Effect Of Rolling And Annealing On Microstructure And Properties Of Ferritic Stainless Steel

With the increasing development of automobile manufacturing industry,there exists the negative impact on environment.With the implementation of automobile exhaust emission by gradation,the engine fuel efficiency is continuously improved,resulting in an increased exhaust gas temperature.The temperature of the exhaust emission system in some cars will reach to 1000?,even to 1050?.A new generation of automobile exhaust system with ferritic stainless-steel materials is urgently needed,which should meet the requirement for higher temperature oxidation resistance and high temperature strength and higher thermal anti-fatigue ability.The materials used in this thesis is 444-type ferritic stainless steel which were designed with multi-elemental alloying.The aim of this research is to reveal the relationship among the rolling process,annealing,microstructure,mechanical property and formability.It will provide theoretical basis for the processing technology and the subsequent study on the welding and high temperature oxidation resistance.The main contents and results are summarized as follows.(1)The cold rolling and annealing processes of ferritic stainless steels with different contents of W and Mo were studied by metallographic observation and hardness measurement.The changes of the microstructure and hardness were obtained with cold rolling and annealing processes.The optimal annealing process is 1050 ?×1 min for the cold rolled steel sheet.Also,the addition of W in ferritic stainless steel is more effective in increasing recrystallization temperature than the Mo addition.(2)A study was made on the effect of hot rolling process on microstructure and properties of two kinds of 444 ferritic stainless steels.By reducing the finishing temperature of hot rolling,the work hardening rate and the deformation stored energy can be improved for the hot rolling sheet,and the annealing grains can be refined.As for the texture,reducing the finishing temperature can lower the ?-fiber texture intensity of hot-rolling sheet and make the strong position move downwards.It also makes the texture of hot rolled and annealed sheet dispersed,and weaken the ?-fiber texture and increase the y-fiber texture intensity of cold rolling and annealing sheets.The recrystallization texture is more located nearby the component {111}<121>.A through-thickness uniformly distributed grains can be obtained in the cold rolled and annealed sheet.In this case,the yield and tensile strengths of the finished sheet can be improved,while the formability can be enhanced.The surface quality of the finished sheet can also be increased.Higher W content can improve the formability of cold rolled and annealed sheet,while higher Mo content can increase the tensile and yield strengths of the sheet.(3)A study was also made on the effect of cold rolling process on microstructure and properties of 444 ferritic stainless steels.By increasing the reduction rate of cold rolling,the width of the deformation band of cold rolling sheet was reduced,and the annealing grains can be refined.As for the texture,increasing the reduction rate of cold rolling can weaken y-fiber texture intensity and increase the intensity of ?-fiber texture.The strong position is in{223}<110>components.It also increases the y-fiber texture intensity of cold-rolled and annealing sheet,and weakens deviation from {111}<121>?{111}<112>components.It can also increase the percentage of<111>//ND orientation grain,decrease the percentage of<001>//ND orientation grain.In this case,the tensile strength and yield strength of the finished sheet can be improved,and the formability and surface quality of finished sheet can also be increased.