40MnB round steel (GB/T 3077) is a boron-alloyed medium-carbon steel with a cost-effective composition: 0.37–0.44% carbon (C), 1.10–1.40% manganese (Mn), 0.0005–0.0035% boron (B), ≤0.035% P/S, and no chromium.
Boron significantly enhances hardenability-allowing 40MnB to achieve similar mechanical properties to 40Cr (a chromium-alloyed steel) at 15–20% lower cost. After quenching and tempering, 40MnB has 880–1080 MPa tensile strength and 735 MPa yield strength, matching 40Cr's performance. It is used as a substitute for 40Cr in non-critical components like general machinery shafts and bolts, with Made-in-China listings promoting it as "40MnB Cost-Effective Alternative to 40Cr."