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The extremely slow transformation kinetics of a nanocrystalline bainitic steel makes this novel structure the perfect candidate to determine the carbon content of the bainitic ferrite away from any carbon enriched regions, such as dislocations and boundaries, as the bainite transformation progresses at extremely low temperatures. The purpose of this atom probe tomography study was to systematically track atom distributions during the bainite reaction in a nanocrystalline steel. The results will provide new experimental evidence on the explanation for the incomplete reaction phenomenon and the carbon supersaturation of the bainitic ferrite during transformation, subjects critically relevant to understanding the atomic mechanism controlling bainitic ferrite growth. [ABSTRACT FROM AUTHOR]

The present paper deals with a new vanadium and nitrogen alloyed steel for heavy products applications, which offer improved toughness and strength properties as compare with a commercial heavy product steel. A strong structural refinement, by introducing acicular ferrite microstructure, combined with an optimum cooling process can be utilised to develop tough, high strength thick products. [ABSTRACT FROM AUTHOR]

The goal of the work reported in the present paper is to develop a neural network model for describing the evolution of the compromise (UTS × EL) between ultimate tensile strength (UTS), and elongation (EL) mechanical properties on low carbon sheet steels. The model presented here take into account the influence of 21 parameters describing chemical composition, and thermomechanical processes such as austenite and ferrite rolling, coiling, cold working and subsequent annealing involved in the production route of low carbon steels. The results presented in the present paper demonstrate that this model can help with optimising simultaneously both strength and ductility for the various types of forming operation that the sheets can be subjected to. [ABSTRACT FROM AUTHOR]

The bainite start temperature Bs is defined as the highest temperature at which ferrite can transform by a displacive transformation. A common observation is that the bainite start temperature is very sensitive to the chemical composition, indicating that the influence of solutes is more than just thermodynamic. Empirical linear regression models have long been used to calculate the Bs in a limited range of compositions. This paper attempts to create an empirical model of wider applicability and higher accuracy by means of neural networks. The results are compared with those calculated using the thermodynamic theory for bainite transformation, revealing that in general this theory agrees with the experimental results, but some discrepancies can still be found when the alloys are heavily alloyed. [ABSTRACT FROM AUTHOR]

By using an iron alloy that transforms slowly to bainite at a low temperature, it has been possible to study the development of the upper bainite under the influence of exceptionally large stresses, which are nevertheless below the yield strength of the parent austenite. It is found that a uniaxial stress whose magnitude is below the elastic limit, strongly favours the growth of compliant variants, leading to an organised microstructure. It also accelerates the overall rate of reaction. A comparison between samples transformed with and without an applied stress revealed significant changes in the crystallographic texture, consistent with the observed microstructures. Stress assisted transformation resulted in large blocks of bainite in identical orientation. [ABSTRACT FROM AUTHOR]

Recent work has shown that bainitic ferrite plates produced by transformation at low temperatures can be as thin as 20 nm with a hardness in excess of 650 HV30, tensile strength ~2.3 GPa and toughness ~30 MPa m1/2. Because these properties rely on the fine scale of the microstructure, a study has been carried out in relation to the tempering resistance of steel over the temperature range 350 – 750°C. It is found that significant softening occurs only after the plates of ferrite begin to coarsen. The coarsening process is hindered by the intense precipitation of carbides resulting from decomposition of the carbon enriched retained austenite. The carbides themselves lead to some precipitation strengthening during the early stages of tempering. The ferrite is found to contain excess carbon, beyond its solubility limit, and X-ray analysis indicates that the carbon is associated with heterogeneous strains in the microstructure. It does not readily precipitate until the onset of substantial recovery during annealing. [ABSTRACT FROM AUTHOR]

A theory is developed for the quantitative representation of the strain induced transformation of retained austenite in low alloy, TRIP-assisted steels of the type developed for the automobile industry. It is possible, therefore, to calculate the fraction of austenite as a function of the plastic strain, chemical composition, deformation temperature and the starting amount of austenite. The effect of composition and temperature is expressed through the free energy available for transformation. Good agreement has been obtained with published experimental data. The model can be used to investigate the stability of the austenite during plastic deformation. [ABSTRACT FROM AUTHOR]