High- and medium-Mn (H/M-Mn) base lightweight steels are a class of ultrastrong structural materials with high ductility compared to their low-Mn counterparts with low strength and poor ductility.However, producing th...High- and medium-Mn (H/M-Mn) base lightweight steels are a class of ultrastrong structural materials with high ductility compared to their low-Mn counterparts with low strength and poor ductility.However, producing these H/M-Mn materials requires the advanced or high-tech manufacturing techniques, which can unavoidably provoke labor and cost concerns. Herein, we have developed a facilestrategy that circumvents the strength–ductility trade-off in low-Mn ferritic lightweight steels, by employing low-temperature tempering-induced partitioning (LTP). This LTP treatment affords a typical Fe-2.8Mn-5.7Al-0.3C (wt.%) steel with a heterogeneous size-distribution of metastable austenite embeddedin a ferrite matrix for partitioning more carbon into smaller austenite grains than into the larger austenite ones. This size-dependent partitioning results in slip plane spacing modification and lattice strain,which act through dislocation engineering. We ascribe the simultaneous improvement in strength andtotal elongation to both the size-dependent dislocation movement in austenite grains and the controlleddeformation-induced martensitic transformation. The low-carbon-partitioned large austenite grains increase the strength and ductility as a consequence of the combined martensitic transformation andhigh dislocation density-induced hardening and by interface strengthening. Additionally, high-carbonpartitioned small austenite grains enhance the strength and ductility by planar dislocation glide (inthe low strain regime) and by cross-slipping and delayed martensitic transformation (in the high strainregime). The concept of size-dependent dislocation engineering may provide different pathways for developing a wide range of heterogeneous-structured low-Mn lightweight steels, suggesting that LTP maybe desirable for broad industrial applications at an economic cost.展开更多
A lightweight aggregate concrete-filled steel tube(LACFST) spatial truss beam was tested under bending load. The performance was studied by the analysis of the beam deflection and strains in its chords and webs. Accor...A lightweight aggregate concrete-filled steel tube(LACFST) spatial truss beam was tested under bending load. The performance was studied by the analysis of the beam deflection and strains in its chords and webs. According to the test results, several assumptions were made to deduce the bearing capacity calculation method based on the force balance of the whole section. An optimal dimension relationship for the truss beam chords was proposed and verified by finite element analysis. Results show that the LACFST spatial truss beam failed after excessive deflection. The strain distribution agreed with Bernoulli-Euler theoretical prediction. The truss beam flexural bearing capacity calculation results matched test evidence with only a 3% difference between the two. Finite element analyses with different chord dimensions show that the ultimate bearing capacity increases as the chord dimensions increase when the chords have a diameter smaller than optimal one; otherwise, it remains almost unchanged as the chord dimensions increase.展开更多
New and increasingly sharp laws for the reduction of emissions from commercial vehicles entered into force in the European Union since 1993 with the aim of substan-tially reducing pollutants and emissions from trucks ...New and increasingly sharp laws for the reduction of emissions from commercial vehicles entered into force in the European Union since 1993 with the aim of substan-tially reducing pollutants and emissions from trucks and buses. Thereby the maximum levels for the emission of particulate matter （basically soot particles） as well as oxides of nitrogen （NOx） have been subsequently lowered. To comply with these demands, commercial vehicle producers had to introduce new emission reducing technologies of which exhaust-gas recirculation （EGR）,展开更多
