Effect of Tube Size and Heat Treatment on Formability of Steel Tube for Automobile

Abstract

Effects of the tube size (t/D) and heat treatment on formability of steel tubes for automobiles were studied by using steel tubes of the size 45∅×5.0mm and 75∅×1.6mm (the tensile strength range from 30 to 60Kg/mm²).
The main results obtained are as follows.
(1) Expanding and 90°C flattening formabilities are decreased by the increment of tube size (t/D). In the flare forming of materials having tensile strengths above 50kg/mm², singular behavior is recognized, in comparision with the other forming processes, in that an increase of t/D gives a superior formability.
(2) In flare forming, the circumference of a tube is not deformed into a uniform shape but the “earing” is remarkably increased in tensile strength materials. On the other hand, in expand forming, as the circumference of a tube is deformed into a uniform shape and the ratio of strain of a welded part to that in the circumference of a tube increases; this is considered to be a very severe forming method.
(3) The effect of heat treatments of formability of steel tubes becomes larger by increasing the tensile strength on S content. The effect of heat treatments is also varied by the method of forming. These changes of formability result from the variation of the microstructure, metal flow, and hardness at welded parts affected by the heat treatment.