Characteristics and Formation Mechanism of

metals Article

Characteristics and Formation Mechanism of Inclusions in 304L Stainless Steel during the VOD Refining Process Xingrun Chen 1,2, *, Guoguang Cheng 1, *, Jingyu Li 1 , Yuyang Hou 1 , Jixiang Pan 2 and Qiang Ruan 2 1 2

*

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China; [email protected] (J.L.); [email protected] (Y.H.) Hongxing Iron & Steel Co. Ltd., Jiuquan Iron and Steel Group Corporation, Jiayuguan 735100, China; [email protected] (J.P.); [email protected] (Q.R.) Correspondence: [email protected] (X.C.); [email protected] (G.C.); Tel.: +86-106-233-4664 (X.C. & G.C.)

Received: 27 October 2018; Accepted: 29 November 2018; Published: 5 December 2018

Abstract: The formation and characteristics of non-metallic inclusions in 304L stainless steel during the vacuum oxygen decarburization (VOD) refining process were investigated using industrial experiments and thermodynamic calculations. The compositional characteristics indicated that two types of inclusions with different sizes (from 1 µm to 30 µm) existed in 304L stainless steel during the VOD refining process, i.e., CaO-SiO2 -Al2 O3 -MgO external inclusions, and CaO-SiO2 -Al2 O3 -MgOMnO endogenous inclusions. The calculation results obtained using the FactSage 7.1 software confirmed that the inclusions that were larger than 5 µm were mostly CaO-SiO2 -Al2 O3 -MgO; the similarity in composition to the slag indicated that these inclusions originated from the slag entrapment. The CaO-SiO2 -Al2 O3 -MgO-MnO inclusions that were smaller than 5 µm originated mainly from the oxidation reaction with Ca, Al, Mg, Si, and Mn. The changes in the inclusion composition resulting from changes in the Ca, Al, and O contents, and the temperature during the VOD refining process was larger for the smaller inclusions. Generating mechanisms for the CaO-SiO2 -Al2 O3 -MgO-MnO inclusions in the 304L stainless steel were proposed. Keywords: 304L stainless steel; non-metallic inclusions; formation mechanism; VOD refining

1. Introduction In recent years, 304L stainless steel has been rapidly developed, and it is widely used in shipbuilding, offshore drilling platform construction, metal structures for construction of buildings and bridges, containers and cisterns, flux-cored wire, and in industrial transport machinery engineering, petrochemical engineering, nuclear power engineering, etc. [1–4]. Because of its harsh application environment, the requirements for C, N, P, S, and O elements are stringent, the production is difficult, and the added value of the products is high. In addition to the control of the metal elements, the control of inclusions is also the key to improving the quality of 304L stainless steel. The control of the composition, quantity, and size of the inclusions at the beginning of their formation is likely to become a new and effective method to reduce the harmful influence of the inclusions. Therefore, it is very important to investigate the source and formation mechanism of the inclusions [5–18]. The formation of non-metallic inclusions of Si-killed stainless steel during the GOR (gas oxygen refining) process has been reported by Li et al. [7]. The authors found that the inclusions that were larger than 5 µm and contained more than 30% CaO were attributed to the modification of slag droplets through the oxidation of Si and Al and the collision with deoxidation-type Metals 2018, 8, 1024; doi:10.3390/met8121024

www.mdpi.com/journal/metals

Metals 2018, 8, 1024

2 of 11

inclusions; in addition, the degree of change was larger for the smaller inclusions. A tracer was used by Kim et al. [8] to determine the source of the large inclusions (larger than 20 µm) in 304 stainless steel, and it was found that these inclusions originated from the slag entrapment. This result was confirmed by the study of Ehara et al. [9], who also pointed out that the oxidation of Si and Al on the surface of slag inclusions can lead to the increase in the SiO2 and Al2 O3 contents. Yin et al. [10] reported that after de-oxidation with Si/Mn additions, spherical complex inclusions mainly consisting of calcium silicates were observed. The contents of MgO and Al2 O3 in these inclusions continuously increased as the steel moved from the argon–oxygen decarburization (AOD) through ladle processing to the tundish. Park et al. [11] reported that the inclusions mainly consisted of Al2 O3 when the Al content was higher than 0.05%. Ren et al. [12] pointed out that a high-basicity slag improved the cleanness of stainless steel, whereas a low basicity slag lowered the Al2 O3 content in the inclusions, thereby lowering the melting temperature of the inclusions and improving the deformability of the inclusions. This result was confirmed by the studies of Yan et al. [13,14] and Sakata [15]. However, in previous studies on the formation of inclusions, only a single factor was considered, namely external inclusions or endogenous inclusions. The formation mechanism of both types of inclusions has not been clarified to date. In the present study, the 304L stainless steel was produced by the process route of basic oxygen furnace (BOF)–AOD–vacuum oxygen decarburization (VOD)–ladle furnace (LF)–continuous casting (CC). The characteristics and generating mechanism of the inclusions during the VOD refining process were investigated using industrial experiments and thermodynamic calculations with FactSage 7.1 software; in addition, a formation mechanism for non-metallic inclusions occurring in the VOD refining process was proposed. 2. Experiments 2.1. Experimental Procedure and Sampling The smelting process route of 304L stainless steel consisted of BOF–AOD–VOD–LF–CC. The molten iron from the blast furnace was used as the raw material, and it was directly poured into the AOD furnace for decarburization and denitrification treatments after the dephosphorization pre-treatment; the target values of the carbon and nitrogen content were 0.30–0.50% and less than 0.08%, respectively and ferrosilicon was used for the Cr reduction in the AOD process. The slag was tapped after the AOD process, and then the ladle (the refractory material was magnesia-calcium) was hung to the VOD for deep decarburization and denitrification; ferrosilicon was used for Cr reduction in the VOD process, and subsequently, the liquid steel was poured into the LF furnace. When the temperature and composition of the liquid steel met the requirements, it was transported to the platform for CC. The VOD consists of three stages: (i) oxygen blowing: oxygen was blown onto the melt at 1000–1500 m3 /h for 40–60 min. Argon was injected at 200–600 L/min through two porous bricks at the bottom of the ladle. Lime was charged to form a basic slag. The total pressure was maintained at between 2.6 and 16 kPa; (ii) vacuum carbon deoxidation (VCD): the total pressure was reduced to between 26.6 and 66.5 Pa for 10–15 min without any additions of oxygen, fluxes, or ferroalloys. The bottom stirring argon flow rate was increased to 800–1000 L/min because at this stage of very low carbon contents, the decarburization is controlled by the rate of mass transfer of carbon and oxygen in the bath. The oxygen content was 0.015% after the VCD stage; (iii) reduction: ferrosilicon was added as a reducing agent for the chromium oxide in the slag. The aluminum content of the ferrosilicon was 1.3%. The final steel temperature was 1600 ◦ C. In order to elucidate the formation of the nonmetallic inclusions in the 304L stainless steel, a steel sample was taken at the end of the VOD and was immediately quenched in water. A slag sample was taken after the VOD treatment.

