31CrMoV9 was originally a European brand of high-hardenability medium-carbon alloy steel. When this steel was developed, it was used for gears and later for bearings. The process of smelting 31CrMoV9 steel is electric furnace smelting or vacuum induction melting (EAF/EBT)→outside furnace refining (LF)→vacuum degassing (VD). The smelting method is basically the same as the smelting process of other types of gear steel in Germany. Most gear steels in Germany are chromium-manganese steels, with about a dozen steel types. In order to improve the mechanical properties of the steel, German metallurgists adjust the alloy elements in the steel to meet the required mechanical properties of the gear steel. Among them, 30Cr1Mo1V and 31CrMoV9 are representative typical steel types.
In order to compare the quality gap of finished steel products at home and abroad and improve the smelting level of this type of steel in China, scientific researchers selected 31CrMoV9 steel from different origins to study non-metallic inclusions. Observe the appearance and morphology of the inclusions in the electrolytic steel sample, conduct metallographic observation of the steel sample, and observe the structural changes of the steel sample using a high-temperature confocal microscope. The study found that single-component inclusions are mostly elongated, while composite inclusions are mostly spherical. There is a gap in the quantity and shape control of inclusions between the steel produced in China and the steel produced in Germany selected during the research process. The steel produced in China has more inclusions per unit area, the size of the inclusions is uneven, and the shape control is not ideal.
In terms of composition, the inclusions in German steel are mainly composite inclusions composed of Al2O3, CaO, MnS, etc., while the inclusions in Chinese steel have compositional fluctuations. During the confocal microscopy test, it was found that inclusions will spheroidize during the heating process. During the cooling process, the temperature at which the steel sample begins to form a martensite structure is different. The Chinese steel sample is 580°C, and the German steel sample is 450°C. ℃, which is related to the composition of its own crystal. The final structure is martensite + carbide particles + retained austenite.
