| Improvement of the yield ratio(YR) of a high strength structural steel has been required to enhance the resistance to seismic motion in building. Microstructural modification through thermomechanical control processing(TMCP) was attemped to lower the YR of a structural steel with 600㎫ tensile strength. In the direct quench and tempering(DQT) processing condition, finish-rolling temperature(FRT) was changed in the wide range to modify the appearence of austenite before quenching. When FRT`s were lower than 940℃ the microstructures consisted of ferrite and bainite. However, when the specimen was finish-rolled at 940℃ the microstructure was characterized as bainite phase. When the direct quench, lamellarizing and tempering(DQLT) process was applied, the mixed structure comprising continuous fine ferrite and martensite was obtained and the YR was lowered to 75%. The prior microstructure before lamellarizing heat treatment, significantly affected the YR and the final microstructures. When the bainite phase was prior microstructure, the ferrite phase acted as a continuum phase after DQLT process, and the low YR was obtained, |
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