High Uniform Elongation Research Articles

An investigation on microstructure evolution and mechanical properties of cryogenic steel rebars under different cooling conditions

In this study, industrial trial production of the microstructure evolutions and resultant mechanical properties and finite element model of the temperature variation in the cryogenic temperature rebar subjected to the TempCore processes were presented. The microstructure of tested steel rebar is composed of ferrite, pearlite and bainite when the cooling rate is less than or equal to 5 °C s−1. The pearlite disappears when the cooling rate is 10 °C s−1, and the microstructure is composed of granular bainite and a small amount of ferrite. The microstructure consists of martensite and bainite at cooling rate of 20°. Compared with the tested steel rebar with water flow rate of 450 m3 h−1, the tested steel rebar with water flow rate of 100 m3 h−1 has an optimum mechanical strength and a higher uniform elongation at room temperature, and has a higher NSR(notch sensitivity ratio) at −165 °C. The uniform elongation is 7.3% at room temperature and NSR is 1.14 at −165 °C. The ductile-brittle transition temperature of the tested steel rebar with water flow of 100 m3 h−1 is about −30 °C, while which of 450 m3 h−1 is 0 °C. The impact energy of the tested steel bar with water flow of 100 m3 h−1 is 2 ∼ 3 times that of 450 m3 h−1 at or below 0 °C, which is mainly due to its ultrafine acicular ferrite and bainite containing high dislocation density and sub-grains.

Strong and ductile steel via high dislocation density and heterogeneous nano/ultrafine grains

High strength and high ductility are critical to metallic materials and have always been pursued. A novel strategy to produce a medium Mn stainless steel with high dislocation density and heterogeneous nano/ultrafine-grains was designed and outstanding mechanical properties were achieved. The special heterogeneous microstructure was fabricated by cold rolling followed by rapid reverse annealing at intercritical temperature and a final step of low-temperature tempering. The results of this study demonstrated that the heterogeneous microstructure consisted of 74% reversed austenite, 16% deformed austenite and 10% tempered martensite. The dislocation density in austenite of the final microstructure had a high value of 1.12 × 1015 m−2 and the average grain size was 0.63 μm. The high yield strength (1324 MPa) and tensile strength (1401 MPa) of the steel were achieved and combined with high uniform elongation of 34.7%. The combination of high dislocation density (dislocation strengthening) and heterogeneous nano/ultrafine grains (fine-grain strengthening) resulted in such high yield strength. Simultaneously, twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effect occurred within the heterogeneous nano/ultrafine grains leading to high strain hardening rate and good uniform elongation in the steel.

Concurrent enhancement of strength and ductility for Al-Si binary alloy by refining Si phase to nanoscale

The present study suggests a novel method of simultaneous improvement of strength and ductility for Al-7 wt% Si binary alloy by tailoring the precipitation of Si phase via high pressure solution treatment (HPST) and subsequent aging treatment. The coarse Si flakes are fully dissolved into Al matrix owing to highly enhanced solubility under HPST, and plenty of nanoscale Si particles re-precipitate during aging treatment. The massive spherical and plate-like Si precipitates effectively improve the yield strength as per Orowan mechanism, and then plastic relaxation including prismatic punching and particle rotation takes place. The prismatic punching near spherical precipitates results in rapid dislocation accumulation, while the rotation of plate-like precipitates leads to dislocation annihilation at interfaces. The dislocation density would be balanced when the proportion of two types of precipitates is reasonable, and then high uniform elongation is achieved. Therefore, introducing two different types of particles acting as dislocation sources and sinks into coarse-grained alloys is crucial to balance dislocation density and achieve excellent mechanical properties.

