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 were calculated. Then, the tensile temperature.fractures Fourof the tensile specimens tests were were observed repeated by for SEM. each heat treatment procedure, and the corresponding fracturesaverage values of the were specimens calculated. were Then, observed the tensile by SEM. fractures of the specimens were observed by SEM. 3. Results and Discussion 3. Results and Discussion 3.1. Isothermal Transformation 3.1. Isothermal Transformation 3.1.1. Dilatation 3.1.1. Dilatation 3.1.1. DilatationFigure 2 shows the dilatation versus temperature during the entire austempering treatments Figure(400 °C) 22 for showsshows the Cr-free thethe dilatationdilatation and Cr-added versusversus steels. temperaturetemperatur The measurede duringduring Ac1 and thethe Ac entireentire3 temperatures austemperingaustempering for the treatmentsCr-freetreatments (400 ◦steelC) for are the about Cr-free 777 °C and and Cr-added 906 °C, respectively, steels. The measuredand those for Ac Cr-addedand Ac steeltemperatures are about 775 for °C the and Cr-free (400 °C) for the Cr-free and Cr-added steels. The measured Ac11 and Ac3 temperatures for the Cr-free 905 °C, respectively.◦ Full austenite◦ microstructures can be obtained at 1000 °C in the two steels,◦ steel are about 777 °CC and 906 °C,C, respectively, and those for Cr-added steel are about 775 °CC and ◦ because the austenization temperature (1000 °C) is higher than Ac3. Chromium◦ addition has little 905 °C,C, respectively.respectively. Full Full austenite austenite microstructures microstructures can can be obtained be obtained at 1000 at 1000C in the°C twoin the steels, two because steels, effect on the Ac1 and Ac3 temperatures.◦ In addition, the dilatation versus temperature curves are the austenization temperature (1000 C) is higher than Ac3. Chromium addition has little effect on becausestraight the linesaustenization during the temperatcooling processure (1000 from °C) 1000 is °C higher to 400 than°C at Ac30 3°C/s,. Chromium indicating addition that austenite has little the Ac and Ac temperatures. In addition, the dilatation versus temperature curves are straight effect on1 the Ac31 and Ac3 temperatures. In addition, the dilatation versus temperature curves are does not decompose before the isothermal◦ holding at◦ 400 °C. Moreover,◦ the apparent increase in the straightlinesdilatation during lines the during at cooling400 °C the is processcausedcooling by fromprocess the bainit 1000 fromicC transformation to1000 400 °CC to at 40030 during °CC/s, at isothermal indicating30 °C/s, holding.indicating that austenite that austenite does not ◦ doesdecompose not decompose before the before isothermal the isothermal holding at holding 400 C. at Moreover, 400 °C. Moreover, the apparent the increaseapparent in increase the dilatation in the ◦ dilatationat 400 C isat caused 400 °C byis caused the bainitic by the transformation bainitic transformation during isothermal during isothermal holding. holding.

Figure 2. The dilatation versus temperature during the entire austempering treatment (400 °C) for the Cr-free and Cr-added steels: (a) Cr-free steel and (b) Cr-added steel.

Figure 2. TheThe dilatation dilatation versus temperature during the entire austempering treatment (400 ◦°C)C) for the Cr-free and Cr-added steels: (a) Cr-free steelsteel andand ((b)) Cr-addedCr-added steel.steel.

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The dilatation (representing the transformation amount) versus time during the isothermal The dilatation (representing the transformation amount) versus time during the isothermal holding at 400 °C, 430 °C, and 450 °C for the two steels is presented in Figure 3a. Chromium hinders holding at 400 ◦C, 430 ◦C, and 450 ◦C for the two steels is presented in Figure3a. Chromium hinders the bainitic transformation kinetics, and decreases the final amount of bainitic transformation at all the bainitic transformation kinetics, and decreases the ﬁnal amount of bainitic transformation at all three temperatures. It is interesting to find that the hindrance of Cr on bainitic transformation is more three temperatures. It is interesting to ﬁnd that the hindrance of Cr on bainitic transformation is more obvious at higher temperatures (430 °C and 450 °C) compared with that at a lower temperature obvious at higher temperatures (430 ◦C and 450 ◦C) compared with that at a lower temperature (400 ◦C). (400 °C). The time-temperature-transformation (TTT) curves of the two steels in Figure 3b, which are The time-temperature-transformation (TTT) curves of the two steels in Figure3b, which are calculated calculated by software JMatPro, also demonstrate that the kinetics of bainitic transformation are by software JMatPro, also demonstrate that the kinetics of bainitic transformation are hindered by Cr hindered by Cr addition. The time required to initiate bainitic transformation (2% relative addition. The time required to initiate bainitic transformation (2% relative transformation fraction) transformation fraction) is listed in Table 2. The experimentally measured values are compared with is listed in Table2. The experimentally measured values are compared with the calculated values the calculated values (JMatPro). The comparison shows that the calculated values are larger than the (JMatPro). The comparison shows that the calculated values are larger than the measured values, measured values, so that the kinetics of bainitic transformation calculated by JMatPro are slower so that the kinetics of bainitic transformation calculated by JMatPro are slower compared to the compared to the experimentally measured one. experimentally measured one.

FigureFigure 3. ((a)) The dilatation versus time during the isot isothermalhermal holding at 400 °C,◦C, 430 °C,◦C, and 450 °C◦C forfor the the two two steels; steels; (b (b) Calculated) Calculated time-temperatu time-temperature-transformationre-transformation (TTT) (TTT) curves curves of the of theCr-free Cr-free and andCr- addedCr-added steels steels showing showing the the start start of oftransformations transformations from from austenite austenite A A to to ferrite ferrite FF and and from from austenite austenite to to bainitebainite B. B. It shows that the kinetics of bainit bainiticic transformation are hindered by Cr addition.

TableTable 2. TheThe time time needed needed to to initiate initiate bainitic transfor transformationmation (2% relative transformation fraction).

