Abstract
Keywords - Formability, high strength steel, dual phase steel, mild steel, testing
Abstract - The interest for high strength dual phase steel in the automotive industry has increased significantly in recent years. Although they have been produced commercially for over 20 years there still seems to be a large potential for high strength dual phase steels, mainly motivated by weight savings on the vehicle and increased focus on passenger safety. High strength steel offers a possibility to reduce weight and increase passenger safety at a price often equal or lower than that of conventional material solutions.
The term dual phase steel refers to steels with a microstructure of ferrite and martensite. Simply spoken the ferrite, which is a soft phase, gives a low yield point and the martensite, which is hard, gives the strength. The strain hardening of dual phase steels is much higher than for HSLA steels, which is beneficial both in terms of formability and energy absorbtion capacity.
The formability of high strength dual phase steels is in general inferior to mild steel. But the concept of formability is very complex and it is perhaps better to say that the formability is different instead of inferior, and to point out the differences.
Today, formability is still described by a number of comparative tests, such as the Erichsen test and the limiting dome height test. A more recent formability measure is the forming limit diagram. In this paper a series of formability tests has been used on high strength dual phase steels ranging in tensile strength from 600 MPa to 1000 MPa. The same tests have been performed on the deep drawing steel DC04 and an ultra high strength martensitic steel with tensile strength of 1400 MPa. The tests include limiting dome height, limiting drawing ratio, minimum bending radius, hole expansion capacity and forming limit diagram. To make a comparison possible the same thickness gauge has been used for all materials.
The results show that the high strength dual phase steels have inferior formability when compared to the deep drawing steel, but that the formability still is significant when the high strength levels of the steels are considered. The results clearly show that high strength dual phase steels indeed are formable and thus very suitable for weight savings and applications related to passenger safety.