SOFTENING IN WELD HEAT AFFECTED ZONE OF DUAL PHASE STEEL SHEET FOR AUTOMOTIVE WHEEL RIM

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.