TransformationTransformation ExperimentallyExperimentally SteelsSteels CalculatedCalculated Data Data (s) (s) TemperatureTemperature (◦ C)(°C) MeasuredMeasured Data Data (s) (s) Cr-free 400 6.0 39.4 Cr-free 400 6.0 39.4 Cr-added 400 19.4 79.8 Cr-added 400 19.4 79.8 Cr-freeCr-free 430430 17.9 17.9 31.0 31.0 Cr-addedCr-added 430430 45.0 45.0 74.3 74.3 Cr-freeCr-free 450450 15.3 15.3 35.8 35.8 Cr-addedCr-added 450450 30.1 30.1 129.8 129.8

AccordingAccording toto thethe displacive displacive approach approach of theof the bainitic bainitic transformation transformation proposed proposed by Bhadeshia by Bhadeshia [19,20], [19,20],bainitic bainitic transformation transformation is expected is expected to occur to when: occur when:

ΔGm < GN (1) ∆G < G (1) γ→αm N ΔG < −GSB (2) γ→α γ→α ∆G < −G (2) where ΔG is the driving force from γ to α, GSB is the storedSB energy of bainite (about 400 J/mol), ΔGm is the maximumγ→α driving force for the nucleation of bainitic ferrite with paraequilibrium carbon where ∆G is the driving force from γ to α, GSB is the stored energy of bainite (about 400 J/mol), N partitioning,∆Gm is the maximum and G drivingis the universal force for thenucleation nucleation function. of bainitic Equation ferrite with(1) ensures paraequilibrium that there carbon is a detectable nucleation rate, and Equation (2) ensures that a diffusionless growth of bainite can occur partitioning, and GN is the universal nucleation function. Equation (1) ensures that there is a detectable γ→α [19,20].nucleation The rate,values and of EquationΔGm and Δ (2)G ensures versus that temperature a diffusionless in Cr-free growth and of Cr-added bainite can steels occur are [ 19calculated,20]. The γ→α γ→α m ΔG usingvalues the of ∆MUCGGm and 83∆ program,G versus and temperature the results are in Cr-free shown and in Figure Cr-added 4. Δ steelsG and are calculateddecrease using with the γ→α CrMUCG addition, 83 program, so that it and is more the results difficult are to shown meet inthe Figure requirements4. ∆Gm and of bainitic∆G transformationdecrease with Cr(Equations addition, (1) and (2)) in Cr-added steel. As a result, bainitic transformation is hindered by Cr addition. In

Metals 2017, 7, 263 5 of 13 so that it is more difficult to meet the requirements of bainitic transformation (Equations (1) and (2)) in Metals 2017, 7, 263 5 of 13 Cr-added steel. As a result, bainitic transformation is hindered by Cr addition. In addition, it is shown Metals 2017, 7, 263 5 of 13 → ◦ in Figureaddition,4 that the it is∆ shownGγ α indecreases Figure 4 fromthat the−1012 ΔGγ→ J/molα decreases to − from900 J/mol−1012 byJ/mol Cr to addition −900 J/mol at 400by CrC, and it ◦ decreasesaddition,addition from it − atis 864 400shown J/mol°C, and in to Figureit −decreases753 4 J/molthat from the at − 864 430ΔGγ →J/molαC. decreases The to − decreased753 J/molfrom at−1012 driving430 °C.J/mol The forces to decreased −900 account J/mol driving for by11% Cr and 13% ataddition 400forces◦C ataccount and 400 430 °C, for ◦and C,11% respectively,it anddecreases 13% at from indicating400 °C−864 and J/mol that430 °C,to the −respectively,753 decreased J/mol at drivingindicating430 °C. The force that decreased accountsthe decreased driving for a larger driving force accounts for a larger proportion at a higher transformation temperature. A similar result proportionforces ataccount a higher for transformation11% and 13% attemperature. 400 °C and 430 A similar°C, respectively, result can indicating be obtained that for the∆ decreasedGm. Therefore, drivingcan be force obtained accounts for Δ forGm a. Therefore,larger proportion the hind atrance a higher of Cr transformation on bainitic transformation temperature. is more A similar obvious result the hindranceat higher of transformation Cr on bainitic temperatures. transformation is more obvious at higher transformation temperatures. can be obtained for ΔGm. Therefore, the hindrance of Cr on bainitic transformation is more obvious at higher transformation temperatures.

→ Figure 4. The values of ΔGm and ΔGγ α versus temperature in Cr-free and Cr-added steels. Figure 4. The values of ∆G and ∆Gγ→α versus temperature in Cr-free and Cr-added steels. m 3.1.2. Microstructures → Figure 4. The values of ΔGm and ΔGγ α versus temperature in Cr-free and Cr-added steels. 3.1.2. Microstructures The typical microstructures of the Cr-free and Cr-added steels treated by austempering are The3.1.2.shown typical Microstructures in microstructuresFigures 5 and 6. ofThe the microstructures Cr-free and Cr-added contain lath-like steels treated bainite byferrite austempering (BF), RA, and are shown martensite (M). A selected region is magnified and presented in Figure 5c,d. It shows that ultra-fine in FiguresThe5 and typical6. The microstructures microstructures of the contain Cr-free lath-like and Cr-added bainite steels ferrite treated (BF), RA,by austempering and martensite are (M). BF is separated by film RA. No carbides are observed due to high silicon content [20–22]. When the shown in Figures 5 and 6. The microstructures contain lath-like bainite ferrite (BF), RA, and A selectedtransformation region ismagnified temperature and is 400 presented °C (Figure in 5), Figure the amount5c,d. and It shows morphology that ultra-fine of bainite in BF the is two separated by filmmartensitesteels RA. Noshow (M). carbides no A significant selected are observedregion difference. is magnified due However, to high and the siliconpresented amount content of in bainite Figure [20 obviously– 5c,d.22]. It When shows decreases the that transformation andultra-fine the ◦ temperatureBF amountis separated is of 400 martensite byC film (Figure RA.obviously No5), carbides the increases amount are with obse and Crrved content morphology due toat high430 °C silicon of and bainite 450content °C in (Figure [20–22]. the two 6). When This steels is the show no significanttransformationconsistent difference. with temperature the dilatation However, is results400 °C the (Figure(Figure amount 3). 5), ofthe bainiteamount obviouslyand morphology decreases of bainite and thein the amount two of steels show no significant difference. However, the amount of bainite obviously decreases and the martensite obviously increases with Cr content at 430 ◦C and 450 ◦C (Figure6). This is consistent with amount of martensite obviously increases with Cr content at 430 °C and 450 °C (Figure 6). This is the dilatation results (Figure3). consistent with the dilatation results (Figure 3).

