Method for representing segregation degree of C and Ti in NS6500 alloy cast ingot

Abstract

The invention discloses a method for characterizing the segregation degree of C and Ti in an NS6500 alloy ingot, belongs to the technical field of metal material detection, and aims to solve the technical problem that the segregation degree of low-content elements in the NS6500 alloy ingot cannot be accurately and quantitatively characterized by existing direct component detection means EDS and EPMA. The method comprises the following steps: step 1, preparing a sample; cutting a metallographic sample from the NS6500 alloy cast ingot to be detected; 2, heat treatment induced precipitation; performing heat treatment on the metallographic specimen through heat treatment equipment, and taking out and cooling the metallographic specimen after heat treatment; step 3, microscopic structure observation; 4, evaluating the segregation degree; the segregation degree of C and Ti elements in the cast ingot is evaluated qualitatively or quantitatively according to the number, size and distribution information of dendritic precipitated phases; the more precipitated phases are, the denser the precipitated phases are, the more serious the segregation of C and Ti is. According to the invention, the content of carbon (C) and titanium (Ti) elements is low (Clt; 0.1%, Tilt; 1%) can be accurately measured.

Description

Technical Field

[0001] This invention belongs to the field of metal material testing technology, and in particular relates to a method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots. Background Technology

[0002] NS6500 alloy is a nickel-copper corrosion-resistant alloy that is strengthened by Al and Ti aging, and it has been widely used in high-end equipment fields such as marine engineering and petrochemicals.

[0003] As related equipment becomes increasingly larger, the required size of alloy ingots continues to grow, making element segregation in the as-cast microstructure more prominent. In particular, the microscopic segregation of C and Ti elements significantly affects the age-hardening effect, corrosion resistance, and mechanical properties of the alloy.

[0004] Currently, the characterization of ingot segregation primarily relies on microscopic composition detection methods such as electron probe microanalysis (EPMA) or energy dispersive spectroscopy (EDS). However, due to the low C and Ti content in NS6500 alloy (typically C < 0.1%, Ti < 1%), and the extremely small actual compositional differences between dendrites and dendrites, existing semi-quantitative detection methods are limited by resolution and accuracy, making it difficult to accurately distinguish the differences in C and Ti content between dendrites and dendrites. This results in the inability to effectively quantify the degree of segregation, and a lack of reliable evidence for comparing segregation in ingots from different processes and with different ingot shapes. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots, in order to solve the technical problem that existing direct composition detection methods cannot accurately and quantitatively characterize the degree of segregation of low-content elements in NS6500 alloy ingots.

[0006] The objective of this invention is mainly achieved through the following technical solutions:

[0007] This invention provides a method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots, comprising the following steps:

[0008] Step 1, Sample preparation;

[0009] Metallographic samples were cut from the NS6500 alloy ingot to be tested;

[0010] Step 2: Heat treat the metallographic sample;

[0011] The metallographic sample is heat-treated using heat treatment equipment, and then removed and cooled.

[0012] Step 3: Microscopic tissue observation;

[0013] The heat-treated metallographic specimens were successively ground and polished; after polishing, the microstructure of the interdendritic region of the heat-treated metallographic specimens was observed using an optical microscope or a scanning electron microscope to obtain information on the quantity, size and distribution of precipitates.

[0014] Step 4: Evaluation of the degree of segregation;

[0015] Based on the quantity, size, and distribution of interdendritic precipitates, the degree of segregation of C and Ti elements in the ingot can be qualitatively or quantitatively evaluated; the more numerous and denser the precipitates, the more severe the segregation of C and Ti.

[0016] Furthermore, in step 1, the central area of ​​the ingot is selected as the sampling location.

[0017] Furthermore, in step 1, when it is impossible to sample from the central region of the ingot due to experimental constraints, samples are taken from any region of the ingot. In this case, the characterization results will reflect the local segregation of the sampled area.

[0018] Furthermore, in step 1, the dimensions of the metallographic sample are: 10-20 mm in length × 10-20 mm in width × 5-7 mm in thickness.

[0019] Furthermore, in step 3, after microscopic observation, energy dispersive spectroscopy (EDS) is performed on the precipitated phase to confirm that the precipitated phase is TiC.

[0020] Furthermore, in step 4, the degree of segregation of the ingot is evaluated using a metallographic microscope, and a photomicrograph is taken at 100x magnification.

[0021] Furthermore, in step 4, at least three non-overlapping representative fields of view are selected, and the area fraction of the TiC precipitate between dendrites is statistically analyzed using Image-Pro image analysis software.

[0022] Furthermore, in step 4, the process of using Image-Pro image analysis software to statistically determine the area fraction of the TiC precipitates between dendrites is as follows: by setting the grayscale threshold of the precipitates and the identification parameters of the morphological features, the TiC phase and the matrix are automatically segmented. Then, the ratio of the total area of ​​the TiC phase in each field of view to the total area of ​​the field of view is calculated, or its percentage is calculated. Finally, the average value of the results of each field of view is taken as the final area fraction.

[0023] Furthermore, in step 4, the quantitative evaluation criteria for the degree of segregation are as follows: when the area fraction of the TiC precipitate is 0-0.5%, it is determined that there is no obvious segregation; when the area fraction of the TiC precipitate is 0.5-3%, it is determined that there is slight segregation; when the area fraction of the TiC precipitate is greater than 3%, it is determined that there is significant segregation, thereby achieving quantitative characterization of the degree of segregation of Ti and C elements in NS6500 alloy ingots.

