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Article Effect of Addition on As-Cast Structure and High-Temperature Tensile Property of 20Cr-8Ni Stainless for Heavy

Qiming Wang, Guoguang Cheng * and Yuyang Hou

State Key Laboratory of Advanced , University of Science and Technology Beijing, Beijing 100083, ; [email protected] (Q.W.); [email protected] (Y.H.) * Correspondence: [email protected]; Tel.: +86-010-6233-4664

 Received: 21 March 2020; Accepted: 17 April 2020; Published: 20 April 2020 

Abstract: 20Cr-8Ni is used to manufacture heavy castings in industrial practice. Owing to the slow solidification rate of heavy castings, the as-cast structure is usually coarse, which reduces the mechanical properties. To refine the solidification structure, the effect of titanium addition on the as-cast structure and high-temperature tensile property was investigated in the laboratory. The ingots with 0.0036, 0.2, and 0.45 percent titanium were produced in the laboratory. On the basis of experiment and thermodynamic calculation through Thermo-Calc software, the typical inclusions changed from dual Si-Mn-Ti to pure TiN or complex Ti2O3 + TiN with titanium content increasing. The equiaxed grain ratio of ingot increased from 51 percent to nearly 100 percent, and the size of equiaxed grain decreased owing to heterogeneous nucleation of δ-Fe. Besides, the equilibrium solidification model changed from the FA model to the F model, and the mass fraction of ferritic phase in ingots increased from 24.8 to 42.6 percent. As a result, the strength and ultimate tensile strength of ingots increased gradually, but the tensile elongation changed little owing to the increase of ferritic phase mass fraction and decrease of equiaxed grain size.

Keywords: stainless steel; as-cast structure; ; tensile property; titanium content

1. Introduction 20Cr-8Ni stainless steel is used to manufacture heavy castings owing to its perfect resistance and mechanical properties [1,2], such as pump casing for nuclear power. However, owing to the slow solidification rate of heavy castings, the as-cast structure is usually coarse, which reduces the mechanical properties. is usually used to improve mechanical properties owing to the intrinsic solution hardening effect [3,4]. However, nitrogen gas pores are usually formed during the process of heavy castings [5,6]. Therefore, it is necessary to refine the as-cast structure of heavy castings, and titanium addition is a common method to refine the solidification structure. It is well known that titanium addition in steel would cause problems. Oxides containing titanium are the typical inclusions formed in steel firstly [7–11]. Seo et al. concluded that the typical inclusions changed from MnSiO3 inclusions, (Mn, Ti)- , to Ti2O3 with titanium content increasing in Ti-containing steel weld metals [12]. Besides oxides rich in titanium, is the other typical inclusions in titanium-contained steel. Ozturk et al. investigated the thermodynamics of titanium in Fe-Cr alloys and formation of TiN in Fe-Cr-N-Ti alloys [13]. Yin et al. found pure TiN particles and complex TiN with Al2O3-MgO-TiOx cores in Ti-stabilized 17Cr austenitic stainless steel. Stringer-shaped TiN inclusions were also observed in the rolled bar [14]. On the basis of oxide metallurgy, the oxides rich in titanium and TiN could refine the solidification structure through heterogeneous nucleation [15–17]. Bramfitt found the misfit factor between TiN and

Metals 2020, 10, 529; doi:10.3390/met10040529 www.mdpi.com/journal/metals Metals 2020, 10, 529 2 of 15

δ-Fe was small, and TiN was very effective as a nucleating agent for δ-Fe [18]. Park found that TiN and MgAl2O4-TiN complex inclusions promoted the formation of fine equiaxed grains when Ti was added in ferritic stainless steel [19]. Nakajima et al. studied TiN, Al2O3, and Ti2O3 on heterogeneous nucleation in pure Fe and Fe-Ni alloys. They found that only TiN had a practical effect as a catalyst on the triggered nucleation of the primary crystal of δ phase, but none of them had a practical effect on the nucleation of the primary crystal of γ phase [20]. They also investigated TiN and Al2O3 on heterogeneous nucleation in pure Fe-Ni-Cr alloys, and TiN was the greater catalyst compared with Al2O3 [21]. It is known that mechanical properties could be improved through refining grains of steel [22–24]. Schino et al. investigated the effect of grain size on the properties of a low austenitic stainless steel. They found that the micro- increased with the decreasing grain size, as did the yield and tensile strength [25]. Li et al. also concluded that the Hall–Petch dependency for both yield strength and tensile strength was valid for the grain size range in the nickel-free high nitrogen austenitic stainless steel [26]. In addition to forming compounds, titanium could be presented as a solid solution state in steel. It is known that Ti was a strong ferrite forming element, and could promote formation of ferrite in austenitic stainless steel or increase ferrite fraction in . Jang et al. investigated Cr contents on tensile and corrosion behaviors of 0.13 pct N-containing CD4MCU (0Cr26Ni5Mo2Cu3) cast duplex stainless [27]. They found that the tensile behavior of duplex stainless steel increased in a nonlinear manner with increasing Cr content. They believed the major reasons for this trend appeared to be the increase of ferrite fraction and the second precipitates of fine austenitic phase in the ferrite matrix rather than the intrinsic effect. They also investigated different Mo contents on tensile and corrosion behaviors of CD4MCU cast duplex stainless steels [28]. Son et al. investigated N addition on the tensile and corrosion behaviors of CD4MCU cast duplex stainless steels [29]. They concluded that the initial increase in yield strength with N addition was the result of the beneficial shape change of austenitic phase, as well as the intrinsic solid solution hardening effect of N. However, the yield strength decreased with further addition of N owing to the increase in the fraction of austenitic phase, which had inherently lower strength than the ferritic phase. So, it is necessary to investigate the effect of titanium addition on the as-cast structure and mechanical properties of 20Cr-8Ni stainless steel in the laboratory. In the present work, the as-cast structure and microstructure of ingots with different Ti content were investigated firstly. Moreover, the evolution of typical inclusions in 20Cr-8Ni stainless steel was analyzed. The mechanical property of ingot was detected. On the basis of the calculation result via Thermo-Calc software (version 2017a, Thermo-Calc software, Stockholm, Sweden), the effect of Ti on the as-cast structure and mechanical property of 20Cr-8Ni stainless steel was investigated.

