A method for determining the grain size of a lath martensitic steel by combining metallographic detection and EBSD detection

Abstract

The application discloses a method for determining the grain size of lath martensite steel by combining metallographic detection with EBSD detection, which comprises the following steps: determining the original austenite grain size grade of the martensite steel sample to be detected by the metallographic method; obtaining the maximum diameter of the fitting ellipse or the maximum Feret diameter by using the area weighted average value of the EBSD detection; converting the maximum diameter of the fitting ellipse or the maximum Feret diameter into the corresponding martensite lath grain size grade; and then establishing a quantitative corresponding relationship between the original austenite grain size grade and the martensite lath grain size grade. In the subsequent batch detection, the original austenite grain size grade of the sample to be detected is determined by the metallographic method, and then the martensite lath grain size grade can be obtained through the established quantitative corresponding relationship between the original austenite grain size grade and the martensite lath grain size grade. The detection cycle is shortened, the use amount of high-risk chemicals is significantly reduced, the detection cost is reduced, and the urgent needs of large-scale industrial production for the detection of a large amount of martensite steel grain size are met.

Description

Technical Field

[0001] This invention belongs to the field of lath martensitic steel testing technology, specifically relating to a method for determining the grain size of lath martensitic steel by combining metallographic testing and EBSD testing. Background Technology

[0002] The discovery of martensitic steel dates back to the 1890s, when it was first identified in mineral studies by German metallurgist Adolf Martens (1850–1914). Martensitic steel is an important microstructure in iron-carbon alloys, and its high strength, hardness, and wear resistance make it widely used in machinery manufacturing, aerospace, and other fields.

[0003] When assessing grain size using metallographic corrosion observation, only the original austenite grains can typically be evaluated. Although some corrosion methods can reveal the martensitic lath structure, the mixed and interwoven martensitic laths make it difficult for metallographic methods to distinguish the orientation of the martensitic lath grains, and the size of the martensitic laths is also difficult to measure. Since the mechanical properties of martensitic steel are significantly affected by the morphology and grain size of the martensitic laths, the assessment results cannot fully reflect product quality.

[0004] Therefore, in the grain size assessment of martensitic steels, EBSD is usually required to determine the grain size. According to the national standard GB / T 36165-2018, the determination of the average grain size of metals by electron backscatter diffraction (EBSD) requires a certain number of grains and regions to be assessed. At the same time, the EBSD method has high requirements for sample preparation, high equipment requirements, and difficulties in post-processing data. Assessing the grain size of a single sample may require a lot of time and resources, or a considerable amount of experimental equipment and hazardous chemicals (such as perchloric acid, methanol, phosphoric acid, chromic anhydride, etc.).

[0005] Extensive use of EBSD for grain size testing may lead to either failure to meet the practical requirements of industry, especially large-scale production, in terms of time and cost, or adverse impacts on safety and the environment. It also increases the actual economic cost of testing, significantly limiting its application in industrial production. Table 1 compares the time, equipment, and chemicals required for metallographic grain size assessment according to national standard GB / T 6394-2017 and EBSD testing according to national standard GB / T 36165-2018.

