Q345NH's overall performance - including strength, corrosion resistance, low‑temperature toughness, weldability - is directly controlled by its chemical composition under GB/T 4171. Below is a clear breakdown of each key element's effect.
1. Carbon (C) ≤ 0.16%
Strengthening: Increases yield and tensile strength.
Toughness & weldability: Highly negative if too high.Higher carbon causes brittle microstructure, worsens low‑temperature impact toughness, and increases cold-cracking risk during welding.
Corrosion resistance: Slightly negative; excess carbides reduce uniformity of the protective rust layer.
Design purpose: Kept low to balance strength, weldability and toughness.
2. Manganese (Mn) 0.70–1.50%
Strength: Major solid‑solution strengthener.
Low‑temperature toughness: Strongly positive.Refines ferrite grains, neutralizes harmful sulfur, lowers ductile‑brittle transition temperature.
Weldability: Moderate Mn improves hot ductility; excessive Mn increases hardenability.
Overall: Critical for achieving Q345 strength level without losing toughness.
3. Silicon (Si) 0.25–0.75%
Strengthening: Improves strength via solid‑solution hardening.
Toughness: Slightly negative at high levels; may coarsen grains.
Corrosion resistance: Helps form a dense rust layer in early stages.
Controlled to avoid embrittling the steel.
4. Phosphorus (P) ≤ 0.035%
Corrosion resistance: Strongly positive; promotes compact protective patina.
Toughness: Extremely negative; segregates at grain boundaries and causes brittleness, especially at low temperatures.
Q345NH limits P much lower than old weathering steels (like 09CuPCrNi‑A) to improve toughness.
5. Sulfur (S) ≤ 0.030%
Almost entirely harmful.Forms MnS inclusions that act as crack initiation points, reducing transverse toughness and corrosion uniformity.
Strictly limited to ensure weldability and impact performance.
6. Copper (Cu) 0.25–0.55%
Core weathering element.Enriches in the inner rust layer, blocks oxygen and ion penetration, significantly improves atmospheric corrosion resistance.
Slightly benefits strength without harming toughness.
7. Chromium (Cr) 0.40–0.70%
Enhances corrosion resistance significantly.Stabilizes the dense α‑FeOOH patina, especially in industrial and humid environments.
Mildly improves strength and high-temperature oxidation resistance.
8. Nickel (Ni) ≤ 0.65%
Greatly improves low‑temperature toughness and corrosion resistance.
Lowers ductile‑brittle transition temperature and offsets the embrittling effect of P and S.
Improves adhesion of the rust layer in coastal or polluted air.
9. Microalloy elements (Nb, V, Ti) trace
Refine grains strongly, improving both strength and low‑temperature toughness simultaneously.
Precipitate fine carbides/nitrides to increase strength without brittleness.
10. Aluminum (Al) ≥ 0.015%
Deoxidizes the steel, purifying the melt.
Forms AlN to pin grains and refine microstructure, boosting toughness.
