DEFORMATION MICROSTRUCTURES AND THE SHEAR STRAIN-RATE OF TYPE 304 STAINLESS-STEEL SHEET IN CYLINDRICAL DEEP-DRAWING OF WARM WORKING

A deep-drawing test was conducted under a single temperature or two temperatures on both sides of type 304 stainless steel sheet, 0.8 mm thick. The different results produced by these processes are described. A lower rate of shear straining is generated at 100 degrees C in the two-temperature process than in the single-temperature process, and a much larger rate is produced with room-temperature working. Their microstructures at the failure point and the flange portion nearing the die throat were studied by optical microscopy and TEM. An intermixed structure of lath and blocky alpha'-martensites appeared at the failure point in cold working. By warm working at a single temperature, homogeneous shear in a single direction and coarser slip bands were generated. Heterogeneous shear straining on both sides of the specimen occurred in the two-temperature process, and the microstructure of the failure point was observed as twin-twin intersections. The microstructures generated in the flange portion corresponding to the individual process, appeared to be blocky martensite nucleation and growth at the shear band intersections, twin-twin band or twin-slip band intersections, respectively. The strengths of these microstructures at different positions are compared with each other and the drawability is explained.