Development of high strength steels with high press formability and fatigue property

High strength steels with good press formability and high fatigue strength were developed by NSSMC. They are hot-rolled steels of tensile strength grade of 590-780 MPa. They have high elongation and fatigue property as DP steels, and have higher hole expansion ratio than the DP steels and conventional HSLA steels. In this study, press formed automotive parts made from the developed steels were compared with those by using the DP steels and the conventional HSLA steels. To compare the press formability, the steels were pressed into lower-arms by bend flange with a pad method. Height of flange was changed to control the effort required for the press forming. In the case of flange height of 14.5mm, the developed steels were able to be formed without cracks. However, there were some cracks when the DP steels and the conventional HSLA steels were used. Since the cracks were initiated at the region where stretch-flangeability was needed, the developed steels are thought to be applicable to the parts which require high stretch-flangeability. To compare the fatigue strength, plane bending fatigue tests and coaxial low cycle fatigue tests were conducted. The plane bending fatigue tests showed that fatigue strength of the DP steels and the developed steels were superior to those of the conventional HSLA steels between 10⁴ to 10⁷ cycles. Low cycle fatigue tests indicated that although the conventional HSLA steels exhibited fatigue softening, the DP steels and the developed steels showed fatigue hardening.