Microstructure Evolution During the Production of Dual Phase and Transformation Induced Plasticity Steels Using Modiﬁed Strip Casting Simulated in the Laboratory

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Article Microstructure Evolution During the Production of Dual Phase and Transformation Induced Plasticity Steels Using Modiﬁed Strip Casting Simulated in The Laboratory

Zhiping Xiong 1,2 , Andrii G. Kostryzhev 2, Yanjun Zhao 1,* and Elena V. Pereloma 2,3

1 Guangxi Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi University, Nanning 530004, China; [email protected] 2 School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of Wollongong, Wollongong NSW 2522, Australia; [email protected] (A.G.K.); [email protected] (E.V.P.) 3 Electron Microscopy Centre, University of Wollongong, Wollongong NSW 2519, Australia * Correspondence: [email protected]

Received: 16 February 2019; Accepted: 10 April 2019; Published: 16 April 2019

Abstract: Instead of conventional steel making and continuous casting followed by hot and cold rolling, strip casting technology modified with the addition of a continuous annealing stage (namely, modified strip casting) is a promising short-route for producing ferrite-martensite dual-phase (DP) and multi-phase transformation-induced plasticity (TRIP) steels. However, at present, the multi-phase steels are not manufactured by the modified strip casting, due to insufficient knowledge about phase transformations occurring during in-line heat treatment. This study analysed the phase transformations, particularly the formation of ferrite, bainite and martensite and the retention of austenite, in one 0.17C-1.52Si-1.61Mn-0.195Cr (wt. %) steel subjected to the modified strip casting simulated in the laboratory. Through the adjustment of temperature and holding time, the characteristic microstructures for DP and TRIP steels have been obtained. The DP steel showed comparable tensile properties with industrial DP 590 and the TRIP steel had a lower strength but a higher ductility than those industrially produced TRIP steels. The strength could be further enhanced by the application of deformation and/or the addition of alloying elements. This study indicates that the modified strip casting technology is a promising new route to produce steels with multi-phase microstructures in the future.

Keywords: strip casting; heat treatment; DP steel; TRIP steel; phase transformation

1. Introduction The strip casting technology allows to produce solid metal strips from liquid metals with minimal amount of deformation in one hot rolling stand continuously operating with a casting unite [1]. This technique has been applied in industry for the production of aluminium, lead, carbon steel, silicon steel and stainless steel [2–4]. In laboratory the ferrite-martensite dual-phase (DP) steel [5–7], multi-phase transformation-induced plasticity (TRIP) steel [8,9] and twinning-induced plasticity steel [10] have also been successfully produced by strip casting. In comparison with a traditional way to produce steels by the sequence of steel-making, continuous casting, hot rolling and cold rolling, the strip casting technology requires a shorter production line, due to the strips being directly manufactured from liquid metals. Therefore, it is an eﬃcient way to save energy, reduce CO2 emission and increase proﬁtability of a metallurgical enterprise with a minimal impact on environment [1,11].

Metals 2019, 9, 449; doi:10.3390/met9040449 www.mdpi.com/journal/metals Metals 2019, 9, 449 2 of 14

Typical DP steels contain soft ferrite and hard martensite and they are manufactured in industry by hot or cold rolling followed by annealing [12]. The presence of martensite leads to a continuous yielding behaviour and a moderate work hardening capability [12,13]. Strength and ductility can be adjusted via controlling the volume fraction of martensite. An increasing martensite fraction decreases the ductility but increases strength [14]. For example, when the martensite fraction is 40%, the ultimate tensile strength (UTS) is 660 MPa with a total elongation (TE) of 46.7%; when the martensite fraction is 50%, the UTS is 670 MPa while the TE is 45.0% [14]. After cold rolling, the inter critical annealing in the ferrite-austenite two-phase region can control the ferrite fraction and the austenite fraction. During cooling to room temperature, the austenite will transform to martensite. Thus, the martensite fraction can be controlled through the adjustment of annealing temperature and time. A higher inter critical annealing temperature and longer holding time lead to a larger austenite fraction followed by a larger fraction of martensite after cooling to room temperature [13,14]. Multi-phase TRIP steels consist of ferrite, bainitic ferrite, granular bainite and retained austenite (RA); sometimes a small amount of martensite may be present [15–17]. These steels attract attention of many investigators and researchers all over the word because of their combination of high strength and good ductility. The reason for TRIP steels have a good balance between strength and ductility originates from the presence of RA grains. During deformation, the RA grains transform to martensite (namely, the TRIP effect) leading to enhanced ductility prior to fracture. This transformation in homogeneously occurs in a range of strain levels. Strains required to initiate the austenite-to-martensite transformation vary with the austenite stability ascribed to the austenite grain morphology, carbon content and type of neighbouring phases [17–19]. The TRIP effect delays the development of the local stress concentration and enhances the strain hardening ability, which results in an increased ductility [17,20,21]. Thus, the control of amount and stability of RA grains is a key factor for the enhancement of mechanical properties in TRIP steels [22,23]. The first stage of heat treatment schedule applied to obtain a TRIP steel microstructure is the inter critical annealing in the ferrite-austenite two-phase region. The temperature and holding time in the two-phase region can control the fraction and grain size of ferrite [15,24]. The second stage of heat treatment induces the bainite transformation, during which austenite is enriched in carbon, partitioning from bainite. The fraction, morphology and stability of RA grains can be controlled by adjusting the bainite transformation temperature, which affects the amount and morphology of bainite [16,20,25]. Bainitic ferrite comprises film RA clamped between bainitic ferrite laths, while granular bainite includes blocky RA surrounded by the irregular-shaped ferrite [17]. During the final cooling to room temperature, the austenite which is stable enough can be retained, while the unstable austenite transforms to martensite. DP and TRIP steels are industrially produced by hot rolling or cold rolling followed by annealing [26]. In the automotive industry they have been successfully applied to manufacture various car parts such as B-pillar reinforcements, longitudinal beams and cross members [27]. Recent studies carried out by the present authors produced low-alloyed ferrite-martensite DP [8] and multi-phase TRIP steels [8] using strip casting simulated in the laboratory. Although tensile properties comparable with industrially hot/cold rolled DP and TRIP steels have been achieved, the microstructure is not uniform through strip thickness, due to the cooling rate gradient [8,9]. To avoid this problem, a furnace was introduced into the processing line of strip casting (Figure1) for the first time in this work, namely the modified strip casting. An interrupted cooling in the first zone of the furnace should result in formation of a ferrite-pearlite microstructure, instead of bainite and/or martensite obtained in the conventional strip casting due to fast cooling. This stage requires further researches on the acceleration of pearlite formation in the future in order to shorten the furnace in industry. Following this, the second step of heat treatment allowed to produce the homogeneous microstructures of DP and TRIP steels. The phase transformations occurring during the simulated heat treatments for the modified strip casting technology have been investigated and presented below (Figure1). Metals 2019, 9, 449 3 of 14 Metals 2019, 8, x FOR PEER REVIEW 3 of 14 Metals 2019, 8, x FOR PEER REVIEW 3 of 14

