Method for accurately determining crystal structure of nano carbide in bainite steel

Abstract

The invention provides a method for accurately determining a nanometer carbide crystal structure in bainite steel, and belongs to the technical field of steel and iron material microscopic crystal structure determination. According to the invention, a series of organically combined experimental methods such as a matrix crystal face diffraction light splitting method, an extremely slow sample rotation method and a magnetic evasion method are adopted. And the electron diffraction pattern of the nano carbide in the bainite structure is successfully obtained. Moreover, aiming at the same nano carbide, multiple sets of diffraction spectrums can be obtained, and each set of diffraction spectrums has translation symmetry and rotational symmetry at the same time. On the basis of a large amount of experimental data, through accurate analysis of a series of electron diffraction patterns, the crystal structure and lattice parameters of the nano carbide are determined, and theoretical basis and technical support are provided for research and development of novel high-quality and high-strength bainite steel.

Description

Technical Field

[0001] The invention belongs to the technical field of microscopic crystal structure determination of steel materials, and in particular relates to a method for accurately determining the crystal structure of nano-carbides in bainite steel. Background Art

[0002] Bainitic steel is the most practical steel among all steel materials. It has low manufacturing costs, can balance mechanical properties such as strength, plasticity and toughness, and has good weldability. Therefore, bainitic steel is widely used in various fields, such as ship and submarine hulls, high-speed rail tracks, bridge construction, engineering machinery, etc. Utilizing the induction tempering heat treatment method, that is, a rapid heating method, it is possible to modulate a high density of nanocarbides in the low-carbon bainite structure. This type of tiny nano-precipitate phase can significantly enhance the strength of the bainite structure and can make the yield strength of bainitic steel reach above 1000MPa. The significance of studying nanocarbides in bainitic steel lies in determining the physical mechanism by which carbides strengthen bainite, and providing a theoretical basis and direction for the intelligent design of steel.

[0003] Studying bainitic nanocarbides using transmission electron microscopy presents several challenges. First, the tiny carbides are deeply embedded within the bulk of the bainite structure, making electron diffraction methods for studying the carbide crystal structure exponentially more difficult and complex. Second, bainite typically exhibits strong magnetism, which greatly complicates capturing electron diffraction spectra of the nanocarbides in different directions by tilting the sample. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for accurately determining the crystal structure of nanocarbides in bainitic steel. This method can obtain information such as the crystal structure and lattice parameters of nanocarbides in bainitic structure. Furthermore, using this information, the strengthening mechanism of nanocarbide precipitation relative to the matrix structure can be revealed, providing a theoretical basis and technical support for the research and development of new high-quality, high-strength bainitic steel.

[0005] To achieve the above object, the technical solution adopted by the present invention is:

[0006] A method for accurately determining the crystal structure of nanocarbides in bainitic steel, comprising:

[0007] 1) Preparation of TEM samples of bainitic steel using the electrolytic double-spray method;

[0008] 2) Using the diffraction contrast mode in a transmission electron microscope, observe the bainite structure in the thin area of ​​the sample and look for the complete bainite structure;

[0009] 3) Focusing the incident electron beam of a transmission electron microscope, irradiating the carbide precipitation phases in sequence in the intact bainite structure, and selecting nanocarbides with more Kikuchi bands and brighter Kikuchi bands as target carbides;

[0010] 4) By adjusting the angles of the double tilting platform in the α and β directions, the target carbide is tilted to a specified Kikuchi pole;

[0011] 5) Under the Kikuchi pole of step 4), photograph the diffraction spot pattern of the target carbide with translational periodicity and rotational symmetry, obtain the first set of electron diffraction spectra of the target carbide, and record the values ​​of the two angles α and β of the double tilt stage.

[0012] Then a series of electron diffraction spectra of the target carbide are obtained.

[0013] Preferably, step 1) specifically includes:

[0014] 1-1) Cutting bainitic steel into 0.4 mm thick metal sheets;

[0015] 1-2) Use sandpaper to thin the metal sheet to 150 μm thickness;

[0016] 1-3) Punching out a metal sheet with a diameter of 3 mm from the metal sheet using a punch;

[0017] 1-4) Grind the stamped 3mm metal disc to 70-100μm;

[0018] 1-5) Electrolytic double spraying was performed on the metal disc at -25°C using an electrolytic double spraying apparatus.

[0019] Preferably, in step 2), a complete bainite structure is searched in the thin region of the sample with a thickness ranging from 50 nm to 70 nm.

[0020] Preferably, in step 3), nanocarbides are selected outside the range of 100 nm from the grain boundary to eliminate the influence of the grain boundary.

[0021] Preferably, in step 4), the α direction is first used to control the angle of the carbide to tilt, and each tilt is 1°. The changes in the morphology of the carbide and the Kikuchi band are observed in the transmission mode and the diffraction mode respectively. When the Kikuchi band is found to be deviated or lost, the α direction is adjusted in the opposite direction to retrieve the Kikuchi band. Then, the rotation of the carbide is controlled by means of the β direction. Similarly, the β direction is only tilted 1° each time, and the changes in the morphology of the carbide and the Kikuchi band are observed in the transmission mode and the diffraction mode respectively. The double tilt table angle is repeatedly adjusted to finally obtain a Kikuchi pole of the nanocarbide.