Metals 2018, 8, 1024

3 of 11

2.2. Composition Analysis and Inclusion Characterization The chemical composition of the steel samples were determined by a direct reading of the spectrum (ARL4460, Thermo Fisher Scientific, Waltham, MA, USA). The contents of C and S were analyzed by a C/S analyzer (CS-800, ELTRA, Haan, Germany). Cylinders (Φ 5 mm × 5 mm) were machined for the measurement of the total oxygen contents, which were analyzed using the inert gas fusion–infrared absorptiometry method. The acid-soluble Al and Ca contents in the steel were determined using the inductively coupled plasma optical emission spectroscopy method (ICP-OES). The composition of the slags was analyzed by an X-ray fluo-rescence spectrometer (ARL PERFORM’X, Thermo Fisher Scientific, Waltham, MA, USA). The morphologies of the inclusions in the specimens were observed using scanning electron microscopy (Merlin Compact, Zeiss, Gottingen, Germany) (SEM). The 15 mm × 15 mm × 10 mm samples for the SEM analysis were made by cutting, grinding, and polishing. The chemical compositions of the inclusions were analyzed with an energy dispersive spectrometer (X-Max 80, Oxford Instruments, High Wycombe, UK) (EDS) to determine the inclusion type. A quantitative analysis of the inclusions was performed using the INCA software (Inca Energy 250, Oxford Instruments, High Wycombe, UK) of the scanning electron microscope. To ensure good accuracy for the automated EDS analysis of the inclusions, the size of the inclusions was larger than 1 µm because the interaction volume may spread into the steel and excite electrons from the environment surrounding the inclusions if the diameters are smaller than 1 µm. 3. Results 3.1. Composition of the Molten Steel and Slag The averages of the chemical compositions of the molten steel and slag after the VOD stage are listed in Tables 1 and 2, respectively. It can be seen that after the VOD treatment, the contents of C and N in steel were 0.008% and 0.015% respectively. The oxygen content and sulfur content were higher because of deep degassing. The S content after VOD reached 30 ppm, indicating that the oxygen potential was higher in the molten steel, which was consistent with the conclusion drawn from the test results of the slag. Conversely, the slag and steel continuously reacted with each other during the VOD refining process, and the slag–steel balance was finally achieved. The contents of Cr2 O3 and FeO in the slag were much higher after VOD refining, and were 0.43% and 0.15%, respectively. Table 1. Chemical composition of 304L after vacuum oxygen decarburization (VOD) treatment (wt %). Stage

C

Si

Mn

P

S

Ni

Cr

Ca

Al

N

Mg

O

VOD

0.008

0.22

1.14

0.015

0.003

8.04

18.00

0.002

0.004

0.015

0.0005

0.007

Table 2. Chemical composition of 304L slag after VOD treatment (wt %). Stage

CaO

SiO2

MgO

Al2 O3

Cr2 O3

FeO

VOD

59.16

29.12

5.97

1.87

0.43

0.15

3.2. Characterization of the Inclusions The morphology and the compositions of the inclusions in the molten steel after the VOD process are shown in Figure 1. It was observed that two types of inclusions existed in the 304L steel after the VOD process. The first type of inclusions were spherical CaO-SiO2 -Al2 O3 -MgO with sizes ranging from several to tens of microns at the end of the VOD smelting (Figure 1a). These inclusions contained a small amount of aluminum. The second type consisted of endogenous inclusions with a size smaller than 5 µm, and a different composition. The common types of inclusions were CaO-SiO2 -Al2 O3 -MgO-MnO, which represented the dominant type, as shown in Figure 1b.

Metals 2018, 8, x FOR PEER REVIEW

4 of 11

Metals 2018, 8, 1024 x FOR PEER REVIEW

(a1)

4 4ofof1111

(a2)

(a3)

(a2)

(a1)

(a3)

Wt% O:46.7 Ca:28.5 Wt% Si:17.9 O:46.7 Al:0.7 Ca:28.5 Mg:6.2 Si:17.9 Al:0.7 Mg:6.2

(b1)

(b3)

(b2)

(b3)

(b2)

(b1)



Wt% O:46.0 Mn:1.0 Wt% Ca:28.2 O:46.0 Si:14.8 Mn:1.0 Al:6.1 Ca:28.2 Mg:3.9 Si:14.8 Al:6.1 Mg:3.9



Figure 1. Morphology of typical inclusions encountered in the samples: (a) CaO-SiO2-MgO-Al2O3; (b) 2-Al2O3-MgO-MnO. CaO-SiO Figure 1. Morphology Morphology of typical inclusions encountered in the samples: (a) (a) CaO-SiO CaO-SiO22-MgO-Al -MgO-Al22OO3;3 (b) ; (b) CaO-SiO22-Al -Al22OO3-MgO-MnO. 3 -MgO-MnO.

