| How to further enhance the very high cycle fatigue(VHCF)properties of high-strength steel,in order to better meet the needs of the security service and long life of components are the new challenges faced by researchers studied iron and steel materials in recent years.Based on the ‘Natural Science Foundation of China – Study on VHCF Behavior and Mechanism of Non-inclusion Induced Crack Initiation of Bainite/Martensite Duplex-phase High Strength Steel' as the main content and research objectives of the work,the paper studied the behavior of very high cycle fatigue and characteristics of series of bainite/martensite multiphase steels,and explored the mechanism of VHCF fracture of bainite/martensite multiphase steels,especially for the internal non-inclusion initiation;also studied Nb microallying and quenching plus partitioning(Q&P)heat treatment on strength and toughness and VHCF properties of bainite/martensite multiphase steels.With ultrasonic fatigue test technique(loading frequency 20 kHz),we studied here VHCF behavior of more than ten different types of bainite/martensite multiphase steels with four different composites.Taking alloy composition and microstructure design,electroslag remelting(ESR)clean process,Nb microalloying and advanced heat treatment(BQ&P,BQ&T),in order to reduce the size of maximum inclusion in steel,refine the microstructure,increase the amount of nano-film-like retained austenite,and ultimately optimize and dramatically improved the VHCF properties of bainite/martensite multiphase steels.The ratio of fatigue limit to tensile strength(?w9/Rm)of the steel with Nb microalloying and Q&P heat treatment,both are reached 0.55,which exceeds other high strength advanced steels.Meanwhile,the paper analyzed the mechanism of all factors that enhanced VHCF strength of high strength steels.The study found that compared to other high strength steels,the sensitivity of bainite/martensite multiphase for inclusions is lower,and crack initiating in martx is one of the main failure form of VHCF in high strength steels in very high cycle fatigue loading process.This paper foused on the mechanism of non-inclusion induced crack initiation phenomenon,and proposed microvoids generate in coarse soft phase(bainite or ferrite),and then accumulate in the grain boundary to form microcracks in thenon-inclusion of initiation,and finally established a non-inclusion initiation model.Refinement of microstructure and purification is equally important to improve VHCF properties of the material.Based on a lot of experiments,we proposed‘competitive control' mechanism between microstructure and inclusion for VHCF performance,basically comprehend the laws of inclusion and non-inclusion crack initiation appeared in the bainite/martensite multiphase steels,proposed the idea to get the best VHCF properties by metallurgy and heat treatment process.Experimental confirmative methods confirmed that the beneficial influence of the film-like retained austenite to VHCF properties.B/M multiphase high strength steel with the special structure including soft and hard phase,and the retained austenite,can refine the microstructure,hinder fatigue crack initiation,and improve the threshold value of crack growth. |
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