As thin slab direct rolling technologies are moving to the production of higher quality steel grades, chemical compositions based on Nb-Ti and Nb-Mo become a good option. However, with the use of multiple microalloying additions, the as-cast austenite conditioning becomes more complex. This paper analyzes some of the microstructural features that should be taken into account during the as-cast austenite conditioning in Nb-Ti and Nb-Mo microalloyed steel grades. In the case of Nb-Ti grades, it has been observed that the process parameters during solidification and post-solidification steps affect the austenite evolution during hot rolling. This is due to the differences in the size and volume fraction of TiN particles that can be formed. Fine TiN precipitates have been shown to be able to delay recrystallization kinetics. Moreover, the solute drag effect of Ti cannot be ignored in the case of hyperstoichiometric Ti/N ratios. It is observed that Nb-Ti grades tend to have lower non-recrystallization temperatures compared to Nb grades, which means that pancaking of the austenite is more difficult for these steels. The opposite is observed for the Nb-Mo grades, although in both cases the behavior is affected by the nominal content of Nb. (C) The Minerals, Metals & Materials Society and ASM International 2016

This work focuses on the validation of a method for reconstructing the fcc crystallographic data from martensite orientation electron backscatter diffraction (EBSD) maps based on the "gamma nuclei identification" and "gamma nuclei spreading strategy." To that end, an Fe-30Ni alloy was employed. The martensite transformation start temperature (M (s) ) of this material is close to or below room temperature; therefore, during hot deformation and after water quenching, it presents an fcc austenitic microstructure, while after subzero quenching, austenite-to-martensite transformation takes place. Accordingly, the reconstruction procedure can be applied to the martensitic EBSD crystallographic data, and the morphological and orientation results of the reconstruction can be validated by comparison with the original crystallographic fcc data. Torsion tests were performed to produce recrystallized and deformed austenite microstructures. Although applying the Kurdjumov-Sachs orientation relationship (OR) resulted in reconstructed area fractions larger than 75 pct, the reconstruction quality improved significantly when other ORs closer to the Greninger-Troiano OR were applied. The analysis carried out on the recrystallized microstructure shows that the method is robust against variation in the different parameters involved in the reconstruction. Good angular and morphological reconstruction results were obtained in both recrystallized and deformed microstructures, including the ability to reconstruct twins. (C) The Minerals, Metals & Materials Society and ASM International 2017.

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 degrees 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.

The influence of strain, strain rate, and temperature on deformation-induced transformation (DIT) in a low-alloy medium carbon steel is studied. The strain promotes the nucleation of ferrite (deformation-induced ferrite) and also pearlite (deformation-induced pearlite), this last being characterized by a fine interlamellar spacing and morphological instability. At strains epsilon > 0.5, intragranular nucleation activates and further ferrite nucleation over the newly created alpha/gamma interface takes place, which gives rise to the precipitation of cementite (deformation-induced cementite) at the ferrite boundaries. Soft annealing treatments have been performed on the microstructures obtained by DIT, and the degree of spheroidization has been quantified by image analysis techniques. In comparison to non-deformed conditions, the application of DIT results in a higher degree of spheroidization after soft annealing. Moreover, the EBSD analysis denotes that ferrite grain size refinement is achieved with respect to non-deformed conditions. The degree of spheroidization is highly influenced by the applied strain level and subsequent holding temperature.

During hot rolling, austenite recrystallization determines the grain size evolution and the extent of strain accumulation, and therefore, it can be used to control the microstructure and improve the mechanical properties of the final product. However, at the moment, experimental data and models describing the recrystallization kinetics of high-Mn steels are scarce, and they do not take into account the effect of the different C and Mn alloying contents usually present in these steels. The aim of this work is to provide a quantitative model for the determination of the static recrystallization kinetics and recrystallized grain size that is valid for a wide range of high-Mn steel compositions. In order to do this, softening data determined in previous works for steels with different Mn (20 to 30%), Al (0 to 1.5%) and C (0.2 to 1%) levels were considered. In addition, new tests were carried out to determine the effect of deformation conditions on the static softening kinetics and the recrystallized grain size. The static recrystallization kinetics of the high-Mn steels follows Avrami's law, with n Avrami exponents which are temperature dependent and lower than those determined for low C steels. A dependence of the 2'0.5 (time for 50% fractional softening) on the carbon content has been observed and it was incorporated into an equation for the calculation of this parameter. An expression that is valid for predicting the recrystallized grain size as a function of deformation conditions is also proposed.

