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<p>Recrystallisation annealing, a repeated heat treatment between different stages of cold rolling of martensitic chromium steel strip, is successful when neither high rolling forces nor wear of the working rolls occur during the subsequent cold rolling. Mechanical properties as tensile strength, yield, elongation or hardness have been, by tradition, the criteria that described the quality of the annealing process. In recent years, the development of the measurement equipment in the rolling mills and of the instruments for material investigations has accentuated more and more the role played by the microstructural properties in the evaluation of the heat treatment. Two microstructural characteristics of the degree of annealing are, firstly and most important, the recrystallisation degree, and, secondly, the secondary carbide density.</p><p>The sample manufacturing and heat treatment, modelling and microstructure investigations by light optical- (LOM) and scanning electron microscopy (SEM) described in this article were carried out at Sandvik Materials Technology’s R&D Department and Bell Furnace Line in Sandviken, Sweden, while microstructure investigations and evaluation by scanning electron microscopy with field emission gun (FEG-SEM) and electron back scatter diffraction (EBSD) were done at the Corrosion and Metals Research Institute (KIMAB) in Stockholm, Sweden.</p><p>The first part of this work shows that, in contrast to the traditional methods LOM and SEM, that use chemical etching for the preparation of the samples, EBSD can successfully characterise recrystallised structures in annealed martensitic chromium steels. Unlike conventional microscopy with LOM and SEM, EBSD is able to reveal the grain geometry, as well as to separate and identify the different phases in this kind of steels (ferrite, M23-, M6-carbides). Important parameters such as grain size, particle size and recrystallised fraction can be measured with high accuracy. This information can be used to understand, evaluate, control and even predict the recrystallisation annealing of martensitic chromium steel.</p><p>The second part of this work presents how the results from microstructure description by EBSD can be directly used in relatively simple empirical models for determination of recrystallisation degree as function of the annealing parameters and the deformation history. EBSD was applied to evaluate the degree of recrystallisation in a series of annealing tests, with the purpose to model recrystallisation temperature in two types of martensitic chromium steel strip, a traditional one and one alloyed with molybdenum, cold rolled with different amounts of reduction and annealed with different temperatures, soaking times and heating rates. The empirical quadratic models were built with Umetrics’ software for experimental design, MODDEÒ 8.0 and they defined the recrystallisation degree (limits for LAGB and HAGB were set to 1.5° and 7.5° for the first grade and 2.5° and 10° for second one) and the secondary carbides density as functions of annealing temperature, soaking time and cold reduction (the factor heating rate was removed as nonsignificant). To be observed that these empirical models were fit much better for the recrystallisation degree than for the secondary carbides density.</p><p>The modelling work described above, together with the implementation of online physical temperature models in the bell annealers may lead to an increased productivity in the production plant by shortening the annealing cycle and minimising scrap and thus to an economical gain of ca 1,5 MSEK per year at Sandvik Materials Technology.</p><p> </p>

