New approach to evaluatioN of formability of higher-streNgth steel strips

The cold formability of sheets is defined as the ability of material to continuously change its shape and dimensions due to an action of external forces without its failure under particular conditions. 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. The formability (pressability) of steel sheets is mostly evaluated according to their behaviour during the tensile test. As the pressability criterion, the yield point, the tensile strength Rm, the elongation A, the uniform deformation Ag and the strain hardening exponent n are used, as well as other criteria derived from them, such as Re/Rm, (Re/Rm)·A, etc. These criteria only apply to material and to the uniaxial scheme of main stress. The pressability of sheets under particular conditions is evaluated using technological tests, such as the 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 The paper analyses the material characteristics of deep-drawability of steel sheets with the thickness from 1 to 1,8 mm, the yield point above 330 MPa and the loading rate up to 150 m/min. The analysis resulted in the finding that with the loading rate up to ca 10 m/min the formability characteristics slightly increase and after exceeding this value these characteristics significantly decrease. In practice, the strain rates are below 1 s–1, therefore the material characteristics determined using standard tests can be used to determine the limit states up to the above-mentioned rate.