[0022] Preferably, during the tilting process of step 4), if the target carbide is lost due to magnetic interference, the angle of the dual tilt stage needs to be adjusted back to the previous tilt angle, and the image in the field of view needs to be adjusted to the clearest state in the diffraction contrast mode to find the target carbide; if the magnetic influence still cannot be avoided, it is necessary to select a new bainite structure suitable for observation and repeat step 4) to obtain the Kikuchi pole of the nanocarbide.

[0023] Preferably, step 5) specifically includes: under the Kikuchi pole of step 4), using the selected area electron diffraction method, presenting the electron diffraction pattern of the target carbide on the fluorescent screen, observing whether the diffraction spots of the carbide at this time have complete translational periodicity, and also observing whether the brightness of each diffraction spot of the carbide at this time is symmetrical with the central transmission beam as the center of symmetry and has rotational symmetry, that is, whether the brightness is the same; if the diffraction spots of the target carbide have translational periodicity and rotational symmetry, then select appropriate brightness to shoot the diffraction spot pattern.

[0024] More preferably, if the diffraction spot of the target carbide does not satisfy any one of the conditions of translational periodicity and rotational symmetry, return to the diffraction contrast mode and fine-tune the angle of the target carbide so that its diffraction spot satisfies both translational periodicity and rotational symmetry. The direction of fine-tuning the angle is to tilt the brighter side of the diffraction spot to the weaker side of the diffraction spot. If the fine-tuning fails to achieve the expected effect, repeat steps 3), 4), and 5) until a diffraction spectrum with complete translational periodicity and rotational symmetry is obtained.

[0025] Preferably, in step 5), when the angle of the dual-tilt stage exceeds the angle range allowed by the instrument, the dual-tilt stage angle is adjusted back to the 0-degree position, and then, based on the Kikuchi band on which the tilt is based, the sample is rotated in a direction opposite to the previous tilt direction to find more Kikuchi poles and obtain more diffraction patterns; when all the Kikuchi poles on the Kikuchi band are found within the angle range allowed by the dual-tilt stage, all the diffraction spectra of the nanocarbides within the angle range allowed by the instrument under the Kikuchi band are obtained.

[0026] More preferably, in order to obtain more diffraction patterns, a new Kikuchi band can be selected, and all previous operations can be repeated to photograph and record all diffraction spectra under this Kikuchi band.

[0027] Deciphering the crystal structure of nanocarbides in steel is a challenging task due to several limitations: 1) magnetism; 2) extremely small size; and 3) easy loss during rotation. This invention addresses these challenges by solving the following technical problems:

[0028] 1) The problem of tilting the positive zone axis of nanocarbides

[0029] In transmission electron microscopy, tilting the positive band axis of nanocarbides has always been a technical challenge. The fundamental reason is that the size of the nanocarbides limits their ability to diffract the electron beam. On the one hand, an incident electron beam of moderate intensity is required to make the diffraction spots of the nanocarbides sharper and brighter, and to ensure that the intensity of the transmitted electron beam is not too higher than the intensity of the diffracted electron beam, which would dilute the display brightness of the diffraction beam on the fluorescent screen; on the other hand, an electron beam of moderate intensity is also required to stimulate more Kikuchi bands in the nanocarbides. Moreover, they can be displayed on the fluorescent screen for easy observation by the experimenter.

[0030] Solution: Using the matrix crystal plane diffraction spectrometry method, the transmitted electron beam is redistributed into a new transmitted electron beam and a diffracted beam using the matrix crystal plane diffraction method. The diffraction ability of the matrix crystal plane to the transmitted electron beam changes with the thickness of the thin area. After a large number of experiments, it was found that when the thickness of the thin area is in the range of 50nm to 70nm, the intensity of the new electron beam incident on the nanocarbide after diffraction by the matrix crystal plane is the optimal intensity for the experiment. Under these conditions, it is possible to obtain nanocarbide Kikuchi bands that are convenient for experimental use, as well as nanocarbide diffraction spectra with similar intensities between the diffracted beam and the transmitted beam.

[0031] 2) The influence of grain boundaries when the tilt angle is too large

[0032] Capturing electron diffraction spectra along the positive zone axis of nanocarbides requires tilting the sample. Furthermore, accurately analyzing the crystal structure of nanocarbides requires more than just one set of electron diffraction spectra. The sample must be continuously tilted to obtain multiple sets of electron diffraction spectra along different positive zone axes. This inevitably leads to a problem: when the tilt angle increases, the bainite grain boundaries overlap with the nanocarbides. This excessive tilt angle can interfere with the electron diffraction spectrum.

[0033] Solution: The dual tilt stage of a transmission electron microscope (TEM) has a maximum tilt angle. Typical analytical TEMs have a wide range of tilt angles. For example, the F20 TEM's dual tilt stage has angles of α±40° and β±30°, while the JEM-2000FX TEM's has even wider ranges, reaching α±60° and β±40°. The high-resolution JEM-2010 TEM's dual tilt stage has a much narrower range of α±20° and β±15°. Based on the thickness and area of ​​the sample observation area in actual experiments, it can be assumed that the observed bainite interface is a plane with a height of approximately 60nm to 80nm (H) and a length of approximately 15μm. Initially, this interface is parallel to the incident electron beam. Therefore, the spatial angle conversion formula can be used to calculate the vertical projection range of the bainite interface after tilting the dual tilt stage at a certain angle: L = Hcos(α)cos(β). Considering the safety of the experiment, we chose 2L as the safe range length. In this experiment, 2L is approximately 100nm. In this experiment, the nanocarbide is selected outside the 100nm range of the grain boundary to eliminate the influence of the grain boundary.

