Medium Mn Research Articles

Elevated Temperature Tensile Behavior of a Nb-Mo Microalloyed Medium Mn Alloy under Quasistatic Loads

The elevated temperature tensile behavior of a Nb-Mo microalloyed medium steel was investigated over the −50 to 150 °C temperature range. The ultimate tensile strength was significantly reduced with increasing deformation temperature, but both YS (yield strength) and EI (total elongation) values changed slightly. The best product of UTS (ultimate tensile strength) and EI (~59.5 GPa·%) can be achieved at the deformation temperature of 50 °C, implying an excellent combination of strength and ductility. Furthermore, the change in strain hardening rate as a function of deformation temperature was further explained by the following two aspects: the dependence of mechanical stability of retained austenite on deformation temperature as well as the dependence of deformation mechanism on deformation temperature. Theoretical models and experimental observations demonstrate that the dominant deformation mechanism of the present medium Mn steel changed from the single transformation-induced plasticity (TRIP) effect at −50 to 50 °C to the multiple TRIP + TWIP (twinning-induced plasticity) effect at 50–150 °C.

Modeling of Flux Reaction and Mixing in Continuous Casting Mold of Medium Mn Steel

Modeling of Flux Reaction and Mixing in Continuous Casting Mold of Medium Mn Steel

Austenite Decomposition of a Lean Medium Mn Steel Suitable for Quenching and Partitioning Process: Comparison of CCT and DCCT Diagram and Their Microstructural Changes.

The present work deals with the dilatometric study of a hot-rolled 0.2C3Mn1.5Si lean medium Mn steel, mainly suitable for the quenching and partitioning (Q&P) heat treatment in both hot-rolled or cold-rolled condition, subjected to a variation of austenitization temperature. These investigations were performed in a temperature range of 800–1200 °C. In this context, the martensite transformation start temperature (Ms) was determined as a function of austenitization temperature and in turn obtained prior austenite grain size (PAGS). The results show rise in prior austenite grain size due to increasing austenitization temperature, resulting in elevated Ms temperatures. Measured dilatation curves were confronted with the metallographic analysis by means of scanning electron microscopy (SEM). The present paper also focuses on the construction of a continuous cooling transformation (CCT) and deformation continuous cooling transformation (DCCT) diagram of the investigated lean medium Mn steel in a range of cooling rates from 100 to 0.01 °C/s and their subsequent comparison. By comparing these two diagrams, we observed an overall shift of the DCCT diagram to shorter times compared to the CCT diagram, which represents an earlier formation of phase transformations with respect to the individual cooling rates. Moreover, the determination of individual phase fractions in the CCT and DCCT mode revealed that the growth stage of ferrite and bainite is decelerated by deformation, especially for intermediate cooling rates. Microstructural changes corresponding to cooling were also observed using SEM to provide more detailed investigation of the structure and present phases identification as a function of cooling rate. Moreover, the volume fractions obtained from the saturation magnetization method (SMM) are compared with data from X-ray diffraction (XRD) measurements. The discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements. In that regard, the volume fraction of retained austenite increased with a decrease of cooling rate as a result of larger volume fraction of ferrite and bainite. The hardness of the samples subjected to the deformation was slightly higher compared to non-deformed samples. The reason for this was an evident grain refinement after deformation.

Interactions of nanoprecipitates, metastable austenite, and Lüders banding in an ultra-low carbon medium Mn steel with abnormal strength dependence of hydrogen embrittlement

