The Characteristics of Deformation of Ss Sheet Metal
The material of the magneto shield is 1Crl8Ni9, and the material thickness is 0.2～0.3mm. It is an ss sheet reformed and rolled. 1Crl8Ni9 belongs to austenitic stainless steel, which has the characteristics of tolerance, hardness, adhesion, and strong cold-work hardening. During the rolling process, with the continuous increase of the deformation degree of the rolling, the internal austenite structure is continuously induced to martensite. This will bring more significant difficulties to the reform of rolling and forming.
The common defects of ss sheet metal reformed-rolled parts are wrinkles, scars, cracks, and ruptures.
Wrinkle Generation and Correction Method
2.1 Flange Wrinkles
When the sheet metal passes through the die, the flange part is not only subjected to the blank holder force but also subjected to radial tensile stress and tangential tensile stress. When the tangential compressive stress is large, plastic instability and tangential bending may occur in the flange part of the ss sheet material, causing the flange part of the sheet material to appear slightly wavy in the vertical direction, that is, the flange is wrinkled. The cause of flange wrinkles is that the blank holder force is too small and the die fillet radius is inappropriate.
When the blank holder force is too small, the outer edge of the sheet will become thicker due to the vertical compressive force and radial tensile force in the direction of the fillet. It can be remedied by adding and modifying the bead, and the second is to adjust the size of the blank holder force. The blank holder force when the shield is reformed in our factory is 0.3～0.5MPa.
The radius of the die fillet directly affects the quality of the rolled parts. Because it can affect the internal stress of the ss sheet metal and the size of the required blank holder force, the formation of wrinkles, the thinning of the ss sheet metal wall thickness, and the number of rolling changes. The smaller the fillet radius, the greater the internal stress generated by the sheet metal, the greater the blank holder force, and the more serious the wrinkles of the modified rolling piece.
The larger the radius of the fillet, the smaller the blank holder force required, the reduction of the compressed part of the sheet and the increase of the compression force in the circumferential direction, and the larger the periphery of the flange wrinkles of the modified rolling piece. Therefore, it is necessary to carefully grind the rounded corners to make the surface smoother, which helps prevent flange wrinkles.
2.2 Wall Wrinkles
The wall wrinkles of the ss sheet metal reformed-rolled parts are mainly caused by the flange wrinkles being reformed to the cylinder wall. The correction method is to appropriately adjust the rolling gap and at the same time give a tension in the direction perpendicular to the rolling direction.
Scar
3.1 Change the Scar
When the modified rolling piece passes through the rounded part of the die, small slip scars may appear, called scratches. Since the rounded part of the die is where the sheet material produces the greatest deformation force, and the ss sheet has a strong viscosity during the deformation process, this defect will become more and more serious. It will scratch the product and affect its smoothness. There will also be scratches, pitting, dents, etc. on the working surface of the punch and die. The actual production process can be overcome by chrome-plating the punch, adjusting the rolling gap, and using a suitable lubricant.
3.2 Mold Scratches
Because the surface roughness of the working surface and rounded part of the convex and concave molds does not meet the requirements, and the gap is uneven due to poor assembly of the mold, bright spots are often formed on the surface of the product, which is called a scratch. This kind of defect can be solved by repairing the mold.
Cracks and Ruptures
4.1 Bottom Tap
In the rolling process, the bottom of the product is a common defect. The prevention method is to increase the radius of the convex and concave mold fillet, reduce the anti-wrinkle blank holder force, strengthen lubrication, and finish machining the working surface of the concave mold. Prevent it.
4.2 The Bottom is Sunken
When the modified rolling piece has not broken yet, but the material in contact with the round corners of the punch has necked due to the modified rolling, the bottom of the product is dented due to the existence of residual stress after the processing is completed. The correction method is to adjust the corner radius of the convex and concave molds so that the sheet metal is in a reasonable stress state during the rolling process.
The Choice of Lubricant
When selecting stainless steel lubricants, the following two points should be considered first:
(1) The lubricant should have a certain relative thermal stability, and maintain a good lubricating effect when the rolling is heated.
(2) When the temperature rises at the moment of rolling, the lubricant can still adhere to the surface of the blank and the mold evenly without accumulating or forming a thick film.
Stainless steel can be re-rolled with re-rolling oil, high-concentration filler-containing emulsion, powdered graphite suspending agent, etc. After long-term practical use, our factory has achieved good results by using re-rolling oil to coat the re-rolled surface of the blank.
Concluding Remarks
The re-rolling forming of ss sheets is a relatively complicated process. It is necessary to carefully study its forming characteristics and influencing factors, explore and accumulate experience in the actual production process, so as to take corresponding measures to eliminate various defects and improve the quality of products.