On the Effect of Filling Rate on Positive Macrosegregation Patterns in Large Size Cast Steel Ingots

applied sciences

Article On the Effect of Filling Rate on Positive Macrosegregation Patterns in Large Size Cast Steel Ingots

Chunping Zhang 1,* , Abdelhalim Loucif 1, Mohammad Jahazi 1,*, Rami Tremblay 2 and Louis-Philippe Lapierre 2 1 Department of Mechanical Engineering, École de Technologie Supérieure, 1100 Notre-Dame Street West, Montreal, QC H3C 1K3, Canada; [email protected] 2 Finkl Steel-Sorel (Sorel Forge Co.), 100 Rue McCarthy, Saint-Joseph-de-Sorel, QC J3R 3M8, Canada; [email protected] (R.T.); lplapierre@ﬁnkl.com (L.-P.L.) * Correspondence: [email protected] (C.Z.); [email protected] (M.J.); Tel.: +1-514-396-8800-7868 (C.Z.); +1-514-396-8974 (M.J.) Received: 13 September 2018; Accepted: 3 October 2018; Published: 11 October 2018

Abstract: The effect of filling velocity on positive macrosegregation in large size steel ingots was studied. Macrosegregation and macro/microstructures were characterized on the hot-tops and a portion of the upper section of two ingots. The measurements revealed that segregation features in the two ingots varied as a function of the alloying elements, and that the severity of positive macrosegregation in the casting body was reduced when the filling rate was increased. It was also found that at the higher filling rate, grain morphologies in the first solidified zones of the ingot changed from columnar to equiaxe, and secondary dendrite arm spacing (SDAS) became slightly smaller in the intermediate and final solidified zones. The experimental findings were analyzed in the framework of diffusion and convection-controlled solidification, as well as liquid metal flow theories. The solute dependence of segregation features was related to the difference in the solid-liquid partition coefficient and diffusion capability of each element in the liquid iron. Calculation of Reynolds numbers (Re) during the filling process, for both ingots, showed that higher filling velocity caused more instable movement of the liquid metal in the initial solidification stage, resulting in the modification of grain morphology, as well as accelerated solidification rate.

Keywords: medium-carbon low-alloy steel; large size ingot; hot-top; ﬁlling rate; positive macrosegregation

1. Introduction Ingot casting is the only method for the production of large size mono-block high-strength steels used in the energy and transportation industries. Various forms of casting defects, however, may stem from the liquid metal filling stage, and then remain during the solidification process. Among them, macrosegregation, defined as chemical heterogeneity on the scale of the entire ingot, is one of the most important, particularly in high alloyed steels. Such compositional variation over large distances results in local changes in mechanical properties, which may lead to reduced ingot quality and sometimes even to ingot rejection [1,2]. While the severity of macrosegregation could be significantly reduced in small to medium size ingots through subsequent homogenization and forging operations; the situation is more complex when it comes to large size ingots. Therefore, a better understanding of macrosegregation mechanisms, with the aim to control its extent in heavy castings, is an important scientific challenge with direct industrial implications. This is particularly true when it comes to high value-added products, such as large size casting of high strength steels used for turbine shaft applications.

Appl. Sci. 2018, 8, 1878; doi:10.3390/app8101878 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1878 2 of 11

Macrosegregation patterns in heavy low-alloy steel ingots is often characterized by intense positive segregation in the upper part, negative segregation at the bottom, and A- and V-shaped channel segregations in the body [3]. The pioneering work of Flemings et al., developed in the 1960s [4,5], set the theoretical bases for studying the mechanisms of macrosegregation. Since then, intensive research has been underway, and some critical process parameters have been identified and used as ‘countermeasures’ to produce sound ingots with improved quality and limited macrosegregation [6–9]. Specially, stringent control of the superheat [7], accelerating the solidification process in the mold [8], and improving ingot dimensions and configurations [9] have been investigated experimentally and numerically by researchers. In recent years, the influence of filling process parameters has been studied by several researchers. Lee et al. [10] numerically studied the velocity, stress, and temperature fields of pure metal during mold filling and the solidification process, showing the important effect of the filling process on fluid flow and heat transfer at the early stages of the casting process. Im et al. [11] modeled the effects of wall temperature and filling flow on solidification, and pointed out that the residual flow due to the filling stage had an important effect on flow characteristics and the liquid metal solidified more slowly in higher flow situations. Ravindran et al. [12] presented a model to investigate the effect of mold filling on cold shuts formation and metal front migration, and incorporated inter-dendritic flow models to simulate the effect of solidification on the filling patterns and temperature fields. Yadav et al. [13] modeled the filling and solidification of a Pb–15 wt % Sn alloy in a small side-cooled cavity (50 × 60 mm2), and investigated melt superheat and filling velocity on the evolution of mushy regions and macrosegregation. It was found that residual flow due to filling effects significantly affected the shape of the mushy zone and delayed the development of solutal convection, resulting in less intense macrosegregation near the cold wall and a farther distance of A-segregates from the cold wall. Kermanpur et al. [14] simulated a 6-ton Cr–Mo low alloy steel to study the effect of bottom pouring rate and mold dimensions on solidification behavior and crack susceptibility during subsequent hot forging. Results showed that pouring the melt at a constant rate with a lower mold slenderness ratio would improve the riser efficiency and thereby possibly reduce crack susceptibility during subsequent hot forging. However, the effect of filling velocity on macrosegregation in large size steel ingots has not yet been reported, either numerically or experimentally. In the present work, the effect of the filling rate on positive macrosegregation patterns in two large size cast ingots was studied. Various experimental techniques were used to examine the chemical composition, macro- and microstructure evolution of the hot-top and a portion of the upper section of the ingot. The results were analyzed in the framework of diffusion and convection controlled solidification, as well as liquid metal flow theories. The obtained results are expected to contribute to a better understanding of macrosegregation mechanisms in large size ingots. The results could also be used as an input data in numerical simulation codes which still suffer from lack of accurate determination of material characteristics [15].

