Review on the plastic instability of medium -Mn steels for identifying the formation mechanisms of Lüders and Portevin -Le Chatelier bands

Plastic instability,including both the discontinuous yielding and stress serrations,has been frequently observed during the tensile deformation of medium-Mn steels(MMnS)and has been intensively studied in recent years.Unfortunately,research results are controversial,and no consensus has been achieved regarding the topic.Here,we first summarize all the possible factors that affect the yielding and flow stress serrations in MMnS,including the morphology and stability of austenite,the feature of the phase interface,and the deformation parameters.Then,we propose a universal mechanism to explain the conflicting experimental results.We conclude that the discontinuous yielding can be attributed to the lack of mobile dislocation before deformation and the rapid dislocation multiplication at the beginning of plastic deformation.Meanwhile,the results show that the stress serrations are formed due to the pinning and depinning between dislocations and interstitial atoms in austenite.Strain-induced martensitic transformation,influenced by the mechanical stability of austenite grain and deformation parameters,should not be the intrinsic cause of plastic instability.However,it can intensify or weaken the discontinuous yielding and the stress serrations by affecting the mobility and density of dislocations,as well as the interaction between the interstitial atoms and dislocations in austenite grains.

Toughening ultrastrong low-density steel by textured δ-ferrite lamellas

By both the Charpy V-notched impact and the projectile tests, we here investigated the dynamic fracture behavior of a recently developed ultrastrong lightweight steel comprising a hierarchical martensitic matrix, dispersed ultra-fine-retained austenite grains and oriented δ-ferrite lamellas, the latter being due to high Al and Si contents employed for low-density design. This steel shows a superior combination of specific ultimate tensile strength and impact toughness to other ultrastrong steels and has successfully arrested a real steel-cored bullet shot. These are attributed to the densely textured δ-ferrite lamellas that can deflect the propagating cracks until they are trapped and enclosed besides austenite-to-martensite transformation crack closure, leading to more energy consumed before failure. These results suggest a new pathway for toughening ultrastrong lightweight steels.

High Temperature Ductility Loss of 16MnCr5 Pinion Steels

A wide ductility trough covering from 700 to 1100 is observed in the curve of Reduction of Area (RA) vs.tempetature for 16MnCr5 pinion steel. At 750℃, corresponding to the minimum of RA, it is grain boundary slidingthat controls its hot ductility rather than usual Deforming Induced Ferrite (DIF), which can only appear just below750℃ and slightly improve hot ductility. The volume fraction of ferrite is dependent on the strain and strain rate.Firstly a critical strain must be necessary for formation of DIF then with strain rate increasing, the volume fractionof DIF decreases but RA is elevated. In the γ phase region, hot ductility is seriously deteriorated because of grainboundary sliding promoted by oxidcs and sulfides at the grain boundary and recovered because of dynamic recrystal-lization at higher temperature; when strain rate increases, ductility is improved as there is insufficient time for cracksto propagate along the γ grain boundary as well as dynamically precipitating, and ductility trough becomes narrowerbecause the temperature for onset of dynamic recrystallization decreases. In addition, γ→α phase transformationintroduced by temperature drop before the tensile test encourages precipitation of AlN and impairs ductility.

Ultrahigh tensile strength achieved in a lightweight medium Mn steel via prominent work hardening

We develop a new ultrastrong medium Mn steel with a density reduced to 7.39 g cm^(-3).It has a novel tri-phase microstructure comprising a hierarchical martensitic matrix(α’),dispersed ultra-fine-retained austenite grains(γ),and both compressed and{200}orientedδ-ferrite lamellas,the latter’s formation is due to the alloying of high Al and Si contents for reducing density.As a result,both ultrahigh ultimate tensile strength of 2.1 GPa and good ductility of 16%are achieved after an extraordinary plastic strain hardening increment of about 1.4 GPa.The in-situ synchrotron-based high-energy(HE)X-ray diffraction(XRD)examinations during the tensile deformation revealed that the initial presence of residual com-pressive stress inδ-ferrite could increase the stress required to initiate the plastic tensile deformation of the specimen,leading to the isolatedδ-ferrite lamellas mostly deformed elastically to coordinate the plastic deformation of the martensitic matrix during yielding.During the plastic deformation,the gradual release of residual compressive stress inδandα’,the dislocation multiplication in all the three phases and the successiveγ-to-α’transformation all contribute to such a prominent work hardening increment.This study facilitates the development of novel strategies for fabricating ultrastrong but light steels.

