Effect of hot forming temperature on the microstructure and mechanical properties of high strength square tubes

To overcome the challenge of traditional roll forming process and equipment on high strength square tubes at room temperature,hot roll forming process using local induction heating was proposed to produce high strength square tubes.A series of tests including tensile testing,fracture morphology,scanning electron microscopy(SEM) and X-ray diffraction were performed to investigate the effect of hot forming temperature on the microstructure and mechanical properties of high strength square tubes.Experimental results show that as the temperature rises,the mechanical properties of the corners improve significantly.The fracture morphology gradually changes from cleavage fracture at room temperature to ductile fracture.The microstructures at the corners are developed from lath bainite to granular bainite and the polygonal ferrite begins to grow.Moreover,the circumferential and longitudinal residual stresses on the outside surface of the square tubes are reduced and reasonably redistributed.Based on the analyses of experimental results,the optimum forming temperature for high strength square tubes is 650 ℃.