Effect of Nitrogen on the Properties of Steel Tubes

Under general conditions, the main hazards of nitrogen are as follows:

(1) The precipitation of Fe4N results in the ageing of steel.

(2) Reducing the cold working performance of steel;

(3) Brittleness of HAZ.

When vanadium, aluminium, titanium, niobium and other elements exist in the steel, stable nitrides can be formed with nitrogen, and the strength of the steel can be improved, which is beneficial to the performance of the steel.

When studying the brittleness of steel, it is found that manganese can reduce the brittleness sensitivity of steel, while chlorine can increase the brittleness sensitivity of steel. The order of increasing embrittlement sensitivity of steels produced by different methods is as follows: silicon and aluminium killed blast furnace steel, aluminium killed blast furnace steel and semi-killed steel, oxygen-blown boiling steel or semi-killed steel. This shows the importance of free nitrogen in steel.

According to relevant information, the nitrogen content of LD converter steel is 0.003%~0.006%. The nitrogen content of EAF steel ranges from 0.008% to 0.0016%. The highest solubility of nitrogen in a-Fe is about 0.1% at 590 C, and it drops below 0.001% at room temperature. When the steel with high free nitrogen content is cooled faster from high temperature, the ferrite will be saturated. If the steel is stationary at room temperature, ammonia will precipitate in the form of Fe4N with the increase of time, which will increase the strength and hardness of the steel and decrease the plasticity and toughness, i.e. aging.

In order to reduce the brittle transition temperature of steel, it is necessary to reduce the nitrogen and silicon contents in steel. In order to use grain refiners (Al, V, Ti) for stabilizing nitride elements, especially in non-grain refined steels, the total ammonia content should be the lowest. In grain refinement steel, helium content should be the largest, which is conducive to the formation of nitrides and the reduction of free nitrogen content.

For high carbon steels, vanadium is an element that can refine and strengthen the grain size of steels. The brittle transition temperature FATT is -5 C when the free nitrogen content is 0.008% by controlled rolling. The content of free nitrogen is 0.002% and the brittle transition temperature is - 45 C after normalization treatment.

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