Mn Transformation-induced Plasticity Research Articles

The effect of double intercritical annealing on micromechanical behavior of a medium Mn transformation induced plasticity steel investigated by in situ high energy X-ray diffraction

The micromechanical behavior of a medium Mn transformation induced plasticity (TRIP) steel processed by intercritical annealing (IA) and double intercritical annealing (DIA) has been investigated by using in situ high energy X-ray diffraction during uniaxial tensile deformation. The results show that the ultimate tensile strength (UTS) and total elongation are 777.1 MPa and 39.6 % for IA and 953.6 MPa and 59.1 % for DIA, and that the product of UTS and total elongation is 30.8 GPa·% for IA and 56.4 GPa·% for DIA, respectively. Discontinuous phase transformation from austenite to martensite leads to Portevin-Le Châtelier (PLC) band propagation for both IA and DIA. The pronounced TRIP effect takes place in DIA step by step during deformation and contributes to higher strain hardening, which results in almost doubling the product of UTS and total elongation. The stepwise phase transformation highly corresponds to the lattice strain evolution of ferrite and austenite, indicating that load partitioning occurs during the deformation. The change of grain structure generated by different heat-treatments correlate with the load partitioning, thereby inducing large discrepancy in micromechanical behavior. The present investigation provides a deeper fundamental understanding of the effect of double inter-critical annealing on micromechanical behavior and effective engineering strategies for improving the product of UTS and total elongation for the medium Mn TRIP steels.

Metallurgical Factors Affecting the Tensile Properties of Weld Heat-Affected Zone in Medium Mn Transformation-Induced Plasticity Steel

The role of retained austenite in the mechanical property changes in the medium Mn transformation-induced plasticity (TRIP) steel heat-affected zone (HAZ) was elucidated throug a comparison with conventional TRIP steel. The two steel grades had similar microstructures consisting of ferritic phases with a high fraction of retained austenite in the as-received state; however, the difference in properties after welding was significant. The microstructures of both steels were dramatically changed to a martensitic matrix in the coarse-grained HAZ region. The medium Mn TRIP steel HAZ can have a higher retained austenite fraction than the conventional one; hence, an excellent combination of tensile strength and elongation can be obtained. The decisive difference between the two steels was determined in terms of austenite stability and the initial grain size.

Characterization and Analysis of Plastic Instability in an Ultrafine‐Grained Medium Mn TRIP Steel

Herein, the mechanical and magnetic behavior of an ultrafine‐grained (UFG) medium manganese (Mn) transformation‐induced plasticity (TRIP) steel is focused on in its plastic instability. The in situ methods of digital image correlation (DIC) and magnetic Barkhausen noise (MBN) are used to macroscopically characterize the propagation of the Lüders band (stretcher–strain marks) and the evolution of MBN activities during quasistatic tensile deformation. The evolution of microstructure during the plastic instability is investigated ex situ using X‐Ray diffraction (XRD) and transmission electron microscopy (TEM) for selected plastic strain states. It is showed in the results that the plastic instability of this steel is associated with an increase of hardness and enrichment of dislocation density, which can also amplify the MBN signal, while the derived coercivity behaves reversely on an overall trend due to work hardening. The different stress response of the medium Mn steel is closely related to the kinetic martensite microstructure, which in turn modifies the domain–structure response. Thus, the MBN can be used as a potential means for nondestructive evaluation (NDE) for the strengthening of the UFG medium Mn TRIP steel.

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.

Relationship between crystallographic orientation, microstructure characteristic and mechanical properties in cold-rolled 3.5Mn TRIP steel

The relationship between crystallographic orientation, microstructure characteristic and mechanical properties in 3.5Mn transformation induced plasticity (TRIP) steel that subjected to different cold rolling reduction and annealing time was elucidated. Before the hot-rolled plates were cold rolled to different thicknesses, large-fractioned Mn-enriched cementite particles dispersed in ferrite matrix were deliberately prepared to enhance Mn-partitioning and improve austenite stability. The evolution of microstructure was performed by electron probe micro-analyzer (EPMA), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD), and the mechanical properties of the intercritical annealed samples were evaluated by uniaxial tensile test at room temperature. The results show that the cementite dissolution, ferrite recrystallization and austenite formation are strongly coupled and interdependent, resulting in a complex ultra-fine microstructure composed of retained austenite, ferrite and cementite in the intercritical annealed samples. Moreover, the Schmid factor (SF) distribution within the range of 0.25–0.5 was studied to understand its correlation with the crystallographic orientation and mechanical property of the test steel. It is concluded that the mechanical properties of 3.5Mn TRIP steel can be improved when the annealing period, orientation and austenite stability are optimized.