3.3. Corresponding Relation between the Size and Composition of the Inclusions 3.3. Corresponding Relation between the SizeSize and and Composition of the Inclusions 3.3. Corresponding Relation between Composition theinclusions Inclusionsof various sizes were The mass fractions of CaO, SiO2the , Al2O3, MgO, and MnO ofofthe The mass mass fractions of CaO, CaO, SiOderived , Al Ofrom ,MgO, MgO, and MnO theinclusions inclusions various sizeswere were calculated using the mass fraction the EDS, as shown in Figure 2. Itvarious can be sizes clearly seen The fractions of SiO 22, Al22O3,3 and MnO ofofthe ofof calculated using the mass fraction derived from the EDS, as shown in Figure 2. It can be clearly that the inclusion composition exhibited changes with the increase in size from 1 μm to 30 μm. Most calculated using the mass fraction derived from the EDS, as shown in Figure 2. It can be clearly seen seen thatinclusion the inclusion composition exhibited changes with thebased increase in size µm to 30 µm. inclusions were smaller than 5exhibited μm. We created two categories on the relationship between the that the composition changes with the increase in size from 1 from μm to1 30 μm. Most Most inclusions were smaller than 5 µm. We created two categories based on the relationship between size and mass composition, i.e., larger 5 μm based and smaller 5 μm. The inclusions inclusions werepercent smallerof than 5 μm. We created twothan categories on the than relationship between the the size and mass percent of composition, i.e., larger than 5 µm and smaller than 5 µm. The inclusions that were larger than 5 μm had almost the same CaO, SiO 2 , MgO, and Al 2 O 3 contents, but the MnO size and mass percent of composition, i.e., larger than 5 μm and smaller than 5 μm. The inclusions that werewere larger than µmhad had almost same CaO, SiO and Alsmaller but the contents very low. The composition differed for the that were than 5 μm. The 2 , MgO, 2 O3 contents, that were larger than 5 5μm almost thethe same CaO, SiOinclusions 2, MgO, and Al 2O3 contents, but the MnO MnO contents were very low. The composition differed for the inclusions that were smaller than contentswere of SiO 2, CaO, and composition Al2O3 fluctuated in afor wide range from 0 to more than than 30%.5 The contents very low. The differed the inclusions that were smaller μm. MnO The 5contents µm. The contents of SiO , CaO, and Al O fluctuated in a wide range from 0 to more than 30%. contents increased markedly from 0 to more than 10% as the size of the inclusions decreased. To 2 2 3 of SiO2, CaO, and Al2O3 fluctuated in a wide range from 0 to more than 30%. The MnO The MnO contents increased markedly from 0 to more than 10% as the size of the inclusions decreased. correlateincreased the compositions of from inclusions (size than smaller than 5 μm), weredecreased. plotted onTo the contents markedly 0 to more 10% as the sizethe of inclusions the inclusions To correlate compositions of inclusions (size smaller 5inclusion µm), thequantity inclusions were plotted on ternary system, as shown of in inclusions Figure 3. It(size was shown thatthan increased with the correlate thethe compositions smaller than 5the μm), the inclusions were plotted on the the ternary system, as shown in Figure 3. It was shown that the inclusion quantity increased with the inclusion size decreasing. The contents of SiO 2 , CaO, and Al 2 O 3 fluctuated in a wide range, but the ternary system, as shown in Figure 3. It was shown that the inclusion quantity increased with the inclusion size decreasing. The contents of SiO22,, CaO, CaO, and Al Al22OO3 3fluctuated fluctuatedininaawide widerange, range,but butthe the contents of MgO and MnO hadcontents smallerof variation of range. inclusion size decreasing. The SiO and contents and MnO MnO had had smaller smaller variation variation of of range. range. contents of of MgO MgO and

Figure 2. Cont.

8, x FOR PEER REVIEW MetalsMetals 2018, 2018, 8, 1024 Metals 2018, 8, x FOR PEER REVIEW

5 of 511of 11 5 of 11

Figure 2. The relationship between inclusion size and inclusion content. Figure Figure2.2.The Therelationship relationshipbetween betweeninclusion inclusionsize sizeand andinclusion inclusioncontent. content.

Figure 3. Composition distributions (mass fraction) of inclusions smaller 5 μm). Figure 3. Composition distributions (mass fraction) of inclusions (size(size smaller than than 5 µm). Figure 3. Composition distributions (mass fraction) of inclusions (size smaller than 5 μm).

