Isasti-Gordobil, N. (Nerea)
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- Effect of carbon content and cooling rate on the microstructure and hardness of TiC-Fe-Cr-Mo cermets(2024) Isasti-Gordobil, N. (Nerea); Lozada-Cabezas, L. (Lorena); Sánchez-Moreno, J.M. (José Manuel); Ibarreta-Lopez, F. (Federico); Martinez-Pampliega, R. (Roberto); Navarrete-Cuadrado, J. (Jazmina); Soria-Biurrun, T. (Tomás)TiC-FeCrMo cermets have been obtained in fully dense form by Sinter HIP at 1400 degrees C. Significant microstructural changes have been observed in these materials for relatively small variations in their carbon content after sintering. In the cermets with higher carbon content Cr-rich likely M7C3 carbides are observed to precipitate at the (Ti1-x,Mo-x)(y)C-z - metal interface. In addition, these cermets present a significant amount of retained austenite as part of the metal matrix. No retained austenite and many fewer M7C3 carbides are found in alloys with a reduction of 0.2 wt% in the total C content. Continuous cooling diagrams have been obtained from an austenitizing temperature of 950 degrees C. Hardness increases by 30% with respect to that of as sintered specimens after cooling at 1 degrees C/s confirming that these TiC-FeCrMo cermets are suitable for hardening by air-quenching. At this cooling rate, it is observed that the relatively small carbon changes mentioned before have a significant effect on the bainitic transformation, displacing its onset to higher temperatures as the C content is reduced. Slower cooling rates result in complex microstructures, in which, in addition to martensite, ferritic bainite, M7C3 and M23C6 carbides are also found. Microstructure and hardness of TiC-FeCrMo materials can be modified by the use of standard heat treatments to obtain a wide variety of mechanical properties suitable for certain hot rolling applications.
- An EBSD-based methodology for the characterization of intercritically deformed low carbon steel(Elsevier, 2019) Isasti-Gordobil, N. (Nerea); Rodriguez-Ibabe, J.M. (José María); Uranga-Zuaznabar, P. (Peio); Jorge-Badiola, D. (Denis); Mayo-Ijurra, U. (Unai)Heavy gauge structural plates has been widely rolled in the austenite/ferrite two phase region, in order to meet the demanding market requirements regarding tensile properties. Even though strength levels can be increased by intercritical rolling, toughness properties may be impaired. Therefore, a greater knowledge of how different austenite-ferrite balances affect the microstructural evolution during intercritical deformation is required. With the aim of gaining a deep comprehension of the evolution of the microstructure during intercritical deformation, dilatometry tests were performed simulating intercritical rolling conditions. Different ferrite populations are identified in the resulting microstructures, composed of intercritically deformed ferrite and non-deformed ferrite transformed during final air cooling. In the deformed ferrite grains well defined substructure is clearly noticed, whereas the non-deformed grains formed during air cooling step do not show any evidence of substructure. In the current work, EBSD advanced characterization technique was used to develop a methodology that is able to differentiate the intercritically deformed ferrite from non-deformed ferrite for low carbon steels. Based on the Grain Orientation Spread (GOS) parameter, a threshold value of 2 degrees was defined to distinguish deformed and non deformed ferrite grains. The proposed procedure allows distinguishing both ferrite populations and quantifying microstructural parameters of each family. The effect of the addition of C and austenite-ferrite balance on the microstructural evolution of each ferrite type was analyzed.
- Effect of intercritical deformation on final microstructure in low carbon grades.(2017) Isasti-Gordobil, N. (Nerea); Rodriguez-Ibabe, J.M. (José María); Uranga-Zuaznabar, P. (Peio); Mayo-Ijurra, U. (Unai)Heavy gauge line pipe and structural steel plate materials are often rolled in the two-phase region for strength reasons. However, strength and toughness show opposite trends and the exact effect of each rolling process parameter remains unclear. Even though intercritical rolling has been widely studied, further understanding of the evolution of the microstructure under intercritical conditions and the effect of different austenite-ferrite phase contents at high temperature is needed to define stable processing windows. For that purpose, laboratory thermomechanical simulations reproducing intercritical rolling conditions have been performed in low carbon steels. A methodology able to distinguish deformed ferrite from non-deformed ferrite was developed using Electron Backscattered Diffraction (EBSD) technique. The effect of chemical composition, austenite-ferrite balance and deformation on the final microstructure was evaluated. This analysis is intended to deepen the knowledge of the effect that intercritical rolling has on the microstructural evolution and, by extension, on the mechanical properties.
