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준정적 변형률속도에서 저망간 쌍정유기소성강의 기계적 특성에 미치는 변형쌍정의 영향
송병환 ( Byeong Hwan Song ),김진용 ( Jin Young Kim ),정승훈 ( Seung Hun Jeong ),최일동 ( Il Dong Choi ),이영국 ( Young Kook Lee ) 대한금속재료학회(구 대한금속학회) 2014 대한금속·재료학회지 Vol.52 No.1
The high Mn (≥25 wt%) TWIP steels with low stacking fault energy are currently one of the most attractive materials in the automotive industry due to their unique combination of high strength and good ductility. However, the conventional manufacturing processes have many problems with alloying high Mn. Therefore, many efforts are being made to reduce the Mn content, but the steels could not have an appropriate stacking fault energy (SFE) for twinning with decreasing Mn content. In this study, TWIP steel with lean Mn less than 20 wt% (Fe-18Mn-1.5Al-0.6C) was made. Tensile properties of the lean 18 Mn TWIP steel were measured at a strain rate range of 10.4s.1≤ ε. ≤10.1s.1 to investigate the correlation between twinning behavior and stress at various strain rates. The lean 18 Mn TWIP steel possessed an austenite single phase before and after tensile tests at room temperature and exhibited a total elongation of about 60% and ultimate tensile strength of about 1000 MPa by strain-induced twinning. The stress decreased with an increasing strain rate, showing a negative strain rate sensitivity. That was because the volume fraction of twinned grains decreased with an increasing strain rate at the same strain. Stress was directly related to the volume fraction of twinned grains at quasi-static strain rates.
0.1C-1.5Si-1.5Cu TRIP 형 냉연강판의 성형시 미치는 열처리 효과
김성준,최일동,이창길,송병환 대한금속재료학회(대한금속학회) 2000 대한금속·재료학회지 Vol.38 No.2
The effects of retained austenite on the formability of a 0.1C-1.5Si-1.5Mn-0.5Cu TRIP-aided cold rolled steel sheet were investigated after various heat treatments. Tensile tests and limiting dome height (LDH) tests were conducted, and the changes of retained austenite volume fractions as a function of tensile strain were measured using X-ray diffraction. The results showed plausible relationships between the formability and retained austenite parameters such as stability and initial volume fraction of retained austenite. The formability was improved with the increase of volume fraction of retained austenite. However, when the volume fraction of retained austenite was same, better formability was obtained in the specimens with high stability of retained austenite, meaning that the strain induced transformation of retained austenite to martensite could be stably progressed and resulted in the improvement of formability. Thus, the conditions of intercritical annealing and isothermal treatment in TRIP-aided cold rolled steel sheets should be established in consideration of volume fraction and stability of retained austenite. It was concluded that the effects of retained austenite on the formability of TRIP-aided cold rolled steel sheets could be clearly understood in terms of the stability and the initial volume fraction of retained austenite.
0.15C-1.5Si-1.5Mn 복합조직형 냉연강판의 성형성
김성준,최일동,이창길,송병환 대한금속재료학회(대한금속학회) 1999 대한금속·재료학회지 Vol.37 No.5
The main emphasis of the present study has been placed on investigating the formability and understanding the effects of retained austenite on the formability of a 0.15C-1.5Si-1.5 Mn multi-phase cold-rolled steel sheet (ECO-1 steel sheet). The steel sheet was intercritically annealed at 800℃, and isothermally treated at 400℃ and 430℃. Microstructural observation, tensile tests, and limiting dome height (LDH) tests were conducted, and the change of retained austenite volume fractions as a function of tensile strain was measured. The tensile properties and the formability of the ECO-1 steel sheets were superior to the conventional low carbon cold-rolled steel sheets, due to maintaining high strain hardening rate in the high strain region by the strain induced transformation of retained austenite to martensite. The formability of the ECO-1 steel sheets was dependent on the stability of retained austenite. If the stability of retained austenite was high, the strain induced transformation of retained austenite to martensite can be stably progressed, resulting in the delay of necking to high strain region and the improvement of formability. The effects of retained austenite on formability of the multi-phase cold-rolled steel sheet could be clearly understood in terms of the stability of retained austenite.