Figure 5. The typical microstructures of the Cr-free and Cr-added steels treated by austempering at 400 °C: (a,c) Cr-free steel; (b,d) Cr-added steel. M, martensite; BF, bainite ferrite; RA, retained austenite.

Figure 5. The typical microstructures of the Cr-free and Cr-added steels treated by austempering at Figure400 5. °C:The (a typical,c) Cr-free microstructures steel; (b,d) Cr-added of the Cr-freesteel. M, and martensite; Cr-added BF, steels bainite treated ferrite; by RA, austempering retained at ◦ 400 C:austenite. (a,c) Cr-free steel; (b,d) Cr-added steel. M, martensite; BF, bainite ferrite; RA, retained austenite.

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Figure 6. The typical microstructures of the Cr-free and Cr-added steels treated by austempering at Figure 6. The typical microstructures of the Cr-free and Cr-added steels treated by austempering 430 °C and 450 °C: (a) Cr-free steel, 430 °C; (b) Cr-added steel, 430 °C; (c) Cr-free steel, 450 °C; (d) Cr- at 430 ◦C and 450 ◦C: (a) Cr-free steel, 430 ◦C; (b) Cr-added steel, 430 ◦C; (c) Cr-free steel, 450 ◦C; added steel, 450 °C. (d) Cr-added steel, 450 ◦C.

In addition, the volume fractions of RA (Vγ) in two the steels are calculated according to the integratedIn addition, intensities the volumeof (200)α fractions, (211)α, of(200) RAγ (V, andγ) in(220) twoγ thediffraction steels are peaks calculated based on according the following to the integratedequation [23,24]. intensities of (200)α, (211)α, (200)γ, and (220)γ diffraction peaks based on the following equation [23,24]. 1 == VVi i (3)(3) 1 +GG(Iα// Iγ) α γ where Vi is the volume fraction of austenite for each peak, Iα and Iγ are the corresponding integrated where Vi is the volume fraction of austenite for each peak, Iα and Iγ are the corresponding integrated intensities of ferrite and austenite, and the G value is chosen as follows, 2.5 for Iα(200)/Iγ(200), 1.38 intensities of ferrite and austenite, and the G value is chosen as follows, 2.5 for Iα(200)/Iγ(200), 1.38 for for Iα(200)/Iγ(220), 1.19 for Iα(211)/Iγ(200), and 0.06 for Iα(211)/Iγ(220) [23,24]. The results show thatIα(200)/ theIγ volume(220), 1.19 fraction for Iα(211)/ of RAIγ increases(200), and from 0.06 6.3for vol.Iα(211)/ % toIγ(220) 10.1 vol.[23,24]. % with The theresults addition show ofthat Cr the for austemperingvolume fraction at 400of ◦RAC, andincreases it slightly from increases 6.3 vol. from % 4.0to vol.10.1 % vol. to 4.8 % vol.with % forthe 430addition◦C austempering. of Cr for Thisaustempering is because at the 400 stability °C, and ofit slightly austenite increases is enhanced from by4.0 Crvol. addition. % to 4.8 vol. In addition, % for 430 the °C carbonaustempering. content This is because the stability of austenite is enhanced by Cr addition. In addition, the carbon content in RA (Cγ) is calculated according to the (200)γ and (220)γ diffraction peaks using the method in referencein RA (Cγ [)24 is] andcalculated the results according are given to inthe Table (200)3.γ The and carbon (220)γ content diffraction in RA peaks decreases using with the Cr method addition, in becausereference the [24] amount and the of bainiticresults are transformation, given in Table which 3. The is accompanied carbon content by thein RA partitioning decreases of with carbon Cr fromaddition, bainite because ferrite the to amount RA, is smaller of bainitic in Cr-added transformation, steel. which is accompanied by the partitioning of carbon from bainite ferrite to RA, is smaller in Cr-added steel. Table 3. The tensile results and RA characteristics of two tested steels treated by austempering. YS, yield strength;Table 3. TS,The tensile tensile strength; results and TE, RA total characteristics elongation; PSE, of two product tested of steels strength treated and by elongation. austempering. YS, yield strength; TS, tensile strength; TE, total elongation; PSE, product of strength and elongation. Steel YS (MPa) TS (MPa) TE (%) PSE (GPa%) Vγ (vol. %) Cγ (wt. %) Steel YS (MPa) TS (MPa) TE (%) PSE (GPa%) Vγ (vol. %) Cγ (wt. %) ◦ Cr-freeCr-free (400 C)(400 °C) 772 ± 77216 ± 16 977 977± 36± 36 15.8 15.8 ±± 0.0.33 15.4 15.4 ± 0.79± 0.79 6.3 6.3± 1.1± 1.1 1.18 1.18± 0.11± 0.11 Cr-added(400 ◦C) 649 ± 16 1083 ± 16 18.3 ± 0.4 19.8 ± 0.52 10.1 ± 0.7 1.14 ± 0.08 Cr-added(400 °C) 649 ± 16 1083 ± 16 18.3 ± 0.4 19.8 ± 0.52 10.1 ± 0.7 1.14 ± 0.08 Cr-free (430 ◦C) 620 ± 15 986 ± 13 18.5 ± 0.3 18.2 ± 0.55 4.0 ± 0.2 1.13 ± 0.06 Cr-addedCr-free (430 ◦(430C) °C) 786 ± 62020 ± 15 1185 986± ±26 13 18.5 11.7 ±± 0.0.93 18.2 13.9 ± 0.55± 0.98 4.0 4.8± 0.2± 0.3 1.13 0.99± 0.06± 0.03 Cr-freeCr-added (450 ◦C) (430 °C) 795 ± 78617 ± 20 1051 1185± 19± 26 11.7 13.6 ±± 0.90.6 13.9 14.3 ± 0.98± 0.42 4.8 3.6± 0.3± 0.4 0.99 0.96± 0.03± 0.05 Cr-addedCr-free (450 ◦(450C) °C) 1050 ± 79521 ± 17 1263 1051± 23± 19 13.6 5.7 ± 0.0.36 14.3 7.2 ± ±0.420.58 3.6 2.8± 0.4± 0.2 0.96 0.71± 0.05± 0.03 Cr-added (450 °C) 1050 ± 21 1263 ± 23 5.7 ± 0.3 7.2 ± 0.58 2.8 ± 0.2 0.71 ± 0.03