4. Discussion 4. Discussion 4. Discussion The The compositional characteristics shown in Figures 2 and 3 indicate that that two two types of inclusions compositional characteristics shown in Figures 2 and 3 indicate types of inclusions existed in the 304L stainless steel during the VOD refining process, i.e., the CaO-SiO -Al O 2 3 -MgO The compositional characteristics shown in Figures 2 and 3 indicate that two types of 2inclusions existed in the 304L stainless steel during the VOD refining process, i.e., the CaO-SiO 2-Al 2O3-MgO external inclusions that were larger than 5 µm, and the CaO-SiO -Al O -MgO-MnO endogenous 2 2 2-Al 3 the existed in the 304L stainless steel larger duringthan the VOD i.e., 2-Al2endogenous O3-MgO external inclusions that were 5 μm,refining and theprocess, CaO-SiO 2O3CaO-SiO -MgO-MnO inclusions that that werewere smaller thanlarger 5 µm. The The composition of the sample showed external inclusions thatsmaller were 5 μm, and the CaO-SiO 2-Al2in O3the -MgO-MnO endogenous inclusions than 5than μm. composition ofinclusions the inclusions in same the same sample showed ainclusions large difference. Therefore, the source and the formation process of the inclusions are likely different; were smaller than 5 μm. The composition of the inclusions in of thethe same sample showed a largethat difference. Therefore, the source and the formation process inclusions are likely this is discussed in theinfollowing a large difference. Therefore, the source and the formation different; thisinisdetail discussed detail in sections. the following sections.process of the inclusions are likely

different; this is discussed in detail in the following sections.

Metals 2018, 8, 1024 Metals 2018, 8, x FOR PEER REVIEW

6 of 11 6 of 11

4.1. Generating the Mechanism of CaO-SiO2-MgO-Al2O3 Inclusions 4.1. Generating the Mechanism of CaO-SiO2 -MgO-Al2 O3 Inclusions Figure 4 shows the CaO-SiO2-Al2O3 phase diagram of the inclusions (larger than 5μm) and their Figure 4 shows the CaO-SiO2 -Al2 O3 phase diagram of the inclusions (larger than 5µm) and their composition during the VOD process. The composition of the slag and the average composition of composition during the VOD process. The composition of the slag and the average composition of the the inclusions are listed in the figure. It can be clearly seen that the compositions of the inclusions inclusions are listed in the figure. It can be clearly seen that the compositions of the inclusions were in were in good agreement with slag composition. The inclusions consisted mostly of slag components, good agreement with slag composition. The inclusions consisted mostly of slag components, and they and they originated from slag entrapment. In addition, the Al2O3 contents in the inclusions were very originated from slag entrapment. In addition, the Al2 O3 contents in the inclusions were very low, low, and were very close to that of the slag. The MgO contents in the inclusions in the VOD process and were very close to that of the slag. The MgO contents in the inclusions in the VOD process were were also close to that of the slag. Therefore, it can be concluded that the inclusions that were larger also close to that of the slag. Therefore, it can be concluded that the inclusions that were larger than than 5 μm during the VOD process were mainly derived from the entrapment of the top slag. The 5 µm during the VOD process were mainly derived from the entrapment of the top slag. The strong strong argon stirring in the VOD smelting causes the inclusions to be directly or indirectly in argon stirring in the VOD smelting causes the inclusions to be directly or indirectly in equilibrium equilibrium between the molten steel and slag. Thus, the control of the inclusions can be achieved by between the molten steel and slag. Thus, the control of the inclusions can be achieved by controlling controlling the slag composition. The CaO and SiO2 contents were higher in some inclusions. The the slag composition. The CaO and SiO2 contents were higher in some inclusions. The main reason main reason was that some inclusions such as CaSiO3 contain a higher content of SiO2, and Ca2SiO4 was that some inclusions such as CaSiO3 contain a higher content of SiO2 , and Ca2 SiO4 contains a contains a higher content of CaO precipitated in the cooling process. The oxidations of Si and Al on higher content of CaO precipitated in the cooling process. The oxidations of Si and Al on the surfaces the surfaces of some inclusions led to increases in the SiO2 and Al2O3 contents. of some inclusions led to increases in the SiO2 and Al2 O3 contents.

Figure 4. Chemical composition of the inclusions (larger than 5 µm) and the slag during the VOD process. Figure 4. Chemical composition of the inclusions (larger than 5 μm) and the slag during the VOD process.

Qian [19] reported that the flow velocity at the critical interface between the steel and slag was 0.65Qian m/s.[19] When the flow velocity at the interface was largerinterface than thebetween critical velocity, reported that the flow velocity at the critical the steeldroplets and slagformed was and were entrapped into the molten steel; the average size of the slag droplets from theformed top slag 0.65 m/s. When the flow velocity at the interface was larger than the critical velocity, droplets gradually decreasedinto withthe increasing velocity the interface. and were entrapped molten steel; theataverage size of the slag droplets from the top slag The stirring intensity of the argon blowing during the VOD reduction stage was 800–1000 L/min, gradually decreased with increasing velocity at the interface. which far stronger theargon argonblowing blowingduring intensity 200–400 L/minstage during LF stageL/min, [20,21]; Thewas stirring intensitythan of the theofVOD reduction wasthe 800–1000 therefore, thestronger velocity than at thethe interface betweenintensity the steelof and slag was greater thanthe 2 m/s. The[20,21]; reaction which was far argon blowing 200–400 L/min during LF stage between the slag and steel was very intense during the VOD refining process and the droplets were therefore, the velocity at the interface between the steel and slag was greater than 2 m/s. The reaction stronglythe mixed and steel changed small particles; as a result, the minimum of the droplets between slag and was into veryvery intense during the VOD refining process and size the droplets were during VOD refining was 5 µm. strongly mixed and changed into very small particles; as a result, the minimum size of the droplets

