WO2017149785A1

WO2017149785A1 - Method of visualizing austenite phase in multiphase steel and multiphase steel specimen for microstructure observation

Info

Publication number: WO2017149785A1
Application number: PCT/JP2016/057259
Authority: WO
Inventors: Sarka MIKMEKOVA; Daisuke TSUKIORI; Yuji Tanaka
Original assignee: Jfe Steel Corporation
Priority date: 2016-03-02
Filing date: 2016-03-02
Publication date: 2017-09-08
Also published as: JP2018513347A; JP6308326B1

Abstract

A method of visualizing an austenite phase in a multiphase steel capable of identifying the austenite phase in the multiphase steel at the nanoscale is provided. The method includes the steps of: a) electro-polishing a surface of a specimen of a multiphase steel in an electrolyte solution containing a corrosive acid and an organic solvent to form a native oxide layer on the surface of the specimen, b) after completion of the electro-polishing, continuously retaining the specimen in the electrolyte solution to form a corrosion product layer selectively only on a domain of the native oxide layer covering an austenite phase, and c) subjecting the surface of the specimen to scanning electron microscopy observation and providing a contrast between the corrosion product layer and remaining surface to visualize the austenite phase as a darkest area.

Description

DESCRIPTION

Title of Invention :METHOD OF VISUALIZING AUSTENITE PHASE IN MULTIPHASE STEEL AND MULTIPHASE STEEL SPECIMEN FOR MICRO STRUCTURE OBSERVATION

Technical Field

[0001] The disclosure generally relates to a method of visualizing an austenite phase in a multiphase steel and a multiphase steel specimen for microstructure observation.

Background

[0002] Knowledge of the fraction, morphology and the distribution of austenite in the matrix of multiphase steel are crucial for getting a better understanding of the relationship between the microstructure and the mechanical properties. In order to clearly distinguish the phases, a wide range of characterization techniques have been used.

[0003] There are several known techniques enabling to visualize an austenite phase in multiphase steels at different scale levels. The most frequently used technique is a selective chemical etching of a steel surface and its observation by an optical microscopy (OM). However, due to a demand for a higher accuracy, the traditional etching methods are considered to be insufficient, and other novel etching techniques enabling more precise phase separation have been developed. For example, PTL 1 discloses an improved etching technique for multiphase steel. This technique, however, has a drawback that austenite cannot be separated from martensite islands.

[0004] The others developed techniques for optical microscopy are described in NPL 1. The objective of this extensive study is to develop the methodology for the quantitative and qualitative determination of phases in multiphase steels. There are described extensive etching methods for an optical microscopy (OM), such as conventionally used etching techniques (i.e. simple electro- etching, Nital, Picral, le Pera and Klemm etching) and also advanced two step etch methods. All of the above-mentioned techniques have been applied on multiphase steels and it can be concluded that:

- conventional electro-polishing, Nital etching and Picral etching are - - not able to provide a contrast between secondary phases and matrix in the OM;

- although the conventional methods using the Le Pera and Klemm etchants enable to differentiate between the secondary phases and matrix based on their specific color in the OM, separation of austenite and martensite is impossible (But NPL 2 describes that only Klemm etchants can be utilized for the retained austenite visualization in the OM); and

- two steps etching techniques utilizing two reagents (i.e. Picral + sodium bisulfate, Nital + sodium bisulfate and V2A + Klemm) are able to differentiate the secondary phases from the matrix and moreover there is a possibility to distinguish an austenite phase from a martensite phase because of their different color in the OM micrographs.

[0005] Thus, a combination of color etching and the OM enables to visualize an austenite phase in multiphase steels, but, on the other hand, all techniques utilizing the optical microscopy are limited by poor spatial resolution (200nm or less) of this technique, and accuracy of phase identification depends on an experience of the operator.

