In the present study,a simple but effective two-step annealing processing strategy via manipulating the austenite reversion path is proposed to obtain a large fraction of retained austenite in low-Mn medium-Mn steels....In the present study,a simple but effective two-step annealing processing strategy via manipulating the austenite reversion path is proposed to obtain a large fraction of retained austenite in low-Mn medium-Mn steels.Initially,the Fe-3Mn-0.2C-1.5Si(wt%)steel is intercritically annealed to form Mn-enriched lamellar martensite precursors.Subsequently,the austenite reversion transformation is manipulated to occur within the martensite lamellae during the second annealing process,resulting in an ultra-fine duplex microstructure of laminated austenite and ferrite.This process can not only allow a large fraction of austenite to be retained in low-Mn medium-Mn steels,but also increase the elongation by up to 41%without sacrificing the strength level compared to the conventional annealing.展开更多
In this study,we investigated the correlation between the pre-strain and hydrogen embrittlement(HE)mechanisms in medium-Mn steel.Intercritically annealed Fe-7Mn-0.2C-3Al(wt.%)steel,which showed a two-phase microstruct...In this study,we investigated the correlation between the pre-strain and hydrogen embrittlement(HE)mechanisms in medium-Mn steel.Intercritically annealed Fe-7Mn-0.2C-3Al(wt.%)steel,which showed a two-phase microstructure comprising α ferrite and γR retained austenite,was used as a model alloy.As the pre-strain level increased from 0%to 45%,the volume fraction of γR gradually decreased owing to the strain-inducedα′martensite transformation accompanied by an increase in dislocation density.The HE resistance decreased with increasing the pre-strain level because the sample with a higher pre-strain level revealed a higher amount of dissolved hydrogen,combined with a more extensive brittle fracture region owing to the enhanced diffusion and permeation of hydrogen from the reduced γR fraction.Ad-ditionally,the H-assisted crack in the sample without pre-strain was initiated and propagated from the γR grains when the strain-induced α′phase was formed,because most of the dissolved hydrogen was concentrated in the γR grains,and these grains were predominantly deformed compared to the other phases.However,the pre-strained sample showed more pronounced multiple H-assisted cracking at the constituent phases,such as α and α′,because it exhibited relatively well-dispersed hydrogen atoms and reduced microstrain localization at the γR grains,due to the reduced γR fraction.展开更多
An in situ high-energy X-ray diffraction(HE-XRD) technique was mainly used to investigate the micromechanical behavior of medium-Mn Fe-0.12 C-10.16 Mn-1.87 Al(in wt%) transformation-induced plasticit(TRIP) steel subje...An in situ high-energy X-ray diffraction(HE-XRD) technique was mainly used to investigate the micromechanical behavior of medium-Mn Fe-0.12 C-10.16 Mn-1.87 Al(in wt%) transformation-induced plasticit(TRIP) steel subjected to intercritical annealing at 625℃, 650℃, 675℃ and 700℃ for 1 h. As the intercritical annealing temperature increased, the volume fraction of retained austenite(RA) and ultimate tensilstress(UTS) increased, while the Lüders strain and yield stress(YS) decreased. The incremental workhardening exponent of experimental steel increased with increasing intercritical annealing temperatureThe overall trend of the transformation kinetics of the RA with respect to the true strain followed thsigmoidal 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 thRA decreased with increasing intercritical annealing temperature, the load undertaken by the martensitincreased. 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 thimedium-Mn TRIP steel.展开更多
The martensitic hot-rolled 0.3 C-6 Mn-1.5 Si(wt%)steel was annealed at 630℃for 24 h to improve its cold rollability,followed by cold rolling and annealing at 670℃for 10 min.The annealing process was designed based o...The martensitic hot-rolled 0.3 C-6 Mn-1.5 Si(wt%)steel was annealed at 630℃for 24 h to improve its cold rollability,followed by cold rolling and annealing at 670℃for 10 min.The annealing process was designed based on the capacities of industrial batch annealing and continuous annealing lines.A duplex submicron austenite and ferrite microstructure and excellent tensile properties were obtained finally,proved the above process is feasible."Austenite memory"was found in the hot-rolled and annealed sample which restricted recrystallization of lath martensite,leading to lath-shaped morphology of austenite and ferrite grains."Austenite memory"disappeared in the cold-rolled and annealed sample due to austenite random nucleation and ferrite recrystallization,resulting in globular microstructure and refinement of both austenite and ferrite grains.The austenite to martensite transformation contributed most of strain hardening during deformation and improved the uniform elongation,but the dislocation strengthening played a decisive role on the yielding behavior.The tensile curves change from continuous to discontinuous yielding as the increase of cold-rolled reduction due to the weakening dislocation strengthening of austenite and ferrite grains related to the morphology change and grain refinement.A method by controlling the cold-rolled reduction is proposed to avoid the Lüders strain.展开更多
Advanced high-strength steels have been widely used to improve the crashworthiness and lightweight of vehicles.Different from the popular cold stamping,hot forming of boron-alloyed manganese steels,such as 22MnB5,coul...Advanced high-strength steels have been widely used to improve the crashworthiness and lightweight of vehicles.Different from the popular cold stamping,hot forming of boron-alloyed manganese steels,such as 22MnB5,could produce ultra-high-strength steel parts without springback and with accurate control of dimensions.Moreover,hot-formed medium-Mn steels could have many advantages,including better mechanical properties and lower production cost,over hot-formed 22MnB5.This paper reviews the hot forming process in the automotive industry,hot-formed steel grades,and medium-Mn steel grades and their application in hot forming in depth.In particular,the adaptabilities of medium-Mn steels and the presently popular 22MnB5 into hot forming were compared thoroughly.Future research should focus on the technological issues encountered in hot forming of medium-Mn steels to promote their commercialization.展开更多