A hierarchical microstructure, containing an ultrafine-grained matrix with mixed lamellar and equiaxed morphology, metastable austenite, and intermetallic nanoprecipitate, was designed to improve the hydrogen embrittlement (HE) resistance of ultra-low carbon medium Mn steels. Compared with the warm-rolled (WR) state, the warm-rolled-tempering (WRT) sample shows a simultaneous enhancement of yield strength (YS) and tensile ductility, but lower total elongation loss (∼3%) after hydrogen charging, thus exhibiting an abnormal strength dependence of HE. The microstructural analysis with numerical calculation proves that an additional tempering not only introduces a heterogeneous distribution of Ni(Al, Mn) precipitations, but also results in a reverse γ→α transformation with Mn partitioning, thus contributing to a self-stabilization of austenite with refined grain size and enriched element stabilizer. On one hand, the introduction of intermetallic Ni(Al, Mn) precipitations could be beneficial for the intrinsic hydrogen capture and storage, although its limited number density only contributes little to impeding hydrogen diffusion. Moreover, the introduction of nanoprecipitate can improve the strain compatibility and suppress the strain concentration in ferrite phases, which is consistent with a lower kernel average misorientation (KAM) value of the WRT sample after deformation. Meanwhile, the increased austenite stability leads to a shorter yield point elongation (YPE) and an effective transformation-induced plasticity (TRIP) effect at large strains, and the transformation product of nano-lamellar α’-martensite from self-stabilized austenite contributes little to the hydrogen-accelerated strain localization in weak α/γ interfaces of the WRT sample. In contrast, the low HE resistance of the WR sample is mainly attributed to the enhanced martensitic transformation process during the propagation of Lüders banding, which leads to a strong localized stress concentration in α/α’ interfaces, and the premature failure due to hydrogen-enhanced decohesion effect (HEDE). The interactions of nanoprecipitate, metastable austenite, and Lüders banding provide new insight into the design of high-strength steel with excellent HE resistance.

Revealing the interdependence of microstructure evolution, micromechanics and macroscopic mechanical behavior of multi-phase medium Mn steels

The mechanical response, on a microscopic and macroscopic level, and the deformation-induced martensitic transformation (DIMT) were investigated in multi-phase medium Mn steels (MMnS) with 6, 8 and 9 wt% Mn using in situ high-energy synchrotron x-ray diffraction during tensile loading. Prior to the in-situ analysis, a similar heat treatment finishing with an intercritical annealing was imposed on all MMnS. The initial microstructure prior to tensile loading was investigated by electron backscatter diffraction analysis. The volume fraction of austenite (γ) after the heat treatment decreases from 60.2% to 50.7%, and 23.6% with increasing Mn content from 6 to 8 and 9 wt% Mn, respectively. This is mainly due to the difference in the formation of athermal α′-martensite. Athermal ε-martensite also formed in the MMnS with 8 and 9 wt% Mn, whereas no athermal ε-martensite formed in the MMnS with 6 wt% Mn. The alloys have quite different deformation behavior due to the different microstructures, and the majority of the load is carried by the phase that forms a continuous network throughout the steel, which in turn influences the DIMT. These results reveal the importance of assessing both phase-specific strain/stress and the inherent mechanical stability of the austenite in order to predict the macroscopic mechanical properties of the steel. As an example, this is witnessed by the comparison of MMnS9 and MMnS8. Austenite in MMnS9 bears about half the load as compared to austenite in MMnS8 during early deformation due to a continuous network of athermal α′-martensite resulting in significant load partitioning from austenite to α′-martensite. Thus, the mechanical driving force for DIMT in MMnS9 is reduced and therefore causes lower DIMT kinetics in MMnS9 than in MMnS8, even though MMnS9 has lower inherent austenite stability.

A novel process combining thermal deformation and intercritical annealing to enhance mechanical properties and avoid Lüders strain of Fe-0.2C–7Mn TRIP steel

This paper proposed a novel process combining thermal deformation and intercritical annealing (IA) to enhance mechanical properties and avoid Lüders strain of Fe-0.2C–7Mn steel. The experiment results show that the prior microstructure, including prior austenite grain (PAG) size, dislocation density, carbides precipitation and stored deformation energy before IA have great impacts on the nucleation and growth of austenite during IA, and ultimately influence the morphology, grain size, volume fraction and stability of retained austenite (RA) after IA. The low temperature and relatively high strain rate deformation condition sample (referred as 1-LT/RHS) with film-like and block-like mixture morphology structure after IA exhibits the largest product of ultimate tensile strength and total elongation (UTS × TE) value (53 GPa∙%), due to the double-stage discontinuous transformation-induced plasticity (TRIP) effects. Block-like austenite dominates the 1st-stage plastic deformation, while film-like austenite dominates the 2nd-stage. Because the film-like austenite is more stable than the block-like, which is confirmed by the interrupted tensile tests and nanoindentation investigation. Besides, compared to the cold-rolled sample (referred as 3-CR) with single block-like structure after IA, the film-like ferrite in sample 1-LT/RHS initiate dislocation plasticity earlier than the block-like ferrite, resulting in the disappearance of Lüders strain. This is because the different formation mechanisms, leading to the initial dislocation density before deformation in film-like grains much higher than in block-like grains. Moreover, the procedure of this novel process is simple with low industrial cost, which can provide a new guidance for development of lower cost fabricating technology of medium Mn TRIP steel.