2. Materials and Experimental Process Two 40 metric ton (MT) cylindrical shape steel ingots, 250 cm in height and 150 cm in mean diameter, were cast in big-end-up cast iron molds using identical conditions, except for the filling rate. The hot-top, as an extension of the ingot that was 70 cm in height, was made in the mold lined inside with insulating tiles. During the production process, molten steel with the nominal composition listed in Table1 was first produced by melting scrap material in an electric arc furnace, and then teemed into a ladle before ladle refining and vacuum degassing in an argon atmosphere. Then it was smoothly bottom poured into the mold at 1570 ◦C, until the mold was filled to the top of the insulating tiles. Two filling times were used: the first ingot named LFR (Low Filling Rate ingot), with a filling rate of about 0.0018 m/s (30 min for the 40 MT ingot), and the second one, HFR (High Filling Rate ingot), with a filling rate of about 0.0024 m/s (22 min). The liquidus temperature for the current material was determined to be 1495 ◦C, using the thermodynamic software Thermo-Calc® (Thermo-Calc Software, Appl. Sci. 2018, 8, 1878 3 of 11

Pittsburgh, PA, USA) [16]. Based on this value, the initial superheat (the excess temperature above the melting point) for both ingots was determined to be 75 ◦C.

Table 1. Nominal chemical composition of the investigated steel ingots, wt.%.

C Mn Cr P S Si Ni Mo Fe 0.36 0.85 1.82 0.01 0.0023 0.4 0.16 0.45 balance

After solidification, a block comprised of the hot-top and 30 cm thick section of the ingot’s main body was transversely cut off for investigation. Then, two plates (130 × 70 × 1.5 cm3) were sliced on each side of the axial plane. One slice was sectioned at regularly-spaced intervals into over 250 samples (6.5 × 4.5 × 1.5 cm3). All the faces in the centerline plane along the longitudinal axis were ground, and then chemically mapped using the Thermo Scientific the ARLTM 4460 Optical Emission Spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA). The axial face of the other slice was prepared by grinding and macroetched in a 50% HCl solution at 50 ◦C to reveal macrosegregation patterns. Some selected regions were metallographically prepared and etched in Oberhoffer’s etchant, and then optically observed using a binocular microscope. It should be noted that hot-tops are commonly used in the ingot casting industry in order to provide a reservoir of molten steel for feeding the shrinkage zone as the ingot solidifies [1]. They are generally cut off from the ingot body after solidification. However, there is a growing interest in reserving at least part of the hot-top for further use [17]. Thus, in the present study, positive segregation in the hot-top was also characterized, along with the upper section of the casting body, to evaluate and validate such possibility. It is clear that, if successful, the findings could result in significant material and energy saving in the ingot casting industry. The chemical composition of each specimen was obtained by averaging out 3 random spectrometer measurements and calculated using the relation [18]:

Ri = (ωi − ω0,i)/ω0,i,

Here, Ri is the segregation ratio for solute element i, ωi is the solute’s local concentration and ω0,i is its nominal concentration value. A positive Ri value corresponds to positive segregation, and conversely, a negative Ri to negative segregation. Segregation ratio patterns of different elements in the longitudinal section of the entire block were then reconstructed using MATLAB® (The MathWorks Inc., Natick, MA, USA) [19] by ﬁlling the areas between the isolines using constant colors corresponding to the local segregation intensities.

