| In order to solve the problem of energy consumption and environmental pollution caused by automobiles,many countries have put forward the concept of automobile lightweight.The use of high-strength steel instead of the traditional low-carbon steel in automobile body structure can be more economical and convenient to achieve the weight reduction of automobiles under the premise of ensuring the safety.Therefore,in the last decades a large scale R&D works of high-strength automotive steels have been carried out around the world.Among them,NanoSteel company developed the homonymous alloy materials with a new design ideas "the addition of high content of P group elements",and new strengthening mechanisms "Static Nanophase Refinement mechanism and Dynamic Nanophase Strengthening mechanism".The new alloy has drawn the attention of researchers.However,the details have not yet been disclosed due to the protection of their commercial patents.Exploration and research on this new alloy will provide some new ideas and guidance for the development of high strength steels.In this paper,typical alloy from NanoSteel's 2012 patent will be taken as an example.By melting the experimental alloy and observe the material microstructure in various states,as well as test the mechanical properties of the material to understand and demonstrate the specific details of NanoSteel.The main contents and research results are as follows:(1)Ten kilos of large size ingot were smelted by medium frequency induction furnace and cast in conventional die,compared with 100g button ingot melted by vacuum electromagnetic stirring electric arc furnace.The microstructure of the as-cast alloy was observed by optical microscope and scanning electron microscope.Combined with the composition analysis of electron probe and the phase analysis of electron backscatter diffraction technology,the effect of solidification rate on the as-cast microstructure was studied.(2)Based on the thermodynamic equilibrium phase diagram calculated by Thermo-Calc,Dictra was used to simulate phase transformation in the continuous heating and continuous cooling of the experimental alloy and the results were verified by the phase transition and hot-rolling experiments.Due to the difference of the diffusion behavior of the substitutional alloying elements in austenite to ferrite and ferrite to austenite transformation,the ferrite in the as-cast microstructure could transform to austenite during heating but the high temperature austenite was stable to room temperature during continuous cooling.(3)The microstructure evolution of the heat treatment process was studied,and the influence rules of the heat treatment temperature on the micro-phase composition were especially investigated.The results showed that the heat treatment temperature indirectly affects the micro-phase composition after heat treatment by influencing the diffusion rate of substitutional alloying elements and the driving force of austenite to ferrite transformation.(4)Tensile mechanical properties of the experimental alloys were tested in a series of ways.Combined with the phase identification results obtained by X-ray diffraction and the fine structure observetion by a transmission electron microscope,the strengthening behavior in the stretching process was analyzed.The dynamic transformation behavior of austenite to ferrite(Transformation-induced plasticity)was mainly studied.However,Nano-dynamic precipitates belonging to the#186 and#190 space groups described in the NanoSteel grant patent were not observed.(5)From the perspective of reducing the cost of the alloy,an alloy reduction experimental steel which still retains a large amount of P group elements was designed.The microstructure of the steel was analyzed by means of optical microscope,electron microscope and electron probe.It was found that the boride and matrix grains of the steel were coarse and the mechanical behavior was brittle. |
PDF Full Text Request