Study on the effect of different heat treatment processes on microstructure and properties of a cold rolled complex-phase steel
With the development of the automotive industry,the application of advanced high strength steel has become more and more extensive.A low-carbon cold rolled complex-phase steel containing Cr-Mo-Nb-Ti was designed.Intercritical heating,slow cooling and overaging heat treatment process were adopted.By optical microscope(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and tensile testing machine,the microstructure and mechanical properties of experi-mental steel under different heat treatment processes were studied.The results show that the microstructure of the experimental steel is ferrite+bainite+martensite under different heat treatment processes.With the increase of annealing temperature,the degree of re-crystallization of ferrite is higher,the dislocation density of ferrite decreases,the super-cooled austenite increases,and more bainite is formed in the overaging stage.At lower overaging temperature,martensite is partially tempered to form tempered martensite.Under the annealing temperature of 800 ℃ and overaging temperature of 350 ℃,the experimental steel has optimum recrystallization degree and bainite/martensite tempering state.The number of large angle grain boundaries accounts for 44.7％,the yield strength of the experi-mental steel is 613 MPa,the tensile strength is 799 MPa,the hole expansion rate is 67％,and the tensile fracture presents large and deep dimples.The excellent formability of the experimental steel has been obtained.
complex-phase steelheat treatmentmicrostructure and propertyhole expansion ratefracture
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