초고장력 강판의 2단 성형 스프링백 해석에 관한 연구

Abstract

In this thesis, two different double-stage forming techniques were proposed to reduce the amount of springback of a U-shaped channel made of advanced high strength steel (AHSS) sheets. Both techniques were based on the U-bending process but the first consisted of a double punch stroke, while the second made use of a modified tooling, namely, a punch head with a shallow groove and a bottom die plate with a bulgy shape. Two 1.2 mm thick sheet samples, a dual-phase (DP) steel and a twinning-induced plasticity (TWIP) steel, both with an ultimate tensile strength of 980 MPa were investigated. The performance of the two techniques in reducing the amount of springback was compared with that of the conventional U-draw bending process. The two processing techniques were simulated using a finite element (FE) analysis in which plastic anisotropy, described by different yield conditions and the degradation of the elastic modulus with increasing accumulated strains were taken into account. In addition, anisotropic hardening was considered to capture the Bauschinger effect and the associated hardening transients during strain-path changes. In particular, two different hardening types were investigated in details, namely, isotropic-kinematic (Yoshida–Uemori) and distortional plasticity (HAH) models. The stress-strain behaviors of the two materials were characterized using uniaxial tension, in-plane balanced biaxial tension and compression-tension tests to assess the basic mechanical properties of the two sheet samples and determine the constitutive coefficients needed for the FE modeling. For the two forming techniques, the experimental and FE simulated results were compared and analyzed in order to understand the influence of anisotropic hardening on springback under non-proportional loading and to interpret the mechanisms of springback reduction.