Isothermal heat treatments below the martensite start temperature are carried out in a 0.25C-1.4Mn-1.4Si-0.32Mo steel (in wt%). The microstructural observation reveals a combination of tempered lath and plate martensite, accompanied by bainite and retained austenite. The temperature variation of the austenitic carbon content under different constrained equilibrium models is calculated and compared with experimental measurements. Neither of the existing models seems to predict the behavior of the measured austenitic carbon content in the temperature range studied. The results suggest that the diffusional formation of bainite governs the carbon content of austenite at temperatures nearby the M_S.

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This study focuses on the effect of non-conventional annealing strategies on the micro-structure and related mechanical properties of austempered steels. Multistep thermo-cycling (TC) and ultrafast heating (UFH) annealing were carried out and compared with the outcome obtained from a conventionally annealed (CA) 0.3C-2Mn-1.5Si steel. After the annealing path, steel samples were fast cooled and isothermally treated at 400 °C employing the same parameters. It was found that TC and UFH strategies produce an equivalent level of microstructural refinement. Neverthe-less, the obtained microstructure via TC has not led to an improvement in the mechanical properties in comparison with the CA steel. On the other hand, the steel grade produced via a combination of ultrafast heating annealing and austempering exhibits enhanced ductility without decreasing the strength level with respect to TC and CA, giving the best strength–ductility balance among the studied steels. The outstanding mechanical response exhibited by the UFH steel is related to the formation of heterogeneous distribution of ferrite, bainite and retained austenite in proportions 0.09–0.78–0.14. The microstructural formation after UFH is discussed in terms of chemical hetero-geneities in the parent austenite.

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Four titanium alloys (Ti-Ta, Ti-Ta-Sn, Ti-Ta-Mn, and Ti-Nb-Sn) were synthesized by mechanical alloying (MA) in a planetary mill in different times between 2 h and 100 h. The microstructure characterization was made by X-ray diffraction (XRD), in which the Rietveld method was applied to analyze the diffraction patterns. The study demonstrated that after short milling times between 2 h and 30 h, the fraction of hexagonal close-packed (hcp) phase decreases; at the same time, the formation of body-centered cubic (bcc) and face-centered cubic (fcc) Ti phases are promoted. Additionally, after 30 h of MA, the full transformation of hcp-Ti was observed, and the bcc-Ti to fcc-Ti phase transformation took place until 50 h. The results suggest that the addition of Ta and Sn promotes the fcc-Ti phase formation, obtaining 100% of this phase at 50 h onwards, whereas Nb and Mn show the opposite effect.

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The microstructure and mechanical properties of an Fe-0.24C-1.4Mn-1.4Si steel were investigated after combining ultrafast heating (UFH) at a heating rate of 500 °C/s followed by fast cooling to room temperature (DQ) or quenching and partitioning processes (Q&P). Two peak temperatures were studied, annealing into the intercritical range and above the A_C3 temperature. After ultrafast heating and quenching, the resulting microstructures revealed that intercritical annealing led to the formation of a banded ferritic-martensitic microstructure. On the other hand, heating above the intercritical range led to an even distribution of allotriomorphic ferrite grains upon fast cooling and a complex phase microstructure, consisting mainly of martensite, was produced. Q&P steel grades exhibit an enhanced mechanical behavior compared to their DQ counterparts, where yield strength, uniform elongation, and total elongation increased after partitioning at 400 °C. The ultimate tensile strength of the Q&P steels decreased compared to the DQ steels annealed at the same peak temperature. However, the final strength-ductility balance of the studied Q&P steels was superior to the DQ steel grades. Moreover, considerable strength and improved ductility were obtained through the combination of peak annealing above the A_C3 temperature followed by Q&P. These results are attributed to an interplay between a sustainable TRIP effect and effective strain-stress partitioning among the microconstituents resulted after the Q&P process.

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The final carbon content of austenite in equilibrium with tempered martensite can be estimated by the so-called constrained carbon equilibrium in the presence of carbide (CCEθ) model. However, the linear predictions under CCEθ deviate from both the initial and the experimentally measured carbon content. A modified approach to the CCEθ model is proposed, which predicts an increase of the carbon content in austenite with the decrease of temperature below the onset of martensitic transformation.