[0024] Furthermore, in step 4, the qualitative evaluation criterion for the degree of segregation is: if TiC precipitates are observed in the microstructure, it can be determined that Ti and C element segregation exists in the corresponding part of the ingot.

[0025] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0026] (1) This invention transforms the problem of segregation of low-content C and Ti elements (C<0.1%, Ti<1%), which is difficult to measure directly and accurately, into a problem of precipitate distribution that can be macroscopically observed and quantified through the key step of "heat treatment-induced precipitation". Its working principle is as follows: During the solidification process of NS6500 alloy ingots, C and Ti elements undergo dendritic segregation, that is, they accumulate in the interdendritic region. When the prepared metallographic sample is heat-treated at a specific temperature (800℃~900℃) and time (8~24h), these C and Ti elements segregated in the interdendritic region combine to form a stable titanium carbide (TiC) precipitate. Since the formation of the TiC precipitate is directly related to the degree of segregation of the original C and Ti elements, and its size and quantity are much larger than a single C or Ti atom, the quantity, size, and distribution information of the interdendritic TiC precipitate can be conveniently and intuitively obtained through microstructural observation and image analysis techniques. The greater the quantity and density of precipitated phases, the more severe the segregation of C and Ti elements in the original ingot. This indirect characterization method avoids the limitations of existing direct component detection methods in terms of accuracy and resolution in the quantitative characterization of segregation of low-content elements, thus achieving an accurate evaluation of the degree of C and Ti segregation.

[0027] (2) Indirect characterization and reliable results: This invention induces the formation of TiC precipitates by heat treatment of C and Ti segregated between dendrites. The quantity and distribution of the precipitates indirectly reflect the degree of segregation, thus avoiding the problem of insufficient accuracy of direct component detection.

[0028] (3) Easy to operate and highly repeatable: Only heat treatment equipment and metallographic observation equipment are required, making it suitable for industrial production and laboratory research.

[0029] (4) Strong applicability: The characterization method of the present invention is applicable to the segregation comparison analysis of NS6500 alloy ingots with different ingot shapes, different compositions and different smelting processes.

[0030] (5) The present invention can indirectly, accurately and repeatably characterize the degree of C and Ti segregation in NS6500 alloy, which is of great significance for optimizing smelting process, controlling ingot quality and improving material performance.

[0031] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained from what is particularly pointed out in the description and drawings. Attached Figure Description

[0032] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0033] Figure 1a The image shows the 50X microstructure of the NS6500 alloy in Example 1 after aging at 850℃ for 16 hours.

[0034] Figure 1b The image shows the 100X microstructure of the NS6500 alloy in Example 1 after aging at 850℃ for 16 hours.

[0035] Figure 2a The image shows the 500X SEM image of the NS6500 alloy in Example 1 after aging at 850℃ for 16 hours.

[0036] Figure 2b The image shows the 1000X SEM image of the NS6500 alloy in Example 1 after aging at 850℃ for 16 hours.

[0037] Figure 3a The image shows the SEM image of the interdendritic precipitates in the experimental steel NS6500 alloy after aging at 850℃ for 16h, characterized by EDS.

[0038] Figure 3b This is the data at point 1 (matrix) in the EDS characterization diagram of the interdendritic precipitates in the experimental steel NS6500 alloy after aging at 850℃ for 16h in Example 1.

[0039] Figure 3c The image shows the data at point 2 (TiC) in the EDS characterization diagram of the interdendritic precipitates in the experimental steel NS6500 alloy after aging at 850℃ for 16h in Example 1.

[0040] Figure 4a SEM image of the NS6500 alloy provided in Example 1 after aging at 850℃ for 16h;

[0041] Figure 4b A comparison image of the segmented TiC phase and the matrix segmentation obtained by processing the NS6500 alloy provided in Example 1 after aging at 850℃ for 16h using Image-Pro software.

[0042] Figure 4c A statistical histogram of the TiC precipitate area of ​​the NS6500 alloy provided in Example 1 after aging at 850℃ for 16h.

[0043] Figure 5 SEM image of the as-cast microstructure of NS6500 alloy in Comparative Example 1;

[0044] Figure 6a The image shows the EPMA surface scan characterization results of the as-cast NS6500 alloy in Comparative Example 1.

[0045] Figure 6b The image shows the EPMA surface scan characterization results of Ti element in the as-cast NS6500 alloy in Comparative Example 1.

[0046] Figure 6c The image shows the EPMA surface scan characterization results of Al in the as-cast NS6500 alloy of Comparative Example 1.

[0047] Figure 6d The image shows the EPMA surface scan characterization results of Fe in the as-cast NS6500 alloy of Comparative Example 1.

[0048] Figure 6e The image shows the EPMA surface scan characterization results of C element in the as-cast NS6500 alloy of Comparative Example 1.

[0049] Figure 6f The image shows the EPMA surface scan characterization results of Ni in the as-cast NS6500 alloy of Comparative Example 1.

[0050] Figure 6g The image shows the EPMA surface scan characterization results of Cu in the as-cast NS6500 alloy of Comparative Example 1.

[0051] Figure 7a The image shows the 50X microstructure of the NS6500 alloy in Example 2 after aging at 850℃ for 16 hours.

[0052] Figure 7b The image shows the 100X microstructure of the NS6500 alloy in Example 2 after aging at 850℃ for 16 hours.

[0053] Figure 8a The image shows the 500X SEM image of the NS6500 alloy after aging at 850℃ for 16 hours in Example 2.

[0054] Figure 8b The image shows the 1000X SEM image of the NS6500 alloy after aging at 850℃ for 16 hours in Example 2.

