Product Of Strength And Elongation Research Articles

The new design to improve the stability of retained austenite and mechanical properties in super martensitic stainless steel

The quenching and partitioning (QP) process has proven to be an effective technique for stabilizing retained austenite (RA). The N element partitioning process has replaced the traditional C element partitioning method, resulting in improved stability of RA in super martensitic stainless steel. To enhance these benefits, an innovative combination of intercritical annealing (A) and QP has been developed, known as the QAP process. This technique utilizes interstitial N atoms and substitutional Ni, Mn atoms to co-stabilize RA, leading to excellent comprehensive mechanical properties. The findings indicate that the QAP samples display a yield strength (YS) that is 75–121 MPa higher than the QP sample. At intercritical annealing temperatures of 500 °C and 550 °C, the QAP steel demonstrates a product of strength and elongation (PSE) of 36.61 GP·% and 31.51 GP·%, respectively, surpassing the QP steel (26.52 GP·%). Furthermore, the presence of secondary martensite increases with higher intercritical annealing temperatures in QAP samples. However, excessive secondary martensite significantly diminishes toughness.

Preparation of high-strength and high-ductility medium Mn steel with chemistry heterogeneous via a two-step intercritical annealing process

Recently, the retained austenite (RA) with chemically heterogeneous has attracted great attention in medium Mn steel. In this work, we designed a two-step intercritical annealing (TIA) heat treatment process to prepare high-strength medium Mn steel with the chemical composition of Fe–0.20C–8.0Mn–1.6Si–1.3Al (wt.%). Compared with the normal intercritical annealing (IA) heat treatment process, the TIA process can not only increase the volume fraction of RA but also obtain the RA with inhomogeneous Mn distribution. Meanwhile, it was found that the Mn content on the outer side of this structured RA was higher than that on the inner side, forming a nanoscale Mn-enriched layer on the outer side of RA, which played a significant role in improving the stability of RA. Finally, we used the TIA process to obtain medium Mn steel with an ultimate tensile strength (UTS) of 1611.83 MPa, a total elongation (TE) of 45.02%, and a product of strength and elongation (PSE) of 72.56 GPa·%, which achieves a superior combination of high strength and high ductility in medium Mn steel.

Research on deformation behaviors at different temperatures of lean duplex stainless steel S32101 produced by direct cold rolling process

In this paper, the mechanical properties, work hardening characteristics and deformation mechanism of lean duplex stainless steel (LDSS) S32101 produced by direct cold rolling were studied under different temperature conditions. The yield strength (YS) and tensile strength (TS) of S32101 decreased with the increase of deformation temperature, while the elongation increased first and then decreased. The TS of S32101 at −70 °C was 1052.6 MPa, the elongation remained 57.2%, and the product of strength and elongation (PSE) was as high as 60.2 GPa·%, which was mainly due to the synergistic strengthening of transformation induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects in austenite at cryogenic temperatures. The efficiency of grain refinement at cryogenic temperature was improved by TRIP effect and deformation twins (DTs), indicating that it had the potential to be applied at low temperature (−70 ∼ −30°C). Moreover, the high density nanotwins and the strong interaction between the dislocation and nanotwin bundles also contributed to strain hardening at low temperatures and thus better comprehensive mechanical properties could be achieved.

Effect of quenching temperature on the austenite stability and mechanical properties of high-strength air-cooled TRIP steel prepared with hot-rolled C–Si–Mn sheets

This paper systematically studies the effect of quenching temperature on the microstructure evolution and the relationship between the microstructure and mechanical properties of air-cooled TRIP steel prepared with C–Si–Mn hot-rolled sheet. The results indicate that when the quenching temperature is lower than (200 °C) or close to (250 °C) the Ms temperature, the martensite transformation mainly occurs during quenching or at early stage of air cooling. When the quenching temperature reaches 300 °C, lathy bainite begins to form. As the quenching temperature increases, the morphology of bainite is transformed from lathy to granular. Bainite transformation during air cooling can promote the C enrichment in austenite, thus improving the thermal stability of the austenite. However, martensite transformation also occurs during subsequent air cooling, and the start temperature of martensite transformation first decreases and then increases with the increase of quenching temperature from 300 °C to 450 °C. The yield and tensile strength of TRIP specimens first decrease and then increase, while the retained austenite content, elongation and product of strength and elongation (PSE) first increase and then decrease. When the quenching temperature is 400 °C, the best elongation and PSE are obtained. The best elongation of 400TRIP specimen is mainly attributed to its highest content and suitable mechanical stability of retained austenite, which is favorable for the expansion of the TRIP effect range. The TRIP specimens prepared with hot-rolled sheet all exhibit ultra-high tensile strength (over 1050 MPa), which is related to the ultrafine ferrite grains, martensite transformation during air cooling and TRIP effect of retained austenite.