The interaction between recovery, recrystallization, and strain-induced precipitation in two high-Mn steels, one of them microalloyed with Nb (0.1 pct) was investigated using mechanical testing and advanced microscopy techniques. Double-hit torsion tests were carried out in the 1373 K to 1173 K (1100 A degrees C to 900 A degrees C) temperature range in order to characterize the fractional softening behavior. Quenched specimens were analyzed using electron backscatter diffraction and transmission electron microscopy to determine the recrystallized fraction, the precipitation state, and the austenite microstructure evolution. At the highest temperature, 1373 K (1100 A degrees C), similar softening kinetics were found in both steels. However, at temperatures lower than 1273 K (1000 A degrees C) for the Nb steel, strain-induced precipitation was observed to take place resulting in significant softening retardation. For the base steel at all the temperatures investigated, and for the Nb steel in the absence of strain-induced precipitation, the mechanical softening corresponded well with the recrystallized fraction. However, when strain-induced precipitation took place, a major deviation was observed denoting a significant contribution of recovery to the fractional softening. Within the deformed grains, a substructure consisting of "subgrain bands" or microbands was developed. The precipitates were found mainly on the elongated subgrain boundaries, or at dislocations within the subgrains. This configuration was maintained after the migration of the recrystallization front. (C) The Minerals, Metals & Materials Society and ASM International 2015

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.

Double-hit torsion tests were performed in order to study the effect of high Al levels (up to 2 wt.%) and Nb microalloying (up to 0.07 wt.%) on the static softening kinetics of 0.2%C-2%Mn steels. The addition of 1%Al leads to a delay in the softening kinetics due to solute-drag effect, equivalent to that exerted by 0.027%Nb. For the 2%Al steels, at temperatures below 1000 degrees C, gamma -> alpha phase transformation occurs after deformation, resulting in a larger retardation of the softening kinetics. At temperatures higher than 1000 degrees C, Nb in solid solution also contributes to the retardation of the static softening kinetics, and at lower temperatures NbC strain-induced precipitation leads to incomplete softening for the 1%Al steel, and to a complex interaction between softening, phase transformation, and NbC strain-induced precipitation for the 2%Al-Nb steels. The effect of Al on the static softening kinetics was quantified and introduced in a model developed in previous works for the prediction of the austenite microstructural evolution. In order to validate the results of the model, multipass torsion tests were carried out at conditions representative of hot strip and plate rolling mills. Model predictions show reasonable agreement with the results obtained at different deformation conditions.

The effect of Al addition on the static softening behavior of C-Mn steels was investigated. Double-hit torsion tests were performed at different deformation temperatures ranging from 1198 K to 1338 K (925 degrees C to 1065 degrees C) with pass strains of epsilon = 0.2 and 0.35. It was found that solute Al produced a significant delay on the static softening kinetics. Additionally, at the lowest temperatures [1198 K to 1238 K (925 degrees C to 965 degrees C)] and highest Al level (2 wt pct), austenite to ferrite phase transformation was found to be concurrent with softening, leading this to higher softening retardation. The softening kinetics of the steels investigated were analyzed using a physically based model which couples recovery and recrystallization mechanisms. The main parameters of the model were identified for the present alloys. An expression for the grain boundary mobility of the base C-Mn steel was derived and the retarding effect of Al in solid solution on the static recrystallization kinetics was introduced in the model. Reasonable agreement was obtained between model and experimental results for a variety of deformation conditions. (C) The Minerals, Metals & Materials Society and ASM International 2013

The application of high heating rates in tempering treatments can provide a valuable tool for refining carbide sizes, mainly those located at high angle grain boundaries. This work analyses the influence of heating rates ranging from 1 to 300 degrees C/s during the tempering treatment of a 0.42%C low alloy steel. The results indicate that when high heating rates are combined with short holding times, predicting hardness will require the inclusion of the heating up and cooling down cycles in addition to the holding time and temperature used in the definition of the conventional Hollomon-Jaffe tempering parameter (TP). The effect of heating rate on carbide size distribution has been quantified, distinguishing between particles located at high (HAB) and low (LAB) misorientation angle boundaries. The former correspond to those carbides nucleated at prior gamma grain, martensite block or packet boundaries whereas the latter refer to those nucleated within martensite laths and at lath boundaries. The refinement obtained has been evaluated from the point of view of hardness behaviour. (C) 2014 Elsevier Ltd. All rights reserved.

