influence of fe oxidation on selective oxidation behavior

© 2014 ISIJ 664

ISIJ International, Vol. 54 (2014), No. 3, pp. 664–670

Influence of Fe Oxidation on Selective Oxidation Behavior of Si and Mn Added in High Strength Sheet Steel

Yusuke FUSHIWAKI,* Yasunobu NAGATAKI, Hideki NAGANO, Wataru TANIMOTO and Yoshiharu SUGIMOTO

Steel Research Laboratory, JFE Steel Corp., 1 Koukan-cho, Fukuyama, Hiroshima, 721-8510 Japan.

(Received on June 19, 2013; accepted on October 11, 2013; originally published in Tetsu-to-Hagané,Vol. 99, 2013, No. 3, pp. 221–227)

In the process of hot dip galvanizing high tensile strength sheet steels containing Si and Mn, selectivesurface oxidation of Si and Mn causes coating defects. One promising method for overcoming this prob-lem is an oxidation-reduction process. When the steel surface is exposed to an oxidizing atmosphere, itwill react primarily by forming an Fe oxide, which can be reduced by hydrogen in a reduction process thatfollows. It has been explained that good wettability can be obtained due to the formation of pure iron.However, the mechanism of suppression of selective surface oxidation has not been clearly understoodin detail yet.

In order to reveal this mechanism, the present study focused on both Mn and Fe oxidation behavior dur-ing the oxidation-reduction process for a cold-rolled sheet steel containing 0.25mass%Si-1.8mass%Mn.Surface and cross-sectional analyses were performed by using secondary electron microscopy and trans-mission electron microscopy. Selective surface oxidation behavior was investigated by glow dischargeoptical emission spectroscopy.

The main results obtained are as follows.First, selective surface oxidation of Mn was suppressed even if soaking was continued after completion

of the reduction of the Fe oxide.Second, as the reduction process proceeded, Mn was trapped as an internal oxide under the Fe oxide

layer. Moreover, depletion of solute Mn was observed in the matrix.From these results, depletion of solute Mn is supposed to suppress the outer diffusion of Mn during

soaking. Therefore, selective surface oxidation of Mn is suppressed even after Fe oxide reduction is com-pleted.

KEY WORDS: oxidation-reduction process; external oxidation; internal oxidation; selective surface oxida-tion; depletion.

1. Introduction

In recent years, concern about reducing automotive bodyweight and improving collision safety has increased dramat-ically, encouraging active use of high-tensile steel sheets forautomobiles.1,2) In manufacturing high tensile steel sheets,various elements including Si and Mn are added to obtainhigh strength and high ductility.3,4) These high-tensile steelsheets have also been applied to galvannealed componentsto achieve excellent anti-corrosion properties.5)

In the manufacturing process of steel sheets for automo-biles, recrystallization annealing is generally performed inorder to reduce surface oxides of Fe and to recrystallize thesteel sheet structures. Recrystallization annealing is typical-ly carried out in a continuous furnace controlled to a dewpoint of less than –20°C and annealing temperature ofaround 800°C under a N2 gas atmosphere, which usuallycontains around 10 vol% of H2. Fe oxide can be reduced in

this annealing atmosphere. Thermodynamically, however,both Si and Mn can be oxidized under these conditions.6)

Therefore, Si and Mn tend to diffuse from the steel matrixto the surface and be oxidized as selective surface oxides. Itis well recognized that these selective surface oxides of Siand Mn deteriorate the wettability of molten zinc on the sur-face of the steel in the hot dip galvanizing process immedi-ately after annealing. As a result, coating defects are subjectto be observed.7–10) Moreover, in the galvannealing process,it has been reported that these selective surface oxides pre-vent reaction between Zn in the coating and Fe in the steelsubstrate.11,12) Therefore, it is important to clarify the selec-tive surface oxidation behavior of Si- and Mn-bearing steelfrom the viewpoint of active utilization of galvannealedhigh tensile steel sheets.