Η συγκεκριμένη εργασία συνιστά προϊόν μικροσκοπικής εξέτασης και ανάλυσης 50 δειγμάτων (λεπτές τομές) τα οποία συλλέχθηκαν από χαλαζιτικούς ορίζοντες της Φυλλιτικής-Χαλαζιτικής σειράς της νότιας Πελοποννήσου, περιοχή Ταϋγέτου και Πάρνωνα, των Κυθήρων και τέλος, της κεντρικής και δυτικής Κρήτης. Κύριος σκοπός της εργασίας είναι η μελέτη των μικροδομών, όπως αυτές προέκυψαν από τη δράση των μηχανισμών παραμόρφωσης στα πετρώματα της ενότητας, καθώς και η ερμηνεία των αποτελεσμάτων της μικροσκοπικής εξέτασης, η οποία αφορά το μέγεθος των κόκκων του χαλαζία (grain size) και κατ' επέκταση την τάση ροής (flow stress). Σε πρώτη φάση γίνεται εκτεταμένη αναφορά, στη χωρική κατανομή (γεωγραφική και κατακόρυφη) της φυλλιτικής-χαλαζιτικής σειράς στον ελλαδικό χώρο, στη μελέτη τεκτονικών κυρίως, αλλά και πετρογραφικών δεδομένων, τα οποία συντελούν στην κατανόηση του τρόπου και των συνθηκών σχηματισμού των πετρωμάτων της ενότητας αυτής. Ακολουθεί εργαστηριακή εξέταση, με τη βοήθεια οπτικού μικροσκοπίου και σχολιασμός των χαρακτηριστικών των δειγμάτων. Τέλος, γίνεται αναφορά στον τρόπο λήψης των μετρήσεων, στην επεξεργασία των δεδομένων με γραφήματα, στη συσχέτιση και ερμηνεία των αποτελεσμάτων. / -

The annealing behaviour, including studies of recrystallisation kinetics and development of crystallographic texture, of two low carbon steels after different cold rolling reductions have been investigated using Optical Microscopy (OM), Electron Back-Scatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). The primary recrystallisation behaviour of 20, 50, 70 and 90% cold rolled Interstitial Free (IF) and High Strength Low Alloy (HSLA) steels was studied. The HSLA was initially processed to give a volume fraction of about 0.2 of fine pearlite colonies, which acted as mechanically hard particles. The presence of such particles on the HSLA steel significantly reduced the temperature needed for recrystallisation by enhancing the recrystallisation and acting as nucleation sites by the Particle Stimulated Nucleation (PSN) mechanism. The inhomogeneous deformation and the local orientation changes introduced in the neighbourhood of the carbide particles (i.e. the particle deformation zone) were observed using TEM and selected area electron diffraction. The JMAK model was used to analyse the recrystallisation kinetics of the two steels. The experimental data plotted according to the JMAK model could be represented by straight lines with a JMAK exponent n falling in the range from 1.4 to 2.0. The development of crystallographic textures after cold rolling reductions and subsequent recrystallisation has been investigated. The texture development is shown to be largely dependent on the rolling reduction. With increasing rolling reduction, the annealing texture show gradual intensification of α- and γ-fibre components. However, the intensity of both fibres is weaker in the HSLA steel. Despite the dominance of PSN in HSLA steel, the recrystallisation textures were similar to that of the rolling textures with weaker intensity of both fibres. To help clarify the reasons for that, in-situ EBSD experiments of recrystallising HSLA steel deformed to 50% and 70% have been carried out. It shows that the formation of the nucleus seems to occur within the deformation zones in regions away from the particle surface leading to recrystallisation textures similar to that of the rolling textures (i.e. both α- and γ-fibre exist). The validity of this assumption has been confirmed by the use of Monte Carlomodelling. This model was used to simulate, in general way, and study the phenomenon of PSN during the recrystallisation process. The simulation shows the discontinuous evolution of the subgrains in the deformation zone to form recrystallisation nuclei around the particle. It shows also the subsequent growth of these nuclei to consume the matrix region around the particle. The simulation results are shown to match with the experimentally observed features of the recrystallisation phenomena in low carbon steel containing coarse cementite particles.