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3.1.3.3.1.3. Mechanical Mechanical Properties Properties TheThe tensile tensile results results of of Cr-free Cr-free and and Cr-added Cr-added steels steels treated treated by by austempering austempering are are given given in in Table Table 3.3. The typical comparison of the engineering strain-s strain-stresstress curves between Cr-free Cr-free and Cr-added steels treatedtreated by by austempering austempering is is shown shown in in Figure Figure 77.. ItIt showsshows thatthat whenwhen thethe transformationtransformation temperaturetemperature isis ◦ lowerlower (400 (400 °C),C), Cr-added Cr-added steel steel shows shows lower lower yield yield stre strengthngth (YS) (YS) compared compared with with Cr-free Cr-free steel steel due due to to moremore amounts ofof softersofter phasephase (austenite) (austenite) in in Cr-added Cr-added steel. steel. However, However, the the tensile tensile strength strength (TS) (TS) and and the thetotal total elongation elongation (TE) (TE) increase increase with with Cr addition,Cr addition, resulting resulting in the in increasethe increase in the in productthe product of strength of strength and andelongation elongation (PSE, (PSE, TS ×TSTE). × TE). One One reason reason for for the the increase increase in in tensile tensile strength strength is is thatthat CrCr actsacts as the role ofof solution strengthening. In In addition, addition, it it is is known known that that RA RA transforms to to ma martensitertensite during during a a tensile test,test, and and thus thus improves improves the the strength strength and and plasticity plasticity simultaneously, simultaneously, which which is is generally termed termed as as the the transformationtransformation induced induced plasticity plasticity (TRIP) (TRIP) effect effect [25,26 [25,26]. There There is is more more RA RA in in Cr-added Cr-added steel steel treated treated by by ◦ austemperingaustempering at at 400 400 °C,C, so so that that the the strength strength and and plasticity of of the the steel are higher. Moreover, Moreover, when ◦ ◦ thethe transformation transformation temperatures temperatures are are higher higher (430 (430 °CC and and 450 450 °C),C), the the yield yield strength strength and and the tensile strengthstrength increase, increase, whereas the total elongation elongation sign signiﬁcantlyificantly decreases with Cr addition. The The increase increase inin strength strength is is mainly mainly caused caused by by the the obvious obvious increase increase in in martensite martensite amount amount (Figure (Figure 66)) andand thethe solutionsolution strengtheningstrengthening of of Cr. Cr. However, However, more amounts of mart martensiteensite decreases the the plasticity obviously. obviously. As As a a result,result, the the PSE PSE decreases. decreases. Therefore, Therefore, at at a a lower lower tran transformationsformation temperature, temperature, Cr Cr addition addition increases increases the the comprehensivecomprehensive property property of of the the steel, steel, whereas whereas it itde decreasescreases the the comprehensive comprehensive property property of ofthe the steel steel at higherat higher transformation transformation temperatures. temperatures. Moreover, Moreover, in inthe the present present study, study, Cr-free Cr-free steel steel shows shows a a better ◦ comprehensivecomprehensive property atat 430430 °C,C, whereaswhereas Cr-addedCr-added steel steel shows shows a a better better comprehensive comprehensive property property at ◦ at400 400C. °C. Therefore, Therefore, the the austempering austempering temperature temperature should should be lowerbe lower for for Cr-added Cr-added steel steel than than for for Cr-free Cr- freesteel steel in order in order to obtain to obtain a better a better comprehensive comprehensive property. property.

FigureFigure 7. TypicalTypical comparisoncomparison of of the the engineering engineering strain-stress strain-stress curves curves between between the Cr-freethe Cr-free andCr-added and Cr- addedsteels treatedsteels treated by austempering. by austempering.

Figure 8 shows the typical morphologies of tensile fractures in the two steels treated by Figure8 shows the typical morphologies of tensile fractures in the two steels treated by austempering at 400 °C. Many dimples are observed in the two steels, indicating that the fracture is austempering at 400 ◦C. Many dimples are observed in the two steels, indicating that the fracture is ductile. The fracture morphologies in the two steels show no significant difference. ductile. The fracture morphologies in the two steels show no signiﬁcant difference.

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Figure 8. Morphologies of tensile fractures in two steels treated by austempering at 400 °C: (a) Cr-free Figure 8. Morphologies of tensile fractures in two steels treated by austempering at 400 ◦C: (a) Cr-free steel; (b) Cr-added steel. steel; (b) Cr-added steel.