[0034] 3) The problem of magnetic influence of bainite structure

[0035] Steel samples for transmission electron microscopy (TEM) are generally magnetic. In particular, structures such as bainite and martensite exhibit stronger magnetism. This magnetism can significantly interfere with TEM experiments. At certain angles, the strong coupling between the magnetic properties of the steel sample and the objective lens pole piece can even cause the sample to be directly drawn into the objective lens pole piece.

[0036] Solution: Secure the sample with a dual-tilt stage. While tilting the stage to locate the nanocarbide Kikuchi pole, if a sudden strong interaction occurs between the TEM sample's magnetic field and the pole piece's magnetic field, resulting in a sudden blurring of the image in the thin area of ​​the sample, immediately stop rotating the sample, adjust the sample height to regain the field of view, and re-tilt the sample. If this strong magnetic interference persists in that area, replace it with a new, intact bainite grain to continue the experiment.

[0037] The beneficial effects of the present invention are:

[0038] 1) Through the organic combination of a series of methods such as matrix crystal plane diffraction spectroscopy, extremely slow sample rotation method, and magnetic avoidance method, the electron diffraction pattern of nanocarbide under the positive zone axis can be obtained;

[0039] 2) A series of electron diffraction patterns of nanocarbides under different positive band axes can be obtained, and in each set of diffraction patterns, the position and intensity of the nanocarbides diffraction spots have both translational symmetry and rotational symmetry;

[0040] 3) Through the precise analysis of a series of electron diffraction patterns, the experimental results obtained provide strong experimental evidence for the precise determination of the crystal structure of nanocarbides, and thus provide a theoretical basis and technical support for the research and development of new high-quality, high-strength bainitic steels. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is the morphology and distribution of nanocarbides captured in Example 1 of the present invention.

[0042] Figure 2 These are six electron diffraction patterns of nanocarbide captured in Example 1 of the present invention.

[0043] Figure 3 6 electron diffraction patterns of Example 1 of the present invention Vector annotations and their corresponding calibration results.

[0044] Figure 4 This is the energy spectrum experimental result of the nanocarbide precipitation phase in Example 1 of the present invention. DETAILED DESCRIPTION

[0045] The present invention will be further described below by means of specific examples. The examples are for illustrative purposes only, and the protection scope of the present invention is not limited to these examples.

[0046] The present invention takes low-carbon bainite steel (C: 0.02wt.% to 0.06wt.%, Mn<1wt.%, Si<0.5wt.%) as an example, and obtains a large amount of nano-carbides in bainite through induction tempering heat treatment.

[0047] Example 1

[0048] This embodiment provides a method for accurately determining the crystal structure of nanocarbides in bainitic steel. The main operating steps are as follows:

[0049] 1) Preparation of TEM samples of bainitic steel using an electrolytic double-spray method, specifically including:

[0050] 1-1) Cutting low-carbon bainite steel into 0.4 mm thick metal sheets;

[0051] 1-2) Use sandpaper to thin the metal sheet to 150 μm thickness;

[0052] 1-3) Punching out a metal sheet with a diameter of 3 mm from the metal sheet using a punch;

[0053] 1-4) Grind the stamped 3mm metal disc to 70-100μm;

[0054] 1-5) Electrolytic double spraying was performed on the metal disc at -25°C using an HG-2000 electrolytic double spraying apparatus.

[0055] 2) In the transmission electron microscope, use the diffraction contrast mode to observe the bainite structure in the thin area of ​​the sample. Try to find a complete feather-like bainite structure. Usually, a large number of carbide precipitates with a size of about 60nm (length) × 20nm (width) are evenly distributed inside the bainite structure after induction tempering, such as Figure 1 .

[0056] When at the edge of the thin zone, the beam intensity of the transmitted electron beam will greatly interfere with the diffraction beam of the carbide. Usually, at the edge of the thin zone, the weak diffraction beam of the carbide can hardly be observed. Therefore, a relatively thick area must be selected to diffract the transmitted electron beam with the help of the matrix to weaken the influence of the transmitted electron beam. In this way, the weak diffraction spots of the nanocarbide can be better revealed. Therefore, a complete bainite structure is selected to avoid the edge of the thin zone. After a large number of experimental accumulations, it was found that when the thickness of the thin zone is in the range of 50nm to 70nm, the new electron beam intensity incident on the nanocarbide obtained after diffraction of the matrix crystal plane is the best selected intensity for the experiment. Under this condition, it is possible to obtain a nanocarbide Kikuchi band that is convenient for experimental use, and also to obtain a nanocarbide diffraction spectrum with a diffraction beam similar to the transmitted beam intensity.

[0057] 3) Focus the incident electron beam of the transmission electron microscope and irradiate the carbide precipitation phase in the complete bainite structure in sequence. Each time a nanocarbide is irradiated, it is necessary to adjust to the diffraction mode and observe the Kikuchi band pattern of the carbide. If the carbide has more Kikuchi bands and the brightness of the Kikuchi bands is relatively bright, then the carbide can be used as the target carbide for subsequent electron diffraction experiments. Here, it must be pointed out that due to the small size of carbides, their Kikuchi bands are usually very weak and the distribution range is extremely small. The Kikuchi bands of nanocarbides can only appear near the transmitted electron beam. Therefore, when it is found that the brightness and distribution range of the Kikuchi bands of a certain carbide are better than those of other carbides, then this carbide is very suitable as a target carbide.

