Optimizing Microstructure and Property by Ausforming in a Medium-Carbon Bainitic Steel

ISIJ International, Vol. 60 (2020),ISIJ International, No. 9 Vol. 60 (2020), No. 9, pp. 2007–2014 Optimizing Microstructure and Property by Ausforming in a Medium-carbon Bainitic Steel Guanghui CHEN,1) Haijiang HU,1,2)* Guang XU,1) Junyu TIAN,1,2) Xiangliang WAN1) and Xiang WANG2) 1) The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081 China. 2) Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, ON L8S4L7 Canada. (Received on January 28, 2020; accepted on March 23, 2020) The transformation behavior and microstructure in a medium-carbon bainitic steel were investigated by combination of metallography and dilatometry. The fine micro-structural units of carbide-free bainite in non-ausformed and ausformed materials were measured by a transmission electron microscope. Mechanical stabilization of austenite in deformed material and its effect on property were analyzed by nanoindentation and tensile tests. Ausforming with a strain of 0.2 at 573 K can not only accelerate bainite transformation, but also improve the comprehensive properties. The strength and ductility of nanostructured bainitic steel can be simultaneously enhanced by ausforming, which should be attributed to the refinement of bainite and the enhanced volume fraction of retained austenite. Compared to the non- deformed material, the mechanical stabilization of austenite can be optimized by ausforming, resulting in good transformation-induced plasticity effects. Also a very important advantage was that, the bainite transformation time could be minimized into practical scale by prior ausforming compared to traditional low-temperature austempering. KEY WORDS: ausforming; bainite transformation; retained austenite; property; mechanical stabilization. theory indicates that a high level of substitutional solute was 1. Introduction required at a low-carbon steel to produce nanostructured Automobile industries are continuously challenged to bainite where the difference between the bainite and the Ms reduce weight and improve fuel efficiency due to economic then vanishes.11) Also at low carbon concentrations, experi- and environmental requirements. For that purpose, advanced mentally observations show that the thin platelets of bainite high strength steels were paid great attention to, such as tend to coalesce into coarse grains which are detrimental quenching and partitioning (Q&P) steel,1) medium Mn to strength and toughness.8) The prospects therefore look transformation-induced plasticity (TRIP) steel,2) TRIP aided promising for the design of a medium-carbon nanostruc- bainitic ferrite (TBF) steel,3) etc. Carbide-free bainite usu- tured bainite. ally formed in Si-rich steels accompanying with blocky and As for the control of RA, a multi-step bainitic austemper- film-like retained austenite (RA), which can contribute to ing process was developed to reduce blocky microstructure excellent combination of strength, ductility and toughness.4) and refine the subunits.12) During the first step isothermal To obtain nano-scaled bainite, composition design with holding, a higher austempering temperature was normally high-carbon high-alloy and very low temperature austem- adopted to partially form the relatively coarse bainitic pering were initially utilized.5) However, it took several ferrite plates. Extra carbon content can be rejected from hours even a couple of days to complete transformation, newly formed bainite to surrounding austenite, leading to which is unpractical from the viewpoint of production. its chemical stabilization and a decrease in Ms. Thus, one Also the coarse blocky austenite with low stability usually can use a deep-cold isothermal bainite transformation at transforms partially to brittle martensite during cooling after a lower temperature to obtain finer bainite. The secondly isothermal bainite transformation, resulting in deteriora- formed plates can also divide the untransformed austenite tion of ductility and toughness.6) Many efforts have been into pieces which further facilitates the stability of RA. In done to solve above problems, including optimal composi- addition, an inverted multi-step bainitic austempering route tion and processing design.7–10) Decreasing carbon content was proposed,13) in which the second-step temperature was can shorten the incubation of bainite nucleation, but raise higher than that in the first step. Compared to the previous martensite starting temperature (Ms) simultaneously. The strategy, a higher temperature in the second step can accelerate the bainite transformation. Although both above two * Corresponding author: E-mail: [email protected] multi-step methods contribute to the refinement of bainite DOI: https://doi.org/10.2355/isijinternational.ISIJINT-2020-054 and RA, leading to good comprehensive properties, the 2007 © 2020 ISIJ ISIJ International, Vol. 60 (2020), No. 9 procedure is complicated and still takes more than two hours to complete transformation. Fortunately, a low temperature