Carbide-free bainitic steel with ultra-fine bainitic ferrite and film-like RA is developed by CaballeroCarbide-free et al. [12–15]. bainitic Cr steel is withadded ultra-fine in this bainiticadvanced ferrite bainitic and film-like steel. They RA isfound developed that when by Caballero Mn is etreplaced al. [12– 15by]. Cr, Cr the is added strength in this of the advanced steel with bainitic 0.2 wt. steel. % TheyC decreases found that significantly when Mn during is replaced a continuous by Cr, the strengthcooling process of the steel [15]. with However, 0.2 wt. isothermal % C decreases treatment significantly is not studied during in a continuoustheir studies. cooling Sugimoto process and [ 15his]. However,co-workers isothermal [27,28] investigated treatment is notthe studiedeffects inof their alloying studies. elements Sugimoto (Cr, and Mo, his co-workersNi, etc.) on [27 ,the28] investigatedmicrostructure, the the effects retained of alloying austenite, elements and the (Cr, mech Mo,anical Ni, etc.) properties on the microstructure,of bainitic steels the treated retained by austenite,austempering. and theTheir mechanical results showed properties that compared of bainitic to steels the treatedCr-free bysteel, austempering. Cr addition increases Their results the showedcomprehensive that compared property to (PSE) the Cr-free of the steel, steels Cr at addition all studied increases austempering the comprehensive temperatures. property The authors’ (PSE) of theprevious steels study at all studied[18] also austempering reported that Cr temperatures. addition improves The authors’ the comprehensive previous study property [18] also of reportedthe steel ◦ thataustempered Cr addition at 350 improves °C. However, the comprehensive the present study property finds of that the steelat a lower austempered transformation at 350 temperatureC. However, ◦ the(400 present °C), Cr study addition finds thatincreases at a lower the transformationcomprehensive temperatureproperty of (400 the C),steel, Cr whereas addition increasesCr addition the comprehensivedecreases the comprehensive property of the property steel, whereas of the Crsteel addition at higher decreases transformation the comprehensive temperatures property (430 °C of and the ◦ ◦ steel450 °C). at higher The difference transformation between temperatures the previous (430 resultC and[18,27,28] 450 C). and The the difference present result between may the beprevious because resultthe austempering [18,27,28] and temperatures the present result used mayin previous be because studies the austempering are relatively temperatures lower. In addition, used in the previous effect studiesof Cr on are bainitic relatively transformation lower. In addition, was not the studied effect ofin Crreferences on bainitic [27,28]. transformation The authors’ was previous not studied study in references[18] reported [27 ,that28]. TheCr addition authors’ increases previous the study amount [18] reported of isothermal that Cr bainitic addition transformation increases the amountat 350 °C, of ◦ isothermalbecause high bainitic temperature transformation ferritic at transformation 350 C, because highoccurs temperature in Cr-free ferritic steel transformationand consumes occurssome inuntransformed Cr-free steel andaustenite consumes before some austempering, untransformed wherea austenites no ferritic before transformation austempering, occurs whereas in noCr-added ferritic transformationsteel. In the present occurs study, in Cr-added there is no steel. ferritic In the transformation present study, before there isothermal is no ferritic bainitic transformation transformation, before isothermaland it is found bainitic that transformation, Cr addition decreases and it is the found bainite that amount Cr addition due decreases to a decreased the bainite chemical amount driving due toforce a decreased for bainitic chemical transformation, driving force and for bainiticthis effect transformation, is more significant and this effectat higher is more transformation significant at highertemperatures. transformation temperatures. 3.2. Continuous Transformation 3.2. Continuous Transformation 3.2.1. Dilatation 3.2.1. Dilatation The dilatation versus temperature during cooling process for the two steels treated by continuous coolingThe is presenteddilatation inversus Figure temperature9a. Ferrite transformation during cooling occurs process at a relatively for the hightwo temperature steels treated due by to continuous cooling is presented◦ in Figure 9a. Ferrite transformation occurs at a relatively high a slow cooling rate (0.5 C/s). The calculated MS temperatures for Cr-free and Cr-added steels are 376temperature and 353 ◦dueC, respectively,to a slow cooling so that rate the (0.5 region °C/s). between The calculated a and bM inS temperatures the Cr-free steel for Cr-free and the and region Cr- betweenadded steels c and are d 376 in the and Cr-added 353 °C, respectively, steel contain so not th onlyat the bainitic region transformation,between a and b but in the also Cr-free martensitic steel transformation.and the region between The transformation c and d in the start Cr-added temperature steel contain decreases not fromonly bainitic 537 ◦C transformation, (point a) to 450 but◦C also martensitic transformation. The transformation start temperature decreases from 537 °C (point (point c) with Cr addition. In addition, D1 and D2 (Figure9a) are dilatations caused by bainitic and a) to 450 °C (point c) with Cr addition. In addition, D1 and D2 (Figure 9a) are dilatations caused by martensitic transformations. D2 is larger than D1, indicating that the volume fractions of bainitic and martensiticbainitic and transformations martensitic transformations. increase with CrD2 addition.is larger than This mayD1, indicating be because that the the bainitic volume and fractions martensitic of transformationsbainitic and martensitic in Cr-added transformations steel occur increase in lower with temperature Cr addition. regions, This somay that be thebecause undercooling the bainitic of theand transformationmartensitic transformations is larger. In addition, in Cr-added the continuous steel occur cooling in lower transformation temperature (CCT)regions, curves so that for the undercooling of the transformation is larger. In addition, the continuous cooling transformation (CCT) curves for the two steels are calculated using the software JMatPro (Figure 9b). It is obvious that the