[0058] Since not every carbide exhibits distinct Kikuchi bands, searching for observable nanocarbides is essential. Typically, more than one observable nanocarbide can be found within the same bainite structure. In such cases, it is best to observe nanocarbides close to the interior of the bainite structure. This prevents the influence of bainite grain boundaries on the captured carbide diffraction pattern when the tilt angle is too large.

[0059] The dual tilt stage of a transmission electron microscope (TEM) has a maximum tilt angle. Typical analytical TEMs have a wide range of tilt angles. For example, the F20 TEM's dual tilt stage has angles of α±40° and β±30°, while the JEM-2000FX TEM's has even wider ranges, reaching α±60° and β±40°. The high-resolution JEM-2010 TEM's dual tilt stage has a much narrower range of α±20° and β±15°. Based on the thickness and area of ​​the sample observation area in actual experiments, it can be assumed that the observed bainite interface is a plane with a height of approximately 60nm to 80nm (H) and a length of approximately 15μm. Initially, this interface is parallel to the incident electron beam. Therefore, the spatial angle conversion formula can be used to calculate the vertical projection range of the bainite interface after tilting the dual tilt stage at a certain angle: L = Hcos(α)cos(β). Considering the safety of the experiment, we chose 2L as the safe range length. In this experiment, 2L is approximately 100nm. In this experiment, the nanocarbide is selected outside the 100nm range of the grain boundary to eliminate the influence of the grain boundary.

[0060] 4) For the target carbide, use its Kikuchi band to tilt it to a specified Kikuchi pole. This tilting process is relatively slow. The tilting is completed using the double tilt stage of the transmission electron microscope. The angle of sample rotation is precisely controlled by the α and β directions of the double tilt stage. During the tilting process, a bright Kikuchi band is the basis for rotating the crystal direction. First, use the α direction to control the angle of the carbide tilt, tilting 1° at a time. Observe the morphology of the carbide and the changes in the Kikuchi band in the transmission mode and diffraction mode respectively. When it is found that the Kikuchi band is deviated or lost, it is necessary to control the rotation of the carbide with the help of the β direction. Similarly, the β direction is only tilted 1° at a time. Observe the morphology of the carbide and the changes in the Kikuchi band in the transmission mode and diffraction mode respectively. By repeatedly adjusting the angle of the double tilt stage in this way, a Kikuchi pole of the nanocarbide can be obtained.

[0061] Currently, observation or analysis of magnetic steel samples is prohibited in transmission electron microscopy, particularly in the field of spherical aberration. This is because the sample's magnetism not only affects the transmission electron microscope's pole piece and optical path, but can also cause magnetic samples secured to the sample holder to be drawn away by the pole piece during actual experiments, causing the microscope to malfunction. Securely securing the sample is essential before beginning any experiment.

[0062] During the tilting process, because bainite is typically magnetic, this magnetism can at some point interfere with the experiment and cause the target carbides to be lost. If this occurs, first return the dual tilt stage to its previous tilt angle and, in diffraction contrast mode, maximize the image clarity within the field of view to re-identify the target carbides. Then, proceed to step 4) to tilt the sample. If the magnetic influence cannot be avoided after performing step 4), select a new bainite structure suitable for observation and repeat step 4) to obtain the Kikuchi pole of the nanocarbide.

[0063] In the experiment, we chose to tilt 1° each time, taking two points into consideration: First, although the influence of the intensity of the transmitted electron beam on the carbide Kikuchi band is reduced, the brightness of the carbide Kikuchi band is still weak. A large tilt angle will cause the loss of the previous state (the state refers to the distribution pattern of the Kikuchi band). In this way, it is impossible to determine whether the tilt is along the same Kikuchi band. Therefore, a smaller rotation angle is selected each time to ensure the continuity of the experimental transition state. This can trace the source without losing the observation target; secondly, tilting 1° each time can avoid the sudden influence caused by the magnetism of the bainite structure. That is, if the tilt angle is too large at a certain time, the sudden magnetic influence will cause the observation target to be lost instantly, and it will be difficult to trace the source.

[0064] 5) Under this Kikuchi pole, use the selected area electron diffraction method to present the electron diffraction pattern of the target carbide on the fluorescent screen. Observe whether the diffraction spots of the carbide at this time have complete translational periodicity. And also observe whether the brightness of each diffraction spot of the carbide at this time is symmetrical with the central transmission beam and has rotational symmetry, that is, whether the brightness is the same. Usually, when the center of the Kikuchi pole of the target carbide completely coincides with the center of the incident electron beam, the diffraction spots of the target carbide must have translational symmetry and rotational symmetry. If the diffraction spots of the target carbide have translational periodicity and rotational symmetry, you can choose a suitable brightness to shoot the diffraction spot pattern. If the diffraction spots of the target carbide do not meet any of the conditions of translational periodicity and rotational symmetry, you need to return to the diffraction contrast mode and fine-tune the angle of the target carbide so that its diffraction spots meet both translational periodicity and rotational symmetry. The direction of fine-tuning the angle is to tilt the brighter side of the diffraction spot to the weaker side of the diffraction spot. If fine-tuning fails to achieve the desired effect, steps 3), 4), and 5) need to be repeated until a diffraction spectrum with complete translational periodicity and rotational symmetry is obtained.

