Ductility (measured by elongation) decreases as carbon content rises. Low-carbon round steel (e.g., SAE 1018, 0.18% C) exhibits 25-30% elongation, allowing bending and forming. Medium-carbon (SAE 1045, 0.45% C) drops to 15-20% elongation, suitable for limited forming. High-carbon (SAE 1095, 0.95% C) has <10% elongation, making it brittle but hard. This trend occurs because carbon forms iron carbides, which restrict dislocation movement-essential for plastic deformation. For applications requiring both strength and ductility (e.g., automotive parts), medium-carbon grades with heat treatment are preferred.