In this work, we investigate the sensitivity of the microstructure and mechanical properties of an ultrafast heat treated low carbon-steel to the peak temperature. In all studied cases, the steel was heated within the intercritical temperature range (i.e. between the AC1 and AC3 temperatures). Both the peak temperature and soaking time were varied, and their effect on the size, the fraction of individual microstructural constituents and their tensile mechanical response were investigated. It is shown that the increasing peak temperature and soaking time promote austenite formation and recrystallization processes in the ferritic matrix. The highest nanohardness is shown by martensitic grains, while recovered ferrite demonstrated slightly higher nanohardness compared to recrystallized ferrite. The applied heat treatment parameters have a strong effect on the nanohardness of martensite, whereas the nanohardness of ferrite microconstituents is not sensitive to variations of peak temperature and soaking time. The non-recrystallized ferrite is harder than its recrystallized counterpart due to the higher dislocation density of the former. Increasing peak temperatures promote strengthening in the material at the expense of its ductility mainly due to increased martensite fraction. The steel demonstrates enhanced strain hardening ability independently of the peak temperature. Analysis of the experimental results showed that the industrial processing window of ±10 °C may lead to some heterogeneity of the local microstructure in the ultrafast heat treated sheets. However, the latter should not have any negative effect on the overall mechanical behavior of the ultrafast heat treated steel sheets on the macro-scale.@en

The effect of ultra-fast heating on the microstructures of steel has been thoroughly studied over the last year as it imposes a suitable alternative for the production of ultra high strength steel grades. Rapid reheating followed by quenching leads to fine-grained mixed microstructures. This way the desirable strength/ductility ratio can be achieved while the use of costly alloying elements is significantly reduced. The current work focuses on the effect of ultra-fast heating on commercial dual phase grades for use in the automotive industry. Here, a cold-rolled, low-carbon, medium-manganese steel was treated with a rapid heating rate of 780 °C/s to an intercritical peak temperature (760 °C), followed by subsequent quenching. For comparison, a conventionally heated sample was studied with a heating rate of 10 °C/s. The initial microstructure of both sets of samples consisted of ferrite, pearlite and martensite. It is found that the very short heating time impedes the dissolution of cementite and leads to an interface-controlled α → γ transformation. The undissolved cementite affects the grain size of the parent austenite grains and of the microstructural constituents after quenching. The final microstructure consists of ferrite and martensite in a 4/1 ratio, undissolved cementite and traces of austenite while the presence of bainite is possible. Finally, it is shown that the texture is not strongly affected during ultra-fast heating, and the recovery and recrystallization of ferrite are taking place simultaneously with the α → γ transformation.@en

The main objective of this study is to understand the effect of the soaking time during the ultrafast heat treatment of a low carbon steel on its complex multi-phase microstructure, tensile mechanical behavior and properties of individual microconstituents. Tensile tests were performed to determine the macro-mechanical properties. Nanoindentation testing was carried out on individual microconstituents (martensite, recrystallized ferrite and non-recrystallized ferrite) identified a priori via EBSD analysis to measure their properties. It is shown that ultrafast heating combined with short soaking times results in improved macro-mechanical properties due to finer grain size and higher fraction of non-recrystallized ferrite, that has a higher nanohardness than recrystallized ferrite. Prolonged soaking times eliminate the advantages of the ultrafast heat treatment. This occurs because, even though a long soaking time promotes a higher volume fraction of martensite than a short one, it also induces substantial grain growth and complete recrystallization of the ferritic matrix. On the micro-scale, the ferritic grains show two different types of mechanical response. The recrystallized ferritic grains are prone to show pop-in events on the nanoindentation curves that are associated to dislocation nucleation events as a consequence of their low dislocation density, while non-recrystallized ferritic grains demonstrate a continuous response. The relationship between microstructure and mechanical properties on the macro- and micro-scales is discussed with respect to the microstructure, which in turn strongly depends on the applied heating rate and soaking time. A general recipe for microstructural design to improve the tensile mechanical behavior of low carbon steels implementing controlled heating and soaking conditions is outlined.@en

