| The paper is the main part of the'Study to improve the quality and life of iron and steel'for the'973 Program - Fundamental Research on New generation of iron and steel material in China'. With the technology of ultrasonic fatigue, the present work studied the very high cycle fatigue (VHCF) behaiviors, failure mechanism and crack propagation of Mn-Si-Cr series Bainite/Martensite (B/M) duplex-phase high strength low-alloys steels.The present work designed a new type Mn-Si-Cr B/M steel microalloyed with Nb. The processing map of the Nb-microalloying B/M steel was built as well as the optimized processing technology. With the heating rate of 100℃/s and water or oil quenching, the cycling phase transformation was carried out on the steel, and fine prior austenite grain of about 5μm can be reached. The tensile strength and Charpy impact energy are 1500-1700MPa and 90-100J respectively, which means outstanding coordination of strength and toughness. The VHCF strength under 108 cycles of the steel is 760MPa, which can achieve the goal of the 973 program.The present work designed 5 different chemical compositions and 10 types of B/M steels with different microstructures. The paper conducted systematic studies on the VHCF behaviors of these B/M steels, aiming at the influence of ultrosonic fatigue technology, microstructure homogeneity, microsturcture purity, microsturcture refinement, and the amount of retained austenite on the VHCF behaviors. The results show the microstructure homogeneity and microsturcture purity are of equal importance to the VHCF properties. According to the VHCF behaviors of these B/M steels, the paper defined the conception of microstructure sensitivity and inclusion sensitivity for the VHCF properties. And the competition mechanism between microstructure sensitivity and inclusion sensitivity is addressed and proved, and this competition mechanism can give a satisfactory explanation about the relation between the inclusion and non-inclusion induced crack initiation. Based on this competition relationship, in order to improve the VHCF properties of high strength steels, the necessary coordination of physical and chemical metallurgy is also discussed.More important, the inclusion in B/M steels is less sensitive to the VHCF loading than that in other high strength steels, and the non-inclusion induced crack initiation is one of the main failure forms. Different microstructures of the Nb-microalloying B/M steels were deliberately designed to disclose the VHCF failure mechanism of non-inclusion induced crack initiation, and'soft or coarsely soft structure induced fatigue crack'should be responsible for the primary VHCF failure. In addition, the conception of Optical Dark Area (ODA) in non-inclusion induced crack initiation is defined, and the difference from the ODA in inclusion induced crack initiation is disscussed.The microstructure feature of most B/M high strength steels is bainite/martensie duplex-phase structure with retained austenite films in the prior austenite grain boundary, the interlath of bainite and boundary of martensite. During the fatigue crack propagation, the B/M microstructure coordinated with retained austenite can lead to the fatigue crack tip blunting and reduce the stress concentration in the crack tip, which can effectively improve the△Kth and recuce the da/dN of the B/M high strength steels. However, the amount of retained austenite films almost has no influence on the VHCF behaviors of B/M steels.
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