localized repair without stopping the furnace to reduce the loss of production downtime.

• Replacement of the entire refractory lining: systematic replacement of the entire refractory lining according to a predetermined cycle or degree of wear and tear.

Conclusion

Behind the seemingly miraculous phenomenon of steelmaking furnaces being able to carry high-temperature molten steel without melting down, the wisdom of materials science and engineering technology has coalesced. From high-performance refractory materials to advanced heat insulation system, from precise structural design to scientific maintenance management, the heat protection system of modern steelmaking furnace is a complex system engineering.

Refractories, as the "unsung hero" of the steel industry, not only guarantee production safety, but also support the development of the entire modern industrial system. As technology advances, future refractory materials will become more efficient, durable and environmentally friendly, and will continue to play an irreplaceable role in the high-temperature industry.

Understanding the science behind these materials not only answers initial questions, but also gives us a deeper understanding of the subtleties of modern industrial technology. Behind every steel smelting is a perfect fusion of materials science and engineering technology.