Effect of austempering treatment on the microstructure and mechanical properties of 0.4C–1.5Si-1.5Mn TRIP-aided bainitic ferrite steel

In this study, the effects of austempering treatment on the microstructure, mechanical properties, and fracture mechanisms of Fe-0.4C-1.5Si-1.5Mn transformation-induced plasticity (TRIP)-aided bainitic ferrite (TBF) steel were investigated. When the austempering time increased from 0 to 3600 s, the volume fraction of retained austenite and its carbon concentration initially increased and then decreased. The increase in carbon concentration during austempering may be attributed to the formation of fine bainitic ferrite, representing the matrix and in turn the formation of retained austenite with a film-type morphology. When the austempering time was increased from 0 to 1000 s, the TBF steel showed excellent mechanical properties such as high yield and tensile strengths, and uniform and total elongations. Meanwhile, void density increased significantly in the initial stages and then slightly decreased; in contrast, the trend observed for mean crack length was opposite to that of void density leading to the change in the fracture mode of the TBF steels from intergranular to quasi-cleavage and dimple fractures. It could be inferred that the observed improvement in mechanical properties, especially the good ductility of TBF steels, at the optimal austempering time is due to the suppression of crack propagation and crack blunting to form voids owing to the effective deformation-induced transformation of metastable retained austenite.

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.

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.

Transformations During Intercritical Annealing and Their Implications for Microstructure and Mechanical Properties of Medium Mn Transformation-Induced Plasticity Steel in Continuous Annealing Line

Phase transformation during intercritical annealing (IA) and the relationship between microstructure and mechanical properties of the intercritical annealed 0.2C-5MnTRIP steels were investigated using a combined method of dilatometry, scanning electron microscope, transmission electron microscope, x-ray diffraction, and tensile testing. With the increase in IA temperature and holding time, the fraction of reversed austenite increases while the chemical stability of reversed austenite decreases, leading to the transformation of reversed austenite to martensite in the cooling stage. IA temperature is found to have a greater influence on the stability of austenite than IA time. The microstructures mainly comprised of ferrite and retained austenite, with granular carbide and martensite also observed in some specimens. The carbide precipitates formed during hot rolling remain after cold rolling and annealing process and gradually dissolve at higher IA temperature or with longer IA time. The optimal IA parameter, i.e., 650 °C for 15 min, contributes to a good product of tensile strength and elongation (997 MPa × 29.25%) due to an excellent TRIP effect. The present work gives a clear insight in controlling the mechanical properties of medium Mn TRIP steel in the continuous annealing line by a reasonable design of the IA process.

Quantitative Description of External Force Induced Phase Transformation in Silicon-Manganese (Si-Mn) Transformation Induced Plasticity (TRIP) Steels.

Transformation Induced Plasticity (TRIP) steels with silicon–manganese (Si–Mn) as the main element have attracted a lot of attention and great interest from steel companies due to their low price, high strength, and high plasticity. Retained austenite is of primary importance as the source of high strength and high plasticity in Si–Mn TRIP steels. In this work, the cold rolled sheets of Si–Mn low carbon steel were treated with TRIP and Dual Phase (DP) treatment respectively. Then, the microstructure and composition of the Si–Mn low carbon steel were observed and tested. The static tensile test of TRIP steel and DP steel was carried out by a CMT5305 electronic universal testing machine. The self-built true stress–strain curve model of TRIP steel was verified. The simulation results were in good agreement with the experimental results. In addition, the phase transformation energy of retained austenite and the work borne by austenite in the sample during static stretching were calculated. The work done by austenite was 14.5 J, which was negligible compared with the total work of 217.8 J. The phase transformation energy absorption of retained austenite in the sample was 9.12 J. The role of retained austenite in TRIP steel is the absorption of excess energy at the key place where the fracture will occur, thereby increasing the elongation, so that the ferrite and bainite in the TRIP steel can absorb energy for a longer time and withstand more energy.

