high-Mn Steel Research Articles

A correlation between grain boundary character and deformation twin nucleation mechanism in coarse-grained high-Mn austenitic steel

In polycrystalline materials, grain boundaries are known to be a critical microstructural component controlling material’s mechanical properties, and their characters such as misorientation and crystallographic boundary planes would also influence the dislocation dynamics. Nevertheless, many of generally used mechanistic models for deformation twin nucleation in fcc metal do not take considerable care of the role of grain boundary characters. Here, we experimentally reveal that deformation twin nucleation occurs at an annealing twin (Σ3{111}) boundary in a high-Mn austenitic steel when dislocation pile-up at Σ3{111} boundary produced a local stress exceeding the twining stress, while no obvious local stress concentration was required at relatively high-energy grain boundaries such as Σ21 or Σ31. A periodic contrast reversal associated with a sequential stacking faults emission from Σ3{111} boundary was observed by in-situ transmission electron microscopy (TEM) deformation experiments, proving the successive layer-by-layer stacking fault emission was the deformation twin nucleation mechanism, different from the previously reported observations in the high-Mn steels. Since this is also true for the observed high Σ-value boundaries in this study, our observation demonstrates the practical importance of taking grain boundary characters into account to understand the deformation twin nucleation mechanism besides well-known factors such as stacking fault energy and grain size.

High strength-high ductility medium Mn steel obtained through CALPHAD based alloy design and thermomechanical processing

There is considerable research interest in developing medium Mn steels as part of the 3rd generation of advanced high strength steels, mainly due to the possibility to achieve high tensile strength-high ductility combination at an affordable cost. In the present work, we have designed a steel chemistry and its thermomechanical processing route based on a computational approach based on CALPHAD with an objective to achieve tensile strength and uniform elongation in excess of 1000 MPa and 20%, respectively. The influence of alloying elements on factors such as weldability, coatability and formability are also taken into account while designing the steel chemistries. The alloy chemistry was optimised to achieve at least 50% of retained austenite and a stacking fault energy in the range 12–20 mJ m −2 to activate transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effects. The designed steel was cast and thermomechanically processed to produce ultrafine-grained ferrite and austenite microstructure with ~50% of retained austenite. Analysis of local composition by atom probe tomography revealed preferential partitioning of Mn and C to austenite during intercritical annealing, thereby enhancing its stability. The optimised microstructure resulted in tensile strength and uniform elongation in excess of 1300 MPa and 26%, respectively. The stress-strain curves revealed serrations and a staircase type of strain hardening. A detailed study of the strain hardening behaviour showed that this can be attributed to the occurrence of discontinuous TRIP effect and deformation twinning in the austenite. This was further corroborated by transmission electron microscopy of the deformed samples which showed the presence of nano-twins in the austenite phase while the XRD and EBSD of the deformed samples showed a significant drop in the austenite fraction post deformation.

Influence of annealing temperature on both mechanical and damping properties of Nb-alloyed high Mn steel

Influence of annealing temperature on both mechanical and damping properties of Nb-alloyed high Mn steel

Investigation of solidification modes of high Manganese steels

ABSTRACT The solidification modes of high manganese steels were measured by experiments and comparatively calculated by different predicted methods. The DSC experiments indicate the same phase transformation sequence as the isothermal solidification experiments. It is demonstrated that the solidification modes of the Fe–0.028C–21.1Mn–5.72Al, Fe–0.28C–21.0Mn–5.80Al and Fe–0.64C–21.0Mn–5.80Al alloys are F mode, FA1 mode and FA2 mode. The change of heating/cooling rate influences the position and shape of the peaks in the DSC curves but has no effect on the solidification mode. By comparison with different calculation methods, the equilibrium model is shown to be a valuable tool to predict the solidification sequence of the high-Mn steel, from which the critical carbon range for the Fe–C–20Mn–5Al alloy is calculated to be 0.062∼0.427 wt-%.

On the mechanism of cross-slip induced dislocation substructure formation in an high-Mn steel

The underlying mechanism of dislocation substructure formation in a tensile deformed fine-grained high-Mn steel is reported using transmission electron microscopy. A cross-slip assisted dislocation truncation mechanism was revealed that formed strings of dislocation loops at early strain, which were also retained at fracture strain. Planar glide producing a Taylor lattice was also observed. The dislocation plasticity based on a cross-slip based mechanism delayed dynamic recovery in stage C to manifest an uniform strain hardening. Such a mechanism producing good combination of high strength and ductility is observed for the first time in high-Mn steel.

