In the current steel structures of high-rise buildings, high heat input welding techniques are used to improve productivity in the construction industry. Under the high heat input welding, however, the microstructures of the weld metal (WM) and heat-affected zone (HAZ) coarsen, resulting in the deterioration of impact toughness. This study focuses mainly on the effects of fine TiN precipitates dispersed in steel plates and B addition in welding materials on grain refinement of the HAZ microstructure under submerged arc welding (SAW) with a high heat input of 200 kJ/cm. The study reveals that, different from that in conventional steel, the γ grain coarsening is notably retarded in the coarse grain HAZ (CGHAZ) of a newly developed steel with TiN precipitates below 70 nm in size even under the high heat input welding, and the refinement of HAZ microstructure is confirmed to have improved impact toughness. Furthermore, energy dispersive spectroscopy (EDS) and secondary-ion mass spectrometry (SIMS) analyses demonstrate that B is was identified at the interface of TiN in CGHAZ. It is likely that B atoms in the WM are diffused to CGHAZ and are segregated at the outer part of undissolved TiN, which contributes partly to a further grain refinement, and consequently, improved mechanical properties are achieved.

Abstracts
 1. 서 론
 2. 실험방법
  2.1 실험 재료
  2.2 용접 방법
  2.3 미세조직 분석 및 기계적 물성 평가
 3. 결과 및 고찰
  3.1 미세조직 관찰 및 석출물 분석
  3.2 석출물 성분분석 및 B 영향
  3.3 미세조직 분석 및 기계적 물성 평가
 4. 결 론
 References

• 박진성(순천대학교 신소재공학과) | Jin-seong Park (Department of Advanced Materials Engineering, Sunchon National University)
• 황중기(동명대학교 기계공학부) | Joong-Ki Hwang (School of Mechanical Engineering, Tongmyong University)
• 조재영(포스코 기술연구원) | Jae Young Cho (POSCO Technical Research Laboratories)
• 한일욱(포스코 기술연구원) | Il Wook Han (POSCO Technical Research Laboratories)
• 이만재(포항산업과학연구원) | Man Jae Lee (Analysis and Assessment Group, Research Institute of Industrial Science and Technology (RIST))
• 김성진(순천대학교 신소재공학과) | Sung Jin Kim (Department of Advanced Materials Engineering, Sunchon National University) Corresponding author