2. Materials and Methods The ingots with different titanium content were produced in the laboratory. The high-purity materials including , chromium blocks, nickel blocks, and so on were firstly melted in a induction furnace. After those materials were melted completely at 1650 ◦C, the titanium blocks were added into the melt. For homogenization of the composition, the melt was held for 5 min. Next, the melt was cast into mold when the temperature decreased to 1520 ◦C. The mold was made of , and its size is shown in Figure1a. The ingot was tapped after 5 min and put into the bucket full of water to quench to room temperature. The temperature of the water was around 20 ◦C. There were three ingots with different Ti content investigated in this work. The contents of , , nitrogen, and total were analyzed by the inert gas fusion-infrared absorptiometry method. The contents of , , , and titanium were determined by the inductively coupled optical emission spectrometry method. Moreover, the contents of chromium and nickel were detected through discharge atomic emission spectrometric analysis. The composition of three ingots is shown in Table1. Metals 2020, 10, 529 3 of 15 Metals 2020, 10, x FOR PEER REVIEW 3 of 16

Figure 1. (a) The size of cast iron mold and the sampling method; (b) tensile specimen from ingot; Figure 1. (a) The size of cast iron mold and the sampling method; (b) tensile specimen from ingot; (c) (c) sample from tensile specimen after testing. sample from tensile specimen after testing. Table 1. Composition of ingots, mass%. Table 1. Composition of ingots, mass%. Ingot C Si Mn S Cr Ni Mo Ti N O Ingot C Si Mn S Cr Ni Mo Ti N O S1 0.0170 1.60 1.32 0.0080 20.11 8.55 0.4 0.0036 0.0190 0.0056 S1 0.0170 1.60 1.32 0.0080 20.11 8.55 0.4 0.0036 0.0190 0.0056 S2 0.0076 1.59 1.38 0.0071 20.10 8.46 0.4 0.20 0.0170 0.0012 S3S2 0.0074 0.0076 1.651.59 1.37 1.38 0.00770.0071 20.1520.10 8.46 8.58 0.4 0.4 0.20 0.45 0.0170 0.0081 0.0012 0.0017 S3 0.0074 1.65 1.37 0.0077 20.15 8.58 0.4 0.45 0.0081 0.0017

TheThe ingot ingot was was cut cut into into two two parts parts verticality verticality throughthrough the diameter. One One part part was was etched etched by by aqua aqua regiaregia solution solution to revealto reveal the the as-cast as-cast structure. structure. The The tensile tensile specimen specimen was was processed processed from from thethe otherother partpart of ingotof ingot shown shown in Figure in Figure1b. The 1b. tensile The tensile test was test carried was carried out by theout electronicby the electronic universal universal testing machinetesting at 350machine◦C. Theat 350 tensile °C. The property tensile atproperty 350 ◦C at is 350 a performance °C is a performance index of index castings of castings in industrial in industrial practice. Tensilepractice. specimens Tensile werespecimens heated were up heated to the up experiment to the experiment temperature temperature with a heatingwith a heating rate of rate 10 ◦ ofC /10min. The°C/min. specimens The specimens were pulled were to fracture pulled to with fracture 1 10 with3 strain 1 × 10 rate−3 strain at the rate tensile at the test tensile temperature, test temperature, followed × − byfollowed air cooling. by Afterair cooling. the tensile After test, the the tensile sample test, was the cut sample near thewas fracture cut near of the tensilefracture specimen of the tensile shown in Figurespecimen1c. shown in Figure 1c. TheThe sample sample from from each each tensile tensile specimenspecimen waswas polishedpolished and and etched etched by by mixed mixed solution solution of of2 g 2 g potassium metabisulfite, 20 mL hydrochloric , and 50 mL H2O at 50 °C to reveal the potassium metabisulfite, 20 mL , and 50 mL H2O at 50 ◦C to reveal the microstructure andmicrostructure grain size of ingot.and grain The microstructuresize of ingot. The was mi observedcrostructure through was observed Leica DM4000M through opticalLeica DM4000M (OM,optical made microscope in Leica, (OM, Beijing, made China). in Leica, After Beijing, that, China). the grain After size that, and the ferritegrain size fraction and ferrite were fraction analyzed were analyzed through Image-Pro Plus (version 6.0, Media Cybernetics, Bethesda, MD, USA). The through Image-Pro Plus (version 6.0, Media Cybernetics, Bethesda, MD, USA). The element distribution element distribution between ferrite and was detected by electron probe (EPMA, made in between ferrite and austenite was detected by electron probe (EPMA, made in Shimadzu, , Shimadzu, Hong Kong, China). The typical inclusions in samples from each tensile specimen were China). The typical inclusions in samples from each tensile specimen were observed and analyzed observed and analyzed through scanning electron microscopy (SEM, made in FEI, Hillsboro, OR, through scanning electron microscopy (SEM, made in FEI, Hillsboro, OR, USA) equipped with energy USA) equipped with energy dispersive spectroscopy (EDS). Inclusion statistics was conducted dispersivethrough spectroscopyautomatic scanning (EDS). electr Inclusionon microscope statistics was(EVO18-INCAsteel, conducted through ZEISS automatic Co. Ltd., scanningJena, Germany). electron microscopeThe maximum (EVO18-INCAsteel, diameter of the inclusion ZEISS Co. was Ltd., defined Jena, as the Germany). size of inclusions. The maximum Thermo-Calc diameter software of the inclusion(version was 2017a, defined Thermo-Calc as the size software, of inclusions. Stockholm, Thermo-Calc Sweden) was software used (versionto analyze 2017a, the evolution Thermo-Calc of software,typical Stockholm,inclusions and Sweden) solidification was used process to analyze through the Poly evolution and Scheil of typical model. inclusions and solidification process through Poly and Scheil model. 3. Results 3. Results 3.1. As-Cast Structure of Ingots 3.1. As-Cast Structure of Ingots After etching, the as-cast structures of three ingots are shown in Figure 2. As shown in Figure 2a,After the as-cast etching, structure the as-cast was structurescomposed ofof threecolumnar ingots grains are shown and equiaxed in Figure grains2. As in shown ingot inS1 Figurewithout2a, the as-cast structure was composed of columnar grains and equiaxed grains in ingot S1 without Metals 2020, 10, 529 4 of 15 MetalsMetals 2020 2020, ,10 10, ,x x FOR FOR PEER PEER REVIEW REVIEW 44 of of 16 16 titaniumtitaniumtitanium addition. addition. The The The equiaxed equiaxed equiaxed grain grain grain ratio ratio ratio of of ingo ingo of ingottt S1 S1 was was S1 measured wasmeasured measured to to be be 51 51 to percent percent be 51 percent through through through Image- Image- ProImage-ProPro Plus Plus software. software. Plus software. When When the Whenthe titanium titanium the titanium content content content incr increasedeased increased to to 0.2 0.2 mass tomass 0.2 percent, masspercent, percent, the the equiaxed equiaxed the equiaxed grain grain grainratio ratio ofratioof ingotingot of ingot S2S2 increasedincreased S2 increased upup toto up nearlynearly to nearly 100100 100 percent.percent. percent. TheThe The samesame same resultresult result waswas was obtainedobtained obtained inin in ingotingot ingot S3 S3 with with the thethe additionadditionaddition of ofof 0.45 0.45 massmass percentpercent titanium.titanium. It ItIt could couldcould be bebe concluded concluded that that titanium titanium addition addition increased increased the the equiaxedequiaxedequiaxed grain grain ratio ratio of of ingot. ingot.

