Preparation of High-Nitrogen Ductile Iron by Injecting Nitrogen Gas in Molten Iron

materials

Article Preparation of High-Nitrogen Ductile Iron by Injecting Nitrogen Gas in Molten Iron

Lifeng Tong *, Shichao Liu, Jinchuan Jie and Tingju Li Key Laboratory of Solidiﬁcation Control and Digital Preparation Technology, School of Material Science and Engineering, Dalian University of Technology, Dalian 116000, China; [email protected] (S.L.); [email protected] (J.J.); [email protected] (T.L.) * Correspondence: [email protected]; Tel./Fax: +86-411-8470-6220

Received: 8 April 2020; Accepted: 27 May 2020; Published: 31 May 2020

Abstract: High-nitrogen ductile iron (DI) was prepared by a new method of injecting nitrogen gas into molten iron and nodularizing treatment. The microstructure and mechanical properties of the as-prepared DI for diﬀerent nitrogen gas injection periods were characterized. The graphite morphology gradually deteriorated with the increase in the nitrogen gas injection time. The maximum nitrogen and pearlite contents were obtained after 20 min of nitrogen gas injection, and the corresponding tensile strength and elongation of the DI were calculated as 492 MPa and 9.5%, respectively, which were 9.3% and 22% higher than those of the DI prepared without the nitrogen gas injection treatment, respectively.

Keywords: ductile iron; nitrogen gas; graphite morphology; tensile strength

1. Introduction It is usually believed that nitrogen increases the aging tendency and cold brittleness of steel and also significantly reduces its plasticity, toughness, and weldability [1–5]. Therefore, nitrogen is considered a harmful element for steel. However, the advent of high-nitrogen steel has changed this perception. It is found that when the nitrogen content reaches a certain amount, it can strengthen and refine steel and also increase the stability of passivation films, thus improving the yield strength, tensile strength, and corrosion resistance of steel [6–13]. Ductile iron (DI) is widely used in manufacturing industries due to its excellent casting performance, superior wear resistance, good hardenability, high thermal conductivity, and low thermal expansion coefficient [14–18]. It is of great significance to further improve the corresponding properties of DI. In the present paper, high-nitrogen DI was prepared by a new method of injecting nitrogen gas in molten iron and nodularizing treatment. The effects of different nitrogen gas injection periods on the nitrogen content, microstructure, and mechanical properties of the as-prepared DI were investigated.

2. Experimental Section A schematic representation of the high-nitrogen DI production process by injecting nitrogen gas into the molten iron is displayed in Figure1. The raw material, including 85% pig iron and 15% scrap steel, was melted into molten iron in an intermediate-frequency electric furnace at 1500–1600 ◦C. High-purity nitrogen gas (purity = 99.99%) was stored in a cylinder. Nitrogen gas first entered the dryer after the switch was turned on, and the desiccant absorbed the moisture from nitrogen gas. Nitrogen gas then entered a piezometer and, subsequently, a boron nitride vent pipe and a boron nitride vent plug. Finally, nitrogen gas was injected in molten iron through some small holes in the plug that were placed at the bottom of the furnace. During the injection process, the pressure intensity of nitrogen gas was controlled at 0.1 MPa by a piezometer, and the gas flow was controlled at 3 L/min by a flow

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Materials 2020, 13, 2508 2 of 9 of nitrogen gas was controlled at 0.1 MPa by a piezometer, and the gas flow was controlled at 3 L/min by a flow meter. Nitrogen gas was injected for the periods of 0 min, 20 min, 40 min, and 60 min, and meter.then molten Nitrogen iron gas treated wasinjected at 1470–1490 for the °C periods was po ofured 0 min, into 20 a min, tundish 40 min, where and the 60 min, nodularizer and then and molten the ironinoculant treated were at 1470–1490 pre-placed.◦C wasThepoured nodularizing into a tundish treatment where occurred the nodularizer between andmolten the inoculant iron and were the pre-placed.nodularizer, The and nodularizing Mg in the nodularizer treatment occurredremoved between harmful moltenelements iron (such and as the O nodularizer, and S); consequently, and Mg in thespherical nodularizer graphite removed was generated harmful in elements molten iron. (such Subsequently, as O and S); consequently, molten iron was spherical poured graphite into a sand was generatedmold. in molten iron. Subsequently, molten iron was poured into a sand mold.

