Materials Science Forum Vols. 539-543

Abstract: Seamless tubes are manufactured, as it is well known, via continuous mandrel rolling process. A billet is first pierced, then the hollow is reduced and rolled with mandrel at temperatures higher than 1000oC. After mandrel extraction, the hollow can either cool down to room temperature or be directly charged in an intermediate furnace for re-austenitization. Finally, the tube is finished in a stretch-reducing mill to several gauges. The thermomechanical process is complex allowing little flexibility along the line. The aims of this paper are a) to describe the industry schedule in terms of simple process variables and b) to simulate this process via numerical and physical models. The latter uses torsion testing as experimental technique. It is shown that good agreement is obtained between industry results and predictions from the numerical model. Torsion experiments have produced somewhat larger predictions for ferrite grains sizes, however. This is mostly attributed to the necessary simplifications made to the mechanical testing experiments due to restrictions in maximum strain rates and shortest dwell times achievable with thermomechanical simulators.

Abstract: This work presents a unique approach for the modelling of the austenitisation of martensite in dual-phase steels within the phase-field method. Driving forces for nucleation and growth are derived from thermodynamic databases. Routines for nucleation are based on a discretisation of the classical nucleation theory. Validation is given via dilatometric experiments.

Abstract: The orientation relationships that apply to phase transformations in high-performance TRIP and TWIP steels were characterised by orientation imaging and EBSD techniques. The results are presented in the fundamental zones of Rodrigues-Frank space that correspond to the specific phase transformation under consideration (cubic to cubic or cubic to hexagonal). The use of Rodrigues-Frank space enables straightforward comparison to be made with orientation relationships proposed in the literature. The observations indicate that the active slip systems in the parent phase play important roles in variant selection.

Abstract: Deformations at temperatures 900 °C, 860 °C, 810 °C and 780 °C in the consecutive amounts 24%, 20%, 19% and 18.5% were applied to low carbon HN5MVNb bainitic steel using hot compression testing in dilatometer Bähr 805 followed by continuous cooling. The results show clearly that the kinetics of the austenite decomposition were depended on local equilibrium conditions between recovery, recrystallization and phase transformation processes for a given cooling rate. Bainite transformation was accelerated when sample was cooled after deformation at cooling rate 60 °C/s. At lower cooling rates than 5 °C/s down to 0.5 °C/s, bainite transformation was postpone when comparing its kinetics with those for non deformed steel. The bainitic transformation cannot be fitted to a single transformation mechanism owing to the formation of carbides. Different behavior was observed for austenite to ferrite transformation. Usually it was accelerated with consecutive deformations of the steel for all cooling rates used in experiments.

Abstract: The first step in the heat-treatment processes for a vast majority of commercial steels is austenitization. There is much less research put in this field comparing to the cooling transformation, but the interest is continuously increasing especially in view of the development of TRIP and Dual-phase steels. The microstructural evolution during continuous heating experiments has been studied for a series of C-Mn steels with carbon contents in the range 0.35-0.45 wt. % using optical and scanning electron (SEM) microscopy. It is shown that the formation of the austenitic phase is possible in pearlitic as well as in ferritic regions, although in the former it proceeds at a much faster rate due to the shorter diffusion distances. Thus a considerable overlap in time of the ferriteto- austenite and the pearlite-to-austenite transformations is likely to occur. Another observation that was made during the experiments is that depending on the heating rate, the pearlite-to-austenite transformation can proceed in either one or two steps. At low heating rates (0.05 °C/s) ferrite and cementite plates transform simultaneously. At higher heating rates (20 °C/s) it is a two-step process: first ferrite transforms to austenite within pearlite grains and then the dissolution of the cementite lamellae takes place.

Abstract: In order to develop an economical production method of high Si steel sheet, 6%Si (by weight percent, unless specified otherwise) steel was prepared by a combined process of conventional casting and hot- and cold-rolling. Tension and nano-hardness tests and TEM analysis were carried out to examine the effects of ordered phases, Si-content, and testing temperature on cold workability. By optimizing the successive processes of casting, hot-rolling, heat treatment, and cold-rolling, 0.5 mm thick 6%Si steel sheet was successfully produced without crack formation. As Si content increased from 3% to 6%, core loss (W10/50) of the 0.5 mm thick Si steel sheet decreased from 1.36 W/kg to 0.89 W/kg.

Abstract: Fe and Fe-C based alloys present the exceptional feature that the processing route can be adapted to lead to various phases that present antagonist mechanical properties ranging from soft ferrite to high strength martensite. Among the different deformation mechanisms that can be exhibited by these phases, the TRIP effect brings about large enhancements of the work-hardening rate. The current TRIP-assisted multiphase steels present a ferrite-based matrix with a distribution of islands of bainite and retained austenite obtained at the end of specific thermal or thermomechanical treatments. The present study aims at characterising the interactions occurring between ferrite recrystallisation and austenite formation during the intercritical annealing of cold rolled Nb-added TRIP-aided steels. It is shown that the addition of niobium retards the ferrite recrystallisation during heating. As a consequence, ferrite may not be completely recrystallised before the nucleation and growth of the austenite grains. Strong interactions between these phenomena can then be observed, i.e. a strong hindering of the ferrite recrystallisation due to the austenite formation. Furthermore, the heating rate from room temperature to the intercritical temperature range influences the thermodynamic conditions prevailing at the ferrite / austenite interface and dictates the phase proportions.

Abstract: A novel concept of two-step austempering in a magnetic field has been conceived by this investigator. This twostep process involves first quenching the alloy to a lower temperature after austenitizing and then immediately rising the temperature of the quenching media to a higher temperature and the whole austempering process is carried out in the presence of a high magnetic field. In this investigation, Austempered Ductile Iron (ADI) was processed by this novel two-step austempering process in a high magnetic field of 20 Tesla. The microstructure and the mechanical properties of the ADI processed in a magnetic field has been characterized and compared with ADI processed by conventional process. The results of this investigation indicate significant improvement in the mechanical properties of ADI when Austempered in a high magnetic field. Both yield and tensile strengths were higher in the samples processed in the presence of a magnetic field. Very fine grain ferrite and austenite was obtained in the microstructure. The ferrite content was also significantly higher.