FigureFigureFigure 2. 2. As-cast As-castAs-cast structure structurestructure of ofof three threethree ingots: ingots:ingots: ( (aa)) S1 S1 with withwith no nono Ti TiTi addition; addition;addition; ( ((bbb))) S2 S2S2 with withwith 0.2 0.20.2 mass% mass%mass% Ti; Ti;Ti; and andand (((ccc))) S3 S3S3 with withwith 0.45 0.45 mass% mass% Ti Ti (online (online version version in in ). color).

AfterAfterAfter polishingpolishing and andand etching, etching,etching, the thethe microstructuresmicrostructures of of threeof threethree samples samplessamples from fromfrom each eacheach tensile tensiletensile specimen specimenspecimen after after350after◦ C 350350 tensile °C°C tensiletensile testing testingtesting with low withwith magnification lowlow magnificationmagnification are shown areare inshownshown Figure inin3 . FigureFigure The austenite 3.3. TheThe austenite locatedaustenite on locatedlocated the grain onon theboundarythe graingrain boundaryboundary (BA) or in (BA) the(BA) grain oror inin interior thethe grain grain (IA). interior interior The microstructures (IA). (IA). The The microstructuresmicrostructures of three samples ofof threethree were samplessamples all composed werewere allall of composedequiaxedcomposed grains of of equiaxed equiaxed owing grains grains to the owing samplingowing to to the the location sampling sampling of the lo location tensilecation of specimenof the the tensile tensile in ingotsspecimenspecimen shown in in ingots ingots in Figure shown shown1b. inHowever,in Figure Figure 1b. 1b. the However, However, grains in the S1the weregrains grains obviously in in S1 S1 were were larger obviously obviously than S2larger larger and than S3.than After S2 S2 and and statistics S3. S3. After After through statistics statistics Image-Pro through through Image-ProPlus,Image-Pro the results Plus, Plus, the aboutthe results results grain about about sizeand grain grain size size and and are density showndensity inare are Figures shown shown4 inandin Figures Figures5. 4 4 and and 5. 5.

FigureFigureFigure 3. 3.3. Microstructure Microstructure of ofof three threethree samples samples with with low low magnification: magnification:magnification: ( ((aaa))) S1; S1;S1; ( ((bbb))) S2; S2;S2; and andand ( ((ccc))) S3. S3.S3.

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FigureFigure 4. 4.Grain Grain diameterdiameter distribution of of each each sample. sample. Figure 4. Grain diameter distribution of each sample.

FigureFigure5. 5. GrainGrain quantity of each each sample. sample.

