Effects of Heat-Treatment Temperature on the Microstructure ... - MDPI

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Effects of Heat-Treatment Temperature on the Microstructure and Mechanical Properties of Steel by MgO Nanoparticle Additions Yutao Zhou 1 , Shufeng Yang 1, *, Jingshe Li 1 , Wei Liu 1 and Anping Dong 2, * 1 2

*

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (Y.Z.); [email protected] (J.L.); [email protected] (W.L.) School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Correspondence: [email protected] (S.Y.); [email protected] (A.D.); Tel.: +86-10-6233-4277 (S.Y.)

Received: 9 August 2018; Accepted: 9 September 2018; Published: 13 September 2018

Abstract: The characteristics and formation mechanisms of intragranular acicular ferrite (IAF) in steel with MgO nanoparticle additions were systematically investigated for different isothermal heat-treatment temperatures, and its influence on mechanical properties was also clarified. The results indicate that the inclusions were finely dispersed and refined after adding MgO nanoparticles. In addition, with decreasing heat-treatment temperature, the microstructure changed from grain boundary ferrite (GBF) and polygonal ferrite (PF) to intragranular acicular ferrite. Moreover, the steel with MgO additions had excellent mechanical properties in the temperature range of 973 to 823 K and an average Charpy absorbed energies value of around 174 J at 873 K due to the significant refinement of the microstructure and nucleation of intragranular acicular ferrite. Keywords: microstructure; oxide metallurgy; mechanical properties; intragranular acicular ferrite

1. Introduction With the development of a new generation of steel materials, the cleanliness of steel has been significantly improved [1–3]. However, the complete removal of nonmetallic inclusions from liquid steel during the steelmaking process is almost impossible [4,5]. Therefore, Takamura et al. [6] and Mizoguchi et al. [7] proposed a new concept of oxide metallurgy, which uses small-sized inclusions and precipitates particles as nucleation sites of intragranular acicular ferrite (IAF) or pins austenite grains to improve the final properties of steel. Further research and applications of this technique have shown that nonmetallic inclusions induce the formation of IAF, which becomes the core point [8]. Up to now, these inclusions have been formed either by an internal precipitation method or by an external addition method [9]. Numerous studies have focused on the internal precipitation method, which mainly forms oxide inclusions through strong deoxidizer and alloying elements such as Mg [10,11], Ti [12,13], V [14], and Ce [15], and these inclusions become nucleation sites of IAF during the austenite (γ) → ferrite (α) transformation [16]. In addition, some researchers have suggested that inclusions such as ZrO2 [17], TiN [18], and MnS [19] can also act as heterogeneous nuclei for IAF. However, this method requires precise control of the refining process within a narrow operating window. Moreover, due to the complexity of the metallurgical chemical reaction and the instability of the steelmaking process, it is difficult to obtain the desired inclusion characteristics, especially the inclusion type and size. Therefore, in the past few decades, some researchers have proposed an external addition method, which mostly uses artificially added oxide powder such as Ti2 O3 and ZrO2 to produce the desired inclusions types [20]. Kiviö et al. [21] reported that they had added to steel different additives with metallic Ti and TiO2 . Samples brought for subsequent hot rolling and

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heat-treatment experiments to find out the grain-refining effect eventually formed IAF. Mu et al. [22] and Xuan et al. [23] investigated different powders such as Ti2 O3 and TiN, which were used for steel samples to act as nucleation sites of IAF formation. Furthermore, recent studies have attempted to use various methods of externally adding different particles to obtain small-sized and uniformly distributed inclusions [24,25]. In the previous research of the author and his coworkers [26–28], a novel method for predispersing MgO nanoparticles with AlSi or AlTi alloy nanoparticles was applied. Small-sized inclusions could be obtained by adding nanoparticles to molten steel. For laboratory experiments, these nanoparticles could be added to liquid steel with a molybdenum rod in a tubular resistance furnace [26]. For industrial production, the nanoparticles could be added to liquid steel within the mold in the vicinity of the submerged nozzle outlet [27]. Meanwhile, several studies have reported the formation mechanism of IAF induced by the addition of nanoparticles. Gao et al. [28] determined that the Mn-depleted zone (MDZ) can be found around Ti2 O3 and MgO–Al2 O3 inclusions in steels by MgO addition, which can promote ferrite nucleation by increasing the chemical driving force. However, Mu et al. [29] reported that the MDZ was not formed around inclusions in steels by Ti2 O3 addition. Similar conclusions were also drawn by Kang et al. [30]. Hence, it is of great significance to systematically investigate the formation mechanism of IAF in steel. Although the addition of nanoparticles is very popular for the induction of IAF nucleation to improve the mechanical properties of steels, few researchers have systematically studied the nucleation of IAF and how to refine the microstructure, in particular, the effect of phase transformation temperatures on nucleation and growth of IAF during γ → α transformation. Therefore, the current study, based on previous studies, investigated the effects of isothermal treatment temperature on microstructure and mechanical properties. Then, the critical size of the inclusions for IAF nucleation was analyzed using theoretical calculations, and the mass fraction of the inclusions during solidification was predicted using FactSage software. Finally, the mechanism of IAF formation and the refinement of IAF to microstructure are discussed. 2. Experimental 2.1. Experimental Procedure The low-carbon steel used in the present study was melted in a 6-kg vacuum induction furnace (Shenyang Vacuum Technology Institute Co., Ltd., Shenyang, China), and the mixed MgO nanoparticles were poured into the liquid steel through a charging hopper located under the furnace cover. In order to ensure that the MgO nanoparticles added to the steel melt were fully dispersed, another type of nanoparticle material, AlTi alloy, was used as a predispersion medium. The predispersion process of MgO nanoparticles was performed using a planetary ball mill (IKN Mechanical Equipment Co., Ltd., Berlin, Germany) with a weight ratio of MgO and AlTi alloy of 4:1. The rotation speed was maintained at 6000 rpm during the 3-h process which was conducted under low-oxygen and low-temperature conditions. The microstructure and crystal structure of MgO and AlTi alloy nanoparticles are shown in Figure 1. The detailed operational parameters and processes of the nanoparticles have been reported by previous studies by the author and his coworkers [26]. Then, the melt was cast into ingots and was cooled to room temperature in the air. Subsequently, the ingots were heated at 1473 K for 1 h to homogenize the elements, hot forged into a rectangular shape with the size of 40 × 120 × 120 mm3 at a temperature ranging from 1323 to 1073 K, and then air cooled to room temperature. The chemical composition of the steel was determined using optical emission spectroscopy and is given in Table 1. To evaluate the effect of the different phase transformation temperatures on IAF, first, thermodynamic software was used to predict the phase transformation temperatures under equilibrium conditions to provide theoretical guidance for the experiment. The phase transformation temperatures, Ae1 and Ae3 , were calculated by FactSage7.1 software (Centre for Research in Computational Thermochemistry, Montreal, QC, Canada,) (Figure 2a) to be 723 and 1058 K, respectively. The TTT

