1. Thermal-Mechanical Processing: The Core Driver of Microstructure
a. Controlled Rolling & Cooling (Thermo-Mechanical Control Process, TMCP)
Mechanism: TMCP involves rolling the steel at a specific temperature range (typically 800–950°C, the austenite recrystallization zone) and controlling post-rolling cooling rate. This process refines austenite grains, which later transform into finer ferrite-pearlite grains during cooling.
Finer grains = better low-temperature toughness: Smaller ferrite grains increase the number of grain boundaries, which act as barriers to crack propagation during low-temperature impact loading. For example, reducing ferrite grain size from 50 μm to 10 μm can double the 0°C impact energy of S355J0WP (from the minimum 27 J to over 50 J).
Cooling rate control: Slow cooling (air cooling) avoids the formation of hard, brittle phases like martensite or bainite, which are prone to brittle fracture at low temperatures. Conversely, overly rapid cooling (e.g., water quenching) can induce martensite, raising the ductile-brittle transition temperature (DBTT) by 30–50°C.
b. Normalizing Heat Treatment
Application scenario: For thick S355J0WP plates (e.g., >20 mm), rolling alone may cause uneven grain growth in the core. Normalizing (heating to 900–950°C, holding to homogenize austenite, then air cooling) eliminates segregation, refines grains, and ensures uniform ferrite-pearlite distribution.
Impact on properties: Normalized S355J0WP exhibits 15–20% higher low-temperature impact toughness than non-normalized material, as it reduces "banded structures" (alternating ferrite and pearlite layers) that act as crack paths at low temperatures.
2. Internal Defects: Hidden Risks for Low-Temperature Brittleness
a. Non-Metallic Inclusions
Types and impacts:
Sulfide inclusions (e.g., MnS): Even with low sulfur content (≤0.015%), residual MnS inclusions (elongated along the rolling direction) create stress concentrations. At low temperatures, these inclusions separate from the matrix, initiating cracks that propagate rapidly.
Oxide inclusions (e.g., Al₂O₃): Hard, angular Al₂O₃ inclusions (from deoxidation) act as "micro-notches," reducing the steel's ability to absorb impact energy.
Mitigation: Using calcium treatment during smelting modifies MnS inclusions into spherical CaS-CaO complexes, which are less likely to initiate cracks. This can improve low-temperature impact toughness by 25–30%.
b. Porosity and Shrinkage Cavities
Formation: Porosity (small gas bubbles) or shrinkage cavities (from incomplete solidification) form during casting. These defects reduce the effective load-bearing area and concentrate stress-at low temperatures, they can grow into macroscopic cracks under even moderate stress.
Impact: A porosity volume fraction of >0.5% can lower the 0°C impact energy of S355J0WP by 40%, failing the "J0" grade requirement.
c. Residual Stresses
Origins: Residual stresses form during rolling (uneven cooling) or welding (thermal expansion/contraction). Tensile residual stresses (e.g., >200 MPa) in the surface or near-weld regions combine with low-temperature brittleness, accelerating crack initiation.
Example: S355J0WP plates with high residual tensile stress may exhibit brittle fracture at -10°C, even if their DBTT is theoretically 0°C. Stress relief annealing (heating to 550–600°C, holding, then slow cooling) can reduce residual stresses by 60–80%, restoring low-temperature toughness.
3. Material Thickness: A Critical Factor for Low-Temperature Performance
a. Microstructural Heterogeneity
Thick plates (e.g., >30 mm) cool more slowly in the core than the surface during rolling, leading to coarser grains in the core. Coarse grains have lower toughness: the 0°C impact energy of a 40 mm-thick S355J0WP plate may be 30–40% lower than a 10 mm-thick plate of the same composition.
b. Triaxial Stress State
Under impact loading, thick materials experience a triaxial stress state (tensile stress in three directions) near the impact site, whereas thin materials experience more uniform planar stress. Triaxial stress restricts plastic deformation (the main way to absorb impact energy) and promotes brittle fracture-even if the microstructure is refined.
Standard requirement: EN 10025-5 allows lower impact energy for thicker S355J0WP plates (e.g., 27 J for 16–40 mm, vs. 34 J for <16 mm) to account for this effect.
4. Service Environment: Accelerating Degradation of Low-Temperature Properties
a. Atmospheric Corrosion
Mechanism: S355J0WP relies on a dense, adherent rust layer (containing Cu, Cr oxides) for corrosion resistance. However, in cold, humid environments (e.g., coastal cold regions), repeated freeze-thaw cycles cause the rust layer to crack. Moisture penetrates the cracks, leading to pitting corrosion (localized metal loss).
Impact on properties: Pits act as sharp notches, concentrating stress. At low temperatures, these notches reduce the steel's fracture toughness (KIC) by 20–30%, making it prone to brittle failure under static or dynamic loads.
b. Hydrogen Absorption (Hydrogen Embrittlement)
Sources: Hydrogen can enter S355J0WP during welding (moisture in electrodes), pickling (acidic solutions), or service (humid air with H₂S). At low temperatures, hydrogen atoms diffuse to grain boundaries and form hydrogen molecules (H₂), creating high internal pressure.
Consequence: Hydrogen embrittlement reduces low-temperature impact toughness by 50–70% and can cause "delayed brittle fracture"-sudden failure under constant stress (e.g., structural loads) even at temperatures above the DBTT.
