The stress-strain behavior of and martensite volume fraction vs. plastic strain curve for type 304 stainless steel at room temperature were investigated by uniaxial tension and compression experiments. The equivalent stress-equivalent plastic strain curve was not affected by hydrostatic pressure up to 0.2 of equivalent plastic strain, and the equivalent stress under compressive loading becomes larger than that under tensile loading at large strains. Similarly, such a hydrostatic pressure dependence was observed in the martensite volume fraction vs. equivalent plastic strain curve. For better understanding of microstructural aspects, TEM (Transmission Electron Microscope) and EBSD (Electron Backscattering Diffraction) observations were performed on the deformed specimens. From the TEM images, it was found that more shear bands and those intersections, which were the dominant nucleation sites of α'-martensitic transformation, were induced under compression than those under tension. From the EBSD images, it was found that orientations stable against deformation-induced martensitic transformation (γ and γ directions) are the preferred orientations under tensile loading. In addition, material parameters in the strain-induced martensitic transformation model proposed by Olson and Cohen imply that shear bands and their intersections are more likely to be formed under compression as corroborated by the results of TEM observation.
The coefficients of friction of uncoated high-strength steel sheets and galvannealed high-strength steel sheets were investigated by sliding tests at elevated temperatures using uncoated tools and TD-coated tools with a VC layer on their surface. The temperatures in the tests ranged from room temperature to 700°C. The coefficients of friction of the tested steel sheets at elevated temperatures are higher than those at room temperature. The coefficients of friction of the uncoated steel sheets are much higher than those of galvannealed steel sheets. The phenomena during the sliding tests were clarified by observing the surfaces and cross sections of the tested steel sheets. The sliding behavior of the uncoated high-strength steel sheets at high temperatures depends on adhesion to the tools and the scale on the steel sheets. For the galvannealed high-strength steel sheets, the sliding behavior at high temperatures depends on the strength of the coating layer. The effects of different factors such as lubricant, surface conditions, adhesion to the tools, and material strength on the coefficient of friction were also examined in this study and presented in this paper.
The effect of microstructural parameters, particularly, ferrite grain size, on the quality of a sheared edge in piercing was examined in 0.02C ferrite steel with grain sizes of 13μm and 0.7μm, 0.002C ferrite steel with a grain size of 0.7μm, and 0.3C ferrite+pearlite steel. The quality, component ratio of rollover, burnished surface and fracture surface was influenced by both clearance during piercing and microstructure. The ratio of rollover decreased with a decrease in clearance, on the other hand, the ratio of the burnished surface increased with a decrease in clearance, irrespective of microstructure and grain size. The ratio of rollover was markedly influenced by grain size. The ratios of rollover in ultrafine-grained ferrite steels were very small compared with those in large-grained and ferrite+pearlite steels. This is because ultrafine-grained steels have a small work hardening ability. The ratio of the burnished surface decreased with grain refinement, owing to smaller elongation. However, the ratio of the burnished surface of 0.02C ultrafine-grained steel was as high as 40%, although the elongation was as small as 5%. The ultrafine-grained ferrite steel has a large reduction in area in spite of its small elongation. This indicates that the ratio of the burnished surface is influenced by reduction in area rather than elongation.
In the first experiment, steel sheets with a high tensile strength of approximately 1,000 MPa were trimmed. For a steel sheet with a free end, a highly precise trimmed surface was obtained by shearing even at a small tool clearance of 5-10%t (t: steel sheet thickness), whereas for a steel sheet with both ends fixed, a highly precise trimmed surface was obtained by shearing when the tool clearance was increased to approximately 15%t. Next, bent formed products were trimmed. As the angle of the side wall of the steel sheet with respect to the tool end surface increases, the quality of the trimmed surface deteriorates because of the friction between sheared surfaces and other factors. The poor quality is caused by the difference in the timing of separation from the tool depending on the position of the steel sheet. Therefore, we developed a new punch with a shape that enables simultaneous separation over the entire steel sheet. It was experimentally demonstrated that defects related to the cut surface are prevented and high-precision sheared surfaces are obtained by trimming using the punch developed.
In this paper, we propose a novel roller burnishing method, which achieves rolling and sliding effects simultaneously at a burnishing point to accomplish a finish of superior surface integrity. The circumferential surface of the workpiece, which is rotated by the main spindle of the lathe, was targeted. The sliding effect is obtained by changing the rotation axis of the burnishing roller with respect to the workpiece. The principle of this method was examined theoretically, and the processing features of the method were evaluated experimentally to compare it with conventional methods. The experiments confirm that the sliding effect can be obtained using the developed novel method. Moreover, the processing features of this method were verified by evaluating the surface roughness, surface profile, and microstructure at the layer of burnished surfaces. In particular, the superiority of this method to conventional methods was observed from the point of view of burnishing the minute unevenness on preliminary surfaces.
To reduce the weight of and to increase impact safety in a car body, the spring-back behavior of laser-welded tailored blanks (TBs) in draw bending has been examined both experimentally and numerically. The materials used in laser-welded TB are a mild steel sheet (270MPa strength) and high-strength steel sheet (440MPa strength). The thicknesses of the sheets are 0.8 mm (thick) and 1.2 mm (thick). Results have indicated that spring-back of all TB combinations (the same thickness and different strength, different thickness and the same strength, different thickness and different strength) were nearly equal to the spring-back of a single thick sheet or low strength sheet. Elastic restoration of one part of TBs was controlled by the other part of TBs.