1. Chemical Composition
: 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℃ than that with 1.10% Mn.
: 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.
: Copper (Cu) and chromium (Cr) (added for corrosion resistance) also slightly improve toughness by refining microstructure.
2. Microstructure (Heat Treatment-Dependent)
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.
: Refines grains to 5–15 μm, creating uniform ferrite-pearlite. Q355NHD in normalized state reaches 45–55 J at -20℃.
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℃ (below the 27 J standard).
3. Internal Defects
: Non-metallic particles (e.g., MnS, Al₂O₃) weaken the matrix. Large inclusions (>=50 μm) can lower impact energy by 30–40%.
: Small voids or shrinkage gaps expand under impact, cutting toughness by 15–20%.
: Uneven element distribution (e.g., P at grain boundaries) creates brittle zones, reducing low-temperature toughness by 25–30%.
