Preferable Resistance against Hydrogen Embrittlement of Pearlitic Steel Deformed by Caliber Rolling

Abstract

High strengthened steel bar with elongated pearlite was prepared by caliber rolling at room temperature and its hydrogen embrittlement was studied. A 0.6%C - 2%Si - 0.2%Mn - 1%Cr (in mass%) steel was caliber rolled to a reduction of area of 88%, resulting in the evolution of the elongated lamellar structure similar to those in cold drawn pearlitic steel. Mean lamellar spacing of the elongated pearlite was 70 nm and the texture of the rolled steel consisted of strong fiber texture with the rolling direction parallel to <110>. The rolled steel exhibited high yield strength of 1.4 GPa with large reduction of area at fracture (51%). It was demonstrated by both conventional and slow strain tensile tests (CSRT and SSRT) with circumferentially-notched bar specimens that the rolled steel with elongated pearlite performed higher resistance to hydrogen embrittlement than other thermo-mechanically treated steels in which fundamental microstructure is martensite. The possible mechanism for this desirable property is discussed with linking between the cracking path and the significant characters of the microstructure such as anisotropic morphology of lamellar structure and crystallographic texture.

1. Introduction

Drawn pearlitic steel is one of the most important steels because it has the highest tensile strength in bulk steels.^1,2) Many research works have been conducted to clarify the physical phenomena behind the superior strength of deformed pearlite, such as the morphological change during drawing^3,4) or cold rolling,⁵⁾ texture evolution,^3,6,7) cementite decomposition,^8,9) interaction between solute-carbon and delamination,^10,11) stress partitioning examined by in-situ diffraction.^12,13) These research works have brought deeper understanding about the strength of deformed pearlite, with the recent progress of the microstructural characterization of as-transformed pearlite.^14,15,16,17)

The aggressive activity of the research for the deformed pearlite must be motivated by additional engineering benefits, one of which is the superior hydrogen embrittlement (HE) resistance.^2,18) HE is the serious problem for high strengthened steels for long time.¹⁹⁾ The mechanism of the superiority of pearlitic steel has been studied by several researchers: one of the previous papers has pointed out that the fiber structure in the drawn pearlite prevents the hydrogen-induced cracking from propagation,¹⁸⁾ and actually this suggestion inspired the invention of the superior HE properties of the tempformed steels with deformed lath martensite structure.^20,21) Tempforming means the thermomechanical process characterized by deformation of carbon steel with a martensitic structure at warm temperatures. Lately, the study with the small-scale test in the electron microscope clarified that the HE resistance of the cold-drawn pearlite shows anisotropic behavior of fracture, supporting the importance of lamellar structure.²²⁾ Other paper discussed the characteristic lattice defects is the dominant factor for the HE resistance of the cold-drawn pearlite.²³⁾ However, still the detailed mechanism for the superior HE resistance has not been clarified adequately. One of the physical reasons for this incomprehensibility lies on the small diameter (< 5 mm²³⁾) of the drawn pearlitic steels, which prevents the mesoscale analysis such as the notched tensile test with hydrogen charged steels²¹⁾ or the relationship between the crystallographic orientation and fracture.^24,25) Although Kim et al.²⁶⁾ studied the HE of cold drawn pearlitic steel bars with relatively large diameter (7.25 mm), the detailed analysis about the cracking path has not been clarified so far.

The caliber rolling is one of the manufacturing processes suitable for intense straining of metallic bar.^27,28,29) For example, Yin et al.²⁷⁾ clarified that the caliber rolling of the low carbon steel at warm temperature evolves the strong fiber texture with the <110>//rolling direction (RD). The texture component of <110>//RD was also found in the drawn pearlitic steel wire.^3,6,7) This similarity of the texture means that the caliber rolling has potential to bring the bar with elongated pearlite similar to that in the drawn pearlitic steel wire and this upscaling should be desirable for the HE study. The aim of this work is to clarify the microstructure and HE properties of the pearlitic steel bar deformed by caliber rolling.

