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Reduction of oxide scale on hot-rolled steel by hydrogen at low temperature
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 5 1 1 6 e1 5 1 2 4
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Reduction of oxide scale on hot-rolled steel by hydrogen at low temperature Chuang Guan a, Jun Li b, Ning Tan b, Yong-Quan He c, Shu-Guang Zhang a,* a
School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai, 200240, China Central Research Institute of Baosteel Group, Shanghai, 201900, China c State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, 110819, China b
article info
abstract
Article history:
This study was carried out with an intention to remove the oxide scale on hot-rolled steel by
Received 22 January 2014
gaseous reduction instead of traditional acid pickling method with an aim to reduce the
Received in revised form
pollution. The reduction of iron oxide scale by hydrogeneargon mixture was studied by
9 June 2014
thermogravimetric tests in the temperature range of 370e550 C. The rate controlling process
Accepted 4 July 2014
was discussed according to the AvramieErofe'ev equation generalized method. The analysis
Available online 4 August 2014
suggests that the reduction of scale is controlled by two- and/or three-dimensional growth of nuclei in the whole temperature range investigated. The apparent activation energy exhibit a
Keywords:
sudden decrease from 78.8 to 31.8 kJ/mol at temperature higher than 410 C. Morphological
Oxide scale
structure of the reduced scale was investigated by scanning electron microscope.
Hydrogen reduction
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Kinetics Thermogravimetric Hot-rolled steel
Introduction During the final rolling and subsequent coiling, a thin oxide film, also known as oxide scale, formed on the surface of hotrolled strip steel. Prior to the following process, the scale should be removed to produce a desirable surface. Presently, acid pickling using chloric acid or sulfonic acid is the most common descaling method in continuous production. However, usage of acid causes serious environmental problems, such as air pollution, acid sewage and hazardous residue [1]. Under high pressure of environmental protection, new acid free descaling processes have long been desired by steel companies. In recent years, one alternative to conventional acid
pickling is mechanical treatment such as shot blasting [2]. The other method is gaseous reduction of oxide scale in reducing atmosphere. Although considerable work has been investigated on the reduction of iron oxides [3e6], only a few studies are concerned with the reduction of hot-rolled strip scale (HRSS). The first report on descaling of hot-rolled steel by hydrogen was published as late as 1985. In the research, Hudson [7,8] experimentally evaluated the reduction of HRSS with H2 or H2eN2 mixture by packing the scale specimens in a stainless steel box to simulate the box annealing. The results suggested that the coil temperature should higher than 1175 F (635 C) for more than 24 h. Primavera et al. [1] simulated the continual annealing of HRSS reduction and revealed that reaction rate is highly dependent on the H2O content in atmosphere. The influence of
* Corresponding author. Tel./fax: þ86 21 5474 4246. E-mail addresses: [email protected], [email protected] (S.-G. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2014.07.024 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Table 1 e Selected gasesolid reaction kinetic equation. Mechanisms One-dimensional diffusion Two-dimensional diffusion Three-dimensional diffusion (Jander equation) Phase boundary reaction (infinite slab) Phase boundary reaction (contracting cylinder) Phase boundary reaction (contracting sphere) Random nucleation (First order) Two-dimensional growth of nuclei (AvramieErofe'ev equation) Three-dimensional growth of nuclei (AvramieErofe'ev equation)
variations in the steel grade is not significant except for stainless steel. Shi et al. [9] studied the reduction of oxide scale by carbon monoxide and suggested that the optimum reduction temperature is 750 C. Recently, Saeki et al. [10] reported the reduction of oxide scale by pure hydrogen. The reduction became fast as temperature increasing. However, reaction rate slows down between 823 and 873 K and showed a minimum at 873 K. They considered the negative increase of reaction rate in terms of morphology and type of the reduction product. Similar phenomenon were also reported in literatures [11e13]. He et al. [14] studied the hot-dip galvanizing on the reduced scale. The scale was cold-rolled before annealing in order to accelerate the reduction. The overlay zinc coating is rough but have superior adherence. All these studies suggested that gaseous reduction of HRSS is a feasible way to replace traditional acid pickling process. Several patents have been published in this field [15,16]. A commercial Acid-Free Scale Removal (AFSR) line has already been constructed by Danieli Wean United. Although AFSR has shown a promising prospect, the scale reduction rate and efficiency still limited the wide application of this technology. Pervious researches on the reduction of HRSS mostly focus on the engineering parameters, such as the reaction temperature and hydrogen or water concentration. Report on the underlying reduction mechanism of the scale was hardly found. Meanwhile, as the reduction rate and processes of iron oxides is highly depend on the starting raw material [17,18], the reduction mechanism and kinetic data of HRSS may different from the widely investigated iron oxides powder or bulk oxide ores. In this paper, we focus on the reduction of oxide scale of hot-rolled steel by hydrogeneargon mixture in the low temperature range of 370e550 C. The thermogravimetric (TG) analysis method was broadly used in the investigation of oxidation and reduction mechanism of metal and metal oxides for its high precision and flexibility [19,20]. The kinetic data of the scale reduction were investigated by isothermal TG tests. Controlling mechanism comparison was played using the AvramieErofe'ev equation generalized method. The oxide scale and reduced specimens were examined by scanning electron microscope (SEM).
Materials and methods Scale samples The scale specimens were obtained from a commercial hotrolled strip steel (grade: DT-3CA DI, 2 mm thick) furnished
Symbol
a ð1 aÞlnð1 aÞ þ a ½1 ð1 aÞ1=3 2 a 1 ð1 aÞ1=2 1 ð1 aÞ1=3 lnð1 aÞ ½lnð1 aÞ1=2 ½lnð1 aÞ1=3
D1 D2 D3 R1 R2 R3 F1 A2 A3
m
gðaÞ 2
(5) (6) (7) (8) (9) (10) (11) (12) (13)
0.62 0.57 0.54 1.24 1.11 1.07 1 2 3
by Baosteel. The final rolling temperature is 874 C and coiling temperature is 707 C. The chemical composition of substrate steel is 0.037C-0.231Mn-0.011Si (wt.%). The sample weight for each reduction experiment was between 2.5 g and 2.8 g. Oxide scale composition was examined by surface X-ray diffraction (XRD, Rigaku Rint-2200/PC) analysis.