FigureFigure4 shows 4 shows the the grain grain diameter Figurediameter 5. distributionGrain distribution quantity of ofof the theeach threethree sample. samples.samples. The The grain grain diameter diameter was was divideddivided into into four four groups, groups, including including 0–0.4 0–0.4 mm,mm, 0.4–0.80.4–0.8 mm, 0.8–1.2 0.8–1.2 mm, mm, and and 1.2–1.6 1.2–1.6 mm. mm. Compared Compared Figure 4 shows the grain diameter distribution of the three samples. The grain diameter was withwith S1, S1, the the proportion proportion of of grains grains smaller smaller thanthan 0.80.8 mm increased, and and the the proportion proportion of of grains grains larger larger dividedthanthan 0.8 0.8into mm mm four decreased decreased groups, in includingin S2. S2. TheThe 0–0.4 distributiondistribution mm, 0.4–0.8 of of grains’ grains’ mm, diameter diameter0.8–1.2 mm, in in S3 S3and was was 1.2–1.6 similar similar mm. to tothat Compared that in inS2. S2. withFigureFigure S1,5 showsthe 5 shows proportion the the grain grain of density grainsdensity (the smaller (the number number than of 0.8of grains grmmains per increased, per unit unit area) area) and and andthe average proportionaverage grain grain of diameter diametergrains larger of of the than 0.8 mm decreased in S2. The distribution of grains’ diameter in S3 was similar to that in S2. threethe samples.three samples. It was It obvious was obvious that thethat grain the grain density dens inity S2 in was S2 was larger larger than than in S1, in andS1, and the averagethe average grain Figure 5 shows the grain density (the number of grains per unit area) and average grain diameter of diametergrain diameter in S2 was in smallerS2 was thansmaller in S1.than The in resultsS1. The in results S3 were in similarS3 were to similar those into S2.those The in conclusion S2. The the three samples. It was obvious that the grain density in S2 was larger than in S1, and the average couldconclusion be drawn could that be the drawn size that of grains the size decreased of grains when decreased the titanium when the content titanium increased content increased up to 0.2 massup grain diameter in S2 was smaller than in S1. The results in S3 were similar to those in S2. The percent,to 0.2 mass but the percent, grain but size the decreased grain size little decrease withd the little further with increasethe further of increase titanium. of titanium. conclusion could be drawn that the size of grains decreased when the titanium content increased up to3.2. 0.23.2. Microstructure mass Microstructure percent, and butand Element theElement grain Distribution Distribution size decrease d little with the further increase of titanium. The microstructures of the three samples from each tensile specimen with high magnification 3.2. MicrostructureThe microstructures and Element of the Distribution three samples from each tensile specimen with high magnification areare shown shown in in Figure Figure6. 6. The The bright bright phase phase was was austenite, austenite, andand thethe grey phase was was ferrite. ferrite. The The element element mapping of austenite and ferrite is shown in Figure 7. It was obvious that ferrite was rich in chromium, mappingThe microstructures of austenite and of ferrite the isthree shown samples in Figure from7. Itea wasch tensile obvious specimen that ferrite with was high rich magnification in chromium, but lacked nickel. Figure 6a,c,e shows the austenite on the grain boundary, and Figure 6b,d,f shows arebut shown lacked nickel.in Figure Figure 6. The6a,c,e bright shows phase the austenitewas austenite, on the and grain the boundary, grey phase and was Figure ferrite.6b,d,f The shows element the the austenite in the grain interior. This figure clearly demonstrates that the microstructure of 20Cr- mappingaustenite of in austenite the grain and interior. ferrite This is shown figure in clearly Figure demonstrates7. It was obvious that that the ferrite microstructure was rich in of chromium, 20Cr-8Ni 8Ni stainless steel varied significantly with different titanium contents. As shown in Figure 6a,b, the butstainless lacked steel nickel. varied Figure significantly 6a,c,e shows with the di austenitfferente titanium on the grain contents. boundary, As shown and Figure in Figure 6b,d,f6a,b, shows the size of austenite was small in S1. With increasing titanium content up to 0.2 mass percent, the size of the austenite in the grain interior. This figure clearly demonstrates that the microstructure of 20Cr- sizeaustenite of austenite and the was mass small fraction in S1. of With ferrite increasing increased. titanium With thecontent further addition up to 0.2 of mass titanium percent, up to the 0.45 size 8Ni stainless steel varied significantly with different titanium contents. As shown in Figure 6a,b, the ofmass austenite percent, and the the morphology mass fraction of aust of ferriteenite changed increased. little, With but the the mass further fraction addition of ferrite of titanium increased up to to size of austenite was small in S1. With increasing titanium content up to 0.2 mass percent, the size of 0.45a higher mass percent, degree. theAfter morphology statistics th ofrough austenite at least changed 25 fields little, in each but sample, the mass the fraction mass fraction of ferrite of increased ferrite austeniteto a higher and degree. the mass After fraction statistics of throughferrite increased. at least 25 With fields the in eachfurther sample, addition the massof titanium fraction up of to ferrite 0.45 mass percent, the morphology of austenite changed little, but the mass fraction of ferrite increased to a higher degree. After statistics through at least 25 fields in each sample, the mass fraction of ferrite

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Metals 2020, 10, x FOR PEER REVIEW 6 of 16 in samples with different titanium contents is shown in Figure8. It was obvious that the ferrite mass fractionin increased samples with with different the increasing titanium contents titanium is show content.n in Figure Moreover, 8. It was the obvious mass that fraction the ferrite of ferritemass could fraction increased with the increasing titanium content. Moreover, the mass fraction of ferrite could increase up to 42.6 percent with the increase of titanium content to 0.45%. increase up to 42.6 percent with the increase of titanium content to 0.45%.

Figure 6.FigureMicrostructure 6. Microstructure of the of the three three samples samples withwith high high magnification: magnification: (a,b)( S1;a,b ()c S1;,d) S2; (c, dand) S2; (e,f and) S3 (e,f) S3 (online version in color). (onlineMetals 2020 version, 10, x FOR in color).PEER REVIEW 7 of 16

FigureFigure 7. Element 7. Element mapping mapping of of ferrite ferrite and and austenite austenite by energy energy dispersive dispersive spectroscopy spectroscopy (EDS) (EDS) (online (online versionversion in color). in color).

Figure 8. Mass fraction of ferrite in samples with different Ti content.

After point detection by EPMA, the titanium distribution between ferrite and austenite is shown in Figure 9. The detected titanium content in ferrite and austenite increased with titanium addition in ingot. Besides, the titanium content in S1 was low, which made no difference between titanium content in ferrite and austenite. In S2 and S3, the titanium content in ferrite was higher than in austenite.

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Figure 7. Element mapping of ferrite and austenite by energy dispersive spectroscopy (EDS) (online Metals 2020, 10, 529 7 of 15 version in color).

FigureFigure 8. 8. MassMass fraction fraction of of ferrite ferrite in in samp samplesles with with different different Ti Ti content. content.

After point detection by EPMA, the titanium distribution between ferrite and austenite is shown After point detection by EPMA, the titanium distribution between ferrite and austenite is shown in Figure9. The detected titanium content in ferrite and austenite increased with titanium addition in in Figure 9. The detected titanium content in ferrite and austenite increased with titanium addition ingot. Besides, the titanium content in S1 was low, which made no difference between titanium content in ingot. Besides, the titanium content in S1 was low, which made no difference between titanium in ferrite and austenite. In S2 and S3, the titanium content in ferrite was higher than in austenite. contentMetals 2020 in, 10ferrite, x FOR andPEER austenite. REVIEW In S2 and S3, the titanium content in ferrite was higher than8 of in 16 austenite.

FigureFigure 9. 9. TitaniumTitanium distribution distribution in in ferrite ferrite and and austenite austenite by by electron electron probe (EPMA).

