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Title: Elemental segregation and subsequent precipitation during solidification of continuous cast Nb–V–Ti high-strength low-alloy steels

In this study, elemental segregation during solidification and subsequent precipitation behaviour in a continuous cast Nb–V–Ti high-strength low-alloy steel was investigated by optical microscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and thermodynamic modelling. It is known that for steels with low carbon contents the pearlite that forms on slow cooling does so where the interdendritic liquid was present prior to final solidification. The alloying elements of Nb, Ti, Mn and V segregate into the interdendritic liquid during solidification, while Al preferentially segregates into the solidifying solid phase. The composition analysis on the slab samples verified the predicted element segregation behaviour, with a smaller difference in the concentrations of Mn and V in the pearlite and dendritic ferrite regions being observed compared to the Nb levels. Small (30–100 nm) spherical or irregular shaped Nb-rich precipitates (Nb(C, N) and (Nb, V)(C, N)) were mainly found in the pearlite regions, while angular Al-rich (60–300 nm) precipitates were found in the dendritic ferrite regions, in the form of AlN and complex AlN–V(C, N) precipitates. Small isolated ferrite regions surrounded by pearlite were observed in the microstructure and has two origins: one type is dendritic ferrite that appears as an isolated island duemore » to a sectioning effect when observing the two-dimensional microstructure; the other is a ferrite idiomorph that forms in the interdendritic region due to the low carbon content of the steel. Accordingly, in these isolated ferrite islands two different precipitation behaviours are found; predominantly Al-rich particles in the dendritic regions or predominantly Nb-rich precipitates in the interdendritic ferrite idiomorphs. No Al-rich precipitates were observed in the interdendritic regions (pearlite or isolated ferrite idiomorphs) despite the Thermo-Calc predictions indicating a higher volume fraction of AlN in these regions compared to the dendritic regions. This is believed to be due to back diffusion of nitrogen after solidification reducing the available nitrogen, which reacts with the high Nb levels present. - Highlights: • Nb-rich precipitates (Nb(C, N) and (Nb, V)(C, N)) were mainly found in pearlite. • Angular Al-rich precipitates (AlN and AlN–V(C, N)) were found in dendritic ferrite. • Precipitation behaviour in isolated ferrite islands within pearlite was revealed. • Segregation of substitutional elements during solidification is retained. • Back diffusion of interstitial elements (C, N) occurred.« less
Authors:
 [1] ; ;  [2]
  1. School of Materials and Metallurgy, Northeastern University, Shenyang 110819 (China)
  2. Department of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT (United Kingdom)
Publication Date:
OSTI Identifier:
22403538
Resource Type:
Journal Article
Resource Relation:
Journal Name: Materials Characterization; Journal Volume: 95; Other Information: Copyright (c) 2014 Elsevier Science B.V., Amsterdam, The Netherlands, All rights reserved.; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; ALUMINIUM NITRIDES; CARBON; COMPARATIVE EVALUATIONS; CONCENTRATION RATIO; DENDRITES; DIFFUSION; FERRITE; LOW ALLOY STEELS; MICROSTRUCTURE; NIOBIUM ALLOYS; NITROGEN; OPTICAL MICROSCOPY; PEARLITE; PRECIPITATION; SCANNING ELECTRON MICROSCOPY; SOLIDIFICATION; TITANIUM ALLOYS; TWO-DIMENSIONAL SYSTEMS; VANADIUM ALLOYS; X-RAY SPECTROSCOPY