Measures to improve the level of inclusion control
In recent years, the industry has put forward higher requirements for inclusion control, requiring strict control of the maximum size of inclusion on the basis of reducing the total amount of inclusion, and greatly improving the performance of steel.
In order to achieve this goal, the following technical measures should be taken: first, vacuum carbon deoxidation instead of alloy precipitation deoxidation to reduce deoxidation inclusions; The second is to inhibit or reduce the slag steel reaction in refining to avoid the formation of large particle denaturation, slag rolling and desulphurization inclusions; The third is to strengthen the protection of molten steel to avoid the secondary oxidation of molten steel in casting, resulting in new inclusions.
The key technology to change the deoxidation process is to increase the reaction speed of the vacuum cyclic degassing (RH) interface, so that the carbon oxygen reaction is close to the gas phase equilibrium under vacuum. The specific measures are as follows: first, expand the vacuum chamber surface area; The second is to blow argon gas in vacuum chamber to speed up the updating speed of liquid steel surface and increase the interface flow rate; The third is the use of micro-bubble technology (PERM method), before RH refining, blowing H2 or N2 to increase the gas content in the steel, the use of vacuum degassing energy, in the later stage to form a large number of micro-bubbles, promote the decarbonization of the molten pool and conducive to the inclusion floating.
Tundish metallurgy plays an important role in the control of inclusions in steel. Its goal is not to further purify molten steel, but to strictly control the pollution in the casting process to avoid secondary oxidation, slag rolling and denatured inclusions. The concrete measures include: using large capacity tundish, reasonably designing tundish shape to avoid turbulent flow of molten steel; Strengthen the sealing of tundish and control the oxygen content PO2 in tundish atmosphere ≤0.1mol%; The inclusion area ratio in the inclusion enrichment zone of the surface layer of the billet can be reduced from 1% after 5 minutes of sedation to 0.2% after 10 minutes of sedation. Using slag free casting process, such as Japan Sanyang special steel plant to produce ultra-pure bearing steel, using slag free casting process, can control the maximum inclusion size in steel ≤12μm.
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