FigureFigure 1. 1.A A schematic schematic presentation presentation of of the the modified modiﬁed strip strip casting. casting. Figure 1. A schematic presentation of the modified strip casting. 2. Materials2. Materials and and Experimental Experimental Procedures Procedures 2. Materials and Experimental Procedures 2.1. Materials2.1. Materials 2.1. Materials 3 TheThe received received plates plates of of 36 36 ×36 36 × 1.2 mm mm3 werewere cast cast using using a dip a diptester tester (in‐house (in-house manufactured manufactured at × × at thethe University UniversityThe received of of Deakin, Deakin,plates of Victoria,Victoria, 36 × 36 × Australia), 1.2 mm3 were which which cast can using can simulate simulate a dip testerrapid rapid (insolidification‐house solidiﬁcation manufactured during during strip at strip castingthecasting [University28]. [28]. Thus, Thus, of the Deakin, the features features Victoria, of of the theAustralia), microstructuremicrostructure which can in in thesimulate the strip strip castingrapid casting solidification can can be simulated be simulated during in strip the in the laboratorycastinglaboratory [ 5[28].,8]. [5,8]. TheThus, The technical the technical features parameters parameters of the microstructure of of dip dip tester tester can canin bethe be found foundstrip incasting in Ref. Ref. [ [28].can28]. be The simulated steel steel studied studied in the in in this laboratory [5,8]. The technical parameters of dip tester can be found in Ref. [28]. The steel studied in workthis (contained work (contained 0.172) C, 1.520Si,0.172) C, 1.610Mn, 1.520Si, 0.195Cr1.610Mn, (wt. 0.195Cr %) and (wt. balanced %) and Fe.balanced The cast Fe. microstructure The cast this work (contained 0.172) C, 1.520Si, 1.610Mn, 0.195Cr (wt. %) and balanced Fe. The cast consistedmicrostructure of martensite consisted and of bainite martensite as shown and bainite in Figure as shown2. in Figure 2. microstructure consisted of martensite and bainite as shown in Figure 2.