3. Results and Discussions

3.1. General Macrosegregation Patterns The chemical distribution patterns of the principal alloying elements on the axial plane in the two ingots are shown in Figure1. The observed section is illustrated in grey in the upper right corner of the ﬁgure. The patterns on the left are for LFR and those on the right are for HFR. Regions with no segregation (Ri = 0) are colored with light blue, negative segregation (Ri < 0) with dark blue, and the hot-top/casting body separation position is marked with a black dashed line. It can be seen that all the solute segregation ratio maps appear symmetric about the ingot central axis, even though minor differences are present. A gradient in solute concentration is observed over the ingot’s radius from the periphery to the center and from the lower to the upper region. This indicates that the slow rise in concentration of solute in the solid was the result of bulk liquid becoming progressively concentrated in solute elements. Appl. Sci. 2018, 8, 1878 4 of 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 11

FigureFigure 1. SegregationSegregation ratio ratio patterns patterns in inthe the two two ingots ingots (LFR (LFR (Low (Low Filling Filling Rate Rateingot) ingot) and HFR and (High HFR (HighFilling Filling Rate ingot)) Rate ingot)) for the for three the analyzed three analyzed solutes: solutes: (a,b) C; (a ,(bc,)d C;) Mn; (c,d )(e Mn;,f) Cr. (e ,f) Cr.

AA weakweak differencedifference fromfrom nominalnominal compositioncomposition (negative(negative segregation)segregation) is observedobserved in thethe regionregion nextnext toto thethe surfacesurface layerlayer ofof thethe hot-tops.hot-tops. This difference could bebe attributedattributed toto thethe increasedincreased locallocal solidificationsolidification timetime duedue toto thethe presencepresence ofof thethe refractoryrefractory lininglining inin thethe hot-tophot-top wall.wall. TheThe presencepresence ofof thethe refractoryrefractory lininglining decreasesdecreases thethe coolingcooling rate,rate, asas comparedcompared toto thethe moldmold wall,wall, andand thereforetherefore reducesreduces thethe segregationsegregation inin these these regions regions [18 [18].]. On On top top of of this, this, some some concentration concentration islands islands were were observed, observed, which which can becan associated be associated with with possible possible segregation segregation channels. channels. ItIt isis alsoalso notednoted inin FigureFigure1 1 that that segregation segregation features features inin the the two two ingots ingots werewere found found toto varyvary asas a a functionfunction ofof alloyingalloying elements.elements. Carbon is thethe elementelement withwith thethe highesthighest segregationsegregation ratio.ratio. TheThe mostmost intenseintense positivepositive segregationsegregation ratio ratio of of carbon, carbon, manganese, manganese, and and chromium chromium in in both both ingots ingots was was found found to beto sequencedbe sequenced in decreasing in decreasing order, order, and reached and reached 1.316, 0.225, 1.316, and 0.225, 0.212, and respectively. 0.212, respectively. The above sequenceThe above is consistentsequence is with consistent the descending with the orderdescending of the solid-liquidorder of the partitionsolid-liquid coefficient partitionk (thecoefficient ratio of k the (the solute ratio contentof the solute in the content solid CinS thecompared solid CSto compared the solute to contentthe solute in content the liquid in theCL liquidin equilibrium, CL in equilibrium,k = CS/C kL =) ofCS/ eachCL) of solute. each solute. This coefficient This coefficient was determined was determined for each for element each element based onbased linearized on linearized binary binary phase diagrams,phase diagrams, with respect with torespect iron calculated to iron calculated using Thermo-Calc using Thermo-Calc® (Thermo-Calc® (Thermo-Calc Software, Pittsburgh, Software, PA,Pittsburgh, USA) [16 PA,], and USA) the results[16], and are shownthe results in Figure are 2a.shown The smallerin Figure the partition2a. The coefficientsmaller the of partition a solute element,coefficient the of more a solute solute theelement, solid willthereject more into solute the liquid the solid during will solidification, reject into and the the liquid more intenseduring solidification, and the more intense the resulting segregation will be [20]. Although the k value of manganese lies between carbon and chromium, as seen in Figure 2a, its slightly lower diffusion coefficient in the liquid iron (as displayed in Figure 2b [21]) results in a similar positive segregation Appl. Sci. 2018, 8, 1878 5 of 11

the resulting segregation will be [20]. Although the k value of manganese lies between carbon and Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 11 Appl.chromium, Sci. 2018, 8, as x FOR seen PEER in Figure REVIEW2a, its slightly lower diffusion coefﬁcient in the liquid iron (as displayed5 of 11 in Figure2b [ 21]) results in a similar positive segregation pattern to chromium. In addition, as shown pattern to chromium. In addition, as shown in Figure 1, the most intense segregations of all the patternin Figure to 1chromium., the most intenseIn addition, segregations as shown of in all Figu the solutesre 1, the (the most red intense color in segregations the maps) in of LFR all werethe solutes (the red color in the maps) in LFR were observed at the top center of the hot-top, in contrast solutesobserved (the atred the color top in center the maps) of the in hot-top, LFR were in contrast observed to theat the lower top locationscenter of the of positive hot-top, segregatesin contrastof to the lower locations of positive segregates of manganese and chromium in HFR. tomanganese the lower locations and chromium of positive in HFR. segregates of manganese and chromium in HFR.