[0055] Figure 9aThe image shows the SEM image of the interdendritic precipitates in the NS6500 alloy of the test steel in Example 2 after aging at 850℃ for 16h, characterized by EDS.

[0056] Figure 9b This is the data point 1 (matrix) in the EDS characterization diagram of the interdendritic precipitates in the experimental steel NS6500 alloy after aging at 850℃ for 16h in Example 2.

[0057] Figure 9c The image shows the data at point 2 (TiC) in the EDS characterization diagram of the interdendritic precipitates in the experimental steel NS6500 alloy after aging at 850℃ for 16h in Example 2.

[0058] Figure 10a SEM image of the NS6500 alloy provided in Example 2 after aging at 850℃ for 16h;

[0059] Figure 10b The image shows a comparison of the segmented TiC phase and the matrix segmentation obtained by processing the NS6500 alloy provided in Example 2 after aging at 850℃ for 16h using Image-Pro software.

[0060] Figure 10c A statistical histogram of the TiC precipitate area of ​​the NS6500 alloy provided in Example 2 after aging at 850℃ for 16h.

[0061] Figure 11 OM image of the as-cast microstructure of NS6500 alloy in Comparative Example 2;

[0062] Figure 12a The image shows the 50X microstructure of the NS6500 alloy in Example 3 after aging at 850℃ for 16 hours.

[0063] Figure 12b The image shows the 100X microstructure of the NS6500 alloy in Example 3 after aging at 850℃ for 16 hours.

[0064] Figure 13a The image shows the 500X SEM image of the NS6500 alloy in Example 3 after aging at 850℃ for 16 hours.

[0065] Figure 13b The image shows the 1000X SEM image of the NS6500 alloy in Example 3 after aging at 850℃ for 16 hours.

[0066] Figure 14a This is a SEM image of the interdendritic precipitates in the NS6500 alloy after aging at 1850℃ for 16 hours in Example 3.

[0067] Figure 14bThis is a comparison image of the segmented TiC phase and the matrix segmentation obtained by processing with Image-Pro software after aging the NS6500 alloy at 850℃ for 16 hours in Example 3.

[0068] Figure 15a SEM image of the as-cast microstructure of NS6500 alloy in Comparative Example 3;

[0069] Figure 15b SEM image 2 shows the as-cast microstructure of the NS6500 alloy in Comparative Example 3;

[0070] Figure 15c Figure 1 shows the as-cast microstructure of the NS6500 alloy in Comparative Example 3.

[0071] Figure 15d Figure 2 shows the as-cast microstructure of the NS6500 alloy in Comparative Example 3.

[0072] Figure 16a The image shows the EPMA surface scan characterization results of the as-cast NS6500 alloy in Comparative Example 3.

[0073] Figure 16b The image shows the EPMA surface scan characterization results of Ni in the as-cast NS6500 alloy of Comparative Example 3.

[0074] Figure 16c The image shows the EPMA surface scan characterization results of Cu in the as-cast NS6500 alloy of Comparative Example 3.

[0075] Figure 16d The image shows the EPMA surface scan characterization results of Fe in the as-cast NS6500 alloy of Comparative Example 3.

[0076] Figure 16e The image shows the EPMA surface scan characterization results of Al in the as-cast NS6500 alloy of Comparative Example 3.

[0077] Figure 16f The image shows the EPMA surface scan characterization results of Ti element in the as-cast NS6500 alloy of Comparative Example 3.

[0078] Figure 16g The image shows the EPMA surface scan characterization results of C element in the as-cast NS6500 alloy of Comparative Example 3. Detailed Implementation

[0079] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not intended to limit the scope of the present invention.

[0080] In existing technologies, the characterization of the segregation degree of carbon (C) and titanium (Ti) elements in NS6500 alloy ingots mainly relies on microscopic composition detection methods such as electron probe microanalysis (EPMA) or energy dispersive spectroscopy (EDS). However, due to the low content of C and Ti elements in NS6500 alloy (C < 0.1%, Ti < 1%), and the small actual compositional differences between dendrites and dendrites, existing semi-quantitative detection methods are limited by resolution and accuracy, making it difficult to accurately distinguish the differences in C and Ti content between dendrites and dendrites. Consequently, they cannot effectively quantify the degree of segregation, resulting in a lack of reliable basis for comparing segregation ingots from different processes and with different ingot shapes.

[0081] This invention provides a method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots, used to indirectly characterize the degree of C and Ti element segregation in NS6500 alloy ingots. The method includes the following steps:

[0082] Step 1, Sample preparation;

[0083] Metallographic samples were cut from the NS6500 alloy ingot to be tested, and the central area of ​​the ingot was selected as the sampling location.

[0084] It should be explained that the central region of the ingot has the lowest cooling rate at the end of solidification and is the part of the ingot where segregation is most significant. Selecting samples from this location can best reflect the overall degree of segregation and elemental distribution characteristics of the ingot.

[0085] In section 1 above, when it is impossible to sample from the central region due to experimental conditions (such as ingot size and sampling equipment limitations), samples are taken from any region of the ingot. In this case, the characterization results will reflect the local segregation of the sampled area.

[0086] In step 1 above, the dimensions of the metallographic sample are: 10-20 mm long × 10-20 mm wide × 5-7 mm thick. This size range can meet the requirements of uniformity of the sample temperature field during subsequent aging treatment, and also facilitates subsequent metallographic sample preparation (such as grinding, polishing, and etching) and microstructure observation.

[0087] Step 2: Heat treat the metallographic sample;

[0088] In step 2 above, the metallographic sample is heat-treated using a heat treatment device. After heat treatment, the metallographic sample is removed and cooled. The purpose of heat treatment is to induce the precipitation of precipitates.

