16Mn and Q345 are both grades of steel commonly used in structural applications, with the main difference lying in their chemical composition and mechanical properties:

16Mn:

It is a low-alloy steel with carbon content around 0.16%.

Primarily used for welded structures such as bridges, ships, vehicles, boilers, and pressure vessels.

Tensile strength typically ranges from 490 MPa to 660 MPa.

Q345:

Q345 is classified into three quality grades: Q345A, Q345B, and Q345C.

It is a low-alloy high-strength structural steel with additional elements like Nb, V, and Ti compared to 16Mn.

Widely used in construction and engineering structures due to its higher strength and better performance.

Tensile strength typically ranges from 470 MPa to 630 MPa.

In summary, while both 16Mn and Q345 are used for structural purposes, Q345 is generally considered to possess higher strength and better comprehensive mechanical properties due to its alloying elements, making it suitable for more demanding applications.