The chemical composition of S275JR is strictly defined to ensure its mechanical properties and weldability. A critical feature is that the permissible limits for certain elements, particularly carbon (C), phosphorus (P), and sulfur (S), vary depending on the product thickness. This is because thicker sections cool more slowly after hot rolling, which can affect the final microstructure and properties. The composition is designed to compensate for this.
The following table summarizes the key compositional limits according to EN 10025 for different thickness ranges, primarily based on melt analysis:
| Element | Thickness ≤ 16 mm | Thickness > 16 mm ≤ 40 mm | Thickness > 40 mm |
|---|---|---|---|
| Carbon (C) | ≤ 0.21 % | ≤ 0.21 % | ≤ 0.22 % |
| Manganese (Mn) | ≤ 1.50 % | ≤ 1.50 % | ≤ 1.50 % |
| Phosphorus (P) | ≤ 0.035 % | ≤ 0.035 % | ≤ 0.045 % |
| Sulfur (S) | ≤ 0.035 % | ≤ 0.035 % | ≤ 0.045 % |
| Nitrogen (N) | ≤ 0.012 % | ≤ 0.012 % | ≤ 0.009 % |
Other Elements: Silicon (Si) is typically limited to ≤ 0.35%. Copper (Cu) may be present up to 0.55%, and limits for Chromium (Cr), Nickel (Ni), and Arsenic (As) are sometimes specified in mill-specific standards to control residual elements.
Key Points on Chemistry:
Low Carbon Content: The maximum carbon content is relatively low (0.17-0.22%). This is the primary factor ensuring good weldability, as it reduces the risk of forming hard, brittle microstructures in the heat-affected zone (HAZ) during welding.
Manganese (Mn): Manganese contributes significantly to strength and hardenability. Its content is balanced to achieve the required strength without compromising toughness or weldability.
Impurity Control (P & S): Phosphorus and sulfur are harmful impurities. Phosphorus can cause cold shortness (embrittlement at low temperatures), while sulfur forms manganese sulfide inclusions that can reduce ductility and impact toughness, particularly in the through-thickness direction (Z-direction). The tighter limits for thinner products reflect the need for better cold-forming capabilities.
Carbon Equivalent Value (CEV): While not always a mandatory check for S275JR, it is a calculated value (e.g., CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15) that predicts the steel's weldability. A typical maximum CEV for S275JR is around 0.40% to 0.44% for certain thicknesses, ensuring it remains easily weldable with common techniques.