Effect of manganese pre-partitioning process on the microstructure and mechanical properties of low carbon dual phase steel

A low carbon (0.11 wt.%) steel with 1.78% Mn was subjected to pre-partitioning quenching and partitioning (PQ&P), quenching and tempering (Q&T), quenching and partitioning (Q&P) to develop ferrite-martensite dual phase (DP) steel. The study suggested that all the treatments resulted into ferrite, martensite (or tempered martensite) and retained austenite microstructure. The amount of retained austenite was ∼ 5%, when the steel was treated by PQ&P process with partitioning time of 100 s, because of Mn pre-partitioning stage increased in the stability of retained austenite. But only ∼ 1% for Q&T and Q&P processes. With the increase of partitioning time from 10 to 100 s in the PQ&P process, the amount of retained austenite was increased, the tensile strength decreased, and yield strength and elongation were both increased. Among the different heat treatment processes, PQ&P sample clearly yields most attractive combination of strength and ductility compared to Q&T and Q&P samples. The experimental steel after pre-partitioning quenching and partitioning (PQ&P) process had higher uniform elongation of 16.3% and strength-ductility balance (UTS × TEL) of 17013 MPa.%. In contrast to conventional Q&T process, 27.3% higher uniform elongation and 7.4% greater strength-ductility balance were obtained.

Ferrite-Martensite Dual-Phase Steel Produced by a Modified Quenching and Partitioning Process

One industrially C-Mn steel was studied in the present work. The Mn pre-partitioning process during continuous annealing was used. The results showed that the stability of retained austenite is increased because of the Mn pre-partitioning. The amount of retained austenite was about 5%, when increase the partition time to 100s. With the increase of the partition time, the amount of retained austenite increases, the tensile strength decreases, the yield strength and elongation both increase. Compared with quenching and tempering (Q&T) process, the tested steel after pre-partitioning quenching and partitioning (PQ&P) process has higher uniform elongation and the product of strength and elongation (UTS×TEL).

Promising Tensile and Fatigue Properties of Commercially Pure Titanium Processed by Rotary Swaging and Annealing Treatment.

The effect of the grain refinement and texture on tensile and fatigue properties in commercially pure titanium (grade 2) processed by rotary swaging (RS) and an annealing treatment is investigated. The as-processed sample consists of band-like grains on the longitudinal section and equiaxed grains on the transversal section and revealed an obvious <10-10> fiber texture with respect to the rod axis. Through this technique, a sample with a high tensile strength of 870 MPa, a high uniform elongation of 8.5%, and a high fatigue limit of 490 MPa can be achieved, and the tensile and fatigue properties are almost the same as those of a conventional Ti-6Al-4V alloy. The enhanced mechanical properties and plastic deformation mechanism are discussed in terms of the observed ultrafine-grained microstructure and strong fiber texture.

Design for strength-ductility synergy of 316L stainless steel with heterogeneous lamella structure through medium cold rolling and annealing

Superior strength-ductility synergy of 316L stainless steel was achieved through medium cold rolling and subsequent annealing, which successfully fabricated heterogeneous lamella structure (HLS), reduced processing cost and improved preparation controllability. The HLS in 316L stainless steel processed by lower original rolling strain was more stable, and its microstructure was characterized with lamella recrystallized grain clusters and lamella coarse grains that were sandwiched by the mixtures of nano-sized twin bundles and ultrafine grains. The satisfactory strength-ductility synergy was mainly determined by the contents of various microstructures as well as the soft/hard interfaces in HLS. The high strength was obtained from hard ultrafine grains and nano-twin bundles. Meanwhile, the back stress induced by soft/hard interfaces also contributed to the high strength. For the favorable ductility, it was ascribed to the generation of plentiful forest dislocations in soft structures and geometrically necessary dislocations around soft/hard interfaces. It is specially noted that high yield strength (∼820 MPa) and good uniform elongation (∼10.6%) can be simultaneously obtained for the annealed 70% cold rolled 316L stainless steel. These results may markedly boost the industrial application of HLS 316L stainless steel.

Microstructures and mechanical properties of the non-equiatomic FeMnNiCoCr high entropy alloy processed by differential speed rolling

High-ratio differential speed rolling was applied to a non-equiatomic Fe41Mn25Ni24Co8Cr2 high entropy alloy. After subsequent annealing, the processed alloy exhibited a superior combination of high strength and high uniform elongation compared with the one processed by conventional rolling, which was achieved through the development of a bimodal grain size distribution, composed of recrystallized fine grains and unrecrystallized nano (sub)grains.

