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Zn-assisted Liquid Metal Embrittlement of High Mn Austenitic Steels

고 Mn 오스테나이트계 강의 용융아연에 의한 취성

정근수 (Jung Geunsu, 포항공과대학교)

원문보기

  • 주제(키워드) TWIP , LME , resistance spot welding , Zn coating
  • 발행기관 포항공과대학교 철강대학원
  • 지도교수서동우
  • 발행년도2016
  • 학위수여년월2016. 2
  • 학위명박사
  • 학과 및 전공철강대학원 철강학과
  • 세부전공computational metallurgy laboratory
  • 원문페이지149
  • 본문언어영어
  • 저작권포항공과대학교 논문은 저작권에 의해 보호받습니다.
초록 moremore
The application of TWIP steel in the automotive industry is still limited due to many pending problems. Liquid metal embrittlement caused by Zn is one of the obstacles to be overcome, which is particularly accented when welding TWIP steels. The current study tackles the issue with experimental metho...
The application of TWIP steel in the automotive industry is still limited due to many pending problems. Liquid metal embrittlement caused by Zn is one of the obstacles to be overcome, which is particularly accented when welding TWIP steels. The current study tackles the issue with experimental method as well as numerical one. LME resistivity of different high strength steel grades were investigated for comparative study. It is found that Zn embrittles all the Fe based alloy system regardless of constituent phases. The critical stress for LME has been found to be the one corresponding to strain of 0.4 %, which will cause LME at temperatures over 700 °C. TEM observation revealed that thermo-mechanically assisted diffusion of Zn can occur along grain boundaries of the substrate when strained at high temperatures, which might cause LME. A finite element model has been developed for stress and temperature analysis in welding conditions. It is suggested from FE simulation that tensile stress arises during the cooling stage of the welding, which might cause LME on the weld surface. The level of the tensile stress is dependent on the thermal expansion coefficient of the weld metal, explaining why TWIP steels are vulnerable to LME during resistance spot welding.
목차 moremore
Table of contents
1 Introduction 9
2 Literature review 12
...
Table of contents
1 Introduction 9
2 Literature review 12
2. 1 TWIP steel 12
2. 2 Zn coating process 15
2. 2. 1 Metallurgy of galvanizing 15
2. 2. 2 Metallurgy of galvannealing 22
2. 3 Resistance spot welding 28
2. 3. 1 Weld parameters 29
2. 3. 2 Spot weld cracking of Zn-coated TWIP steel 32
2. 4 Liquid metal embrittlement (LME) in general 33
2. 4. 1 Introduction 33
2. 4. 2 Influencing factors on LME 35
2. 4. 3 Mechanisms of LME 40
2. 5 LME of Zn coated TWIP steel 48
2. 5. 1 Gleeble test 48
2. 5. 2 Residual stress analysis of drawn cup 51
2. 5. 3 A criterion for LME occurrence 52
3 Part 1 : experimental study 55
3. 1 Introduction 55
3. 2 LME cracking during spot welding of Zn-coated TWIP steels 57
3. 2. 1 Spot welding of Zn-coated TWIP steel 57
3. 2. 2 Fracture study 61
3. 2. 3 Interrupted welding 65
3. 2. 4 Discussion 68
3. 3 LME of high strength steels, comparative study 71
3. 2. 1 Experimental method 71
3. 2. 2 Dilatometry and micrstructure study 75
3. 2. 3 Hot tensile test 78
3. 2. 4 Behavior of liquid Zn at high temperatures 80
3. 2. 5 Modified hot tensile test 85
3. 2. 6 Grain boundary study 89
3. 2. 7 Spot welding of Zn-coated high strength steel 95
3. 2. 8 Discussion 97
4 Part 2 : finite element study 99
4. 1 Introduction 99
4. 2 FEM of spot welding, review 101
4. 3 FE study on spot welding oh Zn-coated steel 106
4. 3. 1 Parameters 110
4. 3. 2 Simulation results and discussion 111
4. 3. 3 consideration of phase transformation 121
5 Study on prevention of LME 131
5. 1 Heat treatment of the Zn-coated steel 131
5. 2 Adjustment in welding schedule 137
6 Conclusions 139
References 142