Complex Approach Mechanical Properties and Formability Assessment of Selected Deep-drawing Steels

The demands for properties of deep-drawing materials are continuously growing, while new processing technologies and testing procedures to assess the deep-drawing properties are looked for. The formability of materials is influenced by all factors participating in the forming process, namely the sheet material, the product type, the tool and the production technology of a pressing. Indicative information of deep-drawing properties of steel sheets can be obtained from their behaviour during the tensile test. As the pressability criterion, the yield point Re, the tensile strength Rm, the elongation A, the strain uniform deformation Ag, and the exponent n are used, as well as other criteria hardening derived from them, such as Re/Rm, (Re/Rm), A, etc. These criteria only apply to the material and to the uniaxial scheme of main stress. The pressability of sheets under particular conditions is evaluated using technological tests, such as the Erichsen cupping test. In this test, the formability criterion is the depth at which a crack is formed when a pushing a punch with a spherical end of a prescribed radius into the sheet, so-called Erichsen number IE, but also the appearance of the impressed spherical cap. The material characteristics of steel sheet in the forming process are also significantly influenced by external factors. The strain rate is a significant external factor and the intensity of its influence on the behaviour of material during the forming process, and hence on the material characteristics, is a function of its internal structure. With increasing the strain rate, the critical slip stress increases, the yield point intensively increases, the ultimate tensile strength increases and the material deformation characteristics change. As a result, the values of formability criteria derived from these characteristics also change. One of possibilities to increase the productivity during cold forming consists in increasing the forming (pressing) rate. Therefore it is necessary to know the behaviour of material in the forming process at increased rates, as well as its material characteristics. The compactness of the surface zinc layer is an important requirement for these materials, mainly at high loading rates. This information can be provided by modified dynamic Erichsen tests, or cyclic loading tests. Measuring