A new ductile fracture criterion for skew rolling and its application to evaluate the effect of number of rolls

摘要

A new ductile fracture criterion for skew rolling has been proposed in this paper. The prediction of internal fractures is crucial given that internal fractures cause loss of product strength and are difficult to detect. The proposed criterion considers the effect of cyclic shear stress on void evolution in a cylindrical coordinate system. This paper covers the following four points: (1) evaluation of the predictability of the conventional ductile fracture criteria for the internal fracture in three-roll skew rolling, (2) clarification of the internal fracture mechanism and proposal of a new criterion, (3) application of the criterion to predict internal fractures in two-roll skew rolling, and (4) relationship between hot workability and damage value determined using the proposed criterion. Given that the hot rolling experimental results did not agree with the prediction provided by the conventional ductile fracture criteria, composed of stress invariants, we evaluated the stress and strain state of the rolled material through elasto-plastic finite element analysis. The results of the numerical analysis led to the hypothesis that the cyclic shear stress is effective in determining the internal fractures caused by skew rolling. The proposed criterion, based on the above hypothesis, accurately predicted the internal fracture initiation, which is specific to skew rolling, regardless of the number of rolls.