Method for measuring austenite grain size of steel

Abstract

This invention relates to a method for measuring the austenite grain size of steel. For steel with a carbon content of 0.35-0.45%, a thermal simulation testing machine is used to heat the steel to 850-1200℃ and hold it for 10-60 minutes. The cooling rate is precisely controlled, with the round bar cooled to room temperature at a rate of 5-15℃ / s. The resulting sample forms a uniform network of ferrite, clearly and accurately revealing the austenite grains. This invention can accurately measure the austenite grain size and has high reproducibility, meeting the needs of scientific research and production guidance.

Description

Technical Field

[0001] This invention relates to a method for measuring the austenite grain size of steel, belonging to the field of grain size detection technology. Background Technology

[0002] Methods for determining the austenite grain size of steel include carburizing, ferrite network method, direct hardening method, and cementite network method. For carbon steel with a carbon content >1%, the cementite network method is generally used; for carbon steel with a carbon content of 0.60% < carbon and ≤1%, martensitic structure can be obtained through quenching, and the direct hardening method is commonly used; for carbon steel with a carbon content <0.25%, carburizing is generally used. For alloy steel, the direct hardening method and ferrite network method are mainly used.

[0003] Since carbon steel with a carbon content of 0.25% < 0.60% has poor hardenability, the most commonly used method for measuring austenite grain size is the ferrite mesh method. The so-called ferrite mesh method involves heating the steel sample to above 850°C, holding it at that temperature for at least 30 minutes, and then air-cooling, oil-cooling, or water-cooling it. After polishing and etching, the austenite grains are revealed by the ferrite mesh distributed along the grain boundaries. The disadvantages of this method are: (1) The cooling rate cannot be precisely controlled and adjusted, making the transformation of the mesh ferrite uncontrollable, which will ultimately affect the accuracy of austenite grain size measurement. For example: the mesh ferrite is discontinuous (as shown in the attached figure). Figure 1 As shown), excessive growth of network ferrite intruding into the grain interior (as shown in the attached image). Figure 2 (2) During the cooling process, the cooling rate from the surface to the center of the sample is inconsistent, which may lead to inconsistent morphology of the network ferrite from the surface to the center, or the microstructure from the surface to the center may be martensite, network ferrite + pearlite and pearlite + blocky ferrite, which will make it impossible to assess the uniformity of the grains. (3) The air cooling, oil cooling and water cooling methods used in this method are greatly affected by the environment, and the reproducibility of the measurement results is poor.

[0004] Existing methods for measuring the austenite grain size of carbon steel, such as the invention disclosed in application number 201711285133.8, involve heating the sample to 850-1200℃ using a thermal simulation test, holding it at that temperature for 10-60 minutes, then cooling it at a rate of 30 K / s to 700-720℃, holding it at that temperature for 10 minutes, and finally cooling it to room temperature. This method causes a network of cementite to form at the austenite grain boundaries in the high-silicon, high-carbon steel. The austenite grain boundaries are then revealed through etching with sodium picrate solution, and finally, the austenite grain size is measured using an optical microscope. However, research has found that this method is only suitable for high-silicon, high-carbon steel wire rods (carbon content exceeding 0.87%), measuring the austenite grain size through the network of cementite formed at the austenite grain boundaries. This method is not applicable to medium and low carbon steels (carbon content ≤ 0.60%), mainly because a network of cementite cannot be formed. At the same time, the use of sodium picrate for corrosion is detrimental to environmental protection and the health of laboratory personnel.

[0005] Therefore, there is a need to provide a method for measuring the austenite grain size of steel to solve the above problems. Summary of the Invention

[0006] This invention provides a method for measuring the austenite grain size of steel, which can accurately measure the austenite grain size and has high test reproducibility.

