Hot-rolled medium-Mn Steel Research Articles

Intercritically Annealed Medium-Manganese Steel: Insights into Microstructural and Microtextural Evolution, Strain Distribution, and Grain Boundary Characteristics.

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%).

Microstructure-mechanical properties evolution of hot-rolled medium Mn steel: Effects of short-time holding and low-temperature tempering following ultrafast heating

In this work, the mechanical properties of hot-rolled 7Mn steel are significantly sensitive to the short-time isothermal holding after ultrafast heating (UFH). These samples also exhibited significant disparities in sensitivity of mechanical properties to subsequent low-temperature tempering. Through a combination of multi-scale experimental characterization, calculations of yield strength, and numerical simulations of elemental diffusion, we discussed the sensitivities described above. The results indicate that mechanical properties of the samples subjected to ultra-fast heating is not sensitive to subsequent low-temperature tempering. This due to the preferential segregation of C atoms at dislocation sites during the ultra-fast heating, leading to less noticeable C atoms segregation during low-temperature tempering. Short-time isothermal holding after ultrafast heating results in the precipitation of numerous nanoscale carbides within the martensite, thereby reducing solid solution strengthening and significantly decreasing the yield strength. Simultaneously, the low-temperature tempering process induces the segregation of C atoms towards dislocations, grain boundaries, or phase interfaces. This significantly mitigates the Portevin-Le Chatelier (PLC) effect in the sample, increasing the elongation rate of the steel from about 10% to 19%. Additionally, the combined effects of strengthening induced by Cottrell atmosphere and phase boundary segregation contribute to an elevation in yield strength.

Effect of Si on the dislocation state within martensite of ultra-high strength hot-rolled medium Mn steel with good ductility

Medium Mn steels are promising candidates for the third-generation advanced high-strength steels due to their good combination of ultimate tensile strength (UTS) and total elongation after fracture (TEL). Most investigations of medium Mn steels focused on the volume fraction of retained austenite (RA) and stability of RA; however, the strengthening phase martensite was paid little attention to. The present study systematically investigated the effect of Si on the deformation behavior of martensite in 0.2C–6.8Mn-(0, 2, 4)Si (wt. %) hot-rolled medium Mn steels without any heat treatment. The UTS and TEL of the 4Si hot-rolled medium Mn steel are 2163 MPa and 11.7%, respectively, which is rare in the existing hot-rolled medium Mn steels. All three hot-rolled steels had practically equal fraction and mechanical stability of RA, so that the influence of Si on the evolution of martensite fine structure and the nature of the ultra-high UTS and good ductility during deformation could be revealed. By combining the X-ray and neutron diffraction techniques, it was found that the addition of Si increased the dislocation density, especially screw dislocations, and decreased the crystallite size within martensite. The ultra-high UTS originates from the enhanced strain hardening rate caused by the Si alloying, high dislocation density and possibly increased number of martensite lath boundaries and dislocation cell formation. The good ductility mainly stems from the formation of dislocation cell structure due to the addition of Si causing dislocations to be mostly of screw type. The dislocation cells can also improve the strain hardening rate. This work opens a new pathway towards tailoring the dislocation state within martensite for substantially enhanced the mechanical properties of hot-rolled medium Mn steels.

Formation of core-shell austenite as promoted by alloying solutes in hot-rolled medium Mn steel

Recently, there has been a new trend to design chemically heterogeneous austenite by adjusting the various thermal treatment processes in medium Mn steel. In this work, we found that the single or synergistic addition of ferrite stabilizers (e.g., Al) accelerated the heterogeneous distribution of Mn and the formation of a core-shell austenitic structure compared with the addition of an austenite stabilizer (e.g., Cu) under the conventional quenched-annealing process. In addition, the Mn-enriched austenite shell played a significant role in suppressing the martensitic transformation of the Mn-depleted austenite core upon cooling. The sharp Mn gradient distribution corresponded to the heterogeneous kinetics of the deformation-induced martensitic transformation, resulting in the gradual transformation-induced plasticity (TRIP) effect.

