세립 dual phase 강의 공정 조건에 따른 미세조직 진화 및 μ-DIC 기반 국부 변형 거동에 관한 연구

다국어 초록 (Multilingual Abstract)

Fine-grained dual-phase (DP) steels are widely known to have an excellent combination of strength, ductility, and low yield ratio, which is beneficial for seismic-resistant structural applications. In contrast to conventional DP steels, the fine-grained DP steel in this study did not exhibit the strength increase that generally accompanies the higher martensite fraction at elevated inter-critical annealing temperatures. That is, the yield strength decreased as the inter-critical annealing temperature increased, and the ultimate tensile strength and elongation remained nearly constant. To understand this mechanical properties behavior, the microstructural evolution and the phase-specific local strain behavior were investigated. Microstructural analysis under various inter-critical annealing conditions indicated that the prior-austenite grain (PAG) size remained constant with increasing annealing temperature, whereas the fractions of globular-type martensite and island-type martensite increased. The globular-type martensite at PAGBs and the island-type martensite within grain are considered to influence strength through mechanisms that differ from the mechanism associated with PAG growth. To identify the local deformation behavior, μ-DIC analysis method using Ag sputtering based nano-patterning was applied. The local deformation was mainly concentrated in the softer ferrite phase, whereas much less deformation occurred in the harder martensite phase. In addition, the ferrite adjacent to grain-boundary globular martensite exhibited larger deformation compared with the ferrite near island-type martensite within the grains.

목차 (Table of Contents)

• Ⅰ. 서 론 1
• Ⅱ. 이론적 배경 3
• 1. Dual phase 강 3
• 2. 열처리 공정 4
• 3. 세립 DP 강 5
• 4. 합금 원소 6
• Ⅲ. 실험 방법 7
• Ⅳ. 결과 및 고찰 10
• 1. 공정조건에 따른 물성평가 10
• 1.1. Inter-critical 어닐링 열처리 온도에 따른 세립 DP 강재의 인장 물성 10
• 2. 공정조건에 따른 초기 미세조직 분석 12
• 2.1. 초기 미세조직 12
• 2.2. Manganese 편석 메커니즘 15
• 3. 공정조건에 따른 미세조직 거동 19
• 3.1. Inter-critical 어닐링 열처리 온도에 따른 미세조직 거동 19
• 4. μ-DIC를 통한 국부 변형률에 따른 변형량 거동 분석 22
• 4.1. Sputtering 기반 μ-DIC 분석 전략 22
• 4.2. Martensite 및 ferrite 상 구분 24
• 4.3. Nano-patterning을 통한 μ-DIC 분석 25
• 1) Nano-patterning 조건 선정 25
• 4.4. RTA 열처리에 따른 열 영향 27
• 4.5. μ-DIC 분석 28
• 4.6. Inter-critical 어닐링 온도 증가에 따른 항복강도 감소 33
• Ⅴ. 결론 35
• 참고문헌 37