Secondary refinement pertains to the refining processes carried out outside the conventional steelmaking furnaces, such as open-hearth furnaces, refining furnaces, and electric furnaces. The primary objectives of secondary refinement encompass deoxidation, desulfurization, dephosphorization, decarburization, inclusion removal, alloying, and precise composition adjustment.
Global crude steel production statistics reveal a fluctuating yet consistent upward trend over the past three decades. With the increasing continuous casting ratio, the yield rate has also risen. Presently, LD steel mills contribute to 60% of the world's total crude steel output, while electric furnace steel mills account for nearly 40%.
Over the last 15 years, the vacuum processing capacity within the 15 EU member states has steadily expanded. Currently, over 80% of their steel output undergoes vacuum treatment. The ladle furnace, initially introduced in electric furnace steel plants for secondary refining, has seen a growing adoption in BOF steel plants in recent years. Presently, more than 30% of the steel output from these 15 EU member states is refined in ladle furnaces.
In fact, continuous casting technology was pioneered in Japan and Europe in the early 1970s and subsequently introduced into steelmaking plants. At present, the continuous casting ratio in these two regions has reached approximately 95%, mirroring the trend in the United States, which achieved a similar ratio in just 3-4 years. It is anticipated that the continuous casting ratio in these countries will continue to rise in the coming years. According to statistics, continuous casting currently accounts for about 85% of global steel output, a figure that may climb to around 90% in the near future.
To achieve composition and temperature homogenization, a gas stirring process within the ladle was developed.
The most notable aspect is the utilization of secondary refining, which significantly enhances both steel output and quality.
But what are the main tasks and purposes of secondary refining? They include separating the molten steel and slag phases during tapping and continuous casting, deoxidizing the molten steel, performing alloying based on the endpoint, adjusting the injection temperature, improving the cleanliness of the molten steel, modifying inclusions, removing dissolved [H] and [N] from the molten steel, decarburization, desulfurization, and ensuring uniform composition and temperature of the molten steel.
The introduction of vacuum decarburization has enabled a substantial reduction in carbon content. Currently, molten steel with a carbon content as low as 20 ppm is achievable, and it is expected that special-purpose steel will reach a carbon content of 10 ppm in the future. It is anticipated that the phosphorus (P) content in special-purpose steel will be reduced to 30 ppm in the future. By removing sulfur (S) from molten iron and steel, the S content can now stably reach 10 ppm, with no immediate need to reduce it further. Special steel will require a nitrogen content of 20 ppm in the future. Steel with a total oxygen content of 15 ppm is currently obtainable, with a minimum oxygen content requirement of about 10 ppm in the future. Vacuum treatment can reduce the hydrogen content to approximately 1 ppm.
The current steelmaking process is highly flexible and tailored to the product range of the steelmaking plant. Generally, it commences with the pre-desulfurization of molten iron, followed by converter steelmaking. Subsequently, vacuum treatment (RH treatment or ladle furnace degassing treatment) is conducted, and if necessary, the molten steel can also be heated, either in the ladle furnace or via the aluminothermic method. At the conclusion of secondary refining, calcium-containing materials are added for inclusion modification treatment. Finally, the molten steel is cast using a continuous casting process.
We are a professional electric furnace manufacturer. For further inquiries, or if you require submerged arc furnaces, electric arc furnaces, ladle refining furnaces, or other melting equipment, please do not hesitate to contact us at susie@aeaxa.com
