JP6365771B1 - Separation visualization method of austenite phase, martensite phase and bainite-ferrite matrix in bainite steel and bainite steel slab for microstructure observation - Google Patents

Abstract

Disclosed is a method for separating and visualizing an austenitic phase, a martensite phase, and a bainite-ferrite matrix in a bainite steel, wherein the austenite phase and the martensite phase in the bainite steel can be distinguished on a nanoscale. According to the method of the present invention, a) a surface of a bainite steel piece is electropolished in an electrolyte solution containing a corrosive acid and an organic solvent, and a natural oxide layer is formed on the surface of the bainite steel piece. And b) after completion of the electropolishing, the bainitic steel pieces are continuously immersed in the electrolyte solution, respectively, on the regions covering the martensite phase and austenite phase of the natural oxide layer. Selectively forming first and second corrosion product layers; and c) subjecting the surface of the bainite steel piece to a scanning electron microscope observation with a backscattered electron detector to provide the first and second corrosion products. By obtaining the contrast between the product layer and the remaining surface, the austenite phase is the darkest region, the bainite-ferrite matrix is the brightest region, and the martensite phase is It includes a step of visualizing, as the brightness of the region between the c) step is performed the incident energy of the primary electrons as follows 3 keV. [Selection] Figure 1

Description

  The present invention generally relates to a method for separating and visualizing an austenite phase, a martensite phase, and a bainite-ferrite matrix in bainite steel, and a bainite steel slab for structure observation.

  Accurate and accurate knowledge of bainite steel structure is essential for the advancement of bainite steel development. The mechanical properties of bainite steel are greatly influenced by carbon-rich residual island martensite (MA). Characterization of nanobainite steel is a particularly difficult technique. Nanobainite steel contains a very fine second phase that is heterogeneously dispersed within a bainite-ferrite matrix, such as a bainite matrix. Detailed knowledge of the fraction, size, and morphology of island martensite is extremely important in understanding the mechanisms that affect the properties of bainite steel.

  There are several conventional methods that allow the characterization of island martensite in bainite steel, but these methods have the following dilemmas. In other words, it is a dilemma whether to prioritize the high spatial resolution required for visualization of the nanophase or to support the wide area inspection required when the fraction of inhomogeneously dispersed island martensite is low . There has been no technology that can satisfy these requirements at the same time.

Non-Patent Document 1 describes a conventional technique and the latest technique for optical microscopy. The purpose of the extensive research described here is to establish a methodology for quantitative and qualitative determination of each phase in duplex steels. Various types for optical microscopy (OM) such as conventional etching techniques (ie simple electrolytic etching, nital etching, picral etching, repeller etching, and Krem etching) and the latest two-step etching method Various etching methods are described. The above-mentioned techniques are all applied to TRIP (Transformation Induced Plasticity) steel, and the following conclusion can be made.
-Conventional electropolishing, nital etching, and picral etching do not provide a contrast between the second phase and the matrix in the OM.
-In the conventional method using the repeller etchant and the Krem etchant, the second phase and the matrix can be distinguished in the OM based on their specific colors, but the separation of austenite and martensite is Impossible. (However, Non-Patent Document 2 describes that only the Krem etching solution can be used for visualization of retained austenite in OM.)
-The two-phase etching technique with two reagents (ie picral + sodium bisulfate, nital + sodium bisulfate, and V2A + Krem) can distinguish the second phase from the matrix, as well as the OM image There is a possibility that the austenite phase can be distinguished from martensite due to the difference in the colors inside.

  Non-Patent Document 1 also describes various experiments in which a color etching technique and a scanning electron microscope (SEM) technique are combined. Since SEM technology exhibits high spatial resolution, it is possible to visualize nano-scale features. Unfortunately, the experimental results of Non-Patent Document 1 show that martensite cannot be separated from austenite.

  The above technique uses an optical microscope, but has a limited spatial resolution (about 200 nm). These techniques can not only visualize each phase in island martensite, but can also separate them, but they are not sufficient for the characterization of modern bainite steel containing nano-sized island martensite. .

