Medium Mn Steels

What are medium Mn steels ?

Medium Mn steels establish an imnportant class of novel alloys in the 3rd generation of advanced high strength steels (AHSS). 
These alloys have a manganese content between 3-12 wt.% and often multiple phase-like microstructure ingredients and phases.
They 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 and sometimes also delta ferrite. 
Especially 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.
 

EBSD IQ-phase maps of a medium Mn steel taken from the normal direction of (a) the hot-rolled and annealed (HRA) specimen and (b) the cold-rolled and annealed (CRA) specimen. Austenite is in green. Ferrite is in red; Acta Materialia 122 (2017) 199. EBSD IQ-phase maps of a medium Mn steel taken from the normal direction of (a) the hot-rolled and annealed (HRA) specimen and (b) the cold-rolled and annealed (CRA) specimen. Austenite is in green. Ferrite is in red; Acta Materialia 122 (2017) 199.

 

 

 

What is the role of prior austenite grain boundaries and microstructural on the impact toughness of medium Mn steels ?

We studied the effects of prior austenite grain boundaries and microstructural morphology on the impact toughness of an annealed Fe-7Mn-0.1C-0.5Si medium Mn steel for two different types of  microstructure states, namely, hot-rolled and annealed specimens and cold-rolled and annealed specimens. 
Both types of specimen microstructures had a dual-phase microstructure consisting of retained austenite and ferrite after intercritical annealing at 640°C for 30 min. The phase fractions and the chemical composition of the retained austenite were almost identical in both types of specimens. However, their microstructural morphology was different. The hot-rolled and annealed specimens had lath-shaped morphology and the cold-rolled and annealed specimens specimens had globular-shaped morphology. We observed that both types of specimens showed 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 hot-rolled and annealed specimens had higher ductile to brittle transition temperature and lower low-temperature impact toughness compared to the cold-rolled and annealed specimens. 
This was due to intergranular cracking in the HRA specimens along prior austenite grain boundaries decorated by C, Mn and P. In the CRA specimen intergranular cracking occurred along the boundaries of the very fine martensite grains. The results reveal that cold working prior to intercritical annealing promotes the elimination of the solute-decorated boundaries of coarse prior austenite grains through the recrystallization of martensite prior to reverse transformation, hence improving the low-temperature impact toughness of medium Mn steel.

Charpy impact absorbed energy vs. impact test temperature curves of both, the hot-rolled and annealed (HRA) specimen and the cold-rolled and annealed (CRA) specimen; Acta Materialia 122 (2017) 199. Charpy impact absorbed energy vs. impact test temperature curves of both, the hot-rolled and annealed (HRA) specimen and the cold-rolled and annealed (CRA) specimen; Acta Materialia 122 (2017) 199.
Effects of prior austenite grain boundaries and microstructural morphology on the impact toughness of intercritically annealed medium Mn steel
Acta Materialia 122 (2017) 199
Fe Acta 2016 Han Medium Mn Steel prior a[...]
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Do medium Mn steels reveal microstructural size effects ? 

Medium Mn steels containing reverted nanoscale austenite islands or films dispersed in a martensitic or ferritic matrix show excellent strength, ductility
and toughness. The underlying microstructural mechanisms responsible for these improvements are not yet fully understood, but are observed to be strongly connected to the island or film size of the (reverted) nanoscale austenite. Two main micromechanical effects are conceivable in this context:

 

(i) interaction of reverted nanoscale austenite with microcracks from the matrix (crack blunting or arresting);

 

and

 

(ii) deformation-induced phase transformation of reverted nanoscale austenite to martensite (TRIP effect).

 

We studied the latter phenomenon. To investigate size effects on reverted nanoscale austenite transformation independent of other factors that can influence austenite stability (composition, crystallographic orientation, defect density, surrounding phase, etc.), a model (TRIP-maraging steel) microstructure is designed with support from diffusion simulations (using DICTRA modeling) to have the same, homogeneous chemical composition in all reverted nanoscale austenite grains. Characterization is conducted by in-situ tension and bending experiments in conjunction with high-resolution electron backscatter diffraction mapping and scanning electron microscopy imaging, as well as post-mortem transmission electron microscopy and synchrotron X-ray diffraction analysis. Results reveal an unexpected “smaller is less stable” effect due to the size-dependent competition between mechanical twinning and deformation-induced
phase transformation.