The influence of silicon additions on the deformation behavior of austenite-ferrite duplex medium manganese steels

The present study systematically investigated the effect of Si additions from 0 to 3 wt.% on the deformation mechanisms of a 0.2C-10Mn-3Al medium Mn steel. Two austenite-ferrite duplex microstructures, characterized by the different austenite characteristics (i.e. fraction and stability), were produced for different Si-alloyed steels by intercritical annealing and subsequently subjected to tensile testing. The influence of Si content on the activation and kinetics of transformation-induced plasticity (TRIP) effect, the formation of deformation twins, as well as the strain partitioning between phase constituents during deformation was studied in detail. For the low austenite fraction (∼30%) duplex structure with a high austenite stability (low Ms temperature), the tensile strength was only slightly changed with Si levels, whereas the uniform elongation was significantly influenced, with first an increase followed by a decreasing trend with higher Si contents. This was attributed to changes in strain partitioning between austenite and ferrite and to different extent of deformation twinning in austenite, which were highly dependent on the solute Si contents and the related microstructural changes. It was found that strain partitioning in this type of microstructure can result in a substantially high strain hardening rate and high uniform elongation. Conversely, for the samples with a high austenite fraction (∼45%) and low stability, the tensile properties were insensitive to the Si content, being mainly controlled by the TRIP effect. In this structure, a higher fraction of martensite was formed at the beginning of the plastic deformation, which decreased the strain partitioning between austenite and ferrite. The TRIP effect after the Lüders strain is believed to be the main factor influencing the strain hardening rate of this type of structure.

A Fine Grain, High Mn Steel with Excellent Cryogenic Temperature Properties and Corresponding Constitutive Behaviour.

A Fe-34.5 wt % Mn-0.04 wt % C ultra-high Mn steel with a fully recrystallised fine-grained structure was produced by cold rolling and subsequent annealing. The steel exhibited excellent cryogenic temperature properties with enhanced work hardening rate, high tensile strength, and high uniform elongation. In order to capture the unique mechanical behaviour, a constitutive model within finite strain plasticity framework based on Hill-type yield function was established with standard Armstrong-Frederick type isotropic hardening. In particular, the evolution of isotropic hardening was determined by the content of martensite; thus, a relationship between model parameters and martensite content is built explicitly.

Microstructure and Mechanical Properties Evolution of the Al, C-Containing CoCrFeNiMn-Type High-Entropy Alloy during Cold Rolling.

The effect of cold rolling on the microstructure and mechanical properties of an Al- and C-containing CoCrFeNiMn-type high-entropy alloy was reported. The alloy with a chemical composition (at %) of (20–23) Co, Cr, Fe, and Ni; 8.82 Mn; 3.37 Al; and 0.69 C was produced by self-propagating high-temperature synthesis with subsequent induction. In the initial as-cast condition the alloy had an face centered cubic single-phase coarse-grained structure. Microstructure evolution was mostly associated with either planar dislocation glide at relatively low deformation during rolling (up to 20%) or deformation twinning and shear banding at higher strain. After 80% reduction, a heavily deformed twinned/subgrained structure was observed. A comparison with the equiatomic CoCrFeNiMn alloy revealed higher dislocation density at all stages of cold rolling and later onset of deformation twinning that was attributed to a stacking fault energy increase in the program alloy; this assumption was confirmed by calculations. In the initial as-cast condition the alloy had low yield strength of 210 MPa with yet very high uniform elongation of 74%. After 80% rolling, yield strength approached 1310 MPa while uniform elongation decreased to 1.3%. Substructure strengthening was found to be dominated at low rolling reductions (<40%), while grain (twin) boundary strengthening prevailed at higher strains.

Additively manufactured hierarchical stainless steels with high strength and ductility.

