Research On Microstructures And Mechanical Properties Of Hot Rolled Quenching And Partitioning Steels With Trip Effect

It is an effective way for cost reduction, energy saving, environmental protection and automobile safety to exploit high strength-steels, which have low cost, high strength and excellent security. Therefore, in order to develop advanced high strength steels which possess high strength with corresponding plasticity, low cost, short productive cycle and good quality steels, we put forward one heat treatment method: Quenching and Partitioning (Q&P) for low alloy hot rolled steels. The new type of high strength automotive steel with TRIP effect was obtained through Q&P process. The product of tensile strength and elongation can get17244.2MPa.%. The performances of steels meet the requirements of excellent properties of the third generation high strength automobile steel and cut down alloy content, reduced production cost, combined hot rolling and heat-treating process and improved production efficiency.The steels were characterized by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and mechanical performance test. The main contents and important results are showed as follows.Firstly, the composition of low alloy Q&P steels was designed and the relationship between quenching temperature and retained austenite was obtained by CCE model, and two temperatures were determined. The temperature of austenitizing and martensite formation and finishing were calculated by empirical formulas and the temperature of austenitizing, the temperature of quenching and the time of distribution for this research were designed by the composition of test steels.Secondly, the effects of quenching and partitioning temperature on microstructure and mechanical properties of the steels were researched. The samples after heat treatment were characterized by OM, SEM, TEM, XRD and mechanical performance test. The experimental result indicates that the microstrueture of the steels after quenching and partitioning process mainly consists of lath martensite and retained austenite, and there have large dislocation density in lath martensite. The certain content of retained austenite were got after two kinds of heat treatment process, and the content were all higher than6.4%, the highest can reach9.78%. The retained austenite transforms into martensite in the deformation process, the TRIP effect was created and the mechanical properties of the steels were better than other heat treatment. The tensile strength of the samples was higher than897.06MPa, the elongation was higher than14.6%, the tensile strength and elongation can get17244.2MPa.%and the mechanical properties at210℃quenching and400"C partitioning was better than260℃quenching and400℃partitioning.Lastly, the effects of partitioning time on microstrueture and mechanical properties of the steels were researched. The samples after heat treatment were characterized by the same characterization methods. The experimental result indicates that the microstrueture of the steels after heat treatment mainly consists of lath martensite and retained austenite, and there have large dislocation density in lath martensite. The lath width of martensite in20s was dramatically narrower than in40s and the carbide precipitation at inside of martensite when the partitioning time was40s. The certain content of retained austenite was got after two kinds of partitioning time, and the highest can reach9.78%. The TRIP effect was created in the deformation process and the mechanical properties of the steels were better than other heat treatment. The tensile strength of the samples was higher than958MPa after two kinds of heat treatment and the highest was974MPa in40s. The elongation of the steels was higher than11.47%and the highest can reach18%.