Study On Microstructure Evolution And Mechanical Properties Of Ti Microalloyed Hot Rolled Dual Phase Steel

With the continuous deepening study of energy conservation,emission reduction and automobile lightweigh,the high strength steel has been widely used in automobile industry.As a kind of advanced high strength steel,hot rolled dual phase steel has been widely used in automobile manufacturing because of its excellent comprehensive performance.Due to the addition of high content of Cr,Mo and other alloying elements,traditional hot rolled dual phase steel production costs higher,while C-Mn-Nb-Ti hot rolled dual phase steel not only high cost,At the same time,the rolling force is larger.Because Ti resource is abundant in China,the cost of Ti is lower,compared with Nb has obvious price advantage,in line with the concept of energy conservation and environmental protection,compositional design using Ti instead of Nb to reduce the cost,while combined with controlled rolling process and controlled cooling of hot rolled dual phase steel has an important theoretical and practical significance.In this paper,the hot rolled dual phase steel with C-Mn-Ti composition system is taken as the research object.The austenite deformation behavior and under-cooling austenite transformation behavior of the experimental steel are studied,and the laboratory hot rolling experiment is carried out.The main work and research results are as follows:(1)The deformation behavior of high temperature austenite in experimental steel was studied by single channel compression test.The influence of deformation parameters on deformation resistance is given.The coefficient of constitutive equation of deformation resistance under different deformation degrees is fitted.A mathematical model for deformation resistance of high temperature austenite is established,and the accuracy of the obtained model is verified through comparison.(2)Using phase transformation instrument,the phase transition behavior of under-cooled austenite was studied,and the dynamic CCT curve was drawn.The results show that the smaller cooling rate can promote the ferrite transformation of under-cooled austenite and inhibit the formation of hard phase,while the larger cooling rate can refine the ferrite grains.The Ar3 temperature decreases as the cooling rate increases,.(3)The cooling path of under-cooled austenite was simulated to study the influence of process parameters on the micro structure evolution of under-cooled austenite.The results show that the smaller cooling rate of the first stage is favorable for the precipitation of ferrite and the reduction of the martensite size at the same time when the holding temperature is maintained.The temperature of ferrite change ccnose temperature" is about 650?.The content of ferrite increases as the holding time increase.The curve of ferrite transformation with holding time is similar to that of"S".(4)The experimental steel was subjected to hot rolling test of stepped sample to study the influence of the reduction and the relative deformation on the microstructure evolution of the experimental steel.The results show that pearlite transformation occurs in the experimental steel when the relative deformation reaches 73%,and the micro structure is a mixed phase structure of ferrite+pearlite+martensite,the other relative deformations are ferrite+martensite dual phase structure.Larger reduction is beneficial for grain refinement and increase ferrite content.At same reduction conditions,larger relative amount of deformation can increase the ferrite content.(5)The laboratory hot rolling experiments were conducted to study the influence of air cooling time on the microstructure and mechanical properties of the experimental steel in different finishing rolling temperature range.The results show that,(a)in the range of low finishing temperature(about 810?),the microstructure is ferrite+martensite at room temperature when the air cooling time is short,and the pearlite transformation occurs when the air cooling time is longer(40 s).The longer the air cooling time,the more the ferrite content,and the larger the grain size,the smaller the martensite grain size.When the air cooling time is prolonged,the experimental steel tensile strength and yield strength increased to the highest value at first,TiC precipitates gradually increased,(b)In the high finishing temperature range(about 880?),when the air cooling time is 1 s,the microstructure is almost completely martensite at room temperature,no TiC precipitates are observed.When the air cooling time is prolonged,ferrite transformation took place,the microstructure is ferrite+martensite.Yield strength decreased rapidly,then remained unchanged within a certain range,tensile strength has been slowly reduced.