[0006] Summary of the Invention

[0007] The purpose of this invention is to provide a method for determining the grain size of lath martensitic steel by combining metallographic testing and EBSD testing. This method effectively shortens the testing cycle, significantly reduces the use of high-risk chemicals (such as picric acid, chromic anhydride, and other corrosive agents), and reduces reliance on expensive and precision equipment (such as field emission scanning electron microscopes (FE-SEM) equipped with electron backscatter diffraction (EBSD) detectors), thus meeting the urgent need for large-scale industrial production to detect the grain size of martensitic steel in large quantities.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows: A method for determining the grain size of lath martensitic steel by combining metallographic testing and EBSD testing specifically includes the following steps: S1: Perform EBSD testing on the martensitic steel sample to be tested to obtain EBSD data including grain crystallographic orientation and morphology; and determine its original austenite grain size grade by metallographic method. S2: Based on the above EBSD data, identify grain boundaries, reconstruct each independent martensitic lath grain, and for each reconstructed grain, obtain the area-weighted average of the maximum diameter of the fitted ellipse or the area-weighted average of the maximum Freret diameter. S3: Obtain the corresponding martensitic lath grain size grade based on the weighted average of the area of ​​the maximum diameter of the fitted ellipse or the weighted average of the area of ​​the maximum Freret diameter; S4: Perform a correlation analysis between the original austenite grain size grade and the corresponding martensite lath grain size grade to establish a quantitative correspondence between the two. The martensite lath grain size grade = original austenite grain size grade + 1 / 2n grade, where n is an integer. S5: In batch testing, the original austenite grain size grade of the sample to be tested is determined by metallographic method, and the martensite lath grain size grade of the sample to be tested is calculated based on the quantitative correspondence established in step S4.

[0009] Preferably, in step S1, EBSD testing is performed according to national standards, and preferably, the national standard is GB / T36165-2018.

[0010] Preferably, the orientation difference threshold used when identifying grain boundaries is ≥15°.

[0011] Preferably, in step S3, the weighted average of the maximum diameter area of ​​the fitted ellipse or the weighted average of the maximum Freret diameter area is used to obtain the corresponding martensitic lath grain size grade according to the national standard GB / T 6394-2017.

[0012] Preferably, in step S4, the number of samples used to establish the quantitative correspondence is no less than 20.

[0013] Preferably, in step S5, during batch testing, at least one sample is randomly selected from every 10 to 20 batches of products for EBSD testing to determine the martensite grain size grade, in order to verify the effectiveness of the quantitative correspondence between the original austenite grain size grade and the martensite lath grain size grade.

[0014] Preferred, , in: d i No. i The maximum Freette diameter or the maximum diameter of the fitted ellipse of each grain, in μm; A i For the first i The area corresponding to each grain, in μm 2 ; n To reconstruct the total number of grains.

[0015] The grain size of lath martensite is not a single scalar quantity, but a composite parameter determined by its three-level structure of laths, blocks, and bundles. Its mechanical behavior is highly dependent on the length, width, orientation difference, and spatial arrangement of the laths. The current national standard GB / T 36165–2018 uses EBSD data analysis, but its core still relies on the statistical framework of equivalent circle diameter. While it can obtain fitted values ​​with small standard deviations, it cannot capture the extension direction and true geometric morphology of the laths. Numerous studies (such as Morris et al., Acta Materialia, 2003; Kitahara et al., Materials Science and Engineering A, 2006) have shown that the aspect ratio of martensitic laths is typically between 5:1 and 20:1, and its strengthening mechanism mainly stems from dislocation pile-up at high-angle grain boundaries, with grain boundary density directly related to lath length. Therefore, characterizing it solely by area or equivalent diameter will severely underestimate the scale of the actual load-bearing unit. At the same time, although the obtained grain size results are small, reflecting the characteristics of fine lath martensite, the coefficient of variation (CV = standard deviation / mean) is abnormally high (the measured CV often reaches more than 300%, as shown in Table 2), resulting in a large statistical dispersion of the results, making regression impossible. The sample changes little under different original austenite grain sizes, causing the final results to deviate significantly from the actual state.

[0016] Traditional grain size evaluation methods are mostly based on equivalent circle diameter, intercept method, or area averaging method. These methods work well when dealing with equiaxed grains, but when faced with highly non-equiaxed, slender, and orientation-sensitive lath martensite, they often lose key structural information due to oversimplification. This invention introduces a geometric parameter system that more closely matches the actual morphology and combines it with crystallographic boundary identification criteria to achieve high-fidelity quantification of the martensite microstructure.