[0007] The technical solution adopted by this invention to solve its technical problem is: A method for measuring the austenite grain size of steel, for steel with a carbon content of 0.35~0.45%, includes the following steps: Step S1: Process the steel into several round bars, each round bar being a sample. The length of each round bar is 80±20mm and the diameter is 8±2mm. Step S2: Place the sample on a thermal simulation tester and heat it to 850~1200℃. Hold it at that temperature for 10~60 minutes and continue to cool the round bar to room temperature at a cooling rate of 5~15℃ / s. Step S3: Continue to cut the sample laterally to obtain a sample with a length of 5-8 mm; Step S4: Select a suitable hot mounting material for metallographic testing, turn on the mounting machine, and hot mount the sample. After mounting is completed, turn off the mounting machine and wait for the sample to cool to room temperature before taking out the fully mounted sample. Step S5: Fix the inlaid sample on the grinding equipment. Make sure the observation surface of the sample is in uniform contact with the sandpaper surface. Turn on the grinding equipment and first use coarse sandpaper to grind the sample until there are no obvious protrusions on the observation surface. Then continue to use medium sandpaper until the coarse scratches on the observation surface disappear. Finally, use fine sandpaper until the surface of the observation surface is smooth and flat. Step S6: Wipe the sample clean after fine grinding in step S5, place the observation surface of the sample against the surface of the polishing cloth, turn on the polishing machine, and let the sample rotate at a constant speed with the turntable until the observation surface reaches a mirror gloss. Step S7: Immerse the observation surface of the polished and cleaned sample in a nitric acid alcohol solution for etching. After etching, rinse the observation surface with water and alcohol, and dry it with a hair dryer. Step S8: Take photographs using a metallographic microscope to measure the austenite grain size; Furthermore, in step S4, the thermal embedding parameters are set, with the temperature controlled at 180±5℃, the pressure at 200±5 bar, and the heat and pressure holding time at 5.0±0.5 min. Furthermore, step S5, the step of polishing the inlaid sample, includes: Step S51: Select 200-grit coarse sandpaper, turn on the grinding equipment, set the grinding equipment speed to 200±10 r / min, and grind the sample. During the grinding process, continuously rinse the observation surface with clean water until there are no obvious protrusions on the observation surface. Step S52: Replace with 400-grit medium sandpaper, rotate the sample 90°, set the grinding equipment speed to 220±10 r / min, and continue grinding the observation surface of the sample. Keep rinsing with clean water until the rough scratches on the observation surface disappear. Step S53: Replace with 800-grit sandpaper, rotate the sample 90°, set the grinding equipment speed to 240±10 r / min, and continue grinding the observation surface of the sample. Keep it rinsed with clean water to reduce scratches on the observation surface. Step S54: Replace with 1200-grit sandpaper, rotate the sample 90°, set the grinding equipment speed to 260±10 r / min, and continue grinding the observation surface of the sample. Keep rinsing with clean water until there are no visible scratches on the observation surface. Furthermore, in step S6, the polishing machine speed is set to 310±10 r / min, and polishing agent is continuously added during the polishing process. The polishing time is controlled to be 3~5 min. A metallographic polishing agent with a roughness of 3~5 μm is selected. Furthermore, in step S7, the nitric acid concentration in the nitric acid alcohol solution is 4~6%, and the corrosion time is 20±10s; Furthermore, in step S8, the austenite grain size is measured, specifically: Step S81: Place the sample under a metallographic microscope for imaging to obtain several typical tissue morphology photographs; Step S82: Import the metallographic image software, use the cross-section method to draw four cross-sections in each image, count the number of austenite grains passed through each cross-section, and calculate the average intercept of the austenite grains. Step S83: The average value of the average intercepts obtained from several photographs is then calculated and used as the austenite grain size of the sample. Furthermore, the steel using the aforementioned austenite grain size measurement method also includes the following chemical composition: Si ≤0.35%, Mn: 0.60~0.90%, P ≤0.030%, S ≤0.030%, Cr: 0.02~0.30%, with the remainder being Fe and unavoidable impurities.

[0008] By employing the above technical solutions, the present invention has the following beneficial effects compared to the prior art: 1. The method for measuring the austenite grain size of steel provided by the present invention uses a thermal simulation testing machine to precisely control the cooling rate, and the resulting sample forms a uniform network of ferrite, which can clearly and accurately show the austenite grains. 2. The austenite grain size measurement method for steel provided by this invention relies on precise control of the cooling rate. At the same time, the entire metallographic sample preparation process, from hot mounting, step-by-step grinding, mechanical polishing, to nitric acid alcohol etching, metallographic microscopic observation, and grain size measurement using the cross-section method, achieves reproducibility and standardization of the operation process, ensuring the authenticity and integrity of the metallographic morphology and maximizing the accuracy of the measurement. Attached Figure Description

[0009] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0010] Figure 1 This is an image of excessive growth of network ferrite; Figure 2 It is an image of discontinuous network ferrite; Figure 3 It is an image of a uniform network of ferrite. Detailed Implementation

[0011] To make the objectives, technical solutions, and advantages of this invention clearer, the design scheme of the steel smelting components and the corresponding preparation process of this invention will be described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention. Raw materials, equipment, and operating steps not specifically described herein are all conventional technologies in the field of steel smelting.