[0006] NPL 1 also describes a rich series of experiments utilizing a scanning electron microscopy (SEM) technique. There are mentioned two possibilities how to visualize an austenite phase in multiphase steels by the SEM. There are also mentioned two techniques enabling to differentiate the austenite phases from other phases. The first technique uses an SEM micrographs created by secondary electrons (SE), which carry information about a specimen topography. A conventional electro-polishing technique and a Nital etching technique were used for specimen preparations. The second technique uses SEM micrographs created by back-scattered electrons (BSE). This technique combines the SEM with color etching and various Klemm and le Pera etched samples of the multiphase steel were investigated.

[0007] Another technique used for characterization of multi-phase steels is an electron backscattered diffraction technique (EBSD). A method for quantitatively evaluating retained austenite in steel by EBSD is described in PLT 2, and a method for calculating three phase ratio of hot rolled multiphase steel in described in PTL 3. Citation List

Patent Literature

[0008] PTL 1 : CN 101382494 A

PTL 2: CN 102735703 A

PTL 3 : CN 102353690 A

Non Patent Literature

[0009] NPL 1 : E. Leunis et al., Quantitative phase analysis of multi-phase steels - PHAST, Final Report, EUR 22387 EN, European Communities, 2006 NPL 2: K. Radwanski et al. : Role of the advanced micro structure characterization in modeling of mechanical properties of AHSS steels, Materials Science and Engineering A 639 (2015) 567-574

Summary

Technical Problem

[0010] The specimen prepared by the conventional electro-polishing method enabled to separate secondary phases from the matrix in the SEM SE micrograph (martensite and austenite are standing out of the ferrite matrix) but did not differentiate between martensite and austenite. The specimen surface prepared by the Nital etching technique enabled not only to differentiate between the secondary phases and matrix but also distinction between them because of the internal structure, visible in martensite grains. This technique is, however, not suitable for identification of very fine phases and does not enable precise and accurate separation of the austenite in martensite-austenite constituents. Furthermore, it has been found that this technique is almost impossible to define standard procedure or the like due to high sensitivity to many factors such as etching time, freshness of the reagents, the solution temperature and the like. Moreover, a highly experienced operator is required.

[0011] In the SEM BSE micrographs, there is no possibility to differentiate the austenite phase from martensite. Moreover, carbides rich phases appear the same contrast and contribute to second phase fraction. Thus, there are not any suitable method for visualization of a fine austenite phase in a multiphase steel at the nanoscale.

[0012] Despite of the fact that the EBSD technique enables to separate the - - austenite phase accurately, there are several disadvantages inseparably connected with this technique. Spatial resolution of the technique is 20 nm or less, which is not sufficient for characterization of advanced steels. Moreover, the technique is not adequate for large area mapping and does not offer a reasonable statistical data.

[0013] In this way, the above mentioned techniques encounter a dilemma: either high spatial resolution (which is necessary for characterization of the austenite phase in advanced multiphase steels) or possibility of large area mapping (but without insufficient resolution). There are not any techniques enabling to visualize the austenite phase in multi-phase steels at the nanoscale and simultaneously to investigate large area in a reasonably short time.

[0014] An object of the present disclosure is, therefore, to provide a method of visualizing an austenite phase in a multiphase steel capable of clearly and visually identifying the austenite phase in the multiphase steel at the nanoscale. Another object of the present disclosure is to provide a multiphase steel specimen for microstructure observation capable of clearly and visually identifying the austenite phase in multiphase steel at the nanoscale.

Solution to Problem

[0015] As a result of conducting intensive study, the present inventors have discovered that, when a surface of a specimen of a multiphase steel is electro-polished in a certain electrolyte solution and thereafter the specimen of the multiphase steel is continuously retained in the electrolyte solution, a corrosion product layer is formed selectively on an austenite phase of the surface of the specimen. The present inventors have thus found that when the surface of the specimen of a multiphase steel is observed with a scanning electron microscopy (SEM), a contrast is provided due to a corrosion product layer selectively formed on an austenite phase of the surface of the specimen, which enables to visually observe the austenite phase can be clearly visualized.

