Medium manganese steels

what is medium manganese steel?
advantage of medium mn steel
microstructure
mechanical properties
designing criteron
plastic yielding

1. Medium Manganese Steels Mini Project by, Sohini Mondal (UG 2 nd Year) Enrollment ID - 511119028

2. Introduction: Medium Mn steels establish an important class of novel alloys in the 3rd generation of advanced high strength steels (AHSS). Medium Mn steels usually contain 0.05~0.4 wt.% C and 3~10 wt.% Mn. Medium Mn steels show a good combinations of high to ultra-high strength at quite high total elongation. Ultra-fine grained microstructures can be achieved via intercritical annealing, resulting in a complex multi-phase microstructure consisting of several phases such as- Different types of austenite (retained, partition, stabilized, reverted) Martensite Ferrite Sometimes also delta ferrite.

3. C and Mn are both well-known austenite stabilizers in AHSSs, and thus increasing Mn and C content in steels is expected to be an effective route to enhance the fraction of Retained austenite. The different types of austenite in medium Mn steels promote enhanced ductility due to the higher strain hardenability enabled by dislocation accumulation, TRIP effect or even the TWIP effects. Adjusting alloy composition and adequate selection of the intercritical annealing temperature and pre-deformation results in the formation of different types of beneficial microstructures at room temperature. Compared with Quenching and Partitioning steels also having a tempered martensite matrix, the ultrafine-grained ferrite in medium Mn steels is softer. Therefore, the strength of medium Mn steels is usually somewhat lower than that of Q&P steels, but they have a much better elongation due to a higher RA fraction.

4. Advanced High Strength Steels: Advanced high-strength steels (AHSS) constitute a class of high-strength, formable metallic alloys that are designed mainly as sheet products for the transportation sector. AHSS have often very complex and hierarchical microstructures consisting of ferrite, austenite, bainite, or martensite matrix or of duplex or even multiphase mixtures of these constituents, sometimes enriched with precipitates. This complexity makes it challenging to establish reliable and mechanism-based microstructure–property relationships. different types of AHSS : dual-phase steels, complex phase steels, transformation-induced plasticity steels, twinning-induced plasticity steels, bainitic steels, quenching and partitioning steels, press hardening steels etc.

5. The strength-elongation ranges for the AHSSs: The strength-elongation envelopes for the various types of AHSSs: the strength-elongation envelopes for the various types of AHSSs, classified by their microstructure:

6. The Previous figure clearly illustrates a higher mechanical performance require both a more complex matrix microstructure and an increasing contribution of the RA. Crashworthiness is another important factor for automotive materials and Dual phase steels have very good crashworthiness. Compared to RA free DP steels at the same strength level, RA in TRIP , Q&P and medium-Mn steels can further increase the energy absorption at collisions and improve the crashworthiness through the TRIP effect. In the figure- IF: interstitial free steel HSLA: high strength low alloyed steel; DP: dual phase steel; CP: complex phase steel; Mart: martensitic steel; TRIP: transformation-induced plasticity steel; TWIP: twinning induced plasticity steel; CFB: carbide-free bainitic steel; Q&P: quenching and partitioning steel.

7. The advantage of medium Mn steels compared to other high strength steels : Medium Mn steels with 3.12 wt % Mn content emerge as strong candidate alloys for the 3rd generation of advanced high strength steels, due to: their excellent strength-ductility combination (product of tensile strength and total elongation up to ~70 GPa %) simple heat treatment process (e.g. intercritical annealing (IA)), and low-cost alloy ingredients The composite-like microstructure (normally ferrite and metastable austenite) combined with submicron grain scale differentiates such materials from other types of alloys.

8. The typical microstructures of hot-rolled and cold-rolled medium Mn steels: Medium Mn steels usually have an ultrafine dual phase microstructure, containing 20~50 vol. % C- and Mn enriched RA and a ferrite matrix (i.e. a heavily tempered martensite)

9. Desired microstructures and required chemical composition:

10. Fig. a and b show the effects of C and Mn additions on the phase diagram of Fe-C-Mn alloy, respectively. C addition narrows the intercritical annealing temperature region and promotes carbides precipitation at low temperatures. Mn addition shifts the intercritical annealing region to a lower temperature region. Fig c and d show the effects of Si and Al additions on the phase diagram of Fe-0.2C-5 Mn alloy, respectively. Besides suppressing carbide formation, Si and Al both increase the Ae1 and Ae3 temperatures. Si addition enhances the tensile strength via solid solution strengthening, while it deteriorates the surface quality due to the formation of Si enriched oxide, which negatively influences the Zn coating process. High content of Al causes the formation of coarse δ-ferrite during solidification.

