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Steelmaking – Refining – Control of inclusions in continuous casting steel

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With the rapid development of the national economy, the demand for steel has gradually increased, and the quality requirements for steel have continued to increase. Pipelines, bridges, automobiles, shipbuilding, pressure vessels and other industrial production steels require a large amount of steel, and there are also high requirements for the control of inclusion levels. Therefore, it is necessary to strengthen the control of steel purity requirements to improve the overall quality and quality of steel.


1 Basic concept of clean steel

There is no clear definition of clean steel, nor is there a scientific definition method. Generally speaking, clean steel contains a small amount of phosphorus, oxygen, nitrogen, hydrogen and sulfur impurities, so it is necessary to strengthen the reasonable control of non-metallic inclusions, such as sulfides and oxides. Generally speaking, clean steel has the following three characteristics: First, the oxygen content in steel is relatively small; second, the size and number of inclusions contained are controlled within the ideal range, and the distribution of impurities is good; third, the content of brittle inclusions is very small. Therefore, in order to improve the quality and performance of clean steel, it is necessary to have good purification technology and strengthen the introduction of advanced equipment and advanced technology. Since the early 1980s, the application of purification technology in the production activities of continuous casting, steelmaking and refining has significantly improved the cleanliness of steel. Around 2000, the number of harmful elements contained in clean steel produced in Japan accounted for only 0.005%; the clean steel produced by Baoshan Iron and Steel Co., Ltd. in my country also controlled harmful elements to around 0.008%. At present, with the increasing requirements for steel in transportation construction, national defense construction and special construction, the requirements for clean steel are also increasing, so steel companies are required to continuously improve the cleanliness of clean steel.

2 Hazard Analysis of Inclusions in Steel

Inclusions in steel are non-metallic compounds like oxides, sulfides, and nitrides. These inclusions cause uneven steel structure. Factors like geometric shape, chemical composition, and physical properties of inclusions reduce steel’s mechanical and fatigue properties.

2.1 Alumina inclusions

Al2O3 inclusions greatly impact killed steel performance. They are brittle and non-deformable. Their thermal deformation differs from the base material. Under hot working stress, many Al2O3 inclusions break and form sharp corners. These inclusions form a chain distribution on the base. They can create cracks and stress concentration points under cyclic stress.

2.2 Silicate inclusions

When molten steel solidifies, some silicates remain as supercooled liquids. This happens due to high temperature and delayed crystallization. When the temperature rises from 800°C to 1300°C, plasticity changes rapidly. Silicate and aluminate inclusions are complex and stay spherical during rolling. In low-carbon steel, especially boiling steel, these inclusions cause plasticization. They reduce toughness and cause strip peeling.

3 Causes and distribution of inclusions

Steel inclusions have two main sources. First, inclusions form during smelting. Deoxidizers from ferroalloy steel mix into the steel during tapping. Secondary oxidation products from molten steel and air mix during pouring. Second, external factors introduce foreign inclusions. These are usually irregular in shape, large, and unevenly distributed.

Endogenous inclusions occur under specific conditions. During smelting, incomplete deoxidation or lower temperatures during pouring can cause issues. Deoxidation products may not float up and remain in the steel matrix as small particles. Some particles aggregate into larger ones, like Al2O3, or remain in solid solution, like MnO and FeO. These inclusions cause internal defects and surface cracks in ingots.

During steel tapping and pouring, molten steel exposure to air causes oxidation. Oxygen combines with elements in the steel, producing secondary oxides that remain. Continuous casting generates many inclusions and loose defects, reducing product quality. When molten steel solidifies, FeS and FeO form due to selective crystallization, precipitating between grain boundaries and dendrites.

In the late 1960s, Kohnvertral investigated inclusion sources in continuous casting ingots. They found issues with slag systems, refractory materials, and secondary oxides from molten steel and air. In the 1970s, Ueda Haoye studied continuous casting billets. They concluded that tundish steel and ladle casting strands oxidized from the atmosphere. Ladle linings penetrated steel liquid, and protective slag entered the steel liquid in the crystallizer.

