Fe-Mn-C계 TWIP강의 기계적 성질과 지연파괴 특성 = Mechanical Properties and Delayed Fracture Characteristics of Fe-Mn-C TWIP Steels

다국어 초록 (Multilingual Abstract)

New design concepts for the construction of advanced light-weight and crash resistant transportation system require the development of high strength and supra-ductile steels combined with enhanced energy absorption and reduced specific weight. A variety of AHSS（advanced high strength steel) including DP, TRIP and TWIP steel have been used in automobile body. Among these high strength steels, TWIP steel containing 15∼20%Mn and additions of carbon and aluminum are regarded as much attractive one due to the excellent mechanical properties, which result from high strain hardening due to the extensive twin formation during plastic deformation. The excellent ductility (over 50%) and the enhanced tensile strength (over 700MPa) enable complex deep drawing or stretch forming operations of sheets and the fabrication of crash absorbing frame structures. Mechanical twins, strain induced ɛ-martensite(hcp) or α' martensite(bct) may be formed depending on the chemical composition and test temperature. The mechanical properties of metastable austenitic TWIP steel largely depend on the characteristics of deformation modes such as slip, deformation twinning, transformation to ɛ and/or α' martensite, which are to be closely related to the stacking fault energy (SFE) of austenite.
The aim of this study is to investigate the effect of alloying elements on the phase transformation, deformation behavior and mechanical properties in Fe-Mn-C hot rolled TWIP steels for the development of a high strength (over 800MPa) and large ductility (over 50%) steel sheets. In a second step, this article focuses on the cold rolled and annealed Fe-18Mn-0.6C TWIP steels with addition of Al, Ti and Ni in order to investigate the effects of mechanical properties on the annealing temperature and alloying elements in applications for car body parts in the automotive industry. Also, this research is to investigate the delayed fracture of high strength TRIP/TWIP steels with a tensile strength of 700MPa to 1000MPa grade using cathodically hydrogen charged specimens.
The main results and conclusions from the work carried out are as follows :
1. The ductility of Fe-Mn-C TWIP steels with high manganese concentrations (12 to 18%) and additions of carbon (0.15 to 0.6%) and aluminum (0.5 to 1.5%) is decreased due to the strain induced transformation of metastable austenite to α' and/or ɛ martensite. However, the autenitic TWIP steels of the composition with C󰁅 0.45% and Mn 󰁅 18% shows high elongation (over 68%) due to the formations of mechanical twin during plastic deformation.
2. The dominant deformation mode shifts from TRIP to TWIP mode as the amount of carbon, manganese and aluminum additions is increased. The addition of Al to Fe-Mn-C TWIP steels increases the stacking fault energy of the austenite and suppress the martensitic γ→ε transformation, this leads to enhance the ductility. Also, addition of 0.2 %Mo to TWIP steels shows that the tensile strength increase to 100MPa without sacrificing the ductility. Also, Cu (0.3 to 0.46%) added steels result in a significant increase of ductility due to the grain refining.
3. The chemical composition range exhibiting the excellent mechanical properties in Fe-Mn-C TWIP steels is 0.45∼0.6%C, 18%Mn and 1.0∼1.5%Al. The tensile strength and elongation of these hot rolled steel sheets exhibit about 800∼1000MPa and 60∼70%, respectively.
4. In Fe-18Mn-0.6C-1.5Al TWIP steel with 0.123%Ti content, the average recrystallized grain size was reduced to 2.5㎛ by cold rolled and annealed at 800℃ for 5 min, because of the pinning effect of the fine TiC carbides on grain coarsening. The tensile strength was decreased and the ductility was improved with increasing annealing temperature. However, the reversion of hardness and yield strength happened between 750℃ and 800℃ due to the TiC and M3C type precipitation.
5. The Fe-18Mn-0.6C TWIP steel with 0.56%Ni content exhibited relatively lower yield strength, because Ni precipitates were not formed during annealing process. However, when the specimen was annealed at 800℃ for 5min, the tensile strength and elongation revealed 1096MPa and 61.8%, respectively.
6. The TWIP steel with the stable austenite structure shows a lower hydrogen content than that of TRIP steels. The uniform distribution of strong traps throughout the matrix as the austenite is considered beneficial to reduce the delayed fracture of TWIP steels. Moreover, austenite structure with very fine deformation twins formed during plastic deformation could also improve the ductility and reduce the notch sensitivity. In U-bend test and deep drawing cup test, TWIP steel shows a good resistance of delayed fracture compared with the TRIP steel.

목차 (Table of Contents)

• Abstract Ⅳ
• 제1장 서론1
• 참고문헌 4
• 제2장 연구배경 6
• 2.1 Fe-Mn-C계 TWIP강 6
• 2.1.1 연구개발 동향 6
• 2.1.2 합금설계 방안 11
• 2.1.3 상변태 및 기계적 성질 12
• 2.1.4 오스테나이트의 안정성과 변형기구 17
• 2.2 수소 지연파괴 25
• 2.2.1 강재내의 수소와 지연파괴 거동 25
• 2.2.2 기계적 성질에 미치는 수소의 영향 36
• 2.2.3 지연파괴 발생기구 39
• 2.2.4 지연파괴 민감도 평가 42
• 참고문헌 51
• 제3장 Fe-Mn-C계 열연 TWIP강의 미세조직과 기계적 성질 57
• 3.1 서언 57
• 3.2 실험방법 59
• 3.3 결과 및 고찰 61
• 3.3.1 미세조직과 오스테나이트의 안정성 61
• 3.3.2 기계적 성질에 미치는 첨가원소의 영향 70
• 3.3.3 연성에 미치는 가공방법의 영향 80
• 3.4 요약 83
• 참고문헌 84
• 제4장 Fe-Mn-C계 냉연 TWIP강의 재질특성에 미치는 소둔온도와 첨가원소의 영향 86
• 4.1 서언 86
• 4.2 실험방법 88
• 4.3 결과 및 고찰 89
• 4.3.1 소둔 온도에 따른 미세조직의 변화 89
• 4.3.2 재질특성에 미치는 첨가원소의 영향 98
• 4.4 요약 107
• 참고문헌 108
• 제5장 Fe-Mn-C계 TWIP강의 지연파괴 특성 110
• 5.1 서언 110
• 5.2 실험방법 112
• 5.3 결과 및 고찰 114
• 5.3.1 수소농도에 미치는 미세조직의 영향 114
• 5.3.2 TWIP강의 지연파괴 거동 117
• 5.3.3 굽힘 시편과 딥드로우잉컵의 지연파괴 거동 125
• 5.4 요약 130
• 참고문헌 131
• 제6장 결론 133