1. Chemical Composition

The main components of carbon steel are iron (Fe) and carbon (C), with low content of other elements. The classification is as follows:

Type Carbon (C) Manganese (Mn) Silicon (Si) Sulfur (S) Phosphorus (P)

Low Carbon 0.05%–0.25% 0.3%–0.6% ≤0.3% ≤0.05% ≤0.04%

Medium Carbon 0.25%–0.60% 0.6%–1.0% ≤0.4% ≤0.04% ≤0.04%

High Carbon 0.60%–1.0% 0.3%–0.9% ≤0.4% ≤0.03% ≤0.03%

Notes:

Sulfur (S) and phosphorus (P) are impurity elements. High-quality carbon steel requires strict control of their content (e.g., S ≤ 0.03%, P ≤ 0.03%).

Manganese (Mn) enhances strength and toughness, with its content adjusted according to carbon content.

2. Mechanical Properties

Mechanical properties are closely related to carbon content and heat treatment conditions:

Parameter Low Carbon (e.g., Q235) Medium Carbon (e.g., 45#) High Carbon (e.g., T8)

Tensile Strength 300–500 MPa 500–800 MPa 600–1000 MPa

Yield Strength 200–300 MPa 300–550 MPa 400–800 MPa

Elongation (%) 20%–35% 10%–20% 5%–15%

Hardness (HB) 120–160 160–250 200–350

Notes:

Low Carbon Steel: Excellent plasticity, weldability, and formability, but lower strength. Commonly used in structural components (e.g., buildings, bridges).

Medium Carbon Steel: Offers balanced performance after quenching and tempering. Used for mechanical parts (gears, shafts).

High Carbon Steel: High hardness and wear resistance but brittle. Requires heat treatment (e.g., spheroidizing annealing) to improve machinability. Used for tools, springs, etc.