3.3.3.3. Typical Typical Inclusions Inclusions AsAs shown shown in in Figure Figure 66,, the inclusions in ingot S1 were black and spherical, which were speculated asas oxides. After After titanium titanium addition, addition, the the inclusions inclusions in in ingots ingots S2 S2 and and S3 S3 were rectangular and orange TiN through OM. The inclusions were mostly located in grain interior. Meanwhile, they distributed inin austenite, austenite, ferrite, ferrite, or or the the interface interface of of two phases phases.. In In order order to to investigate investigate the the influence influence of of titanium titanium contentscontents on on inclusions, the the inclusions inclusions in in sample sampless from from each each tensile specimen were were observed and analysedanalysed through SEM andand EDS.EDS. TheThe morphologymorphology and and composition composition of of typical typical inclusions inclusions are are shown shown in inFigure Figure 10 10(the (the numbers numbers beside beside the the inclusion inclusion are ar atomice atomic percentages percentages determined determined by by EDS EDS analysis). analysis). AsAs shownshown inin Figure Figure 10 A1,A2,10A1,A2, the the typical typical inclusions inclus inions S1 withoutin S1 without titanium titanium addition addition were spherical were sphericalor subspherical or subspherical dual phase dual oxides. phase On oxides. the basis On ofthe the basis morphology of the morphology of oxides, itof could oxides, be it speculated could be speculatedthat those oxides that those were oxides liquid were at melting liquid temperature.at melting temperature. With temperature With temperature decreasing decreasing during the during casting the casting process, the oxides changed from liquid to solid gradually. Finally, the dual phase oxides with different compositions formed in ingots. On the basis of EDS analysis, the grey area was oxide- rich in titanium and manganese, and the black area was oxide-rich in silicon. The element mapping of a typical dual phase oxide is shown in Figure 11a. The elements’ distribution was consistent with results shown in Figure 10A1,A2. With titanium content increasing up to 0.20 mass percent, the typical inclusion in ingot S2 was pure TiN, as shown in Figure 10B1. Besides, there were also complex inclusions observed in ingot S2, as shown in Figure 10B2. On the basis of EDS analysis, the grey area was TiN, and the black area was titanium oxide owing to the higher content of oxygen. The element mapping of a typical dual phase inclusion is shown in Figure 11b. The black area lacked nitrogen, but was rich in oxygen, which was consistent with result shown in Figure 10B2. With further addition of titanium up to 0.45 mass percent, the typical inclusions in S3 were similar to those in S2, as shown in Figure 10C1,C2.

Metals 2020, 10, 529 8 of 15 process, the oxides changed from liquid to solid gradually. Finally, the dual phase oxides with different compositions formed in ingots. On the basis of EDS analysis, the grey area was oxide-rich in titanium and manganese, and the black area was oxide-rich in silicon. The element mapping of a typical dual phaseMetals 2020 oxide, 10, isx FOR shown PEER in REVIEW Figure 11a. The elements’ distribution was consistent with results shown9 of in16 MetalsFigure 2020 10,A1,A2. 10, x FOR PEER REVIEW 9 of 16

Figure 10. Morphology and type of typical inclusions in samples: (A1,A2) S1; (B1,B2) S2; and (C1,C2) FigureS3. 10. Morphology andand typetype of of typical typical inclusions inclusions in in samples: samples: (A1 (A1,A2,A2) S1;) S1; (B1 (B1,B2,B2) S2;) S2; and and (C1 (,C1C2,)C2 S3.) S3.

Figure 11. Elemental mapping of ( a)) a typical dual phase oxide formed in S1 condition; ( b) a typical Figure 11. Elemental mapping of (a) a typical dual phase oxide formed in S1 condition; (b) a typical complex TiN formed in S2 (online version inin color).color). complex TiN formed in S2 (online version in color). After statistics, the inclusion size and density of each samples are shown in Figure 12. The inclusionsAfter instatistics, ingot S1 the were inclusion mainly size oxides and rich density in Si, of Mn, each and samples Ti. The are density shown of inoxides Figure was 12. about The inclusions123/mm2. Nitride in ingot was S1 werethe main mainly inclusion oxides in rich ingot in Si,S2 andMn, S3,and and Ti. theThe density ofwere oxides about was 195 about and 123/mm208/mm2., separately.Nitride was the main inclusion in ingot S2 and S3, and the densities were about 195 and 208/mm2, separately.

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With titanium content increasing up to 0.20 mass percent, the typical inclusion in ingot S2 was pure TiN, as shown in Figure 10B1. Besides, there were also complex inclusions observed in ingot S2, as shown in Figure 10B2. On the basis of EDS analysis, the grey area was TiN, and the black area was titanium oxide owing to the higher content of oxygen. The element mapping of a typical dual phase inclusion is shown in Figure 11b. The black area lacked nitrogen, but was rich in oxygen, which was consistent with result shown in Figure 10B2. With further addition of titanium up to 0.45 mass percent, the typical inclusions in S3 were similar to those in S2, as shown in Figure 10C1,C2. After statistics, the inclusion size and density of each samples are shown in Figure 12. The inclusions in ingot S1 were mainly oxides rich in Si, Mn, and Ti. The density of oxides was about 123/mm2. Nitride Metalswas the 2020 main, 10, x inclusionFOR PEER REVIEW in ingot S2 and S3, and the densities were about 195 and 208/mm2, separately.10 of 16

FigureFigure 12. 12. InclusionInclusion size size and and density density of of each each sample. sample. 3.4. Tensile Properties 3.4. Tensile Properties Table2 shows the e ffect of titanium content on the tensile property of 20Cr-8Ni stainless steel at Table 2 shows the effect of titanium content on the tensile property of 20Cr-8Ni stainless steel at 350 ◦C. Comparing ingot S2 with S1, the yield strength (YS) value increased from 207 MPa to 229 MPa. 350With °C. the Comparing titanium content ingot S2 further with S1, increasing the yield up strength to 0.45 mass(YS) value percent, increased the YS valuefrom 207 increased MPa to to 229 238 MPa. MPa. WithSimilarly, the titanium the ultimate content tensile further strength increasing (UTS) valueup to 0.45 increased mass graduallypercent, the with YS thevalue increasing increased titanium to 238 MPa.content. Similarly, However, the the ultimate tensile tensile elongation strength changed (UTS) little value with increased the increasing gradually titanium with content. the increasing titanium content. However, the tensile elongation changed little with the increasing titanium content.