The microstructural effects on the mechanical response of polycrystals: A comparative experimental-numerical study on conventionally and additively manufactured metallic materials

Metal additive manufacturing (AM) provides an unprecedented opportunity to explore extreme microstructural regimes and their associated effects on the mechanical response of polycrystals. In this work, a single alloy (composition) belonging to the high-Mn steels family was processed by a conventional thermo-mechanical treatment and two different AM processes. This resulted in three different materials with strikingly diverse microstructures with respect to the microstructural size and polarity. The influence of disparities in the statistical microstructural features arising from the different processing conditions on the (macroscopic) mechanical response was investigated through an experiment-simulation approach. The (as-processed/as-built) microstructures were analyzed on the meso-scale for derivation of a set of parameters, namely meso-structure descriptors, which adequately describe the meso-structural heterogeneity features. To link the microstructure and mechanical properties, we used a physics-based crystal plasticity modeling approach in the framework of full-field polycrystal homogenization. The meso-structure descriptors together with the submeso-scale/constitutive microstructural parameters were used to simulate the (anisotropic) mechanical response of the aforementioned materials. Then, for a reduced-order quantitative assessment of the effect of microstructural polarity on the anisotropic mechanical response, the plastic anisotropy indices were defined and calculated using the simulation results. This study provides new insights into the primary microstructural effects (the size and polarity effects) on the mechanical (stress and strain hardening) response of polycrystals as well as the propensity of micro-mechanisms accommodating polycrystal plasticity in highly different microstructural regimes, particularly those typically obtained by metal AM.

Effects of Al addition on tensile properties of partially recrystallized austenitic TRIP/TWIP steels

Abstract Austenitic high-Mn steels are regarded as a promising candidate for high-strength cold-rolled steels because their combination of strength and ductility improves greatly by combined effects of twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP). Although it is well known that Al plays key roles in tensile properties of austenitic TWIP steels, its effects in austenitic TRIP or TRIP/TWIP steel are still unclear yet. In this study, three austenitic steels (composition; 0.4C–15Mn–1Si-(0,0.5,1)Al-0.3Mo-0.5V (wt.%)) were fabricated, and the effect of Al alloying on microstructures and tensile properties were investigated in relation to the deformation behavior with TRIP and TWIP mechanisms. A partial recrystallization was conducted for enhancing the yield strength, which was characterized with electron backscatter diffraction (EBSD) grain orientation spread maps. The present steels showed 1 GPa of yield strength achieved by partial recrystallization with the precipitation of (V + Mo) complex carbides. Particularly in the non-Al-alloyed steel, the e-martensite formed in the early deformation stage, and the martensitic transformation continued until the failure, thereby resulting in the highest tensile strength (1.5 GPa) along with the highest strain hardening.

AlN-Assisted Internal Oxidation Behavior in an Al-Containing High Mn Steel

The role of AlN on the internal oxidation behavior was investigated in an Al-containing high Mn steel. The sequential formation of internal oxidation and AlN+γ-matrix layers on the γ-matrix was observed using an EPMA, an SEM, and a TEM after the heat-treatment in air. The heterogeneous nucleation of MnAl2O4 on the AlN was identified in between the internal oxidation and AlN+γ-matrix layers. MnAl2O4 shows the [2-1-10]AlN//[110]MnAl2O4 and (0002)AlN//(-11-1)MnAl2O4 orientation relationship with the pre-existing AlN by sharing Al atoms. The coherency between MnAl2O4 and AlN and the evolution of oxides in an Al-containing high Mn steel were further discussed.

Embrittlement Mechanism in Simulated Heat-Affected Zone of Cryogenic High-Mn Steel

Embrittlement Mechanism in Simulated Heat-Affected Zone of Cryogenic High-Mn Steel

Temperature-induced wear transition in ceramic-metal composites

Wear degradation is a key parameter to evaluate the performance of ceramic-metal composites applied in manufacturing and mining industries. We selected a typical composite (high Mn steel + 50 wt.% TiC) as the model system and studied its wear behavior. The integrity and damage of TiC ceramic particles were analyzed under different temperatures and atmospheres. A critical temperature of ~125 °C at which transitions of wear mechanism occurred was observed for the first time. Below this temperature, hardness dominated the wear behavior as the presence of oxygen induced surface decarburization of ceramic particles with an increased wear rate. The thermodynamics of oxygen-assisted decarburization were investigated using the CALPHAD (CALculation of PHAse Diagram) approach, and the resultant hardness reduction of TiC particles was clarified using density functional theory (DFT) calculations. At temperatures above 125 °C, toughness became the governing factor as the ceramic-metal deformation incompatibility induced fracture of ceramic particles. The formation of nano-crystalline oxide tribolayers improved the fracture toughness of ceramic particles and restrained the wear loss, which was elucidated using DFT calculations. Thus, the current work has delineated the competitive micro-mechanisms consisting of surface decarburization and toughening of ceramic particles, which could be used to interpret the wear behavior of ceramic-metal composites.