2. Experimental Procedure

A medium-carbon Si-bearing steel (0.62%C - 2.02%Si - 0.23%Mn - 0.001%P - <0.001%S - 1.01%Cr - Fe) (in mass%) was examined. The reason for the choice of this steel is that several studies have already evaluated the mechanical properties of this steel with different microstructures such as tempered martensite,³⁰⁾ deformed martensite³⁰⁾ and bainite.^31,32) Some of these data enable the comparison to reveal the microstructural effect on the HE. The round bar with a dimension of 40 mm diameter × ~100 mm length was austenitized at 1123 K for 1.8 ks followed by air cooling to obtain fully pearlite structure. The heat-treated bar was lubricated in order to reduce the rolling load, and it was caliber rolled at room temperature to reduce the dimension of cross-section to 14 mm × 14 mm square bar as schematically shown in Fig. 1(a). The rolled bar was cooled by water at every several passes in order to prevent the self-heating by plastic deformation. Concerning the textual expression for the directions, the plane normal direction of the rolled bar is referred as the transverse direction (TD), so as to clarify the difference from the normal plane in rolling.²⁹⁾ As presented later with Fig. 3, two TDs (TD1 and TD2 in Fig. 1(a)) were microstructurally equivalent to each other. The rolled bar was rotated 90° around the RD for each pass and total reduction of area was 88%. The detailed condition about the shape of caliber rolls was just the same as mentioned in the previous papers.^29,30,33)

Fig. 1.

Dimensions of the rolled bar (a), the round bar tensile specimen (b) and the notched bar specimen (c). The notch radius (R) of the notched specimen (R) is 0.1 mm for the stress concentration factor, Kt = 4.9.

Fig. 3.

Orientation color maps of the BCC phase showing orientation parallel to TD1 (a) and RD (b). The black line in these color maps indicates the high angle boundaries where the misorientation angle is higher than 15°. The texture measured from the relatively large area (0.3 mm × 0.8 mm) was presented by the 001 pole figure (PF) (c) and the ϕ₂ = 45° ODF section (d). {hkl}<uvw> means the plane and the normal direction of the TD1 plane and the RD, respectively. TD1, RD and TD2 were selected as the reference direction to obtain Euler angle with Bunge’s notation. (Online version in color.)

Microstructural observation was conducted at the center region of the bars by scanning electron microscope (SEM) (JEOL 7000F) equipped with electron back scattered diffraction (EBSD) measurement system (TSL OIM data collection). The observation surface was prepared by electrochemical polishing with the mixture of 10% perchloric acid and 90% acetic acid and the accelerating voltage was 15 kV. The EBSD scan was conducted at a scan step of 0.2 μm or 0.5 μm.

Tensile properties (Table 1) were evaluated with the round bar specimen as shown in Fig. 1(b). According to the JIS-14A, the tensile specimen has a dimension with a gage diameter of 6 mm and a gage length of 30 mm. Tensile test was conducted at a constant cross head speed of 0.85 mm/min at room temperature. The tensile direction was parallel to the RD.

Table 1. Tensile properties of the 0.6%C - 2%Si - 0.2% Mn - 1%Cr steel with various microstructures.

Thermo-mechanical treatment	Microstructure	Yield strength [GPa]	Tensile strength [GPa]	Uniform elongation (%)	Total elongation (%)	Reduction of area (%)
Pearlite transformation and cold-rolling	Elongated pearlite	1.44	1.67	1.6	8.7	51
Quenching and tempering at 773 K (QT)30)	Equiaxed martensite	1.38	1.58	5.9	10.3	34
Quenching and tempforming at 773 K (TF)30)	Elongated martensite	1.46	1.54	6.9	12.7	39

Hydrogen entry of the rolled bar was evaluated by the immersion test (Fig. 4). Cylindrical specimens with a diameter × length of 7 mmϕ × 20 mm were cut from the rolled bar and they were immersed at 303 K in an aqueous solution of 0.5 mol/L NaCl and 0.01 mol/L HCl (pH = 2.0) for 72 to 432 hours (3 to 18 days) to evaluate the hydrogen entry from an atmospheric corrosive environment.³⁴⁾ Additionally, for the assessment of trapped hydrogen, the apparent activation energy for the evolution of hydrogen from the trapped sites was evaluated by so-called Choo-Lee plot³⁵⁾ (or Kissinger plot³⁶⁾) (Fig. 5). In this assessment, the plate-shaped sample with a thickness of 1 mm × a width of 5 mm × a length of 15 mm was cut from the rolled bar and charged with hydrogen for 48 hour in a 0.1 N NaOH aqueous solution at current density of 25 A/m². Both the cylindrical and the plate-shaped samples were stored in liquid nitrogen until thermal desorption spectrometry (TDS) in which cylindrical or plate-shaped sample was heated at a heating rate of 100 to 300 K/hour and desorbed hydrogen was detected with a quadrupole mass spectrometer. These conditions were decided according to the previous work.²¹⁾

Fig. 4.

