EFFECT OF NOTCH GEOMETRY ON HOT DUCTILITY OF LOW-ALLOY STEELS

In order to understand the effect of oscillation marks on surface cracking, hot deformation of some low carbon low alloy and austenitic stainless steels has been investigated by hot tensile tests using specimens with a round notch at temperatures from 800 to 1,100 degree C at average strain rates from 10** minus **4 to 10** minus **1 s** minus **1. Although total elongation decreased with strengthening by the notches, such notch effects were reduced by lowering either deformation temperature or the average strain rate or by Nb addition. This can be explained in terms of dynamic precipitation behavior of carbonitrides such as NbC and/or AlN, i. e. , the precipitation can be suppressed by the increase of true strain rate which arises from local straining in the notched region, resulting in the ductility improvement. The ductility loss due to the notch is determined by the depth and does not depend on the initial sharpness, since the shape can change in the early stage of deformation. Therefore, control of the oscillation mark depth is important to prevent surface cracking of CC slabs.