High Cycle Fatigue Behavior Of Carbide-free Bainite/martensite Multiphase Steel

In this paper,the new heat treatment of BQP process was applied to the Mn-Si-Cr carbon free bainite/martensite multiphase steel,and the best matching of strength,plasticity and toughness was obtained by optimizing the process.The rotating-bending fatigue test was carried out with BQP process,conventional bainite austempering(BAT)process and bainite-based quenching plus tempering(BQT)process.The fatigue properties of the steels with different heat treatment processes and different inclusion level were studied.The experimental steel treated by BQP process was obtained the best conventional mechanical properties,which quenched at 200 ?,the partition time is 45 min.Tensile strength is 1688 MPa,the total elongation is 25.2%and PSE reached to 42.6 GPa%.Compared with the BAT process and BQT process,the microstructure of BQP process contains more(22%)stable submicron and nanoscale film-retained austenite,and because the prior austenite grain was separated by the needled lower bainite,the best mechanical properties were obtained.Five groups of rotating-bending fatigue tests were carried out.The fatigue limit of BQP process with ESR was 968 MPa and the fatigue ratio was 0.573.The fatigue limit of BAT process with ESR was 721 MPa and the fatigue ratio was 0.479.The fatigue limit of BQT process with ESR was 997 MPa,the fatigue ratio was 0.523.The fatigue limit of BQP without ESR was 828 MPa,the fatigue ratio was 0.491,the fatigue limit of BAT without ESR was 729 MPa,and the fatigue Strong ratio of 0.484.The size of the surface defects of BQP samples is from 10 to 42 ?m,the corresponding stress intensity factor is from 4 to 8 MPa(?),the larger the stress intensity factor of the surface defects is,the lower the fatigue life of the sample will be.The surface defects which are slender flaw has greater damage the fatigue performance than the notched defects specimen,and the size of defects which leads to surface defects decrease with the increase of stress amplitude.The BQP fatigue limit increased dramatically with the decrease of inclusion level,however BAT fatigue has a little change.It is more less sensitive to inclusions when the smaller the size of inclusions and the inclusions closer to the internal location.The critical inclusion of BQP,BAT and BQT are 17 ?m,20 ?m,9 ?m,respectively.The compatibility of CFB/M multiphase high strength steel with inclusions is much better than that of the same level of tempered martensite steel.The crack of internal matrix crack samples formed at the grain boundary or phase boundary,and formed little facets,the angle of the facets with the horizontal impacts on the fatigue life.The fatigue crack thresholds for the BQP,BAT and BQT processes are 3.77 MPay(?),4.67 MPa(?)and 4.16 MPa(?)respectively.The rate of fatigue crack propagation the GBF region of the fatigue fracture ranged is from 2×10-9 to 10-6,and with the increase of fatigue life,the crack growth rate is decrease.The critical condition for the transformation from surface cracking to internal cracking is that the stress intensity factor of internal crack initiation expands to the threshold of fatigue crack first.