[0065] 6) After obtaining the first set of electron diffraction spectra of the target carbide using the above method, record the values ​​of the double tilt stage angles α and β. Next, repeat steps 4) and 5) to obtain a second set of electron diffraction spectra of the target carbide. Repeat this experimental procedure to obtain at least two sets of diffraction patterns of the target carbide. Based on these patterns, the crystal structure of the nanocarbide can be accurately analyzed.

[0066] When the angle of the double-tilt stage exceeds the angle range allowed by the instrument, adjust the double-tilt stage angle back to the 0-degree position. Then, based on the Kikuchi band that you rely on when tilting, rotate the sample in the direction opposite to the previous tilting direction. In this way, you can find more Kikuchi poles and obtain more diffraction patterns. When all the Kikuchi poles on the Kikuchi band are found within the angle range allowed by the double-tilt stage, you will get all the diffraction spectra of the nanocarbide under the Kikuchi band within the angle range allowed by the instrument. In order to obtain more diffraction patterns, you can select a new Kikuchi band and repeat all the previous operations to record all the diffraction spectra under this Kikuchi band.

[0067] By the method of the present invention, 6 electron diffraction patterns of nano-carbides in bainite structure under different positive band axes are obtained. Figure 2 As shown. It was determined that the positions and intensities of the electron diffraction spots in the six diffraction patterns all satisfy translational symmetry and rotational symmetry. From these six diffraction patterns, 6 groups of 12 interplanar spacings were obtained in sequence: (0.2082nm, 0.688nm), (0.3411nm, 0.2766nm), (0.3033nm, 0.2032nm), (0.3368nm, 0.2060nm), (0.2032nm, 0.2091nm), (0.2426nm, 0.2031nm). Next, the crystal structure of the carbide was determined by analyzing these six patterns.

[0068] You can try to calibrate with known crystal structures first. The intermetallic compounds that Fe and C can form are as follows: Fe3C (θ-Fe3C), ε-Fe3C, Fe7C3, Fe 23 After comparing the four crystal structures of C6, the above 12 interplanar spacing values ​​are closest to those of Fe3C. Therefore, we can first try to use the crystal structure of Fe3C to interpret these 6 diffraction patterns.

[0069] Fe3C has an orthorhombic structure. The diffraction pattern of this crystal is difficult to calibrate. In the specific calibration process, we grasped two key breakthroughs: 1. There is a rectangular pattern in the diffraction spectrum, that is, in the reciprocal space. and The two vectors are perpendicular to each other; 2. Four diffraction patterns are obtained along the same Kikuchi pole, and there are common atomic planes in these four patterns.

[0070] First analyze the electron diffraction spectrum in 2a. In this diffraction spectrum, find the basic reciprocal vector and like Figure 3 As shown in a. After calculation, The vector corresponds to the {001} family of Fe3C crystal planes. This family of crystal planes has two planes, (001) and (00-1). Vector with The angle between the two vectors proves that the directions of the two basic vectors are perpendicular to each other. According to the formula for the angle between the crystal planes of the orthorhombic crystal system, the crystal plane index of the family perpendicular to the {001} crystal plane family is the {hk0} crystal plane family. Among these indices, the crystal plane spacing corresponding to the diffraction spot with the largest crystal plane spacing is 0.2082nm, which is consistent with The interplanar spacing corresponding to the vectors coincides. Therefore, The diffraction spots corresponding to the {210} crystal plane family. There are four crystal planes in this crystal plane family, namely (210)(-2-10)(-210)(2-10). The crystal plane indices of the two vectors and the angle between them can be used to determine the crystal zone axis of the diffraction pattern. Figure 2 There are two cases for the calibration of the diffraction pattern in a: 1. The first basic vector (001), the second basic vector (210), the angle between them is a right angle, and the crystal axis is [1-20]; 2. The first basic vector (001), the second basic vector (-210), the angle between them is a right angle, and the crystal axis is

[120] . It should be noted here that Figure 2 There are two main features of the a diffraction spectrum: 1. The electron diffraction spectrum captures the maximum interplanar spacing of the nanocarbide, which is 0.688 nm. This interplanar spacing must be the (001) plane in Fe3C; 2. The two vectors are perpendicular to each other. These two key data can fully prove the accuracy of the diffraction spectrum calibration. Figure 3 a is the electron diffraction spectrum result calibrated according to the

[120] crystal zone axis.

[0071] Figure 2 b electron diffraction spectrum Two basic vectors such as Figure 3 As shown in b. The calculated interplanar spacings corresponding to the two basic vectors are 0.3411nm and 0.2766nm respectively. There are relatively few Fe3C crystal planes with similar interplanar spacings to these two. 0.3411nm may correspond to a crystal plane in the {002}{110} crystal plane family, while 0.2766nm can only correspond to a crystal plane in the {102} crystal plane family. Using the crystal plane indices in these crystal plane families, the angle between the interplanar spacing of 0.3411nm and the interplanar spacing of 0.2766nm can be calculated. The actual measured The vector angle is 66.31° (obtuse angle 114.2°). The angle value calculated theoretically is consistent with the actual By comparing the vector angle values, we can find similar angles and determine the crystal indices of the two crystal planes with a crystal plane spacing of 0.3411nm and a crystal plane spacing of 0.2766nm.