(a) (b) (a) (b) Figure 2. (a) Partition coefficients of different alloying elements determined based on linearized FigureFigure 2. 2.(a()a )Partition Partition coefficients coefﬁcients ofof differentdifferent alloying alloying elements elements determined determined based based on linearizedon linearized binary binary phase diagrams with respect to iron; (b) Evolution of diffusion coefficients of solutes in liquid binaryphase phase diagrams diagrams with respectwith respect to iron; to (iron;b) Evolution (b) Evolution of diffusion of diffusion coefﬁcients coefficients of solutes of solutes in liquid in ironliquid as a iron as a function of temperature. ironfunction as a function of temperature. of temperature. 3.2. Effect of Filling Rate on Segregation Severity 3.2.3.2. Effect Effect of ofFilling Filling Rate Rate on on Segregation Segregation Severity Severity Frequency distribution of segregation ratios were examined in the hot-top and the upper FrequencyFrequency distribution distribution of of segregation ratiosratios were were examined examined in thein the hot-top hot-top and theand upper the upper section section of the casting body for the two investigated ingots. The results are plotted in histograms in sectionof the of casting the casting body forbody the for two the investigated two investigated ingots. ingots. The results The results are plotted are plotted in histograms in histograms in Figure in3 . Figure 3. It can be seen in Figure 3a that in the hot-top of ingot HFR, the intensity of positive FigureIt can 3. be It seen can in be Figure seen3 ain that Figure in the 3a hot-top that in of the ingot hot-top HFR, of the ingot intensity HFR, of the positive intensity segregation of positive of all thesegregation analyzed of elements all the analyzed is more pronounced elements is (histogramsmore pronounced with R (histograms> 0), especially with in R higheri > 0), especially segregation in segregation of all the analyzed elements is more pronounced (histogramsi with Ri > 0), especially in regionshigher segregation (R > 0.1); regions solute concentrations(Ri > 0.1); soluteclose concentrations to the nominal close to composition the nominal arecomposition less pronounced are less higher segregationi regions (Ri > 0.1); solute concentrations close to the nominal composition are less (histogramspronounced (histograms in the region inR the= region 0). In R contrast,i = 0). In contrast, in the upper in the section upper section of the castingof the casting body ofbody ingot of pronounced (histograms in thei region Ri = 0). In contrast, in the upper section of the casting body of HFRingot (FigureHFR (Figure3b), the 3b), positive the positive segregation segregation is less intense is less (histograms intense (histograms with R > 0)with and R morei > 0) solutesand more are ingot HFR (Figure 3b), the positive segregation is less intense (histogramsi with Ri > 0) and more presentsolutes are with present concentrations with concentratio near the nominalns near the composition nominal composition (R = 0). (Ri = 0). solutes are present with concentrations near the nominal compositioni (Ri = 0).

(a) (b) (a) (b) Figure 3. Frequency distribution of segregation ratios: ( a)) In the hot-top; ( b) In the upper section of Figure 3. Frequency distribution of segregation ratios: (a) In the hot-top; (b) In the upper section of ingot body. ingot body.

The above-mentioned tendency, more severe segregation in the hot-top of HFR and lessless severe The above-mentioned tendency, more severe segregation in the hot-top of HFR and less severe segregation in the casting body of HFR, was more remarkable when the evolution of carbon and segregation in the casting body of HFR, was more remarkable when the evolution of carbon and manganese was examined along the the central central axis, axis, as as shown shown in in Figures Figure4 a,b.4a,b. The The axial axialconcentration concentration manganese was examined along the central axis, as shown in Figures 4a,b. The axial concentration examination of chromium presented less severe segregation in both the casting body and the hot-top examination of chromium presented less severe segregation in both the casting body and the hot-top of HFR, as shown in Figure 4c. Moreover, it can be seen that in both ingots, the axial concentrations of HFR, as shown in Figure 4c. Moreover, it can be seen that in both ingots, the axial concentrations of carbon present a stepped monotonic increase from the cutting section to the top center of the of carbon present a stepped monotonic increase from the cutting section to the top center of the hot-top. In contrast, positive segregation of manganese and chromium were found to increase hot-top. In contrast, positive segregation of manganese and chromium were found to increase Appl. Sci. 2018, 8, 1878 6 of 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 11

