Medium Mn Research Articles

Superior strength-ductility combination achieved via double heterogeneities of microstructure and composition: An example of medium manganese steel

The double heterogeneities of microstructure and composition are introduced in a medium Mn steel via the partially austenite reversed transformation (PART). The role of these heterogeneities in the mechanical properties is revealed. A heterogeneous microstructure consisting of both the granular and lath-typed ferrite/austenite grains is obtained in the PART sample. During plastic deformation, the lath-typed microstructures bear more deformation than the granular ones due to their unique morphological geometry, leading to strain partitioning. This mechanical incompatibility gives rise to significant hetero-deformation induced (HDI) hardening and thus a remarkable enhancement of the mechanical properties. The compositional heterogeneity, i.e. Mn concentration in the lath-typed austenite is higher than that in the granular austenite, is also obtained in the PART steel. Such heterogeneity gives rise to different austenite stabilities and thus sustainable martensitic transformation at different stages of deformation, which also contributes to the enhancement of the mechanical properties. These results provide new insights into the design of advanced metallic materials with outstanding mechanical properties via the double heterogeneity of microstructure and composition.

A Scale-up Study on Chemical Segregation and the Effects on Tensile Properties in Two Medium Mn Steel Castings

Two ingots weighing 400 g and 5 kg with nominal compositions of Fe–8Mn–4Al–2Si–0.5C–0.07V–0.05Sn were produced to investigate the effect of processing variables on microstructure development. The larger casting has a cooling rate more representative of commercial production and provides an understanding of the potential challenges arising from casting-related segregation during efforts to scale up medium Mn steels, while the smaller casting has a high cooling rate and different segregation pattern. Sections from both ingots were homogenized at 1250 ^{circ} C for various times to study the degree of chemical homogeneity and delta -ferrite dissolution. Within 2 hours, the Mn segregation range (max–min) decreased from 8.0 to 1.7 wt pct in the 400 g ingot and from 6.2 to 1.5 wt pct in the 5 kg ingot. Some delta -ferrite also remained untransformed after 2 hours in both ingots but with the 5 kg ingot showing nearly three times more than the 400 g ingot. Micress modeling was carried out, and good agreement was seen between predicted and measured segregation levels and distribution. After thermomechanical processing, it was found that the coarse untransformed delta -ferrite in the 5 kg ingot turned into coarse delta -ferrite stringers in the finished product, resulting in a slight decrease in yield strength. Nevertheless, rolled strips from both ingots showed >900 MPa yield strength, >1100 MPa tensile strength, and >40 pct elongation with <10 pct difference in strength and no change in ductility when compared to a fully homogenized sample.

Austenite Stability and Deformation Behavior in Medium Mn Steel Processed by Cyclic Quenching ART Heat Treatment.

This study investigated the austenite stability and deformation behavior of cyclic quenching-austenite reverse transformation processed Fe-0.25C-3.98Mn-1.22Al-0.20Si-0.19Mo-0.03Nb medium Mn steel. A number of findings were obtained. Most importantly, the extent of the TRIP effect was mainly determined by an appropriately retained austenite stability rather than its content. Simultaneously, chemical elements were the key factors affecting austenite stability, of which Mn had the greatest impact, while the difference of retained austenite grain size and Mn content resulted in different degrees of retained austenite stability. Additionally, there were still large amounts of strip and granular-retained austenite shown in the microstructure of the CQ3-ART sample after tensile fracture, revealing that the excessively stable, retained austenite inhibited the generation of an extensive TRIP effect.

The role of retained austenite on the stress-strain behaviour of chemically patterned steels

Two medium Mn steels with ghost pearlite microstructures and different C contents are compared, to reveal the effect of C on the stress-strain behaviours. Results indicate that with similar mechanical stability of austenite, higher C leads to stronger austenite and TRIPed martensite, thus providing more pronounced strain hardening behaviour.