[0089] In step 2 above, a box-type resistance furnace is selected as the heat treatment equipment, and the temperature is raised from room temperature to 800℃~900℃ at a heating rate of 20~30℃ / min. After the furnace temperature stabilizes, the metallographic sample is placed in and kept at that temperature for 8-12 minutes to ensure that the temperature inside and outside the metallographic sample is uniform.

[0090] In step 2 above, after the temperature inside and outside the metallographic sample is uniform, it is kept at a temperature range of 800℃ to 900℃ for 8 to 24 hours to provide sufficient diffusion and reaction conditions for the Ti and C elements that segregate between dendrites, so as to promote their combination to form a precipitate phase mainly composed of stable TiC.

[0091] In step 2 above, after the heat preservation is completed, the metallographic sample is taken out and cooled by air cooling or water cooling.

[0092] In step 2 above, preferably, the temperature at which the metallographic sample is heat-treated is 840℃~860℃.

[0093] Compared with existing technologies, this invention precisely limits the heat treatment temperature range to 840℃~860℃ when inducing C and Ti elements to form precipitates through heat treatment. This allows for more accurate control of the nucleation and growth process of the TiC precipitates, ensuring that C and Ti elements fully combine between dendrites to form a stable TiC phase. Simultaneously, it avoids adverse effects such as grain coarsening, excessive precipitate growth or dissolution caused by excessively high temperatures, and insufficient precipitation caused by excessively low temperatures. This optimization improves the controllability and uniformity of precipitate formation, thereby making the segregation characterization results more accurate, sensitive, and reproducible.

[0094] In step 2 above, preferably, the holding time for heat treatment of the metallographic sample is 12h to 18h. By precisely limiting the holding time to 12h to 18h when inducing the formation of C and Ti precipitates through heat treatment, sufficient time is ensured for C and Ti elements to diffuse and combine in the interdendritic region to form TiC precipitates, guaranteeing the completeness of the precipitation reaction. Simultaneously, it avoids excessive growth, agglomeration, or uneven distribution of the precipitates that may result from excessive holding time. This allows the size and distribution of the precipitates to more accurately and sensitively reflect the degree of segregation of the original C and Ti elements, improving the accuracy and reliability of the characterization.

[0095] In step 2 above, the present invention uses a box-type resistance furnace as a heat treatment device. This device is in communication with air and does not require a vacuum environment. It has the advantages of strong versatility and simple operation.

[0096] Step 3: Microscopic tissue observation;

[0097] In step 3 above, the heat-treated metallographic sample is successively ground and polished.

[0098] In step 3 above, the selection of the observation surface and the grinding process are as follows: A 10-20 mm × 10-20 mm surface of the heat-treated metallographic sample is selected as the observation surface. Gradually, 80-grit, 150-grit, 320-grit, 600-grit, and 1000-grit sandpaper are used for grinding. Each time a new sandpaper is used, the grinding direction must be perpendicular to the previous sandpaper's grinding direction to ensure uniform surface grinding. After polishing, the sample surface reaches a state without obvious scratches, which provides a good foundation for subsequent observation of the TiC precipitate phase under a metallographic microscope.

[0099] In step 3 above, after grinding, the observation surface is polished with diamond metallographic polishing agent until no obvious scratches are visible on the surface of the metallographic sample when it is placed under a metallographic microscope. The polished metallographic sample is then placed in a potassium permanganate solution heated to 50°C in a water bath for 10–30 seconds to clearly reveal the microstructure morphology, providing a clear microstructural contrast for subsequent observation of the TiC precipitate phase.

[0100] It should be noted that the polished metallographic sample was placed in a potassium permanganate solution heated to 50°C in a water bath for 10-30 seconds to etch it, utilizing the oxidizing properties of the potassium permanganate solution to treat the surface of the metallographic sample. Within this temperature and time range, the potassium permanganate solution can moderately etch the sample surface, creating a clear contrast between the TiC precipitate phase and the matrix, thereby clearly revealing the microstructure morphology.

[0101] In step 3 above, the diamond microparticles in the diamond metallographic polishing agent have a particle size of 3 μm. Using diamond microparticles of this particle size can ensure polishing efficiency while obtaining a relatively smooth and flat metallographic sample surface.

[0102] The microstructure of the interdendritic region of the heat-treated sample was observed using an optical microscope or a scanning electron microscope to obtain information on the quantity, size and distribution of precipitated phases.

[0103] After microscopic observation, energy dispersive spectroscopy (EDS) was performed on the precipitated phase to confirm that the precipitated phase was TiC.

[0104] The precipitated phases mentioned above are mainly TiC phases, which are formed by the combination and precipitation of C and Ti that are segregated between dendrites during the aging process.

[0105] Given that C and Ti combine to form a TiC precipitate, this invention introduces EDS (Energy Dispersive Spectroscopy) analysis to accurately determine the composition of the precipitate observed under a microscope, confirming that its main components are Ti and C, thus verifying that the precipitate is indeed TiC. This verification step eliminates interference from other possible precipitates, enhances the accuracy and reliability of the characterization results, and ensures that the physical basis of the indirect characterization is correct.

[0106] Step 4: Evaluation of the degree of segregation;

[0107] The degree of segregation of C and Ti elements in the ingot can be qualitatively or quantitatively evaluated based on the quantity and density of interdendritic precipitates; the more and denser the precipitates, the more severe the segregation of C and Ti.

