Investigation on corner thickening mechanism of Q460NH square tube using "Ω" new pass design in the hot roll forming

Large-size thick-walled square tubes offer high strength and stability for large-scale construction projects. Traditional cold roll forming often induces corner thinning and microcrack, affecting local mechanical properties. This study developed a new "Ω" hot roll forming model and experiment platform investigating corner thickening, outer corner radius in square tubes and the microstructure evolution on forming quality. Plates with residual heat from hot rolling were directly fed into the "Ω" hot roll forming machine innovatively, developing a short-process progressive hot roll forming technique. The results demonstrate excellent agreement between numerical simulations and experiment data. The 850°C "Ω" hot roll process achieved a 1.7% increase in corner thickness ratio and a 5.2% reduction in outer corner radius compared to traditional cold forming. Microstructure analysis show greater grain refinement at the outer corner edge than the inner edge, KAM analysis confirm dislocation accumulation at grain boundaries, indicating enhanced plastic deformation. Vickers hardness measurements reveal higher values at the outer edge, reaching a maximum at 850°C. This strengthening effect is primarily attributed to the grain refinement and increased dislocation density. The "Ω" hot roll forming not only reduces forming passes but also enhances the mechanical properties of the corner area. This study provides important theoretical and practical support for the hot roll forming of Q460NH weathering steel square tubes.