Hole Expansion Characteristics of Advanced High Strength Steel (AHSS) Grades and Their Effects on Manufacturability in Automotive Industry 2022-28-0350

Currently, automotive industries are using Advanced High-Strength Steels (AHSS) sheet grades to achieve key requirements like light weighting and improved crash performance. But forming of AHSS grades becomes key challenge due to its lesser ductility and edge fracturing tendency during forming. In general, most of the automotive components undergoes shearing operations like blanking and punching which affects the edge ductility of the steel. AHSS grades possess limited edge ductility compared with conventional steel grades which results in edge fracturing due to tensile strain during stretch flanging operation.

Stretch flange-ability is an important formability characteristic, which aids in material selection to avoid edge fracturing of complex shaped parts. Material with better stretch flange-ability possess better edge ductility and hence perform better in stretch flanging of sheet metal. Various test methods have been developed to predict stretch flange-ability behavior of the steel grades. Among that, Hole Expansion test is a one of the effective method to predict the stretch flange-ability of sheet metal. The present work investigates the effect of microstructure and tensile properties on hole expansion characteristics of different AHSS with different process conditions. Steel grades with varying chemical composition, varying tensile strength and varying microstructure are evaluated: Dual phase steel grades (DP590, DP780, DP980), Transformation Induced Plasticity Steel (TRIP 780), Micro-alloyed High Strength Low alloy grades (E34, E46, SPFH590, S700MC). Hole Expansion Ratio (HER) measurement is done as per the ISO16630 standard. Results shows that the HER% decreases linearly with increase in tensile strength below 700MPa. Steel with tensile strength greater than 700-1000 MPa, results in HER of 20-30%. Also, the importance of HER in the AHSS grades selection for the manufacturability of complex parts is demonstrated.