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

Abstract

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 difference 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 effect 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.

Tensile and impact behaviour of a microalloyed medium carbon steel: Effect of the cooling condition and corresponding microstructure

“…3c) and Al 2 O 3 inclusions, as shown in Fig. 3d and e), as also reported in [19][20][21][22][23][24][25]. Chemical composition of the previously mentioned particles, obtained by EDS analysis, are shown in Fig.…”

“…In microalloyed steels, with both low and medium C, an increase of impact toughness can be obtained through a reduction of the grain size, that is controlled by a fine dispersions of stable particles, such as TiN, that restrict austenite grain growth during high temperature deformation. However, many studies [20][21][22][23][24][25]19] pointed out that these particles could be potent initiation sites for cleavage fracture, counteracting the positive effect on austenite grain refinement.…”

Delamination Fracture Related to Tempering in a High-Strength Low-Alloy Steel

“…The deformation under plane-strain conditions is so highly constrained that the stresses in this rigid zone can reach 2 to 3 times the yield stress. [32] This rigid plane-strain zone is located in the center of the neck in a cylindrical tensile specimen, as was shown by Bridgman some years ago. [33] Bridgman deduced that the stress state in the neck of a tensile specimen consists of a uniform axial stress plus a hydrostatic tensile component.…”

9-12Cr Heat-Resistant Steels

“…Inclusions of several microns will be large enough to initiate the brittle cleavage fracture on the rigid zone ahead of the notch, where the stress is concentrated (Yan et al 2007). The fractography analysis on the fracture surface of the broken impact specimens is conducted by using scanning electron microscopy.…”