Multiscale Study on Low Temperature Crack Resistance Mechanism of Steel Slag Asphalt Mixture

The objective of this paper was to study low temperature crack resistance mechanism of steel slag asphalt mixture(SAM).Thermal stress restrained specimen test(TSRST)and three-point bending test were carried out to evaluate the low-temperature crack resistance of SAM and basalt asphalt mixture(BAM).Based on the digital image correlation technique(DIC),the strain field distribution and crack propagation of SAM were analyzed from the microscopic point of view,and a new index,crack length factor(C),was proposed to evaluate the crack resistance of the asphalt mixture.The crystal phase composition and microstructure of steel slag aggregate(SA)and basalt aggregate(BA)were studied by X-ray diffraction(XRD)and scanning electron microscopy(SEM)to explore the low-temperature crack resistance mechanism of SAM.Results show that the low-temperature crack resistance of SAM is better than that of BAM;SAM has good integrity and persistent elastic deformation,and its bending failure mode is a hysteretic quasi-brittle failure;The SA surface is evenly distributed with pores and has surface roughness.SA has the composition phase of alkaline aggregate-calcite(CaCO3),so it has good adhesion to asphalt,which reveals the mechanism of excellent low-temperature crack resistance of SAM.

Impact of Mo/Ni alloying on microstructural modulation and low-temperature toughness of high-strength low-alloy steel

The high-strength low-alloy steel plates with varying Ni/Mo contents were manufactured using the thermos-mechanical control process.The investigation was conducted to explore the effect of Ni/Mo microalloying on microstructure evolution and mechanical properties of the steel.The results revealed that the increase in Ni content from 1 to 2 wt.%reduced the transition temperature of ferrite and the growth range of ferritic grain was narrowed,which promoted grain refinement.The optimized combination of grain size,high-angle grain boundaries(HAGBs),and martensite-austenite(M-A)islands parameter contributed to the excellent impact toughness of S1 steel at-100℃(impact absorbed energy of 218.2 J at-100℃).As the Mo increases from 0 to 2 wt.%,the matrix structure changes from multiphase structure to granular bainite,which increases the average effective grain size to~4.62 pm and reduces HAGBs proportion to~36.22%.With these changes,the low-temperature impact toughness of S3 steel is weakened.In addition,based on the analysis of the characteristics of crack propagation path,it was found that M-A islands with low content(~2.21%)and small size(~1.76 pm)significantly retarded crack propagation,and the fracture model of M-A islands with different morphologies was further proposed.Furthermore,correlation between behaviour of delamination and toughness was further analysed by observing delamination size and impact energy parameters.

Effect of short-time low-temperature austenitizing on microstructure and mechanical properties of DT300 ultra-high strength steel fabricated by laser powder bed fusion

To address the inhomogeneous microstructure and improve the mechanical properties of DT300 ultra-high strength steel specimens fabricated by laser powder bed fusion,different post-heat treatment schedules are performed.With the increase in austenitizing temperature and time,the migration rate of austenite grain boundaries continuously increases with the dissolution of nano-carbides,and the formation of nano-oxides and twin martensite is also inhibited accordingly.The rapid growth in the size of prior austenite grains and martensite laths,as well as the decrease in the content of nano-oxides and twin martensite,led to a rapid decrease in the strength(yield strength and ultimate tensile strength)from HT2 to HTF specimens.The HT1 specimens(austenitizing at 830℃for 30 min,then oil quenching and tempering at 300℃for 120 min and finally air cooling)display excellent mechanical properties of yield strength of 1572 MPa,ultimate tensile strength of 1847 MPa,elongation of 9.84%,and fracture toughness of 106 MPa m^(1/2),which are counterparts to those of conventional DT300 steel forgings after heat treatment.

Medium-Mn steels for hot forming application in the automotive industry

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.

Multiphase-field simulation of austenite reversion in medium-Mn steels

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.

Effect of low-temperature ausforming on bainitic transformation and mechanical properties in ultrahigh-strength bainitic steel

Low-temperature ausforming(LT-AF)prior to bainitic transformation leads to a noticeable acceleration of bainitic transformation kinetics;however,the effect of LT-AF on the retained austenite(RA)features and the resulting mechanical properties is still unclear.LT-AF was applied to ultrahigh-strength bainitic steel before austempering.The deformation behavior and the resulting dislocation substructures were investigated by thermomechanical simulator and transmission electron microscopy(TEM).The planar dislocation structures produced during deformation at 350℃ accelerate the bainitic transformation kinetics during isothermal holding.The effect of LT-AF on the bainitic transformation kinetics and the features of RA was elucidated via dilatometer measurement,TEM,scanning electron microscopy,and X-ray diffraction.It is observed that LT-AF not only retains more RA content but also facilitates improved RA stability.This trend is mainly due to the large amounts of planar dislocations in RA and bainitic laths inherited from undercooled austenite caused by LT-AF,the decrease in bainitic sheaves size,and the increase in filmy RA content compared to the sample without ausforming.A large fraction of filmy RA with high stability and the refinement of bainitic sheaves obtained by LT-AF remarkably enhance the strain hardening capacity and achieve significantly better ductility compared to the directly austempered sample.

Recent progress in microstructural evolution,mechanical and corrosion properties of medium-Mn steel

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.

