Revistas
Revista:
JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS
ISSN:
0304-8853
Año:
2022
Vol.:
555
Págs.:
169265
The experimental magnetic susceptibility curves of several soft steel grades, like interstitial free (IF) or low carbon (LC) steels, can be interpreted within the framework of universal scaling functions obtained for systems with quenched disorder, such as the one described by the random field Ising model with supercritical disorder. Mean-field theory (both scalar and site-dependent) is used to explore the behaviour at the proximity of the coercive field, and explicit expressions are derived. As a result, the susceptibility values close to the coercive field can be approximated to a good extend by a Lorentzian function. Theoretical results are compared against a number of experimental curves obtained from interstitial free (IF) and extra low carbon (ELC) steels subjected to isothermal annealing.
Autores:
Vilanova, M. (Autor de correspondencia); Garciandia, F.; Sáinz, Shandra; et al.
Revista:
JOURNAL OF MATERIALS PROCESSING TECHNOLOGY
ISSN:
0924-0136
Año:
2022
Vol.:
300
Págs.:
117398
Inconel 738LC (IN738LC) is a nickel-based superalloy specially used in the hot section components of turbine engines due to its outstanding hot corrosion resistance and mechanical properties under high temperatures. However, one of the main drawbacks of this superalloy is its susceptibility to cracking when it is manufactured by Laser Powder Bed Fusion (LPBF). This paper describes the effect of 400 W laser power and 90 degrees rotation strategy on the formation of cracks and the capability of Hot Isostatic Pressing (HIP) post-treatment to substantially reduce them in the LPBF manufactured IN738LC samples. Based on the characterization of the cracks, the most important finding from this research work was the identification of the limit of crack width at 6 mu m, beyond which the HIP treatment is unable to effect crack healing. Furthermore, this research shows that the HIP treatment leads to microstructural changes in the IN738LC samples with a massive precipitation of gamma ' phase. Indeed, the formation of precipitates implied an increase in the microhardness of up to 23 %, which demonstrates that the HIP treatment also affects the mechanical properties of the IN738LC superalloy. It was therefore shown that the HIP treatment could be a crucial process to substantially reduce the defects of the additively manufactured parts.
Revista:
MATERIALS CHARACTERIZATION
ISSN:
1044-5803
Año:
2019
Vol.:
147
Págs.:
31 - 42
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.
Revista:
JOURNAL OF MATERIALS SCIENCE
ISSN:
0022-2461
Año:
2019
Vol.:
54
N°:
6
Págs.:
5044 - 5060
The development of multiphase steels to obtain an optimum balance between strength and ductility is a very active topic of research. In particular, carbide-free bainitic steels have shown promising mechanical properties, making them good candidates for replacing well-established first-generation steels in the automotive industry. In this work, a detailed analysis of the microstructures attainable through overaging treatments is tackled in two bainitic steels with different Si contents. The focus has been put onto the mechanical characterization, via nanoindentation, of the phases that are generated as a consequence of the change in the bainitic treatment conditions and the final cooling to room temperature. The results show the suitability of the nanoindentation technique for gaining knowledge about the underlying transformation-related phenomena and for measuring the relative difference in hardness of the various micro-constituents. The latter is a key factor in understanding the origin of the damage in this kind of steels.
Revista:
METALS
ISSN:
2075-4701
Carbide-free bainitic (CFB) steels belong to the family of advanced high strength steels (AHSS) that are struggling to become part of the third-generation steels to be marketed for the automotive industry. The combined effects of the bainitic matrix and the retained austenite confers a significant strength with a remarkable ductility to these steels. However, CFB steels usually show much more complex microstructures that also contain MA (Martensite-Austenite) phase and auto-tempered martensite (ATM). These phases may compromise the ductility of CFB steels. The present work analyzes the substructure evolution during tensile tests in the necking zone, and deepens into the void and crack formation mechanisms and their relationship with the local microstructure. The combination of FEG-SEM imaging, EBSD, and X-ray diffraction has been necessary to characterize the substructure development and damage initiation. The bainite matrix has shown great ductility through the generation of high angle grain boundaries and/or large orientation gradients around voids, which are usually found close to the bainite and MA/auto-tempered martensite interfaces or fragmenting the MA phase. Special attention has been paid to the stability of the retained austenite (RA) during the test, which may eventually be transformed into martensite (Transformation Induced Plasticity, or TRIP effect).
