Materials Science Forum Vols. 500-501

Abstract: Bimodal grain size distributions were found in continuously cast slab and thermomechanical controlled rolled (TMCR) samples of Nb-microalloyed steel. Scanning electron microscopy (SEM) revealed inhomogeneous distributions of Al- and Nb-containing precipitates, which were found to pin prior austenite grain boundaries during reheating. An effort has been made to establish parameters to quantify the extent of bimodality of reheated and rolled microstructures. Quantification of bimodality using peak grain size range, (PGSR) and peak height ratio, (PHR), is found to match closely with the visual observation of bimodality. Thermo-Calc software was used to predict the sequence of precipitation for different compositions and that could explain the formation of bimodality during reheating.

Abstract: In Fe-C-Mn steels, the carbon atoms in solution can be either completely free in the iron matrix or in interaction with manganese atoms. In this context, a methodology based on the combined use of thermoelectric power and internal friction measurements was developed in order to evaluate quantitatively these two populations of carbon atoms. This methodology was used to determine the binding energy of the C-Mn dipoles and to follow the precipitation kinetics of the two populations of carbon atoms and/or their segregation kinetics to the dislocations during an isothermal treatment. Lastly, the influence of each population of carbon atoms on the strain aging of extra-mild steels was discussed.

Abstract: The solubility limit of copper in iron at temperature lower than 700°C is not precisely known because copper diffusion is too slow to reach an equilibrium with classical experimental techniques involving long range diffusion. However, fine precipitation of copper can lead to an equilibrium in a reasonable ageing time. Hence, coupling ThermoElectric Power and Small Angle X-ray Scattering techniques leads to a precise estimation of this solubility limit in the temperature range 500°C-700°C. Values obtained are confirmed by Tomographic Atom Probe and give results much higher than what is usually extrapolated from high temperature experiments.

Abstract: Magnetic Barkhausen noise measurements have been carried out to characterize ferritemartensite duplex microstructures and industrial Dual-Phase steels. We have first studied ferritemartensite duplex steels, for which the volume fraction and the carbon content of martensite were higher than for industrial Dual-Phase steels. We found linear evolutions between ferrite peak parameters and its proportion. We applied these results to industrial Dual-Phase steels and show that Barkhausen noise measurement can be successfully used for Dual-Phase steels characterization, and in particular for assessment of ferrite proportion.

Abstract: The recovery and recrystallization kinetics of a cold rolled interstitial free (IF) steel were studied during isothermal annealing. Magnetic methods based on coercive field measurements, hardness tests and metallography were applied so as to follow the kinetics experimentally. The coercive field measurement technique reveals a higher degree of resolution for monitoring recovery than conventional hardness determination and also allows the recrystallization progress to be monitored. The results obtained are compared to those previously determined for a non-stabilized extra low carbon (ELC) steel. The observed differences are discussed in terms of the presence of microalloying elements, Ti and Nb, which slow down recovery and delay recrystallization.

Abstract: In this work, we show that the measurement of the Barkhausen noise allows the residual stresses in each of the two phases of ferrite-martensite steels to be characterized. We have first studied the effect of a tensile and a compressive stress on the Barkhausen noise signature. We observed that for a ferrite-martensite steel, the application of a tensile stress increases the Barkhausen activity of the martensite and ferrite phases, whereas a compressive one reduces it. In a second time, we induced residual stresses by applying a plastic deformation to ferrite-martensite steels. After a tensile plastic deformation, we observed that (i) compressive residual stresses appear in ferrite, and (ii) tensile residual stresses appear in martensite. An opposite behavior is observed after a compressive plastic deformation. These results show that the Barkhausen noise measurement makes it possible to highlight in a nondestructive way the distribution of the stresses in each of the two phases of a ferrite-martensite steel. This result could be used to characterize industrial Dual- Phases steels that are plastically deformed during mechanical processes.

Abstract: Precipitation strengthening is an important parameter controlling the mechanical properties of low carbon steels. These precipitates are very fine and are normally analyzed using either thin foils or carbon extraction replicas under a transmission electron microscopy (TEM). In this work, field emission gun scanning electron microscope (FE-SEM) was applied successfully in the characterization of niobium (Nb) carbo-nitride (C,N) precipitates using carbon extraction replicas. FE-SEM observation of high strength linepipe steel replicas before and after aging at 400°C for 1 hr confirmed the presence of Nb(C,N) precipitates in ferrite. The FE-SEM could analyze small particles (below 50 nm) embedded in the steel but the analysis had to be carried out at low voltages to maximize spatial resolution resulting in a poor signal. However, carbon extraction replicas in the FE-SEM can be analyzed using high voltages, since the interaction volume effect is no longer a problem.

Abstract: Niobium is a strong carbide forming element which is often used in microalloyed steels to control the grain size during thermomechanical treatments and to provide strengthening through precipitation processes. A detailed microscopic investigation is one of the keys for understanding the first stages of the precipitation sequence, thus Transmission Electron Microscopy (TEM) is required. The main difficulty of TEM studies is due to the nanometre scale dimensions of the particles, which makes their detection, structural and chemical characterization delicate. Model Fe- (Nb0.06%,C0.05%) and Fe-(Nb0.05%,C0.03%,N0.03%) ferritic alloys subjected to isothermal annealing treatments have been investigated. High Resolution TEM (HRTEM) and conventional TEM (CTEM) were used to characterise the morphology, nature and location of precipitates. Volume fraction measurements and a statistical approach to the determination of precipitate size histograms have been investigated using Energy Filtered TEM (EFTEM) and High Angle Annular Dark Field (HAADF) imaging. Chemical compositions were quantified by Electron Energy Loss Spectroscopy (EELS). The evolution of precipitate composition with time and temperature is compared with previous simulations obtained from new thermodynamic models based on equilibrium boundary conditions.

Abstract: In this contribution strain induced precipitation of niobium carbides has been analyzed making use of different hot-rolling simulators and combining the advanced precipitation characterization methods of selective chemical extraction and transmission electron microscopy. A laboratory cast Fe-0.1C-0.07Nb alloy has been employed for the study. Thermomechanical simulations were carried out by torsion, plastodilatometry and plane strain compression techniques. The results have shown that, in spite of the different deformation modes a relatively good correlation is obtained between the measurements of the precipitate size and the amount of Nb precipitated in the different experiments.

Abstract: The investigation was focused on the detailed description of precipitation/formation processes of TiN, TixSy, Ti4C2S2, TiC, and (Fe,Ti)P particles occurring in the production of thin sheets of titanium-bearing interstitial-free steels. The knowledge concerning this reactions was gained based on the thermodynamic calculation and physical simulations conducted on Gleeble 3800 thermal – mechanical simulator. The formation process of titanium carbosufide of Ti4C2S2 is the crucial reaction for the thin sheets properties. The investigation confirmed the in-situ mechanism of this phase formation, involving carbon diffusion in the TiS and TixSy sulfides’ crystal lattice. Thus, the precipitation of the substantial fraction of these sulfides is a prerequisite for further intense carbosulfide formation. Titanium sulfides are formed at the rough rolling stage of the transfer bar rolling process. The most intense precipitation of the sulfides occurs at around 1000°C. Besides the effect on the carbosulfide formation, it was found out that the sulfides substantially retard the recrystallization and grain growth in the IF steels. The most favourable Ti4C2S2 formation conditions prevail in the temperature range of approximately 950 - 920°C, which corresponds to the plate finish rolling stage. Both reactions, i.e. sulfides and carbosulfide formation, are deformationinduced processes.