during VOD refining was 5 μm. 4.2. Generating the Mechanism of CaO-SiO2 -Al2 O3 -MgO-MnO Inclusions 4.2. Generating Mechanism of CaO-SiO 2-Al2O3-MgO-MnO Inclusions Aachen, Germany) was used for Factsagethe (FactsageTM7.1, Thermfact/CRCT & GTT-Technologies, the thermodynamic calculations of the inclusions in the 304 L stainless steel during the VOD refining Factsage (FactsageTM7.1, Thermfact/CRCT & GTT-Technologies, Aachen, Germany) was used process; the databases FTmisc and FToxid were used, and the calculation module was Equilib with for the thermodynamic calculations of the inclusions in the 304 L stainless steel during the VOD 100 g of molten steel. Due to the local non-uniformities of composition and temperature in molten refining process; the databases FTmisc and FToxid were used, and the calculation module was steel during the VOD refining process, the elements with local fluctuations in concentrations in the Equilib with 100 g of molten steel. Due to the local non-uniformities of composition and temperature molten steel had a wide range of concentrations and temperatures compared to the other components in molten steel during the VOD refining process, the elements with local fluctuations in determined after the VOD. Figure 5 shows the inclusion composition as a function of the oxygen concentrations in the molten steel had a wide range of concentrations and temperatures compared to concentration when the Al content, Ca content, Mg content and the temperature is 0.004%, 0.002%, the other components determined after the VOD. Figure 5 shows the inclusion composition as a function of the oxygen concentration when the Al content, Ca content, Mg content and the

2018, 8, 1024 FOR PEER PEER REVIEW REVIEW Metals 2018, xx FOR

of 11 of 11 777 of

temperature is is 0.004%, 0.004%, 0.002%, 0.002%, 0.0005%, 0.0005%, and and 1600 1600 °C °C respectively. respectively. The The CaO CaO concentration concentration exhibits exhibits aa temperature ◦ C respectively. The CaO concentration exhibits a steady increasing trend with a 0.0005%, and 1600 steady increasing trend with a decrease in the oxygen concentration, whereas the MgO and SiO SiO22 steady increasing trend with a decrease in the oxygen concentration, whereas the MgO and decrease in the oxygen concentration, whereas the MgO and SiO concentrations exhibit a stable trend, 2 concentrations exhibit exhibit aa stable stable trend, trend, and and the the MnO MnO concentration concentration decreases. However, However, the the Al Al22O O33 concentrations decreases. and the MnO concentration decreases. However, the Al O concentration remains steady at an oxygen 2 3 concentration remains steady at an oxygen concentration range from 0.015% to 0.011%, and then concentration remains steady at an oxygen concentration range from 0.015% to 0.011%, and then concentration range with from decreasing 0.015% to 0.011%, thenat decreases withof oxygen gradually decreases decreases with decreasing oxygen and content atgradually oxygen concentrations concentrations ofdecreasing less than than 0.011%. 0.011%. gradually oxygen content oxygen less content at oxygen concentrations of less than 0.011%. The oxygen concentration is at 150 ppm after The oxygen oxygen concentration concentration is is at at 150 150 ppm ppm after after the the VOD VOD refining refining process, process, and and at at this this stage, stage, the the elements elements The the VOD with refining and at this stage, the elements compete with each other for oxidation as compete with eachprocess, other for for oxidation as ferrosilicon ferrosilicon is added added for deoxidation. deoxidation. During the alloying alloying compete each other oxidation as is for During the ferrosilicon is added for deoxidation. During the alloying deoxidation, strong oxidizing elements such deoxidation, strong strong oxidizing oxidizing elements elements such such as as Ca Ca and and Al Al react react with with and and consume consume some some of of the the deoxidation, as Ca and Al react with and consume some of the oxygen. However, since the concentration of Si in the oxygen. However, since the concentration of Si in the molten steel is high, the deoxidation reaction oxygen. However, since the concentration of Si in the molten steel is high, the deoxidation reaction molten steel is high, the deoxidation reaction of Si also takes precedence. The Mg concentration is at a of Si also takes precedence. The Mg concentration is at a low level, resulting in a low concentration of Si also takes precedence. The Mg concentration is at a low level, resulting in a low concentration low level, resulting in a low concentration of MgO in the inclusion. Moreover, the remaining oxygen of MgO MgO in in the the inclusion. inclusion. Moreover, Moreover, the the remaining remaining oxygen oxygen reacts reacts with with weak weak oxidizing oxidizing elements elements such such of reacts with weak oxidizing elements such as Mn, leading to a small amount of MnO in the inclusions. as Mn, leading to a small amount of MnO in the inclusions. With continuing deoxidation, the Al O33 as Mn, leading to a small amount of MnO in the inclusions. With continuing deoxidation, the Al22O With continuing deoxidation, the Al O and MnO concentrations decrease, triggering an increase in and MnO MnO concentrations concentrations decrease, decrease, 2triggering triggering an increase increase in in the the CaO. CaO. During During the the VOD VOD refining refining 3 and an the CaO. During the VOD refining process, the oxygen content fluctuates with the deoxidation reaction, process, the the oxygen oxygen content content fluctuates fluctuates with with the the deoxidation deoxidation reaction, reaction, causing causing the the fluctuations fluctuations in in each each process, causing the fluctuations in each component of the endogenous inclusions that are smaller than 5 µm. component of the endogenous inclusions that are smaller than 5 μm. component of the endogenous inclusions that are smaller than 5 μm.

5. Inclusion function of of the Figure 5. Inclusion composition composition as as aa function Figure the O O content. content.

