Microstructure and Mechanical Properties of an Ultrahigh-Strength 40SiMnNiCr Steel during the One-Step Quenching and Par
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INTRODUCTION
IN order to reduce the weight of steel parts, save raw materials, and keep or even improve safety standards, advanced high-strength steels (AHSSs), among them especially the high-strength low-alloy steels, dual-phase steels, complex-phase steels, martensitic steels, transformation-induced plasticity (TRIP) steels, and twinninginduced plasticity steels, were developed in the past several decades. In recent years, a new generation of nanostructured bainitic steel with high carbon (0.8 to ~1.0 wt pct) and high silicon has been developed, which is made up of the thin carbide-free bainite laths in the range of 20 to ~40 nm and the fine-scale dispersion of carbon-enriched retained austenite between the laths with an ultimate tensile strength (UTS) of 1770 to ~2200 MPa, a yield strength (YS) of 1200 to ~1500 MPa, and 5 to ~30 pct elongation (EL).[1–4] The development of high-strength steels, which is increasingly demanded in industry, generally involves a tradeoff between high strength and substantial ductility. The mechanical properties of high-strength steels are not only dependent on their composition, but also on the morphology and distribution of their microstructure. In general, the high strength of the steel is provided by the fine lath martensite or lower bainite; meanwhile, the H.Y. LI and X.W. LU, Doctoral Student, W.J. LI, Graduate Student, and X.J. JIN, Professor, are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, P.R. China. Contact e-mail: [email protected] Manuscript submitted August 11, 2009. Article published online February 12, 2010 1284—VOLUME 41A, MAY 2010
good ductility of the steel results from the TRIP phenomenon of strain-induced transformation from carbon-enriched retained austenite to martensite during deformation. To achieve the aim of obtaining a certain amount of carbon-enriched retained austenite in steel at room temperature, a traditional method is to increase the carbon content in steel to reduce the martensite start (Ms) temperature.[5] However, high carbon content in steel is believed to be detrimental to ductility and weldability.[6] In this context, a fundamentally new heat treatment called quenching and partitioning (Q&P) has been proposed[7,8] to improve the mechanical properties of high-strength steels during fabricating a controlled amount of carbon-enriched retained austenite at room temperature. This treatment includes: (1) adequate austenitizing heat treatment above the Ac3 temperature, (2) fast quenching to a temperature below the martensite start temperature (Ms) but above the martensite finish temperature (Mf), to form a controlled volume fraction of supersaturated martensite and untransformed austenite, (3) a subsequent partitioning treatment at the quenching (one-step treatment) or above the Ms temperature (two-step treatment), to accomplish the complete diffusion of carbon from martensite to residual austenite in the absence of carbide precipitation by alloying with Si or Al, and (4) quenching to room temperature.
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