A method for the visualization and quantitative analysis of small-angle grain boundaries in high-strength steel

Abstract

The present application relates to a kind of high-strength steel low-angle grain boundary display and quantitative analysis method, the method comprises the following steps: step 1: sample preparation: the sample to be measured is inlaid, polished, polished, and then vibration polished, cut into 0.25-2mm thin section after being made into ordinary metallographic sample;Step 2: the sample prepared is placed in scanning electron microscope sample chamber, step 3: through EBSD acquisition crystal diffraction pattern;Step 4: utilize software to process data and show low-angle grain boundary;Step 5: utilize software to process data and quantitatively analyze low-angle grain boundary.This method displays a large number of low-angle grain boundaries in high-strength steel, and quantitatively analyzes the percentage content of different angle grain boundaries, which fills the gap in the existing microstructure observation method that cannot clearly display low-angle grain boundaries and conduct quantitative analysis, and provides important theoretical support for high-strength steel organization refinement strengthening mechanism research and on-site cooling process development.

Description

TECHNICAL FIELD

[0001] The application relates to an analysis method, in particular to a method for displaying and quantitatively analyzing small-angle grain boundaries in high-strength steel, and belongs to the technical field of high-strength steel material analysis. BACKGROUND

[0002] High-strength structural steel is increasingly used in the fields of automobiles, engineering machinery, railway vehicles and containers to replace traditional ordinary structural steel, and the grade of the steel is increasingly high, so as to reduce the weight of components and equipment, thereby reducing the fuel consumption of the equipment, improving the working efficiency and reducing the exhaust emission. At present, a TMCP process based on ultrafast cooling technology (UFC) has been increasingly applied in the development and industrial production of high-grade steel, and compared with a traditional TMCP process, the ultrafast cooling process can significantly refine the grains, thereby improving the structure and performance of the steel. The ultrafast cooling technology is an effective way to realize the weight reduction of steel production, and through flexible control of the cooling path of a hot-rolled strip, the refined ferrite grains are beneficial to improve the strength and toughness of the steel. The high-grade steel produced by the ultrafast cooling process has a large number of small-angle grain boundaries and subgrains in the grains, and the traditional metallographic corrosion method can only display the large-angle grain boundaries with an angle greater than 15°, and the small-angle grain boundaries and subgrains are unclear, which is not conducive to the structure analysis and strengthening mechanism research of the high-strength steel. The electron backscatter diffraction (EBSD) technology is an ideal tool for measuring the grain size, can make a point-by-point crystallographic analysis on the submicron microstructure on a bulk sample, is a new technology for analyzing the structure information of a sample through a diffraction Kikuchi line, and can make the microstructure, micro-area composition and crystallographic data analysis on a scanning electron microscope be connected, so that a large amount of information about the spatial distribution of the crystal orientation can be obtained, thereby realizing the phase identification of the crystal / grain, the grain size and shape analysis, the crystal and grain orientation measurement and the like.

[0003] A Chinese patent application with the application publication No. CN102353690A discloses a method for distinguishing the bainite in hot-rolled TRIP steel and calculating three ratios thereof, the method uses an electron backscatter diffraction device arranged on a scanning electron microscope to quantitatively measure the FCC phase (residual austenite) and the BCC phase (ferrite + bainite) of the hot-rolled TRIP steel, and processes the EBSD pattern data, and the bainite structure is distinguished from the BCC phase by adding the small-angle grain boundaries, in the method, the BCC phase is added with the small-angle grain boundaries greater than 1° and the large-angle grain boundaries greater than 15°, so as to be distinguished from the ferrite structure due to the large number of small-angle grain boundaries in the bainite structure, and the content of each item is calculated through the statistical calculation of the pattern. The difference from the application is that only the large-angle and small-angle grain boundaries are distinguished, and the content of the different grain boundary angles is not displayed.