Constitutive and transformation kinetics modeling of [formula omitted]-, [formula omitted]-Martensite and mechanical twinning in steels containing austenite

In the present study, a physically based constitutive and transformation kinetics model of stress assisted (SA) and strain induced (SI) ε-, α′-Martensite and twinning in steels containing austenite, is presented. The deformation behavior of γ-Austenite, α′-Martensite, α- and δ-Ferrite is modeled in terms of the dislocation density. The macroscopic elastoplastic response of the material in uniaxial tension is calculated with respect to the partitioned stress and strain in each phase using a homogenization method. Inelastic strain accommodation below the yield strength of austenite is considered due to the SA transformation. The kinetics of SA and SI ε-, α′-Martensite and twinning in austenite, are calculated in terms of the ε-, α′- and twin embryo nucleation rate and the subsequent growth of shear bands, promoted by the applied stress and strain, respectively. Heterogeneous nucleation of α′-Martensite only on ε and twin shear band intersections is considered, presenting a stress state dependence. Austenite stability against TRIP and TWIP is modeled as a function of the austenite stacking fault energy and the separation distance of Shockley partial dislocations. Fitting the model to experimental stress-strain data allows for the prediction of α′-, ε-Martensite and mechanical twin fractions, without a prior notion of the transformation kinetics. A MATLAB implementation and an Extended Methodology section are provided as Supplementary Material. The model could be used to aid in the design of novel alloys with exceptional properties, like medium Mn steels.

Phase transition and defect relaxation behavior in a medium manganese steel

Phase transition and defect relaxation behavior of a Fe-7Mn-0.1C medium manganese steel have been systematically studied mainly by using internal friction (IF) technique. From temperature-dependent IF curves in both heating and cooling processes, two relaxation IF peaks (the P1 located at ~ 200 °C on heating and the P2 at ~ 550 °C on cooling) and two phase transition IF peaks (the Ptr1 located at ~ 670 °C on heating and the Ptr2 at ~ 300 °C on cooling) were observed. The P1 peak disappears when the cementite (θ) particles and/or retained austenite (γR) exist in the microstructure, and is associated with the Snoek-Kê-Köster relaxation (SKK relaxation). The P2 peak is suggested to be originated from grain boundary relaxation in austenite (γ). The formation mechanisms of the Ptr1 and the Ptr2 come from reverse austenitic transformation (RAT) and martensitic transformation (MT), respectively. In addition, the apparent asymmetry of the Ptr1, the variation of peak height, the difference between the Ptr1 and Ptr2 peaks, and the effect of IF studies on the mechanical properties are discussed in detail in the present work. These results provide a valuable reference for the design of high-performance medium Mn steels.

Non-uniform distribution and strengthening effect of Cu precipitates enclosed in austenite during intercritical annealing in a medium Mn steel

The evolution of Cu particles which form in a Cu-bearing medium Mn steel, whose composition is Fe-0.16C-5.62Mn-2.01Cu-1.95Ni-0.65Al-0.49Si in mass%, was investigated with focus on Cu particles in the reversed austenite. Almost all Cu particles were formed in the martensite matrix during heating to annealing temperature, and calculation of phase boundaries and miscibility gaps in the multi-component alloy indicated that Cu particles in annealed martensite were probably incorporated into reversed austenite with the migration of α/γ interface during annealing. DICTRA simulation and STEM-EDX analysis revealed that the partitionless mode of austenite growth prevailed at an earlier stage and it switched to a mode accompanying alloy partitioning. Cu particles in the ferrite (or annealed martensite) matrix partially dissolved and the rest coarsened during annealing while Cu particles in austenite were on the way to total dissolution, but the rate was slow. As a result, the size and volume fraction were greater near the α/γ interface in austenite and they were opposite in the center of reversed austenite. During deformation, retained austenite provided a remarkable TRIP effect. Cu particles in the ferrite matrix caused strengthening significantly and those in austenite could have a function of delaying the decreasing in strength due to slow diffusivity of alloy elements and dissolution rate.

