Bainitic Transformation and Properties of Low Carbon Carbide-Free Bainitic Steels with Cr Addition

metals Article Bainitic Transformation and Properties of Low Carbon Carbide-Free Bainitic Steels with Cr Addition Mingxing Zhou, Guang Xu * , Junyu Tian, Haijiang Hu and Qing Yuan The State Key Laboratory of Refractories and Metallurgy, Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology, Wuhan 430081, China; [email protected] (M.Z.); [email protected] (J.T.); [email protected] (H.H.); [email protected] (Q.Y.) * Correspondence: [email protected]; Tel.: +86-027-6886-2813 Received: 28 June 2017; Accepted: 6 July 2017; Published: 10 July 2017 Abstract: Two low carbon carbide-free bainitic steels (with and without Cr addition) were designed, and each steel was treated by two kinds of heat treatment procedure (austempering and continuous cooling). The effects of Cr addition on bainitic transformation, microstructure, and properties of low carbon bainitic steels were investigated by dilatometry, metallography, X-ray diffraction, and a tensile test. The results show that Cr addition hinders the isothermal bainitic transformation, and this effect is more significant at higher transformation temperatures. In addition, Cr addition increases the tensile strength and elongation simultaneously for austempering treatment at a lower temperature. However, when the austempering temperature is higher, the strength increases and the elongation obviously decreases by Cr addition, resulting in the decrease in the product of tensile strength and elongation. Meanwhile, the austempering temperature should be lower in Cr-added steel than that in Cr-free steel in order to obtain better comprehensive properties. Moreover, for the continuous cooling treatment in the present study, the product of tensile strength and elongation significantly decreases with Cr addition due to more amounts of martensite. Keywords: chromium; bainitic transformation; microstructure; mechanical properties; retained austenite 1. Introduction Silicon (Si), manganese (Mn), molybdenum (Mo), and chromium (Cr) are important alloying elements in advanced high strength steels (AHSS), such as dual phases (DP) steel, quenching and partitioning (Q&P) steel, quenching-partitioning-tempering (QPT) steel, and carbide-free bainitic steel [1–5]. The amount of these elements in AHSS influences not only the transformation behavior, but also the microstructure and properties of the steel. Therefore, the optimization of the chemical composition in AHSS is always an interesting subject [6–8]. There is a continuous interest in the effects of Cr in AHSS. Han et al. [9] reported that DP steel with more Cr presents better elongation and a lower yield ratio. Li et al. [10] investigated a Cr-bearing low carbon steel treated by a Q&P process. A lath martensite, retained austenite, and lower bainite triplex microstructure was obtained, in which the tensile strength ranges from 1200 MPa to 1300 MPa and the total elongation ranges from 10% to 15%. Jirková et al. [11] found that the yield strength and elongation increase with Cr content in a Q&P steel. Ou et al. [6] investigated the microstructure and properties of QPT steels with Cr addition from a 1.35 wt. % to a 1.65 wt. %. They found that with increasing Cr content, the yield strength decreases, and the elongation first increases and subsequently decreases. Optimal microstructures and properties of steels were obtained at 1.45 wt. % and 1.55 wt. % Cr, respectively, in their study. In addition, Cr is often added in bainitic steel to increase its hardenability, so that a relatively smaller cooling rate can be used to avoid high temperature ferritic transformation and bainite can be obtained more easily [12–16]. However, this does not mean that Cr promotes bainitic Metals 2017, 7, 263; doi:10.3390/met7070263 www.mdpi.com/journal/metals Metals 2017, 7, 263 2 of 13 transformation itself, because the increase in hardenability is mainly caused by the inhabitation of Cr on ferritic transformation. So far, only a few studies have discussed the effect of Cr on bainitic transformation. Bracke and Xu [17] stated that Cr reduces the kinetics of lower bainitic transformation in a continuous cooling process. But the effect of Cr on isothermal bainitic transformation, which is an important process to produce advanced high strength bainitic steel, is rarely reported. Therefore, it is necessary to further investigate the effects of Cr on bainitic transformation, microstructure, and the properties of bainitic steels. In the authors’ previous study, the effect of Cr on isothermal bainitic transformation was investigated [18]. However, high temperature ferritic transformation occured before isothermal bainitic transformation due to a lower cooling rate and only one heat treatment procedure was used. In the present study, two kinds of heat treatment procedure are designed, i.e., isothermal bainitic transformation (including three transformation temperatures) and continuous transformation. The effects of Cr on bainitic transformation, microstructure, and the mechanical properties of low carbon bainitic steels were investigated. The present study is more comprehensive, and some new findings different from the authors’ previous study are obtained. The results are useful to the optimization of the chemical composition of low carbon bainitic steels. 