Figure 2. As‐cast microstructure consisting of martensite and bainite observed by scanning electron FigureFiguremicroscopy 2. As-cast 2. As (SEM).‐cast microstructure microstructure consisting consisting ofof martensitemartensite and and bainite bainite observed observed by scanning by scanning electron electron microscopy (SEM). microscopy (SEM). 2.2. Processing Schedules 2.2. Processing Schedules 2.2. ProcessingThe processing Schedules routes to obtain DP and TRIP steels are illustrated in Figure 3a,b, respectively. The processing routes to obtain DP and TRIP steels are illustrated in Figure 3a,b, respectively. TheThey processing include two routes steps of to heat obtain treatments DP and in TRIP order steels to simulate are illustrated the modified in Figure strip 3castinga,b, respectively. in the They include two steps of heat treatments in order to simulate the modified strip casting in the Theylaboratory: include two (1) the steps first of step heat of heat treatments treatment in is order to produce to simulate a ferrite the‐pearlite modified microstructure strip casting and (2) in the laboratory:the second (1)step the of first heat step treatment of heat istreatment to produce is to microstructures produce a ferrite of‐ pearliteDP and microstructure TRIP steels. The and heat (2) laboratory: (1) the first step of heat treatment is to produce a ferrite-pearlite microstructure and (2) the thetreatment second was step carried of heat out treatment using a Theta is to Dilatronicproduce microstructures III dilatometer (Thetaof DP andInc., TRIPNew York,steels. NY, The USA) heat second step of heat treatment is to produce microstructures of DP and TRIP steels. The heat treatment treatmentunder a vacuum was carried of ~6.7 out × 10using−2 Pa. a Flat Theta samples Dilatronic of 14 III × 6dilatometer × 1 mm3 size (Theta were Inc., cut usingNew York,wire cutting. NY, USA) To was carriedundersimulate a vacuum out the usingmodified of ~6.7 a Theta strip × 10 casting,−2 Dilatronic Pa. Flat the samples samples III dilatometer of were 14 × 6heated × 1 (Theta mm at3 30size Inc.,Ks were−1 to New 1250cut using York, °C and wire NY, held cutting. USA) for 300 underTo s a 2 3 vacuumsimulateand ofthis ~6.7 ledthe tomodified10 an− averagePa. strip Flat graincasting, samples size the ofof samples 14prior6 austenite were1 mm heated (80size ± at27 were 30 μ m)Ks cut −being1 to using 1250 similar wire°C and cutting.to heldthat whichfor To 300 simulate is s × × × 1 the modifiedandusually this observed stripled to casting, an in average the the strip samplesgrain casting size were [8].of priorAfter heated austeniteholding, at 30 the Ks(80− samples± to27 μ 1250m) were ◦beingC and rapidly similar held cooled forto that 300 at which s~90 and Ks thisis−1 led −1 to anusuallyto average different observed grain holding size in thetemperatures of priorstrip casting austenite (T F[8].), held (80 After for 27holding, differentµm) being the times samples similar (tF) to were to study that rapidly the which ferrite cooled is usually and at ~90pearlite observedKs to different holding temperatures (TF), held ±for different times (tF) to study the ferrite1 and−1 pearlite in thetransformation strip casting [8during]. After the holding, modified the strip samples casting were and rapidly helium cooled quenched at ~90 at Ks ~140− to Ks diff erentto room holding transformationtemperature. To during simulate the modifiedthe production strip castingof DP andsteels, helium the quenchedsamples withat ~140 a ferriteKs−1 to‐pearlite room temperatures (TF), held for different times (tF) to study the ferrite and pearlite transformation during temperature.microstructure To were simulate heated atthe 30 productionKs−1 to a selected of DP temperature steels, 1the of samples TIA (750 andwith 780 a °C)ferrite in the‐pearlite two‐ the modified strip casting and helium−1 quenched at ~140 Ks− to room temperature. To simulate the microstructurephase region, held were for heated a selected at 30 timeKs periodto a selected of tIA (120,temperature 180, 240 ofand TIA 300 (750 s) andto allow 780 °C)formation in the two of a‐ production of DP steels, the samples with a ferrite-pearlite microstructure were heated at 30 Ks 1 to a phase region, held for a selected time period of tIA (120, 180, 240 and 300 s) to allow formation of −a selected temperature of TIA (750 and 780 ◦C) in the two-phase region, held for a selected time period of tIA (120, 180, 240 and 300 s) to allow formation of a certain volume fraction of austenite and helium quenched to room temperature to obtain martensite as the second phase. To simulate the production Metals 2019Metals, 9 2019, 449, 8, x FOR PEER REVIEW 4 of 144 of 14

certain volume fraction of austenite and helium quenched to room temperature to obtain martensite as the second phase. To simulate the production of TRIP steels, the samples with ferrite‐pearlite of TRIP steels, the samples with ferrite-pearlite microstructure were heated at 750 ◦C for 240 s, which microstructure were heated at 750 °C for 240 s, which resulted in formation of ~50% ferrite and ~50%1 resulted in formation of ~50% ferrite and ~50% austenite and then quickly cooled at ~50 Ks− to the austenite and then quickly cooled at ~50 Ks−1 to the isothermal bainite transformation temperature of isothermal bainite transformation temperature of TIBT (400, 450 and 500 ◦C), held at this temperature TIBT (400, 450 and 500 °C), held at this temperature for 900 s (15 min) to simulate coiling and finally 1 for 900helium s (15 min)quenched to simulate at ~140 Ks coiling−1 to room and temperature. ﬁnally helium quenched at ~140 Ks− to room temperature.

FigureFigure 3. Schemes 3. Schemes of processing of processing schedules schedules applied applied toto simulate the the production production of ( ofa) (Duala) Dual Phase Phase (DP) (DP) and (band) Transformation-Induced (b) Transformation‐Induced Plasticity Plasticity (TRIP) (TRIP) steels steels usingusing the modified modiﬁed strip strip casting. casting.

