| As the requirements for both lightweight and high security in automobile industry are being gradually enhanced, the3rd generation automobile steel must take both advantages of the first and second generation automobile steels, and further reduces cost and possesses a better combination of high strength and good plasticity. The3rd generation automobile steel is designed with the idea that transformation strengthening, grain refinement and dislocation strengthening are in full use in order to enhance strength, and that multi-phases and meta-stable microstructures which exhibit transformation induced plasticity (TRIP) or twinning induced plasticity (TWIP) under flow stress are controlled to improve the form-ability properties. In2003, a thermal processing route, Quenching and Partitioning (Q&P) was proposed by Speer et al. as a new concept to produce martensitic microstructure containing enhanced levels of retained austenite with the TRIP effect, which made the development of advanced high strength steel towards the combination of strength and ductility. This paper focused on the effects of heat treatment process parameters of Q&P on the microstructures and properties. The microstructures of the Q&P steels were characterized by means of optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microstructural evolution and the correlation between the features, distribution and fraction of retained austenite and the parameters had been studied. In addition to this, parameters were determined to produce the high-strength Q&P steel with the product of tensile strength and elongation greater than20,000MPa%. The chief original work and results are as follows:(1) At the same partitioning temperature, martensitic lath gradually coarsened with the time prolonged, and the characteristic of the martensitic lath was not obvious because of migration of interface between martensite and retained austenite.(2) With the increase of holding time during partitioning, the number of carbides in the martensitic lath increased. Although carbides still existed in the two-step Q&P process when the partitioning temperature is450℃, the number of carbides decreased. Therefore, it can be inferred that the transition carbides formed at the low partitioning temperature, and were suppressed at450℃.(3) Most of the retained austenite appeared as thin films of50-200nm width, and distributed between the martensitic laths. With partitioning time increasing, the width of retained austenite films decreased due to the effects of martensitic transformation and morphology. And during isothermal treatment, martensitic transformation continued so that the width of martensitic laths continuously increased. For equal partitioning temperature and time were equal, samples treated with two-step Q&P process had more retained austenite than those treated with one-stept Q&P. Therefore, partitioning efficiency can be improved by optimizing heat treatment process parameters.(4) When the Copper-containing steel were partitioned at300℃for300seconds, the product of tensile strength and elongation can reach21216MPa-%with1326MPa tensile strength and16%total elongation. With partitioning time increasing, the tensile strength decreased but the total elongation enhanced.(5) Tensile strength was higher than1500MPa while the total elongation was relatively low for short partitioning time with no obvious tempering feature.More retained austenite can be obtained with a long partitioning time. Although TRIP effect of retained austenite contributes to the increasing of tensile strength, it cannot compensate for strength reduce related to the matrix softening.(6) By means of XRD, it is found that the volume fraction of retained austenite increased with the increasing partitioning time, indicating that carbon partitioning was more efficient so that retained austenite with higher carbon concentration can be obtained due to the higher stability. |
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