Semi High Speed Steels for Roughing Rolls with improved Thermal Fatigue Resistance

Thermal fatigue represents one of the most significant causes for roughing mill roll deterioration and possible failure.The materials ability to withstand thermal cycling is strictly correlated to its microstructural properties, which are the result of the solidification process and following heat treatment.Rolls for hot strip rolling mills are usually produced by centrifugal casting, without subsequent forging.A crucial role is played by the eutectic carbide network developing on solidification, which represents a preferential pattern for crack propagation.For this reason conventional highly alloyed HSS are not a popular substitute for traditional Chromium steel rolls in rougher stands.The so -called "semi-HSS", containing less C (0.7-0.9 %C) and alloying elements are usually preferred by users requiring improved roll material.Specially developed "HSS for roughers" are being tested in various mills, but they are out of the scope of this paper.The present work is part of a continuing effort by INNSE Cilindri (RIVA Group) and Trento University to improve the "semi HSS" grades.In particular the thermal fatigue resistance of a new class of low-C semi HSS is evaluated.For purpose of comparison, both a conventional semi-HSS and a HSS grade were also considered.The test is based on induction heating (T max =670°C) and water cooling of cylindrical discs.Experimental results confirm the lower susceptibility towards heat checking of the new grades, containing reduced amount of eutectic carbides, the best behaviour being exhibited by a steel with a mixed martensite-bainite matrix microstructure.