Microstructure characterization of nanometer carbides heterogeneous precipitation in Ti–Nb and Ti–Nb–Mo steel

The influences of micro-alloying elements and hot deformation on the precipitation morphology of Ti–Nb and Ti–Nb–Mo steels were investigated. The nanometer sized carbide particles randomly dispersed in the ferrite matrix are attributed mainly to severe deformation at high temperature and low isothermal holding temperature. Of the two steels with different combinations of the micro-alloying elements, Ti–Nb and Ti–Nb–Mo, the steel with Ti–Nb–Mo was more effective in precipitating hardening due to its slower carbide coarsening rate. Based on observations of micrographs, the nano-sized TiMoC and TiNbC precipitated in polygonal ferrite grains when the Ti–Nb–Mo and Ti–Nb steels were isothermally treated at 650 °C for 3 min and 180 min. The smaller of the two carbides, TiMoC, precipitated in the ferrite grain, and the hardness of Ti–Nb–Mo steel was higher than that of Ti–Nb steel. Moreover, the tiny ferrite grains and high dislocation density in the Ti–Nb–Mo steel were found to provide an attractive combination of strength and toughness.     

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity steel

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity (TRIP) steel was investigated by hot compression tests on Gleeble 3500D thermo-mechanical simulator in the temperature ranges of 900–1100 °C and the strain rate ranges of 0.01–10 s⁻¹. The results show that the flow stress is sensitively dependent on deformation temperature and strain rate, and the flow stress increases with strain rate and decreases with deformation temperature. The peak stress during hot deformation can be predicted by the Zener–Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 387.84 kJ/mol. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and decreasing of Z value lead to more adequate proceeding of DRX.     

Constitutive analysis of the hot deformation behavior of Fe–23Mn–2Al–0.2C twinning induced plasticity steel in consideration of strain

In this study, constitutive analysis has been carried out on Fe–23Mn–2Al–0.2C twinning induced plasticity (TWIP) steel. For this purpose, hot compression tests were conducted on a Gleeble-3500 thermo-mechanical simulator in the temperature range of 900–1150 °C and the strain rate range of 0.001–20 s⁻¹. The effects of deformation heating and friction on flow stress were analyzed and corrected. On the basis of Sellars–Tegart–Garofalo equation, the strain-dependent constitutive equations of the steel were derived. The results show that deformation heating has a significant influence on the flow stress at lower temperatures and higher strain rates, while the frictional effect is slight even at the highest strain level investigated. Comparison of the calculated flow stress with the experimental data suggests that the developed constitutive equations can adequately describe the relationships between the flow stress, strain rate, temperature and strain of the steel during hot deformation. This is supported by a high correlation coefficient (R = 0.996) and a low average absolute relative error (AARE = 3.31%) for the entire deformation condition range investigated.     

Influences of Nb-microalloying on microstructure and mechanical properties of Fe–25Mn–3Si–3Al TWIP steel

The Fe–25Mn–3Si–3Al TWIP steel was microalloyed by niobium in this paper, and the appropriate heat treatment and cold rolling processes were drafted in order to improve the poor yield strength of the steel. The results show that the yield strength of the steel increases from 320 MPa to 445 MPa, and the tensile strength increases from 680 MPa to 795 MPa, but the uniform elongation decreases from 65% to 55%. Nb addition can strongly hinder the growth of recrystallized grains, moreover Nb atoms react with C atoms to form nanoscale NbC precipitations, and these precipitations can block the dislocation motion, and then the yield strength and initial work hardening ability of Fe–25Mn–3Si–3Al steel is clearly improved. Furthermore, the strain-induced twinning is still a major deformation mechanism for the Nb-microalloying TWIP steel, and the twinning induced plasticity (TWIP) effect ensures a satisfactory ductility for the steel. Finally, the modified TWIP steel obtains a better match between the strength and plasticity by the joint action of precipitation strengthening and TWIP effect.     

An investigation into the mechanical behavior of a new transformation-twinning induced plasticity steel

In recent years, the transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels have been the focus of great attention thanks to their excellent tensile strength-ductility combination. Accordingly the mechanical behavior of an advanced microalloyed TRIP–TWIP steel, the compression tests were conducted from 25 to 1000 °C. This experimental steel shows a high compressive strength of 1280 MPa with the yield strength of 385 MPa as well as an outstanding strain hardening rate of about 14,000 MPa at the 25 °C. In addition the results indicate that the plastic deformation in the range of 25–150 °C is controlled by both the strain-induced martensite formation and mechanical twinning. However the mechanical twinning has been speculated as the only deformation mechanism in the temperature range of 150–1000 °C. This as well has led to an outstanding grain refinement via grain partitioning. The occurrence of mechanical twinning at such high temperatures is a novel observation in this grade of TRIP–TWIP high manganese steels.     

