1. Alloying Elements That Enhance Impact Toughness
(1) Nickel (Ni): The Most Effective Low-Temperature Toughness Enhancer
Mechanism: Ni lowers the ductile-brittle transition temperature (DBTT) of Q355GNH. It stabilizes the austenite phase at low temperatures, delays the transformation of austenite to brittle martensite, and promotes the formation of fine, uniform ferrite-pearlite microstructures.
Impact: Q355GNH typically contains 0.20–0.50% Ni. A 0.30% Ni content can shift the DBTT from -20°C (without Ni) to -40°C, significantly improving impact energy at -40°C (e.g., from <27J to ≥34J, meeting GB/T 4171 requirements).
Note: Excessive Ni (>0.60%) is unnecessary, as it marginally improves toughness but increases material cost.
(2) Manganese (Mn): Balances Strength and Toughness
Mechanism: Mn dissolves in ferrite to refine grain size (via inhibiting grain growth during heating) and enhances the uniformity of the ferrite-pearlite structure. It also offsets the brittling effect of sulfur (S) by forming MnS inclusions (which are less harmful than FeS).
Impact: Q355GNH requires 0.45–1.60% Mn. A moderate Mn content (1.0–1.4%) ensures yield strength ≥355MPa while maintaining good toughness; too low Mn (<0.60%) leads to coarse grains and reduced toughness, while too high Mn (>1.60%) may form hard bainite, increasing brittleness.
(3) Copper (Cu) and Chromium (Cr): Synergistic Protection with Controlled Toughness Impact
Mechanism: Cu (0.20–0.60%) and Cr (0.30–0.80%) are primary weathering elements for Q355GNH, but they also indirectly support toughness. They promote the formation of a dense, adherent rust layer (α-FeOOH) that prevents corrosion-induced microcracks (which degrade toughness).
Impact: When kept within standard ranges, Cu and Cr do not harm toughness. However, excessive Cr (>0.80%) may form hard Cr-rich carbides (e.g., Cr₇C₃) at grain boundaries, increasing brittleness; excessive Cu (>0.60%) can cause "hot shortness" (cracking during processing) and reduce ductility.
2. Impurities That Degrade Impact Toughness
(1) Phosphorus (P): A Major Brittleness Promoter
Mechanism: P segregates strongly at ferrite grain boundaries, weakening intergranular bonding. It increases the DBTT sharply and reduces impact energy at low temperatures (e.g., 0.03% P can lower -40°C impact energy from 40J to 20J).
Control Requirement: GB/T 4171 mandates P ≤0.035% for Q355GNH. For applications in extremely cold climates (e.g., -40°C service), P is often controlled to ≤0.025% to ensure toughness.
(2) Sulfur (S): Forms Harmful Inclusions
Mechanism: S reacts with Fe to form FeS, a low-melting-point inclusion that accumulates at grain boundaries. FeS causes "cold brittleness"-it cracks easily under impact loads, especially at low temperatures.
Control Requirement: S must be ≤0.035% (GB/T 4171). In practice, S is often controlled to ≤0.020% by adding Mn (to form MnS, which is more ductile and less harmful to toughness).
(3) Carbon (C): Balanced Strictly to Avoid Brittleness
Mechanism: C strengthens steel by forming carbides but reduces toughness by increasing pearlite content (pearlite is harder and less ductile than ferrite). Excessive C promotes the formation of brittle martensite during cooling.
Control Requirement: C ≤0.19% for Q355GNH. A low C content (0.12–0.16%) ensures a ferrite-rich microstructure (≥60% ferrite), maintaining high impact toughness; C >0.19% increases pearlite content and lowers toughness.
3. Trace Elements That Fine-Tune Toughness
Aluminum (Al): Added as a deoxidizer (total Al ≥0.020%), Al forms AlN particles that pin grain boundaries, preventing grain coarsening during heat treatment. Fine grains significantly improve low-temperature toughness.
Niobium (Nb) or Titanium (Ti): Optional additions (Nb: 0.015–0.060%; Ti: 0.02–0.10%), they form carbides/nitrides that refine grains and strengthen the matrix without reducing toughness-ideal for thick Q355GNH plates (e.g., >50mm) where grain coarsening is a risk.
