Product Of Strength And Elongation Research Articles

High-strength steel structure and mechanical properties after Q-n-P-integrated heat treatment

The article considers the effect of the newly designed Q-n-P-integrated («Quenching and Partitioning») heat treatment modes on the microstructure and mechanical properties changes of cost-saving alloyed high-strength steel 55Si3Mn2CrVMoNbА. It has been found that the application of Q-n-P-integrated technological schemes, with elongated soaking at 160°C, after previous bainitization, or thermal cycling, greatly increase the mechanical properties of 55Si3Mn2CrVMoNbА steel as compared to standard quenching and tempering (at 200-300°С). It has been found that the structure-free ferrite formation, contrary to presupposition, doesn’t increase the ductility of steel as compared to other investigated heat treatment modes. The most effective results showed the heat treatment according to TCT/Q-n-P mode (three cycles 160«250°С) which provided the formation in steel 55Si3Mn2CrVMoMbA high-strength state (TS = 1941-2116 MPa, 50-54 HRC) while maintaining high total elongation (10-16%) and impact toughness (KCU = 74 J/cm2), which results in a 5-fold increase in PSE («Product of Strength and Elongation») level (up to 31,5 GPа%) of treated steel as compared to quenching and tempering respectively. Heat treatment according to TCT/Q-n-P and В-Q-n-P modes in the investigated steel provides increased impact toughness level (up to 74-79 J/cm2) which makes it possible to use this steel with high cyclic and shock loads. Besides,according to the results shown in this article, the 55Si3Mn2CrVMoNbA steel properties in the state of Q-n-P-integrated treatment are in good agreement with the requirements for the 3rd generation high-strength steels. The study suggests that high mechanical properties are achieved due to the formation of the multiphase structure consisting of tempered martensite, lower bainite and significant amount of carbon enriched residual austenite. The obtained results demonstrate the promise of this technology for better mechanical properties of cost-saving alloyed machine-building and tool steels

Dependence of austenite stability and deformation behavior on tempering time in an ultrahigh strength medium Mn TRIP steel

In the present study, the microstructure, tensile property and deformation behavior were investigated in Fe-8Mn-8Al-0.8C steel after quenching and tempering treatment (Q&T). The steel exhibited a ferrite-austenite-martensite mixed microstructure, and a good combination of ultrahigh ultimate tensile strength (UTS) of ~1500 MPa and total elongation (TE) of ~30%. The tensile property was improved after tempering at 200 °C, which is mainly due to an improvement of austenite stability. Tempering process promoted the uniform distribution of carbon element in austenite, resulting in an elimination of the serrated behavior in the strain hardening rate curve. The element partitioning from the supersaturated martensite and δ-ferrite to austenite during tempering, as well as its contributing effects to austenite stability, was also discussed. Two methods were used to quantitatively monitor austenite stability as a function of tempering time with regard to transformation-induced plasticity (TRIP) effect. After an optimal tempering treatment at 200 °C for 60 min, the continuous strain-induced martensite transformation resulting from an optimized austenite stability led to an ultrahigh tensile strength and a product of strength and elongation (PSE) of 41 GPa·%. A fracture transition from brittle cleavage to ductile fracture was observed, which is due to variation of the local stress distribution between coarse δ-ferrite and adjacent austenite.

An ultrahigh strength and enhanced ductility cold-rolled medium-Mn steel treated by intercritical annealing

An ultrahigh strength and enhanced ductility cold-rolled medium-Mn steel was manufactured via intercritical annealing. The fine lamellar and equiaxed duplex microstructure, austenite (40.5 vol%) and ferrite, was observed due to austenite reversed transformation and ferrite recrystallization. The ultimate tensile strength (1090.0 MPa), total elongation (56.3%) and product of strength and elongation (PSE) (61.4 GPa·%) were achieved, which were comparable and superior to the reported medium-Mn steels. The excellent properties were attributed to high dislocation density, transformation induced plasticity, twinning induced plasticity, grain refinement strengthening and solution strengthening effects. Additionally, the dependence of PSE on austenite fraction was discussed.