  Patent Document 1 describes a color etching technique for visualizing island martensite in a laser confocal microscope with improved spatial resolution. Unfortunately, this technique cannot separate the martensite and austenite phases in island martensite. Although the technique described in Patent Document 1 enables visualization of island martensite by increasing the spatial resolution as compared with the technique using an optical microscope, it is still insufficient for nanoscale characterization. is there.

  The next candidate that may allow visualization of island martensite in bainite steel is the use of electron backscatter diffraction technology (EBSD) in SEM. In this technique, the austenite phase can be identified by the difference in crystal lattice (face-centered cubic (FCC) type crystal lattice). The martensite phase can be separated because the diffraction pattern has a very low quality. The EBSD technology visualizes island martensite in bainite steel with high spatial resolution (about 20 nm) and makes it possible to separate the martensite and austenite phases.

Chinese Patent Publication No. 102175191

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

  There are two main drawbacks to EBSD technology. First, the spatial resolution of this technology is about 20 nm, but nano-sized island martensite characters such as the characterization of foil austenite between the ferrite lath and the fine austenite phase inside the island martensite. It is not enough for tectarization. Second, this technique is not optimal for wide area mapping because data collection is very slow in nanoscale mapping.

  Although transmission electron microscopy exhibits higher spatial resolution, it requires careful preparation of the sample and does not support wide-area mapping, so that representative and statistically important data cannot be obtained.

  Therefore, separation visualization of island martensite including austenite and martensite phases in bainite steel that enables high-speed data collection that provides high spatial resolution (less than 10 nm) and completes wide-area mapping at an appropriate time. Until now, there was no technology to achieve this.

  Therefore, the object of the present invention is to provide a method for separating and visualizing the austenite phase and martensite phase in bainite steel, which can clearly and visually distinguish the austenite phase and martensite phase in bainite steel, respectively. It is to be. Another object of the present invention is to provide a bainite steel slab for structure observation that can clearly and visually distinguish the austenite phase and the martensite phase in the bainite steel on a nanoscale.

  When the present inventors diligently studied, after electrolytic polishing of the surface of the bainite steel piece in a specific electrolyte solution, when the bainite steel piece was subsequently immersed in the electrolyte solution, the martensite phase on the surface of the bainite steel piece and It was found that a corrosion product layer is selectively formed on the austenite phase, and that the thickness of the corrosion product layer depends on the carbon concentration of the corrosion phase. Therefore, if the surface of the bainite steel slab thus treated is observed with a scanning electron microscope (SEM) and a backscattered electron (BSE) detector, the surface of the bainite slab is selectively on the martensite phase and austenite phase. Since the contrast in the SEM-BSE image is obtained due to the difference in the thickness of the corrosion product layer formed on the surface, it was found that the austenite phase and the martensite phase can be visually observed individually.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
(1) A method for separating and visualizing austenite phase, martensite phase, and bainite-ferrite matrix in bainite steel,
a) electropolishing the surface of a bainite steel slab in an electrolyte solution containing a corrosive acid and an organic solvent to form a natural oxide layer on the surface of the bainite steel slab;
b) After completion of the electropolishing, the bainite steel pieces are continuously immersed in the electrolyte solution, and the first and second regions are respectively formed on regions of the natural oxide layer covering the martensite phase and austenite phase. Selectively forming a corrosion product layer of
c) The surface of the bainite steel slab is subjected to scanning electron microscope observation with a backscattered electron detector to obtain a contrast between the first and second corrosion product layers and the remaining surface, whereby the austenite phase The darkest area, the bainite-ferrite matrix as the brightest area, and the martensite phase as a medium brightness area,
Have
The step c) is performed by setting the incident energy of primary electrons to 3 keV or less.

  (2) The method according to (1), wherein the backscattered electron detector is an in-lens detector.

  (3) The said backscattered electron detector is a method as described in said (1) which consists of the concentric segment which is located under the pole piece and can operate | move separately.

  (4) The method according to (1), wherein the backscattered electron detector is located below the pole piece and has a working distance of more than 10 mm.

  (5) The method according to any one of (1) to (4), wherein an incident energy of the primary electrons is 1 keV or less.

  (6) The method according to any one of (1) to (5), wherein an incident energy of the primary electrons is 0.5 keV or more.

  (7) The method according to any one of (1) to (6), wherein the average thickness of the first corrosion product layer is 1 nm or more and 9 nm or less.

