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Effect of annealing temperature on the surface and subsurface microstructure of Al-added TWIP steel
Surface & Coatings Technology 386 (2020) 125479
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Effect of annealing temperature on the surface and subsurface microstructure of Al-added TWIP steel Xinyan Jina,b, Yong Zhonga,b, Li Wanga,b, Hua Wangc,
T
⁎
a
Baosteel Research Institute, Shanghai 201900, China State Key Laboratory of Development and Application Technology of Automotive Steels, Baosteel, Shanghai 201900, China c Key Laboratory for Material Microstructures of Shanghai University, 200444, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: TWIP steel Selective oxidation Annealing temperature Dew point
Annealing experiments were carried out in a N2-5 vol% H2 atmosphere with a dew point of +10 °C to investigate the effect of annealing temperatures ranging from 600 °C to 850 °C on the surface and subsurface microstructure of Al-added TWIP steel. The depth profiles of Fe, Al, Mn, O and C in the annealed samples along a 3 μm depth from the surface were measured by GD-OES. High-resolution cross sections were prepared by FIB and characterized by TEM. It was found that the annealing temperature plays an important role in the surface and subsurface formation of Al-added TWIP steel. Due to the abundant formation of external MnO and decarburization, the content of Mn and C in the subsurface area decreases to a critical low level and results in the formation of a continuous ferrite layer. In addition, the type, shape, size and distribution of the internal oxides in the subsurface area are greatly affected by the annealing temperature. When the annealing temperature increases from 700 °C to 800 °C, the film-like internal MnAl2O4 along the grain boundaries grows coarser, and some large internal MnO is formed. Furthermore, a layer of protruded ferrite grains without any internal oxides inside the grains or along the grain boundaries appears just beneath the external MnO layer, the amount of which increases with the annealing temperature.
1. Introduction Reducing the weight of vehicles is well recognized as an effective way to reduce carbon dioxide emissions and enhance fuel efficiency. In the last two decades, varieties of advanced high-strength steel (AHSS) and ultrahigh-strength steel (UHSS) have been developed and widely used in the body-in-white. High-Mn twinning-induced plasticity (TWIP) steel a new type of UHSS structural steel and has received great attention. TWIP steel is characterized by a sustained high strain hardening rate, which results in higher observed elongation and tensile strength [1]. The use of TWIP steel may therefore lead to a considerable reduction in weight of steel components, a reduction in material use and improved press forming behavior [2]. To obtain a desirable microstructure and mechanical properties, C, Mn, Al and, in some cases, Si are the main element added to TWIP steel [2]. A sufficient content of Mn, usually in the range of 12–30 mass%, is required to obtain a fully austenitic microstructure [3]. C addition, typically in the range of 0.4–1.0 mass%, also results in the stabilization of the austenite phase and solid-solution strengthening [2]. Al addition, typically < 3 mass% [3], shows beneficial effects on the suppression of
⁎
the delayed fracture phenomenon of TWIP steel [2] and a slight reduction in density [3]. However, the selective oxidation of the key elements added to TWIP steel during the continuous annealing process or hot-dip galvanizing process may lead to the deterioration of the surface state of the steel. A series of studies on the selective oxidation of TWIP steels under a variety of conditions were performed by the De Cooman [4–7] group. When TWIP steel was annealed at 800 °C in a N2 + 10% H2 gas atmosphere with dew points of −17 °C and −3 °C, a continuous layer of MnO was found to form on the surface, and a Mn-depleted zone was found to transform from austenite to ferrite in the subsurface. A thicker external Mn oxidation layer and a deeper Mn-depleted zone were found when the dew point was increased from −17 °C to −3 °C. When the dew point was −17 °C, the characteristics of the MnO layer/steel matrix interface were also influenced by the addition of Al to the matrix. In the case of Al-free TWIP steel, crystalline c-xMnO·SiO2 particles and amorphous a-xMnO·SiO2 particles were found. In the case of Al-added TWIP steel, amorphous a-xMnO·SiO2, crystalline c-xMnO·Al2O3 and Kirkendall voids were found. Large internal MnO particles were observed in the Mn-depleted ferrite zone when the dew point was −3 °C.
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.surfcoat.2020.125479 Received 6 December 2019; Received in revised form 13 February 2020; Accepted 16 February 2020 Available online 19 February 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.
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follows:
The selective oxidation of Al-added TWIP steel and its effect on the wettability of TWIP steel by liquid Zn-0.23 mass% Al were studied by Kim et al. [8,9]. It was also noted that the type of surface oxides and the thickness of the oxide layer were determined by the dew point [8]. When the dew point was −20 °C, a 100-nm-thick MnO layer formed uniformly on the steel surface, and an ~15-nm-thick MnAl2O4 layer formed with small particles of Mn2SiO4 of ~70-nm diameter under the MnO layer. When the dew point was −40 °C, a 5-nm-thick MnAl2O4 layer formed across most of the surface, and ~50-nm-thick Mn2SiO4 particles were formed irregularly on some parts of the surface. Because the thick MnO layer was replaced by relatively thin MnAl2O4 when the dew point decreased from −20 °C to −40 °C, the formation of intermetallic compounds occurred easily, and the wettability of TWIP steel by liquid Zn improved [9]. Nickel precoating is another way to prevent the formation of surface oxides and thus to improve the Zn-coating properties of TWIP steel [10]. It was found that a nonuniform thin MnO layer instead of a uniform thick MnO layer was formed on the surface of a sample with ~50nm-thick nickel precoating. Preoxidation was also confirmed to be a useful tool for the hot-dip galvanizing of TWIP steel based on the study of Blumenau et al. [11]. It was reported that the amount of metallicbond Fe within the MnO·Femetal layer increased considerably when preoxidation was performed at 600 °C for 10 s in 1.8% O2-N2. By this process, Fe2Al5 formation on top of the MnO·Femetal layer was considerably intensified and was accompanied by significantly improved liquid Zn wetting. The interface conditions in high-Mn TWIP steel after the initial stage of the oxidation process were studied by Chen [12]. It was found that an obvious oxide layer did not appear until 600 °C, MneAl oxides formed below the interface along the grain boundary, and aluminum oxides formed in deeper positions. As the soaking time increased from 0 s to 180 s, a more uniform surface formed without a significant change in layer thickness. It was also found that the H2 content of the annealing gas changed the oxide type and thickness due to the different external oxygen potentials. The importance of external oxide morphology on reactive wetting during the hot-dip galvanizing of different Mn-alloyed steels has been studied. It was reported by Sagl et al. [13,14] that crystalline oxides ensure the contact of Al and Zn with Fe in the steel substrate, which is not the case for amorphous oxides. Bath-dissolved Al can diffuse into the grain boundaries of crystalline oxides and form an FeeAl intermetallic phase, while the process is suppressed in amorphous oxides that offer no efficient diffusion path for dissolved Al in the zinc bath. A recent series of studies by Pourmajidian and McDermid [15–17] determined that a decreased external MnO thickness obtained by decreasing the annealing temperature and the modification of external oxides to a fine and globular morphology by Sn addition can improve the reactive wetting of 0.1C-6Mn-2Si steels. Several mechanisms, such as the aluminothermic reduction of MnO, oxide bridging and oxide liftoff by the molten galvanizing bath, were proposed. A summary of the steel compositions and the key annealing parameters in TWIP steel selective oxidation studies in the literature is listed in Table 1. Some common conclusions drawn from the literature are as
1. In most cases, the surface of the annealed TWIP steel was covered with dense external MnO, and the austenite in the subsurface was transformed into ferrite due to Mn depletion and decarburization. In the case of a low-dew point annealing atmosphere [8,9] or with Ni precoating [10], external MnO and subsurface ferrite formation were suppressed. 2. It was found that there were different types of oxides at the interface of MnO/ferrite and internal oxides in the subsurface, including amorphous a-xMnO·SiO2, crystalline c-xMnO·SiO2 and cxMnO·Al2O3, Mn2SiO4, MnAl2O4 and Al2O3. The type, size and distribution of the oxides were affected by the composition and process conditions. In some cases [5,7,11], voids were observed in the interface oxides. 3. The dew point is considered to have a great effect on the selective oxidation of TWIP steel; the thickness of the external MnO and subsurface ferrite layers and the amount and depth of internal oxides increased with the dew point. The existing studies [4–12] on the selective oxidation of TWIP steel mainly focused on the effect of the dew point, and in most cases, the annealing temperature was fixed to 800 °C. The influence of annealing temperature on the selective oxidation of TWIP steel is still not very clear. Therefore, laboratory annealing experiments at different annealing temperatures in an atmosphere with a relatively high dew point were conducted on Al-added TWIP steel, and both the surface and subsurface were characterized by means of GD-OES and FIB in the current work. The primary aim of this study is to determine the effect of annealing temperature on the surface and subsurface microstructure of TWIP steel. 2. Experimental procedures The material used in the present work is industrially cold-rolled TWIP steel with the following composition: 0.7 mass% C, 16 mass% Mn and 1.5 mass% Al. The thickness was 1.4 mm, and blanks of 120 mm × 220 mm were cut from a cold-rolled coil. All samples were degreased with industrial degreaser, which mainly contained 2 mass% NaOH, rinsed with hot water and dried with compressed N2 before annealing. The annealing experiments were carried out in an IWATANI hot-dip galvanizing process (HDGP) simulator. The thermal cycles are shown in Fig. 1. The samples were heated to different annealing temperatures ranging from 600 °C to 800 °C with a fixed heating rate of 5 °C/s and were soaked for 120 s in a N2-5 vol% H2 atmosphere. Following the soaking period, the samples were cooled to room temperature with 100% N2 at a cooling rate of −20 °C/s. For the purpose of obtaining a relatively high oxygen partial pressure to promote the formation of internal oxides, the dew point of the atmosphere was set to +10 °C, which is much higher than the value commonly used in industrial hot-dip galvanizing lines. A summary of the oxygen partial pressure in the process atmosphere at each annealing temperature is provided in Table 2. The partial pressure of water vapor (pH2O) at the
Table 1 A summary of the steel composition and key annealing parameters for the TWIP steel selective oxidation studies in literature [5–12]. No.