Revista:
MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING
ISSN:
0921-5093
Año:
2017
Vol.:
691
Págs.:
42 - 50
Annealing after cold rolling brings about the activation of recovery and recrystallization in microalloyed steels. The importance of recovery has been most often associated with its effects on the recrystallization kinetics. However, recovery has gained particular importance as an alternative heat treatment under the name "back annealing" or "recovery annealing". In the present work, various annealing treatments were applied to a Nbmicroalloyed steel in the range of temperatures and times where recrystallization is not complete. As a consequence, a large set of tensile strength-ductility pairs was obtained, even for conditions in which recrystallization was avoided. Through non-destructive magnetic coercive field measurements, recovery and partial recrystallization were monitored for each annealing treatment. Magnetic softening is significantly greater than mechanical softening. The variation in recovery in terms of temperature and time is highly affected by the presence of Nb in solution in the hot band (before cold rolling). At low recovery annealing temperatures, 350-450 degrees C, Nb solute drag on dislocations is the main mechanism that controls recovery, whereas at 550 degrees C, Nb strain induced precipitation leads to a recovery plateau in terms of coercive field.
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2017
Vol.:
48A
N°:
6
Págs.:
2943 - 2948
Very often Nb contributes to the strength of a microalloyed steel beyond the expected level due to the grain size strengthening resulting from thermomechanical processing. Two different mechanisms are behind this phenomenon, and both of them have to do with the amount of Nb remaining in the solution after hot rolling. The first of them is the increase of the hardenability of the steel due to Nb, and the second one is the fine precipitation of NbC in ferrite. The contribution of the precipitates to the work hardening of two thermally and thermomechanically processed microalloyed steels is addressed in this work and this contribution has been integrated into previously developed models by the authors for ferrite-pearlite microstructures. An L (eff) is considered through the effective spacing associated to the different obstacles and their interactions with the moving dislocations. The model obtained shows good agreement with the experimental tensile curves from the end of yield point elongation to the onset of necking. (C) The Minerals, Metals & Materials Society and ASM International 2017
Revista:
JOM
ISSN:
1047-4838
Año:
2016
Vol.:
68
N°:
1
Págs.:
215 - 223
High-resolution electron backscattered diffraction (HR-EBSD) is a powerful tool to describe microstructures at the sub-micrometric scale that achieves a higher degree of angular accuracy compared with conventional EBSD. However, such an EBSD technique is time-consuming and requires data-intensive computing to save and postprocess the results obtained after each scan. In the current work, a simple strategy to transform conventional results into high-resolution results is put forward in an averaging statistical layout. This makes it possible to measure the misorientations more precisely and, subsequently, the geometrically necessary dislocations by lowering the typical noise generated from Hough transformation-based conventional EBSD. Different steel microstructures are analyzed in light of this strategy. The calculated dislocation densities for those microstructures are used as input values for evaluating the initial dislocation density contribution to the yield strength in a newly developed mechanical model.
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2016
Vol.:
47A
N°:
6
Págs.:
3150 - 3164
Boron is added to steels to increase hardenability, substituting of more expensive elements. Moreover, B acts as a recrystallization delaying element when it is in solid solution. However, B can interact with N and/or C to form nitrides and carbides at high temperatures, limiting its effect on both phase transformation and recrystallization. On the other hand, other elements like Nb and Ti are added due to the retarding effect that they exert on the austenite softening processes, which results in pancaked austenite grains and refined room microstructures. In B steels, Nb and Ti are also used to prevent B precipitation. However, the complex interaction between these elements and its effect on the austenite microstructure evolution during hot working has not been investigated in detail. The present work is focused on the effect the B exerts on recrystallization when added to microalloyed steels. Although B on its own leads to retarded static recrystallization kinetics, when Nb is added a large delay in the static recrystallization times is observed in the 1273 K to 1373 K (1000 A degrees C to 1100 A degrees C) temperature range. The effect is larger than that predicted by a model developed for Nb-microalloyed steels, which is attributed to a synergistic effect of both elements. However, this effect is not so prominent for Nb-Ti-B steels. The complex effect of the composition on recrystallization kinetics is explained as a competition between the solute drag and precipitation pinning phenomena. The effect of the microalloying elements is quantified, and a new model for the predictions of recrystallization kinetics that accounts for the B and Nb+B synergetic effects is proposed.
