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마르텐사이트계 열간단조용 비조질강의 미세조직이 강도 및 인성에 미치는 영향
주웅용,이덕락,박성운 대한금속재료학회(대한금속학회) 1996 대한금속·재료학회지 Vol.34 No.5
The development of microstructures in micro alloyed forging steels was controlled by applying different cooling rate after hat rolling. The water quenching led to the formation of lath and auto-tempered martensite while the air cooling resulted in bainite and M-A constituents. As the cooling rate increased, the size of M-A constituents decreases and the constituents elongated to the specific direction. Since high carbon twinned martensites in the air cooled specimen caused the brittle fracture, the air cooled specimen showed a low toughness.
주웅용,권숙인,홍순택 한국열처리공학회 1999 熱處理工學會誌 Vol.12 No.2
Grain size of steels is one of the most important parameters which influence yield strength and fracture toughness. Ultrasonic wave propagating in polycrystalline materials is mostly attenuated by scattering at grain boundary. Effect of ultrasonic attenuation on average ferrite grain size of carbon steels with tensile strength 40∼60kgf/㎟ consisting of multi phases such as ferrite+pearlite and ferrite+pearlite+bainite was evaluated. The attenuation of these steels rapidly increased with average ferrite grain diameter. Average ferrite grain diameter (D_(av), ㎛) could be expressed as 1.79+22.97* a^(1/2.03), where a is attenuation with unit of nepers/㎝. From this study, it was confirmed that nondestructive ultrasonic method could be used in measuring average ferrite grain size indirectly.
주웅용,이덕락,Yue, Steve 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.6
Inferior drawability of AISI4135 steel rod is sometimes due to the presence of hard constituents such as bainite and martensite in the microstructure of as-hot rolled state. The amount of hard phases can be minimized by controlled rolling, thus refining the austenite grains, which, in turn, shift the CCT diagram towards shorter times. For the same cooling rate, the austenite to ferrite-plus-pearlite transformation is accordingly enhanced. In conducting numerous hot torsion tests, equations describing the recrystallization behavior of AISI4135 steel during hot deformation were developed in order to find the best condition for grain refinement. Austenite grain sizes predicted by the equations are in good agreement with data from commercial rod rolling.