To To determine determine the the effect effect of of Ca Ca on on the the inclusions, inclusions, the the inclusion inclusion composition composition as as aaa function function of of To determine the effect of Ca on the inclusions, the inclusion composition as function of increasing Ca content was calculated using the FToxid and FTmisc databases of FactsageTM7.1 when increasing Ca Ca content content was was calculated calculated using using the the FToxid FToxid and and FTmisc FTmisc databases databases of of FactsageTM7.1 FactsageTM7.1 when when increasing ◦ C, the O content, Al content, Mg content, and the temperature is 0.007%, 0.004%, 0.0005%, 0.0005%, and and 1600 1600 °C, the O content, Al content, Mg content, and the temperature is 0.007%, 0.004%, the O content, Al content, Mg content, and the temperature is 0.007%, 0.004%, 0.0005%, and 1600 °C, respectively It was observed that an increase in the Ca content results an increase respectively (Figure (Figure 6). 6). It It was was observed observed that that an an increase increase in in the the Ca Ca content content results in in an an increase increase in in the the respectively (Figure 6). results in in the CaO and SiO contents, and a decrease in the Al O content; however, the MgO and MnO contents 2 2 3 CaO and and SiO SiO22 contents, contents, and and aa decrease decrease in in the the Al Al22O O33 content; content; however, however, the the MgO MgO and and MnO MnO contents contents CaO did not change much. The Ca Ca was was mainly mainly derived derived from from the the metal–slag metal–slag reaction, reaction, the the metal–refractory metal–refractory did not change much. The did not change much. The Ca was mainly derived from the metal–slag reaction, the metal–refractory reaction, this resulted in in fluctuations in reaction, and and the theCa Cafrom fromthe theFeSi FeSialloy alloyduring duringthe theVOD VODrefining refiningprocess; process; this resulted fluctuations reaction, and the Ca from the FeSi alloy during the VOD refining process; this resulted in fluctuations the Ca content. This is one reason for the fluctuations in the composition of the endogenous inclusions in the the Ca Ca content. content. This This is is one one reason reason for for the the fluctuations fluctuations in in the the composition composition of of the the endogenous endogenous in that are smaller than 5 µm. than 5 μm. inclusions that are smaller inclusions that are smaller than 5 μm.

Figure 6. Inclusion composition as a function of the Ca content. Figure 6. 6. Inclusion Inclusion composition composition as as aa function function of of the the Ca Ca content. content. Figure


8 of 11 8 of 11

The inclusion composition as a function of the Al content is presented in Figure 7 when the O temperatures are 0.007%, 0.007%, 0.002%, 0.002%, 0.0005%, and 1600 ◦°C content, Ca content, Mg content, and the temperatures C Al content content results results in in an an increase increase in in the theAl Al22O O33 respectively. It was observed that an increase in the Al content and andaadecrease decrease CaO 2 contents; however, the MgO and MnO contents did not content in in CaO andand SiOSiO contents; however, the MgO and MnO contents did not change 2 changeWith much. With to regard to the described results described Qian [18], it isnoting worth that noting the Al content much. regard the results by Qianby [18], it is worth thethat Al content comes comesthe from the ferrosilicon. large part of the aluminum with oxygen Al2O3 from ferrosilicon. A large A part of the aluminum reacts withreacts oxygen to form Al2to O3form inclusions, inclusions, and thealuminum remainingisaluminum involved in the steel–slag reaction, thereby in and the remaining involved inisthe steel–slag reaction, thereby resulting in theresulting fluctuation thethe fluctuation of the Aliscontent. is another for theinfluctuations in the compositions of the of Al content. This anotherThis reason for the reason fluctuations the compositions of the endogenous endogenous inclusions thatthan are 5smaller inclusions that are smaller µm. than 5 μm.

Figure 7. Inclusion composition composition as as aa function Figure 7. Inclusion function of of the the Al Al content. content. ◦ C at the beginning of the VOD process, the highest temperature The temperature was was1550 1550°C The temperature at the beginning of the VOD process, the highest temperature was ◦ at the oxygen blowing stage, and the final steel temperature was 1600 ◦ C; therefore, was 1650 1650 °C at C the oxygen blowing stage, and the final steel temperature was 1600 °C; therefore, the the temperature changed during VOD refiningprocess. process.The Therelationship relationshipbetween between the the inclusion temperature changed during thethe VOD refining inclusion composition and the temperature was calculated by using the FToxid and FTmisc composition and the temperature was calculated by using the FToxid and FTmisc databases databases of of FactsageTM7.1 when the O content, Ca content, Mg content, and the Al content were 0.007%, 0.002%, FactsageTM7.1 when the O content, Ca content, Mg content, and the Al content were 0.007%, 0.002%, 0.0005% and 0.004%, 0.004%, respectively, respectively, and and the the result result are are shown in Figure 0.0005% and shown in Figure 8. 8. It It can can be be seen seen that that each each inclusion composition changes with the changing temperature. The CaO content in the inclusion first inclusion composition changes with the changing temperature. The CaO content in the inclusion first ◦ ◦ C, increases The Al O33 and and MgO MgO contents contents first first decrease decrease from from 1650 1650 °C C to to 1610 1610 °C, increases and and then then decreases. decreases. The Al22O and of of MgO andand SiOSiO inclusion are just The MnO 2 in2the and then thenincrease. increase.The Thechange changetrends trends MgO in the inclusion are the justopposite. the opposite. The content increases gradually with the decrease in the temperature. The oxidation reaction with Ca, Al, MnO content increases gradually with the decrease in the temperature. The oxidation reaction with Mg, Si, and are exothermic, so that allsoofthat the all reactions occurwill as the temperature decreases. Ca, Al, Mg, Mn Si, and Mn are exothermic, of the will reactions occur as the temperature However, since Ca is a strong oxidizing element, and the concentration of Si in the molten steel is decreases. However, since Ca is a strong oxidizing element, and the concentration of Si in the molten high, deoxidation reaction of Ca and takes The CaO and SiO contents in the steel isthe high, the deoxidation reaction of CaSiand Si precedence. takes precedence. The CaO and2 SiO 2 contents in inclusion increase, while the Al O and MgO contents in the inclusion decrease. With continuing 2 3 the inclusion increase, while the Al2O3 and MgO contents in the inclusion decrease. With continuing deoxidation, concentrations decrease and deoxidation, the the CaO CaO and and SiO SiO22 concentrations decrease relatively, relatively,triggering triggeringan anincrease increasein inAl Al22O O33 and MgO. VOD refining process is one important reason for the MgO. The The change changein inthe thetemperature temperatureduring duringthe the VOD refining process is one important reason for fluctuations in the composition of the endogenous inclusions that are smaller than 5 µm. the fluctuations in the composition of the endogenous inclusions that are smaller than 5 μm.