[0017] This invention specifically optimizes the grain size assessment for the complex microscopic characteristics of lath martensite, including its high anisotropy, hierarchical (lath-block-packet) structure and its three-dimensional staggered arrangement. Through systematic EBSD data comparison, it demonstrates that parameters that better reflect the lath geometry should be prioritized as grain size evaluation indicators. These include parameters such as the maximum Feret diameter (the longest distance between parallel tangents at grain boundaries) or the major axis of the fitted ellipse (the length of the major axis after fitting an individual lath grain to an ellipse using the least squares method). Such parameters not only reflect the maximum elongation of the grains but also establish a more direct physical connection with the effective grain size in the Hall-Petch relation. This allows for a more realistic and accurate reflection of the essential state of the grain size distribution and presents the actual size of martensite grains in a more intuitive and physically meaningful way, thereby significantly improving the correlation between grain size characterization and material mechanical properties (such as yield strength, fracture toughness, and fatigue life).

[0018] Furthermore, combining the crystallographic grain boundary orientation difference threshold (usually set above 15° to distinguish high-angle grain boundaries and exclude subgrain boundary interference) can further ensure that the statistical objects are truly independent martensitic lath units, avoiding the false increase in grain number caused by misjudgment of substructure.

[0019] Conventional metallographic observation is limited to two-dimensional cross-sections, while the interlacing, stacking, and penetrating arrangement of laths in three-dimensional space often results in what appear to be "small grains" on the surface actually being truncated fragments of larger laths. Regardless of whether the laths appear randomly arranged in the EBSD image ( Figure 1 , Figure 2 ), parallel arrangement ( Figure 3 ) or staggered stacking ( Figure 4 These localized small areas can significantly interfere with statistical results based on arithmetic mean, leading to systematic underestimation. Although three-dimensional grain size reconstruction (such as through focused ion beam SEM (FIB-SEM) tomography or X-ray microtomography) can partially solve this problem, a single detection can take tens of hours, the equipment cost exceeds ten million yuan, and the data processing is complex, making it difficult to apply to production line-level quality control.

[0020] If the arithmetic mean and standard deviation calculation method in GB / T 36165–2018 is directly adopted, even if the maximum Freret diameter or the maximum diameter of the fitted ellipse is introduced, its coefficient of variation is lower than that of the equivalent circle diameter, but it is still too high (the measured CV is still about 170%). The data dispersion is still very large, which weakens the correlation between martensite grain size and macroscopic mechanical properties, as shown in Table 2 (based on the statistical average of 100 samples). This indicates that the statistical results are still seriously affected by small grains.

[0021] To address this issue, this invention introduces the area-weighted mean as the core rating criterion, and its calculation formula is as follows: , in: d i No. i The maximum Freette diameter or the maximum diameter of the fitted ellipse of each grain, in μm; A i For the first i The area corresponding to each grain, in μm 2 ; n To reconstruct the total number of grains.

[0022] This method assigns higher weight to large-sized grains, effectively suppressing statistical bias caused by a large number of small-section grains (mostly slices of three-dimensional laths), thus more accurately reproducing the overall scale distribution of martensitic laths. As shown in Table 2, after using area-weighted averaging, the coefficient of variation significantly decreased to below 40%, and the data centrality and statistical robustness were significantly improved. This approach also conforms to the recommendations in the ASTM E112 appendix regarding weighted statistics for non-equiaxed grains and is highly consistent with the concept of "effective grain size" proposed in advanced international research (such as Gao et al., Scripta Materialia, 2020).

[0023] Traditional metallographic methods typically use the prior austenite grain (PAG) as the rating object, classifying grain size according to GB / T 6394 or ASTM E112. However, PAG no longer exists after martensitic transformation and can only be revealed through special corrosion (such as saturated picric acid + corrosion inhibitor), with the clarity greatly affected by the corrosion process, alloy composition, and cooling rate. More importantly, the actual service performance of a material is mainly determined by the transformed lath martensite structure, not the PAG itself. Therefore, there is no direct physical correlation between PAG rating and mechanical properties, leading to a disconnect in engineering applications where rating is conducted but not used, or used but not rated. Furthermore, due to the limitations of the aforementioned GB / T 36165–2018 method in EBSD data processing, its output results are difficult to establish a stable mapping with metallographic rating, forcing long-term reliance on EBSD for lath martensite microstructure rating, thus restricting testing efficiency and cost control.