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The influence of the heating rates from 10 to 1000 °C/s and annealing temperatures on the microstructure and mechanical properties of two 0.2%C, 1.9%Mn, 1.4%Si cold-rolled steels with and without the addition of carbide-forming elements (Mo, Nb, and Ti) have been investigated. Results show that the increase of the heating rate above 100 °C/s refines the parent austenitic grains in both alloys. The increment of the heating rate led to carbon heterogeneities in the austenite, which after subsequent cooling promoted the formation of a complex mixture of fine-grained constituents. As expected, at the lower heating rates the presence of Nb and Ti-rich carbides and carbonitrides controls the austenite grain growth during the annealing treatment. The tensile test results reveal that high heating rates do not have a significant influence on the tensile strength of the alloy with carbide-forming elements. On the other hand, both the ultimate tensile strength (UTS) and total elongation of the alloy without carbide-forming elements decrease, due to the formation of bands of ferrite and high carbon martensite. However, samples treated at heating rates above 100 °C/s show a combination of UTS in the range of 1400–1600 MPa, and 12–18% of total elongation. The results suggest that the microstructure heterogeneity obtained after high heating rates, especially the ferrite content, has the major effect on the mechanical behavior of the studied steels.

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The effect of the microstructure on the principal strain paths (uniaxial, plane, and biaxial) in the formability processes of ferritic stainless steel AISI 430 sheets is studied. The Marciniak test (determination of the plastic strain of sheet metal with a flat tip punch) is applied to determine the forming limit curves and different strain levels in the strain paths by the digital image correlation technique. The formability is discussed in light of the microstructure, standard mechanical properties, work hardening behavior, and anisotropy measurements (R-value). Electron backscatter diffraction analysis is carried out to determine the texture of the selected strain paths. The texture evolution shows a marked γ (<111>// normal direction [ND]) fiber and cube ({001} <100>) texture component under the biaxial strain mode, whereas the α (<110>// rolling direction [RD]) fiber is somewhat favored under uniaxial plane strain. The results are compared with texture simulations performed under the fully constrained Taylor model, finding reasonable agreement with the experimentally measured main components.

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The austenitizing heat treatment of the 75Cr1 tool steel was optimized in order to obtain high yield strength and toughness. Samples were austenitized at different temperatures and soaking times, and subsequently quenched and tempered. The phase transformation characteristics during heating and quenching were studied by dilatometry. Optical microscopy, electron backscatter diffraction, x-ray diffraction and transmission electron microscopy were used to characterize the microstructure, while tensile tests and toughness tests were employed to determine the mechanical properties after various heat treatments. It was found that both yield strength and toughness decrease with increasing austenitizing temperature. Thermodynamic and kinetic calculations with Thermocalc and Dictra software were used to study the movement of the austenite-carbide interface and the compositional gradients in the microstructure at different austenitizing conditions. Based on the calculations it was found that the rate of dissolution of carbides into austenite is mainly controlled by the partitioning of chromium and manganese between carbides and austenite. The compositional gradients in the microstructure from the Dictra calculations were confirmed by energy dispersive spectrum (EDS) measurements in transmission electron microscope. No significant changes in mechanical properties and microstructure (carbide fraction) could be observed after more than 15 min soaking. However a significant prior austenite grain size growth and as a consequence a larger martensite grain (block) size is observed for long soaking times.

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In this study an Fe-0.28C-1.91Mn-1.44Si cold-rolled steel was subjected to conventional (10 °C/s) and ultrafast (100 °C/s - 700 °C/s) heating peak annealing treatments, followed by quenching and partitioning (Q&P). The microstructural characterization results showed that grain refinement of the parent austenite and its transformation products occurred with the increment of the heating rate from 10 °C/s to 100 °C/s, without further refining at 700 °C/s. The formation of complex microstructures after the end of the thermal treatment, accompanied by the reduction in the retained austenite carbon content, suggested that local chemical heterogeneities in austenite appear upon ultrafast heating. Regardless of the prior heating rate, similar mechanical properties and strain hardening were measured, revealing that both, the microstructure development and the extent of austenite stabilization during quenching and partitioning stage play a fundamental role on the mechanical behavior of the peak annealed Q&P steels.

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Single and two-step heat treatments were performed on hot-rolled 0.2C-2Mn-1.4Si-0.3Mo steel. The microstructure was characterized by means of optical and scanning electron microscopy and X-ray diffraction measurements. The results showed that the main microstructural components are tempered martensite, bainite, and retained austenite. The thermodynamic analysis showed that the constrained carbon equilibrium in the presence of cementite CCEθ model fits the experimental data for the isothermal heat treatment at a temperature well below the M_S. Neither of the existing models for predicting the equilibrium between martensite and austenite (i.e., constrained carbon equilibrium CCE and CCEθ) seems to predict the measured austenitic carbon content nearby the M_S, which might indicate that local equilibrium (LE) conditions during bainitic transformation govern the final stage of both single and two-phase heat treatments.

