High strength-elongation product of Nb-microalloyed low-carbon steel by a novel quenching-partitioning-tempering process

In this article, a novel quenching-partitioning-tempering (Q-P-T) process was applied to a Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt%) hot-rolled steel, and its optimized parameters were obtained by a Gleeble-3500 thermal simulator and salt baths, respectively. Mechanical property results of the as-treated Q-P-T samples show that the Nb-microalloyed low-carbon steels subjected to Q-P-T processes cover a wide spectrum of strength (1200-1500 MPa) and elongation (14-18%), and exhibit excellent product of strength and elongation (21,000-22,000 MPa%). Microstructural characterization indicates that high strength results from dislocation-type martensite laths and dispersively distributed fcc NbC or hcp ?-carbides in martensite matrix and good ductility is attributed to transformation induced plasticity (TRIP) effect from plenty of retained austenite flakes between martensite laths.     

Tensile deformation of a duplex Fe-20Mn-9Al-0.6C steel having the reduced specific weight

The room temperature deformation characteristics of a duplex Fe-20Mn-9Al-0.6C steel with the reduced specific weight of 6.84 g/cm³ in the fully solutionized state were described in conjunction with the deformation mechanisms of its constituent phases. The phase fraction was insensitive to annealing temperature in the range of 800-1100 °C. The ferrite grain size was also nearly unaltered but the austenite grain size slightly increased with increasing annealing temperature. This revealed that there is little window to control the microstructure of the steel by annealing. The steel exhibited a good combination of strength over 800 MPa and ductility over 45% in the present annealing conditions. Ferrite was harder than austenite in this steel. Strain hardening of both phases was monotonic during tensile deformation, but the strain hardening exponent of austenite was higher than that of ferrite, indicating the better strain hardenability of austenite. In addition, the strain hardening exponent of austenite increased but that of ferrite remained unchanged with increasing annealing temperature. The overall strain hardening of the steel followed that of austenite. Considering element partitioning by annealing, the stacking fault energy of austenite of the steel was estimated as ∼70 mJ/m². Even with the relatively high stacking fault energy, planar glide dominantly occurred in austenite. Neither strain induced martensite nor mechanical twins formed in austenite during tensile deformation. Ferrite exhibited the deformed microstructures typically observed in the wavy glide materials, i.e. dislocation cells. The mechanical properties of the present duplex steel were compared to those of advance high strength automotive steels recently developed.     

时效态Fe-Mn-Al-C钢的性能和变形机制

采用OM、SEM、XRD、EBSD以及TEM等手段分析时效温度对Fe-30Mn-9Al-0.9C-0.45Mo钢中奥氏体的晶粒尺寸和力学性能的影响.结果 表明:时效处理对Fe-30Mn-9Al-0.9C-0.45Mo钢的组织和性能有较大的影响.在450℃时效的实验钢其抗拉强度为863 MPa、断后伸长率为56.1％、强...     

High strain rate effects on direct tensile behavior of high performance fiber reinforced cementitious composites

Direct tensile behavior of high performance fiber reinforced cementitious composites (HPFRCCs) at high strain rates between 10 s⁻¹ and 30 s⁻¹ was investigated using strain energy frame impact machine (SEFIM) built by authors. Six series of HPFRCC combining three variables including two types of fiber, hooked (H) and twisted (T) steel fiber, two fiber volume contents, 1% and 1.5%, and two matrix strengths, 56 MPa and 81 MPa, were investigated. The influence of these three variables on the high strain rate effects on the direct tensile behavior of HPFRCCs was analyzed based on the test results. All series of HPFRCCs showed strongly sensitive tensile behavior at high strain rates, i.e., much higher post cracking strength, strain capacity, and energy absorption capacity at high strain rates than at static rate. However, the enhancement was different according to the types of fiber, fiber volume content and matrix strength: HPFRCCs with T-fibers produced higher impact resistance than those with H-fibers; and matrix strength was more influential, than fiber contents, for the high strain rate sensitivity. In addition, an attempt to predict the dynamic increase factor (DIF) of post cracking strength for HPFRCCs considering the influences of fiber type and matrix strength was made.     