Martensite Reversion Duality Behavior in a Cold-Rolled High Mn Transformation-Induced Plasticity Steel

The martensite reversion behavior of a cold-rolled high Mn transformation-induced plasticity (TRIP) steel was investigated through an elaborate continuous annealing procedure. The dilatometry analysis was employed to ensure a precise heating rate (50 °C/s) control during annealing along with acquiring the critical temperature range of the martensite reversion. The magnetic measurement was used for $$ \alpha^{\prime} $$-martensite quantification through annealing, and electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM) analyses were also performed to characterize the microstructure. The dilatometry results revealed the dual temperature ranges for the $$ \alpha^{\prime} $$-martensite reversion in the experimental TRIP steel; the reversion occurred in two separate temperature regions of 540 °C to 730 °C and 730 °C to 850 °C. Although the inhomogeneity in the morphology of $$ \alpha^{\prime} $$-martensite (lath type and dislocation cell type), which was detected in the ECCI image of the cold-rolled structure, might be responsible for the reversion temperature range broadening, the discontinuity of the reversion was mainly attributed to the partitioning of Mn from $$ \alpha^{\prime} $$-martensite to austenite through annealing. The microstructural characterizations indicated that the duality would also exist in the case of the martensite reversion mechanism. Accordingly, the reversion of lath-type martensite appeared to occur merely through an athermal mechanism, while the dislocation cell-type martensite might be reversed to austenite via both shear and diffusional mechanisms.

Microscopic strain partitioning in Lüders band of an ultrafine-grained medium Mn steel

This paper is focused on the determination and understanding of the microscopic strain partitioning in the Lüders band of an ultrafine-grained medium Mn transformation-induced plasticity (TRIP) steel with austenite-ferrite dual phase microstructures. First, a microscopic digital image correlation (μ-DIC) method is developed that enables high-resolution strain mapping at the subgrain scale. Thanks to the local Lüders strain assessment, the strain partitioning between ferrite and retained austenite is quantified. It reveals that the average Lüders strain in ferrite is about 10.1% and that in austenite about 9.1% at a macroscopic Lüders strain 9.5%, thus it indicates that both phases are important to the plastic deformation in the Lüders band. Besides the strain partitioning between the two phases, a more striking finding is the significant heterogeneity of Lüders strain within the same phase. The microstructural origins for the strain partitioning are investigated using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) in combination with the strain field analysis. The results show that the main cause for strain heterogeneity formation in Lüders band can be attributed to the heterogenous distribution of mobile dislocations in the initial microstructure before tensile deformation. And the effects of grain size and crystallographic texture are proofed to be the secondary factors that may only have minor influence on Lüders strain distribution.

Austenite morphology and resistance to hydrogen embrittlement in medium Mn transformation-induced plasticity steel

Influence of austenite morphology on the hydrogen embrittlement in medium Mn transformation-induced plasticity (TRIP) steel was investigated. With nearly the same volume fraction and mechanical stability, the lath-type austenite is found to have more beneficial effect in relieving the hydrogen susceptibility compared to the equi-axed one. Slower hydrogen uptake and diffusion are confirmed in the steel with the austenite of lath-type morphology, which is possibly led by more connections between austenite grains obstructing the migration of hydrogen.

Interactions of negative strain rate sensitivity, martensite transformation, and dynamic strain aging in 3rd generation advanced high-strength steels

Medium Mn transformation-induced plasticity (TRIP) steels, a subgroup of third generation advanced high-strength steels (AHSS), often present a combination of both dynamic strain aging (DSA) and TRIP. It has been previously shown that TRIP coincides with the passage of strain bands due to DSA, but their interdependence has not been well-established. This study seeks to explore the interactions between the two phenomena as well as the commonly-observed negative strain rate sensitivity. Digital image correlation (DIC) and in-situ magnetic measurements of the retained austenite fraction were employed to demonstrate the variations in mechanical behavior with increasing strain rate. Strain rate jump tests provided measurements of the strain rate sensitivity and were combined with either DIC to characterize PLC bands or with magnetic measurements to measure variations in the martensite fraction during the jumps. Finally, the micromechanics defining the peculiar bursts of martensite transformation in response to strain rate jumps are discussed in terms of a competition between dislocation mobility and austenite stability.

Extraordinary Lüders-strain-rate in medium Mn steels

An innovative experimental means based on high-speed digital image correlation (DIC) is employed in this work to investigate the Lüders band formation mechanisms in a medium Mn transformation-induced plasticity (TRIP) steel, allowing precise assessment of local strain rate inside the Lüders band. The stress-controlled instead of conventional strain-controlled mode is adopted during tensile test such that the instantaneous Lüders strain development right under the yield stress can be well captured. A surprisingly high Lüders-strain-rate on the order of 1 s−1 is observed in the tensile test at 25 °C, which is three orders of magnitude above the mean strain rate and about 20 times higher than that obtained in a low-carbon steel. The theoretical analysis shows that the mobile dislocations must be rapidly multiplied so as to support the observed high Lüders-strain-rate. This theoretical viewpoint is confirmed experimentally by transmission electron microscopy (TEM) observations, showing a large number of new dislocations generated in the Lüders band. It is revealed that the martensitic transformation is neither a cause nor a direct effect of Lüders band. Instead, they are two independent events that occur simultaneously. Nevertheless, the martensitic transformation does have the ability to influence a Lüders deformation process when it is involved. It accelerates the Lüders band formation by generating more dislocations and in the meantime raising their velocities, leading to an enhanced Lüders-strain-rate.