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Ultrafast annealing (UFA) is considered a feasible alternative processing route for third-generation advanced high strength steels (AHSSs). In fact, significant grain refinement occurs in low carbon steels (0.1 wt%) when heating rates (example, 1000 K/s) considerably higher than the conventional heating rates are applied. Additionally, high heating rates result in multiphase microstructures containing ferrite, retained austenite, and martensite/bainite. The mixture of structural constituents is induced by carbon gradients resulting from the short time available for diffusion during ultrafast annealing of the steel. Quasi-static and high strain rate tensile tests, with strain rates ranging from 0.0033 s⁻¹ to 600 s⁻¹, revealed that the tensile strength of both conventional and UFA samples increases with increasing strain rates, whereas the strain to fracture decreases. However, compared with the conventionally heat-treated samples (heating rate: 10 K/s), the UFA samples exhibited less ductility deterioration at high strain rates. The samples were investigated using light optical microscopy, scanning electron microscopy, electron backscatter diffraction (EBSD), and X-ray diffraction. The results indicated that the improved tensile properties of the UFA steels (compared with those of the conventional steels) resulted mainly from the presence of retained austenite (up to 5%) in conjunction with a lower carbon martensite/bainite fraction. Owing to adiabatic heating associated with dynamic conditions, the stability of retained austenite shifted to higher strain values than those encountered under static conditions. The TRIP effect of the UFA samples appeared at higher strain values than in the conventionally heat-treated samples. The delayed transformation is considered essential for realizing improved preservation of the deformation capacity and, in turn, improved crashworthiness.

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In this work an experimental method to determine the limit strain by means of digital image correlation is proposed. This method analyzes the variation of the thickness of a sheet metal through the temporal evolution of the strains, assuming incompressibility during the plastic strain process. To achieve this objective, the center and the edge of the necking area are identified with the strain history and the strain rate, respectively. The limit strain is thus determined when a non-homogeneous decrease in the thickness necking area begins, in contrast with existing methods based on performing analysis of surface strains. The proposed method is applied to determine the forming limit curve of commercial stainless steel and compared with other approaches. The results indicate that the proposed methodology is more reliable for determining the forming limit curve of the sheet, because it captures the heterogeneity of the strain distribution at the local necking site. The microstructure and the textures of the initial material are characterized via optical microscopy, x-ray diffraction and electron backscattering diffraction. The mechanical properties of the material are discussed in the light of the microstructural analysis.

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The aim of the present study is to evaluate the impact of heating rate on the microstructure and tensile properties of cold-rolled low and medium carbon steels. For this purpose, cold-rolled low and medium carbon steels were subjected to short peak-annealing experiments at 900 and 1100 °C under three heating rates (10, 450 and 1500 °C/s). The microstructure reveals a mixture of phases and microconstituents (ferrite, bainite, and as-quenched martensite) which are related to the carbon heterogeneities in austenite. The microstructural characterization suggests that the grain refinement achieved after ultrafast heating has a minor effect on the yield and ultimate tensile strength, compared to the relative microstructural distribution. It is suggested that the interplay of various strengthening mechanisms in samples subjected to ultrafast heating rates are responsible for the observed increase in strength and ductility.

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Texture and microstructure formation during annealing of a 75% cold rolled AHS steel, reheated at a rate of 10°C/s to ~3000°C/s were studied with the purpose to investigate the interaction between the recrystallization and the austenite formation as well as the possibility of grain refinement. It was found that at ultra-high heating rates, the bcc-fcc phase transformation starts before the completion of recrystallization and the deformation texture is partially retained in the samples after annealing. The crystallographic texture observed in the martensitic phase, which is the product of intercritical austenite transformation in as-quenched samples, i.e., after the double α-γ-α' transformation, is similar to the initial cold rolling texture, which is an indication for the texture memory effect. After ultra-fast reheating with average reheating rates of 1000°C/s and higher, a significant grain refinement was observed with an average calculated ferrite grain diameter of ~2 μm. The obtained final ferrite grain size depends significantly on both the reheating temperature and the heating rate. The same effect is observed in 75% cold rolled quenching and partitioning steels for heating rates of 400°C/s, 800°C/s and 1000°C/s. In both cases the combination of fine grains, and weak RD/ND-fiber textures produce an increase of strength and ductility compared to material annealed using a conventional heating rate of ~10°C/s.@en

Ultrafast heating (UFH) experiments have been carried out in cold rolled ultra low carbon (ULC) steel, followed by quenching. The selected heating rates are in the range between 10 and 800 °C s⁻¹. The recrystallization curves are slightly shifted to higher temperatures as the heating rate is increased. The average recrystallized grain size and texture are virtually unaffected by increasing the heating rate. Nucleation took place at grain boundaries as well as inside deformed grains. Electron backscattered diffraction (EBSD) analysis reveals that the texture of grains nucleated inside deformed ferrite grains prevails at later stages of recrystallization. The results obtained in this work demonstrate that similar microstructure and textures are obtained after very short annealing cycles.

