Influence of TiN Inclusions on the Cleavage Fracture Behavior of Low-Carbon Microalloyed Steels

Influence of TiN Inclusions on the Cleavage Fracture Behavior of Low-Carbon Microalloyed Steels

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Infuence oF TiN Inclusions on the Cleavage ±racture Behavior oF Low-Carbon Microalloyed Steels W. YAN, Y.Y. SHAN, and K. YANG Toughness is a major concern for low-carbon microalloyed steels. In this work, the impact fracture behavior of two low-carbon Ti-V microalloyed steels was investigated in order to better understand the role of TiN inclusions in the toughness of the steels. The steels had similar chemical compositions and were manufactured by the same rolling process. However, there was an obvious diFerence in the ductile brittle transition temperature (DBTT) in the Charpy V-notch (CVN) impact tests of the two steels; one (steel 1) possessing a DBTT below –20 ° C, while the DBTT of the other (steel 2) was above 15 ° C. Scanning electron microscopy (SEM) fractography revealed that there were TiN inclusions at the cleavage fracture initiation sites on the fracture surfaces of steel 2 at both low and room temperatures. It is shown that the TiN inclusions had nucleated on Al 2 O 3 particles and that they had pre-existing interior flaws. A high density of TiN inclusions was found in steel 2, but there was a much lower density in steel 1. Analysis indicates that these inclusions were responsible for the shift of DBTT to a higher temperature in steel 2. A mechanism is proposed for understanding the eFect of the size and density of TiN inclusions on the fracture behavior, and the cleavage fracture initiation process is analyzed in terms of the distribution and development of stresses ahead of the notch tip during fracture at both low and room temperatures. DOI: 10.1007/s11661-007-9161-2 Ó The Minerals, Metals & Materials Society and ASM International 2007 I. INTRODUCTION I T has long been known that body center cubic (bcc) metals always exhibit a ductile brittle transition behav- ior because their yield strengths are sensitive to temper- ature, increasing rapidly with decreasing temperature, while their fracture stresses are rather insensitive to temperature. Therefore, when the temperature is low enough, the stress will ±rst 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. ²or example, some acicular ferrite linepipe steels have been shown to have excellent toughness even at –80 ° C.
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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.

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Influence of TiN Inclusions on the Cleavage Fracture Behavior of Low-Carbon Microalloyed Steels

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