Influence of stress triaxiality and strain rate on the failure behavior of a dual-phase DP780 steel

To better understand the in-service mechanical behavior of advanced high-strength steels, the influence of stress triaxiality and strain rate on the failure behavior of a dual-phase (DP) 780 steel sheet was investigated. Three flat, notched mini-tensile geometries with varying notch severities and initial stress triaxialities of 0.36, 0.45, and 0.74 were considered in the experiments. Miniature specimens were adopted to facilitate high strain rate testing in addition to quasi-static experiments. Tensile tests were conducted at strain rates of 0.001, 0.01, 0.1, 1, 10, and 100 s⁻¹ for all three notched geometries and compared to mini-tensile uniaxial samples. Additional tests at a strain rate of 1500 s⁻¹ were performed using a tensile split Hopkinson bar apparatus. The results showed that the stress–strain response of the DP780 steel exhibited mainly positive strain rate sensitivity for all geometries, with mild negative strain rate sensitivity up to 0.1 s⁻¹ for the uniaxial specimens. The strain at failure was observed to decrease with strain rate at low strain rates of 0.001–0.1 s⁻¹; however, it increased by 26% for an increase in strain rate from 0.1 to 1500 s⁻¹ for the uniaxial condition. Initial triaxiality was found to have a significant negative impact on true failure strain with a decrease of 32% at the highest triaxiality compared to the uniaxial condition at a strain rate of 0.001 s⁻¹. High resolution scanning electron microscopy images of the failure surfaces revealed a dimpled surface while optical micrographs revealed shearing through the thickness indicating failure occurred via ductile-shear. Finite element simulations of the tests were used to predict the effective plastic strain versus triaxiality history within the deforming specimens. These predictions were combined with the measured conditions at the onset of failure in order to construct limit strain versus triaxiality failure criteria.     

Influence of initial microstructures on intercritical annealing behaviour in a medium Mn steel

ABSTRACT

The present work reports the effect of different initial microstructures on reverse transformation kinetics and morphologies of austenite formed during intercritical annealing in Fe-0.14C-7Mn-1Si (wt-%) medium Mn steel. Three different initial microstructures were produced by cold-rolling and cold-rolling followed by austenitisation at 820°C and 900°C. The specimen austenitised at higher temperature shows lath-type austenite after intercritical annealing. The difference in austenitisation temperature leads to different Mn distribution in martensitic initial microstructures, thereby leading to a difference in morphology of austenite. The inhomogeneous Mn profiles in initial microstructures also affect reverse transformation kinetics of austenite upon intercritical annealing. The presence of Mn-enriched regions accelerates austenite growth at an early stage of intercritical annealing but retards the transformation kinetics afterwards.

This paper is part of a Thematic Issue on Medium Manganese Steels.     

Investigation of cold rolling variables on the formation of strain-induced martensite in 201L stainless steel

In this work, effects of cold rolling variables including strain, strain rate, strain path, initial austenite grain size and rolling temperature on the formation of strain-induced martensite in AISI 201L stainless steel are investigated. Cold rolling was carried out at −40, −10, and 25 °C with strain rates of 0.1–1.2 s⁻¹ and thickness reductions of 0–95%. The results showed that saturation strain of martensite formation during cold rolling at room temperature with the strain rate of 0.5 s⁻¹ was about 0.5. Increasing the strain, strain rate, and initial austenite grain size, decreasing rolling temperature, and the use of cross rolling resulted in an increase in the volume fraction of strain-induced martensite and a decrease in the saturation strain value. It was found that effect of decreasing rolling temperature and cross rolling was more effective on the formation of strain-induced martensite compared to other parameters, leading to a reduction of saturation strain from 0.5 to 0.28.     