Effect of Mo and Nb on mechanical properties and hydrogen embrittlement of hot-rolled medium-Mn steels

The effects of Mo and Nb on the mechanical properties and hydrogen embrittlement (HE) of hot-rolled medium-Mn steel were investigated. The diffusion behavior of hydrogen was examined using an electrochemical hydrogen permeation test. The mechanical properties and HE were studied using a slow strain rate test. The results show that most Mo and Nb served as solid-solution atoms in the hot-rolled steels, and little carbide content was observed. Both Mo and Nb increased the volume fraction of retained austenite (VA), decreased the starting temperature of martensitic transformation (MS), and retarded the martensite transformation. The effect of Nb was more pronounced. Simultaneous addition of Mo and Nb had a synergistic effect on increasing VA, decreasing MS, and retarding martensite transformation. The elongation of the samples increased with an increase in VA. The strength exhibited the opposite trend. A small amount of Nb (0.04 wt%) significantly improved the HE resistance of steels to a much greater extent than Mo (1 wt%). Filmy retained austenite formed along the primary austenite grain boundaries because of the Nb addition; it inhibited hydrogen-induced cracks, leading to better HE resistance. Blocky retained austenite, formed by the simultaneous addition of Mo and Nb, deteriorated the HE resistance.

Effect of Different Heat Treatment Processes on Microstructure Evolution and Tensile Properties of Hot-Rolled Medium-Mn Steel

The effects of different heat treatment processes on microstructure evolution and tensile properties of Fe-0.11C-5.23Mn-1.11Al-0.10Si medium-Mn steel were investigated. The uniaxial tensile properties of tension specimens after heat treatment were tested, and the relationship between different annealed microstructures and tensile properties was analyzed; furthermore, the optimum heat treatment parameters were explored. Besides, the water-quenched sample exhibited an extensive TRIP effect than the furnace-cooled sample. The optimum microstructure and mechanical properties can be obtained when the sample is intercritically annealed at 625 °C for 4 h and then water-quenched. At the same time, the work hardening rate of the experimental steel is expounded, which provides theoretical reserve and technical support for the practical application of this kind of steel.

Microstructural Characteristics and Tensile Behavior of a Hot-Rolled Medium-Mn Steel (0.25C-8.5Mn-0.5Si-2.5Al) Processed by Intercritical Annealing Treatment

Microstructural characteristics and tensile behavior of a novel hot-rolled medium-Mn steel, 0.25C-8.5Mn-0.5Si-2.5Al (wt.%), under different intercritical annealing temperatures (TIA) were evaluated. The results show that an ultrafine duplex lamellar microstructure with a maximum volume fraction of retained austenite (RA) of 65 vol.% was obtained at TIA of 750 °C. It was found that both the stability of the RA and its stability range play critical role in controlling the sustainable TRIP effect. The product of ultimate tensile strength and total elongation as high as ~ 66 GPa% was obtained at TIA of 725 °C, which is much higher than those of previously investigated medium-Mn steels with similar Mn contents, indicating a great potential for the tested steel. The analysis of the fracture surface revealed that the steel exhibited a kind of micro-delamination fracture, which was found to be primarily nucleated at the interfaces of the transformation-induced martensite and ferrite, and the micro-delamination fracture became more significant with increasing TIA.

Analysis of Plastic Deformation Instabilities at Elevated Temperatures in Hot-Rolled Medium-Mn Steel.

The study addressed the microstructure and mechanical properties of hot-rolled advanced high-strength medium manganese steel. Some of the curves that were obtained in static tensile tests at deformation temperatures of 20–200 °C showed the occurrence of the heterogeneous plastic deformation phenomenon, called the Portevin-Le Chatelier (PLC) effect. The deformation temperature significantly influenced a serration character. The correlations between the deformation temperature, serration range, microstructural features, and fracture behavior were investigated. The curves showed no Lüders elongation as a result of the thermomechanical processing applied. The serrated flow phenomenon was observed at 60 and 140 °C. The serration type was different and the most enhanced at 140 °C, where the PLC effect was present in both uniform and post-uniform elongation ranges. The disappearance of serrations at 200 °C was related to the increased diffusion intensity.

Correlation between cementite precipitation and Portevin-Le Chatelier effect in a hot-rolled medium Mn steel

Abstract The correlation between cementite precipitation and plastic instability phenomena especially Portevin-Le Chatelier (PLC) effect was investigated for a hot-rolled medium Mn steel. The microstructure demonstrated lamellar morphology of austenite and ferrite. Cementite precipitations could be retained by controlling certain intercritical annealing (IA) process. The initial high density of dislocations in ferrite contributed to the continuous yielding. Cementite precipitations lowered solute carbon content in ferrite and the dynamic interaction with dislocations was eliminated. A moderate product of strength and elongation (PSE) grades hot-rolled medium Mn steel without PLC effect during tensile test was obtained by controlling the IA process.