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respectively. The TTT (Time, Temperature, Transformation) diagram was established by JMatPro respectively. The TTT (Time, Temperature, Transformation) diagram was established by JMatPro software (Version 10.0, Sente Software Ltd., London, UK) as shown in Figure 2b, and the tip (Time, Temperature, Transformation) diagram was established JMatPro software software (Version 10.0, Sente Software Ltd., London, UK) asby shown in Figure 2b,(Version and the10.0, tip temperature was 919 K during γ → α transformation. The isothermal heat-treatment processes were Sente Software Ltd., London, UK) as shown in Figure 2b, and the tip temperature was 919 K during γ temperature was 919 K during γ → α transformation. The isothermal heat-treatment processes were as follows: Specimens with a size of 12 × 120 × 50 mm3 from forged steels were heated to 1323 K at a 3 from forged transformation. The isothermal processes were as follows: → as α follows: Specimens with a size of heat-treatment 12 × 120 × 50 mm steels wereSpecimens heated to with 1323 aKsize at a rate of 10 K/min and 3held at this temperature for 20 min before being cooled to the corresponding of 12 × 120 × 50 mm from forged steels were heated to 1323 K at a rate of 10 K/min and held at this rate of 10 K/min and held at this temperature for 20 min before being cooled to the corresponding salt bath temperature at the same rate. Subsequently, the isothermal treatment temperatures based temperature for 20 minatbefore being cooled to the corresponding salt treatment bath temperature at the based same salt bath temperature the same rate. Subsequently, the isothermal temperatures on the TTT diagram were carried out at 973, 923, 873, and 823 K for 30 min, respectively. For rate. Subsequently, thewere isothermal temperatures based diagram were carried on the TTT diagram carriedtreatment out at 973, 923, 873, and 823onKthe forTTT 30 min, respectively. For convenience, the specimens were orderly denoted as M1, M2, M3, and M4, respectively. Lastly, the out at 973, 923, 873, and 823 K for 30 min, respectively. For convenience, the specimens were orderly convenience, the specimens were orderly denoted as M1, M2, M3, and M4, respectively. Lastly, the heat-treated specimens were water-cooled to room temperature. In addition, the forged specimens denoted as M1, M2, M3, and respectively. heat-treated were water-cooled to heat-treated specimens wereM4, water-cooled toLastly, room the temperature. In specimens addition, the forged specimens were heated to 1323 K at a rate of 10 K/min and held at this temperature for 20 min, followed by room temperature. In addition, the forged specimens were heated to 1323 K at a rate of 10 K/min and were heated to 1323 K at athe rate of 10 K/min and at this temperature 20 min, of followed by water-quenching to reveal prior austenite grainheld boundaries. The detailsfor schematic the heatheld at this temperature for the 20 min, water-quenching to reveal theschematic prior austenite water-quenching to reveal priorfollowed austenitebygrain boundaries. The details of thegrain heattreatment process is shown in Figure 3. boundaries. The details schematic of the treatment process is shown in Figure 3. heat-treatment process is shown in Figure 3.

Figure 1. Morphology and crystal structure of MgO and AlTi nanoparticles. (a) Morphology of MgO Figure Morphologyand andcrystal crystalstructure structure MgO AlTi nanoparticles. (a) Morphology of Figure 1. 1. Morphology of of MgO andand AlTi nanoparticles. (a) Morphology of MgO nanoparticles, (b) morphology of AlTi nanoparticles, (c) crystal structure of MgO nanoparticles, and MgO nanoparticles, (b) morphology of AlTi nanoparticles, (c) crystal structure of MgO nanoparticles, nanoparticles, (b) morphology of AlTi nanoparticles, (c) crystal structure of MgO nanoparticles, and (d) crystal structure of AlTi nanoparticles. and (d) crystal structure of AlTi nanoparticles. (d) crystal structure of AlTi nanoparticles.

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Table 1. 1. Chemical composition of experimental experimental steel steel (wt. (wt. %).

C C Si Si Mn Mn Nb Nb TiTi NiNi Al Al SS NN 0.09 0.38 1.58 0.04 0.01 0.25 0.025 0.008 0.003 0.09 0.38 1.58 0.04 0.01 0.25 0.025 0.008 0.003

Figure 3. Schematic of forging process and isothermal heat treatment of specimens. Figure 3. Schematic of forging process and isothermal heat treatment of specimens.

2.2. Characterization of Inclusions 2.2. Characterization of Inclusions Different specimens were cut into small cubes (10 × 10 × 10 mm3 ). Subsequently, they were Different specimens werefinally, cut into (10 ×pastes. 10 × 10Thereafter, mm3). Subsequently, they were polished by SiC papers and, bysmall 1-µmcubes diamond the morphology and polished by SiC papers and, finally, by 1-μm diamond pastes. Thereafter, the morphology and composition of inclusions were observed with scanning electron microscopy (SEM, Phenom proX composition of inclusions were observed with scanning electron microscopy (SEM, Phenom proX scanning electron microscopy, Eindhoven, The Netherlands) and energy-dispersive spectroscopy (EDS) scanning electron microscopy,The Eindhoven, The Netherlands) and energy-dispersive spectroscopy (PANalytical B.V., Eindhoven, Netherlands). The characteristics of the inclusions were analyzed (EDS) (PANalytical B.V., Eindhoven, The Netherlands). The characteristics of the inclusions were by Image-Pro-Plus software (Version 6.0, Media Cybernetics, Inc., Rockville, MD, USA). analyzed by Image-Pro-Plus software (Version 6.0, Media Cybernetics, Inc., Rockville, MD, USA). 2.3. Mechanical Properties and Microstructural Characteristics 2.3. Mechanical Properties and Microstructural Characteristics Tensile specimens were machined to a length of 60 mm and diameter of 5 mm, which were Tensile were machined a length of 60 mm testing and diameter 5 mm, which were conducted atspecimens room temperature using ato computerized tensile system.ofStandard specimens 3 conducted temperature a computerized tensile testing system. specimens with a size at of room 10 × 10 × 55 mm using were machined for Charpy impact tests whichStandard were conducted at with a size of 10 × 10 The × 55fracture mm3 were machined for Charpy tests wereinconducted at ambient temperature. surfaces were examined byimpact SEM and thewhich inclusions the dimples ambient temperature. The fracture surfaces were examined by SEM and the inclusions in the dimples were analyzed by EDS. All the specimens tested by Rockwell Hardness and the values were were analyzed by EDS. of Allfive themeasurements specimens were tested by and values were reported as an average to reduce theRockwell error. To Hardness investigate thethe microstructure reported as an average of five measurements to reduce the error. To investigate the microstructure characteristics by optical microscopy (OM, Leica, Wetzlar, Germany) and SEM, the polished specimens characteristics by optical (OM,solution. Leica, Wetzlar, Germany) and SEM, the polished were slightly etched for 15 smicroscopy with 4 vol% nital In addition, the forged specimens were etched specimens were etched for distilled 15 s withwater, 4 vol% nital solution. addition, the the forged by a solution (3 gslightly picric acid, 50 mL and 2 mL HCl) atIn 333 K to reveal priorspecimens austenite were etched by a solution (3 observed g picric acid, 50 mLX-ray distilled water, and 2 mL HCl) at 333 K to reveal the grain boundaries and then by OM. diffraction (XRD, Rigaku Corporation, Tokyo, prior austenite observed by OM. X-ray diffraction (XRD, Rigaku Japan) patterns ofgrain all theboundaries specimens and were then obtained with Cu-Ka radiation. Corporation, Tokyo, Japan) patterns of all the specimens were obtained with Cu-Ka radiation. 3. Experimental Results 3. Experimental Results 3.1. Characteristics of Inclusions