An approach of optimizing the intercritical annealing path in a 0.2C-5Mn medium-Mn steel is presented by introducing precursor microstructure prior to normal austenite reverted transformation(ART)annealing.The steel i...An approach of optimizing the intercritical annealing path in a 0.2C-5Mn medium-Mn steel is presented by introducing precursor microstructure prior to normal austenite reverted transformation(ART)annealing.The steel is fi rstly pre-annealed at diff erent intercritical temperatures to form designed precursor microstructures.Then,they are employed for subsequent conventional ART annealing processing.It is found that pre-annealing at relative high intercritical temperatures can promote precipitation and dissolution of the carbide in the steel and re-distribute the C and Mn in the microstructures.The produced microstructural precursors show excellent merits in accelerating the austenite reversions in subsequent normal ART processing and assisting the RA formation.Tensile testing reveals that the excellent strength-elongation balance can be achieved in the heat-treated samples using diff erent microstructural precursors,which suggests the potential applicability in producing the medium-Mn steels with shortened processing period.展开更多
Medium-manganese(Mn)steel(MMS)has remarkable characteristics of high strength,strong work-hardening capacity,and wear resistance,being a promising third-generation advanced high-strength steel with lower raw material ...Medium-manganese(Mn)steel(MMS)has remarkable characteristics of high strength,strong work-hardening capacity,and wear resistance,being a promising third-generation advanced high-strength steel with lower raw material cost compared with other generations of advanced high-strength steel.The chemical composition and processing route play critical roles in determining the microstructural evolution of the MMS,and the microstructure composition significantly influences the mechanical,corrosion and wear properties of the steel.Hence,a lot of research work focus on exploring the direct relation between microstructural evolution and mechanical/corrosion/wear properties,and the progress has the following crucial aspects:(1)alloying design on the phase composition and carbide precipitation,(2)processing route on regulating microstructure evolution and twinning-induced plasticity and/or transformation-induced plasticity strengthening mechanism,(3)work-hardening,corrosion,and corrosion resistance of the regulated MMS,and(4)fracture and failure mechanism of MMS under tensile,corrosion and wear damages,as well as the improvement strategies.展开更多
Medium-Mn steels have attracted immense attention for automotive applications owing to their outstanding combination of high strength and superior ductility.This steel class is generally characterized by an ultrafine-...Medium-Mn steels have attracted immense attention for automotive applications owing to their outstanding combination of high strength and superior ductility.This steel class is generally characterized by an ultrafine-grained duplex microstructure consisting of ferrite and a large amount of austenite.Such a unique microstructure is processed by intercritical annealing,where austenite reversion occurs in a fine martensitic matrix.In the present study,austenite reversion in a medium-Mn alloy was simulated by the multiphase-field approach using the commercial software MICRESS®coupled with the thermodynamic database TCFE8 and the kinetic database MOBFE2.In particular,a faceted anisotropy model was incorporated to replicate the lamellar morphology of reversed austenite.The simulated microstructural morphology and phase transformation kinetics(indicated by the amount of phase)concurred well with experimental observations by scanning electron microscopy and in situ synchrotron high-energy X-ray diffraction,respectively.展开更多
This work demonstrated the viability of friction stir welding for the welding of medium-Mn steels when used as cryogenic vessel materials for liquefied gas storage.We used an intercritically annealed Fe-7 Mn-0.2 C-3 A...This work demonstrated the viability of friction stir welding for the welding of medium-Mn steels when used as cryogenic vessel materials for liquefied gas storage.We used an intercritically annealed Fe-7 Mn-0.2 C-3 Al(wt.%)steel with a dual-phase(α'martensite andγ_(R) retained austenite)nanolaminate structure as a base material and systematically compared its microstructure and impact toughness after friction stir and tungsten inert gas welding.The friction stir welded specimen exhibited a large amount ofγ_(R) phase owing to a relatively low temperature during welding,whereas the tungsten inert gas welded specimen comprised only theα'phase.Furthermore,the friction stir welded steel exhibited a tuned morphology of nanoscale globular microstructure at the weld zone and did not exhibit any prior austenite grain boundary due to active recrystallization caused by deformation during welding.The preserved fraction ofγ_(R) and morphological tuning in the weldment improved the impact toughness of the friction stir welded steel at low temperatures.In the steel processed by tungsten inert gas welding,the notch crack propagated rapidly along the prior austenite grain boundaries-weakened by Mn and P segregations-resulting in poor impact toughness.However,the friction stir welded steel exhibited a higher resistance against notch crack propagation due to the slow crack propagation along the ultrafine ferrite/ferrite(α/α)interfaces,damage tolerance by the active transformation-induced plasticity from the large amount ofγR,and enhanced boundary cohesion by suppressed Mn and P segregations.展开更多
The microstructure and mechanical properties of a V-microalloyed Al-containing medium-Mn steel after hot rolling and intercritical annealing(IA)are explored.The tested steel exhibits a fne multiphase microstructure co...The microstructure and mechanical properties of a V-microalloyed Al-containing medium-Mn steel after hot rolling and intercritical annealing(IA)are explored.The tested steel exhibits a fne multiphase microstructure consisting of bimodal sizes of ferrite and retained austenite plus considerable amount of fne VC and/or M3C precipitates.Physical-chemical phase analysis shows that about 71.0%of the total V is in VC phase and more than 93%of VC particles is less than 5 nm.The calculated precipitation strengthening values of VC are^347 and^234 MPa for the specimens intercritically annealed at 625 and 750℃,respectively.An excellent combination of strength and ductility as high as^50 GPa%and yield strength(YS)of 890 MPa was obtained at intercritical temperature(TIA)of 725℃,although it does not correspond to the maximum precipitation strengthening of VC phase.Therefore,it is suggested that an optimization of TIA corresponding to both excellent combination of strength and ductility and high YS should be further explored through chemical composition and IA process optimization.展开更多
