Manganese (1.00–1.60% in S355JR) enhances strength via two key mechanisms:
Solid Solution Strengthening: Manganese atoms dissolve into the iron lattice, creating distortions that resist dislocation movement (the cause of plastic deformation). This raises yield strength to 355 MPa and tensile strength to 470–630 MPa.
Grain Refinement: During hot rolling, manganese promotes smaller, uniform grain formation. Smaller grains have more boundaries, which block dislocations, boosting both strength and toughness.Manganese also counteracts sulfur by forming manganese sulfides-less brittle than iron sulfides-preventing "hot shortness" during rolling. Importantly, it maintains ductility (21% elongation) when within the 1.00–1.60% range, avoiding the brittleness of high-carbon steels. Without manganese, S355JR could not achieve its high strength while retaining workability.