The surface and internal properties of sheet metal have great influence on the quality of stamping products. The requirements for stamping materials are:
① The thickness is accurate and uniform. The die used for stamping is precise and has small gap. If the sheet metal thickness is too high, the deformation force will be increased, and the material will be stuck, and even the die will crack. If the sheet metal is too thin, the quality of the finished product will be affected, and even the drawing crack will occur.
② The surface is smooth and smooth, without spots, scars, abrasions and surface cracks. All surface defects will remain on the surface of the finished product, and the crack defects may extend to the depth and width in the process of bending, drawing and forming, resulting in waste products.
③ The yield strength is uniform without obvious directionality. Anisotropy (see plastic deformation of sheet metal in the process of drawing, flanging, bulging and other stamping processes, due to the occurrence of anisotropic yield, the amount of plastic deformation is inconsistent, which will cause uneven deformation, resulting in inaccurate forming, resulting in defective or scrap products.
④ The uniform elongation is high. In the tensile test, the elongation before the appearance of thin neck is called uniform elongation. In the process of drawing, the deformation of any area of the sheet metal should not exceed the uniform extension range of the material, otherwise the uneven deformation will occur.
⑤ The ratio of flexion to strength is low. The ratio of yield limit to strength limit is called yield ratio. Low yield ratio can not only reduce the deformation resistance, but also reduce the tendency of wrinkling during drawing, reduce the springback after bending, and improve the accuracy of bending parts.
⑥ Low work hardening. The work hardening after cold deformation will increase the deformation resistance of the material and make it difficult to continue to deform, so the plate with low hardening index is generally used. However, the plastic deformation stability of the material with high hardening index is good (i.e. the plastic deformation is more uniform), and the local tensile cracking is not easy to occur.
In practical production, the stamping properties of materials are often inspected by technological tests similar to the stamping process, such as drawing performance test and bulging performance test, so as to ensure the quality of finished products and high qualified rate