Xiong Jing believed large inclusions formed from refractory corrosive substances and steel liquid oxidation. In the 1980s, Byrne, Cramb, and Fenide analyzed inclusion causes in continuous casting billets. They found that converter slag entering the ladle caused large slag droplets to float out, reducing slag beads in the casting. High argon gas flow rates increased inclusions and oxygen in the ladle.

Chen Hongyu pointed out that foreign inclusions merged with small particle inclusions during continuous casting. Large inclusions came from ladle slag and protective slag, while small inclusions resulted from secondary oxidation. Composite inclusions formed after deoxidizers and steel liquid interacted with refractory materials.

4 Control inclusions

Usually, impurities are considered harmful components, but in reality, inclusions can be transformed into beneficial components. For example, high-sulfur steel will produce sulfide inclusions, which may reduce the strength of the steel, but for free-cutting steel, it is a brittle inclusion in the form of thin strips or spindles, with low hardness, which can greatly increase the cutting speed. If there is a small amount of calcium in the steel, the original inclusion CaO・SiO2 can be dissolved in Al2O3, or converted into 3CaO・2SiO2, which can be deposited on the surface of the tool during the cutting process, covering the surface of the tool to avoid friction between the chips and the workpiece and the front and back surfaces of the tool.

5 Technical measures to reduce steel inclusions

To control and reduce inclusions, steelmaking and continuous casting use various measures. These include deoxidation purification, ladle refining, and filtration purification. They also use vacuum treatment technology and electromagnetic purification.

5.1 Deoxidation method

If the oxygen content in the steel is too high, a large amount of oxides and macro inclusions will be formed, which will have an adverse effect on the quality of the steel. With the progress of modern science and technology, deoxidizers have transitioned from single deoxidizers to composite deoxidizers. The use of composite deoxidizers will cause compounds to form between products or mutual dissolution, thereby reducing the actual activity of the products. Deoxidizer is a molten compound with a low melting point and is easy to generate liquid deoxidizers. The composite deoxidation method can reduce the interfacial tension of the deoxidizer in the molten steel and accelerate the deoxidation rate of the deoxidizer. The composition of the composite deoxidizer is relatively complex, and there are more than 150 known deoxidizers. Calcium, as an excellent deoxidizer, can achieve deep deoxidation and deep desulfurization.

5.1.1 Application of composite deoxidizer

At present, there are many deoxidizer materials, such as alkaline earth metal composite materials, ferrosilicon rare earth alloys and zirconium binary alloys. In the specific application process, the quaternary deoxidizer can give full play to the affinity of rhenium element with oxygen, nitrogen and sulfur elements in steel. Adding calcium to aluminum-deoxidized steel can cause part of the calcium to dissolve in the steel and react with solid alumina to produce impurities to form calcium aluminate. During the smelting process, with the addition of multiple substances, the total amount of calcium oxide will be accelerated to enrich to lower the liquidus temperature. In addition, calcium reacts with sulfur to form calcium sulfide. If there is more manganese in the steel, manganese sulfide will also be produced. The composite deoxidizer can remove most impurities and has a remarkable effect.

5.1.2 Application of calcium

In the purification process of molten steel, calcium has a relatively good purification effect, which can not only deeply desulfurize it, but also achieve deep deoxidation effects. After the molten steel is deoxidized, the oxygen content is low, and the calcium deoxidation reaction is not effective at this time. When there are many alumina inclusion particles, as the calcium element diffuses, calcium reacts with aluminum to replace the aluminum element, so that the amount of calcium oxide on the surface of the alumina inclusions will continue to increase. If the calcium oxide content is greater than 25%, liquid calcium aluminate will appear and float on the surface of the molten steel, while the remaining non-floating inclusion particles will remain in the steel in smaller quantities. The application of calcium can not only effectively solve the deoxidation problem, but also remove alumina inclusions, improve the fluidity of molten steel, and improve problems such as nozzle blockage during the pouring process.

