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Based on the experiment and first-principles calculation methods, the evolution of microstructure, mechanical properties and electrical conductivity of low-carbon steel were investigated at various annealing conditions. The results show that with the increase of annealing temperature, the morphology of ferrite matrix grain undergoes a transformation from narrow and long fibrous to, accompanied by the continuous growth in size. The strength of α-fibre texture is reduced continuously while the strength of γ-fibre texture increases evidently. The tensile and conductivity testing results indicate that with the increase of annealing temperature, both the yield strength and tensile strength of the steel decrease, while the elongation exhibits an opposite tendency due to the continuous growth of ferrite grains and dissolution of tertiary cementites. Additionally, the electrical conductivity increases because the reduction in the number of grain boundaries weakens their scattering ability to electronic waves. The first-principles calculation results indicate that with increasing distance to the interface, the segregation energy of Nb atom in ferrite matrix is reduced, as well as the interfacial energy of the structure is also decreased. The interface segregation preference of Nb atoms induces the transfer of charge, which is beneficial to enhance the conductivity of the ferrite matrix.

Absence of strain-induced precipitation of NbC is observed in a conventional hot rolled Nb-Ti microalloyed X90 strip. Quantitative analysis using transmission electron microscopy and 3D atom probe tomography indicates that epitaxial growth of NbC on pre-existing TiN particles with a short inter-particle spacing of 300 ± 65 nm suppresses the occurrence of strain-induced precipitation of NbC on dislocations. The model for predicting kinetics of strain-induced precipitation of NbC on dislocations is incorporated into a criterion for the end of nucleation of strain-induced precipitation to estimate the effects of process parameters on suppression of strain-induced precipitation of NbC. TiN particle inter-spacing is identified as a key parameter influencing the precipitation behavior of NbC during thermomechanical controlled processing. TiN-NbC composite precipitates offer an alternative approach to strain-induced precipitation of NbC for high temperature processing of Nb microalloyed steels with high toughness. The significance and possible implication of the TiN-NbC composite precipitate engineering are discussed. [ABSTRACT FROM AUTHOR]

In order to study the influence of composition and thermo-mechanical processing schedule on the kinetics of austenite recrystallization, strain induced precipitation and final microstructural evolution in Nb-microalloyed steels, thermo-mechanical processing simulations have been carried out inside Gleeble® by varying the number of deformation passes (2-pass vs. 6-pass), deformation temperatures (1000–800 °C) and inter-pass times. Low-C high-Mn steel (LCHMn) has been found to offer finer ferrite grain size and finer Nb-precipitation which contributed to superior hardness to that steel, compared to high-C low-Mn steel (HCLMn). Among the deformation schedules applied, 6-pass schedule has been found to be superior over 2-pass schedule in terms of precipitation strengthening and hardness. This study also proposes a mathematical framework to explain the effect of composition and processing schedule in Nb-microalloyed steels following Dutta and Sellars approach on precipitation-recrystallization interaction. [ABSTRACT FROM AUTHOR]

In order to control the microstructure and nanoscale Nb(C, N) precipitation in a Nb microalloyed steel, three deformation schedules were employed, followed by cooling process to study the transformation and precipitation behaviors of the tested steel. It was found that plastic deformation in the non-recrystallization region of austenite accelerates the onset of transformation from austenite to ferrite, while the progress of ferrite transformation is retarded. The deformation also refines the ferrite grain size. The transformation behavior for deformation temperature of 850 and 910 °C are almost the same due to the similar deformed austenite. Although the latter deformation temperature is 60 °C higher, the deformation can also significantly refine the transformation microstructure. With the decrease in deformation temperature and increase in cooling rate, the precipitation start temperature decreases, and the nucleation zone of precipitation is transformed from austenite to ferrite or bainite under continuous cooling condition. When the cooling rate reaches 10 °C s−1, the nucleation of precipitation is totally inhibited during cooling process. The precipitate size is significantly refined by increasing cooling rate due to the increase of precipitation driving force. [ABSTRACT FROM AUTHOR]

There are concerns about the use of Nb microalloyed steels in the normalizing rolling of plate steels, as this thermomechanical treatment is finished at a temperature range where Nb(CN) precipitation can promote partial recrystallization of austenite. The aim of this work was to verify the effectiveness of Nb in retarding austenite recrystallization and to determine conditions under which Nb microalloyed steels can be processed through normalizing rolling fulfilling, as much as possible, the metallurgical recommendations proposed to this thermomechanical treatment, mainly the full austenite recrystallization between consecutive rolling passes [ABSTRACT FROM AUTHOR]

Abstract: A new elliptical function for predicting the area fraction of the nucleated sub-grains during static recrystallization has been proposed which considers the fractal based correlation between the sub-grain size and the prior austenite grain size for Nb microalloyed steel. To derive the correlation, fractal analysis of microstructural images of Nb microalloyed steel was performed. The proposed elliptical model in conjunction with the fractal based grain size model improves the prediction of the final grain size distribution for the Nb microalloyed steel and gives excellent correlation with experimental results. [Copyright &y& Elsevier]

In the present study, an artificial neural networks-based model (ANNs) was developed to predict the Vickers microhardness of low-carbon Nb microalloyed steels. Fourteen parameters affecting the Vickers microhardness were considered as inputs, including the austenitizing temperature, cooling rate, initial austenite grain size, different chemical compositions and Nb in solution. The network was then trained to predict the Vickers microhardness amounts as outputs. A Multilayer feed-forward back-propagation network was developed and trained using experimental data form literatures. Five low-carbon Nb microalloyed steels and one low-carbon steel without Nb were investigated. The effects of austenitizing temperature (900-1,100°C) and subsequent cooling rate (0.15-227°C/s) and initial austenite grain size (5-130 μm) on the Vickers microhardness of steels were modeled by ANNs as well. The predicted values are in very good agreement with the measured ones, indicating that the developed model is very accurate and has the great ability for predicting the Vickers microhardness. [ABSTRACT FROM AUTHOR]

The thermosimulation test, measurement of hardness and transmission electron microscopy were applied to investigate the variation of hardness and precipitation behaviour during aging at 525°C after relaxation in deformed Nb bearing microalloyed steel. The experiment results indicate two hardness peaks found during the aging time after relaxation. The growth and coarsening of previously existing strain induced precipitates formed in deformed austenite contribute to the formation of the first hardness peaks, while the second hardness peaks result from the further precipitates which nucleate in α phase after γ→α transformation. With the aid of transmission electron microscopy analysis, the two kinds of precipitates can be distinguished. [ABSTRACT FROM AUTHOR]