[0012] As described in the background section, the existing methods for measuring the austenite grain size of wire rod are mainly for high-carbon steel (carbon content ≥ 0.87%), which forms a network of cementite at the austenite grain boundaries. However, for medium and low-carbon steel (carbon content ≤ 0.60%), the network of cementite cannot be obtained due to the low carbon content. Obviously, this method cannot achieve the technical objective of measuring the austenite grain size.

[0013] To address the aforementioned problems, this application provides a method for measuring the austenite grain size of steel. The main innovation lies in the precise control of the cooling rate for steel with a C content of 0.35~0.45%, enabling the formation of a uniform network of ferrite in the sample. This clearly and accurately reveals the austenite grains, ultimately achieving precise measurement of the austenite grain size to meet the needs of scientific research and production guidance. Preferably, the steel for which this measurement method is applied has a chemical composition including Si ≤0.35%, Mn: 0.60~0.90%, P ≤0.030%, S ≤0.030%, Cr: 0.02~0.30%, with the remainder being Fe and unavoidable impurities.

[0014] The method for measuring the austenite grain size of steel includes the following steps: Step S1: Process the steel into several round bars, each round bar being a sample. The length of each round bar is 80±20mm and the diameter is 8±2mm.

[0015] Step S2: Place the sample on a thermal simulation tester and heat it to 850~1200℃. Hold it at that temperature for 10~60 minutes and continue to cool the round bar to room temperature at a cooling rate of 5~15℃ / s.

[0016] Step S3: Continue to cut the sample laterally to obtain a sample with a length of 5~8mm.

[0017] Step S4: Select a suitable hot mounting material for metallographic testing, turn on the mounting machine, set the hot mounting parameters: temperature controlled at 180±5℃, pressure at 200±5 bar, and holding time at temperature and pressure at 5.0±0.5 min. After mounting is completed, turn off the mounting machine and wait for the sample to cool to room temperature before removing the fully mounted sample. After removal, check whether the mounting is intact, free of cracks and bubbles, and whether the observation surface is flat. If defects are found, remount the sample.

[0018] Step S5: Fix the inlaid sample onto the polishing equipment, ensuring the sample's observation surface is evenly in contact with the sandpaper surface. Turn on the polishing equipment, first using coarse sandpaper to polish the sample until there are no obvious protrusions on the observation surface. Continue using medium sandpaper until the coarse scratches on the observation surface disappear. Finally, use fine sandpaper until the observation surface is smooth and flat. The purpose of polishing in this step is to remove the rough surface layer and residual inlay material from the inlaid sample's observation surface, laying the foundation for subsequent polishing. As one of the innovative points, relying on precisely controlled cooling rates, it follows the principle of polishing from coarse to fine, step by step, avoiding problems such as excessively deep scratches and structural deformation. The specific steps include: Step S51: Select 200-grit coarse sandpaper, turn on the grinding equipment, set the grinding equipment speed to 200±10 r / min, and grind the sample. During the grinding process, continuously rinse the observation surface with clean water until there are no obvious protrusions on the observation surface. Step S52: Replace with 400-grit medium sandpaper, rotate the sample 90°, set the grinding equipment speed to 220±10 r / min, and continue grinding the observation surface of the sample. Keep rinsing with clean water until the rough scratches on the observation surface disappear. Step S53: Replace with 800-grit sandpaper, rotate the sample 90°, set the grinding equipment speed to 240±10 r / min, and continue grinding the observation surface of the sample. Keep it rinsed with clean water to reduce scratches on the observation surface. Step S54: Replace with 1200-grit sandpaper, rotate the sample 90°, set the grinding equipment speed to 260±10 r / min and continue grinding the observation surface of the sample. Keep rinsing with clean water until there are no visible scratches on the observation surface.