As one promising method of overcoming these issues, anoxidation-reduction process has been suggested.13–15) Thesteel surface will be primarily exposed to an oxidizing atmo-sphere to form an Fe oxide layer (Fe pre-oxidation), whichcan be reduced by hydrogen in the reduction process thatfollows (post-reduction). It has been explained that good

* Corresponding author: E-mail: [email protected]: http://dx.doi.org/10.2355/isijinternational.54.664

ISIJ International, Vol. 54 (2014), No. 3

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wettability can be obtained due to the formation of pureiron. However, the mechanism of suppression of selectivesurface oxidation has not been clearly understood in detailyet. Y. F. Gong et al. reported the results of transmissionelectron microscopy (TEM) observation of the surface andsubsurface regions of both Si- and Mn-bearing steels in anoxidation-reduction process, and investigated the propertiesof the oxides generated in detail.16) However, they did notdescribe the mechanism responsible for suppressing theselective surface oxidation of Si and Mn during the oxidation-reduction process in detail. Thus, there are few studieswhich give consideration to the phenomena of selective sur-face oxidation in relation to internal oxidation, depletion ofsolute elements in the matrix, and outer diffusion in the sub-surface of the steel during an oxidation-reduction process.

Therefore, in this study, the selective surface oxidationbehaviors in each stage of an oxidation-reduction processwere investigated and associated with internal oxidation,depletion and outer diffusion of solute elements in the sub-surface of the steel in order to clarify the mechanism of sup-pression of selective surface oxidation.

2. Experimental Method

The steel samples used in the present study were full hardcold-rolled Si- and Mn-bearing steels with a thickness of1.0 mm. The composition was 0.095mass%C, 0.25mass%Siand 1.8mass%Mn, as shown in Table 1. The samples werepre-cleaned by electric degreasing, which was performed inan alkaline solution of 3.4mass%NaOH at 500 A·m–2 for 10 s.The samples were then pickled in an acidic solution of5mass%HCl at 60°C for 6 s, after which the samples wereannealed. Annealing was carried out in two types of gasatmospheres. As shown in Fig. 1, Fe pre-oxidation was con-ducted in an oxidizing atmosphere of N2 + 0.1vol%O2 at adew point of 10°C. The sample heating rate was 13°C·s–1,and the Fe pre-oxidation temperature was 650°C. After pre-oxidation, post-reduction was carried out in an atmosphereof N2 + 5vol%H2 at a dew point of –35°C. The heating ratewas 13°C·s–1 up to 650°C, and 2°C·s–1 up to maximum tem-perature. The soaking time was 1–240 s at maximum tem-perature. After annealing, the samples were rapidly cooledto room temperature with 100vol%N2 with a flow rate of

200 L·min–1.After cooling, the annealed samples were analyzed as fol-

lows. Selective surface oxidation behavior was investigatedby glow discharge optical emission spectroscopy (GD-OES). The conditions utilized for measurement were a cur-rent of 20 mA and an Ar gas flow rate of 8.3 mL·s–1. Thesputtering time was 30 s, and the sputtering rate was approx-imately 0.008 μm·s–1. The amount of selective surface oxi-dation of the annealed samples was quantified by measuringthe concentration profiles of Mn at the steel surface by GD-OES. In these GD-OES profiles, the integrated Mn intensity(arbitrary unit) of 0–0.04 μm was defined as the amount ofselective surface oxidation of Mn. The peak of the intensityin 0–0.04 μm, which was higher than that of the matrix andwas more than 0.2 μm away from the surface, was definedas a rich layer.

The surfaces of the annealed samples were observed byscanning electron microscopy (SEM). The conditions usedin this observation were an acceleration voltage of 5 kV andworking distance of 15 mm. Fourier transform infra-redspectroscopy (FT-IR) and X-ray photoelectron spectroscopy(XPS) were used to characterize the selective surface oxideson the annealed samples. In FT-IR measurement, a high-sensitivity unit was utilized and the incidence angle wasadjusted to 70°. The conditions used in XPS were an X-raysource of Al kα and a measured region of 600 mmϕ. Thecross-sectional annealed samples were prepared by focusedion beam (FIB) milling to a thickness of 100 nm. Thesesamples were observed by SEM under conditions of anacceleration voltage of 3 kV and working distance of 5 mm.The samples were also investigated by TEM to determinethe morphology and distribution of the oxides in the subsur-face region. Determination of the composition of the inter-nal and surface oxides and the amount of solute Mn wasperformed by energy dispersive spectroscopy (EDS).

