lacnhceT000iataDs・tcudorPH：0.24m/sL：0.12m/sDrawing 10 Micrograph of SKD11Source：Daido Steel （Co., Ltd.）Drawing 12 Micrograph of SKH51Source：Daido Steel （Co., Ltd.）Drawing 11 Micrograph of DC53 （Improved SKD11）MicrographSource：Daido Steel （Co., Ltd.）Drawing 13 Micrograph of DRM2Source：Daido Steel （Co., Ltd.）50℃100℃130℃100℃100℃50℃150℃100℃200℃200℃50℃100℃110℃110℃200℃Stroke ofpunch28％tStroke ofpunch35％tAverage temperature of cut（℃）400Sharing frequency｝300200100（Al1.17t）（Cu1.20t）（Bs1.18t）（BsPb1.21t）（SUS1.0t）Upward temperature（℃）600400200205401060158020100Stroke of punch（％t）25Cutting time（ms）Hardness（HRC）DC53686664General working condition of DC 53：＞62HRC6260585654General working condition of SKD11：60HRC5250100200300400Tempering temperature（℃）SKD11500Drawing 6 Isotherm of upward temperature for working piece（t＝7.8mm mild steel plate, Cl＝0.3mm, crankshaftpunching is applied）（YANAGIBALA, SAITO, NAKAGAWA）Source：Machining Database of Punching（Nikkan Kogyo Shimbun Ltd.）Drawing 8 The average temperature of cut for eachworking piece during punching（YANAGIBALA, SAITO）Source：Machining Database of Punching（Nikkan Kogyo Shimbun Ltd.）Drawing 7 Upward temperature of working piece including peak value（t＝7.8mm mild steel plate, Cl＝0.3mm, crankshaft punching is applied）（YANAGIBALA, SAITO, NAKAGAWA）Source：Machining Database of Punching（Nikkan Kogyo Shimbun Ltd.）Drawing 9 The hardness of cold-forging Daido SteelSource：Daido Steel（DC53：Iimproved SKD11）Drawing 14 Micrograph of DRM3Source：Daido Steel （Co., Ltd.）1119■Case 3For SKD11, high shearing force processing will expedite the wear of punch.During machining, the heat caused by shearing force will anneal the punch, and expedite the wear of punch. This is very common when SKD11 is applied as punch material. Diagrams 6.7 indicate the increase of heat when a mild steel plate of 7.8mm thickness is machined with 0.3mm clearance （4% of the plate thickness）. And Diagram 8 indicates the average heat of kerf when different work speeds are applied for punching different kinds of working piece. Obviously, the heat of punch occasionally exceeds 300℃. Usually, SKD11 is tempered at 200℃, and it will be annealed in 300℃ or higher. This will expedite the wear of punch. For prevention, stamping oil is applied for cooling, or SKD11 is improved by high-temperature tempering. The tempering curve of improved SKD11 is shown in Diagram 9. And the hardness is about 62HRC, which is high than normal SKD11. On the other hand, the punch material is unchanged when PVD coating is applied （operating temperature：about 500℃）. （improved SKD11 is applied for Punch Industry’s production）. ■Case 4Small burr or abnormal kerf happens during high-precision machining.This is caused by macro-carbides on the tip （tip defection）. As shown in Pictures 11～14, macro-carbides are hard and fragile, and usually exist in tool steel （punch material）. In Picture 10 of SKD11 and Picture 11 of DC53 （improved SKD11）, the macro-carbides are white. And compared to SKD11, the macro-carbides in DC53 are tiny. This can effectively prevent tip defection. In addition, Picture 12 is for SKH51, Picture 13 is for super-high strength steel DRM2, and Picture 14 is for super-high strength steel DRM3 （the macro-carbides are black）. With the features of macro-carbide free, high tenacity and lower price than P/M high speed steel, super-high strength steel is applicable when Case 4 happens. DRM2 is applied for excessive defects solution, and DRM3 is applied for wearability solution. Macro-carbidesMacro-carbides
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