Medium Mn Research Articles

Critical role of Lüders banding in hydrogen embrittlement susceptibility of medium Mn steels

The effect of Lüders banding on hydrogen embrittlement (HE) susceptibility of medium Mn steels was investigated by tuning the degree of yield point elongation (YPE). It was found that the steel with a larger YPE was more susceptible to HE. A larger YPE led to a higher local flow stress and a higher fraction of strain-induced martensite, which in turns resulted in more H-induced cracks in the Lüders banding area and the consequent catastrophic failure. It thus suggests that Lüders banding and the associated localized deformation play a key role in influencing the overall HE susceptibility of medium Mn steels.

Microstructure-mechanical property evaluation and deformation mechanism in Al added medium Mn steel processed through intercritical rolling and annealing

The present work investigates the microstructure evolution, mechanical properties and deformation mechanism in medium Mn high Al steel processed through intercritical rolling and subsequent intercritical annealing treatment at different temperatures. The annealed samples possessed a multi-phase microstructure consisting of intercritical ferrite/martensite, austenite and δ-ferrite. However, the morphology of the phases varied with annealing temperature. The result shows that annealing at a temperature of 730° and 780 °C led to the development of bimodal grain structure consisting of fine laths and equiaxed ultra-fine-grains (UFG) of ferrite-austenite; whereas annealing at 830 °C led to fully equiaxed coarse ferrite-austenite grains. An excellent combination of strength and ductility (product of ultimate tensile strength and elongation) as high as 56 GPa% was obtained in the 780 °C annealed sample. The chemical composition and grain size of austenite was found to be critical factors governing its stability. A mixture of lath and equiaxed austenite grains, having appropriate stability, in 780 °C annealed sample led to sustained four-stage strain hardening during deformation. Multiple work hardening mechanisms involving transformation induced plasticity (TRIP) effect, twin induced plasticity (TWIP) effect and discontinuous TRIP effect were found to occur sequentially in the equiaxed and lath-type austenite during the deformation that led to the extraordinary strength ductility combination.

Temperature-Dependent Macroscopic Mechanical Behaviors and Their Microscopic Explanations in a Medium Mn Steel

Plastic instability (e.g., Luders band and PLC band) is a phenomenon frequently observed in cold-rolled medium Mn steels. In this study, through a number of quasi-static uniaxial tensile tests with digital image correlation (DIC) measurement of the strain rate field, we found the medium Mn steel (Fe-0.14C-7.14Mn-0.23Si wt. pct with approximately 33 vol pct of retained austenite and 67 vol pct of ferrite) exhibited distinct competition/coordination between the Luders band and the PLC band at different deformation temperatures: (1) at 25 °C and 55 °C, a Luders band was followed by several PLC bands; (2) at 100 °C, fracture occurred right after the Luders band formation, without the presence of a PLC band; (3) at 250 °C, only PLC bands were observed. We propose that these macroscopic behaviors can be explained by the temperature-dependent conditions for the formation/appearance and propagation of Luders bands and PLC bands. The propagation of Luders bands becomes difficult when the temperature increases as less hardening is provided by martensitic transformation. In contrast, the appearance and propagation of PLC bands become easier when the temperature increases and/or the plastic strain becomes large (16.5 and 18.9 pct in this material at the deformation temperatures of 25 °C and 55 °C, respectively), as the difference in the average velocities of solute atom diffusion and dislocation gliding is reduced. Either condition for Luders bands or for PLC bands will be satisfied at different deformation temperatures or plastic strains, thus leading to the competition/coordination between them.

The study on martensite morphology in the stir zone and its influence to impact toughness during friction stir welding medium–Mn ultrahigh strength steel

A 6 mm thickness medium-Mn ultrahigh strength steel was friction stir welded (FSW) with various rotation speed at constant of welding speed in this study. The microstructural evolution and impact toughness of the stir zone were investigated in a systematic manner. The microstructural observation results suggested that the metastable austenite in base metal disappeared after FSW, and the stir zone of FSW samples contained various morphology of martensite (i.e., well tempered martensite, less tempered martensite and common martensite). The content of well tempered martensite and less tempered martensite in the stir zone was decreased with the increase of rotation speed, and the martensite blocks and prior austenite grain size were gradually coarsened. It is suggested that the plastic deformation and the small prior austenite grain size promoted the formation of well tempered martensite in the stir zone at low rotation speed. The well tempered martensite could effectively release the stress concentration by interfaces separation with other martensite of different morphology during impact test, which greatly benefited the impact toughness. Furthermore, the small martensite blocks could also improve the impact toughness. The impact toughness of the stir zone was decided by the martensite blocks size and the content of different morphology of martensite phase.