[0108] In step 4 above, the degree of segregation is evaluated using a metallurgical microscope at 100x magnification to capture micrographs, selecting at least three non-overlapping representative fields of view. Image-Pro image analysis software is then used to statistically analyze the area fraction of the TiC precipitates between dendrites. Specifically, by setting the grayscale threshold of the precipitates and the morphological feature recognition parameters, the TiC phase and matrix are automatically segmented. Then, the ratio of the total area of ​​the TiC phase in each field of view to the total area of ​​the field of view is calculated, or its percentage is calculated. Finally, the average of the results from each field of view is taken as the final area fraction.

[0109] In step 4 above, the present invention selects at least three non-overlapping representative fields of view to ensure the objectivity of the statistics.

[0110] The qualitative evaluation standard for the degree of segregation is: if TiC precipitates are observed in the microstructure, it can be determined that Ti and C elements are segregated in the corresponding part of the ingot.

[0111] The quantitative evaluation standard for the degree of segregation is as follows: if TiC precipitates are observed in the microstructure, and the higher the area fraction of the TiC precipitates, the more severe the segregation of Ti and C elements. Specifically, when the area fraction of the TiC precipitates is 0-0.5%, it is judged as no obvious segregation; when the area fraction of the TiC precipitates is 0.5-3%, it is judged as slight segregation; and when the area fraction of the TiC precipitates is greater than 3%, it is judged as significant segregation. This achieves quantitative characterization of the degree of segregation of Ti and C elements in NS6500 alloy ingots.

[0112] This invention transforms the problem of segregation of low-content C and Ti elements (C < 0.1%, Ti < 1%), which is difficult to measure directly and accurately, into a problem of precipitate distribution that can be macroscopically observed and quantified, through the key step of "heat treatment-induced precipitation". Its working principle is as follows: During the solidification process of NS6500 alloy ingots, C and Ti elements undergo dendritic segregation, i.e., enrichment in the interdendritic region. When the prepared metallographic sample is heat-treated at a specific temperature (800℃~900℃) and time (8~24h), these interdendritic segregated C and Ti elements combine to form stable titanium carbide (TiC) precipitates. Since the formation of TiC precipitates is directly related to the degree of segregation of the original C and Ti elements, and its size and quantity are much larger than individual C and Ti atoms, the quantity, size, and distribution information of the interdendritic TiC precipitates can be conveniently and intuitively obtained through microstructural observation and image analysis techniques. The greater the quantity and density of precipitated phases, the more severe the segregation of C and Ti elements in the original ingot. This indirect characterization method avoids the limitations of existing direct component detection methods in terms of accuracy and resolution in the quantitative characterization of segregation of low-content elements, thus achieving an accurate evaluation of the degree of C and Ti segregation.

[0113] This method is highly applicable and suitable for the comparative analysis of segregation in NS6500 alloy ingots with different shapes, compositions, and smelting processes.

[0114] Example 1

[0115] Using the method described above for characterizing the degree of C and Ti segregation in NS6500 alloy ingots, this embodiment aims to illustrate how to characterize the degree of C and Ti segregation in NS6500 alloy ingots using this method. Specifically, it includes the following steps:

[0116] Step 1, Sample preparation;

[0117] The chemical composition of a batch of 380mm NS6500 alloy ingots prepared by the VIM+ESR dual process is shown in Table 1.

[0118] Table 1. Measured chemical composition (wt%) of test steel 1

[0119]

[0120] Metallographic samples were cut from the NS6500 alloy ingot to be tested, and the central area of ​​the ingot was selected as the sampling location. The dimensions of the metallographic sample were: 20mm long × 20mm wide × 7mm thick.

[0121] Step 2: Heat treatment to induce precipitation;

[0122] A box-type resistance furnace was used to uniformly raise the temperature from room temperature to 830℃ at a heating rate of 22℃ / min. After the furnace temperature stabilized, the metallographic sample was placed in and held at that temperature for 9 minutes to ensure uniform temperature throughout the sample. Once the temperature was uniform, the sample underwent a 16-hour holding aging treatment at 830℃, followed by air cooling. During this process, the C and Ti elements segregated between dendrites in the ingot combine to form TiC precipitates.

[0123] Step 3: Microscopic observation and confirmation of tissue structure:

[0124] A 15mm × 15mm surface of the heat-treated metallographic sample was selected as the observation surface. It was progressively ground using 80-grit, 150-grit, 320-grit, 600-grit, and 1000-grit sandpaper. Each time a new sandpaper was used, the grinding direction was perpendicular to the previous sandpaper to ensure uniform surface grinding. After grinding, the observation surface was polished using a diamond metallographic polishing agent with 3μm diamond microparticles until no obvious scratches were visible on the surface when the sample was observed under a metallographic microscope. Subsequently, the polished sample was placed in a potassium permanganate solution heated to 50℃ in a water bath for 15 seconds to etch, clearly revealing the microstructure morphology and providing clear microstructural contrast for subsequent observation of the TiC precipitate phase.

[0125] The microstructure of the interdendritic region of the heat-treated sample was observed using optical microscopy and scanning electron microscopy, respectively. The corresponding results are as follows: Figure 1a , Figure 1b and Figure 2a and 2b As shown in the figure. The observation results show that after aging treatment, a large number of TiC precipitates were formed in the interdendritic region, which directly verifies the nature of the precipitates induced by heat treatment and provides a reliable basis for segregation evaluation.

[0126] The composition of these precipitated phases was analyzed by EDS energy dispersive spectroscopy, such as Figures 3a to 3c As shown, its main components were confirmed to be Ti and C, thus confirming that the precipitated phase was TiC.

