Investigation Of Microstructure,Properties And Deformation Behavior Of UFG Low Carbon Medium Manganese Transformation Induced Plasticity(TRIP)Steel Produced By Heavy Warm Rolling

Low carbon（C）medium manganese（Mn）transformations induced plasticity（TRIP）steels exhibit exceptional mechanical properties with a combination of high strength and high ductility.With development of automotive industry and pressure due to environment and resources in recent years,developing inexpensive high strength steel with good enough mechanical properties has become one of the important tasks for material scientists and engineers.Energy conservation,environmental safety,lower emission of vehicles and passenger safety are also main targets for automotive-manufacturers for achieving steels with excellent strength,ductility and good formability performance.In this regards,Low-C medium Mn-TRIP steels have been paid much scientific and technological attentions because these steels fulfill all above requirements and have been declared as the 3^rdgeneration advanced high strength steels（AHSS）.Moreover,it is well understood that microstructural refinement is an effective way to improve both strength and ductility of the steels.Medium temperature thermo-mechanical treatment（MT-TMT）was found an effective way for refining theγ-austenite grains as well as fine martensitic microstructure after transformation,thus improving the strength of low-C medium Mn-TRIP steels.Therefore,MT-TMT has become preferred path for making high strength low-C medium Mn-TRIP steels.However,most of the medium Mn-TRIP steels processed by MT-TMT focused on dealing with intercritical annealing treatment.And only a few studies have systematically investigated the effect of annealing on medium-Mn steels processed by MT-TMT.The microstructure evolution,strengthening mechanism and tensile deformation behavior of medium Mn-TRIP steels processed by MT-TMT are yet not well understood.Moreover,annealing treatment of medium-Mn steels generally shows some discontinuous yielding or pronounced Lüders-like strain phenomena with large yielding strain.Among the observed discontinuous yielding phenomena,two important Lüders-like phenomena have commonly been observed;the first shows an absence or very small work hardening after yielding,while the second shows a pronounced upward curvature in the work hardening curve after yielding.The significance of the relationship between retainedγ-austenite and back stresses evolution at different strains and strain hardening due to back stress hardening of such tensile behaviors/curves in medium Mn-TRIP steels has rarely been undertaken.In the present study,on the basis of the newly developed design,three kinds of medium Mn-TRIP steels,0.15C-0.3Si-1.0Al-7Mn,0.2C-7Mn and 0.2C-7Mn-6.8Ni were selected for research materials.The heavily warm rolling（HWR）process was employed to fabricate the ultrafine-grained medium Mn-TRIP steels.The effect of annealing on microstructural evolution,mechanical properties variation and deformation behavior and strengthening mechanism were systematically studied.The occurrence of the special tensile behaviors in the present study was further explored.In order to study the evolution of retainedγ-austenite and back stress during deformation and strain hardening behavior after yielding due to back stress hardening for these special tensile behaviors,the load-unload reload（LUR）test was performed to evaluate the back stress hardening effect during tensile.The main contents of the research include three parts.In the first part,microstructures and mechanical properties of hot rolled（HRed）and heavily warm rolled（HWRed）steels were analyzed.In the second part,only the HWRed steels were annealed at different annealing temperatures and their microstructures were examined by OM,SEM and TEM.The XRD was performed to identify the coexisting phases in the steels.Tensile test and microhardness test were performed in order to evaluate the mechanical properties of steels.In the third part,in order to study the back stress hardening for the special tensile deformation behaviors of medium Mn-TRIP steels having very unique yielding phenomena,load-unload-reload（LUR）and some other special tensile tests were carried out.In this study,an ultrafine-grained（UFG）low-C medium Mn-TRIP steel（0.15C-0.3Si-1.0Al-7Mn）was fabricated by the heavily warm rolling（HWR）and subsequent quenching,and the effect of annealing temperatures on microstructure and mechanical properties of the UFG heavily warm rolled（HWRed）steel were investigated.Results showed that the HWRed steel exhibited simultaneous improvements in strength,uniform elongation and work hardening,which is mainly attributed to the refined martensitic microstructures.The HWRed steels comprised of onlyα-phase for annealing at temperatures below 550℃ and above 700℃.Whereas,UFGγ-austenite is formed by reverse transformation when HWRed steel was annealed at intermediate temperatures from550 to 700℃ and the volume fraction ofγ-austenite increases with increasing annealing temperatures,consequently resulting in a dramatic increase in ductility of the annealed-HWRed steels.It was found that transformed UFGγ-austenite andα-ferrite remained～500nm and～800 nm in size when HWRed steel was annealed at 650 and 700℃ for 1 h,respectively,showing an excellent thermal stability.The mechanical stability ofγ-austenite decreases with increasing annealing temperature from 580 to 700℃.Moreover,the HWRed steel annealed at 650℃ exhibits high strength-ductility combination with a