11. Starting microstructures and processing routes: The starting microstructure of medium Mn steels is usually fully martensitic due to the high hardenability of such steels but some medium Mn steels with a very high Mn content can even contain a small amount of pre-existing austenite in the martensite matrix. The initial martensitic microstructures can be generally divided into two types: hot-rolled and cold-rolled microstructures, which leads to different final microstructures and mechanical properties. The typical microstructure of hot-rolled medium Mn steels after austenite reversion treatment consists of lath-shaped austenite and ferrite. For cold-rolled medium Mn steels, recrystallization of the heavily deformed martensite microstructure will proceed simultaneously with austenite formation during ART, leading to ultrafine globular austenite and ferrite. Due to the recrystallization, the dislocation density in ferrite of cold-rolled medium Mn steels is often lower than that for hot-rolled grades

12. In general, two kinds of reverted austenite morphologies, i.e. lath and globular, can be observed, and the final morphology is strongly affected by the initial microstructure. In addition to hot rolling and cold rolling processes, the warm rolling process was also adopted to control the morphology and sizes of reverted austenite through partial recrystallization. In the conventional ART(Austenite reversion treatment), the selection of intercritical annealing temperature is essential to control the fraction and stability of the RA. At a higher ART temperature, the kinetics of austenite reversion is fast and as a result the reverted austenite has a relatively low C and Mn content and a larger grain size. This will lead to a low stability of the reverted austenite, of which some will transform into fresh martensite during quenching. At a lower ART temperature, the fraction of reverted austenite is relatively low, although the stability is increased due to higher degree of C and Mn enrichment and a smaller grain size. At an optimized ART temperature, a desirable balance between the reverted austenite fraction and its thermomechanical stability is achieved, and a maximum amount of austenite upon cooling to ambient temperature can be retained.

13. It is important to note that Mn segregation band during solidification is inevitable in medium Mn steels, which often results in anisotropic mechanical properties and thus deteriorate the strength or elongation of steels. Several variants of the conventional ART, e.g. double annealing, cyclic-ART, flash-ART, quenching-ART, intercritical annealing Q&P and two-step intercritical annealing have also been proposed to process medium Mn steels. Cementite precipitation could also occur during the processing of medium Mn steels. Taking the advantage of cementite precipitation before austenite reversion, it was proposed that a two-step intercritical annealing process to obtain retained austenite in medium Mn steels. In the two-step intercritical annealing process, cementite precipitation is carefully tailored via annealing at a lower temperature before austenite reversion, and then reverted austenite can nucleate at the cementite/martensite interfaces during a shorter intercritical annealing at a higher temperature. The reverted austenite is partially transformed into martensite during quenching to ambient temperature, which results in a considerable amount of retained austenite adjacent to martensite. It was found that such a microstructure is beneficial to both the strength and the ductility of medium Mn steels.

14. Double annealing treatment : a first intercritical annealing at a higher temperature is performed to facilitate C and Mn partitioning into the reverted austenite, resulting in the reverted austenite having a large grain size. Due to the insufficient stability, the austenite reverted during the first intercritical annealing would partially transform into fresh martensite during quenching to room temperature. The microstructure after the first intercritical annealing consists of fresh martensite and the recrystallized globular ferrite. Subsequently, during the second annealing at a lower temperature, austenite reverts primarily from the C and Mn enriched fresh martensite, and the newly formed austenite is stabilized by further C and Mn enrichment. Medium Mn steels processed by the double annealing treatment have a hierarchical microstructure consisting of coarse globular ferrite, ultrafine-grained lath-shaped ferrite and RA. The double annealing treatment, which fine tunes the stability of the reverted austenite, was also found to be effective in eliminating the Lüders band phenomenon in medium Mn steels

15. Austenite reversion from martensite or martensite-austenite mixture: The kinetics of austenite growth (e.g. the martensite/austenite interface migration) and alloying elements partitioning during austenite reversion of medium Mn steels usually have been simulated using the LE model . Fig.a shows the kinetics of austenite reversion in an Fe-0.2C-5 Mn medium Mn steel simulated by the LE (Local equilibrium) model.

