In this study, we developed a basic test method of reproducing the slide damage of surface coating layers on dies during actual press working. To simulate actual production processes, dies made of SKD11 on which TiC, TiN, CrN, or VC was coated by PVD, CVD or TRD were used. The sliding damage of the coatings was quantitatively evaluated by this new method, and results showed good agreement with the results of the X-ray diffraction technique and surface roughness measurement. In the case of galvanized blank steel sheets, it was found that the surface damage was caused by the adhesion of zinc onto the coated die. Excellent resistance against the sliding damage was obtained for the VC coating layer by TRD.
A square steel pipe is reshaped from a welded round pipe by roll forming. The effect of roll diameter on the cross-sectional size of the square steel pipe was investigated by experiment and three-dimensional finite element simulation. When the diameter of the top roll is larger than that of the side roll, the width of a corner part of the formed pipe is larger than the height. The square steel pipe was formed by offsetting the small roll to the upstream side in order to make the width and height of a corner part equal. The result of offsetting is affected by the longitudinal contact distance between a roll and a pipe. Geometric contact length and relative offset (= offsetting distance / geometric contact length) were defined. The optimum value of the relative offset was clarified, which increased with the expansion of the contact length of a top roll, the roll gap, and the wall thickness of a round pipe.
A square steel pipe is reshaped from a welded round pipe by roll forming. The pipe end deformation after cutting the square steel pipe was investigated by experiment and three-dimensional finite element simulation. Each direction of the residual stresses composed of longitudinal stress and in-plane shear stress becomes reverse with respect to the outer and inner surfaces, respectively. A large opening deformation arises at the back end of the product by the release of those residual stresses. The side part receives bending deformation in the surrounding area of the contact zone with a roll. The residual stresses composed of longitudinal stress and in-plane shear stress arise as a result of the reverse bending deformation downstream of the roll center position. The absolute value of residual stress decreases as the initial wall thickness decreases.
Iron hollow spheres (IHSs) were manufactured by the reduction of iron oxides. The compressive deformation behavior and structures of IHS were systematically investigated. The strength of IHSs is proportional to the product of the IHS diameter and the shell thickness of IHSs, and is equivalent to the buckling strength of the IHS structure. A plateau stress region is observed during the compression of the IHS structure. The plateau stress is proportional to the strength of IHSs. The bonding of IHSs is indispensable to the occurrence of the plateau stress region. The average plateau stress does not depend on the distribution of the IHS strength but depends on the average IHS strength. The plateau stress decreases in the inclined compression of the IHS structure. The plateau stress increases with an increase in the strain rate during the compression. Its increasing ratio is larger than that of the porous aluminum.
A new concept rolling mill for flat products, i.e. the NSC intelligent mill (NIM), has been proposed. The NIM concept states that the mill can estimate and control roll force distribution across the width between the work roll and the rolled material during rolling. The roll force distribution reflects longitudinal stress distribution in the rolled material, which has a direct relationship with the flatness of the rolled material. Hence, the NIM enables real-time flatness detection and control, and the NIM can eliminate the limitations of conventional flatness control technology as follows: delay of control response mainly due to transport time from the rolling mill to the flatness sensor, noncontrol portion of the head end of the rolled material, and limited flatness control accuracy due to the low accuracy of the flatness sensor. A pilot mill on the laboratory scale was constructed, and the concept of NIM, as well as feasibility of the real-time estimation of roll force distribution, was verified.
A new rolling mill for flat products, i.e. the NSC intelligent mill (NIM), is developed. The NIM concept states that the mill can estimate and control roll force distribution across the width between the work roll and the rolled material during rolling. Owing to this concept, real-time flatness detection and control of rolled materials can be realized. A commercial scale prototype NIM with a flatness control system has been constructed to verify the feasibility of the real-time estimation and control of roll force distribution. The prototype NIM has successfully realized real-time flatness detection and control of rolled materials, and has revealed an extremely powerful ability for improving flatness of rolled materials.
Many types of punching process have been utilized for the production of automobile parts and other related components. In the normal punching process that uses a punch and a die, both shear and fracture surfaces usually exist on the cutting surface. It is important to estimate the ratio of the shear surface to the cutting surface to produce highly accurate parts. To realize a smooth cutting surface of products, we have to apply the most appropriate tools and punching conditions to the process, taking account of the costs. The cutting surface of punching processes has been analyzed to study the ratio of the shear surface to the fracture surface by the finite element method (FEM). The criteria for fracture initiation on cutting surfaces have been proposed by several researchers. It is too difficult to identify the fracture criteria on cutting surfaces by simple experiments such as tensile tests, because the punching process has many complicated steps. In the present study, the finite element method is applied to several punching processes to evaluate the criteria of the ductile fracture functions proposed by Oyane and Cockcroft and Latham. As a result, the shape of cutting surfaces is affected by the clearance between the punch and the die, blank holding conditions, and the ductile fracture criterion.
The effectiveness of high temperature annealing on the cold formability of commercial AZ31 and AZ61 magnesium alloy sheets was investigated. The elongation, n-value and r-value for the improvement in formability were increased by the high temperature annealing for both sheets. The minimum radius ratio in the V-shaped bending of the annealed sheets at 500 °C was improved to 2 from 3 and 4 for the as-received AZ31 and AZ61 sheets, respectively. Although the limiting drawing ratios for circular and square cups of the annealed AZ31 sheet were 1.61 and 1.45, respectively, the high temperature annealing for the AZ61 sheet was insufficient, particularly for the square cups. In addition, a square cup having a bead at the bottom was formed by two-stage cold stamping. In the 1st stage, a cup having a large corner radius was formed by deep drawing using a punch having a large corner radius to prevent the occurrence of fracture. In the 2nd stage, a bead at the bottom of the cup was formed by a rubber punch and grooved die. An AZ31 square cup having a bottom bead was successfully formed.
A punching process of a small hole using local resistance heating of a shearing region for a die-quenched steel sheet was developed. The shearing region in the sheet was locally heated by a pair of electrodes to decrease the flow stress in the punching. The effects of the distance between the electrodes, the length of the electrode and the current on the temperature distribution generated by the resistance heating were examined. A uniform distribution of temperature in the hoop direction of the hole was obtained by optimizing the heating conditions. The punch load and the burnished surface area for the heating at 500 °C were about 1/3 smaller and 2 times larger than those for the cold punching, respectively. In addition, the occurrence of delayed fracture around the punched hole was prevented by decreases in tensile residual stress and hardness for the heating above 500 °C.