Microstructural Evolution of Nb-V-Mo and V Containing TRIP-assisted Steels during Thermomechanical Processing

doi:10.1016/j.jmst.2016.08.019

Abstract

Abstract:

The microstructural evolution and precipitation behaviour of Nb-V-Mo and single V containing transformation induced plasticity assisted steels were investigated during thermomechanical processing. A plane strain compression testing machine was used to simulate the thermomechanical processing. Microstructures were characterised by optical microscopy, scanning-transmission electron microscopy and microanalysis, and X-ray diffraction analysis, and Vickers hardness was obtained from the deformed specimens. The resulting microstructure of both Nb-V-Mo and V steels at room temperature primarily consisted of an acicular/bainitic ferrite, retained austenite and martensite surrounded by allotriomorphic ferrite. The TEM analysis showed that a significant number of Nb(V,Mo)(C,N) precipitates were formed in the microstructure down to the finishing stage in Nb-V-Mo steel (i.e. 830 °C). It was also found that the V(C,N) precipitation primarily occurred in both ferrite and deformed austenite below the finishing stage. The results suggested that Nb-Mo additions considerably increased the temperature stability of microalloy precipitates and controlled the microstructural evolution of austenite. However, the microalloy precipitation did not cause a significant precipitation strengthening in both Nb-V-Mo and V steels at room temperature.

Key words: Microalloyed TRIP-assisted steel, Thermomechanical processing, Precipitation behaviour, Microstructural evolution

Abbasi Erfan, Mark Rainforth William. Microstructural Evolution of Nb-V-Mo and V Containing TRIP-assisted Steels during Thermomechanical Processing[J]. J. Mater. Sci. Technol., 2017, 33(4): 311-320.

Figures/Tables 12

Table 1 Chemical composition of the investigated steels (wt%)

Material	C	Mn	Si	V	Nb	Mo	N	S	P	Fe
Steel 1	0.12	1.47	1.54	0.16	0.04	0.08	0.0042	0.005	0.018	Bal.
Steel 2	0.12	1.49	1.51	0.16	-	<0.01	0.0042	0.005	0.017	Bal.

Fig. 1. (a) The thermomechanical processing schedules used to prepare the samples by PSC machine, (b) illustration of sectioned area from the deformed PSC sample for microstructural characterisation. ND: normal direction, ED: elongation direction, TD: traverse direction, WQ: water quenching.

Fig. 2. Optical micrographs: (a) and (b) deformation region of the hot rolled and controlled rolled Steel 1, respectively, (c) and (d) deformation region of the hot rolled and controlled rolled Steel 2, respectively, (e) and (f) undeformed region of the hot rolled and controlled rolled Steels 1 and 2, respectively, (g) and (h) water quenched specimens, showing the prior austenite grains in Steel 1 (with an average grain size of 121 μm; grain boundaries are highlighted by ImageJ software) and Steel 2 (with an average grain size of 38 μm), respectively.

Fig. 3. SEM micrographs: (a) and (b) hot rolled microstructure of Steels 1 and 2, respectively, (c) and (d) controlled rolled microstructure of Steels 1 and 2, respectively, (e) and (f) water quenched microstructure of Steels 1 and 2, respectively. α: ferrite, γ: retained austenite.

Fig. 4. Typical XRD pattern showing the peaks corresponding to the ferrite and retained austenite.

Table 2 Retained austenite parameters characterised by the XRD

Material	Thermomechanical processing	Volume fraction (vol.%)	Carbon content (wt%)	Lattice parameter (nm)
Steel 1	Hot rolled	9.0	1.36	0.36101 ± 0.00012
	Controlled rolled	11.3	1.33	0.36090 ± 0.00017
	Water quenched	11.9	1.35	0.36097 ± 0.00009
Steel 2	Hot rolled	5.2	1.32	0.36085 ± 0.00009
	Controlled rolled	5.8	1.32	0.36083 ± 0.00015
	Water quenched	1.9	1.35	0.36098 ± 0.00015

Fig. 5. Selected thin-foil transmission electron micrographs of Steels 1 and 2: (a) and (b) bright field images and typical EDS spectra, showing the presence of precipitates on the dislocations in bainitic ferrite and adjacent to retained austenite in Steel 2, (c) a dark field image and typical EDS spectrum, showing the presence of precipitates at a grain boundary in Steel 2, (d) a dark field image and typical EDS spectrum, showing the presence of precipitates in acicular/bainitic ferrite in water quenched Steel 1 (arrows indicate microalloy precipitates), (e) bright-dark field micrographs and corresponding selected area electron diffraction pattern, showing the retained austenite in Steel 2.

Fig. 6. TEM replica micrographs: (a), (b) and (c) selected TEM image and corresponding typical EDS and EELS spectra, showing microalloy precipitates in Steel 1, (d), (e) and (f) selected TEM image and corresponding typical EDS and EELS spectra, showing microalloy precipitates in Steel 2, (arrows indicate precipitates), (g) and (h) selected HRTEM images of Nb-V-Mo and V carbonitride precipitates with FCC crystal structure, respectively.

Fig. 7. Precipitate size distribution of hot rolled, controlled rolled and water quenched specimens of Steels 1 and 2, measured from carbon extraction replica samples: (a) Steel 1, (b) Steel 2.

Table 3 Average number density and average diameter of microalloy precipitates (measured from TEM replica micrographs)

Material	Thermomechanical processing	Precipitate number density (precipitate μm^-3)	Average diameter of precipitates (nm)
Steel 1	Hot rolled	1538	9.5
	Controlled rolled	1306	14
	Water quenched	1400	11.5
Steel 2	Hot rolled	2400	12
	Controlled rolled	913	15
	Water quenched	229	14.5

Fig. 8. PSC stress-strain curves corresponding to the rolling pass at 830 °C in the hot rolling and controlled rolling simulations.

Fig. 9. Hardness testing results, (a) selected 3D optical micrograph, showing a topographic image of micro-hardness indentation, (b) average Vickers hardness values as a function of thermomechanical processing.

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