What does the JIS steel designation “S45C” stand for?

Answer: B. A carbon steel for machine structural use with about 0.45% carbon

In S45C, S stands for steel and C for carbon, and the number in between gives the nominal carbon content in units of 0.01%, so it is a carbon steel for machine structural use containing about 0.45% carbon (roughly 0.42–0.48%), close to AISI 1045. Its strength can be adjusted by quenching and tempering, and it is widely used for shafts and gears. In SS400 (JIS), the rolled steel for general structure, the number gives not the carbon content but the guaranteed tensile strength (at least 400 MPa), so the choice reading S45C as rolled steel for general structure with a tensile strength of at least 450 MPa confuses how the two designations are read. Above 2.14% carbon the material is no longer steel but cast iron, and 4.5% is the mistake of reading the number in units of 0.1%. In the hypoeutectoid range, more carbon means higher hardness and strength but lower ductility and weldability.