Extra work hardening in room-temperature quenching and partitioning medium Mn steel enabled by intercritical annealing

The present work applies an intercritical annealing process on a room-temperature quenching and partitioning (RT Q&P) medium Mn steel. It is found that the intercritical annealing maintains the amount of austenite while improves its mechanical stability. The reverse transformation during the intercritical annealing competes with the martensitic transformation during the quenching, resulting in a similar quantity of austenite after the intercritical annealing. Both reverse transformation and martensitic transformation help to refine the austenite grain size and facilitate C enrichment, enhancing the austenite stability. The intercritical annealing improves both ultimate tensile strength and uniform elongation of RT Q&P medium Mn steel, which is ascribed to an extra work hardening resulted from the improved austenite stability.

Microstructure evolution and enhanced mechanical properties of a Nb–Mo microalloyed medium Mn alloy fabricated by a novel cyclic quenching treatment

Microstructure evolution and austenite stability of a Fe-6.0Mn-0.2C-1.0Al-0.05Nb-0.20Mo alloy fabricated by a novel cyclic quenching treatment were investigated. According to the experimental results, the influences of cyclic quenching treatment on retained austenite could be described as follows: (1) turning austenite grain morphology from a mixture of lamellar and equiaxed grain to the lamellar grain with the increase in cyclic quenching passes, (2) increasing austenite stability for the grain refinement (up to 0.21μm) and higher C/Mn content in austenite. In addition, cyclic quenching treatment can increase the yield strength (YS), ultimate tensile strength (UTS) and total elongation (TE) of the experimental steel up to 172 MPa, 89 MPa and 8%, respectively. The average product of UTS and TE of the experimental steel after cyclic quenching treatment could reach up to 66.6 GPa·%. Moreover, the yield point elongation increased with the increase in cyclic quenching passes.

Tuning austenite stability in a medium Mn steel and relationship to structure and mechanical properties

We study here the underlying factors that govern the stability of austenite in a medium Mn (Fe–0.18C–11Mn–3.8Al) (wt-%) steel. In this regard, a novel heat treatment involving intercritical quenching and tempering was designed to obtain high total elongation (TEL) and high ultimate tensile strength (UTS) in the cold-rolled steel. And the UTS and TEL approached 920–1150 MPa and 35–65%, respectively. The product of TEL and UTS (PSE) exceeded 40 GPa%, with a maximum value of 60 GPa%. A detailed analysis of microstructure before and after tensile deformation revealed that the TRIP effect occurred and the stability of austenite was predominantly governed by the grain sizes of austenite rather than the orientation of austenite grains. The theoretical analysis of work hardening data suggested that the superior elongation of medium Mn TRIP steel is related to the high stability of austenite and the cooperative deformation of ferrite.

On the Variants of Thermal Process in Developing Strong and Ductile Medium Mn Steel

In this contribution, we investigate the influence of three heat treatments, including intercritical annealing (IA), quenching and partitioning (Q&P) and combination of IA and Q&P (IA-Q&P), on microstructure and mechanical properties of the medium Mn steel. The steel treated by IA process has largest volume fraction of austenite, which is responsible for the longest elongation and largest energy absoprtion with operation of both transformation-induced plasticity (TRIP) effect and twinning-induced plasticity (TWIP) effects. The medium Mn steel produced by IA-Q&P treatment has a mixture microstructure with retained austenite, ferrite and lath martensite. The existence of martensite in the IA-Q&P steel makes the higher yield strength than that of the IA steel. The better uniform elongation of the IA-Q&P steel than that of the Q&P steel is ascribed to the larger volume fraction of retained austenite and higher mechanical stability of austenite. The Q&P steel has highest yield stress due to its martensite matrix. It is expected that the tensile properties of medium Mn steel can be tuned by different thermal processes (IA, Q&P and IA-Q&P) to facilitate its broad automotive applications.