Table 2. Tensile properties of ingots with different Ti content at 350 ◦C. YS, yield strength; UTS, ultimate Tabletensile 2. strength. Tensile properties of ingots with different Ti content at 350 °C. YS, yield strength; UTS, ultimate tensile strength. Ingot Ti Contents (mass%) YS (MPa) UTS (MPa) Tensile Elongation (%) Ti S1 0.0036YS 207.41UTS 452.31Tensile Elongation 31.20 S2Ingot Contents 0.20 229.11 471.24 30.92 S3 0.45(MPa) 238.89 (MPa) 494.87(%) 31.20 (mass%) S1 0.0036 207.41 452.31 31.20 4. Discussion S2 0.20 229.11 471.24 30.92 4.1. Formation of TiNS3 and Effect0.45 on As-Cast 238.89 Structure 494.87 31.20

4. DiscussionTo investigate the effect of titanium on the inclusions and as-cast structure, the equilibrium solidification process of ingots with different titanium content was calculated through Thermo-Calc 4.1.software. Formation The of composition TiN and Effect of on ingots As-Cast is shown Structure in Table1. The calculated temperature ranged from 1000 ◦C to 1600 ◦C, and the calculated results are shown in Figure 13. To investigate the effect of titanium on the inclusions and as-cast structure, the equilibrium Those figures showed that the inclusions during solidification process included oxides, TiN, MnS, solidification process of ingots with different titanium content was calculated through Thermo-Calc and Ti4C2S2. MnS and Ti4C2S2 would not be investigated in this article. As shown in Figure 13a, software. The composition of ingots is shown in Table 1. The calculated temperature ranged from the typical oxides were Si-Mn-Ti-O in ingot S1, which was consistent with the results shown in 1000 °C to 1600 °C, and the calculated results are shown in Figure 13. Figure 10A1,A2 and Figure 11a. Seo et al. also found MnSiO3 inclusions and (Mn, Ti)-spinel oxide in Those figures showed that the inclusions during solidification process included oxides, TiN, MnS, and Ti4C2S2. MnS and Ti4C2S2 would not be investigated in this article. As shown in Figure 13a, the typical oxides were Si-Mn-Ti-O in ingot S1, which was consistent with the results shown in Figure 10A1 and A2 and Figure 11a. Seo et al. also found MnSiO3 inclusions and (Mn, Ti)-spinel oxide in Ti- containing steel weld metals [12]. TiN would form at 1300 °C when the ingot solidified completely, but was not observed in ingot S1. With titanium content increasing up to 0.20 mass percent, the calculated result was shown in Figure 13b. The calculated oxide changed to Ti2O3, and TiN formed during cooling and solidification process. The pure Ti2O3 was not observed in ingot S2. However, the complex Ti2O3 + TiN inclusion was detected shown in Figure 10B2 and Figure 11b. On the basis of Hou’s research, the disregistry between TiN and Ti2O3 was 13.61, and Ti2O3 could promote the formation of TiN [30]. In ingot S3, the calculated inclusions were similar with the results in ingot S2.

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Ti-containing steel weld metals [12]. TiN would form at 1300 ◦C when the ingot solidified completely, but was not observed in ingot S1. Metals 2020, 10, x FOR PEER REVIEW 11 of 16

Figure 13.13. EquilibriumEquilibrium thermodynamic thermodynamic analysis analysis in in the the solidification solidification process process of 20Cr-8Ni of 20Cr-8Ni stainless stainless steel: (steel:a) S1; ( (ab) )S1; S2; (b and) S2; (c and) S3. (c) S3.

WithBramfitt titanium indicated content TiN increasing was very up effective to 0.20 massas a percent,nucleating the agent calculated for δ result-Fe based was shownon misfit in Figure 13b. The calculated oxide changed to Ti O , and TiN formed during cooling and solidification calculation results [18]. Park concluded TiN 2and3 MgAl2O4-TiN complex inclusions promoted the process.formation The of purefine equiaxed Ti2O3 was grains not observed when Ti in was ingot added S2. However,in ferritic thestainless complex steel Ti 2[19].O3 + HouTiN found inclusion the was detected shown in Figures 10B2 and 11b. On the basis of Hou’s research, the disregistry between Ti2O3-TiN complex nucleus increased equiaxed grain ratio of Ti bearing Fe-18Cr ferritic stainless steel [30,31]. To evaluate the effect of TiN and Ti2O3-TiN on the as-cast structure of 20Cr-8Ni stainless steel, the scheil solidification process was calculated. The result is shown in Figure 14. The formation of oxides was not taken into consideration during the scheil solidification process. As shown in Figure 14a, TiN formed at the end of solidification process in ingot S1 owing to element segregation, which was different from Figure 13a. Similarly to Figure 13b,c, TiN and Ti2O3-TiN complex inclusions

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TiN and Ti2O3 was 13.61, and Ti2O3 could promote the formation of TiN [30]. In ingot S3, the calculated inclusions were similar with the results in ingot S2. Bramfitt indicated TiN was very effective as a nucleating agent for δ-Fe based on misfit calculation results [18]. Park concluded TiN and MgAl2O4-TiN complex inclusions promoted the formation of fine equiaxed grains when Ti was added in ferritic stainless steel [19]. Hou found the Ti2O3-TiN complex nucleus increased equiaxed grain ratio of Ti bearing Fe-18Cr ferritic stainless steel [30,31]. To evaluate the effect of TiN and Ti2O3-TiN on the as-cast structure of 20Cr-8Ni stainless steel, the scheil solidification process was calculated. The result is shown in Figure 14. The formation of oxides was not taken into consideration during the scheil solidification process. As shown in Figure 14a, TiN formed at the end of solidification process in ingot S1 owing to element segregation, which was diffMetalserent 2020 from, 10, x Figure FOR PEER 13 a.REVIEW Similarly to Figure 13b,c, TiN and Ti2O3-TiN complex inclusions12 formed of 16 before and during the solidification process in ingot S2 and S3, as shown in Figure 14b,c. As a result, theformed heterogeneous before and nucleation during the did solidification not happen process in ingot in S1. ingot In ingotS2 and S2, S3, the as ratioshown of in equiaxed Figure 14b,c. grains As was a result, the heterogeneous nucleation did not happen in ingot S1. In ingot S2, the ratio of equiaxed nearly 100 percent, as shown in Figure2b, owing to heterogeneous nucleation. Besides, the size of grains was nearly 100 percent, as shown in Figure 2b, owing to heterogeneous nucleation. Besides, equiaxed grains in ingot S2 was smaller than S1 shown in Figures3 and5. Villafuerte et al. also found the size of equiaxed grains in ingot S2 was smaller than S1 shown in Figure 3 and Figure 5. Villafuerte the size of equiaxed grains decreased with increasing titanium contents above 0.18 mass percent in full et al. also found the size of equiaxed grains decreased with increasing titanium contents above 0.18 penetration gas- arc welds on ferritic stainless steel plates [32]. In ingot S3, the solidification mass percent in full penetration gas-tungsten arc welds on ferritic stainless steel plates [32]. In ingot processS3, the was solidification similar to ingotprocess S2. was The similar equiaxed to ingot zone ratioS2. The was equiaxed also nearly zone 100 ratio percent, was also but nearly the equiaxed 100 grainspercent, size but changed the equiaxed little compared grains size withchanged S2. Althoughlittle compared thetitanium with S2. Although content in the ingot titanium S3 was content higher thanin ingot in ingot S3 was S2, the higher nitride than densities in ingot inS2, S2 the and nitride S3 were densities the same, in S2 asand shown S3 were in Figurethe same, 12. as Moreover, shown in the massFigure fraction 12. Moreover, of TiN in the S3 mass was nearlyfraction the of TiN same in as S3 S2, was as nearly shown the in same Figure as S2,13. as This shown was in caused Figure by 13. the lowerThis contentwas caused of nitrogen by the lower in ingot content S3 than of nitrogen in ingot in S2. ingot S3 than in ingot S2.