The influence of microstructural characteristics on Lu¨ders strain and mechanical properties was explored by means of altering thermo-mechanical circumstances in an intercritical annealing(IA)medium-Mn Fe-11Mn-0....The influence of microstructural characteristics on Lu¨ders strain and mechanical properties was explored by means of altering thermo-mechanical circumstances in an intercritical annealing(IA)medium-Mn Fe-11Mn-0.09C-0.25Si(wt.%)steel.By IA of cold-rolled samples with severe plastic deformation,exclusively equiaxed dual phases were obtained because of active recovery and recrystallization.The equiaxed austenite(gamma E)with a larger size and inadequate chemical concentration was more readily transformed into martensite,and subsequent transformation-induced plasticity(TRIP)effect was triggered actively at relatively higher IA temperature,lessening localized deformation.In addition,grown-in dislocations were prone to multiply and migrate around a broad mean free path for coarser equiaxed ferrite(alpha E)due to weakening dynamic recovery;therefore,it was the ensuing increased mobility of dislocations instead of reserving plentiful initial dislocation density that facilitated the propagation velocity of Luders bands and the accumulation of work hardening.In contrast,the bimodal-grained microstructure with lath-like and equiaxed austenite(gamma L+gamma E)satisfactorily contributed to a smaller yield point elongation(YPE)without compromise of comprehensive mechanical properties on the grounds that austenitic gradient stability gave rise to discontinuous but sustainable TRIP effect and incremental work hardening.Hence,Luders strain is closely related to the absence of work hardening in the region which yields locally.It follows that the decreased stability of retained austenite,favorable mobility of dislocations and the bimodal-grained structure all prominently make up for the insufficiency of work hardening,thereof resulting in a limited YPE.展开更多
Often,the addition of more than 4 wt.%Mn to medium-Mn steels is necessary to enhance the thermal stability of intercritical austenite for achieving sufficient amounts of retained austenite(RA)at room tem-perature.In t...Often,the addition of more than 4 wt.%Mn to medium-Mn steels is necessary to enhance the thermal stability of intercritical austenite for achieving sufficient amounts of retained austenite(RA)at room tem-perature.In this paper,a medium-Mn steel with Mn content as low as 2.7 wt.%was designed via alloying with a small amount of Al,and the microstructure and mechanical properties of the steel,subjected to intercritical annealing(IA)at 745°C for different times followed by oil quenching,were investigated.Results show that the volume fraction of RA increases first and then decreases with IA time,with the maximum of 0.36 obtained at IA time of 50 min.It is demonstrated that Al addition slows down the in-terface migration and growth kinetics of reverted austenite via retarding C diffusion in ferrite during IA,which,hence,decreases the amount and size of the reverted austenite and partitions more C and Mn into it.This suggests that Al plays a favorable role in enhancing the thermal stability of reverted austenite and increasing the amount of austenite retained at room temperature.Due to the presence of large amounts of RA and the strong transformation-induced plasticity effect generated during plastic deformation,the steel exhibits persistent high strain hardening and superior mechanical properties,comparable to those of reported medium-Mn steels containing higher Mn content.The present result offers a new insight into the role of Al in adjusting microstructure-property relationships and opens a promising way for designing low-cost,high performance medium-Mn steels with low Mn content for industrial applications.展开更多
As a representative of steels available in the market,medium-Mn steel shows vast application prospects in lightweight automobile fields.This review details the research progress of medium-Mn steels,focusing on the fol...As a representative of steels available in the market,medium-Mn steel shows vast application prospects in lightweight automobile fields.This review details the research progress of medium-Mn steels,focusing on the following aspects.The roles of common adding elements,rolling technologies,and various heat treatments on the microstructure and mechanical properties of medium-Mn steel are analyzed,thus pro-viding references for designing tailored medium-Mn steel with excellent performance.Considering that hydrogen embrittlement is a challenge faced in the development of high-strength steel,the hydrogen embrittlement behavior of medium-Mn steel is also discussed,particularly emphasizing the influence of microstructure,hydrogen concentration,strain,etc.Furthermore,practical strategies to improve resistance to hydrogen embrittlement are summarized.Finally,this review provides prospects for the development and research prospects of medium-Mn steel.展开更多
A medium-Mn steel （0.2C5Mn） was processed by intercritical annealing at different temperatures （625 ℃ and 650 ℃ ）. An ultrafine-grained micro-duplex structure consisting of alternating austenite and ferrite lath...A medium-Mn steel （0.2C5Mn） was processed by intercritical annealing at different temperatures （625 ℃ and 650 ℃ ）. An ultrafine-grained micro-duplex structure consisting of alternating austenite and ferrite laths was de- veloped by austenite reverse transformation （ART） during intercritical annealing after forging and hot rolling. Ultra- high ductility with a total elongation higher than 30% was achieved in the temperature range from -196 ℃ to 200 ℃, and high impact toughness no less than 200 J at -40 ℃ was obtained. Based on the analysis of microstructure and mechanical properties, it was found that the enhanced ductility was determined by the phase transformation effect of austenite （TRIP effect）, while the delayed ductile to brittle transition was controlled by austenite stability.展开更多