Metals 2018, 8, x FOR PEER REVIEW

9 of 11


9 of 11 9 of 11

Figure 8. Inclusion composition as a function of the temperature.

The schematic illustration of the formation mechanism of the CaO-SiO2-Al2O3-MgO-MnO inclusions is shown in Figure oxidation of Si and and the result in the modification Figure 9. 8. Inclusion composition as a aAl function of the Figure 8. The Inclusion composition as function ofcollision the temperature. temperature. of the smaller-sized inclusions; the reactions are shown in Equations (1)–(3). The source of the total The schematic of the mechanism ofofthe the CaO-SiO -Al 2of 22O contents of Ca and Mg illustration mainly comes the addition of FeSi and reduction CaO and MgO The schematic illustration of from the formation formation mechanism the CaO-SiO 2-Al O33-MgO-MnO -MgO-MnO inclusions is shown in Figure 9. The oxidation of Si and Al and the collision result in the modification in inclusions the slag or is refractory. appearance of MnOofinSithe inclusions is collision related toresult the VOD the shown in The Figure 9. The oxidation and Al and the in theprocess; modification of the smaller-sized inclusions; the reactions are shown in Equations (1)–(3). The source of the high oxygen and manganese contents in the liquid steel in part lead to these inclusions. of the smaller-sized inclusions; the reactions are shown in Equations (1)–(3). The source of the total total contents the reduction of of CaO and MgO in contents of of Ca Ca and andMg Mgmainly mainlycomes comesfrom fromthe theaddition additionofofFeSi FeSiand and the reduction CaO and MgO [Si]+2[O]=SiO (1) the slag or refractory. The appearance of MnO in the inclusions is related to the VOD process; the high 2 in the slag or refractory. The appearance of MnO in the inclusions is related to the VOD process; the oxygen and manganese contents in theinliquid steel in part these inclusions. high oxygen and manganese contents the liquid steel inlead partto lead to these inclusions. 4[Al]+3SiO2 = 2Al2O3 + 3[Si] (2) (1) [Si ] + 2[O] = SiO2 2 [Si]+2[O]=SiO (1)

2x[Al]+y[Si]+(3x+2y)[O]=xAl 4[Al] + 3SiO2 = 2Al2 O3 +2O 3[3Si⋅] ySiO2 4[Al]+3SiO2 = 2Al2O3 + 3[Si]

2x[Al] + y[Si] + (3x + 2y)[O] = xAl2 O3 · ySiO2

(3)(2) (2) (3) (4) (3) (4)

3[Me]+Al 3[Me] + Al22O O33 ==3MeO+2[Al] 3MeO + 2[Al] 2[Me]+SiO2 = 2MeO+[Si]

(5)(5) (4)

2[Me]+SiO2 = 2MeO+[Si] 2x[Al]+y[Si]+(3x+2y)[O]=xAl 2[Me] + SiO2 = 2MeO + [Si2O ] 3 ⋅ ySiO2

3[Me]+Al2O3 = 3MeO+2[Al]

(5)

Figure 9. Schematic illustration the formation mechanism the CaO-SiO Figure 9. Schematic illustration of theofformation mechanism of the of CaO-SiO 2-Al2O3-MgO-MnO 2 -Al2 O3 -MgOMnO inclusions. inclusions.

5. Conclusions 5. Conclusions Figure 9. Schematic illustration of the formation mechanism of the CaO-SiO2-Al2O3-MgO-MnO (1) The compositional characteristics indicated that two types of inclusions with different sizes inclusions. existed in the 304L stainless steel during the VOD refining process, namely, CaO-SiO2 -Al2 O3 -MgO 5. Conclusions external inclusions with sizes ranging from several to tens of microns, and CaO-SiO2 -Al2 O3 -MgO-MnO

Metals 2018, 8, 1024

10 of 11

endogenous inclusions with sizes smaller than 5 µm. The main inclusion type was CaO-SiO2 Al2 O3 -MgO-MnO. (2) The inclusion composition changed with an increasing size of the inclusions from 1 µm to 30 µm. Most of the inclusions were smaller than 5 µm. (3) The inclusions that were larger than 5 µm were mostly CaO-SiO2 -Al2 O3 -MgO; the similarity in composition to the slag indicated that these inclusions originated from slag entrapment. The CaO-SiO2 -Al2 O3 -MgO-MnO inclusions that were smaller than 5 µm mostly originated from an oxidation reaction with Ca, Al, Mg, Si, and Mn. The changes in the inclusion composition resulting from changes in the Ca, Al, and O contents, and the temperatures during the VOD refining process were larger for the smaller inclusions. Author Contributions: Conceptualization, G.C.; Methodology, X.C. and J.L.; Software, J.L. and Y.H.; Validation, X.C., J.L. and Y.H.; Formal Analysis, X.C.; Investigation, X.C.; Resources, X.C.; Data Curation, X.C.; Writing-Original Draft Preparation, X.C; Writing-Review & Editing, Y.H.; Visualization, X.C.; Supervision, G.C.; Project Administration, J.P. and Q.R. Funding: This research received no external funding. Acknowledgments: The authors gratefully express their appreciation towards the National Nature Science Foundation of China (Grant No. 51374020), the State Key Laboratory of Advanced Metallurgy at University of Science and Technology Beijing (USTB), and Jiuquan Iron and Steel Group Corporation for supporting this work. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14.

