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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.
Yong Hwan Cho,Jaeeun Lee,Wung Yong Choo,Juseok Kang,Heung Nam Han 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.6
The effect of separation on the fracture surface of ferrite–bainite dual-phase pipeline steels during a drop weight tear test(DWTT) was investigated via microstructural analysis and fracture surface examination. Two specimens (ST1 and ST2) weredesigned to have a dual-phase microstructure, with different ferrite and bainite fractions, by controlling the reduction ratioand holding time in the ferrite–austenite two-phase region during the thermomechanical process. Notably, ST2, which had alonger holding time for the retained austenite to absorb carbon, exhibited a harder bainite but softer ferrite phase comparedto ST1. The greater hardness difference between those phases in ST2 induced the strain incompatibility frequently evidentat the phase boundaries, resulting in lower ductility during the tensile test. Owing to the strain incompatibility, ST2 alsogenerated more separation over a wider temperature range during DWTT. At low temperatures, where the brittle fracture wasprominent, it was observed that ST2 generated separations on its fracture surface, whereas ST1 did not. These separationsformed a local shear lip around themselves, thereby obstructing cleavage fracture propagation from the notch. Hence, it wasconfirmed that ST2 had a higher DWTT shear area than ST1 over a temperature range near the ductile–brittle transitiontemperature (DBTT). As a result, the DBTT of ferrite–bainite dual phase steel could be improved by increasing the amountof separation during DWTT.
주웅용,김창영 대한금속재료학회(대한금속학회) 1994 대한금속·재료학회지 Vol.32 No.6
To find out the critical strain value for recrystallization, strain distribution in cylindrical specimen after hot compression test was simulated by Finite Element Method(FEM) and evaluated by observing the microstructure of hot compressed specimen. FEM analysis results show that effective strain distribution depends on the position in specimen. Core and edge region of specimen shows higher effective strain than other region. Especially the magnitude of strain at core is twice larger than the mean strain value. Mode of effective strain distribution also changed with the magnitude of applied strain and localized phenomena. In hot compression test, the shape change in barreling region and recrystallized state of austenite grains were investigated. The calculated result of strain distribution and microstructure observation result of austenite were correlated to find out the critical strain for recrystallization of austenite, because the shape of hot compresses specimen was well coincident with that of FEM calculation. In case of 304 stainless steel, 17% strain was required was for recrystallization at 1050℃. For practical application, one or two hot compression test instead of dozen of hot compression tests is enough to obtain the critical strain for recrystallization by simulating the strain distribution in specimen.
주웅용 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.11
In 21st century, a lot of changes is expected in industrial and social circumstance due to the fast development in technology and high performance structural steels are required to response to the new environment more efficiently. Remakable improvements in strength, toughness and weldability and good recyclability are discussed as the quality requirements for new high performance structural steels. Among various metallurgical methods, grain refinement to 1㎛ size was selected as the most promising method to meet the quality requirement of new structural steels. Multi-variant, multi-axis deformation, deformation induced transformation and the use of high magnetic field have been discussed as the new technologies for ultra grain refining. In this paper, theoretical background and recent experimental results of new technologies are critically reviewed and the future research and developing direction of ultra fine grain steel was discussed.
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.
이상우,주웅용,서동한 대한금속재료학회(대한금속학회) 1998 대한금속·재료학회지 Vol.36 No.11
As a preliminary study of developing 800MPa tensile strength grade structural steel plates, the ferrite grain refinement was investigated in a plain low carbon steel. Steels with the composition of 0.15%C-1.1%Mn-0.25%Si were heavily deformed in the uniaxial compression mode and cooled down by using a Gleeble 1500 hot deformation simulator. When a specimen was deformed by 80% at a strain rate of 10/s at 775℃ which was higher than Ar3 temperature of the undeformed specimen by 10℃ and then cooled to room temperature at 10℃/sec, a fine microstructure comprising small ferrite grains of about 2㎛ in size and cementites precipitating at grain boundaries was developed. This compressed specimen showed a high Vickers hardness of 210 which could be converted to tensile strength of 720MPa. Ferrite grains exhibited a random crystallographic orientation and misorientation angles between two neighboring grains were mostly larger than 15 degrees. Some grains divided by subgrain boundaries were also observed. From various experimental results, it could be concluded that these fine ferrites were formed by deformation induced ferrite transformation and dynamic recrystallization of ferrite. Ferrite grains formed by heavy deformation grew very fast during holding time after compression. It was also found that ferrite grains became coarser with decreasing strain rate.