Figure 1. SchematicSchematic diagram diagram of high-nitrogen DI, manufactured by injecting nitrogen gas.

The nitrogen content of DI was measured by a nitrogen–hydrogen–oxygen analyzer (TCH600, USA). The carbon and sulfur contents contents were were determined determined by a a carbon–sulfur carbon–sulfur analyzer analyzer (CS-8800, (CS-8800, China), China), and other elementselements werewere analyzedanalyzed byby aa fluorescencefluorescence analyzeranalyzer (XRF-1800,(XRF-1800, Japan).Japan). MetallographicMetallographic specimens were ground with emery papers (80–15 (80–150000 grits) and polished with with a a diamond paste. paste. Microstructural observation of the as-prepared DI after etching in 3% nital solution was performed by an optical microscope microscope (OM, (OM, MEF-4, MEF-4, German). German). OLYCIA OLYCIA m3 m3 image image analysis analysis software software was was used used to toanalyze analyze the the phase phase content content of the of theinvestigated investigated mate materialsrials and andthe morphology the morphology of graphite of graphite nodules. nodules. The The micrographs ( 100) of DI samples without etching were captured by a metallographic microscope micrographs (×100)× of DI samples without etching were captured by a metallographic microscope and analyzed by the graphite analysis function function of of OLYCIA OLYCIA m3 software to to calculate the the nodularity, nodularity, nodule size, nodule count, and area fractionfraction ofof graphite.graphite. The nodularity and the graphite nodule morphology were defined defined from the specimens withoutwithout etching according to the ASTM A247-17 Standard Test MethodMethod forfor EvaluatingEvaluating thethe MicrostructureMicrostructure ofof GraphiteGraphite inin IronIron Castings.Castings. The pearlite content was obtained by subtracting the graphite co contentntent in the microstructure without etching from the pearlite and graphite contents inin thethe microstructuremicrostructure afterafter etching.etching. Tensile teststests (specimen (specimen diameter diameter= 10= 10 mm mm and and original original gauge gauge length length= 50 = mm) 50 mm) were were conducted conducted in an electronicin an electronic universal universal test machine test machine (DNS100, (DNS100, China) withChina) a crosshead with a crosshead speed of speed 2 mm/ min.of 2 mm/min. The hardness The ofhardness DI was of measured DI was meas by aured Brinell by hardnessa Brinell hardness tester (THBS-3000E, tester (THBS-3000E, China; ball China; diameter ball diameter= Ø10) under = ∅10) a loadingunder a forceloading of 3000 force N of and 3000 a dwell N and time a dwell of 15 s.time Each of mechanical 15 s. Each performancemechanical performance test was conducted test was at leastconducted three timesat least at three room times temperature at room to temperature ensure the accuracyto ensure of the the accuracy obtained of results. the obtained results. 3. Results and Discussion 3. Results and Discussion 3.1. Microstructure 3.1. Microstructure Graphite morphology is an important index to evaluate the microstructural and mechanical Graphite morphology is an important index to evaluate the microstructural and mechanical properties of DI. The graphite morphologies of the DI without etching for different nitrogen gas properties of DI. The graphite morphologies of the DI without etching for different nitrogen gas injection periods are displayed in Figure2. The diameter, area fraction, nodularity, and nodule count injection periods are displayed in Figure 2. The diameter, area fraction, nodularity, and nodule count of graphite nodules of the investigated DI for different nitrogen gas injection periods are presented of graphite nodules of the investigated DI for different nitrogen gas injection periods are presented in Figures3–6, and its corresponding chemical compositions are depicted in Table1. The graphite in Figures 3–6, and its corresponding chemical compositions are depicted in Table 1. The graphite morphology of the untreated DI was mainly spherical or nearly spherical (Figure2a). The untreated morphology of the untreated DI was mainly spherical or nearly spherical (Figure 2a). The untreated DI had a nodule diameter of 81.03 µm, a graphite nodule area fraction of 7.5%, a nodularity of 92%, DI had a nodule diameter of 81.03 µm, a graphite nodule area fraction of 7.5%, a nodularity of 92%, a nodule count of 248.4/mm2, and carbon content of 3.50%. However, the graphite morphology of a nodule count of 248.4/mm2, and carbon content of 3.50%. However, the graphite morphology of the the DI deteriorated after 20 min of nitrogen gas injection. Some large graphite nodules were formed, DI deteriorated after 20 min of nitrogen gas injection. Some large graphite nodules were formed, and and a large number of finely-divided graphite nodules appeared in the matrix (Figure2b). The graphite a large number of finely-divided graphite nodules appeared in the matrix (Figure 2b). The graphite

