The firing process to bond an enamel frit onto a steel substrate is conducted under a high-temperature condition of approximately 830°ýC for 3min, which leads to the thermal deformation of the enameled steel product. This thermal deformation of the enameled steel is one of the main problems with the firing process. The thermal deformation is also affected by the residual stress of the enamel coating, which is generated by the mismatch between the thermal expansion coefficient of the enamel and that of the steel substrate. The objective of this study was to predict the deformation and residual stress of enameled steel using an analytical model and finite element analysis. To verify the results from the prediction methods, firing and cooling experiments were performed using the same conditions as such methods, and the experimental results were compared to those of the prediction methods. The residual stress was measured using a crack indentation technique based on a pyramidal diamond indenter. The magnitude of the residual stress could be determined by comparing the crack lengths in the unstressed and stressed specimens. The results of the prediction methods were in good agreement with the experimental results with respect to the deformation and residual stress.

The objective of this study is to design a stamping process for polymer-coated metal (PCM) sheets to prevent defects such as delamination and tearing. A defect diagram composed of the delamination limit curve and the tearing limit curve of the PCM sheet is developed for determining the safe region. Similar to the forming limit diagram, the defect diagram is constructed using the critical major and minor strains at which defects occur in various deformation modes. Experiments are performed for establishing the defect diagram of the PCM sheet by means of the limit dome height test and square cup deep-drawing test. A cross-hatched specimen is employed for estimating the delamination of the polymer layer from the steel substrate. Based on the defect diagram, a finite element analysis (FEA) of the stamping process is conducted to determine a forming condition without delamination and tearing. The effectiveness of the stamping process designed on the basis of the developed defect diagram is verified by a forming experiment. Comparison of the FEA results and experimental results reveals that the defect diagram is efficient and suitable for the design of the actual stamping process using PCM sheets.

The cracking of enamel coating often occurs because of excessive tensile stress under the product usage environment. The cracks loosen the enamel coating, leading to spalling and peeling and subsequent corrosion of the metal substrate. The cracking is also affected by the residual stress, which complicates the prediction of cracking initiation. The objective of this study is to predict the cracking initiation of the enamel coating by the FE-analysis. The brittle fracture properties of the enamel coating in the FE-analysis were determined by an experimental Vickers indentation fracture (VIF) test and a uniaxial tensile test. The effect of the residual stress of the enamel coating generated by the firing and cooling process on the cracking was considered and analyzed with a four-point bending model by the FE-analysis. To verify the predicted displacement up to the cracking initiation of the enamel from the FE-analysis, a four-point bending test was also performed using the same conditions as the simulation and the results were in agreement with those of the simulation.

In this paper, a new evaluation and prediction method for coating delamination during sheet metal forming is presented. On the basis of the forming limit diagram (FLD), the current study evaluates the delamination of PET coating by using a cross-cut specimen, dome test, and rectangular-cup drawing test. Dome test specimens were subjected to biaxial, plane strain, and uniaxial deformation modes. Rectangular cup-drawing test specimens were subjected to the deep-drawing deformation mode, and compression deformation mode. A vinyl-coated metal (VCM) sheet consists of three layers of polymer on the sheet metals: a protective film, a PET layer and a PVC layer. The areas with coating delamination were identified, and the results of the evaluation were plotted according to major and minor strain values, depicting coating delamination. The constructed delamination limit diagram (DLD) can be used to determine the forming limit of VCM during the complex press-forming process. ARGUS (GOM) was employed to identify the strain value and deformation mode of the delaminated surface after the press forming. After identifying the areas of delamination, the DLD of the PET coating can be constructed in a format similar to that of the FLD. The forming limit of the VCM sheet can be evaluated using the superimposition of the delamination limit strain of the coating onto the FLD of VCM sheet. The experimental results showed that the proposed test method will support the sheet metal forming process design for VCM sheets. The assessment method presented in this study can be used to determine the delamination limit strain under plastic deformation of other polymer coated metals. The experimental results suggested that the proposed testing method is effective in evaluating delamination for specific applications.

Porcelain enamel coatings were widely applied for the protection of steel products because they offered high corrosion protection, resistance to heat and abrasion, high hardness, hygiene and ease of cleaning. The typical process to produce enameled steels is roughly divided into two stages: the first stage consists of a forming process to give the desired shape to a steel substrate, and the second stage consists of a firing process to bond enamel frits on the substrate. This firing process requires a high temperature above 800 °C, which may lead to austenitic transformation and severe thermal deformation of the steel substrate. The aim of this study is to develop a finite element analysis (FE analysis) technique to predict the mechanical and thermal deformations of the enameled steels during forming and any further enameling process, including firing. The FE analysis involves analyzing the strain history of the steel substrate, which comprises the stress and thickness distributions of the substrate and its deformed shape, and the high-temperature behavior of the enamel coating layer. The validity of the FE analysis is verified through the U-bending test and firing test with various numbers and positions of enamel coating layers on the substrate. The results reveal that the FE analysis results agree well with the experimental results with 8% error.

An aluminum foil-laminated sheet is a laminated steel sheet on which aluminum foil is adhesively bonded. It is usually used on the outer panel of home appliances to provide an aluminum feeling and appearance on the surface of the product. The delamination of aluminum foil is one of the main problems during the stretch forming process. The purpose of this study is was to determine the delamination limit of an aluminum foil-laminated sheet in the stretch forming process. The delamination was dependent on the bonding strength between aluminum foil and steel sheet. The fracture behavior of the interface between the aluminum foil and the steel sheet was described by a cohesive zone model. A finite element was conducted with the cohesive zone model to analyze the relationship between the delamination limit and the bonding strength of the interface. The interface bonding strength was evaluated by lap shear and T-peel test. The delamination limit of the aluminum foil-laminated sheet was determined by using the bonding strength of the steel sheet. The delamination limit was also verified by the Erichsen test.

A VCM sheet is a metal sheet on which PET/PVC is coated for outer panels of home appliances. The purpose of this study is to obtain methods for suppressing PET tearing that occurs during the press forming of the VCM sheet. In order to identity the factors that minimize PET tearing, an FE analysis was performed. The occurrence of PET tearing cannot be predicted using the conventional forming limit diagram. PET is torn by friction between a die and sheet, which is caused by the thickening of material at a die corner. To reduce the thickening of material, the blank shape was re-designed and the thickened material at a flange was removed by a trimming process. The results of the FE-analysis involving modified process parameters showed that the thickness of the product at a die corner is distributed within the clearance of drawing and flange-forming process. A forming experiment was conducted to verify the proposed process parameters. A good final product was obtained without PET tearing of the VCM sheet.