Composition/mass%
Soaking temperature/°C
Soaking time/s
H2 content vol%
Dew point/°C
1 2 3 4 5 6 7 8
18Mn-0.1Si-0.6C 18Mn-0.1Si-0.6C 18Mn-0.2Si-1Al-0.5C 17Mn-0.1Si-1.2Al-0.5C 17Mn-0.1Si-1.2Al-0.5C 17Mn-0.1Si-1.2Al-0.5C 23Mn-0.3Si-0.6C 17Mn-0.06Si-1.95Al-0.66C
800 800 800 800 800 800 800 500–650
162 162 162 43 43 43 60 0–180
10 10 10 15 15 15 5 0–20
−17, −3 −17 −17 −40 −20 −20 −30 −20
2
Note
Ni pre-coating Pre-oxidation
Literature [6] [7] [5,7] [8] [9,10] [10] [11] [12]
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microscopy (HR-STEM) was performed on cross-sections of the FIBprepared specimens using an FEI Talos F200X TEM operated at 200 keV. The chemical composition was analyzed using a scanning transmission electron microscope-energy dispersive spectrometer (STEM-EDS). Much attention was paid to the selective oxidation of the alloy elements Mn and Al and the resulting change in microstructure in the subsurface area.
800
600
o
Temperature ( C)
700
T,120 s
500 400
3. Results and discussion o
o
-20 C/s
5 C/s
300
The concentration profiles of Fe, Mn, O, Al, and C obtained by GDOES are shown in Fig. 2. The depth profiles of each element at different annealing temperatures show similar shapes, and all the profiles show at least three areas from the external surface to the substrate. In the surface area, which is approximately 0.2 to 0.4 μm thick, the elemental profiles show a high concentration of O and Mn and a low concentration of Fe and C. These results suggest that significant external oxidation of Mn occurred on the steel surface, in which a small amount of Fe ranging from 2 to 10 mass% was also included. However, almost no Al was detected in the surface area, meaning no external oxidation of Al occurred. On the basis of the current state of knowledge about the selective oxidation of TWIP steel [4,6,7,9,11], the observed external oxidation compound should be identified as MnO or (Mn, Fe) O. The Fe within the external MnO layer should be a small amount of metallic-bond Fe when annealing is conducted at an elevated H2O/H2 ratio [11]. It is possible that Fe was homogenously distributed in MnO because of the high dew point used in the current work. In the subsurface area, which is approximately 0.4 to 1.0 μm thick, the amounts of both Mn and C are not only much lower than those in the surface area but also lower than those in the substrate. Therefore, the subsurface area must be a decarburized and Mn-depleted zone. In contrast, the amounts of Al and O in the subsurface area are slightly higher than those in the substrate. Therefore, it is believed that the subsurface area must contain Al internal oxides. Because the decrease in Mn and C is much larger than the increase in Al and O in the subsurface area, the balance Fe content in the same area is the highest compared to those in the surface area and the substrate. It is well known that TWIP steel has a fully austenite microstructure due to its high Mn and C content [1,2]. Because the Mn and C contents in the subsurface area decreased to critically low values, the initial austenite in the subsurface is believed to have transformed into ferrite [4,6]. The Mn profiles were selected and some feature points on the profiles were highlighted to quantitatively study the effect of annealing temperature on the surface and subsurface areas. As shown in Fig. 3, Mn0, Mn1 and Mn2 were defined as the Mn content at different depths, and t1 and t2 were defined as the thickness of the surface and subsurface areas, respectively. Considering possible pollution on the surface, the carbon content in the surface area detected by GD-OES is not very accurate, but the carbon content in the subsurface and substrate areas is believable. Therefore, similar to the definition of Mn content, the carbon contents in the substrate and subsurface areas were defined as C0 and C2, respectively. The effects of annealing temperature on Mn1, Mn2, C2, t1 and t2 are shown in Fig. 4. When the annealing temperature T increases from 600 °C to 850 °C, the external oxide Mn content (Mn1) does not change significantly, and most of the values are very close to 40 mass%. Mn1 reaches the maximum value when T is 650 °C and then decreases slightly. This result suggests that the composition of the external oxidation layer does not vary with annealing temperature. In contrast, the Mn content and C content in the subsurface layer (Mn2 and C2, respectively) show the same tendency; that is, both of them decrease when T increases from 600 °C to 700 °C and then increase when T further increases from 700 °C to 850 °C. As a result, it seems that the annealing temperature of 700 °C is a turning point for the concentrations of Mn and C in the subsurface layer. In addition, this temperature is a turning point for the thicknesses of
200 100 0
0
50
100
150
200
250
300
350
400
Time (s) Fig. 1. Thermal cycle of the annealing experiments. Table 2 Experimental process atmosphere as a function of annealing temperature. Annealing temperature (T)/ °C
Dew point (DP)/°C
Oxygen partial pressure (pO2)/ atm
600 650 700 750 800 850
+10 +10 +10 +10 +10 +10
6.09 2.56 7.35 1.52 2.36 2.88
× × × × × ×
10−26 10−24 10−23 10−21 10−20 10−19
dew point (DP) was calculated according to Eq. (2-1), which was drawn from the ThermoData database [18]:
log pH2O =
7.58DP − 2.22 DP > 0°C 240 + DP
(2-1)
where the units of pH2O and DP are in atmosphere and °C, respectively. Then, the partial pressure of oxygen (pO2) at equilibrium was calculated according to Eq. (2-2) [19]:
log pO2 = 6 −
26176 ⎛ pH O ⎞ + 2 log ⎜ 2 ⎟ p T ⎝ H2 ⎠
(2-2)
in which T is the absolute temperature in Kelvin and pH2 is the partial pressure of H2 in the atmosphere. Specimens for microanalysis were cut from areas with uniform temperature in the annealed samples. A LECO 750 A glow discharge optical emission spectrometry (GD-OES) instrument was employed to detect the element distribution in the depth direction. A spot 4 mm in diameter on the specimen surface was analyzed with plasma determined by argon pressure. The pressure was controlled by applying a DC voltage of 700 V and a 20 mA current. At least 3 spots on the upper and lower sides of each sample were measured to confirm repeatability, and then one representative result was selected for each sample. The concentrations of Mn, O, Al, C and Fe both on the external surface and at the subsurface were examined, and several features of the depth profiles were extracted to study the effect of annealing temperature on the surface and subsurface composition. The depth between the inflection points on the Mn profiles obtained by differentiation was used to represent the thickness of different zones. Two typical samples annealed at 700 °C and 800 °C were selected for cross-sectional metallographic analysis. An FEI Helios Nanolab 600 DualBeam™ focused ion beam (FIB) scanning electron microscope (SEM) was used to prepare the cross-sectional TEM foils. Pt was deposited prior to FIB milling to protect the surface oxides from damage during Ga-ion milling. High-resolution scanning transmission electron 3
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5
100
80
4
80
4
60
3
60
3
5
2 Al C
20
1.0
1.5
2.0
0 3.0
2.5
2 Al C
20
0 0.0
Mn O
0.5
1.0
Depth (um)
2.0
0 3.0
b) 5
5
100
80
4
80
4
60
3
60
3
100
Fe
2 Al C
20
0 0.0
Mn O
0.5
1.0
1.5
2.0
2.5
1
Al and C (mass %)
40
Fe, Mn and O (mass %)
Fe
40
20
0 0.0
0 3.0
2 Al 1
C Mn O
2
0.5
1.0
1.5
2.0
2.5
1
0 3.0
Depth (um)
Depth (um)
c)
d) 5
100
80
4
80
4
60
3
60
3
100
5 Fe
Al 1
20
0 0.0
C Mn O
2
0.5
1.0
1.5
2.0
2.5
1
Al and C (mass %)
2
40
Fe, Mn and O (mass %)
Fe
Fe, Mn and O (mass %)
2.5
Depth (um)
a)
Fe, Mn and O (mass %)
1.5
1
Al and C (mass %)
0.5
40
0 3.0
Al C
1
20
0 0.0
Depth (um)
2
40
Mn O
2
0.5
1.0
1.5
2.0
2.5
1
Al and C (mass %)
0 0.0
1
Mn O
Fe, Mn and O (mass %)
40
Fe
Al and C (mass %)
Fe, Mn and O (mass %)
Fe
Al and C (mass %)
100
0 3.0
Depth (um)
e)
f)
Fig. 2. GD-OES depth profiles of Fe, Mn, O, Al and C in samples annealed at different temperatures: a) 600 °C, b) 650 °C, c) 700 °C, d) 750 °C, e) 800 °C, and f) 850 °C.