A new medium-Mn steel was designed to achieve unprecedented tensile properties,with a yield strength beyond 1.1 GPa and a uniform elongation over 50%.The tensile behavior shows a heterogeneous deforma-tion feature,whi...A new medium-Mn steel was designed to achieve unprecedented tensile properties,with a yield strength beyond 1.1 GPa and a uniform elongation over 50%.The tensile behavior shows a heterogeneous deforma-tion feature,which displays a yield drop followed by a large Lüders band strain and several Portevin-Le Châtelier bands.Multiple strain hardening mechanisms for excellent tensile properties were revealed.Firstly,non-uniform martensite transformation occurs only within a localized deformation band,and ini-tiation and propagation of every localized deformation band need only a small amount of martensite transformation,which can provide a persistent and complete transformation-induced-plasticity effect dur-ing a large strain range.Secondly,geometrically necessary dislocations induced from macroscopic strain gradient at the front of localized deformation band and microscopic strain gradient among various phases provide strong heter-deformation-induced hardening.Lastly,martensite formed by displacive shear trans-formation can inherently generate a high density of mobile screw dislocations,and interstitial C atoms segregated at phase boundaries and enriched in austenite play a vital role in the dislocation multipli-cation due to the dynamic strain aging effect,and these two effects provide a high density of mobile dislocations for strong strain hardening.展开更多
Fatigue crack growth(FCG)tests were conducted on a medium-Mn steel annealed at two intercritical annealing temperatures,resulting in different austenite(γ)to fe rrite(α)phase fractions and differentγ(meta-)stabilit...Fatigue crack growth(FCG)tests were conducted on a medium-Mn steel annealed at two intercritical annealing temperatures,resulting in different austenite(γ)to fe rrite(α)phase fractions and differentγ(meta-)stabilities.Novel in-situ hydrogen plasma charging was combined with in-situ cyclic loading in an environmental scanning electron microscope(ESEM).The in-situ hydrogen plasma cha rging increased the fatigue crack growth rate(FCGR)by up to two times in comparison with the reference tests in vacuum.Fractographic investigations showed a brittle-like crack growth or boundary cracking manner in the hydrogen environment while a ductile transgranular manner in vacuum.For both materials,the plastic deformation zone showed a reduced size along the hydrogen-influenced fracture path in comparison with that in vacuum.The difference in the hydrogen-assisted FCG of the medium-Mn steel with different microstructures was explained in terms of phase fraction,phase stability,yielding strength and hydrogen distribution.This refined study can help to understand the FCG mechanism without or with hydrogen under in-situ hydrogen charging conditions and can provide some insights from the applications point of view.展开更多
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 plastic...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.展开更多
A new model on predicting the density of hot-rolled multi-phased medium-Mn steel has been presen ted on the basis of thermodynamic calculations. This is an integrated model, which includes one for calculating the reta...A new model on predicting the density of hot-rolled multi-phased medium-Mn steel has been presen ted on the basis of thermodynamic calculations. This is an integrated model, which includes one for calculating the retained austenite （RA） fraction and the other for volume expansion during the aus tenite-to martensite transformation, because both of them are key parameters for calculating the den- sity of steel at ambient temperature. The existing empirical equations for calculating Mx temperature and lattice constants of both martensite and austenite have been all rcassessed by the XRD measure ments on the microstructures of seven hot-rolled medium-Mn steels. Finally, the densities ot seven steels were calculated merely from compositions and compared with the measured ones. The differ ence between them is no more than 1 %, suggesting that the presented model should be of good value in designing the low density steels.展开更多
The microstructural evolution of a cold-rolled and intercritical annealed medium-Mn steel(Fe-0.10C-5Mn)was investigated during uniaxial tensile testing.In-situ observations under scanning electron microscopy,transmiss...The microstructural evolution of a cold-rolled and intercritical annealed medium-Mn steel(Fe-0.10C-5Mn)was investigated during uniaxial tensile testing.In-situ observations under scanning electron microscopy,transmission electron microscopy,and X-ray diffraction analysis were conducted to characterize the progressive transformation-induced plasticity process and associated fracture initiation mechanisms.These findings were discussed with the local strain measurements via digital image correlation.The results indicated that Lüders band formation in the steel was limited to 1.5%strain,which was mainly due to the early-stage martensitic phase transformation of a very small amount of the less stable large-sized retained austenite(RA),which led to localized stress concentrations and strain hardening and further retardation of yielding.The small-sized RA exhibited high stability and progressively transformed into martensite and contributed to a stably extended Portevin-Le Chatelier effect.The volume fraction of RA gradually decreased from 26.8%to 8.2%prior to fracture.In the late deformation stage,fracture initiation primarily occurred at the austenite/martensite and ferrite/martensite interfaces and the ferrite phase.展开更多
基金support from the National Natural Science Foundation of China(Grant Nos.52321001 and 52071322).
文摘In the present study,a simple but effective two-step annealing processing strategy via manipulating the austenite reversion path is proposed to obtain a large fraction of retained austenite in low-Mn medium-Mn steels.Initially,the Fe-3Mn-0.2C-1.5Si(wt%)steel is intercritically annealed to form Mn-enriched lamellar martensite precursors.Subsequently,the austenite reversion transformation is manipulated to occur within the martensite lamellae during the second annealing process,resulting in an ultra-fine duplex microstructure of laminated austenite and ferrite.This process can not only allow a large fraction of austenite to be retained in low-Mn medium-Mn steels,but also increase the elongation by up to 41%without sacrificing the strength level compared to the conventional annealing.
