Researh On Microstructures,Mechanical Properties And Austenite Stability In Medium Mncontent Steels With High Strength And Ductility

In recent years,energy conservation,environment protection and increased safety standards are the driving forces behind the quest to develop advanced high-strength steels for automotive applications.Transformation induced plasticity(TRIP)steels are being considered as an important and new class of automotive steels with exceptional formability and excellent combination of high strength and high ductility.Many studies have been carried out on medium Mn-content(4?12%)steels with PSE(product of UTS and TEL)greater than 30 GPa·%,and it was suggested that those steels are considered as promising third generation automotive steels.In TRIP steel,the TRIP effect is depending on the volume fraction and stability of austenite,and the mechanical properties are mainly influenced by the TRIP effect.Thus,the austenite with high amount and appropriate stability can be obtained in the TRIP steel by optimizing the alloy design,heat-treatment and processing means,and the strength and ductility can be enhanced simultaneously.Different kinds of medium Mn-content TRIP steels are designed in this study.In order to study the effect of alloying element on the microstructural evolution and mechanical properties of TRIP steels.In this investigation,the microstructures and mechanical properties of Fe-6/8.5Mn-1.5/3Al-0.2C TRIP steels were studied.The factors related to austenite stability and the relationship between austenite stability and TRIP effect were studied in this investigation.Furthermore,the effect of different degree of pre-strain on the yield elongation,and the microstructural evolution and mechanical properties after pre-strain were also studied.The main achievements are expressed below.(1)The 6Mn-1.5Al steels subjected to Q&T(quenching and tempering)process were characterized by the maximum austenite fraction of 57.1%.The steel after quenching from 630 ? exhibited an excellent mechanical property combination of ultimate tensile strength(UTS)1082 MPa,total elongation(TEL)39.2%and the product of UTS and TEL reaches the maximum of 42.4 GPa·%.While the maximum austenite fraction of steels subjected to ART(austenite reverted transformation)process were only characterized by 34.0%.With the austenitization temperature decrease to 800 ?,the ultimate tensile strength is 945 MPa,total elongation is 28.0%and the product of UTS and TEL is 26.5 GPa%.As a result,the Q&T process was more effective to obtain superior mechanical properties in hot-rolled steels as compared to ART process.Compared to the hot-rolled steels,the grain sized of both ferrite and austenite are decreased in the cold-rolled steels,and the microstructures are more refined.Mechanical properties were improved in cold-rolled steels.Compared with other medium Mn-content steels,the experimental steels have better combinations of mechanical properties,furthermore,the heat treatment adopted in the present case is more convenient.(2)During long time annealing in ART process,excessive Mn were enriched in austenite,which made austenite highly stable and significantly reduced TRIP effect.Furthermore,low dislocation density was present after long time annealing and weakened the work-hardening rate.In the case of the steels subjected to Q&T process,Mn-concentration was not excessively concentrated in austenite,which made austenite properly stable and induced strong TRIP effect.Furthermore,the high volume fraction of austenite was responsible for the distinct TRIP effect in the steels using Q&T process.The microstructures with high dislocation density were presented in the steels after short time annealing in Q&T process,which contributed to the work-hardening rate.(3)The increase in Mn-content in the medium Mn-content TRIP steels increased the volume fraction of austenite,which promoted the TRIP effect and enhanced the work hardening rate.As a result,the steels with a higher Mn-content are characterized by better mechanical properties.And the increase in Mn-content was responsible for the diversification of austenite morphology.The increase in Al element in TRIP steels facilitates the presence of 8-ferrite and contributes to excellent total elongation.Increase in Al-content made the austenite morphology more uniform.However,the increase in Al-content in the steels decreased the volume fraction of austenite,which weakened the TRIP effect and reduced the work hardening rate.Thus,the increase in Al-content in the experimental steels decreased the ultimate tensile strength and the product of UTS and TEL.(4)The lathy austenite with different lengths resulted in different stability.Shorter lathy austenite with higher stability than longer lathy austenite is attributed to the higher content of C and more uniform distribution of Mn.Compared to blocky austenite,higher C and Mn-contents present in lathy austenite,can explain higher stability in lathy austenite.The stability of austenite is decreasing from granulous austenite,lathy austenite to blocky austenite.(5)Formation of yield elongation was associated with TRIP effect and cooperative dislocation glide in TRIP steels.Pre-strain led to a decrease in austenite grain size and increase in the average stability of retained austenite,which delayed the occurrence of TRIP effect and suppressed the initiation strain of yield elongation.The increased dislocation density in ferrite caused by pre-strain improved the work-hardening rate,which rendered yield elongation to end earlier.Thus,the yield elongation formation was gradually decreased with the increase in degree of pre-strain.As the amount of pre-strain was increased to 10%,the stability of retained austenite and the dislocation density in ferrite increased sharply,which was responsible for the elimination of yield elongation.(6)The dislocation density in ferrite and fraction of transformed martensite increased with the increase in the amount of pre-strain,which contributed to yield strength.The ultimate tensile strength increased continuously with increase in pre-strain primarily because of increase in the fraction of transformed martensite induced during pre-strain.Decrease in volume fraction of austenite and excessive increase in stability of retained austenite weakened the TRIP effect.Moreover,increased dislocation density in ferrite and the increased volume fraction of transformed martensite induced by pre-strain were responsible for reduced ductility of the steels.The steel with 10%pre-strain had lower yield/tensile ratio(0.79).