3. Results

3.1. Effect of Fe Pre-oxidation and Soaking Time onSelective Surface Oxidation

Figure 2 shows GD-OES depth profiles of the samplesannealed at 650°C for 1 s and at 800°C for 20 s and 240 swith and without Fe pre-oxidation, respectively. In the casewithout Fe pre-oxidation, the intensity of Si- and Mn-richlayers increased as the reducing temperature and the soakingtime increased. The thickness of the rich layers was 0.01–0.02 μm at 650°C and 0.05–0.06 μm at 800°C for 240 s,respectively. With Fe pre-oxidation, although the intensityof the rich layers increased slightly as the reducing temper-ature and soaking time increased, the rate of increase wasmuch lower than that without Fe pre-oxidation. A peak ofMn was observed in the matrix, which was 0.05–0.20 μmbelow the surface, but this was thought to be due to internaloxidation.

Figure 3(a) shows the results of an investigation of theeffect of the reducing temperature on selective surface oxi-dation of Mn for 1 s soaking time with and without Fe pre-oxidation. Figure 3(b) shows the effect of the soaking timeon selective surface oxidation of Mn at a reducing temper-ature of 800°C. Without Fe pre-oxidation, the amount ofselective surface oxidation of Mn increases monotonously

Table 1. Chemical compositions of specimens (in mass%).

C Si Mn P S Al N O

0.095 0.25 1.8 0.032 0.0010 0.038 0.0031 0.0020

Fig. 1. Heat pattern of Fe pre-oxidation and post-reduction.

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as the reducing temperature and soaking time increase. Incontrast, with Fe pre-oxidation, although less selective sur-face oxidation of Mn was observed at the 800°C reducing

temperature, it tended to increase with increasing soakingtime at 800°C. It should be noted that the amount of selec-tive surface oxidation of Mn at 800°C for 240 s with Fe pre-oxidation was almost the same as that at 650°C for 1 swithout Fe pre-oxidation.

As shown in Figs. 2 and 3, Fe pre-oxidation drasticallyaltered selective surface oxidation behavior. Therefore, thecomposition of the selective surface oxide was investigatedby FT-IR. Figure 4 shows FT-IR reflection spectra of thesamples annealed at 800°C for 20, 80 and 240 s with andwithout Fe pre-oxidation. Without Fe pre-oxidation, the

Fig. 2. GD-OES depth profiles of samples annealed at 650°C for 1 s and 800°C for 20 and 240 s. (Sputtering rate is0.008 μm·s–1.)

(a)

(b)

Fig. 3. (a) Effect of reducing temperature on selective surface oxi-dation of Mn (soaking time; 1 s). (b) Effect of soaking timeon selective surface oxidation of Mn (reducing temperature;800°C).

Fig. 4. IR reflection spectra of samples annealed at 800°C for 20,80, and 240 s with/without Fe pre-oxidation.


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selective surface oxide comprised mainly Mn2SiO417,18) at

each soaking time. On the other hand, with Fe pre-oxidation,hardly any peaks of Mn2SiO4, MnSiO3 and SiO2 wereobserved. The peak observed at around 500 cm–1 was pre-sumed to be MnO2.19) Figure 5 shows the XPS spectra ofthe surfaces of the same samples. Without Fe pre-oxidation,spectra of both Mn–O and SiOx20) were clearly detected,whereas, with Fe pre-oxidation, although Mn–O spectrawere observed, no Si–Ox spectra were found at each soak-ing time.

SEM observation was performed in order to characterizethe selective surface oxide. Figure 6 shows SEM images of

the surfaces of the samples annealed at 800°C for 20, 80 and240 s with and without Fe pre-oxidation. With Fe pre-oxidation, although there were fewer selective surfaceoxides at 20 s of soaking time, a few surface oxides withdiameters in the range of 0.1–0.3 μm were observed at 80 s,as shown in Fig. 6. Moreover, the amount of the selectivesurface oxide tended to increase as the soaking timeincreased. However, without Fe pre-oxidation, the morpholo-gy of the oxide was different from that with Fe pre-oxidationand was not clearly granulous.

3.2. Effect of Reducing Temperature and Soaking Timeon Reduction of Fe Oxides and Internal Oxidation/Depletion of Solute Mn

Cross sections of the annealed samples with Fe pre-oxidation at various post-reduction temperatures for varioussoaking times were observed by SEM, and their composi-tions were investigated by EDX. These results are shown inFig. 7. A layer with a thickness of about 0.5 μm wasobserved on the surface of the sample at the Fe pre-oxida-tion temperature of 650°C. EDX analysis confirmed thatthis layer was composed of Fe oxide. When post-reductionwas carried out after Fe pre-oxidation, reduction of the Feoxide was promoted and the amount of Fe oxide which wasreduced increased with increases in both reducing tempera-ture and soaking time. The Fe oxide disappeared when post-reduction was performed at 800°C for 20 s. Furthermore,numerous precipitates with a diameter of < 1 μm wereobserved in the matrix at around 1 μm below the surface.

