| Lightweight is significant for energy-saving,environmental protection and even the sustainable development of the automotive industry.High-strength automotive steel is an important way to achieve vehicle lightweight and improve safety performance.However,high-strength automotive steel is easy to crack during the cold forming process,especially the local forming of complex parts(such as flanging,bending,etc),which seriously restricts its popularization and application.In TBF(TRIP-Aided Bainitic Ferrite)steel,bainite is the matrix and retained austenite is the second phase.Due to avoiding the soft-hard phase hardness problem such as existing in dual phase steel,TBF steel has good strong plasticity matching and local forming performance at same time.In this paper,TBF steel in low-carbon Si-Mn-Nb system is the object of study,it makes systematical research of bainite phase transformation behavior,microstructure evolution,mechanical properties and local forming properties during the continuous annealing process.The results of the research are as follows:1.The dynamic law of bainite phase transformation with different austenite grain size was studied by means of phase change apparatus.The results show that with the austenite grain size refinement,austenite stability,need higher cooling rate to avoid the ferrite region;The kinetics of bainite transformation is accelerated with the increasing of heat preservation temperature in bainite region;And under the same heat preservation temperature,the kinetics of bainite phase transformation will be slower with the decreasing of austenite grain size.2.The effects of annealing process parameters on the morphology of matrix,the morphology,volume fraction and distribution of retained austenite and the influence on the ductility were studied.Experimental results show:(1)When the soaking temperature was 860?,the microstructure was ferrite+island martensite+bainite+retained austenite,and the retained austenite was mainly distributed in the ferrite grain boundary;When the soaking temperature was increased to 900?,the micro structure was lath bainite+lump martensite+small amount of ferrite,massive retained austenite decreased,and sTRIPed retained austenite increased;When the soaking temperature is 950?,the content of ferrite is further reduced,and the retained austenite is mainly distributed in the bainite slab in the form of film.With the increasing of soaking temperature,the fraction of ferrite and martensite in the microstructure will decrease,the fraction of slab bainite will increase and the yield strength increases significantly.However,the tensile strength,elongation and the volume fraction of retained austenite do not change obviously.(2)Different isothermal time and isothermal temperature,the type of slab bainite+a small amount of massive martensite+a small amount of ferrite and retained austenite.With the increasing of the isothermal time,the retained austenite changed from granular to thin film,and the tensile strength first decreased and then decreased gradually.The yield strength and the austenite volume fraction increased first and then decreased.At the same isothermal time,increased isothermal temperature,tensile strength decreased,yield strength increased.When the isothermal temperature is 400?,the isothermal time is 1 80s,the strong plastic product reaches 21111MPa%.(3)When the heating rate is 5?/s,the retained austenite is mainly being the thin film morphology and distributes between the bainite slab;When the heating rate is 50?/s,the retained austenite is mainly being the block morphology and distributes in the ferrite boundaries.With the increasing of heating rate,the volume fraction of ferrite and martensite increase,the volume fraction of bainite decreases,the yield strength decreases obviously and the tensile strength decreases slightly.But the elongation and the volume fraction of retained austenite increases obviously.When the heating rate is 50?/s,with the increasing of the isothermal time all strength decrease,the elongation and the volume fraction of retained austenite increase.The isopyclastic product is 24075 MPa%and the elongation is 23.58%,when the isothermal time is 250s.3.The local forming properties,crack formation and propagation behavior of TBF steel are investigated by hole expanding and three-point bending tests.Research indicates:(1)When the soaking temperature is 860?,the micropore density is the largest,and mainly forms at the interface of the ferrite/martensite phase and gathers into microcracks.The microcracks encounter hard phase martensite during the expansion process.The ferrite turn to continue to expand;With the increasing of soaking temperature,micropore density decreases,and the micro-cracks in the expansion process encountered hard phase bainite/martensite will turn to tear ferrite or along the original expansion Directional tearing bainite continues to expand.(2)Bainite's isothermal temperature is 400?.With the increasing of isothermal time,the expanding rate will increase firstly and then decrease.When the isothermal time is 50s,the microstructure near the crack tip is the maximal and the hard phase bainite will be torn apart during the process of crack propagation.With the increasing of the isothermal time,the plastic deformation in the vicinity of the crack will be more and more serious,and the density of the micropores will be smaller.(3)When the heating rate increases from 5?/s to 50?/s,the local forming performance will decrease significantly.When the heating rate is 50?/s,the micropores near the crack tip are mainly forming at the ferrite/martensite phase interface.And the crack mainly extend along the martensite/ferrite phase interface.(4)After two steps of heating treatment,the local forming performance of the material is maximal,namely the exapnding rate is 54%and in could bending 180° along the rolling direction without cracking.Micropores' density is very small near the microcracks,and the deformation degree of microstructure is serious. |
PDF Full Text Request