Abstract
Recently, strong consideration has been given to the use of steel tubes for automotive components because of the associated weight saving, which eventually leads to a reduction of carbon dioxide emissions. The application of steel tube instead of steel bar is expected to greatly reduce weight while maintaining component stiffness. However, the number of application examples of steel tube as an alternative to steel bar is relatively small due to the following problems associated with the conventional tube-forming process: 1) High-frequency electric resistance welded (HFW) tube with a very small diameter and heavy wall thickness is not available in the conventional tube forming process; 2) Seamless tube has lower dimensional accuracy compared to the HFW tube, which sometimes leads to product quality problems; 3) Seamless tube has a larger surface decarburizing depth compared to the HFW tube, which sometimes causes insufficient hardness after quench hardening.
To solve these problems, a new type of HFW tube has been developed utilizing rapid induction heating of the tube followed by hot stretch reducing. The primary features of the modified HFW tube called hot stretch reduced (HSR) tube are described below. 1) Tube with thickness/diameter ratio of up to 25% is available by using the hot stretch reducing process. 2) Relatively high dimension accuracy can be obtained by using a 4-roll type stretch-reducing mill. 3) The surface decarburizing depth is smaller compared to the seamless tube because of rapid tube induction heating, resulting in uniform quenching hardness throughout the thickness. 4) High r-value and ductility can be obtained by crystal texture control in the hot stretch reducing process.
The recently developed HSR tubes have been applied to various automotive parts, such as a stabilizer, drive shaft, and seat frame. The stabilizer is the component that takes the greatest advantage of the features of this application because it has a small diameter and is bent in a small bending radius. The synergetic effect of even thickness and smaller decarburization layer thickness means the HISTORY® tube has good fatigue properties for use as a hollow drive shaft.
In addition, the HSR tube has excellent electromagnetic properties developed by crystal texture control through the hot stretch reducing process. This ultra-low carbon HSR steel tube is suitable for electromagnetic parts, such as shielding parts and actuators. The applicability of these newly developed hot stretch reduced steel tubes was confirmed by evaluation tests performed by customers including weld seam reliability.
KEYWORDS – hot stretch reducing, electric resistance welded tube, stabilizer, drive shaft, electromagnetic property