1. Increases Brittle Tendency and Raises Brittle Transition Temperature (DBTT)
Carbon's role: As summarized in Summary 3 and Summary 4, an increase in carbon content enhances the steel's yield point and tensile strength but significantly reduces its plasticity (e.g., elongation) and impact toughness. At low temperatures, carbon promotes the formation of brittle phases (such as coarse carbides) and increases the tendency for "deformation twinning" (a key factor in brittle fracture, per Summary 4), making the steel more prone to sudden brittle failure under small stresses.
Quantitative constraint: To mitigate this risk, S355J0WP strictly limits carbon content to ≤0.12% (Summaries 1 and 6). This is far lower than the 0.2% upper limit for general low-temperature steel (Summary 4) and aligns with the global trend of using "low-carbon (<0.15%)" materials for low-temperature resistance (Summary 4). By controlling carbon, the steel's ductile-brittle transition temperature (DBTT) is reduced, ensuring it maintains toughness even at near-0°C (consistent with the "J0" grade requirement for impact resistance at 0°C, per Summary 6).
2. Weakens Low-Temperature Impact Toughness
Mechanism: High carbon content leads to the precipitation of fine carbide particles (e.g., Fe₃C) at grain boundaries. At low temperatures, these carbides act as stress concentration points, preventing plastic deformation of the matrix and causing cracks to initiate and propagate rapidly during impact loading (Summary 3).
Contrast with alloying elements: While S355J0WP contains nickel (Ni) and manganese (Mn) to improve low-temperature toughness (Ni enhances toughness at -100°C or lower, Mn refines grains to reduce brittleness, per Summaries 1 and 4), excessive carbon would offset these benefits. For example, even with 1.0–1.5% Mn (Summary 1), a carbon content exceeding 0.12% would still raise the DBTT and lower impact energy below the required threshold for the J0 grade.
3. Impairs Weldability, Indirectly Affecting Low-Temperature Joint Performance
Carbon equivalent (CET) control: As highlighted in Summary 2 (for S355J0, a material with similar low-alloy properties), controlling carbon content is critical to limiting the carbon equivalent (CET ≤0.40%), which avoids the formation of hard, brittle martensite in the heat-affected zone (HAZ) during welding. For S355J0WP, the ≤0.12% carbon limit ensures the CET remains low, preventing HAZ brittleness and ensuring the welded joint retains toughness at low temperatures (consistent with the requirement for "welded constructions" in Summary 6).
Avoiding cold brittleness: High carbon also increases the steel's "cold brittleness" (Summary 3)-a phenomenon where toughness drops sharply at low temperatures, especially in welded areas with residual stress. The low carbon design of S355J0WP minimizes this risk, ensuring the entire structure (including joints) performs stably in low-temperature environments.
4. Reduces Atmospheric Corrosion Resistance, Indirectly Compromising Low-Temperature Durability
Carbon's negative effect: As noted in Summary 3, high carbon content reduces the steel's atmospheric corrosion resistance-high-carbon steel rusts more easily in open yards. In low-temperature, high-humidity environments (e.g., cold coastal areas), rust layers can crack due to thermal expansion/contraction, exposing the matrix to further corrosion. This corrosion weakens the steel's cross-section and creates additional stress concentration points, accelerating low-temperature brittle fracture.
Synergy with alloying elements: The low carbon content (≤0.12%) allows alloying elements like copper (Cu) and chromium (Cr) to function effectively (Summary 1): Cu promotes the formation of a dense, self-protective rust layer, while Cr stabilizes the oxide film. This ensures the steel maintains both corrosion resistance and mechanical integrity in low-temperature, corrosive environments.
