Revistas
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2022
Vol.:
53
N°:
6
Págs.:
2270 - 2285
In this work, multipass torsion tests followed by coiling simulations under different conditions have been performed with a reference Nb (0.03 pct) and a high Ti (0.1 pct)-Nb-microalloyed (0.03 pct) steel. In the case of the high Ti steel, estimated yield strengths close to or over 700 MPa were obtained for some of the conditions researched. However, a very significant effect of previous austenite grain size and strain accumulation on precipitation strengthening has also been observed. As a result, depending on deformation sequence and final cooling conditions, the coiling simulation temperatures that lead to the highest mechanical strength varied from 600 degrees C to 500 degrees C. The effect of increasing strain accumulation was mainly related to higher phase transformation temperatures, which led to a lower driving force for precipitation and higher microalloying element diffusivity, resulting in the formation of less and coarser precipitates.
Autores:
Amorim-Pessoa, R.; Rangel-Porcaro, R. (Autor de correspondencia); Candido, L. C.; et al.
Revista:
MATERIALS RESEARCH
ISSN:
1516-1439
Año:
2022
Vol.:
25
Págs.:
e20220132
High-carbon steels have been used to high-strength steel wire and strands for prestressing concrete. The necessity of high-strength levels at increasingly larger diameters of wire rods is a technological challenge. Two steels with and without Nb were obtained in a steel mill, submitted to detailed microstructural (previous austenitic grain size, pearlite interlamellar spacing and colony size) and mechanical characterization through tensile tests and hardness. Hot torsion and dilatometry tests were performed to simulate steels processing and to verify the influence of Nb on phase transformation. Adding Nb to steel resulted in a refinement in austenitic grain size and pearlite colonies but had no effect on pearlitic interlamellar spacing. There was a decrease at the start transformation temperature austenite/pearlite and therefore an increase in the hardenability of the Nb-added steel. Finally, Nb addition proved to be a technical and economical way to increase tensile strength and to reduce the variability of the mechanical properties.
Revista:
METALS
ISSN:
2075-4701
Año:
2020
Vol.:
10
N°:
2
Págs.:
165
This work has focused on the study of hot working behavior of Ti-Nb microalloyed steels with high Ti contents (> 0.05%). The role of Nb during the hot deformation of low carbon steels is well known: it mainly retards austenite recrystallization, leading to pancaked austenite microstructures before phase transformation and to refined room temperature microstructures. However, to design rolling schedules that result in properly conditioned austenite microstructures, it is necessary to develop models that take into account the effect of high Ti concentrations on the microstructural evolution of austenite. To that end, in this work torsion tests were performed to investigate the microstructural evolution during hot deformation of steels microalloyed with 0.03% Nb and different high Ti concentrations (0.05%, 0.1%, 0.15%). It was observed that the 0.1% and 0.15% Ti additions resulted in retarded softening kinetics at all the temperatures. This retardation can be mainly attributed to the solute drag effect exerted by Ti in solid solution. The precipitation state of the steels after reheating and after deformation was characterized and the applicability of existing microstructural evolution models was also evaluated. Determined recrystallization kinetics and recrystallized grain sizes reasonably agree with those predicted by equations previously developed for Nb-Ti microalloyed steels with lower Ti concentrations (<0.05%).
Revista:
MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING
ISSN:
0921-5093
Año:
2019
Vol.:
748
Págs.:
386 - 395
Steels alloyed with high Ti addition levels present an interesting combination of high strength and formability, principally due to the high levels of precipitation hardening that can be attained. However, their mechanical properties can be highly sensitive to variations in the processing route. In this work, dilatometry tests were performed to study the effect of coiling conditions on the microstructures and hardening mechanisms of a reference Nb microalloyed steel (0.03%Nb) and two high Ti-Nb steels (0.05%Ti-0.03%Nb, 0.1%Ti-0.03%Nb). Coiling temperatures from 550 degrees C to 675 degrees C and cooling rates of 0.01 degrees C/s and 0.03 degrees C/s were considered. A significant increase in hardness was observed for the high Ti-Nb steel samples. While the grain size and dislocation hardening were similar for all steels, much higher precipitation strengthening values in the range of 69-163 MPa and 100-307 MPa were calculated for the 0.05%Ti and 0.1%Ti steels, respectively. As a consequence, high yield strength values (over 700 MPa) were estimated for coiling temperatures greater than 625 degrees C for the Ti10Nb3 steel. However, it was also observed that the mechanical behavior of this steel greatly depended on coiling conditions: maximum mechanical strength was achieved at 625-650 degrees C, while it decreased significantly for temperatures between 550 degrees C and 600 degrees C and at 675 degrees C. The small size and density of the precipitates detected with TEM support the large precipitation hardening effect calculated in these conditions.
