How to define secondary refinement

Secondary refining refers to the refining outside the traditional steelmaking furnace (such as open hearth, refined furnace, electric furnace). The main tasks of refining are deoxidation, desulfurization, dephosphorization, decarburization, removal of inclusions, alloying and fine-tuning of composition.

Statistics of world crude steel output show that in the past 30 years, the world's crude steel output has continued to grow in ups and downs. As the continuous casting ratio increases, the yield rate is increased. At present, crude steel produced by LD steel mill accounts for 60% of the world's total steel output. Electric furnace steel mills produce nearly 40% of steel.

In the past 15 years, the vacuum processing capacity of the 15 EU member states has continued to increase. Currently, more than 80% of the steel output can be vacuum treated. The ladle furnace used for secondary refining was first introduced into the electric furnace steel plant. In recent years, the use of ladle furnaces in BOF steel plants has also been increasing steadily. At present, more than 30% of the steel output of the 15 EU member states is refined in the ladle furnace.

In fact, continuous casting technology was developed in Japan and Europe in the early 1970s. It was later introduced into a steelmaking plant. At present, the continuous casting ratio of these two regions has reached about 95%. The development trend in the United States is the same. It only took 3-4 years to make the current continuous casting ratio reach about 95%. In the next few years, the continuous casting ratio of these countries will definitely continue to increase. According to statistics, the current world continuous casting output accounts for about 85% of the steel output. In the next few years, this figure may rise to about 90%.

In order to homogenize the composition and temperature, a gas stirring process in the ladle appeared. The most striking thing is the use of secondary refining, which greatly improves the output and quality of steel. However, what is the main task and purpose of secondary refining? Separate molten steel and slag phase during tapping and continuous casting, deoxidize molten steel, perform alloying according to the end point, adjust injection temperature, improve the cleanliness of molten steel, denaturation of inclusions, Removal of [H] and [N] dissolved in molten steel, decarburization, desulfurization, and uniform molten steel composition and temperature.

The introduction of vacuum decarburization makes it possible to greatly reduce the carbon content. At present, molten steel with a carbon content of 20 ppm is available, and it is expected that steel for special purposes can reach a carbon content of 10 ppm. It is expected that P for special purpose steel will be reduced to 30ppm in the future. Now, by removing S from molten iron and molten steel, the content of S can stably reach 10ppm. At present, there is no need to drop s to a lower level. In the future, special steel requires a nitrogen content of 20 ppm. Now steel with a total oxygen content of 15 ppm can be obtained. In the future, the minimum oxygen content is required to be about 10 ppm. Vacuum treatment can reduce the hydrogen content to about 1 ppm.

The current steelmaking process is quite flexible and depends on the product range of the steelmaking plant. Generally, it starts with the pre-desulfurization of molten iron, followed by converter steelmaking. Then vacuum treatment (RH treatment or ladle furnace degassing treatment), if necessary, the molten steel can also be heated. It can be heated in the ladle furnace or the aluminothermic method. At the end of the secondary refining, calcium-containing materials are added for inclusion denaturation treatment. Finally, the molten steel is cast using a continuous casting process.