progressively in a fluctuating way. The fluctuation was more distinct in HFR. The most intense examination of chromium presented less severe segregation in both the casting body and the hot-top positive segregation regions of manganese and chromium in HFR were found to lie about 5cm of HFR, as shown in Figure4c. Moreover, it can be seen that in both ingots, the axial concentrations of below the corresponding regions in the low filling rate case, LFR. carbon present a stepped monotonic increase from the cutting section to the top center of the hot-top. In contrast,Appl. Sci. positive2018, 8, x FOR segregation PEER REVIEW of manganese and chromium were found to increase progressively6 of 11 in a fluctuatingprogressively way. in The a fluctuating fluctuation way. was The more fluctuatio distinctn inwas HFR. more The distinct most in intense HFR. The positive most segregationintense regionspositive of manganese segregation and regions chromium of manganese in HFR and were chromium found to in lie HFR about were 5cm found below to thelie correspondingabout 5cm regionsbelow in the the lowcorresponding filling rate regions case, LFR. in the low filling rate case, LFR.

(a) (b)

(c)

Figure 4. Segregation ratio profiles along the ingot central axis of the two ingots: (a) carbon; (b) manganese; (c) chromium. (c) An examination of the concentration distribution along the cutting line of the two ingots (i.e., 30 cmFigure belowFigure 4. the Segregation 4.hot-top/casting Segregation ratio ratio body proﬁlesprofiles interface), along the the as ingot sh ingotown central central in Figureaxis axisof the5, of revealed two the ingots: two that ingots: (a) thecarbon; concentration (a) ( carbon;b) manganese; (c) chromium. profiles(b) manganese; of all the (threec) chromium. solutes in the casting body of HFR are below the LFR case and closer to the nominal composition (Ri = 0), presenting less severe segregation. An examinationAn examination of of the the concentration concentration distribution along along the the cutting cutting line of line the of two the ingots two (i.e., ingots 30 (i.e., cmThe below above the hot-top/castingresults indicate body that interface), the filling as sh velocityown in Figurehas a 5,clear revealed influence that the on concentration the resultant 30 cm below the hot-top/casting body interface), as shown in Figure5, revealed that the concentration macrosegregation,profiles of all the atthree least solutes in the instudied the casting zones, body of large of HFR size are ingots. below The the obtainedLFR case resultsand closer confirm to the the proﬁles of all the three solutes in the casting body of HFR are below the LFR case and closer to the observationsnominal composition of Yadav (etRi al.= 0), [13] presenting in small less size severe ingo segregation.ts, despite the fact that in large size ingots the nominalfilling compositiontimeThe isabove very resultsshort (Ri = compared 0),indicate presenting thatto the the less total filling severe solidification velocity segregation. has time. a clear influence on the resultant macrosegregation, at least in the studied zones, of large size ingots. The obtained results confirm the observations of Yadav et al. [13] in small size ingots, despite the fact that in large size ingots the filling time is very short compared to the total solidification time.

(a) (b)

Figure 5. Cont. (a) (b) Appl. Sci. 2018, 8, 1878 7 of 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 11

(c)

FigureFigure 5. Segregation5. Segregation ratio ratio proﬁles profiles along along the the bottom bottom of the of cuttingthe cutting section section of the of two the ingots:two ingots: (a) carbon; (a) (b)carbon; manganese; (b) manganese; (c) chromium. (c) chromium.

