The mechanical properties dependence on the microstructure was reviewed and analyzed,and the ultrafine grained duplex microstructure of BCC matrix and large fractioned austenite was given as one of the optimum structures to develop the third generation steel with high strength and high ductility.The medium-Mn steels with different carbon contents processed by austenite reverted transformation(ART-annealing) were studied thoroughly to fabricate the ultrafine duplex steels with large fractioned austenite.The lamellar typed ultrafine structure,the granular typed ultrafine duplex structure and the corresponding mechanical properties of the medium-Mn steels processed by ART-annealing were demonstrated in this paper.It was revealed that the duplex structure with large fraction of austenite and ultrafine grain structure is capable of producing steels with excellent combination of strength and ductility,i.e.,Rm A about 30-50 GPa%,which is about two times of that of the conventional automobile steels and close to that of the TWIP steels.It was concluded that the ART-annealing of the medium-Mn steels would be at least one of the promising ways to fabricate the third generation automobile steels in the near future.
CAO WenQuanWANG ChangWANG CunYuSHI JieWANG MaoQiuDONG HanWENG YuQing
用膨胀法和高温激光共聚焦显微镜分别对CrNi3Si2MoVNb钢经淬火和碳再配分(Q&P,Quenching and Partitioning)工艺的相变及马氏体相变的组织不均匀性进行表征,讨论了组织不均匀性对Q&P工艺钢微观组织的影响。结果表明,CrNi3Si2MoVNb钢第一次淬火过程中先形成马氏体在某些区域的批次增加,将原奥氏体晶粒分割成尺寸不同的若干区域,尺寸较大的未转变奥氏体稳定性较差,最终淬火过程中容易发生马氏体转变成二次淬火马氏体,而尺寸较小的更易于成为残留奥氏体。
In this work,DIFT technology and Q&P process were combined in order to introduce ultrafine-grained ferrite into the matrix of martensite and retained austenite to develop a new kind of advanced high strength steel,and two kinds of steels were investigated by this novel combined process.The newly designed process resulted in a sophisticated microstructure of a large amount of ferrite(about 5 m in diameter),martensite and a considerable amount of retained austenite for TRIP 780 steel.The ultimate tensile strength can reach about 1200 MPa with elongation above 16% for TRIP 780,that is much higher than the one solely treated by Q&P process.Tensile tests showed that both steels with the novel combined process achieved a good combination of strength and ductility,indicating that the new process is promising for the new generation of advanced high strength steels.
