Reduction in Weight of Steel Wheels by Development of 780 MPa-Grade Hot Rolled Steel Sheets
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.