formation of internal Mn oxides at 850 °C. The arrow marked “1” in Fig. 2f suggests that an obvious second peak value of Mn appears, and the arrow marked “2” in the same figure indicates a second Mn depletion zone deeper in the subsurface. The opposite trend of t1 and t2 compared with that of Mn2 and C2 indicates that there might be a certain relationship between the concentration of Mn or C and the thickness of the surface or subsurface layer. The formation of a thicker external manganese oxidation layer requires more Mn, which can only be provided from the subsurface. When the annealing temperature is increased, the diffusion coefficient
the surface layer and subsurface layer (t1 and t2, respectively). When T is 700 °C, the maximum thickness of the external oxidation layer is ~0.4 μm, and the maximum thickness of the subsurface Mn depletion zone is ~1 μm. It is worth noting that the maximum t2 achieved at 700 °C is approximately twice the t2 values at a T of 600 °C or 750–850 °C. It is also necessary to mention that the thicknesses of the external oxidation layer at 700 °C and 850 °C are almost equal, but the thickness of the Mn depletion zone in the subsurface layer does not follow the same trend: the thickness of the Mn depletion zone is not thicker at 850 °C. This result can be explained by the increased 4
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of Mn in the matrix increases, and a large amount of Mn diffuses to the surface, leading to the formation of a thicker Mn external oxidation layer. When the annealing temperature is further increased, the oxygen partial pressure is significantly increased to a level at which a considerable amount of internal Mn oxides can form, so the Mn2 detected by GD-OES increases. The similar shapes of C and Mn in the subsurface area indicate that the selective oxidation behavior of C might be similar to that of Mn. As a result, the Mn depletion zone in the subsurface area is a decarburization zone as well. However, the selective behavior of Al is totally different from that of either Mn or C in the current work. Because the dew point of the annealing atmosphere in this work was +10 °C, the oxygen partial pressure might be high enough to form internal Al oxides instead of external oxides. According to the results of GD-OES, two typical samples were selected to prepare cross-sectional TEM foils by FIB. One was the sample annealed at 700 °C, which had the thickest surface and subsurface layers. The other was the sample annealed at 800 °C, which represented the high-temperature annealed samples. From the cross-sectional morphologies with different magnifications shown in Fig. 5, detailed differences between the above two samples were observed. From the low-magnification images in Fig. 5a and b, three layers, including the surface, subsurface and substrate, were clearly observed. This result agrees well with the different alloy concentrations of the three layers in the GD-OES profiles shown in Figs. 2 and 3. Therefore, the surface layer is a continuous external oxidation layer of Mn, the subsurface layer is composed of ferrite grains and internal oxide particles, and the substrate is purely austenite. The thicknesses of the
Mn0: Mn in the substrate
40
Mn1: Mn in the external Mn oxides
Mn (mass %)
Mn2: Mn in the subsurface layer d1,d2: inflection points on the curve
30
t1: thickness of the external Mn oxidation layer t2: thickness of the Mn depletion zone
20
10
0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
Depth (um) Fig. 3. Feature points extracted from the depth profile of Mn. Mn0, Mn1 and Mn2 are the average Mn content in the substrate area, the maximum Mn content in the surface area and the minimal Mn content in the subsurface area, respectively. d1 and d2 are the inflection points on the curve determined by the differentiation method. t1 and t2 are used to indicate the thickness of the external Mn oxide layer and the Mn depletion zone, respectively.
20
60 Mn1
Mn0
Mn content (mass %)
Mn content (mass %)
Mn2
Mn0
50 40 30 20 10 0
600
650
700
750
800
Annealing Temperature (
15
10
5
0
850
600
)
650
a)
800
850
)
1.6
0.9
C2
0.8
C0
t1
1.4
t2
1.2
0.7
Thickness (um)
C content (mass %)
750
b)
1.0
0.6 0.5 0.4 0.3 0.2
1.0 0.8 0.6 0.4 0.2
0.1 0.0
700
Annealing Temperature (
600
650
700
750
800
Annealing Temperature (
0.0
850
)
600
650
700
750
800
Annealing Temperature (
c)
850
)
d)
Fig. 4. Effect of annealing temperature on a) Mn content in the external oxidation layer, b) Mn content and c) C content in the subsurface area, and d) thickness of the external oxidation layer and Mn depletion zone in the subsurface. 5
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External oxidation
External oxidation Surface
Surface Subsurface
Subsurface Ferrite Ferrite
Austenite
Austenite
Substrate
Substrate
Internal oxidation
Internal oxidation a)
b)
Surface
Surface
Zone I
Zone I
Subsurface Zone II
Zone II
Subsurface Zone III
Zone III c)
d)
Zone I
Zone I
Zone II Zone II
Zone III
Zone III
e)
f)
Fig. 5. Images of cross-sections of samples annealed at a, c, e) 700 °C and b, d, f) 800 °C.
very fine ferrite grains. In addition, the interface of MnO/Zone I is rough; therefore, the fine ferrite grains in zone I appear to grow locally into the MnO layer, and the thickness of zone I ranges from ~30 nm to ~250 nm. It should be mentioned that the coverage of zone I in Fig. 5d is much larger than that in Fig. 5c, indicating that the annealing temperature has a certain effect on zone I when the dew point is high. Similar Fe nodule extrusion from the substrate was also observed by McDermd and Norden in the selective oxidation of low-alloy TRIP steels rich in Si and Al [20,21]. Zone II, which is ~100 nm to ~300 nm below zone I, has extremely fine ferrite grains and internal oxides. A mixture of grain boundary and internally precipitated oxides is observed, where the fine grain size is likely a result of the grain boundary oxides pinning the grain boundaries during annealing. Such morphologies have also been documented in studies on TRIP steels [22] and Med-Mn steels [20]. In the local enlargement shown in Fig. 5e and f, it was found that the thickness of zone II and the size of the oxide particles increased when the annealing temperature was increased from 700 °C to 800 °C.
surface and subsurface areas were measured in the metallographic images. The thicknesses of both the surface and subsurface layers of the sample annealed at 700 °C are larger than those of the sample annealed at 800 °C. The former sample has a 0.4–0.5 μm surface layer thickness and an ~1 μm subsurface layer thickness, while the latter sample has a 0.2–0.3 μm surface layer thickness and an ~0.5 μm subsurface layer thickness. The results of the metallographic measurements are basically consistent with the results of GD-OES. Moreover, the microstructures of the subsurface areas are also different. The subsurface ferrite grains in Fig. 5a are columnar, and the grain boundary network internal oxides extending from the surface to the substrate continuously are almost vertical. The subsurface ferrite grains in Fig. 5b are much more equiaxed, and the internal oxides are large, discontinuous and disordered. From the medium- and large-magnification images in Fig. 5c–f, the subsurface layer can be further divided into three zones, as indicated by arrows I to III. Zone I, which is just underneath the external oxidation layer, is absolutely free of internal oxide particles and only consists of 6
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a)
b)
c)
External MnO oxides
Internal MnAl2O4 oxides
d)
e)
f)
Fig. 6. STEM-EDS mapping of Fe, O, Mn, and Al for the sample annealed at 700 °C.