5.1.3 Rare earth applications

At the beginning of the 20th century, research showed rare earths change the morphology of inclusions in steel. By the 1970s, research found controlling the sulfur-to-rare-earth ratio reduces inclusions. Rare earths have a good affinity with harmful elements in steel. They produce compounds, remove impurities, and purify molten steel. Research shows adding rare earths with 25%-33% sulfur and rare earths ratio refines inclusions. Rare earth sulfides can replace manganese sulfide and aluminum oxide.

5.2 Smelting process

5.2.1 Process of off-furnace purification using the ray method

There are two types of off-furnace refining using the ray method: blowing and wire feeding. Powder spraying technology is the addition of calcium and its main components to molten steel in the 1960s; wire feeding technology can improve the accuracy of microalloying and improve product quality. The cored wire used in steelmaking can be divided into two parts: the core and the outer layer. It is mainly composed of deoxidizer and powder alloy. The outer layer is made of low-carbon steel with a thickness of 0.3 to 0.4 mm. In actual production, the staff uses a speed controller and a length counter to control the feed rate and feed amount.

5.2.2 Argon stirring ladle

Injecting argon into molten steel generates argon bubbles. The partial pressures of oxygen, nitrogen, and hydrogen in these bubbles are zero. Dissolved gases in the molten steel discharge after diffusion and polymerization. This process makes the temperature and composition of the melt more uniform.

Argon blowing equipment comes in three types. The first type uses refractory sealing, typically with a top-blowing argon gun. The second type uses multi-hole argon blowing, effectively solving the issue of excessive concentration from single-hole argon jets. The third type involves argon blowing through air-permeable bricks installed at the bottom of the ladle.

5.3 Protective casting process

To reduce inclusions in steel, continuous casting protective technology and tundish metallurgy are essential. We seal the molten steel with argon or use a refractory box to prevent contact with air when it enters the tundish from the ladle. Before pouring, we purge the tundish with argon to reduce secondary oxidation caused by the pouring process.

To prevent oxidation in the tundish, we add the tundish covering agent in time and ensure the amount added. This creates a physical barrier between the molten steel and the atmosphere. The crystallizer is the final area for inclusion removal. Inclusions float to the protective slag by adsorbing on argon bubbles or through the crystallizer flow field.

For steel grades with strict inclusion requirements, we control the argon amount in the plug rod and slide plate. This ensures casting while preventing oxide inclusions from forming due to air inhalation.

5.4 Filtration technology

In the 1980s, Western developed countries aimed to improve steel cleanliness. They strengthened scientific analysis of molten steel filtration and treatment technology. They also improved filters made from mullite, zirconia, and alumina. Common filters included ceramic foam filters and ceramic vacuum filters. Ceramic vacuum filters required porosity below 50%, while ceramic foam filters needed porosity above 70%.

Filtration technology uses mechanical principles to remove impurities. Different filtration methods include high bed filtration, stacking filtration, and screen filtration. In screen filtration, metal liquid flows through the filter surface. Solid inclusions get intercepted and blocked above the screen gap, removing large particles and purifying the liquid.

In stacking filtration, metal liquid flows through the filter surface. It separates from solid inclusions, which stagnate and accumulate as block stacking layers. High bed filtration requires filter apertures larger than the inclusion particles. As metal liquid flows through, the filter’s internal network adsorbs inclusions, achieving purification.

6 Conclusion

The presence of inclusions will have a great impact on the mechanical properties of the material. With the development of the modern economy, people have put forward higher requirements for the purity of steel products.Therefore, steel mills must understand the source, distribution, and hazards of inclusions in steel. They should adopt scientific treatment and purification processes. This coordination ensures performance and economic benefits of steel products. Meeting relevant use standards promotes enterprise development.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

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