基金supported by the Korea Institute for Advancement of Technology(KIAT)grant funded by the Korean Government(MOTIE)(P0023676,HRD Program for Industrial Innovation)the National Research Foundation of Korea(NRF)(No.2021R1A6A3A13045008).
文摘In this study,we investigated the correlation between the pre-strain and hydrogen embrittlement(HE)mechanisms in medium-Mn steel.Intercritically annealed Fe-7Mn-0.2C-3Al(wt.%)steel,which showed a two-phase microstructure comprising α ferrite and γR retained austenite,was used as a model alloy.As the pre-strain level increased from 0%to 45%,the volume fraction of γR gradually decreased owing to the strain-inducedα′martensite transformation accompanied by an increase in dislocation density.The HE resistance decreased with increasing the pre-strain level because the sample with a higher pre-strain level revealed a higher amount of dissolved hydrogen,combined with a more extensive brittle fracture region owing to the enhanced diffusion and permeation of hydrogen from the reduced γR fraction.Ad-ditionally,the H-assisted crack in the sample without pre-strain was initiated and propagated from the γR grains when the strain-induced α′phase was formed,because most of the dissolved hydrogen was concentrated in the γR grains,and these grains were predominantly deformed compared to the other phases.However,the pre-strained sample showed more pronounced multiple H-assisted cracking at the constituent phases,such as α and α′,because it exhibited relatively well-dispersed hydrogen atoms and reduced microstrain localization at the γR grains,due to the reduced γR fraction.
基金supported by the National Key Research and Development Program of China (No. 2017YFA0403804)the National Natural Science Foundation of China (NSFC) (Nos. 51471032 and 51527801)+3 种基金the Fundamental Research Funds for the Central Universities (Nos. 06111020 and 06111040)the State Key Laboratory for Advanced Metals and Materials (Nos. 2016Z-01, 2016Z-12, and 2016Z-19)financial support from the Chinese Scholarship Council (CSC)supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (No. DE-AC02-06CH11357)
文摘An in situ high-energy X-ray diffraction(HE-XRD) technique was mainly used to investigate the micromechanical behavior of medium-Mn Fe-0.12 C-10.16 Mn-1.87 Al(in wt%) transformation-induced plasticit(TRIP) steel subjected to intercritical annealing at 625℃, 650℃, 675℃ and 700℃ for 1 h. As the intercritical annealing temperature increased, the volume fraction of retained austenite(RA) and ultimate tensilstress(UTS) increased, while the Lüders strain and yield stress(YS) decreased. The incremental workhardening exponent of experimental steel increased with increasing intercritical annealing temperatureThe overall trend of the transformation kinetics of the RA with respect to the true strain followed thsigmoidal 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 thRA decreased with increasing intercritical annealing temperature, the load undertaken by the martensitincreased. 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 thimedium-Mn TRIP steel.
基金financially supported by the National Natural Science Foundation of China(Grant No.51722402)by the Fundamental Research Funds for the Central Universities(Grant No.2007012)+1 种基金111 Project(No.B16009)the Liaoning Revitalization Talents Program(No.XLYC1907128)。
文摘The martensitic hot-rolled 0.3 C-6 Mn-1.5 Si(wt%)steel was annealed at 630℃for 24 h to improve its cold rollability,followed by cold rolling and annealing at 670℃for 10 min.The annealing process was designed based on the capacities of industrial batch annealing and continuous annealing lines.A duplex submicron austenite and ferrite microstructure and excellent tensile properties were obtained finally,proved the above process is feasible."Austenite memory"was found in the hot-rolled and annealed sample which restricted recrystallization of lath martensite,leading to lath-shaped morphology of austenite and ferrite grains."Austenite memory"disappeared in the cold-rolled and annealed sample due to austenite random nucleation and ferrite recrystallization,resulting in globular microstructure and refinement of both austenite and ferrite grains.The austenite to martensite transformation contributed most of strain hardening during deformation and improved the uniform elongation,but the dislocation strengthening played a decisive role on the yielding behavior.The tensile curves change from continuous to discontinuous yielding as the increase of cold-rolled reduction due to the weakening dislocation strengthening of austenite and ferrite grains related to the morphology change and grain refinement.A method by controlling the cold-rolled reduction is proposed to avoid the Lüders strain.
基金The authors acknowledge the financial supports from the National Natural Science Foundation of China(Nos.51861135302 and 51831002)Fundamental Research Funds for the Central Universities,China(No.FRF-TP-18-002C2).
文摘Advanced high-strength steels have been widely used to improve the crashworthiness and lightweight of vehicles.Different from the popular cold stamping,hot forming of boron-alloyed manganese steels,such as 22MnB5,could produce ultra-high-strength steel parts without springback and with accurate control of dimensions.Moreover,hot-formed medium-Mn steels could have many advantages,including better mechanical properties and lower production cost,over hot-formed 22MnB5.This paper reviews the hot forming process in the automotive industry,hot-formed steel grades,and medium-Mn steel grades and their application in hot forming in depth.In particular,the adaptabilities of medium-Mn steels and the presently popular 22MnB5 into hot forming were compared thoroughly.Future research should focus on the technological issues encountered in hot forming of medium-Mn steels to promote their commercialization.