Apparatus and procedure Isothermal reduction tests were carried out in thermogravimetric analyzer (TGA, SETARAM). After evacuation, the reaction chamber was filled with high purity Ar and heated up to the target temperature at the rate of 40 C/min. In order to achieve thermal equilibrium, 5 min of soaking period was employed before reduction. Then Ar was replaced by 80% Ar þ 20% H2 mixture and the weight loss of the specimens were recorded immediately. To eliminate the influence of external gas mass transfer on the reduction, the maximum gas flow rate 200 ml/min was employed. Experiments were continued until the specimen ceased to register further weight loss. The reaction chamber was then flushed with high purity Ar to cool down the reaction system. The crosssection of the scale before and after reduction was photographed using SEM.
Kinetic models For gasesolid reactions, the reaction rate can be written as: da ¼ kf ðaÞ (1) dt where a is the reaction extent of the solid reaction, t is the reaction time, k is the rate constant and f(a) is a function indicating the reaction mechanism. Integration Eq. (1) yields. Za gðaÞ ¼ 0
da ¼ kt f ðaÞ
(2)
Eq. (1) and Eq. (2) are the general equations for gasesolid reaction. In order to confirm the reaction rate controlling steps, the AvramieErofe'ev equation generalized method was employed. According to Avrami equation [21]: a ¼ 1 expð btm Þ Then,
(3)
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mechanism of isothermal solid-state reactions [24e26]. The apparent activation energy were obtained from the Arrhenius plots.
Results and discussion Scale
Fig. 1 e Surface XRD pattern of DT-3CA DI hot-rolled steel.
lnðlnð1 aÞÞ ¼ ln b þ m ln t
(4)
where a is the reaction extent, b and m are both constants. These equations were first derived in the modeling of nucleation and growth processes and developed into a generalized equation for solid-state reaction kinetics by Erofe'ev [22]. Hancock and Sharp [23] pointed out that the ln (ln(1a)) against ln t give rise to approximately linear plots for reactions following any of the equations listed in Table 1 if a is limited in the range of 0.15e0.5. The slope of the fitting line indicates the controlling mechanism. According to Table 1, the mechanism of reaction rate controlling can be divided into three groups. For 0.54 m 0.62, 1.07 m 1.24 and 2 m 3, the reaction are diffusion controlled, phase boundary reaction controlled and nuclei growth controlled, respectively. This method has been proved very convenient in comparing the controlling
Fig. 1 shows the pattern of surface XRD analysis. It can be seen from Fig. 1 that the scale is mainly composed of Fe3O4, with a little a-Fe2O3 and iron. XRD phase concentration analysis conveys that a-Fe2O3 and iron takes up 16% wt. and 4% wt., respectively. Fig 2 displays the cross-section SEM morphology of the scale. It can be calculated that the scale is 7 mm thick on average. The scale mainly contains two distinguishable layer: (1) a homogeneous Fe3O4 layer and (2) an inner lamellar structure layer. Oxide scale formed at the hot-rolling temperature mainly consisted of FeO and this phase is not stable at temperature lower than 571 C. The formation of lamellar structure layer (Fig. 2) can be attributed to the eutectoid transformation or disproportionation of FeO into Fe3O4 and Fe (4FeO ¼ Fe þ Fe3O4) below the eutectoid temperature [27]. This is consistent with XRD result above. As far as the little content of a-Fe2O3 detected in XRD analysis (Fig. 1), it may be ascribed to the extreme thin layer located at the steel surface [28]. Similar phenomena was also observed in previous report [9].
Results of TG analysis As illustrated in the previous Section Scale, the scale is mainly composed of a-Fe2O3 and Fe3O4. It is known to us that a-Fe2O3 was reduced into Fe3O4 first, and then Fe3O4 was reduced in further [18,29]. The weight proportion of a-Fe2O3 on both sides is approximately 16%. That is to say, the weight loss directly caused by the transformation from a-Fe2O3 to Fe3O4 is quietly low, only 1.9% of the total weight loss. Meanwhile the most weight loss can be attributed to the reduction of Fe3O4: Fe3 O4 þ 4H2 ¼ 3Fe þ 4H2 O
(14)
Therefore, we focus on the reduction of Fe3O4 and the Fe3O4 reduction was deemed to start at 1.9% of total weight loss. In the following, the “weight loss” and “reduction extent” will mainly indicate the values in the Fe3O4 reduction step. TG experiments were performed in the temperature range of 370e550 C. Fig. 3 groups the normalized reduction extent of Fe3O4 as function of time at different temperatures. The reduction extent, a, was defined by: a¼
Fig. 2 e Cross-section SEM morphology (backscattering) of DT-3CA DI hot-rolled steel.
DW DWa¼1
(15)
where DW is the weight loss at time t and DWa¼1 is the weight loss during the reduction step of Fe3O4 into metallic iron. It can be seen from Fig. 3 that all these curves exhibit a similar sigmoid shape. At the initial stage, the reaction rate is quite slow. This period lasts nearly 10 min at 370 C and decrease to less than 1 min at 550 C. In the following time, reaction rate increases rapidly. This stage is usually called acceleratory period. A decaying period is observed closely before the end of reaction. The reduction rate is also found to
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Fig. 3 e Reduction extent a vs. time for the reduction of oxide scale by 80% Ar þ 20% H2 mixture.
be function of temperature. As increasing reaction temperature, the time for totally reduction decreases. The entire reduction time is more than 45 min at 370 C and less than 8 min at 550 C. A sharp decrease of reaction rate is observed at temperature lower than 415 C, which implies the apparent activation energy played an important role in the reduction. It will be discussed in Section Apparent activation energy.
Kinetics study Rate controlling process The AvramieErofe'ev equation generalized method was introduced to find the rate controlling process. The m values in Eq. (4) were calculated by using the data of Fig. 3. Exemplary data of ln (ln (1a)) against ln t in the a range of 0.15e0.5 as well as the regression lines are displayed in Fig. 4. The slopes of the regression lines, i.e. the m values, are evaluated in Table 2. All the logarithmic data present perfect linearity with the correlation coefficients, R2, higher than 0.998. As listed in Table 2, all the m value at the six temperatures are around 2. Similar results were obtained in all the other TG experiments.