[0024] This invention, by constructing a grain size parameter calibration model based on area-weighted average, achieves for the first time a quantitative correlation between the true martensitic lath grain size grade characterized by EBSD and the metallographic observable characteristics (original austenitic grain size rating). Initially, the model was fitted using multiple linear regression, principal component analysis, or machine learning algorithms (such as support vector regression). The fitting results showed minimal discrepancies with simple linear regression results, with a deviation ≤5%. The regression method used was a standard method, which will not be elaborated here. After 20-50 rounds of sample training and cross-validation under different process conditions, covering samples with different heat treatment regimes, alloy compositions, and rolling / forging processes, a robust mapping model was constructed. The results of different fitting methods were used to rate the martensitic grain size according to GB / T 6394 or ASTM E112, aiming to establish a direct, linear, and simple correspondence with the original austenitic grain size rating in metallographic methods, adaptable to industrial applications.

[0025] Establishing a quantitative correspondence between the original austenitic grain size grades and the martensitic lath grain size grades allows for the widespread adoption of traditional metallographic methods for rapid grading in subsequent production (single sample testing time <20 minutes). This is further validated through low-frequency (e.g., every 10–20 batches) EBSD sampling, significantly reducing testing costs, shortening delivery cycles, and decreasing reliance on hazardous chemicals and high-end equipment while maintaining testing accuracy and process control. This approach retains the convenience, economy, and universality of metallographic methods while inheriting the microscopic accuracy and physical rigor of EBSD, providing an efficient and green feasible path for large-scale quality control of lath martensitic steel. It is particularly suitable for high-end manufacturing sectors with urgent needs for high-strength, high-reliability batch testing. This mechanism complies with the relevant provisions of ISO / IEC 17025 "General requirements for the competence of testing and calibration laboratories" regarding alternative method validation and uncertainty assessment, demonstrating significant potential for standardization and industrialization.

[0026] This invention does not impose significantly higher requirements on EBSD detection technology. Operational requirements, area selection, and data post-processing all comply with GB / T 36165–2018. There are no restrictions on the type, model, or source of EBSD data acquisition equipment. No higher or special requirements are needed for existing operators; no special data analysis software or learning process is required. More accurate martensite lath grain size values ​​can be obtained simply by changing the data analysis method and parameters, and a strong linear correlation is established between these values ​​and the original austenite grain size.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows: First, this invention proposes to use the weighted average of the area of ​​the largest Freret diameter or the weighted average of the area of ​​the largest diameter of the fitted ellipse to determine the grain size grade of martensite laths. This overcomes the geometric distortion problem of the traditional equivalent circle diameter method when dealing with non-equiaxed, slender lath martensite, making the grain size statistical results more closely match the true microstructure of the material. As a result, the coefficient of variation (CV) of the grain size statistical results is significantly reduced from more than 300% in the traditional method to less than 40%, greatly reducing data dispersion and improving the reliability and repeatability of the results. This makes the correlation between grain size and material mechanical properties (such as strength and toughness) more direct and reliable.

[0028] Secondly, this invention innovatively establishes a stable quantitative correspondence (linear formula) between the martensite lath grain size measured by EBSD and the original austenite grain size measured by metallography. This allows for the determination of the corresponding martensite lath grain size grade of a sample in routine production simply by using metallography to determine the original austenite grain size grade. The accuracy of the martensite grain size rating obtained using this method is ≥99%. Furthermore, it reduces the detection time for a single sample from several hours required by EBSD to less than 20 minutes by metallography, while simultaneously reducing detection costs and significantly increasing throughput.