Simple route to produce excellent mechanical properties of 9Mn steel with heterostructure induced by pre-existing austenite with sub-rapid solidification

Medium-Mn steels have garnered significant interest due to their unique combination of high strength and exceptional ductility. However, a high Mn content inevitably causes the macro-segregation of Mn, which can significantly compromise the mechanical properties. This study effectively prevented the formation of Mn macro-segregation by employing the sub-rapid solidification. It resulted in the distribution of Mn micro-segregation bands within dendritic gaps in Fe-9Mn-2Si-0.5Al-0.1C steel. Mn micro-segregation bands results in a high proportion of pre-existing austenite (γpre) with small size and suitable stability in the as-cast strip, forming a lamellar structure with alternating distribution of lath martensite and austenite on the micrometer scale. The as-cast strip necessitates only basic simple hot rolling control without the need for any tempering treatment to achieve excellent mechanical properties comparable to conventional lengthy processes (ultimate tensile strength (UTS) of 1762 MPa, total elongation (TE) of 20 %, and product of strength and elongation (PSE) up to 35 GPa%). Utilizing this structure as a foundation, a simple austenite reverse tempering (ART) treatment can lead to additional enhancements in comprehensive mechanical characteristics with the UTS of 1529 MPa, TE of 25 % and PSE of 38 GPa%.

Improving laser welded joint performance of medium-Mn steel through C and Mn partition by intercritical annealing process

In this paper, the fracture failure mechanism of laser welded (LW) joints of medium-Mn steel is studied. And the mechanical properties of the joints were greatly improved by designated intercritical annealing (IA) process. Experimental results showed that the elongation of the as-welded joint of medium-Mn steel is very low, and the fracture shows typical brittle fracture characteristics. Electron probe micro-analyzer (EPMA) test showed that there were many Mn segregation bands in the joint, and electron backscatter diffraction (EBSD) surface scanning test further demonstrated that many Mn23C6 carbides were produced at the martensite grain boundary (GB) of the fusion zone (FZ), which was the main reason for the occurrence of brittle fracture in the FZ. The IA process improves the segregation of Mn element at the GB of the joint and reduces the intergranular brittleness. At the same time, more austenite was distributed by C and Mn elements, and more Fe3C phases were found near austenite in the EBSD diagram, which provided nucleation centers for the formation of austenite. The product of strength and elongation (PSE) of the IA treated joint was dramatically increased by 2693.65%.

High-Strength 430 ferritic stainless steel fabricated by selective laser melting process

This study investigates the microstructure and mechanical properties of 430 ferritic stainless steel (FSS) fabricated via the selective laser melting (SLM) process. The mechanical properties and fracture morphologies of SLM-fabricated 430 FSS and conventional 430 FSS were compared in this paper. It was found that 430 FSS produced by SLM exhibits significant microstructural anisotropy, with higher dislocation density and the presence of oxides of silicon, manganese and aluminum. Regarding mechanical properties, 430 FSS fabricated via SLM process displays higher tensile strength and yield strength, as well as superior product of strength and elongation (PSE) and yield ratio, compared to those produced by conventional methods. Fracture morphology analysis revealed that SLM-fabricated 430 FSS primarily exhibited characteristics of cleavage fractures, whereas conventional 430 FSS showed typical features of ductile fractures.

