Study Of P Alloying Low Carbon Bainitic Weathering Steels

Ultra-low carbon bainitic steels for marine applications have attracted more and more attention, because of their special composition designs, excellent mechanical properties and low cost. In this paper, the main purpose is to develop a new steel with low cost, high strength, excellent low-temperatures toughness and corrosion resistance. The effects of alloying elements on the mechanical properties and microstructures of ultra-low carbon bainitic steels were investigated, Furthermore, the continuous cooling transformation curves and hot ductility were studied through thermal simulation tests. The parameters during the hot working process were determined in order to provide the theoretical and experimental basis for producing the steels which have extraordinary mechanical performances and corrosion resistance as the marine applications.Reference to the compositions of the steels and their mechanical properties in the literatures, and on the basis of the previous studies in our research group, two different compositions of the steels in this paper were designed. The dilatometric curves of experimental steels at different cooling rates were detected on gleeble-2000thermo-mechanical simulator. The CCT curves of hot deformation were determined. Using thermal dilation measurement, microstructure observation and hardness measurement, the influences of cooling rate on microstructure and hardness of the steel were studied. The microstructure of ferrite and pearlite is obtained at the cooling rate below1℃/s. Small amount of granular bainite is observed at the cooling rate of1℃/s. Volume fraction of bainite increases while the volume fractions of ferrite and pearlite decrease as the cooling rate increases. The microstructure mainly consists of bainite when the cooling rate ranges between3℃/s and25℃/s. All these provide the references for productive practice to adjust the parameters of rolling and cooling.The ingots were hot rolled to plates with the thickness of12mm. One method is TMCP process with directly water quenching and air cooling after rolling, and the other one is TMCP+RPC process, with40s holding time after the rolling and then water quenching. Tensile properties at room temperature and impact toughness of the steel with different compositions after the two methods of hot rolling were investigated. Microstructures of the steels after the hot rolling were observed. As well as the effects of the morphologies of M/A on the mechanical properties of the steels were studied through Scanning electron microscope (SEM).Microstructure observation results showed that granular bainite and quasi-polygonal ferrite are mainly found in the condition of air cooling. The microstructure mainly consists of bainite when the process with directly water quenching after rolling. The fraction of the M/A islands increase when the process with40s holding time after the rolling and then water quenching. When the deformation temperature decrease to about830℃. The M/A islands are distributed more dispersedly and the size of M/A islands decrease.The mechanical properties results show that, the tensile strengths of the C2steels after are all higher than C1steels, the impact toughness of C1are all highter than C1steels. Through comparing the influence of different kinds of processes, we concluded that strength and toughness will be improved under higher cooling rate. With the relaxation time after deformation, the impact toughness of the experimental steels are higher, while the tensile strengths are lower. Little change in the tensile strength at low rolling temperature, but the impact toughness and the ductility will be improved. Considering the tensile strength, impact toughness, experimental steel has the best combined mechanical performance when the process with directly water quenching after rolling at low temperature.Hot ductility of the continuously casting slabs were tested by gleeble-2000thermo-mechanical simulator. Characteristics of the tensile strength and ductility of the steel under the high temperature condition are clarified. The first brittle zone of casting slab at high temperature-more than1300℃, and the third brittle zone of casting slab was between750℃～800℃, but the second brittle zone was not exist. To avoid the zone of low ductility in the third brittle zone, secondary cooling zone adopt weak cooling as far as possible, enhancing the temperature over800℃during bending and strengthening was helpful in improving ductility of steel and avoiding surface cracks of slabs.