Common carbon hot-rolled steel strip, widely used in engineering structures, exhibits mechanical properties closely related to its carbon content range. Carbon, as a core element in steel, directly influences the material's strength, hardness, plasticity, and toughness through solid solution strengthening and precipitation strengthening mechanisms. The carbon content of common carbon hot-rolled steel strip is typically controlled between 0.06% and 0.25%, and choosing this range is crucial for balancing material properties and processing feasibility.
In the low-carbon range (carbon content ≤ 0.15%), common carbon hot-rolled steel strip exhibits excellent plasticity and toughness. At this level, carbon exists in a solid solution form within the ferrite matrix, resulting in a weaker hindering effect on dislocations, making the material easier to cold-bend, stamp, and perform other forming processes. For example, when cold-bending a steel strip with a carbon content of 0.08% by 180°, the ratio of the mandrel diameter to the sample thickness can be as low as 0.5, indicating good cold-working performance. Simultaneously, the elongation of low-carbon steel strip generally exceeds 30%, meeting the deformation requirements of complex structural components. This characteristic makes it the preferred material for automotive body panels, appliance housings, and other applications requiring deep forming.
Increasing carbon content promotes the precipitation of cementite (Fe₃C) at ferrite grain boundaries, forming fine second-phase particles. This precipitation strengthening mechanism improves the tensile strength of the material. For example, steel strip with 0.22% carbon content can achieve a tensile strength of 450 MPa, about 40% higher than lower carbon steel, but the elongation decreases to around 24%. This performance change makes it more suitable for structural components that need to bear certain loads, such as building steel sections and mechanical supports. While the cold bending performance of the steel strip is somewhat reduced, it still meets conventional processing requirements.
The effect of carbon content on the hardness of common carbon hot-rolled steel strip shows a linear increasing trend. The hardness of low-carbon steel strip is typically between 120-150 HBW, while medium-carbon steel strip can reach 160-190 HBW. The increase in hardness stems from the distortion effect of carbon atoms on the ferrite lattice and the hindrance of dislocation movement by cementite particles. However, increased hardness also leads to increased brittleness. During cold bending, the bending radius must be strictly controlled to avoid stress concentration and cracking. For example, steel strip with a carbon content of 0.20% can remain crack-free on the surface when the ratio of the mandrel diameter to the sample thickness is 1:1, but microcracks may appear at the edges when the ratio is reduced to 0.8:1.
The toughness of common carbon hot-rolled steel strip is extremely sensitive to changes in carbon content. Low-carbon steel strip maintains high impact absorption energy even at -20°C, while the toughness of medium-carbon steel strip decreases significantly at the same temperature. This difference stems from the influence of carbon content on ferrite grain size and cementite distribution.
The fine ferrite grains and uniformly distributed cementite in low-carbon steel help absorb impact energy, while the coarse cementite network in medium-carbon steel can become a pathway for crack propagation. Therefore, low-carbon steel strip should be preferred in cold regions or applications with high impact loads.
Carbon content also indirectly affects the overall mechanical properties of common carbon hot-rolled steel strips by influencing weldability. When welding low-carbon steel strips, the hardness change in the heat-affected zone is smaller, making them less prone to cold cracking and suitable for various welding processes. However, after welding medium-carbon steel strips, the hardness of the heat-affected zone may increase due to carbide precipitation, requiring measures such as preheating before welding and slow cooling after welding to control welding stress. For example, when welding steel strips with a carbon content of 0.20%, the preheating temperature needs to be controlled at 100-150℃ to avoid cracking.
The carbon content range of common carbon hot-rolled steel strips comprehensively affects the material's strength, hardness, plasticity, and toughness through mechanisms such as solid solution strengthening and precipitation strengthening. Low-carbon materials meet forming requirements with excellent plasticity and toughness, while medium-carbon materials adapt to load-bearing scenarios through increased strength. In practical applications, the carbon content range must be rationally selected based on the processing method, usage environment, and performance requirements to achieve the optimal balance between material performance and cost.
