Prevention of Surface Cracks on Billets by Refining Austenite Grain of Bloom Surface before Rolling

Abstract

This study was carried out in order to make clear the crack generation mechanism of case-hardening steels.
It was suggested from the average oxide particle radius around surface cracks of the rolled billets that most of the surface cracks should be generated at the bloom rolling process.
From the point of view of surface crack generation at the rolling process, austenite grain sizes just before rolling were pointed out. In general, in order to prevent surface cracks at the rolling process, bainite or ferite/bainite structure before reheating is suitable for fine grain structure just before rolling. In order to get these structures, the cooling rate of the bloom surface should be higher than 2.0K/s under the cooling start temperature of 1023-1173K. It was proved that refining austenite grain in the bloom surface just before rolling was effective to decrease the crack generation.