Hydrogen desorption rate curves obtained from TDS analysis of the immersed specimen at a heating rate of 100 K/h (a) and the apparent hydrogen content corresponding to the 1st and the 2nd peaks (b). Cylindrical specimens with a diameter × length of 7 mmϕ × 20 mm were cut from the rolled bar and they were immersed at 303 K in an aqueous solution of 0.5 mol/L NaCl and 0.01 mol/L HCl (pH = 2.0) for 72 to 432 hours (3 to 18 days). (Online version in color.)

Fig. 5.

Hydrogen desorption rate curves at different heating rate, φ, (a) and plots of φ and peak temperature, T_c dependent parameter ln(φ/T_c²) as a function of 1/T_c for determination of the activation energy, E_a of hydrogen desorption (b) in the 1-mm-thick plate. The plate was charged with hydrogen for 48 hour in a 0.1 N NaOH aqueous solution at current density of 25 A/m². (Online version in color.)

HE susceptibility was evaluated by tensile test of the specimen with a circumferential notch as shown in Fig. 1(c), of which the stress concentration factor, Kt, is 4.9.³⁷⁾ Before the tensile test, the specimen was catholically charged with hydrogen for 72 hour in a 0.1 N NaOH aqueous solution or in a 3% NaCl + 0.3% NH₄SCN aqueous solution at a current density of 0.2–16 A/m². The tensile test was conducted at 0.005 mm/min or 1 mm/min. These conditions are referred as the slow-strain-rate-testing (SSRT) or conventional-strain-rate-testing (CSRT), respectively. Both the SSRT and CSRT were frequently adopted for the evaluation of the HE susceptibility in previous works.^38,39) In order to prevent the release of hydrogen from the SSRT samples, the specimen was electroplated with Cd plating. The diffusible hydrogen content of the charged specimen was examined by the same TDS as used for the assessment of hydrogen entry, being mentioned in the previous paragraph.

3. Results

3.1. Microstructure, Texture and Tensile Properties

Figure 2 shows the SEM images of the steel austenitized followed by air-cooling (a), and the rolled bar observed in the direction parallel to the TD (b) or the RD (c). The as-heat treated steel showed almost fully pearlitic structure with a few area% of proeutectoid ferrite. In the caliber rolled steel, the observation in the TD revealed lamellar pearlite structure which is typically found in the intensely deformed pearlitic steels.^3,4,5,6) Mean lamellar spacing was 70 nm and the bundle of the elongated cementite and ferrite with similar morphology constitutes the elongated unit whose thickness were a few μm. The evolution of this heterogeneity with some microstructural unit was well studied by the previous studies.^4,5) With the change of the observation direction from the TD to the RD, the appearance of the deformed pearlite changes drastically, as shown in Fig. 2(c). The pearlite lamella bent to different directions and the microscopic metal flow appeared turbulent. This appearance is the same as reported in the observation by the previous study.⁶⁾

Fig. 2.

SEM images of the as-transformed sample (a), the transverse section (b) and the cross section (c) of the rolled sample.

The results of the EBSD analysis of the rolled steel were shown in Fig. 3. The color maps (a, b) show the orientations parallel to the TD1 (a) and the RD (b) with the color key presented with the standard triangle. The black line indicated by the high angle boundary at which the adjacent EBSD scanning points has a misorientation angle higher than 15°. Green color indicating RD//<110> covers the RD map (b) dominantly, indicating intense development of the deformation texture. In addition, the texture colony, which is the domain having an identical texture component, had elongated morphology and these sizes were corresponded with the elongated blocks found in Fig. 2(b). The 001 pole figure (PF) (c) and the orientation distribution function (ODF) (Bunge’s notation) section (d) was obtained from the EBSD scan data with relatively large area (300 μm × 800 μm) to keep statistical accuracy. The 001 pole figure shows the strong fiber texture as illustrated by dotted lines including the components of {110}<1-10> and {001}<1-10>. The ODF section more clearly indicates the typical rolling texture, so-called α-fiber, characterized by RD//<110>. However, the α-fiber in cold-rolled steel sheet generally limits in the top-left part of the ODF section indicating the orientation from {001}<1-10> to {111}<1-10>,⁴⁰⁾ so that the significant presence of {110}<1-10> orientation is characteristic for the caliber rolled steel bar. As described previously,²⁹⁾ this is due to the 90° rotation around the RD for every pass because the 90° rotation convert {001}<1-10> to {110}<1-10>. This conversion brings strong texture component characterized by RD//<110> and it should be noted that RD//<110> is the dominant component in the cold drawn pearlitic steel wire.^6,7)