[0016] The present disclosure has been made based on these findings, and its gist is as follows:

(1 ) A method of visualizing an austenite phase in a multiphase steel comprising the steps of: - - a) electro-polishing a surface of a specimen of a multiphase steel in an electrolyte solution containing a corrosive acid and an organic solvent to form a native oxide layer on the surface of the specimen,

b) after completion of the electro-polishing, continuously retaining the specimen in the electrolyte solution to form a corrosion product layer selectively only on a domain of the native oxide layer covering an austenite phase, and

c) subjecting the surface of the specimen to scanning electron microscopy observation and providing a contrast between the corrosion product layer and remaining surface to visualize the austenite phase as a darkest area.

[0017] (2) The method according to foregoing (1), wherein a mean thickness of the corrosion product layer is between 2 nm and 10 nm.

[0018] (3) The method according to foregoing (1) or (2), wherein the step c) is performed with a landing energy of the primary electrons between 0.2 keV and 4.0 keV to observe a scanning electron microscopy micrograph created by secondary electrons.

[0019] (4) The method according to foregoing (1) or (2), wherein the step c) is performed with a landing energy of the primary electrons between 0.25 keV and 2.0 keV to observe a scanning electron microscopy micrograph created by back-scattered electrons.

[0020] (5) The method according to any one of foregoing (1) to (4), wherein the corrosive acid is selected from a group consisting of an oxidizing acid and an organic acid.

[0021] (6) The method according to any one of foregoing (1) to (5), further comprising: d) evaluating quantitative and/or qualitative characterization of the visualized austenite phase.

[0022] (7) A multiphase steel specimen for microstructure observation comprising:

a multiphase steel base having a plurality of phases including an austenite phase,

a native oxide layer formed on a surface of the multiphase steel base, a corrosion product layer formed selectively only on a domain of the native oxide layer covering the austenite phase. - -

[0023] (8) The multiphase steel specimen for microstructure observation according to foregoing (7), wherein a mean thickness of the corrosion product layer is between 2 nm and 10 nm.

Advantageous Effect

[0024] The method of visualizing an austenite phase in a multiphase steel and the multiphase steel specimen for microstructure observation according the present disclosure enable to clearly and visually identify the austenite phase in the multiphase at the nanoscale. Brief Description of Drawings

[0025] In the accompanying drawing:

Fig. 1 (a) is an image of a TRIP (Transformation Induced Plasticity) steel surface prepared according to the present disclosure and captured at 1 keV landing energy with an in-lens type of the SE detector, and Fig. 1 (b) is a corresponding EBSD phase map of the identical area.

Fig. 2(a) is an STEM (HAADF (High-Angle Annular Dark-Field)) image of the oxide layer covering the ferrite phase, and Fig. 2(b) is an STEM (HAADF) image of the austenite phase after preparation of the specimen according to the present disclosure.

Fig. 3(a) is an SEM SE image of the specimen prepared by a conventional electro-polishing technique, and Fig. 3(b) is an SEM SE image of the specimen prepared by the technique according to the present disclosure.

Figs. 4(a) and (b) are SEM SE images of the TRIP steel obtained at 5 keV, and 1 keV, respectively, landing energy of the primary electrons by the in-lens detector, Fig. 4(c) is a corresponding EBSD phase map.

Fig. 5(a) is an image of a TRIP steel surface prepared according to the present disclosure imaged at 1 keV landing energy using the in-lens type of the SE detector, and Fig. 5(b) is an image of a TRIP steel surface prepared according to the present disclosure imaged at 1 keV landing energy using the out-lens type of the SE detector.

Fig. 6 is an SEM BSE image of the TRIP steel specimen prepared according to the present disclosure obtained at 1 keV of landing energy.

Fig. 7 is a table showing a retained austenite phase fraction in the TRIP steel measured by an X-Ray diffraction and by the technique according - - to the present disclosure.

Fig. 8 shows histograms of austenite phase size and shape distributions.

Figs. 9(a) to 9(g) are SEM BSE images of Example 7 with using different magnitudes of landing energy of the primary electrons, and Fig. 9(h) is a corresponding EBSD phase map of the identical area.