[0072] Table 1

[0073] Crystal plane family crystal face Selected crystal plane Crystal plane family crystal face Angle (unit: degree) Absolute value of the angle difference {102} (102) (102) {002} (002) 36.6957 (-10-2) (00-2) 143.3043 (-102) {110} (110) 63.4746 2.8354 (10-2) (-1-10) 116.5254 2.3254 (-110) 116.5254 2.3254 (1-10) 63.4746 2.8354 (-102) {002} (002) 36.6957 (00-2) 143.3043 {110} (110) 116.5254 2.3254 (-1-10) 63.4746 2.8354 (-110) 63.4746 2.8354 (1-10) 116.5254 2.3254

[0074] Since the calculated angle is slightly different from the actual measured value, Figure 2 The possibility of calibration results for b increases. Preliminary judgment suggests that there should be four calibration results: 1. The first basic vector is (110), the second basic vector is (102), the angle is acute, and the crystal axis is [-221]; 2. The first basic vector is (1-10), the second basic vector is (102), the angle is acute, and the crystal axis is [22-1]; 3. The first basic vector is (-1-10), the second basic vector is (-102), the angle is acute, and the crystal axis is [2-21]; 4. The first basic vector is (-110), the second basic vector is (-102), the angle is acute, and the crystal axis is

[221] .

[0075] Figure 2 c diffraction pattern, Two basic vectors such as Figure 3 c. After calculation, the interplanar spacings corresponding to the two reciprocal vectors are 0.3033nm and 0.2032nm respectively. The 0.3033nm crystal plane value corresponds to the {111} crystal plane family of Fe3C (a crystal plane that can be uniquely determined). The 0.2032nm crystal plane value has many matching cementite crystal planes, such as {210}{013}{022}{103}{211}. Next, we need to determine the specific crystal plane index, which requires a detailed list of the crystal planes in the above crystal plane family and the calculation of the angles between the crystal planes. In the orthorhombic crystal system, the indices of the crystal plane family {hkl} can change the positive and negative signs, but the order of the indices cannot be changed. Therefore, the {111} crystal plane family of the Fe3C orthorhombic crystal system has only eight crystal planes: (111)(-1-1-1), (-111)(1-1-1), (-1-11)(11-1), (1-11)(-11-1). In this calibration, the crystal plane (1-11) is selected. Two basic vectors The measured value of the included angle is 83.31° (the measured value of the obtuse angle is 95.79°).

[0076] Table 2

[0077] Crystal plane family crystal face Select crystal plane Crystal plane family crystal face Angle (unit: degree) Absolute value of the angle difference {111} (111) (1-11) {210} (210) 68.3319 (-1-1-1) (-2-10) 111.6681 (2-10) 31.5988 (-111) (-210) 148.4012 (1-1-1) {013} (013) 80.2107 3.0993 (0-1-3) 99.7893 3.9993 (1-11) (0-13) 49.4596 (-11-1) (01-3) 130.5404 {022} (022) 101.7767 (11-1) (0-2-2) 78.2233 (-1-11) (0-22) 41.9191 (02-2) 138.0809 {103} (103) 45.6798 (-10-3) 134.3202 (-103) 84.0221 0.7121 (10-3) 95.9779 0.1879 {211} (211) 61.0283 (-2-1-1) 118.9717 (-211) 133.0767 (2-1-1) 46.9233 (2-11) 18.9681 (-21-1) 161.0319 (21-1) 77.1938 (-2-11) 102.8062

[0078] The crystal planes whose theoretical calculated angles differ from the measured angles by more than 3° can be excluded. The crystal planes whose theoretical calculated angles differ from the measured angles by less than 1° are retained. Finally, the calculated angles are compared with the measured angles to determine the The crystal plane index of the vector is (-103)(10-3). Therefore, Figure 2 The calibration result of c is: the first basic vector is (1-11), the second basic vector is (-103) and the crystal zone axis is

[341] .

[0079] Figure 2 d, determine the diffraction spectrum Vector. The interplanar spacings of the crystal planes corresponding to these two reciprocal vectors are 0.3368nm and 0.2060nm respectively. Since the tilting process is along the same Kikuchi zone, it can be determined that Figure 2 The 0.2060nm interplanar spacing in the d diffraction spectrum corresponds to the (-103)(10-3) crystal plane. Next, determine the crystal plane corresponding to the 0.3368nm interplanar spacing. By comparing the values ​​with the standard Fe3C crystal plane, it is found that the 0.3368nm interplanar spacing likely corresponds to a plane in the {110}{002} family of planes. Then, by comparing the angles, determine which specific crystal planes correspond to the 0.3368nm interplanar spacing. The actual measured angle between the two basic vectors is 70.43° (obtuse angle 108.79°).

[0080] Table 3

[0081] Crystal plane family crystal face Determined crystal plane Crystal plane family crystal face Angle (unit: degree) Absolute value of the angle difference {103} (103) (-103) {110} (110) 109.4227 0.6327 (-10-3) (-1-10) 70.5773 0.1473 (-103) (1-10) 109.4227 0.6327 (10-3) (-110) 70.5773 0.1473 {002} (002) 26.4200 (00-2) 153.5800

[0082] From the perspective of comparing the difference in the table, Figure 2 There are two calibration results for d: 1. The first basic vector is (-1-10), the second basic vector is (-103), the angle is acute, and the crystal axis is [3-31]; 2. The first basic vector is (-110), the second basic vector is (-103), the angle is acute, and the crystal axis is

[331] . Both calibration results meet Figure 2 The diffraction spectrum of d can be uniquely determined by the recorded double tilt table angle value. Figure 2 d calibration results.