Figure 14. Cont.

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formed before and during the solidification process in ingot S2 and S3, as shown in Figure 14b,c. As a result, the heterogeneous nucleation did not happen in ingot S1. In ingot S2, the ratio of equiaxed grains was nearly 100 percent, as shown in Figure 2b, owing to heterogeneous nucleation. Besides, the size of equiaxed grains in ingot S2 was smaller than S1 shown in Figure 3 and Figure 5. Villafuerte et al. also found the size of equiaxed grains decreased with increasing titanium contents above 0.18 mass percent in full penetration gas-tungsten arc welds on ferritic stainless steel plates [32]. In ingot S3, the solidification process was similar to ingot S2. The equiaxed zone ratio was also nearly 100 percent, but the equiaxed grains size changed little compared with S2. Although the titanium content in ingot S3 was higher than in ingot S2, the nitride densities in S2 and S3 were the same, as shown in Figure 12. Moreover, the mass fraction of TiN in S3 was nearly the same as S2, as shown in Figure 13. This was caused by the lower content of nitrogen in ingot S3 than in ingot S2.

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Figure 14. Scheil solidification calculation result for 20Cr-8Ni stainless steel: (a) S1; (b) S2; and (c) S3.

4.2. Effect of Titanium on Microstructure To evaluate the effect of titanium content on the microstructure of 20Cr-8Ni stainless steel, it was necessary to discuss the formation of auetenite durnig the solidification process or after the solidification process. On the basis of the equilibrium solidification process shown in Figure 13, the austenite formed through peritectic or eutectic reaction at the end of solidification process in ingot S1. The equilibrium solidification model of ingot S1 was the FA model (liquid liquid + δ-ferrite liquid → → + δ-ferrite + γ-austenite δ-ferrite + γ-austenite). Liang et al. also indicated that the equilibrium → solidification model of AISI 321 stainless steel was the FA model, according to the ratio of Ni equivalent and Cr equivalent [33]. In ingot S2 and ingot S3, the austenite formed after solidification, and the equilibrium solidification model was the F model (liquid liquid + δ-ferrite δ-ferrite). During the → → scheil solidification process shown in Figure 14, the austenite in ingot S1, S2, and S3 formed during solidification owing to element segregation. However, the solid fraction at which austenite precipitated increased with titanium addition. It could be concluded that titanium prevented the precipitation of austenite in 20Cr-8Ni stainless steel. According to Hammar and Svensson equivalents shown in Equations (1) and (2), the solidification models of ingot S1, S2, and S3 were all the F model [34]. This meant that equilibrium solidification calculated through Thermo-Calc software was more reliable. On the basis of the equilibrium solidification process, the soluble titanium content in ferrite and austenite at different temperatures was calculated. The results are shown in Figure 15. Figure 15a shows the soluble titanium content in ferrite when austenite began to form. TA was the temperature at which austenite began to form. With soluble titanium content in ferrite increasing through S1, S2, and S3, the stability of ferrite increased gradually. Moreover, transformation of ferrite to austenite retarded. Figure 15b shows the soluble titanium content in ferrite and austenite at 1100 ◦C. The solution treatment temperature for 20Cr-8Ni stainless steel in industrial practice was around 1100 ◦C.The content of soluble titanium in ferrite increased through S1, S2, and S3, and was higher than austenite, which was consistent with the EPMA result shown in Figure9. Figure 16 showed the calculated ferrite fraction at 1100 ◦C, and the calculated result was consistent with the experiment one.

Creq = (%Cr) + 1.37(%Mo) + 1.5(%Si) + 2(%Nb) + 3(%Ti) (1)

Nieq = (%Ni) + 22(%C) + 14.2(%N) + 0.31(%Mn) + (%Cu) (2) Metals 2020, 10, x FOR PEER REVIEW 13 of 16

Figure 14. Scheil solidification calculation result for 20Cr-8Ni stainless steel: (a) S1; (b) S2; and (c) S3.

4.2. Effect of Titanium on Microstructure To evaluate the effect of titanium content on the microstructure of 20Cr-8Ni stainless steel, it was necessary to discuss the formation of auetenite durnig the solidification process or after the solidification process. On the basis of the equilibrium solidification process shown in Figure 13, the austenite formed through peritectic or eutectic reaction at the end of solidification process in ingot S1. The equilibrium solidification model of ingot S1 was the FA model (liquid → liquid + δ-ferrite → liquid + δ-ferrite + γ-austenite → δ-ferrite + γ-austenite). Liang et al. also indicated that the equilibrium solidification model of AISI 321 stainless steel was the FA model, according to the ratio of Ni equivalent and Cr equivalent [33]. In ingot S2 and ingot S3, the austenite formed after solidification, and the equilibrium solidification model was the F model (liquid → liquid + δ-ferrite → δ-ferrite). During the scheil solidification process shown in Figure 14, the austenite in ingot S1, S2, and S3 formed during solidification owing to element segregation. However, the solid fraction at which austenite precipitated increased with titanium addition. It could be concluded that titanium prevented the precipitation of austenite in 20Cr-8Ni stainless steel. According to Hammar and Svensson equivalents shown in Equations (1) and (2), the solidification models of ingot S1, S2, and S3 were all the F model [34]. This meant that equilibrium solidification calculated through Thermo-Calc software was more reliable. On the basis of the equilibrium solidification process, the soluble titanium content in ferrite and austenite at different temperatures was calculated. The results are shown in Figure 15. Figure 15a shows the soluble titanium content in ferrite when austenite began to form. TA was the temperature at which austenite began to form. With soluble titanium content in ferrite increasing through S1, S2, and S3, the stability of ferrite increased gradually. Moreover, transformation of ferrite to austenite retarded. Figure 15b shows the soluble titanium content in ferrite and austenite at 1100 °C. The solution treatment temperature for 20Cr-8Ni stainless steel in industrial practice was around 1100 °C.The content of soluble titanium in ferrite increased through S1, S2, and S3, and was higher than austenite, which was consistent with the EPMA result shown in Figure 9. Figure 16 showed the calculated ferrite fraction at 1100 °C, and the calculated result was consistent with the experiment one.