Fabrication of a biodegradable Fe-Mn-Si alloy by field assisted sintering

Biodegradable metals are emerging as novel implant materials by overcoming the drawbacks of the existing materials used commercially. This work investigates the suitability of Fe-35Mn-5Si as a biodegradable implant by examining its mechanical and corrosion behavior. The processing involves High Energy Ball Milling (HEBM) followed by Spark Plasma Sintering (SPS) and heat treatment at optimized conditions to develop a single-phase austenitic alloy. The heat-treated (HT) samples exhibited low magnetic susceptibility of 3.47x10⁻⁷ due to the austenitic phase formation. Yield strength of 500 MPa, UTS of 712 MPa, Young’s modulus of 110 GPa, and hardness of 380 HV along with 9.5% elongation was obtained in the optimized samples, which are comparable to Ti alloy and 316L stainless steel metallic implants. The corrosion tests yielded degradation rates of 0.025 mm/year for the alloy in standard Hank’s solution. This alloy could pave the way for the fabrication of low-cost biodegradable implants using the simple powder metallurgy route.     

Elastic strain evolution and ε-martensite formation in individual austenite grains during in situ loading of a metastable stainless steel

The (hcp) ε-martensite formation and the elastic strain evolution of individual (fcc) austenite grains in metastable austenitic stainless steel AISI 301 has been investigated during in situ tensile loading up to 5% applied strain. The experiment was conducted using high-energy X-rays and the 3DXRD technique, enabling studies of individual grains embedded in the bulk of the steel. Out of the 47 probed austenite grains, one could be coupled with the formation of ε-martensite, using the reported orientation relationship between the two phases. The formation of ε-martensite occurred in the austenite grain with the highest Schmid factor for the active {111}<12¯1> slip system.     

Mechanical and microstructural analysis of 2205 duplex stainless steel under hot working condition

Hot deformation characteristics of 2205 duplex stainless steel were analyzed by performing hot compression tests at a temperature range of 950–1200 °C and a strain rate of 0.001–1 s⁻¹. Flow stress was modeled by the constitutive equation of hyperbolic sine function. The constants of n, A, α, and the apparent activation energy were determined at different strains. They were then fitted by polynomial equations. Using the hyperbolic sine function and the relations derived between constants and strain flow curves were successfully modeled. Microstructural evolutions were characterized using optical microscopy and electron back scattered diffraction techniques. The results showed that dynamic recovery in ferrite is accelerated at higher temperatures followed by transformation to continuous dynamic recrystallization. Dynamic recrystallization in austenite was postponed by the accommodation of strain in ferrite and very few internal boundaries in austenite. At high strain rates, dynamic recovery in ferrite and dynamic recrystallization in austenite are very slow. Consequently, the total recrystallized fraction decreases. At low temperatures this situation may cause flow instabilities. At low strain rates, softening processes dominate in austenite and ferrite whereas at intermediate strain rates, the formation of substructures is observed in both phases.     

Effect of Mg17Al12 precipitates on the microstructural changes and mechanical properties of hot compressed AZ91 magnesium alloy

During hot compression, Mg₁₇Al₁₂ (β) precipitates show strong influence on the microstructural changes of 415 °C-24 h homogenized AZ91 alloy. When compressed at 300 °C and 350 °C, dynamic recrystallization (DRX) only occurs near grain boundaries with discontinuous β precipitate pinning at the newly DRXed grain boundaries. With increasing compression temperature and decreasing strain rate, the β-precipitating region expands; however, the amount of pinning precipitates decreases, resulting in increases in the DRX ratio and average DRXed grain size. With a compression ratio of only 50%, the specimen compressed at 350 °C and a strain rate of 0.2 s⁻¹ (designated 350 °C-0.2 s⁻¹ compressed specimen) shows an ultimate tensile strength (UTS) of 334 MPa, a 0.2% proof stress (PS) of 195 MPa and an enough elongation of 17.9%. After a subsequent aging treatment at 180 °C, due to the large number of β precipitates, the strength of the compressed specimens are further improved, and the specimen peak aged after compression at 400 °C and 0.2 s⁻¹ shows UTS of 364 MPa and PS of 248 MPa with a moderate elongation of 7.7%.     

