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Unformatted text preview: Effect of TiN Inclusions on the Impact Toughness of Low-Carbon Microalloyed Steels W. YAN , Y.Y. SHAN, and K. YANG Microalloying with various elements, including titanium, coupled with thermomechanically controlled processing, has become a major technology for the manufacture of high-quality steel plate. In this research, the inuence of TiN inclusions on the impact toughness of low-carbon plate steels microalloyed with titanium, vanadium, and boron was investigated. The three experimental steels had Ti/N ratios of 2.44, 3.5, and 4.2, and all three had a granular bainite microstructure. However, Charpy V-notch testing showed that steel A had very high toughness at both room temperature and 20 C, whereas steels B and C showed very low toughness at 20 C and moderate toughness at room temperature. Scanning electron microscope fractography revealed that coarse TiN inclusions had acted as cleavage fracture initiation sites in steels B and C. The effect of Ti and N levels on TiN formation and growth is analyzed using alloy thermodynamics. It is shown that not only is the Ti/N ratio important, but also the product of total Ti and N plays a most important role in TiN formation and growth. It is concluded that the product of the total Ti and N contents should not be greater than the solubility product of TiN at the solidus temperature to prevent the precipitation of TiN particles before solidification. Furthermore, the ratio of Ti to N should also be maintained lower than the stoichiometric ratio of 3.42 to ensure a low coarsening rate for the TiN inclusions during soaking before rolling. I. INTRODUCTION I N the modern manufacture of microalloyed steel plate, it has been recognized that high strength, high toughness, and thereby long service life are of primary interest. Over the past years, the improvements in the toughness of this type of steel, through microalloying and thermomechanical controlled processing, have been remarkable. Recent stud- ies of low- and ultra-low-carbon bainite microstructures has shown that by the addition of microalloying elements such as Nb, Ti, V, and B, the formation of polygonal ferrite can be inhibited, and granular bainite, which has excellent toughness as well as improved strength, can be achieved even under air-cooling conditions in low- or ultra-low- carbon steels. [1,2,3] Therefore, microalloying elements have been widely applied to improve the properties of ultra-low- carbon steels. The present work is also focused on this type of steel. It is well known that the addition of Ti can result in fine TiN precipitates, which can inhibit austenite grain growth at high temperatures because of their strong high-temper- ature stability....
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This note was uploaded on 10/08/2009 for the course CME MAT E 630 taught by Professor Dr. during the Fall '09 term at University of Alberta.
- Fall '09