Decreasing yield ratio of 70 GPa·% grade hot-rolled medium Mn steel by weakening multi-strengthening effects

The yield ratio of investigated steel decreased significantly from 0.93 to 0.59 by prolonging intercritical annealing duration to weak multi-strengthening effects. The contribution of each effect was qualified by theoretical calculation. The grain refinement strengthening and dislocation strengthening dominate the yield strength, while the reduction of precipitation strengthening and martensite strengthening mainly leads to the decrease of yield strength.

Effect of Al on the Microstructure and Mechanical Properties of Hot-Rolled Medium-Mn Steel

The present investigation was made to study the effect of Al on the microstructure and mechanical properties of hot-rolled medium-Mn TRIP steel (abbreviated as Al-TRIP). As a contrast, a Si-added medium-Mn TRIP steel (abbreviated as Si-TRIP) was also studied. Addition Al in medium-Mn steel can raise Ac3 temperature, which will restrain austenite transformation and expand the two-phase region, promoting Mn and C elements enriched in austenite. In-depth microstructure and mechanical properties analysis were carried out for the hot-rolled Al-TRIP and Si-TRIP steels in this study. The microstructure was characterized by scanning electron microscope (SEM) and electron probe microanalyzer (EPMA). Volume fraction of retained austenite was measured by D/max2400 X-ray diffractometer (XRD). A dual-phase microstructure consisting of ultra-fine grained intercritical ferrite (IF) and lath-like retained austenite (RA) with high mechanical stability was obtained after annealing at 630°C for 2h for Al-TRIP steel. As prolonging the intercritical annealing time, the stability of RA decreased primarily due to the increase of grain size. The tensile test results indicated that the Al-TRIP steel possessed a better combination of tensile strength and elongation compared to Si-TRIP steel. Excellent mechanical properties with yield strength of 790MPa, tensile strength of 1050MPa, total elongation of 35% and UTS×TEL of 39GPa·% was obtained for the Al-TRIP steel.

Microstructure and mechanical properties of a novel hot-rolled 4% Mn steel processed by intercritical annealing

The study described here focuses on the microstructure and mechanical properties of a novel hot-rolled medium-Mn steel (Fe–0.25C–4Mn–1.88Al–0.6Si–0.04Nb–0.08V, wt%) that contained only 4 wt% Mn and modest Al content of 1.88 wt%. It was found that a relatively high content (25–53 vol%) of retained austenite was obtained by intercritical annealing process. With increase in intercritical annealing temperature from 700 to 740 and to 760 °C, retained austenite fraction increased from 25.1 to 53.2% and then decreased to 46.1%. Besides, mechanical stability of retained austenite in 700–760 °C intercritically annealed steels decreased with increase in intercritical annealing temperature. The 720 °C intercritically annealed steel yielded excellent mechanical properties with yield strength of 766 MPa, tensile strength of 951 MPa, total elongation of 48.6% and PSE of 46.22 GPa·%, achieved by a high volume fraction of retained austenite (46%) with relatively high mechanical stability. Thus, the 4% Mn steel present in this study indicated excellent mechanical properties of medium-Mn steels can be achieved using low alloy content.

Microstructure and mechanical properties of hot-rolled medium-Mn steel containing 3% aluminum

The present investigation was made to study the variations of microstructure and mechanical properties of hot-rolled 3%Al containing medium-Mn steel at different intercritical annealing temperatures. The results show suitable intercritical annealing could obtain a duplex microstructure of lathy retained austenite (RA) and bimodal size distributed lathy ferrite. The ultimate tensile strength (UTS) increases while the yield strength decreases with increasing annealing temperature, and the total elongation (TEL) initially increases, but then notably decreases with increasing annealing temperature after a maximum value at 730–750°C. An excellent combination of UTS of 922MPa, TEL of 61% and UTS×TEL of 56GPa% could be obtained when annealed at 750°C. These values are significantly superior to a number of previously studied medium-Mn steels with similar Mn content. The stability of RA decreases with increasing annealing temperature primarily due to the increase of the RA size. It is suggested that the RA with an intermediated size of lath width of ~0.51–0.70µm has suitable stability from the standpoint of obtaining large TRIP effect during tensile testing.