Search Results (40)

The volume fractions of martensite and ferrite in dual-phase steel affect its strength and plasticity. In this study, the effect of heat treatment on the structure morphology and volume fractions of martensitic and ferrite was studied in rare earth, micro-alloyed dual-phase steel, and the strain-hardening behaviour of the experimental steel under various process conditions was determined. The results show that a uniform structure with an alternating distribution of ferrite and martensite could be obtained by complete quenching before critical annealing, and the martensitic phase content increased from 60% to 93% with a rise in annealing temperature. With the growth in the martensitic phase content, the strength of dual-phase (DP) steel gradually increased, and elongation gradually decreased. However, the strength–plasticity product remained at approximately 17 GPa∙%, showing good comprehensive mechanical properties, and the mechanical properties were better at 780 and 820 °C annealing temperatures. When the martensite content was higher, the strain-hardening ability of the DP steel was stronger. The results show that the failure mode of the DP steel was a typical ductile fracture, and only a small amount of cleavage pattern was observed in the samples annealed at 840 °C. No obvious interfacial disbonding was seen in the tensile fracture, and only a few cracks formed. By optimizing the heat treatment process, the microstructural uniformity was improved, and the ferrite phase was strengthened to some extent, which better coordinated the deformation of ferrite and martensite, thereby delaying fracture. The modification effect of rare earth elements on inclusions in the DP steel was obvious. Full article

Solutions and new processes are continually being developed to produce components demonstrating high strength and elongation. This paper focuses on medium manganese steel with a composition of 0.2% carbon, 3% manganese, and 2.15% aluminium (by weight percent). The mechanical properties of the steel and the effect of aluminium and manganese on the microstructure are investigated. The steel sheets are shaped into omega profiles using a press tool, followed by the intercritical annealing of the samples to enhance their ductility. Before the experiment, the anticipated values were a tensile strength (UTS) of approximately 1100 MPa and elongation within 30–35%. A key objective was to achieve a microstructure that incorporates residual austenite. The experimental parameters were carefully derived from an extensive exploration to identify potential weaknesses in the experiment. The main parameters selected were the intercritical annealing (IA) temperature and IA dwell time. The results revealed that the highest recorded UTS was 1262 ± 6 MPa, while the maximum elongation achieved was 16 ± 1%. Full article

In this paper, the nucleation and growth of austenite are controlled through a two-step annealing process to achieve multi-scale distribution and content increase of retained austenite in low manganese series medium-Mn steel. Combining SEM, EBSD, AES, and other experimental equipment, the evolution rules of the microstructure, properties, and element distribution behavior of the test steel during the annealing process are studied. Compared with one-step annealing, the two-step annealing significantly broadens the size distribution range of retained austenite. In the first step, after annealing at a higher intercritical temperature (760 °C), the ferrite and the M/A island are obtained, completing the initial partition of Mn and the refinement of microstructures. During the second step of annealing (720 °C), the primary Mn-rich martensite region provides higher nucleation driving force and finer dispersed nucleation sites, promoting the nucleation and growth of reverse transformation austenite. At the same time, the metastable-retained austenite formed after the first step of annealing continues to grow through interface movement. Furthermore, a high proportion (23.4%) of retained austenite with multi-scale distribution is formed in the final microstructure, and the product of strength and elongation increased from 21.8 GPa·% by the one-step annealing process to 30.1 GPa·%. Full article

Aluminum-incorporated medium-manganese steel (MMnS) has potential for lightweight transport applications owing to its impressive mechanical properties. Increasing the austenite volume fraction and making microstructural changes are key to manufacturing MMnS. However, the grain boundary character and strain distribution of intercritically annealed low-density MMnS have not been extensively scrutinized, and the effects of crystallographic texture orientation on tensile properties remain ambiguous. Therefore, in this study, the microstructure, microtexture, strain distribution, and grain boundary characteristics of a hot-rolled medium-Mn steel (Fe–0.2 C–4.3 Al–9.4 Mn (wt%)) were investigated after intercritical annealing (IA) at 750, 800, or 850 °C for 1 h. The results show that the 800 °C annealed sample exhibited the highest austenite volume fraction among the specimens (60%). The duplex microstructure comprised lath-type γ-austenite, fine α-ferrite, and coarse δ-ferrite. As the IA temperature increased, the body-centered cubic phase orientation shifted from <001> to <111>. At higher temperatures, the face-centered cubic phase was oriented in directions ranging from <101> to <111>, and the sums of the fractions of high-angle grain boundaries and coincidence–site–lattice special boundaries were significantly increased. The 800 °C annealed sample with a high austenite content and strong γ-fiber {111}//RD orientation demonstrated a noteworthy tensile strength (1095 MPa) and tensile elongation (30%). Full article