Influence of refined hierarchical martensitic microstructures on yield strength and impact toughness of ultra-high strength stainless steel

The hierarchical martensitic features in ultra-high strength stainless steel(UHSSS),including the prior austenite grains,martensite packets,blocks and laths with the descending size,were refined to various extents by employing different thermomechanical processes and then carefully characterized.Their relation to yield strength and impact toughness was analyzed.We conclude that the refinement of martensitic structures could lead to the significant increase of yield strength,which follows the Hall-Petch relation with the effect grain size defined by high angle boundaries(HABs).Impact toughness of UHSSS depends on the frequency and capability for retained austenite(RA)grains at both HABs and martensite lath boundaries to trap the propagating cracks via strain-induced transformation,in which the film-like RA grains at lath boundaries appear to make the greater contribution.

Effect of warm rolling process on microstructures and tensile properties of 10 Mn steel

The influence of warm rolling processes on the microstructures and tensile properties of 100 Mn steel was studied.Strength appeared to increase with the rolling temperature but strengthening mechanisms varied.The increase of warm rolling temperature from 250℃ to 600℃ leads to enhanced recrystallization in martensite during the intercritical annealing(IA) at 620℃ for 5 h.As a result,both ultimate tensile strength(UTS) and total elongation(TE) increase.However,the size of relatively coarse recrystallized austenite grains and the resultant yield strength(YS) remain almost constant in this temperature range.The further increase of rolling temperature to 700-800℃ causes a considerable amount of pearlite to be formed during the IA,and then martensite is formed after the IA,resulting in dramatical increases in both YS and UTS but at the great loss of ductility.The warm rolling at 600℃ with 63% thickness reduction can produce the steel with the best mechanical combination of 1.2 GPa UTS and 35% TE,due to the formation of many ultrafine austenite grains and strain-induced cementite precipitates.This demonstrates that the mechanical combination of non-V-alloyed medium Mn steel can be improved to an equivalent level of 0.7% V alloyed 10 Mn steel just via the economic strain-induced cementite precipitation.

A strong and ductile medium Mn steel manufactured via ultrafast heating process

Ultrafast heating(UFH)at the rates of 10-300℃/s was employed as a new strategy to anneal a coldrolled 7 wt%Mn steel,followed by the immediate cooling.Severely deformed strain-induced martensite and lightly-deformed thermal martensite,both had been already enriched with C and Mn before,transformed to fine and coarse austenite grains during the UFH,leading to the bimodal size distribution.Compared with the long intercritical annealing(IA)process,the UFH processes produced larger fraction of RA grains(up to 37%)with a high density of dislocation,leading to the significant increase in yield strength by 270 MPa and the product of strength and elongation up to 55 GPa%due to the enormous work hardening capacity.Such a significant strengthening is first attributed to high density dislocations preserved after UFH and then to the microstructural refinement and the precipitation strengthening;whilst the sustainable work hardening is attributed to the successive TRIP effect during deformation,resulting from the large fraction of RA instantly formed with the bimodal size distribution during UFH.Moreover,the results on the microstructural characterization,thermodynamics calculation on the reverse transformation temperature and the kinetic simulations on the reverse transformation all suggest that the austenitization during UFH is displacive and involves the diffusion and partition of C.Therefore,we propose that it is a bainite-like transformation.

