Effect Of Hierarchical Martensite Microstructure On Fatigue Crack Propagation Behavior Of 1500 MPa Grade Hot-stamped Steel

Fatigue failure is one of the most important failure modes in the service process of automobile components,which is very harmful.The fatigue life of components is mainly composed of the initiation and propagation of fatigue cracks.The study of the propagation and variation behavior of fatigue cracks has important guiding significance for the accurate prediction and evaluation of fatigue life.In this paper,the multi-layer characteristics of martensite and the variation law of mechanical properties were analyzed for 22 Mn B5 hot stamping steel through hot stamping test.The fatigue crack propagation behavior was analyzed by using standard compact(CT)fatigue crack propagation test,and the following conclusions were drawn:(1)The hot stamping process parameters were optimized through conventional heat treatment experiments,and the hot stamping experiments of 22 Mn B5 were carried out on flat die and U die.It is found that the strength of hot stamping steel increases greatly,the mechanical properties of U-shaped steel are better than that of flat steel,and the microstructure of U-shaped steel is lath martensite.At 830?,the mechanical properties are the lowest.At 860? and 890?,the mechanical properties are close to 1500 MPa.At 920?,the mechanical properties decrease again due to the increase of grain size.(2)In the in-situ tensile process,micropore cracks initiate at the high stress concentration and grain boundaries with inclusions,destroy along the martensite grain boundaries,and the inclusions are debonded with the matrix,and the microcracks expand and grow up at the center and aggregate,finally leading to the fracture of the material.(3)SEM,EBSD and TEM were used to characterize the microstructure of hot stamping steels under different processing conditions.It was found that the average primary austenite grain size,packet,block and lath width all increased with the increase of temperature.(4)The fatigue life of hot stamping steel at 920? is 128988 cycles while that at 860? is only 103346 cycles.The stable growth zone of fatigue crack in hot stamping steel can be described by Paris formula.By analyzing the fatigue crack growth rate,the size of the reverse plastic zone and the fatigue crack path,it is inferred that the block is an effective substructure to control the fatigue crack growth behavior of 22 Mn B5 hot stamping steel.(5)The fatigue fracture of stable-growing region was typical fatigue striations along with secondary cracks,and the number of secondary cracks decreased with increasing ?K.The fatigue fractography of rapid-growing region was characterized by dimples and crack closure phenomenon.Ductile fracture mechanism gradually became predominant as the fatigue crack moved on.