[0083] Figure 2 Two basic vectors of diffraction spectrum in e The corresponding interplanar spacings are 0.2032nm and 0.2091nm respectively. This spectrum is also obtained by rotating the Kikuchi pole in the same Kikuchi direction. Figure 2The 0.2032nm interplanar spacing in e corresponds to the (-103) and (10-3) planes. The interplanar spacing of 0.2091nm corresponds to the plane indices {210}{022}{013}. Because the interplanar spacing of the plane {103}{211} is too small compared to 0.2091nm, it was not selected. The interplanar spacing of the plane {210}{121} is slightly larger than the measured interplanar spacing and was not selected either. Because Figure 2 The (001) interplanar spacing obtained in a is larger than the calculated result. Therefore, if the interplanar spacing calculated in the calibration is larger than the actual measured interplanar spacing, it is unlikely to be the required crystal plane. The actual measured value of the angle between the two vectors is 65.43° (obtuse angle 114.26°).

[0084] Table 4

[0085]

[0086]

[0087] As can be seen from the table, the angle calculation value is not much different from the measured value. Figure 2 There are also two calibration results of the diffraction spectrum in e: 1. The first basic vector is (-2-10), the second basic vector is (-103), the angle is acute, and the crystal zone axis is [3-61]; 2. The first basic vector is (-210), the second basic vector is (-103), the angle is acute, and the crystal zone axis is

[361] .

[0088] Figure 2 The diffraction spectrum in reciprocal space in f Two basic vectors such as Figure 3 f. Their corresponding interplanar spacings are 0.2426nm and 0.2031nm respectively. Among them, the interplanar spacing of 0.2031nm still corresponds to the (-103)(10-3) crystal plane. The crystal planes corresponding to 0.2426nm may be {112} and {021}. The crystal plane index is determined by calculating the crystal plane angle. The angle between the two vectors is 66.93° (obtuse angle 113.29°).

[0089] Table 5

[0090]

[0091]

[0092] After angle comparison, it was found that the diffraction spectrum also has two calibration results: 1. The first basic vector is (112), the second basic vector is (-103), the angle is acute, and the crystal axis is [3-51]; 2. The first basic vector is (1-12), the second basic vector is (-103), the angle is acute, and the crystal axis is

[351] .

[0093] In summary, the unified calibration results of the 6 diffraction patterns are as follows:

[0094] Figure 3 a

[221] (-110)(-102) The actual recorded angles α=15.50°,β=-8.58°

[0095] Figure 3 b

[341] (1-11)(-103) The actual recorded angles α=3.96°,β=2.53°

[0096] Figure 3 c

[331] (-110)(-103) The actual recorded angles α=6.08°,β=-6.54°

[0097] Figure 3 d

[361] (-210)(-103) The actual recorded angles α=5.67°,β=16.11°

[0098] Figure 3 e

[351] (1-12)(-103) The actual recorded angles α=4.21°,β=10.83°

[0099] Figure 3 The actual recorded angles of f

[120] (001)(-210) are α=-2.06° and β=25.19°.

[0100] Using the orthorhombic crystal orientation angle calculation formula, the angle between two adjacent crystal axes can be calculated. The calculated angle value is compared with the actual angle calculated:

[0101] Table 6

[0102] Calculation results of orthorhombic crystal orientation cosθ=cosΔαcosΔβ 13.2053° 15.9664° 8.1215° 9.3124° 18.3927° 22.6535° 4.3814° 5.4776° 13.7774° 15.6427°

[0103] The calculated result of the crystal orientation angle is very close to the actual tilt angle. This proves the correctness and reliability of the above calibration results. Therefore, the crystal structure of the nanocarbide should be cementite Fe3C structure. Since the energy spectrum results detected that the carbide contains a certain amount of Cr element, such as Figure 4 Therefore, the actual carbide crystal structure should be an orthorhombic structure of (Fe, Cr)3C composite.

[0104] Fe3C has an orthorhombic structure, and the unit cell parameters are a=0.45235nm, b=0.50890nm, c=0.67433nm, α=β=γ=90 degrees. There are 16 atoms in its unit cell, 12 iron atoms and 4 carbon atoms. The lattice parameters of (Fe, Cr)3C cementite containing Cr will change. Among the 6 diffraction patterns, the maximum interplanar spacing is 0.688nm, which is about 2% higher than the maximum interplanar spacing of Fe3C of 0.6741nm. Therefore, when Fe3C contains Cr elements, its lattice will expand. Through the existing measured interplanar spacing, it can be determined that the entire lattice parameters will expand slightly through previous crystal plane comparison. After correction, the lattice parameters of (Fe, Cr)3C cementite are obtained:

[0105] a (Fe，Cr)3C =a Fe3C *(1+2%), b (Fe，Cr)3C =b Fe3C *(1+2%), c (Fe，Cr)3C =c Fe3C *(1+2%)

[0106] a (Fe，Cr)3C =0.46140nm, b (Fe，Cr)3C =0.51908nm, c (Fe，Cr)3C =0.68782nm

[0107] In summary, this method enables the capture of a series of electron diffraction patterns of nanocarbides. By analyzing these patterns, the crystal structure of the nanocarbides can be precisely determined. This provides valuable information for in-depth study of the orientation, coherence, and strengthening relationships between the nanocarbides and the matrix. These results represent a major breakthrough in the study of nanocarbides in bainitic steel.

[0108] It should be noted that those skilled in the art may make various modifications and improvements without departing from the scope of the present invention, and these modifications and improvements fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be based on the appended claims.