In order to clarify the morphology and composition of theoxides, cross-sectional TEM observations were performed.At the same time, the composition of the oxide was inves-tigated by EDX. The results are shown in Fig. 8. Manyinternal oxides with a diameter of less than 0.1 μm weredetected in the matrix, i.e., in the range of 0.2–0.4 μm belowthe surface. However, some of the internal oxides had a

Fig. 5. XPS spectra of samples annealed at 800°C for 20, 80 and240 s with/without Fe pre-oxidation.

Fig. 6. SEM images of surface of samples annealed at 800°C for20, 80 and 240 s with/without Fe pre-oxidation.

Fig. 7. SEM images and EDX analytical results of cross section ofsamples annealed at 650°C for 1 s and 800°C for 1, 20, 80and 240 s with Fe pre-oxidation.

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length of about 0.2 μm. Based on the results of EDX anal-ysis of the sample annealed for 80 s, these oxides were pre-sumed to be MnSiO3. On the other hand, the results of EDXof the sample annealed for 20 s indicated that the internaloxides located along the grain boundaries of the matrix atabout 0.5 μm below the surface were SiO2. Moreover, exter-nal oxides with a diameter of 0.1–0.2 μm were clearlyobserved on the surface of the sample annealed at 800°C for240 s. The EDX results indicated that these external oxideswere MnO.

At the same time as this TEM observation of oxides, EDXanalyses of the subsurface area, except for the part whereinternal oxides were observed, were carried out in order tomeasure the amount of solute Mn in the subsurface areadown to 2 μm below the surface. As a result, Fig. 9 showsthe dependence of the amount of solute Mn on the depthfrom the surface of the samples annealed at 800°C for 20,80 and 240 s. This investigation clarified the fact that theamount of solute Mn in the subsurface area, i.e., down to

1 μm below the surface, was less than that in the matrix, i.e.,more than 2 μm below the surface of the sample annealedat 800°C for 20 s. Here, the area where the amount of soluteMn is less than 1.5 mass% is defined as the “depletion zoneof solute Mn”. In this case, the thickness of the depletionzone of solute Mn was about 0.77 μm. Furthermore, thethickness of the depletion zone of solute Mn decreased withincreasing soaking time, about 0.63 μm at the soaking timeof 80 and about 0.37 μm at 240 s. It should be noted, how-ever, even when the depletion zone was formed, the amountof solute Mn was about 0.1 mass% at a soaking time of 20 sand was about 0.5 mass% and 1.4 mass% at 80 s and 240 s,respectively. That is to say, the amount of solute Mnincreased with increasing soaking time.

4. Discussion

4.1. Mechanism of Selective Surface and Internal Oxi-dation with Fe Pre-oxidation

Selective surface oxidation is a phenomenon in which Siand Mn in steel are oxidized by oxygen provided by thethermodynamic deviation of H2O in the atmosphere on theinterface between the steel surface and the atmosphere.21) Inthe case without Fe pre-oxidation, as shown in Figs. 4 and5, the selective surface oxides are assumed to be composedof mainly Mn2SiO4. On the other hand, as shown in Figs. 5and 8, in the case with Fe pre-oxidation, the selectivesurface oxides are mainly MnO. As shown in Fig. 6, thedifference of the surface morphology with and without Fepre-oxidation is thought to be due to the composition of theselective surface oxides.

Here, let us consider the mechanism of selective surfaceand internal oxidation with Fe pre-oxidation on the basis ofthe thermodynamic theory. It is necessary to consider thefollowing reactions in connection with the oxidation of Si-and Mn-bearing steel in an atmosphere with H2–H2O.