3.1. Characteristics of Inclusions Before studying the effect of heat-treatment temperature on the microstructure, it is necessary to clarify the inclusion in experimentaltemperature steels. Figureon 4 shows the elemental it distribution of Before studyingcharacteristics the effect of heat-treatment the microstructure, is necessary the typical inclusions in steel with the MgO addition. Figure 4a,b shows that the inclusions mainly to clarify the inclusion characteristics in experimental steels. Figure 4 shows the elemental consisted of of MgO–Al MgO–Al irregular in shape 2 O3 and 3 –TiO 2 . The distribution the typical inclusions in2 O steel with the single MgO inclusion addition. was Figure 4a,b shows that and the most of the inclusions were less than 3 µm in size. Moreover, in Figure 4b, it can be seen that TiO inclusions mainly consisted of MgO–Al2O3 and MgO–Al2O3–TiO2. The single inclusion was irregular2 was formed theofsurface of MgO–Al the Moreover, inclusions in had a spherical tendency. 3 inclusions in shape and on most the inclusions were2 O less than 3 μmand in size. Figure 4b, it can be seen Figure 4c shows the complex MgO–Al O –TiO –TiN inclusion. Based on the elemental mapping, 2 3 2 that TiO2 was formed on the surface of MgO–Al2O3 inclusions and the inclusions had a spherical

tendency. Figure 4c shows the complex MgO–Al2O3–TiO2–TiN inclusion. Based on the elemental


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the central area contains the elements Al, O, and Mg, and the outermost layer consists of mostly Ti and mapping, the central area contains the elements Al, O, and Mg, and the outermost layer consists of N.mostly Figure Ti 4dand shows the complex MgO–Al2 O3 –TiO inclusion, and MnS and TiN were found 2 –TiN–MnS N. Figure 4d shows the complex MgO–Al 2O3–TiO2–TiN–MnS inclusion, and MnS and to TiN precipitate randomly at the corners of the corecorners of MgO–Al discussion of the 3 –TiO 2 . A detailed were found to precipitate randomly at the of the2 O core of MgO–Al 2O3–TiO2. A detailed microstructures is presented later. discussion of the microstructures is presented later.

Figure Morphologyand andcomposition compositionof oftypical typical inclusions. inclusions. (a) 2O3O -TiO2; Figure 4. 4. Morphology (a) MgO–Al MgO–Al22OO3;3(b) ; (b)MgO–Al MgO–Al 2 3 -TiO2 ; 2O3–TiO2–TiN; (d) MgO–Al2O3–TiO2–TiN–MnS. MgO–Al (c)(c) MgO–Al 2 O3 –TiO2 –TiN; (d) MgO–Al2 O3 –TiO2 –TiN–MnS.

Thecomposition composition and of of oxide inclusions in experimental steels are shown Figure in The and distributions distributions oxide inclusions in experimental steels areinshown 5. The solidsolid lineslines andand isotherm of the TiO 2–MgO–Al 2O3 O ternary system were calculated using Figure 5. The isotherm of the TiO –MgO–Al ternary system were calculated using 2 2 3 FactSage7.1 software, and the composition is the mass fraction and the size chart of the inclusions is FactSage7.1 software, and the composition is the mass fraction and the size chart of the inclusions represented by the color map. The percentage of inclusions smaller than 3 μm was greater than 80%, is represented by the color map. The percentage of inclusions smaller than 3 µm was greater than with most of the having a sizeabetween 1 and 12 and μm, 2 and theand large-sized inclusions were 80%, with most of inclusions the inclusions having size between µm, the large-sized inclusions not observed in all specimens. This indicates that the inclusions were finely dispersed and refined were not observed in all specimens. This indicates that the inclusions were finely dispersed and after adding MgO nanoparticles, which is consistent with the findings by Gao et al. [28]. Further, Lee refined after adding MgO nanoparticles, which is consistent with the findings by Gao et al. [28]. et al. [31] found that larger inclusions have a greater potential for the nucleation of ferrite, and when Further, Lee et al. [31] found that larger inclusions have a greater potential for the nucleation of ferrite, the inclusion size is 1.1 μm in steel, the probability of nucleation is 1. Therefore, the size distribution and when the inclusion size is 1.1 µm in steel, the probability of nucleation is 1. Therefore, the size of inclusions in experimental steel was optimal. distribution of inclusions in experimental steel was optimal.

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Figure Size distributions of oxide inclusionsin inexperimental experimentalsteels. steels. Figure5.5.Size Sizedistributions distributionsof ofoxide oxideinclusions Figure

3.2. Microstructural Microstructural Characteristics Characteristics 3.2. 3.2. Microstructural Characteristics Figure 66 displays displays the the microstructure microstructure of of the the experimental experimental steel steel and and shows shows that that the the microstructure microstructure Figure Figure 6 displays the microstructure of the experimental steel and shows that the microstructure of the experimental steel after hot forging mainly consisted of pearlite and ferrite. The white area was was ofthe theexperimental experimentalsteel steelafter afterhot hotforging forgingmainly mainlyconsisted consistedof ofpearlite pearliteand andferrite. ferrite.The Thewhite whitearea area of was ferrite and the black area was pearlite. Figure 6b presents the prior austenite grains of experimental ferriteand andthe theblack blackarea areawas waspearlite. pearlite.Figure Figure6b 6bpresents presentsthe theprior prioraustenite austenitegrains grainsof ofexperimental experimental ferrite steels quenched at 1323 K. It is clear that the prior austenite grain size was slightly fine and uniform. uniform. steelsquenched quenchedatat1323 1323K. K.ItItisisclear clearthat thatthe theprior prioraustenite austenitegrain grainsize sizewas wasslightly slightlyfine fineand and steels uniform. The mean prior austenite grain size of the specimen was measured to be 45 µm using the intercept ntercept Themean meanprior prioraustenite austenitegrain grainsize sizeof ofthe thespecimen specimenwas wasmeasured measuredto tobe be45 45μm μmusing usingthe theiintercept The method, which which is is similar similar to to the the one one reported reported Wan et al. [32]. method, Wan et al. [32]. method, which is similar to the one reported Wan et al. [32]. Figure 77 shows showsthe thetypical typicalmicrostructure microstructure of M1, M4 specimens. It is from seen Figure ofthe thethe M1, M2,M2, M3,M3, andand M4specimens. specimens. seen Figure 7 shows the typical microstructure of M1, M2, M3, and M4 ItItisisseen from from Figure 7a that the microstructure in M1 was composed of coarse grain boundary ferrite (GBF), Figure 7a 7a that that the the microstructure microstructure in in M1 M1 was was composed composed of of coarse coarse grain grain boundary boundary ferrite ferrite (GBF), (GBF), Figure polygonal ferrite (PF), and little Widmanstätten ferrite (WF). When the heat-treatment temperature polygonalferrite ferrite(PF), (PF),and andlittle littleWidmanstätten Widmanstättenferrite ferrite(WF). (WF).When Whenthe theheat-treatment heat-treatmenttemperature temperature polygonal was 923 923 K, K, the amounts of GBF GBF and PF were significantly decreased, and and the the amount amount of of IAF IAF was was was the amounts of and PF were significantly decreased, was 923 K, the amounts of GBF and PF were significantly decreased, and the amount of IAF was increased (Figure 7b). Figure 7a,b shows that the PF in the latter was much finer than in the former. increased(Figure (Figure7b). 7b).Figure Figure7a,b 7a,bshows showsthat thatthe thePF PFin inthe thelatter latterwas wasmuch muchfiner finerthan thanin inthe theformer. former. increased The microstructures of M3 and M4 are shown in Figure 7c,d, respectively. It was found that the The microstructures microstructures of of M3 M3 and and M4 M4 are are shown shown in in Figure Figure 7c,d, 7c,d, respectively. respectively. ItIt was was found found that that the the The dominant microstructure for M3 and M4 was IAF, and a small amount of bainite was also observed dominantmicrostructure microstructurefor forM3 M3and andM4 M4was wasIAF, IAF,and andaasmall smallamount amountof ofbainite bainitewas wasalso alsoobserved observed dominant in M4. M4. Although Although IAF IAF existed existed in in M4, M4, they they were were much much coarser coarser than than those those in in M3. M3. Therefore, Therefore, it it can can be be in in M4. Although IAF existed in M4, they were much coarser than those in M3. Therefore, it can be inferred that the nucleation and growth of IAF were closely related to the isothermal heat treatment. inferredthat thatthe thenucleation nucleationand andgrowth growthof ofIAF IAFwere wereclosely closelyrelated relatedto tothe theisothermal isothermalheat heattreatment. treatment. inferred

