Chemical composition
Carbon content: Q235NH has a controlled low carbon content (≤0.18%). Lower carbon content can reduce the brittleness of the steel, avoid the formation of excessive hard and brittle cementite, and thus improve elongation. If the carbon content exceeds the standard due to smelting errors, the ductility of the steel plate will decrease significantly, and the elongation at break will be lower than the specified value.
Alloy elements for corrosion resistance: The addition of Cu (0.25–0.55%), Cr (0.40–0.80%), and Ni (≤0.50%) in Q235NH is mainly to promote patina formation. These elements are added in moderate amounts and will not significantly reduce elongation; on the contrary, proper Ni addition can refine grains and slightly improve ductility. However, excessive addition of alloy elements will increase the hardness of the steel and reduce elongation.
Impurity elements: Sulfur (S ≤0.045%) and phosphorus (P ≤0.045%) are harmful impurities. Sulfur will form low-melting sulfide inclusions, which cause hot brittleness; phosphorus will increase the cold brittleness of the steel. Both will reduce the elongation at break of the steel plate, so their content must be strictly controlled during smelting.
Internal microstructure
The elongation at break of Q235NH is determined by its ferrite-pearlite dual-phase microstructure:
Ferrite is a soft and ductile phase, which is the main contributor to the elongation of the steel plate. A higher proportion of ferrite (generally 70–80% in Q235NH) can improve ductility.
Pearlite is a hard and brittle phase composed of ferrite and cementite lamellae. An appropriate amount of pearlite can ensure the strength of the steel, but if the proportion of pearlite is too high (exceeding 30%), the elongation of the steel plate will decrease.
Grain size also affects elongation: finer grains can increase the grain boundary area, improve the coordination of deformation between grains, and thus increase the elongation at break. Coarse grains caused by improper heat treatment will reduce ductility.
Processing technology
Rolling process: Q235NH steel plates are usually produced by hot rolling. Reasonable rolling parameters (such as controlled rolling temperature and reduction ratio) can refine grains and homogenize the microstructure. For example, rolling in the austenite recrystallization zone can break coarse austenite grains, and subsequent cooling can form fine ferrite-pearlite, which is beneficial to improving elongation. If the rolling temperature is too high or the reduction ratio is insufficient, coarse grains will be formed, leading to lower elongation.
Hot-forming and post-forming heat treatment: If Q235NH steel plates undergo hot-forming within the optimal temperature range (900–1100°C) and adopt air cooling, the microstructure will remain refined, and the elongation will not be significantly affected. However, overheating (temperature >1100°C) will cause grain coarsening, reducing elongation; rapid cooling after forming will introduce residual stress and even form martensite, which will sharply reduce ductility. Normalizing treatment after overheating can refine grains and restore the original elongation level.
Test conditions
Test temperature: The elongation at break of Q235NH decreases with the decrease of test temperature. At room temperature (20°C), its elongation is generally ≥22%. In low-temperature environments (e.g., below -20°C), the steel will show cold brittleness, and the elongation will decrease significantly, which is why Q235NH is not recommended for long-term use in extremely cold areas.
Sample size and direction: According to national standards, the elongation test of steel plates usually uses standard tensile samples. The elongation value of samples taken along the rolling direction is slightly higher than that taken along the transverse direction, because the rolling process will make the grains and inclusions elongated along the rolling direction, showing anisotropy.