Warm deformations have been applied to a low-alloy medium carbon steel (AISI 5140) to promote faster spheroidization during soft annealing treatments. The application of warm deformation leads to the fragmentation of cementite lamellae and the formation of defects on both cementite and the matrix. This induces faster lamellae break-up according to a boundary splitting mechanism, which is responsible for the improved spheroidization after annealing. The substructure developed in the matrix enhances pipe diffusion through the sub-boundaries, which helps the lamellae terminations to coarsen and causes lamellae fast splitting and finally yields a coarse cementite particle distribution. When deforming up to epsilon = 0.3, almost fully spheroidized microstructures are obtained after annealing at 993 K (720 A degrees C), independently of the initial pearlite features. By means of the EBSD technique, it has been observed that the applied warm deformation, in addition to enhancing the degree of spheroidization, allows a much finer microstructure to be formed after annealing. Grain refinement takes place as a consequence of a continuous recrystallization process, which is directly related to cementite spheroidization in the long term. (C) The Minerals, Metals & Materials Society and ASM International 2013

The effect of Nb (up to 0.07 wt%) and high Al content (up to 2 wt%) on the multipass deformation behaviour of steels with 0.2% C and 2% Mn was studied with the aid of hot torsion simulations. From the tests, the critical Non-Recrystallisation (T-nr), Recrystallisation Limit and Stop Temperatures (RLT and RST) and the ferrite phase transformation start temperature (A(r3)) were determined. It was observed that an increase in Al content from 1% to 2% or a microalloying addition of 0.03% Nb to 1% Al steel both led to a significant increase in the recrystallisation critical temperatures, which is greater than 100 degrees C in the case of the T-nr However, the value of the T-nr was not affected when 0.03% or 0.07% Nb was added to the 2% Al steel. Specimens quenched after several deformation passes were examined by optical and TEM means in order to study the interaction between static recrystallisation, strain-induced precipitation and gamma ->alpha phase transformation, and determine the mechanisms leading to strain accumulation in the steels investigated. The results suggest that for the 1% Al steels, the Al and Nb solute drag effect is the main mechanism leading to the increase in the critical recrystallisation temperatures, while for the 2% Al steels the occurrence of gamma ->alpha phase transformation at temperatures close to the T-nr is the main mechanism involved in softening retardation, with a limited contribution of Nb. However, gamma ->alpha phase transformation taking place at temperatures close to the T-nr resulted in a loss of hot ductility, which can limit the industrial applicability of the 2% Al steels. (c) 2014 Elsevier B.V. All rights reserved.

Different recrystallization models are now available in the literature for plain carbon and microalloyed steels. However, the attempts made to unify these models have been unsuccessful due to the considerable scatter in experimental results that support the corresponding equations. In fact, kinetics results can be separated into different groups depending on the testing procedure. In order to cope with this problem and improve the reliability of the different models, the different methods that are currently applied at laboratory need to be re-analyzed. The present work concentrates on the determination of recrystallization/softening kinetics of austenite from torsion tests. To this purpose the softening results obtained by the application of double hit torsion tests to two Nb microalloyed steels and the fraction recrystallized as determined by metallography have been subjected to a direct and an inverse analysis that takes into account the specificity of the torsion test. (C) 2013 Elsevier B.V. All rights reserved.

Although a lot of data are available in the literature on the recrystallization behavior of plain carbon and microalloyed steels, comparisons are often difficult to make due to the effect of different experimental techniques and the type of tests used to obtain these data. Few systematic comparisons can be found in the literature to correlate the different techniques and methods. A previous paper by the present authors concentrates on the analysis of torsion methods. The present study focuses on the recrystallization kinetics determined by using double hit and stress relaxation tests in plane strain compression mode. The specificity of the test and the strain distribution across the section has been incorporated into the analysis, by means of finite element methods. A good correlation has been obtained between the kinetics for stress relaxation and double hit tests. However, some rules should be respected in order to determine the true reaystallization/softening kinetics comparable with those obtained for other deformation modes like torsion. (C) 2013 Elsevier B.V. All rights reserved.