and grain boundary diffusion. Therefore, Al2O3 appears deeper in the substrate than the grain-boundary MnAl2O4. It was noted that AlN precipitates were found in the subsurface of annealed Al-added TRIP steels. However, Al2O3 instead of AlN is dominant in the present study, which may be because of the very high partial oxygen pressure. Some voids at the grain boundaries deep inside the 800 °C sample were observed. These voids might be caused by the combined Kirkendall effect [5] and decarburization. Although the types of oxides are the same for the samples annealed at 700 °C and 800 °C, the annealing temperature has a great effect on the shape and size of the internal oxides in the subsurface. In the case of 800 °C, some large internal MnO grains with a diameter in the range of 300–600 nm, as indicated by the arrows in Fig. 7d, appear in zone III. Because the size of these MnO grains is almost as large as that of the surrounding ferrite grains, it seems that some of the ferrite grains in the subsurface area are replaced by these large internal manganese oxides. Such enhanced internal oxidation of Mn should be attributed to the increased oxygen partial pressure and the increased annealing temperature. The increased diffusivity of O, Mn and Al at a higher annealing temperature promotes the growth rate of internal oxides. The increased size of the subsurface MnO precipitates is largely due to temperature rather than partial oxygen pressure effects. In addition, a large amount of excess Mn becomes available when some of the austenite grains transform into ferrite during the annealing process at 800 °C. Therefore, a ferrite + austenite two-phase subsurface layer promotes the formation of large Mn grains at a distance deeper than that of external MnO. The formation of internal MnO in the subsurface layer can explain why the thickness of the external oxidation layer of Mn is slightly reduced when the annealing temperature is 800 °C. Fig. 6e and Fig. 7e show that the fine ferrite grains in zone I in the subsurface layer contain a small amount of aluminum, while Fig. 6d and Fig. 7d show that the manganese content is very limited in these ferrite grains. These results are likely determined by the different solid solutions of Al and Mn in ferrite. Based on the above GD-OES and cross-sectional results, a schematic illustration of the effect of annealing temperatures of 700 °C and 800 °C
Zone III, which is the main part of the subsurface area, consists of both ferrite grains and relatively coarse internal oxides. In this zone, the size of the ferrite grains is greater than that of the grains in zones I and II but still smaller than that of the austenite grains in the substrate. It should be noted that the shape and size of the internal oxides in Fig. 5e and f are totally different from each other even though the annealing temperature only differs by 100 °C. When the annealing temperature is 700 °C, the continuous film-like internal oxides in Fig. 5e grow vertically along the ferrite grain boundaries into the substrate. The bulk of the ferrite grains and the interface of ferrite/austenite are almost free of internal oxides. When the annealing temperature is 800 °C, most of the internal oxides still grow along the grain boundaries, but they grow randomly in all directions. Internal oxides are easily found in the bulk of the ferrite grains and at the interface of ferrite/austenite. In addition, the size of the internal oxides in Fig. 5f is larger than that in Fig. 5e, indicating that a higher annealing temperature promotes the growth of internal oxides. In contrast, the depth of zone III and the depth of the internal oxidation layer in Fig. 5f are smaller than those in Fig. 5e, suggesting that the transformation of austenite to ferrite in zone III is suppressed when the annealing temperature is 800 °C. The elemental mappings of the cross sections of samples annealed at 700 °C and 800 °C, as shown in Figs. 6 and 7, respectively, reveal the distribution of Mn, Al, O, and Fe in the surface and subsurface layers. The enrichment of O and Mn in the surface layer and the depletion of Mn in the subsurface layer are completely consistent with the GD-OES results. Three types of oxides are observed in the present work. The external oxides were identified as MnO through EDS analysis. The internal oxides in zone II and zone III along the grain boundaries were identified as MnAl2O4. In some deep areas, internal Al2O3 was found. The grain boundary MneAl complex oxides probably have a core-shell structure similar to those of the similar grain boundary internal MneSi complex oxides revealed by De Cooman23) and McDermid [16], where Al might be enriched in the oxide core, while both Mn and Al are present in the outer shell surrounding the Al-rich core. Al2O3 is more thermodynamically stable than aluminate, and aluminate also requires diffusive transformation arising from the slower Mn migration via bulk 7
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a)
b)
External MnO oxides
Large internal MnO oxides
c)
Internal MnAl2O4 oxides
Internal Al2O3 oxides
d)
voids
e)
f)
Fig. 7. STEM-EDS mapping of Fe, O, Mn, and Al for the sample annealed at 800 °C.
oxides to form in the subsurface layer, the more noble solvent metal (in this case Fe) may be extruded to the surface (in this case, the MnO/ subsurface interface) through dislocation channels, resulting in nodules of pure solvent metal at the surface [26]. Notably, the amount of ferrite grains increases with annealing temperature. Zone II is composed of ultrafine ferrite grains and fine internal oxides, and a similar transition zone with many oxide precipitates is also observed and considered to be caused by the earlier nucleation and crystallization process in this zone [24] or the pinning effect of grain boundary oxides [20,22]. Zone III is composed of fine ferrite grains and relatively large internal oxides. In zone III, both the depletion of Mn, which is an austenite stabilizer, and decarburization result in the transformation of austenite into ferrite [7]. Third, the annealing temperature plays an important role in the surface and subsurface formation of Al-added TWIP steel. The annealing temperature not only affects the thickness of both the surface and subsurface layers but also affects the type, shape, size and distribution of internal oxides. The type of external oxide remains the same in the range from 600 °C to 850 °C, but its thickness increases with annealing temperature below 700 °C and decreases with annealing temperature above 700 °C. Internal oxidation is promoted when the annealing temperature is increased. For example, when the annealing temperature was increased from 700 °C to 800 °C, the film-like internal oxides along the grain boundaries grew coarser, and large MnO internal oxides formed. Moreover, the amount of protruded ferrite grains in zone I increases with annealing temperature, which is also considered to be the result of the increased internal Al oxidation when the annealing temperature is increased. The effect of annealing temperature on the selective oxidation of Mn and Al is considered to be determined by the partial oxygen pressure and the diffusion coefficients of O, Mn and Al in the substrate.
Fig. 8. Schematic illustration of the difference in surface and subsurface microstructure of Al-added TWIP steel annealed in a N2 + 5 vol% H2 gas atmosphere with a dew point of +10 °C at 700 °C and 800 °C.
on the surface and subsurface microstructure of Al-added TWIP steel annealed in a N2 + 5 vol% H2 gas atmosphere with a dew point of +10 °C is shown in Fig. 8. Although external MnO and Mn-depleted ferrite zones with internal oxides have already been reported in the literature [5–11], in the current work, the microstructure in the subsurface layer was revealed in further detail, and the effect of annealing temperature was highlighted. First, similar to all the results in the literature, the surface of the annealed TWIP steel in the current work is also covered by a compact and continuous layer of external MnO, and the subsurface of the steel is composed of ferrite grains and a variety of internal oxides. The main cause of ferrite formation is the significant depletion of Mn and C in the subsurface layer due to the external oxidation of Mn and decarburization. Second, the subsurface area between the external MnO and the austenite substrate can be further subdivided into three zones. Zone I is composed of fine ferrite grains without any internal oxides inside the grains or along the grain boundaries, and the formation of this zone could be attributed to the protrusion of iron atoms from the subsurface due to the significant volume expansion of internal Al oxides when the dew point is high enough [24,25]. To make room for these internal
4. Conclusions The effect of annealing temperature on Al-added TWIP steel in a N25 vol% H2 atmosphere with a dew point of +10 °C was studied. The conclusions listed below can be drawn: 1. The annealing temperature shows a significant effect on the external 8
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and internal oxidation of Mn and Al in TWIP steel and therefore obviously changes the subsurface microstructure. 2. Due to the abundant formation of external MnO and decarburization, the content of Mn and C in the subsurface area decreases to a critical level and results in the formation of a continuous ferrite layer that is transformed from austenite. 3. The type of external MnO does not change with annealing temperature. However, the type, shape, size and distribution of internal oxides in the subsurface area are greatly affected by annealing temperature. The film-like internal MnAl2O4 along the grain boundaries grows coarser and some large internal MnO forms when the annealing temperature increases from 700 °C to 800 °C. 4. A layer of protruded ferrite grains without any internal oxides inside the grains or along the grain boundaries is found just beneath the external MnO layer, the amount of which increases with annealing temperature.
[7] [8]
[9]
[10]
[11]
[12] [13] [14]
Author contribution
[15]
All authors contributed equally to this manuscript. [16]
Declaration of competing interest [17]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[18] [19]
Acknowledgement [20]
The authors would like to thank the National Key R&D Program of China (No. 2017YFB0304402) for the financial support.