[0127] Step 4: Evaluation of the degree of segregation;

[0128] Three representative non-overlapping photomicrographs were taken using a metallurgical microscope at 100x magnification, such as... Figure 4a As shown, the image was processed using Image-Pro image analysis software: First, the software accurately identified the TiC precipitate phase and distinguished it from the matrix tissue (processing effect as shown). Figure 4b As shown in the image, this ensures effective separation of the TiC precipitate; further, the software is used to calculate the area fraction of the interdendritic TiC precipitate in each image. Figure 4cThe histograms of TiC precipitate area distribution corresponding to three metallographic photographs are used. The average statistical result of these three photographs shows that the TiC precipitate accounts for 7.27% of the total area of ​​the photographs, which is greater than 3%, indicating that the segregation of Ti and C elements in this ingot is relatively significant. This specific area fraction value enables a quantitative characterization of the degree of Ti and C element segregation in the corresponding parts of the NS6500 alloy ingot.

[0129] Comparative Example 1

[0130] This comparative example uses the same as the cast sample in Example 1, the only difference being that the heat treatment to induce precipitation in step 2 was not performed.

[0131] In the aforementioned untreated cast specimens, the microstructure is as follows: Figure 5 As shown, it exhibits a distinct dendritic morphology with no fine TiC precipitates between the dendrites. Its EPMA surface scan (as shown) Figures 6a to 6g Although the results show that C and Ti have a positive segregation trend between dendrites, the spot scan results (as shown in Table 2) fluctuate greatly, making it difficult to accurately quantify the segregation differences.

[0132] Table 2 EPMA spot scan results

[0133]

[0134] The as-cast microstructure of the NS6500 alloy after treatment in Example 1, with the large number of TiC precipitates formed between the dendrites, clearly indicates the significant segregation of C and Ti in the ingot.

[0135] By comparing Example 1 and Comparative Example 1, it is illustrated that the "heat treatment-induced precipitation" method of the present invention has significant advantages in characterizing the segregation degree of C and Ti elements in NS6500 alloy ingots. In Comparative Example 1, without heat treatment-induced precipitation, the microstructure of the as-cast sample showed dendrites with no fine TiC precipitates between the dendrites. EPMA surface scanning showed a positive segregation trend of C and Ti between the dendrites, but the spot scanning results fluctuated greatly, making it difficult to accurately quantify the segregation difference. This reflects the limitations of existing direct detection methods in the quantitative characterization of segregation of low-content elements. In Example 1 of the present invention, after the heat treatment-induced precipitation step of the present invention, a large number of TiC precipitates were formed between the dendrites. Through subsequent microstructure observation and confirmation, as well as segregation degree evaluation, the precipitate situation can be observed intuitively. The area fraction of the TiC precipitates can be accurately counted using Image-Pro image analysis software, achieving quantitative characterization of the segregation degree of Ti and C elements in the corresponding parts of the ingot, clearly indicating that the segregation of Ti and C elements in this ingot is relatively severe.

[0136] This comparison fully demonstrates the effectiveness and accuracy of the method of this invention. It successfully transforms the problem of segregation of low-content C and Ti elements, which is difficult to measure directly and accurately, into a problem of precipitate distribution that can be macroscopically observed and quantified, avoiding the shortcomings of existing methods and providing reliable technical support for the quality control and performance evaluation of NS6500 alloy ingots.

[0137] Example 2

[0138] Using the method described above for characterizing the degree of C and Ti segregation in NS6500 alloy ingots, this embodiment aims to illustrate how to characterize the degree of C and Ti segregation in NS6500 alloy ingots using this method. Specifically, it includes the following steps:

[0139] Step 1, Sample preparation;

[0140] The chemical composition of a batch of 205mm NS6500 alloy ingots prepared by the VIM+ESR dual process is shown in Table 3.

[0141] Table 3. Measured chemical composition (wt%) of test steel 2

[0142]

[0143] Metallographic specimens were cut from the NS6500 alloy ingot to be tested, with the central area of ​​the ingot selected as the sampling location. The dimensions of the metallographic specimens were: 15mm long × 15mm wide × 6mm thick.

[0144] Step 2: Heat treatment to induce precipitation;

[0145] A box-type resistance furnace was used as the heat treatment equipment, and the temperature was uniformly increased from room temperature to 870℃ at a heating rate of 23℃ / min. After the furnace temperature stabilized, the metallographic sample was placed in and held at that temperature for 10 minutes to ensure uniform temperature throughout the sample. Once the temperature was uniform, the sample was held at 870℃ for 20 hours to provide sufficient diffusion and reaction conditions for the Ti and C elements segregated between dendrites, promoting their combination to form a stable TiC-dominant precipitate. After the holding period, the metallographic sample was removed and cooled using air cooling or water cooling. During this process, the C and Ti elements segregated between dendrites in the ingot will combine to form the TiC precipitate.

[0146] Step 3: Microscopic observation and confirmation of tissue structure:

[0147] A 15mm × 15mm surface of the heat-treated metallographic sample was selected as the observation surface. It was progressively polished using 80-grit, 150-grit, 320-grit, 600-grit, and 1000-grit sandpaper. Each time a new sandpaper was used, the polishing direction was perpendicular to the previous sandpaper's direction to ensure uniform surface polishing. After polishing, the observation surface was polished with a diamond metallographic polishing agent with 3μm diamond microparticles until no obvious scratches were visible when the sample was observed under a metallographic microscope. The polished sample was then etched in a potassium permanganate solution heated to 50℃ for 25 seconds, clearly revealing the microstructure morphology and providing clear microstructural contrast for subsequent observation of the TiC precipitate phase.

