The chemical composition of Q355B is strictly defined in GB/T 1591-2018 to ensure its weldability, strength, and toughness. It is a low-alloy steel where elements like Manganese (Mn) and Silicon (Si) are primary strengthening agents, with possible micro-additions of Niobium (Nb), Vanadium (V), and Titanium (Ti) for grain refinement and precipitation strengthening. The composition limits vary slightly with product thickness to guarantee uniform properties. Key elements and their typical roles/limits are: Carbon (C): Typically ≤ 0.20-0.24% max (depending on thickness). Carbon increases strength but harms weldability and toughness. Silicon (Si): ≤ 0.50-0.55%. It is a deoxidizer and solid solution strengthener. Manganese (Mn): ≤ 1.60%. A primary strengthener and toughness enhancer. Phosphorus (P) and Sulfur (S): Both are controlled to low levels (P≤0.030%, S≤0.030% for Grade B) as they are detrimental impurities causing brittleness. The "B" grade specifies a maximum Carbon Equivalent (CEV) to ensure good weldability; CEV is calculated using the IIW formula: CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. For Q355B, CEV is typically required to be ≤ a specified value (e.g., 0.43-0.45 depending on thickness) for preheating calculation in welding. Compared to Q355A, Grade B has tighter controls on Phosphorus and Sulfur and mandatory impact testing.
*Table: Typical Chemical Composition Ranges for Q355B (GB/T 1591-2018, thickness ≤ 40mm example)*
| Element | C (max) | Si (max) | Mn (max) | P (max) | S (max) | Nb (max) | V (max) | Ti (max) | Cu (max) | CEV (max, approx.) |
|---|---|---|---|---|---|---|---|---|---|---|
| Q355B | 0.24% | 0.55% | 1.60% | 0.030% | 0.030% | 0.05% | 0.15% | 0.20% | 0.40% | 0.43-0.45 |