Revista:
ISIJ INTERNATIONAL
ISSN:
0915-1559
Año:
2015
Vol.:
55
N°:
9
Págs.:
1963 - 1972
In this paper a multi-linear regression analysis is developed to predict continuous cooling (CCT) diagrams in low carbon Nb and Nb-Mo microalloyed steels. The inputs to the analysis include the weight percentage of alloying elements, the prior austenite grain size, the retained strain and the cooling rate. To develop the model, 11 steels with different combinations of Nb and Mo were considered. In some cases, the resulting equations have been validated with external data from the literature. Additionally, the model was also employed to predict hardness and ferrite grain size with the aim of providing a tool to link microstructural features with mechanical property predictions. Both Nb and Mo additions promote a reduction of ferrite and bainite start temperatures, where the effect is more pronounced for Nb in the bainitic region. Both microalloying elements contribute to an increase in hardness and a refinement of the microstructure.
Revista:
METALS AND MATERIALS INTERNATIONAL
ISSN:
1598-9623
Año:
2014
Vol.:
20
N°:
5
Págs.:
807 - 817
The influence of coiling temperature on the final microstructure and precipitation has been analyzed in several low carbon Nb and Nb-Mo microalloyed steels. A throughout characterization of the complex microstructures has been performed using electron backscattered diffraction, measuring low and high angle unit sizes, microstructural substructure, as well as quantifying the homogeneity. An important microstructural refinement is observed for all compositions as the coiling temperature decreases. Regarding precipitation, the coiling temperature strongly modifies the size and density of the fine precipitates, being 550 degrees C the optimal coiling temperature for the Nb-Mo steels. The addition of Mo to Nb steels provides a refinement of the precipitates and, therefore, enhances their contribution to strengthening. Considering all the microstructural and precipitation quantification data, the yield strength was estimated and the contribution of the different mechanisms calculated. The grain size contribution is proven to be the most important factor regarding strengthening, followed by dislocation density and precipitation especially at low coiling temperatures and Nb-Mo steels.
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2014
Vol.:
45A
N°:
11
Págs.:
4960 - 4971
Low carbon Nb-Mo microalloyed steels show interesting synergies between the "micro"-alloying elements when high strength-high toughness properties are required. Strain accumulation in austenite is enhanced, and therefore grain sizes are refined in the final microstructures. The presence of Mo facilitates the presence of non-polygonal phases, and this constituent modification induces an increment in strength through a substructure formation as well as through an increase in the dislocation density. Regarding fine precipitation and its strengthening effect, the mean size of NbC is reduced in the presence of Mo and their fraction increased, thus enhancing their contribution to yield strength. In this paper, a detailed characterization of the microstructural features of a series of microalloyed steels is described using the electron-backscattered diffraction technique. Mean crystallographic unit sizes, a grain boundary misorientation analysis, and dislocation density measurements are performed. Transmission electron microscopy is carried out to analyze the chemical composition of the precipitates and to estimate their volume fraction. In this first part, the contribution of different strengthening mechanisms to yield strength is evaluated and the calculated value is compared to tensile test results for different coiling temperatures and compositions.
