high-Mn Steel Research Articles

Strengthening Triggered by Deformation Twins in a Hot-Rolled High-Mn Steel: The Role of Stacking Fault Energy

Strengthening Triggered by Deformation Twins in a Hot-Rolled High-Mn Steel: The Role of Stacking Fault Energy

Recent Progress in Laser Powder Bed Fusions Processes of Advanced High-Strength Steels

This review is focused on the perspectives of the application of Advanced High Strength Steels (AHSSs) in the field of additive technologies directed at the laser powder bed fusion/selective laser melting processes. In principle, AHSSs require significant attention due to their promising mechanical properties for usage in the automotive industry towards reducing the weight of vehicles. Although additive manufacturing represents a promising perspective towards expanding the industrialization of AHSSs in a wider area of their applications, they have not been sufficiently investigated concerning their usage in LPBF/SLM processes. AM techniques enable the fabrication of complex machine parts, including those with a cellular structure, which can contribute to further reducing the weight of vehicles or structures. Maraging steels have recently attracted the attention of researchers, and today are a common grade of steel produced by LPBF techniques. The other group of AHSSs are high-Mn steels with an austenitic matrix characterized by the TRIP and TWIP effects. Less published research has been conducted on medium-Mn steels, which require additional intercritical annealing and preheating during printing. Moreover, the advanced bainitic steels and low-density, high-strength steels represent a new window for further research into the use of the LPBF processes for their fabrication.

Internal oxidation behavior and its effect on the surface crack formation in a Fe-Mn-Al-C high Mn steel

The correlation between internal oxidation and surface crack was investigated in high Mn steel. Time- and temperature-dependent internal oxidation behaviors are examined in the temperature ranges of 1000– 1250 °C, revealing faster and deeper oxidation along grain boundaries than in grain. Oxides at the internal oxidation layer are confirmed to be MnO and MnAl2O4. The grain boundary penetration depths of the internal oxide increase from 13 to 34 μm as the temperature increases. The tensile crack on the surface of the specimen was evaluated after 5 % straining with 10−3/s strain. Temperature-dependent crack formation probability (LAC/LT) increases also from 0.4 to 1.6 % as the temperature increases. The evolution of the internal oxidation layer during the hot compression test was investigated to clarify its role in surface crack formation. The transition of the internal oxide layer to the external oxide layer was observed. Exfoliation of the external oxidation layer was also confirmed by further compression. Driving force calculation on the internal and external oxides reveals that the abundant oxygen supply originating from the cracking of oxide and thickness reduction of the internal oxide layer transforms the internal oxide layer to scale during the compression. High-resolution analysis of the grain boundary crack reveals the cracking of grain boundary MnAl2O4. The comparison between the grain boundary oxides’ penetration depth and the crack formation behavior clarifies the grain boundary internal oxide’s crucial role in the surface crack formation in a high Mn steel. This study provides vital insights into the surface crack formation mechanism in a Fe-Mn-Al-C-based high Mn steel during hot-rolling processes, highlighting the significance of grain boundary internal oxidation in production.

Automated detection and mapping of crystal tilt using thermal diffuse scattering in transmission electron microscopy

Quantitative interpretation of transmission electron microscopy (TEM) data of crystalline specimens often requires the accurate knowledge of the local crystal orientation. A method is presented which exploits momentum-resolved scanning TEM (STEM) data to determine the local mistilt from a major zone axis. It is based on a geometric analysis of Kikuchi bands within a single diffraction pattern, yielding the center of the Laue circle. Whereas the approach is not limited to convergent illumination, it is here developed using unit-cell averaged diffraction patterns corresponding to high-resolution STEM settings. In simulation studies, an accuracy of approximately 0.1 mrad is found. The method is implemented in automated software and applied to crystallographic tilt and in-plane rotation mapping in two experimental cases. In particular, orientation maps of high-Mn steel and an epitaxially grown La0.7Sr0.3MnO3-SrTiO3 interface are presented. The results confirm the estimates of the simulation study and indicate that tilt mapping can be performed consistently over a wide field of view with diameters well above 100 nm at unit cell real space sampling.