Correlation between Primary and Secondary Recrystallization Texture Components in Low-temperature Reheated Grain-oriented Silicon Steel

Low-temperature slab-reheated grain-oriented silicon steel is characterized by a sharp {411}〈148〉 primary recrystallization texture. To date, the influence of this texture on secondary recrystallization is not clear. Microtextures in primary and secondary reerystallized sheets of low-temperature reheated grain-oriented silicon steel were examined using electron backscatter diffraction. By comparing the textures and microstructures of specific primary reerystallized grains neighboring secondary grains with those of other primary grains, the influences of primary re- crystallization textures and microstructures on the orientations of secondary grains were investigated. Results show that for low-temperature reheated graiworiented silicon steel, the primary recrystallization sheet comprises { 411 } 〈148〉, {111}〈112〉, and {001}〈120〉 texture componems. During secondary recrystallization, the {111}〈112〉 primary recrystallized grains were easily consumed by abnormally grown Goss, deviated Goss, Brass, or {210}〈001〉grains ;the { 411 }〈148〉 primary recrystallized grains were more resistant to being swallowed; and the {001} 〈120 grains were the most resistant to being consumed. For a particular primary grain, the distribution of its surrounding grain boundaries determined how easily it is consumed during secondary recrystallization. Primary grains surrounded by 20°- 45° grain boundaries were consumed much earlier than those having grain boundaries above 45°, which is in accordance with high-energy grain boundary theory. In addition, special ∑9 boundaries between {411}〈148〉 and Goss grains move more slowly than ∑9 boundaries between {111 }〈112〉 and Goss grains, which is attributed to the different positions of 〈110〉 rotation axis with respect to the normals of grain boundaries.

Deformation mechanisms for a new medium-Mn steel with 1.1 GPa yield strength and 50% uniform elongation

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.

Roles of Al in enhancing the thermal stability of reverted austenite and mechanical properties of a medium-Mn TRIP steel containing 2.7 Mn

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.

Pretreatment of metallurgical sewage via vacuum distillation driven by low-temperature exhausted gas from steel plants

The metallurgical sewage has very complex component and a significant environmental perniciousness and needs high treatment costs. In addition, too much low-temperature waste heat is emitted owing to the lack of suitable users. Considering these concerns, a low-temperature-driven pretreatment method via vacuum distillation was proposed to treat the sewage from the metallurgical production. It uses the sensible heat carried by low-temperature exhausted gases to drive the distillation of sewage. The distilled water can be reused into the process as new water supply, while the enriched wastewater is discharged into the sewage treatment center for subsequent treatment. Converter dust removal sewage was chosen to perform an experimental observation. The variations of chemical oxygen demand, ammonia nitrogen, suspended solids, electrical conductivity, and pH of the condensate under different vacuum degrees and evaporation rates were mainly investigated. It can be found that the quality of the condensate gets better under certain conditions, which validates the feasibility of the proposed approach. Furthermore, by comprehensively analyzing the water quality indices and their influencing factors, the optimal vacuum degree was suggested to be controlled between 0.07 and 0.09 MPa, and the best evaporation rate was between 40 and 60%.

The microstructure evolution and tensile properties of medium-Mn steel heat-treated by a two-step annealing process

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.

Deformation-induced martensitic transformation kinetics and correlative micromechanical behavior of medium-Mn transformation-induced plasticity steel

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.

Improvement of Mechanical Properties of a Medium-Mn TRIP Steel by Precursor Microstructure Control

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.

Microstructure and mechanical properties of hot-rolled V-microalloyed Al-containing medium-Mn steel

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.

Effect of microstructure evolution on Luders strain and tensile properties in an intercritical annealing medium-Mn steel

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.

High Ductility and Toughness of a Micro-duplex Medium-Mn Steel in a Large Temperature Range from—196 ℃ to 200 ℃

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.

Improving toughness of medium-Mn steels after friction stir welding through grain morphology tuning

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.

Evaluation of hydrogen effect on the fatigue crack growth behavior of medium-Mn steels via in-situ hydrogen plasma charging in an environmental scanning electron microscope

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.

Strengthening and toughening mechanism of a Cu-bearing high-strength low-alloy steel with refined tempered martensite/bainite(M/B)matrix and minor inter-critical ferrite

The microstructure–mechanical property relationship of a Cu-bearing low-carbon high-strength low-alloy steel,subjected to a novel multistage heat treatment including quenching(Q),lamellarization(L)and tempering(T),is presented.Yield strength of 989.5 MPa and average toughness at-80℃of 41 J were obtained in this steel after quenching and tempering(QT)heat treatments.Specimen QLT gained a little lower yield strength(982.5 MPa),but greatly enhanced average toughness at-80℃(137 J).To further clarify the strengthening and toughening mechanisms in specimen QLT,parameters of microstructural characteristic and crack propagation process were compared and analyzed for specimens Q,QL,QT and QLT.The microstructure of tempered martensite/bainite(M/B)in specimen QT changed to refined tempered M/B matrix mixed with minor IF(inter-critical ferrite)in specimen QLT.Cu-rich precipitates existed in tempered M/B for both specimens QT and QLT,as well as in IF.Compared with QT,adding a lamellarization step before tempering made the effective grains of specimen QLT refined and also led to coarser Cu-rich precipitates in tempered M/B matrix.The weaker strengthening effect of coarser Cu-rich precipitates should be a key reason for the slightly lower yield strength in specimen QLT than in specimen QT.No austenite was found in all specimens Q,QL,QT and QLT.Specimen QLT showed purely ductile fracture mode at-80℃due to the refined effective grains.The greatly improved toughness is mainly attributed to the enhanced energy of crack propagation.The combination of refined microstructure,softened matrix and deformation of minor'soft'IF during crack propagation led to the most superior toughness of specimen QLT among all specimens.