| With resource crisis going more and more severe, steel industries are having more challenging work to reduce the addition of micro-alloying elements in steel products, which may, however, negatively affect the strength of steels. In order to meet the requirements of reduction in alloy cost and maintain strength, cementite in steel is viewed as a viable option to replace micro-alloying precipitates because it is economical and common second phase constituent in steels. If cementites can be effectively refined to the scale of a few nanometers, it can also generate very strong precipitation strengthening effects to replace the strengthening role of the precipitates of micro-alloying elements.Given that the cementites in carbon steels are usually to form lamellar pearlite structure in the near-equilibrium conditions, rather than to form nanoscale particles precipitation, and they tend to significantly coarsen at high temperatures after hot rolling. Therefore, the non-equilibrium precipitation of cementites can be realized by increasing the cooling rate after hot rolling. Therefore, the ultra fast cooling (UFC) technology developed by RAL lab, which can break the cooling capacity limit of traditional rolling product line, was applied after the hot strip rolling experiments for the research of cementite precipitation and strengthening mechanism in hypoeutectoid steels with four different carbon contents.Moreover, the spheroidization behavior of cementites in carbon structural steel was also investigated by UFC process and annealing treatment for the improvement of plasticity and formability. And the microstructure consisting of spheroidized cementites distributed finely in a ferrite matrix was expected for the excellent hole-expansion property.The main contents and innovative achievements of this dissertation are as follows:(1) The thermal simulation experiment was carried out to investigate the continuous cooling transformation (CCT) behavior of undercooled austenite in carbon steels by a MMS-300 thermo-mechanical simulator. The CCT curves were determined by using thermal dilation method combined with microstructure observation. The effects of carbon content, hot-temperature deformation and cooling rate on continuous cooling transformation behaviors were also analyzed. The results showed that, with the increasing of cooling rate and carbon content in steel, the dynamic transformation temperature of undercooled austenite decreased gradually, and hot-temperature deformation could promote the ferrite transformation temperature and refine the grain. With the cooling rate increasing, the volume fraction of ferrite in the microstructure was reduced significantly, and the microstructure changed from the ferrite-pearlite phase to the ferrite-pearlite-bainite phase, and to the ferrite-bainite phase at last.(2) The UFC technology was applied in the cooling process after hot strip rolling, and precipitation and strengthening behaviors of cementites in hypoeutectoid experimental steels with 4 different carbon contents were investigated by controlling cooling temperature. The experimental results demonstrated that the refinement of ferrite grains and pearlite lamellar spacing was mainly responsible for the strengthening of 0.04%C and 0.5%C steels, respectively, with no nano-scale cementite precipitation having been formed. On the other hand, a large number of dispersed nano-scale cementite precipitates with the average size of 20-30 nm have been formed in 0.17%C and 0.33%C steels. The nanoscale precipitation of cementite was realized in the microstructure without the microalloy addition by UFC. Both the yield strength and tensile strength of experimental steels increased gradually with the stop temperature of UFC decreasing, and the yield strength increments of 0.04%C,0.17%C, 0.33%C and 0.5%C steel were 30MPa, 100MPa, 100MPa and 130MPa respectively, when the stop temperature of UFC decreased from 890℃ to 600℃. Thus, it is clear that the strength of steels can be enhanced by the UFC process, with the enhancement being greater for high carbon steels than for low carbon steels.(3) According to the austenitic transformation mechanism of KRC and LFG model in Fe-C alloys, the transformation driving force of the undercooled austenite was calculated systematically in a thermodynamic view, and thermodynamic feasibility and regularity for the formation of nanoscale cementite precipitates during cooling were determined. Based on the calculation results, the driving force of degenerated pearlitic transformation is the most negative at the same under-cooled temperature, which theoretically indicates that the degenerated pearlitic transformation of under-cooled austenite can easily occur to form cementite and ferrite with the equilibrium concentrations. In practical manufacturing, the diffusion of carbon atoms could be restrained by decreasing the temperature in short time in the application of UFC, as a result that cementites would most likely dispersed in the form of nano-scaled particles directly, rather than being fully grown up into lamellar pealites. Carbon content and the degree of under-cooling are the main factors to control the nano-scale cementite precipitation. Moreover, there were a lot of carbon-rich areas in the microstructure of undercooled austenite, based on the equilibrium concentration calculation, and this part austenite with the high carbon concentration was apt to decompose and form the precipitation of nano-scaled cementites likely.(4) The thermomechanical treatment (TMT) after UFC can increase evidently the dislocation density for cementite nucleation, and it will be a feasible way to realize the uniform precipitation of nano-scale cementite in the entire miscrostructure for the further improvement of strengthening effect. The experimental results demonstrated that, when the UFC stop temperature decreased from 660℃ to 500℃, the microstructure of experimental steel changed from two phases of the proeutectoid ferrite and degenerated pearlite to the bainite phase with nano-scale cementite precipitation, and the yield strength of 0.17%C experimental steel reached 700MPa. The strength increased with the stop temperature of UFC decreasing, and decreased with the holding time increasing.(5) The effects of cooling path on the microstructure and hole-expansion property of annealing 0.33%C steel were investigated. The results show that finely dispersed spherical cementite could be formed in a ferrite matrix after the UFC process and annealing treatment. With the fininsh rolling temperature and UFC stop temperature decreasing, the spheroidized cementites after annealing were more homogeneous and dispersed finely. During hole-expansion test, cracks were observed to form usually in the edge region around the punched hole when the tangential elongation exceeded the forming limit, and cracks are mainly formed in the way of micro-voids coalescence. Fine and homogeneous microstructure comprised of ferrite and spheroidized cementite could improve the elongation of the experimental sheets, suppressing the coalescence of micro-voids, and improving the hole-expansion property. The limit hole-expansion value of experimental steels could reach 165.8%. |
PDF Full Text Request