[0148] Using an optical microscope (e.g.) Figure 7a and Figure 7b (as shown) and scanning electron microscope (e.g.) Figure 8a and Figure 8b The microstructure of the interdendritic region of the heat-treated sample was observed (as shown). The observation results show that TiC precipitates successfully formed in the interdendritic region of the sample after aging treatment, but the amount of TiC precipitates in this example was less than that in the NS6500 alloy of Example 1. Analysis of the experimental conditions suggests that this phenomenon is mainly due to two factors: firstly, the C content in Example 2 was lower than that in Example 1; secondly, the ingot shape of Example 2 was smaller than that of Example 1, resulting in a less severe overall segregation during solidification, and a less severe segregation of Ti and C elements in the interdendritic region.

[0149] Through EDS energy dispersive spectroscopy analysis (e.g.) Figures 9a to 9c As shown, compositional analysis of these precipitates confirmed that their main components were Ti and C, thus confirming that the precipitates were TiC.

[0150] Step 4: Evaluation of the degree of segregation;

[0151] Three representative non-overlapping photomicrographs were taken using a metallurgical microscope at 100x magnification, such as... Figure 10a As shown, the image was processed using Image-Pro image analysis software: First, the software accurately identified the TiC precipitate phase and distinguished it from the matrix tissue (processing effect as shown). Figure 10b As shown in the image, this ensures effective separation of the TiC precipitate; further, the software is used to calculate the area fraction of the interdendritic TiC precipitate in each image. Figure 10c The histograms show the area distribution of TiC precipitates corresponding to three metallographic images. The average of the statistical results from the three images shows that the TiC precipitate accounts for 2.73% of the total area of ​​the images, falling between 0.5% and 3%, indicating slight segregation.

[0152] Comparative Example 2

[0153] This comparative example uses the same as the cast sample in Example 2, the only difference being that the heat treatment to induce precipitation in step 2 was not performed.

[0154] In the untreated as-cast specimens of this comparative example, the microstructure is as follows: Figure 11 As shown, the sample exhibits a distinct dendritic morphology with no fine TiC precipitates between the dendrites. However, after the heat treatment induced by this invention, a smaller number of TiC precipitates formed between the dendrites, as shown in Example 2, were obtained. The TiC precipitate accounted for 2.73% of the total area of ​​the photograph, falling between 0.5% and 3%, which is considered slight segregation. This indicates slight segregation of Ti and C elements in the as-cast sample.

[0155] A comparison of Example 2 and Comparative Example 2 demonstrates the advantages of the "heat treatment-induced precipitation" method of this invention. In Comparative Example 2, without heat treatment-induced precipitation, the as-cast sample exhibited a dendritic morphology, with no fine TiC precipitates between the dendrites, making it difficult to visually determine the segregation of Ti and C elements. However, in Example 2, after the specific heat treatment-induced precipitation step of this invention, TiC precipitates successfully formed between the dendrites. Through subsequent microstructural observation and confirmation, as well as segregation degree evaluation, not only can the presence of the TiC precipitates be visually observed, but the area fraction of the TiC precipitates can also be accurately calculated using professional Image-Pro image analysis software. Based on the area fraction, it can be accurately determined that the Ti and C elements in the ingot are in a state of slight segregation.

[0156] This further illustrates that the method of the present invention can effectively solve the problem of quantitative characterization of low-content element segregation using existing direct detection methods. It transforms the problem of Ti and C element segregation, which is difficult to directly observe and quantify, into a problem that can be accurately judged by macroscopic observation of the precipitate distribution and quantification of the area fraction. This transformation provides a more reliable and effective method for quality control and performance evaluation of NS6500 alloy ingots.

[0157] Example 3

[0158] Using the method described above for characterizing the degree of C and Ti segregation in NS6500 alloy ingots, this embodiment aims to illustrate how to characterize the degree of C and Ti segregation in NS6500 alloy ingots using this method. Specifically, it includes the following steps:

[0159] Step 1, Sample preparation;

[0160] The chemical composition of a batch of 890mm NS6500 alloy ingots prepared by the VIM+ESR dual process is shown in Table 4.

[0161] Table 4. Measured chemical composition (wt%) of test steel 3

[0162]

[0163] Metallographic specimens were cut from the NS6500 alloy ingot to be tested, with the central area of ​​the ingot selected as the sampling location. The dimensions of the metallographic specimens were: 20mm long × 20mm wide × 6mm thick.

[0164] Step 2: Heat treatment to induce precipitation;

[0165] A box-type resistance furnace was used as the heat treatment equipment, and the temperature was uniformly increased from room temperature to 890℃ at a heating rate of 28℃ / min. After the furnace temperature stabilized, the metallographic sample was placed in and held at that temperature for 10 minutes to ensure uniform temperature throughout the sample. Once the temperature was uniform, the sample was held at 890℃ for 22 hours to provide sufficient diffusion and reaction conditions for the Ti and C elements segregated between dendrites, promoting their combination to form a stable TiC-dominant precipitate. After the holding period, the metallographic sample was removed and cooled using air cooling or water cooling. During this process, the C and Ti elements segregated between dendrites in the ingot will combine to form the TiC precipitate.

[0166] Step 3: Microscopic observation and confirmation of tissue structure:

[0167] The 18mm × 18mm surface of the heat-treated metallographic sample was selected as the observation surface. It was progressively polished using 80-grit, 150-grit, 320-grit, 600-grit, and 1000-grit sandpaper. Each time a new sandpaper was used, the polishing direction was perpendicular to the previous sandpaper's direction to ensure uniform surface polishing. After polishing, the observation surface was polished with a diamond metallographic polishing agent with 3μm diamond microparticles until no obvious scratches were visible when the sample was observed under a metallographic microscope. The polished sample was then etched in a potassium permanganate solution heated to 50℃ for 28 seconds, clearly revealing the microstructure morphology and providing clear microstructural contrast for subsequent observation of the TiC precipitate.