Correlation of austenite grain size with ε-/α′-martensitic transformation and mechanical properties in a high-manganese damping steel

One of the core studies on low stacking fault energy (SFE) high-manganese steel is to maintain excellent damping capacity while achieving high strength and toughness. The austenite grain size (AGS) in high-Mn steel affects its SFE and even the strength-plasticity and damping capacity of the steel, but the detailed mechanism related to martensitic transformation has not been clarified. In this study, Fe-17Mn high-Mn damping steel is used as the material to obtain austenite grains of different sizes by controlling the cold rolling and annealing processes. By using in-situ tensile testing, damping capacity testing, and various microscopic analytical techniques, the influence of AGS on the thermal/deformation induced martensitic transformation, work hardening behavior, mechanical properties and damping capacity of the steel is mainly studied. The results indicate that the refinement of austenite grain increases the SFE of high-Mn damping steel and reduces the amount of thermal induced ε-martensite (εth) and stacking faults (SFs). The deformation induced martensitic transformations (DIMTs) in high-Mn damping steel during tensile deformation include γ→εD, εth→α′ and εD→α′ transformation. The deformation induced ε-martensitic transformation is more helpful to improve the work hardening rate than deformation induced α′-martensitic transformation. Due to the concentration of strain partitioning at grain boundaries, twin boundaries and phase boundaries, refinement of austenite grain can promote deformation induced ε-martensitic transformation and improve the steel strength. However, excessively small austenite grains can suppress deformation induced α′-martensitic transformation leading to a significant decrease in elongation. Under the synergetic effects of grain refinement strengthening and multiple martensitic transformations, when the average AGS is about 4.0 μm, the steel has high strength and high plasticity, with a tensile strength of 919 MPa, a yield strength of 518 MPa and an elongation of 46.4 %. The product of tensile strength and elongation is up to 42.6 GPa·%. At low or high strain amplitudes, the damping capacity of this steel shows an opposite trend with the refinement of AGS. The damping capacity of the steel gradually increases with grain refinement at low strain amplitudes of 0.01 %–0.065 %, mainly due to the reduction of the content of weak pinning points. At high strain amplitudes of 0.065 %–0.1 %, the damping capacity of the steel gradually decreases for the reduction of damping sources with the refinement of grain size.

Temperature Effect on Deformation Mechanisms and Mechanical Properties of Welded High-Mn Steels for Cryogenic Applications.

High-manganese steel (high-Mn) is valuable for its excellent mechanical properties in cryogenic environments, making it essential to understand its deformation behavior at extremely low temperatures. The deformation behavior of high-Mn steels at extremely low temperatures depends on the stacking fault energy (SFE) that can lead to the formation of deformation twins or transform to ε-martensite or α'-martensite as the temperature decreases. In this study, submerged arc welding (SAW) was applied to fabricate thick pipes for cryogenic industry applications, but it may cause problems such as an uneven distribution of manganese (Mn) and a large weldment. To address these issues, post-weld heat treatment (PWHT) is performed to achieve a homogeneous microstructure, enhance mechanical properties, and reduce residual stress. It was found that the difference in Mn content between the dendrite and interdendritic regions was reduced after PWHT, and the SFE was calculated. At cryogenic temperatures, the SFE decreased below 20 mJ/m2, indicating the martensitic transformation region. Furthermore, an examination of the deformation behavior of welded high-Mn steels was conducted. This study revealed that the tensile deformed, as-welded specimens exhibited ε and α'-martensite transformations at cryogenic temperatures. However, the heat-treated specimens did not undergo α'-martensite transformations. Moreover, regardless of whether the specimens were subjected to Charpy impact deformation before or after heat treatment, ε and α'-martensite transformations did not occur.