2. Materials and Methods 2.1. Materials Two low carbon bainitic steels with different Cr contents were designed and their compositions are given in Table1. The steels were refined and cast in the form of 50 kg ingots using a laboratory-scale vacuum furnace followed by hot-rolling and then air-cooling to room temperature. Table 1. Chemical compositions of two steels. Steels C Si Mn Cr Mo N P S Cr-free 0.218 1.831 2.021 / 0.227 <0.003 <0.006 <0.003 Cr-added 0.221 1.792 1.983 1.002 0.229 <0.003 <0.006 <0.003 2.2. Thermal Simulation Experiments Thermal simulation experiments were conducted on a Gleeble 3500 simulator (DSI, New York, NY, USA). Cylindrical specimens with a diameter of 6 mm and a length of 70 mm were used. As shown in Figure1, two kinds of heat treatment were designed for the two steels, i.e., austempering and continuous cooling. During austempering treatment, the specimens were heated to 1000 ◦C to obtain full austenite structure, followed by fast cooling to the isothermal bainitic transformation ◦ ◦ ◦ ◦ temperatures (400 C, 430 C, and 450 C) at 30 C/s. The bainite start temperature (BS) and martensite ◦ ◦ start temperature (MS) for Cr-free steel are calculated to be 524 C and 376 C, respectively, using the MUCG 83 program developed by Bhadeshia at Cambridge University, and they are 501 ◦C and 353 ◦C, respectively, for Cr-added steel, so that the austempering temperatures are set as 400, 430, and 450 ◦C. ◦ ◦ Meanwhile, 1000 C is higher than the Ac3 point, and the cooling rate of 30 C/s is fast enough to avoid ferritic and pearlitic transformations (Section 3.1.1). In addition, during continuous cooling treatment, the specimens were first heated to 1000 ◦C and then held for 900 s, followed by cooling to room temperature at a rate of 0.5 ◦C/s. The dilatations along the radial direction were measured for all of the specimens during the entire experimental process. Metals 2017, 7, 263 3 of 13 Metals 2017, 7, 263 3 of 13 Metals 2017, 7, 263 3 of 13 Figure 1. Experimental procedures. Figure 1. Experimental procedures. Figure 1. Experimental procedures. 2.3. Microstructure Examinations and Tensile Tests 2.3. Microstructure Examinations and Tensile Tests 2.3. Microstructure Examinations and Tensile Tests AfterAfter the thermalthe thermal simulation simulation experiments, experiments, thethe microstructurescrostructures of of the the simulated simulated specimens specimens were were examinedAfterexamined by the a thermalby Nova a Nova 400 simulation 400 Nano Nano scanning experiments,scanning electron electron the microscope microstructures (SEM) (SEM) of (FEI, the(FEI, Hillsboro, simulated Hillsboro, OR, specimens OR,USA). USA). In were In orderexaminedorder to bydetermineto adetermine Nova 400the Nanothe volume volume scanning fraction fraction electron ofof retained microscope austeniteaustenite (SEM) (FEI,(RA), (RA), Hillsboro,X-ray X-ray diffraction OR,diffraction USA). (XRD) In(XRD) order experimentsto determineexperiments thewere volumewere carried carried fraction out out on ofon retaineda aBRUKER BRUKER austenite D8 ADVANCE (RA), X-ray diffractometer diffractometer diffraction (XRD) (Bruker, (Bruker, experiments Karlsruhe, Karlsruhe, were Germany),carriedGermany), out onusing a BRUKERusing unfiltered unfiltered D8 ADVANCECu Cu Ka Ka radiation radiation diffractometer andand operating (Bruker, at at Karlsruhe,40 40 kV kV and and 40 Germany), 40mA. mA. In addition,In using addition, unfiltered tensile tensile Cu testsKa radiation testswere were carried and carried operating out out on on ata UTM-4503 40a UTM-4503 kV and 40electronic electronic mA. In addition, universal tensile tensile tensile tests tester tester were (SUNS, (SUNS, carried Shenzhen, outShenzhen, on aChina) UTM-4503 China) with a cross-head speed of 1 mm/min at room temperature. Four tensile tests were repeated for each withelectronic a cross-head universal speed tensile of tester 1 mm/min (SUNS, at Shenzhen, room temperature. China) with Four a cross-head tensile tests speed were of 1 repeated mm/min for at roomeach heat treatment procedure, and the corresponding average values were calculated. Then, the tensile heat treatment procedure, and the corresponding average values

Load more