...................... (1)

.............. (2)

...................... (3)

........................ (4)

Here, pH2O/pH2 is defined as the ratio of the partial pressureof H2O (pH2O) and that of H2(pH2). pH2O/pH2 of eachoxide, that is SiO2, MnSiO3, MnO and FeO, was calculatedconsidering the above Eqs. (1) through (4) for an Fe-1.8mass%Mn-0.25mass%Si alloy annealed in an atmo-sphere of 5vol%H2-N2.22) The results are shown in Fig. 10.In this study, these calculations were made on the supposi-tion that the activities of Mn and Si were equal to the atomicratio of each element.22) log(pH2O/pH2) could be calculatedat about –2.5 in the annealing atmosphere, which had a dewpoint of –35°C and hydrogen concentration of 5 vol%. Asshown in Fig. 10, when log(pH2O/pH2) equals to –2.5, thereaction in Eq. (4) should proceed to the left, that is, thereaction is reducing. On the other hand, the reactions of (1)–(3) proceed to the right, that is, these reactions are oxidizing.Therefore, at the beginning of annealing at 800°C with Fepre-oxidation, reduction of the Fe oxide started on the sur-

Fig. 8. TEM images and EDX analytical results of cross section ofsamples annealed at 800°C for 20, 80 and 240 s with Fepre-oxidation.

Fig. 9. Dependence of amount of solute Mn (TEM-EDX) on depthfrom surface of samples annealed at 800°C for 20, 80 and240 s.

Si H O SiO H2+ = +2 22 2

Mn SiO+ + = +2 3 2H O MnSiO H2

Mn H O MnO H2+ = + 2

Fe H O FeO H2+ = + 2


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face, and simultaneously, internal oxidation of Si and Mn,which are much more easily oxidized than Fe, occurred dueto the oxygen supplied by the Fe oxide at the interfacebetween the Fe oxide and the steel substrate. As a result, asshown in Fig. 3(b), the amount of selective surface oxida-tion of Mn was small during a comparatively short soakingtime up to 80 s at 800°C. On the other hand, with a relativelylong soaking time of 240 s, MnO tended to form on the steelsurface because the reduction of Fe oxides had been com-pleted at the surface, and as a result, outer diffusion of Mnbegan, which was followed by selective surface oxidation ofthe diffused Mn. However, as shown in Fig. 3(b), theamount of selective surface oxidation of Mn in this case wasless than one-quarter of that without Fe pre-oxidation. Thereason for this will be discussed in detail in the followingChapter 4.2.

As shown in Fig. 8, in the case of a relatively long soak-ing time of 240 s, the selective surface oxides should beMnO, and the internal oxides which formed in the matrix ofthe steel in the range of 0.2–0.4 μm below the surfaceshould be MnSiO3. The internal oxides located along thegrain boundaries of the matrix at about 0.5 μm below thesurface were supposed to be SiO2. Therefore, the oxideswere formed in the order of MnO, MnSiO3 and SiO2 fromthe surface of the steel into the matrix. As shown in Fig. 10,the order of pH2O/pH2 at 800°C was also MnO, MnSiO3 andSiO2, which corresponds to the order of the oxides formedin the steel, as mentioned above. In summary, it was foundthat oxides whose equilibrium partial pressure of oxygenwas low formed firstly in the matrix of the steel, and then,as their equilibrium partial pressure of oxygen became high-er, the depth at which each oxide formed came closer to thesurface of the steel. Furthermore, as shown in Fig. 7, thethickness of the Fe oxide was about 0.5 μm, so that of thereduced Fe was presumed to be about 0.2 μm on the suppo-sition that the Fe oxide was Fe3O4 and the Fe oxide wascompletely reduced to reduced Fe. Therefore, the Fe oxidewas reduced on the surface of the steel annealed at 800°C.At the same time, in the interface region, SiO2 was supposedto form as an internal oxide at around 0.3 μm from the Feoxide. That is, oxygen diffused through a distance of 0.3 μmfrom the Fe oxide. In the same way, MnSiO3 were supposedto form at around 0.2 μm from the Fe oxide. It seems rea-sonable that the formation of internal oxides should finish

if the reduction of Fe oxides is completed. This is becausethe oxygen potential in the steel should decrease, and theposition where the internal oxides form should come closerto the surface as the reduction of the Fe oxide proceeds.Therefore, it is suggested that internal oxidation of chemicalcomponents such as Si and Mn should be attributed to innerdiffusion of oxygen originated from the Fe oxide.

