Effect of local induction heating on secondary roll-forming of high strength square tubes

The formability of corners was improved by heating the corner area of high strength square tubes with induction coils.Under the premise of not impacting the undeformed zone,we obtained the ideal sectional dimension,microstructure and properties of products. In the hot roll forming process,the billet accumulated at the outside area of square corners without mold restrictions. With rising temperature,the accumulation became more serious. At the same time,microcracks deteriorated on the surface of the inner tube,while the cracks grew up in a form of arborization. A self-tempering process would be produced by residual temperature after hot roll forming,which could significantly reduce the residual stress. The residual stress sharply decreased as the temperature increased.When the heating temperature was above 650℃,the influence of self-tempering on the residual stress was greater than the extended capability of macrocracks and played a leading role; there was no crack when the distance was larger than 2/3 of diagonal length at flattening test.