| High strength cold rolled products is an important direction of microalloyed steel research. The actual strength of steel is contributed by many strengthening mechanism at the same time. Ferrite precipitation strengthening is one way to improve the strength of cold rolled products. In this paper, fine grain strengthening and precipitation strengthening are the main research contents. The experiments mainly include the behavior and process control of the precipitation particles in the ferrite. On the one hand, ferrite and martensite dual phase is obtained at the room temperature microstructure of microalloyed experimental steel, with the purpose to refine the ferrite grain size. On the other hand, the ferrite is strengthened by controlling the precipitation behavior. In this way, the strength grade and the forming property of the cold experimental steel can be greatly improved. By many advanced methods, such as the nano indentation test technology and the electron microscopic analysis technology, through the control of hot rolling parameters and cold rolling/continuous annealing parameters, this paper discusses the relationship between the properties/microstructure and the parameters, and the function of micro alloying elements in the material strengthening and its change regularity at different stages. The main works and results of this paper are as follows:(1)By the single pass compression experiment of the experimental steels, high temperature deformation behavior of austenitic is researched. Meanwhile, the author analyzes the process of strain induced precipitation and isothermal precipitation of the high temperature deformation austenite. At900℃,40%deformation has obvious strain induced precipitation process, and the strain induced precipitation has its order. The isothermal precipitation experiment for Nb,V,Ti microallyed steel and Cu microallyed steel shows that, at a certain temperature, with the extension of isothermal time, the number and size of precipitates gradually increased and finally tends to be stable.(2)By austenite continuous cooling transformation experiment, CCT curves were drawn and the precipitation law in continuous cooling transformation was analyzed. During continuous cooling, with the cooling rate increasing, the number of precipitates in the ferrite gradually reduce and the precipitates diameter becomes smaller.(3)Hot rolling experiments is done by Φ450rolling mill with two different experimental parameter for the same steel in RAL. The results of the precipitation in hot rolling process is different obviously. When the cooling rate is low, large amount of precipitation particles distribute in ferrite dispersively. The experimental steel microstructures and the mechanical properties testing results show that, after rolling50℃/s cooling rate can increase the strength more than150MPa and the grain size is reduced to half. The ferrite content in microstructure is more than80%, and the extension rate can achieve20%.(4)The author has made cold rolling and continuous annealing experiments in cold rolling mill and continuous annealing furnace in RAL. By the observation of the microstructure and the mechanical forming properties test, the author has analyzed the strengthening effect of precipitates on the cold rolling experimental steels. After continuous annealing, the microstructure of cold rolled steel is ferrite and martensite dual phase. Nano indentation experiment results show that the nano hardness of martensite matrix is1.34～1.85times as the nano hardness of ferrite matrix.(5)The tensile and reaming experiment shows that, such as these tensile performance index, yield strength and tensile strength ratio/strain hardening exponent/plastic strain ratio can be consistent characterization of reaming performance. Cold rolled and annealed steel strip has many suitable mechanical characteristics for forming, such as continuous yield feature, low yield strength and tensile strength ratio, high work hardening rate and high elongation. |
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