Materials 2020, 13, 2508 3 of 9 Materials 2020, 13, x FOR PEER REVIEW 3 of 9 size, nodule count, nodularity, area fraction of graphite nodules, and carbon content of the DI treated size, nodule count, nodularity, area fraction of graphite nodules, and carbon content of the DI treated with nitrogen gas decreased significantly. After 40 min of nitrogen gas injection, spherical graphite with nitrogen gas decreased significantly. After 40 min of nitrogen gas injection, spherical graphite nodules disappeared (Figure 2c). Large irregular graphite nodules appeared, and the number of nodules disappeared (Figure2c). Large irregular graphite nodules appeared, and the number of finely finely divided graphite nodules decreased, thus increasing the overall diameter of graphite nodules divided graphite nodules decreased, thus increasing the overall diameter of graphite nodules (Figure3). (Figure 3). After 60 min of nitrogen gas injection, only finely divided graphite nodules existed (Figure After 60 min of nitrogen gas injection, only finely divided graphite nodules existed (Figure2d), and the 2d), and the nodularity, nodule count, nodule diameter and area fraction, and carbon content of the nodularity, nodule count, nodule diameter and area fraction, and carbon content of the DI all reached DI all reached the minimum, which may be attributed to graphite floatation in molten iron due to the the minimum, which may be attributed to graphite floatation in molten iron due to the injection of injection of nitrogen gas. Therefore, the nodularity, nodule count, diameter and area fraction of nitrogen gas. Therefore, the nodularity, nodule count, diameter and area fraction of graphite nodules graphite nodules of the investigated DI declined gradually with the increase in the nitrogen gas of the investigated DI declined gradually with the increase in the nitrogen gas injection time. injection time.

Figure 2. GraphiteGraphite nodule nodule morphology morphology (unetched) (unetched) of DI DI af afterter injecting injecting nitrogen nitrogen gas for for different diﬀerent time periods: ( a)) 0 0 min; min; ( b) 20 min; ( c) 40 min; ( d) 60 min.

Figure 3. DiameterDiameter of of graphite graphite nodules nodules in in DI DI after after injecting nitrogen gas for different diﬀerent time periods.

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. .. Figure 4. Area fraction of graphite in DI after injecting nitrogen gas for different time periods. FigureFigure 4.4. Area fraction of graphite inin DIDI afterafter injectinginjecting nitrogennitrogen gasgas forfor didifferentﬀerent timetime periods.periods.

Figure 5. NodularityNodularity of of DI by injecting nitrogen gas at different diﬀerent time periods. Figure 5. Nodularity of DI by injecting nitrogen gas at different time periods.

Figure 6. NoduleNodule count count of of DI DI after injecting nitrogen gas for different diﬀerent time periods. Figure 6. Nodule count of DI after injecting nitrogen gas for different time periods.

TheTable nitrogen 1. Chemical contents compositions of the investigated of DI after injecting DI for dinitrogenﬀerent gas nitrogen for different gas injectiontime periods. periods are Table 1. Chemical compositions of DI after injecting nitrogen gas for different time periods. presented in Figure7. The nitrogen content in the untreated DI was 0.0032%. However, in the Time N% O% H% C Si Cr Mn S P Fe nitrogen-gas-treatedTimeTime N% N% DI, the nitrogen O% O% H% H%content Cﬁrst C increased Si Si Cr andCr then Mn Mn decreased S S to some P P extent, Fe Fe with the min m/m m/m ug/g wt% wt% wt% wt% wt% wt% wt% increase inmin themin nitrogen m/mm/m gas injectionm/mm/m ug/g timeug/g [12ww].t%t% The w maximumwt%t% wwt%t% nitrogen wwt%t% contentwwt%t% ofww 0.0096%t%t% ww wast%t% obtained 0 0.0032 0.0005 7.5 3.50 2.97 0.03 0.25 0.024 0.015 Bal. after 20 min00 of nitrogen0.00320.0032 gas 0.00050.0005 injection. 7.57.5 3.50 3.50 2.972.97 0.030.03 0.250.25 0.0240.024 0.0150.015 Bal.Bal. 20 0.0096 0.0032 5.5 2.59 2.09 0.01 0.21 0.027 0.014 Bal. 2020 0.0096 0.0096 0.00320.0032 5.55.5 2.59 2.59 2.092.09 0.010.01 0.210.21 0.0270.027 0.0140.014 Bal.Bal. 40 0.0084 0.0022 4.5 2.56 2.22 0.01 0.24 0.027 0.013 Bal. 4040 0.0084 0.0084 0.00220.0022 4.54.5 2.56 2.56 2.222.22 0.010.01 0.240.24 0.0270.027 0.0130.013 Bal.Bal. 60 0.0075 0.0014 4.6 2.4 2.38 0.01 0.31 0.014 0.024 Bal. 6060 0.0075 0.0075 0.00140.0014 4.64.6 2.4 2.4 2.38 2.38 0.010.01 0.310.31 0.0140.014 0.0240.024 Bal.Bal.

