Q1: What are the characteristics of the chemical composition of Q345D steel pipe?
Q345D belongs to low-alloy high-strength structural steel, and its carbon content is controlled below 0.18% to ensure good welding performance and toughness. The manganese content is relatively high (1.00%~1.60%), which helps to improve strength and hardenability. Phosphorus and sulfur, as harmful elements, are limited to less than 0.030% respectively to reduce cold brittleness and hot brittleness. In addition, Q345D usually adds trace alloying elements (such as vanadium, niobium, titanium, etc.) to refine the grains and improve low-temperature impact toughness. These composition controls enable it to maintain good mechanical properties in low-temperature environments.
Q2: What are the mechanical properties of Q345D steel pipe?
The yield strength of Q345D steel pipe is ≥345MPa, the tensile strength is between 470~630MPa, the elongation is ≥22%, and it has high plasticity and toughness. Its most notable feature is low-temperature impact toughness, requiring -20℃ impact energy ≥34J, and some standards even require -30℃ impact energy ≥27J. Compared with Q345B (only room temperature impact is required), Q345D is more suitable for use in cold areas. In addition, its yield strength ratio (yield strength/tensile strength) is ≤0.83, ensuring that the material has sufficient plastic deformation capacity when subjected to force.
Q3: What are the main differences between Q345D and Q345B?
The phosphorus and sulfur content of Q345D is lower than that of Q345B (P≤0.030%, S≤0.030%) to reduce low-temperature brittleness. Q345D is required to be delivered in normalized state, while Q345B can be delivered in hot-rolled state, so the structure of Q345D is more uniform. The low-temperature impact toughness of Q345D is strictly required (-20℃ impact ≥34J), while Q345B has no mandatory requirement for low-temperature impact. In addition, Q345D has a higher degree of microalloying, and elements such as niobium and vanadium are usually added to improve the overall performance. These differences make Q345D more suitable for low temperature and high stress environments.
Q4: What temperature range is Q345D steel pipe suitable for?
The design temperature range of Q345D steel pipe is -20℃~+350℃, and it can still maintain good toughness in low temperature environment. Below -20℃, it is necessary to ensure that its impact energy meets the standard requirements (such as -30℃ impact ≥27J). If used at high temperature (>350℃) for a long time, its strength will gradually decrease, and heat-resistant steel (such as 15CrMoG) should be used at this time. The low temperature performance of Q345D is worse than ordinary carbon steel (such as Q235B), but not as good as special low temperature steel (such as 09MnNiDR). Therefore, its applicability needs to be strictly evaluated when used in extremely cold areas (such as the Arctic and high altitudes).
Q5: What are the requirements for the delivery status of Q345D steel pipe?
Q345D steel pipes must be delivered in normalized (N) state to ensure uniformity of structure and low temperature toughness. Normalizing process is usually heated at 880~920℃ and then air-cooled, and the holding time is calculated according to the wall thickness (1.5min/mm). Thick-walled pipes (>40mm) need to extend the holding time to prevent uneven core structure. Compared with hot-rolled state, normalizing can significantly improve impact toughness and reduce residual stress. In addition, heat treatment records and mechanical properties test reports are required when leaving the factory.