Mesoscopic nature of serration behavior in high-Mn austenitic steel

We have thoroughly clarified the mesoscopic nature of serration behavior in a high-Mn austenitic steel in connection with its characteristic localized deformation. A typical high-Mn steel, Fe-22Mn-0.6C (wt. %), with a face centered cubic (FCC) single-phase structure was used in the present study. After 4 cycles of repeated cold-rolling and annealing process, a specimen with a fully recrystallized microstructure having a mean grain size of 2.0 μm was obtained. The specimen was tensile tested at room temperature at an initial strain rate of 8.3 × 10−4 s−1, during which the digital image correlation (DIC) technique was applied for analyzing local strain and strain-rate distributions in the specimen. Obtained results indicated that a unique strain localization behavior characterized by the formation, propagation and annihilation of deformation localized bands, so-called Portevin–Le Chatelier (PLC) bands, determined the global mechanical response appearing as serration on the stress-strain curve. In addition, the in-situ synchrotron XRD diffraction during the tensile test was utilized to understand what was happening in the material with respect to the PLC banding. Lattice strain of (200) plane nearly perpendicular to the tensile direction dropped when every PLC band passed through the beam position, which indicated a stress relaxation occurred inside the PLC band. At the same time, the dislocation density increased drastically when the PLC band passed the beam position, which described that the material was plastically deformed and work-hardened mostly within the PLC band. All the results obtained consistently explained the serration behavior in a mesoscopic scale. The serration behavior on the stress-strain curve totally corresponded to the formation, propagation and annihilation of the PLC band in the 22Mn-0.6C steel, and the localized deformation, i.e., the PLC banding, governed the characteristic strain hardening of the material.

Effect of Nb addition on the solidification structure of Fe–Mn–C–Al twin-induced plasticity steel

AbstractThe solidification microstructure of an Fe–Mn–C–Al high-Mn steel with added Nb was studied by optical microscopy and scanning electron microscopy. The results showed that the size of the columnar and equiaxed grains in the solidified structure of the high-Mn steel without added Nb was large. When Nb was added, the size of the equiaxed and columnar grains in the solidification structure of the high-Mn steel decreased. The twin-induced plasticity in the high-Mn steel with added Nb clearly refined the grain size of the solidified structure and simultaneously increased the equiaxed crystallization rate. The high-melting-point second-phase particles increased the heterogeneous nucleation rate, which was beneficial for grain refinement. A large amount of Nb precipitates from the solidification process increased the equiaxed crystallization rate of the solidified microstructure. The increased crystallization rate affected the growth direction of the columnar and equiaxed grains during solidification and indirectly decreased the probability of crack formation, which improved the quality of the twin-induced plastic steel.

Characterization of inclusions and second-phase particles in high-Mn TWIP steels microalloyed with Ti, Ti/B, Nb, V and Mo, in as-solutioned condition

AbstractIn recent years there has been an increase in the field of research of advanced steels that have excellent mechanical properties combining high strength with excellent ductility. Within this range of advanced steels are the stable austenitic phase steels at room temperature of twinning induced plasticity known as TWIP. An important aspect to highlight about TWIP steels is their addition with different microalloying elements, generally less than 0.20 wt. %, which are forming precipitated phases such as carbides, nitrides and carbonitrides, and directly or indirectly control and/or modify microstructure and mechanical properties in these steels. Microalloying elements can cause a higher degree of hardening due to the formation of precipitates and grain refinement. The present research work studies the inclusions and second-phase particles formed in Fe–21Mn–1.3Si–1.6Al TWIP steels microalloyed with Ti, Nb, V, Mo and Ti/B in as-solution condition. TWIP steels melted in induction furnace were homogenized and hot-rolled at 1200 °C with reduction of 60 %. Subsequently, rolled plates were solubilized at 1100 °C followed by water quench. Thermodynamics-based predictions of inclusions and second-phases of different TWIP steels were carried out using JMatPro®V.9.1.2. Metallographic characterization was carried out by light optical and scanning electron microscopies (LOM, SEM), while second-phase particles characterization was performed using energy dispersion spectroscopy (SEM-EDS). Also, Vickers microhardness tests were carried out in accordance to ASTM E92 standard. In general, results showed the formation of inclusions of AlN and MnS at higher temperatures, which act as nuclei points for the precipitation particles of each type of microalloying element (TiN, TiC, Nb (C, N), VC and MoC) at lower temperatures. The studied TWIP steels exhibit similar microhardness values, since the microalloying elements are mostly dissolved in solid solution. The TWIP steels microalloyed with V and Ti exhibited the highest microhardness values.

