Study On Microstructure And Mechanical Properties Of Friction Stir Welding Of TRIP590 High Strength Steel Sheet For Automobile

The microstructure of transformation induced plasticity（TRIP）steel is composed of hard bainite,soft ferrite and metastable retained austenite.In the process of plastic deformation,the transformation of retained austenite into martensite was induced by TRIP effect,through strain induced transformation,resulting in significant work hardening,which leaded to significant work hardening,so as to improve the strength and plasticity of steel.Friction stir welding（FSW）is a new solid-state welding technology.With the development of stirring tools and the improvement of welding process,the application of FSW in various steel materials was promoted.Taking TRIP590 high-strength steel as the research object,FSW experiments were carried out by butt welding and lap welding,and the samples after welding were treated by cryogenic treatment to explore the change law of retained austenite and the effect of retained austenite on the microstructure and mechanical properties of the joints.The main conclusions are as follows:The change of retained austenite during FSW butt welding of TRIP590 steel sheets with thickness of 1.4 mm was investigated in the welding speed of 100 mm/min and200-500 rpm.Martensite was formed in the stir zone and thermomechanical affected zone of the welded joints,the heat affected zone softens and the content of retained austenite decreased.When the rotation speed was 200-300 rpm and the peak temperature in the stir zone was between Ac₁ and Ac₃,partial austenitization occurred.With the increase of temperature,the content of retained austenite gradually decreased.Due to the higher grain refinement at lower temperature,the stability of austenite was improved and some retained austenite was retained after welding.When the rotation speed was 400-500 rpm,the peak temperature was higher than Ac₃,complete austenitization occurred,and the size of austenite was large.With the increase of temperature,the size of austenite increased and the stability was low,resulting in the disappearance of retained austenite after welding.Compared with the base metal,the tensile strength and elongation of the joints were reduced.The tensile test of small samples in the stir zone showed that the tensile strength increased with the increase of rotation speed,and the elongation decreased with the increase of rotation speed due to the decrease of retained austenite content.The effect of retained austenite on microstructure and properties was investigated at a constant speed of 400 rpm and a welding speed of 50-150 mm/min.Because the peak temperature of the stir zone exceeded Ac₃ and complete austenitization occurred,the poor stability of austenite leaded to the reduction of the content of retained austenite and the elongation.Due to the formation of a large amount of martensite,the hardness and tensile strength increased.In the lapping experiment,taking 100 mm/min as the constant welding speed,the speed of 300-1000 rpm was used to study.The retained austenite in the stir zone of all joints was significantly less than that of the base metal.Due to the different heat input during the welding of the upper and lower plates,when the welding peak temperature was between the two-phase zone,the grain refinement degree was higher when the temperature was low.Although the amount of austenite was less,it had high stability and the content of retained austenite was relatively high.When the rotation speed was300-500 rpm,the content of retained austenite decreased gradually with the increase of rotation speed.When the rotation speed was higher than 600 rpm,complete austenization occurred,the austenite size was large and the stability was poor,and the retained austenite basically disappeared after cooling.In the 300-500 rpm shear tensile test,it broke at the base metal and the interface broke at 600 rpm.With the increase of rotation speed,the interface bonding performance decreased.The butt welding samples at 200 rpm and 300 rpm were subjected to deep cryogenic treatment,and part of the retained austenite was transformed into martensite.With the extension of deep cryogenic treatment time,the content of retained austenite decreased further and the content of martensite increased.However,the average grain size in the stir zone was further refined.Due to the effect of grain refinement,the hardness,strength and elongation of the stir zone were improved.