Study Of A New Type Bainitic Steel And It's Heat Treatment

A new type of bainitic steel containing suitable contents of Mn,Si and Cr has been designed and manufactured.With the method of dilatometric change together with microscopic test and hardness measurement,different expanding curves of Continuous Cooling Transformation were constructed,and CCT(continuous cooling transformation)diagram was obtained using DIL805A thermal expansion fast phase transition instrument.Changes of the morphology and the austenite transforming of the bainitic steel are analyzed.According to the CCT curve of the steel,a kind of special quenching medium was developed.The content of the lower bainite structure in the steel was controlled by changing the temperature and the density of the quenching medium.In addition,the evolving rules of the bainite structure obtained in the self-designed quenching medium were summarized.Production basement of this bainitic steel was provided.The main conclusions in this article are as follows:1,The CCT curves of the new testing steel with different constitutions were obtained with the method of heat expansion as well as metallography and hardness.The martensite was usually obtained in the cooling rate range of 8℃/s～12℃during the continuous cooling process. The bainite was usually obtained in the cooling rate range of approximately 0.3℃/s～12℃/s.2,A kind of cheap special quenching medium was discovered.The microstructure of the steel can be adjusted by changing the temperature and density of the quenching medium,and it is convenient to obtain the combination of microstructures and the properties.3,The change rules of the bainite structure during the heat treatment was summarized.(1)The structure of steel was martensite when the quenching medium temperature is 25℃and the density is within 1.1 g/cm~3to 1.6g/cm~3.(2)The structure of steel was martensite when the quenching medium temperature is 40℃and the density is between 1.1 g / cm~3 and 1.4g/cm~3;when the density is between 1.4g/cm~3 and 1.5 g/cm~3,it was a mixture of martensite and bainite;and when the density lay between 1.5 g/ cm~3 and 1.6 g/cm~3,lower bainite can be obtained.(3)When the quenching medium temperature was 50℃and the density is from 1.1 g/cm~3 to 1.3 g / cm~3,the structure of steel was martensite;it was a mixture of martensite and bainite when the density lay between 1.3 g/cm~3 and 1.4g/cm~3;it was lower bainite when the density lay between 1.4 g/cm~3 and 1.6 g/cm~3. (4)When the quenching medium temperature was 60℃and the density is within 1.1 g / cm~3 to 1.2g/cm~3,the structure of steel was martensite;it was lower bainite when the density lay between 1.2g/cm~3 and 1.6g/cm~3.(5)When the quenching medium temperature was 70℃and the density is from 1.1 g/cm~3 to 1.6 g/cm~3,the structure of steel was lower bainite.(6)When the quenching medium temperature was 80℃and the density lay between 1.1 g / cm~3 and 1.5 g/ cm~3,the structure of steel was lower bainite and when the density is within 1.5 g/ cm~3 to 1.6 g/ cm~3,it was pearlite.(7)When the quenching medium temperature was 90℃and the density lay between 1.1 g/cm~3 and 1.4 g/ cm~3,the structure of steel was lower bainite and when the density lay between 1.4 g/ cm~3 and 1.6 g/ cm~3,it was pearlite.4,A new type of low-cost bainitic steel with low content of value metal was developed and the techniques of preparation is simple and convenient.