[0168] Using optical microscopes (e.g.) Figure 12a and Figure 12b (as shown) and scanning electron microscope (e.g.) Figure 13a and Figure 13b As shown in the figure, the microstructure of the interdendritic region of the heat-treated sample was observed. The observation results show that no TiC precipitate phase was found in the interdendritic region of the sample after the above aging treatment.

[0169] Step 4: Evaluation of the degree of segregation;

[0170] After software processing and manual verification, the micrographs of the metallographic specimens in this embodiment (such as...) Figure 14a No TiC precipitates were detected in the dendrites (as shown in the image). Figure 14bAs shown in the figure, there is no need to perform area fraction statistics. Therefore, after heat treatment to induce precipitation, the area fraction of the TiC precipitate in this ingot is between 0-0.5%, which indicates that there is no obvious segregation.

[0171] Comparative Example 3

[0172] This comparative example uses the same as the cast sample in Example 3, the only difference being that the heat treatment to induce precipitation in step 2 was not performed.

[0173] In the aforementioned untreated cast specimens, the microstructure is as follows: Figures 15a to 15d As shown, it exhibits a distinct dendritic morphology with no fine TiC precipitates between the dendrites. Its EPMA surface scan (as shown) Figures 16a to 16g The segregation trends of C and Ti are not obvious, indicating that the segregation degree of the ingot is not serious.

[0174] In summary, by comparing the quantity, size, and distribution of precipitated phases in different samples, this invention can qualitatively and quantitatively evaluate the degree of segregation of C and Ti elements, thereby overcoming the limitations of existing direct detection methods in characterizing segregation of low-content elements and verifying the effectiveness of this method.

[0175] It should be reiterated that this method is highly applicable and can be used for comparative analysis of segregation in NS6500 alloy ingots with different shapes, compositions, and smelting processes. This helps to better grasp the quality status of ingots in actual production, optimize production processes, and improve product quality and stability.

[0176] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots, characterized in that, Includes the following steps: Step 1, Sample preparation; Metallographic samples were cut from the NS6500 alloy ingot to be tested; Step 2: Heat treat the metallographic sample; The metallographic sample is heat-treated using heat treatment equipment, and then removed and cooled. Step 3: Microscopic tissue observation; The heat-treated metallographic specimens were successively ground and polished; after polishing, the microstructure of the interdendritic region of the heat-treated metallographic specimens was observed using an optical microscope or a scanning electron microscope to obtain information on the quantity, size and distribution of precipitates. Step 4: Evaluation of the degree of segregation; Based on the quantity, size, and distribution of interdendritic precipitates, the degree of segregation of C and Ti elements in the ingot can be qualitatively or quantitatively evaluated; the more numerous and denser the precipitates, the more severe the segregation of C and Ti.

2. The method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots according to claim 1, characterized in that, In step 1, the central area of ​​the ingot is selected as the sampling location.

3. The method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots according to claim 2, characterized in that, In step 1, if it is impossible to take a sample from the central region of the ingot due to experimental constraints, then a sample is taken from any region of the ingot. In this case, the characterization results will reflect the local segregation of the sampled area.

4. The method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots according to claim 3, characterized in that, In step 1, the dimensions of the metallographic sample are: 10-20 mm in length × 10-20 mm in width × 5-7 mm in thickness.

5. The method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots according to claim 1, characterized in that, In step 3, after microscopic observation, energy dispersive spectroscopy (EDS) is performed on the precipitated phase to confirm that the precipitated phase is TiC.

6. The method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots according to claim 1, characterized in that, In step 4, the degree of segregation of the ingot is evaluated using a metallographic microscope, and a photomicrograph is taken at 100x magnification.

7. The method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots according to claim 6, characterized in that, In step 4, at least three non-overlapping representative fields of view are selected, and the area fraction of TiC precipitates between dendrites is statistically analyzed using Image-Pro image analysis software.

8. The method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots according to claim 7, characterized in that, In step 4, the process of using Image-Pro image analysis software to calculate the area fraction of TiC precipitates between dendrites is as follows: by setting the grayscale threshold of the precipitates and the identification parameters of the morphological features, the TiC phase and the matrix are automatically segmented. Then, the ratio of the total area of ​​the TiC phase in each field of view to the total area of ​​the field of view is calculated, or its percentage is calculated. Finally, the average value of the results of each field of view is taken as the final area fraction.

9. The method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots according to claim 8, characterized in that, In step 4, the quantitative evaluation criteria for the degree of segregation are as follows: when the area fraction of TiC precipitates is 0-0.5%, it is determined that there is no obvious segregation; when the area fraction of TiC precipitates is 0.5-3%, it is determined that there is slight segregation; when the area fraction of TiC precipitates is greater than 3%, it is determined that there is significant segregation, thereby achieving quantitative characterization of the degree of segregation of Ti and C elements in NS6500 alloy ingots.

10. The method for characterizing the degree of C and Ti segregation in NS6500 alloy ingots according to any one of claims 1 to 9, characterized in that, In step 4, the qualitative evaluation criterion for the degree of segregation is: if TiC precipitates are observed in the microstructure, it can be determined that Ti and C elements segregate in the corresponding part of the ingot.

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