Investigaton of parameters that affect formability of steel sheets and investigation of forming limit diagrams

Sheet metals are used in industry for manufacturing the most varied components and products. During forming operations, especially when subjected to combinations of stretching and drawing, they may fail due to localized necking and/or cracking or deviate in shape from the required part geometry. Scrapping caused by such failures affect the economy, continuity and quality of the production in a negative way. Analyzing these operations is very important to reduce wastes and optimize the processes. Formability is the ability of the sheet metal to be formed without developing any failure, and it depends on several interacting factors such as material flow properties, ductility, die geometry, die material, lubrication, press speed, etc. Evaluation of formability is mostly made according to the results of tensile tests and simulation tests. First estimation about the formability of sheet metals is possible on the basis of tensile and drawing test results. However, mechanical and chemical features of sheet metals such as yield strength, tensile strength, or microstructure are not alone sufficient to account for their formability. Therefore new techniques are needed to use the sheet metals effectively in production processes. A useful technique for overall consideration of sheet metal formability is the Forming Limit Diagram (FLD), which represents all combinations of deformations during forming processes for complex parts. With the FLD, the behaviours of sheet metal materials which are subjected to two-dimensional tensile and compression tests can be analyzed. So, the use of FLD allows estimation of sheet metal formability and serves to choose suitable process solutions. This study was carried out for understanding of the formability of steel sheets, which are formed in the shapes from various in industrial applications and that's why in this study, parameters that affect formability were investigated primarily. Formability of ten qualities has been characterized by FLDs and the effects of material properties on FLDs were evaluated. It was found that the limit curve of the grade which have a higher r value was determined to be higher than the other grade's curve, so more safe region to work was found for this grade, increased thickness is determined to be a positive effect on formability and the presence of the Nb and Ti elements in the steel composition that have a positive impact on formability by the forming limit diagrams obtained. On the other hand the increase of the carbon content and mechanical properties were found to be negatively effect the ability to shape.