  (8) The method according to any one of (1) to (7), wherein an average thickness of the second corrosion product layer is 2 nm or more and 10 nm or less.

  (9) The method according to any one of (1) to (8), wherein the corrosive acid is selected from the group consisting of an oxidizing acid and an organic acid.

  (10) d) The method according to any one of (1) to (9), further including a step of performing quantitative and / or qualitative characterization of the visualized austenite phase and the martensite phase. Method.

(11) a bainite steel base having a plurality of phases including an austenite phase, a martensite phase, and a bainite-ferrite matrix;
A native oxide layer formed on the surface of the bainite steel base;
A first corrosion product layer formed on a region of the natural oxide layer covering the martensite phase;
A second corrosion product layer formed on a region of the natural oxide layer covering the martensite and austenite phase and having a thickness greater than the thickness of the first corrosion product layer;
A bainite steel slab for structure observation characterized by comprising:

  (12) The bainite steel slab for structure observation according to the above (11), wherein the average thickness of the first corrosion product layer is 1 nm or more and 9 nm or less.

  (13) The bainite steel slab for structure observation according to (11) or (12) above, wherein an average thickness of the second corrosion product layer is 2 nm or more and 10 nm or less.

  According to the present invention, the austenite phase, martensite phase in bainite steel, and the bainite steel slab for microstructure observation of the austenite phase, martensite phase, and bainite-ferrite matrix, respectively, the austenite phase and martensite phase in bainite steel are clearly clarified at the nanoscale. And can be visually identified.

(A) is an SEM of bainite steel containing island martensite produced by conventional electropolishing, taken by a BSE detector located below the pole piece (lens pole piece) with an incident energy of primary electrons of 1 keV. It is a BSE image, and (b) is a bainite steel containing island martensite produced according to the present invention, which is captured by a BSE detector located below a pole piece (lens pole piece) with an incident energy of primary electrons of 1 keV. It is a SEM-BSE image of. (A)-(c) are the SEM-BSE images of the austenite phase in the bainite-ferrite matrix obtained with the incident energy of primary electrons being 1 keV, 3 keV, and 5 keV, respectively. (A) to (c) are SEM images obtained by an SE in-lens detector, an SE out-lens Everhart-Thornley detector, and a BSE detector, respectively, with the incident energy of primary electrons being 1 keV. (A) is an electron backscatter diffraction (EBSD) phase map showing the austenite phase in green and the martensite phase and bainite-ferrite matrix in red, and (b) showing the martensite phase in black. (C) is an SEM-BSE image of the same region obtained by a BSE detector with the incident energy of primary electrons being 1 keV. (A) is an EBSD image quality map, (b) is a phase map, and (c) is a BSE image of the same region according to the present invention. It is a figure which shows the area fraction of the austenite phase and martensite phase obtained by the method according to this invention, and the volume fraction of the austenite phase obtained by the XRD technique. It is a figure which shows the influence of the annealing temperature with respect to the size, shape, and morphology of an austenite phase and a martensite phase. It is a SEM-BSE image obtained with a BSE in-lens detector with the incident energy of primary electrons being 1 keV.

  According to an embodiment of the present invention, a method for separating and visualizing an austenite phase, a martensite phase, and a bainite-ferrite matrix in a bainite steel is performed by performing a specific treatment on the surface of the bainite steel base (step a and step b). ), Giving a sample to be observed, and then subjecting the surface of the sample to SEM observation with a backscattered electron detector to separate and visualize the austenite phase, martensite phase, and bainite-ferrite matrix under specific conditions (process) c). Performing quantitative and / or qualitative characterization of the visualized austenite and martensite phases (step d) may be included.

  A bainite steel slab is produced as follows. First, a bainite steel slab is prepared. Then, the surface of the bainite steel piece is mechanically polished and mirror finished. Any conventional abrasive can be used for the mechanical polishing. Preferably, diamond particles having a particle size of about 1 μm are used. Thus, a bainite steel base having a polished surface is obtained.

