Research On The Influence Mechanisms Of V/TI Micro-alloying To The Microstructure And Mechanical Properties Of Medium Carbon Steel

More attention about the development of micro-alloying medium-carbon steel is paid on the progress in production and application at present, Extra efforts are necessary to study the influence of micro-alloying elements on the microstructure and mechanical properties of medium-carbon.In response to these problems, the medium-carbon axle steel heat-treated by double normalizing and tempering is applied as sample in the current research. Not only the influence of V,Ti micro-alloying elements on the microstructure of the sample is studied, their existing morphology, quantity and distribution feature, as well as changes of the mechanical properties are also analized. The main results are as follows:We defined the composition of sample steel by the method of spectral analysis, through the metallographic microscope and the scanning electron microscope, the microstructure was observed and the grain size was measured. With the aid of the transmission electron microscope, we found the dispersed precipitation phases on the ferrite which was around the fragmentation cementite lamellar after the micro-alloying. Used the method of physicochemical phase analysis, we studied the composition, structure and content of the precipitates. The X-ray diffraction analysis showed that the precipitates were mainly composed of the carbonitride of micro-alloying elements vanadium, titanium. By the size distribution measurement of the precipitates, we know that adding different amounts of vanadium to micro-alloying will obtain different size of precipitates, but adding vanadium, titanium together can receive more fine precipitates.The mechanical indexs of sample steel with micro-alloying elements have been improved differently. With instrumented impact test under series of temperature, we studied the dynamic fracture process of the samples and found that they inclined to brittle fracture, the crack initiation energy can be significantly improved by micro-alloying, thus making the total impact energy increased.The above analysis shows that the main strengthening mechanisms of the micro-alloying medium carbon steel is the fine grain strengthening and dispersion strengthening, the grain refinement can improve the strength while improving the toughness, it is the most important way of strengthening and toughening.