[0004] Chinese patent application with publication number CN103334069A discloses "heat treatment method for improving performance of 7085 type aluminum alloy", using the method, the grain size of 7085 type aluminum alloy is reduced from 100 microns to micron level, the proportion of small angle grain boundary is increased by 2-3 times, the invention uses EBSD analysis method, shows the EBSD analysis pictures before and after using the method, the grain size is obviously refined, the small angle grain boundary is obviously increased, 2-3 times is estimated, and there is no specific quantitative analysis.

[0005] There are also related reports of grain analysis using EBSD technology in published literature, but most of them are for measurement of large angle grain boundary grain size, and there is no analysis, measurement and quantitative analysis of small angle grain boundary, and due to the particularity of small angle grain boundary, it is necessary to eliminate false images and show the true small angle grain boundary and content. The present application discloses a method for displaying and quantitatively analyzing small angle grain boundaries in high strength steel, which effectively solves the above problems. SUMMARY

[0006] The present application is aimed at the problems in the prior art, and provides a method for displaying and quantitatively analyzing small angle grain boundaries in high strength steel. The technical scheme adopts EBSD (electron backscattering diffraction) technology to display a large number of small angle grain boundaries in high strength steel and quantitatively analyze the percentage content of grain boundaries with different angles. The method fills the gap in the prior art that the microstructure observation method cannot clearly display small angle grain boundaries and quantitatively analyze them, and provides important theoretical support for the study of high strength steel organization refinement strengthening mechanism and the development of on-site cooling process.

[0007] In order to achieve the above purpose, the technical scheme of the present application is as follows: a method for displaying and quantitatively analyzing small angle grain boundaries in high strength steel, the method comprising the following steps:

[0008] Step 1: sample preparation: the sample to be tested is inlaid, polished and polished to form a general metallographic sample, and then vibratory polishing is performed to make the surface to be tested flat and free of residual stress. After protecting the surface of the sample, cut it into a 0.25-2mm thin sheet. According to the above scheme, step (1): 1) in order to eliminate the deformation layer and residual stress formed on the surface of the sample during metallographic grinding and polishing, the metallographic sample is vibratory polished. The principle of vibratory polishing is low frequency and long time. The polishing liquid is selected as 0.05μm alumina suspension. The vibration frequency is 500-1000r / s, and the vibration polishing time is 5-8h. 2) The sample is cut into a 0.25-2mm thin sheet. During EBSD detection, the moving space of the sample is small. In order to obtain a higher counting rate, the surface to be tested needs to be as close to the probe as possible. Cutting the sample into a small sheet ensures a relatively large activity space of the sample, avoids touching the expensive probe, and also helps to improve the counting rate, reduce the odd points and noise. The odd point is a point with no specific orientation difference from the surrounding point in the calibration result, and the noise point is a point that cannot be analyzed in the scanning result. 3) When cutting the sample into a 0.25-2mm thin sheet, the surface of the sample needs to be protected to prevent contamination and damage to the surface to be tested during cutting. The protective film should be fully adhered to the surface of the sample and not easy to fall off, and easy to peel off without contaminating the sample.

[0009] Step 2: Put the prepared sample into the scanning electron microscope sample chamber. After the vacuum reaches the required value, set reasonable electron microscope conditions such as acceleration voltage, beam current, tilt correction, etc., and select the appropriate area to obtain the sample image.

[0010] Step 3: Collect the crystal diffraction pattern by EBSD. According to the above scheme, step (3): 1) open the EBSD controller and extend the EBSD probe into the sample chamber to the specified position. 2) Open the data acquisition software, select reasonable Binning(4x4), Gain(high) and Time per frame(not more than 100) values, get the appropriate brightness of the background, and import the appropriate calibration data. 3) Determine the imaging parameters such as step size, number of measurement points in X / Y direction, magnification, etc. Because the grain size of high-strength steel is small, the subgrain and small-angle grain boundary is very small, so the incident electron beam diameter needs to be as small as possible, the energy needs to be as high as possible, and the scanning step needs to be as small as possible. Under the above conditions, the electron beam diameter is 100-200nm, the step is 0.1-0.2um, and the magnification is 1000-2000 times.

