Effects of Edge Heating and Strain Gradient on Stretch Flange Deformation Limit of Steel Sheet

Abstract

To improve the stretch flange deformation limit of ultrahigh-strength steels, a partial edge heating method was investigated. The effect of the strain gradient on the stretch flange deformation limit when a partial edge heating method was applied was investigated by the hole expansion test and FEM analysis. The stretch flange formability was improved by application of a partial edge heating method because of the recovery of the shearing microstructure and the reduction of the hardness difference of microstructure. It was found that the improvement of the deformation limit strain for the strain gradient with edge heating depends on the strain gradient region. In the low strain gradient region, the improvement margin of the deformation limit strain with edge heating was small. On the other hand, in the high strain gradient region, the deformation limit strain was markedly improved compared with that without heating. It was confirmed that the strain gradient in the direction perpendicular to the maximum principal strain could be used for the prediction of the deformation limit and also the determination of the stretch flange fracture by the FEM analysis of actual parts even under edge heating conditions.

Comparative Investigation of the Stretch-flange Cracking Mechanism of Ultrahigh-strength Steels with Different Microstructures and Hole-Expansion Ratios

“…The hole-expansion limit λ obtained from the hole-expansion tests in accordance with International Organization for Standardization (ISO) standards is generally used as an indicator of the stretch-angeability of materials. According to previous reports, the in uencing factors for the decrease in λ related to machining techniques were the signi cant work-hardening of the sheared edge surface [3][4][5][6][7][8][9] and the high roughness of the edge surface [7,[9][10][11]. The in uencing factors for the decrease in λ related to microstructures were the high heterogeneity due to the hardness difference between soft and hard phases in the microstructure [3,4,[12][13][14][15][16][17][18], the low fracture toughness, which represents the resistance to crack formation and propagation [19][20][21][22], and the low local ductility [9,23].…”

Effect of Microstructural Transformation upon Edge Heat Treatment on Stretch Flangeability of Ultrahigh-Strength Steel with Sheared Edge

“…This is consistent with the result reported by Tobita et al, which shows that the partial heating by laser on the sheared edge causes microstructural variations in thickness direction and the microstructural distribution affects . 39) Ihara et al's 40) investigation of the sheared edge surface of an ultrahigh-tensile-strength DP steel sheet by the electrical heating method revealed that more than 120% of the value of was obtained even by electrical heating for 1 s with the heating range of 1.0 mm. Therefore, similarly in laser heating, if sufficient tempering is provided by increasing the heating time, it is conjectured to be possible to obtain the improvement of to the same level as whole heating.…”