Metals 2017, 7, 263 9 of 13 two steels are calculated using the software JMatPro (Figure9b). It is obvious that the temperatures of ferritic,Metals 2017 pearlitic,, 7, 263 and bainitic transformations all decrease with Cr addition, which is9 consistent of 13 withMetals the experimental2017, 7, 263 results. As shown in Figure9b, the calculated bainite transformation9 of 13 start temperaturestemperatures for of Cr-free ferritic, and pearlitic, Cr-added and bainitic steels are transformations 482 ◦C and 451 all decrease◦C, respectively, with Cr addition, for a cooling which rate of temperatures of ferritic, pearlitic, and bainitic transformations all decrease with Cr addition, which 0.5 ◦isC/s. consistent The calculated with the value experimental (451 ◦C) isresults. almost As the shown same asin theFigure measured 9b, the value calculated (450 ◦ C,bainite Figure 9a) transformationis consistent withstart temperaturesthe experimental for Cr-free results. and AsCr-added shown◦ steels in Figureare 482 °C9b, and the 451 calculated °C, respectively, bainite for Cr-addedtransformation steel, start whereas temperatures the calculated for Cr-free value and (482 Cr-addedC) is steels obviously are 482 lower °C and than 451 °C, the respectively, measuredvalue (537for◦C, a Figurecooling9 a)rate for of Cr-free0.5 °C/s. steel. The calculated value (451 °C) is almost the same as the measured value (450for a °C, cooling Figure rate 9a) of for 0.5 Cr-added °C/s. The steel, calculated whereas value the (451calculated °C) is almostvalue (482 the same°C) is asobviously the measured lower valuethan In addition, the relative change of transformation fraction versus temperature during the cooling the(450 measured °C, Figure value 9a) for (537 Cr-added °C, Figure steel, 9a) whereasfor Cr-free the steel. calculated value (482 °C) is obviously lower than processthe measured fromIn addition, point value the ato relative(537 b in°C, Cr-freechange Figure of9a) steel, transformation for Cr-free and from steel. fraction point cversus to d temperature in Cr-added during steel, the are cooling calculated usingprocess theIn lever addition,from rule point the and a torelative theb in resultsCr-free change aresteel, of transformation shown and from in Figurepoint fraction c to10 db. inversus An Cr-added example temperature steel, is givenare during calculated in the Figure cooling using9a. At a certaintheprocess temperature,lever from rule andpoint the a to results relative b in Cr-free are volume shown steel, in fraction and Figure from of10b. point transformation An c exampleto d in Cr-added is given is determined steel,in Figure are calculated 9a. as ON/MNAt a certain using [29 ,30]. In general,temperature,the lever the rule transformation theand relative the results volume are kinetics shown fraction arein Figureof hindered tran 10b.sformation An by example Cr is addition determined is given (Figure in as Figure ON/MN9b). 9a. This At[29,30]. a certain is because In the lowergeneral,temperature, transformation the transformation the relative temperature volume kinetics fractionare regionhindered of makestran bysformation Cr addition the diffusion is (Figure determined of 9b). carbon This as isON/MN slow because down [29,30].the lower during In the general, the transformation kinetics are hindered by Cr addition (Figure 9b). This is because the lower nucleationtransformation process temperature of bainitic transformationregion makes the [diffus20]. Inion addition, of carbon the slow slope down of during the transformed the nucleation fraction transformation temperature region makes the diffusion of carbon slow down during the nucleation curvesprocess (df /d ofT bainitic) is shown transformation in Figure [20].10c. In The addition, slope the represents slope of the the transformed transformation fraction rate. curves There (df/d isT a) peak isprocess shown of in bainitic Figure transformation 10c. The slope [20].represents In addition, the tr theansformation slope of the rate. transformed There is fractiona peak value curves in (d eachf/dT ) value in each slope curve, which corresponds to the maximum transformation rate. First, the maximum slopeis shown curve, in Figure which 10c. corresponds The slope representsto the maximu the transformationm transformation rate. There rate. isFirst, a peak the value maximum in each transformation rate appears later in Cr-added steel, indicating that the transformation is delayed by Cr transformationslope curve, whichrate appears corresponds later in Cr-addedto the maximu steel, indicatingm transformation that the transformation rate. First, the is delayedmaximum by addition.Crtransformation addition. Second, Second, therate maximum appearsthe maximum later transformation in transformation Cr-added steel, rate rate indicating in Cr-added Cr-added that steelthe steel transformation is larger is larger than than that is delayed in that Cr-free in by Cr-free steel.steel.Cr The addition. The faster faster Second, transformation transformation the maximum rate rate may transformationmay be caused rateby by martensite in martensite Cr-added transformation. steel transformation. is larger than that in Cr-free steel. The faster transformation rate may be caused by martensite transformation.

Figure 9. (a) The dilatation versus temperature during cooling process for the two steels treated by FigurecontinuousFigure 9. (a 9.) The(a cooling;) The dilatation dilatation (b) Calculated versus versus temperaturecontinuous temperature cooling during during transfor co coolingolingmation process process (CCT) for forcurvesthe thetwo for twosteels the steels twotreated steels. treated by by continuouscontinuous cooling; cooling; (b) ( Calculatedb) Calculated continuous continuous cooling transfor transformationmation (CCT) (CCT) curves curves for the for two the steels. two steels.

Figure 10. (a) The example of lever rule; (b) The relative change of transformation fraction versus FiguretemperatureFigure 10. (10.a) The( aduring) The example example the cooling of of lever leverprocess rule; rule; (cooling ((bb)) TheThe rate: relative relative 0.5 °C/s) change change from ofpoint oftransformation transformation a to b (Figure fraction 9a) fractionin Cr-free versus versus temperaturesteel,temperature and duringfrom during point the the coolingc to cooling d (Figure process proc 9a)ess (cooling in(cooling Cr-added rate:rate: steel; 0.5 0.5 °C/s)◦ (C/s)c) Thefrom from slope point point of a theto a b to transformed(Figure b (Figure 9a) in9 fractiona) Cr-free in Cr-free steel, and from point c to d (Figure 9a) in Cr-added steel; (c) The slope of the transformed fraction steel,curves and from(df/dT point). c to d (Figure9a) in Cr-added steel; ( c) The slope of the transformed fraction curves (df/dT). curves (df /dT). 3.2.2. Microstructure 3.2.2. Microstructure 3.2.2. MicrostructureThe typical microstructures of the Cr-free and Cr-added steels treated by continuous cooling are shownThe in typical Figure microstructures 11. The microstructure of the Cr-free is signand ificantlyCr-added different steels treated between by continuous the two steels. cooling The are microstructureshownThe typical in Figure microstructures in Cr-free11. The steel microstructure contains of the Cr-freelath-like is sign andbainiteificantly Cr-added (LB), different granular steels between bainite treated (GB),the by twopolygon continuous steels. ferrite The cooling are shown(PF),microstructure and in martensite/austenite Figure in Cr-free11. The steel microstructure contains(M/A) constitution lath-like is significantlybainite, whereas (LB), the granular different microstructure bainite between (GB), in the Cr-addedpolygon two steels.ferrite steel The microstructure(PF), and martensite/austenite in Cr-free steel contains (M/A) lath-like constitution bainite, whereas (LB), granular the microstructure bainite (GB), in polygonCr-added ferrite steel (PF),

