(1) The role of manganese

Manganese and nickel are both alloying elements that promote the formation of austenite, so it can replace nickel (or part of it), but manganese itself is not corrosion resistant, so manganese cannot be used alone as an alloying element in stainless steel.

The effect of manganese on austenite is similar to that of nickel. It does not lie in the formation of austenite, but reduces the critical cooling rate of steel, increases the stability of austenite during cooling, and inhibits the decomposition of austenite, so that the formation of austenite at high temperatures Austenite can be maintained to room temperature. In improving the corrosion resistance of steel, manganese has little effect. This is because manganese has little effect on increasing the electrode potential of iron-based solid solution, and the protective effect of forming an oxide film is also very low. Therefore, although manganese alloying is available in the industry Austenitic steels (such as 7Mn15Cr2A13V2WMo, 5Mn15Cr8Ni5Mo3V240Mn18Cr4 50Mn18Cr4WN, etc.), but they cannot be used as stainless steel, so they are not stainless steel.

The complete replacement of nickel in stainless steel with manganese cannot solve the corrosion resistance of steel. This is because chromium-manganese steel wants to obtain a completely austenitic structure. In addition to the appropriate content of chromium and manganese, the carbon content should also be considered. When the carbon content in the steel is low, it will be difficult to obtain a completely austenitic structure. If the carbon content in the steel is increased, the corrosion resistance of the steel will be reduced. Furthermore, after the chromium-manganese stainless steel is heated at 500~800℃, the steel's resistance to intergranular corrosion will be very low, and the addition of titanium and niobium will not reduce its sensitivity to intergranular corrosion. Therefore, the current applications are mostly based on the 18-8 chromium-nickel austenitic stainless steel, replacing part of nickel with manganese or using the method of adding manganese and nitrogen to replace all nickel, and replacing nickel with manganese and nitrogen. Low-nickel stainless steel and nickel-free chromium-manganese-nitrogen stainless steel, such as 12Cr17Mn6Ni5N (201), 20Cr15Mn15Ni2N and 26Cr18Mn12Si2N. Moreover, it has now been included in the national standard and has been applied, and some can replace the traditional 18-8 type chromium-nickel stainless steel, such as 10Cr17Mn9Ni4N and 20Cr13Mn9Ni4, which have good atmospheric corrosion resistance.

(2) The role of nitrogen

Nitrogen is an austenite stabilizing element and can increase the strength of steel. In austenitic stainless steel or austenitic-ferritic stainless steel, it can increase the resistance to pitting corrosion and crevice corrosion, and reduce the chance of precipitation of intermetallic σ phase at high temperature or during welding. At the same time, replacing expensive nickel with a composite alloy of nitrogen and manganese also has the advantage of improving the solubility of nitrogen. The combined effect of manganese and nitrogen can also replace nickel in stainless steel, thereby innovating and developing a new type of CrMn-N stainless acid-resistant steel. Low-nickel stainless steel with manganese and nitrogen substituted for nickel and nickel-free chromium-manganese-nitrogen stainless steel have been used in industry. The typical grade of this type of steel is 12Cr17Mn6Ni5N (201), which has higher strength and corrosion resistance than the austenitic stainless steel grade of 06Cr19Ni10 (304).