Cooling process and mechanical properties design of hot-rolled low carbon high strength microalloyed steel for automotive wheel usage

For the purpose of developing Nb–V–Ti microalloyed, hot rolled, high strength automotive steel for usage in heavy-duty truck wheel-discs and wheel-rims, appropriate cooling processes were designed, and microstructures and comprehensive mechanical properties (tension, bending, hole-expansion, and Charpy impact) of the tested steels at two cooling schedules were studied. The results indicate that the steel consists of 90% 5 μm polygonal ferrite and 10% pearlite when subjected to a cooling rate of 13 °C/s and a coiling temperature of 650 °C. The yield strength, tensile strength, and hole-expansion ratio are 570 MPa, 615 MPa, and 95%, respectively, which meet the requirements of the wheel-disc application. The steel consists of 20% 3 μm polygonal ferrite and 80% bainite (granular bainite and a small amount of acicular ferrite) when subjected to a cooling rate of 30 °C/s and a coiling temperature of 430 °C. The yield strength, tensile strength, and hole-expansion ratio are 600 MPa, 655 MPa, and 66%, respectively, which meet the requirements of the wheel-rim application. Both the ferrite–pearlite steel and ferrite–bainite steel possess excellent bendability and Charpy impact property. The precipitation behavior and dislocation pattern are characterized and discussed.     

Influence of processing parameters on hot workability and microstructural evolution in a carbon–manganese–silicon steel

In this work, the influence of processing variables such as strain, strain rate, temperature and cooling medium, on workability, microstructural evolution and mechanical properties of a carbon–manganese–silicon (C–Mn–Si) steel have been studied. Hot deformation of the C–Mn–Si steel has been carried out using compression testing over a domain 1223–1473 K and 0.001–10 s^− 1 where the steel is in austenitic phase field. The effect of cooling medium on the microstructural evolution has been studied by carrying out post-deformation cooling of the specimens in air and water media. Influence of the cooling medium on properties of the steel has been evaluated by comparing the hardness and Charpy impact test results. Based on the flow behavior analysis and microstructural examinations the optimum domain for the hot deformation of C–Mn–Si steel is found to be in the ranges of 1273–1350 K and 3–10 s^− 1. Flow instability in C–Mn–Si steel is manifested in the form of deformation bands in the microstructure. The signature of instability is not influenced by the phase transformation. The hardness of the material is dependent on the temperature of deformation and influenced by cooling medium. However, it does not show any correlation with deformation strain rate.     

Effect of the thermal cycle on the hot ductility and fracture mechanisms of a C–Mn steel

Transverse cracking on the surface of continuously cast steel products has been one of the main problems of this stage in steelmaking for many years. The incidence of this problem has been found in microalloyed steels as well as in some plain carbon steels containing residual elements. In this work, the hot ductility and fracture mechanisms of a C–Mn steel containing 0.6%Cu and 0.053%Sn as residual elements have been evaluated. To simulate the thermo-mechanical conditions of the straightening operation, tensile tests were carried out at temperatures ranging from 700 to 1100 °C with an initial strain rate of 5 × 10⁻³ s⁻¹. Specimens were subjected to three different reheating temperatures prior to the hot ductility test, including 1100 °C, 1330 °C and melting. After each test, the reduction in area of the samples tested to fracture was used as a measure of the hot ductility. The fracture surfaces were then examined by scanning electron microscopy. The widest and deepest ductility trough was obtained for the specimens tested after melting; for these conditions brittle fractures are interdendritic showing very low ductility. After reheating at 1330 °C, fracture features showed intergranular fracture combined with some plastic deformation corresponding to the test temperature. Reheating at 1100 °C produced a finer microstructure and the fracture features showed a mixture of intergranular with some interdendritic features. Also, ductile behaviours were associated with void coalescence. The different results obtained depending on the thermal cycle can be attributed to the presence of the residual elements in the steel through different segregation and precipitation patterns.     