Article Preview The applicability of a physical model to describe the austenite microstructure evolution after hot deformation in High-Mn steels was investigated. Double-hit torsion tests were carried out to determine the softening behaviour of two High-Mn steels, one of them microalloyed with 0.11 wt% Nb. The values of the unknown parameters included in the model were determined by fitting experimental results. The model describes adequately the softening evolution of the steels. At high temperatures recovery and recrystallization contribute to mechanical softening, the latter having the main contribution. In contrast, when strain-induced precipitation occurs recovery has a larger effect.

A spheroidization kinetic study has been carried out in a low alloy medium carbon steel by means of image analysis techniques. Two different initial pearlite microstructures, coarse and fine pearlite, have been generated at two different transformation temperatures of 700 and 630ºC. The effect of a deformation application once the steel is completely transformed has been analyzed and compared with that observed in non deformed samples. The deformation accelerates spheroidization kinetics and leads to a higher spheroidization degree. Several phenomena that take place during the spheroidization treatment contribute to the matrix softening. Artículo premiado con el 2012 Gilbert R. Speich Award (AIST). PR-264-049 - 2011

The effect of Al addition on the static softening behavior of C-Mn steels was investigated. The compositions of the steels studied are representative of the recently developed TRIP-assisted steels: a base composition of 0.2%C, 2%Mn, 50ppm N and three different Al levels, 0.03 (base steel), 1 and 2%. Double-hit torsion tests were performed at different deformation temperatures, in the range 950°C to 1100°C, and pass-strains, =0.2 and 0.35. It was found that solute Al produced a significant retardation on static recrystallization kinetics, equivalent to that exerted by 0.026%Nb for the 1%Al steel and to 0.05%Nb for the 2%Al steel. Additionally, at the lowest temperatures (950-1000°C) and 2%Al level, austenite to ferrite phase transformation was found to be concurrent with softening, enhancing retardation in the mechanical softening.

HSLA steels constitute one of the main types of steels produced technologies (TSDR). Among microalloying elements, the most widely used different roles during the austenite evolution in TSDR. Regarding austenite worldwide by Thin Slab Direct Rolling are V, Nb and Ti. These elements play conditioning before transformation, the limitations in the total reduction that can be applied in TSDR technologies need to be considered when composition/process parameters are selected. In this context, whereas an important number of studies have been focused on Nb microalloyed grades, a less systematic analysis has been performed concerning the role of vanadium on austenite conditioning. This paper analyzes these singularities taking into account different process parameter conditions, such as total reduction and initial rolling temperature.

Top25 Hottest articles en Materials Science and Engineering A durante el período enero-marzo 2011 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 degrees C/s to about 200 degrees 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.

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.

Plane strain compression tests of two V microalloyed steels and one plain C-Mn steel have been done to analyse the influence of the deformation temperature, in the warm working range, on the final microstructure and subsequent mechanical behaviour. In the case of V microalloyed steels, the reheating temperature has an effect on the amount of vanadium in solution prior to deformation. This factor influences the austenite evolution during warm deformation and the transformation during cooling. As a consequence, in the microalloyed steels complex multiphase microstructures are obtained that lead to a wide range of strength-toughness combinations. In contrast, in the case of the plain C-Mn steel minor effects are observed in the deformation range from 800 to 870 A degrees C.

In the present investigation, the effect of both: rolling parameters (2 reduction rates and 3 cooling rates) and chemical elements such as: C, Mn, Nb, Ti, Mo, Ni, Cr, Cu and B, has been studied in relation to strength properties in low-carbon microalloyed steels with high niobium contents (up to 0,12 wt.% Nb). For this purpose, an experimental set-up was designed based on an intelligent design of experiments (DoE), resulting in 26 casts (laboratory casts). A combination of metallography, Electron Back-Scattered Diffraction (EBSD) and tensile tests have been performed to study how processing parameters and chemical composition affect the strength. The results, where the proof stress, tensile strength, uniform and fracture elongations are the response variables, have been analysed statistically by means of multiple linear regression technique, leading to response equations. From the results, it was found that the effectiveness of niobium increasing the strength is reduced as carbon content increases.

Nowadays there is a continuous demand, particularly from the automotive industry, for cheaper, lighter and more reliable components. It is not surprising then that steel research has been focused during the last decades in new qualities and processes. This paper is dealing with the use of vanadium microalloyed steels on one of those new processes, warm forging. For its low precipitation temperature and its recognised ability to strengthen steel microstructures via austenite grain growth control, precipitation hardening and interference of the static recrystallization process, vanadium in microalloyed steels seem to be an appropriate candidate for warm forging.

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.