The tensile properties of the rolled samples were summarized in Table 1. For comparison, this table includes the reference data³⁰⁾ for the quenched and tempered specimen and the tempformed specimen of the 0.6%C - 2%Si -1%Cr steel. The tempforming is the thermomechanical process including the warm caliber rolling of martensite and the resultant microstructure is elongated martensite.³⁰⁾ These three samples listed in Table 1 shows similar yield stress and tensile strength, while the rolled pearlitic steel is characterized by larger local elongation (difference between total and uniform elongations) and better reduction of area.

3.2. Hydrogen Entry

Figure 4 shows the hydrogen desorption curves obtained by the TDS analysis of the immersed specimens with the cylindrical shape. At any immersing condition, the curve shows two peaks: the 1st peak locates at around 400 K and the 2nd peak is found at the higher temperature. These two-peak curves have been reported as typical for the cold-drawn pearlitic steels.^23,41,42) The curve for the sample immersed for 240 hour shows extremely larger desorption rate at the 2nd peak. The possible reason for this large rate should be the incomplete removement of the corrosion product at the surface, although the precise reason has not been clarified yet. The apparent hydrogen content evaluated by the separation of these two-peak curves at the intermediate minimum were shown in Fig. 4(b). The hydrogen content of the 1st peak appears to increase slightly with increasing of the immersing time. In other words, it is difficult to find the saturated condition of the hydrogen content at the 1st peak in this work.

In order to analyze the hydrogen trapping states, the apparent activation energy for the evolution of hydrogen from trapping sites, E_a, was evaluated with the TDS analysis of the plate-shaped samples. According to the method proposed by Choo and Lee,³⁵⁾ E_a is evaluated with data of the heating rate, ϕ, and the corresponding peak temperature, T_c, at 1st or 2nd peaks using Kissinger’s equation,³⁶⁾   

$$\frac{\partial \text{ln(}\varphi /T_c{}^2\text{)}}{\partial (1/T_c)}=-E_a/\text{R}$$(1)

where R is the gas constant (8.31 J/(mol∙K)). Namely, E_a can be evaluated with the slope of the regression line for each peak in Fig. 5. The E_a at the 1st and the 2nd peaks was 23.2 kJ/mol and 44.2 kJ/mol, respectively. These two E_a appeared smaller than the previously reported data by Doshida and Takai²³⁾ (37 kJ/mol and 90 kJ/mol for the 1st and 2nd peak), while these were larger than the data documented by Enos and Scully⁴¹⁾ (21 kJ/mol and 30 kJ/mol). It is consistent to their findings that the E_a at the 1st peak is relatively smaller than that at the 2nd peak. This means that the hydrogen desorption around the 1st peak from the rolled sample in this work can be regarded as being due to the diffusible hydrogen as the identification by Choo and Lee.³⁴⁾ The diffusible hydrogen content, H_D was evaluated by the integration of the hydrogen desorption rate from the ambient temperature to the minimum point between the 1st and the 2nd peaks. Although the atmospheric hydrogen entry is difficult to be evaluated due to the evolution of corrosion as found in Fig. 4, H_D at the longest immersing time (432 hour in Fig. 4) keeps as small as 0.14 mass ppm. This value appears similar to those for the other thermomechanically treated steels listed in Table 1.^21,30)

3.3. HE Susceptibility

The examples of the desorption rate curves obtained by TDS analysis of the charged bar specimens were shown in Fig. 6. As shown in the immersion test in Fig. 4, two distinct hydrogen desorption peaks were observed, so that the content of H_D was evaluated by the integration of the desorption rate from the room temperature to the minimum point between the two peaks, which is the same as discussed with Fig. 4. The difference between the results of the tensile test samples (Fig. 6) and the immersed specimens (Fig. 4) can be found with the fact that the desorption rate at the 1st peak is as large as, or larger than, that at the 2nd peak. This is because the total contents of the charged hydrogen in the tensile test samples were much larger than that for the immersion test to simulate atmospheric hydrogen entry.²³⁾

Fig. 6.