Impact of Si and Al on Microstructural Evolution and Mechanical Properties of Lean Medium Manganese Quenching and Partitioning Steels

Herein, the investigation of the tensile behavior of lean medium Mn Quenching and Partitioning (Q&P) steels containing 0.2 wt% C, 4.5 wt% Mn, and additions of 1.5 wt% Si or 1.3 wt% Al is concentrated upon. By the variation of the quenching temperature (TQ), different volume fractions of primary martensite (α′prim) are adjusted, influencing the subsequent microstructural evolution during Q&P processing. Using scanning electron microscopy (SEM), the final microstructure consisting of tempered martensite (α″), retained austenite (RA), partially bainitic ferrite (αB), and final martensite (α′final) is characterized. Furthermore, interrupted tensile tests at gradually increased strains are conducted to investigate the stability of RA against strain‐induced martensite transformation (SIMT) and overall tensile behavior of lean medium Mn Q&P steels. The investigations manifest the formation of larger amounts of αB and consequently lower RA contents, when Si is substituted by Al. As an aftermath, the tendency to form α′final is significantly lower, compared with the Si‐alloyed composition, reflected in the overall stress–strain behavior. Especially in case of the Si‐alloyed samples containing RA fractions exceeding 15 vol%, an over‐accelerated SIMT is observed, inducing failures occurring prior to necking, whereas for the Al‐alloyed samples, a wider process window is obtained.

Effect of Grain Structure on the Mechanical and Corrosion Behavior of Advanced Medium Mn Stainless TWIP Steel

The influence of recrystallization annealing on the microstructure, mechanical properties and corrosion behaviour of an austenitic TWIP steel bearing medium Mn and V alloying was investigated. The steel undergone a heavy cold deformation of 65 % reduction. Subsequently, recrystallization annealing at 1000 °C and 1100 °C for 15 min was conducted to achieve different grain structures. The microstructural evolution was studied using optical microscopy and electron backscatter diffraction technique. Mechanical properties were determined using tensile tests at room temperature. Corrosion behaviour was measured by cyclic potentiodynamic polarization at 3.5 pct NaCl. For comparison, austenitic stainless-steel Type 201 was used in this study. It was observed that at 1000 °C, a non-uniform austenitic grain structure with vanadium carbides distributed throughout the matrix was obtained. However, a coarse grain structure without carbides was induced at 1100 °C. The fine-grained structure enhanced at 1000 °C exhibited higher strength and good ductility. Contrary to this, the corrosion results showed that a significant deterioration in the corrosion resistance could be observed in sodium chloride solution for the achieved structure at 1000 °C compared to 201 stainless steel.

Homogeneous elasto-plastic deformation and improved strain compatibility between austenite and ferrite in a co-precipitation hardened medium Mn steel with enhanced hydrogen embrittlement resistance

A co-precipitation hardened medium-Mn steel with respective NiAl and Cu-rich particles in dual phases (austenite and ferrite) has been designed by using a quenching-partitioning-tempering (QPT) treatment. The influences of co-precipitates on the elasto-plastic transition and strain compatibility has been investigated based on the micro-mechanical behavior of constituent phases, the evolution of lattice strain and stress components. Compared to the Cu-Free steel, in which single NiAl precipitates form in ferrite, co-precipitation in the Cu-Added steel results in an improved strain compatibility between austenite (γ) and ferrite (α). The lattice strains of both γ and α phases show similar evolutions during the entire strain regime, and a homogeneous elasto-plastic deformation occurs together with a continuous yielding and weak transient of strain hardening rate. The improved strain compatibility leads to an alleviation of strain localization in the phase interfaces, so that the overall back stress increases slowly with the tensile strain increasing during the whole plastic deformation process, although the transformation-induced plasticity (TRIP) effect of the Cu-Added steel only occurs at low strains (<5%) due to the low mechanical stability of the retained austenite. In contrast, a non-continuous yielding of the constituent phases occurs in the Cu-Free steel leads to strong strain transfer across the α/γ interfaces due to a low deformation compatibility between austenite and ferrite. High internal stress in the phase interfaces results in easy damage-crack formation, but it can be accommodated by the TRIP effect which dominates in large strains, resulting in back stress decreasing and effective stress hardening during the subsequent deformation stage. However, the introduction of hydrogen can aggravate the dislocation accumulation and strain transfer between austenite and ferrite; thus, hydrogen-induced cracks (HICs) are prone to nucleate at the α/γ interfaces while the TRIP effect is not significant, leading to premature failure and significant elongation loss (85.4%) of the Cu-Free steel. Therefore, the improved strain compatibility of the Cu-Added steel contributes to a better resistance to hydrogen embrittlement, although both medium Mn steels possess similar tensile strength (1 GPa) and elongation (25%) values.