Fig. 10 is a graph showing a relationship between the magnitude of landing energy of the primary electrons and the contrast ratio in SEM SE images.

Fig. 11 is an SEM SE image at IkeV landing energy of Example 9.

Detailed Description

[0026] A method of visualizing an austenite phase in a multiphase steel according to one embodiment of the present disclosure includes: processing a surface of a multiphase steel base with certain treatments (steps a and b) to give a specimen to be observed, and then subjecting the surface of the specimen to SEM observation to visualize an austenite phase (step c). The method may also include evaluating quantitative and/or qualitative characterization of the visualized austenite phase (step d).

[0027] The multiphase steel specimen is prepared by the following manner. First, a piece of a multiphase steel is provided. A surface of the multiphase steel piece is then mechanically polished to mirror finish. Any conventional abrasives may be used for the mechanical polishing. Diamond particles having a particle size of about 1 μιη are preferably used. A multiphase steel base having a polished surface is thus obtained.

[0028] (Step a: Electro-polishing process)

The multiphase steel base is then subjected to an electro-polishing process in an electrolyte solution. It will be appreciated that any suitable electro-polishing process known in the art may be used herein. The electrolyte solution consists essentially of a corrosive acid and an organic solvent. The term "a corrosive acid" as used herein refers to any acids which can corrode the surface of the multiphase steel base, and preferably contains chlorine atoms. The corrosive acid may be an oxidizing acid. The term "an - - oxidizing acid" as used herein refers to an acid contains an anion with an oxidation potential higher than the potential of H⁺ ion. Preferably, the oxidizing acid is at least one selected from a group consisting of, but not limited to, perchloric acid, sulfuric acid and phosphoric acid. The corrosive acid oxides and dissolves the surface of the multiphase steel base to form a native oxide layer having a generally uniform thickness throughout the surface of the multiphase steel base. The concentration of the corrosive acid in the electrolyte solution is preferably 0.30 mol/L or more, and more preferably 0.45 mol/L or more, and most preferably about 0.62 mol/L. When the concentration of the corrosive acid is lower than 0.3 mol/L, a polishing time significantly increases or the surface of the multiphase steel base is not sufficiently corroded due to a low reaction rate of the acid with the multiphase steel base, which may deteriorate the surface smoothness of the multiphase steel base. The upper limit of the concentration of the corrosive acid depends on a type of the corrosive acid used. For example, in case of perchloric acid, the concentration in the electrolyte solution is preferably 0.90 mol/L or less, more preferably 0.75 mol/L or less. Since perchloric acid is unstable and can be explosive at room temperature, the electrolyte solution including prechloric acid higher than 0.90 mol/L is difficult to be handled. Also, the surface of the multiphase steel base may be over-etched, which diminishes the smoothness of the surface. If the surface of the multiphase steel base is under-etched or over-etched, a distinctive contrast cannot be obtained in the subsequent process.

[0029] The organic solvent is used for adjusting a viscosity of the electrolyte solution, which may affect the thickness of the native oxide layer. Examples of the organic solvent include, but not limited to, lower alcohols having 1 to 10 carbon atoms such as methanol, glycerol, and butyl glycol, ethers such as 2-n-butoxyethanol, acetic acid, and mixture thereof. The concentration of the organic solvent in the electrolyte solution may be determined based on the kind and the surface smoothness of the multiphase steel base to be etched. If the viscosity is too low, the electrolyte solution does not sufficiently stay on the surface of the multiphase steel base, and the surface is not uniformly polished.

[0030] The temperature of the electrolyte solution affects the reaction rate of - - the corrosion. The lower temperature will need more processing time, which is practically not favorable. The higher temperature will accelerate the corrosion but may cause an over-etching. Also, the higher temperature renders the corrosive acid more unstable. In view of these, the temperature of the electrolyte solution is preferably from 283 to 308 K, and more preferably from 293 to 298 K.

