Charpy-impact-toughness prediction using an “effective” grain size for thermomechanically controlled rolled microalloyed steels |
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Authors: | D Bhattacharjee J F Knott C L Davis |
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Affiliation: | (1) Tata Iron & Steel Co., 831 001 Jamshedpur, India;(2) Present address: the Department of Metallurgy and Materials, The University of Birmingham, Edgbaston, B15 2TT Birmingham, United Kingdom |
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Abstract: | Thermomechanically controlled rolling of steel plate can involve substantial straining in the intercritical temperature region,
which may result in the final ferrite grains not fully recrystallizing, and, hence, the presence of low-angle grain boundaries.
It is shown in this article that a Nb-microalloyed thermomechanically controlled rolled (TMCR) steel can contain a high proportion
of low-angle grain boundaries (the extent depending on the thermomechanically controlled rolling schedule) and that during
toughness testing, the crack front ignores boundaries with less than a 12 deg misorientation. Thus, the average microstructural
unit experienced by the crack front (i.e., the cleavage facet) is significantly larger than the average metallographic, two-dimensional grain size. Consequently, use
of the metallographic grain size gives a poor prediction of the impact transition temperature (ITT) and fracture stress for
these steels. It is also shown that the micromechanism of crack initiation and propagation involves grain-boundary carbides
and groups of closely aligned grains that act as single “effective” grains. |
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Keywords: | |
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