Hydrogen desorption rate curves obtained from TDS analysis of the notched bar specimens with different diffusible hydrogen content, H_D. Before the tensile test, the specimen was catholically charged with hydrogen for 72 hour in a 0.1 N NaOH aqueous solution or in a 3% NaCl + 0.3% NH₄SCN aqueous solution at a current density of 0.2–16 A/m². (Online version in color.)

Figure 7 shows the notched tensile strength, σ_NB, of the rolled pearlitic steels charged with H_D. The results by both the CSRT and the SSRT revealed the decreasing of σ_NB with the increasing of H_D. When compared at the similar H_D, the σ_NB degradation with the SSRT is a little larger than that with the CSRT. This difference is well consistent with the previous papers for the conventional quench and tempered steels.³⁸⁾ In addition to the rolled pearlitic steel, the SSRT data for the tempformed (TF) and the quenched and tempered (QT) steels,³⁰⁾ whose mechanical properties were described previously in Table 1, were presented for comparison. Both σ_NB of the TF sample and the QT sample show relatively large decrease with the increase of H_D, while the TF sample shows smaller HE susceptibility than the QT sample at any value of H_D. Although Fig. 7 apparently indicates the superior HE susceptibility of the rolled pearlitic steels, the precise comparison among these three samples need the consideration for the difference between the results by SSRT and that by CSRT. This point describes later in the discussion section of this paper.

Fig. 7.

Notch tensile strength,σ_NB of the rolled pearlite as a function of diffusible hydrogen content, H_D. The strength obtained by CSRT or SSRT were presented separately. The data for TF and QT samples²⁹⁾ were also shown for reference.

The characterization of the fracture appearance in the rolled pearlitic steels was conducted and the results are shown in Figs. 8, 9, 10. The fracture surfaces after the CSRT with different H_D were observed by SEM (Fig. 8), indicating that all of these can be characterized by significant delamination with shear fracture. This constituent of delamination is similar to that found in the TF samples,^21,30) indicating that the cracking path has the key to reveal the HE behavior of the rolled pearlitic steel.

Fig. 8.

Fracture appearance of the specimens after the CSRT with H_D content of 0 (a), 1.4 (b) and 5.8 (c) mass ppm.

Fig. 9.

Orientation color maps (a,b) of the fractured sample after SSRT with H_D of 4.8 mass ppm. The orientation parallel (a) or perpendicular (b) to RD was illustrated. The scanned surface was prepared by cutting of the plane parallel to RD and the area at the tip of the crack was scanned as denoted in (c). (Online version in color.)

Fig. 10.

SEM images of the fractured samples after the SSRT with H_D of 4.8 mass ppm. The area including a crack was observed at relatively lower (a) and higher (b) magnifications. The arrows indicate the steps along the crack.

Figures 9(a), 9(b) shows the orientation color maps of the fractured samples after the SSRT with H_D = 4.8 mass ppm. In the preparation for these maps, the fractured sample was cut to make the plane parallel to the tensile direction (//RD) so as to reveal the cracking path around the notch surface as shown in Fig. 9(c). The area around the tip of crack was selected for EBSD scanning and the crystallographic orientations parallel (a) or perpendicular (b) to the RD was illustrated as the color maps. The black part in these color maps indicates the opening cracks. The color map (a) shows the strong concentration of <110>//RD and the elongated morphology of textured colony appears in the color map (b). Concerning of the cracks, the dominant parts of opening cracks are elongated parallel to the RD. Additionally, the parallel parts of cracking path appear to propagate along the boundaries of the texture colony.

The relationship between the morphology of microstructure and the cracking path can be confirmed with the SEM images in Fig. 10. These images were observed in the fractured samples with H_D = 4.8 mass ppm as the same as in Fig. 9. The cracking path had the small steps with around 1 μm in height some of which were found at the allowed positions in Fig. 10(a). The observation at higher magnification (Fig. 10(b)) clarified that the steps consist of largely bended pearlite.