2.3. Microstructure2.3. Microstructure Characterization Characterization The heat-treatedThe heat‐treated samples samples were were cut cut through through thickness,thickness, mounted, mounted, mechanically mechanically polished polished and and electropolishedelectropolished using using an an electrolyte electrolyte containing containing 330 330 mL mL methanol, methanol, 330 mL 330 butoxyethanol mL butoxyethanol and 40 mL and 40 mL perchloricperchloric acid.acid. Then, the the samples samples were were etched etched using using 2 2vol vol % %nital. nital. The The microstructures microstructures were were characterized using a Leica DMR research optical microscope (OM) (Leica, Wetzlar, Germany) and a characterized using a Leica DMR research optical microscope (OM) (Leica, Wetzlar, Germany) and JEOL JSM‐7001F field emission gun scanning electron microscope (FEG SEM) (Jeol, Tokyo, Japan) a JEOLoperating JSM-7001F at an field accelerating emission voltage gun scanning of 15 kV. On electron the basis microscope of the pixel (FEG quantities SEM) (Jeol,of different Tokyo, grey Japan) operatingscales at in an the accelerating micrographs, voltage the ferrite of 15 fraction kV. On was the basiscalculated of the using pixel ImageJ quantities (Image of‐ diProfferent Plus, greyMedia scales in theCybernetics micrographs, Inc., the Rockville, ferrite fraction MD, USA). was calculatedIn order to using distinguish ImageJ between (Image-Pro austenite, Plus, Mediamartensite Cybernetics and Inc., Rockville,bainite, colour MD, etching USA). Inwas order employed to distinguish according between to Ref. austenite, [28]. A PANalyticalX’pert martensite and bainite,‐Pro MRD colour etchinggoniometer was employed (Malvern according Panalytical, to Ref. Eindhoven, [28]. A PANalyticalX’pert-ProNetherlands), equipped MRDwith goniometerNi‐filtered Cu (Malvern Kα Panalytical,radiation Eindhoven, source scanning Netherlands), over a range equipped of 2θ = 60–110°, with was Ni-filtered used to measure Cu Kα radiationthe RA fraction. source The scanning RA fraction was calculated using the direct comparison method; the details are presented in Ref. [8]. over a range of 2θ = 60–110◦, was used to measure the RA fraction. The RA fraction was calculated using the direct comparison method; the details are presented in Ref. [8]. 2.4. Mechanical Properties 2.4. MechanicalThe dog Properties‐bone tensile samples (with a gauge length of 4.9 mm, width of 2.1 mm and thickness of 1 mm) were cut from the heat‐treated samples. The uniaxial tensile testing was carried out using an The dog-bone tensile samples (with a gauge length of 4.9 mm, width of 2.1 mm and thickness of 1 in‐house modified Kammrath and Weiss GmbH tensile stage (Kammrath & Weiss GmbH, Dortmund, mm) wereGermany) cut fromat a constant the heat-treated cross‐head samples.speed of 2 μ Them/s, uniaxial which resulted tensile in testing an initial was strain carried rate of out 4 × using 10‐4 an in-houses−1. modified Kammrath and Weiss GmbH tensile stage (Kammrath & Weiss GmbH, Dortmund, 4 1 Germany) at a constant cross-head speed of 2 µm/s, which resulted in an initial strain rate of 4 10− s− . 3. Results × 3. Results 3.1. The Formation of Ferrite and Pearlite Microstructure during the First Step of Simulated Heat 3.1. TheTreatments Formation in the of Ferrite Modified and Strip Pearlite Casting Microstructure during the First Step of Simulated Heat Treatments in the Modified Strip Casting The microstructure evolution with holding temperature (TF) and time (tF) (Figure 3) is shown in TheFigure microstructure 4 and the corresponding evolution withfractions holding of ferrite temperature are listed (inT FTable) and 1. time Figure (tF )3a (Figure was observed3) is shown by in Figure 4 and the corresponding fractions of ferrite are listed in Table1. Figure3a was observed by OM where black is pearlite, white is ferrite and grey is martensite. Figure3b–f were obtained by SEM where dark grey is ferrite, light grey is martensite and white is pearlite. With an increase in holding temperature from 620 to 650 ◦C, for a constant holding time of 180 s, the fraction of ferrite increased Metals 2019, 9, 449 5 of 14 from 0.14 0.02 to 0.46 0.02 (Figure4a,d,f) due to a shortened incubation time for ferrite nucleation ± ± and a correspondingly increased time for ferrite growth. With increasing the holding temperature up to 670 ◦C the fraction of ferrite decreased (as listed in Table1 for the holding time of 60 or 300 s) due to an increased incubation time for ferrite nucleation. It means that the nose temperature (namely, having shortest incubation time for phase transformation) of ferrite transformation field of the continuous phase transformation diagram is 650 ◦C. When the holding temperature is 650 ◦C, the ferrite fraction increases with an increased holding time from 60 to 180 s (Table1) due to an increased time for ferrite nucleation and growth. When the holding time was 180 s, a minor fraction of pearlite was observed at 620 ◦C (Figure4a). A larger fraction of pearlite was obtained after holding at 630 ◦C (Figure4c,d)due to a shortened time for pearlite nucleation. Whereas, only little pearlite was observed at a higher temperature of 650 ◦C (Figure4e,f) due to an increased time for pearlite nucleation. Therefore, the nose temperatureMetals 2019 of, 8, pearlitex FOR PEER transformation REVIEW field was 630 ◦C. 6 of 14

Figure 4. (a) Optical microscopy (OM) and (b–f) SEM microstructures after holding at (a) 620 °C for Figure 4. (a) Optical microscopy (OM) and (b–f) SEM microstructures after holding at (a) 620 C for 180 s, (b) 670 °C for 300 s, (c,d) 630 °C for 180 s and (e,f) 650 °C for 180 s. F is ferrite, P is pearlite and◦ 180 s, (b) 670 C for 300 s, (c,d) 630 C for 180 s and (e,f) 650 C for 180 s. F is ferrite, P is pearlite and M M is martensite.◦ ◦ ◦ is martensite. Table 1. The fraction of ferrite as a function of holding temperature and time.

Time 620 °C 630 °C 650 °C 670 °C 60 s ‐ ‐ 0.08 ± 0.02 0.02 ± 0.001 180 s 0.14 ± 0.02 0.33 ± 0.02 0.46 ± 0.02 ‐ 300 s ‐ 0.46 ± 0.04 0.56 ± 0.02 0.50 ± 0.03

Since the nose temperatures of ferrite and pearlite transformation field were 650 and 630 °C, respectively, the holding for ferrite and pearlite formation was conducted between these two temperatures, namely 635 °C. This was supposed to enhance the austenite decomposition. Figure 5 shows that the corresponding microstructure consists of ferrite (0.75 ± 0.02) and pearlite. This microstructure was suitable for further processing to produce DP and TRIP steels because ferrite is a

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Table 1. The fraction of ferrite as a function of holding temperature and time.

