| Weight lightening and safety improvement are becoming the main consideration of the development of the automobile industries, which requires the new generation automobile steels assuming high strength(Rm), high ductility(A) and thus high product of strength to ductility (RmxA).The mechanical properties of the martensitic steels with relative improved strength and ductility could be obtained by the conventional quenching and tempering techniques, which needs to be improved further for its application in auto industries. Recently a new heat treatment conception of Quenching and Partitioning process (Q&P) was proposed by American researchers, which was shown to be capble of controlling the volume fraction of retained austenite in martensite microstructure and in turn improving the ductility without significant loss of strength. So far this research was mainly conducted based on the consideration of C partition in conventional martensite steels but no study was reported on other elements stabilizing austenite phase or suppressing the precipitation of carbides, such as Mn. Si and Al.This thesis performed Q & P process treatment experiment on medium manganese steels with different carbon content. The microstructure and mechanical properties dependences on the austenization temperature (AT), quenching temperature and time (QT, t2) and partitioning temperature and time (PT, t3) were studied in details. In addition, the new steels with further addition of Si and Al in medium manganese steels were developed in our Lab. The effects of Si and Al on the microstructure and mechanical properties of the steels processed by Q&P were studied, it was found that the addition of Al and Si could suppress the precipitation of carbides, stabilize the residue austenite and improve the mechanical properties further.Microstructure examinations of the Q&P processed medium manganese steels were carried by SEM, EBSD and XRD. It was found that the microstructure of the Q&P processed steels is mainly composed of firstly-formed martensite, fresh martensite and residual austenite structure. The morphologies, quantities and distributions of these phases were strongly affected by austenization temperature, quenching temperature and time, and partition temperature and time. Intercritical austenization results in a continuous lath structure with relative high austenite fraction, which weren't affected by followed quenching and partitioning. However, after single phase austenization at relative high temperature, the microstructure was strongly affected by the followed heat treatment parameters. It was found that a relative high austenite fraction could be obtained when the quenching and partitioning was carried at relative high temperature. Furthermore the in-continuous laths and polygonal shapes were the main morphologies of the multiphases after Q&P processing when austenization carried out in the single phase region.The mechanical properties of the Q&P processed medium manganese steels indicate that the mechanical properties were strongly dependent on the heat treatment parameters. Intercritical austenization gives the maximum RmxA but relative lower Rm. The analysis on the mechanical properties of the steels processed by single phase austenization indicates a better combination of both strength and ductility when quenching and partitioning were carried out at relative high temperature. Furthermore, the addition of Si and Al could improve the mechanical property clearly, which gives an excellent combination of tensile strength about 1500MPa and ductility of about 20%. Based on the analysis on the relationship between mechanical properties and microstructure, it was proposed that increasing the volume fraction of the austenite is the main factor to improve the ductility of the steels processed by Q&P process. This study provides the ideas of material alloying and heat treatment design for the new generation automobile steels with high strength and high ductility. |
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