[0011] Step 4: data processing is carried out by using software to display small-angle grain boundaries; according to the above scheme, in step (4): 1) the data collected by EBSD is imported into the analysis software Manager-Data; 2) the grain boundaries data information in the extracted data information is superimposed with the band slope data information, and the grains with a grain boundary angle less than 2° are manually removed, because the grain boundaries less than 2° are mostly caused by sample scratching or uneven sample surface, which is a sample preparation artifact; 3) the processed data is output to form a grain boundary diagram, the picture properties are modified, different colors are displayed according to different angles of the grain boundaries, and the positions and shapes of the grain boundaries with different angles are observed.

[0012] Step 5: data processing is carried out by using software to quantitatively analyze the small-angle grain boundaries; according to the above scheme, in step (5): 1) the grain boundaries superimposed with the band slope data are imported into an EXCEL table; 2) the exported data is optimized and processed, and the grains with a grain boundary angle less than 2° are manually removed; 3) the content of the grain boundaries with different grain boundary angles is statistically analyzed, and the grains in different angle ranges can be counted according to the research needs, and the percentage of the grain boundaries in a range of 1° can be output, so that the percentage of the grain boundaries with any angle can be calculated and compared with the content of the grain boundaries with different angles in different steels.

[0013] Compared with the prior art, the present application has the following advantages: 1) the present application provides a method for displaying and quantitatively analyzing small-angle grain boundaries in high-strength steel, which fills the gap in the existing microstructure observation method that cannot clearly display small-angle grain boundaries and quantitatively analyze them, and provides important theoretical support for the research on the microstructure refinement and strengthening mechanism of high-strength steel and the development of on-site cooling process; 2) the present application efficiently and accurately determines the content of small-angle grain boundaries with different angles by effectively removing the original collected data and combining theoretical analysis and experimental analysis to establish the best data processing method and image display result, solving the problems that the conventional metallographic corrosion method cannot display small-angle grain boundaries and subgrains, and the conventional EBSD method has too many artifacts and cannot effectively display small-angle grain boundaries; 3) the present application achieves the purpose of extracting and quantitatively analyzing grain boundaries with any angle as needed, avoiding the shortcomings of the conventional metallographic corrosion method that small-angle grain boundaries and subgrains cannot be clearly displayed and quantitatively analyzed, and solving the problems of the conventional EBSD analysis method that it cannot distinguish between large-angle and small-angle grain boundaries and has large errors in displaying small-angle grain boundaries, providing a more intuitive and quantitative analysis tool for the research on the substructure of high-strength steel, and having a wide application prospect; 3) the operation process of the present application can realize standardized operation, good sample repeatability, wide adaptability, and easy popularization. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 , Figure 2 is a DP600 different grain boundary angle display map produced by the ultrafast cold hot rolling process of specific embodiment 1.

[0015] Figure 3 , Figure 4 is a DP600 different grain boundary angle display map produced by the conventional hot rolling process of specific embodiment 1.

[0016] Figure 5 , Figure 6 is a BS700 small angle grain boundary and large angle grain boundary display map of 2-15° of specific embodiment 2. DETAILED DESCRIPTION

[0017] In order to deepen the understanding of the present application, the present embodiment will be described in detail below in conjunction with the drawings.

