Q1: What are the key steps in the production process of Q390 steel pipe?
The production of Q390 steel pipe mainly includes steelmaking, continuous casting, rolling, heat treatment and testing. The LF+VD refining process is required in the steelmaking stage to strictly control the sulfur and phosphorus content (both ≤0.030%). Electromagnetic stirring is used during continuous casting to reduce segregation and improve the quality of steel billets. The controlled rolling and controlled cooling (TMCP) process is used in the hot rolling stage to optimize the microstructure and properties. Normalizing or quenching and tempering treatment is a mandatory requirement to improve strength and toughness. Finally, non-destructive testing (UT, MT, etc.) and mechanical properties testing are carried out to ensure that the product is qualified.
Q2: Why must Q390D/E steel pipe be normalized?
Normalizing treatment can eliminate residual stress after hot rolling and prevent low-temperature brittle fracture. A uniform ferrite + pearlite structure can be obtained by heating to the austenite zone (880~920℃) and air cooling. Normalizing refines the grains (grain size ≥ 8 levels) and significantly improves the impact toughness of the steel pipe. Compared with the hot-rolled state, the low-temperature impact energy of Q390D/E after normalizing is higher, meeting the use requirements of -20℃ or -40℃. In addition, normalizing can improve the processing performance and welding performance, and reduce the need for subsequent heat treatment.
Q3: What are the special requirements for the production of Q390 welded steel pipes?
Q390 welded steel pipes must adopt low-hydrogen welding processes (such as submerged arc welding SAW or high-frequency welding HFW) to reduce the risk of cold cracks. The welding material must match the performance of the parent material, and nickel-containing welding wire (such as H08Mn2NiA) is recommended to improve low-temperature toughness. After welding, local or overall normalizing treatment is required to eliminate welding residual stress. The weld must be tested by ultrasonic (UT) and radiographic (RT) to ensure that there are no defects. For low-temperature uses, a -20℃ or -40℃ impact test is also required to ensure that the toughness of the weld and heat-affected zone meets the standard.
Q4: How to ensure the quality of thick-walled Q390 steel pipes (>50mm)?
Thick-walled steel pipes need to strictly control the rolling process to ensure that the compression ratio is ≥3:1 to prevent the core from being loose. The holding time (2.4min/mm) needs to be extended during normalizing to ensure that the core structure is fully transformed. Ultrasonic testing (UT) needs to focus on checking core defects (such as inclusions and delamination). Mechanical property testing requires sampling at 1/4 of the wall thickness to ensure uniform overall performance. In addition, thick-walled pipes need to undergo a water pressure test before leaving the factory to verify the pressure bearing capacity.
Q5: What are the main production standards for Q390 steel pipes?
Domestic standards include GB/T 1591 (low alloy steel), GB/T 8162 (seamless pipes for structures), and GB/T 9711 L415M (pipeline pipes). International standards include EN 10210 (S355J2H) and ASTM A572 Gr.60 (additional impact requirements are required). Export products must comply with CE, PED and other certifications, and provide low-temperature impact reports. Different standards have slightly different requirements for chemical composition, mechanical properties, and testing, so the appropriate standard must be selected according to the intended use.