Metals 2017, 7, 263 10 of 13 Metals 2017, 7, 263 10 of 13 containsand martensite/austenite LB, a large amount (M/A) of M, constitution, and a small whereas amount the of microstructurePF and RA. No in carbides Cr-added are steel observed. contains The LB, volumea large amount fractions of M,of different and a small phases amount are of determin PF and RA.ed using No carbides the software are observed. Image-Pro The volumePlus 6.0 fractions (Media Cybernetics,of different phases Rockville, are determined MD, USA) using according the software to the Image-Prograyscale and Plus morphology 6.0 (Media Cybernetics, of different Rockville, phases. ThereMD, USA) is about according 48 vol. to % the bainite grayscale (GB + and LB) morphology in Cr-free steel, of different whereas phases. the volume There fraction is about of 48 bainite vol. % (mainlybainite (GB LB) + in LB) Cr-added in Cr-free steel steel, is whereasabout 24 the vol. volume %. This fraction is consistent of bainite with (mainly the result LB) in obtained Cr-added by steel the leveris about rule 24 (Figure vol. %. 10b), This in is which consistent the volume with the fraction result obtainedbainite is by about the lever22 vol. rule % in (Figure Cr-added 10b), steel. in which The amountthe volume of PF fraction decreases bainite with is aboutCr addition 22 vol. due % in to Cr-added the solute-drag steel. The effect amount of Cr of[31]. PF The decreases amount with of CrM significantlyaddition due increases to the solute-drag because the effect transformation of Cr [31]. The temperature amount of Mregion significantly decreases increases by Cr addition because and the largetransformation amount of temperature transformation region occurs decreases below by M CrS temperature addition and (Figure large amount 9a). PF ofpresents transformation more irregular occurs morphology.below MS temperature The regions (Figure showing9a). PF presentsbainite moremorpho irregularlogies morphology. in the two Thesteels regions are showingmagnified bainite and presentedmorphologies in Figure in the 11c,d. two steels It is obvious are magnified that bainite and presented is coarse inin FigureCr-free 11 steel,c,d. Itwhereas is obvious it is that significantly bainite is refinedcoarse in in Cr-free Cr-added steel, steel whereas due to it isthe significantly lower transforma refinedtion in Cr-added temperature. steel dueIn addition, to the lower XRD transformation results show thattemperature. the volume In addition,fraction of XRD RA results decreases show from that 8.7 the vol. volume % to fraction2.3 vol. of% RAwith decreases the addition from of 8.7 Cr. vol. This % tois because2.3 vol. %there with is the more addition ferrite of and Cr. Thisbainite is because in Cr-free there steel. is more It is ferriteknown and that bainite carbon in Cr-freediffuses steel. into Itthe is surroundingknown that carbon untransformed diffuses into austenite the surrounding during ferritic untransformed and bainitic austenitetransformation, during which ferritic stabilizes and bainitic the untransformedtransformation, whichaustenite stabilizes [20]. the untransformed austenite [20].

Figure 11.11. TheThe typical typical microstructures microstructures of theof the Cr-free Cr-free and Cr-addedand Cr-added steels treatedsteels treated by continuous by continuous cooling: (cooling:a,c) Cr-free (a,c) steel; Cr-free (b, steel;d) Cr-added (b,d) Cr-added steel. LB, steel. lath-like LB, lath-like bainite; bainite; GB, granular GB, granular bainite; bainite; PF, polygon PF, polygon ferrite; ferrite;M/A, martensite/austenite. M/A, martensite/austenite.

3.2.3. Mechanical Properties The tensile results of Cr-free and Cr-added steels treated by continuous cooling are given in The tensile results of Cr-free and Cr-added steels treated by continuous cooling are given in Table 4. The engineering strain-stress curves of the two steels are shown in Figure 12. Compared with Table4. The engineering strain-stress curves of the two steels are shown in Figure 12. Compared with Cr-free steel, the yield strength and tensile strength in Cr-added steel is increased because there is Cr-free steel, the yield strength and tensile strength in Cr-added steel is increased because there is more hard martensite in Cr-added steel. However, more amounts of martensite in Cr-added steel more hard martensite in Cr-added steel. However, more amounts of martensite in Cr-added steel significantly decreases the elongation of the steel. As a result, the PSE in Cr-added steel obviously signiﬁcantly decreases the elongation of the steel. As a result, the PSE in Cr-added steel obviously decreases. Moreover, the tensile fractures of the two steels are shown in Figure 13. The fracture in the decreases. Moreover, the tensile fractures of the two steels are shown in Figure 13. The fracture in Cr-free steel mainly consists of dimples, whereas a large amount of quasicleavage morphologies are the Cr-free steel mainly consists of dimples, whereas a large amount of quasicleavage morphologies observed in the Cr-added steel, indicating that the plasticity is better in Cr-free steel. Therefore, Cr are observed in the Cr-added steel, indicating that the plasticity is better in Cr-free steel. Therefore, addition is harmful for the properties, especially ductility, of low carbon bainitic steel under a Cr addition is harmful for the properties, especially ductility, of low carbon bainitic steel under a continuous cooling treatment. continuous cooling treatment.

