In this study, three kinds of bainitic steels are fabricated by controlling the contents of vanadium and boron. High vanadium steel has a lot of carbides and nitrides, and so, during the cooling process, acicular ferrite is well formed. Carbides and nitrides develop fine grains by inhibiting grain growth. As a result, the low temperature Charpy absorbed energy of high vanadium steel is higher than that of low vanadium steel. In boron added steel, boron segregates at the prior austenite grain boundary, so that acicular ferrite formation occurs well during the cooling process. However, the granular bainite packet size of the boron added steel is larger than that of high vanadium steel because boron cannot effectively suppress grain growth. Therefore, the low temperature Charpy absorbed energy of the boron added steel is lower than that of the low vanadium steel. HAZ (heat affected zone) microstructure formation affects not only vanadium and boron but also the prior austenite grain size. In the HAZ specimen having large prior austenite grain size, acicular ferrite is formed inside the austenite, and granular bainite, bainitic ferrite, and martensite are also formed in a complex, resulting in a mixed acicular ferrite region with a high volume fraction. On the other hand, in the HAZ specimen having small prior austenite grain size, the volume fraction of the mixed acicular ferrite region is low because granular bainite and bainitic ferrite are coarse due to the large number of prior austenite grain boundaries.

Abstract
1.서 론
2. 실험 방법
    2.1 시편 제조
    2.2 미세조직 분석 및 기계적 특성 평가
3. 실험 결과
    3.1 모재의 미세조직과 기계적 특성
    3.2 HAZ 시편의 미세조직과 기계적 특성
4. 결과 고찰
    4.1 미세조직 형성에 미치는 바나듐과 보론의 영향
    4.2 미세조직과 기계적 특성의 상관관계
5.결 론
References

• 황원구(울산대학교 첨단소재공학과) | Yuanjiu Huang (School of Materials Science and Engineering, University of Ulsan)
• 이훈(울산대학교 첨단소재공학과) | Hun Lee (School of Materials Science and Engineering, University of Ulsan)
• 조성규(현대제철 기술연구소) | Sung Kyu Cho (Technical Research Center, Hyundai Steel Company)
• 서준석(현대제철 기술연구소) | Jun Seok Seo (Technical Research Center, Hyundai Steel Company)
• 권용재(울산대학교 첨단소재공학과) | Yongjai Kwon (School of Materials Science and Engineering, University of Ulsan)
• 이정구(울산대학교 첨단소재공학과) | Jung Gu Lee (School of Materials Science and Engineering, University of Ulsan)
• 신상용(울산대학교 첨단소재공학과) | Sang Yong Shin (School of Materials Science and Engineering, University of Ulsan) Corresponding author