High-temperature oxidation behavior and subsurface phase transformation of high-Mn high-Al steel

High-Mn steels are widely used in engineering applications because of their high strength, high hardness, and excellent toughness. In the industrial production of high-Mn steel, the steel is easy to be oxidized during the homogenization process, which may affect the properties of high-Mn steel. In this work, high-Mn high-Al steel with composition of (25–32)wt-%Mn-(8–11)wt-%Al-1wt-%C-Fe was prepared, the oxidation behavior and subsurface phase transformation of steel at 800 °C–1200 °C were investigated by thermogravimetric analysis, scanning electron microscope and X-ray diffraction. The results show that the curve of oxidation weight gain with time is in good agreement with linear equations below 900 °C, which obey the parabolic rate law in the temperature range of 1000 °C–1200 °C. The steel matrix would be covered by the oxide scale earlier at higher temperatures, which decreases the oxidation reaction rate and makes the turning point of the oxidation reaction rate reach faster at higher temperatures. The Mn-depleted zone resulting from the selective oxidation of Mn is formed in the steel matrix epidermis, leading to the subsurface phase transformation, which may deteriorate the steel properties.

Numerical simulation and experiment on hot forging of high-Mn steel turnout core

The core of railway turnout is generally cast from high manganese steel, but the casting turnout core will produce porosity, cracks, and other casting defects. By analyzing the structure characteristics and forming difficulties of the turnout core, finite element software simulates the forging process, and the influence of forging parameters on the distribution law of the medium effect field, temperature field, die load, and billet damage is systematically studied. In addition, the main parameters (forging temperature, forging speed, and friction between billet and die) are designed and optimized by the response surface method, and the comprehensive optimal solutions are determined. Using the optimum parameters to forge, the forging force of the die and the damage of the forging are reduced. Besides the forming effect, the mechanical properties of the forging are also improved in different amplitudes, and the comprehensive mechanical properties are far better than the core of the casting turnout.

Study on Mn Volatilization Behavior During Vacuum Melting of High-manganese Steel

As an alloying element in steel, manganese can considerably enhance the mechanical properties of structural steel. However, the Mn volatilisation loss in vacuum melting is severe because of the high saturated vapour pressure, resulting in an unstable Mn yield and Mn content fluctuation. Therefore, a systematic study of the volatilisation behaviour of Mn in vacuum melting is required to obtain a suitable Mn control process to achieve precise control of Mn composition, thereby providing a theoretical basis for industrial melting of high-Mn steel. In order to explore the Mn volatilization behavior, the volatilization thermodynamics and volatilisation rate of Mn, as well as the influence factors are discussed in this study. The results shows that Mn is extremely volatilised into the vapour phase under vacuum, the equilibrium partial pressure is closely related to Mn content and temperature. With an increase in the Mn content, a higher C content has a more obvious inhibitory effect on the equilibrium partial pressure of Mn. The maximum theoretical volatilisation rate of Mn shows a linear upward trend with an increase in Mn content. However, a higher C content has a more obvious effect on the reduction of the maximum theoretical volatilisation rate with the increase of Mn content. This study provides an improved understanding of Mn volatilisation behaviour as well as a theoretical foundation for consistent Mn yield control during the vacuum melting process of high-Mn steel.

Hot Ductility of High-Mn Steel with Niobium and Titanium

Hot Ductility of High-Mn Steel with Niobium and Titanium

The Dominant Role of Recrystallization and Grain Growth Behaviors in the Simulated Welding Heat-Affected Zone of High-Mn Steel.

Single-pass-welding thermal cycles with different peak temperatures (Tp) were reproduced by a Gleeble 3800 to simulate the heat-affected zone (HAZ) of a Fe-24Mn-4Cr-0.4C-0.3Cu (wt.%) high manganese austenitic steel. Then, the effect of Tp on the microstructure and mechanical properties of the HAZ were investigated. The results indicate that recrystallization and grain growth play dominant roles. Based on this, the HAZ is proposed to categorize into three zones: the recrystallization heat-affected zone (RHAZ) with a Tp of 700~900 °C, the transition heat-affected zone (THAZ) with a Tp of 900~1000 °C, and the coarse grain heat-affected zone (CGHAZ) with a Tp of 1000~1300 °C. The recrystallization fraction was 29~44% in the RHAZ, rapidly increased to 87% in the THAZ, and exceeded 95% in the CGHAZ. The average grain size was 17~19 μm in the RHAZ, slightly increased to 22 μm in the THAZ, and ultimately increased to 37 μm in the CGHAZ. The yield strength in the RHAZ and THAZ was consistent with the change in recrystallization fraction, while in the CGHAZ, it satisfied the Hall-Petch relationship with grain size. In addition, compared with the base material, the Charpy impact absorbed energy at -196 °C decreased by 22% in the RHAZ, but slightly increased in the CGHAZ. This indicates that the theory of fine grain strengthening and toughening is not entirely applicable to the HAZ of the investigated high-Mn steel.