Microstructure evolution through heavy compression aided by thermodynamic calculations

The induced martensite transformation in a dual-phase bainitic ferrite–austenite steel during heavy compression was studied by thermodynamic computations. Compression tests were conducted at temperatures of 298 and 573 K on the rectangle samples at the strain rate of 0.001 s⁻¹. The samples were deformed to 40 and 70% of their original thickness. It was found that 70% compression of the steel at room temperature resulted in transformation of retained austenite to martensite, which is in agreement with thermodynamic calculations. Additionally, heavy compression resulted in the formation of fine grains with high angle grain boundaries which confirms grain refinement.     

Fabrication of lotus-type porous carbon steel via continuous zone melting and its mechanical properties

Lotus-type porous carbon steel (lotus carbon steel) AISI1018 rods with long cylindrical pores aligned in one direction were fabricated using the continuous zone melting technique under nitrogen gas pressure of 2.5 MPa. The porosity decreased with increasing transference velocities of 40–160 μm s⁻¹. Tensile tests of the fabricated lotus-type carbon steel rods were performed. The elongation of lotus carbon steel increased after normalizing at 1200 K. The tensile strength and the Young's modulus decreased with increasing porosity. In contrast, the yield strength of lotus carbon steel did not decrease, even with a porosity of 20%, compared with that of non-porous carbon steel. This superior characteristic is attributed to solid-solution strengthening by solute nitrogen.     

High temperature flow behavior and constitutive model of alloy transition layer in composite steel tube prepared by centrifugal self-propagating high-temperature synthesis (SHS)

Centrifugal self-propagation high-temperature synthesis (SHS) is a newly developed composite preparation technique by which a ceramic-alloy-carbon steel multilayer composite tube can be prepared. The hot deformation behaviors of the alloy steel layer at 800 °C–1000 °C, strain rates of 0.01 s⁻¹, 0.1 s⁻¹, 1.0 s⁻¹ and 10 s⁻¹ were studied by Gleeble-1500 thermal simulator. Rheological curve characteristics were analyzed under different thermal compression processes and a phenomenological hyperbolic sinusoidal Arrhenius constitutive equation was established to characterize the rheological mechanics of the material. The results show that the alloy steel is sensitive to temperature and strain rate, and its value of true stress decreases with the increase of temperature and strain rate. Thermal deformation process is the interaction between work hardening and dynamic softening, which is accompanied by the increase and extinction of dislocations. Under the strain rate of 10 s⁻¹, the stress-strain curve has a significant decrease when the strain exceeds 0.5. According to the observation of microstructure, this phenomenon can be attributed to the micro-crack generated by the local instability flow in the denatured zone. With the strain rate decreases, the softening mechanism of the alloy changes from dynamic recovery to dynamic recrystallization. The calculation results of the Arrhenius constitutive equation (AARE = 6.54 %, R = 0.99452) indicate that the model can predict the flow stress of the alloy accurately.     

The hot deformation behavior and processing map of Ti–47.5Al–Cr–V alloy

The high temperature flow behavior of as-extruded Ti–47.5Al–Cr–V alloy has been investigated at the temperature between 1100 °C and 1250 °C and the strain rate range from 0.001 s^− 1 to 1 s^− 1 by hot compression tests. The results showed that the flow stress of this alloy had a positive dependence on strain rate and a negative dependence on deformation temperature. The activation energy Q was calculated to be 409 kJ/mol and the constitutive model of this material was established. By combining the power dissipation map with instability map, the processing map was established to optimize the deformation parameters. The optimum deformation parameter was at 1150 °C–1200 °C and 0.001 s^− 1–0.03 s^− 1 for this alloy. The microstructure of specimens deformed at different conditions was analyzed and connected with the processing map. The material underwent instability deformation at the strain rate of 1 s^− 1, which was predicted by the instability map. The surface fracture was observed to be the identification of the instability.     

