The Source of Nitrogen
Nitrogen in steel is mainly inhaled and dissolved in the exposed process of steel.Electric furnace steelmaking, including secondary refining arc heating, accelerates the dissociation of gas, so the content of nitrogen is high. Long smelting time in open hearth furnace increases nitrogen content. Improper control of converter reblowing and not timely switching between nitrogen and argon will also increase the nitrogen content. Nitrogen from ferroalloys, scrap iron and slag will also be carried into the molten steel with the charge.
The Form of Nitrogen
A part of nitrogen in steel is in the form of metal nitrides or carbides.Most of the elements added to special alloy steels form nitride under the right conditions.These elements include manganese, aluminum, boron, chromium, vanadium, molybdenum, titanium, tungsten, niobium, tantalum, zirconium, silicon and rare earth. Considering that many nitride forming elements have several simple or complex nitrides, up to 70 kinds of nitrides may be formed in steel.The other part of the nitrogen is dissolved in the steel as nitrogen atoms .In rare cases, nitrogen forms molecules in bubbles or adsorbs on the surface of the steel.
The Effect of Nitrogen
Nitrogen cannot be generalized as a harmful gas element because some special steels are purposefully added with nitrogen. All steels contain nitrogen, and the amount depends on how the steel is produced, the type and amount of alloying elements and how they are added, how the steel is cast. Some brands of stainless steel, appropriately increase the content of N, can reduce the use of Cr, this method can effectively reduce the cost. Nitrogen in iron and steel is mostly in the form of metal nitrides. For example, after being stored for some time, the steel will undergo strain aging and cannot be processed by deep drawing (such as automobile protection plate), because the steel will be torn and cannot be evenly stretched in all directions. This is due to the large grain size and the deposition of Fe4N on the grain interface.
Another example: in stainless steel, the formation of chromium nitride (Cr2N) at the grain interface depletes the chromium contained in the interface and causes so called intergranular corrosion. Adding titanium, which gives preference to titanium nitride, prevents this harmful effect.
