Application of a Continuous Annealing Process to the Production of Superplastic High-Carbon Steels

Abstract

High-carbon steels with carbon content of more than 0.8wt% have been known to be superplastic by refining their microstructure. Many methods such as warm rolling have been proposed to refine the microstructure. But most of them are considered to be difficult to use as an actual production method. To make high-carbon steels superplastic and economical, application of a cold rolling and continuous annealing process has been examined. p]Changes in microstructure of high-carbon hot rolled steels during the cold rolling and continuous annealing was investigated. By cold rolling with a reduction of more than 50%, a lamellar pearlite structure is deformed and heavily sheared, and a large strain energy is stored. Rapid annealing at temperatures just below the A₁ temperature(1000K) is shown to spheroidize the sheared pearlite and to recrystallize the ferrite grains. After processing, the final structure consists of very fine cementite particles with 0.05–0.2µm diameter in a very fine equiaxed ferrite matrix with 0.1–0.5µm diameter. The processed steel shows total elongations of more than 700% and a strain rate sensitivity index of about 0.5 when deformed at 973K.