[0031] The voltage applied to the electrolyte solution and the duration of the electro polishing process also affect the formation of the native oxide layer. The voltage is preferably in a range of 15 to 45 V, and more preferably 35 to 40 V. When the voltage is less than 15 V, the surface will be etched rather than polished. On the other hand, when the voltage is more than 45 the surface will be destroyed due to high current density. The duration can be determined based on process conditions such as temperature, voltage, concentration of corrosive acid and the like. Typical duration is, but not limited to, 2 seconds.

[0032] The thickness of the native oxide layer is preferably 2 to 4 nm. The thickness of the native oxide layer can be measured from the cross-section STEM (HAADF) images such as shown as Figs. 2(a) and 2(b).

[0033] (Step b: Retaining process)

After the completion of the electro-polishing process, the bias is switched off and the multiphase steel base is continuously retained in the electrolyte solution for a given time period. Then, the multiphase steel base is removed from the electrolyte solution and cleaned in methanol and subsequently in ethanol. The surface of the multiphase steel base is corroded and becomes matte. Various masks having different sizes may be used to cover the surface of the multiphase steel base, but a smaller mask such as one providing an electro-polished surface area of 0.1 -0.3 cm² is preferably used. An ultra-sonic cleaner may be used for stirring the electrolyte solution.

[0034] The duration of this retaining process may depend on the compositions of the multiphase steel, the concentration of the corrosive acid in the electrolyte solution, the processing temperature, and the like. In some cases, the duration can be as short as one second, but typically it is 2-10 seconds.

[0035] During this retaining process, a corrosion product layer is formed selectively only on the austenite phase of the surface of the specimen. It is - - believed that the austenite phase has a carbon content higher than remaining surface (e.g., a martensite phase, ferrite) which facilitates an additional corrosion during the retaining process. The corrosion product layer is porous and therefore thicker than the native oxide layer.

[0036] The thickness of the corrosion product layer greatly affects the below-discussed contrast in SEM micrographs. The contrast in the SEM images increases as the thickness of the corrosion product layer increases, and thus a thicker corrosion product layer will give a better result. In order to secure clear contrast in the SEM images, a mean thickness of the corrosion product layer is preferably at least 2 nm. The mean thickness of the corrosion product layer can be measured from the cross-section STEM (HAADF) image. As clearly visible in Fig.2, there is a difference in the internal structures between the native oxide layer and the corrosion product layer. The native oxide layer is compact whereas the corrosion product layer has a porous structure. In addition, these layers have different chemical compositions; the native oxide layer essentially consists of iron oxides and the corrosion product layer is a mixture of oxides and hydroxides. The interface between the layers in the STEM (HAADF) images can be determined by a difference of the signal electrons yield from the oxide and corrosive layer.

[0037] (Step c: Visualizing process)

In this way, a multiphase steel specimen having a multiphase steel phase, a native oxide layer, and a corrosion product layer is prepared. The specimen is then subjected to scanning electron microscopy (SEM) observation. When an electron beam is irradiated on the surface of the specimen, a contrast is provided between the corrosion product layer and remaining surface due to the larger thickness of the corrosion product layer. This enables the austenite phase to be visualized as a darkest area in an SEM micrograph. Preferable operation conditions securing the contrast highly suited to quantitative and qualitative image analysis without extensive image post processing are reached by detection of the signal electrons carrying information about the specimen surface composition. It is also advantageous to operate the SEM at low landing energies securing high surface sensitivity. Combination of these operation conditions enables to obtain clear contrast between the austenite phase and the other phases in the multiphase steel. - -

[0038] Information about compositional contrast is carried dominantly by the backscattered electrons (BSE), but the material contrast can be also found in the secondary electrons (SE) mode. As reported in Image Formation in Low-voltage SEM (Ludwig Reimer, SPIE Press, 1993), the SE are more sensitive to the material contrast at low landing beam energies (less than 1 keV). Thus, both types of the signal electrons (i.e., BSE and SE) can be used for the visualization of an austenite phase, and the landing energy plays a critical role for providing the contrast in the SEM micrographs. In principle, the higher contrast will facilitate the identification of the austenite phase in the SEM micrographs. Here, a contrast ratio C (%) is defined as

C = (S_F - S_y) / (S_F + S_y) * 100

where S_y is average luminance of the corrosion product layer, and Sp is average luminance of the remaining surface in the SEM micrograph. The contrast ratio is preferably at least 1.00%, more preferably at least 1.25%, and most preferably at least 1.50%.

