The chemical composition characteristics of Q345R

1. High carbon
The carbon content of Q345R steel is very high, generally 0.95-1.15, which belongs to hypereutectoid steel. A part exists in the martensite matrix to strengthen the martensite; another part forms a sufficient number of carbides to obtain the required wear resistance. But too high carbon content will increase the inhomogeneity of carbide distribution, and it is easy to generate network carbide and reduce the performance of Q345R steel.

2. The main addition of alloying element Cr
The role of Cr is to improve the hardenability of Q345R steel, form alloy cementite, and improve the corrosion resistance of bearing steel. The alloy cementite (Fe,Cr) 3C formed by part of chromium dissolves slowly during quenching and heating, which can reduce the tendency of overheating. After heat treatment, a finer structure can be obtained, and the carbide can be evenly distributed in the Q345R steel matrix with fine particles In the microstructure, it can not only improve the tempering stability of Q345R steel, but also increase the hardness of Q345R steel, and then improve the wear resistance and contact fatigue strength of Q345R steel.
The suitable Cr content is 0.40-1.65. When the Cr content is higher than 1.65, it will increase the retained austenite, reduce the hardness and dimensional stability of Q345R steel, and increase the inhomogeneity of carbides, reducing the toughness of the steel.

3. Add silicon, manganese, vanadium, etc. to further improve hardenability
More alloying elements need to be added to Q345R steel for large bearings to improve hardenability. Usually, Mn and Si are added to improve hardenability. An appropriate amount of Si (0.40-0.60) can also significantly improve the strength and elastic limit of Q345R steel ; Part of adding V dissolves in austenite to improve hardenability, and the other part forms vanadium carbide VC to improve the wear resistance of Q345R steel and prevent overheating. Vanadium is usually found in chrome-free steels.

4. Reduce S and P content, reduce the number of oxide and silicate inclusions, and improve metallurgical quality
Since the contact fatigue performance of bearing steel is very sensitive to the small defects of Q345R steel, it is required that S<0.02, P≤0.027. Generally, technologies such as electroslag remelting, electric furnace smelting and vacuum smelting are used to reduce the number of inclusions.