This paper is concerned with modeling of a ductile fracture criterion for sheet metal considering anisotropy to predict the sudden fracture of advanced high strength steel (AHSS) sheets during complicated forming processes. The Lou−Huh ductile fracture criterion is modified using the Hill’s 48 anisotropic plastic potential instead of the von Mises isotropic plastic potential to take account of the influence of anisotropy on the equivalent plastic strain at the onset of fracture. To determine the coefficients of the model proposed, a two dimensional digital image correlation (2D-DIC) method is utilized to measure the strain histories on the surface of three different types of specimens during deformation. For the derivation of an anisotropic ductile fracture model, principal stresses (𝜎₁,𝜎₂, 𝜎₃) are expressed in terms of the stress triaxiality, the Lode parameter, and the equivalent stress (𝜂_𝐻, 𝐿,) based on the Hill’s 48 anisotropic plastic potential. The proposed anisotropic ductile fracture criterion was quantitatively evaluated according to various directions of the maximum principal stress. Fracture forming limit diagrams were also constructed to evaluate the forming limit in sheet metal forming of AHSS sheets over a wide range of loading conditions.

The current study was conducted to elucidate the effect of cementite morphology and matrix-ferrite microstructure on sliding wear behavior in spheroidized high carbon (1wt. % C) steel. The high carbon steel was initially heat treated to obtain a full pearlite or a martensite microstructure before the spheroidization. The spheroidizing heat treatment was performed on the full pearlitic steel for 100 hours at 700℃ and tempering was performed on the martensitic steel for 3 hours at 650℃. A spheroidized cementite phase in a ferrite matrix was obtained for both the full pearlite and the martensite microstructures. Sliding wear tests were conducted using a pin-on-disk wear tester with the heat treated steel as the disk specimen. An alumina(Al₂O₃) ball was used as the pin counterpart during the test. After the spheroidizing heat treatment and the tempering, both pearlite and martensite exhibited similar microstructures of spheroidized cementite in a ferrite matrix. The spheroidized pearlite specimens had lower hardness than the tempered martensite; however, the wear resistance of the spheroidized pearlite was superior to that of the tempered martensite.

The aim of the current study was to develop an advanced prediction model for the slab width spread during hot rough rolling. Rough rolling consists of both vertical rolling using a set of profiled edger rolls and horizontal rolling using a set of plain work rolls. FE-simulations were performed to investigate the influences of process variables such as initial slab width, initial thickness, sizing draft, edger roll draft and work roll draft on the final slab width variation. From a statistical analysis of the simulation results, an advanced model, which can predict the slab width spread during the edger rolling and horizontal rolling, was developed. The experimental hot rolling trials showed that the newly developed model provided fairly accurate predictions on the slab width spread during hot rough rolling process using a profiled edger rolls.

In the current study, automatic tool compensation is accomplished by using a finite element stamping analysis for a center roof rail made of UHSS in order to satisfy the specifications for shape accuracy. The initial blank shape is calculated from a finite element inverse analysis and potential forming defects such as tearing and wrinkling are determined by the finite element stamping analysis based on the initial tool shape. The blank shape is optimized to meet the shape requirements of the final product with the stamping analysis, and die compensation is determined with the information about springback. The specifications for shape accuracy were successfully achieved by the proposed die compensation scheme using the finite element stamping analysis. The current study demonstrates that the compensation tendency is similar when the proposed scheme is used or when the compensation is performed by trial and error in the press-shop. This similarity verifies that the automatic compensation scheme can be used effectively in the first stage of tool design especially for components made from UHSS.

Press-brake bending is used to shape flat and thick plates into a targeted circular configuration without excessive localized thinning or thickening. A brake bending press called 'a knife press bending apparatus' has been widely adopted to manufacture thick, large and long pipe from initially thick plate. Submerged Arc Welded (SAW) pipes are also produced by employing press-brake bending. These pipes are mainly used for oil, natural gas and water pipelines. The principal process variables for press-brake bending can be summarized as stroke of the press-brake knife, the distance between both roll in the lower die, and the feeding length of the plate. Many combinations of these process variables are available, thus various pipe diameters can be realized. In the current study, a series of repetitive numerical simulations by feeding a thick plate with initial thickness of 25.4mm were conducted with the consideration of elastic recovery. Furthermore, an index for SAW pipe production is proposed which can be widely used in industry.

Recently, global railway car makers are competing desperately in developing high-speed railway vehicles. Ensuring passenger safety during a crash is essential. The design and the manufacturing of energy absorbing components are becoming more and more important. A deformation tube is a typical passive energy absorbing component for railway cars. In the current study the slab method was used to predict the energy absorbing capability of a deformation tube during the early design stage. The usefulness of the prediction method is verified through the comparisons between the results of FE simulations and those of the prediction method.

Rolled products often have residual stresses or strip waves that are beyond the customer’s tolerance. To resolve this problem, skin-pass rolling is widely used during post-processing of such products. Because a short contact length compared to the strip width is a characteristic of skin-pass rolling, several numerical analyses have been previously conducted based on a two-dimensional approach. In the current study, a series of simulations was conducted using numerical analysis of three-dimensional elastic-plastic finite element method.

Although the finite element method is a good tool to analyze skin-pass rolling, it is hard to be applied in the field because of its long calculation time. In the current study, simple numerical models were developed for the prediction of roll force and residual stress profiles along the strip width. These models are based on finite element analysis and a coupled solution of Sims’ equation and Hitchcock’s formula. The results indicate that plastic strains can be represented as in simple equations of the deformed roll profile and the initial thickness of the strip.