Interaction of austenite reversion with precipitation/dissolution during aging in a medium Mn steel alloyed with Cu, Ni and Al

Controlling the amount of retained austenite and precipitate particles allows for tailoring of mechanical properties in medium Mn steel alloyed with Cu, Ni and Al. The austenite reversion and precipitation/dissolution occur simultaneously during aging treatment at 550 °C being lower than Ac 1 . The degree of austenite transformation depends on the diffusion of austenite stabilizing elements (Mn, Cu and Ni), and the dissolution of the precipitate particles promotes the diffusion. Small angle neutron scattering (SANS) measurements were performed at the China Spallation Neutron Source (CSNS) to investigate the precipitation/dissolution behaviors. A large number of precipitate particles form at the aging time of 1 h, and then the volume fraction of the particles continuously decreases, and the average particle size has little change as holding time increases, indicating the dissolution of the precipitate particles. Atom probe tomography and transmission electron microscopy were used to study the type, composition and distribution of the precipitate particles. The particles contain Cu and NiAl-type precipitates, which have BCC and B2 structures, respectively. The characterization and modeling of austenite reversion kinetics indicate that austenite transformation is mainly controlled by the diffusion of Cu, which is promoted by the dissolution of Cu particles. This work offers significant insights towards an austenite-precipitate cooperative design for controlling the mechanical properties of the steel. • Austenite reversion occurs at aging temperature being lower than Ac 1 . • The diffusion of austenite stabilizing elements controls growth of austenite. • The dissolution of Cu particles facilitates the austenite transformation. • Austenite reversion and precipitation/dissolution control mechanical properties.

Influence of co-existing medium Mn and dual phase steel microstructures on ductility and Lüders band formation

The mechanical property spectrum of steels can be expanded to new limits through combining the characteristic microstructures of different steel grades in a single steel. Here, we demonstrate this microstructural grading concept in a single steel sheet that possesses microstructures representative of two steel grades, i.e., the duplex medium Mn steel and the dual-phase steel, in the form of a multi-layered structure. This graded steel was produced by applying the surface mechanical attrition treatment to a duplex medium Mn steel with metastable retained austenite. The resulting graded steel collects the respective advantages of the two steels: high ductility of the former, and the Lüders-band-free plastic flow of the latter. By carrying out systematic experiments and finite element simulations, we reveal that the suppression of Lüders band formation in this graded steel is due to the high early-stage strain hardening rate of the dual-phase steel microstructure, which offsets the post-yield strain softening tendency of the medium Mn steel microstructure. The ductility, on the other hand, results from the transformation-induced plasticity effect of the medium Mn steel microstructure, which enhances the overall strain hardening rate and suppresses the necking of the dual-phase steel microstructure at large strain levels.

Influence of lamellar and equiaxed microstructural morphologies on yielding behaviour of a medium Mn steel

Additional austenitization followed by quenching was introduced before the intercritical annealing (IA) of cold rolled medium Mn steel to produce the duplex microstructures having both lamellar and equiaxed morphologies, whose respective fractions vary with austenitization temperature. When it increased from 850°C to 1050°C, more martensite was formed during quenching and then transformed to the lamellar grains after the IA. At the same time, the higher austenitization temperature resulted in the greater microplasticity before macro-yielding, the decreases in both upper yield point and yield drop and the shorter yield point elongation. The relationship between all the yielding behavior and the microstructural morphologies were discussed on the basis that the lamellar and the equiaxed phase interfaces could have the different capability of emitting either partial or full dislocations during deformation.

A new combinatorial processing route to achieve an ultrafine-grained, multiphase microstructure in a medium Mn steel