Metals 2017, 7, 263 11 of 13 Metals 2017, 7, 263 11 of 13

Table 4. The tensile results and RA characteristics of two tested steels treated by continuous cooling. Table 4. The tensile results and RA characteristics of two tested steels treated by continuous cooling. Metals 2017, 7, 263 11 of 13 SteelSteel YS YS (MPa) TS TS (MPa) (MPa) TE TE (%) (%) PSE PSE (GPa%) (GPa%) V Vγ γ(vol.(vol. %) %) Cγ C(wt.γ (wt. %) %) Table 4. The tensile results and RA characteristics of two tested steels treated by continuous cooling. Cr-freeCr-free 662 ± ± 13 1054 1054 ± 1515 13.2 13.2 ±± 0.80.8 13.9 13.9 ±± 0.560.56 8.7 8.7 ±± 0.70.7 1.05 1.05 ± 0.05± 0.05 Cr-addedCr-addedSteel 812 YS ±± 15(MPa) 1145 1145 TS (MPa)± 2121 6.9 6.9TE ± ±(%) 0.20.2 PSE 7.9 7.9 (GPa%) ±± 0.150.15 Vγ (vol. 2.3 2.3 ± %)± 0.20.2 Cγ (wt. 0.82 0.82 %) ± 0.04± 0.04 Cr-free 662 ± 13 1054 ± 15 13.2 ± 0.8 13.9 ± 0.56 8.7 ± 0.7 1.05 ± 0.05 Cr-added 812 ± 15 1145 ± 21 6.9 ± 0.2 7.9 ± 0.15 2.3 ± 0.2 0.82 ± 0.04

FigureFigure 12. 12. TheThe engineering engineering strain-stress strain-stress curves curves of Cr-fr ofee and Cr-free Cr-added and Cr-addedsteels treated steels by continuous treated by Figure 12. The engineering strain-stress curves of Cr-free and Cr-added steels treated by continuous cooling. continuouscooling. cooling.

Figure 13. The tensile fractures of two steels treated by continuous cooling: (a) Cr-free steel; (b) Cr- Figure 13. The tensile fractures of two steels treated by continuous cooling: (a) Cr-free steel; Figureadded 13. The steel. tensile fractures of two steels treated by continuous cooling: (a) Cr-free steel; (b) Cr- (b) Cr-added steel. added steel. 4. Conclusions 4. Conclusions 4. ConclusionsTwo kinds of heat treatment procedure (austempering and continuous cooling) are designed. TwoThe effects kinds of of Cr heat addition treatment on the procedure bainitic transformation, (austempering the andmicrostructure, continuous and cooling) the properties are designed. of Two kinds of heat treatment procedure (austempering and continuous cooling) are designed. The effectslow carbon of Cr carbide-free addition on bainitic the bainitic steels are transformation, investigated. The the following microstructure, conclusions and can the be properties drawn: of low The effects of Cr addition on the bainitic transformation, the microstructure, and the properties of carbon carbide-free bainitic steels are investigated. The following conclusions can be drawn: low carbon(1) Chromium carbide-free addition bainitic hinders steels the are isothermal investigated. bainitic The transformation following conclusions kinetics and can decreases be drawn: the (1) Chromiumamount of addition isothermal hinders transformation the isothermal due to the bainitic decrease transformation in chemical driving kinetics force andfor nucleation decreases the (1) Chromiumand growth addition of bainite. hinders The the hindrance isothermal of Cr bainitic on bainitic transformation transformation kinetics is more and significant decreases at the amount of isothermal transformation due to the decrease in chemical driving force for nucleation amounthigher of isothermaltransformation transformation temperatures due because to the the de decreasedcrease in chemicalchemical driving driving force force accounts for nucleation for and growth of bainite. The hindrance of Cr on bainitic transformation is more signiﬁcant at and growtha larger proportion.of bainite. The hindrance of Cr on bainitic transformation is more significant at higher transformation temperatures because the decreased chemical driving force accounts for a higher(2) Chromium transformation addition temperatures increases the becausestrength anthed decreasedelongation chemicalsimultaneously driving for forceaustempering accounts for larger proportion. a largertreatment proportion. at a lower temperature. However, when the austempering temperature is higher, the (2) Chromiumstrength additionincreases increasesand the elongation the strength obviousl andy elongationdecreases by simultaneously Cr addition, resulting for austempering in the (2) Chromium addition increases the strength and elongation simultaneously for austempering treatmentdecrease at in a lowerthe product temperature. of tensile However,strength and when elongation. the austempering In addition, temperaturethe austempering is higher, treatment at a lower temperature. However, when the austempering temperature is higher, the the strengthtemperature increases should be and lower the in elongationCr-added steel obviously than that in decreases Cr-free steel by in Cr order addition, to obtain resulting better in strength increases and the elongation obviously decreases by Cr addition, resulting in the the decreasecomprehensive in the properties. product of tensile strength and elongation. In addition, the austempering decrease(3) For continuous in the product cooling oftreatment tensile instrength the presen andt study, elongation. the amount In addition,of RA decreases, the austempering and the temperature should be lower in Cr-added steel than that in Cr-free steel in order to obtain better temperatureyield strength should and be tensile lower strength in Cr-added increases, steel bu thant the thattotal in elongation Cr-free steel obviously in order decreases to obtain in Cr better comprehensive properties. comprehensive properties. (3) For continuous cooling treatment in the present study, the amount of RA decreases, and the yield strength and tensile strength increases, but the total elongation obviously decreases in Cr

Metals 2017, 7, 263 12 of 13

(3) For continuous cooling treatment in the present study, the amount of RA decreases, and the yield strength and tensile strength increases, but the total elongation obviously decreases in Cr added steel due to more amounts of martensite. The product of tensile strength and elongation signiﬁcantly decreases.

Acknowledgments: The authors gratefully acknowledge the financial supports from National Natural Science Foundation of China (NSFC) (No. 51274154), the National High Technology Research and Development Program of China (No. 2012AA03A504), and the Special Fund of Wuhan University of Science and Technology for Master Students’ Short-Term Studying Abroad. Author Contributions: Guang Xu and Mingxing Zhou conceived and designed the experiments; Mingxing Zhou and Junyu Tian performed the experiments; Mingxing Zhou, Junyu Tian, and Haijiang Hu analyzed the data; Qing Yuan contributed materials tools; and Mingxing Zhou wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

Abbreviations The following abbreviations are used in this manuscript:

SEM scanning electron microscope XRD X-ray diffraction BF bainite ferrite RA retained austenite M martensite LB lath-like bainite GB granular bainite PF polygon ferrite YS yield strength TS tensile strength TE total elongation PSE product of strength and elongation TRIP transformation induced plasticity

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