It seems that the reaction is controlled by two-dimensional growth of nuclei (A2 model) in all cases studied according to Table 1. It worth mention that, however, the concentration of aFe2O3 may be amplified considering the system error of XRD phase composition analysis and the insufficient detection depth of the X-ray. This error may result in the imprecise selection of start point of Fe3O4 reduction. It is to say that the practical m values may slightly higher than the calculate ones. The reaction is more likely to be controlled by growth of nuclei (A2 and A3 model) other than phase-boundary reaction (R1, R2 and R3 model). Further comparison between the nuclei growth models and phase-boundary control models using the general rate equation support this viewpoint. According to Eq. (2), g(a) function should be proportional to the time reacted. Mathematical modeling of g(a) as function of time with an extent a range of 0.05e0.95 was performed. Growth of nuclei (A2, A3) and Phase-boundary reaction (R1, R2, R3) models were all calculated and fitted. Fig. 5(aec) presents the results of 370, 415 and 475 C. The reaction rate constant (k) for each model was listed in Table 3. It is evident that better linearity was obtained by using A2 or A3 model in all the cases investigated. The modeling suggests that the reduction is controlled by the two and/or three dimensional growth of nuclei. In summary, one may conclude from the above discussion that the reaction closely obey the AvramieErofe'ev equation for two and/or three dimensional growth of nuclei in the temperature range of 370e550 C. The controlling step of iron oxides reduction suggested by several researches in the
Table 2 e Fitting parameters of AvramieErofe'ev equation for the reduction of oxide scale. Temperature/ C
Fig. 4 e Plots of ln (¡ln (1¡a)) vs. ln t for the reduction of oxide scale.
370 400 415 445 475 505
Time for half reduction/min
m
Correlation coefficients R2
23.0 10.7 8.9 6.7 5.7 4.9
2.18 1.71 1.83 1.99 2.06 2.04
0.9999 0.9985 0.9996 0.9998 0.9999 0.9999
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Fig. 5 e Mathematical modeling of thermogravimetric data at (a) 370 C, (b) 415 C and (c) 475 C.
similar temperature range are summarized in Table 4. In most of these researches, the reaction is controlled by growth of nuclei at lower temperature (<400 C) and diffusion or phase boundary reaction controlled at higher temperature (430e600 C). The different controlling mechanism above 430 C may attribute to the different composition and structure of raw oxide materials.
Apparent activation energy As mentioned in Section Results of TG analysis, the apparent activation energy, Ea, played an important role in the scale reduction. Here, the apparent activation energy was also calculated. It was obtained from the Arrhenius plot, ln k vs. 1/
Table 3 e Modeling parameters of thermogravimetric data. 370 C
A2 A3 R1 R2 R3
415 C 2
475 C 2
k
R
k
R
k
R2
0.0380 0.0267 0.0254 0.0204 0.0163
0.9998 0.9955 0.9843 0.9950 0.9883
0.0980 0.0691 0.0665 0.0527 0.0418
0.9976 0.9983 0.9920 0.9908 0.9800
0.1604 0.1130 0.1081 0.0861 0.0684
0.9978 0.9980 0.9915 0.9915 0.9807
T, as shown in Fig. 6. Reaction rate constant was calculated though the linearity range (0.2 < a < 0.6) of the TG curves present in Fig. 3. It can be seen from Fig. 6 that ln k is a strong function of temperature and the plot is characterized by a modification of slope around 410 C. Regression analysis of the Arrhenius diagram revealed that Ea values are about 78.8 kJ/ mol and 31.8 kJ/mol at temperature lower and higher than 410 C, respectively. The sudden transition of Ea at a certain temperature or the non-Arrhenius behavior have been reported in many previous literatures [33e37]. Decrease of activation energy usually implies a lower activation energy chemical degradation pathway dominating at high temperatures [38]. The reason for the transition was discussed. In most cases, the variation of reaction or phase transformation controlling process resulted in abrupt change of activation energy [33e35]. However, as discussed above, the reaction rate is controlled by growth of nuclei at all the temperatures investigated. There is no change of reaction controlling mechanism in the whole temperature range investigated. That is to say the sudden change of Ea was not caused by variation of controlling step. Modification of apparent activation energy may also attribute to the change of physical or chemical properties of the
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Table 4 e Reaction rate controlling mechanisms of the iron oxides reduction. Reducing gas
Reduction step
Temperature range/ C
Rate controlling mechanisms
Fe2O3 Fe2O3 Fe3O4 Fe2O3
H2 H2 H2eH2OeN2 H2
Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe
<417 417e683 400e500 250e350
Fe3O4 Fe3O4 FeO Fe2O3 HRSS
CO CO H2 H2 H2eAr
Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe
<430 >430 270e397 TRPb 370e550
Two- and three-dimensional growth of nuclei Phase boundary reaction (infinite slabs) Phase boundary reaction (contracting sphere) AvramieErofe'ev phase change model (two- or three-dimensional) Two- and/or three-dimensional nuclei growth Diffusion Two- and/or three-dimensional growth of nucleia Two-dimensional growth of nuclei Two- and/or three-dimensional growth of nuclei
Raw materials
a b
Ref. [18] [18] [30] [31] [17] [17] [32] [29] This study
Calculated from original data. Temperature-programmed reduction.