[0018] Embodiment 1: see Figure 1 , Figure 2 A method for displaying and quantitatively analyzing small angle grain boundaries in high-strength steel, the method comprising the following steps: Step 1: taking a sample: embedding, polishing and polishing the sample to be measured to make a general metallographic sample, then performing vibration polishing to make the surface to be measured flat and free of residual stress, and cutting the sample into 0.25-2mm thin slices. According to the above scheme, in step 1: in order to eliminate the deformation layer and residual stress formed on the surface of the sample during metallographic grinding and polishing, the metallographic sample is vibration polished, the principle of vibration polishing is low frequency and long time, the polishing liquid is selected as 0.05μm alumina suspension, the vibration frequency is 500-1000r / s, and the vibration polishing time is 5-8h. 2) Cut the sample into 0.25-2mm thin slices, when EBSD detection, the moving space of the sample is small, in order to get higher counting rate, the surface to be measured needs to be as close to the probe as possible, cutting the sample into small slices ensures the relatively large activity space of the sample, avoids touching the expensive probe, and is also beneficial to improve the counting rate, reduce the noise points and the noise points. The noise points are points that cannot be analyzed in the scanning result. 3) When cutting the sample into 0.25-2mm thin slices, the surface of the sample needs to be protected to prevent contamination and damage to the surface to be measured during cutting. The protective film should be fully adhered to the surface of the sample and not easy to fall off, and easy to peel off without contaminating the sample.

[0019] Step 2: Put the prepared sample into the scanning electron microscope sample chamber, after the vacuum reaches the required value, set reasonable electron microscope conditions such as acceleration voltage, beam current, tilt correction, etc., and select appropriate area to get sample image.

[0020] Step 3: Collecting crystal diffraction pattern by EBSD. According to the above scheme, in step 3: 1) turn on the EBSD controller, and extend the EBSD probe into the sample chamber to a specified position. 2) turn on the data acquisition software, select reasonable Binning (4x4), Gain (high), and Time per frame (not more than 100) values, and import appropriate calibration data to obtain a suitable brightness background. 3) determine imaging parameters such as step size, number of measurement points in X / Y direction, magnification, etc. Since the grain size of high-strength steel is small, and subgrains and small-angle grain boundaries are very small, it is necessary to use an electron beam with the smallest possible diameter, the highest possible energy, and the smallest possible scanning step. Under the above conditions, the electron beam diameter is 100-200 nm, the step size is 0.1-0.2 um, and the magnification is 1000-2000 times.

[0021] Step 4: Using software to process and display small-angle grain boundaries. According to the above scheme, in step 4: 1) import the data collected by EBSD into the analysis software Manager-Data; 2) extract the grain boundaries data information in the data information, and in order to display the small-angle grain boundaries, superimpose the band slope data information, and manually remove the grains with a grain boundary angle less than 2°, because the grain boundary angle less than 2° is mostly caused by sample scratching or uneven sample surface, which is a sample artifact; 3) output the processed data to form a grain boundary map, modify the picture properties, and display different colors according to different angles of the grain boundaries, and observe the position and morphology of the grain boundaries with different angles.

[0022] Step 5: Using software to process and quantitatively analyze the small-angle grain boundaries. According to the above scheme, in step 5: 1) import the grain boundaries superimposed with band slope data into an EXCEL table; 2) optimize the exported data and manually remove the grains with a grain boundary angle less than 2°; 3) statistically analyze the content of grain boundaries with different grain boundary angles, and according to the research needs, the grains within different angle ranges can be counted, and the percentage of grain boundaries within 1° range can be output, so that the percentage of grain boundaries with any angle can be calculated and compared with the content of grain boundaries with different angles in different steels. Specific embodiment 1:

[0024] Specific embodiment 1: A low-cost DP600 hot-rolled dual-phase steel with a chemical composition by weight percentage of C: 0.052%, Si: 0.16%, Mn: 1.04%, P: 0.012%, S: 0.002%, Cr: 0.50%, and Al: 0.037% is selected, and the steel is produced by ultra-fast cooling hot rolling process and conventional hot rolling process, respectively. The distribution and content of large-angle and small-angle grain boundaries under the two hot rolling processes are compared and analyzed by the method of the present application, and the specific analysis process is as follows:

[0025] (1) The sample to be tested is inlaid, polished and polished to make a general metallographic sample, and then vibration polishing is performed to make the surface to be tested flat and free of residual stress. The polishing liquid for vibration polishing is 0.05 μm alumina suspension, the vibration frequency is 1000 r / s, and the vibration polishing time is 6 h. After protecting the sample surface, cut into 1 mm thin slices.