Comparison of Microstructure and Mechanical Properties of a High‐Carbon Bainitic Steel Treated by Long‐Time Bainitic Austempering and Short‐Time Austempering Plus Tempering Processes

Herein, the microstructure and mechanical properties of a high‐carbon bainitic steel treated by long‐time bainitic austempering and short‐time austempering plus tempering processes are compared. The multiphase microstructures are characterized by dilatometry, X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy to correlate with mechanical properties. Results show that although long‐time austempering treatment can reduce the volume fraction of brittle martensite, no significant improvement is observed in fracture damage resistance. Besides, the cementite is prone to precipitation from the austenite at the later period of the long‐time austempering process. The cementite precipitation in austenite decreases the carbon content in retained austenite (RA) and consequently reduces the mechanical stability of RA. In contrast, the cementite has not been able to precipitate from austenite after short‐time austempering treatment, whereas the martensite is softened and the stability of RA is improved during subsequent tempering. Therefore, excellent mechanical properties are obtained in the samples treated by short‐time austempering plus tempering process: ultimate tensile strength, 1489 MPa, yield strength, 1014 MPa, total elongation, 33.2%, and the product of strength and elongation (PSE) of 48.4 GPa%, where PSE is increased by 27% compared with the sample after long‐time bainitic austempering.

Enhancing strength and ductility in the nugget zone of friction stir welded 7Mn steel via tailoring austenitic stability

The martensite often appears in the nugget zone (NZ) of friction stir welding (FSW) 7 wt.% Mn steel due to low austenite stability, deteriorating ductility and toughness. In this work, a 7 wt.% Mn steel was subjected to FSW, and preheating was used to tailor the austenitic stability to greatly improve the strength–ductility combination of the NZ. The austenitic deformation behavior and strain hardening mechanism in the NZ were systematically investigated. The microstructure of the as-welded NZ was composed of ultrafine blocky ferrite, austenite, and small amounts of martensite, whereas the as-preheated NZ contained ultrafine blocky ferrite and austenite, and the concentration of Mn in austenite was increased from 8.4 wt.% to 10.7 wt.%. This enhanced the austenitic stability, resulting in a significant increase in the volume fraction of austenite in the as-preheated NZ from 37.3% to 66.4%. The product of strength and elongation (PSE) in the as-preheated NZ increased dramatically from 42.6 GPa% to 67.1 GPa%, depending on a persistent high strain hardening rate (SHR). Multiple strain-hardening mechanisms were revealed. The austenite with enhanced stability can provoke sustained transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects, and massive dislocation multiplication occurs during tension, resulting in strong strain hardening.

On balanced strength and ductility synergy in low alloy steels through multiphase heterostructure involving cumulative process of hot rolling, coiling and tempering

We present here a new pathway of combining hot rolling, coiling and tempering for multiphase heterostructure to achieve a balanced combination of strength and ductility in low alloy steels. The multiphase heterostructure was composed of bainite/martensite and retained austenite (RA), which was tuned by coiling and tempering. The steel coiled at 450 °C exhibited superior mechanical properties compared with the steels coiled at 350 and 550 °C. After tempering at 300 °C, the steel achieved excellent combination of strength and ductility, with twice the elongation and a corresponding product of strength and elongation (PSE) of 25.1 GPa·%. After coiling, 17 vol% of RA was retained. The RA fraction decreased slightly to 14 vol% by tempering, the steel still sustained considerable austenite. Tempering further enabled carbon partitioning and improved the mechanical stability of RA. The continuous transformation induced plasticity (TRIP) effect from stable RA over the entire strain range resulted in good ductility of the steel.

Achieving 1.5 GPa grade medium Mn steel with high ductility via interrupted intercritical annealing process

In this study, we proposed an interrupted intercritical annealing (IIA) strategy to produce high-strength medium Mn steel with a nominal chemical composition of Fe–0.20C–8.0Mn–1.6Si–1.3Al (wt.%), which can promote the dissolution of carbides and make the C and Mn elements fully diffuse into austenite to enhance the stability of austenite. Furthermore, the content of retained austenite (RA) was significantly improved while avoiding excessive growth of RA by controlling the nucleation and growth of austenite. The concentration gradient of Mn was established to form the core-shell structured RA which had a higher Mn content in the shell than in the core. We used the IIA process to prepare medium Mn steel with an ultimate tensile strength (UTS) of 1536.93 MPa, a total elongation (TE) of 43.26 %, and a product of strength and elongation (PSE) of 66.49 GPa·%, which exhibits excellent mechanical properties. This study provides a theoretical reference for the preparation of high-strength medium Mn steel.