Chen, X.; Pan, J. Analysis on microstructure and inclusions of slab surface layer in 304L stainless steel. Contin. Casting 2018, 43, 44–48. Momeni, A.; Abbasi, S.M. Repetitive Thermomechanical processing towards ultra fine grain structure in 301, 304 and 304L stainless steels. J. Mater. Sci. Technol. 2011, 27, 338–343. [CrossRef] Amine, T.; Kriewall, C.S.; Newkirk, J.W. Long-term effects of temperature exposure on SLM 304L stainless steel. JOM 2018, 70, 384–389. [CrossRef] Padhy, N.; Ningshen, S.; Panigrahi, B.K.; Mudali, U.K. Corrosion behaviour of nitrogen ion implanted AISI type 304L stainless steel in nitric acid medium. Corros. Sci. 2010, 52, 104–112. [CrossRef] Mizuno, K.; Todoroki, H.; Noda, M.; Tohge, T. Effects of Al and Ca in ferrosilicon alloys for deoxidation on inclusion composition in type 304 stainless steel. Iron Steelmak. 2001, 28, 93–101. Park, J.H.; Kang, Y.B. Effect of ferrosilicon addition on the composition of inclusions in 16Cr-14Ni-Si stainless steel melts. Metall. Mater. Trans. B 2006, 37, 791–797. [CrossRef] Li, L.; Cheng, G.; Hu, B.; Wang, C.; Qian, G. Formation of Non-metallic Inclusions of Si-killed Stainless Steel during GOR Refining Process. High Temp. Mater. Proc. 2018, 37, 521–529. [CrossRef] Kim, J.W.; Kim, S.K.; Kim, D.S.; Lee, Y.D.; Yang, P.K. Formation mechanism of Ca-Si-Al-Mg-Ti-O inclusions in type 304 stainless steel. ISIJ Int. 1996, 36, S140–S143. [CrossRef] Ehara, Y.; Yokoyama, S.; Kawakami, M. Control of formation of spinel inclusion in type 304 stainless steel by slag composition. Tetsu-to-Hagané 2007, 93, 475–482. [CrossRef] Yin, X.; Sun, Y.H.; Yang, Y.D.; Bai, X.F.; Deng, X.X.; Barati, M.; McLean, A. Inclusion evolution during refining and continuous casting of 316L stainless steel. Ironmak. Steelmak. 2016, 43, 533–540. [CrossRef] Park, J.H.; Lee, S.B.; Kim, D.S. Inclusion control of ferritic stainless steel by aluminum deoxidation and calcium treatment. Metall. Mater. Trans. B 2005, 36, 67–73. [CrossRef] Ren, Y.; Zhang, L.; Fang, W.; Shao, S.; Yang, J.; Mao, W. Effect of Slag Composition on Inclusions in Si-Deoxidized 18Cr-8Ni Stainless Steels. Metall. Mater. Trans. B 2016, 47, 1024–1034. [CrossRef] Yan, P.; Huang, S.; Pandelaers, L.; Van Dyck, J.; Guo, M.; Blanpain, B. Effect of the CaO-Al2 O3 based top slag on the cleanliness of stainless steel during secondary metallrugy. Metall. Mater. Trans. B 2013, 44, 1105–1119. [CrossRef] Yan, P.; Huang, S.; Guo, M.; Blanpain, B. Desulphurisation and inclusion behaviour of stainless steel refining by using CaO-Al2 O3 based slag at low sulphur levels. ISIJ Int. 2014, 54, 72–81. [CrossRef]

Metals 2018, 8, 1024

15. 16. 17. 18. 19. 20.

21.

11 of 11

Sakata, K. Technology for production of austenite type clean stainless steel. ISIJ Int. 2006, 46, 1795–1799. [CrossRef] Park, J.H.; Lee, S.B.; Gaye, H.R. Thermodynamics of the formation of MgO-Al2 O3 -TiOx inclusions in Ti-stabilized 11Cr ferritic stainless steel. Metall. Mater. Trans. B 2008, 39, 853–861. [CrossRef] Kang, Y.B.; Lee, H.G. Inclusions chemistry for Mn/Si deoxidized steels: Thermodynamic predictions and experimental confirmations. ISIJ Int. 2004, 44, 1006–1015. [CrossRef] Qian, G.; Qu, Z.; Cheng, G. Effect of Al in FeSi alloy on continuous casting and surface defect of stainless steel hot-rolled sheet. Iron Steel 2016, 51, 76–81. Qian, G. The Technologies and Theory of Desulfurization and Inclusion Control for 304 Stainless Steel on GOR Process. Ph.D. Thesis, University of Science and Technology Beijing, Beijing, China, 2015. Swinbourne, D.R.; Kho, T.S.; Langberg, D.; Blanpain, B.; Arnout, S. Understanding stainless steelmaking through computational thermodynamics Part 2–VOD converting. Miner. Process. Extr. Metall. 2010, 119, 107–115. [CrossRef] Wei, J.H.; Li, Y. Study on Mathematical Modeling of Combined Top and Bottom Blowing VOD Refining Process of Stainless Steel. Steel Res. Int. 2015, 86, 189–211. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).