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During the past few years, an application of high strength steels to wheels of passenger cars has been developed. Two big problems associated with the materials were noted with attempts to apply the high strength steels to wheels. One relates to properties required for wheel use and other relates to consistency of the properties. Reserch on the properties required for wheel use revealed that good formability is required for disc use materials and good formability both at base metal and flash-butt welded portion is required for rim use materials. It was found that good consistency of the mechanical properties over the entire portion of a coil and among coils is very important to reduce scrap ratio in wheel manufacturing process. Based on the results, high strength steels for wheel use were developed. This paper describes the properties and manufacturing process of the steels.

Two new types of high-strength 780 MPa-grade hot rolled steel sheets were developed for use in wheel discs and rims, which are the structural parts of the wheel. The fundamental composition of the disc material is 0.08%C-1.5%Si-1.75%Mn-0.1%Ti. The basic material is a dual phase, ferrite-martensite steel, with precipitation strengthening of the soft ferrite phase by TiC. The aim of hardening the ferrite phase is to suppress the difference in hardness between the ferrite and martensite phases, and thus secure good stretch flangeability. The fundamental composition of the rim material is 0.09%C-1.7%Mn-0.05%Nb-0.15%Ti. This steel is based on a ferrite-bainite material and is precipitation strengthened using NbC and TiC. Although 590 MPa-grade steel had been considered to represent the limit of formability for wheels, a 30% increase in strength was achieved in the newly developed materials, while maintaining good formability.