4.2. Influence of Depletion and Outer Diffusion of Mnon Selective Surface Oxidation of Mn with Fe Pre-oxidation

Let us now consider the behavior of depletion and diffu-sion in order to elucidate the selective surface oxidationbehavior of Mn. As shown in Fig. 9, the thickness of thedepletion zone of solute Mn (td) in the subsurface of thesamples annealed for 20, 80 and 240 s is about 0.77 μm,0.63 μm and 0.37 μm, respectively. From the results in Figs.7 and 8, the reason why the depletion zone of solute Mnforms is thought to be a decrease in the amount of solute Mnaround the internal oxide, which was MnSiO3 in the subsur-face in the range of 0.2–0.4 μm below the surface. Afterreduction of the Fe oxide and formation of the internaloxide, solute Mn diffusing out of the steel to the surfaceshould be oxidized as the selective surface oxide. On thesupposition that reduction of the Fe oxide and formation ofthe internal oxide were completed at a soaking time of 20 s,the diffusion distance of solute Mn (dMn) in the samplesannealed for 80 and 240 s can be calculated from thefollowing Eqs. (5) and (6) at 0.16 μm and 0.39 μm, respec-tively.23,24)

............................... (5)

........................ (6)

D: diffusion coefficientt: diffusion timeD0: temperature-independent pre-exponential (0.35×10–4

m2·s–1)Q: activation energy (221 kJ·mol–1)R: gas coefficientT: absolute temperature

Table 2 summarizes the calculated dMn from a soakingtime of 20 s to 80 s and to 240 s in comparison with theamount of reduction of td (Δtd) based on the results in Fig.9. These results reveal that the calculated dMn agrees almostperfectly with the measured Δtd from a soaking time of 20 sto 80 s and to 240 s. Thus, the reduction of td may be con-trolled by dMn. In the case with Fe pre-oxidation, the reasonwhy selective surface oxidation of Mn can be suppressed isthought to be that the depletion zone of solute Mn is suffi-ciently formed and td is much longer than dMn during anneal-ing in this study. Therefore, as shown in Fig. 3(b), theamount of selective surface oxidation of Mn in this casewith a comparatively long soaking time of 240 s is recog-nized to be less than one-quarter of that without Fe pre-oxidation. Strictly speaking, however, the influence ofdiffusions in the grain boundary and dislocations aroundprecipitates in the subsurface of the steel should also be con-sidered.

Consequently, the mechanism by which selective surface

Fig. 10. Equilibrium pH2O/pH2 of various oxides on sample withannealing in H2–H2O mixed gas atmosphere.

dMn Dt= 2

D D = −0exp RT( Q )/

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oxidation of Mn is suppressed/promoted can be shown sche-matically in Fig. 11.

5. Conclusion

In order to understand more clearly the mechanismresponsible for suppression of selective surface oxidation inhigh-strength steels containing added Si and Mn, the selec-tive surface oxidation behaviors in each stage of the oxidation-reduction process were investigated and associated withinternal oxidation and depletion/outer diffusion of solute Mnin the subsurface of a cold-rolled sheet steel containing1.8mass%Mn-0.25mass%Si. The results of the present studymay be summarized as follows.

(1) In the case with Fe pre-oxidation, the amount ofselective surface oxide is less than that without Fe pre-

oxidation. With a comparatively long soaking time of 240 s,MnO forms as a selective surface oxide, and its amountincreases with time. This is attributed to the fact that reduc-tion of Fe oxide on the surface has been completed, and out-er diffusion and selective surface oxidation of Mn begins.

(2) In this case, the oxides form in the order of MnO,MnSiO3 and SiO2 from the surface of the steel into thematrix. This result indicates that oxides whose equilibriumpartial pressure of oxygen is low form firstly in the matrixof the steel, and then, as their equilibrium partial pressureof oxygen becomes higher, the position where each oxideforms comes closer to the surface of the steel.

(3) Internal oxidation of Si and Mn is attributable to theinner diffusion of oxygen from the Fe oxide.

(4) With Fe pre-oxidation, the amount of selective sur-face oxidation of Mn at a comparatively long soaking timeof 240 s is less than one-quarter of that without Fe pre-oxidation. This may be because the depletion zone of soluteMn is sufficiently formed after this extended soaking time,and as a result, the thickness of the depletion zone of soluteMn becomes much larger than the diffusion distance of Mnduring annealing, thereby suppressing selective surface oxi-dation of Mn.

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Table 2. Comparison of amount of reduction of td based on TEM-EDX analysis and calculation results.

Soaking (800°C)time/s

Thickness of thedepletion zone ofsolute Mn, td/μm

The amount ofreduction of td,

Δ td/μm

Diffusiondistance of Mn,

dMn/μm

Based on TEM-EDX analysis Calculation

20 0.77 – –

80 0.63 0.14 0.16

240 0.37 0.40 0.39

Fig. 11. Schematic illustrations of mechanism of suppression/pro-motion of selective surface oxidation of Mn.

influence of fe oxidation on selective oxidation behavior

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