Currently, the power circuit breaker and switchgear market are continuously growing. Accordingly, the demand for heavy electric machines more than 300 kW is increasing. In order to prevent the occurrence of overcurrent in such a heavy electric machine, a high-performance bimetal material for magnetic switch parts is used. Currently, there is no manufacturing technology in domestic, so manufacturing technology is required. A bimetal is a bonded form of two or more metals with different coefficients of thermal expansion, and the curvature changes as the temperature changes. Alloys with various compositions such as Fe-Ni-Cr, Fe-Ni-Mn and Fe-Ni-Mo are used for high thermal expansion alloys for bimetal production and Invar alloys, which are well known for their low thermal expansion coefficient, are used as low thermal expansion alloys.
Bimetals are conventionally manufactured by explosive welding, twin roll casting and rolling. Among them, the cladding process through rolling is widely used in consideration of productivity and economy. However, in conventional rolling, the amount of deformation introduced to the surface and center of the rolled material is not uniform, properties of bimetals through the thickness are different. In order to introduce a uniform deformation for through the thickness to the rolled sheet, differential speed rolling was performed.
Differential speed rolling was performed under two conditions such as 1 pass or 2 pass for high thermal expansion alloy and low thermal expansion alloy with the initial thickness of 1.5mm, respectively. The roll speed of the upper roll was varied from 5 to 10 RPM and the speed of the lower roll was fixed at 5 RPM. Rolling was performed with a reduction ratio of 50% for 1 pass and then a reduction ratio of 50% for 2 pass with a total reduction ratio of 75%. To evaluate deformation behavior according to roll speed ratios, shear strain and equivalent strain were calculated through the pin embedded method. In addition, texture evolutions of bimetals manufactured by differential speed rolling according to roll speed ratios were analyzed by Electron BackScattered Diffraction.