Study On Microstructure Regulation And Mechanical Behavior Of Medium-Carbon-Manganese Aluminum-containing TRIP Steel

As a representative of advanced third-generation automotive high-strength steel?AHSS?,medium-manganese steel?Mn content:4 to 12 wt.%?has been favored by many researchers due to its enhanced strength and plasticity by transformation induced plasticity?TRIP?effect.In recent years,on the basis of the medium manganese steel system,C-Mn-Al series medium manganese steels with a product of strength and elongation?PSE?of30-60GPa·%have developed by adding aluminum?Al content is 2 to 5 wt.%?and optimizing the composition of the alloy and the manufacturing process.Considering weldability and austenite stability,most of the current researches mainly focus on low-carbon medium manganese steels?C and Mn contents of 0.01 to 0.2 wt.%and 8 to 12wt.%,respectively?,and rather limited attentions have been paid on the medium-carbon-manganese TRIP steel systems containing aluminum?C content of0.35-0.4wt.%?.In the work,a medium-carbon-mangaese TRIP steel containing aluminum?C content:0.35-0.36%,Mn content:5-7wt.%,Al content:3.0-3.5wt.%?is designed and prepared.Microstructure and microcomponent of experimental steel are regulated through intercritical annealing treatment?water quenching after incubation at 650⁸00°C?and low-temperature tempering?air cooling after heat preservation at 200°C?.And the microstructure and mechanical behavior of studied steel subjected to heat treatment are also analyzed.Besides,the effects of factors such as C/Mn element allocation and retained austenite grain size on the TRIP effect and mechanical properties were studied.The main results of this paper are summarized as follows:?1?The calculated results of the Thermol Cal show:the two-phase region expands and the stability of austenite improves with with the increase of Mn contents,which is consistent with the experimental results;as the annealing temperature increases,the retained austenite?RA?contents gradually increases and reach the maxiumum of 90vol.%at 950?of 5Mn steel and 100vol.%at 870?of 7Mn steel,respectively,which is quite different from the experimental results and the main reason is that the simulation calculation did not consider the effect of grain size on the stability of RA.?2?X-ray diffraction and microstructure analysis showed that after intercritical annealing at 650°C of both 5Mn and 7Mn steel,a large amount of martensite and particulate carbide remained;at 700°C and above,the microstructure mainly contained ferrite and laths austenite?hot-rolled?or block austenite?cold-rolled?;the highest austenite content of 5Mn steel after heat treatment of is 43.9 vol.%?750°C×1h,hot-rolled?and 28.9 vol.%?750°C×1h,cold-rolled,respectively?;the highest austenite content in the 7Mn steel can reach 54.3 vol.%?750°C×1h,hot-rolled?and 36.1 vol.%?800°C×1h,cold-rolled?.?3?Tensile test results show that:yield strength?YS?,ultimate tensile strength?UTS?and total elongation?TE?of 5Mn hot-rolled steel were 400-650MPa,810-1150MPa and31-70%,respectively.7Mn hot-rolled steel demonstrated a YS and UTS range of650MPa-780MPa and 890-1155MPa respectively and the TE of 49-65%;5Mn hot-rolled experimental steels annealed at 750°C for 1 hour obtain an excellent comprehensive mechanical property and the PSE reaches 65GPa·%;5Mn cold-rolled steels annealed at750°C for 1h can obtain tensile the UTS of 923MPa and TE of 57%;7Mn cold-rolled steel annealed at 750°C for 1 hour can obtain YS and UTS of 780MPa and 1118MPa,respectively,and TE of 67%,with the PSE can reach 75GPa·%,which is at the advanced level at home in similar components.?4?Based on the study of the 7Mn cold-rolled steels after various IA time at 750?,It is founded that the austenite content fluctuates around 32%and is insensitive to the holding time.With the extension of the holding time,the austenite grain grows.The degree of partitioning of elements such as C and Mn increases,which has an important influence on austenite stability.?5?Through the study of strain hardening behavior of different compositions and states steels,it is found that the rapid decline of the S₁ stage is mainly due to the softening deformation of ferrite and the slow rise of the stage 2 corresponds to both effect of ferrite deformation and TRIP effect,at which the ferrite deformation can effectively delay the transformation of austenite.And the fluctuations in the stage 3,because of the discontinuous TRIP effect,was a consequence of different degree of RA,which introduces relaxation of localized stress and martensitic hardening simultaneously.