reactants or products. Since the controlling mechanism is growth of nuclei, the reduction rate may also impacted by crystals defects in the oxides grains [39,40]. Although handled carefully, the deformation are inevitable during the preparing of the specimens. Moreover, during the heating or cooling process, different thermal expansion coefficients at the substrate/scale interface can also introduce great deal of defects in the thin oxide film. The recovery of crystal defects may lead to the changing of nucleus creation and growth rate of new iron phase and reflect in the apparent activation energy. To check the effect of crystal defects on the reaction kinetics, a group of experiments were designed. Specimens were first heated up to 500 C and held for 10 min in pure argon atmosphere to reduce the defects in the scale. After that, these recovered specimens were cooled down to temperatures below 410 C and reduced isothermally as normal experiments. The results showed no great difference with the unrecovered reductions. One may conclude that the recovery or of crystal defects in the scale has little or no influence on the reaction rate. Zielinski et al. [41] examined the reduction product of aFe2O3 by hydrogenewater mixture. The formation of FeO
below the eutectoid point (571 C) was detected by ex situ XRD analysis at high water concentration cases. Pineau et al. [17,18] also found the transition of Ea around 420 C for the iron oxides reduction. They analyzed the solid phase composition during reduction by in situ XRD analysis and found FeO peaks in the temperature range of 450e570 C. They related the transition of Ea to the formation of FeO. The presence of intermediate product changed the direct transformation (Fe3O4eFe) into a two-step (Fe3O4eFeOeFe) reaction thus providing a different pathway of reaction to bring down the Ea. One may note that, although the internal diffusion and phase boundary reaction are not the controlling mechanism, they have effect on the chemical and phase equilibrium at the reaction front. As broadly recognized that the reduction of metal oxides by hydrogen is strongly influenced by the water concentration in the reduction atmosphere [11,42]. In the condition of our experiment, the water content of the gas source is extremely low (<10 ppm). However, the gaseous reduction product, H2O, may accumulate at the reaction front caused by the nonideal or even terrible internal diffusion in the porous reduced iron (Fig. 7). Enrichment of water may change the chemical and phase equilibrium at the reaction interface as the temperature increasing and eventually lead to the formation of metastable FeO below the eutectoid temperature. It is to say that the sudden change of Ea can be explained by the variation in chemical and phase equilibrium at the reaction interface caused by the insufficient internal diffusion. Additionally, considering the experiment conditions in our study are similar to that of Pineau's and the transition temperature (Tt) of Ea is also extremely close (Table 5), we believe that the Ea modification could be caused by the presence of FeO as intermediate during the reduction at temperature lower than 571 C.
Morphology study
Fig. 6 e Arrhenius plot of the reduction of oxide scale by 80% Ar þ 20% H2.
Generally speaking, the descaling process is not the final step for strip steel production. It is usually followed by cold-rolling, hot-galvanizing, coating, etc. As one would expect from practical considerations, the microstructure of reduced scale is often crucial to the performances of final products.
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Fig. 7 e SEM micrographs of reduced scale (cross-section) at (a) 370 C, (b) 400 C, (c) 460 C and (d) 550 C.
The reduced specimens were examined by surface XRD analysis and residual oxides were not detected in all the cases. Fig. 7 exhibits the cross-section SEM morphology of the reduction products at different temperatures. The scale are reduced into sponge iron and no obvious residue oxide is observed in all the cases. As the temperature increasing from 370 to 550 C, the pore structure of the reduced scale becomes progressively coarser. For reduction at temperature 370 and 400 C (Fig. 7(a), (b)), the pores are extremely fine that even cannot discern clearly at this magnification. Large scale holes with a star like shape appear separately in the surface of the scale reduced at 370 C but hardly seen at higher temperatures. While for specimens reduced at 460 and 550 C (Fig. 7(c), (d)), the pores are much larger and the characteristics of agglomeration was observed. It also can be found that the pore structure is courser and denser at the outside surface than inner part of the reduced scale for reduction at 460 and 550 C. It would be easy to
understand when considering the different structures of original oxides. As can be seen from Fig. 2, the outmost surface of the scale consists of pure Fe3O4 comparing with the FeO transferred Fe3O4/iron eutectoid phase in the inner part. The different composition and structure of oxides produced the different reduction product appearance. Besides, the time sequence of reduction may also raise this phenomenon since the surface layer was the first part being reduced into iron. However, further tests showed that prolonged preservation in reducing atmosphere after fully reduction had little effect on the size or distribution of the pore structure. This outcome agreed with the research of Turkdogan and Vinters [4]. In term of energy-dispersive X-ray (EDX) analysis, element mapping revealed that trace oxygen is present in both the reduced iron layer and the steel substrate. The oxygen concentrations in the two positions are also found to be equal, which indicates that the oxygen was introduced during the sample preparation.
Table 5 e Comparison of Ea and transition temperature in literature. Raw material Fe2O3 Fe2O3 Fe2O3 Fe3O4 Fe3O4 Oxide scale a b
Reducing gas
Reduction step
Ea1a/kJ/mol
Tt/ C
Ea2b/kJ/mol
Ref.
H2 H2eN2 CO H2 CO H2eAr
Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe
88 103 114 71 150.3 78.8
417 415 439 390 428 410
39 36 40 44 63.8 31.8
[18] [18] [18] [17] [17] This work
Activation energy below Tt. Activation energy above Tt.
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Different from acid pickling, the surface of reduced scale is rough and porous. This kind of structure increases the surface roughness as well as the specific surface area. The roughness of substrate material may result in surface roughening of the galvanizing coating. It also has significant impact on the coating quality when deformation [43,44]. The latter one enhances the reactivity of steel surface [40] and aluminum consumption [45] during hot-dip galvanizing, thus claims adjustment to the composition of zinc melt and/or hot dipping processes.
Conclusions Reduction of HRSS in the temperature range of 370e550 C were investigated by thermogravimetric experiments. With increasing temperature, the time for complete reduction decreases. The scale with a thickness of 7 mm can be mostly reduced by 80% Ar þ 20% H2 gas mixture within 8 min at 550 C. Mathematical modeling of thermogravimetric data indicate that the reaction closely obey the AvramieErofe'ev equation for two and/or three dimensional growth of nuclei in the temperature range of 370e550 C. A sharp decrease of apparent activation energy was observed at temperatures higher than 410 C. For temperature lower and higher than 410 C, Ea decrease from 78.8 kJ/mol to 31.8 kJ/mol. The variation of apparent activation energy probably caused by the formation of FeO as intermediate product. SEM study indicated that the pore structure of the reduced scale becomes progressively coarser as the temperature increasing. Agglomeration of reduced iron, especially at the surface layer, was observed at temperature higher than 430 C.
Acknowledgments The experiments of this work were performed in the State Key-laboratory of Rolling and Automation, Northeastern University, China. The authors wish to thank Professor Liu Zhenyu for his help. This research was funded by the National Twelfth Five-year Science and Technology Support Program of China (Grant No. 2011BAE13B04) and National Natural Science Foundation of China under contract No. 51027005.
