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Vibration damping steel sheets (VDSS), which have sandwich structures with intermediate layers of resin, have been studied. The most important characteristics of VDSS for inner panels of automotive vehicles are the vibration damping properties, press formability and spot weldability. Vibration damping properties, which are quantified by loss factor,η, were influenced by both tanδ, which indicates damping capacity of resins, and elastic modulus of core resin. From a view point of vibration damping properties, resins with larger tanδ and relatively lower elastic modulus were favorable. Because these mechanical characteristics vary considerably with temperature, it is important to select the most suitable resin for the service temperature range. The relationship between noise reduction effect and loss factor of VDSS were also studied. It was experimentally confirmed that noise reduction effect of VDSS is proportional to the logarithm of their loss factor.

Flash butt welding process has been widely used in the fabrication of wheel-rims in the automobile industry because of its high production efficiency. On the other hand, a DC butt welding process has been recently developed and put into practice for wheel-rim production instead of the flash butt welding process. The reasons seem to be higher production efficiency, energy savings and clean environment (no flash). There are many reports1)∼3) on flash butt welding, but there are little, if any reports on DC butt welding. In this report, the DC butt welding process was studied from the view point of comparison with the flash butt welding process. The weld defect formed in flash butt welding is mainly oxide inclusion (Si-Mn-Al oxide) at weld interface. On the other hand, in DC butt welding the cold weld and overheat cracking due to heat input are main weeld defects.

Dual phase steel offers potential for wheel weight reduction, but has not been completely succeeded, in part, because of a localized necking or a fracture during rim fabrication. This failure should be related to a softening in the weld heat affected zone (HAZ). This paper investigates mechanical properties through microstructural change produced in the HAZ of as-rolled dual phase steel and finally suggests a practical solution. The softening, caused by heating the steel up to Ac1 in the HAZ, is found to be affected not only by tempering of martensite, but also by the variation of hardness in ferrite matrix. Moreover strain aging as well as work hardening due to up-setting stage during the flash welding also concerns this problem. Coiling temperature in manufacturing as-rolled dual phase steel has a large effect on the conditions of ferrite and martensite. Control of the temperature is of great importance to avoid such a localized necking or a fracture in the HAZ.