Medium 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.

A new perspective on Lüders band formation in medium-Mn steels based on Lüders-strain-rate and dislocation evolution

Propagating Lüders bands with pronounced strain localization are often observed in medium-Mn transformation-induced plasticity (TRIP) steels, which adversely limits the application of such alloys in engineering structures. The present work aims to understand this phenomenon from a dislocation-based perspective, with a focus on the specific role of dislocation multiplication. The material studied is a typical dual-phase medium-Mn steel with a ferrite matrix and a second phase of retained austenite. A warm-rolling process was used as a pre-straining approach to adjust the microstructure of the material, such that materials with two different microstructural states can be obtained: as-received material and pre-strained material. They show distinct differences in initial dislocation density. Stress-controlled rather than conventional strain-controlled tensile testing was adopted, which allowed the Lüders band to develop in an unconstrained manner. In this way, the intrinsic Lüders-strain-rate can be obtained. The results show that the Lüders band-related mechanical parameters, including Lüders strain, Lüders-strain-rate and yield stress drop, all depend on the initial microstructure state and the dislocation evolution process during Lüders band formation. In particular, Lüders-strain-rate is a critical microstructure-sensitive factor. It is closely related to the dislocation multiplication in the formation of Lüders band, which can be characterized and quantified using scanning transmission electron microscope and synchrotron X-ray diffraction. Finally, a quantitative relationship between Lüders-strain-rate and dislocation multiplication rate is established, which allows providing a better understanding of the exceptional Lüders band phenomenon in medium-Mn 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.

The Significance of Austenite/Ferrite/Martensite Ratio on Governing Impact Toughness of Fe-0.2C-6Mn-1.5Al Medium-Mn Transformation-Induced Plasticity Steel Studied by a Novel Heat Treatment

In this paper, the scientific challenges associated with obtaining high impact toughness in the medium-Mn TRIP steel, Fe-0.2C-6Mn-1.5Al, have been explored. This was enabled by a novel and effective heat treatment involving the combination of intercritical hardening and tempering to obtain high impact toughness. Electron microscopy and x-ray diffraction studies clearly underscored absence of TRIP effect in Fe-0.2C-6Mn-1.5Al medium manganese steel during impact and the volume fraction of austenite played a determining role in governing impact toughness. The highest impact toughness of 180.17 J/cm2 was obtained when the steel was subjected to intercritical hardening temperature of 630 °C and low tempering temperature of 200 °C. The presence of martensite in the microstructure reduced the impact toughness on quenching from 670 to 700 °C. When the quenching temperature was increased to 750 °C, the impact toughness of steel was slightly increased, which is ascribed to the supersaturated carbon in martensite that precipitated as carbides. The fracture mode was ductile after intercritical hardening in the temperature range of 600-700 °C.

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.

Remarkable improvement in resistance spot weldability of medium-Mn TRIP steel by paint-baking heat treatment

In this study, a remarkable improvement in resistance spot weldability of medium-Mn transformation-induced plasticity (MT) steel through a paint-baking heat treatment was addressed using dissimilar resistance spot weldments between MT and dual phase (DP) steels with various soaking temperatures. Experimental results showed that the cross-tension strength in the dissimilar MT/DP weldment increased up to 128% when the soaking temperature was more than 110 °C, and was at its highest at 210 °C. The improved cross-tension strength is attributed to the failure mode change from a pull-out to a partial pull-out failure. The brittle-to-ductile transition of martensitic matrix in coarse-grained heat-affected zone in MT steel was also considered to be a factor. Austenite reversion and carbide precipitation occurred in the martensitic matrix during the paint-baking heat treatment, whereas there was no significant change in alloying element segregation at the prior austenitic grain boundary. The driving force for the tempering was discussed based on the calculated carbon diffusion distance.

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.

Dynamic Tensile Behavior of Fiber Laser Welds of Medium Manganese Transformation-Induced Plasticity Steel

During a crash situation, tailor-welded blanks (TWBs) are subjected to local high strain rate (HSR) deformations. Hence, understanding the effects of fiber laser welding on the microstructure and subsequent dynamic (1 to 100 s−1) tensile properties of the recently developed medium-Mn transformation-induced plasticity (TRIP) steel is essential. Electron backscatter diffraction (EBSD) analysis indicated that Mn segregation during rapid solidification of the FZ resulted in retention of austenite within interdendritic regions. Moreover, it was found that the lower stability of retained austenite within the intercritical heat-affected zone (HAZ) resulted in double yielding behavior of the welds due to discontinuous yielding (Luders banding). However, by increasing the strain rate, the second yield point decreased and shifted to higher strains. In the dynamic loading range, the ultimate tensile strength (UTS) and elongation of welds showed positive strain rate sensitivity (SRS). In addition, high-speed thermal camera results indicated that the average temperature decreased with increasing strain rate as a consequence of the balance between TRIP effect suppression and HSR adiabatic heating. The overall increase in elongation of the welds at higher strain rates was attributed to the higher localized softening during pre-failure nonuniform deformation.