The composition,morphology,size,and number of inclusions in Fe-22Mn-xAl-0.7C(x=0.5%,5.2%,and 10.5%)lightweight steels after smelting and heat treatment experiments are characterized by scanning electron microscopy and...The composition,morphology,size,and number of inclusions in Fe-22Mn-xAl-0.7C(x=0.5%,5.2%,and 10.5%)lightweight steels after smelting and heat treatment experiments are characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy from both 2D and 3D perspectives.The inclusions are classified into eight categories according to chemistry and morphology.For the steel sample with a low Al content(0.5%),the inclusions are MnS,AlN-MnS,and Al_(2)O_(3)-MnS,among which Al_(2)O_(3)-MnS is the primary type.With the increase in Al content in the steel sample(to 5.2%),the populations of AlN and AlN-MnS inclusions of 1-3μm in diameter increase.A further increase in Al content(to 10.5%)leads to a significant decrease in the amount of AlN inclusions of 1-3μm in diameter,and an increase in the amount of AlN-MnS inclusions.The precipitation behavior during the phase transformation is also studied by FactSage 8.0 thermodynamic software,and a precipitation mechanism is proposed based on the calculation results.During the heat treatment,AlN inclusions re-precipitate out,due to the interactions between Al and dissolved N in the steel matrix.However,AlN inclusions cannot grow large because of unfavorable kinetic conditions.The re-precipitation phenomenon of AlN is predominant under low Al and high N conditions but not at high Al cases.展开更多
The microstructures and mechanical properties of ferrite-based lightweight steel with different compositions were investigated by tensile test,scanning electron microscopy（SEM）,transmission electron microscopy（TEM...The microstructures and mechanical properties of ferrite-based lightweight steel with different compositions were investigated by tensile test,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction（XRD）and thermodynamic calculation（TC）.It was shown that the ferrite-based lightweight steels with 5wt.%or 8wt.%Al were basically composed of ferrite,austenite andκ-carbide.As the annealing temperature increased,the content of the austenite in the steel gradually increased,while theκ-carbide gradually decomposed and finally disappeared.The mechanical properties of the steel with 5wt.%Al and 2wt.%Cr,composed of ferrite and Cr7C3carbide at different annealing temperatures,were significantly inferior to those of others.The steel containing 5wt.%Al,annealed at 820°C for 50sthen rapidly cooled to 400°C and held for 180s,can obtain the best product of strength and elongation（PSE）of 31242MPa·%.The austenite stability of the steel is better,and its PSE is higher.In addition,the steel with higher PSE has a more stable instantaneous strain hardening exponent（n value）,which is mainly caused by the effect of transformation induced plasticity（TRIP）.When theκ-carbide or Cr7C3carbide existed in the microstructure of the steel,there was an obvious yield plateau in the tensile curve,while its PSE decreased significantly.展开更多
The processing of innovative lightweight materials to sheet metal components and assemblies with globally or locally defined properties is the object of this work. It takes a load-dependent design of components and as...The processing of innovative lightweight materials to sheet metal components and assemblies with globally or locally defined properties is the object of this work. It takes a load-dependent design of components and assemblies, for example, based on the composition of different construction materials or a targeted setting of component areas with specified characteristics to fully exploit the lightweight potential when substituting conventionally used materials. Different process chains for the manufacturing of roll-formed long products made of magnesium alloys and high-strength steels with locally defined properties will be presented in this paper. Depending on the kind of material to be formed and the desired product characteristics, different temperature managements are needed for capable processes. Due to limited formability at room temperature, magnesium alloys require a heating of the forming zones above 200–225 °C throughout the bending process in order to activate additional gliding planes and to avoid any failures in the radii. The realization of local hardening effects requires at least one process-integrated heat treatment when roll forming manganese–boron steels. For both processes, it is imperative to realize a heating and cooling down or quenching appropriate for the manufacturing of long products with the required quality. Additionally, proper line speeds that allow a continuously operated economical production have to be considered. Research results including design, FEA, realization and experimentation of the mentioned process chains and strategies will be described in detail.展开更多
