Abstract: A new model of strain induced precipitation in niobium microalloyed austenite is proposed. This is based on the experimental observation of formation of microbands during hot deformation of iron – 30% nickel, which remains austenitic to room temperature. Precipitates are preferentially nucleated on nodes in the dislocation network in the microbands. This geometry enables features of earlier models to be simply explained, and facilitates extension of the model to multipass deformation. It is shown that the model captures all the essential features of previous experimental observations on microalloyed C – Mn steels. Currently, sensitivity analysis of the model and systematic experimental work are being undertaken to quantitatively validate the model.
Abstract: It is shown that the kinetics of softening between mill passes can be modeled more
simply when the normalized strain (reduction) per pass is employed rather than the conventional strain. This method requires a second important input, namely the strain hardening rate at the end of preloading. Using this approach, the number of input parameters and experiments required for their determination are drastically reduced. The use of the Law of Mixtures to describe the behaviors of the recrystallized and unrecrystallized volume fractions is then illustrated. Finally, the approach required for quantifying the precipitation kinetics (in microalloyed steels) is described.
Abstract: TRIP steels containing Mn, Si, Al, Mo, and Nb have been examined using a laboratory simulation of a continuous hot dipped galvanizing line. The evolution of microstructure has been studied as the steel passes through the various stages of CG line processing. Tensile strengths approaching 800 MPa and ductilities approaching 30% have been achieved in the 1.5Mn-0.5Si- 1.0Al-0.015Mo-0.03Nb system.
Abstract: The development of ultrafine grained microstructures in steels has received considerable attention in recent times. In many cases the aim is to produce high strength structural steels with minimal alloying. It is well established that for an equiaxed ferrite with a uniform dispersion of second phase, both the strength and toughness will be markedly improved if the grain size can be reduced to 1-2 µm, from the typical range of 5-10 µm. Means of achieving this through dynamic strain induced transformation are examined here, following a brief overview of some of the key issues encountered when attempting to refine the austenite in existing mill configurations. A number of deformation microstructure maps are developed to aid the discussion.
Abstract: Thin slab direct rolling (TSDR) of microalloyed steels belongs to a new generation of thermomechanical treatments. TSDR shows some metallurgical peculiarities that significantly differentiate this process from the traditional route. This paper reviews some of these singularities, considering in a more detail those processing aspects which need to be optimized in order to obtain final homogeneous microstructures that bring about the strength and toughness levels required for specific applications.
Abstract: The choreography of atoms during the course of the bainite transformation has
major consequences on the development of structure. In particular, the scale and extent of the structure is dependent directly on the fact that the atoms move in a disciplined fashion. This information can be exploited to develop unconventional alloys - for example, rail steels which do not rely on carbides for their properties, and the hardest ever bainite which can be manufactured in bulk form, without the need for rapid heat treatment or mechanical processing.
Abstract: A brief summary is given of the desired effects of precipitation of microalloy nitrides and carbides in austenite and ferrite prior, during and after g−a transformation. Precipitation of microalloy compounds of Nb(C,N), TiC and V(C,N) is discussed in relation to several grain refinement and precipitation strengthening mechanisms. An improved understanding of the thermodynamics and kinetics of precipitation and phase transformations is presented using the approach based on the chemical driving force. Nucleation of intragranular polygonal ferrite on VN particles that grow in austenite and the formation of acicular ferrite inside the austenitic grains in Vsteels is described. The role of carbon in increasing the driving force for nucleation is elucidated and the benefits of using microalloy carbo-nitrides for precipitation strengthening of bainitic steels are reviewed. An expanded view on the role of microalloy carbo-nitrides in grain refinement and precipitation strengthening is presented.
Abstract: Developments related to the use of microalloy additions, primarily of Ti, Nb, and V, and controlled processing are reviewed to illustrate how steels with tailored microstructures and properties are produced from either bar or sheet steels for new automotive components. Microalloying additions are shown to control the necessary strengthening mechanisms to produce high strength materials with the desired toughness or formability for a specific application. Selected examples of direct cooled
forging steels, microalloyed carburizing steels, and advanced high strength sheet (AHSS) steels are discussed.
Abstract: Microalloying elements like Al, B, Nb, Ti ,V can be used to optimise the microstructure evolution and the mechanical properties of advanced high strength steels (AHSS). Microalloying elements are characterised by small additions < 0.1 mass% and their ability to form carbides or nitrides. They can increase strength by grain refinement and precipitation hardening, retard or accelerate transformations and affect the diffusion kinetics as well as the stacking fault energy. Thus, by their addition the AHSS with their high requirements to process control can be adopted to
existing processing lines. Different combinations of microstructural phases and different chemical compositions have been investigated for AHSS in order to combine high strength with excellent formability. The recently developed high manganese steels further improve the formability due to their austenitic microstructure and inherent phase transformations during forming.
Abstract: Low hot ductility of steel at the straightening stage of the continuous casting process is a problem found in steels containing microalloying and/or certain alloying additions. The thermal schedule undergone by the billet surface in the mill has a significant effect on the hot ductility. In this work, thermomechanical processing was employed to alleviate the problem of hot ductility in the Nb-microalloyed steel. Specimens were melted in situ and subjected to the billet surface thermal schedule in order to generate a microstructure similar to that present at the straightening stage of the continuous casting process. Some deformation schedules were incorporated with the thermal schedule at very high temperatures, specifically during solidification, within the d-ferrite region, and during the d®g transformation, and the hot ductility was subsequently evaluated at the end of the thermal schedule where the straightening operation is performed. After the thermal schedule alone, the steel exhibited a very low hot ductility at the straightening stage. It was found that deformation at very high temperatures prior to the straightening stage had a considerable effect on the hot ductility, either detrimental or beneficial, depending on the region in which the deformation has been executed. The mechanisms leading to loss and improvement of hot ductility are explained in this paper.