Stacking fault energy and tensile deformation behavior of high-carbon twinning-induced plasticity steels: Effect of Cu addition

Three experimental fully austenitic high-carbon twinning-induced plasticity (TWIP) steel grades were produced and the stacking fault energy (SFE) was investigated based on the thermodynamic modeling approach. The SFE of Fe–20Mn–xCu–1.3C (x = 0, 1.5 and 3.0) steels varied from 24.36 to 28.74 mJ m⁻² at room temperature. In order to study the correlation between the SFE and the mechanical behavior of TWIP steels, tensile tests were performed at room temperature and the deformed microstructures were examined at different strain levels by transmission electron microscopy. The Cu additions resulted in a remarkable increase in total elongation without a slight loss of tensile strength. In addition, the critical strain for serration start on the tensile stress–strain curves (i.e. required strain to generate mechanical twinning) was found to increase with increasing Cu content. Transmission electron microscope (TEM) observations also indicated that the occurrence of mechanical twinning was suppressed by increasing the Cu addition. The strain hardening mechanism and the superior ductility in deformation are dominated by the interaction of twins and dislocations. The mechanical behavior of TWIP steels is related to the Cu addition, the SFE, the interaction of twins and dislocations.     

Hot deformation and processing map of an as-extruded Mg–Zn–Mn–Y alloy containing I and W phases

The hot deformation characteristics of an as-extruded ZM31 (Mg–Zn–Mn) magnesium alloy with an addition of 3.2 wt.% Y, namely ZM31 + 3.2Y, have been studied via isothermal compression testing in a temperature range of 300–400 °C and a strain rate range of 0.001–1 s^− 1. A constitutive model based on hyperbolic-sine equation along with processing maps was used to describe the dependence of flow stress on the strain, strain rate, and deformation temperature. The flow stress was observed to decrease with increasing deformation temperature and decreasing strain rate. The deformation activation energy of this alloy was obtained to be 241 kJ/mol. The processing maps at true strains of 0.1, 0.2, 0.3 and 0.4 were generated to determine the region of hot workability of the alloy, with the optimum hot working parameters being identified as deformation temperatures of 340–500 °C and strain rates of 0.001–0.03 s^− 1. EBSD examinations revealed that the dynamic recrystallization occurred more extensively and the volume fraction of dynamic recrystallization increased with increasing deformation temperature. The role of element Y and second-phase particles (I- and W-phases) during hot compressive deformation was discussed.     

Small-angle neutron scattering analysis of Mn–C clusters in high-manganese 18Mn–0.6C steel

Nanometer-scale particles (Mn–C clusters) were analyzed quantitatively using small-angle neutron scattering in 18Mn–0.6C (wt.%) austenite high-manganese steel. The size, number, and volume fraction of the particles were determined as a function of strain (0, 5, 15, 30, 45, 50%) at different temperatures (25 and 100 °C). The diameter of the cluster ranges from 2 to 14 nm in the matrix. The total volume fraction of the cluster significantly increases from 2.7 × 10^− 6 to 8.7 × 10^− 6 as the strain increases. Such clustering phenomenon is correlated to the serration behavior under loading in high-manganese steels.     

Modeling of flow stress of 42CrMo steel under hot compression

The compressive deformation behavior of 42CrMo steel was investigated at temperatures from 850 °C to 1150 °C and strain rates from 0.01 s^?1 to 50 s^?1 on a Gleeble-1500 thermo-simulation machine. The results show that the true stress–true strain curves exhibit peak stresses at small strains, then the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in an exponent-type equation. A revised model describing the relationships of the flow stress, strain rate and temperature of the 42CrMo steel at elevated temperatures is proposed by compensation of strain. The stress–strain relations of 42CrMo steel predicted by the proposed models agree well with experimental results.     

Microstructure evolution and flow behavior of hot-rolled aluminum – 5% B4C composite

Differential strain rate compression tests were conducted to study flow behavior of hot rolled Al–5 wt% B₄C composite as a function of sample orientation (longitudinal and transverse) over the temperature and strain rate ranges of 25–500 °C and 10⁻⁴ to 1 s⁻¹, respectively. The longitudinal samples are found to show lower flow stress than that shown by the transverse samples in the temperature range of 25–200 °C. The reverse becomes true at higher temperatures of 300–500 °C. The values of stress exponent (n) and activation energy for deformation (Q), based on applied stress, ranged from 10 to 46 and 307–416 kJ/mol, respectively. However, by considering effective stress, these values were reduced to n = 8 and Q = 126–190 kJ/mol. This stress exponent ofn = 8 is further reduced to n = 5 by considering substructural evolution, which suggests the dislocation climb creep mechanism to be favorable for deformation.     