[21]
References [22] [1] B.C. De Cooman, Y. Estrin, S.K. Kim, Twinning-induced plasticity (TWIP) steels, Acta Mater. 142 (2018) 283–362. [2] B.C. De Cooman, O. Kwon, K.G. Chin, State-of-the-knowledge on TWIP steel, Mater. Sci. Technol. 28 (2012) 513–527. [3] L. Chen, Y. Zhao, X. Qin, Some aspects of high manganese twinning-induced plasticity (TWIP) steel, a review, Acta Metall Sinica. (English Letters) 26 (2013) 1–15. [4] Y.F. Gong, H.S. Kim, S.K. Kim, B.C. De Cooman, Selective oxidation and sub-surface phase transformation during austenitic annealing of TWIP steels, Mater. Sci. Forum 654 (2010) 258–261. [5] Y.F. Gong, B.C. De Cooman, Kirkendall void formation during selective oxidation, Metall. Mater. Trans. A 41 (2010) 2180–2183. [6] Y.F. Gong, B.C. De Cooman, Selective oxidation and sub-surface phase
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transformation of TWIP steel during continuous annealing, Steel Res. Int. 82 (2011) 1310–1318. L. Cho, B.C. De Cooman, Selective oxidation of TWIP steel during continuous annealing, Steel Res. Int. 83 (2012) 391–397. Y. Kim, J. Lee, K.S. Shin, S. Jeon, K.G. Chin, Effect of dew point on the formation of surface oxides of twinning-induced plasticity steel, Mater. Charact. 89 (2014) 138–145. Y. Kim, K.S. Shin, S.H. Jeon, K.G. Chin, J. Lee, The influence of the dew point on the wettability of twinning-induced-plasticity steels by liquid Zn–0.23-wt% Al, Corros. Sci. 85 (2014) 364–371. Y. Kim, J. Lee, K.S. Shin, S.H. Jeon, K.G. Chin, Effect of nickel precoating on wettability of twinning-induced plasticity steels by liquid Zn-0.23 wt% Al, Metall. Mater. Trans. A 47 (2016) 4960–4969. M. Blumenau, M. Norden, F. Friedel, K. Peters, Use of pre-oxidation to improve reactive wetting of high manganese alloyed steel during hot-dip galvanizing, Surf. Coat. Tech. 206 (2011) 559–567. W. Chen, A Study on Interface Condition in High Mn TWIP Steel After Initial Stage of Oxidation Process, Seoul National University, 2019. R. Sagl, A. Jarosik, D. Stifter, G. Angeli, The role of surface oxides on annealed highstrength steels in hot-dip galvanizing, Corros. Sci. 70 (2013) 268–275. R. Sagl, A. Jarosik, G. Angeli, T. Haunschmid, G. Hesser, Tailoring of oxide morphology and crystallinity on advanced high-strength steel surfaces prior hot-dip galvanizing, Acta Mater. 72 (2014) 192–199. M. Pourmajidian, J.R. Mcdermid, Effect of annealing temperature on the selective oxidation and reactive wetting of a 0.1 C-6Mn-2Si advanced high strength steel during continuous galvanizing heat treatments, ISIJ Int. 58 (2018) 1635–1643. M. Pourmajidian, J.R. McDermid, Selective oxidation of a 0.1 C-6Mn-2Si third generation advanced high-strength steel during dew-point controlled annealing, Metall. Mater. Trans. A 49A (2018) 1795–1808. M. Pourmajidian, J.R. McDermid, On the reactive wetting of a medium-Mn advanced high-strength steel during continuous galvanizing, Surf. Coat. Tech. 357 (2019) 418–426. Thermodata, Electronic data bank for thermodynamic quantities, available online at the address, 2005. http://thermodata.online.fr. A. Rist, M.F. Ancey-moret, C. Gatellier, P.V. Ribound, Équilibres thermodynamiques dans l’élaboration de la fonte et de l’acier, Techniques de L’ingénieur. Matériaux Métalliques M1730 (1974) 1–42. E.M. Bellhouse, J.R. McDermid, Selective oxidation and reactive wetting during galvanizing of a CMnAl TRIP-assisted steel, Metall. Mater. Trans. A 42A (2011) 2753–2768. M. Norden, M. Blumenau, E. Zimmermann, K.J. Peters, Local equilibrium and nodule formation on Al-TRIP-aided steel surfaces during intercritical annealing, Steel Res. Int. 83 (2012) 870–877. Y.F. Gong, H. Kim, B.C. De Cooman, Internal oxidation during Intercritical annealing of CMnSi TRIP steel, ISIJ Int. 49 (2009) 557–563. Y.F. Gong, H.S. Kim, B.C. De Cooman, Formation of surface and subsurface oxides during ferritic, intercritical and austenitic annealing of CMnSi TRIP steel, ISIJ Int. 48 (2008) 1745–1751. H. Wang, X.Y. Jin, G.K. Hu, Y. He, Changing oxide layer structures with respect to the dew point prior to hot-dip galvanizing of δ-TRIP steel, Surf. Coat. Tech. 337 (2018) 260–269. X.Y. Jin, G.K. Hu, H.W. Qian, H. Wang, Effect of dew point on galvanizability in 4 mass% Al added low density steel, ISIJ Int. 58 (2018) 1584–1591. D.L. Douglass, A critique of internal oxidation in alloys during the post-Wagner era, Oxid. Met. 44 (1995) 81–111.
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Effect of annealing temperature on the surface and subsurface microstructure of Al-added TWIP steel Xinyan Jina,b, Yong Zhonga,b, Li Wanga,b, Hua Wangc,
T
⁎
a
Baosteel Research Institute, Shanghai 201900, China State Key Laboratory of Development and Application Technology of Automotive Steels, Baosteel, Shanghai 201900, China c Key Laboratory for Material Microstructures of Shanghai University, 200444, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: TWIP steel Selective oxidation Annealing temperature Dew point
Annealing experiments were carried out in a N2-5 vol% H2 atmosphere with a dew point of +10 °C to investigate the effect of annealing temperatures ranging from 600 °C to 850 °C on the surface and subsurface microstructure of Al-added TWIP steel. The depth profiles of Fe, Al, Mn, O and C in the annealed samples along a 3 μm depth from the surface were measured by GD-OES. High-resolution cross sections were prepared by FIB and characterized by TEM. It was found that the annealing temperature plays an important role in the surface and subsurface formation of Al-added TWIP steel. Due to the abundant formation of external MnO and decarburization, the content of Mn and C in the subsurface area decreases to a critical low level and results in the formation of a continuous ferrite layer. In addition, the type, shape, size and distribution of the internal oxides in the subsurface area are greatly affected by the annealing temperature. When the annealing temperature increases from 700 °C to 800 °C, the film-like internal MnAl2O4 along the grain boundaries grows coarser, and some large internal MnO is formed. Furthermore, a layer of protruded ferrite grains without any internal oxides inside the grains or along the grain boundaries appears just beneath the external MnO layer, the amount of which increases with the annealing temperature.
1. Introduction Reducing the weight of vehicles is well recognized as an effective way to reduce carbon dioxide emissions and enhance fuel efficiency. In the last two decades, varieties of advanced high-strength steel (AHSS) and ultrahigh-strength steel (UHSS) have been developed and widely used in the body-in-white. High-Mn twinning-induced plasticity (TWIP) steel a new type of UHSS structural steel and has received great attention. TWIP steel is characterized by a sustained high strain hardening rate, which results in higher observed elongation and tensile strength [1]. The use of TWIP steel may therefore lead to a considerable reduction in weight of steel components, a reduction in material use and improved press forming behavior [2]. To obtain a desirable microstructure and mechanical properties, C, Mn, Al and, in some cases, Si are the main element added to TWIP steel [2]. A sufficient content of Mn, usually in the range of 12–30 mass%, is required to obtain a fully austenitic microstructure [3]. C addition, typically in the range of 0.4–1.0 mass%, also results in the stabilization of the austenite phase and solid-solution strengthening [2]. Al addition, typically < 3 mass% [3], shows beneficial effects on the suppression of
⁎
the delayed fracture phenomenon of TWIP steel [2] and a slight reduction in density [3]. However, the selective oxidation of the key elements added to TWIP steel during the continuous annealing process or hot-dip galvanizing process may lead to the deterioration of the surface state of the steel. A series of studies on the selective oxidation of TWIP steels under a variety of conditions were performed by the De Cooman [4–7] group. When TWIP steel was annealed at 800 °C in a N2 + 10% H2 gas atmosphere with dew points of −17 °C and −3 °C, a continuous layer of MnO was found to form on the surface, and a Mn-depleted zone was found to transform from austenite to ferrite in the subsurface. A thicker external Mn oxidation layer and a deeper Mn-depleted zone were found when the dew point was increased from −17 °C to −3 °C. When the dew point was −17 °C, the characteristics of the MnO layer/steel matrix interface were also influenced by the addition of Al to the matrix. In the case of Al-free TWIP steel, crystalline c-xMnO·SiO2 particles and amorphous a-xMnO·SiO2 particles were found. In the case of Al-added TWIP steel, amorphous a-xMnO·SiO2, crystalline c-xMnO·Al2O3 and Kirkendall voids were found. Large internal MnO particles were observed in the Mn-depleted ferrite zone when the dew point was −3 °C.
Corresponding author. E-mail address: [email protected] (H. Wang).
https://doi.org/10.1016/j.surfcoat.2020.125479 Received 6 December 2019; Received in revised form 13 February 2020; Accepted 16 February 2020 Available online 19 February 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.