The significant impact of pre-strain on the structure-mechanical properties relationship in cold-rolled medium manganese TRIP steel

We elucidate here the impact of pre-strain on the structure on mechanical properties of cold-rolled Fe-0.2C-5.5Mn-1.0Al (wt%) transformation induced plasticity (TRIP) steel. The steel was intercritically hardened at 650°C and exhibited excellent combination of total elongation (TEL) of 45.2%, ultimate tensile strength (UTS) of 1100MPa, and UTS×TEL of 50.2GPa%. Pre-strain increased both yield strength (YS) and ultimate tensile strength (UTS), while it decreased total elongation (TEL). Product of strength and elongation (PSE) was similar with 10% pre-strain (50.4GPa%) and 0% pre-strain (50.2GPa%). However, the 10% pre-strain had lower yield-to-tensile ratio. The increase in YS is attributed to the increased dislocation density and transformed martensite, while the continuous increase in UTS was associated with increase in the fraction of transformed martensite and enhanced work-hardening rate. On the other hand, the increase in the average stability of retained austenite, increased density of dislocations in ferrite and increased volume fraction of transformed martensite induced by pre-strain led to decreased ductility. The desired mechanical properties can be obtained by optimizing the amount of pre-strain.

Microstructures and mechanical properties of ferrite-based lightweight steel with different compositions

The microstructures and mechanical properties of ferrite-based lightweight steel with different compositions were investigated by tensile test, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermodynamic calculation (TC). It was shown that the ferrite-based lightweight steels with 5 wt. % or 8 wt. % A1 were basically composed of ferrite, austenite and κ-carbide. As the annealing temperature increased, the content of the austenite in the steel gradually increased, while the κ-carbide gradually decomposed and finally disappeared. The mechanical properties of the steel with 5 wt.% A1 and 2 wt. % Cr, composed of ferrite and Cr7C3 carbide at different annealing temperatures, were significantly inferior to those of others. The steel containing 5 wt. % Al, annealed at 820 °C for 50 s then rapidly cooled to 400 °C and held for 180 s, can obtain the best product of strength and elongation (PSE) of 31242 MPa · %. The austenite stability of the steel is better, and its PSE is higher. In addition, the steel with higher PSE has a more stable instantaneous strain hardening exponent (n value), which is mainly caused by the effect of transformation induced plasticity (TRIP). When the κ-carbide or Cr7C3 carbide existed in the microstructure of the steel, there was an obvious yield plateau in the tensile curve, while its PSE decreased significantly.

Comparative study on microstructure and mechanical properties of a C-Mn-Si steel treated by quenching and partitioning (Q&P) processes after a full and intercritical austenitization

A C-Mn-Si steel with lean microalloy elements was treated by a series of quenching and partitioning (Q&P) processes after a full or intercritical austenitization (hereinafter referred as F-QP and I-QP process, respectively). The discrepancy in microstructure and mechanical properties of the samples under these two heat treatment processes was investigated. The results indicate that the austenite grains are mainly film-like in the F-QP samples, while blocky austenite grains account for the majority in the I-QP samples. And the later contains a larger amount of retained austenite than the former. In addition, the reason why the austenite fraction decreases with the increasing of partitioning time after the peak value can be related to the bainite transformation due to the dynamic change of phase region in the continue cooling transformation (CCT) diagram of untransformed austenite with carbon enrichment. The I-QP samples show higher product of strength and elongation (PSE) but relatively lower strength, while the F-QP samples are just the reverse. Notably, the one-step I-QP samples show an excellent strength–ductility balance, of which the tensile strength and total elongation are 1140–1280MPa and 17–22%, respectively. The exponential function and Olson-Cohen (O-C) model were utilized to fit the variation of retained austenite fraction with strain. By comparison, retained austenite in the I-QP sample is more sensitive to the increasing strain, i.e., lower mechanical stability.

Precipitation Behavior and Microstructural Evolution of Vanadium-Added TRIP-Assisted Annealed Martensitic Steel

The precipitation behavior and microstructural evolution of vanadium-added TRIP-assisted annealed martensitic (TAM) steel are investigated using scanning electron microscopy, transmission electron microscopy, and three-dimensional atom probe. The results indicate that final microstructure after heat treatment is refined with the addition of vanadium. The yield strength and ultimate tensile strength of the TAM steel increase obviously, while the total elongation slightly decreases with increase of V-content. The product of strength and elongation (PSE) increases from 28.0 to 30.1 GPa% with the increase of V-content. According to the analysis of three-dimensional atom probe, remarkable concentration peak of V occurs within retained austenite and at the interface between retained austenite and BCC phases; additionally, it is concluded that the granular areas are VC precipitates, and V and C atoms distribute along dislocation within retained austenite. The addition of V leads to the increase of the volume fraction of retained austenite and the decrease of the carbon content in retained austenite.