Claims

1. A method for accurately determining the crystal structure of nanocarbides in bainitic steel, characterized in that: include: 1) Preparation of TEM samples of bainitic steel using the electrolytic double-spray method; 2) Using the diffraction contrast mode in a transmission electron microscope, observe the bainite structure in the thin area of ​​the sample and look for the complete bainite structure; 3) Focusing the incident electron beam of a transmission electron microscope, irradiating the carbide precipitation phases in sequence in the intact bainite structure, and selecting nanocarbides with more Kikuchi bands and brighter Kikuchi bands as target carbides; 4) By adjusting the angles of the double tilting platform in the α and β directions, the target carbide is tilted to a specified Kikuchi pole; 5) Under the Kikuchi pole of step 4), the diffraction spot pattern of the target carbide with translational periodicity and rotational symmetry is photographed to obtain the first set of electron diffraction spectra of the target carbide, and the values ​​of the two angles α and β of the double tilt table are recorded to obtain a series of electron diffraction spectra of the target carbide.

2. The method for accurately measuring the crystal structure of nanocarbides in bainite steel according to claim 1, characterized in that: Step 1) specifically includes: 1-1) Cutting bainitic steel into 0.4 mm thick metal sheets; 1-2) Use sandpaper to thin the metal sheet to 150 μm thickness; 1-3) Punching out a metal sheet with a diameter of 3 mm from the metal sheet using a punch; 1-4) Grind the stamped 3mm metal disc to 70-100μm; 1-5) Electrolytic double spraying was performed on the metal disc at -25°C using an electrolytic double spraying apparatus.

3. The method for accurately measuring the crystal structure of nanocarbides in bainite steel according to claim 1, characterized in that: In step 2), a complete bainite structure is searched in the thin area of ​​the sample with a thickness ranging from 50 nm to 70 nm.

4. The method for accurately measuring the crystal structure of nanocarbides in bainitic steel according to claim 1, characterized in that: In step 3), nanocarbides are selected outside the range of 100 nm from the grain boundary to eliminate the influence of the grain boundary.

5. The method for accurately measuring the crystal structure of nanocarbides in bainite steel according to claim 1, characterized in that: In step 4), the α direction is first used to control the angle of the carbide to tilt, tilting 1° each time, and observing the changes in the morphology of the carbide and the Kikuchi band in the transmission mode and diffraction mode respectively. When the Kikuchi band is found to be deviated or lost, the α direction is adjusted in the opposite direction to retrieve the Kikuchi band. Then, the rotation of the carbide is controlled with the help of the β direction. Similarly, the β direction is only tilted 1° each time, and the changes in the morphology of the carbide and the Kikuchi band are observed in the transmission mode and diffraction mode respectively. The double tilt table angle is repeatedly adjusted to finally obtain a Kikuchi pole of the nanocarbide.

6. A method for accurately measuring the crystal structure of nanocarbides in bainitic steel according to claim 1 or 5, characterized in that: During the tilting process in step 4), if the target carbide is lost due to magnetic interference, it is necessary to adjust the angle of the dual tilt stage back to the previous tilt angle and adjust the image in the field of view to the clearest state in the diffraction contrast mode to find the target carbide. If the magnetic influence cannot be avoided, it is necessary to select a new bainite structure suitable for observation and repeat step 4) to obtain the Kikuchi pole of the nanocarbide.

7. The method for accurately measuring the crystal structure of nanocarbides in bainite steel according to claim 1, characterized in that: Step 5) specifically includes: under the Kikuchi pole of step 4), using the selected area electron diffraction method, presenting the electron diffraction pattern of the target carbide on the fluorescent screen, observing whether the diffraction spots of the carbide at this time have complete translational periodicity, and also observing whether the brightness of each diffraction spot of the carbide at this time is symmetrical with the central transmission beam as the center and has rotational symmetry, that is, whether the brightness is the same. If the diffraction spots of the target carbide have translational periodicity and rotational symmetry, then selecting an appropriate brightness to shoot the diffraction spot pattern.

8. The method for accurately measuring the crystal structure of nanocarbides in bainite steel according to claim 7, characterized in that: If the diffraction spot of the target carbide does not meet any of the conditions of translational periodicity and rotational symmetry, return to the diffraction contrast mode and fine-tune the angle of the target carbide so that its diffraction spot satisfies both translational periodicity and rotational symmetry. The direction of fine-tuning the angle is to tilt the brighter side of the diffraction spot to the weaker side of the diffraction spot. If the fine-tuning fails to achieve the expected effect, repeat steps 3), 4), and 5) until a diffraction spectrum with complete translational periodicity and rotational symmetry is obtained.

9. The method for accurately measuring the crystal structure of nanocarbides in bainite steel according to claim 1, characterized in that: In step 5), when the angle of the double-tilt stage exceeds the angle range allowed by the instrument, the double-tilt stage angle is adjusted back to the 0-degree position. Then, based on the Kikuchi band on which the tilt is based, the sample is rotated in a direction opposite to the previous tilt direction to find more Kikuchi poles and obtain more diffraction patterns. When all the Kikuchi poles on the Kikuchi band are found within the angle range allowed by the double-tilt stage, all the nanocarbide diffraction spectra within the angle range allowed by the instrument under the Kikuchi band are obtained.

10. The method for accurately measuring the crystal structure of nanocarbides in bainite steel according to claim 9, characterized in that: In order to obtain more diffraction patterns, you can select a new Kikuchi belt, repeat all previous operations, and record all diffraction spectra under this Kikuchi belt.

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