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follows:
The selective oxidation of Al-added TWIP steel and its effect on the wettability of TWIP steel by liquid Zn-0.23 mass% Al were studied by Kim et al. [8,9]. It was also noted that the type of surface oxides and the thickness of the oxide layer were determined by the dew point [8]. When the dew point was −20 °C, a 100-nm-thick MnO layer formed uniformly on the steel surface, and an ~15-nm-thick MnAl2O4 layer formed with small particles of Mn2SiO4 of ~70-nm diameter under the MnO layer. When the dew point was −40 °C, a 5-nm-thick MnAl2O4 layer formed across most of the surface, and ~50-nm-thick Mn2SiO4 particles were formed irregularly on some parts of the surface. Because the thick MnO layer was replaced by relatively thin MnAl2O4 when the dew point decreased from −20 °C to −40 °C, the formation of intermetallic compounds occurred easily, and the wettability of TWIP steel by liquid Zn improved [9]. Nickel precoating is another way to prevent the formation of surface oxides and thus to improve the Zn-coating properties of TWIP steel [10]. It was found that a nonuniform thin MnO layer instead of a uniform thick MnO layer was formed on the surface of a sample with ~50nm-thick nickel precoating. Preoxidation was also confirmed to be a useful tool for the hot-dip galvanizing of TWIP steel based on the study of Blumenau et al. [11]. It was reported that the amount of metallicbond Fe within the MnO·Femetal layer increased considerably when preoxidation was performed at 600 °C for 10 s in 1.8% O2-N2. By this process, Fe2Al5 formation on top of the MnO·Femetal layer was considerably intensified and was accompanied by significantly improved liquid Zn wetting. The interface conditions in high-Mn TWIP steel after the initial stage of the oxidation process were studied by Chen [12]. It was found that an obvious oxide layer did not appear until 600 °C, MneAl oxides formed below the interface along the grain boundary, and aluminum oxides formed in deeper positions. As the soaking time increased from 0 s to 180 s, a more uniform surface formed without a significant change in layer thickness. It was also found that the H2 content of the annealing gas changed the oxide type and thickness due to the different external oxygen potentials. The importance of external oxide morphology on reactive wetting during the hot-dip galvanizing of different Mn-alloyed steels has been studied. It was reported by Sagl et al. [13,14] that crystalline oxides ensure the contact of Al and Zn with Fe in the steel substrate, which is not the case for amorphous oxides. Bath-dissolved Al can diffuse into the grain boundaries of crystalline oxides and form an FeeAl intermetallic phase, while the process is suppressed in amorphous oxides that offer no efficient diffusion path for dissolved Al in the zinc bath. A recent series of studies by Pourmajidian and McDermid [15–17] determined that a decreased external MnO thickness obtained by decreasing the annealing temperature and the modification of external oxides to a fine and globular morphology by Sn addition can improve the reactive wetting of 0.1C-6Mn-2Si steels. Several mechanisms, such as the aluminothermic reduction of MnO, oxide bridging and oxide liftoff by the molten galvanizing bath, were proposed. A summary of the steel compositions and the key annealing parameters in TWIP steel selective oxidation studies in the literature is listed in Table 1. Some common conclusions drawn from the literature are as
1. In most cases, the surface of the annealed TWIP steel was covered with dense external MnO, and the austenite in the subsurface was transformed into ferrite due to Mn depletion and decarburization. In the case of a low-dew point annealing atmosphere [8,9] or with Ni precoating [10], external MnO and subsurface ferrite formation were suppressed. 2. It was found that there were different types of oxides at the interface of MnO/ferrite and internal oxides in the subsurface, including amorphous a-xMnO·SiO2, crystalline c-xMnO·SiO2 and cxMnO·Al2O3, Mn2SiO4, MnAl2O4 and Al2O3. The type, size and distribution of the oxides were affected by the composition and process conditions. In some cases [5,7,11], voids were observed in the interface oxides. 3. The dew point is considered to have a great effect on the selective oxidation of TWIP steel; the thickness of the external MnO and subsurface ferrite layers and the amount and depth of internal oxides increased with the dew point. The existing studies [4–12] on the selective oxidation of TWIP steel mainly focused on the effect of the dew point, and in most cases, the annealing temperature was fixed to 800 °C. The influence of annealing temperature on the selective oxidation of TWIP steel is still not very clear. Therefore, laboratory annealing experiments at different annealing temperatures in an atmosphere with a relatively high dew point were conducted on Al-added TWIP steel, and both the surface and subsurface were characterized by means of GD-OES and FIB in the current work. The primary aim of this study is to determine the effect of annealing temperature on the surface and subsurface microstructure of TWIP steel. 2. Experimental procedures The material used in the present work is industrially cold-rolled TWIP steel with the following composition: 0.7 mass% C, 16 mass% Mn and 1.5 mass% Al. The thickness was 1.4 mm, and blanks of 120 mm × 220 mm were cut from a cold-rolled coil. All samples were degreased with industrial degreaser, which mainly contained 2 mass% NaOH, rinsed with hot water and dried with compressed N2 before annealing. The annealing experiments were carried out in an IWATANI hot-dip galvanizing process (HDGP) simulator. The thermal cycles are shown in Fig. 1. The samples were heated to different annealing temperatures ranging from 600 °C to 800 °C with a fixed heating rate of 5 °C/s and were soaked for 120 s in a N2-5 vol% H2 atmosphere. Following the soaking period, the samples were cooled to room temperature with 100% N2 at a cooling rate of −20 °C/s. For the purpose of obtaining a relatively high oxygen partial pressure to promote the formation of internal oxides, the dew point of the atmosphere was set to +10 °C, which is much higher than the value commonly used in industrial hot-dip galvanizing lines. A summary of the oxygen partial pressure in the process atmosphere at each annealing temperature is provided in Table 2. The partial pressure of water vapor (pH2O) at the
Table 1 A summary of the steel composition and key annealing parameters for the TWIP steel selective oxidation studies in literature [5–12]. No.
Composition/mass%
Soaking temperature/°C
Soaking time/s
H2 content vol%
Dew point/°C
1 2 3 4 5 6 7 8
18Mn-0.1Si-0.6C 18Mn-0.1Si-0.6C 18Mn-0.2Si-1Al-0.5C 17Mn-0.1Si-1.2Al-0.5C 17Mn-0.1Si-1.2Al-0.5C 17Mn-0.1Si-1.2Al-0.5C 23Mn-0.3Si-0.6C 17Mn-0.06Si-1.95Al-0.66C
800 800 800 800 800 800 800 500–650
162 162 162 43 43 43 60 0–180
10 10 10 15 15 15 5 0–20
−17, −3 −17 −17 −40 −20 −20 −30 −20
2
Note
Ni pre-coating Pre-oxidation
Literature [6] [7] [5,7] [8] [9,10] [10] [11] [12]
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X. Jin, et al.
900
microscopy (HR-STEM) was performed on cross-sections of the FIBprepared specimens using an FEI Talos F200X TEM operated at 200 keV. The chemical composition was analyzed using a scanning transmission electron microscope-energy dispersive spectrometer (STEM-EDS). Much attention was paid to the selective oxidation of the alloy elements Mn and Al and the resulting change in microstructure in the subsurface area.
800
600
o
Temperature ( C)
700
T,120 s
500 400
3. Results and discussion o
o
-20 C/s
5 C/s
300
The concentration profiles of Fe, Mn, O, Al, and C obtained by GDOES are shown in Fig. 2. The depth profiles of each element at different annealing temperatures show similar shapes, and all the profiles show at least three areas from the external surface to the substrate. In the surface area, which is approximately 0.2 to 0.4 μm thick, the elemental profiles show a high concentration of O and Mn and a low concentration of Fe and C. These results suggest that significant external oxidation of Mn occurred on the steel surface, in which a small amount of Fe ranging from 2 to 10 mass% was also included. However, almost no Al was detected in the surface area, meaning no external oxidation of Al occurred. On the basis of the current state of knowledge about the selective oxidation of TWIP steel [4,6,7,9,11], the observed external oxidation compound should be identified as MnO or (Mn, Fe) O. The Fe within the external MnO layer should be a small amount of metallic-bond Fe when annealing is conducted at an elevated H2O/H2 ratio [11]. It is possible that Fe was homogenously distributed in MnO because of the high dew point used in the current work. In the subsurface area, which is approximately 0.4 to 1.0 μm thick, the amounts of both Mn and C are not only much lower than those in the surface area but also lower than those in the substrate. Therefore, the subsurface area must be a decarburized and Mn-depleted zone. In contrast, the amounts of Al and O in the subsurface area are slightly higher than those in the substrate. Therefore, it is believed that the subsurface area must contain Al internal oxides. Because the decrease in Mn and C is much larger than the increase in Al and O in the subsurface area, the balance Fe content in the same area is the highest compared to those in the surface area and the substrate. It is well known that TWIP steel has a fully austenite microstructure due to its high Mn and C content [1,2]. Because the Mn and C contents in the subsurface area decreased to critically low values, the initial austenite in the subsurface is believed to have transformed into ferrite [4,6]. The Mn profiles were selected and some feature points on the profiles were highlighted to quantitatively study the effect of annealing temperature on the surface and subsurface areas. As shown in Fig. 3, Mn0, Mn1 and Mn2 were defined as the Mn content at different depths, and t1 and t2 were defined as the thickness of the surface and subsurface areas, respectively. Considering possible pollution on the surface, the carbon content in the surface area detected by GD-OES is not very accurate, but the carbon content in the subsurface and substrate areas is believable. Therefore, similar to the definition of Mn content, the carbon contents in the substrate and subsurface areas were defined as C0 and C2, respectively. The effects of annealing temperature on Mn1, Mn2, C2, t1 and t2 are shown in Fig. 4. When the annealing temperature T increases from 600 °C to 850 °C, the external oxide Mn content (Mn1) does not change significantly, and most of the values are very close to 40 mass%. Mn1 reaches the maximum value when T is 650 °C and then decreases slightly. This result suggests that the composition of the external oxidation layer does not vary with annealing temperature. In contrast, the Mn content and C content in the subsurface layer (Mn2 and C2, respectively) show the same tendency; that is, both of them decrease when T increases from 600 °C to 700 °C and then increase when T further increases from 700 °C to 850 °C. As a result, it seems that the annealing temperature of 700 °C is a turning point for the concentrations of Mn and C in the subsurface layer. In addition, this temperature is a turning point for the thicknesses of