Time 620 ◦C 630 ◦C 650 ◦C 670 ◦C 60 s - - 0.08 0.02 0.02 0.001 ± ± 180 s 0.14 0.02 0.33 0.02 0.46 0.02 - ± ± ± 300 s - 0.46 0.04 0.56 0.02 0.50 0.03 ± ± ±

Since the nose temperatures of ferrite and pearlite transformation ﬁeld were 650 and 630 ◦C, respectively, the holding for ferrite and pearlite formation was conducted between these two temperatures, namely 635 ◦C. This was supposed to enhance the austenite decomposition. Figure5 shows that the corresponding microstructure consists of ferrite (0.75 0.02) and pearlite. Metals 2019, 8, x FOR PEER REVIEW ± 7 of 14 This microstructure was suitable for further processing to produce DP and TRIP steels because ferrite is amicrostructural microstructural constituent constituent for for both both of ofthem them and and austenite austenite prefers prefers to tonucleate nucleate at atthe the pearlite pearlite with with a a largelarge driving driving force. force.

FigureFigure 5. 5.Microstructures Microstructures after after austenitizing austenitizing for for 300 300 s at s at 1250 1250◦C °C and and holding holding at 635at 635◦C °C for for 900 900 s (15 s (15 min): (a)min): OM imaging(a) OM imaging and (b) and SEM (b imaging.) SEM imaging.F is ferrite F is ferrite and P andis pearlite. P is pearlite.

3.2.3.2. Production Production of of DP DP Steel Steel Microstructure Microstructure duringduring the Second Step Step of of Simulated Simulated Heat Heat Treatments Treatments in in the the ModifiedModified Strip Strip Casting Casting FollowingFollowing the the first first step step of of simulated simulated heat heat treatments treatments in in the the modified modified strip strip casting casting in in Section Section 3.1 3.1,, the secondthe second step of step simulated of simulated heat heat treatments treatments was was investigated investigated for for the the production production of of DP DP steels steels (Figure (Figure3a). Figure3a). 6Figure shows 6 theshows microstructures the microstructures after inter after critical intercritical annealing annealing at di ff erentat different temperatures temperatures for diff forerent timesdifferent within times the two-phase within the region two‐phase followed region by directfollowed quenching by direct to roomquenching temperature. to room Quenching temperature. after theQuenching inter critical after annealing the intercritical led to theannealing formation led to of the microstructures formation of microstructures consisting of ferrite consisting and of martensite ferrite (Figureand 6martensite). When the (Figure inter critical6). When annealing the intercritical temperature annealing was 750 temperature◦C, the ferrite was fraction 750 °C, decreased the ferrite from 0.79fraction0.01 decreased to 0.42 0.02 from with 0.79 the ± 0.01 increase to 0.42 of ± the 0.02 inter with critical the increase annealing of the time intercritical from 120 toannealing 300 s (Figure time 7). from± 120 to 300± s (Figure 7). When the intercritical annealing time was 300 s, a higher intercritical When the inter critical annealing time was 300 s, a higher inter critical annealing temperature of 780 ◦C ledannealing to a lower temperature ferrite fraction of 780 of °C 0.09 led to0.02 a lower (Figure ferrite6d). fraction of 0.09 ± 0.02 (Figure 6d). With an increased intercritical± annealing time in the austenite‐ferrite temperature region the With an increased inter critical annealing time in the austenite-ferrite temperature region the austenite grains continued to nucleate and grow and this resulted in a lower ferrite fraction (Figure austenite grains continued to nucleate and grow and this resulted in a lower ferrite fraction (Figure7). 7). Similarly, a higher temperature led to a larger driving force for austenite formation resulting in a Similarly, a higher temperature led to a larger driving force for austenite formation resulting in a less less amount of ferrite. Thus, through the control of intercritical annealing temperature and time amount of ferrite. Thus, through the control of inter critical annealing temperature and time during the during the intercritical annealing, DP steels having different fractions of ferrite and martensite can be inter critical annealing, DP steels having different fractions of ferrite and martensite can be produced. produced.

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Figure 6. OM microstructures after intercritical annealing at (a) 750 °C for 120 s, (b) 750 °C for 180 s, (c) 750 °C for 300 s and (d) 780 °C for 300 s. White is ferrite (F) while grey is martensite. Figure 6. OM microstructures after intercritical annealing at (a) 750 °C for 120 s, (b) 750 °C for 180 s, Figure 6. OM microstructures after inter critical annealing at (a) 750 ◦C for 120 s, (b) 750 ◦C for 180 s, (c) 750 °C for 300 s and (d) 780 °C for 300 s. White is ferrite (F) while grey is martensite. (c) 750 ◦C for 300 s and (d) 780 ◦C for 300 s. White is ferrite (F) while grey is martensite.