Figure6.6.Optical Opticalmicrograph micrograph ofexperimental experimental steels. (a) Initial microstructure of steels after forging, micrographof experimentalsteels. steels.(a) (a)Initial Initialmicrostructure microstructureof ofsteels steelsafter afterforging, forging, Figure ◦°C). and (b) prior austenite grain micrograph of experimental steels quenched at 1323 K (1050 C). and (b) prior austenite grain micrograph of experimental steels quenched at 1323 K (1050 °C).

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Figure 7. Typical microstructure of the specimens. (a) M1, (b) M2, (c) M3, and (d) M4. Figure 7. Typical Typical microstructure of the specimens. (a) M1, (b) M2, (c) M3, and (d) M4.

X-ray diffraction patterns of all the specimens are shown in Figure 8. Peaks corresponding to X-ray diffraction ofof allall thethe specimens are are shown in Figure 8. Peaks corresponding diffraction patterns specimens shown 8. corresponding to ferrite and austenitepatterns were indexed based on respective (h k in l) Figure planes of Peaks phases. It can to beferrite clearly and austenite were indexed based on respective (h k l) planes of phases. It can be clearly observed ferrite and that austenite werediffraction indexed based on of respective (h k l) planes of basically phases. Itsimilar. can beAustenite clearly observed the XRD patterns the four specimens were that the XRD diffraction patterns ofpatterns the in four specimens were basically similar. Austenite diffraction observed thatpeaks the XRD the four specimens were basically similar. Austenite diffraction werediffraction clearly present theof XRD diffractogram, but the number was small. The peak peaks were clearly present in the XRD diffractogram, but the number was small. The peak ofangles ferrite diffraction were clearly present in the XRD diffractogram, but the number small. The peakof of ferrite peaks was obvious, exhibiting two strong lines and broad half peaks at thewas diffraction ◦ and was obvious, exhibiting twoitstrong and broad half peaks at diffraction angles of 44.4 of44.4° ferrite was obvious, exhibiting strong and broad peaks at the diffraction angles of and 64.8°. Therefore, can two belines seen thatlines ferrite should behalf thethe main phase. 64.8◦ .and Therefore, it can be seen that should beshould the main 44.4° 64.8°. Therefore, it can be ferrite seen that ferrite be phase. the main phase. 12000 12000

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2θ(degree) Figure 8. X-ray diffraction curves of the specimens. Figure 8. X-ray diffraction curves of the specimens. 8. nucleated X-ray diffraction curves of the specimens. To further investigate Figure the IAF on inclusions at different heat-treatment temperature,

To further investigate the IAF were nucleated on inclusions different heat-treatment the microstructure of four specimens observed by SEM, asatshown in Figure 9. Figure temperature, 9a,b shows To further investigate nucleated on inclusions at different temperature, theseveral microstructure of four specimens were observed by SEM, as shown ina Figure 9. Figure that PF nucleated onthe theIAF surface of the small-sized inclusions andheat-treatment large amount of 9a,b GBFshows and the microstructure of four specimens wereof observed SEM, as shown in Figure 9. Figure 9a,b shows that several PFobserved, nucleated on the the small-sized inclusions and athat large amount of GBF and some IAF were but IAFsurface plates were still by very short, indicating IAF transformation that several PF nucleated on the surface of the small-sized inclusions and a large amount of GBF and some were observed, platesinclusions were still very indicating that IAF transformation did did notIAF occur in the vicinitybut of IAF complex aftershort, quenching at 973 and 923 K. In Figure 9c, some IAF were observed, IAF plates were stillafter veryquenching short, indicating that923 IAFK. transformation not occur in the vicinity but of complex inclusions at 973 and In Figure 9c, itdid can not occur in the vicinity of complex inclusions after quenching at 973 and 923 K. In Figure 9c, it can

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it can be observed thatand IAFPF andnucleated PF nucleated on complex inclusions (approximately 2 µm in size). be observed that IAF on complex inclusions (approximately 2 μm in size). In In contrast, IAF in M3 seemed to be longer, indicating that IAF had a growth in the length direction. contrast, IAF in M3 seemed to be longer, indicating that IAF had a growth in the length direction. Moreover, some intragranular acicular acicular ferrites ferrites (SIAF) (SIAF) were were found found on on the the branches branches of of IAF, IAF, Moreover, some secondary secondary intragranular as indicated with arrows. The formation of SIAF might help promote the final properties of the steel as indicated with arrows. The formation of SIAF might help promote the final properties of the steel by increasing inin M4 was composed of by increasing the the multiple multipleinterlocking interlockingdegree degreeofofIAF IAF[33]. [33].The Themicrostructure microstructure M4 was composed coarse IAF, PF,PF, andand some bainites (B),(B), as shown in Figure 9d. 9d. of coarse IAF, some bainites as shown in Figure Typical elemental mapping images of inclusions inducing IAF nucleation nucleation in in M3 M3 are are shown Typical elemental mapping images of inclusions inducing IAF shown in in Figure 10. –TiN–MnS complex complex Figure 10. It It can can be be found found that that IAF IAF preferentially preferentially nucleates nucleates at atMgO–Al MgO–Al222O O333–TiO –TiO222–TiN–MnS inclusions located located within within the the austenite austenite grain, grain, and and many many IAF IAF plates plates can can be inclusions be clearly clearly observed observed around around the inclusions, which is in good agreement with other studies [34]. The results indicate that that the the inclusions, which is in good agreement with other studies [34]. The results indicate the abovementioned inclusions abovementioned inclusions are are effective effective at at providing providing nucleation nucleation sites sites for for IAF IAF at at 873 873 K. K.