Role of liquid Zn and α-Fe(Zn) on liquid metal embrittlement of medium Mn steel: An ex-situ microstructural analysis of galvannealed coating during high temperature tensile test

Zn assisted liquid metal embrittlement (LME) has characteristics that differ from the general characteristics of other LME systems: the formation of Fe-Zn intermetallic compounds and the high solubility of Zn in Fe. Consequently, there is a need to investigate the influence of the Fe-Zn interaction and resulting phase composition on LME behavior. However, little research has been conducted to reveal such relationships. This paper explores the influence of high temperature phases in galvannealed coating, specifically liquid Zn and α-Fe(Zn) on the LME sensitivity of medium Mn steel using a high temperature tensile test with eight combinations of temperatures (700 °C to 900 °C) and hold times (1 s, 50 s, and 100 s). The test results are categorized as LME, microcrack and LME free cases in terms of LME behavior. The study reveals that the LME sensitivity is governed by the fraction of effective liquid Zn, which is a consequence of isothermal solidification owing to the thermodynamic stability of liquid Zn, diffusivity of Zn in Fe and the partitioning coefficient of liquid Zn. Moreover, LME is influenced by the breaking of α-Fe(Zn) layer, which directs the liquid Zn into the substrate steel. Finally, a water-tank analogy is proposed to demonstrate the effect of the fraction of liquid Zn and the breaking of α-Fe(Zn) layer on LME.

Influence of hydrogen and annealing process on the microstructural evolution and fracture micromechanisms of medium Mn steel: An in-situ TEM investigation

Medium manganese (Mn) steel has excellent strength and ductility and is widely used in the automobile industry and elsewhere; however, hydrogen embrittlement (HE) is considered as one of the most serious issues of its application. In this paper, in situ transmission electron microscopy was used to study the microstructural evolution and fracture micromechanisms of medium Mn steel under the influence of hydrogen and annealing. The results indicated that two specimens with different annealing processes showed transformation-induced plasticity and twinning-induced plasticity, albeit to a varying degree due to different microstructures with distinct mechanical properties. It was shown that hydrogen affected the fracture micromechanisms, which promoted the initiation of microcracks at the boundaries of martensite transformed from austenite. Moreover, the main cracks propagate in a zigzag fashion, following hydrogen charging, due to the hardness of the martensite. These findings provide directions for improving HE resistance through microstructural regulation of medium Mn steel.

Effect of heat treatment processes on hydrogen embrittlement in hot-rolled medium Mn steels

The influence of heat treatment processes, which consist of a quenching partitioning (QP) process, intercritical annealing (IA) process and warm rolling (WR) process, on the hydrogen embrittlement (HE) of medium Mn steels is investigated. The corresponding experimental specimens are labeled QP160, IA810 and WR760, respectively. WR760 reveals the best mechanical properties, and the corresponding product strength and elongation (PSE) is 42.9 GPa•%; nevertheless, HE susceptibility is the lowest due to fine grains and microcrack propagation paths, which are parallel to rolling direction (RD), and they relax the triaxial stress conditions ahead of the main transverse crack's tip. IA810 (PSE = 29.1 GPa•%) shows the highest HE susceptibility due to the extremely unstable γ, which makes the microcrack propagation along the interface of γ(α′)/α-ferrite occur more easily when there is hydrogen. Compared to IA810, QP160 (PSE = 17.6 GPa•%) reveals a lower HE susceptibility due to the higher γ stability.

Quenching and partitioning (Q&P) processed medium Mn steel starting from heterogeneous microstructure

We examine the microstructure and tensile properties of the quenching and partitioning (Q&P) processed medium Mn steels having different starting microstructure. Heterogeneous Mn distribution led by intercritical annealing prior to the Q&P process is more effective in retaining higher fraction of austenite; a major fraction of austenite exists as Mn-rich islands surrounded by martensite. Starting with uneven Mn distribution prior to the Q&P process enhanced the austenite stability by high Mn concentration and shielding effect by neighboring martensite. It contributed to the impressive tensile properties by operating multiple plasticity mechanisms: transformation-induced plasticity (TRIP) with twinning-induced plasticity (TWIP).

Formability of Medium Mn Steel Welded Joints

In this study, a new-generation high-strength and high-ductility medium Mn steel (0.1C-5Mn-Fe) for the automotive industry was joined by the fiber laser and gas tungsten arc welding (GTAW) methods. Formability testing of the welded joints was done by the Erichsen cupping test and finite element (FE) analysis. The results showed that the formability of medium Mn steel welded joints was sensitive to the welding parameters and inferior to that of the base metal (BM). The hardening zone (HZ) was formed in the welded joint, which was composed of the fusion zone and two symmetrical parts of the heat-affected zone. The width of the HZ was one of the primary factors affecting the formability of the welded joints, while the tensile strength and ductility of the HZ were secondary factors. FE simulation of the Erichsen cupping test results indicated that the forming strain of the welded joint with narrow HZ concentrated on the BM, while the forming strain of the welded joint with wide HZ concentrated on the HZ. The HZ strain reached the deformation limit first due to its weak ductility; therefore, the welded joint with wide HZ had the worst formability. It was also observed that adopting high-velocity and high-laser power welding could greatly improve the formability of the welded joint as a result of reducing the width of the HZ.