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Table 1. Chemical compositions of DI after injecting nitrogen gas for diﬀerent time periods. Materials 2020, 13, x FOR PEER REVIEW 5 of 9 Time N% O% H% C Si Cr Mn S P Fe minThe nitrogen m/m contents m/m of ug the/g investigatedwt% wtDI% for differentwt% nitrogenwt% gaswt% injectionwt% periodswt% are presented0 in 0.0032 Figure 0.0005 7. The nitrogen 7.5 content 3.50 in 2.97 the untreated 0.03 DI 0.25 was 0.0032%. 0.024 However, 0.015 in Bal. the nitrogen-gas-treated20 0.0096 0.0032 DI, the nitrog 5.5en content 2.59 first 2.09 increased 0.01 and then 0.21 decreased 0.027 to some 0.014 extent, Bal. with the 40increase 0.0084 in the nitrogen 0.0022 gas 4.5 injection 2.56 time [12]. 2.22 The maximum 0.01 0.24nitrogen 0.027 content 0.013of 0.0096% Bal. was 60 0.0075 0.0014 4.6 2.4 2.38 0.01 0.31 0.014 0.024 Bal. obtained after 20 min of nitrogen gas injection. Materials 2020, 13, x FOR PEER REVIEW 5 of 9

The nitrogen contents of the investigated DI for different nitrogen gas injection periods are presented in Figure 7. The nitrogen content in the untreated DI was 0.0032%. However, in the nitrogen-gas-treated DI, the nitrogen content first increased and then decreased to some extent, with the increase in the nitrogen gas injection time [12]. The maximum nitrogen content of 0.0096% was obtained after 20 min of nitrogen gas injection.

FigureFigure 7. Content7. Content of of DI DI after after injectinginjecting nitrog nitrogenen gas gas for for different diﬀerent time time periods. periods.

The microstructures of the investigated DI for different nitrogen gas injection periods are The microstructures of the investigated DI for diﬀerent nitrogen gas injection periods are displayed in Figure 8, and the corresponding statistical area fraction of pearlite is presented in Figure displayed in Figure8, andFigure the 7. corresponding Content of DI after injecting statistical nitrog areaen gas fractionfor different of time pearlite periods. is presented in Figure9. 9. The microstructure of the untreated DI was composed of ferrite, pearlite (area fraction = 24.8%), The microstructure of the untreated DI was composed of ferrite, pearlite (area fraction = 24.8%), and graphite nodules (Figure 8a). After 20 min of nitrogen gas injection, the ferrite content in the DI and graphite nodulesThe microstructures (Figure8a). of Afterthe investigated 20 min ofDI nitrogen for different gas nitrogen injection, gas injection the ferrite periods content are in the DI matrix greatlydisplayed decreased, in Figure 8, andand the the corresponding pearlite cont statisticalent increasedarea fraction toof pearlite55.8%, is andpresented a small in Figure amount of matrixcementite greatly9. The decreased,was microstructure formed andsimultaneously of the the pearlite untreated (Figure content DI was 8b).composed increased With of the ferrite, to increase 55.8%, pearlite andin (area the a small fractionnitrogen amount = 24.8%), gas injection of cementite was formedtime, ferriteand simultaneously graphite gradually nodules disappeared, (Figure(Figure 8a).8b). After the With 20 pearlite min the of ni increase trogencontent gas in decreased,injection, the nitrogen the ferriteand gasthecontent injectioncementite in the DI time,content ferrite gradually disappeared,matrix greatly the decreased, pearlite and content the pearlite decreased, content andincreased the cementiteto 55.8%, and content a smallsigniﬁcantly amount of increased significantlycementite increased was formed (Figures simultaneously 8c,d and (Figure 10). The 8b). Withreduction the increase of the in graphitethe nitrogen area gas fractioninjection can be (Figureascribed8c,d and time,to the Figure ferrite formation 10gradually). The of adisappeared, reduction large number ofthe the ofpearlite cementite graphite content areagrains. decreased, fraction and can the be cementite ascribed content to the formation of a large numbersignificantly of cementite increased (Figures grains. 8c,d and 10). The reduction of the graphite area fraction can be ascribed to the formation of a large number of cementite grains.