基金supported by the National Natural Science Foundation of China(Grant Nos.52071322,51771192 and U1708252)。
文摘An approach of optimizing the intercritical annealing path in a 0.2C-5Mn medium-Mn steel is presented by introducing precursor microstructure prior to normal austenite reverted transformation(ART)annealing.The steel is fi rstly pre-annealed at diff erent intercritical temperatures to form designed precursor microstructures.Then,they are employed for subsequent conventional ART annealing processing.It is found that pre-annealing at relative high intercritical temperatures can promote precipitation and dissolution of the carbide in the steel and re-distribute the C and Mn in the microstructures.The produced microstructural precursors show excellent merits in accelerating the austenite reversions in subsequent normal ART processing and assisting the RA formation.Tensile testing reveals that the excellent strength-elongation balance can be achieved in the heat-treated samples using diff erent microstructural precursors,which suggests the potential applicability in producing the medium-Mn steels with shortened processing period.
基金supported by the Innovation and Technology Fund(ITF)(ITP/020/20AP)the National Key Research and Development Program of China(2021YFB3701704)+2 种基金the Guangdong Academy of Sciences(GDAS)Project of Science and Technology Development(2022GDASZH-2022010103)Jiangxi Science and Technology Plan Project(S2020CXTD0356)the Young Talent Support Project of Guangzhou Association for Science and Technology(QT20220101075)。
文摘Medium-manganese(Mn)steel(MMS)has remarkable characteristics of high strength,strong work-hardening capacity,and wear resistance,being a promising third-generation advanced high-strength steel with lower raw material cost compared with other generations of advanced high-strength steel.The chemical composition and processing route play critical roles in determining the microstructural evolution of the MMS,and the microstructure composition significantly influences the mechanical,corrosion and wear properties of the steel.Hence,a lot of research work focus on exploring the direct relation between microstructural evolution and mechanical/corrosion/wear properties,and the progress has the following crucial aspects:(1)alloying design on the phase composition and carbide precipitation,(2)processing route on regulating microstructure evolution and twinning-induced plasticity and/or transformation-induced plasticity strengthening mechanism,(3)work-hardening,corrosion,and corrosion resistance of the regulated MMS,and(4)fracture and failure mechanism of MMS under tensile,corrosion and wear damages,as well as the improvement strategies.
基金The authors gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft(DFG)within the Collaborative Research Center(SFB)761‘Steel-ab initio:Quantum mechanics guided design of new Fe-based materials’and the project BL402/49-1.H.W.Luo is thankful for the financial supports from the National Natural Science Foundation of China(Nos.51861135302 and 51831002).Dr.Bernd Böttger at ACCESS e.V.is acknowledged for the helpful discussions.The synchrotron high-energy X-ray diffraction measurements were carried out at the Powder Diffraction and Total Scattering Beamline P02.1 of PETRA III at DESY(No.I-20181007),a member of the Helmholtz Association(HGF),which is gratefully acknowledged.Dr.Martin Etter at DESY is acknowledged for his support of acquiring HEXRD data.
文摘Medium-Mn steels have attracted immense attention for automotive applications owing to their outstanding combination of high strength and superior ductility.This steel class is generally characterized by an ultrafine-grained duplex microstructure consisting of ferrite and a large amount of austenite.Such a unique microstructure is processed by intercritical annealing,where austenite reversion occurs in a fine martensitic matrix.In the present study,austenite reversion in a medium-Mn alloy was simulated by the multiphase-field approach using the commercial software MICRESS®coupled with the thermodynamic database TCFE8 and the kinetic database MOBFE2.In particular,a faceted anisotropy model was incorporated to replicate the lamellar morphology of reversed austenite.The simulated microstructural morphology and phase transformation kinetics(indicated by the amount of phase)concurred well with experimental observations by scanning electron microscopy and in situ synchrotron high-energy X-ray diffraction,respectively.
基金supported by the National Research Foundation of Korea(NRF)(No.2020R1F1A1070808)supported by the Korea Institute for Advancement of Technology(KIAT)grant funded by the Korea Government(MOTIE)(P0008425,The Competency Development Program for Industry Specialist)。
文摘This work demonstrated the viability of friction stir welding for the welding of medium-Mn steels when used as cryogenic vessel materials for liquefied gas storage.We used an intercritically annealed Fe-7 Mn-0.2 C-3 Al(wt.%)steel with a dual-phase(α'martensite andγ_(R) retained austenite)nanolaminate structure as a base material and systematically compared its microstructure and impact toughness after friction stir and tungsten inert gas welding.The friction stir welded specimen exhibited a large amount ofγ_(R) phase owing to a relatively low temperature during welding,whereas the tungsten inert gas welded specimen comprised only theα'phase.Furthermore,the friction stir welded steel exhibited a tuned morphology of nanoscale globular microstructure at the weld zone and did not exhibit any prior austenite grain boundary due to active recrystallization caused by deformation during welding.The preserved fraction ofγ_(R) and morphological tuning in the weldment improved the impact toughness of the friction stir welded steel at low temperatures.In the steel processed by tungsten inert gas welding,the notch crack propagated rapidly along the prior austenite grain boundaries-weakened by Mn and P segregations-resulting in poor impact toughness.However,the friction stir welded steel exhibited a higher resistance against notch crack propagation due to the slow crack propagation along the ultrafine ferrite/ferrite(α/α)interfaces,damage tolerance by the active transformation-induced plasticity from the large amount ofγR,and enhanced boundary cohesion by suppressed Mn and P segregations.
