자동차용 초고강도 철강소재의 부식 유기 수소 취성에 미치는 합금성분의 영향 연구

다국어 초록 (Multilingual Abstract)

The recently developed ultra high strength steel exhibited superior mechanical properties of a high tensile strength of more than 1.5 GPaand a good impact toughness. Nevertheless, its application to auto parts has been severely restricted due mainly to its substantial reduction of the resistance to hydrogen embrittlement (HE) induced by atmospheric/aqueous corrosion. Although there exists a considerable body of literature on HE of a variety of steel alloys, most experimental methods involves the mechanical degradationof steel specimens that has been cathodically charged with hydrogen. This conventional metallurgical approach, however, did not consider the corrosion behaviors on the steel surface and introduction of a small amount of atomic hydrogen into the steel. Therefore, in this study, hydrogen evolution on the steel surface and subsequent hydrogen diffusion into the steel matrix were evaluated using an electrochemical permeation test with no applied cathodic current on the hydrogen charging side. In particular, cyclic operation in the permeation test was also conducted to clarify the corrosion-induced hydrogen evolution behavior. In contrast to the conventional perception that the cathodic reduction reaction on the steel in neutral aqueous environments is oxygen reduction reaction, this study showed that atomic hydrogen is possibly generated on the steel surface by the corrosion reaction, evenin a neutral environment. Although a much lower permeation current density and significant slower diffusion kinetics of hydrogen were observed compared to the results measured in acidic environments, they contributed to the increase in embrittlement index. This suggests that the research on HE of ultra high strength steels should be approached in the view point of corrosion reactions on the steel surface and subsequent hydrogen evolution/diffusion behavior.
Besides, there has been no systematic study on the relationship among alloying elements, corrosion reaction on the steel surface, and hydrogen diffusion behavior into the steel. This study is focused mainly on the effect of the addition of alloying elements such as carbon (C), nickel (Ni) and copper (Cu) on the aqueous corrosion and hydrogen diffusion characteristics of ultra high strength steels. The hydrogen reduction reaction on the steel surface, and its diffusion kinetics in the steel matrix were evaluated using electrochemical polarization and hydrogen permeation tests, respectively. Furthermore, the HE indices of steels with different C, Ni and Cu contents were determined in two aqueous solutions contents were determined in two aqueous solutions (acidic/neutral solution) using the slow strain rate tensile (SSRT) test. This study demonstrates clearly that the corrosion resistance was decreased, and hydrogen diffusion kinetics were decreased significantly, increasing C content. Also, depth profile analysis of the surface significantly, increasing C content. Also, depth profile analysis of the surface of Ni and Cu containing steels shows that metallic enriched layer, approximately 4~5 nm in thickness, is formed on the outermost layer of steel surface, which leads to a lower cathodic reduction rate on the steel surface and slower hydrogen intake kinetics in the steel. Contrary to the beneficial effects on HE, the detrimental effects of Ni alloying can also be obtained when the steel with a Ni content greater than 0.95 wt.% is exposed to a neutral solution (3.5% NaCl + 0.3% NH4SCN solution). These indicate that the ultra high strength steel with superior resistance to corrosion-assisted HE can be produced by making optimal use of Ni element with extremely minute quantities of Ni (0.55~0.75wt.%).

목차 (Table of Contents)

• Ⅰ. 서론 1
• Ⅱ. 이론적 배경 5
• 1. 자동차용 강재의 종류 및 특성 5
• 2. 강재 표면의 부식 현상 12
• 3. 강재의 수소 취화 이론 17
• 4. 합금성분이 내식성 및 수소 취화 현상에 미치는 영향 25
• Ⅲ. 실험 방법 30
• 1. 초고강도강의 합금조성 및 열처리 30
• 2. 미세조직 관찰 33
• 3. 부식 특성 평가 34
• 4. 부식 생성물 분석 37
• 5. 수소 투과 거동 분석 38
• 6. Slow strain rate tensile (SSRT) 실험 45
• Ⅳ. 실험 결과 및 고찰 48
• 1. 열처리 조건에 따른 강종 별 미세조직 관찰 48
• 2. 부식 유기 수소 취성 메커니즘 분석 57
• 3. 부식 거동 분석 68
• 4. 부식 생성물 분석 98
• 5. 수소 투과 거동 분석 112
• 6. SSRT 실험 및 파단면 분석 120
• Ⅴ. 결론 141
• Ⅵ. Summary 147
• Ⅶ. Acknowledgements 150
• Ⅷ. 참고문헌 151