During the conventional continuous casting process of high-aluminum steels(w([Al])>0.5 wt.%),some components of slag,such as SiO_(2),B_(2)O_(3),and TiO_(2),could be reduced by aluminum in molten steel.Therefore,the...During the conventional continuous casting process of high-aluminum steels(w([Al])>0.5 wt.%),some components of slag,such as SiO_(2),B_(2)O_(3),and TiO_(2),could be reduced by aluminum in molten steel.Therefore,the CaO-BaO-Al_(2)O_(3)-CaF2-Li2O non-reactive mold fluxes were designed using the simplex grid method and molecular dynamics to mitigate the slag-metal interface reaction and stabilize the performance of mold fluxes.The results show that the components of nonreactive quinary system are 20-40 wt.% CaO,14-34 wt.% BaO,14-34 wt.% Al_(2)O_(3),4-12 wt.% F,and 4-8 wt.% Li_(2)O.Molecular dynamics simulation results show that[AlO_(4)]-tetrahedron acts as network formers and melt network structure is mainly chain and lamellar in the low-viscosity area.The cross sections of w(F)=8 wt.%,w(Li_(2)O)=8 wt.%and w(F)=12 wt.%,w(Li2O)=8 wt.% are important reference sections for the design of mold flux,with the compositions of 22-40 wt.%CaO,14-34 wt.% BaO,20-34 wt.%Al_(2)O_(3) and 23-40 wt.%CaO,14-34 wt.%BaO,20-28 wt.% Al_(2)O_(3),respectively.展开更多
Precipitation strengthening has been widely adopted in austenitic low-density steel owing to excellent hardened effects.This approach generally employs the coherentκ′carbides and non-coherent B2 particles.Revealing ...Precipitation strengthening has been widely adopted in austenitic low-density steel owing to excellent hardened effects.This approach generally employs the coherentκ′carbides and non-coherent B2 particles.Revealing the precipitation transformation pathway is decisive for further optimizing the microstructures under specific engineering applications.Herein,the detailed precipitation sequence of Fe-28Mn-11Al-1C-5Ni(wt%)austenitic low-density steel as well as its influence on mechanical prop-erties during aging process is systematically investigated.Our results reveal that nano-sizedκ′carbides domains(2 nm)exist in the solution-treated specimen.During aging at 500℃for 1 h,the cuboidalκ′carbides(15-20 nm)uniformly disperse in austenite matrix.However,after aging at 700℃for 15 min,the coarsenκ′carbides(30-35 nm)inhomogeneously distribute and align preferentially along the〈100〉directions.Further,extending the aging time to 60 min,the needle-type B2 particles replace theκ′carbides due to the enrichment of Ni elements at the phase boundaries among the austenite andκ′carbides.After aging at 900℃,κ′carbides entirely dissolve into the austenite matrix,and the intragranular B2 particles are the sole precipitates in the austenite matrix and follow the K-S orientation relationship with austenite.The work hardening capability seriously deteriorates due to the shearing ofκ′carbides by gliding dislocations.While the intragranular B2 particles preserve excellent work hardening rate by dislocations bow-out mechanism.The present work is meaningful for guiding the design of new generation dual-nano precipitation austenitic lightweight steel.展开更多
基金The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.Patent application(Korean Patent application number 10-2020-0172118)has been filed based on the results of this study。
文摘High- and medium-Mn (H/M-Mn) base lightweight steels are a class of ultrastrong structural materials with high ductility compared to their low-Mn counterparts with low strength and poor ductility.However, producing these H/M-Mn materials requires the advanced or high-tech manufacturing techniques, which can unavoidably provoke labor and cost concerns. Herein, we have developed a facilestrategy that circumvents the strength–ductility trade-off in low-Mn ferritic lightweight steels, by employing low-temperature tempering-induced partitioning (LTP). This LTP treatment affords a typical Fe-2.8Mn-5.7Al-0.3C (wt.%) steel with a heterogeneous size-distribution of metastable austenite embeddedin a ferrite matrix for partitioning more carbon into smaller austenite grains than into the larger austenite ones. This size-dependent partitioning results in slip plane spacing modification and lattice strain,which act through dislocation engineering. We ascribe the simultaneous improvement in strength andtotal elongation to both the size-dependent dislocation movement in austenite grains and the controlleddeformation-induced martensitic transformation. The low-carbon-partitioned large austenite grains increase the strength and ductility as a consequence of the combined martensitic transformation andhigh dislocation density-induced hardening and by interface strengthening. Additionally, high-carbonpartitioned small austenite grains enhance the strength and ductility by planar dislocation glide (inthe low strain regime) and by cross-slipping and delayed martensitic transformation (in the high strainregime). The concept of size-dependent dislocation engineering may provide different pathways for developing a wide range of heterogeneous-structured low-Mn lightweight steels, suggesting that LTP maybe desirable for broad industrial applications at an economic cost.