Figure 7. Effect of temperature and time on ferrite fraction during the intercritical annealing. Figure 7. Eﬀect of temperature and time on ferrite fraction during the inter critical annealing. 3.3.Figure Production 7. Effect of TRIP of temperature Steel Microstructure and time during on theferrite Second fraction Step of duringSimulated the Heat intercritical Treatments annealing. in the Modified Strip Casting 3.3. Production of TRIP Steel Microstructure during the Second Step of Simulated Heat Treatments in the Following the first step of simulated heat treatments in the modified strip casting in Section 3.1, Modifiedthe Strip second Casting step of simulated heat treatments has been investigated for the production of TRIP steels (Figure 3b). Based on the published data, ~50% ferrite fraction provides the optimal combination of Following the first step of simulated heat treatments in the modified strip casting in Section 3.1, the second step of simulated heat treatments has been investigated for the production of TRIP steels (Figure 3b). Based on the published data, ~50% ferrite fraction provides the optimal combination of

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3.3. Production of TRIP Steel Microstructure during the Second Step of Simulated Heat Treatments in the Modified Strip Casting Following the first step of simulated heat treatments in the modified strip casting in Section 3.1, theMetals second 2019, 8 step, x FOR of PEER simulated REVIEW heat treatments has been investigated for the production of TRIP9 steelsof 14 (Figure3b). Based on the published data, ~50% ferrite fraction provides the optimal combination of UTSUTS and and TE TE in in the the multi-phase multi‐phase TRIPTRIP steelssteels [[15,25].15,25]. According to Section 3.2,3.2, 0.51 0.51 ± 0.060.06 ferrite ferrite was was ± achievedachieved after after inter intercritical critical annealing at at 750 750 °C◦C for for 240 240 s s (Figure (Figure 88),), thisthis conditioncondition was selected for for furtherfurther processing processing to to obtain obtain bainite bainite andand RARA requiredrequired forfor TRIPTRIP steels.

Figure 8. Microstructures after intercritical annealing at 750 °C for 240 s followed by quenching to Figure 8. Microstructures after inter critical annealing at 750 ◦C for 240 s followed by quenching to roomroom temperature: temperature: ( (aa)) OM OM imagingimaging andand ((b)) SEM imaging. FF isis ferrite ferrite and and MM isis martensite. martensite.

TheThe isothermal isothermal bainitebainite transformationtransformation (IBT) was studied by by holding holding at at 400, 400, 450 450 and and 500 500 °C◦C for for 900900 s s (15(15 min). The The representative representative and and homogenous homogenous microstructures microstructures are areshown shown in Figure in Figure 9 (OM)9 (OM) and andFigure Figure 10 10(SEM), (SEM), indicating indicating the the successful successful production production of oftypical typical TRIP TRIP steel steel microstructures microstructures [26]. [26 ]. FigureFigure9b,d,f 9b,d,f show show the the ferrite ferrite in in grey grey/white,/white, bainite bainite in in dark dark grey grey/bluish,/bluish, martensite martensite in in brown brown and and RA RA in greyin grey/white./white. After After holding holding at 400 at◦ C,400 the °C, microstructure the microstructure consisted consisted of ferrite, of bainite, ferrite, RA bainite, and martensite RA and (Figuremartensite9a,b). (Figure As seen 9a,b). in FigureAs seen 10 ina, Figure bainite 10a, included bainite bainitic included ferrite, bainitic where ferrite, ﬁlm-shaped where film retained‐shaped austeniteretained /austenite/martensitemartensite constituent constituent (RA/M) was(RA/M) clamped was clamped by ferritic by lathsferritic and laths granular and granular bainite, bainite, where globularwhere globular RA/M was RA/M surrounded was surrounded by irregular-shaped by irregular ferrite.‐shaped After ferrite. holding After at a higherholding temperature at a higher of temperature of 450 °C the martensite fraction increased (Figures 9d and 10b). The increasing of 450 ◦C the martensite fraction increased (Figures9d and 10b). The increasing of holding temperature up holding temperature up to 500 °C slightly decreased the martensite fraction (Figure 9f) and promoted to 500 ◦C slightly decreased the martensite fraction (Figure9f) and promoted some pearlite formation (Figuresome pearlite 10c,d). formation It is well known (Figure that 10c,d). pearlite It is iswell harmful known to that the TRIPpearlite steels; is harmful pearlite to consumes the TRIP carbonsteels; andpearlite reduces consumes the carbon carbon content and reduces in austenite. the carbon A decreased content in carbon austenite. content A decreased in austenite carbon decreases content thein austeniteaustenite stability decreases and the the fractionaustenite of stability RA in the and room the temperature fraction of microstructure RA in the room [29,30 temperature]. Due to the microstructure [29,30]. Due to the presence of pearlite after holding at 500 °C this heat treatment presence of pearlite after holding at 500 ◦C this heat treatment condition was regarded as inappropriate condition was regarded as inappropriate for the TRIP steel production and thus the RA fraction was for the TRIP steel production and thus the RA fraction was measured only for the samples held at 400 measured only for the samples held at 400 and 450 °C (Figure 11). Based on the direct comparison and 450 ◦C (Figure 11). Based on the direct comparison method, the RA fraction was measured to be method, the RA fraction was measured to be 5.2% and 4.2% after holding at 400 and 450 °C, 5.2% and 4.2% after holding at 400 and 450 ◦C, respectively. respectively. The critical driving force (Gcritical) for bainite transformation can be expressed by the following equation [31]:

𝐺 𝐴𝑇𝐵 (1) where A and B are positive constants related to the steel chemical compositions [32,33] and T is the temperature. When the temperature decreased from ferrite‐austenite two‐phase region to bainite transformation region, the bainite formed during the holding and the carbon diffused from bainitic laths to austenite. Increasing the IBT temperature from 400 to 450 °C led to an increase in Gcritical. This indicates that the transformation from austenite to bainite becomes more difficult with an increase in temperature, leading to a lower fraction of bainite. For a lower fraction of bainite formation, the content of carbon partitioning from bainitic ferrite laths into austenite was lower and thus the austenite became less enriched in carbon. Therefore, a less stable austenite transformed to martensite

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Metalsduring2019 quenching, 9, 449 from 450 °C (c.f. Figure 9b,d) and a lower RA fraction was obtained. Similar results9 of 14 were reported in Ref. [34].