Figure 9. Morphologies different Figure 9. Morphologies of of intragranular intragranular acicular acicular ferrite ferrite (IAF) (IAF) induced induced by by inclusions inclusions for for different specimens. (a) M1, (b) M2, (c) M3, and (d) M4.

Figure 10. SEM mapping images of inclusion nucleating IAF in M3.

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3.3. Mechanical Properties

To compare the mechanical properties of the experimental steels after different heat-treatment 3.3. Mechanicaltensile Properties temperatures, tests were performed on the specimens and the results are shown in Table 2. It can be that the mechanical heat-treatment temperature a slight effect tensile strength and yield Toseen compare properties of the had experimental steelson after different heat-treatment strength, and the yieldtests strength the tensile variedand fromthe 529 to 540are MPa and 763 to 8032. temperatures, tensile wereand performed on strength the specimens results shown in Table MPa, respectively. Figure 11 shows the variation in the specimens’ macrohardness and the Charpy It can be seen that the heat-treatment temperature had a slight effect on tensile strength and yield absorbed energy room temperature. It is obvious that the macrohardness then strength, and the at yield strength and the tensile strength varied from 529 to 540 first MPadecreased and 763 toand 803 MPa, increased slightly heat-treatment decreased from 973and to the 823 Charpy K. The absorbed average respectively. Figureas11the shows the variationtemperatures in the specimens’ macrohardness Charpy absorbed energy values 298 K for of M1, M2,first M3,decreased and M4 were 157, increased 163, 174, energy at room temperature. It isatobvious thatspecimens the macrohardness and then and 169 as J, respectively, whichtemperatures showed excellent impact absorbed to slightly the heat-treatment decreased fromproperties. 973 to 823 The K. The averageenergy Charpytended absorbed increase in the at order specimen M1,of M2, and M3 because M3 contains a large (Figure energy values 298 of K for specimens M1, M2, M3, and M4 were 157, 163, 174,amount and 169of J, IAF respectively, 7). Thisshowed indicates that the formation of IAF byenergy inclusions was which excellent impact properties. Theinduced absorbed tended to conducive increase in to theimprove order of toughness [35,36]. specimen M1, M2, and M3 because M3 contains a large amount of IAF (Figure 7). This indicates that Figure 12 of shows the macro-fractographs SEM images of the toughness fracture surfaces the formation IAF induced by inclusions wasand conducive to improve [35,36]. of the four specimens from the notch. It canand be clearly observed in Figure 12a–f that the Figureaway 12 shows the V-shaped macro-fractographs SEM images of the fracture surfaces of fracture the four morphology wasfrom a mixture of quasi-cleavage ductileinfracture, which mainly specimens away the V-shaped notch. It can fracture be clearlyand observed Figure 12a–f thatwas the fracture composed of typical riverlike patterns and tongue patterns. In addition, dimples could also be found morphology was a mixture of quasi-cleavage fracture and ductile fracture, which was mainly composed in localriverlike region. patterns The fibrous the secondary fibrous region can be be clearly in of the typical and region tongue and patterns. In addition, dimples could also foundobserved in the local Figure Figureregion 12g,i shows a ductile fracture characterized byclearly microvoid coalescence, and the region.12b,d. The fibrous and the secondary fibrous region can be observed in Figure 12b,d. fracture surface cana be divided into characterized three regions:by a fibrous region, a radial and region, a shear lip Figure 12g,i shows ductile fracture microvoid coalescence, the and fracture surface [37]. size into of unit became and the number dimples increased, can beThe divided threecleavage regions: afacet fibrous region,smaller a radial region, and a shear of lip [37]. The size of unit contributing to the increase in crack propagation absorbed energy [10]. According to the formation cleavage facet became smaller and the number of dimples increased, contributing to the increase in mechanism of ductile fracture [38], theAccording number of unitformation dimples mechanism reflects its of ability tofracture hinder [38], the crack propagation absorbed energy [10]. to the ductile propagation ofunit cracks. In addition, Figure 12itoshows dimples than FigureIn12c,f, which implies the number of dimples reflects its ability hinderdeeper the propagation of cracks. addition, Figure 12i that M3 had a higher toughness than M2 and M1. The fibrous region and riverlike patterns were shows deeper dimples than Figure 12c,f, which implies that M3 had a higher toughness than M2 and apparently observed inand M4,riverlike as shown in Figure 12j,l. M1. The fibrous region patterns were apparently observed in M4, as shown in Figure 12j,l. Table Table 2. 2. Mechanical Mechanical properties properties of of specimens. specimens. Yield Strength (Mpa) (%)(%) YieldStrength Strength(MPa) (MPa) Tensile Tensile Strength (Mpa) Elongation Elongation 535 769 9.6 535 769 9.6 529 763 10.8 529 763 10.8 531 803 13.613.6 531 803 540 775 8.4 8.4 540 775

Impact energy(J)

210

55 Impact energy Hardness

200 190

50 45

180 40

170

35

160 150

Hardness(HRC)

Specimen Specimen M1 M1 M2 M2 M3 M3 M4 M4

M1

M2

Specimen

M3

M4

30

Figure11. 11.Variation Variationof ofmacrohardness macrohardness and and the the Charpy Charpy absorbed absorbed energy energy at at room room temperature. temperature. Figure


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Figure12. 12. Fracture Fracture morphology morphology features M1; (d–f) M2; (g–i) M3;M3; andand (j–l)(j–l) M4.M4. Figure featuresofofspecimens. specimens.(a–c) (a–c) M1; (d–f) M2; (g–i)

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Figure 13shows showsSEM SEMimages images fibrous region on fracture the fracture surface the inclusion Figure 13 of of thethe fibrous region on the surface of the of inclusion in M3. in M3. The inclusions in the dimples were essentially spherical in shape and about 2 µm in EDS size. The inclusions in the dimples were essentially spherical in shape and about 2 μm in size. The The EDS result analysis result indicates the inclusions M3 mainly consisted of MgO–Al 2 O32–TiO 2 –MnS. analysis indicates that thethat inclusions in M3 in mainly consisted of MgO–Al 2O3–TiO –MnS. This This type of appropriately sized inclusion can weaken the cleaving effect and reduce the stress type of appropriately sized inclusion can weaken the cleaving effect and reduce the stress concentration concentration to to improve improve toughness toughness [39]. [39].

Figure 13. SEM images of the fibrous region on the fracture surface of the the inclusion inclusion in in M3. M3.