Calculation and Analysis of the Structure and Viscosity of B2O3-Regulated CaO-Al2O3-Based Mold Fluxes

The high content of aluminum in the steel reacts with the CaO-Si2O-based mold fluxes, resulting in deterioration of the mold slag physical and chemical properties, which cannot be applied to the continuous casting molten slag casting process of high-Mn high-Al steel Herein, the thermodynamic and structural properties of low-reactivity CaO-Al2O3-based mold fluxes were investigated. The thermodynamic properties were studied based on the first principles of quantum mechanics. The results show that the formation of stable structures of B-O and O-B-O in the mold fluxes was beneficial to reduce the probability of structural interconnection, degree of polymerization, and viscosity of the molten slag. The increase in the ratio of CaO/Al2O3 = 0.88–2 led to an increase in the O2− concentration. O2− entered the [AlO4] structure to form a stable structure of [AlO6] and [AlO5], wherein [AlO6] was more stable than [AlO5], reducing the degree of polymerization of the network structure. When cosolvent content B2O3 = 2%–10%, a simple layered structure of [BO3] was formed, and the particle migration resistance, break temperature, and viscous activation energy of the mold fluxes were reduced, while the corrected optical basicity of mold fluxes was gradually increased.

TiC–High Mn Steel‐Bonded Cermets with Improved Strength and Impact Toughness

TiC–high Mn steel‐bonded cermets are prepared by powder metallurgy techniques. The effects of (Ti0.5W0.5)C or WC on the microstructure, hardness, transverse rupture strength (TRS), and impact toughness (IM) are investigated. The results show that regardless of the additives used, the hardness and TRS of cermets increase with the increase in W content. However, IM is not directly proportional to the W content. Peak IM value is obtained at 5% WC addition. The highest IM of 15.12 J cm−2 is measured from the cermet with (Ti0.5W0.5)C additive with a hardness and TRS of 64.8 HRC and 2510 MPa, respectively. Liquid phase sintering occurs at vacuum sintering. Black core–gray outer rim structure is formed in cermets with WC additives and white inner rims appear in cermets with higher W content. White core–gray rim structure is formed in cermets with (Ti0.5W0.5)C additives. The impact fracture mode of the cermets is the intergranular fracture accompanied by the tearing edges of the binder. The tearing of the high Mn steel binder is the main source of IM. IM peaks are attributed to the contradictory effects of decreased fraction and refined binder microstructure.

Micro-segregation Study of Directional Solidified Fe–Mn–C–Al TWIP Steels

Chemical variation occurring during the solidification process can cause undesirable effects on the material property, especially for alloys with high solute content. The current study investigated the distributions of solute elements in directional solidified Fe–Mn–C–Al TWIP steels and compared the micro-segregation with various pulling rates and C contents using the phase field model and the grid analysis based on the data performed by EPMA, which could provide advice for mitigating micro-segregation in high Mn steel. The results indicated that C and Mn preferentially segregated toward the liquid phase, while Al entered the solid phase. The occurrence of solid-state diffusion was observed, which led to a homogenization effect in the solid phase during solidification. The pulling speed increase made a more severe micro-segregation because the change of pulling speeds had more effect on the SDAS than on the local solidification time; thus, the dimensionless diffusion time decreased with increasing pulling rates, representing a more serious micro-segregation. For a C content increasing from 0.06 to 0.68 wt pct, the micro-segregation level of C and Al reduced first and then increased, which was caused by the change in the solidification mode. The continuous accumulation of solute elements and the slower solute element diffusion in austenite in combination led to the presence of more severe micro-segregation in the case of one-phase solidification. Besides, a C content increase could attract Mn, resulting in more serious Mn micro-segregation.

Effect of Heat Treatment and Drawing on High-Manganese Steel Pipe Welded by Gas Tungsten Arc

This study investigated the effect of post-weld processes including annealing and drawing on the microstructure and mechanical properties of high-Mn steel pipes welded by gas tungsten arc welding. The weld metal showed a solidified microstructure having coarse and elongated grains due to coalescence of columnar dendrite into welding heat direction. After post-annealing, the solidified microstructure changed into equiaxed grains due to recrystallization and grain growth. Mn segregation occurred during welding solidification and caused lower stacking fault energy (SFE) in the Mn-depleted region. Although ε-martensite formation in the as-welded state and during deformation was expected due to decreased SFE of the Mn-depleted zone, all regions showed a fully austenitic phase. The annealing process decreased strength due to grain coarsening but increased ductility. The drawing process increased strength of weld metal through work hardening. All pipes showed decreasing strain rate sensitivity (SRS) with deformation and negative SRS after certain strain levels. It was confirmed that negative SRS is related to less formation of mechanical twinning at a higher strain rate. This work provides fundamental insights into manufacturing a high-Mn steel pipe and manipulating its properties with annealing and drawing processes.