Impact fracture behavior at low temperatures was investigated in medium manganese steel with bcc-fcc duplex microstructures. The impact energy was above 150 J (−80~20 °C) and the fractography showed dimples for inter-critical annealing at 630 °C (QHA) because of the high retained austenite stability and low martensite dislocation density. The impact energy was from 180 J (20 °C) to 60 J (−80 °C) and the fractography was intergranular for inter-critical annealing at 610 °C (QLA) because of the low stability of RA and carbides precipitated at the prior austenite grain boundaries. The impact energy was below 60 J (−80~20 °C) and the fractography showed cleavage for direct quenching (DQ) because of the high dislocation density of martensite. Full article

Process maps were developed using a combination of microstructural analysis and DICTRA-based modeling to predict the austenite vol.% as a function of the intercritical annealing parameters and starting microstructure. The maps revealed a strong dependence of the calculated austenite fraction (vol.%) on the Mn content (4–12 wt.%) and intercritical annealing temperatures (600 °C to 740 °C). The calculations were carried out for constant carbon, Al, and Si contents of 0.2 wt.%, 1.5 wt.%, and 1.0 wt.%, respectively. A modified empirical equation proposed by Koistinen and Marburger was employed to calculate the room-temperature retained austenite vol.% as a function of the intercritical annealing temperature, including the effect of the austenite composition. The process maps offer valuable insights for designing intercritical treatments of medium-Mn steels, aiding in the optimization of steel properties for automotive applications. Full article

In this study, the effect of subsequent heat treatment applied to high-strength low-alloy steel (HSLA) on the structure–property relationships was investigated. Tempering and intercritical annealing processes are introduced to elucidate the influence of subsequent heat treatment on mechanical properties of direct-quenched Fe-0.06C-0.2Si-2.0Mn steel from a microstructural perspective. The tempering process results in a typical tempered martensite with uniformly dispersed cementite, whereas the intercritical annealing process forms a dual-phase microstructure composed of soft ferrite and hard martensite for the direct-quenched steel. In the intercritical annealed steel, a number of mobile dislocations at the interphase (martensite/ferrite) boundary significantly decrease the yield strength, and the large difference in strength between ferrite and martensite enhances work hardening. Charpy V-notch impact test results indicate that the tempering and intercritical annealing processes improved the absorbed energy by more than 100 J compared to the direct-quenched steel at room temperature, and at −50 °C, the intercritically annealed steel exhibited the highest absorbed energy of approximately 140 J. Additionally, the high fraction of high-angle grain boundaries and fine grains of the intercritically annealed steel increase the resistance to cleavage crack propagation, thereby reducing the ductile-to-brittle transition temperature. Full article

In this paper, we investigate the effects of vanadium on the strength and ductility of medium-manganese steels by analyzing the microstructural evolution and strain hardening rates and performing quantitative calculations. Two significantly different contents of vanadium, 0.05 and 0.5 wt.%, were independently added to model steel (0.12C-10Mn) and annealed at different intercritical temperatures. The results show that higher vanadium addition increases the yield strength but decreases the ductility. The maximum yield strength can increase from 849 MPa to 1063 MPa at low temperatures. The model calculations reveal that this is due to a precipitation strengthening increment of up to 148 MPa and a dislocation strengthening increment of 50 MPa caused by a higher quantity of V₄C₃ precipitates. However, the high density of vanadium carbides leads them to easily segregate at grain boundaries or phase interfaces, which prevents strain from uniformly distributing throughout the phases. This results in stress concentrations which cause a high strain hardening rate in the early stages of loading and a delayed transformation-induced plasticity (TRIP) effect. Additionally, the precipitates decrease the austenite proportion and its carbon concentrations, rendering the TRIP effect unsustainable. Accordingly, the ductility of high vanadium steels is relatively low. Full article