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This work focuses on the effect of a pre-heating stage on the microstructure evolution during continuous heating of 50% and 75% cold-rolled low carbon steel under conventional and ultrafast heating rates. Peak annealing experiments under two heating rates (10 °C/s and 400 °C/s) are applied to the samples after a pre-heating stage of 10 s. The selected pre-heating temperatures (300 °C and 400 °C) show an influence on the phase fraction of austenite formed after a short holding time in the intercritical (α + γ) phase field, and an effect on the intensity of recrystallized ferrite texture components. A refinement of ferritic average grain diameter is observed after increasing the heating rate from 10 °C/s to 400 °C/s. It is concluded that a pre-heating stage has a negligible impact on the microstructure and texture of cold-rolled low carbon steel. Therefore, a suitable technological window for the application of ultrafast heating rates in steel processing is enabled.@en

The austenite formation in 0.2% C and 0.45% C steels with the initial microstructure of ferrite and pearlite has been studied. The effect of conventional (10 °C/s), fast (50 °C/s–100 °C/s) and ultrafast heating rates (> 100 °C/s) on the austenite nucleation and growth mechanisms is rationalized. Scanning Electron Microscopy (SEM), and Electron BackScatter Diffraction (EBSD) analyses provide novel experimental evidence of the austenite nucleation and growth mechanisms operating at ultrafast heating rates. Two mechanisms of austenite formation are identified: diffusional and massive. It is demonstrated that at conventional heating rates the austenite formation kinetics are determined by carbon diffusion, whereas at ultrafast heating rates formation of austenite starts by carbon diffusion control, which is later overtaken by a massive mechanism. Comprehensive thermodynamic and kinetic descriptions of austenite nucleation and growth are developed based on experimental results.

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Heating experiments in a wide range of heating rates from 10 °C/s to 1200 °C/s and subsequent quenching without isothermal soaking have been carried out on a low carbon steel. The thermal cycles were run on two different cold rolled microstructures, namely ferrite+pearlite and ferrite+martensite. It is shown that the average ferritic grain size, the ferrite grain size distribution, the phase volume fractions and the corresponding mechanical properties (ultimate tensile strength and ductility) after quenching are strongly influenced by the heating rates and the initial microstructure. The ferrite grain size distribution is significantly modified by the heating rate, showing a markedly bimodal distribution after fast annealing. The rise of the heating rate has produced a change in the relative intensities of texture components, favouring those of the cold-deformed structure (RD fibre) over the recrystallization components (ND fibre).

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The microstructure and texture evolution of cold-rolled low carbon steel after ultrafast heating and quenching is investigated. Experiments were carried out at heating rates of 150 °C/s and 1500 °C/s. The recrystallization of ferrite is studied by scanning electron microscopy and electron backscattered diffraction techniques. The texture evolution of cold rolled steel during ultrafast heating was studied, making it possible to estimate the precise effect of heating rate on the orientations of newly formed grains. The experimental results showed that the recrystallization of ferrite was not completed before the full transformation of austenite. The noticeable increase in the fraction of recrystallized grains of diameter less than 1 µm, when the heating rate is increased from 150 °C/s to 1500 °C/s suggests that the increase of the heating rate enhances the nucleation of ferrite. The crystallographic orientations in recrystallized ferrite are strongly influenced by the heating rates. The effect of heating rate in the releasing of stored energy, carbon diffusion and spheroidization of cementite might explain some differences in textures observed in recrystallized ferrite.

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The effect of heating rate on the formation and decomposition of austenite was investigated on cold-rolled low carbon steel. Experiments were performed at two heating rates, 150 ˚C/s and 1500 ˚C/s, respectively. The microstructures were characterized by means of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Experimental evidence of nucleation of austenite in α/θ, as well as in α/α boundaries is analyzed from the thermodynamic point of view. The increase in the heating rates from 150 ˚C/s to 1500 ˚C/s has an impact on the morphology of austenite in the intercritical range. The effect of heating rate on the austenite formation mechanism is analyzed combining thermodynamic calculations and experimental data. The results provide indirect evidence of a transition in the mechanism of austenite formation, from carbon diffusion control to interface control mode. The resulting microstructure after the application of ultrafast heating rates is complex and consists of a mixture of ferrite with different morphologies, undissolved cementite, martensite, and retained austenite.

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