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Reduction of oxide scale on hot-rolled steel by hydrogen at low temperature Chuang Guan a, Jun Li b, Ning Tan b, Yong-Quan He c, Shu-Guang Zhang a,* a
School of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai, 200240, China Central Research Institute of Baosteel Group, Shanghai, 201900, China c State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang, 110819, China b
article info
abstract
Article history:
This study was carried out with an intention to remove the oxide scale on hot-rolled steel by
Received 22 January 2014
gaseous reduction instead of traditional acid pickling method with an aim to reduce the
Received in revised form
pollution. The reduction of iron oxide scale by hydrogeneargon mixture was studied by
9 June 2014
thermogravimetric tests in the temperature range of 370e550 C. The rate controlling process
Accepted 4 July 2014
was discussed according to the AvramieErofe'ev equation generalized method. The analysis
Available online 4 August 2014
suggests that the reduction of scale is controlled by two- and/or three-dimensional growth of nuclei in the whole temperature range investigated. The apparent activation energy exhibit a
Keywords:
sudden decrease from 78.8 to 31.8 kJ/mol at temperature higher than 410 C. Morphological
Oxide scale
structure of the reduced scale was investigated by scanning electron microscope.
Hydrogen reduction
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Kinetics Thermogravimetric Hot-rolled steel
Introduction During the final rolling and subsequent coiling, a thin oxide film, also known as oxide scale, formed on the surface of hotrolled strip steel. Prior to the following process, the scale should be removed to produce a desirable surface. Presently, acid pickling using chloric acid or sulfonic acid is the most common descaling method in continuous production. However, usage of acid causes serious environmental problems, such as air pollution, acid sewage and hazardous residue [1]. Under high pressure of environmental protection, new acid free descaling processes have long been desired by steel companies. In recent years, one alternative to conventional acid
pickling is mechanical treatment such as shot blasting [2]. The other method is gaseous reduction of oxide scale in reducing atmosphere. Although considerable work has been investigated on the reduction of iron oxides [3e6], only a few studies are concerned with the reduction of hot-rolled strip scale (HRSS). The first report on descaling of hot-rolled steel by hydrogen was published as late as 1985. In the research, Hudson [7,8] experimentally evaluated the reduction of HRSS with H2 or H2eN2 mixture by packing the scale specimens in a stainless steel box to simulate the box annealing. The results suggested that the coil temperature should higher than 1175 F (635 C) for more than 24 h. Primavera et al. [1] simulated the continual annealing of HRSS reduction and revealed that reaction rate is highly dependent on the H2O content in atmosphere. The influence of
* Corresponding author. Tel./fax: þ86 21 5474 4246. E-mail addresses: [email protected], [email protected] (S.-G. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2014.07.024 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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Table 1 e Selected gasesolid reaction kinetic equation. Mechanisms One-dimensional diffusion Two-dimensional diffusion Three-dimensional diffusion (Jander equation) Phase boundary reaction (infinite slab) Phase boundary reaction (contracting cylinder) Phase boundary reaction (contracting sphere) Random nucleation (First order) Two-dimensional growth of nuclei (AvramieErofe'ev equation) Three-dimensional growth of nuclei (AvramieErofe'ev equation)
variations in the steel grade is not significant except for stainless steel. Shi et al. [9] studied the reduction of oxide scale by carbon monoxide and suggested that the optimum reduction temperature is 750 C. Recently, Saeki et al. [10] reported the reduction of oxide scale by pure hydrogen. The reduction became fast as temperature increasing. However, reaction rate slows down between 823 and 873 K and showed a minimum at 873 K. They considered the negative increase of reaction rate in terms of morphology and type of the reduction product. Similar phenomenon were also reported in literatures [11e13]. He et al. [14] studied the hot-dip galvanizing on the reduced scale. The scale was cold-rolled before annealing in order to accelerate the reduction. The overlay zinc coating is rough but have superior adherence. All these studies suggested that gaseous reduction of HRSS is a feasible way to replace traditional acid pickling process. Several patents have been published in this field [15,16]. A commercial Acid-Free Scale Removal (AFSR) line has already been constructed by Danieli Wean United. Although AFSR has shown a promising prospect, the scale reduction rate and efficiency still limited the wide application of this technology. Pervious researches on the reduction of HRSS mostly focus on the engineering parameters, such as the reaction temperature and hydrogen or water concentration. Report on the underlying reduction mechanism of the scale was hardly found. Meanwhile, as the reduction rate and processes of iron oxides is highly depend on the starting raw material [17,18], the reduction mechanism and kinetic data of HRSS may different from the widely investigated iron oxides powder or bulk oxide ores. In this paper, we focus on the reduction of oxide scale of hot-rolled steel by hydrogeneargon mixture in the low temperature range of 370e550 C. The thermogravimetric (TG) analysis method was broadly used in the investigation of oxidation and reduction mechanism of metal and metal oxides for its high precision and flexibility [19,20]. The kinetic data of the scale reduction were investigated by isothermal TG tests. Controlling mechanism comparison was played using the AvramieErofe'ev equation generalized method. The oxide scale and reduced specimens were examined by scanning electron microscope (SEM).
Materials and methods Scale samples The scale specimens were obtained from a commercial hotrolled strip steel (grade: DT-3CA DI, 2 mm thick) furnished
Symbol
a ð1 aÞlnð1 aÞ þ a ½1 ð1 aÞ1=3 2 a 1 ð1 aÞ1=2 1 ð1 aÞ1=3 lnð1 aÞ ½lnð1 aÞ1=2 ½lnð1 aÞ1=3
D1 D2 D3 R1 R2 R3 F1 A2 A3
m
gðaÞ 2
(5) (6) (7) (8) (9) (10) (11) (12) (13)
0.62 0.57 0.54 1.24 1.11 1.07 1 2 3
by Baosteel. The final rolling temperature is 874 C and coiling temperature is 707 C. The chemical composition of substrate steel is 0.037C-0.231Mn-0.011Si (wt.%). The sample weight for each reduction experiment was between 2.5 g and 2.8 g. Oxide scale composition was examined by surface X-ray diffraction (XRD, Rigaku Rint-2200/PC) analysis.