(Process a: Electropolishing process)
The bainite steel base is then subjected to electropolishing in an electrolyte solution. Preferably, any conventionally known electropolishing process can be used here. The electrolyte solution basically consists of a corrosive acid and an organic solvent. Here, the term “corrosive acid” means any acid capable of corroding the surface of the bainite steel base, preferably containing chlorine atoms. The corrosive acid can be an oxidizing acid. Here, the term “oxidizing acid” means an acid containing an anion having an oxidation potential higher than that of H ⁺ ions. The oxidizing acid is preferably at least one selected from the group consisting of perchloric acid, sulfuric acid, and phosphoric acid, but is not limited thereto. The corrosive acid oxidizes and dissolves the surface of the bainite steel base to form a native oxide layer having a substantially uniform thickness over the entire surface of the bainite steel base. The concentration of the corrosive acid in the electrolyte solution is preferably 0.30 mol / L or more, 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, the reaction rate of the bainite steel base and the acid is low, so the polishing time increases significantly, and the surface of the bainite steel base is not sufficiently corroded, resulting in the bainite steel. The surface flatness of the base may be deteriorated. The upper limit of the concentration of the corrosive acid depends on the type of the corrosive acid used. For example, in the case of perchloric acid, the concentration in the electrolyte solution is preferably 0.90 mol / L or less, and more preferably 0.75 mol / L or less. Since perchloric acid is unstable and can explode at room temperature, an electrolyte solution containing perchloric acid at a concentration higher than 0.90 mol / L is difficult to handle. Further, the surface of the bainite steel base is over-etched, and the flatness of the surface is impaired. If the surface of the bainite steel base is under-etched or over-etched, no significant contrast is obtained in subsequent processes.

  Organic solvents are used to adjust the viscosity of the electrolyte solution and can affect the thickness of the native oxide layer. Examples of the organic solvent include lower alcohols having 1 to 10 carbon atoms such as methanol, glycerol and butyl glycol, ethers such as 2-n-butoxyethanol and acetic acid, and mixtures thereof. It is not limited to. The concentration of the organic solvent in the electrolyte solution can be determined based on the type and surface flatness of the bainite steel base to be etched. If the viscosity is too low, the electrolyte solution does not remain sufficiently on the surface of the bainite steel base, and the surface is not sufficiently polished.

  The temperature of the electrolyte solution affects the corrosion reaction rate. The lower the temperature, the longer the process time, which is not practically preferable. Higher temperatures accelerate corrosion, but can cause overetching. Further, the corrosive acid becomes unstable as the temperature increases. From these viewpoints, the temperature of the electrolyte solution is preferably 283 to 308K, more preferably 293 to 298K.

  The voltage applied to the electrolyte solution and the holding time of the electropolishing process also affect the formation of the native oxide layer. The voltage is preferably 15 to 45V, more preferably 35 to 40V. If the voltage is less than 15V, the surface will be etched rather than polished. On the other hand, when the voltage exceeds 45V, the current density increases and the surface is destroyed. The holding time can be determined according to process conditions such as temperature, voltage, and concentration of corrosive acid. A typical holding time is 2 seconds, but is not limited thereto.

  The thickness of the natural oxide layer is preferably 2 to 4 nm. The thickness of the natural oxide layer can be measured from a cross-sectional STEM (HAADF) image.

(Process b: Holding process)
After completion of the electropolishing process, the bias is switched off and the bainite steel base is subsequently held in the electrolyte solution for a predetermined period of time. Thereafter, the bainite steel base is removed from the electrolyte solution, washed in methanol, and then washed with ethanol. The surface of the bainite steel base is corroded and becomes dull. Although the surface of the bainite steel base can be coated using masks of various dimensions, a small mask having an electropolishing surface area of 0.1 to 0.3 cm ² can be preferably used. The electrolyte solution may be stirred using an ultrasonic cleaner.

  The holding time of this holding process depends on the composition of the bainite steel, the concentration of the corrosive acid in the electrolyte solution, the process temperature, and the like. In some cases, the retention time can be as short as 1 second, but is typically 2-10 seconds.