Revista:
METALS
ISSN:
2075-4701
Recently, procedures for recovering austenite fcc crystallographic information from EBSD (Electron Back-Scatter Diffraction) data recorded from martensite or other bcc transformation products have been proposed. Due to the difficulties in revealing prior austenite grain boundaries using available etching techniques, these reconstruction methods appear as promising tools for understanding austenite hot deformation behavior in low carbon steels. In a previous work, the accuracy of an in-house developed reconstruction code was validated using an Fe-30Ni alloy. Validation of this method in low carbon steels is more difficult due to the loss of austenitic orientation information. In addition, it is not known how the results of conventional metallography correlate with those of the reconstruction or which scan parameters or post-processing treatments are necessary for obtaining comparable results. To study this, two martensitic specimens obtained from water quenched recrystallized and deformed austenite were characterized in this study, using both conventional metallographic techniques and the above mentioned reconstruction procedure applied to EBSD scans acquired using different parameter values. The comparability of austenite grain size and morphology was analyzed as a function of the post-processing treatment. The results show that in order to obtain comparable results, it is critical to analyze the coherence of the twins present in the microstructure.
Revista:
TRATER PRESS
ISSN:
1888-4423
Año:
2018
Vol.:
Nov.
N°:
67
Págs.:
19 - 25
Revista:
MATERIALS SCIENCE FORUM
ISSN:
0255-5476
Año:
2017
Vol.:
879
Págs.:
1170-1175
Semi-empirical models for predicting the austenite static recrystallization behavior are widely used in designing thermomechanical treatments to improve final mechanical properties. However, a problem with these models is that their utility can be limited to the range of deformation conditions and chemical compositions they were developed for. This work focuses on the study of the applicability of current recrystallization models to the range of low strain conditions and/or high Nb microalloying additions (=0.1%). To do so, the recrystallization behavior of two low carbon Nb-Ti microalloyed steels (0.04 and 0.11% Nb and =0.01% Ti) has been investigated by torsion tests. Experimental results for recrystallization time and recrystallized grain size have been compared to previously developed equations. It has been observed that at low strains (¿ = 0.1) the predictions fail. A dependence of the n Avrami exponent both on temperature and applied strain was also found.
Revista:
MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING
ISSN:
0921-5093
Año:
2017
Vol.:
685
Págs.:
377 - 390
RFCS project [RFSR-CT-2013-00012]
Revista:
MATERIALS SCIENCE FORUM
ISSN:
0255-5476
Año:
2017
Vol.:
879
Págs.:
1465 - 1470
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2017
Vol.:
48A
N°:
11
Págs.:
5258 - 5272
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.
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:
MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING
ISSN:
0921-5093
Año:
2016
Vol.:
651
Págs.:
358 - 369
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.
Revista:
ISIJ INTERNATIONAL
ISSN:
0915-1559
Año:
2016
Vol.:
56
N°:
6
Págs.:
1038 - 1047
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.
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2015
Vol.:
46A
N°:
11
Págs.:
5248 - 5265
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
Revista:
JOURNAL OF MATERIALS ENGINEERING AND PERFORMANCE
ISSN:
1059-9495
Año:
2015
Vol.:
24
N°:
3
Págs.:
1279 - 1293
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.
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2014
Vol.:
45A
N°:
2
Págs.:
934 - 947
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
Revista:
MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING
ISSN:
0921-5093
Año:
2014
Vol.:
600
Págs.:
37 - 46
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.
Revista:
MATERIALS SCIENCE FORUM
ISSN:
0255-5476
Año:
2013
Vol.:
762
Págs.:
398 - 404
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.
Revista:
MATERIALS SCIENCE FORUM
ISSN:
0255-5476
Año:
2013
Vol.:
753
Págs.:
453 - 458
Revista:
MATERIALS SCIENCE FORUM
ISSN:
0255-5476
Año:
2013
Vol.:
753
Págs.:
443 - 448
Revista:
METALLURGICAL AND MATERIALS TRANSACTIONS A-PHYSICAL METALLURGY AND MATERIALS SCIENCE
ISSN:
1073-5623
Año:
2012
Vol.:
43A
N°:
12
Págs.:
4553 - 4570
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 A degrees C to 750 A degrees C) range. Transmission electron microscopy (TEM) showed interphase precipitation of NbC at high coiling temperatures, while at 873 K (600 A degrees C), homogeneous general precipitation took place in ferrite and followed a Baker-Nutting orientation relationship.
Revista:
MATERIALS SCIENCE FORUM
ISSN:
0255-5476
Año:
2012
Vol.:
706 -709
Págs.:
2764 - 2769
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.
Revista:
MATERIALS SCIENCE FORUM
ISSN:
0255-5476
Año:
2010
Vol.:
638-642
Págs.:
687 - 692