Apparatus and procedure Isothermal reduction tests were carried out in thermogravimetric analyzer (TGA, SETARAM). After evacuation, the reaction chamber was filled with high purity Ar and heated up to the target temperature at the rate of 40 C/min. In order to achieve thermal equilibrium, 5 min of soaking period was employed before reduction. Then Ar was replaced by 80% Ar þ 20% H2 mixture and the weight loss of the specimens were recorded immediately. To eliminate the influence of external gas mass transfer on the reduction, the maximum gas flow rate 200 ml/min was employed. Experiments were continued until the specimen ceased to register further weight loss. The reaction chamber was then flushed with high purity Ar to cool down the reaction system. The crosssection of the scale before and after reduction was photographed using SEM.
Kinetic models For gasesolid reactions, the reaction rate can be written as: da ¼ kf ðaÞ (1) dt where a is the reaction extent of the solid reaction, t is the reaction time, k is the rate constant and f(a) is a function indicating the reaction mechanism. Integration Eq. (1) yields. Za gðaÞ ¼ 0
da ¼ kt f ðaÞ
(2)
Eq. (1) and Eq. (2) are the general equations for gasesolid reaction. In order to confirm the reaction rate controlling steps, the AvramieErofe'ev equation generalized method was employed. According to Avrami equation [21]: a ¼ 1 expð btm Þ Then,
(3)
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mechanism of isothermal solid-state reactions [24e26]. The apparent activation energy were obtained from the Arrhenius plots.
Results and discussion Scale
Fig. 1 e Surface XRD pattern of DT-3CA DI hot-rolled steel.
lnðlnð1 aÞÞ ¼ ln b þ m ln t
(4)
where a is the reaction extent, b and m are both constants. These equations were first derived in the modeling of nucleation and growth processes and developed into a generalized equation for solid-state reaction kinetics by Erofe'ev [22]. Hancock and Sharp [23] pointed out that the ln (ln(1a)) against ln t give rise to approximately linear plots for reactions following any of the equations listed in Table 1 if a is limited in the range of 0.15e0.5. The slope of the fitting line indicates the controlling mechanism. According to Table 1, the mechanism of reaction rate controlling can be divided into three groups. For 0.54 m 0.62, 1.07 m 1.24 and 2 m 3, the reaction are diffusion controlled, phase boundary reaction controlled and nuclei growth controlled, respectively. This method has been proved very convenient in comparing the controlling
Fig. 1 shows the pattern of surface XRD analysis. It can be seen from Fig. 1 that the scale is mainly composed of Fe3O4, with a little a-Fe2O3 and iron. XRD phase concentration analysis conveys that a-Fe2O3 and iron takes up 16% wt. and 4% wt., respectively. Fig 2 displays the cross-section SEM morphology of the scale. It can be calculated that the scale is 7 mm thick on average. The scale mainly contains two distinguishable layer: (1) a homogeneous Fe3O4 layer and (2) an inner lamellar structure layer. Oxide scale formed at the hot-rolling temperature mainly consisted of FeO and this phase is not stable at temperature lower than 571 C. The formation of lamellar structure layer (Fig. 2) can be attributed to the eutectoid transformation or disproportionation of FeO into Fe3O4 and Fe (4FeO ¼ Fe þ Fe3O4) below the eutectoid temperature [27]. This is consistent with XRD result above. As far as the little content of a-Fe2O3 detected in XRD analysis (Fig. 1), it may be ascribed to the extreme thin layer located at the steel surface [28]. Similar phenomena was also observed in previous report [9].
Results of TG analysis As illustrated in the previous Section Scale, the scale is mainly composed of a-Fe2O3 and Fe3O4. It is known to us that a-Fe2O3 was reduced into Fe3O4 first, and then Fe3O4 was reduced in further [18,29]. The weight proportion of a-Fe2O3 on both sides is approximately 16%. That is to say, the weight loss directly caused by the transformation from a-Fe2O3 to Fe3O4 is quietly low, only 1.9% of the total weight loss. Meanwhile the most weight loss can be attributed to the reduction of Fe3O4: Fe3 O4 þ 4H2 ¼ 3Fe þ 4H2 O
(14)
Therefore, we focus on the reduction of Fe3O4 and the Fe3O4 reduction was deemed to start at 1.9% of total weight loss. In the following, the “weight loss” and “reduction extent” will mainly indicate the values in the Fe3O4 reduction step. TG experiments were performed in the temperature range of 370e550 C. Fig. 3 groups the normalized reduction extent of Fe3O4 as function of time at different temperatures. The reduction extent, a, was defined by: a¼
Fig. 2 e Cross-section SEM morphology (backscattering) of DT-3CA DI hot-rolled steel.
DW DWa¼1
(15)
where DW is the weight loss at time t and DWa¼1 is the weight loss during the reduction step of Fe3O4 into metallic iron. It can be seen from Fig. 3 that all these curves exhibit a similar sigmoid shape. At the initial stage, the reaction rate is quite slow. This period lasts nearly 10 min at 370 C and decrease to less than 1 min at 550 C. In the following time, reaction rate increases rapidly. This stage is usually called acceleratory period. A decaying period is observed closely before the end of reaction. The reduction rate is also found to
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Fig. 3 e Reduction extent a vs. time for the reduction of oxide scale by 80% Ar þ 20% H2 mixture.
be function of temperature. As increasing reaction temperature, the time for totally reduction decreases. The entire reduction time is more than 45 min at 370 C and less than 8 min at 550 C. A sharp decrease of reaction rate is observed at temperature lower than 415 C, which implies the apparent activation energy played an important role in the reduction. It will be discussed in Section Apparent activation energy.
Kinetics study Rate controlling process The AvramieErofe'ev equation generalized method was introduced to find the rate controlling process. The m values in Eq. (4) were calculated by using the data of Fig. 3. Exemplary data of ln (ln (1a)) against ln t in the a range of 0.15e0.5 as well as the regression lines are displayed in Fig. 4. The slopes of the regression lines, i.e. the m values, are evaluated in Table 2. All the logarithmic data present perfect linearity with the correlation coefficients, R2, higher than 0.998. As listed in Table 2, all the m value at the six temperatures are around 2. Similar results were obtained in all the other TG experiments.