  During this holding process, first and second corrosion product layers are selectively formed on the martensite phase and austenite phase on the surface of the bainite steel slab, respectively. The martensite and austenite phases are both carbon rich (ie, the carbon concentration is higher than the rest of the surface), which is believed to promote additional corrosion during the holding process. The corrosion product layer is porous and therefore thicker than the native oxide layer. In principle, the thickness of the corrosion product layer depends on the carbon concentration of the corrosion phase. In the bainite steel, the most corroded is the austenite phase having the highest carbon concentration, and the thickness of the corrosion product layer formed on the austenite phase is the largest. The martensite phase has a lower carbon concentration than the austenite phase, the corrosion is moderate, and the thickness of the corrosion product layer formed on the phase is also thin. Because the bainite-ferrite matrix has the lowest carbon concentration and high corrosion resistance in the electrolyte solution, no corrosion product layer is formed on the bainite-ferrite matrix.

  The thickness of the corrosion product layer greatly affects the energy of backscattered electrons, and also affects the contrast in the SEM image described later. The contrast in the SEM image increases as the corrosion product layer becomes thicker. Therefore, the thicker the corrosion product layer, the better. In order to ensure a clear contrast in the SEM image, the average thickness of the first and second corrosion product layers is preferably 1 nm or more. However, if the corrosion product layer becomes excessively thick, for example, if the average thickness exceeds 10 nm, surface deterioration becomes remarkable, and it may be difficult to obtain an SEM image having a reasonable image quality. For this reason, the upper limit of the average thickness of the first and second corrosion product layers is preferably 10 nm, respectively. Further, in order to clearly distinguish the first corrosion product layer from the second corrosion product layer, the difference in thickness between the first corrosion product layer and the second corrosion product layer is at least 1 nm. is there. In view of the above, the average thickness of the first corrosion product layer is preferably 1 nm or more and 9 nm or less, and the average thickness of the second corrosion product layer is preferably 2 nm or more and 10 nm or less. The average thickness of the corrosion product layer can be measured from a cross-sectional STEM (HAADF) image. There is a difference in internal structure between the native oxide layer and the corrosion product layer. The natural oxide layer is dense, whereas the corrosion product layer has a porous structure. These layers also have different chemical compositions. That is, the natural oxide layer is basically composed of iron oxide, whereas the corrosion product layer is a mixture of oxide and hydroxide. The interface between the two layers in the STEM (HAADF) image is determined by the difference in signal electron yield from both layers.

(Process c: Visualization process)
In this way, a bainite steel slab having a natural oxide layer and a corrosion product layer is produced. Thereafter, the bainite steel slab is subjected to scanning electron microscope (SEM) observation. When the surface of the bainite steel slab is irradiated with an electron beam, the energy of backscattered electrons passing through the thicker corrosion product layer is lowered. Thus, in the SEM image, the austenite phase with a relatively thick corrosion product layer is shown as the darkest region, and the bainite-ferrite matrix without the corrosion product layer is shown as the brightest region, and the corrosion product layer A relatively thin martensite phase is shown as an intermediate brightness region. Also, backscattered electrons are very susceptible to the chemical composition of the sample. The result is a distinct contrast between the austenite phase and the martensite phase, and between the martensite phase and the rest of the surface, and the austenite and martensite phases in bainite steel are individually separated at the nanoscale. It becomes possible to visualize.

  Suitable operating conditions to ensure adequate contrast for quantitative and qualitative image analysis without extensive post-processing of the image detects signal electrons that provide information about the composition of the sample surface This can be achieved. It is also advantageous to operate the SEM with low incident energy that can ensure sufficient surface sensitivity. By combining these operating conditions, a clear contrast between the austenite phase and the other phases in the bainite steel can be obtained.

In principle, the greater the contrast in the SEM image, the easier the austenite phase is identified. Here, the contrast ratio C (%) is defined as follows.
C = (S _F -Sγ) / (S _F + Sγ) × 100
Here, S [gamma] is the average luminance of the corrosion product layer in the SEM image, S _F is the average brightness of the remaining surface. The contrast ratio is preferably 1.00% or more, more preferably 1.25% or more, and most preferably 1.50% or more.