[0039] The above-described preparation technique ensures that the austenite phase is covered by the corrosion product layer formed on the native oxide layer. Thus, the landing energy should be so low that the dominant part of the interaction volume is situated within the native oxide layer. To this end, the landing energy of SEM-SE images is preferably between 0.2 keV and 4.0 keV, more preferably between 0.25 keV and 2.0 keV, and most preferably between 0.6 keV and 1.3 keV. The landing energy of SEM-BSE images is preferably between 0.25 keV and 2.0 keV, more preferably between 0.3 keV and 1.7 keV, and most preferably between 0.4 keV and 1.5 keV.

[0040] The multiphase steel may be any carbon steel having an austenite phase (also referred to as a "γ phase") therein, such as Transformation Induced Plasticity (TRIP) steel. However, the method is not applicable to stainless steel because stainless steel has a passive layer on its surface which prevents the formation of the corrosion product layer. Other phases present in the multiphase steel may be a martensite phase, bainite phase, ferrite phase, and the like.

[0041] (Step d: Characterization process)

Optionally, the method may further include the step of evaluating quantitative and/or qualitative characterization of the visualized austenite - - phase. For example, the micrographs acquired by the step c may be used as input date for quantitative image analysis. Also, the micrographs acquired by the step c may be transformed to binary image and statistical data for further characterization of the austenite phase.

[0042] The austenite phase appears as the darkest area in the SEM micrographs taken by both BSE and SE micrographs. The SEM micrographs created by the SE is preferable for image analysis due to more uniform background resulting from significant suppression of the channeling contrast between differently oriented ferrite grains.

[0043] Two types of the SE detectors, an out-lens detector and an in-lens detector, are widely used in the art. The former is also known as an Everhard-Thorney detector. Although these detectors detect different types of SE, the both detectors can provide contrasts. Because the in-lens detector is more sensitive to a hydro-carbon contamination, the SE detector used in this method is selected according to a state of the surface of the specimen.

[0044] In this way, the austenite phase can be visualized as the darkest area by both SE and BSE signal electrons, and both detectors can be used in the SEM microscope according an actual state of the surface and a type of the microscope. The SEM technique of this embodiment has a very high spatial resolution with a low landing energy and can map a large surface area on the specimen in a short time. This enables to obtain statistically significant data and to better understand effects of the size, shape and morphology of the austenite phase on the final mechanical properties of multiphase steels such as TRIP steels.

Examples

[0045] The following describes examples of the present disclosure. A TRIP steel (0.21 C, 1.5Si, 2.1Mn, 0.01P, 0.001 S) was provided for a multiphase steel to be observed. A specimen of the multiphase steel was prepared according to the preparation technique described above. The electrolyte solution used for the examples was prepared by mixing 300ml of CH₃OH, 180 ml of 2-n-butoxyethanol (98%), and 30 ml of HC10₄ (60%). The electro-polishing process was conducted at the temperature of 298K at a voltage of 40 V for 2 seconds. After the electro-polishing process, the bias was switched off, and - - the multiphase steel base was continuously retained in the electrolyte solution for another 3 seconds. A mean thickness of the obtained corrosion product layer of each example was 2 nm. The micrographs were collected using two types of SEM. The SE micrographs were obtained in the SEM LEO 1530 (Carl Zeiss), which was equipped with an in-lens and out-lens type of SE detector. The BSE images were obtained by the dual-beam Helios Nano-lab 600i (FEI) using the CBS detector located bellow the pole piece. By applying a negative bias on the surface of the specimen, it was possible to obtain a high signal at a low landing energy.