It seems that the reaction is controlled by two-dimensional growth of nuclei (A2 model) in all cases studied according to Table 1. It worth mention that, however, the concentration of aFe2O3 may be amplified considering the system error of XRD phase composition analysis and the insufficient detection depth of the X-ray. This error may result in the imprecise selection of start point of Fe3O4 reduction. It is to say that the practical m values may slightly higher than the calculate ones. The reaction is more likely to be controlled by growth of nuclei (A2 and A3 model) other than phase-boundary reaction (R1, R2 and R3 model). Further comparison between the nuclei growth models and phase-boundary control models using the general rate equation support this viewpoint. According to Eq. (2), g(a) function should be proportional to the time reacted. Mathematical modeling of g(a) as function of time with an extent a range of 0.05e0.95 was performed. Growth of nuclei (A2, A3) and Phase-boundary reaction (R1, R2, R3) models were all calculated and fitted. Fig. 5(aec) presents the results of 370, 415 and 475 C. The reaction rate constant (k) for each model was listed in Table 3. It is evident that better linearity was obtained by using A2 or A3 model in all the cases investigated. The modeling suggests that the reduction is controlled by the two and/or three dimensional growth of nuclei. In summary, one may conclude from the above discussion that the reaction closely obey the AvramieErofe'ev equation for two and/or three dimensional growth of nuclei in the temperature range of 370e550 C. The controlling step of iron oxides reduction suggested by several researches in the
Table 2 e Fitting parameters of AvramieErofe'ev equation for the reduction of oxide scale. Temperature/ C
Fig. 4 e Plots of ln (¡ln (1¡a)) vs. ln t for the reduction of oxide scale.
370 400 415 445 475 505
Time for half reduction/min
m
Correlation coefficients R2
23.0 10.7 8.9 6.7 5.7 4.9
2.18 1.71 1.83 1.99 2.06 2.04
0.9999 0.9985 0.9996 0.9998 0.9999 0.9999
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Fig. 5 e Mathematical modeling of thermogravimetric data at (a) 370 C, (b) 415 C and (c) 475 C.
similar temperature range are summarized in Table 4. In most of these researches, the reaction is controlled by growth of nuclei at lower temperature (<400 C) and diffusion or phase boundary reaction controlled at higher temperature (430e600 C). The different controlling mechanism above 430 C may attribute to the different composition and structure of raw oxide materials.
Apparent activation energy As mentioned in Section Results of TG analysis, the apparent activation energy, Ea, played an important role in the scale reduction. Here, the apparent activation energy was also calculated. It was obtained from the Arrhenius plot, ln k vs. 1/
Table 3 e Modeling parameters of thermogravimetric data. 370 C
A2 A3 R1 R2 R3
415 C 2
475 C 2
k
R
k
R
k
R2
0.0380 0.0267 0.0254 0.0204 0.0163
0.9998 0.9955 0.9843 0.9950 0.9883
0.0980 0.0691 0.0665 0.0527 0.0418
0.9976 0.9983 0.9920 0.9908 0.9800
0.1604 0.1130 0.1081 0.0861 0.0684
0.9978 0.9980 0.9915 0.9915 0.9807
T, as shown in Fig. 6. Reaction rate constant was calculated though the linearity range (0.2 < a < 0.6) of the TG curves present in Fig. 3. It can be seen from Fig. 6 that ln k is a strong function of temperature and the plot is characterized by a modification of slope around 410 C. Regression analysis of the Arrhenius diagram revealed that Ea values are about 78.8 kJ/ mol and 31.8 kJ/mol at temperature lower and higher than 410 C, respectively. The sudden transition of Ea at a certain temperature or the non-Arrhenius behavior have been reported in many previous literatures [33e37]. Decrease of activation energy usually implies a lower activation energy chemical degradation pathway dominating at high temperatures [38]. The reason for the transition was discussed. In most cases, the variation of reaction or phase transformation controlling process resulted in abrupt change of activation energy [33e35]. However, as discussed above, the reaction rate is controlled by growth of nuclei at all the temperatures investigated. There is no change of reaction controlling mechanism in the whole temperature range investigated. That is to say the sudden change of Ea was not caused by variation of controlling step. Modification of apparent activation energy may also attribute to the change of physical or chemical properties of the
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Table 4 e Reaction rate controlling mechanisms of the iron oxides reduction. Reducing gas
Reduction step
Temperature range/ C
Rate controlling mechanisms
Fe2O3 Fe2O3 Fe3O4 Fe2O3
H2 H2 H2eH2OeN2 H2
Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe
<417 417e683 400e500 250e350
Fe3O4 Fe3O4 FeO Fe2O3 HRSS
CO CO H2 H2 H2eAr
Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe
<430 >430 270e397 TRPb 370e550
Two- and three-dimensional growth of nuclei Phase boundary reaction (infinite slabs) Phase boundary reaction (contracting sphere) AvramieErofe'ev phase change model (two- or three-dimensional) Two- and/or three-dimensional nuclei growth Diffusion Two- and/or three-dimensional growth of nucleia Two-dimensional growth of nuclei Two- and/or three-dimensional growth of nuclei
Raw materials
a b
Ref. [18] [18] [30] [31] [17] [17] [32] [29] This study
Calculated from original data. Temperature-programmed reduction.
