Vol.:(0123456789) 1 3 Acta Metallurgica Sinica (English Letters) (2020) 33:1635–1644 https://doi.org/10.1007/s40195-020-01091-3 Nanostructured Carbide‑Free Bainite Formation in Low Carbon Steel Hamid Mousalou 1  · Sasan Yazdani 1  · Naghi Parvini Ahmadi 1  · Behzad Avishan 2 Received: 9 January 2020 / Revised: 9 March 2020 / Accepted: 3 April 2020 / Published online: 23 June 2020 © The Chinese Society for Metals (CSM) and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract It is an important challenge to reduce the carbon content in nanostructured bainitic steels for commercialization purposes while still being able to gain the desired microstructural characteristics in nanoscale and not to deteriorate the strength– ductility combinations. That is the point at which an appropriate heat treatment procedure design would be an important parameter. This article aims to investigate how to obtain nanostructured bainite in steel with 0.26 wt% carbon content by applying multi-step austempering procedures. One-, two- and three-step austempering processes have been implemented, and proper heat treatment temperatures and approaches were selected based on dilatometry tests. Results indicated that it has become possible to achieve bainitic ferrites and austenite flms with overall thicknesses of 164, 145 and 132 nm and 134, 105 and 90 nm at the end of one-, two- and three-step austempering heat treatments, respectively. Meanwhile, microstructural characteristics resulted in enhanced mechanical properties with ultimate tensile strength (UTS) of 1435, 1455 and 1428 MPa in combination with elongation levels of 15.4, 13.6 and 11.4% after implementing those heat treatments. Finally, it has been shown that applying the multi-step austempering heat treatments resulted in enhanced yield strength and impact toughness values due to the microstructural characteristics and proper heat treatment procedure design. Keywords Bainite · Multi-step austempering · Austenite · Mechanical properties · Impact toughness 1 Introduction Nanostructured low-temperature carbide-free bainitic steels, with outstanding strength and ductility properties, have been developed recently which attained great interest among researchers due to their simple production method [1–12]. An appropriate chemical composition for these advanced materials can be primarily designed using phase transformation theories of steels [13–16] with no need for trial-and-error procedures and simulations, and a simple low-temperature austempering heat treatment (temperature range of 200–400 °C) gives the desired microstructural char- acteristics in nanoscale without any need to apply compli- cated shape deformation methods. Depending on the heat treatment temperature, bainitic ferrites of less than 100 nm thicknesses can be achieved which is the main contributing factor in strength properties [16–18]. Considering the ductil- ity, it is mainly depending on the volume fraction and mor- phology of the high-carbon retained austenite phase being present in the microstructure. Austenite presents in two flmy and blocky morphologies. The former separates the bainitic ferrites in each bainitic sheaf with identical thicknesses to that of bainitic ferrites, and the latter surrounds the sheaves. A high volume fraction of austenite is required for ductil- ity promotion and replacement of austenite to martensite during straining the sample further adds to the resultant strength level due to the replacement of soft austenite with harder martensite [19, 20]. It has been approved that higher mechanical stability of austenite results in higher ductility since more gradual transformation to martensite occurs until later stages of failure [21]. Generally, there is no need to add any expensive alloy- ing elements to the chemical composition to obtain desired mechanical performance or microstructural features and the total amount of alloying elements never exceeds more than 5 wt%. Meanwhile, carbon is the main alloying ele- ment in the composition which is added mainly to reduce Available online at http://link.springer.com/journal/40195. * Sasan Yazdani yazdani@sut.ac.ir 1 Faculty of Materials Engineering, Sahand University of Technology, P. O. Box: 51335/1996, Tabriz, Iran 2 Department of Materials Engineering, Azarbaijan Shahid Madani University, Tabriz, Iran