Analysis of phase transformation thermodynamics and kinetics and its relationship to structure-mechanical properties in a medium-Mn high strength steel

The difference in transformation mechanisms between the normal and reverse phase transformation in medium Mn steel was revealed using thermodynamic and kinetic analysis. The results indicate that the austenite formation during the reverse phase transformation is a fast reaction, significant amount of 46.9 vol% retained austenite is obtained after subjected partial reversion at 650 °C × 1 h. At the nucleation stage, the lath-like structure, dislocations, martensitic packet boundaries, and grain boundaries provide high-density nucleation sites for austenite formation. The critical temperature of austenite formation is reduced by the enrichment of Mn and C. However, no ferrite transformation occurred during the normal phase transformation (650 °C × 6 h isothermal treatment and subsequent furnace-cooling process). At the nucleation stage, the undersaturated Mn and C in the prior austenite grain interior provide insufficient concentration fluctuation, leading to a low nucleation rate. At the phase growth stage, the atomic mobility of Mn and C in the parent austenite (control the ferrite growth) is 1/69 and 1/282 than that in ferrite (control the austenite growth) at 650 °C. The differences in nucleation site density, driving force, and element diffusion rate lead to significant variations in phase transformation behaviors. The excellent mechanical property with the product of strength and elongation (PSE) exceeding 20 GPa % is achieved via the sustained transformation-induced plasticity (TRIP) effect by metastable retained austenite over a large strain range.

Unusual step-like stress flow behavior and mechanisms of a 1.3 GPa grade medium-Mn steel with 51.2 % ductility

A Fe-8.9Mn-1.59Si-1.0Al-0.25C medium-Mn steel have achieved a superior combination of strength and ductility, i.e., the product of strength and elongation (PSE) of 66.5 GPa%. Meanwhile, an unusual step-like stress–strain curve due to heterogeneous deformation bands including Lüders band and multiple Portevin–Le Chatelier (PLC) bands was investigated systematacially. The repeated heterogeneous deformation is due to a combination of extrinsic and intrinsic factors. The former refers a small stress concentration near the grip ends of the tensile sample, and the latter is associated with low density of initial mobile dislocations, limited capacity of mobile dislocation multiplication and the presence of meta-stable austenite. As for meta-stable austenite, stress-assisted martensite transformation (SAMT) was confirmed experimentally, which will facilitate strain concentration and the starting of heterogeneous deformation. Moreover, before strain-induced martensite transformation (SIMT), it is the plastic deformation of austenite grains through the slip of partial dislocation and stacking faults (SFs) that contributes to a larger Lüders strain. Finally, we propose that whether it is possible to make the heterogeneous deformation to be a positive side for certain working conditions, which may open new doors for the application of medium-Mn steels. We will continue to do some exploratory studies in this direction next.

Relationships between Strength, Ductility and Fracture Toughness in a 0.33C Steel after Quenching and Partitioning (Q&P) Treatment

The effect of quenching and partitioning (Q&P) processing on strength, ductility and fracture toughness is considered in a 0.33% C-1.8% Si-1.44 Mn-0.58% Cr steel. The steel was fully austenitized at 900 °C and quenched to 210 °C for 30 s. Partitioning at 350 °C for 600 s produces a martensitic matrix with transition carbides, bainitic ferrite and film-like retained austenite (RA) that is stable against transformation to strain-induced martensite under tension. This processing provided the highest strength and fracture toughness but the lowest ductility and product of strength and elongation (PSE), σB·δ (MPa·%). Partitioning at 500 °C produced RA with a relatively low carbon content and low volume fraction of carbides. The steel after this Q&P processing exhibits the highest ductility and PSE but low YS and Charpy V-notch (CVN) impact toughness. High ductility and PSE correlate with the ability of RA to transform into strain-induced martensite, while high strength and impact toughness are associated with the high-volume fraction of transition carbides in the carbon-depleted martensitic matrix and a lack of transformation of RA to strain-induced martensite. The highest CVN impact energy was attained in the steel exhibiting transgranular quasi-cleavage fracture with the lowest effective grain size for brittle fracture. No correlation between strength, ductility and fracture toughness is observed in Q&P steels if these materials have distinct structural constituents.

