A series of experiments have been carried out using a former to investigate the fatigue behavior of cemented carbide die in the extrusion-upsetting process of a steel wire. The stress amplitude of the die in the process is controlled by changing the wall thickness of the die. The die stress generated by extrusion-upsetting is estimated by FEM simulation and verified through strain measurement using a strain gauge. It is found that a fatigue crack initiates around the inlet of extrusion approach where the tensile stress becomes maximum and the maximum shear stress is also large. It is also recognized that die life increases significantly with a decrease in tensile stress amplitude or maximum shear stress amplitude when the product shape is kept constant, as in the typical S-N curve obtained by the fatigue test. However when the stress state of the die is changed by changing the product shape, die life deviates from the curve when the product shape is kept constant. The fatigue behavior could be estimated well by using the maximum tensile strain amplitude instead of the maximum stress amplitude.
A numerical analysis method for the lubricated skin pass rolling of tin plates with dull work rolls is established considering the surface asperity and elastic deformation of such work rolls. A rolling experiment using dull work rolls of comparable diameter to those used in commercial-scale skin pass rolling mills reveals higher elongation for smaller rolling force and much smaller elongation for larger rolling force compared with elongation behavior in the case of bright work rolls. The results of the numerical analysis exhibit good agreement with the experimental results. The skin pass rolling characteristics with dull work rolls can be explained by the following two factors: (a) localized plastic deformation emanating from the asperity of work rolls and its penetration behavior through the entire thickness of the rolled plate, and (b) geometrically activated friction force due to the asperity of work rolls.
Light gauge channel steel is fabricated from a steel sheet by roll forming. The cut end deformation of the channel steel was investigated by experiment and three-dimensional finite-element simulation. During roll forming, concave, convex and reverse bending deformations on a flange occur and causing bending lines to diverge from the point of contact between the top roll and the corner of the flange. The reverse bending deformation is caused by a bending moment and a twisting moment. These moments remain on the flange. When the channel steel is cut, the release of the bending moment results in an opening at both the top end and the tail end. Then, the release of the twisting moment makes the flange close at the top end and open at the tail end. Deformations at the tail end become widely open with the overlap of the two moments.
A new method for predicting the position of the galling generation in square cup drawing is proposed on the basis of fact that galling at a drawn cup occurs at the position of maximum temperature and at the moment the position is sliding out of the die. Blank of three shapes, square, octagon, and triacontadigon, are used to change the galling position on the drawn cup. FEM simulation shows that galling on the drawn cup occurs at the top of the boundary between the drawn area and the bent area for the square blank and at the top of the middle of the drawn zone for an octagonal or triacontadigonal blank. Demonstrations using a high-tensile-strength steel sheet under a lubricated condition show that galling on the drawn cup does occur as predicted by FEM simulation. Galling on the die surface remains at the path end of the galling position of the blank material. It is also found that the sliding distance after the galling position of the blank attains the maximum temperature markedly, greatly affects the galling generation: the longer the sliding distance is, the easier the galling generation becomes.
To prevent the occurrence of wrinkling in the shrink flanging of ultra-high strength steel sheets, a gradually contacting punch was developed. Compressive stress around the convex corner of the sheet in shrink flanging was reduced by gradually bending the edge from the center to the outside of the sheet using the gradually contacting punch. Compressive strain around the convex corner edge decreased as the contact angle of the punch increased. Although a small camber occurred on the upper surface of the flanged sheet using the conventional flat bottom punch, the camber was large using the gradually contacting punch because of the gradual bending. The limiting flange height of the flanged sheet without wrinkling showed an increase of 26% for the 980 MPa grade ultra-high strength steel sheet.
Lubrication is one of the most important factors for improving the productivity of tandem cold rolling mills, which increases the rolling speed for thin steel strips and prevents chatter when rolling materials with a high deformation resistance. A new hybrid lubrication system is proposed and its effectiveness is clarified. The system is based on a re-circulation system of coolant, combined with a system for flexible lubrication control. The key to realizing the new coolant system is the control of plate-out oil formation on a strip surface under high-speed rolling conditions. In this study, the plate-out oil film formation of emulsions is investigated, and the conditions for achieving sufficient plate-out oil films are clarified. Then, the time-dependent property, oil droplet size, and concentration are found to have significant effects on the plate-out behavior. Also, the application conditions for hybrid lubrication systems are discussed to realize effective plate-out control.