| In face of the challenge from energy, environment and safety, automobile lighting is the most efficient measure. Furthermore, in order to meet the development requirements for new generation automobile industry in the future, we need to improve the comprehensive mechanics performance and accelerate the localization procedure of cold-rolled dual phase steel. This paper took the cold-rolled initial structure of Nb-Ti microalloyed dual phase steel as research object, and developed the continuous cooling transformation behavior after intercritical heating. Meanwhile, microstructure evolution and mechanical properties of Nb-V and Nb-Ti microalloyed dual phase steel during the continuous annealing were researched. Tensile deformation behavior in the uniaxial tension was also investigated. In addition, the good comprehensive mechanical properties of 700MPa grade cold-rolled dual phase weathering steel was carry out under laboratory condition, and the corrosion resistance was investigated by dry/wet cyclic accelerated corrosion experiment. The main work involved as follows:1. Took the cold-rolled initial structure of Nb-Ti microalloyed dual phase steel as research object, and the continuous cooling transformation behavior after intercritical heating was studied, pointing out its importance for the continuous annealing process control.The continuous annealing process optimization is important for the precise control of microstructure, and the continuous cooling transformation is a very good research measure for the precise control of microstructure during the continuous annealing. However, most research reports about continuous cooling transformation of dual phase steel have focused solely on the hot-rolled initial structure or the complete austenitization. In the present paper, considering the technological characteristics of practical conditions, the influence of different soaking temperature on cold-rolled initial structure of the microalloyed steels was investigated. Results showed that increasing soaking temperatures would decrease ferrite start temperature (Fs) and increase martensite start temperature (Ms). This was mainly attributed to the changes in austensite grain size as well as the average carbon content. At the lowest cooling rate of 0.5℃/s, a small amount of martensite structure could be observed. Ms increased smoothly with lower cooling rates and showed a dramatic increase at elevated cooling rates, at which point a coarse structure was readily obtained from transformation of coarse-grained austenite and composed of bainite and martensite structures in the inner and peripheral region, respectively.2. The effect of continuous annealing parameters (soaking temperature, slow-cooling temperature, cooling rate and overaging temperature) on microstructure evolution and mechanical properties were systematically investigated, and the control mechanism of the annealing process on the microstructure and properties of microalloyed dual phase steel was explained.The results showed that the microstructures of test steel were composed of polygonal ferrite, lath martensite and small amount of bainite, besides, a small amount of twin martensite were observed. Increasing the intercritical temperature, the second phase fraction decreased, and the coarse bainite-martensite structures increased. Compared with tranditional C-Si-Mn dual phase steel, the yield strength and tensile strength of microalloyed dual phase steel decreased, total elongation increased and the product of tensile strength and ductility increased as soaking temperature increased. The microstructure evolution of microalloyed dual phase steel was characterized by obvious stages as slow-cooling temperature changes. The second phase (bainite+martensite) fraction is fewer changes as the slow-cooling temperature decreased from 720℃ to 660℃. However, the second phase fraction was obviously changes as the slow-cooling temperature decreased from 660℃ to 620℃. The changing trend of mechanical properties was similarly to the microstructure evolution. It was found that the continuous yield behavior of microalloyed dual phase steel remains until cooling rate was down to 20℃/s, when the cooling rate was lower than 20℃/s, the discontinuous yielding and up-lower yield points return. The overaging test results showed that the microalloyed dual phase steel has the higher temper resistance, the test steel have the continuous yield behavior until the temper temperature up to 350℃.3. The precipitate evolution of the Nb-V and Nb-Ti dual phase steels during the annealing process was investigated, and the precipitates coarsening behavior was also be studied.Took the cold-rolled ferrite/pearlite initial structure as a research object, the precipitate behavior during continuous annealing was studied. Results showed that the sizes of the obtained precipitates were approximately 2nm to 20nm. Furthermore, it was found that the distribution of precipitates was not homogeneous within ferrite grains of annealed plates, and the precipitates regions was observed composed by high density and low density areas. Besides, some growth of the precipitates occurs during annealing, and the precipitates coarsen was more seriously as the increase of soaking temperature.4. Tensile deformation behavior in the uniaxial tension was investigated. The mechanism that the microalloyed dual phase steel has the higher initial work hardening rate was clarified.Compared to the other tranditional C-Si-Mn ferrite/martensite dual phase (FMDP) steels, the microalloyed FMDP steels containing amount of finely nano-sized precipitates leads to an improved work hardening rate. The higher initial work hardening rate in this study was attributed to fine island-like martensite particles and amount of finely dispersed nano-sized precipitates. The results showed that the work hardening behavior of the mciroalloyed dual phase steel was better revealed in the C-J or M-C-J analysis than the Hollomon analysis. The work hardening characteristics of the microalloyed dual phase steel was made up of three stages. The curves of the instantaneous work hardening exponent (n) versus true strain ε were made up with three stages during the uniform plastic deformation, n-was relatively higher at stage I, decreased slowly with ε in stage Ⅱ, and then decreased quickly with ε at stage Ⅲ5. The good comprehensive mechanical properties of 700 MPa grade cold-rolled dual phase weathering steel was carry out under laboratory condition, and the corrosion resistance was investigated by dry/wet cyclic accelerated corrosion tests.The results showed that the good comprehensive mechanical properties of 700MPa grade cold-rolled dual phase weathering steel have been fabricated in laboratory. The mechanical properties fully meet the aim target. After the optimal annealing process, the test steel has the excellent mechanical properties:soaking temperature:780℃, slow-cooling temperature: 660℃, cooling rate:40℃/s, overaging temperature:320℃. Rpo.2=346MPa, Rm=747MPa, δA50=22%, cold-bending properties meet the target. In addition, the weathering resistance properties for the carbon steel Q345, commercial weathering steel SPA-H and independent designed weathering steel NHR were contrastively investigated through the dry/wet cyclic accelerated corrosion tests. Results showed that the corrosion rate of NHR weathering steel was more slowly than the carbon steel Q345. Moreover, increasing the corrosion cycles, the difference was more significant. Among the three contrastive test steels, the corrosion rate of NHR was the slowest, and better than commercial weathering steel SPA-H. After 120 cycles dry/wet cyclic accelerated corrosion tests, the difference in the inner and outer layer of Q345 carbon steel was not significant, inner and outer rust layer were loose and porous. However, the difference in the inner and outer layer of NHR steel was significant, the rust layers were composed of the compacted inner rust layer and loose outer layer. |
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