[0029] The method described in this invention only requires conventional metallographic testing equipment for routine testing, reducing the occupation and dependence on expensive field emission scanning electron microscopes (FE-SEM) and EBSD detectors; it significantly reduces the consumption of high-risk chemicals (such as perchloric acid and chromic anhydride) required to prepare high-quality samples in pure EBSD testing, thereby reducing the environmental pressure, safety risks, and management costs of the laboratory; and it is easy to operate, requiring no special additional skills training for testing personnel, and can be performed using existing metallographic knowledge and standardized EBSD procedures. Attached Figure Description

[0030] Figure 1 EBSD scan image of martensitic heat-resistant steel; Figure 2 EBSD scan image of martensitic heat-resistant steel; Figure 3 EBSD scan image of martensitic heat-resistant steel; Figure 4 This is an EBSD scan of martensitic heat-resistant steel. Detailed Implementation

[0031] The present invention will be further described below with reference to the embodiments and accompanying drawings. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way.

[0032] I. Accuracy Verification of Different Processing Methods This invention uses G115 / P92 martensitic heat-resistant steel as an example and draws conclusions through comparison of multiple sets of data. Accurate and repeatable determination of the original austenite grain size is an important prerequisite for evaluating the service performance and life prediction of this type of material.

[0033] In this invention, the pre-austenite grain size of the martensitic steels was determined by heat treatment followed by Nital etching. Subsequently, the sample surface was selectively etched using standard Nital (nitric acid-ethanol) etchant to clearly reveal the pre-austenite grain boundaries, facilitating subsequent correlation with electron backscatter diffraction (EBSD) analysis data.

[0034] EBSD data were evaluated using EBSD probes from different manufacturers (such as Bruker, Oxford Instruments, EDAX, etc.). To ensure data objectivity and device independence, this invention underwent cross-validation across multiple experimental platforms, covering mainstream commercial EBSD systems. EBSD data was processed and statistically analyzed using data processing software from different manufacturers, according to GB / T 36165-2018. This national standard clearly specifies EBSD data acquisition parameter settings, calibration accuracy requirements, noise filtering strategies, and grain reconstruction criteria, serving as the technical foundation for ensuring the scientific rigor of quantitative microstructure analysis. Each set of data was obtained through cross-comparison using different software (i.e., the probe manufacturer and software manufacturer used for data acquisition do not necessarily need to be identical) to avoid systematic biases or algorithmic preferences introduced by specific hardware and software coupling. For example, a set of data might have been acquired using a Bruker probe, but post-processed using Oxford Instruments' AZtec Crystal and EDAX's OIM Analysis respectively, with data reliability confirmed through result consistency checks. The obtained EBSD data must meet the requirements of GB / T 36165-2018, and the post-processing method must strictly follow the standard process, including but not limited to: minimum confidence index (CI) threshold setting, grain boundary definition (usually taking an orientation difference ≥15° as a large angle grain boundary), grain merging rules and statistical sampling area specifications.

[0035]

[0036] II. Establishing the Relationship between Martensite Grain Size Grade and Original Austenite Grain Size Grade A method for determining the grain size of lath martensitic steel using a combination of metallographic and EBSD testing includes the following steps: S1: EBSD testing was performed on 100 groups of G115 / P92 martensitic steel samples according to the national standard GB / T 36165-2018 to obtain EBSD data including grain crystallographic orientation and morphology; and the original austenite grain size grade was determined by metallographic method. S2: Based on the above EBSD data, grain boundaries are identified, and each independent martensite lath grain is reconstructed. For each reconstructed grain, the area-weighted average of the maximum diameter of the fitted ellipse or the area-weighted average of the maximum Freret diameter is obtained. , in: d i No. i The maximum Freette diameter or the maximum diameter of the fitted ellipse of each grain, in μm; A i For the first i The area corresponding to each grain, in μm2 ; n To reconstruct the total number of grains; S3: According to the national standard GB / T 6394-2017, the area-weighted average value is converted into the corresponding martensitic lath grain size grade; S4: Correlation analysis is performed between the original austenite grain size grade and the corresponding martensite lath grain size grade to establish a quantitative correspondence between the two. Specifically: Martensite grain size grade = original austenite grain size grade + 5 grades. S5: In subsequent batch testing, the original austenite grain size grade of the sample to be tested is determined by metallographic method, and the corresponding martensite lath grain size grade is calculated based on the quantitative correspondence established in step S4.