Insights into Non-free Opening of Ladles: Filler Sand Chemistry Modifications for High Mn Steel Billet Casting

Insights into Non-free Opening of Ladles: Filler Sand Chemistry Modifications for High Mn Steel Billet Casting

Effects of cooling process on the microstructure and mechanical properties of a novel high-V high-Mn TWIP steel for cryogenic storage tank applications

In the present work, a novel high-V high-Mn twinning induced plasticity (TWIP) steel for cryogenic storage tank applications was developed. Furthermore, innovative two-stage cooling processes encompassing ultra-fast cooling and the subsequent furnace cooling were proposed. Effects of cooling process on the microstructure and mechanical properties of the experimental steel were investigated. It was observed that with the change in cooling process (the initial temperature of furnace cooling rised gradually), the dwell time of hot-rolled plates above 600 °C was extended. The extension in dwell time facilitated the precipitation of secondary phase particles, leading to the change in average diameter and volume fraction of secondary phase particles. The yield strength increased with the increasing initial temperature of furnace cooling due to the combined effect of small-sized secondary phase particles inside the grains and large-sized ones located at the grain boundaries. The occurrence of intergranular cracking, primarily caused by the presence of large-sized secondary phase particles at the grain boundaries, was the main factor responsible for the significant reduction in total elongation. The small-sized secondary phase particles inside the grains inhibited deformation twinning, while the large-sized ones at the grain boundaries weaken the binding force between grain boundaries, which ultimately resulted in a decrease in cryogenic impact toughness. The implementation of two-stage cooling processes effectively mitigated the trade-off between yield strength and cryogenic impact toughness, offering a novel approach and valuable insights for optimizing the strength and toughness characteristics of cryogenic high-Mn steels.

Cooling Curves of Concentrated Alloys, Steelmaking Slag, and Lithium Iron Phosphate

AbstractOver the past few years, a variety of melting casting projects have been performed, in collaboration with industries and universities. Concentrated alloys and nonmetallic compounds were studied including high-Si cast iron, high-Mn steel, Al-added stainless steel, high-entropy alloy, Permalloy 80 Ni alloy, Stellite Co alloy, steelmaking slag, and lithium iron phosphate (LFP). A melt synthesis of LFP was developed. Cooling curves were measured using the thermal analysis cup method and differential thermal analysis. In addition, the phase diagrams of alloys were calculated using Thermo-Calc. This paper draws on some examples of the measured cooling curves to illustrate the thermal behaviors of melts, along with a quick review of each project.

The effect of κ-carbides on high cycle fatigue behavior of a Fe-Mn-Al-C lightweight steel

Numerous studies have demonstrated the viability of lightweight Fe-Mn-Al-C steels for exhibiting an improved balance of high strength and high ductility in automotive applications. However, their high-cycle fatigue behaviour has been scarcely studied. This work examines the effect of κ-carbides formed during the aging treatment on the high-cycle fatigue performance of an austenitic Fe-29Mn-8.7Al-1C (wt. %) steel. The material is studied in solution-treated, under-aged, and peak-aged conditions. High-cycle fatigue tests and analysis of fatigue fracture surfaces were performed using SEM and EBSD techniques. The results indicate satisfactory high-cycle fatigue performance in the aged material, somewhat better than for high Mn steels. Fatigue crack formation and growth occur predominantly via a quasi-cleavage mechanism along the [1 1 1] crystallographic planes, which is also a plane for planar glide and the formation of persistent slip bands during plastic deformation. The nanoscale intragranular κ-carbides in the aged samples interact with the gliding dislocations, resulting in the shearing of nanoscale κ-carbides in a weakly coupled regime. The resistance of particles to shearing is determined by their size, volume fraction, and antiphase boundary energy (γAPB), which vary during the aging process. The aged Fe-29Mn-8.7Al-1C steel significantly improves the fatigue strength as the formation of persistent slip bands is delayed due to an additional energy barrier related to the shearing of the κ-carbides. This improvement peaks in the under-aged condition and decreases with further aging time.