16. ( i ) NPLE(Negligible partitioning local equilibrium)-(α′→γ), during which the kinetics of martensite/austenite interface migration is controlled by carbon diffusion in martensite while a concentration spike of Mn forms ahead of the interface. In this stage, the rate of martensite/austenite interface migration is very fast and controlled by carbon diffusion in martensite ( Fig. b). Although the NPLE-(α′→γ) stage is very short (about second), the size of the reverted austenite at the end of this stage is significantly increased and is strongly affected by the initial thickness of martensite lath.  

17. (ii) PLE(Partitioning local equilibrium)-(α′→γ), during which the kinetics of martensite/austenite interface migration is controlled by Mn diffusion in martensite and as a result the Mn concentration in the reverted austenite is gradually enhanced ( Fig. c). A kinetic plateau was predicted to occur due to the NPLE/PLE transition. (iii) PLE-(γ→α′ ), during which the martensite/austenite interface migrates backward into the austenite. Its kinetics is very sluggish and controlled by Mn diffusion in austenite Fig.a & c). Hence, the growing fraction of reverted austenite first exceeds the full equilibrium fraction and then approaches it by shrinking. In general, the LE (Local equilibrium) model can qualitatively predict the basic features of austenite reversion. However, the thickening kinetics of reverted austenite predicted by the LE model, in particular at the early stage of austenite reversion, is usually much faster than that measured in experiments. This could be attributed to the infinite interface mobility of martensite/austenite assumption in the LE model. The mobility of martensite/austenite interface in medium Mn steels may not be infinite either, and thus a certain amount of Gibbs energy would be dissipated due to interface friction.

18. Influence of Intercritical Annealing on Microstructure and Mechanical Properties of a Medium Manganese Steel This fig. displays the initial microstructure of the 50 % cold-rolled medium manganese steel prior to intercritical annealing. After hot-rolling, homogenization annealing, water quenching and cold rolling, the material exhibits a deformed α´ martensitic microstructure as indicated by EBSD-image quality ( IQ) and EBSD phase maps. It is noticeable that the measurements show no austenite or precipitates prior to intercritical annealing.

19. EBSD phase maps of the cold-rolled medium manganese steel intercritically annealed at 555 °C for (a) 1 min, (b) 1 h and (c) 15 h, (bcc structure is indicated in red, fcc structure is indicated in green):

20. After intercritical annealing of the cold-rolled Fe-12Mn-3Al-0.05C medium manganese steel at 555 °C with subsequent quenching the microstructure consists of a α´ martensitic matrix with embedded reversed austenite islands, as evident from the EBSD phase maps, As shown in Fig.a ) an intercritical annealing time of only 1 min results in a small fraction of reversed austenite of approximately 1 %. It is observed that the austenite is formed first in highly deformed regions of the initial microstructure. Dark contrast in the map indicate low indexing and low pattern quality due to strain-induced local lattice rotations. When increasing the intercritical annealing time to 1 h, the volume fraction of the reversed austenite increased significantly to 15.7 % (in Fig. b). The austenite is observed primarily along the high angle α´ martensite grain boundaries, e.g. deformation bands. This is assumed to be due to manganese and carbon partitioning during intercritical annealing. A further increase of the intercritical annealing time to 15 h results in a continued increase of the reversed austenite fraction to approximately 34.0 %, being in good agreement to the calculated reversed austenite fraction in thermodynamic equilibrium. The austenite grains are homogeneously distributed and observed also within the martensite grains. Regardless of the varied intercritical annealing times, no evidence of recrystallization of the initial α´ martensite was observed by the EBSD measurements at an intercritical annealing temperature of 555 °C.

21. Mechanical properties after intercritical annealing This shows the engineering stress-strain curves of cold-rolled medium manganese steel after intercritical annealing. The material intercritically annealed for only 1 min at 555 °C exhibits a yield strength (YS) of 810 MPa, an ultimate tensile strength (UTS) of 840 MPa and a total elongation (TE) of approximately 16 %. A pronounced yield point with a very short yield point elongation of less than 1 % was observed

22. A longer intercritical annealing time, however, results in a flow curve exhibiting only continuous yielding, indicating uniform deformation of the specimen. A substantial increase of the total elongation to 23 % and a slight reduction of the YS and UTS to 740 MPa and 815 MPa, respectively, was observed for material intercritically annealed for 1 h. Further increase of the intercritical annealing time to 5 h shows a decrease of the YS and UTS to 715 Mpa and 795 MPa, respectively, while the TE increases to almost 31 %. The YS, UTS and TE of cold-rolled material intercritically annealed for 15 h amount 640 MPa, 775 MPa, and 30 %, respectively. This indicates a decrease of strength as well as of ductility of the Fe-12Mn-3Al-0.05C medium manganese steel in case of a significant further increase of the annealing duration.