[0046] Example 1

An SEM image was obtained at 1 keV landing energy of the primary electrons using the in-lens type of the SE detector. Fig. 1 (a) shows the SEM image thus obtained. The austenite phase in the SEM micrograph appears the darkest area (marked by the arrows) in Fig. 1 (a). For the purpose of comparison, an EBSD phase map was obtained from the identical area. The EBSD phase map is shown in Fig. 1 (b). EBSD technique is a conventional technique for phase identification in multiphase steels and enables to separate the austenite phase based on their specific crystal lattice (FCC-type). In Fig. 1 (b), the austenite phase is shown in green, and other phases having BCC-type of a crystal lattice (i.e. ferrite, martensite) are in red. It is apparent from Figs. 1 (a) and (b) that the position of the austenite phase in the EBSD phase map is well corresponding with the position of the austenite phase in the SEM micrographs.

[0047] Example 2

A specimen prepared according to the invention is covered by a non-uniform oxide layer, which is specific for the phases. The difference in layer thicknesses is a source of the contrast between the austenite phase and other phases in the SEM micrographs acquired at low landing energies. Figs. 2 (a) and (b) show the cross sectional view of the oxide layer covering the ferrite phase and the austenite phase, respectively, obtained by scanning transmission electron microscopy (STEM). Figs 2 (a) and (b) clearly demonstrate presence of an extraordinary thick porous oxide layer on the austenite phase.

[0048] Presence of the austenite phase contrast in the SEM images is - - inseparably connected with the specimen preparation technique according to the disclosure. Fig. 3(a) shows an SEM micrograph of the specimen prepared by a conventional electro-polishing technique, and Fig. 3(b) shows an SEM micrograph of the specimen prepared by the above-described technique. Both micrographs were collected under identical microscope settings. Fig. 3 (a) has less contrast and therefore cannot differentiate the austenite phase from other phases whereas Fig. 3(b) has larger contrast ration and thus easily identifies the austenite phase.

[0049] Example 3

The contrast present in SEM images can be adjusted by an operational condition of an instrument, particularly by the landing energy. As demonstrated in Example 2, the difference in the layer thicknesses dominantly contributes to the contrast between the phases. Operation of the SEM at low landing energies of the primary electrons secures suitable surface sensitivity and possibility to make the austenite phase visible even with a small difference in oxide layer thickness. Landing energy of 1 keV and lower are suitable for the austenite phase differentiation. Figs. 4(a) and (b) show the SEM SE images of the same area on the TRIP steel specimen surface (prepared according to the present disclosure) obtained at 5 keV and 1 keV, respectively, landing energy of the primary electrons using the in-lens SE detector. The contrast is not visible at 5 keV landing energy due to insufficient surface sensitivity. To verify the presence of the austenite phase, the identical specimen is observed with the EBSD technique. The obtained EBSD phase map is shown in Fig. 4(c) where the austenite phase is shown in green and other phases in red.

[0050] Example 4

Typical SEMs are equipped with two detectors, in-lens detector and out-lens detector, for SE detection. In this example, the identical specimen was observed by both detectors with a landing energy of the primary electrons of 1 keV. The results are shown in Figs. 5(a) and (b). The micrographs obtained by out-lens SE detector (Fig. 5(b)) is more burdened by the topographical contrast, which can be slightly disturbing. The standard detector equipment consists of SE detectors and a detector for BSE. The BSE is also possible to use for visualization of austenite phase, as shown in - -

Fig. 6.

[0051] Example 5

Area fraction of the austenite phase was calculated from the total area of 9655 μπι². 31 pieces of the SEM micrographs acquired by the in-lens SE detector with 5kx magnification were used as input data for image analysis. X-Ray diffraction, which is a routinely used technique of retained austenite phase measurement, was also used to calculate the area fraction of the austenite phase. The results are shown in Fig. 7. As can be seen in Fig. 7, the data obtained by the present disclosure is in a good agreement with data obtained by the X-Ray diffraction. Therefore, the contrast between the austenite and other phases in TRIP steel in the micrographs obtained according to the present disclosure is highly suited to quantitative image analysis.