[0019] Step S6: Wipe the sample clean after fine grinding in step S5. Place the sample observation surface against the polishing cloth surface, turn on the polishing machine, and set the polishing machine speed to 310±10 r / min. During the polishing process, continuously add polishing agent, and control the polishing time to 3~5 min until the observation surface achieves a mirror finish. Use a metallographic polishing agent with a roughness of 3~5μm.

[0020] Step S7: Immerse the observation surface of the polished and cleaned sample in a nitric acid-alcohol solution for etching. After etching, rinse the observation surface with water and alcohol, and dry it with a hairdryer. The purpose of etching is to chemically expose different microstructures within the sample to reveal pearlite and network ferrite, thereby confirming the austenite grains through the morphology of the network ferrite. For optimal etching results, the preferred nitric acid concentration in the nitric acid-alcohol solution is 4-6%, and the etching time is 20 ± 10 seconds. After etching, remove the sample, rinse the observation surface with water, and clean it thoroughly.

[0021] Step S8: The austenite grain size is measured using a metallographic microscope. Specifically: Step S81: Place the sample under a metallographic microscope for imaging to obtain several typical tissue morphology photographs; Step S82: Import the metallographic image software, use the cross-section method to draw four cross-sections in each image, count the number of austenite grains passed through each cross-section, and calculate the average intercept of the austenite grains. Step S83: The average value of the average intercepts obtained from several photographs is then calculated and used as the austenite grain size of the sample.

[0022] This application continues to provide Examples 1, 2, 1, 2, 3, and 4 to better understand this application. Among them, Comparative Example 2 is based on the same wire rod as Example 1, and the sample is prepared using the method provided by the patent with application number 200810227821.3. The method provided by this patent is a commonly used method at present, and the results obtained have good accuracy.

[0023] The chemical composition of the wire rods used in the examples and comparative examples is shown in Table 1. Apart from the components shown in the table, the rest are Fe and unavoidable impurities.

[0024] Table 1 Chemical composition / %

[0025] Example 1:

[0026] The first step is to use hot-rolled wire rod with a diameter of 8mm to process it into round bars with a length of 80mm and a diameter of 8mm. The second step is to heat the round bar to 900℃ on a thermal simulation testing machine, hold it at that temperature for 30 minutes, and then cool it to room temperature at a cooling rate of 15℃ / s. The third step is to cut along the cross-section at the position where the thermocouple is welded to the round bar and at a distance of 8mm from the position where the thermocouple is welded. The fourth step is to perform thermal mounting on the cut sample (the metallographic microscope observation surface is the cross-section at the thermocouple position). The fifth step is to polish using 200-grit, 400-grit, 800-grit, and 1200-grit metallographic sandpaper; Step 6: Polish the sample using a 5μm metallographic polishing agent; Step 7: Etch the sample with a 6% nitric acid alcohol solution for 20 seconds, then rinse and dry with alcohol. Step 8: Take photographs using a metallographic microscope and measure the austenite grain size using the section method in metallographic image software.

[0027] Comparative Example 1: Using the round bar obtained in Example 1, the heat-treated specimens were prepared according to the ferrite mesh method in GB / T6493: the round bar was heated to 900℃ and held for 30 min, then the temperature was lowered to 730℃ and held for 10 min, followed by quenching treatment with oil as the quenching medium. The round bar was then cut along its cross-section, and the resulting samples were hot-mounted and polished using 200-grit, 400-grit, 800-grit, and 1200-grit metallographic sandpaper; the samples were polished using a 5μm metallographic polishing agent; the samples were etched with a 6% nitric acid alcohol solution for 20 seconds, then rinsed with alcohol and dried; the samples were photographed using a metallographic microscope, and the austenite grain size was measured using the section method in metallographic image software.

[0028] Comparative Example 2: The round bar obtained in Example 1 was cut into cylindrical shapes with a diameter of 8 mm and a height of 3.5 mm. The cylindrical shapes were ground and polished according to the metallographic sample preparation method, and then cleaned with acetone in an ultrasonic cleaner and dried. After the sample preparation was completed, it was placed in a heating furnace and heated to 900°C under a high-temperature laser confocal microscope. After holding at that temperature for 30 minutes, the austenite grains could be seen directly. A photograph of the austenite grains was taken, and the size of the austenite grains was measured using the section method of metallographic image software.