Recent progress in medium-Mn steels made with new designing strategies, a review

After summarizing the relevant researches on the medium Mn steels in references, two new targets on the tensile properties have been defined. One is that both transformation-induced（TRIP） and twinninginduced plasticity（TWIP） could be realized for the steel with a relatively low Mn content, which exhibits the similar tensile properties to the classical TWIP steels with higher Mn content. The other is to achieve ultrahigh ultimate tensile strength（〉1.5 GPa） without sacrificing formability. To achieve these goals,new designing strategies was put forward for compositions and the processing route. In particular, warm rolling was employed instead of the usual hot/cold rolling process because the former can produce a mixture of retained austenite grains with different morphologies and sizes via the partial recrystallization. Consequently, the retained austenite grains have a wide range of mechanic stability so that they can transform to martensite gradually during deformation, leading to enhanced TRIP effect and then improved mechanic properties. Finally, it is succeeded in manufacturing these targeted medium Mn steels in laboratory, some of them even exhibit better tensile properties than our expectation.

Yielding behavior of triplex medium Mn steel alternated with cooling strategies altering martensite/ferrite interfacial feature

In this paper, we report the influence of cooling processes on the yielding behavior of a medium Mn steel(MMS) with triplex microstructure, i.e. austenite(γ), ferrite(α) and as-quenched martensite(α’). After the intercritical annealing(IA) at both 725℃ and 750℃, the steel was subjected to the two cooling processes, i.e. air cooling(AC) and water quenching(WQ). It exhibits the discontinuous yielding after the AC following the IA at 750℃ while the continuous yielding after the WQ. Compared with WQ process,both the dilatometry and the microstructural examinations show that the AC process leads to lower Ms temperature, larger retained austenite(RA) fraction and less martensite, the latter is always companied with geometry necessary dislocations(GNDs) generated near the α/α’ interfaces. Considering the complexity of nanosized tri-phases in this steel, the presence of martensite with key features in the resultant specimens was systematically examined by atom probe tomography(APT) on the samples prepared by the specific target lift-out method. The APT results directly revealed the C/Mn co-segregation at the α’/αinterfaces in the AC samples but not in WQ samples. The numerical simulation results further suggest that the segregation of C and Mn at the α’/α interfaces may be due to different mechanisms. We conclude that the yielding of triplex MMS is determined by both the quantity of GNDs generated near theα/α’ interfaces, which increases with martensite fraction, and the extent of their immobilization resulting from the interfacial segregation of solute atoms when the presence of martensite is sufficient. WQ tends to suppress the discontinuous yielding of MMS since the rapid cooling may promote more martensite formed with the increasing quantity of GNDs and prevent the interfacial segregation of both C and Mn.

Improved mechanical properties of V-microalloyed dual phase steel by enhancing martensite deformability

A good combination of ultimate tensile strength(UTS)up to 1365 MPa and total strain to failure(StF)to 15.5%has been achieved due to deformable martensite in the invented vanadium-microalloyed dual-phase(DP)steel,which was manufactured by two-stage annealing of cold rolled steel strip.The employed extensive characterizations revealed that the ductile martensitic phase in this DP steel differentiated from ordinarily low-carbon martensitic lath in both morphology and lattice structure.Complex coherent orientation relationships between ferrite,reverse austenite,martensitic phase and vanadium carbide(VC)do exist,leading to a new martensitic transformation mechanism and resultant dual-phase microstructure.Besides,a detailed characterization including essential phase transformation analysis in combination with in situ TEM observation,shows that,all the essential processing including recrystallization,reverse austenitic and martensitic transformation,in debt to the particular effects of VC,can be recognized as phase transformations with higher thermodynamic driving force and higher kinetic energy barrier as compared to previously common processing,which actually changes the microstructure and,indirectly leads to higher strength and higher ductility.This synergy of thermodynamics and kinetics can be generalized to improve mechanical properties of present steels.