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IT has long been known that body center cubic (bcc) metals always exhibit a ductile brittle transition behavior because their yield strengths are sensitive to temperature, increasing rapidly with decreasing temperature, while their fracture stresses are rather insensitive to temperature. Therefore, when the temperature is low enough, the stress will first reach the fracture stress rather than the yield strength, and thus, brittle cleavage fracture will occur without obvious plastic deformation. This type of brittle fracture can be disastrous and should be avoided. Hence, a low ductile brittle transition temperature (DBTT) is highly desirable. Nowadays, as-rolled plate steels have to face more and more severe service environments, such as extremely cold climates, in applications such as pipe lines, pressure vessels, bridges, and other structures. Therefore, the steels are required to not only have well-matched ambient temperature strength and toughness, but also must have excellent low-temperature toughness. Thus, a low DBTT is necessary, and many modern as-rolled steels possess outstanding low-temperature toughness. For example, some acicular ferrite linepipe steels have been shown to have excellent toughness even at –80
C.[1–4] Many factors, including composition, microstructure, grain size, nonmetallic inclusions, second phases, notch geometry, and loading rate, have been shown to influence the DBTT of steels. In order to obtain a lower W. YAN, Postdoctoral Student, and Y.Y. SHAN and K. YANG, Professors, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: yyshan@ imr.ac.cn Manuscript submitted July 8, 2006. Article published online June 16, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A

DBTT, such measures as reduction of the carbon content, refinement of the grain size, improved purification, and thermomechanical controlled processing (TMCP) have been applied to produce high-performance low-carbon or ultra-low-carbon acicular ferrite steels.[5–7] In the production of high-strength, tough steels, enhancement of strength levels can be achieved by increasing microalloy additions that stimulate precipitation strengthening. However, precipitation strengthening always reduces the impact toughness and so there needs to be an accompanying refinement of the microstructure. The latter is most readily achieved through control of the prior austenite grain size using fine dispersions of stable particles, such as TiN, to restrict grain growth during hot rolling. TiN particles, because of their thermodynamic stability, are often used as the grain refiners in steels. However, the Ti and N levels, as well as the process parameters such as solidification rate, need to be carefully controlled, because TiN particles that form in the molten or semisolid matrix can grow to large sizes.[8–10] Optimum toughness cannot be achieved when TiN forms either in the liquid steel or at higher temperatures in the solid state. Such coarse particles, far larger than the submicron pa