Cr = (%Cr) +1.37(%Mo) +1.5(%Si) +2(%Nb) + 3(%Ti) (1) Metals 2020, 10, 529 13 of 15 Ni = (%Ni) +22(%C) + 14.2(%N) +0.31(%Mn) +(%Cu) (2)

Figure 15. Calculated Ti distribution in phase at different temperature through equilibrium MetalsFigure 2020, 10 15., x FOR Calculated PEER REVIEW Ti distribution in phase at different temperature through equilibrium14 of 16 thermodynamic analysis: (a)T and (b) 1100 C thermodynamic analysis: (a) TAA and (b) 1100 °C◦

Figure 16. Experiment mass fraction of ferrite in ingots and the calculated result through equilibrium Figure 16. Experiment mass fraction of ferrite in ingots and the calculated result through equilibrium thermodynamic analysis. thermodynamic analysis. 4.3. Improvement of Tensile Property 4.3. Improvement of Tensile Property On the basis of the discussion above, the mechanism for tensile property improvement would be discussedOn the basis in this of part.the discussion Comparing above, S1 with the me S2chanism or S3, the for smaller tensile equiaxed property grainimprovement size and higherwould massbe discussed fraction in of this ferrite part. improved Comparing the YSS1 andwith UTS. S2 or Comparing S3, the smaller S2 with equiaxed S3, the grain higher size mass and fraction higher ofmass ferrite fraction caused of ferrite the higher improved YS andthe YS UTS, and although UTS. Comparing the equiaxed S2 with grain S3, the size higher was nearly mass fraction the same. of Jangferrite and caused Son et the al. higher investigated YS and the UTS, effect although of microstructure the equiaxed on tensile grain property size was in nearly CD4MCU the castsame. duplex Jang stainlessand Son et steels al. investigated [27–29]. They the concluded effect of microstruc that the YSture and on UTS tensile increased property with in increasing CD4MCU mass cast fractionduplex ofstainless ferritic steels phase, [27–29]. because They the concluded ferritic phase that had the inherentlyYS and UTS higher increased strength with than increasing the austenitic mass fraction phase. Besides,of ferritic the phase, fine equiaxedbecause the grains ferritic also phase improved had inhe the tensilerently property,higher strength which than was similarthe austenitic to Schino phase. and Li’sBesides, research the fine [25, equiaxed26]. grains also improved the tensile property, which was similar to Schino and Li’s research [25,26]. 5. Conclusions 5. Conclusions To refine the solidification structure of 20Cr-8Ni stainless steel for heavy castings in industrial practice,To refine the eff theect ofsolidification titanium addition structure on of the 20Cr-8Ni as-cast structure stainless and steel high-temperature for heavy castings tensile in industrial property waspractice, investigated the effect in of the titanium laboratory. addition According on the as to-cast the structure analysis of and the high-temperature as-cast structure, tensile microstructure, property inclusionswas investigated type, and in tensilethe laboratory. properties According of ingot with to the diff analysiserent titanium of the content,as-cast st theructure, following microstructure, conclusions wereinclusions obtained: type, and tensile properties of ingot with different titanium content, the following conclusions were obtained: (1) With the increasing titanium content, the typical inclusions changed from dual phase Si-Mn- Ti oxides to pure TiN or complex Ti2O3 + TiN. Meanwhile, the equiaxed grain ratio of ingot increased from 51 percent to nearly 100 percent, and the size of equiaxed grain decreased owing to heterogeneous nucleation of δ-Fe; (2) With the increasing titanium content, the soluble titanium content in ferrite phase increased, retarded the formation of austenite phase, and changed the equilibrium solidification model from FA to F. As a result, the mass fraction of ferrite phase in ingot increased gradually; (3) Owing to increase of the ferrite phase mass fraction and decrease of the equiaxed grain size, the YS and UTS at 350 °C of ingot increased gradually, but tensile elongation changed little.

Author Contributions: Writing—review and editing, Q.W.; writing—original preparation, Q.W.; conceptualization, G.C.; formal analysis, Y.H. All authors have read and agreed to the published version of the manuscript.

Funding: This work was supported by the National Natural Science Foundation of China (No. 51674024).

Acknowledgments: The authors would like to appreciate NCS Testing Technology Co., Ltd. for contributing to the testing and inspection.

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(1) With the increasing titanium content, the typical inclusions changed from dual phase Si-Mn-Ti oxides to pure TiN or complex Ti2O3 + TiN. Meanwhile, the equiaxed grain ratio of ingot increased from 51 percent to nearly 100 percent, and the size of equiaxed grain decreased owing to heterogeneous nucleation of δ-Fe; (2) With the increasing titanium content, the soluble titanium content in ferrite phase increased, retarded the formation of austenite phase, and changed the equilibrium solidification model from FA to F. As a result, the mass fraction of ferrite phase in ingot increased gradually; (3) Owing to increase of the ferrite phase mass fraction and decrease of the equiaxed grain size, the YS and UTS at 350 ◦C of ingot increased gradually, but tensile elongation changed little.

Author Contributions: Writing—review and editing, Q.W.; writing—original draft preparation, Q.W.; conceptualization, G.C.; formal analysis, Y.H. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Natural Science Foundation of China (No. 51674024). Acknowledgments: The authors would like to appreciate NCS Testing Technology Co., Ltd. for contributing to the testing and inspection. Conflicts of Interest: The authors declare no conflict of interest.

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