Effects of in-situ post-weld heat treatment on the microstructure and mechanical properties of the coarse-grained heat-affected zone in a resistance spot weld in medium Mn TRIP steel

The tensile properties of the coarse-grained heat-affected zone (CGHAZ) play an important role in the cross-tension strength of resistance spot weldment of medium Mn transformation-induced plasticity (TRIP) steel. In this paper, an in-situ post-weld heat treatment is proposed as a way to achieve remarkable improvement of the ductility of the CGHAZ. Using four different weld thermal cycles to simulate the heat treatment, thermo-mechanically simulated specimens are prepared and used for mechanical testing. The experimental results show that the enhancement in ductility is based on the prevention of intergranular brittle fracture at the prior-austenite grain boundary because of the tempering of martensite. The tempering of the CGHAZ is affected by both the amount of martensite present immediately before reheating and the peak tempering temperature. Along with existing theory on tempering kinetics, the phenomenon is discussed in terms of carbon diffusion causing nanoscale austenite reversion and carbide precipitation.

Evaluations of tensile properties as a function of austenitizing temperature and springback by V-bending testing in medium-Mn steels

In the present study, tensile and springback properties of medium (3–7 wt%) Mn steels for warm stamping were investigated as functions of Mn content, austenitizing temperature (Taus), V-bending temperature (Tb) and time (tb). For comparison, the tensile and springback properties of 22MnB5 steel used for hot stamping were also evaluated. Whereas medium-Mn steels were air-cooled to room temperature after austenitizing at various Taus, the 22MnB5 steel was water-quenched. The 5Mn steel exhibited the highest tensile properties due to the martensitic matrix with inactive auto-tempering and negligible amount of retained austenite. The springback of medium-Mn specimens was increased by the addition of Mn at a given Tb when tb was 0 s. At the Tb of 300–500 °C, the fraction of martensitic transformation was raised by increasing the Mn content, resulting in high springback. At the Tb of 600 °C, the rise in transformation strain by the addition of Mn raised springback. When Tb was fixed to be 600 °C, the springback of medium-Mn specimens was increased by the addition of Mn due to the increases in transformation strain at the tb of 0 s and the fraction of martensitic transformation during air cooling at the tb of 2 s and over.

Kinetics and Numerical Simulation of Dynamic Recrystallization Behavior of Medium Mn Steel in Hot Working

Thermal deformation leads to controllable and effective grain refinement and microstructure optimization in medium Mn steel. To study the appropriate hot deformation parameters, the dynamic recrystallization (DRX) behavior and grain size evolution of medium Mn steel (0.15C‐7Mn) are investigated by thermal‐compression tests. Two types of DRX flow behaviors are observed on the flow curve, that is, single peak and cyclic behaviors. Based on the linear regression of characteristic points, dynamic recovery (DRV) model and DRX model are established. The predicted flow stress using the model established conforms well to experimental results. The correlation coefficient (R) and average absolute relative error (AARE) are 0.99% and 4.95%, confirming the accuracy of the model. After obtaining the value of DRX grain sizes by metallographic observation, grain size evolution model is developed to estimate the microstructure evolution. In addition, the established models are incorporated into DEFORM‐3D to simulate the kinetics and microstructure evolution of DRX. The simulation values are in good agreement with the experimental results, indicating that the finite element method (FEM) is an effective and accurate way for the analysis of thermal compression.

Fusion Weldabilities of Advanced High Manganese Steels: A Review

A large amount of manganese has been added to next-generation advanced high strength steels for automotive applications. The increased manganese content changes the microstructural and mechanical characteristics by varying both the stacking fault energy and the austenite stability, but it is known to deteriorate weldability. The current review provides a general strategy to address several problems in fusion welds of advanced high manganese steels from the viewpoint of microstructural transitions, plasticity mechanism, and mechanical properties based on the welding metallurgy. The importance of research on the weldabilities of advanced Mn steels was highlighted. Among the representative types of advanced manganese steels, this review focuses on high Mn twinning-induced plasticity steel, high Mn austenitic lightweight steel, and medium Mn transformation-induced plasticity steel. Specifically, this review suggests fundamental concepts for designing each steel with improved weld characteristics by presenting many research results on the correlations between the microstructure and mechanical properties of welds and heat-affected zones for these three different steel groups.