Are there any other factors that affect the impact toughness of Q355NH besides temperature?

1. Chemical Composition

Key elements determine the steel's intrinsic toughness by regulating grain size and brittle phase formation:

Beneficial elements: Manganese (Mn, 1.00–1.60%) refines grains and boosts ductility; microalloying elements like niobium (Nb) form fine carbides to block cracks. Steel with 1.50% Mn can have 25–30% higher impact energy at -20°C than that with 1.10% Mn.

Harmful elements: Excess carbon (C > 0.18%) forms brittle carbides; sulfur (S) and phosphorus (P) (each ≤0.035% per standard) create inclusions or weaken grain boundaries. S/P above limits can cut toughness by 40–50% at low temperatures.

Weathering elements: Copper (Cu) and chromium (Cr) (added for corrosion resistance) also slightly improve toughness by refining microstructure.

2. Microstructure (Heat Treatment-Dependent)

Heat treatment defines the steel's internal structure, which is the direct driver of toughness:

TMCP (Thermo-Mechanical Control Processing): Produces ultra-fine ferrite-bainite (grain size <5 μm), offering the highest toughness. Q355NHE in TMCP state maintains 30–35 J at -40°C.

Normalized (N): Refines grains to 5–15 μm, creating uniform ferrite-pearlite. Q355NHD in normalized state reaches 45–55 J at -20°C.

Hot-Rolled (AR): Coarse grains (20–50 μm) and uneven phases lead to low toughness-Q355NHD in AR state may only hit 22–25 J at -20°C (below the 27 J standard).

3. Internal Defects

Pre-existing flaws act as crack starters, drastically reducing toughness:

Inclusions: Non-metallic particles (e.g., MnS, Al₂O₃) weaken the matrix. Large inclusions (≥50 μm) can lower impact energy by 30–40%.

Porosities/Cavities: Small voids or shrinkage gaps expand under impact, cutting toughness by 15–20%.

Segregation: Uneven element distribution (e.g., P at grain boundaries) creates brittle zones, reducing low-temperature toughness by 25–30%.

In short, optimizing chemical composition, choosing proper heat treatment, and minimizing defects are critical to ensuring Q355NH's impact toughness meets application needs-alongside accounting for temperature effects.