Many traditional approaches for strengthening steels typically come at the expense of useful ductility, a dilemma known as strength-ductility trade-off. New metallurgical processing might offer the possibility of overcoming this. Here we report that austenitic 316L stainless steels additively manufactured via a laser powder-bed-fusion technique exhibit a combination of yield strength and tensile ductility that surpasses that of conventional 316L steels. High strength is attributed to solidification-enabled cellular structures, low-angle grain boundaries, and dislocations formed during manufacturing, while high uniform elongation correlates to a steady and progressive work-hardening mechanism regulated by a hierarchically heterogeneous microstructure, with length scales spanning nearly six orders of magnitude. In addition, solute segregation along cellular walls and low-angle grain boundaries can enhance dislocation pinning and promote twinning. This work demonstrates the potential of additive manufacturing to create alloys with unique microstructures and high performance for structural applications.

Manganese Effect on Q&amp;amp;P CMnSi Steels

The present study is focused on analyzing the effect of Mn amount in two experimental steel compositions, specially designed for Quenching and Partitiong (Q&amp;P), 0.15C-2.5Mn-1.5Si and 0.15C-3Mn-1.5Si without significant contribution of Al. Two-Step Q&amp;P thermal treatments were performed at laboratory scale in a quenching dilatometer Bähr DIL805A/D. The fractions of retained austenite were evaluated by X-ray diffraction techniques. The mechanical properties of the Q&amp;P samples were evaluated, a strong dependence of strength, uniform elongation and strain hardening values on process parameters has been found. Higher uniform elongation were related to higher residual austenite contents. The 0.15C-3Mn-1.5Si steel showed systematically the largest mechanical values with respect to the 0.15C-2.5Mn-1.5Si steel.

High-speed extrusion of dilute Mg-Zn-Ca-Mn alloys and its effect on microstructure, texture and mechanical properties

Three dilute Mg-Zn-Ca-Mn alloys were successfully extruded at 24m/min and the alloy with lowest Zn content (0.21wt%) can even be extruded at 60m/min without any surface defects, which was ascribed to the thermally stable Mg2Ca phase and high solidus temperature (∼620°C). The alloys extruded at die-exit speed ≥6m/min showed a fully dynamically recrystallized (DRXed) microstructure and weak rare earth (RE) texture at the position between [21̅1̅4] and [21̅1̅2] parallel to the extrusion direction. Besides, fine Mg2Ca and α-Mn particles dynamically precipitated during extrusion, acting as effective pinning obstacles against the DRXed grain growth via Zener drag effect. Due to the deformation temperature rise with increasing extrusion speeds, the grain size increased gradually, which can be understood from the relationship between DRXed grain size and Zener-Hollomon parameter. The RE texture contributed to high uniform elongation of ∼23%, but the increased grain size (>30µm) deteriorated post-uniform elongation due to the prevalence of {101̅1} contraction and {101̅1}-{101̅2} double twins during post-uniform deformation.

Effect of Q&amp;P Process on 0.15C-MnSi Steels

The present study is focused on analyzing the effect of Mn amount on two experimental steel compositions, specially designed for Q&P (Quenching and Partitioning), 0.15C-2.5Mn-1.5Si and 0.15C-3Mn-1.5Si without significant contribution of Al. Two-Step Q&P thermal treatments were performed at laboratory scale in a quenching dilatometer Bahr DIL805A/D. The fractions of retained austenite were evaluated by X-ray diffraction techniques. The mechanical properties of the Q&P samples were evaluated, a strong dependence of strength, uniform elongation and strain hardening values on process parameters has been found. Higher uniform elongation were related to higher residual austenite contents. The 0.15C-3Mn-1.5Si steel showed systematically the largest mechanical values with respect to the 0.15C-2.5Mn-1.5Si steel.

Effect of coiling temperature on microstructure and properties of X100 pipeline steel

Multiphase structure of bainite and M/A(martensite/austenite constituent) can be obtained through coiling continuous-HOP (Heating On-line Partitioning) technology, resulting in excellent deformability for X100 pipeline steel. The microstructure and properties of the steel at different coiling temperatures were investigated by means of mechanical properties test, microscopic analysis and X-ray diffraction method. The results show that with the increase of coiling temperature, the strength decreases and the ductility increases because of the widening of bainite laths, the decrease of bainite content and dislocation density. However, because of the formation of martensite, the precipitation of carbides and the decrease of retained austenite at high coiling temperatures, the strength increases and the ductility decreases. Low yield ratio, high uniform elongation and high strain hardening exponent can be achieved at appropriate coiling temperatures through coiling continuous-HOP technology, which endows the steel with good ductility and excellent deformability.