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2014
Vol.:
45A
N°:
11
Págs.:
4972 - 4982
The present paper is the final part of a two-part paper where the influence of coiling temperature on the final microstructure and mechanical properties of Nb-Mo microalloyed steels is described. More specifically, this second paper deals with the different mechanisms affecting impact toughness. A detailed microstructural characterization and the relations linking the microstructural parameters and the tensile properties have already been discussed in Part I. Using these results as a starting point, the present work takes a step forward and develops a methodology for consistently incorporating the effect of the microstructural heterogeneity into the existing relations that link the Charpy impact toughness to the microstructure. In conventional heat treatments or rolling schedules, the microstructure can be properly described by its mean attributes, and the ductile-brittle transition temperatures measured by Charpy tests can be properly predicted. However, when different microalloying elements are added and multiphase microstructures are formed, the influences of microstructural heterogeneity and secondary hard phases have to be included in a modified equation in order to accurately predict the DB transition temperature in Nb and Nb-Mo microalloyed steels.
Revista:
JOURNAL OF MATERIALS SCIENCE
ISSN:
0022-2461
Año:
2013
Vol.:
48
N°:
4
Págs.:
1480-1491
A strain reversal applied for both under hot and cold workings produces a microstructural transient which manifests in different ways and at different scales. The evolution of the dislocation network during strain reversal is the result of the competition between two mechanisms. The first mechanism corresponds to the partial untangling and recovery of previously created dislocation network and the second one is associated to the build-up compatible with the current deformation condition. This study proposes a phenomenological formulation based on an effective equivalent strain concept to describe, in a simple way, the several experimental manifestations of the microstructural transient at grain and intragranular scales and its effect on the static recrystallisation kinetics.
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2013
Vol.:
44A
N°:
8
Págs.:
3552 - 3563
A complete microstructural characterization and phase transformation analysis has been performed for several Nb and Nb-Mo microalloyed low-carbon steels using electron backscattered diffraction (EBSD) and dilatometry tests. Compression thermomechanical schedules were designed resulting in the undeformed and deformed austenite structures before final transformation. The effects of microalloying additions and accumulated deformation were analyzed after CCT diagram development and microstructural quantification. The resulting microstructures ranged from polygonal ferrite and pearlite at slow cooling ranges, to a combination of quasipolygonal ferrite and granular ferrite for intermediate cooling rates, and finally, to bainitic ferrite with martensite for fast cooling rates. The addition of Mo promotes a shift in the CCT diagrams to lower transformation start temperatures. When the amount of Nb is increased, CCT diagrams show little variations for transformations from the undeformed austenite and higher initial transformation temperatures in the transformations from the deformed austenite. This different behavior is due to the effect of niobium on strain accumulation in austenite and its subsequent acceleration of transformation kinetics. This article shows the complex interactions between chemical composition, deformation, and the phases formed, as well as their effect on microstructural unit sizes and homogeneity.
Revista:
MATERIALS SCIENCE FORUM
ISSN:
0255-5476
Año:
2013
Vol.:
753
Págs.:
443 - 448
Revista:
MATERIALS SCIENCE FORUM
ISSN:
0255-5476
Año:
2012
Vol.:
715-716
Págs.:
643 - 648
A reversion of the strain produces a modification of the static recrystallization kinetics. Initially, the reversion increases the recrystallization time, that reaches a maximum at a certain strain, and decreases again for increasing reverse strains. This transient on recrystallization kinetics develops over a strain interval similar to that of the microstructural and stress-strain transients. At strains beyond the transient, the reversion can be regarded as a shift on the strain axis. However, at the authors knowledge there is no formulation able to describe the material behaviour during the transient. The present work introduces an equivalent strain concept based on the substructural dissolution/build-up processes taking place as a result of the strain reversal. This formulation allows including the effect of the strain path on recrystallization models.