Investigation on strength and plasticity enhancement mechanism of 1180 MPa-grade quenching and partitioning steel

A comparative study on microstructures and mechanical properties of a low carbon Fe–Mn–Si steel with and without quenching pretreatment is performed to investigate the strength and plasticity enhancement mechanism. Microstructural analysis and 3D APT observation show that the lamellar microstructure obtained by introducing pre-quenching at 990 °C for 20 min prior to quenching and partitioning (Q&P) process mainly consists of ferritic laths, Mn-enriched tempered martensite and retained austenite with high mechanical stability. Mn enrichment in tempered martensite helps maintain a relatively high tensile strength. The refined alternative laths consisting of ferrite and tempered martensite share the same crystallographic orientation, which should be easier to hinder the crack propagation and delay the fracture. Compared to the polygonal-structured sample treated without quenching pretreatment, both higher frequencies of Σ3 CSL boundaries and higher volume fraction of stable retained austenite allow the pre-quenched sample exhibit an excellent strength-plasticity balance. The plasticity of the pre-quenched sample increases by nearly 90% and the product of strength and elongation (PSE) reaches higher than 24 GPa·%.

Enhancing strength-ductility combination in a novel Cu–Ni bearing Q&P steel by tailoring the characteristics of fresh martensite

The present work aims to study the effect of quenching temperature on phase transformation, microstructural characteristics and mechanical properties of a novel Cu–Ni bearing Q&P steel based on combining the experiments and modeling. In this study, a new constrained carbon equilibrium (CCE) model was proposed by considering the effect of intercritical ferrite and multi-element synergistic partitioning. The optimum quenching temperature and corresponding retained austenite (RA) fraction can be reflected in the new CCE model, which is in good agreement with XRD results. Furthermore, the significant role of fresh martensite in the tensile strength and fracture behavior were elucidated. As the volume fraction of fresh martensite increases from 9.8% to 57.8%, the strengthening contribution to the tensile strength increases from 13.7% to 68.8%. Meanwhile, the fracture mechanism transforms from ductile into quasi-cleavage fracture due to an increased amount of fresh martensite and the absence of tempered martensite. The superior mechanical properties, ultimate tensile strength of 1152 MPa, yield strength of 797 MPa, total elongation of 25%, and the product of strength and elongation (PSE) up to 28.8 GPa% were obtained in QP-200, which is largely due to the combined effect of the better deformation ability of a considerable amount of tempered martensite and the continuous TRIP effect provided by multiscale RA with different morphologies.

Effect of pre-treatment and austempering on microstructure and mechanical properties of 55Si2MnMoV steel

To ensure the high strength of 55Si2MnMoV steel and greatly improve the ductility, three samples with various initial microstructures (annealing, quenching, and austempering) were subjected to intercritical annealing (820 °C) and austempering (325 °C) for 25 min, respectively. The effects of the initial microstructure on the final microstructure and mechanical properties were systematically studied. XRD revealed that the volume of retained austenite (RA) of the three decreased significantly with the location close to the tensile fracture surface. While the conversion proportion of RA in the quenched sample peaked at 16.5%, indicating that RA transforms into martensite to a greater extent during tensile, thus delaying the occurrence of necking. The ferrite grains in the quenched sample were significantly refined and uniformly distributed, and a few were polygonal. Additionally, the grains of the austempered sample were mostly lath because of the inheritance of the initial microstructure. The ultimate tensile strength (UTS) of the quenched sample reaches 1093.61 MPa, which is slightly lower than that of the annealed sample, but the plasticity is greatly improved. The corresponding product of strength and elongation (PSE) reaches the peak value of 67.05 GPa%, which is 26.22% higher than that of the annealed sample, this is mainly attributed to the transformation of RA in a broad range of strain and the uniform equiaxed microstructure. Also, the strength ratio of the three samples fluctuates between 0.67 and 0.71, and their failure modes are all ductile fractures.