Quantitative Evaluation of Hydrogen Effects on Evolutions of Deformation-Induced ε-Martensite and Damage in a High-Mn Steel

We investigated the effects of hydrogen on e-martensite-related damage evolution (crack/void initiation and growth) in Fe-Mn-Si-base austenitic steel using tensile tests after gaseous hydrogen charging at 100 MPa. Specifically, we evaluated the quantitative hydrogen effects on e-martensite fraction and associated damage evolution with different strains and strain rates. Hydrogen charging increased the probability of e-martensite-related damage initiation and deteriorated micro-damage arrestability, which decreased elongation. The primary factor causing the detrimental hydrogen effects on resistance to damage evolution was the promotion of deformation-induced γ-e martensitic transformation. An increasing strain rate from 10−4 to 10−2 s−1 suppressed the γ-e martensitic transformation and correspondingly increased elongation. Interestingly, the e-martensite fraction near the fracture surface did not change with increasing strain rate, but the area fraction of the brittle-like fracture region decreased. This fact implied that the brittle-like fracture at the low strain rate, which had a longer time for damage growth, was assisted by stress-driven hydrogen diffusion near the crack/void tips.

Strategies to increase austenite FCC relative phase stability in High-Mn steels

Several strategies to increase the FCC austenite stability compared to BCC and HCP martensites have been tested and are discussed. The relative stability of the different phases was analyzed by studying the effects of: a) grain size, b) antiferromagnetic ordering of the austenite, c) thermal cycling through the FCC-HCP transition, d) plastic deformation of the austenite and e) combined effects. As a first step, the effect of decreasing the grain size was analyzed in Fe–Mn alloys for Mn contents smaller than 18 wt.%, where BCC and HCP martensites compete in stability. Formation of the BCC phase is inhibited for 15 wt.% and 17 wt.% of Mn for grain sizes smaller than 2 μm. This enabled, for the first time at these compositions, the measurement of the Néel temperature of the austenite using specific heat and magnetic measurements. A comparison of the obtained transition temperatures with accepted models is discussed. The effect of modifying the grain size on the FCC-HCP transition temperatures was also analyzed for 15 wt.% and 17 wt.% Mn contents showing a complete HCP inhibition for grain sizes smaller than 200 nm. A nucleation model for the HCP martensite is considered which includes an additional resistance to the transformation term depending on the austenitic grain size. Additional combined effects on the FCC stabilization are discussed like the interaction between the antiferromagnetic ordering and the introduction of defects by thermal cycling through the martensitic transformation. The analysis can be easily applied to systems with a larger number of components. Results obtained in the Fe–Mn–Cr system are also presented.

Kinetic study of Mn vacuum evaporation from Mn steel melts

Considering the interference of Mn volatilization on the composition control during the vacuum treatment of medium/high Mn steel melt. The kinetic analysis of Mn volatilization from steel melt in various initial Mn contents, holding times, temperature and pressure regimes were investigated in lab scale experiments. The results show that Mn evaporation is accelerated significantly with the increase of temperature and initial Mn content in steel melt or with the decrease of chamber pressure. Furthermore, the logarithm of Mn content in steel melt has a linear function relationship with the holding time, indicating the first-order kinetic reaction of Mn evaporation. In addition, the overall mass transfer coefficient of Mn significantly decreases from 2.176E−05 to 5.932E−06 m s−1 with the increase of the initial Mn content from 2% to 30%. Moreover, high temperature and low partial pressure can obviously improve the overall mass transfer coefficient of Mn. Simultaneously, the apparent activation energy for Mn evaporation from steel melt is calculated to be 324.55 kJ mol−1. The theoretical kinetics model is established to fully analyze the evaporation mechanics of Mn, which is consistent with experimental results. In particular, with the increase of initial Mn content, the rate-limiting step transforms from mass transfer in the steel melt boundary layer to gas phase mass transfer, and mass transfer resistance in the gaseous phase is reached 91.22% with initial Mn content of 30%. Besides, the Mn evaporation process at high temperature and high partial pressure is determined by the mass transfer in the steel melt boundary layer and gas phase, respectively. This work systematically clarifies the factors influencing Mn loss in vacuum refining on high quality Mn steel manufacture.