Medium Mn steel: Images of the deformation sequence of two outlined areas (a) and (b) in Fig. 4a. EBSD phase maps and inverse pole figure (IPF) with respect to tensile axis (TA) are provided for both areas. For area (a), the image quality (IQ) and KAM map Medium Mn steel: Images of the deformation sequence of two outlined areas (a) and (b) in Fig. 4a. EBSD phase maps and inverse pole figure (IPF) with respect to tensile axis (TA) are provided for both areas. For area (a), the image quality (IQ) and KAM map
Smaller is less stable: Size effects on twinning vs. transformation of reverted austenite in TRIP-maraging steels
Acta Materialia 79 (2014) 268
Acta Materialia 79 (2014) 268 reversion [...]
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Is the martensite to austenite reversion in medium Mn steels diffusive or displacive ?

We studied the transition from diffusive to displacive austenite reversion mechanisms in a medium Mn steel, also considering the effect of heating rate
on austenite reversion behavior using a 0.15%C–5%Mn model steel. Austenite reversion temperature first increased gradually with the heating rate owing to the superheating effect and then remained at a constant temperature above a critical heating rate. In response, the austenite formed by rapid heating exhibited a coarse prior austenite grain structure, indicating the occurrence of displacive reversion even in low-alloy steel.

Transition from Diffusive to Displacive Austenite Reversion in Low-Alloy Steel
ISIJ International, Vol. 53 (2013), No. 12, pp. 2275–2277
ISIJ Austenite reversion diffusion displ[...]
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Inheritance of Dislocations and Crystallographic Texture during Martensitic Reversion into Austenite
ISIJ International, Vol. 53 (2013), No. 7, pp. 1286
ISIJ-austenite reversion 53_1286 Nakada [...]
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Study of medium Mn steels by a bulk combinatorial design with specific reference to martensite-to-austenite reversion

We studied the effect of local martensite-to-austenite reversion on microstructure and mechanical properties with the aim of designing ductile medium Mn steels. Following a combinatorial screening with tensile and hardness testing on a matrix of six alloys (0 - 5 wt.% Mn, 0 - 2 wt.% Si, constant 13.5 wt.% Cr and 0.45 wt.% C) and seven martensite tempering conditions (300 - 500 °C, 0 -30 min), investigations were focussed on martensite-to-austenite reversion during tempering as function of chemical composition and its correlation with the mechanical properties. While Mn additions promoted austenite formation (up to 35 vol.%) leading to a martensitic-austenitic TRIP steel with optimum mechanical properties (1.5 GPa ultimate tensile strength and 18 % elongation), Si led to
brittle behaviour despite even larger austenite contents. Combined additions of Mn and Si broadened the temperature range of austenite reversion, but also significantly lowered hardness and yield strength at limited ductility. These drastically diverging mechanical properties of the probed steels are discussed in light of microstructure morphology, dispersion and transformation kinetics of the
austenite, as a result of the composition effects on austenite retention and reversion.

Bulk combinatorial design of ductile martensitic stainless steels through confined martensite-toaustenite reversion
Materials Science & Engineering A 582 (2013) 235
Mater Sc Engin A 582 (2013) 235 bulk com[...]
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Medium Mn steels can have increased resistance to crack propagation

Fatigue failures create enormous risks for all engineered structures, as well as for human lives, motivating large safety factors in design and, thus, inefficient use of resources. Inspired by the excellent fracture toughness of bone, we explored the fatigue resistance in metastability-assisted multiphase steels. We show here that when steel microstructures are hierarchical and laminated, similar to the substructure of bone, superior crack resistance can be realized. Our results reveal that tuning the interface structure, distribution, and phase stability to simultaneously activate multiple micromechanisms that resist crack propagation is key for the observed leap in mechanical response. The
exceptional properties enabled by this strategy provide guidance for all fatigue-resistant alloy design efforts.

Bone-like crack resistance in hierarchical metastable nanolaminate steels
Koyama et al., Science 355, 1055–1057 (2017) 10 March 2017
Koyama Science 355 (6329), 1055-1057.pdf
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What is a 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. Here we propose a new thermo-mechanical treatment route to activate the TRIP effect over a broad strain regime and refer to it as ‘spectral TRIP effect’. It aims at spreading the micro-mechanical stability of reverted austenite grains by widening the austenite nucleation barrier in martensite. To validate the proposed thermo-mechanical treatment route, an as-quenched medium-Mn martensitic steel was cold rolled prior to the reversion treatment at 600°C. Microstructure characterization was carried out by electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). Mechanical tests show that the approach is effective. The spectral TRIP effect improves both, the strength and the ductility due to the well dispersed size distribution and the associated size-dependent deformation and phase transformation behavior of the reverted austenite grains, extending TRIP-related work hardening over a broad strain range.

Spectral TRIP enables ductile 1.1 GPa medium Mn steels
Acta Materialia 111 (2016) 262 martensit[...]
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How can nanolaminate transformation-induced plasticity–twinning-induced plasticity medium Mn steels be designed ?

