Research On Preparation Of Bainite/Martensite Dual Phase High Strength Steel And Mechanism Of Strength And Toughness

Bainite/martensite dual phase steel has an outstanding comprehensive mechanical properties and the advantages of saving resources and energy,reducing the quality of parts and so on,so the demands for this kind of steel are increasing.In order to shorten the production cycle and improve the efficiency of industrial production,using resources rationally,the idea of bainite/martensite dual phase steel is put forward,that idea comes from the conventional DP(martensite/ferrite)steel and then ptimizes it.By using of reasonable composition design,a amount of bainite with good comprehensive mechanical properties is obtained to replace the ferrite in the conventional DP steel by the rapid cooling and slow cooling process after hot rolling,then the bainite will refine the austenite grains and expel the C elements to form a carbon rich region to make the austenite stable and refine martensite grain.In a word,We hope the tested steel could obtain excellent comprehensive mechanical properties.In this thesis,the microstructure evolution during the slow cooling process are studied through the tested steel continuous cooling transformation behavior and high temperature deformation and slow cooling process in the RAL laboratory of Northeastern University,then the best parameters are obtained.On the base of these,the hot rolling experiment is carried out,the cooling parameters to the steel's microstructure and properties are studied.The main contents and conclusions are as follows:(1)In this thesis,0.346%medium carbon steel is designed as the experimental steel.The microstructure evolution behavior of the deformed of 13%austenite under continuous cooling are studied and its continuous cooling transformation curve are drawn.The results show that:during the continuous cooling process,the tested steel only includs the bainite and martensite transformation zone.In the cooling speed range(0.1?/s?1?/s),B microstructure is obtained,with the cooling speed increasing,the starting transition temperature of B begins to drop,at the meantime,the microstructure of B is refined.When the cooling speed is faster than 1?/s,the microstructure of martensite is formed and then the content of it increase with the increasing of the cooling rate.When the cooling speed is faster than 10?/s,its main microstructure is martensite.The results of microhardness test show that:The microhardness value of bainite and martensite increase with the increasing of cooling rate,so there is always a certain value of hardness difference between them:96?148HV,that meet the designing requirements of the experimental steel.(2)The effect of cooling temperature point and cooling speed on the microstructure of steel are studied through high temperature deformation and slow cooling process by thermal simulation testing machine MMS-300.The results show that:in the same cooling speed,the cooling temperature point is higher,the more(Nb,Ti)carbon nitride precipitation and nucleation site,the more content of bainite ferrite forms.When the cooling temperature point reaches 650? and the cooling speed is 0.1?/s,the content of bainite ferrite is the largest(31.9%)and higher than different cooling rate conditions at the other end of the final cooling temperature 7%?20%;When in the same cooling temperature point,with the cooling speed growing faster,the bainite ferrite become smaller and more uniform dispersion,the separated carbon rich region become more dispersed and then the content of retained austenite become higher;When the cooling rate reaches 1?/s,both the content of bainite and the carbon rich region decreases,some less carbon content regions shear into martensite;The content of retained austenite and the carbon content in retained austenite increase firstly and then decrease with the increasing of cooling rate,both the values of them attain the maximum in the speed of 0.5?/s.(3)The mechanical properties of medium carbon low alloy altra high strength steel are studied by different cooling temperature point and slow cooling speed.The results show that:The change of cooling temperature point has little effect on the mechanical properties in the same cooling speed,however,they are significantly effected with the change of cooling speed.When the cooling temperature piont become 650?,their mechanical properties are best in cooling speed of 0.5?/s,tensile strength can reach 2039 MPa and the percentage elongation after fracture is 15.5%,the product of strength and elongation is more than 30GPa%.(4)Hot rolling experiment of experimental steel is carried out on the basis of high temperature deformation and slow cooling process.The results show that:the microstructure of tested steel is bainite/martensite and retained austenite in different final cooling temperature and there is not quite difference in mechanical properties,However,there is greatly improvement comparing with the tested steel that direct quenching to room temperature,the product of strength and elongation increase by 5?6GPa%.When the final cooling temperature is 494? and the cooling rates is 0.3?/s,the tested steel has good comprehensive mechanical properties:tensile strength is 1788MPa,elongation rate is 12.67%,the product of strength and elongation is 22650MPa%,impact Gong reached 41J at-20?,this fully mathes the standard mechanical properties of ultra high strength steel.