- Relation between microstructure and mechanical properties on intercritically deformed low carbon steels.(Elsevier, 2020) Isasti-Gordobil, N. (Nerea); Rodriguez-Ibabe, J.M. (José María); Uranga-Zuaznabar, P. (Peio); Mayo-Ijurra, U. (Unai)Intercritical rolling is often applied in order to improve the mechanical properties of heavy gauge structural steel plates. However, these products have large temperature and deformation gradients both over thickness and width, being the control of the process very complex. Considering this issue, it is important to analyze the microstructural evolution under intercritical conditions and how the different process parameters like deformation temperature, chemical composition etc. influence on the final mechanical properties. For that purpose, plane strain compression tests simulating intercritical rolling conditions were carried out for a CMn and a NbV microalloyed steel. Tensile tests perfomed for various austeniteferrite contents prior to the last deformation, show that the reduction of deformation temperature increases strength properties. However, ferrite fractions higher than 25% prior to the last deformation, reduce significantly the ductility. These analyses give the necessary background for the definition of stable process windows that optimize the strength-ductility property balance of intercritically rolled products. Similarly, the estimation of the contribution of different strengthening mechanisms such as, solid solution, grain size and dislocation density, allows the prediction of the yield strength of an intercritically deformed microstructure.
- Interaction between microalloying additions and phase transformation during intercritical deformation in low carbon steels(MDPI AG, 2019) Isasti-Gordobil, N. (Nerea); Rodriguez-Ibabe, J.M. (José María); Uranga-Zuaznabar, P. (Peio); Mayo-Ijurra, U. (Unai)Heavy gauge line pipe and structural steel plate materials are often rolled in the two-phase region for strength reasons. However, strength and toughness show opposite trends, and the exact effect of each rolling process parameter remains unclear. Even though intercritical rolling has been widely studied, the specific mechanisms that act when different microalloying elements are added remain unclear. To investigate this further, laboratory thermomechanical simulations reproducing intercritical rolling conditions were performed in plain low carbon and NbV-microalloyed steels. Based on a previously developed procedure using electron backscattered diffraction (EBSD), the discretization between intercritically deformed ferrite and new ferrite grains formed after deformation was extended to microalloyed steels. The austenite conditioning before intercritical deformation in the Nb-bearing steel affects the balance of final precipitates by modifying the size distributions and origin of the Nb (C, N). This fact could modify the substructure in the intercritically deformed grains. A simple transformation model is proposed to predict average grain sizes under intercritical deformation conditions.
- Production of a non-stoichiometric Nb-Ti HSLA steel by thermomechanical processing on a steckel mill(2023) De-Souza, A.L. (Altair Lucio); Isasti-Gordobil, N. (Nerea); Rodriguez-Ibabe, J.M. (José María); Rebellato, M.O. (Marcelo Orantes); Uranga-Zuaznabar, P. (Peio); Gorni, A.A. (Antoni Augusto); De-Faria, G.L. (Gerardo Lucio); Martins, C.A. (Cleiton Arlindo); Cohn, J.A.C. (Jorge Adam Cleto); Mayo-Ijurra, U. (Unai)Obtaining high levels of mechanical properties in steels is directly linked to the use of special mechanical forming processes and the addition of alloying elements during their manufacture. This work presents a study of a hot-rolled steel strip produced to achieve a yield strength above 600 MPa, using a niobium microalloyed HSLA steel with non-stoichiometric titanium (titanium/nitrogen ratio above 3.42), and rolled on a Steckel mill. A major challenge imposed by rolling on a Steckel mill is that the process is reversible, resulting in long interpass times, which facilitates recrystallization and grain growth kinetics. Rolling parameters whose aim was to obtain the maximum degree of microstructural refinement were determined by considering microstructural evolution simulations performed in MicroSim-SM (R) software and studying the alloy through physical simulations to obtain critical temperatures and determine the CCT diagram. Four ranges of coiling temperatures (525-550 degrees C/550-600 degrees C/600-650 degrees C/650-700 degrees C) were applied to evaluate their impact on microstructure, precipitation hardening, and mechanical properties, with the results showing a very refined microstructure, with the highest yield strength observed at coiling temperatures of 600-650 degrees C. This scenario is explained by the maximum precipitation of titanium carbide observed at this temperature, leading to a greater contribution of precipitation hardening provided by the presence of a large volume of small-sized precipitates. This paper shows that the combination of optimized industrial parameters based on metallurgical mechanisms and advanced modeling techniques opens up new possibilities for a robust production of high-strength steels using a Steckel mill. The microstructural base for a stable production of high-strength hot-rolled products relies on a consistent grain size refinement provided mainly by the effect of Nb together with appropriate rolling parameters, and the fine precipitation of TiC during cooling provides the additional increase to reach the requested yield strength values.