23. Medium Mn steel design criteria: Hot rolled material : The tensile behavior of hot rolled medium Mn steel depends on the intercritical annealing temperature and time. Austenitization after cold rolling produces a microstructure similar to that of hot rolled samples. The prior austenite texture memory effect of and its spatial alignments are controlling factors influencing the tensile behavior. Since localization of strain is seen within colonies of having the same crystallographic orientation and spatial alignment, the following design criteria are suggested for improving the material’s properties by adjusting specific process parameters without changing its composition. Prior austenite grain size : One way of increasing the yield stress lies in reducing the strain localization areas. The size of colonies of similar spatially aligned are dependent on the martensite packet size within the prior austenite grain. Reducing the prior austenite grain size would thus reduce the resulting martensite packet size. This would in turn reduce the size of colonies with similar spatially aligned and crystallographically oriented . As a result, this approach should help reducing zones of premature strain localization, thus increasing the yield stress.  

24. 2. Crystallographic texture : The texture memory effect of the grains inside the prior austenite grain plays an important role in controlling strain hardening. Breaking the texture memory effect of the in- side the prior austenite grain, rendering it closer to the texture spread observed in the cold rolled material, would increase the strain hard- enability of the hot rolled sample. Cold rolled material: The intercritical annealing temperature and holding times for cold rolled medium Mn steel are critical factors influencing Lüders band propagation and elongation. Approaches for eliminating Lüders bands have been to intercritically anneal the material at high intercritical annealing temperatures where the is thermally less stable resulting in back transformation to α′fresh during the quenching stage. Such treatment would result in loss of ductility though. Intercritically annealing the material at low temperatures prevents equiaxed ferrite formation by martensite recrystallization. Increased holding time results in an increase in volume fraction until compositional partition and volume fraction equilibrium stages are obtained. When a sufficiently large volume fraction of is present, the flow stresses of is lower than that of α′temp resulting in continuous yielding and sufficient strain hardening.  

25. Phase maps of and 𝛼′𝑡𝑒𝑚𝑝 and corresponding inverse pole figure (IPF) maps highlighting of the (a) HRA and (b) CRA material. 3D EBSD IPF snapshots of in (c) HRA and (d) CRA material (Supplementary Movie 1 and 2 in the supplementary data). : reverted austenite; 𝛼′𝑡𝑒𝑚𝑝 : tempered martensite; HRA: hot rolled and intercritically annealed; CRA: cold rolled and intercritically annealed.  

26. Strain partitioning and strain localization in medium manganese steels: Designing medium Mn steels by selecting low intercritical annealing temperatures and long holding times prevents macro- scopical localization of strain in ferrite and would promote the formation of Lüders bands and thus of a pronounced yield point. Apart from strain partitioning, strain localization is observed in hot rolled medium Mn steel samples. Strain is more homogeneously partitioned within the islands in the cold rolled medium Mn steel. This is due to the prior austenite grains had been plastically broken up during cold rolling prior to intercritical annealing. The relative grain size effect in hot rolled samples is assumed to cause the observed lower yield stresses in hot rolled samples compared to the cold rolled samples in cases where both show continuous yielding.  

27. The role of prior austenite grain boundaries and microstructural morphology on the impact toughness of medium Mn steels [Fe-7Mn-0.1C-0.5Si ( wt %)] : Two types of microstructures were produced: hot-rolling plus annealing (HRA) cold-rolling plus annealing (CRA). Both types of specimens had a dual-phase microstructure consisting of retained austenite ( δ ) and ferrite ( α ) after intercritical annealing. Both, the HRA and CRA specimens were characterized by a transition in fracture mode from ductile and partly quasi cleavage fracture to intergranular fracture with decreasing impact test temperature from room temperature to -196°C. The HRA specimens exhibited a higher ductile to brittle transition temperature (DBTT) and lower impact energy at low temperatures below -50°C compared to the CRA specimens.