4. 4. Discussion Discussion 4.1. Effect of Inclusions on the Mechanism of IAF at Different Isothermal Heat-Treatment Temperatures 4.1. Effect of Inclusions on the Mechanism of IAF at Different Isothermal Heat-Treatment Temperatures To determine the capability of inclusions to induce IAF at different heat-treatment To determine the capability of inclusions to induce IAF at different heat-treatment temperatures temperatures [40,41], it is necessary to predict the detailed precipitation behavior of inclusions to [40,41], it is necessary to predict the detailed precipitation behavior of inclusions to clarify the reason clarify the reason for the microstructural evolution. The mass fractions of the inclusion as a function for the microstructural evolution. The mass fractions of the inclusion as a function of temperature of temperature were determined using FactSage7.1 software, as shown in Figure 14. According to were determined using FactSage7.1 software, as shown in Figure 14. According to the the thermodynamic calculation results, the inclusions during solidification were corundum, spinel, thermodynamic calculation results, the inclusions during solidification were corundum, spinel, titania spinel, TiN, and MnS. The corundum phase was a solid solution of TiO2 and Al2 O3 , while titania spinel, TiN, and MnS. The corundum phase was a solid solution of TiO2 and Al2O3, while the the spinel phase was composed predominantly of MgAl2 O4 . When the MgO nanoparticles mixed spinel phase was composed predominantly of MgAl2O4. When the MgO nanoparticles mixed with with AlTi nanoparticles were added to the experimental steel, MgO nanoparticles quickly dispersed AlTi nanoparticles were added to the experimental steel, MgO nanoparticles quickly dispersed within the melt, while Al and Ti dissolved in the liquid steel, which combined with dissolved oxygen within the melt, while Al and Ti dissolved in the liquid steel, which combined with dissolved oxygen to form corundum inclusions due to the strong deoxidizing element of Al and Ti. The strong stirring to form corundum inclusions due to the strong deoxidizing element of Al and Ti. The strong stirring effect within the steel in the mold facilitated the formation of MgO–Al2 O3 –TiO2 inclusions. As the effect within the steel in the mold facilitated the formation of MgO–Al2O3–TiO2 inclusions. As the precipitation temperature decreased to be less than the solidus temperature of the experimental steel, precipitation temperature decreased to be less than the solidus temperature of the experimental steel, MnS and TiN inclusions began forming on the surface of the oxide inclusions, and the Mn and Ti MnS and TiN inclusions began forming on the surface of the oxide inclusions, and the Mn and Ti in in the matrix-inclusion interfacial boundary layer were depleted, resulting in MDZ and transient the matrix-inclusion interfacial boundary layer were depleted, resulting in MDZ and transient zone zone formation near the inclusions [30,42]. Combining the above calculation results and Figure 6, formation near the inclusions [30,42]. Combining the above calculation results and Figure 6, the the evolution behavior of these inclusions provides stable nucleation sites for IAF at heat-treatment evolution behavior of these inclusions provides stable nucleation sites for IAF at heat-treatment temperatures from 973 to 823 K. temperatures from 973 to 823 K.

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TiN Spinel

Corundum Liquid inclusion

0.020 0.015 0.010

Liquidus temperature

MnS Titania spinel

Solidus temperature

Mass fraction(mass pct)

0.025

1212ofof1818

0.005 0.000 800

1000

1200

1400

Temperature(K)

1600

1800 1900

Figure 14. Phase transformation of inclusions in steel cooled from 1900 to 800 K. Figure 14. Phase transformation of inclusions in steel cooled from 1900 to 800 K.

In addition to the composition of the inclusions discussed above, the size of the inclusions In addition to the composition of the inclusions discussed above, the size of the inclusions during during the phase transformation was also an important factor affecting IAF. Therefore, the present the phase transformation was also an important factor affecting IAF. Therefore, the present work work developed a modified method referring to previous models to predict IAF nucleation [43,44]. developed a modified method referring to previous models to predict IAF nucleation [43,44]. The The calculation model for heterogeneous nucleation of IAF on the surface of inclusions was adopted as calculation model for heterogeneous nucleation of IAF on the surface of inclusions was∗adopted ∗ as illustrated in Figure 15. Regarding the relation between the normalized energy barrier (∆Ghet /∆G * * ) hom illustrated in Figure 15. Regarding the relation between ∗ ∗ the normalized energy barrier (ΔGhet/ΔGhom) for IAF nucleation, the smaller the value of ∆Ghet /∆G , the easier IAF formation on the surface * * hom for IAF nucleation, the smaller the value of ΔGhet/ΔGhom , the easier IAF formation on the surface of of inclusion [45]. Through a series of formula iterations, the inclusion size can be expressed as in inclusion [45]. Through a series of formula iterations, the inclusion size can be expressed as in Equations (1)–(4) [46,47]: Equations (1)–(4) [46,47]: 1 1 r 3r cosθθ r r 2 1 1 31 33 ∗ ∗* 33cos ηcos +3 η × ( × () ()23 (2 − −3 cos ψψ++cos ψ )) − ) −(1cos −ψcos (1) ∆Ghet /∆G = * + 1cos Δ Ghom + + ψ − ×× 3 cos cos ( ( )R2 (1 ) ψ) (1) het / Δ G2hom = 22 2 2 2R R R 22 r Rr

− cos θ

− cosθ 2 r 2 cosη = 1 − cosRθ + ( R − cos θ ) r 1 − cos12− θ +Rr (cos−θcosθ )2 cos ψ = q R cos η = q

1 − cos2 θ + ( Rr − cos θ )2

(2)

(2) (3)

r 12σ − γαcosθ (4) cosψ = rK = ∆GR (3) V r 2 2 1 − cos θ +surface ( − cosofθ )inclusion where rK is the critical radius of IAF formation on the (µm), σγα is the interfacial R 2

energy between the austenite and ferrite phases (J/m ), ∆GV is the driving force for ferrite formation ∗ ∗ 2σ γα (J/m3 ), ∆Ghet /∆Ghom is the ratio of the energy barrier rK = of homogeneous nucleation to the energy barrier (4) Δ G Vvariables (deg), r is the radius of IAF (µm), R is of homogeneous nucleation, ψ and η are defined angle the radius of the inclusion (µm), and θ is the contact angle between the ferrite and the surface of the where rK is the critical radius of IAF formation on the surface of inclusion (μm), σγα is the interfacial inclusion (deg). energy between the austenite and ferrite phases (J/m2), ΔGV is the driving force for ferrite formation * * (J/m3), ΔGhet/ΔGhom is the ratio of the energy barrier of homogeneous nucleation to the energy barrier of homogeneous nucleation, ψ and η are defined angle variables (deg), r is the radius of IAF (μm), R is the radius of the inclusion (μm), and θ is the contact angle between the ferrite and the surface of the inclusion (deg).


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Figure15. 15.Model Modelof of IAF IAF formation on Nucleation of of IAF on on the Figure on the the surface surfaceofofaaspherical sphericalinclusion. inclusion.(a)(a) Nucleation IAF surface of inclusion, andand (b)(b) thethe actual SEM image of IAF nucleated on on inclusion. the surface of inclusion, actual SEM image of IAF nucleated inclusion.

