| In order to solve the problem of poor plasticity of hot-formed parts of medium manganese steel,this paper takes Fe-0.18C-4.6Mn-0.23Si as the research object,and creatively puts forward a new process of hot stamping-carbon partition-intercritical annealing on the basis of previous research.This process realizes the integration of plastic forming and heat treatment,in order to directly obtain high strength and toughness to meet the collision safety requirements of the third generation advanced high-strength steel(strong plastic product>30GPa·%)In this paper,firstly,the quenching-partitioning(Q&P)and intercritical annealing(IA)processes are studied.On the basis of the above research results,the effects of carbon and manganese element migration,microstructure morphology and grain size on austenite stability in the new hot stamping-carbon partitioning-intercritical annealing(QA)process are explored,and the influence mechanism of microstructure transformation on strength and toughness is discussed.The results are as follows:(1)The effects of quenching-partitioning(Q&P)process on the properties of medium manganese steel at different quenching temperatures and different partitioning times were studied.The results show that at the quenching temperature of 230℃,the highest strength-plastic product is 24.3 GPa-%and the elongation is 14.1%,which is higher than that of direct quenching,but it still can’t meet the requirement of 30 GPa·%.In order to further improve the properties,the Q&P process was adjusted,and the mechanical properties with elongation of 20.2%and strong plastic product of 26.2 GPa·%were obtained.XRD results showed that the content and stability of retained austenite were improved.(2)The mechanical properties and microstructure transformation of medium manganese steel were studied by intercritical annealing process(IA)at different annealing temperatures.The results showed that the best mechanical properties were obtained by annealing at 680℃ for 60min,with an elongation of 30.0%and a strong plastic product of 30.8 GPa·%.Its microstructure was composed of lath martensite,retained austenite and carbide.The content of retained austenite measured by XRD is 30.78%,and the high content of retained austenite gives full play to the TRIP effect,and finally obtains excellent mechanical properties.(3)The mechanical properties and microstructure transformation of(QA)medium manganese steel under the process of hot stamping-carbon partition-intercritical annealing were studied.The best mechanical properties were obtained by annealing at 680℃ for 30min,with an elongation of 29.5%and a strong plastic product of 29.5 GPa·%;In order to further improve the properties and reach the requirement of 30 GPa·%,the QA process was adjusted and pioneer austenite was introduced.When annealed at 680℃ for 30min,the optimal mechanical properties with elongation of 36.8%and strength-plastic product of 43.6 GPa·%were obtained.A multiphase refined structure composed of lath martensite,retained austenite,fresh martensite and carbide was obtained.The volume fraction of retained austenite was 33.7%,showing two morphologies:thin film and granular.The proportion of fine film was larger,which was more beneficial to mechanical properties,and the fresh martensite produced had higher strength without tempering.The experimental results show that the high strength and toughness medium manganese steel parts can be obtained by the new QA process proposed in this paper,which meets the performance requirements of 30 GPa·%.Compared with Q&P and IA processes,the multiphase microstructure is obtained by this new process.The content of residual austenite in the microstructure is higher and the stability is better.Most of them are in the form of fine films,which can give full play to the TRIP effect.The generated fresh martensite has higher tempering strength,and with the microstructure with smaller residual austenite width and martensite lath width,excellent mechanical properties are finally obtained. |
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