Conventional martensitic medium Mn steels have limited ductility due to insufficient microstructural strain-hardening and damage resistance mecha-
nisms. It was recently demonstrated that the ductility and toughness of martensitic steels can be improved without sacrificing the strength, via partial
reversion of the martensite back to austenite. These improvements were attributed to the presence of the transformation-induced plasticity (TRIP)
effect of the austenite phase, and the precipitation hardening (maraging) effect in the martensitic matrix. However, a full micromechanical understanding of this ductilizing effect requires a systematic investigation of the interplay between the two phases, with regards to the underlying deformation and damage micromechanisms. For this purpose, in this work, a Fe–9Mn–3Ni–1.4Al–0.01C (mass%) medium-Mn TRIP maraging steel is produced and heat-treated under different reversion conditions to introduce well-controlled variations in the austenite–martensite nanolaminate microstructure. Uniaxial tension and impact tests are carried out and the microstructure is characterized using scanning and transmission electron microscopy based techniques and post mortem synchrotron X-ray diffraction analysis. The results reveal that (i) the strain partitioning between austenite and martensite is governed by a highly dynamical interplay of dislocation slip, deformation-induced phase transformation (i.e. causing the TRIP effect) and mechanical twinning (i.e. causing the twinning-induced plasticity effect); and (ii) the nanolaminate microstructure morphology leads to enhanced damage resistance. The presence of both effects results in enhanced strain-hardening capacity and damage resistance, and hence the
enhanced ductility.

Nanolaminate transformation-induced plasticity–twinning-induced plasticity steel with dynamic strain partitioning and enhanced damage resistance
Acta Materialia 85 (2015)
Acta Materialia vol 85 (2015) Nanolamina[...]
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Grain boundary segregation engineering and austenite reversion turn embrittlement into toughness: Example of a 9 wt.% medium Mn steel

We study grain boundary embrittlement in a quenched and tempered Fe–Mn high-purity model martensite alloy using Charpy impact tests and grain boundary characterization by atom probe tomography. We observe that solute Mn directly embrittles martensite grain boundaries while reversion of martensite to austenite at high-angle grain boundaries cleans the interfaces from solute Mn by partitioning the Mn into the newly formed austenite, hence restoring impact toughness. Microalloying with B improves the impact toughness in the quenched state and delays temper embrittlement at 450 °C. Tempering at 600 °C for 1 min significantly improves the impact toughness and further tempering at lower temperature does not cause the embrittlement to return. At higher temperatures, regular austenite nucleation and growth takes place, whereas at lower temperature, Mn directly promotes its growth.

Grain boundary segregation engineering and austenite reversion turn embrittlement into toughness: Example of a 9 wt.% medium Mn steel
Acta Materialia 86 (2015) 182
Acta Materialia 86 (2015) 182 Kuzmina Gr[...]
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Embrittlement and toughness in a 9 wt.% medium Mn steel: DBTT curves of high-purity Mn9 and Mn9+B; Acta Materialia 86 (2015) 182 Embrittlement and toughness in a 9 wt.% medium Mn steel: DBTT curves of high-purity Mn9 and Mn9+B; Acta Materialia 86 (2015) 182

 

 

What is teh role of chemical gradients across phase boundaries between martensite and austenite in medium Mn steel ?

Partitioning at phase boundaries of complex steels is important for their properties. We present atom probe tomography results across martensite/austenite interfaces in a precipitation-hardened maraging-TRIP steel (12.2 Mn, 1.9 Ni, 0.6 Mo, 1.2 Ti, 0.3 Al; at.%). The system
reveals compositional changes at the phase boundaries: Mn and Ni are enriched while Ti, Al, Mo and Fe are depleted. More specific, we observe up to 27 at.% Mn in a 20 nm layer at the phase boundary. This is explained by the large difference in diffusivity between martensite and austenite. The high diffusivity in martensite leads to a Mn flux towards the retained austenite. The low diffusivity in the austenite does not allow accommodation of this flux. Consequently, the austenite grows with a Mn composition given by local equilibrium. The interpretation is based on DICTRA and mixed-mode diffusion calculations (using a finite interface mobility).

Chemical gradients across phase boundaries between martensite and austenite in medium Mn steel studied by atom probe tomography
Acta Materialia 59 (2011) 364–
Acta Materialia 59 (2011) 364 atom probe[...]
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What are linear complexions in medium Mn steels?

For 5000 years, metals have been mankind’s most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We report chemical and structural states confined at dislocations. In a body-centered cubic medium Mn Fe–9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces.The phenomenon resembles interface-stabilized
structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states.

Complexions like structures at dislocations in medium Mn steels
SCIENCE 1080 4 SEPTEMBER 2015 • VOL 349 ISSUE 6252 sciencemag.org
Linear Complexions Science vol 349 (2015[...]
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Acta Mat. 2011, 59, p. 364