CombiningEquations Equations(1)–(3) (1)–(3)with withTable Table3,3, the the relationship relationship between between the the value value of of r/R r/Rand andthe the Combining energybarrier barriertotoheterogeneous heterogeneousnucleation nucleationwas wascalculated, calculated,as asshown shownininFigure Figure16. 16.ItItwas wasfound foundthat that energy ∗ ∗ * ∗ * * ∗ * ∆G /∆G of TiN, TiO , MgO, and MgAl O inclusions were less than 0.50, and the ∆G /∆G 2 4 ΔGhethet /ΔGhom hom of TiN, TiO2, MgO, and MgAl2O24 inclusions were less than 0.50, and the ΔGhethet /ΔGhom hom of ∗ * ∗ * of MgAl O4 inclusions smallest (about 0.16), while values of the /∆G of and MnS MgAl 2O4 2inclusions waswas the the smallest (about 0.16), while the the values of the ΔG∆G ofhom MnS het hom het/ΔG and 0.57 and respectively. Therefore, it can inferred that theTiN, TiN,TiO TiO2,2 , 2 O3 inclusions Al 2O3Al inclusions were were 0.57 and 0.85,0.85, respectively. Therefore, it can be be inferred that the MgO, and MgAl O inclusions in the steel were effective for IAF nucleation, while the Al O and 2 4 4inclusions in the steel were effective for IAF nucleation, while the Al22O33and MgO, and MgAl2O ∗* *∗ individualMnS MnS inclusions inclusions were were not. /∆Ghomcontinuously continuouslydecreased decreased individual not. In In addition, addition, the thevalue valueofof∆G ΔG het het/ΔGhom with decreasing the value of r/R and then tended toward a constant value. Hence, there wasan an with decreasing the value of r/R and then tended toward a constant value. Hence, there was optimal critical size for the inclusions inducing IAF nucleation. Judging from Figure 16, the critical optimal critical size for the inclusions inducing IAF nucleation. Judging from Figure 16, the critical valuer/R r/Rranged rangedfrom from0.01 0.01toto0.1. 0.1. value According to Equation (4) in JMatPro software, it clearly can be seen clearly seen V calculated According to Equation (4) andand ΔGV∆G calculated in JMatPro software, it can be (Figure (Figure the driving force (∆GV ) increased the temperature decreased, increased 17) that 17) the that driving force (ΔG V) increased as the as temperature decreased, whichwhich increased the the capability for IAF nucleation on inclusions. Moreover, critical radius IAFdecreased decreasedwith withthe the capability for IAF nucleation on inclusions. Moreover, thethe critical radius ofof IAF temperature decrease, and it was greatly affected by the value of σ . The critical radius of the IAF temperature decrease, and it was greatly affected by the value of σγαγα. The critical radius of the IAF variedbetween between0.003 0.003and and0.06 0.06μm µmatataatemperature temperatureranging rangingfrom from973 973toto823 823K. K.Therefore, Therefore,the thecritical critical varied radius of inclusions ranged from 0.3 to 0.6 µm, and the values corresponding to critical diameters radius of inclusions ranged from 0.3 to 0.6 μm, and the values corresponding to critical diameters ofof inclusionswere were0.6–1.2 0.6–1.2μm. µm.However, However,these thesecalculation calculationresults resultswere weresmaller smallerthan thanthe theactual actualinclusion inclusion inclusions sizeininthe thespecimens, specimens,which whichmay maybe bedue dueto tothese theseinclusions inclusionsbeing beinglarge largeenough enoughtotobecome becomeeffective effective size nucleation sites for IAF. nucleation sites for IAF. Table 3. The contact angle between the ferrite and the surface of the inclusion. Table 3. The contact angle between the ferrite and the surface of the inclusion. Inclusion Inclusion 2O AlAl 2O 33 MnS MnS TiN TiN TiO 2 2 TiO MgO MgO MgAl2 O4 MgAl2O4

Contact Angle Angle Contact 144 144 118 118 63 63 52 52 35 35 31 31

cosθ References References cosθ –0.809 [48][48] –0.809 –0.469 –0.469 [22][22] 0.450 0.450 [22][22] 0.616 0.616 [49][49] 0.819 [50] 0.819 [50] 0.857 [51] 0.857 [51]


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1.0 0.9 1.0

* ΔG*het/ΔG * hom ΔG*het/ΔG hom

0.8 0.9 0.7 0.8 0.6 0.7

Al2O3

0.5 0.6

Al2O3 MnS TiN MnS

0.4 0.5

TiO TiN 2 TiO MgO 2

0.3 0.4 0.2 0.3 0.1 0.2 0.0 0.1 10-3 0.0 10-3

10-2

10-1

10-2

10-1

100

r/R 100 r/R

MgAl MgO2O4 MgAl2O4 101 102

101

102

Figure 16. Effect of the value of r/R on energy barrier to ferrite at inclusions. Figure 16. Effect of the value of r/R on energy barrier to ferrite at inclusions. Figure 16. Effect of the value of r/R on energy barrier to ferrite at inclusions. 0.36

Critical radius of IAF(rK)(μm) Critical radius of IAF(rK)(μm)

0.36 0.32

Fe-0.09C-0.38Si-1.58Mn-0.04Nb-0.01Ti-0.025Al σγα=0.75[53] 0.32 0.28 [53] [53] σ=0.15 =0.75 σγα γα 0.28 0.24 [53] [47] σ=0.54 =0.15 σγα γα 0.24 0.20 ΔG σγα V=0.54[47] 0.20 0.16 ΔGV 0.16 0.12 0.12 0.08

0.04 0.00 700 0.00 700

-2250 -2500 -2000 -2250 -1750 -2000 -1500 -1750 -1250 -1500 -1000 -1250 -750 -1000 -500 -750

