| The application of high strength and high formability steels in automotive body can decrease thickness of components,thus achieving weight reduction.Density decreasing of high strength and high formability steels could further obtain weight reduction,therefore,the research hotspot of automobile steels are high strength,high formability and low-density steels.The ?-TRIP steel exhibits good strength and toughness,low density due to the aluminium addition,excellent welding performance because of the ? ferrite introduction.So 8-TRIP steel has a good prospect of industrial application and is included in key field of 'Made in China 2025 strategy'.But its strength is limited by the large fraction of ? ferrite.This has been a bottleneck of 8-TRIP steel development.The quenching and partitioning(QP)steel obtains a high strength with a microstructure of martensite and carbon-enriched stable austenite,which stability is enhanced by carbon partitioning from martensite to retained austenite.Meanwhile,thanks to the TRIP effect of austenite,the QP steel has high ductility.However,very sophisticated cooling equipment is required during QP process,resulting from the quenching temperature of QP steel is above ambient temperature.Combined high formability,low density and excellent welding performance of 8-TRIP steel,and the high strength of QP steel,novel?-QP steel has been designed.The novel high strength and ductility ?-QP steel not only achieves low density and excellent welding performance,but also simplifies the production by quenching temperature adjustment.In this paper,the material design,microstructure evolution and strength-toughness mechanism of 8-QP steel have been researched and discussed.(1)The novel quenching-tempering&partitioning(Q-T&P)heat treatment was proposed.based on aluminium-added 8-TRIP steel system,which separated the quenching and partitioning process.The novel ?-QP steel was developed.The mechanism of yield strength enchancement,induced by carbon trapping in ferrite of 8-QP during tempering&partitioning process was explained.The 3.5 wt%aluminum addition in steel permits the existing of a large fraction of ferrite,which allows more carbon enriched in austenite during intercritical annealing.The initial microstructure was adjusted through the Ms temperature by alloy design and intercritical annealing.The carbon is further enriched in austenite by tempering process.The Q-T&P alloy exhibited comparable mechanical properties with conventional Q&P steels.In addition,the addition of aluminium led to an approximately 5%density lower than in the conventional alloy.The ?-Q&P steel,obtained through Q-T&P heat treatment,has a large fraction of ferrite.During the tempering and partitioning process,the carbon partitioned into austenite from supersaturated martensite,meanwhile,carbon also diffused into the ferrite around dislocations,which formed by the quenching martensite transformation stress.The carbon trapping in ferrite for the dislocation locking is responsible for the yield strength enhancement after tempering process.(2)The elements partitioning during heat treatment of 8-Q&P steel was researched by using three-dimensional atom probe tomograohy(APT).The microstructure evolution of BCT martensite to BCC martensite has been verified by transmission EBSD.And the relationship between microstructure and mechanical properties has been discussed.The microstructure of experimental steel was ferrite,austenite and martensite after intercritical annealing,followed by quenching.The crystal structure of martensite was body centered tetragonl(BCT),due to the high carbon content.Then carbon ejected from martensite during the tempering,after which process the intercritical austenite region showed folding structure.The folding structure became more obvious with the tempering temperature increasing,which indicated that the partitioning was more completely.Furthermore,the crystal structure of martensite transformed into body centered cubic(BCC)from body centered tetragonl(BCT).The APT results indicated that carbon partitioned into austenite from martensite during tempering,and manganese enriched into austenite during both intercritical annealing and tempering.The tensile strength was not sensitive with the tempering,but the the yielding strength obviously increased due to carbon partitioned into ferrite.The ductility was improved by partitioning through decreasing hardness of martensite and increasing stability of austenite.(3)An abnormal volume expansion occurred in austenite transformation from mother phases of ferrite plus cementite during heating in high aluminium low-density steels.It has been revealed that the abnormal volume expansion was caused by lattice parameters increasing of austenite,resulting from eutectoid carbon content enhanced by high aluminium addition.The specific volume of austenite increases with its carbon content due to the lattice expansion by the interstitial solutes.The volume expansion or contraction during phase transformation rely on the fraction of ferrite and cementite involved in the austenite formation,since it affects the carbon enrichment in austenite and consequently its specific volume.The carbon content of austenite is determined by eutectoid carbon content,which is significantly influenced by alloy elements.The alloy elements usually move the eutectoid point to left,whilst the aluminium addition moves the eutectoid point to right.Carbon enriched therefore into austenite extensively at the austenite transformation from ferrite plus cementite,inducing the volume expansion.This volume expansion owing to austenite transformation has been proved by lattice parameter and specific volume calculation and dilatometry study.(4)The high aluminium addition has a great influence on cementite dissolution,transformation thermodynamics and kinetics of ferrite to austenite transformation during heating process.Its influence mechanism and microstructure evolution during intercritical annealing has been researched.The hot-rolled microstructure contained more ferrite(68±1vol%)with finer grain size of 8.3±0.8 ?m due to the addition of aluminum compared the steel without aluminium addition consisting of 55±5 vol%of ferrite with 10.4±0.4 ?m grain size.In addition,aluminium addition refined the inter-lamellae structure of pearlite in aluminium-added steel.Aluminium,as strong ferrite stabilizer,elevated both A,and A3 temperatures of steels and enlarged transformation temperature range between A,and A3.Aluminum addition enhanced the unit driving force per ? during heating for the ferrite-austenite transformation associated with cementite dissolution,causing the cementite dissolved in shorter temperature duration.Meanwhile,the kinetics for transformation lowered down owing to the required partitioning of aluminium associated with cementite dissolution,induced the transformation temperature more sensitive with heating rate in this period.In the dual-phase region after full cementite dissolution,aluminium weakened the unit driving force for ferrite-austenite transformation and resulted in a slow transformation rate between Afcl and Ac3.The transformation rate was consequently lowered down and was more sensitive with the heating rate in aluminium added steels during heating in the whole temperature range of austenite formation.(5)The k carbides were easily formed in hot-rolled microstructure of high aluminium added steel,resulting in poor ductility and toughness,which made hot-rolled steels have high risk to generate cracks or to be broken during cold rolling.The origin of the K-pearlite brittleness and the method for improving ductility has been proposed.After heat treatment,the hot-rolled sheet could be cold rolling successfully.The microstructures of hot-rolled were banded structure of ferrite and K-pearlite.The K-pearlite is stronger but more brittle owing to the following two factors:1)refined inter-lamellar spacing,and 2)the high fraction of hard and brittle carbides,which induced poor ductility and hard to be cold rolled.The spheroidization annealing made the lamellar K-carbide in hot-rolled steels breaking in the initial stage of spheroidization,then the carbide coarsening.The ductility was improved,due to the smaller size,uniform distribution and high strength of spheroidization carbide.The spheroidization annealing of hot-rolled steels improved the mechanical properties,and solved the cold rolling problem,making the sheets cold rolling possible. |
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