  According to the sample preparation technique described above, the austenite phase and the martensite phase are coated on the corrosion product layer formed on the natural oxide layer. Therefore, the incident energy should be low so that the bulk of the interaction volume between the primary electrons and the sample is inside the native oxide layer. For this purpose, the incident energy of primary electrons is 3 keV or less, preferably 1 keV or less. The penetration depth of primary electrons in a solid depends on energy. When the incident energy exceeds 3 keV, the bulk of the sample becomes the main source of signal electrons, and surface information (ie, the presence of a corrosion product layer) is lost, thus degrading the contrast in the SEM image. On the other hand, if the incident energy is too small, a sufficient penetration depth of primary electrons in the solid may not be obtained. For this reason, it is preferable that the incident energy of primary electrons is 0.5 keV or more.

  Bainitic steels can be any carbon steel having a nanoscale fine second phase including an austenitic phase (also called a γ phase), a martensitic phase, and a bainite-ferrite matrix, such as transformation induced plasticity (TRIP) steel. can do. However, this method is not applicable to stainless steel. This is because stainless steel has a passive layer on its surface, which inhibits the formation of a corrosion product layer. Examples of other phases contained in the bainite steel include a ferrite phase, a pearlite phase, and a carbide phase.

  Various types of BSE detectors are known in the art and can be used in the present invention. In particular, BSE detectors applicable to the present invention are in-lens detectors and out-lens detectors. An in-lens detector can perform imaging with a very small working distance, and generally provides a high interphase contrast. In-lens detectors were originally developed for the collection of low-angle BSE and can only be seen inside modern equipment. Here, the term “low angle BSE” means backscattered electrons scattered in a direction of about 40 ° or less with respect to the incident direction of the primary electrons.

  Out-lens detectors, also known as Everhard-Thorney detectors, are standard detectors in many SEMs. The out-lens detector is typically installed below the pole piece, and its efficiency is strongly influenced by the working distance. Generally, the longer the working distance, the lower the image quality (that is, the image noise increases). There are several types of detectors. One is a state-of-the-art detector with individually operable concentric segments. State-of-the-art detectors can collect low angle BSE even at relatively short working distances, and can prevent the occurrence of parasitic long angle BSE by switching off the outer segment. Therefore, the detection efficiency is high and the image quality is very good. This detector can also operate as a simple conventional type (ie, all segments can be active) and can detect low angle BSE with a long working distance. However, the image quality may decrease as the working distance increases. Another type is a conventional compact detector with no segments. This type can detect low angle BSE when using a long working distance of more than 10 mm. Thereby, the detection of the high angle BSE is significantly suppressed. Although the interphase contrast can be obtained, the detector efficiency and image quality are lower than those of the in-lens detector and the latest out-lens detector.

(Process d: Characterization process)
The method can optionally further comprise the step of quantitative and / or qualitative characterization of the visualized austenite and martensite phases. For example, the microscope image obtained in step c can be used as input data for quantitative image analysis. Further, the microscopic image obtained in step c may be converted into a binary image or statistical data and used for further characterization of the austenite phase.

  In this way, the austenite phase is visualized as the darkest region and the martensite phase is visualized as an intermediate brightness region. The SEM technique of the present embodiment has a very high spatial resolution with low incident energy, and can map a large surface area on the sample in a short time. Thereby, statistically important data can be obtained, and the influence of the austenite phase size, shape, and morphology on the final mechanical properties of bainite steel such as TRIP steel can be better understood.

Examples of the present invention are shown below. As bainite steel for observation, steel (C: 0.12, Si: 0.02, Mn: 1.5 [mass%]) containing low fractional island martensite (MA constituents) was prepared. A bainite steel slab was prepared according to the above-described manufacturing technique. The electrolyte solution used in each example was prepared by mixing 300 ml CH ₃ OH, 180 ml 2-n-butoxyethanol (98%), and 30 ml HClO ₄ (60%). The electrolytic polishing process was performed under the conditions of a temperature of 298 K, a voltage of 40 V, and 2 seconds. After the electropolishing process, the bias was switched off and the bainite steel base was subsequently held in the electrolyte solution for an additional 2-3 seconds. Thereafter, the sample was removed from the electrolyte and washed in methanol and then in ethanol. The average thickness of the corrosion product layer formed on the austenite phase in each example was 2 nm, and the average thickness of the corrosion product layer formed on the martensite phase in each example was 1 nm. Microscopic images were collected using two types of SEM. The BSE image was obtained using a dual-beam Helios Nano-lab 600i (manufactured by FEI) using a concentric backscatter (CBS) detector located below the pole piece with a working distance of 7 mm. The incident energy of primary electrons was 1 keV. By applying a negative bias to the sample surface, a strong signal was obtained at low incident energy. SE microscopic images were obtained using SEM LEO 1530 (manufactured by Carl Zeiss) equipped with in-lens and out-lens SE detectors. The working distance between the sample surface and the pole piece was 5 mm, and the incident energy of primary electrons was 1 keV.