[0029] Example 2:

[0030] The first step is to use hot-rolled wire rod with a diameter of 6mm to process it into round bars with a length of 60mm and a diameter of 6mm. The second step is to heat the round bar to 850°C on a thermal simulation testing machine, hold it at that temperature for 60 minutes, and then cool it to room temperature at a cooling rate of 10°C / s. The third step is to cut along the cross-section at the position where the thermocouple is welded to the round bar and at a distance of 5mm from the position where the thermocouple is welded. The fourth step is to perform thermal mounting on the cut sample (the metallographic microscope observation surface is the cross-section at the thermocouple position). The fifth step is to polish using 200-grit, 400-grit, 800-grit, and 1200-grit metallographic sandpaper; Step 6: Polish the sample using a 3μm metallographic polishing agent; Step 7: Etch the sample with a 4% nitric acid alcohol solution for 10 seconds, then rinse and dry with alcohol. Step 8: Take photographs using a metallographic microscope and measure the austenite grain size using the section method in the metallographic imaging software.

[0031] Comparative Example 3: The round bar obtained in Example 2 was heated to 850°C on a thermal simulation testing machine, held at that temperature for 60 min, and then cooled to room temperature at a cooling rate of 30°C / s. The bar was then cut along its cross-section at the location where the thermocouple was welded and 5 mm away from the welding point. The resulting sample was then thermally mounted (the metallographic microscope observation surface was the cross-section at the thermocouple location). It was then polished using 200-grit, 400-grit, 800-grit, and 1200-grit metallographic sandpaper. The sample was polished with a 3 μm metallographic polishing agent, etched with a 4% nitric acid alcohol solution, and rinsed and dried with alcohol after 10 seconds. Metallographic microscopy was used to photograph the sample, and the austenite grain size was measured using the section method in metallographic image software.

[0032] Comparative Example 4: The round bar obtained in Example 2 was heated to 850°C on a thermal simulation testing machine and held at that temperature for 60 minutes. It was then cooled to room temperature at a cooling rate of 2°C / s. The bar was then cut along its cross-section at the location where the thermocouple was welded and at a distance of 5 mm from the welding thermocouple. The resulting sample was then thermally mounted (the metallographic microscope observation surface was the cross-section at the thermocouple location). The sample was then polished using 200-grit, 400-grit, 800-grit, and 1200-grit metallographic sandpaper. The sample was polished with 3 μm metallographic polishing agent and etched with 4% nitric acid alcohol solution. After etching for 10 seconds, the sample was rinsed with alcohol and dried. The sample was photographed using a metallographic microscope, and the austenite grain size was measured using the section method in metallographic image software.

[0033] The final austenite grain size of the measured wire rod is shown in Table 2.

[0034] Table 2

[0035] Comparative Example 1 was due to excessive growth of network ferrite (e.g. Figure 1 As shown in the figure, the austenite grain size of Example 1 is 4.14 μm larger than that of Comparative Example 1, and the grain size is reduced by 0.54 grades. The austenite grain size measured in Example 1 differs from that in Comparative Example 2 by 0.17 μm. Therefore, it can be proved that the measurement results of Example 1 are more accurate.

[0036] Because the cooling rate of Comparative Example 3 was too high, the resulting network ferrite was discontinuous (e.g., Figure 2 As shown), this resulted in an overestimation of the austenite grain size measurement. Therefore, the austenite grain size in Example 2 was reduced by 22.71 μm compared to Comparative Example 3, and the grain size was increased by 1.45 grades. However, Example 2 generated a uniform network ferrite (such as...). Figure 3As shown in the figure, the austenite grains are accurately displayed, therefore, compared to Comparative Example 3, the austenite grain size measurement in Example 2 is more accurate. However, compared to Comparative Example 4, Example 2 has a smaller austenite grain size measurement because the cooling rate in Comparative Example 4 was lower, leading to excessive growth of network ferrite. Therefore, it can be proven that the measurement results in Example 2 are more accurate.

[0037] In summary, the austenite grain size measurement method for steel provided in this application allows for precise control of the cooling rate, resulting in a uniform and consistent network of ferrite in the obtained sample, which clearly and accurately reveals the austenite grains. The advantages of this method are its accurate measurement of austenite grain size and high experimental reproducibility, meeting the needs of scientific research and production guidance.

[0038] Those skilled in the art will understand that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as herein.