3.3.The Effect above of Filling results Rate indicateon Fluid Flow that and the Solidification filling velocity Time has a clear influence on the resultant macrosegregation,Considering atthe least large in size the of studied the ingots, zones, the of solid large percentage size ingots. formed The obtainedat the end resultsof filling confirm was not the observationsexpected to of induce Yadav changes et al. [13 in] inthe small melt size density, ingots, temperature, despite the and fact viscosity. that in large Thus, size the ingots influence the filling of timefilling is very velocity short on compared the flows toof thethe totalliquid solidification steel during the time. filling stage (considered as isothermal flow of a single phase material) was analyzed using the dimensionless Reynolds number (Re) [22,23]. The 3.3.proposed Effect of Fillingapproach Rate is on similar Fluid Flowto the and ones Solidification used by Marx Time et al. [22] and Wu et al. [24] for physical modellingConsidering of liquid the melt large flow size in of steel the ingots: ingots, the solid percentage formed at the end of filling was not expected to induce changes in the melt density,ρvd temperature, and viscosity. Thus, the influence Re = of filling velocity on the flows of the liquid steel duringμ the filling stage (considered as isothermal flow ofIn a singlethe above phase equation, material) ρ wasrepresents analyzed the usingdensity the of dimensionless the fluid (kg/m Reynolds3), v the fluid number flow (Re) velocity [22,23 ]. The(m/s), proposed d the characteristic approach is similarlength (m), to the and ones µ the used dynamic by Marx viscosity et al. of [22 the] and fluid Wu (Pa·s). et al. [24] for physical modellingThe ofdensity liquid and melt the flow dynamic in steel viscosity ingots: of the investigated steel at the filling temperature, 1570 °C, were determined using Thermo-Calc® (Thermo-Calc Software, Pittsburgh, PA, USA) [16] and ® ρvd JMatPro (Sente Software Ltd., Guildford, Surrey,Re = UK) [25], respectively. The mean filling speed was taken as the characteristic velocity and the averageµ diameter of the cylinder mold cavity as characteristic length. After calculation with all the determined parameters, Reynolds numbers of 3 3208In for the LFR above and equation, 4375 for HFRρ represents during the the pouring density process of the were fluid obtained. (kg/m ),Thev thehigher fluid value flow of velocityReHFR (m/s),indicatesd the that characteristic more flow length instabilities (m), and andµ movementsthe dynamic were viscosity produced of the inside fluid the (Pa ·molds). under HFR ◦ condition.The density Thisand flow the instability dynamic is viscosity expected of to the result investigated in some residual steel at theflow filling up to temperature, the initial stage 1570 of C, weresolidification determined under using HFR Thermo-Calc condition.® (Thermo-Calc Software, Pittsburgh, PA, USA) [16] and JMatPro® (Sente SoftwareMacroetch Ltd., analysis Guildford, of the Surrey,zones near UK) the [25 surface], respectively. of the ingot The (first mean solidified filling speed zones), was as taken reported as the characteristicin Figure 6, velocityrevealed andcolumnar the average grains in diameter the casting of the body cylinder of LFRmold (Figure cavity 6a) and as characteristicequiaxed grains length. in Afterthe calculationsame position with of allHFR the (Figure determined 6b). The parameters, examined Reynoldsregion is encircled numbers in of red 3208 in forthe LFRupper and right 4375 forcorner HFR duringof Figure the 6. pouring A schematic process views were of the obtained. grain morphologies The higher valueare also of provided ReHFR indicates for clarification that more flow(due instabilities to the large and size movements of the blocks were and producedthe grains, insideit is difficult the mold to capture under high HFR quality condition. images This with flow instabilityuniform islight expected and shade to result in one in single some picture). residual Campbell flow up to [26] the found initial that stage a dampened of solidification fluid flow under HFRpromotes condition. columnar growth, while an unsteady liquid movement promotes the development of equiaxedMacroetch grain analysis structure of the[27]. zones The presence near the of surface the equiaxed of the ingot grain (first morphology solidified in zones), the first as solidified reported in zone in HFR is probably indicative of the presence of higher liquid instabilities at the start of Figure6, revealed columnar grains in the casting body of LFR (Figure6a) and equiaxed grains in the solidification, as also predicted by the Reynolds number calculations. same position of HFR (Figure6b). The examined region is encircled in red in the upper right corner Several researchers have examined the mutual influence of residual flow and solidification on of Figure6. A schematic views of the grain morphologies are also provided for clarification (due to the thermal and solutal behaviors of the ingot. Zhao et al. [28] and Zhu et al. [29] reported that the the large size of the blocks and the grains, it is difficult to capture high quality images with uniform presence of residual flow can homogenize the temperature and solute field in the bulk liquid at the lightearly and stages shade of insolidification, one single picture).while Wang Campbell et al. [30,31] [26] and found Aboutalebi that a dampened [32] related fluid it to flowa decrease promotes of columnartemperature growth, gradient while in an the unsteady pool of liquid. liquid The movement lower temperature promotes the gradient development can prolong of equiaxed the thermal grain structureconvection [27]. time The in presence the cavity, of and the delay equiaxed the development grain morphology of solutal in convection the first solidified [33], because zone of in better HFR is probablymixing indicativeof solute ahead of the of presence the liquidus of higher front liquid and thus instabilities decreased at thedensity start gradients of solidification, [34]. Others as also predicted by the Reynolds number calculations. Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 11 Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 11 reported that delayed solutal convection can significantly affect the distribution of solutes [35–37], andreported hence that affect delayed the evolution solutal ofconvection macrosegregation can signif patternsicantly affect during the the distribution intermediate of andsolutes final [35–37], stages ofand solidification. hence affect theTherefore, evolution more of macrosegregation intense movement patterns of the duringliquid metalthe intermediate in the initial and solidification final stages stage,Appl.of solidification. Sci. caused2018, 8, 1878by Therefore,higher filling more velocity, intense ismovement probably ofthe the source liquid for metal the inobserved the initial reduced solidification solute8 of 11 segregationstage, caused in theby castinghigher fillingbody in velocity, the HFR is ingot probably as compared the source to LFR. for the observed reduced solute segregation in the casting body in the HFR ingot as compared to LFR.