Improving strength–ductility synergy in medium Mn steel by combining heterogeneous structure and TRIP effect

The deformation mechanisms of a novel medium Mn steel (Fe–0.14C–7.8Mn–1.65Al–0.11Zr, wt%) were investigated through mixed multiphase preservation-intercritical annealing route. A heterogeneous duplex microstructure consisting of plate-like and equiaxed austenite and ferrite with multiple morphologies and multiscale and Mn inhomogeneous distribution was obtained after intercritical annealing at 680 °C for 10 min. The heterostructured sample exhibited excellent strength–ductility synergy with a yield strength of 1022 MPa, an ultimate tensile strength of 1280 MPa, and a total elongation of 53%. Comparative analysis revealed that hetero-deformation-induced (HDI) stress was generated during tensile deformation. This stress was primarily responsible for the superhigh yield strength of the heterostructured sample. In addition, the HDI stress could initially trigger premature martensitic transformation of austenite in the grain or phase boundary region and delay the martensite transformation of untransformed austenite to a large strain range due to the shielding effect of the formed core–shell structure, which comprised an untransformed core austenite and a shell α'-martensite. Multiple strengthening and ductility-enhancing mechanisms involving transformation-induced plasticity effect, twin-induced plasticity effect, HDI synergistic hardening, and dislocation strengthening occurred sequentially during deformation, leading to the extraordinary strength–ductility combination of the heterostructured medium Mn steel.

Enhancing the properties of Al–Ni added medium Mn steel by tailoring B2–NiAl precipitates through aging treatment

In the present work, the effect of aging treatment and the role of precipitates in improving the strength-ductility combination of intercritically annealed Fe–8Mn–4Al-3.5Ni-0.6Si-0.2C (wt.%) medium Mn steel has been studied. The steel has been intercritically annealed at 750 °C for 1 h and subjected to two different aging treatments, one-stage aging treatment at 500 °C for 3 h, and two-stage aging comprising first stage at 650 °C for 3 h followed by water quenching and second stage at 500 °C for 3 h. The microstructure, precipitates characteristics, mechanical properties and deformation behavior of the aged samples have been investigated. It was observed that after one-stage and two-stage aging treatment, the strength-ductility product of the steel improved significantly. The one-stage aging led to the formation of nano-sized NiAl precipitates in both ferrite and austenite, which resulted in an increment of yield strength (YS) by 300 MPa, with no notable decrease in ductility, as compared to the intercritically annealed sample. The two-stage aging led to phase-selective precipitation of NiAl of varying sizes in ferrite and austenite during the first and second stages of aging respectively. A combination of TRIP-TWIP effect and precipitation strengthening by the NiAl in austenite phase was observed in the two-stage aged sample, which led to sustained three-stage work-hardening during deformation resulting in improved elongation and strength-ductility combination.

Microstructural evolution and strain hardening behavior of a novel two-stage warm rolled ultra-high strength medium Mn steel with heterogeneous structures

A heterostructured medium Mn steel with duplex phases (ferrite and austenite) and multiple morphologies (lath-typed and granular) is fabricated by a novel two-stage warm rolling process and subsequent intercritical annealing. The obtained sample exhibits excellent mechanical properties with the yield strength and ultimate tensile strength of 905 MPa and 1320 MPa, respectively, and the total elongation of 45.6%, the mechanical properties are higher than those of the hot rolled samples and the one-stage warm rolled samples. The microstructural evolution during the two-stage warm rolling process and annealing are characterized and the strain partitioning behavior of the material is investigated. It is indicated that the outstanding mechanical properties and the strain hardening ability rely on the combination of the transformation induced plasticity (TRIP) effect and the hetero-deformation induced (HDI) hardening.

Heterogeneous Multiphase Microstructure Formation Through Partial Recrystallization of a Warm-Deformed Medium Mn Steel During High Temperature Partitioning

Heterogeneous Multiphase Microstructure Formation Through Partial Recrystallization of a Warm-Deformed Medium Mn Steel During High Temperature Partitioning

Improving Pse in Medium Mn Steel by Synergy of Heterogenous Structure and Trip Effect

Improving Pse in Medium Mn Steel by Synergy of Heterogenous Structure and Trip Effect

Microstructural evolution and mechanical properties of Fe-0.14C-5Mn-1Al-Ce steel during intercritical annealing