Constitutive modeling for flow behavior of GCr15 steel under hot compression experiments

The thermal compressive deformation behavior of GCr15 (AISI-52100), one of the most commonly used bearing steels, was studied on the Gleeble-3500 thermo-simulation system at temperature range of 950–1150 °C and strain rate range of 0.1–10 s⁻¹. According to the experimental results, the stress level decreases with increasing deformation temperature and decreasing strain rate. The peak stresses on the true stress–strain curves suggest that the dynamic softening of GCr15 steel occurs during hot compression tests. To formulate the thermoplastic constitutive equation of GCr15 steel, Arrhenius equation and the Zener–Hollomon parameter in an exponent-type equation were utilized in this paper. In addition, a modified Zener–Hollomon parameter considering the compensation of strain rate during hot compression was employed to improve the prediction accuracy of the developed constitutive equation. Analysis results indicate that the flow stress values predicted by the proposed constitutive model agree well with the experimental values, which confirms the accuracy and reliability of the developed constitutive equation of GCr15 steel.     

Hot deformation behavior of the new Al–Mg–Si–Cu aluminum alloy during compression at elevated temperatures

The hot deformation behavior of the new Al–Mg–Si–Cu aluminum alloy was investigated by compression tests in the temperature range 350 °C–550 °C and strain rate range 0.005 s^− 1–5 s^− 1 using Gleeble-1500 system, and the associated structural changes were studied by observations of metallographic and TEM. The results show that the true stress–true strain curves exhibit a peak stress at a small strain (< 0.15), after which the flow stresses decrease monotonically until high strains, showing a dynamic flow softening. The stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener-Hollomon parameter in an exponent-type equation with the hot deformation activation energy Q of 236 kJ/mol. The substructure in the deformed specimens consists of very small amount and fine precipitates with equaixed polygonized subgrains in the elongated grains and developed serrations in the grain boundaries, indicating that the dynamic flow softening is mainly as the result of dynamic recovery (DR) and recrystallization (RDX).     

Some metallurgical aspects of Dynamic Strain Aging effect on the Low Cycle Fatigue behavior of C–Mn steels

Carbon–Manganese steels and associated welds are commonly used, and sometimes to sustain loads in the Low Cycle Fatigue domain. Nevertheless, the metallurgy of these C–Mn steels is rather complex, due to the interaction of solute atoms (carbon and nitrogen) with dislocations during deformation which leads to metallurgical instabilities: Lüders strain, Static Strain Aging (SSA) and Dynamic Strain Aging (DSA). The DSA phenomenon is an interaction during the test between solute atoms and dislocations which are submitted to an supplementary anchorage if the temperature is sufficient to allow the diffusion of solute atoms leading to a discontinuous plastic deformation localized in bands associated with serrations on the stress–strain curve. In C–Mn, the temperature domain where the phenomenon is present is from 150 °C to 300 °C. If these metallurgical instabilities induce an increase in hardness, unfortunately they produce a decrease of ductility detrimental to components safety. The results of the DSA effect on LCF behavior in C–Mn and Low Alloyed steels reported in the literature are very confused and contradictories. In this study, two C–Mn steels with a different sensitivity to DSA are investigated in the Low Cycle fatigue domain. As reported from some authors, the fatigue life seems enhance or reduce in the temperature domain where the DSA is maximum, but the decrease of the strain rate always decreases the number of cycles to failure.     

Microstructure and mechanical properties of 12 wt.% Cr ferritic stainless steel with Ti and Nb dual stabilization

TCS stainless steel is a 12 wt.% Cr ferritic stainless steel with 0.040 wt.% Ti and 0.096 wt.% Nb dual stabilization. This paper investigated the microstructures and mechanical properties of TCS stainless steel heated at 600–1300 °C for 10 min and followed water quenching. Results show the increasing of both tensile strength and hardness meanwhile the ductility and toughness have experienced the decreasing due to formation of martensitic phase and grain coarsening. In the unheated and heated TCS stainless steel, there are mainly two kinds of particles: Ti-rich particles in size of 2–5 μm; Nb-rich particles in size of 20–50 nm.     