[0037] Table 3 shows the comparison results of traditional metallographic analysis, traditional EBSD analysis and the analysis method of this invention. The results are the average values ​​of 100 sets of data. The ratings are all standardized and converted according to the correspondence between grain size diameter and rating results in GB / T 6394-2017.

[0038]

[0039] Table 3 shows that the coefficient of variation (CV) of the maximum Ferrette diameter is comparable to that of the maximum diameter of the fitted ellipse, indicating no significant difference in statistical dispersion. The martensite grain size obtained using either the area-weighted average of the maximum diameter of the fitted ellipse or the area-weighted average of the maximum Ferrette diameter exhibits a linear relationship with the austenite grain size obtained by metallographic methods. Therefore, the choice between using the area-weighted average of the maximum diameter of the fitted ellipse or the area-weighted average of the maximum Ferrette diameter should be determined by technical personnel based on equipment conditions, software support, and specific application scenarios, ensuring both scientific rigor and engineering practicality while maintaining methodological flexibility.

[0040] In summary, this invention, through multi-platform verification, standard adherence, and parameter optimization, has constructed a technical path suitable for rapid determination of grain size in martensitic heat-resistant steel, providing reliable methodological support for rapid factory inspection of high-temperature structural materials.

[0041] Test Example 1 In this embodiment, G115 martensitic heat-resistant steel (sample number 20) produced in the same batch was used. The original austenitic grain size was evaluated as grade -2. The detailed results of the EBSD method rating in this invention are shown in Table 4. The rating and the overall correspondence are the same, the coefficient of variation is close to 30%, and the maximum diameter of the fitted ellipse method is less than 30%.

[0042]

[0043] Test Example 2 In this embodiment, G115 martensitic heat-resistant steel (30 samples) produced in different batches was used. The original austenitic grain size was evaluated as grade -1.5. The detailed results of the EBSD rating method in this invention are shown in Table 5.

[0044]

[0045] The rating and the overall correlation are the same, with a coefficient of variation close to 30%, and the maximum diameter method of the fitted ellipse is less than 30%.

[0046] Test Example 3 In this embodiment, martensitic heat-resistant steels (G115 and P92) from different batches (67 samples) with different original austenitic grain sizes were used. Detailed results of the EBSD rating method used in this invention are shown in Table 6. The rating and overall correlation are the same, with the coefficient of variation for the maximum Freret diameter method being within 50%. The coefficients of variation for the maximum diameter method of the fitted ellipse are all close to or less than 30%.

[0047]

[0048] Test Example 4 In this embodiment, 103 randomly selected lath martensitic steel samples from different batches were used, with varying original austenite grain sizes. Detailed results of the EBSD rating method used in this invention are shown in Table 7. The rating and overall correlation are the same, with most coefficients of variation less than 50%, and the maximum diameter of the fitted ellipse method showing variation close to or less than 30%. Both methods show an increase in the coefficient of variation when the original austenite grain size is larger than when it is smaller, which is related to the characteristics of lattice shear during martensitic transformation.

[0049]

