Research Of Key Parameters In The Process Of Deformation Induced Ferrite Transformation Of X70HD Pipeline Steel

Relative to the proeutectoid ferrite, the size of deformation induced ferrite transformation ferrite grain is much small. It will be of great significance to refine the grain size if the deformation induced ferrite transformation technique is applied to X70 HD pipeline steel’s production, and makes the deformation induced ferrite replaces the proeutectoid ferrite after finish rolling.X70HD pipeline steel’s static phase transformation point and dynamic phase transition point were determined under certain conditions in this paper. Based on the measured phase transition points, a thermal simulation experiment of two stage large deformation whose accumulated deformation is as high as 80 % was made. We used the two stages of deformation simulate the roughing rolling and finishing rolling respectively.Deformation parameters in the first stage was fixed firstly, then the temperature and the strain rate of deformation in the second stage wer e changed before quenching, and then considered their influences on the generation of deformation induced ferrite and the grain size of ferrite. Through the observation of the metallographic analysis, fine polygon ferrite is found. That ferrite is deformat ion induced ferrite. The lower the strain rate is, the more deformation induced ferrite is found in the low temperature area of the austenite non-recrystallization region. And the higher the strain rate is, the more deformation induced ferrite is found in the high temperature area of the austenite non-recrystallization region.In order to analyze the influence of deformation on deformation induced ferrite transformation, we developed thermal simulation compression test of two phases and three phases. We found that the more amount of deformation in the second stage deformation is, the higher the ferrite content will be under the premise of the cumulative deformation be constant. For two stages of deformation, the amount of deformation induced ferrite will be reduced no matter the amount of deformation in which one phase reduces separately. Relative to the two stages of deformation the three stages of deformation will get more deformation induced ferrite. The fewer rolling passes in the third stage of deformation, the less content of ferrite will be. The content of ferrite will ruduce, if interval time exists in the third stage of deformation. This provides basis for reasonable deformation.To refine the grain size, producing a certain proportion of deformation induced ferrite and avoiding the generation of proeutectoid ferrite in the process of rolling for X70 HD pipeline steel is necessary. By cooling the thermal compression samples at the same rate, the influence of the final cooling temperature on the organiza tion was analyzed. Then the right finish cooling temperature is confirmed. By setting different cooling rates under particular parameter for samples after hot compression deformation, the effect of cooling rate on microstructure was analyzed. Eventually, we obtained a large amount of deformation induced ferrite through regulating the deformation, deformation temperature, strain rate, cooling rate and final cooling temperature and grain refinement and improving organization were achived for X70 HD pipeline steel.