Search Results

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.

New steels were developed to make cold forgeability and induction hardenability compatible with each other. These new grades contain. 0.05%Si, 0.25%Mn, 0.15%Cr and 15ppmB. The carbon contents are the same as conventional carbon steels, 0.48% and 0.53%. Ferrite strengthening elements, Si and Mn; were decreased to reduce the hardness of the spheroidize annealed materials and, as a result, the flow stress during cold forging. Boron was added to maintain induction hardenability. It was confirmed that new grades show cold forgeability superior to that of conventional carbon steels such as SAE 1049 and 1053, while maintaining the same case depth during induction hardening. These new grades have been applied to the production of cuter races for constant velocity joints, in which spheroidize annealed bars are cold forged in five stages followed by induction hardening. In this process, carburizing steels can be replaced by these grades, and an in-line process can be established.

The hardness of microalloyed medium carbon steels is affected significantly by chemical compositions and the conditions in hot forging. Authors investigated first the effect of chemical compositions on the strength of medium carbon steels and introduced “Hardness Equivalent Parameter”, Heq. By this parameter, it becomes possible to design the steels for automobile components with various strength. Then authors examined the effect of hot working conditions on the mechanical properties of the microalloyed steels and obtained the proper processing conditions for them. After the research programs authors carried out the running test of engine and suspension components built in a passenger car and certified that the microalloyed steels possess satisfactory properties as forged.