The mechanical behavior and microstructural evolution of a quenching and partitioning (Q&P) Fe-0.25C-1.5Si-3.0Mn (wt%) steel were investigated in a wide range of strain rates (10⁻⁴–10³ s⁻¹). The static tensile tests (10⁻⁴ and 10⁻² s⁻¹) were conducted using a universal testing machine, while high strain rate tests (500–1000 s⁻¹) were carried out on a split Hopkinson tensile bar system. High speed camera imaging combined with the digital image correlation (DIC) technique were employed to study homogeneity of plastic deformation. Electron backscatter diffraction (EBSD) and scanning electron microscopy were used to characterize the microstructure evolution in the deformed zone and the fracture surface, respectively. The results indicate that the yield strength of the Q&P steel in dynamic tests (500–1000 s⁻¹) is by 200 MPa higher compared to static tests (10⁻⁴ and 10⁻² s⁻¹), while the ultimate tensile strength tends to increase linearly with strain rate. The results of DIC analysis demonstrate that the homogeneity of plastic deformation is similar in static and dynamic test conditions. EBSD characterization shows that the retained austenite (RA) fraction decreases exponentially with the increase of plastic strain during both static and dynamic tensile testing. Additionally, examination of the fracture surfaces reveals the largest dimples in the statically tested specimens.

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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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This work focuses on the effect of soaking time on the microstructure during ultrafast heat treatment of a 50% cold rolled low carbon steel with initial ferritic-pearlitic microstructure. Dilatometry analysis was used to estimate the effect of heating rate on the phase transformation temperatures and to select an appropriate inter-critical temperature for final heat treatments. A thorough qualitative and quantitative microstructural characterization of the heat treated samples is performed using a wide range of characterization techniques. A complex multiphase, hierarchical microstructure consisting of ferritic matrix with embedded martensite and retained austenite is formed after all applied heat treatments. In turn, the ferritic matrix contains recrystallized and non-recrystallized grains. It is demonstrated that the ultrafast heating generally results in finer microstructure compared to the conventional heating independently on the soaking time. There is a significant effect of the soaking time on the volume fraction of martensite of the ultrafast heated material, while in the samples heated with conventional heating rate it remains relatively unchanged during soaking. Recrystallization, recovery and phase transformations occurring during soaking are discussed with respect to the applied heating rate.

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In this work, a Fe-0.25C-3.0Mn-1.5Si-0.023Al-0.015Cr (wt%) steel was subjected to the quenching and partitioning (Q&P) treatment, and its mechanical behavior and microstructure evolution during drop weight impact testing and quasi-static punch testing were thoroughly analyzed. It is shown that the 1 mm thick Q&P steel sheet can withstand 110 J impact energy without any (micro) cracking, which is well above the impact resistance of DP 1180 steel. The local true plastic strain can reach 53.4% in biaxial stretching showing excellent formability of the material. The microstructure characterization shows that the volume fraction of retained austenite decreases exponentially with increasing plastic strain under dynamic biaxial stretching. Scanning electron microscopy analysis of cracks formed after drop weight impact testing with 120 J energy reveals a ductile fracture surface consisting of elongated dimples formed by MnS inclusions which are surrounded by fine chisel-point type dimples. The potential of Q&P treated steels for application in the automotive industry is discussed.

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The effect of ultrafast heating on the microstructure and properties of a low-carbon steel is studied at the microscale. Ultrafast heating results in the formation of a complex multiphase microstructure containing mainly martensite and retained austenite grains embedded into a ferritic matrix. The ferritic matrix exhibits a microstructure consisting of recovered and recrystallized grains. The recrystallized grains are softer and display pop-in events during nanoindentation, while the harder recovered grains show uniform deformation behavior.

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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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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

Themed Issue on Aluminium-based Materials: Processing, Microstructure, Properties, and Recycling.@en

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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Steel with a nominal composition of 0.25C-1.5Si-3Mn-0.023Al (mass %) was subjected to Quenching and Partitioning (Q&P) with varying parameters (quenching temperature, partitioning temperature and partitioning time) resulting in formation of multi-phase microstructure, which was thoroughly studied using X-ray (XRD) and Electron Backscatter Diffraction (EBSD). Mechanical properties of the Q&P steel were measured by tensile tests. Plastic deformation of Q&P steel at micro-scale was investigated by in situ tensile testing and digital image correlation analysis. The effect of Q&P parameters on the microstructure (phase composition, size and volume fraction of micro constituents, texture and carbon content in retained austenite) is discussed. After analyzing the mechanical properties, plastic deformation at the micro-scale and the microstructure, it is shown that the strain partitioning between phases strongly depends on the microstructure of the Q&P steel, which, in turn, can be tuned via manipulation with Q&P parameters.

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