FigureFigure 9.9. OpticalOptical microstructuresmicrostructures afterafter holdingholding atat ((aa,,bb)) 400400 ◦°C,C, ((cc,,dd)) 450450 ◦°CC andand ((ee,,ff)) 500500 ◦°CC forfor 900900 ss (15(15 min)min) followingfollowing holdingholding atat 750750 ◦°CC forfor 240240 s:s: (a,,c,,e)) nital and ((b,,d,,ff)) colourcolour etched microstructures. FerriteFerrite isis shownshown withwith greygrey/white,/white, bainitebainite withwith darkdark greygrey/bluish,/bluish, retainedretained austeniteaustenite withwith greygrey/white/white andand martensitemartensite withwith brownbrown colourscolours inin ((bb,d,d,f,f).). F isis ferrite,ferrite, B isis bainite,bainite, MM isis martensitemartensite andand RARA isis retainedretained austenite.austenite.

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FigureFigure 10. Microstructures 10. Microstructures after after holding holding at at (a ()a )400 400 °C, °C, ((bb)) 450 °C andand ( c(,cd,d) )500 500 °C °C for for 900 900 s (15 s (15min) min) Figure 10. Microstructures after holding at (a) 400 ◦C, (b) 450 ◦C and (c,d) 500 ◦C for 900 s (15 min) following holding at 750 °C for 240 s. F is ferrite, BF is bainitic ferrite, GB is granular bainite, RA/M is followingfollowing holding holding at 750 at 750 °C ◦forC for 240 240 s. F s. isF isferrite, ferrite, BFBF isis bainitic bainitic ferrite, GB GBis is granular granular bainite, bainite,RA /RA/MM is is retainedretainedretained austenite/martensite austeniteaustenite/martensite/martensite constituent, constituent,constituent, M MM is isis martensite martensite martensite and andandP Pis isis pearlite. pearlite.pearlite.

Figure 11. X‐ray diffractograms after holding at 400 and 450 °C for 900 s (15 min).

3.4. TensileFigure PropertiesFigure 11. X 11. ‐rayX-ray diffractograms diﬀractograms after after holding holding at 400 andand 450 450◦ C°C for for 900 900 s (15 s (15 min). min). Ferrite‐martensite DP steels and multi‐phase TRIP steels have been successfully produced in the 3.4. Tensile Properties laboratory using the modified strip casting technology proposed in the above sections. Figure 12 Ferriteshows ‐themartensite engineering DP stress steels‐engineering and multi‐ phasestrain curves TRIP steelsfor a DP have steel been that successfullyhas a ferrite fraction produced of 0.69 in the laboratory± 0.02 (designatedusing the modified as DP) and strip TRIP casting steels held technology at 400 and proposed 450 °C (designated in the above as TC sections. 400 and TC Figure 450, 12 showsrespectively). the engineering The curvesstress‐ engineeringindicate a typical strain continuous curves for yielding a DP steel behaviour that has observed a ferrite in fraction the multi of ‐0.69

± 0.02 (designated as DP) and TRIP steels held at 400 and 450 °C (designated as TC 400 and TC 450, respectively). The curves indicate a typical continuous yielding behaviour observed in the multi‐

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The critical driving force (Gcritical) for bainite transformation can be expressed by the following equation [31]: G = AT B (1) critical − where A and B are positive constants related to the steel chemical compositions [32,33] and T is the temperature. When the temperature decreased from ferrite-austenite two-phase region to bainite transformation region, the bainite formed during the holding and the carbon diﬀused from bainitic laths to austenite. Increasing the IBT temperature from 400 to 450 ◦C led to an increase in Gcritical. This indicates that the transformation from austenite to bainite becomes more diﬃcult with an increase in temperature, leading to a lower fraction of bainite. For a lower fraction of bainite formation, the content of carbon partitioning from bainitic ferrite laths into austenite was lower and thus the austenite became less enriched in carbon. Therefore, a less stable austenite transformed to martensite during quenching from 450 ◦C (c.f. Figure9b,d) and a lower RA fraction was obtained. Similar results were reported in Ref. [34].

3.4. Tensile Properties Ferrite-martensite DP steels and multi-phase TRIP steels have been successfully produced in the laboratory using the modiﬁed strip casting technology proposed in the above sections. Figure 12 Metalsshows 2019 the, 8, engineering x FOR PEER REVIEW stress-engineering strain curves for a DP steel that has a ferrite fraction12 of of 0.69 14 0.02 (designated as DP) and TRIP steels held at 400 and 450 C (designated as TC 400 and TC 450, ± ◦ phaserespectively). steels [35]. The The curves DP indicatesteel showed a typical a higher continuous yield strength yielding behaviour(YS) of 350 observed MPa, a higher in the multi-phaseUTS of 589 MPasteels and [35 ].a lower The DP TE steel of 22.4% showed than a higherthe TRIP yield steels. strength The TRIP (YS) steel of 350 held MPa, at a400 higher °C exhibited UTS of 589 a lower MPa UTSand aof lower 541 MPa TE of and 22.4% a larger than TE the of TRIP 33%, steels. while The the TRIP steel held at 400450 ◦°CC exhibitedhad a higher a lower UTS UTSof 568 of MPa541 MPa and a and lower a larger TE of TE 31%. of 33%,These while two TRIP the TRIP steels steel have held similar at 450 YS ◦ofC 332 had (TC a higher 400) and UTS 328 of (TC 568 MPa450) MPa,and a respectively. lower TE of 31%.The slightly These two larger TRIP TE steels in the have sample similar held YS at of 400 332 °C (TC was 400) probably and 328 (TCascribed 450) MPa,to a higherrespectively. fraction The of RA slightly for this larger processing TE in the condition sample held(5.2% at vs. 400 4.2%),◦C was leading probably to a stronger ascribed TRIP to a higher effect [8,36].fraction of RA for this processing condition (5.2% vs. 4.2%), leading to a stronger TRIP eﬀect [8,36].

