THE METHOD OF PRODUCTION TESTING OF SHEET METAL FROM HIGH-STRENGTH FINE-GRAINED STEELS FOR CRACK RESISTANCE

ZJWEUB

Abstract

The analysis of factors that have a significant impact on the occurrence of defects during cold stamping of critical load-bearing parts of heavy-duty vehicles made of S550MS steel has been carried out. Changes in tensile strength  from 615 to 700 MPa, yield strength    from 530 to 640 MPa. The presence of concentrators on the lateral surface of the sheet leads to a decrease in the elongation d from 28% to 15-17%. The anisotropy of properties across the sheets leads to a variation in elongation of up to 15%, yield strength of up to 15%. Design procedures for developing requirements for samples for testing sheet metal for crack resistance have been completed, taking into account the features of sheet stamping and allowing for a significant increase in the sensitivity of the sample to crack formation. According to the data of the profilographic study of the edge in the dolom zone, ranges of changes in the angles of the depressions a from 20° to 70°, the depth h of the concentrator from 6 to 12 microns and the radius R at the top from 2 to 20 microns have been established.. A crack resistance assessment test with a V-shaped stress concentrator with R=0.5 mm, α=55°, h =0.8-1.2 mm, repeating the shape of the blade according to the technology of blade processing in workshop conditions, has been developed. A three-point bending test procedure is considered, which showed a high sensitivity of the sample to the occurrence of microcracks at bending angles from 70° to 95° on a sheet roll that does not cause cracks when bending without a concentrator up to 170°. Comparative test results of sheet metal samples, which are prone to defects in cold forming of complex-shaped critical parts in large-scale production, are presented, the angle of onset of microcracks was 75°. The crack develops to a length of 3-5 mm at an angle of up to 105°. In sheet metal samples that do not lead to stamping cracks, the angle of occurrence of microcracks is 95 ° with the development of a crack to a length of 3-5 mm at an angle of 125 °, which makes it possible to perform an input sheet roll to assess its tendency to cracks in cold sheet stamping.