Development of AHSS with Improved Hole Expansion Properties

Abstract

The Hole Expansion test is an important method to study the edge stretch-flangeability of sheet steel for automotive applications. In the HE test, a hole is first made in steel sheet, and it expands under a punch until cracks appear at the hole edge. The ratio of the increased diameter to the initial diameter is called ‘hole expansion ratio (HER)’. The higher hole expansion ratio the better stretch-flangeability. The majority of AHSS (Advanced High Strength Steel) such as DP or TRIP steels show poor stretch-flangeability, i.e. they have a low HER value. To understand the reason for this phenomenon, DP and TRIP 780 grade steels were investigated in the present work. To overcome the poor hole expandability, the micro-alloying concept was evaluated. Various factors were found to determine the hole expandability: The sheared edge condition resulting from different blanking methods, the influence of the micro-alloying elements and the microstructure.