Figure 8. Microstructure of DI after injecting nitrogen gas for diﬀerent time periods: (a) 0 min; (b) 20 min;

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MaterialsFigure2020 ,8.13 Microstructure, 2508 of DI after injecting nitrogen gas for different time periods: (a) 0 min; (b) 20 6 of 9 min; (c) 40 min; (d) 60 min.

Figure 9. TheThe amount of pearlite in DI after injectin injectingg nitrogen gas for different diﬀerent time periods.

3.2. Mechanical Property Figure 10 presents the Brinell hardnesshardness values of the investigated DI for didifferentfferent nitrogen gas injection periods.periods. It is clear clear that that the the hardness hardness of DI DI increased increased sharply with the increase in the nitrogen gas injection time. The tensile strength and elongati elongationon of the investigated DI for didifferentfferent nitrogen gas injection periods are presented in Figure 1111,, an andd the corresponding tensile fracture morphologies are exhibited in Figure Figure 12.12. The The untreated untreated DI DI had had a atensile tensile strength strength of of450 450 MPa MPa and and an anelongation elongation of of7.8%, 7.8%, and and obvious obvious ductile ductile tearing tearing and and few few cleaved cleaved surfaces surfaces were were observed observed on on its its tensile fracture surface (Figure(Figure 12 12a),a), thus thus leading leading to to ductile ductile fracture. fracture. The The tensile tensile strength strength and and elongation elongation of the of DI the after DI 20after min 20 of min nitrogen of nitrogen gas injection gas injection were 492were MPa 492 and MPa 9.5%, and respectively,9.5%, respectively, which which were 9.3% were and 9.3% 22% and higher 22% thanhigher those than of those the DI of prepared the DI prepared without without the nitrogen the nitrogen gas injection gas injection treatment, trea respectively.tment, respectively. In addition, In theaddition, cleavage the surfacecleavage area surface in the area tensile in the fracture tensile zone fracture increased zone increased (Figure 12 (Figureb). The presence12b). The ofpresence a large numberof a large of number pearlite of grains pearlite in thegrains matrix in the improved matrix theimproved comprehensive the comprehensive properties properties of the examined of the DI.examined The injection DI. The of injection nitrogen of gas nitrogen purifies gas molten purifies iron. molten Furthermore, iron. Furthermore, interstitial interstitial nitrogen possesses nitrogen apossesses powerful a strengtheningpowerful strengthening effect and effect increases and increase the strengths the ofstrength Fe-based of Fe-based alloys [19 alloys]. Nitrogen [19]. Nitrogen also has aalso significant has a significant effect on grain effect size on hardening. grain size Thehard grainening. size The hardening grain size eff ecthardening increases effect proportionally increases toproportionally the nitrogen to concentration the nitrogen concentration [20]. The reduction [20]. The of reduction the carbon of content the carbon and content the nodule and the count nodule was alsocount beneficial was also to beneficial the mechanical to the propertiesmechanical of properties the examined of the DI. examined However, DI. the However, tensile strength the tensile and elongationstrength and of elongation the investigated of the DIinvestigated decreased withDI de furthercreased increase with further in the increase nitrogen in gas the injection nitrogen time. gas Afterinjection 40 mintime. of After nitrogen 40 min gas injection,of nitrogen the gas elongation injection, of the DI elongation decreased andof DI became decreased almost and the became same asalmost the original the same DI. as Ductilethe original tearing DI. wasDuctile not tearin observed;g was however, not observed; some however, cracks appeared some cracks on the appeared tensile fractureon the tensile surface fracture (Figure surface 12c). After (Figure 60 min12c). of After nitrogen 60 min gas of injection, nitrogen the gas tensile injection, strength the tensile and elongation strength ofand DI elongation decreased of to 221DI decreased MPa and 3.5%,to 221 respectively. MPa and 3.5%, Cleavage respectively. surface becameCleavage the surface dominant became fracture the characteristic,dominant fracture and thecharacteristic, number of cracksand the increased. number Moreover,of cracks increased. a large number Moreover, of structural a large weaknesses number of werestructural formed weaknesses on the tensile were fracture formed surface; on thisthe indicatestensile fracture that the microstructuresurface; this wasindicates destroyed that withthe anmicrostructure increase in the was nitrogen destroyed gas injection with an time. increase The decreasein the nitrogen in pearlite, gasthe injection increase time. in cementite, The decrease and the in morphologypearlite, the increase deterioration in cementite, of graphite and the nodules morpho afterlogy longer deterioration nitrogen of gas graphi injectionte nodules periods after were longer the mainnitrogen reasons gas injection for the degraded periods were mechanical the main properties reasons offor the the investigated degraded mechanical DI. It is evident properties that longerof the nitrogeninvestigated gas injectionDI. It is evident periods that had longer an adverse nitrogen effect gas on injection the DI matrix. periods had an adverse effect on the DI matrix.The injection of nitrogen gas caused the graphite morphology to deteriorate and promoted the formation of pearlite and cementite. The formation of pearlite, the increase in the nitrogen content, and the purification effect of nitrogen gas improved the mechanical properties of the investigated DI. Pearlite and cementite increased hardness. The deteriorated graphite morphology, cementite, and structural weakness were unfavorable to the mechanical properties of the examined DI. Therefore, the mechanical properties of the studied high-nitrogen DI were affected by numerous factors.