0.08 0.04

-250 -500 750 750

800 800

850

900

950

Temperature(K) 850 900 950

5 Gibbs free energy(ΔGV)(10 J/m3) 5 Gibbs free energy(ΔGV)(10 J/m3)

-2500

Fe-0.09C-0.38Si-1.58Mn-0.04Nb-0.01Ti-0.025Al

0 -250 1050 0 1000 1050

1000

Temperature(K) Figure 17. 17. Relationship Relationship between between the the driving driving force and and the the temperature temperature of Figure force of the the ferrite ferrite formation. formation. Figure 17. Relationship between the driving force and theoftemperature of the ferrite formation. 4.2. Refined Microstructure and Improved Mechanical Properties Steel 4.2. Refined Microstructure and Improved Mechanical Properties of Steel Combining the observations of the nucleation of IAF onSteel inclusions in the four specimens, 4.2. Combining Refined Microstructure and Improved the observations of the Mechanical nucleation Properties of IAF onofinclusions in the four specimens, the the refinement mechanism of the microstructure based on these findings is schematically illustrated in refinement mechanism of the microstructure based on findings is schematically illustrated in theheat-treatment observations of the nucleation of these IAF inclusions specimens, the FigureCombining 18. When the temperature was 973 K, it on tended to formina the largefour block of GBF and Figure 18. When the heat-treatment temperature was 973 K, it tended to form a large block of GBF refinement mechanism the microstructure based on these findingshad is schematically illustrated PF on the grain boundaryofbecause the prior austenite grain boundary a high interfacial energyin and PF on the grain boundary because the prior was austenite grain boundary hada large a highblock interfacial Figure 18. When the heat-treatment temperature 973 K, it tended to form GBF (Figure 9a). Byun et al. [45] reported that the inclusion surface was energetically less favorableofthan energy (Figure 9a). Byun et al. [45]because reportedthe thatprior the inclusion surface was energetically less favorable and PF on the grain boundary austenite grain boundary had a high interfacial ferrite nucleation on the prior austenite grain boundaries at high temperatures. The current study than ferrite nucleation onetthe prior austenite grain boundaries at high temperatures. The favorable current energy (Figure Byun al. is, [45] reported that theGBF inclusion surface was energetically results are similar9a). to those, that a large amount of nucleated on the grain boundary,less resulting in study results are similar on to those, thataustenite is, a largegrain amount of GBF nucleated on the grain The boundary, ferrite nucleation the prior boundaries atmicrostructure high temperatures. a than detrimental effect on toughness. As the temperature decreased, the varied fromcurrent GBF resulting in a detrimental effect onthat toughness. Asamount the temperature decreased, the grain microstructure study results are similar to those, is, a large of GBF nucleated on the boundary, and PF to IAF, and IAF began to nucleate at the boundary of the inclusion, while the IAF size at the varied frominGBF and PF to IAF, began toAs nucleate at the boundary of thethe inclusion, while resulting a detrimental effectand on IAF toughness. the to temperature decreased, beginning of nucleation was short. However, it is difficult avoid GBF formation alongmicrostructure the austenite the IAF size the beginning nucleation short. However, is difficult to of avoid formation varied fromatGBF and PF to of IAF, and factor IAFwas began to further nucleate atitthe boundary the GBF inclusion, while grain boundary, which is the limiting for the improvement of mechanical properties. along the austenite grain boundary, which is the limiting factor for the further improvement of the IAF at the beginning of nucleation was However, difficult to avoid GBFsurface formation When thesize temperature was decreased to 873 K, short. a larger numberitofisIAF nucleated at the of mechanical properties. When the temperature was decreased to 873 K, a larger number of IAF along the austenite grain which is MnS, the limiting for grew the further complex oxide inclusions or boundary, between oxides and and thenfactor the SIAF on the improvement IAF in differentof nucleated at the surface ofWhen complex oxide inclusions ordecreased between oxides and MnS, andnumber then the SIAF mechanical properties. the temperature was to 873 K, a larger IAF directions. Further, the inclusions promoted the formation of IAF plates within austenite grainsofand grew on the IAF in different directions. Further, the inclusions promoted the formation of IAF plates nucleatedinhibited at the surface complex oxide or between and grains MnS, and thefiner SIAF effectively GBF of formation, whichinclusions could divide the largeoxides austenite intothen many within austenite grains and directions. effectively Further, inhibited GBF formation, whichthe could divideof the large grew on the IAF in different the inclusions promoted formation IAF plates and separate regions (Figure 9c). The interlocking IAF formed during phase transformation could austenite grains intograins many and finer effectively and separate regions (Figure 9c). The interlocking IAF formedthe during within austenite inhibited GBF formation, which large not only effectively refine the grains to improve the toughness of the steel butcould coulddivide also effectively phase transformation could not only effectively refine the grains to improve the toughness of the steel austenite grains into many finer and separate regions (Figure 9c). The interlocking IAF formed during phase transformation could not only effectively refine the grains to improve the toughness of the steel


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prevent crack propagation [52], which is consistent with the results presented in Figure 12 in the current theeffectively temperature dropped 823 K, the inclusions could induce the IAF butstudy. couldAs also prevent cracktopropagation [52], whichstill is consistent withIAF, the but results presented in Figure 12 in the current study. As the temperature dropped to 823 K, the inclusions still plates appeared to be significantly coarse (Figure 9d). At the same time, a small amount of bainite was could induce IAF, but that the IAF appeared toof beaustenite significantly (Figure 9d). At the sameby time, also observed, indicating theplates transformation intocoarse ferrite had been replaced bainite. a small amount of bainite was also observed, indicating that the transformation of austenite into Based on the above discussion, it can be concluded that the optimal phase transformation temperature ferrite had been replaced by bainite. Based on the above discussion, it can be concluded that the of IAF in specimens was about 873 K, which could refine the microstructure and relatively improve optimal phase transformation temperature of IAF in specimens was about 873 K, which could refine mechanical properties. the microstructure and relatively improve mechanical properties.

18. Schematic illustration of of the mechanism of theofmicrostructure at different FigureFigure 18. Schematic illustration therefinement refinement mechanism the microstructure at heatdifferent treatment temperatures. heat-treatment temperatures.

5. Conclusions 5. Conclusions In this study, the characteristics and formation mechanisms of intragranular acicular ferrite in In this study, the characteristics and formation mechanisms of intragranular acicular ferrite in low-carbon steel were systematically investigated for different isothermal heat-treatment low-carbon steel were systematically investigated for different isothermal heat-treatment temperatures, temperatures, and its influence on mechanical properties was also clarified. The following and itsconclusions influence can on mechanical properties was also clarified. The following conclusions can be drawn. be drawn. 1. After adding MgO nanoparticles withAlTi AlTialloys alloystoto low-carbon steel, a large 1. After adding MgO nanoparticlespredispersed predispersed with low-carbon steel, a large amount of finely dispersed MgO–Al O –TiO inclusions were formed in steel, and MnS or 2 2O 3 3–TiO22 inclusions were formed in steel, and MnS or TiN TiN amount of finely dispersed MgO–Al inclusions precipitated on the surface ofof these whichcould could nucleation inclusions precipitated on the surface theseoxide oxideinclusions, inclusions, which actact as as nucleation sitessites of of intragranular acicular ferrite. intragranular acicular ferrite. By decreasing the heat-treatment temperature, the amount of boundary grain boundary and 2. By 2. decreasing the heat-treatment temperature, the amount of grain ferrite ferrite and polygonal polygonal ferrite was reduced, while the amount of intragranular acicular ferrite increased, ferrite was reduced, while the amount of intragranular acicular ferrite increased, and intragranularand acicular ferrite that nucleated on the inclusions gradually grew up. However, ferriteintragranular that nucleatedacicular on the inclusions gradually grew up. However, intragranular acicular ferrite appears intragranular acicular ferrite appears to be significantly coarsened and a small amount of bainite was to be significantly coarsened and a small amount of bainite was also observed with the further temperature also observed with the further temperature variations from 873 to 823 K. variations from 873 to 823 K. 3. The heat-treatment temperature had a slight effect on the strength of steel with MgO additions 3.but The heat-treatment temperature had aCharpy slight effect on the strength steel MgO additions had a significant effect on the average absorbed energy value,ofand thewith average Charpy but had a significant effectaton the average Charpy absorbed energy value, and impact the average Charpy absorbed energy value 873 K was the maximum value, which showed excellent properties absorbed energy value at 873 K was the maximum value, which showed excellent impact properties at at room temperatures. room Author temperatures. Contributions: Y.Z. conceived and designed the experiments and interpreted the data; Y.Z. and S.Y. wrote the paper; J.L. and A.D. analyzed the data and collected the literatures; and Y.Z. and W.L. performed the

Authorexperiments. Contributions: Y.Z. conceived and designed the experiments and interpreted the data; Y.Z. and S.Y. wrote the paper; J.L. and A.D. analyzed the data and collected the literatures; and Y.Z. and W.L. performed Funding: This research was supported by the National Natural Science Foundation of China (NSFC, No. the experiments. 51734003 and No. 51674023).

Funding: This research was supported by the National Natural Science Foundation of China (NSFC, No. 51734003 and No. 51674023). Acknowledgments: The authors gratefully acknowledge the support by the National Natural Science Foundation of China (NSFC, No. 51734003 and No. 51674023). Conflicts of Interest: The authors declare no conflict of interest.

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3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14.

15. 16. 17.

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