[0026] (2) The prepared sample is placed in the scanning electron microscope sample chamber, and after the vacuum reaches the required value, the electron microscope conditions such as reasonable acceleration voltage, beam current, tilt correction, etc. are set, and the appropriate area is selected to obtain the sample image.

[0027] (3) The EBSD probe is inserted into the sample chamber to the specified position, and the crystal diffraction pattern is collected, and the crystallographic data is saved. The key parameters when collecting data are: Binning value is 4x4, Gain is high, and Time per frame is 100, to get the appropriate brightness of the background; In the imaging parameters, the electron beam diameter is 200 nm, the step is 0.2 um, and the magnification is 1000 times.

[0028] (4) Extract the grain boundary data implied in the software, output and superimpose the grain boundaries and band slope data, and manually remove the grain boundaries with an angle less than 2°. The processed data is output in the Manager-Data software, and the small-angle grain boundaries of 2-15 degrees are displayed with red lines, and the large-angle grain boundaries greater than 15 degrees are displayed with black lines.

[0029] (5) The above processed data is output to an EXCEL table or other data processing software, and the grain boundary content of 2-15°, 16-40°, and 41-62.5° of the two kinds of hot rolling processes are counted respectively, as shown in Table 1.

[0030] This example compares and analyzes the distribution and content of large and small angle grain boundaries of DP600 dual-phase steel under two kinds of hot rolling processes, Figure 1 , Figure 3 , the small-angle grain boundary is 2-15°, Figure 2 , Figure 4 , and the large-angle grain boundary is greater than 15°. As can be seen from the figure, the total amount of grain boundaries and the number of small-angle grain boundaries of the ultra-fast cooling hot rolling process are significantly more than those of the conventional hot rolling process. Table 1 shows the content of grain boundaries of different angles. The content of small-angle grain boundaries of 2-15° of the ultra-fast cooling process is 19.79%, and that of the conventional process is 10.35%, which is twice that of the former.

[0031] Table 1: Content of large and small angle grain boundaries of DP600

[0032] Example 2

[0034] The steel grade of Example 2 is BS700, and its chemical composition is C: 0.07%, Si: 0.15%, Mn: 1.80%, P≤0.015%, S≤0.003%, Cr: 0.50%, Nb: 0.06%, Ti: 0.09%, Mo: 0.12%, Al: 0.030%, the hot rolling process is that the tapping temperature is 1250℃, the rough rolling temperature is 1050℃, the finish rolling temperature is 850℃, and the coiling temperature is 600℃, and the BS700 produced by the above hot rolling process is subjected to the display and quantitative analysis of the low-angle grain boundaries and high-angle grain boundaries by using the method of the present application, and the specific analysis process is as follows:

[0035] (1) The sample to be measured is embedded, polished and polished to form a general metallographic sample, and then vibration polishing is performed to make the surface to be measured flat and free of residual stress, the polishing liquid for vibration polishing is 0.05μm alumina suspension, the vibration frequency is 1000r / s, and the vibration polishing time is 6h. After protecting the sample surface, cut into 2mm thin slices.

[0036] (2) Put the prepared sample into the scanning electron microscope sample chamber, after the vacuum reaches the requirement, set reasonable electron microscope conditions such as acceleration voltage, beam current, tilt correction, etc., and select the appropriate area to obtain the sample image.

[0037] (3) The EBSD probe is inserted into the sample chamber to the specified position, and the crystal diffraction pattern is collected, and the crystallographic data is saved. The key parameters during data collection are: Binning value is 4x4, Gain is high, and Time per frame is 80, to obtain the appropriate brightness background; in the imaging parameters, the electron beam diameter is 1500nm, the step length is 0.1um, and the magnification is 2000 times.