reactants or products. Since the controlling mechanism is growth of nuclei, the reduction rate may also impacted by crystals defects in the oxides grains [39,40]. Although handled carefully, the deformation are inevitable during the preparing of the specimens. Moreover, during the heating or cooling process, different thermal expansion coefficients at the substrate/scale interface can also introduce great deal of defects in the thin oxide film. The recovery of crystal defects may lead to the changing of nucleus creation and growth rate of new iron phase and reflect in the apparent activation energy. To check the effect of crystal defects on the reaction kinetics, a group of experiments were designed. Specimens were first heated up to 500 C and held for 10 min in pure argon atmosphere to reduce the defects in the scale. After that, these recovered specimens were cooled down to temperatures below 410 C and reduced isothermally as normal experiments. The results showed no great difference with the unrecovered reductions. One may conclude that the recovery or of crystal defects in the scale has little or no influence on the reaction rate. Zielinski et al. [41] examined the reduction product of aFe2O3 by hydrogenewater mixture. The formation of FeO
below the eutectoid point (571 C) was detected by ex situ XRD analysis at high water concentration cases. Pineau et al. [17,18] also found the transition of Ea around 420 C for the iron oxides reduction. They analyzed the solid phase composition during reduction by in situ XRD analysis and found FeO peaks in the temperature range of 450e570 C. They related the transition of Ea to the formation of FeO. The presence of intermediate product changed the direct transformation (Fe3O4eFe) into a two-step (Fe3O4eFeOeFe) reaction thus providing a different pathway of reaction to bring down the Ea. One may note that, although the internal diffusion and phase boundary reaction are not the controlling mechanism, they have effect on the chemical and phase equilibrium at the reaction front. As broadly recognized that the reduction of metal oxides by hydrogen is strongly influenced by the water concentration in the reduction atmosphere [11,42]. In the condition of our experiment, the water content of the gas source is extremely low (<10 ppm). However, the gaseous reduction product, H2O, may accumulate at the reaction front caused by the nonideal or even terrible internal diffusion in the porous reduced iron (Fig. 7). Enrichment of water may change the chemical and phase equilibrium at the reaction interface as the temperature increasing and eventually lead to the formation of metastable FeO below the eutectoid temperature. It is to say that the sudden change of Ea can be explained by the variation in chemical and phase equilibrium at the reaction interface caused by the insufficient internal diffusion. Additionally, considering the experiment conditions in our study are similar to that of Pineau's and the transition temperature (Tt) of Ea is also extremely close (Table 5), we believe that the Ea modification could be caused by the presence of FeO as intermediate during the reduction at temperature lower than 571 C.
Morphology study
Fig. 6 e Arrhenius plot of the reduction of oxide scale by 80% Ar þ 20% H2.
Generally speaking, the descaling process is not the final step for strip steel production. It is usually followed by cold-rolling, hot-galvanizing, coating, etc. As one would expect from practical considerations, the microstructure of reduced scale is often crucial to the performances of final products.
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Fig. 7 e SEM micrographs of reduced scale (cross-section) at (a) 370 C, (b) 400 C, (c) 460 C and (d) 550 C.
The reduced specimens were examined by surface XRD analysis and residual oxides were not detected in all the cases. Fig. 7 exhibits the cross-section SEM morphology of the reduction products at different temperatures. The scale are reduced into sponge iron and no obvious residue oxide is observed in all the cases. As the temperature increasing from 370 to 550 C, the pore structure of the reduced scale becomes progressively coarser. For reduction at temperature 370 and 400 C (Fig. 7(a), (b)), the pores are extremely fine that even cannot discern clearly at this magnification. Large scale holes with a star like shape appear separately in the surface of the scale reduced at 370 C but hardly seen at higher temperatures. While for specimens reduced at 460 and 550 C (Fig. 7(c), (d)), the pores are much larger and the characteristics of agglomeration was observed. It also can be found that the pore structure is courser and denser at the outside surface than inner part of the reduced scale for reduction at 460 and 550 C. It would be easy to
understand when considering the different structures of original oxides. As can be seen from Fig. 2, the outmost surface of the scale consists of pure Fe3O4 comparing with the FeO transferred Fe3O4/iron eutectoid phase in the inner part. The different composition and structure of oxides produced the different reduction product appearance. Besides, the time sequence of reduction may also raise this phenomenon since the surface layer was the first part being reduced into iron. However, further tests showed that prolonged preservation in reducing atmosphere after fully reduction had little effect on the size or distribution of the pore structure. This outcome agreed with the research of Turkdogan and Vinters [4]. In term of energy-dispersive X-ray (EDX) analysis, element mapping revealed that trace oxygen is present in both the reduced iron layer and the steel substrate. The oxygen concentrations in the two positions are also found to be equal, which indicates that the oxygen was introduced during the sample preparation.
Table 5 e Comparison of Ea and transition temperature in literature. Raw material Fe2O3 Fe2O3 Fe2O3 Fe3O4 Fe3O4 Oxide scale a b
Reducing gas
Reduction step
Ea1a/kJ/mol
Tt/ C
Ea2b/kJ/mol
Ref.
H2 H2eN2 CO H2 CO H2eAr
Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe Fe3O4eFe
88 103 114 71 150.3 78.8
417 415 439 390 428 410
39 36 40 44 63.8 31.8
[18] [18] [18] [17] [17] This work
Activation energy below Tt. Activation energy above Tt.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 1 5 1 1 6 e1 5 1 2 4
Different from acid pickling, the surface of reduced scale is rough and porous. This kind of structure increases the surface roughness as well as the specific surface area. The roughness of substrate material may result in surface roughening of the galvanizing coating. It also has significant impact on the coating quality when deformation [43,44]. The latter one enhances the reactivity of steel surface [40] and aluminum consumption [45] during hot-dip galvanizing, thus claims adjustment to the composition of zinc melt and/or hot dipping processes.
Conclusions Reduction of HRSS in the temperature range of 370e550 C were investigated by thermogravimetric experiments. With increasing temperature, the time for complete reduction decreases. The scale with a thickness of 7 mm can be mostly reduced by 80% Ar þ 20% H2 gas mixture within 8 min at 550 C. Mathematical modeling of thermogravimetric data indicate that the reaction closely obey the AvramieErofe'ev equation for two and/or three dimensional growth of nuclei in the temperature range of 370e550 C. A sharp decrease of apparent activation energy was observed at temperatures higher than 410 C. For temperature lower and higher than 410 C, Ea decrease from 78.8 kJ/mol to 31.8 kJ/mol. The variation of apparent activation energy probably caused by the formation of FeO as intermediate product. SEM study indicated that the pore structure of the reduced scale becomes progressively coarser as the temperature increasing. Agglomeration of reduced iron, especially at the surface layer, was observed at temperature higher than 430 C.
Acknowledgments The experiments of this work were performed in the State Key-laboratory of Rolling and Automation, Northeastern University, China. The authors wish to thank Professor Liu Zhenyu for his help. This research was funded by the National Twelfth Five-year Science and Technology Support Program of China (Grant No. 2011BAE13B04) and National Natural Science Foundation of China under contract No. 51027005.
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