[0039] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method of measuring austenite grain size of steel, characterized by: For steel with a C content of 0.35~0.45%, also including Si ≤0.35%, Mn: 0.60~0.90%, P ≤0.030%, S ≤0.030%, Cr: 0.02~0.30%, with the remainder being Fe and unavoidable impurities, the process includes the following steps: Step S1: Process the steel into several round bars, each round bar being a sample. The length of each round bar is 80±20mm and the diameter is 8±2mm. Step S2: Place the sample on a thermal simulation tester and heat it to 850~1200℃. Hold it at that temperature for 10~60 minutes and continue to cool the round bar to room temperature at a cooling rate of 5~15℃ / s. Step S3: Continue to cut the sample laterally to obtain a sample with a length of 5~8mm; Step S4: Select a suitable hot mounting material for metallographic testing, turn on the mounting machine, and hot mount the sample. After mounting is completed, turn off the mounting machine and wait for the sample to cool to room temperature before taking out the fully mounted sample. Step S5: Fix the inlaid sample on the grinding equipment. Make sure the observation surface of the sample is in uniform contact with the sandpaper surface. Turn on the grinding equipment and first use coarse sandpaper to grind the sample until there are no obvious protrusions on the observation surface. Then continue to use medium sandpaper until the coarse scratches on the observation surface disappear. Finally, use fine sandpaper until the surface of the observation surface is smooth and flat. Step S6: Wipe the sample clean after fine grinding in step S5, place the observation surface of the sample against the surface of the polishing cloth, turn on the polishing machine, and let the sample rotate at a constant speed with the turntable until the observation surface reaches a mirror gloss. Step S7: Immerse the observation surface of the polished and cleaned sample in a nitric acid alcohol solution for etching. After etching, rinse the observation surface with water and alcohol, and dry it with a hair dryer. Different internal structures of the sample show different degrees of etching, revealing pearlite and network ferrite. Step S8: Take photographs using a metallographic microscope to measure the austenite grain size.

2. The method of measuring austenite grain size of steel according to claim 1, characterized by: In step S4, the thermal embedding parameters are set, with the temperature controlled at 180±5℃, the pressure at 200±5 bar, and the heat and pressure holding time at 5.0±0.5 min.

3. The method of measuring austenite grain size of steel according to claim 1, characterized by: Step S5, the step of polishing the inlaid sample, includes: Step S51: Select 200-grit coarse sandpaper, turn on the grinding equipment, set the grinding equipment speed to 200±10 r / min, and grind the sample. During the grinding process, continuously rinse the observation surface with clean water until there are no obvious protrusions on the observation surface. Step S52: Replace with 400-grit medium sandpaper, rotate the sample 90°, set the grinding equipment speed to 220±10 r / min, and continue grinding the observation surface of the sample. Keep rinsing with clean water until the rough scratches on the observation surface disappear. Step S53: Replace with 800-grit sandpaper, rotate the sample 90°, set the grinding equipment speed to 240±10 r / min, and continue grinding the observation surface of the sample. Keep it rinsed with clean water to reduce scratches on the observation surface. Step S54: Replace with 1200-grit sandpaper, rotate the sample 90°, set the grinding equipment speed to 260±10 r / min, and continue grinding the observation surface of the sample. Keep rinsing with clean water until there are no visible scratches on the observation surface.

4. The method of measuring austenite grain size of steel according to claim 1, characterized by: In step S6, the polishing machine speed is set to 310±10 r / min. During the polishing process, polishing agent is continuously added, and the polishing time is controlled to be 3~5 min. Metallographic polishing agent with a roughness of 3~5 μm is selected.

5. The method of measuring austenite grain size of steel according to claim 1, characterized by: In step S7, the nitric acid concentration in the nitric acid alcohol solution is 4-6%, and the corrosion time is 20±10s.

6. The method of measuring austenite grain size of steel according to claim 1, characterized by: In step S8, the austenite grain size is measured, specifically: Step S81: Place the sample under a metallographic microscope for imaging to obtain several typical tissue morphology photographs; Step S82: Import the metallographic image software, use the cross-section method to draw four cross-sections in each image, count the number of austenite grains passed through each cross-section, and calculate the average intercept of the austenite grains. Step S83: The average value of the average intercepts obtained from several photographs is then calculated and used as the austenite grain size of the sample.

Images