Study On The Intermediate Temperature Transformation,Microstructure And Mechanical Properties For Low Carbon Bainite Steels

Low carbon bainitic steel is a new kind of steel which was developed in20years in world.It has three characteristics:high strength,high ductility,excellent welding property.It is the outcome combing modern metallurgy with physical metallurgy research achievements.At present,a large number of research works are performed on low carbon bainitic steel all over the world.In this paper,technique simulate,heat treatment process and rolling process are studied with their effect on the microstructure and the mechanical properties of low carbon bainitic steel.The most important works are getting the static CCT curves,dynamic CCT curves and the different rolling processes,cooling processes and heat treatment processes by MMS-300multi-function thermo-mechanical simulator.The obtained microstructure is analyzed through the optical microscope,scanning electron microscope and transmission electron microscope.T-he mechanical properties of experimental steels are tested through tensile tests and impact tests.the fracture mechanism and method of the experimental steels are analyzed according to the microscopic and macroscopic pattern of nick.The corresponding results are as follows:(1)The experiment of continuous cooling transformation showed:under static situations,B element and the increase of Mn element both can bring down the beginning and finishing temperature in a certain range of cooling rate and refine the microstructures to some extent.When giving the deformaion of50%,bainite start temperature is brought down.The experimental steels has wider bainitic converting temperatures whether dynamic or static situation,and the requirement to the cooling speeds is very low.when cooling velocity increases, it’s easy for getting bainite ferrite.(2)The experiment of TMCP process showed:the decrease of the final cooling temperature,the deformation temperature and the increase of the deformation, can obtain fine bainite and improve microhardness.The experimental steel can get granular bainite under the condition of laminar cooling,but get the bainite ferrite under the condition of UFC.The experimental steel can get the mixture of granular bainite and bainite ferrite under the condition of the laminar cooling and UFC.The microstructures has the largest microhardness under the condition of the UFC.(3)The experiment of heat treatment process showed:the experimental steel can obtain bainite ferrite whether the first or the second quenching process,and the bainite ferrite get recovery behavior in both processes on which the first quenching process is more serious compared to the second one.A few minor precipitates appear on the both processes.The second quenching process improves the microhardness.The final temperature,the deformation temperature and the tempering process have the same influence laws like the TMCP process with the condition of the second quenching process. With the increase of the tempering heating rate,the size of excess carbon nisidhes become smaller,the excess carbon nisidhes grow in quantity which improve the organization’s microhardness.(4)The experiment of the experiment of TMCP process on laboratory showed:with the lower of the final cooling temperature,the microstructures of the four kinds of the experimental steels transformed from the to acicular ferrite to the granular bainite,and were enhanceed on yield strength and the impact energy with twenty degrees celsius and increased on elongation rate after reducing at first,but had small effect on the tensile strength. The decrease of the beginning and end temperature of steel plate at the finishing mill refine the microstructures and improve the strength,the toughness of experimental steel.The rolling process on experimental steel shows that the improvement of rolling process can reform the mechanical properties.The hardenability were enhanced with the alloying element and the yield strength and tensile strength were improved in the chance of getting bainite with the same finishing cooling temperature,but decreased the elongation rate and the impact energy with twenty degrees celsius.The experimental steels with Cr elements were ductile fracture.