基金This work was supported by the High-level Scientific Research Foundation for the Introduction of Talent of Beijing Jiaotong University(Grant No.M14RC00010).
文摘The microstructure and mechanical properties of a V-microalloyed Al-containing medium-Mn steel after hot rolling and intercritical annealing(IA)are explored.The tested steel exhibits a fne multiphase microstructure consisting of bimodal sizes of ferrite and retained austenite plus considerable amount of fne VC and/or M3C precipitates.Physical-chemical phase analysis shows that about 71.0%of the total V is in VC phase and more than 93%of VC particles is less than 5 nm.The calculated precipitation strengthening values of VC are^347 and^234 MPa for the specimens intercritically annealed at 625 and 750℃,respectively.An excellent combination of strength and ductility as high as^50 GPa%and yield strength(YS)of 890 MPa was obtained at intercritical temperature(TIA)of 725℃,although it does not correspond to the maximum precipitation strengthening of VC phase.Therefore,it is suggested that an optimization of TIA corresponding to both excellent combination of strength and ductility and high YS should be further explored through chemical composition and IA process optimization.
基金the support of the National Key Research and Development Program of Thirteenth Five-Year Plan Period(Grant No.2017YFB0304400)the National Natural Science Foundation of China(Grant No.51574028).
文摘The influence of microstructural characteristics on Lu¨ders strain and mechanical properties was explored by means of altering thermo-mechanical circumstances in an intercritical annealing(IA)medium-Mn Fe-11Mn-0.09C-0.25Si(wt.%)steel.By IA of cold-rolled samples with severe plastic deformation,exclusively equiaxed dual phases were obtained because of active recovery and recrystallization.The equiaxed austenite(gamma E)with a larger size and inadequate chemical concentration was more readily transformed into martensite,and subsequent transformation-induced plasticity(TRIP)effect was triggered actively at relatively higher IA temperature,lessening localized deformation.In addition,grown-in dislocations were prone to multiply and migrate around a broad mean free path for coarser equiaxed ferrite(alpha E)due to weakening dynamic recovery;therefore,it was the ensuing increased mobility of dislocations instead of reserving plentiful initial dislocation density that facilitated the propagation velocity of Luders bands and the accumulation of work hardening.In contrast,the bimodal-grained microstructure with lath-like and equiaxed austenite(gamma L+gamma E)satisfactorily contributed to a smaller yield point elongation(YPE)without compromise of comprehensive mechanical properties on the grounds that austenitic gradient stability gave rise to discontinuous but sustainable TRIP effect and incremental work hardening.Hence,Luders strain is closely related to the absence of work hardening in the region which yields locally.It follows that the decreased stability of retained austenite,favorable mobility of dislocations and the bimodal-grained structure all prominently make up for the insufficiency of work hardening,thereof resulting in a limited YPE.
基金financially supported by the National Natural Science Foundation of China(Nos.52271119 and U1760116)the I nnovation Ability Promotion Program of Hebei(No.22567609H).
文摘Often,the addition of more than 4 wt.%Mn to medium-Mn steels is necessary to enhance the thermal stability of intercritical austenite for achieving sufficient amounts of retained austenite(RA)at room tem-perature.In this paper,a medium-Mn steel with Mn content as low as 2.7 wt.%was designed via alloying with a small amount of Al,and the microstructure and mechanical properties of the steel,subjected to intercritical annealing(IA)at 745°C for different times followed by oil quenching,were investigated.Results show that the volume fraction of RA increases first and then decreases with IA time,with the maximum of 0.36 obtained at IA time of 50 min.It is demonstrated that Al addition slows down the in-terface migration and growth kinetics of reverted austenite via retarding C diffusion in ferrite during IA,which,hence,decreases the amount and size of the reverted austenite and partitions more C and Mn into it.This suggests that Al plays a favorable role in enhancing the thermal stability of reverted austenite and increasing the amount of austenite retained at room temperature.Due to the presence of large amounts of RA and the strong transformation-induced plasticity effect generated during plastic deformation,the steel exhibits persistent high strain hardening and superior mechanical properties,comparable to those of reported medium-Mn steels containing higher Mn content.The present result offers a new insight into the role of Al in adjusting microstructure-property relationships and opens a promising way for designing low-cost,high performance medium-Mn steels with low Mn content for industrial applications.
基金the financial support received from the National Natural Science Foundation of China(Grant No.U1964204).
文摘As a representative of steels available in the market,medium-Mn steel shows vast application prospects in lightweight automobile fields.This review details the research progress of medium-Mn steels,focusing on the following aspects.The roles of common adding elements,rolling technologies,and various heat treatments on the microstructure and mechanical properties of medium-Mn steel are analyzed,thus pro-viding references for designing tailored medium-Mn steel with excellent performance.Considering that hydrogen embrittlement is a challenge faced in the development of high-strength steel,the hydrogen embrittlement behavior of medium-Mn steel is also discussed,particularly emphasizing the influence of microstructure,hydrogen concentration,strain,etc.Furthermore,practical strategies to improve resistance to hydrogen embrittlement are summarized.Finally,this review provides prospects for the development and research prospects of medium-Mn steel.
