1. Manganese (Mn): 0.70–1.50%
Strength
Strong solid-solution strengthener
Helps achieve the Q345 strength level without relying on high carbon
Toughness
Greatly improves low-temperature toughness
Refines ferrite grains, lowers ductile-brittle transition temperature
Combines with sulfur to form MnS, avoiding hot brittleness
Weldability
Moderate Mn is good; excessive Mn increases hardenability and may cause HAZ hardening
Corrosion resistance
Slightly beneficial; improves structural uniformity
2. Silicon (Si): 0.25–0.75%
Strength
Increases strength through solid-solution hardening
Toughness
Slightly harmful at higher levels
Raises cold brittleness, reduces low-temperature impact energy
Corrosion resistance
Helps form a dense protective rust layer in the early stage
Weldability
Moderate Si is acceptable; high Si increases weld cracking tendency
3. Phosphorus (P): ≤ 0.035%
Corrosion resistance
Strongly improves atmospheric corrosion resistance
Promotes formation of a compact, stable patina
Toughness
Extremely harmful
Segregates at grain boundaries, causes grain-boundary embrittlement
Significantly reduces low-temperature impact toughness
Strength
Slightly increases strength but causes severe brittleness
That's why Q345NH limits P much lower than old weathering steels like 09CuPCrNi-A.
4. Sulfur (S): ≤ 0.030%
Almost entirely harmful
Forms MnS inclusions that act as crack initiation sites
Reduces transverse toughness, ductility, and corrosion uniformity
Worsens surface quality and hot workability
Strictly controlled to very low levels.
5. Copper (Cu): 0.25–0.55%
Corrosion resistance
Core weathering element
Enriches in the inner rust layer, blocks oxygen and moisture penetration
Significantly improves atmospheric corrosion resistance
Strength & Toughness
Slightly increases strength without obvious harm to toughness
Hot workability
Excess Cu may cause hot shortness, so controlled within a narrow range
6. Chromium (Cr): 0.40–0.70%
Corrosion resistance
Strongly improves corrosion resistance
Stabilizes the dense α-FeOOH patina, especially in industrial & humid environments
Strength
Mild solid-solution strengthening
Toughness
Basically neutral in the standard range
7. Nickel (Ni): ≤ 0.65%
Toughness
Strongly improves low-temperature toughness
Lowers ductile-brittle transition temperature
Offsets the embrittling effect of P and S
Corrosion resistance
Enhances patina adhesion, especially in coastal or polluted air
Strength
Mild strengthening without brittleness
8. Niobium / Vanadium / Titanium (Nb, V, Ti) – trace amounts
Strength
Strongly increase strength via precipitation strengthening
Toughness
Greatly refine grains, improving both strength and toughness at the same time
Weldability
Help refine HAZ grains, reducing welding embrittlement
9. Aluminum (Al): ≥ 0.015%
Strong deoxidizer → purifies steel
Forms AlN to refine grains
Improves low-temperature toughness and surface quality