A characterization for the flow behavior of 42CrMo steel

In order to evaluate the flow stress and the dynamic softening characteristics of casting 42CrMo steel, isothermal upsetting experiments with height reduction 60% were performed at the temperatures of 1123 K, 1198 K, 1273 K and 1348 K, and the strain rates of 0.01 s⁻¹, 0.1 s⁻¹, 1 s⁻¹ and 10 s⁻¹ on thermal physics simulator Gleeble 1500. The flow behavior of the applied stress as a function of strain, strain rate and temperature exhibits a more pronounced effect of temperature than strain rate, and a typical characteristic of dynamic recrystallization softening. To characterize the flow behavior more factually and accurately, the traditional Fields–Backofen equation was amended, and an innovative mathematical model containing a softening item s, n-value and m-value variable functions was brought forth. The stress–strain curves calculated by the derived flow stress equation are fit with the experimental results well not only at the hardening stage but also at softening stage.     

Hot deformation behavior of an 8% Cr cold roller steel

An 8% Cr cold roller steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s⁻¹. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. Metallographic investigation was performed to evaluate the microstructure evolution and the mechanism of flow instability. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 8% Cr steel; the efficiency of power dissipation decreased with increasing Z value; flow instability was observed at higher Z-value conditions and manifested as flow localization near the grain boundary. The hot deformation equation and the dependences of critical stress for dynamic recrystallization and dynamic recrystallization grain size on Z value were obtained. The suggested processing window is in the temperature range 1050–1200 °C and strain rate range 0.1–1 s⁻¹ in the hot processing of 8% Cr steel.     

Effect of strain rate on tensile behavior of carbon fiber reinforced aluminum laminates

In this paper, the tensile behavior of carbon fiber reinforced aluminum laminates (CRALL) has been determined at a strain rate range from 0.001 s^− 1 to 1200 s^− 1. Experimental results show that CRALL composite is a strain rate sensitive material, and the tensile strength and failure strain both increased with increasing strain rate. A linear strain hardening model has been combined with Weibull distribution function to establish a constitutive equation for CRALL at different strain rates. The analysis of the model shows that the Weibull scale parameter, σ₀, increased with increasing strain rate, but Weibull shape parameter, β, can be regarded as a constant.     

New observations of the twinning effect and austenite stability in intercritical quenched and tempered steel with high strength

To optimize the formability and strength of hot-rolled Fe-10Mn-0.4C-2Al-0.6 V medium Mn steel, intercritical quenching and tempering processes were carried out. The strength of the steel was enhanced, and the Lüdders platform was eliminated. The higher strength of the steel was attributed to the occurrence of a complex twinning effect, martensitic transformation and V-carbide precipitation during tensile deformation. In particular, the twin martensite structure retained after the quenching-tempering process served as another previous twin to accelerate the generation of nanomechanical twins in recrystallized austenite grain. The occurrence of transformation-induced plasticity (TRIP) of austenite with poor stability in non-recrystallized regions stimulated the TRIP and twinning-induced plasticity (TWIP) effects in austenite with high stability in recrystallized regions. Therefore, two pathways to improve the formability and optimize the mechanical properties of medium Mn steel by adjusting the quenching and tempering processes were proposed in this paper: (1) Manufacturing more martensite twin structures and (2) regulating the balance of austenite stability in both recrystallized and non-recrystallized regions.

     

Microstructural evolution and mechanical behaviour of Fe-30Mn-C steels with various carbon contents

A Fe-30Mn-0.6C sheet steel was decarburised and/or annealed to obtain four Fe-30Mn-C alloys with carbon contents of 0.06, 0.2, 0.4 and 0.6 wt-%. The primary deformation products were found to be mechanical twins for the 0.2C, 0.4C and 0.6C alloys and a combination of mechanical twins and ε-martensite for the 0.06C alloy. Both the ε-martensite and mechanical twin formation kinetics increased sigmoidally with true strain such that the final twin volume fraction increased with increasing alloy SFE and C content, where the latter finding disagrees with some of the accepted models for high-Mn twinning induced plasticity steels. Moreover, the activation stress for twin formation was found to increase linearly with alloy SFE, per a model previously proposed by the present authors.