Q1: What are the key requirements for the chemical composition of Q390 steel pipe?
The carbon content of Q390 steel pipe is controlled below 0.20% to ensure good welding performance and toughness. The manganese content is relatively high (1.00%~1.60%), and micro-alloy elements such as niobium (Nb), vanadium (V) or titanium (Ti) are added to improve strength and refine grains. Phosphorus and sulfur, as harmful elements, are limited to less than 0.035% (Q390B) or less than 0.030% (Q390D) to reduce cold brittleness and hot brittleness. Some high-grade Q390 (such as Q390E) also require the nitrogen (N) content to be controlled ≤0.015% to improve low-temperature toughness. These composition optimizations give Q390 steel pipe excellent comprehensive mechanical properties.
Q2: What are the mechanical properties of Q390 steel pipe?
The yield strength of Q390 steel pipe is ≥390MPa, the tensile strength is between 490~650MPa, and the elongation is ≥20%. It has high strength and plasticity. Its impact toughness requirements vary according to the grade: Q390B requires room temperature impact energy ≥34J, Q390D requires -20℃ impact energy ≥34J, and Q390E requires -40℃ impact energy ≥27J. The yield strength ratio (yield strength/tensile strength) of Q390 steel pipe is usually ≤0.85, ensuring that the material has sufficient plastic deformation capacity when subjected to stress. In addition, its cold bending performance is good and can meet the processing needs of complex structures.
Q3: What are the main differences between Q390 and Q345 steel pipes?
The yield strength of Q390 (≥390MPa) is significantly higher than that of Q345 (≥345MPa), and is suitable for structures with higher loads. Q390 has a higher alloy content (such as Nb, V, etc.), while Q345 usually only contains manganese (Mn) as the main alloying element. Q390 has more stringent impact toughness requirements, especially Q390D/E has better low-temperature performance than Q345D/E. In terms of price, Q390 is 10%~15% higher than Q345 because of its higher alloy cost and process requirements. In terms of application, Q390 is mostly used in heavy machinery, bridges and large-span buildings, etc., which require higher strength.
Q4: What temperature range is Q390 steel pipe suitable for?
Q390B is suitable for normal temperature environment, Q390D can be used in low temperature environment of -20℃, and Q390E is suitable for extremely cold conditions of -40℃. In high temperature environment (>350℃), the strength of Q390 will gradually decrease, and heat-resistant steel (such as 15CrMoG) should be used at this time. The low temperature performance of Q390D/E is worse than that of ordinary carbon steel (such as Q235B), but not as good as that of special low temperature steel (such as 09MnNiDR). Therefore, when used in extremely cold areas, it is necessary to strictly evaluate whether its impact toughness meets the design requirements.
Q5: What are the delivery states of Q390 steel pipes?
Q390 steel pipes are usually delivered in hot rolled (HR) or normalized (N) state, depending on the grade and purpose. Q390B can be delivered in hot rolled state, while Q390D/E usually requires normalizing treatment to ensure low temperature toughness. The normalizing temperature is generally 880~920℃, and the holding time is calculated according to the wall thickness (1.5min/mm). For thick wall pipes (>40mm), quenching and tempering (quenching + tempering) process may be required to improve performance. Heat treatment records and mechanical properties test reports are required when leaving the factory.