A new combination of factors enhancing the stabilization of austenite, including pre-existed austenite among quenched martensite, prior deformation, and partitioning at high temperatures is employed to create a multi-component refined microstructure in a medium Mn steel (Fe–4Mn–0.31C–2Ni–0.5Al–0.2Mo, wt.%). The microstructure evolution and phase fraction during the processing are systematically investigated using various characterization methods. The microstructure of the specimen after 0.4 strain deformation of 73% martensite–27% austenite at 250 °C and subsequent partition-annealing at 600 °C for 20 min was composed of several phases including tempered martensite, fresh martensite, pearlite, 10% of retained austenite (RA) and undissolved cementite. By increasing the annealing temperature, the pearlitic transformation was suppressed, whereas recrystallization of the deformed martensite and carbide dissolution occurred following annealing at 650 °C for 20 min resulting in an ultrafine-grained microstructure composed of equiaxed ferrite, 32% RA along with some fresh martensite during final cooling and few carbide precipitates. The results demonstrate that the combinatorial approach accelerated partitioning of alloying elements from martensite and carbides to largely pre-existing austenite is responsible for the improved austenite stabilization during intercritical annealing of the deformed dual-phase specimens. However, competitive processes are also enhanced so that the RA content is not increased by deformation.

The coupled effect of thermal and mechanical stabilities of austenite on the wear resistance in a 0.2C–5Mn-1.6Si steel down to cryogenic temperatures

The effect of testing temperatures on wear-induced austenite-martensite transformation and the resultant wear resistance in metastable austenite-containing steels remains unclear. This issue is addressed in the current study, wherein the wear behavior of medium Mn steel was studied using a ball-on-disk sliding test at 20 °C, 0 °C, −50 °C, and −120 °C. The results demonstrate that the strain hardening and toughness enhancement produced by metastable austenite in austenite-rich steel greatly reduce wear volume at low temperatures. The dominant wear mechanism thereby changes from grooving and ploughing to cutting compared with martensitic steel. The wear resistance of austenite-rich steel depends significantly on the thermal and mechanical stabilities of metastable austenite. This study provides the basis for the development of novel wear resistant steels with relatively low initial hardness applicated at low temperatures.

Cold Formabilities of Martensite-Type Medium Mn Steel

Cold stretch-formability and stretch-flangeability of 0.2%C-1.5%Si-5.0%Mn (in mass%) martensite-type medium Mn steel were investigated for automotive applications. High stretch-formability and stretch-flangeability were obtained in the steel subjected to an isothermal transformation process at temperatures between Ms and Mf − 100 °C. Both formabilities of the steel decreased compared with those of 0.2%C-1.5%Si-1.5Mn and -3Mn steels (equivalent to TRIP-aided martensitic steels), despite a larger or the same uniform and total elongations, especially in the stretch-flangeability. The decreases were mainly caused by the presence of a large amount of martensite/austenite phase, although a large amount of metastable retained austenite made a positive contribution to the formabilities. High Mn content contributed to increasing the stretch-formability.

Microstructure and tensile behaviors for a medium Mn steel with δ-ferrite phase under different annealing temperatures

In this experiment, we systematically investigated the microstructure evolution, mechanical properties, and deformation behaviors as functions of annealing temperature using cold-rolled Fe-0.05C–6Mn–1Al-1.5Si steel. Almost all annealed specimens are composed of the equiaxed and granular α-ferrite (α) grains due to recrystallization, reversed austenite (γ) grains because of reversed transformation from deformed martensite, and fibrous δ-ferrite (δ) grains only undergoing recovery during intercritical annealing except for 760 °C. It is interesting that several annealing twins are formed within γ grains. The outstanding combination of strength and ductility is obtained for annealing at 740 °C. The tensile strength and total elongation are 980 MPa and 32.1%, which are attributed to the ultrafine grains and optimal TRIP effect associated with rational γ content and stability. A phenomenon that yield point elongation (YPE) gradually shortens with increased γ content and decreased γ stability is observed. What's more, the mechanism that active α′ formation can promote the improvement of work hardening rate and further curtail YPE is verified using deep-cryogenic treatment. Based on SEM observation, microscopic strain of δ grains appears nearly consistent with matrix (α and γ grains) during tensile process, although δ phase presents as fibrous morphology with coarse size.