28. The intergranular cracks in HRA specimens propagated primarily along the boundaries of the prior austenite grains with a size of ~35 μ m, but those in the CRA material propagated along the boundaries of the ferrite and α ʹ martensite grains with a much finer size of ~450 nm. The main reason for intergranular cracking along the prior austenite grains in the HRA specimen was the segregation of Mn and P at the grain boundaries occurring during homogenization, hot rolling and air-cooling prior to intercritical annealing. The boundaries of martensitic packets or blocks were decorated only by C, since substitutional diffusion of Mn and P towards the grain boundaries at low temperature below the Ms temperature of ~275°C was too slow. Cold or warm working prior to intercritical annealing of medium Mn steels promotes primary recrystallization of their formable α ʹ martensite matrix prior to reverse transformation so that the solute-segregated boundaries of coarse prior austenite grains can be eliminated. We refer to this effect as ‘prior austenite grain boundary break-up’ mechanism. It results in the improvement of the low-temperature impact toughness of medium Mn steels.

29. Schematic sketch showing the difference in intergranular cracking occurring during the low-temperature impact test in two types of specimens with different microstructural morphologies after annealing:

30. Role of discontinuous plastic yielding in medium Mn steels : A drawback of medium Mn steels is that they often show a discontinuous plastic yielding phenomenon. This is characterized by a yield point drop followed by a stress plateau (also referred to as yield point elongation, YPE) in the tensile stress-strain curves and the formation of Lüders bands. Such localized deformation yielding phenomena can in principle deteriorate the surface quality during sheet forming operations. From a fundamental perspective, it is interesting to study this effect in more detail, since discontinuous yielding has rarely been observed in other multiphase composite-like alloys (e.g. conventional transformation-induced plasticity (TRIP), dual phase and duplex stainless steels).

31. The origin of discontinuous plastic yielding in medium Mn steels: Medium Mn steels with an austenite matrix (austenite fraction ~65 vol%) can exhibit pronounced discontinuous yielding. A combination of multiple in situ characterization techniques from macroscopic (a few millimeters) down to nanoscopic scale (below 100 nm) is utilized to investigate this phenomenon. It is observed that both austenite and ferrite are plastically deformed before the macroscopic yield point. In this microplastic regime, plastic deformation starts in the austenite phase before ferrite yields. The austenite-ferrite interfaces act as preferable nucleation sites for new partial dislocations in austenite and for full dislocations in ferrite. The large total interface area, caused by the submicron grain size, can provide a high density of dislocation sources and lead to a rapid increase of mobile dislocations, which is believed to be the major reason accounting for discontinuous yielding in such steels. The Lüders banding behavior and the local deformation-induced martensite forming inside the Lüders bands. It is found grain size and the austenite stability against deformation-driven martensite formation are two important microstructural factors controlling the Lüders band characteristics in terms of the number of band nucleation sites and their propagation velocity. These factors thus govern the early yielding stages of medium Mn steels, due to their crucial influence on mobile dislocation generations and local work hardening.

32. Spectral TRIP effect in medium Mn steels: Introduction of interlath reverted austenite is an effective method to design ductile lath medium Mn martensitic steels. The challenge in this concept is that all reverted austenite films have similar mechanical stability, hence, they all undergo transformation-induced plasticity (TRIP) at the same strain level. A new thermo-mechanical treatment route is developed t0 render martensite ductile via spreading the micro-mechanical stability of reverted γ grains by widening the γ nucleation barrier in martensite. When annealed a microstructure consisting of γ grains with a wide dispersed size distribution and martensite is developed. This mechanism enables a spectral TRIP effect. The new thermo-mechanical treatment route leads to enhanced mechanical properties of the TRIP steel (Fe-9Mn-3Ni-1.4Al-0.01C, mass %).

33. Compared to as-quenched martensite, cold-rolled martensite (~70% thickness reduction) contains a higher density of martensite grain boundaries with a wide misorientation distribution. The widened γ nucleation barrier in cold-rolled martensite results in a sequential nucleation and growth of γ grains during 600 C annealing. Eventually, a microstructure consisting of martensite and γ grains with a wide dispersed size distribution is successfully obtained in the cold-rolled martensite. The wide size distribution of γ grains results in an active TRIP effect over a wide strain regime (spectral TRIP). The proposed spectral TRIP strategy leads to yield enhanced strain hardening behavior in the cold-rolled material.

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