The Method Of Reducing Oxide Inclusions In Bearing Steel

Inclusion in steel is one of the main factors affecting the contact fatigue life of bearing steel. The key to control the inclusions in steel is to control the oxygen content and oxides in steel. The main ideas to reduce oxide inclusions in steel are as follows.

1、 Control of carbon content at tapping end of primary refining furnace

When the carbon content at the tapping end is too low, the dissolved oxygen in the steel is high, which leads to the consumption of a large amount of aluminum during subsequent deoxidation, and the total amount of inclusions will increase, which brings difficulties to the later refining; In addition, the oxidizability of slag will increase obviously, so it is difficult to adjust and control the composition of slag in later refining. But too high carbon content at tapping end will bring pressure to process control of primary refining furnace. In EAF smelting, the end-point carbon content is generally controlled at about 0.2%.

2、 Selection of suitable deoxidizer

In the production process of bearing steel, the oxygen content in the steel can be reduced and a proper amount of acid soluble aluminum can be obtained by final deoxidation with aluminum. When the content of acid soluble aluminum is too high, the protection of molten steel is not good, which is easy to lead to secondary oxidation, thus increasing the content of brittle Al2O3 inclusions; When the content of acid soluble aluminum is low, the secondary oxidation of silicon and the decrease of molten steel temperature lead to the precipitation of dissolved oxygen, which leads to the formation of coarse silicate inclusions rich in SiO2. According to the research, when the content of acid soluble aluminum in the steel is controlled at 0.02% ~ 0.04%, the grain size of the steel can be refined, so as to obtain higher strength and toughness.

Barium is an ideal deoxidizing modifier. Barium alloy not only has strong deoxidizing ability, but also can denature the residual inclusions in steel. According to the research, after deoxidizing the bearing steel with Si Al Ba alloy, the total oxygen content in the steel rapidly drops to a stable value. Finally, there is no barium containing spheroidal inclusions in the steel, and the residual inclusions in the steel are well denatured. The inclusions are fine, dispersed and evenly distributed.

Magnesium has a significant effect on the modification of Al2O3 inclusions in bearing steel. According to the research, when the content of acid soluble aluminum is 0.03%, there is 210-4% magnesium in the steel, that is to say, the inclusion of Al 2O 3 can be modified into MgO · Al 2O 3, and the content of Mg in the steel is more than 10% μ The results show that the Al 2O 3 inclusions with m are transformed into fine and spherical mg Al spinel inclusions, of which less than 5 μ 46%, the others are 5 ~ 10 μ M.

3、 Optimization of refining slag composition

It is found that when the binary basicity of refining slag increases from 2.0 to 4.5, the total oxygen content at the end of molten steel decreases from 20 10-6 to 11 10-6, and the total number and area of inclusions decrease. The typical inclusions in molten steel refined by high basicity slag are brittle inclusions such as Al2O3 and Al Mg spinel, whose size is less than 5 μ m。 Increasing the content of Al2O3 or adding CaF2 and reducing the content of MgO can significantly improve the speed and ability of refining slag to adsorb inclusions.

4、 Optimization of smelting process

In order to obtain high-quality steel, increasing the amount of molten iron and selecting high-quality scrap for smelting are commonly used methods. In ladle metallurgy process, accurately controlling the content of Al, Si or Ca in steel to promote deoxidation and desulfurization, using protective casting in the whole process, the inclusion content will be greatly reduced. Vacuum melting is an important way to reduce the content of inclusions. In higher vacuum, the oxygen content in steel can be reduced to below 10-6. Electroslag refining can obtain better smelting effect than vacuum smelting. Electroslag refining not only reduces the oxygen content in the steel, but also reduces the size of inclusions in the steel, so that the distribution of inclusions is more uniform. After remelting, it provides opportunities for large inclusions to float up.

At present, vacuum induction and vacuum consumable smelting are widely used in bearing steel for important purposes at home and abroad. The purity of new stainless bearing steel 6Cr14Mo produced by this process has been greatly improved, the oxygen content is only 510-4, and the oxide inclusions are less in quantity, smaller in size and more evenly distributed.

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