Effect of bainitic isothermal transformation plus Q&P process on the microstructure and mechanical properties of 0.2C bainitic steel

A novel bainitic isothermal transformation plus quenching and partitioning (B-QP) process was developed for a 0.2C-1.5Si-1.8Mn bainitic steel. The microstructure and mechanical properties of the steel treated by different heat treatment process were investigated. The results show that the B3-QP (the subscript ‘3’ indicates the isothermal holding of 3min for the bainitic transformation) samples exhibit a multiphase microstructure containing carbide-free bainite, martensite, film-like retained austenite. The microstructure is much finer than that of the Q&T samples with full martensite and the B30-QP samples with a bainite microstructure. Although the Q&T sample has the highest tensile strength (1455MPa), the ductility and the product of strength and elongation (PSE) are lower. In contrast, the B3-QP and B30-QP samples exhibit better ductility and higher PSE values of 26.3GPa% and 28.2GPa%, respectively. Furthermore, the B3-QP sample exhibits maximum impact toughness value of 121 Jcm−2, which is about 20% and 55% higher than those of Q&T sample (101Jcm−2) and B30-QP sample (78Jcm−2) respectively.

Comparative study on a 0.2C steel treated by Q&P and A&T treatments

The effect of various versions of quenching and partitioning (Q&P) and austempering plus tempering (A&T) processes on the combined properties and microstructure of a 0.2C–0.8Si–2.2Mn bainitic steel has been investigated. Results show that the steel exhibits a higher value of product of strength and elongation (PSE) than that reported before with similar compositions. The one-step Q&P process at 230°C and A&T process at 450°C can result in a toughness higher than 80 J cm−2 and a relatively high PSE (above 29.8 GPa%). The alloy design of this steel is suggested to be beneficial for industrial production because there is a big window for similar PSE. The long-partitioning time (1 h) has good effect on combined properties.

The stability of retained austenite at different locations during straining of I&Q&P steel

This paper presents a detailed investigation of the transformation behavior of retained austenite at different locations of intercritical heating, quenching and partitioning (I&Q&P) steel. The consumption of retained austenite at different strains is investigated by X-ray diffraction (XRD). Results indicate that retained austenite can transform into martensite progressively during the whole deformation process, which is in favor of a good combination of strength and ductility, contributing to a high product of strength and elongation (PSE) of 31.9GPa%. The transformation characteristics of retained austenite at different locations after different strains are characterized by electron back scattered diffraction (EBSD) and transmission electron microscopy (TEM). Results show that the transformation preferentially occurs in the retained austenite at ferrite grain boundaries, subsequently the one within ferrite grains (at 10% strain) and eventually the one between martensite laths (at 15% strain). In FCC phase, the average local misorientations are 0.547° and 0.674° at 5% and 10% strain, respectively; however, the values are not more than 0.7° at 15% and 20% strain due to the TRIP effect. For the retained austenite within ferrite grains, the interior part preferentially transforms into twin martensite, while the interface still remains due to alloying elements segregation.

Microstructure and Mechanical Properties of a Cold-Rolled Medium Manganese Steel with Delta Ferrite

In this paper, a quenching and tempering process was applied to the cold-rolled medium Mn steel with the delta ferrite (Fe-0.18C-6.4Mn-2.8Al). Microstructure characterization was carried out by means of optical microscope, scanning electron microscope (SEM) equipped with electron backscattered diffraction (EBSD) and transmission electron microscope (TEM). Mechanical properties tests were carried out by uniaxial tension tests. The microstructure characterization results revealed that the steel possessed a complex microstructure composed of three phases (austenite, martensite and delta ferrite). The volume fractions of austenite before and after a deformation were determined by X-ray diffraction (XRD). The XRD results indicated that the amount of austenite reached up to 20 vol.% and the TRIP effect occurred quite apparently. The mechanical property results showed that the steel possessed adequate ultimate tensile strength of 800MPa and excellent elongation of 25%. The outstanding combination of strength and ductility with the product of strength and elongation (PSE) reaching up to over 20GPa% indicates that the steel has a bright application prospect.