Microstructure and Mechanical Properties of High-Nitrogen Austenitic Stainless Steels Subjected to Equal-Channel Angular Pressing

Three high-nitrogen stainless steels with different N contents were successfully processed by equal-channel angular pressing for one pass, and their microstructures and mechanical properties were investigated. It was found that the microstructure of the billet was heterogeneous across the billet thickness, resulting in the difference in the mechanical properties due to the different deformation conditions. A relatively low strength, high uniform elongation, and high work-hardening rate for the samples at the bottom of the billet was achieved in comparison with those processed at the top. Meanwhile, it was observed that the density of deformation twins increased with the content of N; accordingly, the strength and elongation of the alloys increase with the content of N, resulting in a good strength–ductility combination.

On the performance analysis of AHSS with an application to SET technology – FEM simulations and experimental measurements

Solid expandable tubular (SET) is an innovative breakthrough in the petroleum industry that aims to resolve issues associated with deep wells. It consists of in-place plastic deformation of tubular diameter resulting in larger conduit size that allows drilling deeper. However, due to volume constancy, increasing the tubular diameter results in a decrease in its wall-thickness, and hence even an initially thick-walled tubular may convert into thin-walled; especially at high expansion ratios. Extending the frontier of expandable applications require larger expansion percentage and enhanced tubular integrity after expansion, which exceed the capability of current tubular material. Therefore, the current study aims to investigate new alternative materials to be used as tubular for SET technology. Selected AHSS grades including TRIP, TWIP, and DP steels are analyzed numerically using FEM. Experimental validation of the FEM model was carried out using full-scale expandable tubular testing facility available at Sultan Qaboos University (Muscat, Oman). Interesting results in terms of stress, strain, expansion force, burst and collapse pressures, length shortening and wall-thickness reduction were obtained. Bauschinger effect arises at the collapse pressure rating has been considered in the study. It was found that materials with high true uniform elongation (and therefore with large strain hardening capability) are more capable of counteracting strain intensifications caused by plastic deformation, allowing either larger expansion ratio or larger safety margin for the deformation capability of the material at a given expansion ratio.

Cyclic deformation behaviors of a high strength carbide-free bainitic steel

The cyclic deformation behaviors of low-temperature bainite, lower bainite and upper bainite obtained on a high-strength carbide-free bainitic steel were examined through low-cycle fatigue testing. The relationship between the bainitic microstructure and fatigue behavior was systematically studied using scanning electron microscopy, transmission electron microscopy, atom probe technology, and electron back-scattered diffraction analyses. The results show that low-cycle fatigue undergoes three stages, namely, cyclic hardening, saturation or cyclic softening, and fracturing. The low-temperature bainite exhibits a long fatigue life under the total strain amplitudes, because of its high strength and larger high-angle misorientation distribution of the packets of bainitic ferrite plates. The low-temperature bainite also presents bilinearity in the Coffin–Manson plots. Coordinate deformation and high-uniform elongation lead to prolonged fatigue life at low plastic strain amplitude, whereas phase component primarily affects the fatigue life at high plastic strain amplitude. Highly stable film-like retained austenite is beneficial to arresting fatigue crack propagation. Blocky type retained austenite easily transforms to martensite, resulting in high compatible deformation capability. The hardening ability of the low-temperature bainite is higher than that of the upper bainite, which is attributed to its high pre-existent density of dislocations transformed from movable to immovable during initial hardening stage.

A Novel Strong and Ductile TWIP/Martensite Steel Composite

A novel bi‐layered steel composite consisting of a martensitic steel and a high manganese twinning‐induced plasticity (TWIP) steel has been fabricated by roll bonding at high temperature. Microstructural and mechanical characterizations highlight that the presented bi‐layered steel composite exhibits simultaneously a high yield stress and high uniform elongation, as both the advantages of TWIP steel and martensitic steel are inherited.