저망간 트윕강의 트윈형성 결정립 분율이 기계적 특성에 미치는 영향 = The Effect of Twinned Grain Fraction on Mechanical Properties of Lean Mn TWIP Steels

다국어 초록 (Multilingual Abstract)

Twinning-induced plasticity (TWIP) steels which have a high strength and high elongation use a twinning phenomenon during deformation. TWIP steels with more than 25% Mn can easily reveal the good combination of high strength and ductility with the conventional manufacturing processes without severe plastic deformation and accelerated cooling. However, the steels could not have an appropriate stacking fault energy for twinning with decreasing Mn content. In the present study, it was tried to make steels with lean Mn, that is, Fe-18Mn-1.5Al-0.6C (18Mn TWIP) steel and Fe-12Mn-2Si-0.9C (12Mn TWIP) steel and tested at various temperatures (25℃~ 600℃) and strain rates (10-4/s ~ 102/s) to investigate the effect of temperature and strain rates on the mechanical properties of lean Mn steel. Strength and ductility of 18Mn TWIP steel were increased with decreasing the strain rate at the range of 10-4/s ~ 10-1/s, but those of 12Mn TWIP steel were decreased with decreasing the strain rate at the range of 10-4/s ~ 10-1/s. However, the effect of strain rates over 100/s on strength and ductility of those steels was not significant. Strength and ductility of steels increased with decreasing test temperature at the range of 25℃ ~ 600℃. The deformation mechanisms of lean Mn TWIP steels were studied with the samples deformed with different amounts of strain using the fixture capable of controlling the deformation during tensile test. The volume fraction of twinned grain of 18Mn and 12Mn TWIP steel was increased with increasing strain and the stress also increased with increasing twin formation. Therefore, UTS is directly related with the volume fraction of twinned grain.

목차 (Table of Contents)

• 1. 서론
• 2. 이론적 배경
• 2.1 쌍정유기소성
• 2.2 오스테나이트의 적층결함에너지에 영향을 미치는 인자
• 2.3 고속인장시험
• 2.3.1 고속인장 시험기의 특성
• 2.4 기계적 쌍정 형성의 메카니즘
• 3. 실험방법
• 3.1 TWIP 냉연 강판 제조 및 단상영역 열처리
• 3.2 준정적 인장 시험
• 3.3 고온 인장 시험
• 3.4 고속 인장 시험
• 3.5 변형률 제어 시험
• 3.6 미세조직 관찰
• 3.7 기계적 쌍정이 형성된 결정립의 분율 측정
• 4. 실험결과 및 고찰
• 4.1 Twinned Grain Fraction
• 4.1.1 변형률 조절 시험
• 4.1.2 Twinned Grain Fraction Result
• 4.2 준정적 및 고속인장시험 결과
• 4.2.1 공칭응력-공칭변형률 곡선
• 4.2.2 최대인장강도, 변형률속도민감도와 파단 연신율
• 4.2.3 변형 경화
• 4.2.4 X선 회절 결과
• 4.2.5 광학현미경 조직사진
• 4.2.6 흡수에너지
• 4.3 고온 인장시험 결과
• 4.3.1 공칭응력-공칭변형률 곡선
• 5. 결론
• 6. 참고문헌