The effects of intercritical annealing time on microstructure evolution and mechanical properties of a novel medium Mn steel (Fe-0.14C-5Mn-1Al-Ce) were investigated. The microstructure composed of lamellar ferrite and retained austenite (RA)/α’-martensite mixed phases after intercritical annealing. With the extension of intercritical annealing holding time, the volume fraction of RA first increases and then decreases, and RA is always formed at the high-angle grain boundaries of the ferrite. Both the product of Rm*A and the total elongation increase as the volume fraction of RA increases. The greater volume fraction of RA, the greater total elongation and Rm*A. The enrichment of carbon in RA was investigated by XRD and DICTRA. As intercritical annealing holding time increases, the carbon concentration in austenite decreases, while the change of the carbon concentration will affect the volume fraction of RA after intercritical annealing.

A strong and ductile medium Mn steel manufactured via ultrafast heating process

Ultrafast heating (UFH) at the rates of 10–300 °C/s was employed as a new strategy to anneal a cold-rolled 7wt% Mn steel, followed by the immediate cooling. Severely deformed strain-induced martensite and lightly-deformed thermal martensite, both had been already enriched with C and Mn before, transformed to fine and coarse austenite grains during the UFH, leading to the bimodal size distribution. Compared with the long intercritical annealing (IA) process, the UFH processes produced larger fraction of RA grains (up to 37%) with a high density of dislocation, leading to the significant increase in yield strength by 270 MPa and the product of strength and elongation up to 55 GPa% due to the enormous work hardening capacity. Such a significant strengthening is first attributed to high density dislocations preserved after UFH and then to the microstructural refinement and the precipitation strengthening; whilst the sustainable work hardening is attributed to the successive TRIP effect during deformation, resulting from the large fraction of RA instantly formed with the bimodal size distribution during UFH. Moreover, the results on the microstructural characterization, thermodynamics calculation on the reverse transformation temperature and the kinetic simulations on the reverse transformation all suggest that the austenitization during UFH is displacive and involves the diffusion and partition of C. Therefore, we propose that it is a bainite-like transformation.

Simultaneous Improvement of Yield Strength and Ductility in Medium Mn Steel by Combining Gradient Structure and Trip Effect

Simultaneous Improvement of Yield Strength and Ductility in Medium Mn Steel by Combining Gradient Structure and Trip Effect

Suppression of Serrated Flow in Medium Mn Steel Under Pulsed Electric Current

Suppression of Serrated Flow in Medium Mn Steel Under Pulsed Electric Current

On the mechanism of Mn partitioning during intercritical annealing in medium Mn steels

To understand how solute Mn in the reverted austenite is able to rapidly partition to near-equilibrium concentration values during the typically short intercritical annealing (IA) of medium Mn steels (MMS), reverse transformation kinetics were simulated numerically. We applied a variety of assumptions both for the diffusivity of Mn in austenite and martensite/ferrite and for the interface mobility. These simulation results were compared with measurements of the Mn concentration gradient near the interface as measured by atom probe tomography. Our inclusion of an interface mobility term, with physical origins from the interfacial dissipation energy, reduced the interphase interface migration rate, resulting in much higher values of the interfacial Mn concentration than in the scenario whereby diffusion-controlled transformation kinetics dominate. These results have led us to propose that the austenite/ferrite interface exhibits a fluctuational interface mobility during IA. This new phenomenological description of the interface migration achieves the best fit to the measured fluctuations of the Mn concentration gradients near the interface. We suggest that the fluctuational mobility arises from encounters between the interface and the regions of crystal containing precipitates and/or microstructural defects.

Strengthening Mechanism and Carbide Precipitation Behavior of Nb-Mo Microalloy Medium Mn Steel.

This study investigates the strengthening mechanism and carbide precipitation behavior of medium Mn steel with Nb-Mo microalloy after cyclic quenching and austenite reverse transformation treatment. The results show that the Nb/Mo element not only precipitates (Nb,Mo)C in the grains, hindering the movement of dislocations and increases the strength, but also segregates at the austenite/ferrite grain boundary, thus delaying the transformation from austenite to ferrite. In addition, a large amount of nano-scale cementite is retained after cyclic quenching and austenite reverse transformation, which has a positive effect on the proportion of retained austenite in medium Mn steel. Moreover, the carbides with small size and low Mn content are dissolved, and the decomposed C and Mn content are beneficial to the nucleation of austenite during the intercritical annealing process at a temperature of 690 °C.