基金Item Sponsored by National Natural Science Foundation of China(51371057,51261130091)National Basic Research Program of China(2010CB630803)Danish National Research Foundation(DNRF86-5)
文摘A medium-Mn steel （0.2C5Mn） was processed by intercritical annealing at different temperatures （625 ℃ and 650 ℃ ）. An ultrafine-grained micro-duplex structure consisting of alternating austenite and ferrite laths was de- veloped by austenite reverse transformation （ART） during intercritical annealing after forging and hot rolling. Ultra- high ductility with a total elongation higher than 30% was achieved in the temperature range from -196 ℃ to 200 ℃, and high impact toughness no less than 200 J at -40 ℃ was obtained. Based on the analysis of microstructure and mechanical properties, it was found that the enhanced ductility was determined by the phase transformation effect of austenite （TRIP effect）, while the delayed ductile to brittle transition was controlled by austenite stability.
基金supported by the National Key R&D Pro-gram of China(No.2017YFA0204402)the NSFC Basic Science Cen-ter Program for“Multiscale Problems in Nonlinear Mechanics”(No.11988102)the National Natural Science Foundation of China(Nos.11790293 and 52192591).
文摘A new medium-Mn steel was designed to achieve unprecedented tensile properties,with a yield strength beyond 1.1 GPa and a uniform elongation over 50%.The tensile behavior shows a heterogeneous deforma-tion feature,which displays a yield drop followed by a large Lüders band strain and several Portevin-Le Châtelier bands.Multiple strain hardening mechanisms for excellent tensile properties were revealed.Firstly,non-uniform martensite transformation occurs only within a localized deformation band,and ini-tiation and propagation of every localized deformation band need only a small amount of martensite transformation,which can provide a persistent and complete transformation-induced-plasticity effect dur-ing a large strain range.Secondly,geometrically necessary dislocations induced from macroscopic strain gradient at the front of localized deformation band and microscopic strain gradient among various phases provide strong heter-deformation-induced hardening.Lastly,martensite formed by displacive shear trans-formation can inherently generate a high density of mobile screw dislocations,and interstitial C atoms segregated at phase boundaries and enriched in austenite play a vital role in the dislocation multipli-cation due to the dynamic strain aging effect,and these two effects provide a high density of mobile dislocations for strong strain hardening.
基金financially supported by the Department of Mechanical and Industrial Engineering(MTP),Norwegian University of Science and Technology(NTNU)the financial support of the Deutsche Forschungsgemeinschaft(DFG)within the Collaborative Research Center(SFB)761“Steel-ab initio:Quantum mechanics guided design of new Fe-based materials”。
文摘Fatigue crack growth(FCG)tests were conducted on a medium-Mn steel annealed at two intercritical annealing temperatures,resulting in different austenite(γ)to fe rrite(α)phase fractions and differentγ(meta-)stabilities.Novel in-situ hydrogen plasma charging was combined with in-situ cyclic loading in an environmental scanning electron microscope(ESEM).The in-situ hydrogen plasma cha rging increased the fatigue crack growth rate(FCGR)by up to two times in comparison with the reference tests in vacuum.Fractographic investigations showed a brittle-like crack growth or boundary cracking manner in the hydrogen environment while a ductile transgranular manner in vacuum.For both materials,the plastic deformation zone showed a reduced size along the hydrogen-influenced fracture path in comparison with that in vacuum.The difference in the hydrogen-assisted FCG of the medium-Mn steel with different microstructures was explained in terms of phase fraction,phase stability,yielding strength and hydrogen distribution.This refined study can help to understand the FCG mechanism without or with hydrogen under in-situ hydrogen charging conditions and can provide some insights from the applications point of view.
基金the joint financial support from the Natural Science Foundation of China and Bao Steel Group Co.Ltd(Grant No.U1460203)the International Science&Technology Cooperation Program of China(Grant No.2015DFG51950)
文摘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.
基金financial support from the Natural Science Foundation of China and BaoSteel Group Co.,Ltd.(Grant No.U1460203)the International Science&Technology Cooperation Program of China(Grant No.2015DFG51950)the Fundamental Research Funds for the Central Universities
文摘A new model on predicting the density of hot-rolled multi-phased medium-Mn steel has been presen ted on the basis of thermodynamic calculations. This is an integrated model, which includes one for calculating the retained austenite （RA） fraction and the other for volume expansion during the aus tenite-to martensite transformation, because both of them are key parameters for calculating the den- sity of steel at ambient temperature. The existing empirical equations for calculating Mx temperature and lattice constants of both martensite and austenite have been all rcassessed by the XRD measure ments on the microstructures of seven hot-rolled medium-Mn steels. Finally, the densities ot seven steels were calculated merely from compositions and compared with the measured ones. The differ ence between them is no more than 1 %, suggesting that the presented model should be of good value in designing the low density steels.
基金supported by the National Key R&D Program of China(No.2017YFB0304402)。
文摘The microstructural evolution of a cold-rolled and intercritical annealed medium-Mn steel(Fe-0.10C-5Mn)was investigated during uniaxial tensile testing.In-situ observations under scanning electron microscopy,transmission electron microscopy,and X-ray diffraction analysis were conducted to characterize the progressive transformation-induced plasticity process and associated fracture initiation mechanisms.These findings were discussed with the local strain measurements via digital image correlation.The results indicated that Lüders band formation in the steel was limited to 1.5%strain,which was mainly due to the early-stage martensitic phase transformation of a very small amount of the less stable large-sized retained austenite(RA),which led to localized stress concentrations and strain hardening and further retardation of yielding.The small-sized RA exhibited high stability and progressively transformed into martensite and contributed to a stably extended Portevin-Le Chatelier effect.The volume fraction of RA gradually decreased from 26.8%to 8.2%prior to fracture.In the late deformation stage,fracture initiation primarily occurred at the austenite/martensite and ferrite/martensite interfaces and the ferrite phase.