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.

Deformation-induced martensitic transformation kinetics and correlative micromechanical behavior of medium-Mn transformation-induced plasticity steel

An in situ high-energy X-ray diffraction (HE-XRD) technique was mainly used to investigate the micromechanical behavior of medium-Mn Fe-0.12C-10.16Mn-1.87Al (in wt%) transformation-induced plasticity (TRIP) steel subjected to intercritical annealing at 625 °C, 650 °C, 675 °C and 700 °C for 1 h. As the intercritical annealing temperature increased, the volume fraction of retained austenite (RA) and ultimate tensile stress (UTS) increased, while the Lüders strain and yield stress (YS) decreased. The incremental work-hardening exponent of experimental steel increased with increasing intercritical annealing temperature. The overall trend of the transformation kinetics of the RA with respect to the true strain followed the sigmoidal shape predicted by the Olson and Cohen (OC) model. Load partitioning occurred among the ferrite, austenite and martensite immediately after entering the yielding stage. Because the stability of the RA decreased with increasing intercritical annealing temperature, the load undertaken by the martensite increased. The moderate transformation kinetics of the RA and effective load partitioning among constituent phases were found to contribute to a favorable combination of strength and ductility for this medium-Mn TRIP steel.

Analysis of Hot- and Cold-Rolled Loads in Medium-Mn TRIP Steels

The purpose of this work is to investigate the hot- and cold-rolling requirements to produce third-generation advanced high-strength steels (AHSS). Therefore, five medium-Mn (10 to 14 wt pct Mn) alloys that exhibit transformation-induced plasticity (TRIP) were compared to a commercially produced grade of AISI 1018 using hot- and cold-rolling experiments. Experimental data collected from a STANAT instrumented rolling mill was utilized to measure force and torque during hot- and cold-rolling. Experimental data were processed by a 1D analytical model, based on Orowan model, to determine rolling pressure. It was determined that pressures required to hot-roll TRIP alloys are 1.4 to 1.8 times greater than pressures for rolling AISI 1018 steel. Cold rolling of the medium-Mn TRIP steels was found to be 1.5 to 2.8 times greater than the AISI 1018 steel. Mechanical and microstructural characterization was also performed and the variation in rolling pressure was related to the starting microstructural constituents, and alloys containing greater starting quantities of e-martensite in the microstructure had higher flow stresses at equivalent rolling strains during cold rolling.

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.

A comparison of cross-tension properties and fracture behavior between similar and dissimilar resistance spot-weldments in medium-Mn TRIP steel

The current study characterizes the degradation of cross-tension properties in medium-Mn transformation-induced plasticity (TRIP) steel weldments by comparison of fracture behaviors in both similar and dissimilar weldments with DP980 dual phase steel. The experimental results reveal that the cross-tension strengths of medium-Mn TRIP steel weldments is inferior to those of conventional TRIP steel having lower carbon equivalent. Although the failure modes of similar and dissimilar weldments were different from each other, the brittleness of the martensite was a common key factor governing crack propagation. In similar weldment, the decreased tensile toughness in the coarse grain heat-affected zone due to the insufficient self-tempering of martensite and the decreased retained austenite fraction. Meanwhile, understanding the changes in the carbon equivalent caused by dilution in the dissimilar welds and segregation in the fusion zone is necessary to characterize medium-Mn TRIP steel spot welds.

Correction to: A Novel Shim-Assisted Resistance Spot Welding Process to Improve Weldability of Medium-Mn Transformation-Induced Plasticity Steel

In the Acknowledgements, the following information was missing: H.W. Luo specially thanks the joint financial support from National Natural Science Foundation of China and Bao Steel Group Co. Ltd (No. U1460203).

A Novel Shim-Assisted Resistance Spot Welding Process to Improve Weldability of Medium-Mn Transformation-Induced Plasticity Steel

Medium-Mn transformation-induced plasticity steels have great potential to significantly reduce vehicle weight and improve fuel economy due to their outstanding combination of high strength and excellent ductility. One bottleneck to the application is their poor weldability resulting from their high Mn contents. In this paper, three resistance spot welding set-ups, including no shim, an interstitial-free steel shim at the faying interface (shim-in) and shims against the electrodes (shim-out), were incorporated to investigate the weldability of Fe-7Mn-0.14C medium-Mn steel. Tensile-shear, cross-tension, and microhardness tests were used to evaluate the mechanical properties of the welds. Experimental results demonstrated that the failure mode of the welds transitioned from the interfacial fracture in the case of no shim to the desired nugget pull-out fracture in the shim-out set-up, resulting in dramatical improvements in both peak loads and their corresponding extensions during the tensile-shear and cross-tension tests. In contrast, the shim-in set-up made no improvement. What can contribute to such improvement was then discussed on the basis of observed morphologies and microstructures of welds.