200 100 0
0
50
100
150
200
250
300
350
400
Time (s) Fig. 1. Thermal cycle of the annealing experiments. Table 2 Experimental process atmosphere as a function of annealing temperature. Annealing temperature (T)/ °C
Dew point (DP)/°C
Oxygen partial pressure (pO2)/ atm
600 650 700 750 800 850
+10 +10 +10 +10 +10 +10
6.09 2.56 7.35 1.52 2.36 2.88
× × × × × ×
10−26 10−24 10−23 10−21 10−20 10−19
dew point (DP) was calculated according to Eq. (2-1), which was drawn from the ThermoData database [18]:
log pH2O =
7.58DP − 2.22 DP > 0°C 240 + DP
(2-1)
where the units of pH2O and DP are in atmosphere and °C, respectively. Then, the partial pressure of oxygen (pO2) at equilibrium was calculated according to Eq. (2-2) [19]:
log pO2 = 6 −
26176 ⎛ pH O ⎞ + 2 log ⎜ 2 ⎟ p T ⎝ H2 ⎠
(2-2)
in which T is the absolute temperature in Kelvin and pH2 is the partial pressure of H2 in the atmosphere. Specimens for microanalysis were cut from areas with uniform temperature in the annealed samples. A LECO 750 A glow discharge optical emission spectrometry (GD-OES) instrument was employed to detect the element distribution in the depth direction. A spot 4 mm in diameter on the specimen surface was analyzed with plasma determined by argon pressure. The pressure was controlled by applying a DC voltage of 700 V and a 20 mA current. At least 3 spots on the upper and lower sides of each sample were measured to confirm repeatability, and then one representative result was selected for each sample. The concentrations of Mn, O, Al, C and Fe both on the external surface and at the subsurface were examined, and several features of the depth profiles were extracted to study the effect of annealing temperature on the surface and subsurface composition. The depth between the inflection points on the Mn profiles obtained by differentiation was used to represent the thickness of different zones. Two typical samples annealed at 700 °C and 800 °C were selected for cross-sectional metallographic analysis. An FEI Helios Nanolab 600 DualBeam™ focused ion beam (FIB) scanning electron microscope (SEM) was used to prepare the cross-sectional TEM foils. Pt was deposited prior to FIB milling to protect the surface oxides from damage during Ga-ion milling. High-resolution scanning transmission electron 3
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Fe
Al and C (mass %)
Fe, Mn and O (mass %)
Fe
Al and C (mass %)
100
0 3.0
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e)
f)
Fig. 2. GD-OES depth profiles of Fe, Mn, O, Al and C in samples annealed at different temperatures: a) 600 °C, b) 650 °C, c) 700 °C, d) 750 °C, e) 800 °C, and f) 850 °C.
formation of internal Mn oxides at 850 °C. The arrow marked “1” in Fig. 2f suggests that an obvious second peak value of Mn appears, and the arrow marked “2” in the same figure indicates a second Mn depletion zone deeper in the subsurface. The opposite trend of t1 and t2 compared with that of Mn2 and C2 indicates that there might be a certain relationship between the concentration of Mn or C and the thickness of the surface or subsurface layer. The formation of a thicker external manganese oxidation layer requires more Mn, which can only be provided from the subsurface. When the annealing temperature is increased, the diffusion coefficient
the surface layer and subsurface layer (t1 and t2, respectively). When T is 700 °C, the maximum thickness of the external oxidation layer is ~0.4 μm, and the maximum thickness of the subsurface Mn depletion zone is ~1 μm. It is worth noting that the maximum t2 achieved at 700 °C is approximately twice the t2 values at a T of 600 °C or 750–850 °C. It is also necessary to mention that the thicknesses of the external oxidation layer at 700 °C and 850 °C are almost equal, but the thickness of the Mn depletion zone in the subsurface layer does not follow the same trend: the thickness of the Mn depletion zone is not thicker at 850 °C. This result can be explained by the increased 4
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of Mn in the matrix increases, and a large amount of Mn diffuses to the surface, leading to the formation of a thicker Mn external oxidation layer. When the annealing temperature is further increased, the oxygen partial pressure is significantly increased to a level at which a considerable amount of internal Mn oxides can form, so the Mn2 detected by GD-OES increases. The similar shapes of C and Mn in the subsurface area indicate that the selective oxidation behavior of C might be similar to that of Mn. As a result, the Mn depletion zone in the subsurface area is a decarburization zone as well. However, the selective behavior of Al is totally different from that of either Mn or C in the current work. Because the dew point of the annealing atmosphere in this work was +10 °C, the oxygen partial pressure might be high enough to form internal Al oxides instead of external oxides. According to the results of GD-OES, two typical samples were selected to prepare cross-sectional TEM foils by FIB. One was the sample annealed at 700 °C, which had the thickest surface and subsurface layers. The other was the sample annealed at 800 °C, which represented the high-temperature annealed samples. From the cross-sectional morphologies with different magnifications shown in Fig. 5, detailed differences between the above two samples were observed. From the low-magnification images in Fig. 5a and b, three layers, including the surface, subsurface and substrate, were clearly observed. This result agrees well with the different alloy concentrations of the three layers in the GD-OES profiles shown in Figs. 2 and 3. Therefore, the surface layer is a continuous external oxidation layer of Mn, the subsurface layer is composed of ferrite grains and internal oxide particles, and the substrate is purely austenite. The thicknesses of the
Mn0: Mn in the substrate
40
Mn1: Mn in the external Mn oxides
Mn (mass %)
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t1: thickness of the external Mn oxidation layer t2: thickness of the Mn depletion zone
20
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0 0.0
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Depth (um) Fig. 3. Feature points extracted from the depth profile of Mn. Mn0, Mn1 and Mn2 are the average Mn content in the substrate area, the maximum Mn content in the surface area and the minimal Mn content in the subsurface area, respectively. d1 and d2 are the inflection points on the curve determined by the differentiation method. t1 and t2 are used to indicate the thickness of the external Mn oxide layer and the Mn depletion zone, respectively.
20
60 Mn1
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)
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Annealing Temperature (
0.0
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)
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c)
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)
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Fig. 4. Effect of annealing temperature on a) Mn content in the external oxidation layer, b) Mn content and c) C content in the subsurface area, and d) thickness of the external oxidation layer and Mn depletion zone in the subsurface. 5
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External oxidation
External oxidation Surface
Surface Subsurface
Subsurface Ferrite Ferrite
Austenite
Austenite
Substrate
Substrate
Internal oxidation
Internal oxidation a)
b)
Surface
Surface
Zone I
Zone I
Subsurface Zone II
Zone II
Subsurface Zone III
Zone III c)
d)
Zone I
Zone I
Zone II Zone II
Zone III
Zone III
e)
f)
Fig. 5. Images of cross-sections of samples annealed at a, c, e) 700 °C and b, d, f) 800 °C.
very fine ferrite grains. In addition, the interface of MnO/Zone I is rough; therefore, the fine ferrite grains in zone I appear to grow locally into the MnO layer, and the thickness of zone I ranges from ~30 nm to ~250 nm. It should be mentioned that the coverage of zone I in Fig. 5d is much larger than that in Fig. 5c, indicating that the annealing temperature has a certain effect on zone I when the dew point is high. Similar Fe nodule extrusion from the substrate was also observed by McDermd and Norden in the selective oxidation of low-alloy TRIP steels rich in Si and Al [20,21]. Zone II, which is ~100 nm to ~300 nm below zone I, has extremely fine ferrite grains and internal oxides. A mixture of grain boundary and internally precipitated oxides is observed, where the fine grain size is likely a result of the grain boundary oxides pinning the grain boundaries during annealing. Such morphologies have also been documented in studies on TRIP steels [22] and Med-Mn steels [20]. In the local enlargement shown in Fig. 5e and f, it was found that the thickness of zone II and the size of the oxide particles increased when the annealing temperature was increased from 700 °C to 800 °C.
surface and subsurface areas were measured in the metallographic images. The thicknesses of both the surface and subsurface layers of the sample annealed at 700 °C are larger than those of the sample annealed at 800 °C. The former sample has a 0.4–0.5 μm surface layer thickness and an ~1 μm subsurface layer thickness, while the latter sample has a 0.2–0.3 μm surface layer thickness and an ~0.5 μm subsurface layer thickness. The results of the metallographic measurements are basically consistent with the results of GD-OES. Moreover, the microstructures of the subsurface areas are also different. The subsurface ferrite grains in Fig. 5a are columnar, and the grain boundary network internal oxides extending from the surface to the substrate continuously are almost vertical. The subsurface ferrite grains in Fig. 5b are much more equiaxed, and the internal oxides are large, discontinuous and disordered. From the medium- and large-magnification images in Fig. 5c–f, the subsurface layer can be further divided into three zones, as indicated by arrows I to III. Zone I, which is just underneath the external oxidation layer, is absolutely free of internal oxide particles and only consists of 6
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a)
b)
c)
External MnO oxides
Internal MnAl2O4 oxides
d)
e)
f)
Fig. 6. STEM-EDS mapping of Fe, O, Mn, and Al for the sample annealed at 700 °C.