Dual-phase low-alloy steels combine a soft ferrite phase with a hard martensite phase to create desirable properties in terms of strength and ductility. Nickel additions to dual-phase low-alloy steels can increase the yield strength further and lower the transformation temperatures, allowing for microstructure refining. Determining the correct intercritical annealing temperature as a function of nickel content is paramount, as it defines the microstructure ratio between ferrite and martensite. Likewise, quantifying the influence of nickel on the intercritical temperature and its synergistic effect with the microstructure ratio on mechanical properties is vital to designing dual-phase steels suitable for corrosive oil and gas services as well as hydrogen transport and storage applications. In this work, we used a microstructural design to develop intercritical annealing heat treatments to obtain dual-phase ferritic–martensitic low-alloy steels. The intercritical annealing and tempering temperatures and times were targeted to achieve three different martensite volume fractions as a function of nickel content, with a nominal content varying between 0, 1, and 3-wt% Ni. Mechanical properties were characterized using tensile testing and microhardness measurements. Additionally, the microstructure was studied using scanning electron microscopy coupled with electron backscatter diffraction analysis. Tensile strength increased with increasing martensite ratio and nickel content, with a further grain refinement effect found in the 3-wt% Ni steel. The optimal heat treatment parameters for oil and gas and hydrogen transport applications are discussed. Full article

A hot-rolled medium manganese Q&P steel was annealed at an intercritical annealing temperature (670 °C) and quenched to four partitioning temperatures (300 °C, 280 °C, 260 °C, and 240 °C). The effect of the partitioning temperature on the microstructure and mechanical properties of the steel was studied. The microstructure evolution at different partitioning temperatures was characterized using scanning electron microscopy (SEM), a transmission electron microscope (TEM), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The results showed that, as the partition temperature increased, the fractions and the size of the fresh massive martensite (M₂) increased, the initial lamellar martensite (M₁) was coarsened, and the amount of M₁ decreased. Moreover, the amounts of lath-like retained austenite (RA) decreased and the amounts of bulky austenite increased. The tested steel partitioned at 260 °C had relatively greater fractions of RA, in which there were sufficient contents of carbon diffused from M₁. Therefore, the test steel partitioned at 260 °C showed fine comprehensive mechanical properties. The combination of tensile strength and elongation reached 30.3 GPa.%, and its work-hardening plateau was the longest, showing excellent ductility as it had sufficient fractions of the retained austenite with good stability. Full article

The effect of Cu addition and the intercritical annealing (IA) temperature on the microstructural evolution and mechanical properties of Fe-0.4C-7Mn-4Al (wt%) was investigated via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and nanoindentation tests. The results showed that the volume fraction and the average grain size of austenite, and the fraction of high angle grain boundaries, increased with IA temperature increase in the range of 650 °C to 710 °C. The addition of Cu facilitates the formation of Cu-rich nanoparticles, raises the volume fraction of austenite, and delays the recrystallization of austenite. As IA temperature increased, the yield strength (YS), ultimate tensile strength (UTS), and Lüders bands strain (LBS) decreased in both experimental steels. The Cu addition not only increases the YS of medium Mn steel but also benefits the decrease of LBS. The best comprehensive mechanical properties were obtained at the IA temperature of 690 °C in the studied steel, with Cu addition. According to nanoindentation experiments, the Cu addition raises the hardness of ferrite and austenite from 4.7 GPa to 6.3 GPa and 7.4 GPa to 8.5 GPa, respectively, contributing to the increase of YS of medium-Mn steel. Full article

In the present work, a Cr+Mo+Si low-alloyed low-carbon steel was fabricated at laboratory scale and processed to produce multiphase advanced high-strength steels (AHSS), under thermal cycles similar to those used in a continuous annealing and galvanizing process. Cold-rolled steel samples with a microstructure constituted of pearlite, bainite, and martensite in a matrix ferrite, were subjected to an intercritical annealing (817.5 °C, 15 s) and further isothermal bainitic treatment (IBT) to investigate the effects of time (30 s, 60 s, and 120 s) and temperature (425 °C, 450 °C, and 475 °C) on the resulting microstructure and mechanical properties. Results of an in situ phase transformation analysis show that annealing in the two-phase region leads to a microstructure of ferrite + austenite; the latter transforms, on cooling to IBT, to pro-eutectoid ferrite and bainite, and the austenite-to-bainite transformation advanced during IBT holding. On final cooling to room temperature, austenite transforms to martensite, but a small amount is also retained in the microstructure. Samples with the lowest temperature and largest IBT time resulted in the highest ultimate tensile strength/ductility ratio (1230.6 MPa-16.0%), which allows to classify the steel within the third generation of AHSS. The results were related to the presence of retained austenite with appropriate stability against mechanically induced martensitic transformation. Full article

