The surface and internal properties of the sheet used for stamping have a great influence on the quality of the stamped parts. The requirements for stamping materials are:
① Precision and uniformity of thickness. The die for stamping is precise and the clearance is small. If the thickness of the plate is too large, it will increase the deformation force and cause material jamming, and even burst the convex mold; if the plate is too thin, it will affect the quality of the finished product, and even crack during drawing.
② Surface smooth, no spots, no scars, no scratches, no surface cracks, etc. All surface defects will remain on the surface of the finished workpiece, and crack defects may extend deeper and wider during bending, drawing, forming, etc., resulting in scrap.
③yield strength is uniform and has no obvious anisotropy. Anisotropy (see plate material with plastic deformation in the drawing, bending, bulging and other stamping processes, due to the yield in all directions, there is a difference in the amount of plastic deformation, which will cause uneven deformation, resulting in inaccurate forming and causing secondary products or scrap.
④ Uniform elongation ratio. In the tensile test, the elongation ratio before the specimen begins to neck is called the uniform elongation ratio. In deep drawing, the deformation of any area of the sheet metal should not exceed the uniform elongation range of the material, otherwise, uneven deformation will occur.
⑤) Low yield-to-strength ratio. The yield strength of a material divided by its ultimate strength is called the yield-to-strength ratio. A low yield-to-strength ratio not only reduces the deformation resistance but also reduces the tendency of wrinkling during drawing and the amount of springback after bending, improving the accuracy of the bent part.
⑥ Work hardening is low. The work hardening that appears after cold deformation will increase the deformation resistance of the material, making it difficult to continue deforming, so low-hardenability steel plates are generally used. However, materials with high hardenability have good stability of plastic deformation (i.e., plastic deformation is more uniform) and are less likely to have local tensile cracking.
In actual production, the process performance tests such as drawing performance test, bulging performance test and so on, which are similar to the drawing process, are often used to inspect the drawing performance of materials, so as to ensure the quality of the finished product and the high acceptance rate.