Reconsideration of the dislocation arrest model for dynamic strain aging in C-bearing high Mn steel with low stacking fault energy

In the present study, we investigated the behavior of type A serration in the tensile flow curves measured at temperatures ranging from 273 K to 323 K with a strain rate of 1 × 10−3 /s, using high Mn steels with varying N contents. As a result, it was found that the critical engineering strain for the onset of type A serrations (ecA) increased with an increase in the N content at a given tensile temperature. This finding could not be explained by the conventional dislocation arrest model, which has been used to account for type A serrations occurring in C-bearing high Mn steels without N. Therefore, we attempted to explain the result by introducing the dynamic formation of obstacles to dislocation slip, such as mechanical twins and Lomer-Cottrell locks, during tensile deformation, a factor not considered in the conventional dislocation arrest model. However, the specific reason for the increase in the ecA value due to the addition of N was not elucidated. Therefore, we modified the dislocation arrest model based on the concept that the combination and cross-slip of screw partial dislocations, which accumulate at the obstacles, become more favorable due to an increase in stacking fault energy resulting from the addition of N. The modified dislocation arrest model successfully explained the influences of not only N content but also strain rate and tensile temperature on the ecA value in both C-bearing high Mn steels with and without N.

Cross-slip of extended dislocations and secondary deformation twinning in a high-Mn TWIP steel

Twinning-induced plasticity (TWIP) steels have become important materials in industry owing to the good combination of strength and ductility and high strain hardening rate. The excellent mechanical properties are highly related to the glide of dislocation and deformation twinning. However, the cross-slip behavior of the extended dislocation and the mechanism of deformation twinning are still controversial. Here, the partial dislocation motion and austenite twinning of a high-Mn steel at the early stage of deformation were investigated using in-situ tensile transmission electron microscope (TEM) technique. Results show that a large number of plane glide and cross-slip of extended dislocations can occur at the early stage of deformation. Extended dislocation nodes can be formed as a result of the reaction between adjacent extended dislocations on the same glide plane. In-situ tensile TEM experiments confirm two cross-slip models of partial dislocation: (1) the Friedel-Escaig model, cross-slip based on constriction of extend dislocation and re-dissociation; (2) the Fleischer model, cross-slip involving Lomer-Cottrell dislocation. Based on experimental results and energy calculations, it can be confirmed that the formation mechanism of austenite primary deformation twin induced by partial dislocations is different from that of secondary deformation twin. Grain boundary emits partial dislocation into the grain to form stable stacking fault which induces austenite primary deformation twin. The formation of secondary deformation twin is related to cross-slip of extended dislocation. Only the cross-slip of an extended dislocation containing a 90° partial dislocation can induce the formation of secondary deformation twin by introducing the Frank partial dislocation.

Texture Analyses and Mechanical Behavior of a High-Mn Steel Subjected to Different Pass Number of Simple Shear

Texture Analyses and Mechanical Behavior of a High-Mn Steel Subjected to Different Pass Number of Simple Shear

Enhancing strength–ductility synergy in high-Mn steel by tuning stacking fault energy via precipitation

Deformation-induced twinning or martensitic transformation can improve the work-hardening capability of alloys with face-centered cubic (FCC) structures and suppress strain localization. The stacking fault energy (SFE) of alloys plays a key role in determining deformation mechanisms and mechanical properties. This study developed V-bearing high-Mn steel with a tensile strength of 1288 MPa and uniform elongation of 36 % by tactfully designing the composition. Precipitation of V-carbides was selected to strengthen the steel and tune the global SFE of the matrix by settling carbon. Stronger work-hardening capability due to lower SFE and finer twin/matrix lamellae provided the steel with good ductility, while precipitation strengthened it.

Application of Surface Cracking Process to Improve Impact Toughness of High Mn Steel Using Surface Cracking Process

Application of Surface Cracking Process to Improve Impact Toughness of High Mn Steel Using Surface Cracking Process