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FigureFigure 10. 10. BrinellBrinell hardnesshardness ofof DIDI afterafter injecting injecting ni nitrogennitrogentrogen gas gas for for different didifferentﬀerent timetime periods.periods. Figure 10. Brinell hardness of DI after injecting nitrogen gas for different time periods.

Figure 11. Tensile strength and elongation of DI after injecting nitrogen gas for different time Figure 11. Tensile strength and elongation of DI after injecting nitrogen gas for different time FigureFigure 11. Tensile11. Tensile strength strength and and elongation elongation of DI periods.of after DI afte injecting r injecting nitrogen nitrogen gas for gas di forﬀerent different time periods.time periods.periods.

Figure 12. Tensile fracture surface morphology of DI after injecting nitrogen gas for different time Figure 12. Tensile fracture surface morphology of DI after injecting nitrogen gas for diﬀerent time periods: (a) 0 min; (b) 20 min; (c) 40 min; (d) 60 min. periods: (a) 0 min; (b) 20 min; (c) 40 min; (d) 60 min.

FigureFigure 12.12. TensileTensile fracturefracture surfacesurface morphologymorphology ofof DIDI afteafterr injectinginjecting nitrogennitrogen gasgas forfor differentdifferent timetime periods:periods: ((aa)) 00 min;min; ((bb)) 2020 min;min; ((cc)) 4040 min;min; ((dd)) 6060 min.min.

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4. Conclusions The combination of injecting nitrogen gas in molten iron and nodularizing treatment to prepare high-nitrogen DI is a novel approach to strengthen DI that is different from previous other methods. The data in this study are in a preliminary state and likely to be improved in future studies. The microstructure and mechanical properties of the as-prepared DI for different nitrogen gas injection periods were characterized. The injection of nitrogen gas in molten iron accelerated the formation of pearlite in the DI matrix. The nitrogen and pearlite contents first increased and then decreased with an increase in the nitrogen gas injection time. The maximum nitrogen and pearlite contents were obtained after 20 min of nitrogen gas injection, and the corresponding tensile strength and elongation of the DI were 492 MPa and 9.5%, respectively, which were 9.3% and 22% higher than those of the DI without the nitrogen gas injection treatment, respectively. The graphite morphology gradually deteriorated, and the cementite content gradually increased with the increase in the nitrogen gas injection time, and this phenomenon degraded the microstructure and mechanical properties of the investigated DI.

Author Contributions: Data curation, L.T. and S.L.; Investigation, L.T. and S.L.; Methodology, L.T.; Project administration, J.J. and T.L.; Resources, J.J. and T.L.; Validation, T.L.; Visualization, S.L.; Writing—original draft, L.T.; Writing—review & editing, J.J. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Key Research and Development Program of China (No. 2017YFA0403800). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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