Thermomechanical processing of advanced high strength steels

Vanadium as an important alloying element in steels was initially associated with the properties achieved following tempering. Interest in the microstructure was stimulated by the advent of transmission electron microscopes with a resolution of ∼1 nm together with selected area electron diffraction techniques. A second timely development was that of controlled rolling, particularly of plate and sheet products. The scope of this review will include the historical background on quenched and tempered vanadium steels, precipitation during isothermal aging, conventional controlled rolling and during thin slab direct charging and the development of strength and toughness in vanadium microalloyed steels. The characterisation of microstructure, in particular the methods for the analysis of the chemical composition of precipitates has progressed since the availability of X-ray energy dispersive analysis in the 1970s, and the role played by electron energy loss spectroscopy in providing quantitative analysi...

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Processes, microstructure and properties of vanadium microalloyed steels

Modeling recrystallization of microalloyed austenite: effect of coupling recovery, precipitation and recrystallization

The use of reactive wetting and stir mixing to disperse nano-sized ceramic particles in molten metal was studied using Al2O3 nanoparticle reinforced Al–Cu–Mg composites that were synthesized using gravity casting in a permanent mold and a single sample synthesized by squeeze casting. These experiments have shown that reactive wetting of nanoparticles of Al2O3 in Al–Cu–Mg alloys, combined with stir mixing, can result in significant improvements in the hardness of gravity cast specimens provided that a reaction between the reinforcement and the matrix was either on-going or recently completed at the time of solidification. The results indicate that microstructure and strengthening mechanisms depend on the degree of clustering after reaction completion. It is hypothesized that a small amount of clustering resulted in particle engulfment and Orowan strengthening and increased clustering resulted in particle pushing and grain refinement strengthening described by the Hall–Petch relation. TEM analysis of a squeeze cast sample showed that individual particles and small flocculi were present within the grains rather than at grain boundaries, suggesting that the higher solidification rate of squeeze casting should follow particle pushing models and result in engulfment of larger particles than gravity casting. A comparison of yield stress or hardness variation with the inverse of the square root of grain size or dendrite arm spacing clearly shows the dominant strengthening mechanism as a result of the different process parameters. This method is also used to compare the results of this study to those of other studies.

Microstructure and hardness of Al2O3 nanoparticle reinforced Al–Mg composites fabricated by reactive wetting and stir mixing

The formation and evolution of oxide particles in oxide-dispersion- strengthened ferritic steels during processing

Modelling the kinetics of strain induced precipitation in Nb-microalloyed steels

The evolution of microstructure during the hot working of steels microalloyed with Nb is governed by the recrystallization kinetics of austenite and the recrystallization-precipitation interaction. The present study focuses on the effects of prestrain and deformation temperature on the rectrystallization behavior in these steels. The extent of recrystallization is characterized by a softening parameter calculated from a series of interrupted plane strain compression tests carried out at different deformation temperatures and strain levels. The results indicate that at low temperatures, softening is caused by static recovery, while at higher temperatures, static recrystallization is the predominant mechanism. The recrystallization-stop temperature (T 5pct) and the recrystallization-limit temperature (T 95pct), marking the beginning and end of recrystallization, respectively, are determined as a function of strain. In order to achieve a homogeneous microstructure, finish rolling should be carried out outside the window of partial recrystallization (T 5pct<T<T 95pct), as determined in this study. The Nb(CN) precipitation kinetics have been calculated using a model proposed in an earlier work, and these results are used to estimate the precipitate pinning force under the given processing conditions. Based on these estimations, a criterion has been proposed to predict the onset of recrystallization. The predicted results are found to be in reasonably good agreement with the experimental measurements.

Effect of prestrain and deformation temperature on the recrystallization behavior of steels microalloyed with niobium

A simple analytical model to describe the morphology of a growing dendrite in the presence of an inert particle has been presented. The presence of a particle changes the solute concentration gradient at the tip of a growing dendrite and this, in turn, affects the dendrite tip radius. Results of the analysis show that the dendrite tip radius decreases at a high growth velocity due to the presence of a particle, while there is no influence on the tip radius at both low and intermediate growth velocities. Lower thermal conductivity of the particle decreases the tip radius, while a higher thermal conductivity increases the radius. However, the effect of thermal conductivity on the tip radius is only significant in the cellular growth regime. The analysis shows that the presence of SiC particles in Al-Cu alloys reduces the cell to dendrite transition velocity. Results of directional solidification experiments carried out on an Al-4.5Cu-SiC composite system agree with our model.

Directional dendritic solidification of a composite slurry: Part I. Dendrite morphology

Ultra low-load-dynamic microhardness testing facilitates the hardness measurements in a very low volume of the material and thus is suited for characterization of the interfaces in MMC's. This paper details the studies on age-hardening behavior of the interfaces in Al-Cu-5SiC(p) composites characterized using this technique. Results of hardness studies have been further substantiated by TEM observations. In the solution-treated condition, hardness is maximum at the particle/matrix interface and decreases with increasing distance from the interface. This could be attributed to the presence of maximum dislocation density at the interface which decreases with increasing distance from the interface. In the case of composites subjected to high temperature aging, hardening at the interface is found to be faster than the bulk matrix and the aging kinetics becomes progressively slower with increasing distance from the interface. This is attributed to the dislocation density gradient at the interface, leading to enhanced nucleation and growth of precipitates at the interface compared to the bulk matrix. TEM observations reveal that the sizes of the precipitates decrease with increasing distance from the interface and thus confirms the retardation in aging kinetics with increasing distance from the interface.

Studies on age-hardening characteristics of ceramic particle/matrix interfaces in Al-Cu-SiCp composites using ultra low-load-dynamic microhardness measurements

B. Dutta

Papers

Modelling the kinetics of strain induced precipitation in Nb-microalloyed steels

Effect of prestrain and deformation temperature on the recrystallization behavior of steels microalloyed with niobium

Directional dendritic solidification of a composite slurry: Part I. Dendrite morphology

Studies on age-hardening characteristics of ceramic particle/matrix interfaces in Al-Cu-SiCp composites using ultra low-load-dynamic microhardness measurements

Directional dendritic solidification of a composite slurry: part I - dendrite morphology, part II - particle distribution