1. Normalizing + Tempering (N+T): Enhance Low-Temperature Toughness
Core Purpose: Refine the normalized ferrite-pearlite microstructure further, eliminate tiny residual carbides, and reduce internal stress-ultimately lowering the ductile-brittle transition temperature (DBTT) by 10–15°C compared to single normalizing.
Process Parameters:
First, perform standard normalizing: Heat to 890–950°C, hold (per thickness), and air cool.
Then temper: Reheat to 550–620°C (below the Ac₁ transformation temperature, ~723°C, to avoid austenitization), hold for 1.5–2 hours (per 25mm thickness), and air cool.
Effect: Impact energy at -40°C can increase from ≥27J (single normalizing) to ≥40J; the microstructure becomes finer and more uniform, with reduced risk of brittle fracture.
Applicability: Ideal for thick Q355GNH plates (>30mm) or components like outdoor steel frames, bridge girders, and containers in cold environments.
2. Stress Relieving (SR): Mitigate Post-Welding/Cold-Forming Stress
Core Purpose: Reduce residual stress to ≤200MPa without altering the normalized ferrite-pearlite microstructure, ensuring shape stability and preserving the protective rust layer.
Process Parameters:
Heat to 550–650°C (strictly below Ac₁ to avoid grain growth), hold for 1–2 hours (per 25mm thickness), and cool slowly (≤50°C/hour below 500°C, then air cool).
Key Note: Stress relieving must be performed after normalizing (not instead of it). For example: Q355GNH plates are first normalized to meet strength/toughness requirements, then welded into a structure, and finally stress-relieved to eliminate welding stress.
Applicability: Welded components (e.g., steel towers, storage tanks) and cold-formed parts (e.g., bent profiles for architectural cladding).
3. Controlled Rolling + Accelerated Cooling (CR+ACC): Simplify Production for Thin Plates
Core Purpose: Replace off-line normalizing with on-line processing to save time and energy; the fine-grained microstructure from CR+ACC also improves both toughness and weathering resistance.
Process Parameters:
Controlled rolling: Heat the steel billet to 1100–1200°C, then roll in the austenite recrystallization zone (950–1050°C) and non-recrystallization zone (800–900°C) to refine austenite grains.
Accelerated cooling: After rolling, cool the plate to 550–650°C at a rate of 5–15°C/s (using water sprays), then air cool to room temperature.
Effect: Achieves a fine ferrite-pearlite microstructure (grain size ≥ASTM 8) with yield strength ≥355MPa and -40°C impact energy ≥34J-equivalent to or better than normalizing.
Applicability: Mass-produced thin Q355GNH products, such as roofing sheets, decorative panels, and small structural parts.
4. Surface Heat Treatment: Improve Local Wear/Corrosion Resistance
Induction Surface Hardening: Heat the surface to 850–900°C via induction coils (core remains cool), then water spray cool. Forms a hard martensitic surface layer (hardness ≥50 HRC) to resist wear, while the core retains ductility and weathering performance.
Surface Nitriding: Heat the component to 500–550°C in a nitrogen-rich atmosphere. Nitrogen diffuses into the surface to form hard nitrides (e.g., Fe₄N), improving surface hardness (≥60 HRC) and corrosion resistance (complementary to Q355GNH's inherent weathering ability).
Applicability: Components like weathering steel fasteners, agricultural machinery parts, and marine hardware.
Key Limitations: Methods to Avoid for Q355GNH
Quenching + Tempering (Q+T): Rapid quenching forms brittle martensite, and even tempering cannot restore the ferrite-pearlite microstructure needed for uniform rust layer formation. This method also reduces Cu/Cr segregation at the surface, weakening weathering resistance.
Annealing (Full Annealing): Heating to 800–850°C and slow cooling produces coarse ferrite-pearlite grains, lowering strength (yield strength may drop below 355MPa) and toughness.
