Improvement in Stretch Flange Ability of High-Tensile-Strength Steel Sheets by Piercing under Tension Using Humped Bottom Punch

Abstract

To meet the requirements for the weight reduction of automobiles, high-tensile-strength steel sheets have been applied extensively to the manufacture of auto parts. But their relatively lower press formability has obstructed their further application, and it is strongly demanded that the stretch flange ability of such steel sheets be increased, and cracks that occur when stretching edges during the press forming of such steel sheets be suppressed. Thus the authors have carried out a study to improve the stretch flange ability of high-tensile-strength steel sheets by optimizing the punching method and obtained the following conclusions. Work hardening on a pierced edge is mainly caused by deformation by the punch motion while shearing the material. By attaching a hump of certain shape at the bottom of the punch, the ductile fracture in the material caused by the punch corner is accelerated by the hump's effect to increase stress triaxiality, which reduces the extent of total deformation through shearing, thereby improving the stretch flange ability of the material.

Effect of Composition and Thermal Treatments on Mechanical Properties and Applications of Quenching and Partitioning Steels

“…Edge formability is another mechanical characteristic that is important for automotive components. Several studies have demonstrated a correlation between this aspect and the material fracture toughness [35][36][37][38][39][40][41]. Takahashi et al [35] found that the edge flangeability of hot rolled high strength steels is linearly correlated with their fracture toughness Edge formability is another mechanical characteristic that is important for automotive components.…”

“…Several studies have demonstrated a correlation between this aspect and the material fracture toughness [35][36][37][38][39][40][41]. Takahashi et al [35] found that the edge flangeability of hot rolled high strength steels is linearly correlated with their fracture toughness Edge formability is another mechanical characteristic that is important for automotive components. Several studies have demonstrated a correlation between this aspect and the material fracture toughness [35][36][37][38][39][40][41].…”

“…Takahashi et al [35] found that the edge flangeability of hot rolled high strength steels is linearly correlated with their fracture toughness Edge formability is another mechanical characteristic that is important for automotive components. Several studies have demonstrated a correlation between this aspect and the material fracture toughness [35][36][37][38][39][40][41]. Takahashi et al [35] found that the edge flangeability of hot rolled high strength steels is linearly correlated with their fracture toughness (We), as measured by the hole expansion ratio (HER), according to the ISO 16630 standard.…”

“…Several studies have demonstrated a correlation between this aspect and the material fracture toughness [35][36][37][38][39][40][41]. Takahashi et al [35] found that the edge flangeability of hot rolled high strength steels is linearly correlated with their fracture toughness (We), as measured by the hole expansion ratio (HER), according to the ISO 16630 standard. Fontein et al [40] observed that the HER of a cold rolled steel with different bainite contents increased with increasing crack propagation resistance, which was measured using fracture mechanics.…”

Fundamental Study on Forming Method of Ultrahigh-Strength Steel Sheet to T Model Shape —Development of Forming Method of Ultrahigh-Strength Steel Sheet to L and T Shapes—

“…3,4) Methods for reducing springback by changing product shapes and forming methods have also been developed. 5) To resolve forming failures such as cracks and wrinkles, new methods have been developed, such as a shearing technique to increase the elongation limit of the sheet edge and thus counter the cracking at the ends of the steel material (edge cracking), 6,7) and forming methods for avoiding strain concentrations. 8) However, few countermeasures are effective against cracking and wrinkling when forming a complex shape, where the cross-sectional line changes significantly from the forming.…”