Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

The use of heavy gauge steel sheets for structural applications often requires a combination of high yield strength and adequate toughness. The most cost effective way to achieve high yield strength and high ductility in low alloyed steels is through grain refinement. In industrial practice, such refinement is commonly obtained by thermomechanical controlled processing (TMCP). This approach comprises slab reheating to well defined temperatures, a large amount of hot deformation below the non-recrystallisation temperature Tnr and accelerated cooling. In practice, the Tnr is generally raised by the addition of microalloying elements such as Nb and Ti. As these elements contribute substantially to the alloying costs, optimisation of their use allows for a decrease in production cost. Better understanding of the Tnr assists in tuning the rolling process so that optimum mechanical properties can be produced. One area of importance is to recognise that the concept of the Tnr was originally developed for...

Modern HSLA steels and role of non-recrystallisation temperature

Deformation dilatometry has been used to simulate controlled hot rolling followed by cooling of a Nb–V low carbon steel, looking for conditions corresponding to wide austenite grain size distributions prior to transformation. Recrystallization and non-recrystallization deformation schedules were applied, followed by controlled cooling at rates from 0.1 °C/s to about 200 °C/s, and the corresponding continuous cooling transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling rates to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformations, displacing the CCT curve to higher temperatures and shorter times. However, it was found that the accelerating effect of strain on bainite transformation weakened as the cooling rate diminished and the polygonal ferrite formation was enhanced. Moreover, it was found that plastic deformation had different effects on the refinement of the microstructure, depending on the cooling rate. An analysis of the microstructural heterogeneities that can impair toughness behavior has been done.

Effect of austenite microstructure and cooling rate on transformation characteristics in a low carbon Nb–V microalloyed steel

The dynamic recrystallization as well as meta‐dynamic and static recrystallization of the nickel‐based alloy 80A was investigated by means of electron backscatter diffraction (EBSD). Specimens were hot compressed at a temperature of 1120°C and a strain rate of 0.1/s at varying strain and soak times to describe the recrystallization behaviour. Various approaches were tested in order to differentiate between recrystallized and deformed grains based on EBSD data. The grain orientation spread was clearly found to be the most reliable procedure. A high twinning of the recrystallized grains was observed, and as a consequence the measured grain size was strongly dependent on whether the coherent and incoherent twin boundaries were regarded as genuine boundaries or removed.

Recrystallization behaviour of the nickel based alloy 80A during hot forming

The mechanical properties of Fe-Mn-Al-C steel are significantly enhanced after κ-carbide precipitation via aging; however, most aging treatments are energy demanding because they require relatively high temperatures and extended holding times. This research determined that the precipitation of these carbides can also occur within a few seconds of thermomechanical treatments (TMTs). This behaviour has not been reported post-TMTs for this steel group. Hot compression tests were performed on Fe-21Mn-11Al-1.5C-2Si wt% specimens at test temperatures ranging from 900 °C to 1150 °C and strain rates varying from 0.01 s−1 to 1 s−1. The effects of strain rate and test temperature on dynamic recrystallization behaviour were evaluated. The microstructures were characterized by scanning electron microscope and electron backscatter diffraction. Hardness tests were performed before and after applying processes studied i.e., TMT and aging treatment to determine the change in hardness induced. Particularly, nanoindentation tests were also used to collect indirect evidence about the deformation mechanisms. The load-displacement curves P-h and (P/h)-h showed the occurrence of several pop-ins and slope changes related to the nucleation of dislocations and strain-induced phase transformations. The occurrence of these phenomena is discussed.

Hot deformation of a Fe-Mn-Al-C steel susceptible of κ-carbide precipitation

The influence of the Mo addition on the dynamic recrystallization behavior of Nb microalloyed steels was studied. The initial austenite grain size, the amount of microalloying elements in solid solution and the deformation conditions (temperature and strain rate) affect dynamic recrystallization kinetics of both Nb and Nb-Mo steels. Continuous torsion tests were carried out to characterize the dynamic recrystallization behavior of the microalloyed austenite, after reheating the specimens at different temperatures between 1 100 and 1 460°C, this brought a wide range of initial grain sizes, from 22 to 805 μm. It was observed that decreasing the values of the Zener-Hollomon parameter and of the initial grain size promotes dynamic recrystallization. Mo in solid solution produces a large retardation effect in the dynamic recrystallization and brings higher values in the characteristic critical, e C , and peak, e P , strains, being this effect independent of the Mo content. A corrective factor has been applied to quantify the retardation produced by Mo in solid solution. This means that it was possible to propose a unique relationship to predict the peak strain for Nb, Nb-Ti and Nb-Mo steels.