Magnesium has the lowest density of any structural metal making it a strong candidate for weight savings in the aerospace and automotive industries. However, strong crystallographic textures combine with anisotropic deformation modes to severely limit formability in wrought magnesium alloys. Recently improved formability has been achieved by the addition of small concentrations of solute rare earth elements which reduce the intensity of recrystallisation textures. Developing a mechanistic understanding of this effect is critical in leading alloy design towards a new class of highly formable wrought magnesium alloys. In this study the static recrystallisation mechanism of rolled magnesium rare earth alloys, which causes the texture weakening, is examined with a particular emphasis on the contrasting texture weakening effects in binary and tertiary magnesium rare earth alloys. In binary magnesium-rare earth alloys the `rare-earth' texture is simply a weakened deformation texture, while recrystallisation of magnesium-zinc-rare earth alloys produces unique `rare-earth' texture components. In the binary alloys weakened recrystallisation textures are attributed to the generation of `off-basal' orientations within regions of high strain localisation during deformation. These orientations recrystallise and subsequently dominate the recrystallised texture. Texture weakening by this mechanism is also thought to be observed in non-rare earth magnesium alloys where dynamic recrystallisation is suppressed by cold rolling. The unique rare-earth texture components in magnesium-zinc-rare earth alloys are found to be determined by the orientation of shear bands in the material. Similarly to texture weakening in the binary alloys, nuclei for these orientations are thought to develop during deformation as a result of strain incompatibilities within shear bands. The mechanism forming these orientations remains unclear, however it is postulated that a complex change in recovery behaviour within shear bands, as a result of rare earth and zinc additions, may be the cause. Retarded dynamic recrystallisation is suggested to be of critical importance in the texture weakening mechanisms of all magnesium alloys, both rare earth and non-rare earth. In rare earth alloys dynamic recrystallisation is suppressed by the segregation of rare earth atoms to grain boundaries. A combination of high resolution TEM and EDX shows rare earth atoms form clusters approximately 2nm in diameter on grain boundaries which are expected to retard dynamic recrystallisation through a solute drag mechanism.

Recrystallisation annealing, a repeated heat treatment between different stages of cold rolling of martensitic chromium steel strip, is successful when neither high rolling forces nor wear of the working rolls occur during the subsequent cold rolling. Mechanical properties as tensile strength, yield, elongation or hardness have been, by tradition, the criteria that described the quality of the annealing process. In recent years, the development of the measurement equipment in the rolling mills and of the instruments for material investigations has accentuated more and more the role played by the microstructural properties in the evaluation of the heat treatment. Two microstructural characteristics of the degree of annealing are, firstly and most important, the recrystallisation degree, and, secondly, the secondary carbide density. The sample manufacturing and heat treatment, modelling and microstructure investigations by light optical- (LOM) and scanning electron microscopy (SEM) described in this article were carried out at Sandvik Materials Technology’s R&amp;D Department and Bell Furnace Line in Sandviken, Sweden, while microstructure investigations and evaluation by scanning electron microscopy with field emission gun (FEG-SEM) and electron back scatter diffraction (EBSD) were done at the Corrosion and Metals Research Institute (KIMAB) in Stockholm, Sweden. The first part of this work shows that, in contrast to the traditional methods LOM and SEM, that use chemical etching for the preparation of the samples, EBSD can successfully characterise recrystallised structures in annealed martensitic chromium steels. Unlike conventional microscopy with LOM and SEM, EBSD is able to reveal the grain geometry, as well as to separate and identify the different phases in this kind of steels (ferrite, M23-, M6-carbides). Important parameters such as grain size, particle size and recrystallised fraction can be measured with high accuracy. This information can be used to understand, evaluate, control and even predict the recrystallisation annealing of martensitic chromium steel. The second part of this work presents how the results from microstructure description by EBSD can be directly used in relatively simple empirical models for determination of recrystallisation degree as function of the annealing parameters and the deformation history. EBSD was applied to evaluate the degree of recrystallisation in a series of annealing tests, with the purpose to model recrystallisation temperature in two types of martensitic chromium steel strip, a traditional one and one alloyed with molybdenum, cold rolled with different amounts of reduction and annealed with different temperatures, soaking times and heating rates. The empirical quadratic models were built with Umetrics’ software for experimental design, MODDEÒ 8.0 and they defined the recrystallisation degree (limits for LAGB and HAGB were set to 1.5° and 7.5° for the first grade and 2.5° and 10° for second one) and the secondary carbides density as functions of annealing temperature, soaking time and cold reduction (the factor heating rate was removed as nonsignificant). To be observed that these empirical models were fit much better for the recrystallisation degree than for the secondary carbides density. The modelling work described above, together with the implementation of online physical temperature models in the bell annealers may lead to an increased productivity in the production plant by shortening the annealing cycle and minimising scrap and thus to an economical gain of ca 1,5 MSEK per year at Sandvik Materials Technology.