(a) (b) (a) (b) Figure 6. 6. GrainGrain morphologies morphologies in in the the bordering bordering zones zones (the (the chill chill walls walls are are on on the the left) left) of of(a)( aLFR;) LFR; (b) ( bHFR.) HFR. Figure 6. Grain morphologies in the bordering zones (the chill walls are on the left) of (a) LFR; (b) HFR. InSeveral addition, researchers secondary have dendrite examined arm the spacing mutual (SDA influenceS) measurements of residual flow along and the solidification cutting section on the of thethermal LFRIn addition, andingot solutal revealed secondary behaviors SDAS dendrite ofvalues the ingot. as arm large Zhaospacing as 536 et al.(SDA μ [m28 in]S) and themeasurements Zhu1/2 radius et al. [ 29position along] reported the from cutting that the the center section presence and of 474ofthe residual LFRμm iningot the flow revealedingot can homogenizecenter. SDAS The values two the temperatureobservedas large as regions 536 and μm solute so inlidified the field 1/2 in inradius the the intermediate bulk position liquid from at or the the final early center stage stages and of theof474 solidification, solidificationμm in the ingot while process. center. Wang The The et al.SDAStwo [30 observed, 31values] and Aboutalebiwereregions also so lidifiedin [32 the] related same in the itrange intermediate to a decreasefor the orHFR of final temperature ingot stage but of slightlygradientthe solidification smaller in the pool(530 process. ofμm liquid. and The 470 The SDAS μ lowerm, valuesrespectively, temperature were alsoin gradientthe in correspondingthe cansame prolong range regions), thefor thermalthe asHFR presented convection ingot but in Figuretimeslightly in the7.smaller Lower cavity, (530 andSDAS μ delaym valuesand the 470 developmentare μm, generally respectively, of corre solutal inlated the convection correspondingwith shorter [33], because solidificationregions), of betteras presentedtimes mixing [26]. ofin Furthermore,soluteFigure ahead7. Lower ofas thereported SDAS liquidus byvalues Vives front are andet al.generally thus [35], decreased higher corre fillinglated density rateswith gradients resultshorter in [increased34solidification]. Others dissipation reported times [26]. thatrate ofdelayedFurthermore, the melt solutal superheat as convection reported with by canthe Vives mold significantly et and al. accelerated[35], affect higher the solidification distributionfilling rates result of rate solutes due in increased to [35 the–37 higher], anddissipation hencestrength affect rate of convection.theof the evolution melt superheatTherefore, of macrosegregation with the thefindings mold patterns ofand the accelerated duringshorter thesolidification solidification intermediate time rate and resultingdue final to stages the from higher of higher solidification. strength filling of velocityTherefore,convection. could more Therefore, result intense in theless movement findingstime available ofof thethe liquidforshorter solutes metal solidification to intransport, the initial time and solidification resulting could be fromanother stage, higher source caused filling for by thehighervelocity lower filling could segregation velocity, result in is probablylessintensity time theavailablein sourcethe casting for for so the lutesbody observed to of transport, HFR. reduced It and soluteshould could segregation be be noted another inthat thesource further casting for experimentalbodythe lower in the segregation HFR work ingot on measuring as intensity compared thein to SDASthe LFR. casting values body in different of HFR. locations It should of the be ingots noted will that contribute further toexperimental confirmIn addition, and work better secondary on quantify measuring dendrite the thedifferences arm SDAS spacing values in the (SDAS) in SDAS different measurements values locations for each of along condition. the ingots the cutting will contribute section of theto confirm LFRThe ingotshorter and revealed better solidification quantify SDAS values time the differences and as largethe lower as in 536 the diffusionµ mSDAS in the valuescoefficient 1/2radius for each of positionthe condition. solutes from are the probably center andthe main474 µThe msources in shorter the ingotfor solidification the center. observed The time two lower observedand location the lower regions (~ diffusion5 cm) solidified of coefficientthe in positive the intermediate of manganesethe solutes or arefinaland probably stagechromium of the concentrationsolidificationmain sources process.coresfor the in observed TheHFR SDAS (as reportedlower values location werein Figures also (~5 incm) 1 the and of same 4).the Besides, rangepositive for nomanganese the sufficient HFR ingot anddiffusion but chromium slightly time couldsmallerconcentration be (530 the µorigin mcores and forin 470 HFRtheµm, observed(as respectively, reported higher in in Figuresfluc thetuations corresponding 1 and in 4). the Besides, concentr regions), noations assufficient presented of manganese diffusion in Figure timeand7. chromiumLowercould be SDAS the in originthe values radial for are theand generally observedaxial distributions correlated higher fluc with intuations HFR shorter (as in solidificationseen the concentrin Figuresations times 4 and [of26 5).].manganese Furthermore,The reduced and segregationaschromium reported inin by thethe Vives radialcasting et al.and body [35 axial], of higher HFR,distributions fillingproduced rates in under HFR result (asfast in seen increasedfilling in conditions,Figures dissipation 4 and is probably 5). rate The of thereducedthe meltroot causesuperheatsegregation for withthe in thehigher the casting mold segregation and body accelerated of HFR,intensity produced solidification observed under ratein fast duethe filling tohot-top the conditions, higher of HFR. strength is probablyIndeed, of convection. athe lower root segregationTherefore,cause for thethe level findingshigher in the segregation ofcasting the shorter body intensity solidificationresults inobserved more time partitioning resultingin the hot-top fromof solutes higherof HFR.into filling the Indeed, melt velocity anda couldlower their accumulationresultsegregation in less level timetowards in available the the casting end for ofbody solutes solidification results to transport, in morein the partitioning upper and could part beofof solutesthe another hot-top into source the[4]. meltThe for theaxialand lower theirless severesegregationaccumulation segregation intensity towards of inchromium thethe castingend ofin bodybothsolidification the of HFR. casting Itin should bodythe upper and be noted the part hot-top thatof the further of hot-top HFR experimental could [4]. Thebe due axial work to lessless on availablemeasuringsevere segregation time the for SDAS the of valuesdiffusionchromium in differentof in the both solute. the locations casting of body the ingotsand the will hot-top contribute of HFR to could confirm be due and to better less quantifyavailable the time differences for the diffusion in the SDAS of the values solute. for each condition.