[0050] As clearly shown in Tables 2 to 7, in Examples 1-4, when testing G115 and P92 martensitic heat-resistant steels, it was found that the area-weighted average of the maximum Ferrette diameter and the area-weighted average of the maximum diameter of the fitted ellipse showed a more significant correlation with the changes in grain size under the metallographic rating method than the rating method using the fitted circle diameter and the unweighted average. The rating method using the fitted circle diameter and the unweighted average is difficult to effectively distinguish grain size grades, especially at high magnification, where it is easily affected by local morphological disturbances. Therefore, this invention preferably uses the area-weighted average of the maximum diameter of the fitted ellipse and the area-weighted average of the maximum Ferrette diameter as the core parameters connecting the martensitic lath grain size with the original austenitic grain rating. This parameter uses the least squares method to fit an ellipse to each reconstructed grain and extract its principal axis length, which can more stably reflect the overall dimensional characteristics of the grain and has stronger adaptability to the non-isoaxiality of lath martensite bundles. However, considering the large number of lath martensitic steel grades and the need for compatibility with various parameters in engineering practice, the two models of fitting the ellipse with the maximum diameter area weighted average and the maximum Freret diameter area weighted average require relevant technical personnel to consider the specific product and choose the appropriate EBSD data and linear relationship coefficients. The choice of data type does not affect the determination of the overall evaluation relationship; the goal is to make the relationship between the method of this invention and traditional metallographic grain size evaluation clearer.

[0051] In summary, the fitted ellipse maximum diameter area weighted average method and the Freret maximum diameter area weighted average method are alternatives and have a strong correlation with the results obtained by traditional metallographic methods.

[0052] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention, such as changing the linear relationship coefficients, without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for determining the grain size of lath martensitic steel by combining metallographic testing and EBSD testing, characterized in that, Includes the following steps: S1: Perform EBSD testing on the martensitic steel sample to be tested to obtain EBSD data including grain crystallographic orientation and morphology; and determine its original austenite grain size grade by metallographic method. S2: Based on the above EBSD data, identify grain boundaries, reconstruct each independent martensitic lath grain, and for each reconstructed grain, obtain the area-weighted average of the maximum diameter of the fitted ellipse or the area-weighted average of the maximum Freret diameter. S3: Obtain the corresponding martensitic lath grain size grade based on the weighted average of the area of ​​the maximum diameter of the fitted ellipse or the weighted average of the area of ​​the maximum Freret diameter; S4: Perform a correlation analysis between the original austenite grain size grade and the corresponding martensite lath grain size grade to establish a quantitative correspondence between the two. The martensite lath grain size grade = original austenite grain size grade + 1 / 2n grade, where n is an integer. S5: Batch testing, using metallographic method to determine the original austenite grain size grade of the sample to be tested, and based on the quantitative correspondence established in step S4, to calculate the corresponding martensite lath grain size grade of the sample to be tested.

2. The method for determining the grain size of lath martensitic steel by combining metallographic testing and EBSD testing as described in claim 1, characterized in that, In step S1, EBSD testing is performed according to national standards, preferably GB / T 36165-2018.

3. The method for determining the grain size of lath martensitic steel by combining metallographic testing and EBSD testing as described in claim 1, characterized in that, The orientation difference threshold used when identifying grain boundaries is ≥15°.

4. The method for determining the grain size of lath martensitic steel by combining metallographic testing and EBSD testing as described in claim 1, characterized in that, In step S3, the weighted average of the maximum diameter area of ​​the fitted ellipse or the weighted average of the maximum Freret diameter area is used to obtain the corresponding martensitic lath grain size grade according to the national standard GB / T 6394-2017.

5. The method for determining the grain size of lath martensitic steel by combining metallographic testing and EBSD testing as described in claim 1, characterized in that, In step S4, the number of samples used to establish the quantitative correspondence is no less than 20.

6. The method for determining the grain size of lath martensitic steel by combining metallographic testing and EBSD testing as described in claim 1, characterized in that, In step S5, during batch testing, at least one sample is randomly selected from every 10 to 20 batches of products for EBSD testing to determine the martensite grain size grade, in order to verify the effectiveness of the quantitative correspondence between the original austenite grain size grade and the martensite lath grain size grade.

7. The method for determining the grain size of lath martensitic steel by combining metallographic testing and EBSD testing as described in claim 1, characterized in that, ， in: d i No. i The maximum Freette diameter or the maximum diameter of the fitted ellipse of each grain, in μm; A i For the first i The area corresponding to each grain, in μm 2 ; n To reconstruct the total number of grains.

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