Achievement of High Ductility and Ultra-high Strength of V-Nb Microalloyed Spring Steel by Austempered Multiphase Micros
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INTRODUCTION
THE automobile industry is devoted to improving fuel efficiencies using high-strength and high-ductility lightweight structural materials in car bodies. Medium–high-carbon high-silicon steels are used extensively to manufacture springs, such as 51CrV4, 55SiCrA and 60Si2MnA.[1–4] It is necessary to achieve a mass reduction by increasing the strength and maintaining the ductility because the spring mass can be reduced by ~ 10 wt pct for each 100 MPa increase in strength.[2,5] Researchers suggest that a combination of high strength, excellent ductility and toughness can be obtained by combining the multiphase microstructure of martensite/bainite and an appropriate amount of retained austenite.[6,7] However, the microstructure of the final spring product is tempered martensite that is obtained by traditional quenching–tempering (Q–T) KUI CHEN, ZHOUHUA JIANG, FUBIN LIU, HUABING LI, and CONGPENG KANG are with the School of Metallurgy, Northeastern University, Shenyang 110819, China and also with the State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China. Contact e-mail: [email protected] WENCHAO ZHANG and AO WANG are with the School of Metallurgy, Northeastern University Manuscript submitted November 26, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
heat treatment.[8] The medium-temperature tempered martensite substructures are twins and dislocations. Twins and dislocation structures provide a high hardness, and they make the material brittle. Martensite often suffers from a relatively low toughness and hydrogen embrittlement in a wet environment, so the fatigue resistance decreases significantly.[9] Therefore, it is worth inducing soft phases with a relatively higher toughness in the matrix to buffer stress, hinder crack propagation and provide a better resistance to hydrogen embrittlement. Compared with martensite, large amounts of soft phases of retained austenite (RA) and bainite are obtained by quenching–partitioning (Q–P),[10,11] quenching–partitioning–tempering (Q–P–T)[12–14] and austempering (Q–AT)[15–17] heat treatments. The Q–P heat treatment involves an initial cooling step to a temperature between the martensite start temperature (Ms) and the martensite finish temperature (Mf), to promote carbon transport from martensite to austenite and to obtain more stable retained austenite. This transformation is followed by an isothermal holding process at a partitioning temperature.[10,18,19] After microalloying elements of vanadium (V), niobium (Nb), titanium (Ti), molybdenum (Mo) and boron (B) are added to steels, a tempering step is used after partitioning to promote carbide precipitation, which
increases precipitation strengthening.[12,20,21] Q–AT is a type of special Q–P or Q–P–T as the same temperature is used in tempering, partitioning and quenching. The material retains austenite and bainite generation in steels, and carbides are obtained after a long isothermal treatment. Extensive research has been conducted to improve the steel strength and toughness by Q
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