Increasing yield strength of medium Mn steel by engineering multiple strengthening defects

In general, medium Mn transformation-induced plasticity (TRIP) steels have a low yield strength due to soft ferrite matrix. Here we provide a strategy to improve the yield strength of medium Mn steels by coupling multiple strengthening defects in an austenite matrix. Such strengthening defects include V-precipitates, intra-granular ferrite, forest dislocations and nano-twins. Consequently, the present medium Mn steel has an ultra-high yield strength of 1350 MPa due to the collective contribution from these strengthening defects. Moreover, the present medium Mn steel demonstrates a good uniform elongation of 15%, which is ascribed to enhanced work hardening behavior due to the operation of both TRIP effect and twinning-induced plasticity (TWIP) effect.

Simultaneous Increase of Both Strength and Ductility of Medium Mn Transformation-Induced Plasticity Steel by Vanadium Alloying

In general, microalloying such as vanadium (V) is considered to deteriorate the mechanical stability of austenite grains in medium Mn steel due to the consumption of C content. In this paper, we show that the mechanical stability of austenite grains could be optimized by V-alloying. This is because the V-alloying will refine the austenite grain size and tends to stabilize the austenite grains. The competition between the grain refinement and the reduced C content results in proper mechanical stability of austenite grains, providing continuous transformation-induced plasticity (TRIP) effect. In addition, the V-alloying suppresses the formation of intergranular cracks, leading to a ductile fracture morphology and a large non-uniform elongation.

Temperature dependence of Lüders strain and its correlation with martensitic transformation in a medium Mn transformation-induced plasticity steel

The Lüders deformation behavior in a medium Mn transformation-induced plasticity (TRIP) steel is investigated at different temperatures ranging from 25 to 300 °C. It demonstrates that the Lüders band appears at all testing temperatures but with varied Lüders strains which do not change monotonically with temperature. The martensitic transformation is simultaneously observed within the Lüders band in varying degrees depending on the testing temperature. It is well verified that the martensitic transformation is not responsible for the formation of Lüders band, and a reasonable explanation is given for the non-monotonic variation of Lüders strain with increasing temperature.

Deformation rate controls atomic-scale dynamic strain aging and phase transformation in high Mn TRIP steels

Dynamic strain aging (DSA) in engineering metallic alloys triggers a degradation of the relevant properties of these material. Atomic-scale understanding of DSA is essential for achieving strong and ductile high-Mn martensitic-austenitic transformation induced plasticity (TRIP) steels. Using multiple-scale analytical techniques, we report the influence of carbon addition and strain rates on the mechanical properties and associated deformation-induced phase transformation of these steels. Specific attention is also placed on the origin of DSA at the atomic scale. We find that controlling these parameters (carbon and strain rate) can be used to manipulate the room temperature reverse transformation from martensite to austenite, plastic instability, short-range ordering (SRO), TRIP effect, and strain hardening of these steels. Thus, our results demonstrate that the SRO caused by short-range clustering (SRC) is linked to the DSA, and that high-strain-rate deformation induces an increase in the carbon concentration of SRC, leading to the DSA suppression. Hence, we suggest that manipulating phase transformation and DSA is utilized to achieve strong and ductile steels with continuous and stable flow.

Effects of Mn Content on the Warm Stretch-Flangeabiliy of C-Si-Mn TRIP-Aided Steels

Effects of Mn content on warm stretch-flangeability are investigated using 0.2%C–1.5%Si–(1.5–5.0)% Mn transformation-induced plasticity (TRIP)-aided sheet steels with annealed martensite matrix. Warm hole-expansion at 150–250 °C after hole-punching at 25 °C enhances the stretch-flangeability of 3 and 5% Mn steels. The warm stretch-flangeability increases with the Mn content. The superior warm stretch-flangeability of hole-punched 5% Mn steel is caused by its large localized ductility on expanding, which results primarily from the stabilization of its large quantity of retained austenite and its fine microstructure. Use of hole-drilling instead of hole-punching at 25 °C further increases the stretch-flangeability and raises the optimum expanding temperature to 250–300 °C.