Carbon content directly impacts strength and ductility. Low carbon (≤0.2%) enhances weldability and toughness, ideal for structures requiring flexibility, like bridges in seismic zones. Medium carbon (0.2-0.3%) balances strength and ductility, suited for general construction. High carbon (>0.3%) increases tensile strength but reduces ductility, making H-beams brittle and harder to weld-used only in specialized, low-stress applications. Carbon also influences hardenability: higher levels allow for better heat treatment, but excess can cause cracking during cooling. Manufacturers carefully control carbon to meet standards, ensuring H-beams perform as designed in their intended use cases.