What does 1018-1045 mean in steel strengths?

The numbers 1018 and 1045 refer to specific grades of carbon steel defined by the SAE (Society of Automotive Engineers) system. The first two digits represent the primary alloying element, while the last two digits indicate the carbon content; for example, 1018 contains approximately 0.18% carbon, and 1045 contains about 0.45% carbon. Higher carbon content generally results in greater strength and hardness, but may also reduce ductility. Thus, 1045 steel is typically stronger and harder than 1018 steel.