Microstructure and mechanical properties of a novel medium Mn steel with Cr and Mo microalloying

In this paper, the effects of Cr and Mo microalloying on the microstructure evolution, mechanical properties, and austenite stability of 0.2C-7Mn steel were investigated at different intercritical annealing temperatures plus the same tempering treatment. Both the yield strength and ultimate tensile strength can be enhanced by nano-sized precipitates and solution hardening effect owing to Cr-Mo addition, while the ductility is well retained. The precipitates are mainly distributed on the ferrite matrix of the (Cr, Mo)-added steel, and directly produce a precipitation hardening effect. Due to the precipitation hardening and solution hardening effects of ferrite, the austenite in the (Cr, Mo)-added steel exhibits a lower mechanical stability, and more strain partitioning among the austenite and ferrite can be borne by the austenite than that in the reference steel, resulting in an accelerating transformation of austenite and a prevailed transformation-induced plasticity effect in the (Cr, Mo)-added steel.

On the Prediction of Stacking Fault Energy on Medium MN Steels

On the Prediction of Stacking Fault Energy on Medium MN Steels

The role of austenite stability on the change of fracture mode in a dual-phase medium Mn steel having a lamellar microstructure

Reduction of austenite stability in lamellar-type medium Mn steels consisting of α′-martensite and γ-austenite causes a change in fracture mode from ductile to brittle. In the present study, we investigate the contributing mechanisms to the appearance of such behavior using electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) analyses. The transformation-induced plasticity (TRIP) effect occurs actively during deformation, and the deformation-induced α′ is found to form epitaxially on the pre-existing α′. As a result, the extension of α′ lamella area is observed after tensile deformation. Furthermore, the previous α′/γ interphase boundary remains as a low-angle boundary composed of an array of [110] edge dislocations between the pre-existing α′ and the deformation-induced α′. Increased γ stability by carbon partitioning leaves γ lamella in between neighboring α′ lamellae. Our results show that the variation in fracture mode depending on γ stability can be correlated to different TRIP responses which leads to the changes of dislocation pile-up stresses at the high angle boundaries during tensile deformation. • Improved γ stability changes fracture mode from brittle to ductile mode in a dual-phase medium Mn steel. • TRIP effect is occurred by the migration of α′/γ interface to γ side, leading to the increases in α′ fraction and pile-up length. • The traces of former α′/γ interfaces remain as low angle boundaries characterized by arrays of edge-typed dislocations. • Brittle fracture is a consequence ofhigher flow stress and larger dislocation pile-up length induced by rapid TRIP rate.

Active Layer Morphology Engineering of All-polymer Solar Cells by Systematically Tuning Molecular Weights of Polymer Donors/Acceptors

In all-polymer solar cells (APSCs), number-average molecular weights (Mn) of polymer donors and polymer acceptors play an important role in active layer morphology and photovoltaic performance. In this manuscript, based on a series of APSCs with power conversion efficiency of approaching 10%, we study the effect of Mn of both polymer donor and polymer acceptor on active layer morphology and photovoltaic performance of APSCs. We select poly[4-(5-(4,8-bis(5-((2-butyloctyl)thio)thiophen-2-yl)-6-methylbenzo[1,2-b:4,5-b']dithiophen-2-yl)thiophen-2-yl)-5,6-difluoro-2-(2-hexyldecyl)-7-(5-methylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole] (CD1) as the polymer donor and poly[4-(5-(5,10-bis(2-dodecylhexadecyl)-4,4,9,9-tetrafluuoro-7-methyl-4,5,9,10-tetrahydro3a,5,8,10-tetraaza-4,9-diborapyren-2-yl)thiophen-2-yl)-7-(5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazole] (PBN-14) as the polymer acceptor. The Mn of polymer donor CD1 are 14.0, 35.5 and 56.1 kg mol-1, respectively. The Mn of polymer acceptor PBN-14 are 32.7, 72.4 and 103.4 kg mol-1, respectively. To get the desired biscontinueous fibrous network morphololgy of the polymer donor/polymer acceptor blends, at least one polymer should have high or medium Mn. Moreover, when the Mn of polymer acceptor is high, the active layer morphology and APSC device performance are insensitive to the Mn of polymer donor. The optimal APSC device performance is obtained when the Mn of both the polymer donor and the polymer acceptor are medium. These results provide a comprehensive and deep understanding on the interplay and the effect of Mn of polymer donors and polymer acceptors in high-performance APSCs.

