A 1 shot hot stamping process of ultra-high strength steel parts consisting of rapid resistance heating, forming, shearing and die quenching was developed to eliminate an additional quenching process. A rectangular sheet was resistance-heated to obtain a uniform distribution of temperature, and immediately after the end of heating a sequence of forming, shearing and die quenching was performed by 1 shot to prevent the drop in temperature. An ultra-high strength steel spur gear having a hardness of 540 HV2 was produced by the 1 shot hot stamping consisting of the heating, blanking and die quenching. The surface of the sheared edge of the produced gear was almost burnished, whereas the rollover became comparatively large. An ultra-high strength stainless steel part having a hardness of 580 HV2 was produced by the 1 shot hot stamping consisting of the heating, bending, shearing and die quenching, and no springback and quenching distortion of the produced part were observed by holding at the bottom dead center of the press.
The direct punching of inclined ultra-high strength steel sheets having low ductility was carried out to improve the quality of the sheared edge. In the direct punching of an inclined sheet, the contact between the sheet and punch becomes gradual because of touch from the bottom edge of the punch, and thus the sheared portion tends to bend in the latter half of punching. As the strength of the sheet increased, a burr was formed around the final sheared edge due to the low ductility, and the sheared edge without a burr was not obtained for a 980 MPa grade steel sheet. To prevent the occurrence of burrs, punches having inclined and curved bottoms were developed. In these punches, the contact with the sheet was changed, full contact with the inclined bottom and contact of both edges with the curved bottom. In the punches having the inclined and curved bottoms, small burrs and no burrs were obtained for the 980 MPa grade steel sheet, respectively.
In this study, a piercing method using a large chamfered die is proposed for the improvement in the hole expansion ratio. To validate this method, high-strength steels pierced by a large chamfered die were investigated. The result shows that holes pierced by the large chamfered die present a much higher hole expansion ratio than holes pierced by a conventional die, whereas a small chamfered die does not improve the hole expansion ratio. Additionally, as long as the chamfer value is above a specific value, the effect of the large chamfered die on the hole expansion ratio is independent of the chamfer value and the clearance between the punch and die. To clarify the mechanism of this improvement, the hardness close to the pierced surface and the material deformation process were analyzed. The hardness measurement indicated a decrease in the extent of work hardening when the hole was pierced by the large chamfered die. This contributes to the improvement in the hole expansion ratio. The decrease in the extent of work hardening is caused by material deformation without contact between the material and die, which leads to the suppression of shear deformation around the die edge.
The compression behavior of cemented carbide for cold-forging dies is necessary to calculate the prestress in the die insert by an interference fit. In this paper we describe a simple method of obtaining the compression behavior using the Rockwell hardness test. Six kinds of cemented carbide which are the main materials for cold-forging dies were used in this experiment. The compression property of the cemented carbide was measured precisely by the compression test directly and expressed using an accurate approximation equation. It was found that the coefficients of the equation have a good correlation with Rockwell hardness. It was shown that the stress-strain curves generated using Rockwell hardness have good agreement with the measured ones.
The effects of the thermal conductivity of die materials on hot-forging characteristics were investigated on a servo press. Stellite (Co-28 mass% Cr, thermal conductivity: approximately 15 W·m-1·K-1) and tungsten carbide (WC-20 mass% Co, thermal conductivity: approximately 70 W·m-1·K-1) were used as die materials with low and high thermal conductivities, respectively. In upsetting of a chrome steel at 1273 K, the stellite die reduced forging load by approximately 20% at a forging speed of 5 mm·s-1 because the die with a low thermal conductivity was effective in keeping the specimen at a high temperature during upsetting. Furthermore, the stellite die was effective to fill the specimen into a die cavity part (inlet diameter: φ3 mm, depth: 32 mm) in hot upsetting.
Mash seam welders are not applied to continuous cold rolling lines. The presumption is that rolling fracture occurs under conditions of high reduction because of residual steps at the edge of the lap seam, the defective solid phase bonding line and the defective melted area. The flattening mechanism of residual steps has mainly been investigated, which is a characteristic of mash seam welding. Furthermore, a mash seam welder for preventing double lapped deformation is developed. In the process of development, it is concluded that the potential for rolling fracture at a double lapped defect is increased when the angle of the steps formed increases. On the basis of FEM and experimental analysis, in this report it is argued that rolling fracture is caused by double lapped defects similar to cracks. The conclusions are as follows: (1) The configuration of a rolling fracture caused by a double lapped defect is a shear-type ductile fracture. (2) During rolling in a bite around a double lapped defect, the mean normal stress is in a state of tension and the stress triaxiality is very large. (3) Rolling fracture caused by a double lapped defect is occurred in a roll bite.