yield strength of 906 MPa,ultimate tensile strength（UTS）of 1011 MPa,total elongation（TEL）of 51%and product of strength and elongation（PSE:UTS×TEL）of 52 GPa%.It is believed that these excellent comprehensive mechanical properties are closely associated with the UFGγ-austenite formation by reverse transformation and principally attributed to the transformation-induced plasticity（TRIP）effect.Grain refinement strengthening or grain boundary strengthening and martensitic transformation strengthening are the possible strengthening mechanisms of UFGα-ferrite-γ-austenite steels,but strength of steel mainly depended on the relative volume fractions ofγ-austenite andα-ferrite.In this study,the intercritical annealing treatment at 650℃ and 700℃ results in ultrafine-grained（UFG）dual phase ferrite-austenite medium Mn-TRIP steels（0.15C-0.3Si-1.0Al-7Mn）.The two steels exhibit different and special discontinuous yielding and pronounced Lüders-like strain phenomena with large yielding strain which are related to theirγ-austenite volume fractions and retainedγ-austenite stabilities.The steel annealed at650℃ shows an absence or very small strain hardening while the steel annealed at 700℃ shows an obvious strain hardening upward curvature with increasing strain.The results show that before and during straining,the steel annealed at 650℃ exhibits a mixture of equiaxed and elongated UFGs ofα-ferrite andγ-austenite phases;however,the steel annealed at 700℃ exhibits only elongated UFGs ofα-ferrite andγ-austenite phases.It was found that most of theγ-austenite toα′-martensite transformation occurred at the initial deformation stage and very small or almost no transformation occurred afterwards.This demonstrates that the strain-induced martensite（SIM）transformation（γ-α′）or TRIP effect dominates only at the initial deformation stage.The retainedγ-austenite remained stable and no TRIP effect was observed at the final deformation stage.The load-unload reload（LUR）test was performed to evaluate the back stress（σ_b）hardening effect.It is believed that the pronounced strain hardening behavior at the later deformation stage is mainly associated with theσ_b enhancement induced by the strain partitioning between the soft and hard phases due to the SIM transformation during tensile deformation.Moreover,better strain hardening of the steel annealed at 700℃ is related to its higher back stress values while the poor or small strain hardening of the steel annealed at 650℃ is attributed to its lower back stress values.In this study,low-C medium-Mn 0.2C-7Mn（M70）and 0.2C-7Mn-6.8Ni（M71）TRIP steels were chosen as experimental steels.In case of M70 steel,dual phase（ferrite-austenite）low carbon medium Mn-TRIP steel was fabricated by the heavily warm rolling（HWR）and further annealing treatment.The HWRed M70 steel annealed at 650℃ exhibited the excellent mechanical properties with YS of 835 MPa,UTS of 1242 MPa,microhardness of 330 HV,TEL of 20%and PSE of 25 GPa%.Similarly,in case of M71steel,a dual phase（martensite-austenite）low carbon Nickel-Manganese TRIP steel was fabricated by the heavily warm rolling（HWR）and the effect of annealing on phase fraction,mechanical properties and tensile deformation behavior of the HWRed steel was investigated.The results showed that the reverse transformation of theγ-austenite from martensite occurs and theγ-austenite volume fraction decreases from 91%to 55%as the annealing temperature increases from 400℃ to 800℃.The HWRed M71 steel annealed at400℃ exhibits the high strength-ductility combination with a yield strength of 706 MPa,ultimate tensile strength（UTS）of 1573 MPa,total elongation（TEL）of 21.6%and the product of strength and elongation（PSE:UTS×TEL）of 34 GPa%.These excellent mechanical properties are principally attributed to the formation of large volume fraction ofγ-austenite by the reverse transformation and the subsequent transformation-induced plasticity（TRIP）effect in the tensile deformation.It was found that the HWRed and annealed M71 steels exhibit a special tensile behavior with a large yielding strain followed by the pronounced strain hardening.The tensile curve can be readily divided into three obviously different stages.The SIM transformation（γ-α′）occurs in the early yielding deformation stage and in the intermediate rapidly hardening deformation stage,indicating that the TRIP effect dominates the process of these two stages.However,the retainedγ-austenite remains very stable and no TRIP effect was observed in the final hardening deformation stage.The load-unload-reload（LUR）test was performed to evaluate the backs tress hardening effect during tensile.It is believed that the pronounced strain hardening behavior after yielding is mainly associated with the back stress enhancement induced by the strain partitioning between the soft retained austenite and the hard martensite by SIM transformation during tensile deformation.Moreover,the back stressσ_b is increasing with increasing strain and is positively correlated with the tensile strength of the steel during unloading process.The pronounced“serration”during tensile deformation is closely associated with the strain rate.Similarly,XRD and tensile tests at different strains show that width of each peak at different strains did not change significantly,indicating that the phase stability keep stable during the process.The strain rate sensitivity index（m value）of M71 steel annealed at 400℃ is 0.01747.This shows that flow stress of deformation of this steel is highly sensitive to the change of the tensile strain rates.