| Focused in this thesis are metallurgical mechanisms and industrial technologies on the yield strength of a high strength micro-alloyed steel with a strength of 700MPa (BS700MC).It is required that BS700MC has not only high strength but also good formability, shock resistance and weldability and that the rolling loads are not high in the production line. Since a rational chemistry is a basic to ensure good performance, a microalloyed chemistry system with Nb and Ti bearing steel was introduced. The lower flow stress and higher production strength enabled large production of the steel at Baosteel, as the strengthening mechanism used was the precipitation based assisted with phase transformation and grain size refinement.Ti was employed as the basic strengthening element caused by precipitation. It was proved that an addition of 0.1% Ti is enough to elevate the strength of at least 200 MPa and the strength becomes sensitive to the coiling temperature as increasing the addition. However by adding Mo, the formation of polygonal ferrite could be suppressed and thus allotriomorphic ferrite was introduced at quite high temperature. This not only caused dislocation strengthening but also enhanced the effect of precipitation strengthening. Thus a small amount of Mo addition is enough to cause quite higher strengthening effect.Through single pass thermo-simulation compression experiment, the true stress-strain curves were obtained for BS700MC. The stress increased with increasing the deformation ratio, increasing the deformation degree and decreasing the temperature. As Nb addition was quite high, there was no dynamic recrystallization at the experimental condition. The creep function was employedin building the flow stress model at high temperature. The flow stress was predicted at different deformation rate and different temperature. The experimental results agreed well with the modeling prediction.To demonstrate that the production of BS700MC could be performed at the 2050 mm Hot Strip Rolling Mills at Baosteel, the comparison between the deformation resistance curves of three types of steels by thermo-simulation experiments was done. It showed that the deformation resistance of BS700MC at 900~950°C was close to that of structure steel of 500 MPa at 800~850°C. This proved it feasible to produce BS700MC at the 2050 mm Hot Strip Rolling Mills at Baosteel.The reheating temperature plays a big role on the precipitation based strengthening effect of BS700MC. Experiments displayed that a good reheating temperature should guarantee the complete solution of carbides and nitrides existed after casting in the slabs. The production strength increases about 100 MPa when increasing every 50°C of the reheating temperature at the range of 1150°C~1250°C. However, the final rolling temperature does not influence very much as the result of cooperative effect of grain refinement and precipitation. Furthermore, it is quite important to ensure higher cooling rate and earlier start cooling time, as the coiling temperature is the key to enhance the precipitation and dislocation strengthening effect, which influence the precipitation thermodynamics and phase transformation microstructure.Besides, the precipitation phenomena at both austenite and ferrite zone for BS700MC were studied by using the thermo-simulation and TEM methods.As there exist differences between laboratory experiment and industrial production, a good chemistry and suitable process parameters should be determined based on the deep understanding of the effect of material composition and process conditions. The failure reason in the first and second industrial trials was analyzed. Based on this, the good chemistry and process parameters were determined and thus BS700MC was successfully produced at Baosteel.
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