Study On Microstructures And Mechanical Properties Of Ultra-low Carbon High Strength Bainitic Steel Q690D

Ultra-low carbon bainitic steel with high strength,low cost,high weldability and excellent low temperature toughness of advantages has been widely used in petroleum pipelines,large blower,Petroleum chemical industry,boiler container and other fileds.Requirements,not only high strength,but also high toughness and low costs,have been proposed for Ultra-low carbon bainitic&high strength steel.The article takes the project "Ansteel C,Mn,Nb,Mo,B,Ti ultra-low carbon bainitic steel plate product and process research" as the background,The ultra-low carbon bainitic steel plate product and process had been mainly studied.The chief original works are as follows:(1)Considering strength,toughness and welding performance,the composition design choice C,Mn,Nb,Mo,B,Ti as the best solution.Through the research of recrystallization temperature and the CCT curve,we found out the best process of heating and the best process of controlled rolling and controlled cooling.The best process is heating temperature is 1150℃,initial rolling temperature is 1050℃,finishing temperature is above the recrystallization temperature,cooling speed is 10～15℃/s,final cooling temperature is 550℃.(2)Analysis on the microstructures and properties of the unqualified Q690D steel plate which were industry trial production in Ansteel,The results showed that the main reasons influencing the performances of steel plate are as follows:Low content of alloying elements and element segregation lead to high phase transition temperature and inadequate hardenability;Temperature precise control ability is not enough,Higher red temperature lead to granular bainite,oversize phase of MA and individual local area are small amount of acicular ferrite which affect the strength and toughness of the steel plate.(3)Using MINITAB software analyzed the chemical composition and the parameters of controlled rolling and controlled cooling.The results show that the main reason of Q690D mechanical fluctuation which produced in Ansteel medium plate line is controlled cooling system not stable.Static recrystallization did not lead to the decrease in the strength in the production,Dynamic recrystallization happen or not has no corresponding relation with low yield strength and tensile strength which lead to the unqualified properties.