Example 1
Bainitic steel samples were prepared using conventional electropolishing techniques and techniques according to the present invention. SEM images of both samples were obtained with the same parameter, that is, the incident energy of primary electrons of 1 keV, in order to realize the visualization of the interphase contrast according to the present invention. An SEM image of a bainite steel sample produced by a conventional electrolytic polishing technique is shown in FIG. An SEM image of a bainite steel sample produced by the technique according to the present invention is shown in FIG. As shown in FIG. 1 (a), the sample produced by the conventional electropolishing technique has a strong crystal contrast and the phases cannot be distinguished. On the other hand, as shown in FIG. 1B, in the SEM image of the sample produced by the technique according to the present invention, the austenite phase and the martensite phase can be separated and visualized very clearly.

  2A to 2C show the influence of incident energy on the contrast of the SEM image. In FIG. 2B, the crystal contrast is greatly suppressed because the thicker corrosion product layer is produced as a result of the additional etching in the electrolyte solution as described above.

(Example 2)
To evaluate the effect of incident energy magnitude on the contrast between the austenite phase and the bainite-ferrite matrix, SEM images of the same sample were obtained with varying incident energy magnitudes. The obtained SEM images are shown in FIGS. As the incident energy increases, the contrast between the austenite phase and the bainite-ferrite matrix becomes unclear, and the austenite phase cannot be visually separated from the bainite-ferrite matrix (FIG. 2 (c)).

(Example 3)
SEM images of the same sample were obtained with different detectors. 3 (a)-(c) shows the austenite phase and bainite-ferrite matrix in SEM images obtained by SE detectors (FIGS. 3 (a) and (b)) and BSE detectors (FIG. 3 (c)). The contrast between is shown. 3 (a) to 3 (c), it can be seen that the BSE detector is most suitable for visualization of island martensite in the bainite-ferrite matrix. The SE electron ET detector also shows some contrast, but the contrast is weak and not all island martensite can be visualized. With the in-lens SE detector, a clear contrast could not be obtained.

Example 4
Verification of the technique proposed in the present specification was performed by comparison with a technique conventionally used for phase identification in a duplex steel, that is, EBSD. Using EBSD technology, the austenite phase can be separated from martensite and bainite (ferrite) by a unique crystal lattice. The martensite phase is separated from austenite and bainite (ferrite) by an “image quality” map obtained by EBSD (the martensite phase has the lowest image quality parameter because the dislocation density is orders of magnitude higher). As described above, the EBSD technique can separate all phases, but as described above, it is not optimal for wide area mapping and has insufficient spatial resolution. 4 (a) to 4 (c) show the comparison results between EBSD and the technique according to the present invention. It can be seen that the austenitic phase, martensitic phase, and bainite-ferrite matrix can be distinguished by observing a bainite steel surface made according to the present invention using a SEM instrument operated according to the present invention.

  5A is an EBSD image quality map, FIG. 5B is a phase map, and FIG. 5C is a BSE image of the same region according to the present invention. From these, the method according to the present invention provides higher spatial resolution and surface sensitivity than the conventional EBSD technology, and therefore the present method is extremely effective for the characterization of each phase in fine island martensite. I understand.

(Example 5)
In order to obtain quantitative information regarding the area fraction of austenite and martensite, SEM images of bainite billets according to the present invention were obtained and used as input data for image analysis. Each image was processed by software corresponding to segmentation using threshold processing. FIG. 6 is a graph showing the annealing temperature dependence of the austenite area fraction and martensite area fraction (total inspection area = 2138 μm ² per sample) obtained by the technique according to the present invention. The obtained quantitative information was compared with the measured value of austenite volume fraction by the conventional X-ray diffraction (XRD) technique. The austenite phase volume fraction obtained by the present invention was in good agreement with the XRD technique. Therefore, SEM images showing clear contrast between microscopic images obtained according to the present invention are suitable for quantitative image analysis without image post-processing.