Effect of Mo on Dynamic Recrystallization Behavior of Nb–Mo Microalloyed Steels

One of the options available for coping with current property requirements is to increase microalloying contents. As well as affecting austenite microstructural evolution, this can lead to a larger contribution of precipitation and dislocation density hardening to the mechanical strength. However, it also results in more complex interactions between the strengthening mechanisms, making it difficult to optimize operation parameters. In this work, plane strain compression tests were carried out to produce different austenite microstructures with two low carbon steels microalloyed with different Nb levels (0.04% and 0.11%), followed by coiling simulations in the 500 to 700 °C temperature range. The mechanical properties of the transformed microstructures were determined via tensile testing and the specimens were characterized using optical microscopy, EBSD and TEM. Similar austenite grain sizes and strain accumulation levels were obtained for both steels, and as a result, it was observed that increasing Nb content had a small effect on the grain size of the transformed microstructures. On the other hand, the coiling temperature significantly affected the mechanical strength. The lowest strength values are obtained for 700 °C; these increase significantly at 600 °C (≅90 and 140 MPa increase in the Yield Strength (YS) for the low and high Nb steels, respectively) and decrease slightly at 500 °C. At the conditions investigated, similar dislocation density levels were determined for both steels, which indicates that the larger YS increase observed for the high Nb steel must be mainly due to the effect of precipitates formed during or after phase transformation.

Effect of thermomechanical treatment and coiling temperature on the strengthening mechanisms of low carbon steels microalloyed with Nb

While the role of Nb during the processing of austenite is quite clear, what happens in subsequent stages to the concentration of this element left in solution is subject to some debate. In particular, some uncertainty still subsists concerning the eventual homogeneous precipitation in Nb supersaturated polygonal ferrite. The present work was aimed at clarifying the precipitation sequence of Nb during coiling, through a systematic work and a careful selection of the processing conditions in order to produce different scenarios concerning the initial state of Nb. A Nb-microalloyed steel was thermomechanically processed in the laboratory followed by simulated coiling at different temperatures in the 873 K to 1023 K (600 °C to 750 °C) range. Transmission electron microscopy (TEM) showed interphase precipitation of NbC at high coiling temperatures, while at 873 K (600 °C), homogeneous general precipitation took place in ferrite and followed a Baker–Nutting orientation relationship.

Precipitation of Nb in Ferrite After Austenite Conditioning. Part I: Microstructural Characterization

One of the main problems of empirical and semi-empirical models that are used to predict austenite microstructure evolution during hot rolling is their limited range of applicability regarding chemical compositions. This paper aims to improve a previous microstructural model paying special attention to the development of a more general equation for the prediction of the onset of strain induced precipitation in Nb microalloyed steels. This has been done with the help of new expressions describing the kinetics of strain induced precipitation as a function of the chemical composition. Moreover, the evolution of strain induced Nb precipitation was also modeled in order to determine the amount of Nb precipitated after each strain pass. The predictions obtained with the new model have been compared to results obtained in multipass laboratory tests using several Nb microalloyed steels. The model allows us to better understand the complex interactions occurring between Nb and microstructural softening depending on composition and process parameters.

Improved Model of Kinetics of Strain Induced Precipitation and Microstructure Evolution of Nb Microalloyed Steels during Multipass Rolling

The hot deformation and static softening behavior of various high Mn (20–30 wt%) austenitic steels microalloyed with different V (0.1, 0.2 wt%), C (0.2, 0.6, 1 wt%) and N (0.005–0.025 wt%) contents were investigated. Double-hit torsion tests at temperatures in the range 700–1100 °C were carried out and specimens quenched at selected conditions were examined using advanced microscopy techniques (EBSD-TEM) to characterize the recrystallization and strain-induced precipitation behavior. The results show that precipitation of vanadium at the hot working temperature range is sluggish. It mainly occurs for the combinations of 20%Mn–0.6%C–0.2%V and 30%Mn–1%C–0.1%V. When the carbon content is reduced to 0.2%C, strain-induced precipitation is suppressed at typical hot working temperatures, independently of the N level. The flow stress behavior was affected by the amount of C and by modifying the base composition from 30%Mn to 20%Mn–1.5%Al. However, the effect is complex and depends on deformation conditions. In the absence of strain-induced precipitation, the static softening kinetics was accelerated by increasing C content. However, no effect of Mn or V in solid solution was observed. In those cases where strain-induced precipitation took place, static recrystallization was severely delayed, leading to a major contribution of recovery to softening kinetics.

Beatriz Pereda

Papers

Effect of Mo on Dynamic Recrystallization Behavior of Nb–Mo Microalloyed Steels

Effect of thermomechanical treatment and coiling temperature on the strengthening mechanisms of low carbon steels microalloyed with Nb

Precipitation of Nb in Ferrite After Austenite Conditioning. Part I: Microstructural Characterization

Improved Model of Kinetics of Strain Induced Precipitation and Microstructure Evolution of Nb Microalloyed Steels during Multipass Rolling

Hot deformation and static softening behavior of vanadium microalloyed high manganese austenitic steels