基金Project(51208176)supported by the National Natural Science Foundation of ChinaProjects(2012M511187,2013T60493)supported by the China Postdoctoral Science FoundationProject(2015B17414)supported by the Fundamental Research Funds for the Central Universities,China
文摘A lightweight aggregate concrete-filled steel tube(LACFST) spatial truss beam was tested under bending load. The performance was studied by the analysis of the beam deflection and strains in its chords and webs. According to the test results, several assumptions were made to deduce the bearing capacity calculation method based on the force balance of the whole section. An optimal dimension relationship for the truss beam chords was proposed and verified by finite element analysis. Results show that the LACFST spatial truss beam failed after excessive deflection. The strain distribution agreed with Bernoulli-Euler theoretical prediction. The truss beam flexural bearing capacity calculation results matched test evidence with only a 3% difference between the two. Finite element analyses with different chord dimensions show that the ultimate bearing capacity increases as the chord dimensions increase when the chords have a diameter smaller than optimal one; otherwise, it remains almost unchanged as the chord dimensions increase.
文摘New and increasingly sharp laws for the reduction of emissions from commercial vehicles entered into force in the European Union since 1993 with the aim of substan-tially reducing pollutants and emissions from trucks and buses. Thereby the maximum levels for the emission of particulate matter （basically soot particles） as well as oxides of nitrogen （NOx） have been subsequently lowered. To comply with these demands, commercial vehicle producers had to introduce new emission reducing technologies of which exhaust-gas recirculation （EGR）,
基金This work was supported by the National Natural Science Foundation of China(Grant No.U20A20270).
文摘The composition,morphology,size,and number of inclusions in Fe-22Mn-xAl-0.7C(x=0.5%,5.2%,and 10.5%)lightweight steels after smelting and heat treatment experiments are characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy from both 2D and 3D perspectives.The inclusions are classified into eight categories according to chemistry and morphology.For the steel sample with a low Al content(0.5%),the inclusions are MnS,AlN-MnS,and Al_(2)O_(3)-MnS,among which Al_(2)O_(3)-MnS is the primary type.With the increase in Al content in the steel sample(to 5.2%),the populations of AlN and AlN-MnS inclusions of 1-3μm in diameter increase.A further increase in Al content(to 10.5%)leads to a significant decrease in the amount of AlN inclusions of 1-3μm in diameter,and an increase in the amount of AlN-MnS inclusions.The precipitation behavior during the phase transformation is also studied by FactSage 8.0 thermodynamic software,and a precipitation mechanism is proposed based on the calculation results.During the heat treatment,AlN inclusions re-precipitate out,due to the interactions between Al and dissolved N in the steel matrix.However,AlN inclusions cannot grow large because of unfavorable kinetic conditions.The re-precipitation phenomenon of AlN is predominant under low Al and high N conditions but not at high Al cases.
基金supported by the Shanghai Municipal Natural Science Foundation(Grant No.17ZR1410400)the Shanghai Municipal Science and Technology Commission(Grant Nos.15DZ2260300,15DZ2260301)
文摘The microstructures and mechanical properties of ferrite-based lightweight steel with different compositions were investigated by tensile test,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffraction（XRD）and thermodynamic calculation（TC）.It was shown that the ferrite-based lightweight steels with 5wt.%or 8wt.%Al were basically composed of ferrite,austenite andκ-carbide.As the annealing temperature increased,the content of the austenite in the steel gradually increased,while theκ-carbide gradually decomposed and finally disappeared.The mechanical properties of the steel with 5wt.%Al and 2wt.%Cr,composed of ferrite and Cr7C3carbide at different annealing temperatures,were significantly inferior to those of others.The steel containing 5wt.%Al,annealed at 820°C for 50sthen rapidly cooled to 400°C and held for 180s,can obtain the best product of strength and elongation（PSE）of 31242MPa·%.The austenite stability of the steel is better,and its PSE is higher.In addition,the steel with higher PSE has a more stable instantaneous strain hardening exponent（n value）,which is mainly caused by the effect of transformation induced plasticity（TRIP）.When theκ-carbide or Cr7C3carbide existed in the microstructure of the steel,there was an obvious yield plateau in the tensile curve,while its PSE decreased significantly.