(Example 6)
As described above, the SEM image obtained by the present invention enables highly accurate phase separation and is very suitable for image analysis. The microscopic image can be easily converted into a binary image, and statistical data on the phase shape is obtained. FIG. 7 shows the average size, average roundness, and average ferret diameter of the retained austenite and martensite phases in each sample annealed at different temperatures (total inspection area = 2138 μm ² per sample).

(Example 7)
An electrolyte solution was prepared by mixing 10% by volume of acetylacetone, 1% by weight of tetramethylammonium chloride, and methanol as a solvent. The electropolishing process was performed at room temperature under conditions of a voltage of 100 mV, a charge of 3 C, and 1.5 minutes. After completion of the electropolishing process, the sample was subsequently held in the electrolyte solution for an additional 2 seconds. Then, the sample surface was observed by the same method as described above. An SEM-BSE image obtained using the in-lens type BSE detector is shown in FIG. As in the previous examples, the darkest part in the image shows the austenite phase. In the image, both the austenite phase and the martensite phase are separated and visualized. Therefore, it has been proved that the austenite phase can be selectively corroded by using not only a corrosive acid but also an organic acid as an electrolyte solution.

  According to the present invention, an austenite phase and a martensite phase in bainite steel can be clearly and visually distinguished on a nanoscale.

Claims (13)

A method for separating and visualizing austenite phase, martensite phase, and bainite-ferrite matrix in bainite steel,
a) electropolishing the surface of a bainite steel slab in an electrolyte solution containing a corrosive acid and an organic solvent to form a natural oxide layer on the surface of the bainite steel slab;
b) After completion of the electropolishing, the bainite steel pieces are continuously immersed in the electrolyte solution, and the first and second regions are respectively formed on regions of the natural oxide layer covering the martensite phase and austenite phase. Selectively forming a corrosion product layer of
c) The surface of the bainite steel slab is subjected to scanning electron microscope observation with a backscattered electron detector to obtain a contrast between the first and second corrosion product layers and the remaining surface, whereby the austenite phase The darkest area, the bainite-ferrite matrix as the brightest area, and the martensite phase as a medium brightness area,
Have
The step c) is performed by setting the incident energy of primary electrons to 3 keV or less.   The method of claim 1, wherein the backscattered electron detector is an in-lens detector.   The method of claim 1, wherein the backscattered electron detector comprises concentric segments that are located below the pole piece and are individually operable.   The method of claim 1, wherein the backscattered electron detector is located below the pole piece and has a working distance greater than 10 mm.   The method as described in any one of Claims 1-4 whose incident energy of a primary electron is 1 keV or less.   The method according to claim 1, wherein the incident energy of primary electrons is 0.5 keV or more.   The method according to claim 1, wherein an average thickness of the first corrosion product layer is 1 nm or more and 9 nm or less.   The method according to claim 1, wherein an average thickness of the second corrosion product layer is 2 nm or more and 10 nm or less.   The method according to claim 1, wherein the corrosive acid is selected from the group consisting of an oxidizing acid and an organic acid.   The method according to any one of claims 1 to 9, further comprising the step of d) performing quantitative and / or qualitative characterization of the visualized austenite phase and the martensite phase. A bainite steel base having a plurality of phases including an austenitic phase, a martensitic phase, and a bainite-ferrite matrix;
A native oxide layer formed on the surface of the bainite steel base;
A first corrosion product layer formed on a region of the natural oxide layer covering the martensite phase;
A second corrosion product layer formed on a region of the natural oxide layer covering the martensite and austenite phase and having a thickness greater than the thickness of the first corrosion product layer;
A bainite steel slab for structure observation characterized by comprising:   The bainite steel slab for structure observation according to claim 11, wherein an average thickness of the first corrosion product layer is 1 nm or more and 9 nm or less.   The bainite steel slab for structure observation according to claim 11 or 12, wherein an average thickness of the second corrosion product layer is 2 nm or more and 10 nm or less.