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Synopsis:Fracture surfaces of the rolls have been examined with an electron microscope in relation to the size of cracks and bending stress at breakage, in order to have a technical aspect for the development of materials for slab mill roll. The results obtained are as follows:1. The roll life is increased with the decrease of the proportion of the region of brittle fracture to the fracture surface.2. The fatigue strength reduction factor of the slab mill rolls is smaller than 1.5, and the notch sensitivity of cast steels seems to be very small.3. Following three types of fractures are found;(1) fracture by fatigue crack initiated from heat crack.(2) fracture by thermal stress.(3) fracture originated at the inner defects introduced during casting. (4) fracture originated at the inner defects introduced during overlay welding.

Synopsis:In order to improve the roll material and to prevent the breaking of the slabbing and heavy rougher mill rolls, the thermal fatigue test, rotating bending fatigue test, impact test, and tensile test were carried out for the conventional roll materials. The main results are as follows.(I) Nickel-chromium cast steel (SNI) is superior to chromium cast steel (SCR) and ductile cast iron (DC) in the properties of toughness, thermal fatigue strength and fatigue notch sensitivity. It is appropriate to use the SNI steel as a roll material to prevent the breaking of the roll.(2) The fatigue notch factors of the roll materials are smaller than 1.5.

In order to improve the roll material and to !mvent Ihe breaking of the slabbing and heavy rougher mill rolls, the thermal fatigue test, rotating bending test, impact test, and tensile test were carried out for the conventional roll materials. The main results are as follows. (1) Nickel-chromium cast steel (SN I) is superior to chromium cast steel (SCR) and ductile cast iron (DC) in the properties of toughness, thermal fatigue strength and fatigue notch sensitiviry. It is appropriate to use the SNI steel as a roll material to prevent the breaking of the roll.