FigureFigure 12.12. (a(a)) Engineering Engineering stress-engineering stress‐engineering strain strain curves curves and ( band) comparison (b) comparison of mechanical of mechanical properties propertiesfor the studied for the steels studied and DPsteels and and TRIP DP steels and conventionallyTRIP steels conventionally cold/hot rolled cold/hot in the steelrolled industry in the steel [37]. industry [37]. The tensile properties obtained in the present study are comparable to the properties obtained in industriallyThe tensile cold properties rolled DP obtained and TRIP in steels the present (Figure study 12b). are The comparable studied DP to steel the showed properties a little obtained lower inUTS industrially than DP 590,cold however,rolled DP togetherand TRIP with steels a (Figure little larger 12b). TE. The Obviously, studied DP for steel the showed studied a TRIPlittle lower steels, UTSthe TE than is largeDP 590, enough, however, whereas together the UTSwith isa muchlittle larger lower TE. in comparisonObviously, for with the TRIP studied 690. TRIP Although steels, a thelower TE strengthis large enough, is obtained whereas in the the present UTS study,is much the lower results in indicatecomparison a feasibility with TRIP to produce690. Although DP and a lowerTRIP steelsstrength using is obtained the modiﬁed in the strip present casting. study, Recent the results studies indicate have showna feasibility that the to produce improvement DP and of TRIPtensile steels properties using canthe alsomodified be achieved strip casting. through Recent the deformation studies have in shown the austenite that the temperature improvement region of tensilefollowed properties by accelerated can also cooling be achieved and warm through deformation the deformation in the ferrite in the temperature austenite temperature region, leading region to followed by accelerated cooling and warm deformation in the ferrite temperature region, leading to finer microstructures [38]. In addition, the microalloying with niobium, vanadium and molybdenum can also improve the tensile strength via precipitation and solid solution strengthening [39,40]. Thus, further research on the enhancement of tensile properties in DP and TRIP steels produced by the modified strip casting can be dedicated to the investigation of effects of thermo‐mechanical control process and alloying.

4. Conclusions The modified strip casting with the addition of a continuous annealing stage was shown to be a promising technology for producing ferrite‐martensite DP and multi‐phase TRIP steels. The initial ferrite‐pearlite microstructure was obtained via the adjustment of holding temperature during the first step of heat treatments of the modified strip casting. Following this, to produce DP steels, various temperatures and times of holding during intercritical annealing can result in different fractions of ferrite and martensite; to produce TRIP steels, at first ~50% ferrite should be obtained during intercritical annealing and bainite and retained austenite, together with a small amount of martensite, can be formed as a result of isothermal bainite transformation followed by cooling to room temperature. The tensile strength was lower but the ductility was higher, in the studied steels than those in the conventionally produced DP and TRIP steels. Further enhancement of mechanical properties in strip cast steels can be achieved via the introduction of thermo‐mechanical control process and microalloying, leading to the strengthening from grain refinement and precipitation. These require

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finer microstructures [38]. In addition, the microalloying with niobium, vanadium and molybdenum can also improve the tensile strength via precipitation and solid solution strengthening [39,40]. Thus, further research on the enhancement of tensile properties in DP and TRIP steels produced by the modified strip casting can be dedicated to the investigation of effects of thermo-mechanical control process and alloying.

4. Conclusions The modified strip casting with the addition of a continuous annealing stage was shown to be a promising technology for producing ferrite-martensite DP and multi-phase TRIP steels. The initial ferrite-pearlite microstructure was obtained via the adjustment of holding temperature during the first step of heat treatments of the modified strip casting. Following this, to produce DP steels, various temperatures and times of holding during inter critical annealing can result in different fractions of ferrite and martensite; to produce TRIP steels, at first ~50% ferrite should be obtained during inter critical annealing and bainite and retained austenite, together with a small amount of martensite, can be formed as a result of isothermal bainite transformation followed by cooling to room temperature. The tensile strength was lower but the ductility was higher, in the studied steels than those in the conventionally produced DP and TRIP steels. Further enhancement of mechanical properties in strip cast steels can be achieved via the introduction of thermo-mechanical control process and microalloying, leading to the strengthening from grain refinement and precipitation. These require further investigation. The results presented in this work indicate a strong feasibility to produce homogenous microstructures of DP and TRIP steels in industry using the novel technology of the modified strip casting.

Author Contributions: E.V.P. and A.G.K. conceived the idea. Z.X. carried out the experiments. Y.Z. wrote the draft. All the authors reviewed this paper. Funding: The JEOL JSM-7001F FEG-SEM at the Electron Microscopy Centre of University of Wollongong (Australia) was funded by the Australian Research Council grant LE0882613. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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