| 摘要 |
PROBLEM TO BE SOLVED: To prevent occurrence of chevron cracks inside a workpiece in advance by grasping influences excerted upon a stress condition inside the workpiece from a processing factor and a material factor and finding conditions not exceeding crack generating limit of a tensile stress at a center part of the workpiece in a cold extrusion processing. SOLUTION: In a multi-step cold extrusion processing of metal processing, a workpiece 1 with work hardening coefficient n-value of 0.2 or more is used. Especially in a multi-step extrusion process, the workpiece 1 is used which is heat-treated in a way that the work hardening coefficient n-value becomes 0.2 or more. Additionally, in a processing method when the work hardening coefficient n-value of the workpiece 1 is 0.2 or more, extrusion processing with small face reduction rate is applied after the extrusion processing with large face reduction rate. Especially, this extrusion processing is applied such that the reduction face rate is 30-50% in a first step and 5-15% in the following steps.
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