PURPOSE:To improve delayed breakdown characteristics and mechanical properties by hardening and tempering a PC steel bar then applying minute bending strains-thereto. CONSTITUTION:While a PC steel bar W is conveyed with pinch rolls 1, 4, 11, it is passed through vertical stage rolls 2 for leveling and horizontal stage rolls 3 for leveling whereby it is leveled so as to maintain linearity. After this steel bar is heated to 900-1,000 deg.C hardening temps. with high frequency induction heating coils 5, it is quickly cooled and hardened with cooling water jackets 6, and in succession to these processes it is heated to tempering temps. with high frequency induction heating coils 7. Such steel bar is passed through strain applying devices 8, 9 in the state meintained at the tempering temp. whereby it is applied with about 2% min. strains in vertical and lateral directions; further it is quickly cooled in water cooling jackets 10. The PC steel bar having superior delayed breakdown characteristics and mechanical properties is produced without degrading the uniform elongation characteristics, required for PC steel bar.
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