[0038] (4) Extract the grain boundary data implied in the software, output and superimpose the grain boundaries and band slope data, and manually remove the grain boundaries with an angle less than 2°, and output the processed data in the Manager-Data software, Figure 5 The small-angle grain boundaries are 2-15 degrees, Figure 6 The high-angle grain boundaries are greater than 15 degrees, and other angle grain boundaries can also be selected according to the needs, or even different angle grain boundaries are displayed in different colors in one picture.

[0039] (5) The above processed data is output to the EXCEL table or other data processing software, and the content of different angle grain boundaries is displayed every 1°, as shown in Table 1.

[0040] Table 2 BS700 Different Angle Grain Boundary Content (%)

[0041]

[0042] It should be noted that the above examples are not intended to limit the scope of the present application, and any equivalent variations or substitutions made on the basis of the above technical solutions fall within the scope of the claims of the present application.

Claims

1. A method for displaying and quantitatively analyzing low-angle grain boundaries in high-strength steel, characterized by, The method comprises the following steps: Step 1: sample preparation: the sample to be tested is inlaid, polished and polished to form a general metallographic sample, then vibration polishing is performed to make the surface to be tested flat and free of residual stress, the sample surface is protected, and then the sample is cut into a 0.25-2mm thin slice; Step 2: place the prepared sample into the scanning electron microscope sample chamber, set reasonable acceleration voltage, beam current and tilt correction electron microscope conditions after vacuum reaches the requirement, and select a suitable area to obtain a sample image; Step 3: collect the crystal diffraction pattern by EBSD; Step 4: use software to process data and display low-angle grain boundaries; the specific process is as follows, in step 4: 1) import the data collected by EBSD into the analysis software Manager-Data; 2) extract the grain boundaries data information in the data information, in order to display the low-angle grain boundaries, superimpose the band slope data information, and manually remove the grains with a grain boundary angle less than 2°; 3) output the processed data to form a grain boundary diagram, modify the picture properties, display different colors according to the different angles of the grain boundaries, and observe the position and morphology of the grain boundaries with different angles; Step 5: use software to process data and quantitatively analyze low-angle grain boundaries; use software to process data and quantitatively analyze low-angle grain boundaries, as follows: 1) import the grain boundaries superimposed band slope data into an EXCEL table; 2) optimize the exported data and manually remove the grains with a grain boundary angle less than 2°; 3) statistically analyze the content of grain boundaries with different grain boundary angles, and according to the research needs, count the grains in different angle ranges, output the grain boundary percentage in the range of 1° at least, calculate the grain boundary percentage of any grain boundary angle, and compare the content of grain boundaries with different angles of different steel types.

2. The method of displaying and quantifying small angle grain boundaries in high-strength steel according to claim 1, characterized by, Step 1: sample preparation: as follows: in step 1: the metallographic sample is vibration polished, the principle of vibration polishing is low frequency and long time, the polishing liquid is selected as 0.05um alumina suspension liquid, the vibration frequency is 500-1000r / s, and the vibration polishing time is 5-8h; the sample is cut into a 0.25-2mm thin slice.

3. The method for displaying and quantitatively analyzing low-angle grain boundaries in high-strength steel according to claim 2, characterized by, The step 3: collect the crystal diffraction pattern by EBSD, as follows, in step 3: 1) open the EBSD controller, extend the EBSD probe into the sample chamber to the specified position, 2) open the data acquisition software, select reasonable Binning, Gain (high) and Time per frame values, get the appropriate brightness background, import appropriate calibration data, 3) determine the imaging parameters, including step size, X / Y direction measurement point number, magnification, electron beam diameter is 100-200nm, step size is 0.1-0.2um, magnification is 1000-2000 times.

Images