Normally, nitrogen is considered to be one of the harmful impurities in steel. Although low solubility of nitrogen in liquid steel at atmospheric pressure, these small amounts of nitrogen can lead to age embrittlement of the steel, leading to the development of secondary refining techniques to reduce nitrogen in liquid steel and are continually improving . However, in high nitrogen steels, nitrogen can interact with other alloying elements (such as Mn, Cr, V, Nb, Ti, etc.) in the steel and give the steel many excellent properties. For example, to improve the stability of austenite, so that greatly enhance the mechanical properties of steel to improve the corrosion resistance of steel and so on. A steel containing 0.08% by mass or more of nitrogen in the ferrite matrix or containing 0.4% (mass) or more of nitrogen in the austenite matrix is called a high nitrogen steel.
The main problem with the development of high nitrogen steels
is the development of appropriate processes and equipment to ensure a high and uniform nitrogen concentration throughout the volume range of the metal during solidification. High nitrogen steels are not as easily smelted as other steels, and in order to add a sufficient amount of nitrogen, the alloy composition or smelting process of the steel or both must be adjusted so that the nitrogen solubility is sufficiently high.
The solubility of nitrogen depends on the pressure, temperature and composition of the alloy. This is discussed in more detail in the literature, so it is generally desirable to include as high chromium and manganese as possible in the steel and to smelt at high nitrogen pressures in order to add sufficient nitrogen. The current high nitrogen steel production has the following methods.
Solid nitrogen alloy addition method
The method comprises the following steps: adding nitrogen-containing iron alloy such as nitrogen-containing Mn-Fe, Cr-Fe, V-Fe, nitride Si3N4 or cyanide Ca (CN) 2 to achieve the purpose of nitrogen alloying, Nitrogen can reach 0.5% to 0.6%. However, due to the introduction of nitrogen-containing iron alloy in this method, one is to increase the cost of materials, the second is to reduce the purity of liquid steel, and the concentration of nitrogen increased ups and downs, easy nitrogen nitrogen in the formation of high nitrogen.
(2) booster induction furnace in a large number of nitrogen alloying
A large amount of nitrogen alloying in a stepped induction furnace is effected by the gas phase. At the interface between the molten metal and the gas, N2 is produced by the reaction of N2. The amount of nitrogen absorbed by the metal depends on the contact time and the contact area between the melt and nitrogen. However, due to the operation and processing of a large number of supersaturated nitrogen liquid steel, there are safety issues, it makes large-scale production of the booster induction furnace failed to further develop.
3. A large number of nitrogen alloying in a pressurized plasma furnace
The principle is also achieved by the vapor phase, with the difference that nitrogen is supplied to the liquid metal
in the form of atoms separated in the plasma arc. The amount of nitrogen absorbed by the liquid metal depends on the partial pressure of nitrogen, the rate of melting and the conditions of the plasma arc. Under the action of plasma arc, nitrogen saturation concentration is only 3min, the time is significantly shorter than the resistance furnace, induction furnace, absorption rate of nitrogen, and the solution of low metal impurities, can reduce volatile elements (such as Mn and Cr), a high nitrogen concentration can be obtained without adding a nitrogen-containing alloy. The average nitrogen content in the remelted ingot is about 0.6%, which is equivalent to the Sievert equation. However, the conditions of the plasma are difficult to control, and thus the nitrogen content is difficult to precisely control, and the concentration of nitrogen in the ingot is extremely uneven, either horizontally or longitudinally, due to fluctuations in the bath temperature during the melting process.