and grain boundary diffusion. Therefore, Al2O3 appears deeper in the substrate than the grain-boundary MnAl2O4. It was noted that AlN precipitates were found in the subsurface of annealed Al-added TRIP steels. However, Al2O3 instead of AlN is dominant in the present study, which may be because of the very high partial oxygen pressure. Some voids at the grain boundaries deep inside the 800 °C sample were observed. These voids might be caused by the combined Kirkendall effect [5] and decarburization. Although the types of oxides are the same for the samples annealed at 700 °C and 800 °C, the annealing temperature has a great effect on the shape and size of the internal oxides in the subsurface. In the case of 800 °C, some large internal MnO grains with a diameter in the range of 300–600 nm, as indicated by the arrows in Fig. 7d, appear in zone III. Because the size of these MnO grains is almost as large as that of the surrounding ferrite grains, it seems that some of the ferrite grains in the subsurface area are replaced by these large internal manganese oxides. Such enhanced internal oxidation of Mn should be attributed to the increased oxygen partial pressure and the increased annealing temperature. The increased diffusivity of O, Mn and Al at a higher annealing temperature promotes the growth rate of internal oxides. The increased size of the subsurface MnO precipitates is largely due to temperature rather than partial oxygen pressure effects. In addition, a large amount of excess Mn becomes available when some of the austenite grains transform into ferrite during the annealing process at 800 °C. Therefore, a ferrite + austenite two-phase subsurface layer promotes the formation of large Mn grains at a distance deeper than that of external MnO. The formation of internal MnO in the subsurface layer can explain why the thickness of the external oxidation layer of Mn is slightly reduced when the annealing temperature is 800 °C. Fig. 6e and Fig. 7e show that the fine ferrite grains in zone I in the subsurface layer contain a small amount of aluminum, while Fig. 6d and Fig. 7d show that the manganese content is very limited in these ferrite grains. These results are likely determined by the different solid solutions of Al and Mn in ferrite. Based on the above GD-OES and cross-sectional results, a schematic illustration of the effect of annealing temperatures of 700 °C and 800 °C
Zone III, which is the main part of the subsurface area, consists of both ferrite grains and relatively coarse internal oxides. In this zone, the size of the ferrite grains is greater than that of the grains in zones I and II but still smaller than that of the austenite grains in the substrate. It should be noted that the shape and size of the internal oxides in Fig. 5e and f are totally different from each other even though the annealing temperature only differs by 100 °C. When the annealing temperature is 700 °C, the continuous film-like internal oxides in Fig. 5e grow vertically along the ferrite grain boundaries into the substrate. The bulk of the ferrite grains and the interface of ferrite/austenite are almost free of internal oxides. When the annealing temperature is 800 °C, most of the internal oxides still grow along the grain boundaries, but they grow randomly in all directions. Internal oxides are easily found in the bulk of the ferrite grains and at the interface of ferrite/austenite. In addition, the size of the internal oxides in Fig. 5f is larger than that in Fig. 5e, indicating that a higher annealing temperature promotes the growth of internal oxides. In contrast, the depth of zone III and the depth of the internal oxidation layer in Fig. 5f are smaller than those in Fig. 5e, suggesting that the transformation of austenite to ferrite in zone III is suppressed when the annealing temperature is 800 °C. The elemental mappings of the cross sections of samples annealed at 700 °C and 800 °C, as shown in Figs. 6 and 7, respectively, reveal the distribution of Mn, Al, O, and Fe in the surface and subsurface layers. The enrichment of O and Mn in the surface layer and the depletion of Mn in the subsurface layer are completely consistent with the GD-OES results. Three types of oxides are observed in the present work. The external oxides were identified as MnO through EDS analysis. The internal oxides in zone II and zone III along the grain boundaries were identified as MnAl2O4. In some deep areas, internal Al2O3 was found. The grain boundary MneAl complex oxides probably have a core-shell structure similar to those of the similar grain boundary internal MneSi complex oxides revealed by De Cooman23) and McDermid [16], where Al might be enriched in the oxide core, while both Mn and Al are present in the outer shell surrounding the Al-rich core. Al2O3 is more thermodynamically stable than aluminate, and aluminate also requires diffusive transformation arising from the slower Mn migration via bulk 7
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a)
b)
External MnO oxides
Large internal MnO oxides
c)
Internal MnAl2O4 oxides
Internal Al2O3 oxides
d)
voids
e)
f)
Fig. 7. STEM-EDS mapping of Fe, O, Mn, and Al for the sample annealed at 800 °C.
oxides to form in the subsurface layer, the more noble solvent metal (in this case Fe) may be extruded to the surface (in this case, the MnO/ subsurface interface) through dislocation channels, resulting in nodules of pure solvent metal at the surface [26]. Notably, the amount of ferrite grains increases with annealing temperature. Zone II is composed of ultrafine ferrite grains and fine internal oxides, and a similar transition zone with many oxide precipitates is also observed and considered to be caused by the earlier nucleation and crystallization process in this zone [24] or the pinning effect of grain boundary oxides [20,22]. Zone III is composed of fine ferrite grains and relatively large internal oxides. In zone III, both the depletion of Mn, which is an austenite stabilizer, and decarburization result in the transformation of austenite into ferrite [7]. Third, the annealing temperature plays an important role in the surface and subsurface formation of Al-added TWIP steel. The annealing temperature not only affects the thickness of both the surface and subsurface layers but also affects the type, shape, size and distribution of internal oxides. The type of external oxide remains the same in the range from 600 °C to 850 °C, but its thickness increases with annealing temperature below 700 °C and decreases with annealing temperature above 700 °C. Internal oxidation is promoted when the annealing temperature is increased. For example, when the annealing temperature was increased from 700 °C to 800 °C, the film-like internal oxides along the grain boundaries grew coarser, and large MnO internal oxides formed. Moreover, the amount of protruded ferrite grains in zone I increases with annealing temperature, which is also considered to be the result of the increased internal Al oxidation when the annealing temperature is increased. The effect of annealing temperature on the selective oxidation of Mn and Al is considered to be determined by the partial oxygen pressure and the diffusion coefficients of O, Mn and Al in the substrate.
Fig. 8. Schematic illustration of the difference in surface and subsurface microstructure of Al-added TWIP steel annealed in a N2 + 5 vol% H2 gas atmosphere with a dew point of +10 °C at 700 °C and 800 °C.
on the surface and subsurface microstructure of Al-added TWIP steel annealed in a N2 + 5 vol% H2 gas atmosphere with a dew point of +10 °C is shown in Fig. 8. Although external MnO and Mn-depleted ferrite zones with internal oxides have already been reported in the literature [5–11], in the current work, the microstructure in the subsurface layer was revealed in further detail, and the effect of annealing temperature was highlighted. First, similar to all the results in the literature, the surface of the annealed TWIP steel in the current work is also covered by a compact and continuous layer of external MnO, and the subsurface of the steel is composed of ferrite grains and a variety of internal oxides. The main cause of ferrite formation is the significant depletion of Mn and C in the subsurface layer due to the external oxidation of Mn and decarburization. Second, the subsurface area between the external MnO and the austenite substrate can be further subdivided into three zones. Zone I is composed of fine ferrite grains without any internal oxides inside the grains or along the grain boundaries, and the formation of this zone could be attributed to the protrusion of iron atoms from the subsurface due to the significant volume expansion of internal Al oxides when the dew point is high enough [24,25]. To make room for these internal
4. Conclusions The effect of annealing temperature on Al-added TWIP steel in a N25 vol% H2 atmosphere with a dew point of +10 °C was studied. The conclusions listed below can be drawn: 1. The annealing temperature shows a significant effect on the external 8
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and internal oxidation of Mn and Al in TWIP steel and therefore obviously changes the subsurface microstructure. 2. Due to the abundant formation of external MnO and decarburization, the content of Mn and C in the subsurface area decreases to a critical level and results in the formation of a continuous ferrite layer that is transformed from austenite. 3. The type of external MnO does not change with annealing temperature. However, the type, shape, size and distribution of internal oxides in the subsurface area are greatly affected by annealing temperature. The film-like internal MnAl2O4 along the grain boundaries grows coarser and some large internal MnO forms when the annealing temperature increases from 700 °C to 800 °C. 4. A layer of protruded ferrite grains without any internal oxides inside the grains or along the grain boundaries is found just beneath the external MnO layer, the amount of which increases with annealing temperature.
[7] [8]
[9]
[10]
[11]
[12] [13] [14]
Author contribution
[15]
All authors contributed equally to this manuscript. [16]
Declaration of competing interest [17]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[18] [19]
Acknowledgement [20]
The authors would like to thank the National Key R&D Program of China (No. 2017YFB0304402) for the financial support.
[21]
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