ausforming can be utilized to not only accelerating transformation rate but also promoting the amount of bainite.14,15) Gong et al.16) considered that planar dislocations remain- ing on the active slip planes at 573 K can assist bainite transformation, accompanied by strong variant selection. Anisotropic dilatation during bainite transformation also illustrates the general rule of variant selection, that is, in each austenite grain, the main variant of which domain the bainite should belong to the same Bain group.17) Therefore, one can employ ausforming process to obtain sufficient nanostructured bainite in an expected transformation time. However, the control of RA by ausforming, which plays an important role in determining the property and performance, is still not clear. The relationship between ausformed bainite Fig. 1. Heat treatment and ausforming routes. (Online version in and property should be further clarified. color.) In the present work, the ~0.43 wt.% C bainitic steel was designed to investigate the effect of ausforming on bainite deformation with a strain of 0.2 at the rate of 1 s −1 and the transformation and property. The morphology of bainite and other was free of strain. The load was promptly removed RA in the deformed material is also discussed in details, and after deformation to keep the sample free of external com- efforts are also made to build a link between property and pressive stress during austempering. The deformed sample ausformed bainite. was then isothermally kept at 573 K for 60 min, while the non-deformed one was held at 573 K for 90 min shown as the blue line in Fig. 1. After isothermal holding, both two 2. Experimental Method cases were quenched to ambient temperature. The dilata- The chemical composition of the steel is 0.43C-1.90Si- tion data during the whole process were recorded by a laser 2.83Mn-0.57Al-0.06Cu (wt.%) with Fe balance. Si was dilatometer. To investigate the tensile property of ausformed added to suppress cementite precipitation during bainite bainite, hot rolling and consequent salt bath treatment tests austempering. Mn was added to increase the hardenability to were carried out using 140 mm × 20 mm × 10 mm blocks. obtain bainite and Al is for accelerating transformation. The The schedule was same as that in the thermomechanical material was refined in vacuum induction furnace and casted simulation test. After ausforming, the hot-rolled sample was into a 50 kg ingot. The ingot is homogenized at 1 523 K for put into a salt-bath furnace and isothermally held at 573 K 5 hours, followed by 8 passes hot rolling into a 12.0 mm for certain time. slab. After hot rolling with finishing temperature of 1 173 K, Microstructures were characterized on Keyence optical the steel plate was air-cooled to ambient temperature. The microscope (OM) and a Nova 400 Nano field emission bainite starting temperature (Bs) and Ms of the tested steel scanning electron microscope (FE-SEM) with an accel- were calculated as 707 K and 516 K, respectively, according eration voltage of 20 kV. The metallographical specimens to the following equations:18) were etched with 4% nital. The fine microstructures were observed using a JEM-2100F transmission electron micro- BPSi839xxiC 270 F11exp.33 V ..... (1) B i H X scope (TEM). The samples for TEM examination were mechanically ground down to 30 μm thickness and then MKSi565xxiC 600 F10exp.96 V .... (2) B i H X electrolytic thinned to perforation using an electrolyte where i = Mn, Si, Cr, Ni, and Mo, and the concentration composed of 5% perchloric acid and 95% glacial acetic x in wt.%. acid at ambient temperature, and voltage of 40 V. The volume fractions of RA in the ausformed and non-ausformed Pxii86xxMn 23Si 67xxCr 67 Ni 75xMo ...(3) B i samples after isothermal transformation were determined using an X’Pert diffractometer with CoKα radiation under Kxii31xxMn 13Si 10xxCr 18 Ni 12xMo ... (4) B i the following conditions: acceleration voltage, 40 kV; cur- Samples for thermomechanical simulation tests were rent, 150 mA; and step, 0.06°. Tensile specimens were machined to a dumbbell shape with the central cylinder prepared according to ASTM standard and the strain rate of 8.0 mm diameter and 12.0 mm height. The surface of was ~4×10 −3 s −1. Nanoindentation was performed on the samples was conventionally polished to keep the measure- lightly etched specimens using 2 000 μN load for 30 s in a ment face level and minimize the effect of surface rough- triboindenter TI-900 equipped with scanning probe micro- ness. Ausforming and austempering tests were conducted scope. Each test involved a 5 × 5 array of indents. Vickers on a Gleeble-3500 thermal simulator according to the hardness tests were performed on a HV1000A micro- processing schedules shown in Fig. 1. The specimens were hardness tester (0.2 kg-1 960 mN). The average value of at heated to 1 273 K at 5 K s −1 and kept for 15 min, and then least ten individual measurements was calculated, including cooled to 573 K.

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