(a) (b) (c) (d) (a) (b) (c) (d)

Figure 7. Micrographs along the bottom of the cutting section (30 cm below the hot-top/casting body separation position) of ingots LFR (left) and HFR (right): (a,c) In the 1/2 radius position from the center; (b,d) In the center. Appl. Sci. 2018, 8, 1878 9 of 11

The shorter solidification time and the lower diffusion coefficient of the solutes are probably the main sources for the observed lower location (~5 cm) of the positive manganese and chromium concentration cores in HFR (as reported in Figures1 and4). Besides, no sufficient diffusion time could be the origin for the observed higher fluctuations in the concentrations of manganese and chromium in the radial and axial distributions in HFR (as seen in Figures4 and5). The reduced segregation in the casting body of HFR, produced under fast filling conditions, is probably the root cause for the higher segregation intensity observed in the hot-top of HFR. Indeed, a lower segregation level in the casting body results in more partitioning of solutes into the melt and their accumulation towards the end of solidification in the upper part of the hot-top [4]. The axial less severe segregation of chromium in both the casting body and the hot-top of HFR could be due to less available time for the diffusion of the solute.

4. Conclusions The effect of ﬁlling rate on positive macrosegregation characteristics in large size steel ingots was studied based on the analysis of the hot-top and a small portion of the main ingot body. The following conclusions can be drawn:

1. General macrosegregation patterns presented the solute dependence of segregation features, which was related to the difference in the solid-liquid partition coefficient and diffusion capability of each element in the liquid iron. 2. Frequency distribution of segregation ratios, as well as the axial and horizontal concentration evolutions, presented less severe segregation in the casting body of the ingot with the higher filling rate. 3. One source of the reduced solute segregation in the casting body in the higher filling rate case is the presence of higher liquid instabilities at the start of solidification, which was predicted by the Reynolds number calculations during the filling process, and confirmed by macroetch grain morphology observations in the first solidified zones. 4. Another source for the lower segregation intensity in the casting body of higher filling rate case is the shorter solidification time, resulting in less time available for solutes to transport, which was revealed by the SDAS measurements along the cutting section of the ingot in the regions solidified in the intermediate and final stages of the solidification process.

Therefore, the alleviation of solute segregation in the upper section of the casting body resulting from the accelerated filling process could be due to a combination of the more intense movement of the liquid metal, caused by higher kinetic energy of the liquid metal in the initial solidification stage, and shorter solidification time that reduces the available time for the segregation to build up.

Author Contributions: Conceptualization, M.J. and C.Z.; Methodology, C.Z.; Software, C.Z. and A.L.; Validation, C.Z.; Formal Analysis, C.Z.; Investigation, C.Z. and R.T.; Resources, R.T., M.J. and L.-P.L.; Data Curation, C.Z.; Writing-Original Draft Preparation, C.Z; Writing-Review & Editing, M.J. and A.L.; Visualization, C.Z.; Supervision, M.J.; Project Administration, M.J.; Funding Acquisition, M.J. Acknowledgments: The financial support from the Natural Sciences and Engineering Research Council (NSERC) of Canada in the form of a Collaborative Research and Development Grant (CRDG) under number 470174 is gratefully acknowledged. Finkl Steel-Sorel Co. for providing the material is greatly appreciated. Conflicts of Interest: The authors declare no conflict of interest.

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