Influence of Cooling Process Routes after Intercritical Annealing on Impact Toughness of Duplex Type Medium Mn Steel

To apply the duplex type low-carbon medium-manganese steel to the hot/warm-forging and -stamping products, the influence of cooling process routes immediately after intercritical annealing such as air-cooling (AC) and isothermal transformation (IT) processes on the impact toughness of 0.2%C-1.5%Si-5%Mn (in mass %) duplex type medium-Mn (D-MMn) steel was investigated. Moreover the microstructural and tensile properties were also investigated. The AC process increased the volume fraction of reverted austenite but decreased the thermal and mechanical stability in the D-MMn steel, compared to the IT process. The AC process increased the tensile strength but decreased the total elongation. The Charpy V-notch impact value and ductile-brittle transition temperature were deteriorated by the AC process, compared to the IT process. This deterioration of the impact toughness was mainly related to the reverted austenite characteristics and fracture mode.

Influence of microstructural morphology on the continuous/discontinuous yielding behavior in a medium manganese steel

The continuous and discontinuous yielding behaviors are observed in the medium Mn steels with granular and lath-typed microstructures, respectively. The continuous yielding behavior is attributed to the unique lath-typed microstructure, in which the sub-division of unit cells by laths leads to a more homogeneous distribution of GNDs and thus a higher GND density.

Effect of cold deformation extent and ART annealing duration on the microstructure and mechanical properties of a medium manganese steel

The effect of cold rolling reduction extent and austenite reverted transformation (ART) annealing duration on the microstructure evolution and the ensuing mechanical properties of a medium Mn steel was investigated. Cold rolling reduction was varied from 18.18% to 72.72% followed by ART annealing for different time duration (2–6 h) at a pre-determined temperature of 650 °C. The retained austenite fraction was found to increase with cold deformation extent as well as with the ART annealing duration. With increase in the extent of cold reduction the morphology of microstructure gradually shifted from a lamellar structure to globular, owing to recrystallization of the heavily deformed sample. Cold reduction level of ~36.36% was found to be the threshold limit for morphology transition from lamellar to globular. A two-fold increase in the dislocation density of the retained austenite phase was observed after 6 h of ART annealing of the 18.18% cold deformed specimen in comparison to the 72.72% cold deformed specimen. The higher dislocation density in the less deformed specimen was associated with the recrystallization and a higher austenite to martensite transformation during the quenching post ART annealing process. A greater yield point elongation in samples deformed to a higher extent was revealed, which was completely absent in samples deformed to a lesser extent. Variation in strength-ductility combination was correlated with the retained austenite content and its stability, both of which varied with cold deformation and the duration of ART annealing.

Kinetics Study on Low-Temperature Tempering of Martensitic Phase in Medium Mn Steel Weldment During Paint-Baking Heat Treatment

Kinetics Study on Low-Temperature Tempering of Martensitic Phase in Medium Mn Steel Weldment During Paint-Baking Heat Treatment

A new insight into annealing parameters in tailoring the mechanical properties of a medium Mn steel

Generally, the mechanical properties of medium Mn steels are mainly tailored by adjusting temperature and duration of intercritical annealing (IA). However, in the present study, it is found that the temperature-rise-period in IA also plays a remarkable role in the control of the mechanical properties of the investigated medium Mn steel. A longer temperature-rise-period is conducive to the formation of austenite grains at different temperatures, resulting in more heterogeneous distribution of Mn element within an individual austenite grain and among different austenite grains. This heterogeneous microstructure leads to the difference in the yield strength among austenite laths and a slow strain-induced-martensite-transformation kinetics. Eventually, the yield strength and total elongation of the investigated steel has been significantly increased while high ultimate tensile strength is maintained. This study provides a new insight into comprehensively understanding the role of annealing parameters so as to improve the mechanical properties of medium Mn steels in industrial production.