- Effect of quenching strategy and Nb-Mo sdditions on phase transformations and quenchability of high-strength boron steels(2021) Mohrbacher, H. (Hardy); Isasti-Gordobil, N. (Nerea); Uranga-Zuaznabar, P. (Peio); Detemple, E. (Eric); Zurutuza, I. (Irati); Schwinn, V. (Volker)The application of direct quenching after hot rolling of plates is being employed in the production of ultra-high-strength hot rolled plates. When heavy gauge plates are produced, the complexity involve in achieving high cooling rates in the plate core is increased and the formation of undesirable soft phases within martensite is common. In the current paper, both direct quenching and conventional quenching (DQ and CQ) processing routes were reproduced by dilatometry tests and continuous cooling transformation (CCT) diagrams were built for four different high-strength boron steels. The results indicate that the addition of Mo and Nb-Mo suppresses the ferritic region and considerably shifts the CCT diagram to lower transformation temperatures. The combination of DQ strategy and the Mo-alloying concept provides the best option to ensure hardenability and the formation of a fully martensitic microstructure, and to avoid the presence of soft phases in the center of thick plates.
- Effect of microstructure on post-rolling Induction treatment in a low C Ti-Mo microalloyed steel(MDPI AG, 2018) Isasti-Gordobil, N. (Nerea); Rodriguez-Ibabe, J.M. (José María); Uranga-Zuaznabar, P. (Peio); Larzabal-Primo, G.(Gorka)Cost-effective advanced design concepts are becoming more common in the production of thick plates in order to meet demanding market requirements. Accordingly, precipitation strengthening mechanisms are extensively employed in thin strip products, because they enhance the final properties by using a coiling optimization strategy. Nevertheless, and specifically for thick plate production, the formation of effective precipitation during continuous cooling after hot rolling is more challenging. With the aim of gaining further knowledge about this strengthening mechanism, plate hot rolling conditions were reproduced in low carbon Ti-Mo microalloyed steel through laboratory simulation tests to generate different hot-rolled microstructures. Subsequently, a rapid heating process was applied in order to simulate induction heat treatment conditions. The results indicated that the nature of the matrix microstructure (i.e., ferrite, bainite) affects the achieved precipitation hardening, while the balance between strength and toughness depends on the hot-rolled microstructure.
- Impact of two-phase region rolling on the microstructure and properties distribution in heavy gauge structural steel plate (INCROHSS).(Publications Office of the European Union, 2020) Isasti-Gordobil, N. (Nerea); Caruso, M. (M.); Lorenz, U. (U); Nguyen, M.T. (Minh); Sietsma, J. (J.); Akbary, F.H.(F.H.); Uranga-Zuaznabar, P. (Peio); Duprez, L. (L.); Petrov, R. (R.); Tolleneer, I. (I.); Mayo-Ijurra, U. (Unai)Heavy gauge line-pipe and structural steel plates are often rolled in the two-phase region for strength reasons. However, strength and toughness show opposite trends and the exact effect of each rolling process parameter remains unclear. A stable process window can only be achieved by a more profound understanding of the microstructure development during the intercritical rolling and its relationship with final microstructures and mechanical properties. By means of recently developed microstructure investigation techniques and modeling, the relationship between the temperature gradient, α-γ phase balance at high temperature, strain partitioning between phases and subsequent transformation was studied in detail to allow for wider process windows.
- On the characterization procedure to quantify the contribution of microstructure on mechanical properties in intercritically deformed low carbon HSLA steels(Elsevier, 2020-08) Isasti-Gordobil, N. (Nerea); Rodriguez-Ibabe, J.M. (José María); Uranga-Zuaznabar, P. (Peio); Mayo-Ijurra, U. (Unai)The mechanical properties of intercritically rolled microstructures have been scarcely reported in literature. Although the strengthening effect of intercritical rolling is generally recognized, there is no a clear opinion on its effect on toughness. Therefore, a greater knowledge of how different process parameters affect the mechanical properties during intercritical deformation is required. With the aim of evaluating the relationship between microstructure and mechanical properties on intercritically deformed low carbon steels, plane strain compression tests were carried out. Plane strain compression tests allow for both the characterization of the microstructural features and the evaluation of mechanical properties, via tensile and Charpy tests. Firstly, the intercritically deformed microstructures were characterized using the EBSD technique, and then a discretization methodology was used to distinguish both intercritically deformed and non-deformed ferrite populations. Next, strength and toughness properties were measured by means of tensile and Charpy tests. The results indicate that the reduction of the deformation temperature leads to an increment of yield strength for both steels, but at the same time toughness properties worsen. Deformed ferrite fractions higher than 25% result in a very pronounced loss of ductility. The yield strength was predicted by estimating the contribution of different strengthening mechanisms (solid solution, grain size refinement, dislocation density) corresponding to each ferrite population by considering a nonlinear law of mixtures. Similarly, the impact of different microstructural parameters (solid solution, grain size, microstructural heterogeneity, contribution of dislocation density and secondary phases) on toughness was evaluated and a new equation able to predict ductile to brittle transition temperature for intercritically deformed microstructures was developed.