基金the Federal Government of Germanythe Free State of Saxony namely within the programs European Regional Development Fund and Innovative Regional Growth Cores
文摘The processing of innovative lightweight materials to sheet metal components and assemblies with globally or locally defined properties is the object of this work. It takes a load-dependent design of components and assemblies, for example, based on the composition of different construction materials or a targeted setting of component areas with specified characteristics to fully exploit the lightweight potential when substituting conventionally used materials. Different process chains for the manufacturing of roll-formed long products made of magnesium alloys and high-strength steels with locally defined properties will be presented in this paper. Depending on the kind of material to be formed and the desired product characteristics, different temperature managements are needed for capable processes. Due to limited formability at room temperature, magnesium alloys require a heating of the forming zones above 200–225 °C throughout the bending process in order to activate additional gliding planes and to avoid any failures in the radii. The realization of local hardening effects requires at least one process-integrated heat treatment when roll forming manganese–boron steels. For both processes, it is imperative to realize a heating and cooling down or quenching appropriate for the manufacturing of long products with the required quality. Additionally, proper line speeds that allow a continuously operated economical production have to be considered. Research results including design, FEA, realization and experimentation of the mentioned process chains and strategies will be described in detail.
基金the fund support from the National Natural Science Foundation of China(Project Nos.U20A20270 and U1660204).
文摘During the conventional continuous casting process of high-aluminum steels(w([Al])>0.5 wt.%),some components of slag,such as SiO_(2),B_(2)O_(3),and TiO_(2),could be reduced by aluminum in molten steel.Therefore,the CaO-BaO-Al_(2)O_(3)-CaF2-Li2O non-reactive mold fluxes were designed using the simplex grid method and molecular dynamics to mitigate the slag-metal interface reaction and stabilize the performance of mold fluxes.The results show that the components of nonreactive quinary system are 20-40 wt.% CaO,14-34 wt.% BaO,14-34 wt.% Al_(2)O_(3),4-12 wt.% F,and 4-8 wt.% Li_(2)O.Molecular dynamics simulation results show that[AlO_(4)]-tetrahedron acts as network formers and melt network structure is mainly chain and lamellar in the low-viscosity area.The cross sections of w(F)=8 wt.%,w(Li_(2)O)=8 wt.%and w(F)=12 wt.%,w(Li2O)=8 wt.% are important reference sections for the design of mold flux,with the compositions of 22-40 wt.%CaO,14-34 wt.% BaO,20-34 wt.%Al_(2)O_(3) and 23-40 wt.%CaO,14-34 wt.%BaO,20-28 wt.% Al_(2)O_(3),respectively.
基金supported by the National Natural Science Foundation of China(Nos.52171105 and 51871062).
文摘Precipitation strengthening has been widely adopted in austenitic low-density steel owing to excellent hardened effects.This approach generally employs the coherentκ′carbides and non-coherent B2 particles.Revealing the precipitation transformation pathway is decisive for further optimizing the microstructures under specific engineering applications.Herein,the detailed precipitation sequence of Fe-28Mn-11Al-1C-5Ni(wt%)austenitic low-density steel as well as its influence on mechanical prop-erties during aging process is systematically investigated.Our results reveal that nano-sizedκ′carbides domains(2 nm)exist in the solution-treated specimen.During aging at 500℃for 1 h,the cuboidalκ′carbides(15-20 nm)uniformly disperse in austenite matrix.However,after aging at 700℃for 15 min,the coarsenκ′carbides(30-35 nm)inhomogeneously distribute and align preferentially along the〈100〉directions.Further,extending the aging time to 60 min,the needle-type B2 particles replace theκ′carbides due to the enrichment of Ni elements at the phase boundaries among the austenite andκ′carbides.After aging at 900℃,κ′carbides entirely dissolve into the austenite matrix,and the intragranular B2 particles are the sole precipitates in the austenite matrix and follow the K-S orientation relationship with austenite.The work hardening capability seriously deteriorates due to the shearing ofκ′carbides by gliding dislocations.While the intragranular B2 particles preserve excellent work hardening rate by dislocations bow-out mechanism.The present work is meaningful for guiding the design of new generation dual-nano precipitation austenitic lightweight steel.