The flow behavior and constitutive equation in isothermal compression of FGH4096-GH4133B dual alloy

The electron beam welding of superalloy FGH4096 and GH4133B was conducted, and the cylindrical compression specimens were machined from the central part of the electron beam weldments. Isothermal compression tests were carried out on electron beam weldments FGH4096-GH4133B alloy at the temperatures of 1020–11140 °C (the nominal γ′-transus temperature is about 1080 °C) and the strain rates of 0.001–1.0 s⁻¹ with the height reduction of 50%. True stress–true strain curves are sensitive to the deformation temperature and strain rate, and the flow stress decreases with the increasing deformation temperature and the decreasing strain rate. The true stress–true strain curves can indicate the intrinsic relationship between the flow stress and the thermal-dynamic behavior. The apparent activation energy of deformation at the strain of 0.6 was calculated to be 550 kJ/mol, and the apparent activation energy has a great effect on the microstructure. The constitutive equation that describes the flow stress as a function of strain rate and deformation temperature was proposed for modeling the hot deformation process of FGH4096-GH4133B electron beam weldments. The constitutive equation at the strain of 0.6 was established using the hyperbolic law. The relationship between the strain and the values of parameters was studied, and the cubic functions were built. The constitutive equation during the whole process can be obtained based on the parameters under different strains. Comparing the experimental flow stress and the calculated flow stress, the constitutive equation obtained in this paper can be very good to predict the flow stress under the deformation temperature range of 1020–1140 °C and the strain rate range of 1.0–0.001 s⁻¹.     

Dynamic recrystallization behavior of an as-cast TiAl alloy during hot compression

High temperature compressive deformation behaviors of as-cast Ti–43Al–4Nb–1.4W–0.6B alloy were investigated at temperatures ranging from 1050 °C to 1200 °C, and strain rates from 0.001 s^− 1 to 1 s^− 1. Electron back scattered diffraction technique, scanning electron microscopy and transmission electron microscopy were employed to investigate the microstructural evolutions and nucleation mechanisms of the dynamic recrystallization. The results indicated that the true stress–true strain curves show a dynamic flow softening behavior. The dependence of the peak stress on the deformation temperature and the strain rate can well be expressed by a hyperbolic-sine type equation. The activation energy decreases with increasing the strain. The size of the dynamically recrystallized β grains decreases with increasing the value of the Zener–Hollomon parameter (Z). When the flow stress reaches a steady state, the size of β grains almost remains constant with increasing the deformation strain. The continuous dynamic recrystallization plays a dominant role in the deformation. In order to characterize the evolution of dynamic recrystallization volume fraction, the dynamic recrystallization kinetics was studied by Avrami-type equation. Besides, the role of β phase and the softening mechanism during the hot deformation was also discussed in details.     

Microstructure and compressive flow stress of directionally solidified ternary Ni3(Al,Nb) and quaternary Ni3(Al,Nb, Ti) alloys with duplex phase

The microstructure and compressive flow stress of directionally solidified ternary Ni₃(Al, Nb) and quaternary Ni₃(Al, Nb, Ti) alloys were examined. Three compositions of Ni–16.0 at.%Nb–9.0 at.%Al (Alloy 1), Ni–13.3 at.%Nb–7.5 at.%Al–4.2 at.%Ti (Alloy 2) and Ni–10.7 at.%Nb–6.0 at.%Al–8.3 at.%Ti (Alloy 3) were selected for investigation. Alloy 1 was composed of the L1₂ and the D0_a phases while the constituent phases varied for the D0₂₄ and the D0_a phases for Alloys 2 and 3 with Ti content. The definite crystallographic relationship was observed between the D0₂₄ and the D0_a phases to be (0001)_D024//(010)_D0a and 〈112̄0〉_D024//〈100〉_D0a in Alloy 3. Compression tests were conducted along the growth direction in the temperature ranging from room temperature to 1000 °C. Alloy 1 exhibited high yield stress at low temperatures, but it rapidly decreased above 700 °C. Similar temperature dependence of yield stress was observed in Alloy 2, although the onset temperature of a rapid decrease in yield stress was somewhat lower. Alloy 3 with the highest Ti content showed the lowest compressive strength among the three alloys, while relatively good low-temperature ductility was obtained in Alloy 3. Yield stress of Alloy 3 exhibited anomalous strengthening behaviour accompanied by the basal slip in both D0₂₄ and D0_a phases. Transition in operative slip systems from the basal slip to the prism slip occurred at the peak temperature of yield stress anomaly (600 °C), resulting in a gradual decrease in yield stress. Slip transfer behaviour between the D0₂₄ and the D0_a phases was briefly discussed.