INFLUENCE OF VARIABLE BLANK HOLDING FORCE AND NONMONOTONOUS PROCESS OF DEEP DRAWING ON FORMABILITY OF COATED SHEETS

Abstract

The results of the research of influence of the blank holding force and nonmonotonous process of deep drawing on the formability of low-carbon steel sheet with one-side galvanic coating of zinc are presented in this paper. Identification and influence assesment was done by monitoring: forming force, the distribution of the principal strains in the sheet plane and their relationship to the forming limit diagram, changes of thinning strain and drawing depth as the most significant technological indicator. Based on the analysis of test results, it was concluded that the blank holding force has a significant influence in terms of intensified friction on the flange. Also, in terms of a single-phase monotonous process, the applied type of variable blank holding had a beneficial effect on the results of forming. A nonmonotonous two-phase process with uniaxial tension in the first stage greatly affects the behaviour of the materials. During the first phase, there is an increase of sheet anisotropy, which affects the formability decrease. The applied variable holding force shows favourable effects, espesially when friction was increased, but they were not dominant.