Revista:
STEEL RESEARCH INTERNATIONAL
ISSN:
1611-3683
Año:
2012
N°:
Special Feature Metal Forming 2012
Págs.:
1147 - 1150
In an authors ¿ previous work a new model that allows reproducing accurately the mechanical transient behaviour in two-stage strain reversal has been presented. This model, based on the isotropic and kinematic hardening concepts, takes advantage of the definition of an equivalent effective strain. This equivalent effective strain has been supposed to control the isotropic stress evolution. Meanwhile, a phenomenological approach has been adopted for the evolution of the kinematic stress to describe the Bauschinger effect. Now the exten-sion of the model to cyclic strain reversal is undertaken in order to assess its applicability. Taking into account some physically reasonable assumptions, the model can be generalised without extra parameters. Some multipass strain reversal torsion tests have been performed in a CMn steel. The experimental results of these tests and the predictions of the model have been compared and a good fitting has been achieved
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2011
Vol.:
42A
N°:
9
Págs.:
2633 - 2642
A strain reversal produces microstructural and mechanical transients that cannot be described with conventional models applicable under monotonic deformation conditions. State-of-the-art models incorporating plastic strain path effects are polarized into continuum constitutive models of cyclic plasticity and multiscale constitutive approaches extrapolating crystallographic hardening models for monocrystals to polycrystals. The present work proposes a model midway between these two approaches that describes the mechanical transient through a combined kinematic-isotropic hardening model and the microstructural transient and its effect on the subsequent static recrystallization kinetics. The incorporation of an effective strain concept makes the link among the different submodels and allows for the first time an accurate reproduction with a reduced number of parameters both for mechanical and microstructural experimental data during the transient following a strain reversal.
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2011
Vol.:
42
N°:
12
Págs.:
3729 - 3742
Thermomechanical processing of microalloyed steels containing niobium can be performed to obtain deformed austenite prior to transformation. Accelerated cooling can be employed to refine the final microstructure and, consequently, to improve both strength and toughness. This general rule is fulfilled if the transformation occurs on a quite homogeneous austenite microstructure. Nevertheless, the presence of coarse austenite grains before transformation in different industrial processes is a usual source of concern, and regarding toughness, the coarsest high-angle boundary units would determine its final value. Sets of deformation dilatometry tests were carried out using three 0.06 pct Nb microalloyed steels to evaluate the effect of Mo alloying additions (0, 0.16, and 0.31 pct Mo) on final transformation from both recrystallized and unrecrystallized coarse-grained austenite. Continuous cooling transformation (CCT) diagrams were created, and detailed microstructural characterization was achieved through the use of optical microscopy (OM), field emission gun scanning electron microscopy (FEGSEM), and electron backscattered diffraction (EBSD). The resultant microstructures ranged from polygonal ferrite (PF) and pearlite (P) at slow cooling ranges to bainitic ferrite (BF) accompanied by martensite (M) for fast cooling rates. Plastic deformation of the parent austenite accelerated both ferrite and bainite transformation, moving the CCT curves to higher temperatures and shorter times. However, an increase in the final heterogeneity was observed when BF packets were formed, creating coarse high-angle grain boundary units.
Revista:
MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING
ISSN:
0921-5093
Año:
2010
Vol.:
527
N°:
42494
Págs.:
934 - 940
Multipass torsion tests were performed on an Nb-microalloyed steel at decreasing temperatures so as to analyze the effect of strain reversal - in the strain accumulation region - on the gamma -> alpha transformation kinetics. Strain reversal involved a significant delay in the start of the transformation as well as a microstructural transient compared to monotonic strain conditions. However, after several passes with no reversal performed within the inter-critical temperature region, both types of tests led to similar microstructures. (C) 2009 Elsevier B.V. All rights reserved.
Revista:
ISIJ INTERNATIONAL
ISSN:
0915-1559
Año:
2010
Vol.:
50
N°:
4
Págs.:
546 - 555
Este artículo quedó finalista en el Vanadium Award 2010
Multipass torsion tests were carried on with several V-microalloyed high carbon steels, using different deformation sequences in order to modify the austenite state prior to transformation. Both recrystallized and deformed austenite microstructures were studied. After deformation, different cooling rates were applied. The results show that accumulating strain in the austenite before transformation seems to slightly increase the interlamellar spacing for a given cooling rate, this increase being related to the pearlite transformation taking place at higher temperatures because of the increase in the austenite grain boundary area per unit volume (S(v)). On the other hand, the retained strain significantly contributes to a refinement of the "ferrite units' effect being more significant as vanadium and nitrogen contents rise. A relationship between the mean "ferrite unit" size with S(v) and cooling rate was determined. Similarly, empirical expressions to predict strength as a function of vanadium microalloying addition, S(v) and cooling rate were derived.