In general, the main hazards of Nitrogen are as follows:
1.The precipitation of Fe4N leads to the aging of steel.
2.Reduce the cold workability of steel.
3.The brittleness of the heat-affected zone.
When Vanadium, Aluminum, Titanium, Niobium and other elements are present in the steel, Nitrogen can be used to form stable nitrides, and the strength of the steel can be improved, which is beneficial to the performance of the steel.
According to relevant information, the Nitrogen content of LD converter steel is 0.003% to 0.006%. The Nitrogen content of EAF steel is 0.008% to 0.0016%. The highest solubility of Nitrogen in a-Fe is about 0.1% at 590°C, and drops below 0.001% at room temperature. When steel with a high free Nitrogen content cools faster from high temperatures, the ferrite will saturate. If the steel is stationary at room temperature, ammonia will precipitate in the form of Fe4N over time, which will increase the strength and hardness of the steel, and reduce the plasticity and toughness, with immediate effect.
In order to reduce the brittle transition temperature of steel, the Nitrogen and silicon content in the steel must be reduced. In order to use grain refiners (Al, V, Ti) to stabilize nitride elements, especially in non-grain refined steel, the total ammonia content should be the lowest. In grain-refined steel, the helium content should be the largest, which is conducive to the formation of nitrides and the reduction of free Nitrogen content.
For high carbon steel, vanadium is an element that can refine and enhance the grain size of steel. When the free Nitrogen content obtained by controlled rolling is 0.008%, the brittle transition temperature FATT is -5°C. After the normalization treatment, the free Nitrogen content is 0.002%, and the brittleness transition temperature is -45°C.
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