TRIP-assisted C-Si-Mn steels are usually subjected to austempering with a preliminary intercritical annealing that is targeted at the multi-phase structure with 40–60 vol.% of proeutectoid ferrite. The kinetics and the mechanism of phase-structural transformations can be impacted due to the additional alloying of TRIP-assisted steel by the strong carbide forming elements, thus necessitating an alternative approach for the selection of intercritical annealing parameters. This issue is analyzed in the present work, which investigates the effect of the temperature of intercritical annealing on the “Structure/Properties” correlations in 0.2 wt.% C-Si-Mn-Nb steel additionally alloyed by 0.55 wt.% Cr, 0.20% Mo, and 0.11 wt.% V. The annealing temperature ranged from 770 °C to 950 °C, and austempering was performed at 350 °C for 20 min. It was observed that the addition of the (Cr, Mo, and V) complex significantly improved the steel hardenability. However, the annealing of steel at 770 °C (to gain 50 vol.% of proeutectoid ferrite) resulted in the precipitation of coarse cementite lamellas during bainite transformation, thus lowering the amount of retained austenite (RA) and decreasing the strength and ductility of the steel. At higher annealing temperatures, carbide-free bainite was formed, which presented a 2.5–3.5 times increase in the RA volume fraction and a 1.5 times increase in the RA carbon content. The optimal combination of the mechanical properties (UTS of 1040 MPa, TEL of 23%, V-notch impact toughness of 95 J/cm², PSE of 23.9 GPa·%) referred to annealing at a temperature close to the Ac₃ point, resulting in a structure with 5 vol.% ferrite and 9 vol.% RA (the residue was carbide-free bainite). This structure presented an extended manifestation of the TRIP effect with an enhanced strain hardening rate due to strain-induced martensite transformation. The impact of the alloying elements on the carbon activity in austenite served as the basis for the analysis of structure formation. Full article

In this study, we comparatively study the microstructures and mechanical properties of prequenching—quenching and partitioning (QQ&P) and traditional Q&P samples at different annealing temperatures (intercritical annealing temperatures). When the annealing temperature is 780 °C, the ferrite and retained austenite in QQ&P samples with lath and blocky morphologies. The lath retained austenite is mainly distributed along the lath ferrite. As the annealing temperature increases, the lath ferrite recrystallizes and gradually grows into the blocky (equiaxed) shape, leading to a decrease in the lath retained austenite content. When the annealing temperature increases to 870 °C, the ferrite content decreases significantly, and the retained austenite is mainly blocky and thin film, distributed at the boundaries of prior austenite grains and between martensite laths, respectively. Different from QQ&P samples, the ferrite and retained austenite in Q&P samples are mainly blocky when the annealing temperature is 780 °C or 810 °C. When the annealing temperature is increased to 870 °C, the microstructures of the Q&P sample are basically the same as that of the QQ&P sample. The 780 °C-QQ&P sample and the 810 °C-QQ&P sample have higher total elongation and product of strength and elongations (PSEs) than their counterpart Q&P samples due to the fact that lath ferrite and retained austenite are conducive to carbon diffusion and carbon homogenization in austenite grains, thereby improving the thermal stability and volume fraction of the retained austenite. In addition, the lath structures can release local stress concentration and delay the formation of voids and microcracks. The difference of mechanical properties between QQ&P samples and Q&P samples decreases with the increase in the annealing temperature. The results show that the low annealing temperature combined with prequenching—Q&P heat treatments can significantly improve the elongation and PSE of Q&P steel. Full article

To further improve the mechanical properties of H13 steel at room and high temperatures, its precipitates were regulated based on the Thermo-Calc results. Scanning electron microscope (SEM), electron backscattering diffraction (EBSD), transmission electron microscope (TEM), and X-ray diffraction (XRD) Rietveld refinement were used to study the effect of the intercritical annealing on the microstructure and mechanical properties of H13 steel. The results show that the intercritical annealing at 850~95 °C increased the VC volume fraction from 2.23 to 3.03~3.48%. Increasing the VC volume fraction could inhibit the M₇C₃ precipitation from 10.01 to 6.63~5.72% during tempering. A large amount of VC also promoted the M₂₃C₆ precipitation during tempering at higher dislocation densities. The intercortical annealing simultaneously increased the elongation of H13 steel. An excellent combination (room temperature: ultimate tensile strength (UTS) of 898 MPa and total elongation (TEL) of 19.35%, 650 °C: UTS of 439 MPa, and TEL of 27.80%) could be obtained when intercritical annealing is performed at 900 °C. Meanwhile, after aging at 650 °C for 128 h, the room temperature UTS and TEL decreased by only 31 MPa and 0.52%, respectively. Full article