Microstructure evolution and mechanical properties of Mg-Gd-Sm-Zr alloys

The microstructure evolution and mechanical properties of Mg-Gd-Sm-Zr alloys during heat treatment were systematically investigated. The precipitation sequence of an α-Mg supersaturated solid solution (S.S.S.S., cph) during aging at 225°C is: S.S.S.S.→β″ (D019)→β′ (cbco)→β1 (fcc)→β (fcc), which is different from previously reported Mg-Gd-Sm systems. The observed shortened incubation periods in the Mg-Gd-Sm-Zr alloys can be mainly attributed to the increased amount of Sm present. Peak-aged Mg-Gd-Sm-Zr alloys exhibit mechanical properties superior to that of the WE54 alloy. The product of strength and elongation (PSE) was used to estimate the toughness of Mg-Gd-Sm-Zr alloys, and the PSE value decreases with increasing RE content.

Mechanical Properties of Steels Treated by Q-P-T Process Incorporating Carbide-free-bainite/martensite Multiphase Microstructure

Quenching-partitioning-tempering (Q-P-T) process were applied to a quenched carbide-free bainite/martensite (CFB/M) multiphase steel 40Mn2Si2Cr and its effect on the mechanical properties was studied. The results showed that the partitioning time of Q-P-T treatment can be increased greatly to 90 min or even longer. The tensile strength, elongation and product of strength and elongation (PSE) of the experimental steel can achieve 1650 MPa, about 20% and over 33 GPa · %, respectively. The significant increase of partitioning time and excellent mechanical properties could be related to the CFB microstructure introduced in the quenching process during the Q-P-T treatment.

The Mechanism of High Strength-Ductility Steel Produced by a Novel Quenching-Partitioning-Tempering Process and the Mechanical Stability of Retained Austenite at Elevated Temperatures

The designed steel of Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt pct) treated by a novel quenching-partitioning-tempering (Q-P-T) process demonstrates an excellent product of strength and elongation (PSE) at deformed temperatures from 298 K to 573 K (25 °C to 300 °C) and shows a maximum value of PSE (over 27,000 MPa pct) at 473 K (200 °C). The results fitted by the exponent decay law indicate that the retained austenite fraction with strain at a deformed temperature of 473 K (200 °C) decreases slower than that at 298 K (25 °C); namely, the transformation induced plasticity (TRIP) effect occurs in a larger strain range at 473 K (200 °C) than at 298 K (25 °C), showing better mechanical stability. The work-hardening exponent curves of Q-P-T steel further indicate that the largest plateau before necking appears at the deformed temperature of 473 K (200 °C), showing the maximum TRIP effect, which is due to the mechanical stability of considerable retained austenite. The microstructural characterization reveals that the high strength of Q-P-T steels results from dislocation-type martensite laths and dispersively distributed fcc NbC or hcp e-carbides in martensite matrix, while excellent ductility is attributed to the TRIP effect produced by considerable retained austenite.

Kinetics of fatigue-induced phase transformation in a metastable austenitic 304 L-type steel at low temperatures: Maier, H.J., Donth, B. and Schneeweiss, O. Scr. Metall. Mater. (15 Aug. 1993) 29(4), 521–526

A C-Mn-Si steel with lean microalloy elements was treated by a series of quenching and partitioning (Q&P) processes after a full or intercritical austenitization (hereinafter referred as F-QP and I-QP process, respectively). The discrepancy in microstructure and mechanical properties of the samples under these two heat treatment processes was investigated. The results indicate that the austenite grains are mainly film-like in the F-QP samples, while blocky austenite grains account for the majority in the I-QP samples. And the later contains a larger amount of retained austenite than the former. In addition, the reason why the austenite fraction decreases with the increasing of partitioning time after the peak value can be related to the bainite transformation due to the dynamic change of phase region in the continue cooling transformation (CCT) diagram of untransformed austenite with carbon enrichment. The I-QP samples show higher product of strength and elongation (PSE) but relatively lower strength, while the F-QP samples are just the reverse. Notably, the one-step I-QP samples show an excellent strength–ductility balance, of which the tensile strength and total elongation are 1140–1280 MPa and 17–22%, respectively. The exponential function and Olson-Cohen (O-C) model were utilized to fit the variation of retained austenite fraction with strain. By comparison, retained austenite in the I-QP sample is more sensitive to the increasing strain, i.e., lower mechanical stability.