[0052] Example 6

The SEM micrographs acquired according to the present disclosure are suited to qualitative image analysis without extensive image post processing. The micrographs are easily transformed to binary image and statistical data about the austenite phase size and shape can be obtained. For example, the micrographs can be transformed to the retained austenite phase size and shape distribution as shown in Fig. 8 in a form of histograms.

[0053] Example 7

A specimens of the multiphase steel was prepared according to the procedure as described in Example 1 , and SEM BSE images were obtained with using different magnitudes of landing energy of the primary electrons. Figs. 9(a) to 9(g) are SEM BSE images at landing energies of 0.25 keV, 0.4 keV, 1 keV, 1.5 keV, 2 keV, 5 keV and 25 keV, respectively. As can be seen in these images, the contrast between the austenite phase and the other phases is conditioned by the magnitude of the landing energy. It is caused by a necessity of information depth location mainly within the top layers of the specimen. The contrast is not visible at high landing energies due to dominancy of the specimen bulk (i.e. steel). In this example, present of the austenite phase was best visible at landing energy of 1 keV.

[0054] Example 8

A specimens of the multiphase steel was prepared according to the - - procedure as described in Example 1 , and SEM SE images were obtained with using different magnitudes of landing energy of the primary electrons. The contrast ratio was calculated for each image. The result is shown in Fig. 10. As can be seen in the figure, the contrast drops suddenly at excessive low and high landing energies.

[0055] Example 9

The electrolyte solution was prepared by mixing 10 volume % of acetyl acetone, 1 wt% of tetra-methyl-ammonium chloride, and methanol as solvent. The electro-polishing process was conducted at room temperature at a voltage of 100 mV and electric charge of 3 C for 1.5 minutes. After completion of the electro-polishing process, the specimen continuously retaining in the electrolyte solution for 2 seconds. Then, the specimen surface was observed in the same manner as described above. The obtained SEM SE image using an in-lens type of the SE detector is shown in Fig. 11. The darkest parts in the image represent the austenite phase as in the case of the previous examples. The contrast between the austenite and other phases is visible in the image. Thus, it is proved that, in addition to corrosive acid, organic acids can be used as the electrolyte solution to selectively corrode the austenite phase.

Industrial Applicability

[0056] According to the present disclosure, it is possible to identify a austenite phase in a multiphase steel at the nanoscale.

Claims

[Claim 1] A method of visualizing an austenite phase in a multiphase steel comprising the steps of:

a) electro-polishing a surface of a specimen of a multiphase steel in an electrolyte solution containing a corrosive acid and an organic solvent to form a native oxide layer on the surface of the specimen,

[Claim 2] The method according to claim 1 , wherein a mean thickness of the corrosion product layer is between 2 nm and 10 nm.

[Claim 3] The method according to claim 1 or 2, wherein the step c) is performed with a landing energy of the primary electrons between 0.2 keV and 4.0 keV to observe a scanning electron microscopy micrograph created by secondary electrons.

[Claim 4] The method according to claim 1 or 2, wherein the step c) is performed with a landing energy of the primary electrons between 0.25 keV and 2.0 keV to observe a scanning electron microscopy micrograph created by back-scattered electrons.

[Claim 5] The method according to any one of claims 1 to 4, wherein the corrosive acid is selected from a group consisting of an oxidizing acid and an organic acid.

[Claim 6] The method according to any one of claims 1 to 5, further comprising: d) evaluating quantitative and/or qualitative characterization of the visualized austenite phase.

[Claim 7] A multiphase steel specimen for microstructure observation comprising:

a native oxide layer formed on a surface of the multiphase steel base, a corrosion product layer formed selectively only on a domain of the native oxide layer covering the austenite phase.

[Claim 8] The multiphase steel specimen for microstructure observation according to claim 7, wherein a mean thickness of the corrosion product layer is between 2 nm and 10 nm.