Analysis Of The Causes Of Damage To The Converter Lining

As a key component in steel production, the service life of the converter lining directly affects the productivity, cost, and safety of converter operations. The converter lining operates under harsh conditions, including high temperatures, high pressure, and strong oxidation, and is gradually damaged by a combination of factors. An in-depth analysis of the causes of lining damage is essential for optimizing the maintenance process and extending lining life.

Mechanical Impact and Scouring

Mechanical impact and scouring are among the main causes of furnace lining damage, primarily occurring during the processes of scrap charging and molten iron addition.

During scrap loading, converters typically follow the process of adding scrap first, followed by molten iron. When large scrap is added, it exerts a strong mechanical impact on the upper surface and bottom of the converter. For example, in Ansteel's 100-ton converter, the loading of heavy scrap, internal recycled billets, and other large materials causes direct impact on the furnace lining, easily leading to the crushing and spalling of refractory materials. Especially when the scrap is large (e.g., single pieces weighing over 1 ton), the impact energy can cause cracks on the surface of the furnace lining and even loosen individual bricks.

During the iron charging process, high-temperature molten iron (typically at 1300–1500 °C) strikes the furnace lining at a certain flow rate, creating a continuous scouring effect. In Anshan Iron and Steel's "one ladle to the end" process, the temperature of molten iron is 30–70 °C higher than in traditional mixed-iron processes, which results in stronger abrasive effects and accelerates the wear of the furnace lining's refractory surface. Additionally, the flow and agitation of molten iron within the converter lead to continuous scouring of the furnace bottom and lower wall bricks. This is particularly critical around the bottom-blowing breathable bricks, which are more vulnerable to erosion due to the circulating flow of molten steel.

Chemical Erosion

The chemical erosion of the furnace lining mainly results from chemical reactions between slag and refractory materials, as well as the oxidizing effects of molten steel and furnace gases. This is a core reason for the gradual thinning of the furnace lining.

The working layer of the converter lining is typically made of magnesia-carbon bricks, whose main components are MgO and carbon. During the smelting process, components in the slag such as FeO and SiO₂ react with the magnesia-carbon bricks. In the early stages of smelting, FeO-rich materials such as iron oxide and return ore are added to promote quick slag formation. FeO reacts with MgO to form low-melting-point magnesium-iron solid solutions (e.g., MgO·FeO), which soften the brick surface and cause flaking. Research shows that when the slag basicity (CaO/SiO₂) is lower than 3.5, the erosion rate of magnesia-carbon bricks increases significantly as basicity decreases. This is because low-basicity slag contains higher amounts of SiO₂, which readily reacts with MgO to form low-melting-point magnesium silicate compounds.

Oxidizing gases such as O₂ and CO₂ in the furnace atmosphere also erode magnesia-carbon bricks. The carbon in the bricks reacts with these gases (C + O₂ → CO₂↑, C + CO₂ → 2CO↑), forming a decarburized layer on the brick surface. This decarburized layer has a loose structure and loses its ability to bind MgO particles. As a result, it is easily dislodged under the scouring action of molten steel and slag, thereby accelerating furnace lining erosion. In addition, MgO may be reduced at high temperatures (MgO + C → Mg↑ + CO↑), further contributing to brick degradation.

High-Temperature Thermal Shock and Melt Damage

The high-temperature environment and temperature fluctuations during the converter smelting process cause significant thermal shock to the furnace lining. At the same time, prolonged exposure to high temperatures can directly lead to the melting loss of refractory materials.

The steel temperature in a converter typically ranges from 1600 to 1700 °C, keeping the furnace lining in a high-temperature state for extended periods. Under these conditions, the high-temperature strength and thermal shock resistance of refractory materials gradually deteriorate. Magnesia-carbon bricks expand under high temperatures, but during furnace shutdowns or steel tapping, the lining temperature drops rapidly, causing the bricks to contract. This repeated expansion and contraction leads to the formation of internal cracks. Once these cracks reach a critical size, spalling of the bricks occurs. This phenomenon is especially pronounced in areas such as the furnace cap and trunnion, where temperature fluctuations are more intense and thermal shock damage is more severe.

High temperatures also cause melting and softening of the furnace lining's refractory material. At elevated temperatures, slag becomes more fluid and can more easily penetrate the pores and cracks within the bricks. When the temperature drops, the infiltrated slag solidifies and contracts, further widening the gaps in the brick structure. Additionally, the protective slag layer formed by slag splashing gradually melts at high temperatures. When this layer is less than 5 mm thick, its protective effect on the furnace lining is significantly reduced, exposing the refractory directly to high-temperature molten steel and slag, thereby accelerating erosion.

Operating Process and Furnace Type Factors

Damage to the furnace lining can also be exacerbated by improper operating techniques and design-related furnace conditions, both of which are closely tied to process control during production.

An unreasonable charging system can lead to uneven stress distribution on the furnace lining. For example, if scrap is loaded unevenly, large pieces concentrated in one area can cause localized impact and excessive wear of the lining. Excessive fluctuations in the amount of molten iron charged can lead to frequent changes in the molten pool level, causing repeated scouring at the slag line area. Practical experience from Tangshan Steel's 150-ton converter shows that when the loading deviation exceeds 5%, the erosion rate of the slag line area increases by 15–20%.

Improper control of slag-splashing furnace protection parameters can also reduce furnace lining life. If the final slag has insufficient MgO content (less than 8%) or low basicity, the refractoriness and bonding strength of the splashed slag layer are reduced, making it difficult to form an effective protective layer. Insufficient nitrogen pressure (less than 0.8 MPa) can lead to uneven slag splashing, leaving some areas of the furnace lining inadequately protected.

In addition, bottom bulging is a common issue in the later stages of furnace service life. When bottom rise occurs, the molten pool becomes shallower, reducing the effective operating range of the oxygen lance and accelerating erosion in the trunnion and slag line areas. For example, in Anshan Steel's 260-ton converter with over 5,000 heats, bottom rise resulted in the furnace lining thickness at the trunnion dropping to below 300 mm.

Material and Masonry Factors

The performance of furnace lining materials and the quality of masonry are fundamental factors affecting its service life. Improper material selection or masonry defects can accelerate lining damage.

The quality of magnesia-carbon bricks directly determines the erosion resistance of the furnace lining. If the carbon content in the bricks is insufficient (less than 14%), their oxidation resistance is reduced, resulting in a thicker decarburized layer. Low MgO purity (less than 75%) leads to inadequate high-temperature strength, making the bricks more susceptible to slag erosion. Additionally, an unreasonable brick design can result in excessively wide joints after masonry, allowing slag to penetrate the gaps and cause brick spalling. For example, Lai Steel's 120-ton converter optimized the brick design in the trunnion area and controlled the brick gap to within 2 mm, significantly reducing the risk of steel leakage in this area.

Defects in the masonry process are also a major cause of early furnace lining damage. If there is a gap between the permanent lining and the working lining, it causes uneven stress distribution on the working layer. Misalignment of bricks at the center of the furnace bottom can concentrate stress around the bottom-blown breathable bricks, leading to pit formation. Practical experience from TIANGANG's 120-ton converter shows that adopting the "live bottom, dead masonry" method reduced the erosion rate at furnace bottom joints by 30%, effectively preventing damage caused by steel leakage through these joints.

To sum up, damage to the converter lining results from a combination of mechanical impact, chemical erosion, high-temperature thermal shock, operational factors, and issues related to materials and masonry. In actual production, comprehensive measures-such as optimizing the charging system, improving the slag-splashing process, and selecting high-performance refractory materials-are necessary to maximize lining life, reduce production costs, and ensure safe and stable operation.