4. Discussion

The experimental results showed the possibility of the superior HE susceptibility of the steels with elongated pearlite, which was mainly indicated by the data shown in Fig. 7. However, the HE susceptibility was evaluated in this work by both SSRT and CSRT, while the reference data³⁰⁾ of the steel with the different microstructures were obtained by SSRT. The difference between the results at these testing conditions has been already discussed by several papers^38,39) and the difference is caused mainly by the redistribution of hydrogen by stress-assisted diffusion during the tensile test. This means the importance of local hydrogen content, and actually, the HE susceptibility was consistently assessed at any strain rate with the evaluation of the peak value of maximum principal stress, σ_max*, and the peak value of the concentration of diffusible hydrogen at the vicinity of the notch root under the applied stress, H_C* at fracture.^21,37,39,40) This is reasonable because the HE is triggered by the accumulation of diffusive hydrogen content in front of the notch root under applied stress. Both σ_max* and H_C* were successfully calculated for the steels with the strength similar to the rolled pearlitic steel.^20,36,38,39) Figure 11 presents the relationship between σ_max* and H_C* by using the data in Fig. 7 according to the previous paper.²¹⁾ It should be noted that H_C* is considered to be approximately equal to H_D for CSRT because time for the stress-assisted diffusion is short.^39,40) Figure 11 show the superior HE susceptibility of the rolled pearlitic steel especially at relatively larger H_D, when compared with the high strengthened steels with different microstructures.

Fig. 11.

Peak values of maximum principal stress, σ_max* at fracture of the 0.6%C - 2%Si -0.2%Mn - 1%Cr steel with various microstructures as a function of critical diffusible hydrogen concentration, H_C*.

The reason for the superior HE susceptibility should be due to the relation between the fracture appearance and the microstructure. One of the significant feature of the fracture appearance is that the cracking path in the fractured sample with the elongated pearlite goes dominantly parallel to the loading direction as shown in Figs. 9 and 10. The similar appearance has been found in the tempformed steels as well and it was regarded as beneficial so as to keep the toughness and other mechanical properties.²¹⁾ As far as the microstructural features is concerned, the rolled pearlitic steel has the strong <110>//RD texture component as found in Fig. 3. In addition, the texture colony was elongated to the RD as shown in Fig. 5. According to the previous study,^24,25) the {110} around the prior austenite grain boundaries is preferential plane for the hydrogen-induced cracking. With crystallographic consideration, the direction parallel to <110> accompanies the perpendicular direction parallel to <-110>, so that the strong fiber texture of <110>//RD provides the preferential cracking path around the texture colony boundaries along the loading direction. Furthermore, the cementite was not easily fractured as found in Fig. 10(b). This seems to be consistent with the finding by Tomatsu et al.²²⁾ with their microbending test to clarify the isotropic property of the directional alignment of elongated pearlitic steel. These microstructural features prevent the cracking path from going along the radius direction so that the rolled pearlitic steel shows the preferential HE susceptibility.

5. Summary

High strengthened steel bar with elongated pearlite was prepared by caliber rolling at room temperature and its HE was studied. The 0.6%C - 2%Si - 0.2%Mn - 1%Cr steel with pearlitic structure was caliber rolled to a reduction of area of 88% and the elongated lamellar structure which is typically found in drawn pearlitic steel was evolved. Mean lamellar spacing of the elongated pearlite was 70 nm and the texture of the rolled steel consisted of strong fiber texture with the rolling direction parallel to <110>. The room temperature tensile test clarified that the rolled steel revealed high strength (1.4 GPa) with large reduction of area at fracture (51%). The susceptibility of HE was evaluated by both CSRT or SSRT with a circumferentially-notched specimen. The rolled steel with elongated pearlite performed higher resistance to HE than other high strengthened steels prepared by conventional tempering or the tempformimg. The possible mechanisms for this desirable property can be related to (1) the strong <110>//RD texture and (2) the arrangement of cementite because both features prevent hydrogen-induced cracking from propagation to the radius direction.

Acknowledgments

This study is based on work supported by a Grant-in-Aid for Scientific Research (ID: 19H02468, 20H02488, 21H00109) through the Japan Society for the Promotion of Science (JSPS). We acknowledge the supports of Mr. Iida, Mr. Kobayashi, Mr. Hibaru, Mr. Iwasaki and Ms. Seki in NIMS for their experimental supports.

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