Dependence of strengthing and toughening on retained austenite of quenched and partitioned AISI 430 ferritic stainless steel

The quenching and partitioning (Q&P) process was applied to AISI 430 ferritic stainless steel (FSS). As a comparison, the quenching and tempering (Q&T) process was also applied to the experimental steel. This study evaluated the selection of the optimal interrupted quenching temperature. Through the combination of experiments and theoretical calculations, the functional relationship between the microstructure and the quenching temperature was qualitatively and quantitatively analyzed. When the quenching temperature was close to the intermediate temperature between the martensite start (Ms) point and the Martensite finish (Mf) point, there were more retained austenite could be preserved at room temperature. The model of the microstructure evolution was obtained and carbon diffusion behavior was discussed. The diffusion of carbon to austenite and to precipitation is a competing reaction. The diffusion of carbon into the precipitation can be suppressed by changing the quenching temperature. The effect of quenching temperature on mechanical properties was analyzed by tensile test. The result indicates that the sample quenched at 160 °C exhibits superior tensile properties, with the product of strength and elongation (PSE) of 16.87 GPa%. With the increase of quenching temperature, the yield strength decreased gradually, and the plasticity and PSE first increased and then decreased. Initial fraction and mechanical stability of retained austenite jointly affect ductility. In addition, the quenching temperature affected the formation of the yield platform of the tensile curve, which was mainly related to whether there were enough interstitial carbon atoms in the matrix for pinning dislocations.

In-situ investigation of strengthening and strain hardening mechanisms of Cu-added medium-Mn steels by synchrotron-based high-energy X-ray diffraction

A novel Cu-added medium-Mn steel with a chemical composition of Fe–0.27C–9.1Mn–1.86Al–3.3Cu (wt.%) was designed and subjected to intercritical annealing (IA) temperature range from 620 °C to 680 °C for 1 h. The ultimate tensile strength (UTS) increases and the yielding strength (YS) decreases with the IA temperature increasing. The YS of 824 MPa, UTS of 1222 MPa, total elongation (TE) of 55%, and product of strength and elongation (PSE) of 67.2 GPa·% are achieved after IA at 660 °C. Transmission electron microscopy confirmed that Cu-rich nanoparticles precipitate in the ferrite. The in-situ high-energy X-ray diffraction (HE-XRD) experiments show that at the beginning of plastic deformation, both austenite and ferrite bear the applied load. The load is mainly undertaken by martensite with effective transformation-induced plasticity (TRIP) effect triggered. The YS of ferrite is significantly higher than that of austenite. The individual contribution of solid solution strengthening, grain refinement strengthening, dislocation strengthening, and precipitation strengthening in ferrite and austenite is analyzed. The discrepancy between the YS of ferrite and austenite is mainly attributed to the precipitation strengthening due to the Cu-rich nanoparticles precipitation. The moderate mechanical stability and the collaboration of TRIP and twinning-induced plasticity (TWIP) effects of austenite contributed to the enhanced strain hardening capability and resulted in large ductility.

Q&P Response of a Medium Carbon Low Alloy Steel

An Fe-0.44%C-1.8%Si-1.3%Mn-0.82%Cr-0.28%Mo steel was subjected to quenching followed by low-temperature tempering (Q&T) and quenching and partitioning (Q&P) processing after full austenitization. The Q&P treatment led to an increase in the volume fraction of retained austenite (RA) by factors ranging from 30 to 40 depending on the quenching temperature, Tq, and an additional precipitation of transition η-carbides in the martensitic matrix. The Q&P processing provided a decrease in the yield stress (YS) from 1730 to 1350 MPa and an increase in the ductility by a factor of 3; the product of strength and elongation (PSE) increased from 13.7 to 32 GPa·%. The novelty of the work lies in establishing the origin of the good ductility and high YS of Q&P steel. Blocky-type RA plays a vital role in the effect of Q&P processing on mechanical properties. The main feature of RA is a very high dislocation density proving the strength of ~1000 MPa of this structural component. The strength of RA controls the YS of the steel if its volume fraction is ≥25%. Ductility is provided by the almost full transformation of RA into strain-induced martensite under tension. The localization of plastic deformation in the form of deformation bands is associated with the γ→α′ transformation. Medium carbon Q&P steel with a high volume fraction of RA meets the requirements for advanced high-strength steel (AHSS) belonging to the third generation of AHSS due to the combination of the YS > 1050 MPa with the PSE > 30 GPa·%.